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EO12866_PM NAAQS_2060-AO47_FRN 

Preamble_and_Reg_Text_20121120

6560-50-P

From: Lydia Wegman/RTP/USEPA/US

To: "Frey, Nathan J." <Nathan_J._Frey@omb.eop.gov>, "Mancini, Dominic J." 

<Dominic_J._Mancini@omb.eop.gov>

Cc: Janet McCabe/DC/USEPA/US@EPA, Gina McCarthy/DC/USEPA/US@EPA, Joseph-J 

Dougherty/DC/USEPA/US@EPA, Karen Martin/RTP/USEPA/US@EPA, Beth HassettSipple/RTP/USEPA/US@EPA, John Hannon/DC/USEPA/US@EPA, Erika 

Sasser/RTP/USEPA/US@EPA

Date: 12/14/2012 11:43 AM

Subject: Final compare version of PM NAAQS NFR

Nathan and Dom, 

Here is the redline version of the PM NAAQS rule, compared to the version that was sent to you 

originally. Please let me know if you have any questions. Also, please let me know if you want a 

clean version as well. Thanks.
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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 51, 52, 53 and 58

[EPA-HQ-OAR-2007-0492; FRL-XXXX-X]

RIN 2060-AO47

National Ambient Air Quality Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

SUMMARY: Based on its review of the air quality criteria and the national ambient air 

quality standards (NAAQS) for particulate matter (PM), the EPA is making revisions to 

the suite of standards for PM to provide requisite protection of public health and welfare 

and to make corresponding revisions to the data handling conventions for PM and to the

ambient air monitoring, reporting, and network design requirements. The EPA also is 

making revisions to the prevention of significant deterioration (PSD) permitting program 

with respect to the NAAQS revisions.

With regard to primary (health-based) standards for fine particles (generally 

referring to particles less than or equal to 2.5 micrometers (μm) in diameter, PM2.5), the 

EPA is revising the annual PM2.5 standard by lowering the level to 12.0 micrograms per 

cubic meter (μg/m3) so as to provide increased protection against health effects 

associated with long- and short-term exposures (including premature mortality, increased 

hospital admissions and emergency department visits, and development of chronic 

respiratory disease), and to retain the 24-hour PM2.5 standard at a level of 35 μg/m3. The 

EPA is revising the Air Quality Index (AQI) for PM2.5 to be consistent with the revised 

primary PM2.5 standards. With regard to the primary standard for particles generally less 

than or equal to 10 μm in diameter (PM10), the EPA is retaining the current 24-hour PM10

standard to continue to provide protection against effects associated with short-term 

exposure to thoracic coarse particles (i.e., PM10-2.5). With regard to the secondary
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(welfare-based) PM standards, the EPA is generally retaining the current suite of 

secondary standards (i.e., 24-hour and annual PM2.5 standards and a 24-hour PM10

standard). Non-visibility welfare effects are addressed by this suite of secondary 

standards, and PM-related visibility impairment is addressed by the secondary 24-hour 

PM2.5 standard.

DATES: The final rule is effective on [INSERT DATE 60 DAYS AFTER DATE OF 

PUBLICATION IN THE FEDERAL REGISTER].

ADDRESSESADDRESSES: Section X.B requests comments on an information 

collection request regarding changes to the monitoring requirements. Submit your 

comments, identified by Docket ID No. EPA-HQ-OAR-2007-0492, to the EPA by one of 

the following methods:

www.regulations.gov: Follow the on-line instructions for submitting comments.

Email: a-and-r-Docket@epa.gov

Fax: 202-566-9744 

Mail: Docket No. EPA-HQ-OAR-2007-0492, Environmental Protection Agency, 

Mail code 6102T, 1200 Pennsylvania Ave., NW, Washington, DC 20460. Please 

include a total of two copies. 

Hand Delivery: Docket No. EPA-HQ-OAR-2007-0492, Environmental Protection 

Agency, EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, DC. 

Such deliveries are only accepted during the Docket’s normal hours of operation, 

and special arrangements should be made for deliveries of boxed information.

Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-2007-0492. 

The EPA's policy is that all comments received will be included in the public docket 

Formatted: No underline
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without change and may be made available online at www.regulations.gov, including any 

personal information provided, unless the comment includes information claimed to be 

Confidential Business Information (CBI) or other information whose disclosure is 

restricted by statute. Do not submit information that you consider to be CBI or otherwise 

protected through www.regulations.gov or e-mail. The www.regulations.gov website is 

an “anonymous access” system, which means the EPA will not know your identity or 

contact information unless you provide it in the body of your comment. If you send an email comment directly to the EPA without going through www.regulations.gov your email address will be automatically captured and included as part of the comment that is 

placed in the public docket and made available on the Internet. If you submit an 

electronic comment, the EPA recommends that you include your name and other contact 

information in the body of your comment and with any disk or CD-ROM you submit. If 

the EPA cannot read your comment due to technical difficulties and cannot contact you 

for clarification, the EPA may not be able to consider your comment. Electronic files 

should avoid the use of special characters, any form of encryption, and be free of any 

defects or viruses. For additional information about EPA’s public docket visit the EPA 

Docket Center homepage at http://www.epa.gov/epahome/dockets.htm. Comments on 

this information collection request should also be sent to the Office of Management and 

Budget (OMB). See section X.B below for additional information regarding submitting 

comments to OMB.

Docket: The EPA has established a docket for this action under Docket No. EPAHQ-OAR-2007-0492. All documents in the docket are listed on the www.regulations.gov

website. This includes documents in the rulemaking docket (Docket ID No. EPA-HQFormatted: Indent: First line: 0.5"

Formatted: Font: Not Bold, Italic
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OAR-2007-0492) and a separate docket, established for 2009 Integrated Science 

Assessment (Docket No. EPA-HQ-ORD-2007-0517), that has have been incorporated by 

reference into the rulemaking docket. All documents in these dockets are listed on the 

www.regulations.gov website. Although listed in the index, some information is not 

publicly available, e.g., CBI or other information whose disclosure is restricted by statute. 

Certain other material, such as copyrighted material, is not placed on the Internet and

may be viewed, with prior arrangement, at the EPA Docket Center. Publicly available 

docket materials are available either electronically in www.regulations.gov or in hard 

copy at the Air and Radiation Docket and Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., NW, Washington, DC. The Public Reading Room is 

open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. The 

telephone number for the Public Reading Room is (202) 566-1744 and the telephone 

number for the Air and Radiation Docket and Information Center is (202) 566-1742. For 

additional information about EPA’s public docket visit the EPA Docket Center homepage 

at: http://www.epa.gov/epahome/dockets.htm.

FOR FURTHER INFORMATION CONTACT: Ms. Beth M. Hassett-Sipple, Health 

and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. 

Environmental Protection Agency, Mail code C504-06, Research Triangle Park, NC 

27711; telephone: (919) 541-4605; fax: (919) 541-0237; email:

hassett-sipple.beth@epa.gov.

SUPPLEMENTARY INFORMATION: 

General Information

Availability of Related Information
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A number of the documents that are relevant to this rulemaking are available 

through the EPA’s Office of Air Quality Planning and Standards (OAQPS) Technology 

Transfer Network (TTN) website at 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html. These documents include 

the Plan for Review of the National Ambient Air Quality Standards for Particulate Matter

(U.S. EPA, 2008a), available at

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html, the Integrated Science 

Assessment for Particulate Matter (U.S. EPA, 2009a), available at 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_isa.html, the Quantitative 

Health Risk Assessment for Particulate Matter (U.S. EPA, 2010a), available at 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html, the Particulate Matter 

Urban-Focused Visibility Assessment (U.S. EPA 2010b), available at 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html, and the Policy 

Assessment for the Review of the Particulate Matter National Ambient Air Quality 

Standards (U.S. EPA, 2011a), available at

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. These and other related 

documents are also available for inspection and copying in the EPA docket identified 

above. 

Table of Contents 

The following topics are discussed in this preamble:

I. Executive Summary

A. Purpose of this Regulatory Action

B. Summary of Major Provisions

C. Costs and Benefits

II. Background 

A. Legislative Requirements

B. Review of the Air Quality Criteria and Standards for PM
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1. Previous PM NAAQS Reviews

2. Litigation Related to the 2006 PM Standards

3. Current PM NAAQS Review

C. Related Control Programs to Implement PM Standards

D. Summary of Proposed Revisions to the PM NAAQS

E. Organization and Approach to Final PM NAAQS Decisions

III. Rationale for Final Decisions on the Primary PM2.5 Standards

A. Background

1. General Approach Used in Previous Reviews

2. Remand of Primary Annual PM2.5 Standard

3. General Approach Used in the Policy Assessment for the Current 

Review

B. Overview of Health Effects Evidence

C. Overview of Quantitative Characterization of Health Risks 

D. Conclusions on the Adequacy of the Current Primary PM2.5 Standards

1. Introduction

a. Evidence- and Risk-based Considerations in the Policy 

Assessment

b. CASAC Advice

c. Administrator’s Proposed Conclusions Concerning the 

Adequacy of the Current Primary PM2.5 Standards

2. Comments on the Need for Revision

3. Administrator’s Final Conclusions Concerning the Adequacy of 

the Current Primary PM2.5 Standards

E. Conclusions on the Elements of the Primary Fine Particle Standards 

1. Indicator

2. Averaging Time

3. Form

a. Annual Standard

b. 24-Hour Standard

4. Level

a. General Approach for Considering Standard Levels

b. Proposed Decisions on Level

i. Consideration of Alternative Standard Levels in the 

Policy Assessment

ii. CASAC Advice

iii. Administrator’s Proposed Decisions on the Primary 

PM2.5 Standard Levels

c. Comments on Standard Levels

i. Annual Standard Level

ii. 24-Hour Standard Level

d. Administrator’s Final Conclusions on the Primary PM2.5

Standard Levels

F. Administrator’s Final Decisions on the Primary PM2.5 Standards

IV. Rationale for Final Decision on Primary PM10 Standard 
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A. Background 

1. Previous Reviews of the PM NAAQS

a. Reviews Completed in 1987 and 1997

b. Review Completed in 2006

2. Litigation Related to the 2006 Primary PM10 Standards

3. General Approach Used in the Current Review

B. Health Effects Related to Exposure to Thoracic Coarse Particles

C. Consideration of the Current and Potential Alternative Standards in the 

Policy Assessment

1. Consideration of the Current Standard in the Policy Assessment

2. Consideration of Potential Alternative Standards in the Policy 

Assessment

D. CASAC Advice

E. Administrator’s Proposed Conclusions Concerning the Adequacy of the 

Current Primary PM10 Standard

F. Public Comments on the Administrator’s Proposed Decision to Retain the 

Primary PM10 Standard

G. Administrator’s Final Decision on the Primary PM10 Standard

V. Communication of Public Health Information 

VI. Rationale for Final Decisions on the Secondary PM Standards 

A. Background

1. Approaches Used in Previous Reviews

2. Remand of 2006 Secondary PM2.5 Standards

3. General Approach Used in the Policy Assessment for the Current 

Review

B. Proposed Decisions on Secondary PM Standards

1. PM-related Visibility Impairment

a. Nature of PM-related Visibility Impairment

i. Relationship Between Ambient PM and Visibility

ii. Temporal Variations of Light Extinction

iii. Periods During the Day of Interest for Assessment 

of Visibility

iv. Exposure Durations of Interest

v. Periods of Fog and Rain

b. Public Perception of Visibility Impairment

c. Summary of Proposed Conclusions

i. Adequacy

ii. Indicator

iii. Averaging Time

iv. Form

v. Level

vi. Administrator’s Proposed Conclusions

vii. Related Technical Analysis

2. Other (Non-Visibility) PM-related Welfare Effects

a. Evidence of Other Welfare Effects Related to PM
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b. CASAC Advice

c. Summary of Proposed Decisions Regarding Other Welfare 

Effects

C. Comments on Proposed Rule

1. Comments on Proposed Secondary PM Standard for Visibility 

Protection

a. Overview of Comments 

b. Indicator

i. Comments on Calculated vs. Directly Measured 

Light Extinction

ii. Comments on Specific Aspects of Calculated Light 

Extinction Indicator 

c. Averaging Time

d. Form

e. Level

i. Comments on Visibility Preference Studies

ii. Specific Comments on Level

f. Need for a Distinct Secondary Standard

g. Legal Issues

h. Relationship with Regional Haze Program

2. Comments on the Proposed Decision Regarding Non-Visibility

Welfare Effects

D. Conclusions on Secondary PM Standards 

1. Conclusions Regarding Secondary PM Standards to Address NonVisibility Welfare Effects

2. Conclusions Regarding Secondary PM Standards for Visibility 

Protection

E. Administrator’s Final Decisions on Secondary PM Standards

VII. Interpretation of the NAAQS for PM 

A. Amendments to Appendix N: Interpretation of the NAAQS for PM2.5

1. General

2. Monitoring Considerations

3. Requirements for Data Use and Reporting for Comparison with the 

NAAQS for PM2.5

4. Comparisons with the PM2.5 NAAQS

B. Exceptional Events

C. Updates for Data Handling Procedures for Reporting the Air Quality 

Index

VIII. Amendments to Ambient Monitoring and Reporting Requirements 

A. Issues Related to 40 CFR Part 53 (Reference and Equivalent Methods)

1. PM2.5 and PM10-2.5 Federal Equivalent Methods

2. Use of Chemical Speciation Network (CSN) Methods to Support

the Proposed New Secondary PM2.5 Visibility Index NAAQS 

B. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)
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1. Terminology Changes

2. Special Considerations for Comparability of PM2.5 Ambient Air 

Monitoring Data to the NAAQS

a. Revoking Use of Population-Oriented as a Condition for 

Comparability of PM2.5 Monitoring Sites to the NAAQS

b. Applicability of Micro- and Middle-scale Monitoring Sites 

to the Annual PM2.5 NAAQS

3. Changes to Monitoring for the National Ambient Air Monitoring 

System

a. Background

b. Primary PM2.5 NAAQS

i. Addition of a Near-road Component to the PM2.5

Monitoring Network

ii. Use of PM2.5 Continuous FEMs at SLAMS

c. Revoking PM10-2.5 Speciation Requirements at NCore Sites

d. Measurements for the Proposed New PM2.5 Visibility Index

NAAQS

4. Revisions to the Quality Assurance Requirements for SLAMS, 

SPMs, and PSD

a. Quality Assurance Weight of Evidence

b. Quality Assurance Requirements for the Chemical 

Speciation Network

c. Waivers for Maximum Allowable Separation of Collocated 

PM2.5 Samplers and Monitors

5. Revisions to Probe and Monitoring Path Siting Criteria

a. Near-road Component to the PM2.5 Monitoring Network

b. CSN Network

c. Reinsertion of Table E-1 to Appendix E

6. Additional Ambient Air Monitoring Topics

a. Annual Monitoring Network Plans and Periodic 

Assessment

b. Operating Schedules

c. Data Reporting and Certification for CSN and IMPROVE 

Data

d. Requirements for Archiving Filters

IX. Clean Air Act Implementation Requirements for the PM NAAQS

A. Designation of Areas

1. Overview of Clean Air Act Designations Requirements

2. Proposed Designations Schedules

3. Comments and Responses

4. Final Intended Designations Schedules

B. Section 110(a)(2) Infrastructure SIP Requirements

C. Implementing the Revised Primary Annual PM2.5 NAAQS in 

Nonattainment Areas

D. Prevention of Significant Deterioration and Nonattainment New Source 

Review Programs for the Revised Primary Annual PM2.5 NAAQS
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1. Prevention of Significant Deterioration

a. Transition Provision (Grandfathering)

i. Proposal

ii. Comments and Responses

iii. Final Action

b. Modeling Tools and Guidance Applicable to the Revised 

Primary Annual PM2.5 NAAQS

c. PSD Screening Tools: Significant Emissions Rates, 

Significant Impact Levels, and Significant 

ModelingMonitoring Concentration

d. PSD Increments

e. Other PSD Transition Issues

2. Nonattainment New Source Review

E. Transportation Conformity Program

F. General Conformity Program

X. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory Planning and Review and Executive 

Order 13563: Improving Regulation and Regulatory Review

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination with Indian 

Tribal Governments

G. Executive Order 13045: Protection of Children From Environmental 

Health and Safety Risks

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, 

Distribution, or Use

I. National Technology Transfer and Advancement Act

J. Executive Order 12898: Federal Actions to Address Environmental 

Justice in Minority Populations and Low-Income Populations

K. Congressional Review Act

References 

I. Executive Summary

A. Purpose of this Regulatory Action

Sections 108 and 109 of the Clean Air Act (CAA) govern the establishment, 

review, and revision, as appropriate, of the national ambient air quality standards 

(NAAQS) to protect public health and welfare. The CAA requires periodic review of the 

air quality criteria – the science upon which the standards are based – and the standards 
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themselves. This rulemaking is being done pursuant to these statutory requirements. The 

schedule for completing this review is established by a court order.

In 2006, the EPA completed its last review of the PM NAAQS. In that review, the 

EPA took three principal actions: (1) With regard to fine particles (generally referring to 

particles less than or equal to 2.5 micrometers (μm) in diameter, PM2.5), at that time, the 

EPA revised the level of the primary 24-hour PM2.5 standard from 65 to 35 μg/m3and 

retained the level of the primary annual PM2.5 standard.; (2) With regard to the primary 

standards for particles less than or equal to 10 μm in diameter (PM10), the EPA retained 

the primary 24-hour PM10 standard to continue to provide protection against effects 

associated with short-term exposure to thoracic coarse particles (i.e., PM10-2.5) and 

revoked the primary annual PM10 standard; and (3) the EPA also revised the secondary 

standards to be identical in all respects to the primary standards.

In subsequent litigation, the U.S. Court of Appeals for the District of Columbia 

Circuit remanded the primary annual PM2.5 standard to the EPA because the Agency had 

failed to explain adequately why the standard provided the requisite protection from both 

short- and long-term exposures to fine particles, including protection for at-risk 

populations such as children. The court remanded the secondary PM2.5 standards to the 

EPA because the Agency failed to explain adequately why setting the secondary 

standards identical to the primary standards provided the required protection for public 

welfare, including protection from PM-related visibility impairment. 

The EPA initiated this review in June 2007. Between 2007 and 2011, the EPA 

prepared draft and final Integrated Science Assessments, Risk and Exposure 

Assessments, and Policy Assessments. Multiple drafts of all of these documents were 
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subject to review by the public and were peer reviewed by the EPA’s Clean Air Scientific 

Advisory Committee (CASAC). The EPA proposed revisions to the primary and 

secondary PM NAAQS on June 29, 2012 (77 FR 38890). This final rulemaking is the 

final step in the review process.

In this rulemaking, the EPA is revising the suite of standards for PM to provide 

requisite protection of public health and welfare. The EPA is revising the PSD permitting 

regulations to address the changes in the PM NAAQS. In addition, the EPA is updating

the Air Quality Index (AQI) for PM2.5 and making changes in the data handling 

conventions for PM and ambient air monitoring, reporting, and network design 

requirements to correspond with the changes to the PM NAAQS.

B. Summary of Major Provisions

With regard to the primary standards for fine particles, the EPA is revising the 

annual PM2.5 standard by lowering the level from 15.0 to 12.0 μg/m3so as to provide 

increased protection against health effects associated with long-and short-term exposures.

The EPA is retaining the level (35 μg/m3) and the form (98th percentile) of the 24-hour 

PM2.5 standard to continue to provide supplemental protection against health effects 

associated with short-term exposures. This action provides increased protection for 

children, older adults, persons with pre-existing heart and lung disease, and other at-risk 

populations against an array of PM2.5-related adverse health effects that include 

premature mortality, increased hospital admissions and emergency department visits, and 

development of chronic respiratory disease. The EPA also is eliminating spatial 

averaging provisions as part of the form of the annual standard to avoid potential 

disproportionate impacts on at-risk populations.
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The final decisions for the primary annual and 24-hour PM2.5 standards are within 

the ranges that CASAC advised the Agency to consider. These decisions are based on an 

integrative assessment of an extensive body of new scientific evidence, which 

substantially strengthens what was known about PM2.5-related health effects in the last 

review, including extended analyses of key epidemiological studies, and evidence of 

health effects observed at lower ambient PM2.5 concentrations, including effects in areas 

that likely met the current standards. The revised suite of PM2.5 standards also reflects

consideration of a quantitative risk assessment that estimates public health risks likely to 

remain upon just meeting the current and various alternative standards. Based on this 

information, the Administrator concludes that the current primary PM2.5 standards are not 

requisite to protect public health with an adequate margin of safety, as required by the 

CAA, and that these revisions are warranted to provide the appropriate degree of 

increased public health protection.

With regard to the primary standard for thoracic coarse particles (PM10-2.5,),), the 

EPA is retaining the current 24-hour PM10 standard, with a level of 150 μg/m3and a oneexpected exceedance form, to continue to provide protection against effects associated 

with short-term exposure to PM10-2.5 including premature mortality and increased hospital 

admissions and emergency department visits. In reaching this decision, the Administrator 

concludes that the available health evidence and air quality information for PM10-2.5, 

taken together with the considerable uncertainties and limitations associated with that 

information, suggests that a standard is needed to protect against short-term exposure to 

all types of PM10-2.5 and that the degree of public health protection provided against short-
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term exposures to PM10-2.5 does not need to be increased beyond that provided by the 

current PM10 standard. 

With regard to the secondary PM standards, the Administrator is retaining the 

current suite of secondary PM standards, except for a change to the form of the annual 

PM2.5 standard. Specifically, the EPA is retaining the current secondary 24-hour PM2.5

and PM10 standards to address PM-related non-visibility welfare effects including 

ecological effects, effects on materials, and climate impacts, and is revising only the form 

of the secondary annual PM2.5 standard to remove the option for spatial averaging 

consistent with this change to the primary annual PM2.5 standard. This suite of secondary 

standards addresses PM-related non-visibility welfare effects including ecological effects, 

effects on materials, and climate impacts. With respect to PM-related visibility 

impairment, the Administrator has identified a target degree of protection, defined in 

terms of a PM2.5 visibility index (based on speciated PM2.5 mass concentrations and 

relative humidity data to calculate PM2.5 light extinction), a 24-hour averaging time, and 

a 90th percentile form, averaged over 3 years, and a level of 30 deciviews (dv), which she 

judges to be requisite to protect public welfare with regard to visual air quality. (VAQ).

The EPA’s analysis of monitoring data provides the basis for concluding that the current 

secondary 24-hour PM2.5 standard would provide sufficient protection, and in some areas 

greater protection, relative to this target protection level. Adding a distinct secondary 

standard to address visibility would not affect this protection. Since sufficient protection 

from visibility impairment will be provided for all areas of the country without adoption 

of a distinct secondary standard, and adoption of a distinct secondary standard will not 

change the degree of over-protection of visual air qualityVAQ provided for some areas of 
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the country by the secondary 24-hour PM2.5 standard, the Administrator judges that 

adoption of a distinct secondary standard, in addition to the current suite of secondary 

standards, is not needed to provide requisite protection for both visibility and nonvisibility related welfare effects.

The revisions to the PM NAAQS triggerstrigger a process under which states (and 

tribes, if they choose) will make recommendations to the Administrator regarding 

designations, identifying areas of the country that either meet or do not meet the revised 

NAAQS. States will also review, modify and supplement their existing state 

implementation plans, (SIPs), as needed. With regard to these implementation-related 

activities, the EPA intends to promulgate a separate implementation rule on a schedule 

that provides timely clarity to the states, tribes, and other parties responsible for NAAQS 

implementation. The NAAQS revisions also affect the applicable air permitting 

requirement, but cause no significant change to the transportation conformity and general 

conformity processes. The EPA is revising its PSD regulations to provide limited 

grandfathering from the requirements that result from the revised PM NAAQS for permit 

applications for which the reviewing authority has first published public notice that a 

draft permit or preliminary determination for the permit has been issued prior to the 

effective date of the revised PM NAAQS.

On other topics, the EPA is changing the Air Quality Index (AQI) for PM2.5 to be 

consistent with the revised primary PM2.5 NAAQS. The EPA also is revising the data 

handling procedures for PM2.5 consistent with the revised PM2.5 NAAQS including the 

computations necessary for determining when the standards are met and the measurement 

data that are appropriate for comparison to the standards. With regard to monitoring-
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related activities, the EPA is updating several aspects of the monitoring regulations and 

specifically requiring that a small number of PM2.5 monitors be relocated to be collocated 

with measurements of other pollutants (e.g., nitrogen dioxide, carbon monoxide) in the 

near-road environment.

C. Costs and Benefits

In setting the NAAQS, the EPA may not consider the costs of implementing the 

standards. This was confirmed by the United States Supreme Court in Whitman v. 

American Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001), as

discussednoted in section II.A of this notice. As has traditionally been done in NAAQS 

rulemaking, the EPA has conducted a Regulatory Impact Analysis (RIA) to provide the 

public with information on the potential costs and benefits of attaining several alternative 

PM2.5 standards. In NAAQS rulemaking, the RIA is done for informational purposes 

only, and the final decisions on the NAAQS in this rulemaking are not in any way based 

on consideration of the information or analyses in the RIA. The RIA fulfills the 

requirements of Executive Orders 13563 and 12866. The summary of the RIA, which is

discussed in more detail below in section X.A, estimates benefits ranging from $4,000 

million to $9.100 million at a 3 percent discount rate and $3,600 million to $8,200

million at a 7 percent discount rate in 2020 and costs ranging from $53 million to $350

million per year at a 7 percent discount rate.

II. Background

A. Legislative Requirements

Two sections of the CAA govern the establishment, review and revision of the 

NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to identify and list 
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certain air pollutants and then to issue air quality criteria for those pollutants. The 

Administrator is to list those air pollutants that in her “judgment, cause or contribute to 

air pollution which may reasonably be anticipated to endanger public health or welfare;” 

“the presence of which in the ambient air results from numerous or diverse mobile or 

stationary sources;” and “for which . . . [the Administrator] plans to issue air quality 

criteria...” Air quality criteria are intended to “accurately reflect the latest scientific 

knowledge useful in indicating the kind and extent of all identifiable effects on public 

health or welfare which may be expected from the presence of [a] pollutant in the 

ambient air . . .” 42 U.S.C. 7408(b). Section 109 (42 U.S.C. 7409) directs the 

Administrator to propose and promulgate “primary” and “secondary” NAAQS for 

pollutants for which air quality criteria are issued. Section 109(b)(1) defines a primary 

standard as one “the attainment and maintenance of which in the judgment of the 

Administrator, based on such criteria and allowing an adequate margin of safety, are 

requisite to protect the public health.”1 A secondary standard, as defined in section 

109(b)(2), must “specify a level of air quality the attainment and maintenance of which, 

in the judgment of the Administrator, based on such criteria, is requisite to protect the 

public welfare from any known or anticipated adverse effects associated with the 

presence of [the] pollutant in the ambient air.”2

 

1

The legislative history of section 109 indicates that a primary standard is to be set at 

“the maximum permissible ambient air level . . . which will protect the health of any 

[sensitive] group of the population,” and that for this purpose “reference should be made 

to a representative sample of persons comprising the sensitive group rather than to a 

single person in such a group”.” S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).

2 Welfare effects as defined in section 302(h) (42 U.S.C. 7602(h)) include, but are not 

limited to, “effects on soils, water, crops, vegetation, man-made materials, animals, 

wildlife, weather, visibility and climate, damage to and deterioration of property, and 

hazards to transportation, as well as effects on economic values and on personal comfort 

and well-being.”
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The requirement that primary standards provide an adequate margin of safety was 

intended to address uncertainties associated with inconclusive scientific and technical 

information available at the time of standard setting. It was also intended to provide a 

reasonable degree of protection against hazards that research has not yet identified. See 

Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980); American 

Petroleum Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981;); American Farm 

Bureau Federation v. EPA, 559 F. 3d 512, 533 (D.C. Cir. 2009); Association of Battery 

Recyclers v. EPA, 604 F. 3d 613, 617-18 (D.C. Cir. 2010). Both kinds of uncertainties are 

components of the risk associated with pollution at levels below those at which human 

health effects can be said to occur with reasonable scientific certainty. Thus, in selecting 

primary standards that provide an adequate margin of safety, the Administrator is seeking 

not only to prevent pollution levels that have been demonstrated to be harmful but also to 

prevent lower pollutant levels that may pose an unacceptable risk of harm, even if the risk 

is not precisely identified as to nature or degree. The CAA does not require the 

Administrator to establish a primary NAAQS at a zero-risk level or at background 

concentration levels, see Lead Industries v. EPA, 647 F.2d at 1156 n.51, but rather at a 

level that reduces risk sufficiently so as to protect public health with an adequate margin 

of safety.

In addressing the requirement for an adequate margin of safety, the EPA 

considers such factors as the nature and severity of the health effects involved, the size of 

at-risk population(s), and the kind and degree of the uncertainties that must be addressed. 

The selection of any particular approach to providing an adequate margin of safety is a 

policy choice left specifically to the Administrator’s judgment. See Lead Industries 
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Association v. EPA, 647 F.2d at 1161-62; Whitman v. American Trucking Associations, 

531 U.S. 457, 495 (2001).

In setting standards that are “requisite” to protect public health and welfare, as 

provided in section 109(b), the EPA’s task is to establish standards that are neither more 

nor less stringent than necessary for these purposes. In so doing, the EPA may not 

consider the costs of implementing the standards. See generally, Whitman v. American 

Trucking Associations, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability 

and technological feasibility are not relevant considerations in the promulgation of 

national ambient air quality standards.” American Petroleum Institute v. Costle, 665 F. 2d 

at 1185.

Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year 

intervals thereafter, the Administrator shall complete a thorough review of the criteria 

published under section 108 and the national ambient air quality standards . . . and shall 

make such revisions in such criteria and standards and promulgate such new standards as 

may be appropriate . . . ” Section 109(d)(2) requires that an independent scientific review 

committee “shall complete a review of the criteria . . . and the national primary and 

secondary ambient air quality standards. . . and shall recommend to the Administrator any 

new . . . standards and revisions of existing criteria and standards as may be appropriate . 

. . .” Since the early 1980's, this independent review function has been performed by the 

CASAC.3

 

3

The CASAC PM Review Panel is comprised of the seven members of the chartered 

CASAC, supplemented by fifteen subject-matter experts appointed by the Administrator 

to provide additional scientific expertise relevant to this review of the PM NAAQS. Lists 

of current CASAC members and of members of the CASAC PM Review Panelreview 

panels are available at: 

http://yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/CommitteesandMembership?Op
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B. Review of the Air Quality Criteria and Standards for PM

1. Previous PM NAAQS Reviews

The EPA initially established NAAQS for PM under section 109 of the CAA in 

1971. Since then, the Agency has made a number of changes to these standards to reflect 

continually expanding scientific information, particularly with respect to the selection of 

indicator4and level. Table 1 provides a summary of the PM NAAQS that have been 

promulgated to date. These decisions are briefly discussed below. 

In 1971, the EPA established NAAQS for PM based on the original air quality criteria 

document (DHEW, 1969; 36 FR 8186, April 30, 1971). The reference method specified 

for determining attainment of the original standards was the high-volume sampler, which 

collects PM up to a nominal size of 25 to 45 μm (referred to as total suspended particles 

or TSP). The primary standards (measured by the indicator TSP) were 260 μg/m3, 

24-hour average, not to be exceeded more than once per year, and 75 μg/m3, annual 

geometric mean. The secondary standard was 150 μg/m3, 24-hour average, not to be 

exceeded more than once per year.

In October 1979, the EPA announced the first periodic review of the criteria and 

NAAQS for PM, and significant revisions to the original standards were promulgated in 

1987 (52 FR 24634, July 1, 1987). In that decision, the EPA changed the indicator for 

PM from TSP to PM10, the latter including particles with an aerodynamic diameter less 

than or equal to a nominal 10 μm, which delineates thoracic particles (i.e., that subset of 

 

enDocument. Members of the CASAC PM Review Panel are listed in the CASAC letters 

providing advice on draft assessment documents (Samet, 2009a-f, 2012a-d).

4

Particulate matter is the generic term for a broad class of chemically and physically 

diverse substances that exist as discrete particles (liquid droplets or solids) over a wide 

range of sizes, such that the indicator for a PM NAAQS has historically been defined in 

terms of particle size ranges.
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inhalable particles small enough to penetrate beyond the larynx to the thoracic region of 

the respiratory tract). The EPA also revised the primary standards by (1) replacing the 

24-hour TSP standard with a 24-hour PM10 standard of 150 μg/m3 with no more than one 

expected exceedance per year and (2) replacing the annual TSP standard with a PM10

standard of 50 μg/m3, annual arithmetic mean. The secondary standard was revised by 

replacing it with 24-hour and annual PM10 standards identical in all respects to the 

primary standards. The revisions also included a new reference method for the 

measurement of PM10 in the ambient air and rules for determining attainment of the new 

standards. On judicial review, the revised standards were upheld in all respects. Natural 

Resources Defense Council v. EPA, 902 F. 2d 962 (D.C. Cir. 1990).

In April 1994, the EPA announced its plans for the second periodic review of the 

criteria and NAAQS for PM, and promulgated significant revisions to the NAAQS in 

1997 (62 FR 38652, July 18, 1997). Most significantly, the EPA determined that although 

the PM NAAQS should continue to focus on thoracic particles (PM10), the fine and 

coarse fractions of PM10 should be considered separately. New standards were added, 

using PM2.5 as the indicator for fine particles. The PM10 standards were retained for the 

purpose of regulating the coarse fraction of PM10 (referred to as thoracic coarse particles 

or PM10-2.5).5The EPA established two new PM2.5 standards: an annual standard of 15.0

μg/m3, based on the 3-year average of annual arithmetic mean PM2.5 concentrations from 

single or multiple monitors sited to represent community-wide air quality6and a 24-hour 

 

5

See 40 CFR parts 50, 53, and 58 for more information on reference and equivalent 

methods for measuring PM in ambient air.

6 Monitoring stations sited to represent community-wide air quality would typically be at 

the neighborhood or urban-scale; however, where a population-oriented micro or middlescale PM2.5 monitoring station represents many such locations throughout a metropolitan 

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standard of 65 μg/m3, based on the 3-year average of the 98th percentile of 24-hour PM2.5

 

area, these smaller scales might also be considered to represent community-wide air 

quality [40 CFR part 58, appendix D, 4.7.1(b)].
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Table 1. Summary of National Ambient Air Quality Standards Promulgated for PM 

1971-2006a

Final Rule Indicator Averaging 

Time Level Form

1971

36 FR 8186

April 30, 

1971

TSP

24-hour

260 μg/m3 

(primary)

150 μg/m3

(secondary)

Not to be exceeded more than 

once per year

Annual 75 μg/m3

(primary) Annual average

1987

52 FR 24634

July 1, 1987

PM10

24-hour 150 μg/m3

Not to be exceeded more than 

once per year on average over a 

3-year period

Annual 50 μg/m3 Annual arithmetic mean, averaged 

over 3 years

1997

62 FR 38652

July 18, 1997

PM2.5

24-hour 65 μg/m3 98th percentile, averaged over 3 

years b

Annual 15.0 μg/m3 Annual arithmetic mean, averaged 

over 3 years c,d

PM10 24-hour 150 μg/m3

Initially promulgated 99th 

percentile, averaged over 3 years; 

when 1997 standards for PM10

were vacated, the form of 1987 

standards remained in place (not 

to be exceeded more than once 

per year on average over a 3-year 

period)

Annual 50 μg/m3 Annual arithmetic mean, averaged 

over 3 years

2006

71 FR 61144 

October 17, 

2006

PM2.5

24-hour 35 μg/m3 98th percentile, averaged over 3 

years b

Annual 15.0 μg/m3 Annual arithmetic mean, averaged 

over 3 years c,e

PM10 24-hour 150 μg/m3

Not to be exceeded more than 

once per year on average over a 

3-year period

a When not specified, primary and secondary standards are identical.

b

The level of the 24-hour standard is defined as an integer (zero decimal places) as 

determined by rounding. For example, a 3-year average 98th percentile concentration of 

35.49 μg/m3 would round to 35 μg/m3and thus meet the 24-hour standard and a 3-year 
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average of 35.50 μg/m3 would round to 36 and, hence, violate the 24-hour standard (40 

CFR part 50, appendix N).

c

The level of the annual standard is defined to one decimal place (i.e., 15.0 μg/m3) as 

determined by rounding. For example, a 3-year average annual mean of 15.04 μg/m3

would round to 15.0 μg/m3and, thus, meet the annual standard and a 3-year average of 

15.05 μg/m3 would round to 15.1 μg/m3and, hence, violate the annual standard (40 CFR 

part 50, appendix N).

d

The level of the standard was to be compared to measurements made at sites that 

represent “community-wide air quality” recording the highest level, or, if specific 

requirements were satisfied, to average measurements from multiple community-wide air 

quality monitoring sites (“spatial averaging”). 

e

The EPA tightened the constraints on the spatial averaging criteria by further limiting 

the conditions under which some areas may average measurements from multiple 

community-oriented monitors to determine compliance (See 71 FR 61165 to 61167, 

October 17, 2006). 
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suspended particles or TSP). The primary standards (measured by the indicator TSP) 

were 260 μg/m3, 24-hour average, not to be exceeded more than once per year, and 75 

μg/m3, annual geometric mean. The secondary standard was 150 μg/m3, 24-hour average, 

not to be exceeded more than once per year.

In October 1979, the EPA announced the first periodic review of the criteria and 

NAAQS for PM, and significant revisions to the original standards were promulgated in 

1987 (52 FR 24634, July 1, 1987). In that decision, the EPA changed the indicator for 

PM from TSP to PM10, the latter including particles with an aerodynamic diameter less 

than or equal to a nominal 10 μm, which delineates thoracic particles (i.e., that subset of 

inhalable particles small enough to penetrate beyond the larynx to the thoracic region of 

the respiratory tract). The EPA also revised the primary standards by (1) replacing the 24-

hour TSP standard with a 24-hour PM10 standard of 150 μg/m3 with no more than one 

expected exceedance per year and (2) replacing the annual TSP standard with a PM10

standard of 50 μg/m3, annual arithmetic mean. The secondary standard was revised by 

replacing it with 24-hour and annual PM10 standards identical in all respects to the 

primary standards. The revisions also included a new reference method for the 

measurement of PM10 in the ambient air and rules for determining attainment of the new 

standards. On judicial review, the revised standards were upheld in all respects. Natural 

Resources Defense Council v. EPA, 902 F. 2d 962 (D.C. Cir. 1990).

In April 1994, the EPA announced its plans for the second periodic review of the criteria 

and NAAQS for PM, and promulgated significant revisions to the NAAQS in 1997 (62 

FR 38652, July 18, 1997). Most significantly, the EPA determined that although the PM 

NAAQS should continue to focus on thoracic particles (PM10), the fine and coarse 

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fractions of PM10 should be considered separately. New standards were added, using 

PM2.5 as the indicator for fine particles. The PM10 standards were retained for the purpose 

of regulating the coarse fraction of PM10 (referred to as thoracic coarse particles or PM10-

2.5).7

The EPA established two new PM2.5 standards: an annual standard of 15.0 μg/m3, 

based on the 3-year average of annual arithmetic mean PM2.5 concentrations from single 

or multiple monitors sited to represent community-wide air quality8and a 24-hour 

standard of 65 μg/m3, based on the 3-year average of the 98th percentile of 24-hour PM2.5

concentrations at each population-oriented monitor9 within an area. Also, the EPA 

established a new reference method for the measurement of PM2.5 in the ambient air and 

rules for determining attainment of the new standards. To continue to address thoracic 

coarse particles, the annual PM10 standard was retained, while the form, but not the level, 

of the 24-hour PM10 standard was revised to be based on the 99th percentile of 24-hour 

PM10 concentrations at each monitor in an area. The EPA revised the secondary standards 

by making them identical in all respects to the primary standards.

Following promulgation of the revised PM NAAQS in 1997, petitions for review 

were filed by a large number of parties, addressing a broad range of issues. In May 1998, 

a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit 

issued an initial decision that upheld the EPA’s decision to establish fine particle 

 

7

See 40 CFR parts 50, 53, and 58 for more information on reference and equivalent 

methods for measuring PM in ambient air.

8 Monitoring stations sited to represent community-wide air quality would typically be at 

the neighborhood or urban-scale; however, where a population-oriented micro or middlescale PM2.5 monitoring station represents many such locations throughout a metropolitan 

area, these smaller scales might also be considered to represent community-wide air 

quality [40 CFR part 58, appendix D, 4.7.1(b)].

9

Population-oriented monitoring (or sites) means residential areas, commercial areas, 

recreational areas, industrial areas where workers from more than one company are 

located, and other areas where a substantial number of people may spend a significant 

fraction of their day (40 CFR 58.1).
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standards, holding that "the growing empirical evidence demonstrating a relationship 

between fine particle pollution and adverse health effects amply justifies establishment of 

new fine particle standards." American Trucking Associations v. EPA, 175 F. 3d 1027, 

1055-56 (D.C. Cir. 1999), rehearing granted in part and denied in part, 195 F. 3d 4 (D.C. 

Cir. 1999), affirmed in part and reversed in part, Whitman v. American Trucking 

Associations, 531 U.S. 457 (2001). The panel also found "ample support" for the EPA's 

decision to regulate coarse particle pollution, but vacated the 1997 PM10 standards, 

concluding, in part, that PM10 is a "poorly matched indicator for coarse particulate 

pollution" because it includes fine particles. Id. at 1053-55. Pursuant to the court’s 

decision, the EPA removed the vacated 1997 PM10 standards from the CFR (69 FR 

45592, July 30, 2004) and deleted the regulatory provision [at 40 CFR section 50.6(d)] 

that controlled the transition from the pre-existing 1987 PM10 standards to the 1997 PM10

standards. The pre-existing 1987 PM10 standards remained in place (65 FR 80776, 

December 22, 2000). The court also upheld the EPA’s determination not to establish 

more stringent secondary standards for fine particles to address effects on visibility (175 

F. 3d at 1027).

More generally, the panel held (over a strong dissent) that the EPA’s approach to 

establishing the level of the standards in 1997, both for the PM and for the ozone 

NAAQS promulgated on the same day, effected “an unconstitutional delegation of 

legislative authority.” Id. at 1034-40. Although the panel stated that “the factors EPA 

uses in determining the degree of public health concern associated with different levels of 

ozone and PM are reasonable,” it remanded the rule to the EPA, stating that when the 

EPA considers these factors for potential non-threshold pollutants “what EPA lacks is 

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any determinate criterion for drawing lines” to determine where the standards should be 

set. Consistent with the EPA’s long-standing interpretation and D.C. Circuit precedent, 

the panel also reaffirmed its prior holdings that in setting NAAQS, the EPA is “not 

permitted to consider the cost of implementing those standards.” Id. at 1040-41.

On EPA’s petition for rehearing, the panel adhered to its position on these points. 

American Trucking Associations v. EPA, 195 F. 3d 4 (D.C. Cir. 1999). The full Court of 

Appeals denied the EPA’s request for rehearing en banc, with five judges dissenting. Id. 

at 13. Both sides filed cross appeals on these issues to the United States Supreme Court, 

which granted certiorari. In February 2001, the Supreme Court issued a unanimous 

decision upholding the EPA’s position on both the constitutional and cost issues. 

Whitman v. American Trucking Associations, 531 U.S. 457, 464, 475-76. On the 

constitutional issue, the Court held that the statutory requirement that NAAQS be 

“requisite” to protect public health with an adequate margin of safety sufficiently cabined 

the EPA’s discretion, affirming the EPA’s approach of setting standards that are neither 

more nor less stringent than necessary. The Supreme Court remanded the case to the 

Court of Appeals for resolution of any remaining issues that had not been addressed in 

that court’s earlier rulings. Id. at 475-76. In March 2002, the Court of Appeals rejected all 

remaining challenges to the standards, holding under the statutory standard of review that 

the EPA’s PM2.5 standards were reasonably supported by the administrative record and 

were not “arbitrary and capricious.” American Trucking AssociationsAssociation v. EPA, 

283 F. 3d 355, 369-72 (D.C. Cir. 2002).

In October 1997, the EPA published its plans for the next periodic review of the 

air quality criteria and NAAQS for PM (62 FR 55201, October 23, 1997). After CASAC 

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and public review of several drafts, the EPA’s National Center for Environmental 

Assessment (NCEA) finalized the Air Quality Criteria Document for Particulate Matter

(henceforth, AQCD or the "Criteria Document") in October 2004 (U.S. EPA, 2004) and 

OAQPS finalized an assessment document, Particulate Matter Health Risk Assessment 

for Selected Urban Areas (Abt Associates, 2005), and the Review of the National 

Ambient Air Quality Standards for Particulate Matter: Policy Assessment of Scientific 

and Technical Information, in December 2005 (henceforth, “Staff Paper,” U.S. EPA, 

2005). In conjunction with theirits review of the Staff Paper, CASAC provided advice to 

the Administrator on revisions to the PM NAAQS (Henderson, 2005a). In particular, 

most CASAC PM Panel members favored revising the level of the primary 24-hour PM2.5

standard inwithin the range of 35 to 30 μg/m3 with a 98th percentile form, in concert with 

revising the level of the primary annual PM2.5 standard inwithin the range of 14 to 13 

μg/m3(Henderson, 2005a, p.7). For thoracic coarse particles, the Panel had reservations 

in recommending a primary 24-hour PM10-2.5 standard, and agreed that there was a need 

for more research on the health effects of thoracic coarse particles (Henderson, 2005b). 

With regard to secondary standards, most Panel members strongly supported establishing 

a new, distinct secondary PM2.5 standard to protect urban visibility (Henderson, 2005a, p. 

9). 

On January 17, 2006, the EPA proposed to revise the primary and secondary 

NAAQS for PM (71 FR 2620) and solicited comment on a broad range of options. 

Proposed revisions included: (1) revising the level of the primary 24-hour PM2.5 standard 

to 35 μg/m3; (2) revising the form, but not the level, of the primary annual PM2.5 standard 

by tightening the constraints on the use of spatial averaging; (3) replacing the primary 24-
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-hour PM10 standard with a 24-hour standard defined in terms of a new indicator, PM10-

2.5, which was qualified so as to include any ambient mix of PM10-2.5 dominated by 

particles generated by high-density traffic on paved roads, industrial sources, and 

construction sources, and to exclude any ambient mix of particles dominated by rural 

windblown dust and soils and agricultural and mining sources (71 FR 2667 to 2668), set 

at a level of 70 μg/m3based on the 3-year average of the 98th percentile of 24-hour PM10-

-2.5 concentrations; (4) revoking the primary annual PM10 standard; and (5) revising the 

secondary standards by making them identical in all respects to the proposed suite of 

primary standards for fine and coarse particles.10 Subsequent to the proposal, CASAC 

provided additional advice to the EPA in a letter to the Administrator requesting 

reconsideration of CASAC’s recommendations for both the primary and secondary PM2.5

standards as well as the standards for thoracic coarse particles (Henderson, 2006a).

On October 17, 2006, the EPA published revisions to the PM NAAQS to provide 

increased protection of public health and welfare (71 FR 61144). With regard to the 

primary and secondary standards for fine particles, the EPA revised the level of the 

primary 24-hour PM2.5 standard to 35 μg/m3, retained the level of the primary annual 

PM2.5 standard at 15.0 μg/m3, and revised the form of the primary annual PM2.5 standard 

by adding further constraints on the optional use of spatial averaging. The EPA revised 

the secondary standards for fine particles by making them identical in all respects to the 

primary standards. With regard to the primary and secondary standards for thoracic 

coarse particles, the EPA retained the level and form of the 24-hour PM10 standard (such 

 

10 In recognition of an alternative view expressed by most members of the CASAC PM 

Panel, the Agency also solicited comments on a subdaily (4- to 8-hour averaging time) 

secondary PM2.5 standard to address visibility impairment, considering alternative 

standard levels within a range of 20 to 30 μg/m3in conjunction with a form within a 

range of the 92nd to 98th percentile (71 FR 2685, January 17, 2006).
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that the standard remained at a level of 150 μg/m3 with a one-expected exceedance form

and retained the PM10 indicator) and revoked the annual PM10 standard. The EPA also 

established a new Federal Reference Method (FRM) for the measurement of PM10-2.5 in 

the ambient air (71 FR 61212 to 13). Although the standards for thoracic coarse particles 

were not defined in terms of a PM10-2.5 indicator, the EPA adopted a new FRM for PM10-

-2.5 to facilitate consistent research on PM10-2.5 air quality and health effects and to 

promote commercial development of Federal Equivalent Methods (FEMs) to support 

future reviews of the PM NAAQS (71 FR 61212/2).

Following issuance of the final rule, CASAC articulated its concern that the

“EPA’s final rule on the NAAQS for PM does not reflect several important aspects of the 

CASAC’s advice” (Henderson et al., 2006b, p. 1). With regard to the primary PM2.5

annual standard, CASAC expressed serious concerns regarding the decision to retain the 

level of the standard at 15 μg/m3. Specifically, CASAC stated, “It is the CASAC’s 

consensus scientific opinion that the decision to retain without change the annual PM2.5

standard does not provide an ‘adequate margin of safety... requisite to protect the public 

health’ (as required by the Clean Air Act), leaving parts of the population of this country 

at significant risk of adverse health effects from exposure to fine PM” (Henderson et al., 

2006b, p. 2). Furthermore, CASAC pointed out that its recommendations “were 

consistent with the mainstream scientific advice that EPA received from virtually every 

major medical association and public health organization that provided their input to the 
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Agency” (Henderson et al., 2006b, p. 2).11 With regard to EPA’s final decision to retain 

the 24-hour PM10 standard for thoracic coarse particles, CASAC had mixed views with 

regard to the decision to retain the 24-hour standard and the continued use of PM10 as the 

indicator of coarse particles, while also recognizing the need to have a standard in place 

to protect against effects associated with short-term exposures to thoracic coarse particles

(Henderson et al., 2006b, p. 2). With regard to the EPA’s final decision to revise the 

secondary PM2.5 standards to be identical in all respects to the revised primary PM2.5

standards, CASAC expressed concerns that its advice to establish a distinct secondary 

standard for fine particles to address visibility impairment was not followed and 

emphasized “that continuing to rely on the primary standard to protect against all PM-

-related adverse environmental and welfare effects assures neglect, and will allow 

substantial continued degradation, of visual air quality over large areas of the country”

(Henderson et al, 2006b, p. 2).

2. Litigation Related to the 2006 PM Standards

Several parties filed petitions for review following promulgation of the revised 

PM NAAQS in 2006. These petitions addressed the following issues: (1) selecting the 

level of the primary annual PM2.5 standard; (2) retaining PM10 as the indicator of a 

standard for thoracic coarse particles, retaining the level and form of the 24-hour PM10

standard, and revoking the PM10 annual standard; and (3) setting the secondary PM2.5

standards identical to the primary standards. On February 24, 2009, the U.S. Court of 

Appeals for the District of Columbia Circuit issued its opinion in the case American 

 

11 CASAC specifically identified input provided by the American Medical Association, 

the American Thoracic Society, the American Lung Association, the American Academy 

of Pediatrics, the American College of Cardiology, the American Heart Association, the 

American Cancer Society, the American Public Health Association, and the National 

Association of Local Boards of Health (Henderson et al., 2006b, p. 2).
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Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). The court remanded 

the primary annual PM2.5 NAAQS to the EPA because the EPA failed to adequately 

explain why the standard provided the requisite protection from both short- and long-term 

exposures to fine particles, including protection for at-risk populations such as children. 

American Farm Bureau Federation v. EPA, 559 F. 3d 512, 520-27 (D.C. Cir. 2009). With 

regard to the standards for PM10, the court upheld the EPA’s decisions to retain the 24-

-hour PM10 standard to provide protection from thoracic coarse particle exposures and to 

revoke the annual PM10 standard. American Farm Bureau Federation v. EPA, 559 F. 2d 

at 533-38. With regard to the secondary PM2.5 standards, the court remanded the 

standards to the EPA because the Agency’s decision was “unreasonable and contrary to 

the requirements of section 109(b)(2)” of the CAA. The court further concluded that the 

EPA failed to adequately explain why setting the secondary PM standards identical to the 

primary standards provided the required protection for public welfare, including 

protection from visibility impairment. American Farm Bureau Federation v. EPA, 559 F. 

2d at 528-32.

The decisions of the court with regard to these three issues are discussed further in 

sections III.A.2, IV.A.2, and VI.A.2 below. The EPA is responding to the court’s 

remands as part of the current review of the PM NAAQS.

3. Current PM NAAQS Review

The EPA initiated the current review of the air quality criteria for PM in June 

2007 with a general call for information (72 FR 35462, June 28, 2007). In July 2007, the 

EPA held two “kick-off” workshops on the primary and secondary PM NAAQS, 
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respectively (72 FR 34003 to 34004, June 20, 2007).12 These workshops provided an 

opportunity for a public discussion of the key policy-relevant issues around which the 

EPA would structure this PM NAAQS review and the most meaningful new science that 

would be available to inform our understanding of these issues. 

Based in part on the workshop discussions, the EPA developed a draft Integrated 

Review Plan outlining the schedule, process, and key policy-relevant questions that 

would guide the evaluation of the air quality criteria for PM and the review of the 

primary and secondary PM NAAQS (U.S. EPA, 2007a). On November 30, 2007, the 

EPA held a consultation with CASAC on the draft Integrated Review Plan (72 FR 63177, 

November 8, 2007), which included the opportunity for public comment. The final 

Integrated Review Plan (U.S. EPA, 2008a) incorporated comments from CASAC 

(Henderson, 2008) and the public on the draft plan as well as input from senior Agency 

managers.

13,14

A major element in the process for reviewing the NAAQS is the development of 

 

12 See workshop materials available at: 

http://www.regulations.gov/search/Regs/home.html#home Docket ID numbers EPA-HQOAR-2007-0492-008; EPA-HQ-OAR-2007-0492-009; EPA-HQ-OAR-2007-0492-010; 

and EPA-HQ-OAR-2007-0492-012.

13 The process followed in this review varies from the NAAQS review process described 

in section 1.1 of the Integrated Review Plan (U.S. EPA, 2008a). On May 21, 2009, 

Administrator Jackson called for key changes to the NAAQS review process including 

reinstating a policy assessment document that contains staff analyses of the scientific 

bases for alternative policy options for consideration by senior Agency management prior 

to rulemaking. In conjunction with this change, the EPA will no longer issue a policy 

assessment in the form of an advance notice of proposed rulemaking (ANPR) as 

discussed in the Integrated Review Plan (U.S. EPA, 2008a, p. 3). For more information 

on the overall process followed in this review including a description of the major 

elements of the process for reviewing NAAQS see Jackson (2009). 

14 All written comments submitted to the Agency are available in the docket for this PM 

NAAQS review (EPA-HQ-OAR-2007-0429). Transcripts of public meetings and 

teleconferences held in conjunction with CASAC’s reviews are also included in the 

docket.
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an Integrated Science Assessment. This document provides a concise evaluation and 

integration of the policy-relevant science, including key science judgments upon which

the risk and exposure assessments build. As part of the process of preparing the PM 

Integrated Science Assessment, NCEA hosted a peer review workshop in June 2008 on 

preliminary drafts of key Integrated Science Assessment chapters (73 FR 30391, May 27, 

2008). CASAC and the public reviewed the first external review draft Integrated Science 

Assessment (U.S. EPA, 2008b; 73 FR 77686, December 19, 2008) at a meeting held on 

April 1 to 2, 2009 (74 FR 2688, February 19, 2009). Based on CASAC (Samet, 2009e) 

and public comments, NCEA prepared a second draft Integrated Science Assessment

(U.S. EPA, 2009b; 74 FR 38185, July 31, 2009), which was reviewed by CASAC and the 

public at a meeting held on October 5 and 6, 2009 (74 FR 46586, September 10, 2009). 

Based on CASAC (Samet, 2009f) and public comments, NCEA prepared the final 

Integrated Science Assessment titled Integrated Science Assessment for Particulate 

Matter, December 2009 (U.S. EPA, 2009a; 74 FR 66353, December 15, 2009).

Building upon the information presented in the PM Integrated Science 

Assessment, the EPA prepared Risk and Exposure Assessments that provide a concise 

presentation of the methods, key results, observations, and related uncertainties. In 

developing the Risk and Exposure Assessments for this PM NAAQS review, OAQPS 

released two planning documents: Particulate Matter National Ambient Air Quality 

Standards: Scope and Methods Plan for Health Risk and Exposure Assessment and

Particulate Matter National Ambient Air Quality Standards: Scope and Methods Plan for 

Urban Visibility Impact Assessment (henceforth, Scope and Methods Plans, U.S. EPA, 

2009c,d; 74 FR 11580, March 18, 2009). These planning documents outlined the scope 

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and approaches that staff planned to use in conducting quantitative assessments as well as 

key issues that would be addressed as part of the assessments. In designing and

conducting the initial health risk and visibility impact assessments, the Agency

considered CASAC comments (Samet 2009a,b) on the Scope and Methods Plans made 

during an April 2009 consultation (74 FR 7688, February 19, 2009) as well as public 

comments. CASAC and the public reviewed two draft assessment documents, Risk 

Assessment to Support the Review of the PM2.5 Primary National Ambient Air Quality 

Standards: External Review Draft, September 2009 (U.S. EPA, 2009e) and Particulate 

Matter Urban-Focused Visibility Assessment - External Review Draft, September 2009 

(U.S. EPA, 2009f) at a meeting held on October 5 and 6, 2009 (74 FR 46586, September 

10, 2009). Based on CASAC (Samet 2009c,d) and public comments, OAQPS staff 

revised these draft documents and released second draft assessment documents (U.S.

EPA, 2010d,e) in January and February 2010 (75 FR 4067, January 26, 2010) for 

CASAC and public review at a meeting held on March 10 and 11, 2010 (75 FR 8062, 

February 23, 2010). Based on CASAC (Samet, 2010a,b) and public comments on the 

second draft assessment documents, the EPA revised these documents and released final 

assessment documents titled Quantitative Health Risk Assessment for Particulate Matter,

June 2010 (henceforth, “Risk Assessment,” U.S. EPA, 2010a) and Particulate Matter 

Urban-Focused Visibility Assessment – Final Document, July 2010 (henceforth, 

“Visibility Assessment,” U.S. EPA, 2010b) (75 FR 39252, July 8, 2010).

Based on the scientific and technical information available in this review as 

assessed in the Integrated Science Assessment and Risk and Exposure Assessments, the 

EPA staff prepared a Policy Assessment. The Policy Assessment is intended to help 

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“bridge the gap” between the relevant scientific information and assessments and the 

judgments required of the Administrator in reaching decisions on the NAAQS (Jackson, 

2009, attachment, p. 2). American Farm Bureau Federation v. EPA ̧559 F. 3d at 521. The 

Policy Assessment is not a decision document; rather it presents the EPA staff 

conclusions related to the broadest range of policy options that could be supported by the 

currently available information. A preliminary draft Policy Assessment (U.S. EPA, 

2009g) was released in September 2009 for informational purposes and to facilitate 

discussion with CASAC at the October 5 and 6, 2009 meeting on the overall structure, 

areas of focus, and level of detail to be included in the Policy Assessment. The EPA 

considered CASAC’s comments on this preliminary draft in developing a first draft 

Policy Assessment (U.S. EPA, 2010c; 75 FR 4067, January 26, 2010) that built upon the 

information presented and assessed in the final Integrated Science Assessment and 

second draft Risk and Exposure Assessments. The EPA presented an overview of the first 

draft Policy Assessment at a CASAC meeting on March 10, 2010 (75 FR 8062, February 

23, 2010) and it was discussed during public CASAC teleconferences on April 8 and 9, 

2010 (75 FR 8062, February 23, 2010) and May 7, 2010 (75 FR 19971, April 16, 2010). 

The EPA developed a second draft Policy Assessment (U.S. EPA, 2010f; 75 FR 

39253, July 8, 2010) based on CASAC (Samet, 2010c) and public comments on the first 

draft Policy Assessment. CASAC reviewed the second draft document at a meeting on 

July 26 and 27, 2010 (75 FR 32763, June 9, 2010). The EPA staff considered CASAC 

(Samet, 2010d) and public comments on the second draft Policy Assessment in preparing 

a final Policy Assessment titled Policy Assessment for the Review of the Particulate 

Matter National Ambient Air Quality Standards, April, 2011 (U.S. EPA, 2011a; 76, FR 
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22665, April 22, 2011). This document includes final staff conclusions on the adequacy 

of the current PM standards and alternative standards for consideration. 

The schedule for the rulemaking in this review is subject to a court order in a 

lawsuit filed in February 2012 by a group of plaintiffs who alleged that the EPA had 

failed to perform its mandatory duty, under section 109(d)(1), to complete a review of the 

PM NAAQS within the period provided by statute. American Lung Association and 

National Parks Conservation Association v. EPA, D.D.C. No. 12-cv-00243 (consol. with 

No. 12-cv-00531) Court orders in that case provide that the EPA sign a notice of 

proposed rulemaking concerning its review of the PM NAAQS no later than June 14, 

2012 and a notice of final rulemaking no later than December 14, 2012.

On June 14, 2012, the EPA issued its proposed decision to revise the NAAQS for 

PM (77 FR 38890, June 29, 2012) (henceforth “proposal”). In the proposal, the EPA 

identified revisions to the standards, based on the air quality criteria for PM, and to 

related data handling conventions and ambient air monitoring, reporting, and network 

design requirements. The EPA proposed revisions to the prevention of significant 

deterioration (PSD) permitting program with respect to the proposed NAAQS revisions. 

The Agency also proposed changes to the AQI for PM2.5, consistent with the proposed 

primary PM2.5 standards. The proposal solicited public comments on alternative primary 

and secondary standards and related matters. The proposal is summarized in section II.D 

below.

The EPA held two public hearings to receive public comment on the proposed 

revisions to the PM NAAQS (77 FR 39205, July 2, 2012). One hearing took place in 

Philadelphia, PA on July 17, 2012 and a second hearing took place in Sacramento, CA on 
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July 19, 2012. At these public hearings, the EPA heard testimony from 168 individuals 

representing themselves or specific interested organizations.

The EPA received more than 230,000 comments from members of the public and 

various interest groups on the proposed revisions to the PM NAAQS by the close of the 

public comment period on August 31, 2012. Major issues raised in the public comments 

are discussed throughout the preamble of this final action. A more detailed summary of 

all significant comments, along with the EPA’s responses (henceforth “Response to 

Comments”) can be found in the docket for this rulemaking (Docket No. EPA-HQ-OAR2007-0492) (U.S. EPA, 2012a).

In the proposal, the EPA recognized that there were a number of new scientific 

studies on the health effects of PM that had been published since the mid-2009 cutoff 

date for inclusion in the Integrated Science Assessment.

15 As in the last PM NAAQS 

review, the EPA committed to conduct a provisional review and assessment of any 

significant “new” studies published since the close of the Integrated Science Assessment, 

including studies submitted to the EPA during the public comment period. The purpose 

of the provisional science assessment was to ensure that the Administrator was fully 

aware of the “new” science that has developed since 2009 before making final decisions 

on whether to retain or revise the current PM NAAQS. The EPA screened and surveyed 

the recent health literature, including studies submitted during the public comment 

 

15 For ease of reference, these studies will be referred to as “new” studies or “new” 

science, using quotation marks around the word new. Referring to studies that were 

published too recently to have been included in the 2009 Integrated Science Assessment 

as “new” studies is intended to clearly differentiate such studies from those that have 

been published since the last review and which are included in the 2009 Integrated 

Science Assessment (these studies are sometimes referred to as new (without quotation 

marks) or more recent studies, to indicate that they were not included in the 2004 Air 

Quality Criteria DocumentIntegrated Science Assessment and thus are newly available in 

this review).
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period, and conducted a provisional assessment (U.S. EPA, 2012b) that places the results 

of those studies of potentially greatest policy relevance in the context of the findings of

the Integrated Science Assessment (U.S. EPA, 2009a). This provisional assessment,

including a summary of the key conclusions, can be found in the rulemaking docket

(EPA-HQ-OAR-2007-0492).

The provisional assessment found that the “new” studies expand the scientific 

information considered in the Integrated Science Assessment and provide important 

insights on the relationship between PM exposure and health effects. The provisional 

assessment also found that the “new” studies generally strengthen the evidence that 

long- and short-term exposures to fine particles are associated with a wide range of health 

effects. Some of the “new” epidemiological studies report effects in areas with lower 

PM2.5 concentrations than those in earlier studies considered in the Integrated Science 

Assessment. “New” toxicological and epidemiological studies continue to link various 

health effects with a range of fine particle sources and components. With regard to 

thoracic coarse particles, the provisional assessment recognized that a limited number of

“new” studies provide evidence of an association with short-term PM10-2.5 exposures and 

increased asthma-related emergency department visits in children, but continue to 

provide no evidence of an association between long-term PM10-2.5 exposure and mortality. 

Further, the provisional assessment found that the results reported in “new” studies do not 

materially change any of the broad scientific conclusions regarding the health effects of 

PM exposure made in the Integrated Science Assessment. 

The EPA believes it was important to conduct a provisional assessment in this 

proceeding, so that the Administrator would be aware of the science that developed too 

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recently for inclusion in the Integrated Science Assessment. However, it is also important 

to note that the EPA’s review of that science to date has been limited to screening, 

surveying, and preparing a provisional assessment of these studies. Having performed 

this limited provisional assessment, the EPA must decide whether to consider the “new”

studies in this review and to take such steps as may be necessary to include them in the 

basis for the final decision, or to reserve such action for the next review of the PM 

NAAQS. 

As in prior NAAQS reviews, the EPA is basing its decision in this review on 

studies and related information included in the Integrated Science Assessment, Risk and 

Exposure Assessment, and Policy Assessment, which have undergone CASAC and 

public review. The studies assessed in the Integrated Science Assessment Document, and 

the integration of the scientific evidence presented in that document, have undergone 

extensive critical review by the EPA, CASAC, and the public during the development of 

the Integrated Science Assessment. The rigor of that review makes these studies, and 

their integrative assessment, the most reliable source of scientific information on which 

to base decisions on the NAAQS. NAAQS decisions can have profound impacts on 

public health and welfare, and NAAQS decisions should be based on studies that have 

been rigorously assessed in an integrative manner not only by the EPA but also by the 

statutorily-mandated independent advisory committee, CASAC, and have been subject as 

well to the public review that accompanies this process. As described above, the 

provisional assessment did not and could not provide that kind of in-depth critical review.

This decision is consistent with the EPA’s practice in prior NAAQS reviews. 

Since the 1970 amendments, the EPA has taken the view that NAAQS decisions are to be 
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based on scientific studies and related information that have been assessed as a part of the 

pertinent air quality criteria. See e.g., 36 FR 8186 (April 30, 1971) (the EPA based 

original NAAQS for six pollutants on scientific studies discussed in air quality criteria 

documents and limited consideration of comments to those concerning validity of 

scientific basis); 38 FR 25678, 25679 - 25680 (September 14, 1973) (the EPA revised air 

quality criteria for sulfur oxides to provide basis for reevaluation of secondary NAAQS). 

This longstanding interpretation was strengthened by new legislative requirements 

enacted in 1977, which added section 109(d)(2) of the CAA concerning CASAC review 

of air quality criteria. The EPA has consistently followed this approach. 52 FR 24634, 

24637 (July 1, 1987) (after review by CASAC, the EPA issued a post-proposal addendum 

to the PM Air Quality Criteria Document, to address certain new scientific studies not 

included in the 1982 Air Quality Criteria Document); 61 FR 25566, 25568 (May 22, 

1996) (after review by CASAC, the EPA issued a post-proposal supplement to the 1982 

Air Quality Criteria Document to address certain new health studies not included in the 

1982Air1982 Air Quality Criteria Document or 1986 Addendum). The EPA reaffirmed 

this approach in its decision not to revise the ozone NAAQS in 1993, as well as in its 

final decision on the PM NAAQS in the 1997 and 2006 reviews. 58 FR 13008, 13013 -to

13014 (March 9, 1993) (ozone review); 62 FR 38652, 38662 (July 18, 1997) and 71 FR

61141, 61148 -to 61149 (October 17, 2006) (PM reviews) (The EPA conducted a 

provisional assessment but based the final PM decisions on studies and related 

information included in the air quality criteria that had been reviewed by CASAC).

As discussed in the EPA's 1993 decision not to revise the NAAQS for ozone, 

‘new’ studies may sometimes be of such significance that it is appropriate to delay a 

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decision on revision of NAAQS and to supplement the pertinent air quality criteria so the 

“new” studies can be taken into account (58 FR at, 13013 -to 13014, March 9, 1993). In 

this proceeding, the provisional assessment of recent studies concludes that, taken in 

context, the “new” information and findings do not materially change any of the broad 

scientific conclusions regarding the health effects of PM exposure made in the Integrated 

Science Assessment (U.S. EPA, 2012b). For this reason, reopening the air quality criteria 

review would not be warranted even if there were time to do so under the court order 

governing the schedule for this rulemaking. Accordingly, the EPA is basing the final 

decisions in this review on the studies and related information included in the PM air 

quality criteria that have undergone CASAC and public review. The EPA will consider 

the “new” published studies for purposes of decision making in the next periodic review 

of the PM NAAQS, which will provide the opportunity to fully assess them through a 

more rigorous review process involving the EPA, CASAC, and the public.

C. Related Control Programs to Implement PM Standards

States are primarily responsible for ensuring attainment and maintenance of 

ambient air quality standardsNAAQS once the EPA has established them. Under section 

110 of the CAA and related provisions, states are to submit, for the EPA’s approval, state 

implementation plans (SIPs) that provide for the attainment and maintenance of such 

standards through control programs directed to sources of the pollutants involved. The 

states, in conjunction with the EPA, also administer the PSD permitting program (CAA

sections 160 to 169). In addition, Federalfederal programs provide for nationwide 

reductions in emissions of PM and other air pollutants through the Federalfederal motor 

vehicle and motor vehicle fuel control program under title II of the Act (CAA sections 
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202 to 250) which involves controls for emissions from mobile sources and controls for 

the fuels used by these sources, and new source performance standards (NSPS) for 

stationary sources under section 111 of the CAA.

Currently, there are 35 areas in the U.S. that are designated as nonattainment for 

the current annual PM2.5 standard and 32 areas in the U.S. that are designated as 

nonattainment for the current 24-hour PM2.5 standards. With the revisions to the PM

NAAQS that are being finalized in this rule, the EPA will work with the states to conduct 

a new area designation process. Those states with new nonattainment areas will be 

required to develop SIPs to attain the standards. In developing their attainment plans, 

states will have to take into account projected emission reductions from federal and state 

rules that have already been adopted at the time of plan submittal. A number of 

significant emission reduction programs that will lead to reductions of PM and its 

precursors are in place today or are expected to be in place by the time any new SIPs will 

be due. Examples of such rules include regulations for onroad and nonroad engines and 

fuels, the utility and industrial boilers toxics rules, and various other programs already 

adopted by states to reduce emissions from key emissions sources. States will then 

evaluate the level of additional emission reductions needed for each nonattainment area 

to attain the standards "as expeditiously as practicable" and adopt new state regulations,

as appropriate. Section IX includes additional discussion of designation and 

implementation issues associated with the revised PM NAAQS.

D. Summary of Proposed Revisions to the PM NAAQS

For reasons discussed in the proposal, the Administrator proposed to revise the 

current primary and secondary PM standards. With regard to the primary PM2.5 standards, 
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the Administrator proposed to revise the level of the annual PM2.5 standard from 15.0 

μg/m3to a level within a range of 12.0 to 13.0 μg/m3and to retain the level of the 24-

-hour PM2.5 standard at 35 μg/m3. The Administrator also proposed to eliminate spatial 

averaging provisions as part of the form of the annual standard to avoid potential

disproportionate impacts on at-risk populations. The EPA proposed to revise the AQI for 

PM2.5, consistent with the proposed primary PM2.5 standards,

With regard to the primary coarse particle standard, the EPA proposed to retain 

the current 24-hour PM10 standard to continue to provide protection against effects 

associated with short-term exposure to thoracic coarse particles (i.e., PM10-2.5). 

With regard to the secondary PM standards, the EPA proposed to revise the suite 

of secondary PM standards by adding a distinct standard for PM2.5 to address PM-related 

visibility impairment. The separate secondary standard was proposed to be defined in 

terms of a PM2.5 visibility index, which would use speciated PM2.5 mass concentrations 

and relative humidity data to calculate PM2.5 light extinction, translated to the deciview 

(dv) scale, similar to the Regional Haze Program; a 24-hour averaging time; a 90th

percentile form averaged over 3 years; and a level set at one of two options – either 30 or 

28 dv. The EPA also proposed to retain the current secondary standards generally to 

address non-visibility welfare effects. 

The EPA also proposed to revise the data handling procedures consistent with the 

revised primary and secondary standards for PM2.5 including the computations necessary 

for determining when these standards are met and the measurement data that are 

appropriate for comparison to the standards. With regard to monitoring-related activities, 

the EPA proposed to update several aspects of the monitoring regulations and specifically 
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to require that a small number of PM2.5 monitors be relocated to be collocated with 

measurements of other pollutants (e.g., nitrogen dioxide, carbon monoxide) in the nearroad environment.

E. Organization and Approach to Final PM NAAQS Decisions

This action presents the Administrator’s final decisions on the review of the 

current primary and secondary PM2.5 and PM10 standards. Consistent with the decisions 

made by the EPA in the last review and with the conclusions in the Integrated Science 

Assessment and Policy Assessment, fine and thoracic coarse particles continue to be 

considered as separate subclasses of PM pollution. Primary standards for fine particles 

and for thoracic coarse particles are addressed in sections III and IV, respectively. 

Changes to the AQI for PM2.5, consistent with the revised primary PM2.5 standards, are 

addressed in section V. Secondary standards for fine and coarse particles are addressed in 

section VI. Related data handling conventions and exceptional events are addressed in 

section VII. Updates to the monitoring regulations are addressed in section VIII. 

Implementation activities, including PSD-related actions, are addressed in section IX.

Section X addresses applicable statutory and executive order reviews.

Today's final decisions addressing standards for fine and coarse particles are 

based on a thorough review in the Integrated Science Assessment of scientific 

information on known and potential human health and welfare effects associated with 

exposure to these subclasses of PM at levels typically found in the ambient air. These 

final decisions also take into account: (1) staff assessments in the Policy Assessment of 

the most policy-relevant information in the Integrated Science Assessment as well as a 

quantitative health risk assessment and urban-focused visibility assessment based on that 
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information; (2) CASAC advice and recommendations, as reflected in its letters to the 

Administrator, its discussions of drafts of the Integrated Science Assessment, Risk and 

Exposure Assessments, and Policy Assessment at public meetings, and separate written 

comments prepared by individual members of the CASAC PM Review Panel; (3) public 

comments received during the development of these documents, eitherboth in connection 

with CASAC meetings orand separately; and (4) extensive public comments received on 

the proposed rulemaking.

III. Rationale for Final Decisions on the Primary PM2.5 Standards

This section presents the Administrator’s final decision regarding the need to 

revise the current primary PM2.5 standards and, more specifically, regarding revisions to 

the level and form of the existing primary annual PM2.5 standard in conjunction with 

retaining the existing primary 24-hour PM2.5 standard. As discussed more fully below, the 

rationale for the final decision is based on a thorough review, in the Integrated Science 

Assessment, of the latest scientific information, published through mid-2009, on human 

health effects associated with long- and short-term exposures to fine particles in the 

ambient air. The final decisions also take into account: (1) staff assessments of the most 

policy-relevant information presented and assessed in the Integrated Science Assessment 

and staff analyses of air quality and human risks presented in the Risk Assessment and 

the Policy Assessment, upon which staff conclusions regarding appropriate 

considerations in this review are based; (2) CASAC advice and recommendations, as 

reflected in discussions of drafts of the Integrated Science Assessment, Risk Assessment, 

and Policy Assessment at public meetings, in separate written comments, and in 

CASAC’s letters to the Administrator; (3) the multiple rounds of public comments 
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received during the development of these documents, both in connection with CASAC 

meetings and separately; and (4) extensive public comments received on the proposal. 

In developing this final rule, the Administrator recognizes that the CAA requires 

her to reach a public health policy judgment as to what standards would be requisite --

neither more nor less stringent than necessary -- to protect public health with an adequate 

margin of safety, based on scientific evidence and technical assessments that have 

inherent uncertainties and limitations. This judgment requires making reasoned decisions 

as to what weight to place on various types of evidence and assessments, and on the 

related uncertainties and limitations. Thus, in selecting the final standards, the 

Administrator is seeking not only to prevent fine particle concentrations that have been 

demonstrated to be harmful but also to prevent lower fine particle concentrations that 

may pose an unacceptable risk of harm, even if the risk is not precisely identified as to 

nature or degree. 

As discussed below, as well as in more detail in the proposal, a substantial amount 

of new research has been conducted since the close of the science assessment in the last 

review of the PM2.5 NAAQS (U.S. EPA, 2004), with important new information coming 

from epidemiological studies, in particular. This body of evidence includes hundreds of 

new epidemiological studies conducted in many countries around the world. In its 

assessment of the evidence judged to be most relevant to making decisions on elements 

of the primary PM2.5 standards, the EPA has placed greater weight on U.S. and Canadian 
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studies using PM2.5 measurements, since studies conducted in other countries may reflect 

different demographic and air pollution characteristics.16

The newly available research studies as well as the earlier body of scientific 

evidence presented and assessed in the Integrated Science Assessment have undergone 

intensive scrutiny through multiple layers of peer review and opportunities for public 

review and comment. In developing this final rule, the EPA has drawn upon an 

integrative synthesis of the entire body of evidence concerning exposure to ambient fine 

particles and a broad range of health endpoints (U.S. EPA, 2009a, Chapters 2, 4, 5, 6, 7, 

and 8) focusing on those health endpoints for which the Integrated Science Assessment 

concludes that there is a causal or likely causal relationship with long- or short-term 

PM2.5 exposures. The EPA has also considered health endpoints for which the Integrated 

Science Assessment concludes there is evidence suggestive of a causal relationship with 

long-term PM2.5 exposures. 

The EPA has also drawn upon a quantitative risk assessment based upon the 

scientific evidence described and assessed in the Integrated Science Assessment. These 

analyses, discussed in the Risk Assessment (U.S. EPA, 2010a) and Policy Assessment

(U.S. EPA, 2011a, chapter 2), have also undergone intensive scrutiny through multiple 

layers of peer review and multiple opportunities for public review and comment. 

Although important uncertainties remain in the qualitative and quantitative 

characterizations of health effects attributable to ambient fine particles, progress has been 

made in addressing these uncertainties in this review. The EPA’s review of this 

information has been extensive and deliberate. This intensive evaluation of the scientific 

 

16 Nonetheless, the Administrator recognizes the importance of all studies, including 

international studies, in the Integrated Science Assessment’s considerations of the weight 

of the evidence that informs causality determinations.
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evidence and quantitative assessments has provided a comprehensive and adequate basis 

for regulatory decision making at this time. 

This section describes the integrative synthesis of the evidence and technical 

information contained in the Integrated Science Assessment, the Risk Assessment, and 

the Policy Assessment with regard to the current and alternative standards. The EPA 

notes that the final decision for retaining or revising the current primary PM2.5 standards 

is a public health policy judgment made by the Administrator. The Administrator’s final 

decision draws upon scientific information and analyses related to health effects and 

risks; judgments about uncertainties that are inherent in the scientific evidence and 

analyses; CASAC advice; and comments received in response to the proposal.

In presenting the rationale for the final decisions on the primary PM2.5 standards, 

this section begins with a summary of the approaches used in setting the initial primary 

PM2.5 NAAQS in 1997 and in reviewing and revising those standards in 2006 (section 

III.A.1). The D.C. Circuit Court of Appeals remand of the primary annual PM2.5 standard 

in 2009 is discussed in section III.A.2. Taking into consideration this history, section 

IIIII.A.3 describes the EPA’s general approach used in the current review for considering 

the need to retain or revise the current suite of fine particle standards, taking into account 

public comment on the proposed approach. 

The scientific evidence and quantitative risk assessment were presented in 

sections III.B and III.C of the proposal, respectively (77 FR 38906 to 38917, June 29, 

2012) and are outlined in sections III.B and III.C below. Subsequent sections of this 

preamble provide a more complete discussion of the Administrator’s rationale, in light of 

key issues raised in public comments, for concluding that it is appropriate to revise the 
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suite of current primary PM2.5 standards (section III.D), as well as a more complete 

discussion of the Administrator’s rationale for retaining or revising the specific elements 

of the primary PM2.5 standards, namely the indicator (section III.E.1); averaging time 

(section III.E.2); form (section III.E.3); and level (section III.E.4). A summary of the 

final decisions to revise the suite of primary PM2.5 standards is presented in section III.F. 

A. Background

There are currently two primary PM2.5 standards providing public health 

protection from effects associated with fine particle exposures. The annual standard is set 

at a level of 15.0 μg/m3, based on the 3-year average of annual arithmetic mean PM2.5

concentrations from single or multiple monitors sited to represent community-wide air 

quality. The 24-hour standard is set at a level of 35 μg/m3, based on the 3-year average of 

the 98thpercentile of 24-hour PM2.5 concentrations at each population-oriented monitor 

within an area. 

The past and current approaches for reviewing the primary PM2.5 standards 

described below are all based most fundamentally on using information from 

epidemiological studies to inform the selection of PM2.5 standards that, in the 

Administrator’s judgment, protect public health with an adequate margin of safety. Such 

information can be in the form of air quality distributions over which health effect 

associations have been observed in scientific studies or in the form of concentrationresponse functions that support quantitative risk assessment. However, evidence- and 

risk-based approaches using information from epidemiological studies to inform 

decisions on PM2.5 standards are complicated by the recognition that no population 

threshold, below which it can be concluded with confidence that PM2.5-related effects do 
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not occur, can be discerned from the available evidence.17 As a result, any general 

approach to reaching decisions on what standards are appropriate necessarily requires 

judgments about how to translate the information available from the epidemiological 

studies into a basis for appropriate standards. This includes consideration of how to 

weigh the uncertainties in the reported associations across the distributions of PM2.5

concentrations in the studies and the uncertainties in quantitative estimates of risk, in the 

context of the entire body of evidence before the Agency. Such approaches are consistent 

with setting standards that are neither more nor less stringent than necessary, recognizing 

that a zero-risk standard is not required by the CAA. 

1. General Approach Used in Previous Reviews

The general approach used to translate scientific information into standards in the 

previous PM NAAQS reviews focused on consideration of alternative standard levels that 

were somewhat below the long-term mean PM2.5 concentrations reported in key 

epidemiological studies (U.S. EPA, 2011a, section 2.1.1). This approach recognized that 

the strongest evidence of PM2.5-related associations occurs where the bulk of the data 

exists, which is over a range of concentrations around the long-term (i.e., annual) mean. 

In setting primary PM2.5 annual and 24-hour standards for the first time in 1997, 

the Agency relied primarily on an evidence-based approach that focused on 

epidemiological evidence, especially from short-term exposure studies of fine particles 

judged to be the strongest evidence at that time (U.S. EPA, 2011a, section 2.1.1.1). The 

EPA selected a level for the annual standard that was at or below the long-term mean 

 

17 The term "evidence-based" approach or consideration generally refers to using the 

information in the scientific evidence to inform judgments on the need to retain or revise 

the NAAQS. The term "risk-based" generally refers to using the quantitative information 

in the Risk Assessment to inform such judgments.
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PM2.5 concentrations in studies providing evidence of associations with short-term PM2.5

exposures, placing greatest weight on those short-term exposure studies that reported 

clearly statistically significant associations with mortality and morbidity effects. Further 

consideration of long-term mean PM2.5 concentrations associated with mortality and 

respiratory effects in children did not provide a basis for establishing a lower annual 

standard level. The EPA did not place much weight on quantitative risk estimates from 

the very limited risk assessment conducted, but did conclude that the risk assessment 

results confirmed the general conclusions drawn from the epidemiological evidence that a 

serious public health problem was associated with ambient PM levels allowed under the 

then current PM10 standards (62 FR 38665/1, July 18, 1997).

The EPA considered the epidemiological evidence and data on air quality 

relationships to set an annual PM2.5 standard that was intended to be the “generally 

controlling” standard; i.e., the primary means of lowering both long- and short-term 

ambient concentrations of PM2.5.

18 In conjunction with the annual standard, the EPA also 

established a 24-hour PM2.5 standard to provide supplemental protection against days 

with high peak concentrations, localized “hotspots,” and risks arising from seasonal 

emissions that might not be well controlled by an annual standard (62 FR 38669/3). 

 

18 In so doing, the EPA noted that because an annual standard would focus control 

programs on annual average PM2.5 concentrations, it would not only control long-term 

exposure levels, but would also generally control the overall distribution of 24-hour 

exposure levels, resulting in fewer and lower 24-hour peak concentrations. Alternatively, 

a 24-hour standard that focused controls on peak concentrations could also result in lower 

annual average concentrations. Thus, the EPA recognized that either standard could 

provide some degree of protection from both short- and long-term exposures, with the 

other standard serving to address situations where the daily peaks and annual averages 

are not consistently correlated (62 FR 38669, July 18, 1997). In the circumstances 

presented in that review, the EPA determined that it was appropriate to focus on the 

annual standard as the standard basedbest suited to control both annual and daily air 

quality distributions (62 FR 38670).
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In 2006, the EPA used a different evidence-based approach to assess the 

appropriateness of the levels of the 24-hour and annual PM2.5 standards (U.S. EPA, 

2011a, section 2.1.1.2). Based on an expanded body of epidemiological evidence that was 

stronger and more robust than that available in the 1997 review, including additional 

studies of both short- and long-term exposures, the EPA decided that using evidence of 

effects associated with periods of exposure that were most closely matched to the 

averaging time of each standard was the most appropriate public health policy approach 

for evaluating the scientific evidence to inform selecting the level of each standard. Thus, 

the EPA relied upon evidence from the short-term exposure studies as the principal basis 

for revising the level of the 24-hour PM2.5 standard from 65 to 35 μg/m3to protect 

against effects associated with short-term exposures. The EPA relied upon evidence from 

long-term exposure studies as the principal basis for retaining the level of the annual 

PM2.5 standard at 15 μg/m3to protect against effects associated with long-term exposures. 

This approach essentially took the view that short-term studies were not appropriate to 

inform decisions relating to the level of the annual standard, and long-term studies were 

not appropriate to inform decisions relating to the level of the 24-hour standard. With 

respect to quantitative risk-based considerations, the EPA determined that the estimates 

of risks likely to remain upon attainment of the 1997 suite of PM2.5 standards were 

indicative of risks that could be reasonably judged important from a public health 

perspective and, thus, supported revision of the standards. However, the EPA judged that 

the quantitative risk assessment had important limitations and did not provide an 

appropriate basis for selecting the levels of the revised standards in 2006 (71 FR 61174/1-

2, October 17, 2006). 
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2. Remand of Primary Annual PM2.5 Standard

As noted above in section II.B.2, several parties filed petitions for review in the 

U.S. Court of Appeals for the District of Columbia Circuit following promulgation of the 

revised PM NAAQS in 2006. These petitions challenged several aspects of the final rule 

including the level of the primary PM2.5 annual standard. The primary 24-hour PM2.5

standard was not challenged by any of the litigants and, thus, was not considered in the 

court’s review and decision. 

On judicial review, the D.C. Circuit remanded the primary annual PM2.5 NAAQS 

to the EPA on grounds that the Agency failed to adequately explain why the annual 

standard provided the requisite protection from both short- and long-term exposures to 

fine particles including protection for at-risk populations. American Farm Bureau 

Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). With respect to human health 

protection from short-term PM2.5 exposures, the court considered the different approaches 

used by the EPA in the 1997 and 2006 PM NAAQS decisions, as summarized in section 

III.A.1 above. The court found that the EPA failed to adequately explain why a primary 

24-hour PM2.5 standard by itself would provide the protection needed from short-term 

exposures and remanded the primary annual PM2.5 standard to the EPA “for further 

consideration of whether it is set at a level requisite to protect the public health while 

providing an adequate margin of safety from the risk of short-term exposures to PM2.5.” 

American Farm Bureau Federation v. EPA, 559 F. 3d at 520-24. 

With respect to protection from long-term exposure to fine particles, the court 

found that the EPA failed to adequately explain how the primary annual PM2.5 standard 

provided an adequate margin of safety for children and other at-risk populations. The 
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court found that the EPA did not provide a reasonable explanation of why certain 

morbidity studies, including a study of children in Southern California showing lung 

damage associated with long-term PM2.5 exposure (Gauderman et.al, 2000) and a multicity study (24-Cities Study) evaluating decreased lung function in children associated 

with long-term PM2.5 exposures (Raizenne et al., 1996), did not warrant a more stringent 

annual PM2.5 standard. Id. at 522-23. Specifically, the court found that:

EPA was unreasonably confident that, even though it relied solely upon long-term 

mortality studies, the revised standard would provide an adequate margin of 

safety with respect to morbidity among children. Notably absent from the final 

rule, moreover, is any indication of how the standard will adequately reduce risk 

to the elderly or to those with certain heart or lung diseases despite (a) the EPA’s 

determination in its proposed rule that those subpopulations are at greater risk 

from exposure to fine particles and (b) the evidence in the record supporting that 

determination. Id. at 525.

In addition, the court held that the EPA had not adequately explained its decision 

to base the level of the annual standard essentially exclusively on the results of long-term 

studies and the 24-hour standard level essentially exclusively on the results of short-term 

studies. See 559 F. 3d at 522 (“[e]ven if the long-term studies available today are useful 

for setting an annual standard,..., it is not clear why the EPA no longer believes it useful 

to look as well to short-term studies in order to design the suite of standards that will 

most effectively reduce the risks associated with short-term exposure”); see also idId. at 

523-24 (holding that the EPA had not adequately explained why a standard based on 

levels in short-term exposure studies alone provided appropriate protection from health 

effects associated with short-term PM2.5 exposures given the stated need to lower the 

entire air quality distribution, and not just peak concentrations, in order to control against 

short-term effects). 

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In remanding the primary annual PM2.5 standard for reconsideration, the court did 

not vacate the standard, Id. at 530, so the standard remains in effect and is therefore the 

standard considered by the EPA in this review.

3. General Approach Used in the Policy Assessment for the Current Review

This review is based on an assessment of a much expanded body of scientific 

evidence, more extensive air quality data and analyses, and a more comprehensive 

quantitative risk assessment relative to the information available in past reviews, as 

presented and assessed in the Integrated Science Assessment and Risk Assessment and 

discussed in the Policy Assessment. As a result, the EPA’s general approach to reaching 

conclusions about the adequacy of the current suite of PM2.5 standards and potential 

alternative standards that are appropriate to consider was broader and more integrative 

than in past reviews. Our general approach also reflected consideration of the issues 

raised by the court in its remand of the primary annual PM2.5 standard as discussed in 

section III.A.2 above, since decisions made in this review, and the rationales for those 

decisions, will comprise the Agency’s response to the remand.

The EPA’s general approach took into account both evidence-based and riskbased considerations and the uncertainties related to both types of information, as well as 

advice from CASAC (Samet, 2010c,d) and public comments on the first and second draft 

Policy Assessments (U.S. EPA, 2010c,f). In so doing, the EPA staff developed a final 

Policy Assessment (U.S. EPA, 2011a) which provided as broad an array of policy options 

as was supported by the available information, recognizing that the selection of a specific 

approach to reaching final decisions on the primary PM2.5 standards will reflect the 

judgments of the Administrator as to what weight to place on the various approaches and 
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types of information available in the current review.

The Policy Assessment concluded it was most appropriate to consider the 

protection against PM2.5-related mortality and morbidity effects, associated with both 

long- and short-term exposures, afforded by the annual and 24-hour standards taken 

together, as was done in the 1997 review, rather than to consider each standard 

separately, as was done in the 2006 review (U.S. EPA, 2011a, section 2.1.3).19 As the 

EPA recognized in 1997, there are various ways to combine two standards to achieve an 

appropriate degree of public health protection. The extent to which these two standards 

are interrelated in any given area depends in large part on the relative levels of the 

standards, the peak-to-mean ratios that characterize air quality patterns in an area, and 

whether changes in air quality designed to meet a given suite of standards are likely to be 

of a more regional or more localized nature. 

In considering the combined effect of annual and 24-hour standards, the Policy 

Assessment recognized that changes in PM2.5 air quality designed to meet an annual 

standard would likely result not only in lower annual average PM2.5 concentrations but 

also in fewer and lower peak 24-hour PM2.5 concentrations. The Policy Assessment also 

recognized that changes designed to meet a 24-hour standard would result not only in 

fewer and lower peak 24-hour PM2.5 concentrations but also in lower annual average 

PM2.5 concentrations. Thus, either standard could be viewed as providing protection from 

 

19 By utilizing this approach, the Agency also is responsive to the remand of the 2006 

standard. As noted in section III.A.2, the D.C. Circuit, in remanding the 2006 primary 

annual PM2.5 standard, concluded that the Administrator had failed to adequately explain 

why an annual standard was sufficiently protective in the absence of consideration of the 

long-term mean PM2.5 concentrations in short-term exposure studies as well, and likewise 

had failed to explain why a 24-hour standard was sufficiently protective in the absence of 

consideration of the effect of an annual standard on reducing the overall distribution of 

24-hour average PM2.5 concentrations. 559 F. 3d at 520-24. 
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effects associated with both short- and long-term exposures, with the other standard 

serving to address situations where the daily peak and annual average concentrations are 

not consistently correlated. 

In considering the currently available evidence, the Policy Assessment recognized 

that the short-term exposure studies were primarily drawn from epidemiological studies 

that associated variations in area-wide health effects with monitor(s) that measured the 

variation in daily PM2.5 concentrations over the course of several years. The strength of 

the associations in these data was demonstrably in the numerous “typical” days within the 

air quality distribution, not in the peak days. See also 71 FR 61168, October 17, 2006 and 

American Farm Bureau Federation v EPA, 559 F. 3d at 523, 524 (making the same 

point). The quantitative risk assessments conducted for this and previous reviews 

demonstrated the same point; that is, much, if not most of the aggregate risk associated 

with short-term exposures results from the large number of days during which the 24-

hour average concentrations are in the low-to mid-range, below the peak 24-hour 

concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In 

addition, there was no evidence suggesting that risks associated with long-term exposures 

were likely to be disproportionately driven by peak 24-hour concentrations.20

For these reasons, the Policy Assessment concluded that strategies that focused 

primarily on reducing peak days were less likely to achieve reductions in the PM2.5

concentrations that were most strongly associated with the observed health effects. 

Furthermore, the Policy Assessment concluded that a policy approach that focused on 

 

20 In confirmation, a number of studies have presented analyses excluding higher PM 

concentration days and reported a limited effect on the magnitude of the effect estimates 

or statistical significance of the association (e.g., Dominici, 2006b; Schwartz et al, 1996; 

Pope and Dockery, 1992).
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reducing peak exposures would most likely result in more uneven public health 

protection across the U.S. by either providing inadequate protection in some areas or 

overprotecting in other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). 

This is because, as discussed above, reductions based on control of peak days are less 

likely to control the bulk of the air quality distribution. 

The Policy Assessment concluded that a policy goal of setting a “generally 

controlling” annual standard that will lower a wide range of ambient 24-hour PM2.5

concentrations, as opposed to focusing on control of peak 24-hour PM2.5 concentrations, 

was the most effective and efficient way to reduce total population risk and so provide 

appropriate protection. This approach, in contrast to one focusing on a generally 

controlling 24-hour standard, would likely reduce aggregate risks associated with both 

long- and short-term exposures with more consistency and would likely avoid setting 

national standards that could result in relatively uneven protection across the country, due 

to setting standards that are either more or less stringent than necessary in different 

geographical areas (U.S. EPA, 2011a, p. 2-9).

The Policy Assessment also concluded that an annual standard intended to serve 

as the primary means for providing protection from effects associated with both long- and 

short-term PM2.5 exposures cannot be expected to offer sufficient protection against the 

effects of all short-term PM2.5 exposures. As a result, in conjunction with a generally 

controlling annual standard, the Policy Assessment concluded it was appropriate to 

consider setting a 24-hour standard to provide supplemental protection, particularly for 

areas with high peak-to-mean ratios possibly associated with strong local or seasonal 

sources, or PM2.5-related effects that may be associated with shorter-than-daily exposure 
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periods (U.S. EPA, 2011a, p. 2-10).

The Policy Assessment’s consideration of the protection afforded by the current 

and alternative suites of standards focused on PM2.5-related health effects associated with 

long-term exposures for which the magnitude of quantitative estimates of risks to public 

health generated in the risk assessment was appreciably larger in terms of overall 

incidence and percent of total mortality or morbidity effects than for short-term 

PM2.5-related effects. Nonetheless, the EPA also considered health effects and estimated 

risks associated with short-term exposures. In both cases, the Policy Assessment placed 

greatest weight on health effects that had been judged in the Integrated Science 

Assessment to have a causal or likely causal relationship with PM2.5 exposures, while 

also considering health effects judged to be suggestive of a causal relationship or 

evidence that focused on specific at-risk populations. The Policy Assessment placed 

relatively greater weight on statistically significant associations that yielded relatively 

more precise effect estimates and that were judged to be robust to confounding by other 

air pollutants. In the case of short-term exposure studies, the Policy Assessment placed 

greatest weight on evidence from large multi-city studies, while also considering 

associations in single-city studies. 

In translating information from epidemiological studies into the basis for reaching 

staff conclusions on the adequacy of the current suite of standards, the Policy Assessment

considered a number of factors. As an initial matter, the Policy Assessment considered 

the extent to which the currently available evidence and related uncertainties strengthens 

or calls into question conclusions from the last review regarding associations between 

fine particle exposures and health effects. The Policy Assessment also considered 
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evidence of health effects in at-risk populations and the potential impacts on such 

populations. Further, the Policy Assessment explored the extent to which PM2.5-related 

health effects had been observed in areas where air quality distributions extend to lower 

concentrations than previously reported or in areas that would likely have met the current 

suite of standards. 

In translating information from epidemiological studies into the basis for reaching 

staff conclusions on standard levels for consideration (U.S. EPA, 2011a, sections 2.1.3 

and 2.3.4), the Policy Assessment first recognized the absence of discernible thresholds 

in the concentration-response functions from long- and short-term PM2.5 exposure studies 

(U.S. EPA, 2011a, section 2.4.3).21 In the absence of any discernible thresholds, the 

Agency’s general approach for identifying appropriate standard levels for consideration 

involved characterizing the range of PM2.5 concentrations over which we have the most 

confidence in the associations reported in epidemiological studies. In so doing, the Policy 

Assessment recognized that there is no single factor or criterion that comprises the 

“correct” approach, but rather there are various approaches that are reasonable to 

consider for characterizing the confidence in the associations and the limitations and 

uncertainties in the evidence. Identifying the implications of various approaches for 

reaching conclusions on the range of alternative standard levels that is appropriate to 

 

21 The epidemiological studies evaluated in the Integrated Science Assessment that 

examined the shape of concentration-response relationships and the potential presence of 

a threshold focused on cardiovascular-related hospital admissions and emergency 

department visits associated with short-term PM10 exposures and premature mortality 

associated with long-term PM2.5 exposure (U.S. EPA, 2009a, sections 6.5, 6.2.10.10 and 

7.6). Overall, the Integrated Science Assessment concluded that the studies evaluated 

support the use of a no-threshold, log-linear model but recognized that “additional issues 

such as the influence of heterogeneity in estimates between cities, and the effect of 

seasonal and regional differences in PM on the concentration-response relationship still 

require further investigation” (U.S. EPA, 2009a, section 2.4.3).
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consider can help inform the final decisions to either retain or revise the standards. 

Today’s final decisions also take into account public health policy judgments as to the 

degree of health protection that is to be achieved. 

In reaching staff conclusions on the range of annual standard levels that was 

appropriate to consider, the Policy Assessment focused on identifying an annual standard 

that provided requisite protection from effects associated with both long- and short-term 

exposures. In so doing, the Policy Assessment explored different approaches for 

characterizing the range of PM2.5 concentrations over which our confidence in the nature 

of the associations for both long- and short-term exposures is greatest, as well as the 

extent to which our confidence is reduced at lower PM2.5 concentrations. 

First, the Policy Assessment recognized that the approach that most directly 

addressed this issue considered studies that analyzed confidence intervals around 

concentration-response relationships and in particular, analyses that averaged across 

multiple concentration-response models rather than considering a single concentrationresponse model.22 The Policy Assessment explored the extent to which such analyses had 

been published for studies of health effects associated with long- or short-term PM2.5

exposures. Such analyses could potentially be used to characterize a concentration below 

which uncertainty in a concentration-response relationship substantially increases or is 

judged to be indicative of an unacceptable degree of uncertainty about the existence of a 

continuing concentration-response relationship. The Policy Assessment concluded that 

 

22 This is distinct from confidence intervals around concentration-response relationships 

that are related to the magnitude of effect estimates generated at specific PM2.5

concentrations (i.e., point-wise confidence intervals) and that are relevant to the precision 

of the effect estimate across the air quality distribution, rather than to our confidence in 

the existence of a continuing concentration-response relationship across the entire air 

quality distribution on which a reported association was based. 
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identifying this area of uncertainty in the concentration-response relationship could be 

used to inform identification of alternative standard levels that are appropriate to 

consider. 

Further, the Policy Assessment explored other approaches that considered 

different statistical metrics from epidemiological studies. The Policy Assessment first 

took into account the general approach used in previous PM reviews which focused on 

consideration of alternative standard levels that were somewhat below the long-term 

mean PM2.5 concentrations reported in epidemiological studies using air quality 

distributions based on composite monitor concentrations.23 This approach recognized that 

the strongest evidence of PM2.5-related associations occurs at concentrations around the 

long-term (i.e., annual) mean. In using this approach, the Policy Assessment placed 

greatest weight on those long- and short-term exposure studies that reported statistically 

significant associations with mortality and morbidity effects.

In extending this approach, the Policy Assessment also considered information 

beyond a single statistical metric of PM2.5 concentrations (i.e., the mean) to the extent 

such information was available. Pursuant to an express comment from CASAC (Samet 

2010d, p. 2), the Policy Assessment utilized distributional statistics (i.e., statistical 

characterization of an entire distribution of data) to identify the broader range of PM2.5

concentrations that had the most influence on the calculation of relative risk estimates in 

 

23 Using the term “composite monitor” does not imply that the EPA can identify one 

monitor that represents the air quality evaluated in a specific study area. Rather, the 

composite monitor concentration represents the average concentration across monitors 

within each area with more than one monitor included in a given study as typically 

reported in epidemiological studies. For multi-city studies, this metric reflects 

concentrations averaged across multiple monitors or from single monitors within each 

area and then averaged across study areas for an overall study mean PM2.5 concentration. 

This is consistent with the epidemiological evidence considered in other NAAQS 

reviews.
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both long- and short-term exposure epidemiological studies. Thus, the Policy Assessment

considered the part of the distribution of PM2.5 concentrations in which the data analyzed 

in the study (i.e., air quality and population-level data, as discussed below) were most 

concentrated, specifically, the range of PM2.5 concentrations around the long-term mean 

over which our confidence in the magnitude and significance of associations observed in 

the epidemiological studies was greatest. The Policy Assessment then focused on the 

lower part of the distribution to characterize where the data became appreciably more 

sparse and, thus, where our understanding of the magnitude and significance of the 

associations correspondingly became more uncertain. The Policy Assessment recognized 

there was no single percentile value within a given distribution that was most appropriate 

or “correct” to use to characterize where our confidence in the associations becomes 

appreciably lower. The Policy Assessment concluded that the range from the 25th to 10th

percentiles is a reasonable range to consider as a region where we had appreciably less 

confidence in the associations observed in epidemiological studies.24

In considering distributional statistics from epidemiological studies, the final 

Policy Assessment focused on two types of population-level metrics that CASAC advised 

were most useful to consider in identifying the PM2.5 concentrations most influential in 

 

24 In the PM NAAQS review completed in 2006, the Staff Paper similarly recognized that 

the evidence of an association in any epidemiological study is “strongest at and around 

the long-term average where the data in the study are most concentrated. For example, 

the interquartile range of long-term average concentrations within a study [with a lower 

bound of the 25th percentile] or a range within one standard deviation around the study 

mean, may reasonably be used to characterize the range over which the evidence of 

association is strongest” (U.S. EPA, 2005, p. 5-22). A range of one standard deviation 

around the mean represents approximately 68 percent of normally distributed data, and 

below the mean falls between the 25th and 10th percentiles.
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generating the health effect estimates reported in the epidemiological studies.25

Consistent with CASAC advice, the most relevant information was the distribution of 

health events (e.g., deaths, hospitalizations) occurring within a study population in 

relation to the distribution of PM2.5 concentrations. However, in recognizing that access 

to health event data can be restricted, the Policy Assessment also considered the number 

of study participants within each study area as an appropriate surrogate for health event 

data.

The Policy Assessment recognized that an approach considering analyses of 

confidence intervals around concentration-response functions was intrinsically related to 

an approach that considered different distributional statistics. Both of these approaches 

could be employed to understand the broader distribution of PM2.5 concentrations which 

correspond to the health events reported in epidemiological studies. In applying these 

approaches, the Policy Assessment, consistent with CASAC advice (Samet, 2010d, p.3),

considered PM2.5 concentrations from long- and short-term PM2.5 exposure studies using 

composite monitor distributions. 

In reaching staff conclusions on alternative standard levels that were appropriate 

to consider, the Policy Assessment also included a broader consideration of the 

 

25 The second draft Policy Assessment focused on the distributions of ambient PM2.5

concentrations and associated population data across areas included in several multi-city 

studies for which such data were available in seeking to identify the most influential 

range of concentrations (U.S. EPA, 2010f, section 2.3.4.1). In its review of the second 

draft Policy Assessment, CASAC advised that it “would be preferable to have 

information on the concentrations that were most influential in generating the health 

effect estimates in individual studies” (Samet, 2010d, p.2). Therefore, in the final Policy 

Assessment, the EPA considered population-level data (i.e., area-specific health event 

data and study area population data) along with corresponding PM2.5 concentrations to 

generate a cumulative distribution of the population-level data relative to long-term mean 

PM2.5 concentrations to determine the most influential part of the air quality distribution 

(U.S. EPA, 2011a, Figure 2-7 and associated text).

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uncertainties and limitations of the current scientific evidence. Most notably, these 

uncertainties are related to the heterogeneity observed in the epidemiological studies in 

the eastern versus western parts of the U.S., the relative toxicity of PM2.5 components, 

and the potential role of co-pollutants (U.S. EPA, 2011a, pp. 2-25 to 2-26). The 

limitations and uncertainties associated with the currently available scientific evidence, 

including the availability of fewer studies toward the lower range of alternative annual 

standard levels being considered in this proposal, are summarized in section III.B below 

and further discussed in section III.B.2 of the preamble to the proposed ruleproposal.

The Policy Assessment recognized that the level of protection afforded by the 

NAAQS relies both on the level and the form of the standard. The Policy Assessment 

concluded that a policy approach that used data based on composite monitor distributions 

to identify alternative standard levels, and then compared those levels to concentrations at 

maximum monitors to determine whether an area meets a given standard, inherently has 

the potential to build in some margin of safety (U.S. EPA, 2011a, p. 2-14).26 This 

conclusion was consistent with CASAC’s comments on the second draft Policy 

Assessment, in which CASAC expressed its preference for focusing on an approach 

using composite monitor distributions “because of its stability, and for the additional 

 

26 Statistical metrics (e.g., means) based on composite monitor distributions may be 

identical to or below the same statistical metrics based on maximum monitor 

distributions. For example, some areas may have only one monitor, in which case the 

composite and maximum monitor distributions will be identical in those areas. Other 

areas may have multiple monitors that may be very close to the monitor measuring the 

highest concentrations, in which case the composite and maximum monitor distributions 

could be similar in those areas. As noted in Hassett-Sipple et al. (2010), for studies 

involving a large number of areas, the composite and maximum concentrations are 

generally within 5 percent of each other (77 FR 38905, fn. 30). Still other areas may have 

multiple monitors that may be separately impacted by local sources in which case the 

composite and maximum monitor distributions could be quite different (U.S. EPA, 

2011a, p. 2-14). See further discussion of this issue in section III.E.4.c.i below.
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margin of safety it provides” when “compared to the maximum monitor perspective” 

(Samet, et al., 2010d, pp. 2 to 3). 

In reaching staff conclusions on alternative 24-hour standard levels that are 

appropriate to consider for setting a 24-hour standard intended to supplement the 

protection afforded by a generally controlling annual standard, the Policy Assessment

considered currently available short-term PM2.5 exposure studies. The evidence from 

these studies informed our understanding of the protection afforded by the suite of 

standards against effects associated with short-term exposures. In considering the shortterm exposure studies, the Policy Assessment evaluated both the distributions of 24-hour 

PM2.5 concentrations, with a focus on the 98th percentile concentrations (to the extent 

such data were available) to match the form of the current 24-hour PM2.5 standard, as 

well as the long-term mean PM2.5 concentrations reported in these studies. In addition to 

considering the epidemiological evidence, the Policy Assessment also considered air 

quality information based on county-level 24-hour and annual design values27 to 

understand the policy implications of the alternative standard levels supported by the 

underlying science. In particular, the Policy Assessment considered the extent to which 

different combinations of alternative annual and 24-hour standards would support the 

policy goal of focusing on a generally controlling annual standard in conjunction with a 

24-hour standard that would provide supplemental protection. In so doing, the Policy 

Assessment discussed the roles that each standard might be expected to play in the 

protection afforded by alternative suites of standards.

Beyond these evidence-based considerations, the Policy Assessment also 

 

27 Design values are the metrics (i.e., statistics) that are compared to the NAAQS levels to 

determine compliance.
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considered the quantitative risk estimates and the key observations presented in the Risk 

Assessment. This assessment included an evaluation of 15 urban case study areas and 

estimated risk associated with a number of health endpoints associated with long- and 

short-term PM2.5 exposures (U.S. EPA, 2010a). As part of the risk-based considerations, 

the Policy Assessment considered estimates of the magnitude of PM2.5-related risks 

associated with recent air quality levels and air quality simulated to just meet the current 

and alternative suites of standards using alternative simulation approaches. The Policy 

Assessment also characterized the risk reductions, relative to the risks remaining upon 

just meeting the current standards, associated with just meeting alternative suites of 

standards. In so doing, the Policy Assessment recognized the uncertainties inherent in 

such risk estimates, and took such uncertainties into account by considering the 

sensitivity of the “core” risk estimates to alternative assumptions and methods likely to 

have substantial impact on the estimates. In addition, the Policy Assessment considered 

additional analyses characterizing the representativeness of the urban study areas within a 

broader national context to understand the roles that the annual and 24-hour standards 

may play in affording protection against effects related to both long- and short-term 

PM2.5 exposures.

Based on the approach discussed above, the Policy Assessment reached 

conclusions related to the primary PM2.5 standards that reflected an understanding of both 

evidence-based and risk-based considerations to inform two overarching questions 

related to: (1) the adequacy of the current suite of PM2.5 standards and (2) revisions to the 

standards that were appropriate to consider in this review to protect against health effects 

associated with both long- and short-term exposures to fine particles. When evaluating 
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the health protection afforded by the current or any alternative suites of standards 

considered, the Policy Assessment took into account the four basic elements of the 

NAAQS: the indicator, averaging time, form, and level.

The general approach for reviewing the primary PM2.5 standards described above 

provided a comprehensive basis that helped to inform the Administrator’s judgments in 

reaching her proposed and final decisions to revise the current suite of primary fine 

particle NAAQS and in responding to the remand of the 2006 primary annual PM2.5

standard.

B. Overview of Health Effects Evidence

This section outlines the key information presented in section III.B of the 

proposal (77 FR 38906 to 38911, June 29, 2012) and discussed more fully in the 

Integrated Science Assessment (Chapters 2, 4, 5, 6, 7, and 8) and the Policy Assessment

(Chapter 2) related to health effects associated with fine particle exposures. Section III.B. 

of the proposal discusses available information on the health effects associated with 

exposures to PM2.5, including the nature of such health effects (section III.B.1) and 

associated limitations and uncertainties (section III.B.2), at-risk populations (section 

III.B.3), and potential PM2.5-related impacts on public health (section III.B.4). As was 

true in the last two reviews, evidence from epidemiologicepidemiological, controlled 

human exposure and animal toxicological studies played a key role in the Integrated 

Science Assessment’s evaluation of the scientific evidence. 

The 2006 PM NAAQS review concluded that there was “strong epidemiological 

evidence” for linking long-term PM2.5 exposures with cardiovascular-related and lung 

cancer mortality and respiratory-related morbidity and for linking short-term PM2.5
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exposures with cardiovascular-related and respiratory-related mortality and morbidity 

(U.S. EPA, 2004, p. 9-46; U.S. EPA, 2005, p. 5-4). Overall, the evidence from 

epidemiological, toxicological, and controlled human exposure studies supported “likely 

causal associations” between PM2.5 and both mortality and morbidity from cardiovascular 

and respiratory diseases, based on “an assessment of strength, robustness, and 

consistency in results” (U.S. EPA, 2004, p. 9-48).28

In this review, based on the expanded body of evidence, the EPA finds that:

(1) In looking across the extensive new scientific evidence available in this review, 

our overall understanding of health effects associated with fine particle exposures 

has been greatly expanded. The currently available evidence is largely consistent 

with evidence available in the last review and substantially strengthens what is 

known about the effects associated with fine particle exposures. 

(2) A number of large multi-city epidemiological studies have been conducted 

throughout the U.S., including extended analyses of long-term exposure studies 

that were important to inform decision-making in the last review. The body of 

currently available scientific evidence has also been expanded greatly by the 

publication of a number of new multi-city, time-series studies that have used 

uniform methodologies to investigate the effects of short-term PM2.5 exposures on 

public health. This body of evidence provides a more expansive data base and 

considers multiple locations representing varying regions and seasons that provide 

evidence of the influence of different air pollution mixes on PM2.5-associated 

health effects. These studies provide more precise estimates of the magnitude of 

effects associated with short-term PM2.5 exposure than most smaller-scale singlecity studies that were more commonly available in the last review. These studies 

have reported consistent increases in morbidity and/or premature mortality related 

to ambient PM2.5 concentrations, with the strongest evidence reported for 

cardiovascular-related effects.

(3) In addition, the findings of new toxicological and controlled human exposure 

studies greatly expand and provide stronger support for a number of potential 

biologicbiological mechanisms or pathways for cardiovascular and respiratory 

effects associated with long- and short-term PM exposures. These studies provide 

 

28 The term “likely causal association” was used in the 2004 Criteria Document to 

summarize the strength of the available evidence available in the last review for PM2.5. 

However, this terminology was not based on a formal framework for evaluating evidence 

for inferring causation. Since the last review, the EPA has developed a more formal 

framework for reaching causal determinations with standardized language to express 

evaluation of the evidence (U.S. EPA, 2009a, section 1.5).

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coherence and biological plausibility for the effects observed in epidemiological 

studies.

(4) Using a more formal framework for reaching causal determinations than used in 

prior reviews29,,

30 the EPA concludes that a causal relationship exists between 

both long- and short-term exposures to PM2.5 and premature mortality and 

cardiovascular effects and a likely causal relationship exists between long- and 

short-term PM2.5 exposures and respiratory effects. Further, there is evidence 

suggestive of a causal relationship between long-term PM2.5 exposures and other 

health effects, including developmental and reproductive effects (e.g., low birth 

weight, infant mortality) and carcinogenic, mutagenic, and genotoxic effects (e.g., 

lung cancer mortality).31

(5) The newly available evidence significantly strengthens the link between long- and 

short-term exposure to PM2.5 and premature mortality, while providing indications 

that the magnitude of the PM2.5-mortality association with long-term exposures 

may be larger than previously estimated. The strongest evidence comes from 

 

29 The causal framework draws upon the assessment and integration of evidence from 

across epidemiological, controlled human exposure, and toxicological studies, and the 

related uncertainties that ultimately influence our understanding of the evidence. This 

framework employs a five-level hierarchy that classifies the overall weight of evidence 

and causality using the following categorizations: causal relationship, likely to be causal 

relationship, suggestive of a causal relationship, inadequate to infer a causal relationship, 

and not likely to be a causal relationship (U.S. EPA, 2009a, Table 1-3). The development 

of the causal framework reflects considerable input from CASAC and the public, with 

CASAC concluding that, “The five-level classification of strength of evidence for causal

inference has been systemically applied [for PM]; this approach has provided 

transparency and a clear statement of the level of confidence with regard to causation, 

and we recommend its continued use in future ISAs” (Samet, 2009f, p. 1).

30 The causal framework draws upon the assessment and integration of evidence from 

across epidemiological, controlled human exposure, and toxicological studies, and the 

related uncertainties that ultimately influence our understanding of the evidence. This 

framework employs a five-level hierarchy that classifies the overall weight of evidence 

and causality using the following categorizations: causal relationship, likely to be causal 

relationship, suggestive of a causal relationship, inadequate to infer a causal relationship,

and not likely to be a causal relationship (U.S. EPA, 2009a, Table 1-3). The development 

of the causal framework reflects considerable input from CASAC and the public, with 

CASAC concluding that, “The five-level classification of strength of evidence for causal 

inference has been systemically applied [for PM]; this approach has provided 

transparency and a clear statement of the level of confidence with regard to causation, 

and we recommend its continued use in future ISAs” (Samet, 2009f, p. 1).

31 These causal inferences are based not only on the more expansive epidemiological 

evidence available in this review but also reflect consideration of important progress that 

has been made to advance our understanding of a number of potential biologic modes of 

action or pathways for PM-related cardiovascular and respiratory effects (U.S. EPA, 

2009a, chapter 5).
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recent studies investigating long-term exposure to PM2.5 and cardiovascularrelated mortality. The evidence supporting a causal relationship between longterm PM2.5 exposure and mortality also includes consideration of new studies that 

demonstrated an improvement in community health following reductions in 

ambient fine particles.

(6) Several new studies have examined the association between cardiovascular effects 

and long-term PM2.5 exposures in multi-city studies conducted in the U.S. and 

Europe. While studies were not available in the last review with regard to longterm exposure and cardiovascular-related morbidity, recent studies have provided 

new evidence linking long-term exposure to PM2.5 with an array of cardiovascular 

effects such as heart attacks, congestive heart failure, stroke, and mortality. This 

evidence is coherent with studies of short-term exposure to PM2.5 that have 

observed associations with a continuum of effects ranging from subtle changes in 

indicators of cardiovascular health to serious clinical events, such as increased 

hospitalizations and emergency department visits due to cardiovascular disease 

and cardiovascular mortality.

(7) Extended analyses of studies available in the last review as well as new 

epidemiological studies conducted in the U.S. and abroad provide stronger 

evidence of respiratory-related morbidity effects associated with long-term PM2.5

exposure. The strongest evidence for respiratory-related effects is from studies 

that evaluated decrements in lung function growth, increased respiratory 

symptoms, and asthma development. The strongest evidence from short-term 

PM2.5 exposure studies has been observed for increased respiratory-related 

emergency department visits and hospital admissions for chronic obstructive 

pulmonary disease (COPD) and respiratory infections. 

(8) The body of scientific evidence is somewhat expanded from the 2006 review but 

is still limited with respect to associations between long-term PM2.5 exposures and 

developmental and reproductive effects as well as cancer, mutagenic, and 

genotoxic effects. The strongest evidence for an association between PM2.5 and 

developmental and reproductive effects comes from epidemiological studies of 

low birth weight and infant mortality, especially due to respiratory causes during 

the post-neonatal period (i.e., 1 month -12 months of age). With regard to cancer 

effects, “[m]ultiple epidemiologic studies have shown a consistent positive 

association between PM2.5 and lung cancer mortality, but studies have generally 

not reported associations between PM2.5 and lung cancer incidence” (U.S. EPA 

2009a p. 2-13 ).

(9) Efforts to evaluate the relationships between PM composition and health effects 

continue to evolve. While many constituents of PM2.5 can be linked with differing 

health effects, the evidence is not yet sufficient to allow differentiation of those 

constituents or sources that may be more closely related to specific health 

outcomes nor to exclude any individual component or group of components 

associated with any source categories from the fine particle mixture of concern.
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(10) Specific groups within the general population are at increased risk for 

experiencing adverse health effects related to PM exposures. The currently 

available evidence expands our understanding of previously identified at-risk 

populations (i.e., children, older adults, and individuals with pre-existing heart 

and lung disease) and supports the identification of additional at-risk populations 

(e.g., persons with lower socioeconomic status, genetic differences). Evidence for 

PM-related effects in these at-risk populations has expanded and is stronger than 

previously observed. There is emerging, though still limited, evidence for 

additional potentially at-risk populations, such as those with diabetes, people who 

are obese, pregnant women, and the developing fetus.

(11) The population potentially affected by PM2.5 is large. In addition, large subgroups 

of the U.S. population have been identified as at-risk populations. While 

individual effect estimates from epidemiological studies may be small in size, the 

public health impact of the mortality and morbidity associations can be quite large 

given the extent of exposure. Taken together, this suggests that exposure to 

ambient PM2.5 concentrations can have substantial public health impacts. 

(12) While the currently available scientific evidence is stronger and more consistent 

than in previous reviews, providing a strong basis for decision making in this 

review, the EPA recognizes that important uncertainties and limitations in the 

health effects evidence remain. Epidemiological studies evaluating health effects 

associated with long- and short-term PM2.5 exposures have reported heterogeneity 

in responses between cities and geographic regions within the U.S. This 

heterogeneity may be attributed, in part, to differences in the fine particle 

composition or related to exposure measurement error, which can introduce bias 

and increased uncertainty in associated health effect estimates. Variability in the 

associations observed across PM2.5 epidemiological studies may be due in part to 

exposure error related to measurement-related issues, the use of central fixed-site 

monitors to represent population exposure to PM2.5, models used in lieu of or to 

supplement ambient measurements, and our limited understanding of factors that 

may influence exposures (e.g., topography, the built environment, weather, source 

characteristics, ventilation usage, personal activity patterns, photochemistry). In 

addition, where PM2.5 and other pollutants (e.g., ozone, nitrogen dioxide, and 

carbon monoxide) are correlated, it can be difficult to distinguish the effects of the 

various pollutants in the ambient mixture (i.e., co-pollutant confounding).32

While uncertainties and limitations still remain in the available health effects 

evidence, the Administrator judges the currently available scientific data base to be 

stronger and more consistent than in previous reviews providing a strong basis for 

 

32 A copollutant meets the criteria for potential confounding in PM-health associations if: 

(1) it is a potential risk factor for the health effect under study; (2) it is correlated with 

PM; and (3) it does not act as an intermediate step in the pathway between PM exposure 

and the health effect under study (U.S. EPA, 2004, p. 8-10). 
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decision making in this review. 

C. Overview of Quantitative Characterization of Health Risks

In addition to a comprehensive evaluation of the health effects evidence available 

in this review, the EPA conducted an expanded quantitative risk assessment for selected 

health endpoints to provide additional information and insights to inform decisions on the 

primary PM2.5 NAAQS.33 As discussed in section III.C of the proposal, the approach used 

to develop quantitative risk estimates associated with PM2.5 exposures was built on the 

approach used and lessons learned in the last review and focused on improving the 

characterization of the overall confidence in the risk estimates, including related 

uncertainties, by incorporating a number of enhancements, in terms of both the methods 

and data used in the analyses. 

The goals of this quantitative risk assessment were largely the same as those 

articulated in the risk assessment conducted for the last review. These goals included: (1) 

to provide estimates of the potential magnitude of premature mortality and/or selected 

morbidity effects in the population associated with recent ambient levels of PM2.5 and 

with simulating just meeting the current and alternative suites of PM2.5 standards in 15 

 

33 The quantitative risk assessment conducted for this review is more fully described and 

presented in the Risk Assessment (U.S. EPA, 2010a) and summarized in detail in the 

Policy Assessment (U.S. EPA, 2011a, sections 2.2.2. and 2.3.4.2). The scope and 

methodology for this risk assessment were developed over the last few years with 

considerable input from CASAC and the public as described in section I.C.2 of the 

proposal. II.B.3 above. 

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selected urban study areas34,,

35 including, where data were available, consideration of 

impacts on at-risk populations; (2) to develop a better understanding of the influence of 

various inputs and assumptions on the risk estimates to more clearly differentiate among 

alternative suites of standards; and (3) to gain insights into the distribution of risks and 

patterns of risk reductions and the variability and uncertainties in those risk estimates. In 

addition, the quantitative risk assessment included nationwide estimates of the potential 

magnitude of premature mortality associated with long-term exposure to recent ambient 

PM2.5 concentrations to more broadly characterize this risk on a national scale and to 

support the interpretation of the more detailed risk estimates generated for selected urban 

study areas. 

The expanded and updated risk assessment conducted in this review included 

estimates of risk for: (1) all-cause, ischemic heart disease-related, cardiopulmonary-

 

34 The Risk Assessment concluded that these 15 urban study areas were generally 

representative of urban areas in the U.S. likely to experience relatively elevated levels of 

risk related to ambient PM2.5 exposure with the potential for better characterization at the 

higher end of that distribution (U.S. EPA, 2011a, p. 2-42; U.S. EPA, 2010a, section 4.4, 

Figure 4-17). The representativeness analysis also showed that the 15 urban study areas 

do not capture areas with the highest baseline morality risks or the oldest populations 

(both of which can result in higher PM2.5-related mortality estimates). However, some of 

the areas with the highest values for these attributes had relatively low PM2.5

concentrations (e.g., urban areas in Florida) and, consequently, the Risk Assessment 

concluded failure to include these areas in the set of urban study areas was unlikely to 

exclude high PM2.5-risk locations (U.S. EPA, 2010a, section 4.4.1).

35 The Risk Assessment concluded that these 15 urban study areas were generally 

representative of urban areas in the U.S. likely to experience relatively elevated levels of 

risk related to ambient PM2.5 exposure with the potential for better characterization at the 

higher end of that distribution (U.S. EPA, 2011a, p. 2-42; U.S. EPA, 2010a, section 4.4, 

Figure 4-17). The representativeness analysis also showed that the 15 urban study areas 

do not capture areas with the highest baseline morality risks or the oldest populations 

(both of which can result in higher PM2.5-related mortality estimates). However, some of 

the areas with the highest values for these attributes had relatively low PM2.5

concentrations (e.g., urban areas in Florida) and, consequently, the Risk Assessment 

concluded failure to include these areas in the set of urban study areas was unlikely to 

exclude high PM2.5-risk locations (U.S. EPA, 2010a, section 4.4.1).
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related, and lung cancer-related mortality associated with long-term PM2.5 exposure; (2) 

non-accidental, cardiovascular-related, and respiratory-related mortality associated with 

short-term PM2.5 exposure; and (3) cardiovascular-related and respiratory-related hospital 

admissions and asthma-related emergency department visits associated with short-term 

PM2.5 exposure.

36

The Risk Assessment included a core set of risk estimates supplemented by an 

alternative set of risk results generated using single-factor and multi-factor sensitivity 

analyses. The core set of risk estimates was developed using the combination of modeling 

elements and input data sets identified in the Risk Assessment as having higher 

confidence relative to inputs used in the sensitivity analyses. The results of the sensitivity 

analyses provided information to evaluate and rank the potential impacts of key sources 

of uncertainty on the core risk estimates. In addition, the sensitivity analyses represented 

a set of reasonable alternatives to the core set of risk estimates that fell within an overall 

set of plausible risk estimates surrounding the core estimates. 

The EPA recognized that there were many sources of variability and uncertainty 

inherent in the inputs to its quantitative risk assessment.37 The design of the risk 

assessment included a number of elements to address these issues in order to increase the 

overall confidence in the risk estimates generated for the 15 urban study areas, including 

using guidance from the World Health Organization (WHO, 2008) as a framework for 

 

36 The evidence available for these selected health effect endpoints generally focused on 

the entire population, although some information was available to support analyses that 

considered differences in estimated risk for at-risk populations including older adults and 

persons with pre-existing cardiovascular and respiratory diseases.

37 Variability refers to the heterogeneity of a variable of interest within a population or 

across different populations. Uncertainty refers to the lack of knowledge regarding the 

actual values of inputs to an analysis (U.S. EPA, 2010a, p. 3-63). 
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characterizing uncertainty in the analyses.38

With respect to the sources of variability, the Risk Assessment considered those 

that contributed to differences in risk across urban study areas, but did not directly affect 

the degree of risk reduction associated with the simulation of just meeting current or 

alternative standard levels (e.g., differences in baseline incidence rates, demographics and 

population behavior). The Risk Assessment also focused on factors that not only 

introduced variability into risk estimates across study areas, but also played an important 

role in determining the magnitude of risk reductions upon simulation of just meeting 

current or alternative standard levels (e.g., peak-to-mean ratios of ambient PM2.5

concentrations within individual urban study areas and the nature of the rollback 

approach used to simulate just meeting the current or alternative standards). Key sources 

of potential variability that were likely to affect population risks included the following: 

(1) intra-urban variability in ambient PM2.5 concentrations, including PM2.5 composition;

(2) variability in the patterns of reductions in PM2.5 concentrations associated with 

different rollback approaches when simulating just meeting the current or alternative 

standards; co-pollutant exposures; (3) factors related to demographic and socioeconomic 

status; (4) behavioral differences across urban study areas (e.g., time spent outdoors); (5) 

baseline incidence rates; and (6) longer-term temporal variability in ambient PM2.5

concentrations reflecting meteorological trends as well as future changes in the mix of 

PM2.5 sources, including changes in air quality related to future regulatory actions. 

With regard to uncertainties, single and multi-factor sensitivity analyses were 

combined with a qualitative analysis to assess the impact of potential sources of 

 

38 The extent to which key sources of potential variability were (or were not) fully 

captured in the design of the risk assessment are discussed in section 3.5.2 of the Risk 

Assessment (U.S. EPA, 2010a, pp. 3-67 to 3-69).

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uncertainty on the core risk estimates. Key sources of uncertainty included: (1) 

characterizing intra-urban population exposure in the context of 

epidemiologyepidemiological studies linking PM2.5 to specific health effects; (2) 

statistical fit of the concentration-response functions for short-term exposure-related 

health endpoints; (3) shape of the concentration-response functions; (4) specifying the 

appropriate lag structure for short-term exposure studies; (5) transferability of 

concentration-response functions from study locations to urban study area locations for 

long-term exposure-related health endpoints; (6) use of single-city versus multi-city 

studies in the derivation of concentration-response functions; (7) impact of historical air 

quality on estimates of health risk associate with long-term PM2.5 exposures; and (8) 

potential variation in effect estimates reflecting compositional differences in PM2.5.

Beyond characterizing uncertainty and variability, a number of design elements 

were included in the risk assessment to increase the overall confidence in the risk 

estimates generated for the 15 urban study areas (U.S. EPA, 2011a, pp. 2-38 to 2-41). 

These elements included: (1) use of a deliberative process for specifying components of 

the risk model that reflects consideration of the latest research on PM2.5 exposure and risk 

(U.S. EPA, 2010a, section 5.1.1); (2) integration of key sources of variability into the 

design as well as the interpretation of risk estimates (U.S.EPA, 2010a, section 5.1.2); (3) 

assessment of the degree to which the urban study areas are representative of areas in the 

U.S. experiencing higher PM2.5-related risk (U.S.EPA, 2010a, section 5.1.3); and (4) 

identification and assessment of important sources of uncertainty and the impact of these 

uncertainties on the core risk estimates (U.S.EPA, 2010a, section 5.1.4). Further, 

additional analyses examined potential bias and overall confidence in the risk estimates. 
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Greater confidence is associated with risk estimates based on simulated annual mean 

PM2.5 concentrations that are within the region of the air quality distribution used in 

deriving the concentration-response functions where the bulk of the data reside (e.g., 

within one standard deviation around the long-term mean PM2.5 concentration) (U.S.

EPA, 2011a, p. 2-38).

Key observations and insights from the PM2.5 risk assessment, together with 

important caveats and limitations, were discussed in section III.C.3 of the proposal. In 

general, in considering the set of quantitative risk estimates and related uncertainties and 

limitations related to long- and short-term PM2.5 exposure together with consideration of 

the health endpoints which could not be quantified, the Policy Assessment concluded this 

information provided strong evidence that risks estimated to remain upon simulating just 

meeting the current suite of PM2.5 standards are important from a public health 

perspective, both in terms of severity and magnitude of effects. Patterns of increasing 

estimated risk reductions were generally observed as either the annual or 24-hour 

standard level, or both, were reduced over the ranges considered in the Risk Assessment.

The magnitude of both long- and short-term exposure-related risk estimated to 

remain upon just meeting the current suite of standards as well as alternative standard 

levels was strongly associated with the simulated change in annual mean PM2.5

concentrations. Although long- and short-term exposure-related mortality rates have 

similar patterns in terms of the subset of urban study areas experiencing risk reductions 

for the current suite of standard levels, the magnitude of risk remaining is higher for longterm exposure-related mortality and substantially lower for short-term exposure-related 

mortality. Short-term exposure-related morbidity risk estimates were greater for 
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cardiovascular-related than respiratory-related events and emergency department visits 

for asthma-related events were significant: Furthermore, most of the aggregate risk 

associated with short-term exposures was not primarily driven by the small number of 

days with PM2.5 concentrations in the upper tail of the air quality distribution, but rather 

by the large number of days with PM2.5 concentrations at and around the mean of the 

distribution, that is, the 24-hour average concentrations that are in the low-to mid-range, 

well below the peak 24-hour concentrations (U.S. EPA, 2011a, p. 2-3).

With regard to characterizing estimates of PM2.5-related risk associated with 

simulation of alternative standards, the Policy Assessment recognized that greater overall 

confidence was associated with estimates of risk reduction than for estimates of absolute 

risk remaining (U.S. EPA, 2011a, p. 2-94). Furthermore, the Policy Assessment 

recognized that estimates of absolute risk remaining for each of the alternative standard 

levels considered, particularly in the context of long-term exposure-related mortality, 

may be underestimated

39. In addition, the Policy Assessment observed that in considering 

the overall confidence associated with the quantitative analyses, the Risk Assessment 

recognized that: (1) substantial variability existed in the magnitude of risk remaining 

across urban study areas and (2) in general, higher confidence was associated with risk 

 

39 Based on the consideration of both the qualitative and quantitative assessments of 

uncertainty, the Risk Assessment concluded that it is unlikely that the estimated risks are 

over-stated, particularly for premature mortality related to long-term PM2.5 exposures. In 

fact, the Policy Assessment and the Risk Assessment concluded that the core risk 

estimates for this category of health effects may well be biased low based on 

consideration of alternative model specifications evaluated in the sensitivity analyses 

(U.S. EPA, 2011a, p. 2-41; U.S. EPA, 2010a, p. 5-16; Figures 4-7 and 4-8). In addition, 

the Policy Assessment recognized that the currently available scientific information 

included evidence for a broader range of health endpoints and at-risk populations beyond 

those included in the quantitative risk assessment, including decrements in lung function 

growth and respiratory symptoms in children as well as reproductive and developmental 

effects (U.S. EPA, 2011a, section 2.2.1).
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estimates based on PM2.5 concentrations near the mean PM2.5 concentrations in the 

underlying epidemiological studies providing the concentration-response functions (i.e.g., 

within one standard deviation of the mean PM2.5 concentration reported). Furthermore, 

although the Risk Assessment estimated that the alternative 24-hour standard levels 

considered (when controlling) would result in additional estimated risk reductions beyond 

those estimated for alternative annual standard levels alone, these additional estimated 

reductions were highly variable. Conversely, the Risk Assessment recognized that 

alternative annual standard levels, when controlling, resulted in more consistent risk 

reductions across urban study areas, thereby potentially providing a more consistent 

degree of public health protection (U.S. EPA, 2010a, p. 5-17).

D. Conclusions on the Adequacy of the Current Primary PM2.5 Standards

1. Introduction

The initial issue to be addressed in the current review of the primary PM2.5

standards is whether, in view of the advances in scientific knowledge and other 

information reflected in the Integrated Science Assessment, the Risk Assessment, and the

Policy Assessment, the existing standards should be retained or revised. In considering 

the adequacy of the current suite of PM2.5 standards, the Administrator has considered the 

large body of evidence presented and assessed in the Integrated Science Assessment

(U.S. EPA, 2009a), the quantitative assessment of risks, staff conclusions and associated 

rationales presented in the Policy Assessment, views expressed by CASAC, and public 

comments. The Administrator has taken into account both evidence- and risk-based 

considerations40 in developing final conclusions on the adequacy of the current primary 

 

40 Evidence-based considerations include the assessment of epidemiological, 

toxicological, and controlled human exposure studies evaluating long- or short-term 
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PM2.5 standards.

a. Evidence- and Risk- based Considerations in the Policy Assessment

In considering the available epidemiologicepidemiological evidence in this 

review, the Policy Assessment took a broader approach than was used in the last review. 

This approach reflected the more extensive and stronger body of evidence available since 

the last review on health effects related to both long- and short-term exposure to PM2.5. 

As discussed in section III.A.3 above, this broader approach focused on setting the annual 

standard as the “generally controlling” standard for lowering both short- and long-term 

PM2.5 concentrations and so providing requisite protection to public health. In 

conjunction with such an annual standard, this approach focused on setting the 24-hour 

standard to provide supplemental protection against days with high peak PM2.5

concentrations.

In addressing the question whether the evidence now available in this review 

supports consideration of standards that are more protective than the current PM2.5

standards, the Policy Assessment considered whether: (1) statistically significant health 

effects associations with long- or short-term exposures to fine particles occur in areas that 

would likely have met the current PM2.5 standards ([see American Trucking 

Ass’nsAssociations, 283 F. 3d at 369, 376 (revision of level of PM NAAQS justified 

when health effects are observed in areas meeting the existing standard)),)], and (2) 

 

exposures to PM2.5, with supporting evidence related to dosimetry and potential 

pathways/modes of action, as well as the integration of evidence across each of these 

disciplines, as assessed in the Integrated Science Assessment (U.S. EPA, 2009a) and 

focus on the policy-relevant considerations as discussed in section III.B above and in the 

Policy Assessment (U.S. EPA, 2011a, section 2.2.1). Risk-based considerations draw 

from the results of the quantitative analyses presented in the Risk Assessment (U.S. EPA, 

2010a) and focus on the policy-relevant considerations as discussed in section III.C 

above and in the Policy Assessment (U.S. EPA, 2011a, section 2.2.2).
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associations with long-term exposures to fine particles extend down to lower air quality 

concentrations than had previously been observed. With regard to associations observed 

in long-term PM2.5 exposure studies, the Policy Assessment recognized that extended 

follow-up analyses of the ACS and Harvard Six Cities studies provided consistent and 

stronger evidence of an association with mortality at lower air quality distributions than 

had previously been observed (U.S. EPA, 2011a, pp. 2-31 to 2-32). The original and 

reanalysis of the ACS study reported positive and statistically significant effects 

associated with a long-term mean PM2.5 concentration of 18.2 μg/m3across 50 

metropolitan areas for 1979 to 1983 (Pope et al., 1995; Krewski et al., 2000).41 In 

extended analyses, positive and statistically significant effects of approximately similar 

magnitude were associated with declining PM2.5 concentrations, from an aggregate longterm mean in 58 metropolitan areas of 21.2 μg/m3

in the original monitoring period (1979 

to 1983) to 14.0 μg/m3for 116 metropolitan areas in the most recent years evaluated 

(1999-2000), with an overall average across the two study periods in 51 metropolitan 

areas of 17.7 μg/m3(Pope et al., 2002; Krewski et al., 2009). With regard to the Harvard 

Six Cities Study, the original and reanalysis reported positive and statistically significant 

effects associated with a long-term mean PM2.5 concentration of 18.0 μg/m3for 1980 to 

1985 (Dockery et al., 1993; Krewski et al., 2000). In an extended follow-up of this study, 

the aggregate long-term mean concentration across all years evaluated was 16.4 μg/m3

for 1980 to 198842 (Laden et al., 2006). In an additional analysis of the extended follow-

 

41 The study periods referred to in the Policy Assessment (U.S. EPA, 2011a) and in this 

final rule reflect the years of air quality data that were included in the analyses, whereas 

the study periods identified in the Integrated Science Assessment (U.S. EPA, 2009a) 

reflect the years of health statusevent data that were included.

42 Aggregate mean concentration provided by study author (personal communication 

from Dr. Francine Laden, 2009).
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up of the Harvard Six Cities study, investigators reported that the concentration-response 

relationship was linear and “clearly continuing below the level” of the current annual 

standard (U.S. EPA, 2009a, p. 7-92; Schwartz et al., 2008). 

Cohort studies conducted since the last review provided additional evidence of 

mortality associated with air quality distributions that are generally lower than those 

reported in the ACS and Harvard Six Cities studies, with effect estimates that were 

similar or, in some studies, significantly greater in magnitude than in the ACS and 

Harvard Six Cities studies (see also, section III.D.1.a of the proposal, 77 FR 38918 to 

28919; U.S. EPA, 2011a, pp. 2-32 to 2-33). The Women’s Health Initiative (WHI) study 

reported positive and most often statistically significant associations between long-term 

PM2.5 exposure and cardiovascular-related mortality as well as morbidity effects, with 

much larger relative risk estimates for mortality than in the ACS and Harvard Six Cities 

studies, at an aggregate long-term mean PM2.5 concentration of 12.9 μg/m3for 2000 

(Miller et al., 2007).43

Using the Medicare cohort, Eftim et al. (2008) reported somewhat higher effect 

estimates than in the ACS and Harvard Six Cities studies with aggregate long-term mean 

concentrations of 13.6 μg/m3and 14.1 μg/m3, respectively, for 2000- to 2002. Zeger et al. 

(2008) reported associations between long-term PM2.5 exposure and mortality for the 

eastern region of the U.S. at an aggregated long-term PM2.5 median concentration of 14.0 

μg/m3, although no association was reported for the western region with an aggregate 

 

43 The Policy Assessment noted that in comparison to other long-term exposure studies, 

the Miller et al. (2007) study was more limited in that it was based on only one year of air 

quality data (U.S. EPA, 2011a, p. 2-82). The proposal further noted that the air quality 

data considered were extrapolated from that one single year of air quality data (2000) to 

the whole study, and that the air quality data post-dated the years of health events 

considered (i.e., 1994 to 1998) (77 FR 38918, fn 62).

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Subscript
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long-term PM2.5 median concentration of 13.1 μg/m3(U.S. EPA, 2009a, p. 7-88).44

Premature mortality in children reported in a national infant mortality study as 

well as mortality in a cystic fibrosis cohort including both children and adults reported 

positive but statistically nonsignificant effects associated with long-term aggregate mean 

concentrations of 14.8 μg/m3and 13.7 μg/m3, respectively (Woodruff et al., 2008; Goss 

et al., 2004).

With respect to respiratory morbidity effects associated with long-term PM2.5

exposure, the across-city mean of 2-week average PM2.5 concentrations reported in the 

initial Southern California Children’s Health Study was approximately 15.1 μg/m3

(Peters et al., 1999). These results were found to be consistent with results of crosssectional analyses of the 24-Cities Study (Dockery et al., 1996; Raizenne et al., 1996), 

which reported a long-term cross-city mean PM2.5 concentration of 14.5 μg/m3.

45 In this 

review, extended analyses of the Southern California Children’s Health Study provide 

stronger evidence of PM2.5-related respiratory effects, at lower air quality concentrations 

than had previously been reported, with a four-year aggregate mean concentration of 13.8 

μg/m3across the 12 study communities (McConnell et al., 2003; Gauderman et al., 2004, 

U.S. EPA, 2009a, Figure 7-4). 

In also considering health effects for which the Integrated Science Assessment 

 

44 Zeger et al. (2008) also reported positive and statistically significant effects for the 

central region, with an aggregate long-term mean PM2.5 concentration of 10.7 μg/m3. 

However, in contrast to the eastern and western risk estimates, the central risk estimate 

increased with adjustment for COPD (used as a proxy for smoking status). Due to the 

potential for confounding bias influencing the risk estimate for the central region, the 

Policy Assessment did not focus on the results reported in the central region to inform the 

adequacy of the current suite of standards or alternative annual standard levels (U.S. 

EPA, 2011a, p. 2-32).

45 See American Farm Bureau Federation v. EPA, 559 F. 3d at 525 (noting the 

importance of these studies, as well as EPA’s failure to properly take them into account).
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concluded evidence was suggestive of a causal relationship, the Policy Assessment noted 

a limited number of birth outcome studies that reported positive and statistically 

significant effects related to aggregate long-term mean PM2.5 concentrations down to 

approximately 12 μg/m3(U.S. EPA, 2011a, p. 2-33).

Collectively, the Policy Assessment concluded that currently available evidence 

provided support for associations between long-term PM2.5 exposure and mortality and 

morbidity effects that extend to distributions of PM2.5 concentrations that are lower than 

those that had previously been associated with such effects, with aggregate long-term 

mean PM2.5 concentrations extending to well below the level of the current annual 

standard.

The Policy Assessment also considered the long-term mean PM2.5 concentrations 

in short-term exposure studies in assessing the appropriateness of the level of the current 

annual standard. See American Farm Bureau Federation v. EPA, 559 F. 3d at 522, 523-

24 (remanding 2006 standard because the EPA had not adequately explained its choice 

not to consider long-term means of short-term exposure studies in assessing adequacy of 

primary annual PM2.5 standard). In light of the mixed findings reported in single-city, 

short-term exposure studies, the Policy Assessment placed comparatively greater weight 

on the results from multi-city studies in considering the adequacy of the current suite of 

standards (U.S. EPA, 2011a, pp. 2-34 to 2-35). 

With regard to associations reported in short-term PM2.5 exposure studies, the 

Policy Assessment recognized that long-term mean concentrations reported in new multicity U.S. and Canadian studies provided evidence of associations between short-term 

PM2.5 exposure and mortality at similar air quality distributions to those that had 
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previously been observed in an 8-cities Canadian study (Burnett and Goldberg, 2003; 

aggregate long-term mean PM2.5 concentration of 13.3 μg/m3). In a multi-city time-series 

analysis of 112 U.S. cities, Zanobetti and Schwartz (2009) reported a positive and 

statistically significant association with all-cause, cardiovascular-related (e.g., heart 

attacks, stroke), and respiratory-related mortality and short-term PM2.5 exposure, in 

which the aggregate long-term mean PM2.5 concentration was 13.2 μg/m3(U.S. EPA, 

2009a, Figure 6-24). Furthermore, city-specific effect estimates indicated the association 

between short-term exposure to PM2.5 and total mortality and cardiovascular- and 

respiratory-related mortality was consistently positive for an overwhelming majority (99 

percent) of the 112 cities across a wide range of air quality concentrations (long-term 

mean concentrations ranging from 6.6 μg/m3to 24.7 μg/m3; U.S. EPA, 2009a, Figure 6-

24, p. 6-178 to 179). The EPA staff noted that for all-cause mortality, city-specific effect 

estimates were statistically significant for 55 percent of the 112 cities, with long-term 

city-mean PM2.5 concentrations ranging from 7.8 μg/m

3

to 18.7 μg/m3and 24-hour PM2.5

city-mean 98th percentile concentrations ranging from 18.4 to 64.9 μg/m3(personal 

communication with Dr. Antonella Zanobetti, 2009).46

With regard to cardiovascular and respiratory morbidity effects, in the first 

analysis of the Medicare cohort conducted by Dominici et al. (2006a) across 204 U.S. 

counties, investigators reported a statistically significant association with hospitalizations 

for cardiovascular and respiratory diseases and short-term PM2.5 exposure, in which the 

aggregate long-term mean PM2.5 concentration was 13.4 μg/m3. Furthermore, a subanalysis restricted to days with 24-hour average concentrations of PM2.5 at or below 35 

 

46 Single-city Bayes-adjusted effect estimates for the 112 cities analyzed in Zanobetti and 

Schwartz (2009) were provided by the study authors (personal communication with Dr. 

Antonella Zanobetti, 2009; see also U.S. EPA, 2009a, Figure 6-24). 
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μg/m3indicated that, in spite of a reduced statistical power from a smaller number of 

study days, statistically significant associations were still observed between short-term 

exposure to PM2.5 and hospital admissions for cardiovascular and respiratory diseases 

(Dominici, 2006b).47 In an extended analysis of this cohort, Bell et al (2008) reported a 

positive and statistically significant increase in cardiovascular hospitalizations associated 

with short-term PM2.5 exposure, in which the aggregate long-term mean PM2.5

concentration was 12.9 μg/m3. These results, along with the observation that 

approximately 50 percent of the 204 county-specific mean 98th percentile PM2.5

concentrations in the study aggregated across all years were below the 24-hour standard 

of 35 μg/m3, not only indicated that effects are occurring in areas that would meet the 

current standards but also suggested that the overall health effects observed across the 

U.S. are not primarily driven by the higher end of the PM2.5 air quality distribution (Bell, 

2009a, personal communication from Dr. Michelle Bell regarding air quality data for Bell 

et al., 2008 and Dominici et al., 2006a). 

Collectively, the Policy Assessment concluded that the findings from short-term 

PM2.5 exposure studies provided evidence of PM2.5-associated health effects occurring in 

areas that would likely have met the current suite of PM2.5 standards (U.S. EPA, 2011a, p. 

2-35). These findings were further bolstered by evidence of statistically significant 

PM2.5-related health effects occurring in analyses restricted to days in which 24-hour 

average PM2.5 concentrations were below 35 μg/m3(Dominici, 2006b). 

 

47 This sub-analysis was not included in the original publication (Dominici et al., 2006a). 

The study authors provided sub-analysis results for the Administrator’s consideration as a 

letter to the docket following publication of the proposed rule in January 2006 (personal 

communication with Dr. Francesca Dominici, 2006b). As noted in section III.A.3, this 

study is part of the basis for the conclusion that there is no evidence suggesting that risks 

associated with long-term exposures are likely to be disproportionately driven by peak 

24-hour concentrations.
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In evaluating the currently available scientific evidence, as summarized in section 

III.B of the proposal, the Policy Assessment first concluded that there was stronger and 

more consistent and coherent support for associations between long- and short-term 

PM2.5 exposures and a broad range of health outcomes than was available in the last 

review, providing the basis for fine particle standards at least as protective as the current 

PM2.5 standards (U.S. EPA, 2011a, p. 2-26). Having reached this initial conclusion, the 

Policy Assessment addressed the question of whether the available evidence supported 

consideration of standards that were more protective than the current standards. In so 

doing, the Policy Assessment considered whether there was now evidence that health 

effect associations have been observed in areas that likely met the current suite of PM2.5

standards. As discussed above, long- and short-term PM2.5 exposure studies provided 

evidence of associations with mortality and cardiovascular and respiratory effects both at 

lower ambient PM2.5 concentrations than had been observed in the previous review and at 

concentrations allowed by the current standards (U.S. EPA, 2011a, p. 2-35). 

In reviewing this information, the Policy Assessment recognized that important 

limitations and uncertainties associated with this expanded body of scientific evidence, as 

discussed in section III.B.2 of the proposal, needed to be carefully considered in 

determining the weight to be placed on the body of studies available in this review. 

Taking these limitations and uncertainties into consideration, the Policy Assessment 

concluded that the currently available evidence clearly calls into question whether the 

current suite of primary PM2.5 standards protects public health with an adequate margin 

of safety from effects associated with long- and short-term exposures. Furthermore, the 

Policy Assessment concluded this evidence provides strong support for considering fine 
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particle standards that would afford increased protection beyond that afforded by the 

current standards (U.S. EPA, 2011a, p. 2-35).

In addition to evidence-based consideration, the Policy Assessment also 

considered the extent to which health risks estimated to occur upon simulating just 

meeting the current PM2.5 standards may be judged to be important from a public health 

perspective, taking into account key uncertainties associated with the quantitative health 

risk estimates. In so doing, the Policy Assessment first noted that the quantitative risk 

assessment addresses: (1) the core PM2.5-related risk estimates; (2) the related uncertainty 

and sensitivity analyses, including additional sets of reasonable risk estimates generated 

to supplement the core analysis; (3) an assessment of the representativeness of the urban 

study areas within a national context;48 and (4) consideration of patterns in design values 

and air quality monitoring data to inform interpretation of the risk estimates, as discussed 

in section III.C above. 

In considering the health risks estimated to remain upon simulation of just 

meeting the current suite of standards and considering both the qualitative and 

quantitative assessment of uncertainty completed as part of the assessment, the Policy 

Assessment concluded these risks are important from a public health standpoint and 

provided strong support for consideration of alternative standards that would provide 

increased protection beyond that afforded by the current PM2.5 (U.S. EPA, 2011a, pp. 2-

47 to 2-48). This conclusion reflected consideration of both the severity and the 

magnitude of the effects. For example, the Risk Assessment indicated the possibility that 

 

48 Based on analyses of the representativeness of the 15 urban study areas in the broader 

national context, the Policy Assessment concludes that these study areas are generally 

representative of urban areas in the U.S. likely to experience relatively elevated levels of 

risk related to ambient PM2.5 exposures (U.S. EPA, 2011a, p. 2-42). 
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premature deaths related to ischemic heart disease associated with long-term PM2.5

exposure alone would likely be on the order of thousands of deaths per year in the 15 

urban study areas upon simulating just meeting the current standards49 (U.S. EPA, 2011a, 

pp. 2-46 to 2-47). Moreover, additional risks were anticipated for premature mortality 

related to cardiopulmonary effects and lung cancer associated with long-term PM2.5

exposure as well as mortality and cardiovascular- and respiratory-related morbidity 

effects (e.g., hospital admissions, emergency department visits) associated with shortterm PM2.5 exposures. Based on the consideration of both qualitative and quantitative 

assessments of uncertainty completed as part of the quantitative risk assessment, the Risk 

Assessment concluded that it was unlikely that the estimated risks are over-stated, 

particularly for mortality related to long-term PM2.5 exposure, and may well be biased 

low based on consideration of alternative model specifications evaluated in the sensitivity 

analyses (U.S. EPA, 2010a, p. 5-16; U.S. EPA, 2011a, p. 2-41). Furthermore, the 

currently available scientific information summarized in section III.B of the proposal 

provided evidence for a broader range of health endpoints and at-risk populations beyond 

those included in the quantitative risk assessment (U.S. EPA, 2011a, p. 2-47).

b. CASAC Advice

The CASAC, based on theirits review of drafts of the Integrated Science 

Assessment, the Risk Assessment, and the Policy Assessment, provided an array of 

advice both with regard to interpreting the scientific evidence and quantitative risk 

assessment, as well as with regard to consideration of the adequacy of the current PM2.5

standards (Samet, 2009a b,c,d,e,f; Samet 2010a,b,c,d). With regard to the adequacy of the 

 

49 Premature mortality for all causes attributed to PM2.5 exposure was estimated to be on 

the order of tens of thousands of deaths per year on a national scale based on 2005 air 

quality data (U.S. EPA, 2010a, Appendix G, Table G-1).
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current standards, CASAC concluded that the “currently available information clearly 

calls into question the adequacy of the current standards” (Samet, 2010d, p. i) and that the 

current standards are “not protective” (Samet, 2010d, p. 1). Further, in commenting on 

the first draft Policy Assessment, CASAC noted:

With regard to the integration of evidence-based and risk-based considerations, 

CASAC concurs with EPA’s conclusion that the new data strengthens the 

evidence available on associations previously considered in the last round of the 

assessment of the PM2.5 standard. CASAC also agrees that there are significant 

public health consequences at the current levels of the standard that justify 

consideration of lowering the PM2.5 NAAQS further (Samet, 2010c, p.12). 

c. Administrator's Proposed Conclusions Concerning the Adequacy of the Current 

Primary PM2.5 Standards

At the time of the proposal, in considering the body of scientific evidence, the 

Administrator concluded there was stronger and more consistent and coherent support for 

associations between long- and short-term PM2.5 exposure and a broader range of health 

outcomes than was available in the last review, providing the basis for fine particle 

standards at least as protective as the current PM2.5 standards. In particular, the 

Administrator recognized in section III.D.4 of the proposal that the Integrated Science 

Assessment concluded that the results of epidemiological and experimental studies form 

a plausible and coherent data set that supports a causal relationship between long- and 

short-term PM2.5 exposures and mortality and cardiovascular effects and a likely causal 

relationship between long- and short-term PM2.5 exposures and respiratory effects. 

Furthermore, the Administrator reflected that effects had been observed at lower ambient 

PM2.5 concentrations than what had been observed in the last review, including at 

ambient PM2.5 concentrations in areas that likely met the current PM2.5 NAAQS. With 

regard to the results of the quantitative risk assessment, the Administrator noted that the 

Risk Assessment concludesconcluded that the risks estimated to remain upon simulation 
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of just meeting the current standards arewere important from a public health standpoint in 

terms of both the severity and magnitude of the effects. 

At the time of the proposal, in considering whether the current suite of PM2.5

standards should be revised to provide requisite public health protection, the 

Administrator carefully considered the staff conclusions and rationales presented in the 

Policy Assessment, the advice and recommendations from CASAC, and public comments 

to date on this issue. In so doing, the Administrator placed primary consideration on the 

evidence obtained from the epidemiological studies and provisionally found the evidence 

of serious health effects reported in long- and short-term exposure studies conducted in 

areas that would have met the current standards to be compelling, especially in light of 

the extent to which such studies are part of an overall pattern of positive and frequently 

statistically significant associations across a broad range of studies that collectively 

represent a strong and robust body of evidence. 

As discussed in the Integrated Science Assessment and Policy Assessment, the 

Administrator recognized that much progress has been made since the last review in 

addressing some of the key uncertainties that were important considerations in 

establishing the current suite of PM2.5 standards. For example, progress made since the 

last review provides increased confidence in the long- and short-term exposure studies as 

a basis for considering whether any revision of the annual standard is appropriate and 

increased confidence in the short-term exposure studies as a basis for considering 

whether any revision of the 24-hour standard is appropriate.50

 

50 The EPA notes that this increased confidence in the long- and short-term associations 

generally reflects less uncertainty as to the likely causal nature of such associations, but 

does not address directly the question of the extent to which such associations remain 

toward the lower end of the range of ambient PM2.5 concentrations. This question is 
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Based on her consideration of these conclusions, as well as consideration of 

CASAC’s conclusion that the evidence and risk assessment clearly callcalled into 

question the adequacy of the public health protection provided by the current PM2.5

NAAQS and public comments on the proposal, the Administrator provisionally 

concluded that the current primary PM2.5 standards, taken together, arewere not requisite 

to protect public health with an adequate margin of safety and that revision iswas needed 

to provide increased public health protection. The Administrator provisionally concluded 

that the scientific evidence and information on risk provideprovided strong support for 

consideration of alternative standards that would provide increased public health 

protection beyond that afforded by the current PM2.5 standards.

2. Comments on the Need for Revision

This section addresses general comments based on relevant facts that either 

support or oppose any change to the current suite of primary PM2.5 standards. Comments 

on specific long- and short-term exposure studies that relate to consideration of the 

appropriate levels of the annual and 24-hour standards are addressed in section III.E.4 

below. Many public comments asserted that the current PM2.5 standards are insufficient 

to protect public health with an adequate margin of safety and that revisions to the 

standards are therefore appropriate, indeed necessitated. 

Among those calling for revisions to the current standards were the Children’s 

Health Protection Advisory Committee (CHPAC); major medical and public health 

groups including the American Heart Association (AHA), American Lung Association 

(ALA), American Public Health Association (APHA), American Thoracic Society 

 

central to the Agency’s evaluation of the relevant evidence to determine appropriate 

standards levels, as discussed below in section III.E.4.
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(ATS); the Physicians for Social Responsibility (PSR); major environmental groups such 

as the Clean Air Council, Clean Air Task Force, Earthjustice, Environmental Defense 

Fund (EDF), National Resources Defense Council (NRDC), and Sierra Club; many 

environmental justice organizations as well as medical doctors, academic researchers, 

health professionals, and many private citizens. For example, the American Heart 

Association and other major national public health and medical organizations stated that, 

“[o]ur organizations are keenly aware of the public health and medical threats from 

particulate matter” and called on the EPA to “significantly strengthen” both the annual 

and 24-hour PM2.5 standards “to help us protect the health of our patients and our nation” 

(AHA et al., 2012, pp 1 and 13). AHA et al. and ALA et al., as well as a group of more 

than 350 physicians, environmental health researchers, and public health and medical 

professionals articulated similar comments on the available evidence:

Ample scientific evidence supports adopting tighter standards to protect the health 

of people who are most susceptible to the serious health effects of these 

pollutants. More than 10,000 peer-reviewed scientific studies have been published 

since 1997 when EPA adopted the current annual standard. These studies validate 

and extend earlier epidemiologic research linking both acute and chronic fine 

particle pollution with serious morbidity and mortality. The newer research has 

also expanded our understanding of the range of health outcomes associated with 

PM and has identified adverse respiratory and cardiovascular health effects at 

lower exposure levels than previously reported. As discussed and interpreted in 

the EPA’s 2009 Integrated Science Assessment for Particulate Matter, the new 

evidence reinforces already strong existing studies and supports the conclusion 

that PM2.5 is causally associated with numerous adverse health effects in humans 

at exposure levels far below the current standard. Such a conclusion demands 

prompt action to protect human health. (AHA et al., 2012, pp. 1 to 2; ALA et al., 

pp. 4 to 5; similar comment submitted by Rom et al., 2012, p. 1).

All of these medical and public health commenters stated that the current PM2.5

standards need to be revised, and that even more protective standards than those proposed 

by the EPA are needed to adequately protect public health, particularly for at-risk 

populations. Many environmental justice organizations and individual commenters also 
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expressed such views.

The National Association of Clean Air Agencies (NACAA), the Northeast States 

for Coordinated Air Use Management (NESCAUM), and many State and local air 

agencies and health departments who commented on the PM2.5 standards supported 

revision of the suite of current PM2.5 standards, as did five State Attorneys Generalstate 

attorneys general (Schneiderman et al., 2012) and the National Tribal Air Association 

(NTAA). 

These commenters based their views chiefly on the body of evidence and 

technical analyses presented and discussed in the Integrated Science Assessment, the 

Risk Assessment, and the Policy Assessment finding the available scientific information 

to be stronger and more compelling than in the last review. These commenters generally 

placed much weight on CASAC’s recommendation to revise the PM2.5 standards to 

provide increased public health protection and on the EPA staff conclusions presented in 

the final Policy Assessment.

Some of these commenters specifically mentioned extended analyses of seminal 

long-term exposure studies - the ACS (Krewski et al., 2009), Harvard Six Cities (Laden 

et al., 2006), and Southern California Children’s Health (Gauderman et al., 2004) studies. 

These commenters also highlighted the availability of additional long-term exposure 

studies in this review, specifically a study of premature mortality in older adults (Eftim et 

al., 2008) and the WHI study of cardiovascular morbidity and mortality effects in women 

(Miller et al., 2007) providing stronger evidence of mortality and morbidity effects 

associated with long-term PM2.5 exposures at lower concentrations than had previously 

been observed, including studies of effects in at-risk populations. For example, some 
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commenters asserted:

Evidence during the last review showed clearly that the annual average standard 

needed to be much lower than the standard of 15 μg/m3that was first set in 1997. 

The evidence has only grown since then. Multiple, multi-city studies over long 

periods of time have shown clear evidence of premature death, cardiovascular and 

respiratory harm as well as reproductive and developmental harm at contemporary 

concentrations far below the level of the current (annual) standard (ALA et al., 

2012, p. 39; AHA et al., 2012, p. 10). 

These commenters also highlighted the availability of a number of short-term 

PM2.5 exposure studies as providing evidence of mortality and morbidity effects at 

levelsconcentrations below the level of the current 24-hour PM2.5 standard. Specifically, 

these commenters made note of multi-city studies of premature mortality (Zanobetti and 

Schwartz, 2009) and increased hospitalizations for cardiovascular and respiratory-related 

effects in older adults (Bell et al., 2008). These commenters also asserted the importance 

of many of the single-city studies, arguing that these studies “provide valuable 

information regarding impacts on susceptible populations and on health risk in areas with 

high peak to mean concentration ratios” (ALA, et al., 2012, p. 65). Collectively, 

considering the multi- and single-city short-term exposure studies, these commenters 

asserted “the record clearly supports a more stringent 24-hour standard of 25 μg/m3to 

provide uniform protection in all regions of the country particularly from short-term 

spikes in pollution and from the sub-daily exposures that trigger heart attacks and 

strokes” (ALA et al., 2012, p. 62). A group of more than 350 physicians, environmental 

health researchers, and public health and medical professionals argued, “[s]tudies of 

short-term exposure demonstrate that PM2.5 air pollution increases the risk of hospital 

admissions for heart and lung problems even when you exclude days with pollution 

concentrations at or above the current daily standard of 35 μg/m3. Daily concentrations 

must be capped at lower levels to protect against peak exposure days that occur due to 
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local and seasonal sources of emissions” (Rom et al., 2012, p. 2).

In addition, many of these commenters generally concluded that progress had 

been made in reducing many of the uncertainties identified in the last review, in better 

understanding mechanisms by which PM2.5 may be causing the observed health effects, 

and in improving our understanding of at-risk populations.51 Further, these Further, a 

number of commenters argued that by making the standards more protective, the PM2.5

NAAQS would be more consistent with other existing standards (e.g., California's annual 

average standard of 12 μg/m3) (CARB, 2012; CA OEHHA, 2012). Other commenters 

argued that revising the primary PM2.5 standards would be more consistent with the 

recommendations of the World Health Organization (WHO) and/or Canada (e.g., ALA et 

al., 2012, p. 62; ISEE, 2012, p. 2; MOE-Ontario, 2012, p. 1; ISEE, 2012, p. 2). 

With regard to the scope of the literature reviewed for PM2.5-related health 

effects, some commenters asserted that the EPA inappropriately narrowed the scope of 

the review by excluding a number of categories of relevant studies, specifically related to 

studies of diesel pollution and traffic-related pollution (ALA, et al., 2012, p. 17). These 

commenters argued that, based upon the exclusion of these types of studies, the 

Integrated Science Assessment “came to the erroneous conclusion that the causal 

relationship between PM and cancer is merely suggestive. This conclusion does not 

square with the International Agency Research on Cancer (IARC) finding that diesel 

emissions are a known human carcinogen nor with the conclusions of the extended 

analyses of the [Harvard] Six Cities and ACS cohort studies that report positive and 

 

51 In addition, the EPA notes that CASAC found that the Integrated Science Assessment 

clearly presented and discussed the current scientific information related to potential 

modes of action (Chapter 5) and at-risk populations (Chapter 8) (Samet, 2009e, pp. 7 to 

8; Samet, 2009f, pp. 2 and 11 to 12). 
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statistically significant associations between PM2.5 and lung cancer.” Id.

Some of these commenters also noted the results of the EPA’s quantitative risk 

assessment, concluding that it showed that the risks estimated to remain when the current 

standards are met are large and important from a public health perspective and warrant 

increased protection. For example, ALA et al., noted that the Risk Assessment indicated 

the quantitative risk analyses likely underestimated PM2.5-related mortality (U.S. EPA, 

2010a, p. 5-16) and argued that “the measurements of risk should be treated 

conservatively” (ALA, et al., 2012, p. 73). These commenters also summarized an 

expanded analysis of alternative PM2.5 standard levels that they argued documented the 

need for more protective standards (McCubbin, 2011). 

In general, all of these commenters agreed on the importance of results from the 

large body of scientific studies reviewed in the Integrated Science Assessment and on the 

need to revise the suite of PM2.5 standards as articulated in the EPA’s proposal, while 

generally differing with the EPA’s proposed judgments about the extent to which the 

standards should be revised based on this evidence, specifically for providing protection 

for at-risk populations. 

The EPA generally agrees with these commenters’ conclusion regarding the need 

to revise the current suite of PM2.5 standards. The scientific evidence noted by these 

commenters was generally the same as that assessed in the Integrated Science 

Assessment and the Policy Assessment, and the EPA agrees that this evidence provides a 

strong basis for concluding that the current PM2.5 standards, taken together, are not 

requisite to protect public health with an adequate margin of safety, and they need to be 

revised to provide increased protection. For reasons discussed in section III.E.4.c below, 
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however, the EPA disagrees with aspects of these commenters’ views on the level of 

protection that is appropriate.

The EPA disagrees with these commenters’ views that diesel exhaust studies were 

excluded from the Integrated Science Assessment and were not considered when making 

the causality determination for cancer, mutagenicity, and genotoxicity. As discussed in 

Sectionsection 7.5 of the Integrated Science Assessment, diesel exhaust studies were 

integrated within the broader body of scientific evidence that was considered in reaching 

the causality determination for these health endpoints. Additionally, as discussed in 

section 1.5.3 of the Integrated Science Assessment, the evidence from diesel exhaust 

studies was also considered as part of the collective evidence evaluated when making 

determinations for other, noncancer health outcomes (e.g., cardiovascular and respiratory 

effects).52 Specifically, when evaluating this evidence, the focus was on understanding 

the effects of diesel exhaust particles. 

It is important to recognize that the Integrated Science Assessment focused on 

experimental studies of diesel exhaust that evaluated exposures that were relevant to 

ambient concentrations, i.e., “within one or two orders of magnitude of ambient PM 

concentrations” (U.S. EPA, 2009a, section 1.3). The causal determination for cancer, 

mutagenicity, and genotoxicity presented in the Integrated Science Assessment represents 

an integration of experimental and observational evidence of exposures to ambient PM 

 

52 In developing the second draft Integrated Science Assessment, the EPA reexamined the 

controlled human exposure and toxicological studies of fresh diesel and gasoline exhaust

and considered this. This information, in addition to other considerations, supported a 

change in the causal determinations for ultrafine particles. Specifically, in reevaluating 

the causal determinations for short-term ultrafine particle exposures and cardiovascular 

and respiratory effects, the EPA changed the classification from “inadequate” to 

“suggestive” for both categories of health outcomes (Vandenberg, 2009, p. 3). CASAC 

agreed with the EPA’s rationale for revising these causal determinations (Samet, 2009f, 

p. 10).
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concentrations. The EPA fully considered the findings of studies that assessed these and 

other health effects associated with exposure to diesel particles in reaching causality 

determinations regarding health outcomes associated with PM2.5 exposures. Furthermore, 

CASAC supported the EPA’s change to the causal determination for cancer and longterm PM2.5 concentrations from “inadequate” to “suggestive” (Samet, 2009f, p. 2).

With regard to traffic studies, the EPA disagrees with the commenters’ views that 

traffic studies that focused on exposure indicators such as distance to roadways should 

have been included in the Integrated Science Assessment. These studies were excluded 

from consideration because they did not measure ambient concentrations of specific air 

pollutants, including PM2.5, but instead were studies evaluating exposure to the 

undifferentiated “traffic related air pollution” mixture (ALA et al., 2012, p.17) (U.S.

EPA, 2009a, section 1.3). As a result, these studies do not add to the collective body of 

evidence on the relationship between long- or short-term exposure to ambient 

concentrations of PM2.5 and health effects.

Some of these commenters also identified “new” studies that were not included in 

the Integrated Science Assessment as providing further support for the need to revise the 

primary PM2.5 standards. As discussed above in section II.B.3 above, the EPA notes that, 

as in past NAAQS reviews, the Agency is basing the final decisions in this review on the 

studies and related information included in the PM air quality criteria that have 

undergone CASAC and public review and will consider the “new” studies for purposes of 

decision making in the next PM NAAQS review. Nonetheless, in provisionally 

evaluating commenters’ arguments (see Response to Comments document), the EPA 

notes that its provisional assessment of “new” science found that such studies did not 
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materially change the conclusions in the Integrated Science Assessment (U.S. EPA, 

2012b).

Another group of commenters opposed revising the current PM2.5 standards. 

These views were most extensively presented in comments from the Utility Air 

Regulatory Group (UARG), representing a group of electric generating companies and 

organizations and several national trade associations; the American Petroleum Institute 

(API) representing more than 500 oil and natural gas companies; the National 

Association of Manufacturers (NAM), the American Chemistry Council (ACC), the 

American Fuel & Petroleum Manufacturers (AFPM), the Alliance of Automobile 

Manufacturers, (AAM), and other manufacturing associations; the Electric Power 

Research Institute (EPRI); and the Texas Commission on Environmental Quality (Texas 

CEQ). These commenters generally mentioned many of the same studies that were cited 

by the commenters who supported revising the standards, as well as other studies, but 

highlighted different aspects of these studies in reaching substantially different 

conclusions about their strength and the extent to which progress has been made in

reducing uncertainties in the evidence since the last review. Furthermore, they asserted 

that the evidence that has become available since the last review does not establish a 

more certain risk or a risk of effects that are significantly different in character to those 

that provided a basis for the current standards, nor does the evidence demonstrate that the 

risk to public health upon attainment of the current standards would be greater than was 

understood when the EPA established the current standards in 2006. 

These commenters generally expressed the view that the current standards provide 

the requisite degree of public health protection. In supporting their view, these 
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commenters generally argued that the EPA’s conclusions are inconsistent with the current 

state of the science and questioned the underlying scientific evidence including the causal 

determinations reached in the Integrated Science Assessment. More specifically, this 

group of commenters argued that: (1) the EPA did not apply its framework for causal 

determination consistently across studies or health outcomes and, in the process, the EPA 

relied on a selective group of long- and short-term exposure studies to reach conclusions 

regarding causality; (2) toxicological and controlled human exposure studies do not 

provide supportive evidence that the health effects observed in epidemiological studies 

are biologically plausible; (3) uncertainties in the underlying health science are as great 

or greater than in 2006; (4) there is no evidence of greater risk since the last review to 

justify tightening the current annual PM2.5 standard; and (5) “new” studies not included in 

the Integrated Science Assessment continue to increase uncertainty about possible health 

risks associated with exposure to PM2.5. These comments are discussed in turn below.

(l) Some of these commenters asserted that the EPA did not apply its framework 

for causal determinations consistently across studies or health outcomes (e.g., ACC, 

2012, Attachment A, pp. 1 to 2; API, 2012, Attachment 1, p. 30; NAM et al., 2012, pp. 

22 to 25; Texas CEQ, 2012, pp 2 to 3).53 These commenters argued that the EPA 

downplayed epidemiological studies with null or inconsistent results, inappropriately 

used the Hill criteria when evaluating the epidemiologicepidemiological evidence, and

used the same study and the same underlying database to conclude that there was a causal 

association between mortality and multiple criteria pollutants.

 

53 The EPA notes that the same concerns about the causal determinations presented in the 

Integrated Science Assessment were raised in comments to CASAC on the draft 

Integrated Science Assessments (e.g., UARG, 2009; API, 2009; ACC, 2012, Appendix 

B). CASAC, therefore, had the opportunity to consider these comments in reaching 

consensus conclusions on this issue. 

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The EPA disagrees with these commenters’ views. First, the EPA recognizes that 

the evaluation of the scientific evidence and its application of the causal framework used 

in the current PM NAAQS review was the subject of exhaustive and detailed review by 

CASAC and the public. PriorAs summarized in section II.B.3 above, prior to finalizing 

the Integrated Science Assessment, two drafts were released for CASAC and public 

review to evaluate the scientific integrity of the documents. Evidence related to the 

substantive issues raised by CASAC and public commenters with regard to the content of 

the first and second draft Integrated Science Assessments were discussed at length during 

these public CASAC meetings and considered in developing the final Integrated Science 

Assessment. CASAC supported the development of the EPA’s causality framework and 

its use in the current PM NAAQS review and concluded:

The five-level classification of strength of evidence for causal inference has been 

systematically applied; this approach has provided transparency and a clear 

statement of the level of confidence with regard to causation, and we recommend 

its continued use in future Integrated Science Assessments (Samet 2009f, p. 1). 

These commenters asserted that during the application of the causal framework

the EPA inappropriately relied on a selective group of long- and short-term exposure 

studies in reaching causal inferences (API, 2012, pp 12 to 17; ACC, 2012, Attachment A, 

pp 1 to 2; NAM et al., 2012, pp. 22 to 25; Texas CEQ, 2012, pp 2 to 3). Additionally, 

these commenters expressed the view that the EPA focused on a subset of 

epidemiological studies that reported positive and statistically significant results while 

ignoring other studies, especially those that reported no statistically significant

associations, those that reported potential thresholds, or those that highlighted 

uncertainties and limitations in study design or results. Furthermore, some of these 

commenters argued that epidemiological studies are observational in nature and cannot 
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provide evidence of a causal association.

The EPA disagrees with these commenters’ views on assessing the health effects 

evidence and on the conclusions regarding the causality determinations reached in the 

Integrated Science Assessment. In conducting a comprehensive evaluation of the 

evidence in the Integrated Science Assessment, the EPA recognized the distinction 

between the evaluation of the relative scientific quality of individual study results and the 

evaluation of the pattern of results within the broader body of scientific evidence and 

considered both in reaching causality determinations. The more detailed characterizations 

of individual studies included an assessment of the quality of the study based on specific 

criteria as described in the Integrated Science Assessment (U.S. EPA, 2009a, section 

1.5.3). 

In developing an integrated assessment of the health effects evidence for PM, the 

EPA emphasized the importance of examining the pattern of results across various 

studies and did not focus solely on statistical significance54 as a criterion of study 

strength. This approach is consistent with views clearly articulated throughout the 

epidemiological and causal inference literature, specifically, that it is important not to 

focus on results of statistical tests to the exclusion of other information.55 The concepts 

 

54 Statistical significance is an indicator of the precision of a study’s results, which is 

influenced by a variety of factors including, but not limited to, the size of the study, 

exposure and measurement error, and statistical model specifications. Studies typically 

calculate “p-values” to determine whether the study results are statistically significant or 

whether the study results are likely to occur simply by chance. In general practice, effects 

are considered statistically significant if p values are less than 0.05.

55 For example, Rothman (1998) stated, “Many data analysts appear to remain oblivious 

to the qualitative nature of significance testing [and that]... statistical significance is itself 

only a dichotomous indicator. As it has only two values, significant or not significant 

....Nevertheless, p-values still confound effect size with study size, the two components 

of estimation that we believe need to be reported separately.” As a result, Rothman 

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underlying the EPA’s approach to evaluating statistical associations have been discussed 

in numerous publications, including a report by the U.S. Surgeon General on the health 

consequences of smoking (Centers for Disease Control and Prevention, 2004). This 

report cautions against over-reliance on statistical significance in evaluating the overall 

evidence for an exposure-response relationship. Criteria characterized by Hill (1965) also 

addressed the value, or lack thereof, of statistical tests in the determination of cause: 

No formal tests of significance can answer those [causal] questions. Such tests 

can, and should, remind us of the effects the play of chance can create, and they 

will instruct us in the likely magnitude of those effects. Beyond that, they 

contribute nothing to the ‘proof’ of our hypothesis (Hill, 1965, p. 299). 

The statistical significance of individual study findings has played an important 

role in the EPA’s evaluation of the study’s results and the EPA has placed greater 

emphasis on studies reporting statistically significant results. However, in the broader 

evaluation of the evidence from many epidemiological studies, and subsequently during 

the process of forming causality determinations in integrating evidence across 

epidemiological, controlled human exposure, and toxicological studies, the EPA has 

emphasized the pattern of results across epidemiological studies, and whether the effects 

observed were coherent across the scientific disciplines for drawing conclusions on the 

relationship between PM2.5 and different health outcomes. Thus, the EPA did not limit its 

focus or consideration to just studies that reported positive associations or where the 

results were statistically significant. 

In addition, some commenters asserted that the EPA inappropriately used the Hill 

criteria by failing to consider the limitations of studies with weak associations, thereby 

overstating the consistency of the observed associations (API, 2012, Attachment 1, pp. 30 

 

recommended that p-values be omitted as long as point and interval estimates are 

available. 
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to 35). These commenters argued that risk estimates greater than 3 to 4 reflect strong 

associations supportive of a causal link, while smaller risk estimates (i.e., 1.5 to 3) are 

considered to be weak and require other lines of evidence to demonstrate causality. 

As discussed in section 1.5.3 of the Integrated Science Assessment, the EPA 

thoroughly considered the limitations of all studies during its evaluation of the scientific 

literature (U.S. EPA,, 2009a, p. 1-14). This collective body of evidence, including known 

uncertainties and limitations of the studies evaluated, were considered during the process 

of forming causality determinations as discussed in chapters 6 and 7 of the Integrated 

Science Assessment. For example, the EPA concluded that “a causal relationship exists 

between short-term PM2.5 exposure and cardiovascular effects,” however, in reaching this 

conclusion, the Agency recognized and considered limitations of the current evidence 

that still requires further examination (U.S. EPA, 2009a., in section 6.2.12.1). Therefore, 

the EPA disagrees with these commenters’ views that the Hill criteria were 

inappropriately used in that the limitations of studies were not considered. 

The EPA also disagrees with the commenterscommenters’ assertion that the 

magnitude of the association must be large to support a determination of causality. As 

discussed in the Integrated Science Assessment, the strength of the observed association 

is an important aspect to aid in judging causality and “while large effects support 

causality, modest effects therefore do not preclude it” (U.S. EPA, 2009a, Table 1-2, 

section 1.5.4). The weight of evidence approach used by the EPA encompasses a 

multitude of factors of which the magnitude of the association is only one component 

(U.S. EPA, 2009a, Table 1-3). An evaluation of the association across multiple 

investigators and locations supports the “reproducibility of findings [which] constitutes 
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one of the strongest arguments for causality” (U.S. EPA, 2009a, Table 1-2). Even though 

the risk estimates for air pollution studies may be modest, the associations are consistent 

across hundreds of studies as demonstrated in the Integrated Science Assessment. 

Furthermore, the causality determinations rely on different lines of evidence, by 

integrating evidence across disciplines, including animal toxicological studies and 

controlled human exposure studies.

Furthermore, as summarized in section III.B above and discussed more fully in 

section III.B.3 of the proposal, the EPA recognizes that the population potentially 

affected by PM2.5 is considerable, including large subgroups of the U.S. population that 

have been identified as at-risk populations (e.g., children, older adults, persons with 

underlying cardiovascular or respiratory disease). While individual effect estimates from 

epidemiological studies may be modest in size, the public health impact of the mortality 

and morbidity associations can be quite large given that air pollution is ubiquitous. 

Indeed, with the large population exposed, exposure to a pollutant causally associated at a 

population level with mortality and serious illness has significant public health 

consequences, virtually regardless of the relative risk. Taken together, this information 

indicates that exposure to ambient PM2.5 concentrations has substantial public health 

impacts.

In addition, these commenters believed that the EPA downplayed null or 

inconsistent findings in numerous long-term mortality studies with reported PM2.5

concentrations above and below the level of the current annual standard. The EPA 

disagrees that studies with null or inconsistent findings were not accurately presented and 

considered in the Integrated Science Assessment. For example, as discussed throughout 
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section 7.6 and depicted in Figures 7-6 and 7-7 of the Integrated Science Assessment, the 

EPA presented the collective evidence from all studies that examined the association 

between long-term PM2.5 exposure and mortality. Overall, across these studies there was 

evidence of consistent positive associations in different cohorts. That evidence, in 

combination with the biological plausibility provided by experimental and toxicological 

studies evaluated in sections 7.1 and 7.2 of the Integrated Science Assessment, supported 

a causal relationship exists between long-term PM2.5 exposure and mortality. 

Lastly, some of these commenters argued that in some cases, the EPA used the 

same study and the same underlying database to conclude that there is a causal 

association between mortality and multiple criteria pollutants. These commenters argued, 

“[i]n doing so, EPA attributes the cause of the mortality effects observed to whichever 

criteria pollutant it is reviewing at the time” (API, 2012, pp. 14 to 16). 

The EPA strongly disagrees that the Agency “attributes the cause of mortality 

effects observed to whichever criteria pollutant it is reviewing at the time.” The EPA 

consistently recognizes that other pollutants are also associated with health outcomes, as 

is reflected in the fact that the EPA has established regulations to limit emissions of the 

particulate criteria pollutants as well as other gaseous criteria pollutants. Epidemiological

studies often examine the association between short- and long-term exposures to multiple 

air pollutants and mortality within a common dataset in an attempt to identify the air 

pollutant(s) of the complex mixture most strongly associated with mortality. In evaluating 

these studies, the EPA employs specific study selection criteria to identify those studies 

most relevant to the review of the NAAQS. In its assessment of the health evidence 

regarding PM2.5, the EPA has carefully evaluated the potential for confounding, effect 
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measure modification, and the role of PMPM2.5 as a component of a complex mixture of 

air pollutants (U.S. EPA, 20092009a, p. 1-9). The EPA used a rigorous weight of 

evidence approach to inform causality that evaluated consistency across studies within a 

discipline, evidence for coherence across disciplines, and biological plausibility. 

Additionally, during this process, the EPA assessed the limitations of each study in the 

context of the collective body of evidence. It was the collective evidence, not one 

individual study that ultimately determined whether a causal relationship exists between a 

pollutant and health outcome. In the Integrated Science Assessment, the combination of 

epidemiological and experimental evidence formed the basis for the Agency concluding 

for the first time that a causal relationship exists between short- or long-term exposure to 

a criteria pollutant and mortality (U.S. EPA, 2009, Sectionsections 2.3.1.1 and 2.3.1.2). 

Additionally, while the EPA has evaluated some of the studies used to inform the 

causality determination for PM in the Integrated Science Assessments for the other 

criteria air pollutants, the Agency has done so in the context of examining the collective 

body of evidence for each of the respective criteria air pollutants. As such, the body of 

evidence to inform causality has varied from pollutant to pollutant resulting in the 

association between each criteria air pollutant and mortality being classified at a different 

level of the five-level hierarchy used to inform causation (Seee.g., U.S. EPA, 2008e, U.S.

EPA, 2008f, U.S. EPA, 2010k). 

The EPA notes that the final causality determinations presented in the Integrated 

Science Assessment reflected CASAC’s recommendations on the second draft Integrated 

Science Assessment (Samet, 2009f, pp. 2 to 3). Specifically, CASAC supported the 

EPA’s changes (in the second versus first draft Integrated Science Assessment) from 
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“likely causal” to “causal” for long-term exposure to PM2.5 and cardiovascular effects and 

for cancer and PM2.5 (from “inadequate” to “suggestive”). Id. Furthermore, CASAC 

recommended “upgrading” the causal classification for PM2.5 and total mortality to 

“causal” for both the short- and long-term timeframes. Id. With regard to mortality, the 

“EPA carefully reevaluated the body of evidence, including the collective evidence for 

biological plausibility for mortality effects, and determined that a causal relationship 

exists for short- and long-term exposure to PM2.5 and mortality, consistent with the 

CASAC comments” (Jackson, 20122010). 

(2) With regard to toxicological and controlled human exposure studies, these 

commenters argued that the available evidence does not provide coherence or biological 

plausibility for health effects observed in epidemiological studies (API, 2012, pp. 21 to 

22, Attachment 1, pp. 25 to 29; AAM, 2012, pp. 15 to 16; Texas CEQ, 2012, p. 3). With 

regard to the issue of mechanisms, these commenters noted that although the EPA 

recognizes that new evidence is now available on potential mechanisms and plausible 

biological pathways, the evidence provided by toxicological and controlled human 

exposure studies still does not resolve all questions about how PM2.5 at ambient 

concentrations could produce the mortality and morbidity effects observed in 

epidemiological studies. More specifically, for example, some of these commenters 

argued that:

A review of the Integrated Science Assessment, however, suggests that the 

experimental evidence is inconsistent and not coherent with findings in

epidemiology studies. Specifically, the findings of mild and reversible effects in 

most experimental studies conducted at elevated exposures are not consistent with 

the more serious associations described in epidemiology studies (e.g., hospital 

admissions and mortality). Also, both animal studies and controlled human 

exposure studies have identified no effect levels for acute and chronic exposure to 

PM and PM constituents at concentrations considerably above ambient levels. 
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EPA should consider the experimental findings in light of these higher exposure 

levels and what the relevance may be for ambient exposures (API, 2012, 

Attachment 1, p. 25).

The EPA notes that in the review completed in 1997, the Agency considered the 

lack of demonstrated biologicbiological mechanisms for the varying effects observed in 

epidemiological studies to be an important caution in its integrated assessment of the 

health evidence upon which the standards were based (71 FR 61157, October 17, 2006). 

In the review completed in 2006, the EPA recognized the findings from additional 

research that indicated that different health responses were linked with different particle 

characteristics and that both individual components and complex particle mixtures 

appeared to be responsible for many biologic responses relevant to fine particle 

exposures. Id. Since that review, there has been a great deal of research directed toward 

advancing our understanding of biologic mechanisms. While this research has not 

resolved all questions, and further research is warranted (U.S. EPA, 2011a, section 2.5), it 

has provided important insights as discussed in section III.B.1 of the proposal (77 FR at 

38906 to 38909) and discussed more fully in the Integrated Science Assessment (U.S.

EPA, 2009a, chapterChapter 5). 

As noted in the proposal, toxicological studies provide evidence to support the 

biologicbiological plausibility of cardiovascular and respiratory effects associated with 

long- and short-term PM exposures observed in epidemiological studies (77 FR 38906) 

and provide supportive mechanistic evidence that the cardiovascular morbidity effects 

observed in long-term exposure epidemiological studies are coherent with studies of 

cardiovascular-related mortality (77 FR 38907). The Integrated Science Assessment 

concluded that the new evidence available in this review “greatly expands” upon the 

evidence available in the last review “particularly in providing greater understanding of 
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the underlying mechanisms for PM2.5 induced cardiovascular and respiratory effects for 

both short- and long-term exposures” (U.S. EPA, 2009a, p. 2-17). The mechanistic 

evidence now available, taken together with newly available epidemiological evidence, 

increases the Agency’s confidence that a causal relationship exists between long- and 

short-term exposure to PM2.5 and cardiovascular effects and mortality.56 In addition, 

CASAC supported the Integrated Science Assessment approach and characterization of 

potential mechanisms or modes of action (Samet, 2009e, pp. 7 to 8; Samet, 2009f, p. 11), 

as well as the findings of a causal relationship at the population level between exposure 

to PM2.5 and mortality and cardiovascular effects (Samet, 2009f, pp. 2 to 3).57

Additionally, the EPA disagrees with commenters that the mild and reversible 

effects observed in controlled human exposure studies are inconsistent with the more 

serious effects observed in epidemiologicepidemiological studies. Ethical considerations 

regarding the types of studies that can be performed with human subjects generally limit 

the effects that can be evaluated to those that are transient, reversible, and of limited 

short-term consequence. The relatively small number of subjects recruited for controlled 

exposure studies should also be expected to have less variability in health status and risk 

 

56 See American Trucking Assn’sAssociations v. EPA, 175 F. 3d 1027, 1055-56 (D.C. Cir. 

1999) rev’dreversed in part and affirmed in part sub nom, Whitman v. American 

Trucking Assn’s, Associations, 531 U.S. 457 (2001) holding that the EPA could establish 

NAAQS without identifying a biological mechanism (“To begin with, the statute itself 

requires no such proof. The Administrator may regulate air pollutants "emissions of 

which, in his judgment, cause or contribute to air pollution which may reasonably be 

anticipated to endanger public health or welfare." (emphasis added by the court). 

Moreover, this court has never required the type of explanation petitioners seek from 

EPA. In fact, we have expressly held that EPA's decision to adopt and set air quality 

standards need only be based on ‘reasonable extrapolations from some reliable 

evidence’.... Indeed, were we to accept petitioners' view, EPA (or any agency for that 

matter) would be powerless to act whenever it first recognizes clear trends of mortality or 

morbidity in areas dominated by a particular pathogen.”).

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factors than occurring in the general population. 58 Consequently, the severity of health 

effects observed in controlled human exposure studies evaluating the effects of PM 

should be expected to be less than observed in epidemiologic studies. Nonetheless, that 

effects are observed in relatively healthy individuals participating in controlled exposure 

studies serves as an indicator that PM is initiating health responses and that more severe 

responses may reasonably be expected in a more diverse population. 

It should also be noted that there is a small body of toxicological evidence 

demonstrating mortality in rodents exposed to PM (e.g., Killingsworth et al. 1997; 

Godleski et al. 1996). Overall it is not surprising that lethality is not induced in more 

toxicological research, as these types of studies do not readily lend themselves to this 

endpoint. EpidemiologicEpidemiological studies have observed associations between 

PM and mortality in communities with populations in the range of many thousands to 

millions of people. Clearly, it is not feasible to expose hundreds (if not thousands) of 

animals to ambient PM (potentially over many years) in a laboratory setting to induce 

enough lethalities to distinguish between natural deaths and those attributable to PM. 

 

58 For example, the EPA excludes from its controlled human exposure studies involving 

exposure to PM2.5 any individual with a significant risk factor for experiencing adverse 

effects from such exposure. Thus, the EPA excludes a priori the following categories of 

persons: those with a history of angina, cardiac arrhythmias, and ischemic myocardial 

infarction or coronary bypass surgery; those with a cardiac pacemaker; those with 

uncontrolled hypertension (greater than 150 systolic and 90 diastolic); those with 

neurogenetive diseases; those with a history of bleeding diathesis; those taking betablockers; those using oral anticoagulants; those who are pregnant, attempting to become 

pregnant, or breastfeeding; those who have experienced a respiratory infection within 

four weeks of exposure; those experiencing eye or abdominal surgery within six weeks of 

exposure; those with active allergies; those with a history of chronic illnesses such as 

diabetes, cancer, rheumatologic diseases, immunodeficiency state, known cardiovascular 

disease, or chronic respiratory diseases; smokers. The EPA “Application for Independent 

Review Board Approval of Human Subjects Research: Cardiopulmonary Effects of 

healthy Older GSTM1 Null and Sufficient individuals to Concentrated Ambient Air

Particles (CAPTAIN)”, Nov. 9, 2011, p. 9.
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Furthermore, the heterogeneous human populations sampled in epidemiological studies 

are comprised of individuals with different physical, genetic, health, and socioeconomic 

backgrounds which may impact the outcome. However, in toxicological studies, the 

rodent groups are typically inbred, such that inter-individual variability is minimized. 

Thus, if the rodent strain used is quite robust, PM-induced effects may not be observed at 

low exposure concentrations.

(3) In asserting that the uncertainties in the underlying health science are as great 

or greater than in the last review and therefore do not support revision to the standards at 

this time, commenters in this group variously discussed a number of issues related to: (a) 

confounding, (b) heterogeneity in risk estimates, (c) exposure measurement error, (d) 

model specification, (e) the shape of the concentration-response relationship, and (f) 

understanding the relative toxicity of components within the mixture of fine particles. 

Each of these issues is addressed below and some are discussed in more detail in the 

Response to Comments document. 

In summary, these commenters concluded that the substantial uncertainties 

present in the last review have not been resolved and/or that the uncertainty about the 

possible health risks associated with PM2.5 exposure has not diminished. As discussed 

below, the EPA believes that the overall uncertainty about possible health risks 

associated with both long- and short-term PM2.5 exposure has diminished to an important 

degree since the last review. While the EPA agrees that important uncertainties remain, 

and that future research directed toward addressing these uncertainties is warranted, the 

EPA disagrees with commenters’ views that the remaining uncertainties in the scientific 

evidence are too great to warrant revising the current PM2.5 NAAQS.
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(a) Confounding 

Some commenters have criticized the EPA for not adequately addressing the issue 

of confounding in both long- and short-term exposure studies of mortality and morbidity. 

This includes confounding due to copollutants, as well as unmeasured confounding.59

With regard to copollutant confounding, these commenters asserted that the EPA 

has not adequately interpreted the results from studies that examined the effect of 

copollutants on the relationship between long- and short-term PM2.5 exposures and 

mortality and morbidity outcomes. These commenters contend that the EPA has 

inappropriately concluded that PM2.5-related mortality and morbidity associations are 

generally robust to confounding. The commenters stated that statistically significant 

PM2.5 associations in single-pollutant models in epidemiologicepidemiological studies do 

not remain statistically significant in copollutant models. 

The loss of statistical significance or the reduction in the magnitude of the effect 

estimate when a co-pollutant model is used may be the result of factors other than 

confounding. These changes do not prove either the existence or absence of confounding. 

These impacts must be evaluated in a broader context that considers the entire body of 

evidence. The broader examination of this issue in the Integrated Science Assessment 

included a focus on evaluating the stability of the size of the effect estimates in 

 

59 The Integrated Science Assessment defines confounding as “a confusion of effects. 

Specifically, the apparent effect of the exposure of interest is distorted because the effect 

of an extraneous factor is mistaken for or mixed with the actual exposure effect (which 

may be null) (Rothman and Greenland, 1998)” (U.S. EPA, 2009a, p. 1-16). 

Epidemiological analyses attempt to adjust or control for these characteristics (i.e., 

potential confounders) that differ between exposed and non-exposed individuals (U.S. 

EPA, 2009a, section 1.5.3). Not all risk factors can be controlled for within a study 

design/model and are termed “unmeasured confounders.” An unmeasured confounder is a 

confounder that has not previously been measured and therefore is not included in the 

study design/model. 

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epidemiological studies conducted by a number of research groups using single- and 

copollutant models (U.S. EPA, 2009a, sections 6.2.10.9, 6.3.8.5, and 6.5, Figures 6-5, 6-

9, and 6-15). This examination found that, for most epidemiological studies, there was 

little change in effect estimates based on single- and copollutant models, although the 

Integrated Science Assessment recognized that in some cases, the PM2.5 effect estimates 

were markedly reduced in size and lost statistical significance. Additionally, the EPA 

notes that these comments do not adequately reflect the complexities inherent in 

assessing the issue of copollutant confounding. As discussed in the proposal (77 FR 

38907, 38909, and 38910) and more fully in the Integrated Science Assessment 

(U.S.EPA, 2009a, sections 6.2, 6.3, and 6.5), although copollutant models may be useful

tools for assessing whether gaseous copollutants may be potential confounders, such 

models alone cannot determine whether copollutants are in fact confounders. 

Interpretation of the results of copollutant models is complicated by correlations that 

often exist among air pollutants, by the fact that some pollutants play a role in the 

atmospheric reactions that form other pollutants such as secondary fine particles, and by 

the statistical power of the studies in question inherent in the study methodology. For

example, the every-third or sixth-day sampling schedule often employed for PM2.5

measurements compared to daily measurements of gaseous copollutants drastically 

reduces the overall sample size to assess the effect of copollutants on the PM2.5-morbidity 

or mortality relationship, such that the reduced sample size can lead to less precise effect 

estimates (e.g., wider confidence intervals).

The EPA recognizes that when PM2.5 is correlated with gaseous pollutants it can 

be difficult to identify the effect of individual pollutants in the ambient mixture (77 FR 
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38910). However, based on the available evidence, the EPA concludes epidemiological 

studies continue to support the conclusion that PM2.5 associations with mortality and 

morbidity outcomes are robust to the inclusion of gaseous copollutants in statistical 

models. The EPA evaluated the potential confounding effects of gaseous copollutants 

and, although it is recognized that uncertainties and limitations still remain, the Agency 

concluded the collective body of scientific evidence is “stronger and more consistent than 

in previous reviews providing a strong basis for decision making in this review" (77 FR 

38910/1). 

Several commenters offered detailed comments on the long-term PM2.5 exposure 

studies arguing that associations from mortality studies are subjected to unmeasured 

confounding and as a result are not appropriately characterized as being causal (providing 

evidence of a causal relationship between long-term PM2.5 exposure and mortality (e.g., 

UARG, 2012, pp. 10 to 11, Attachment A, pp. 17 to 23; API, 2012, pp. 13 to 14, 

Attachment 1, pp. 11 to 14, Attachment 7, pp. 2-10; ACC, 2012, p. 18 to 21; AFPM, 

2012, p. 8; TCEQTexas CEQ, 2012, p. 4). Specifically, these commenters cited two 

studies (i.e., Janes et al., 2007 and Greven et al., 2011) that used a new type of statistical 

analysis, in which the study authors to examine associations between annual (long-term) 

and monthly (sub-chronic) PM2.5 exposure and mortality. The commenters interpreted the 

results of the analysisthese analyses as evidence of unmeasured confounding in the longterm PM2.5 exposure-mortality relationship. These commenters interpreted these studies 

as raising fundamental questions regarding the EPA’s determination that a causal 

relationship exists between long-term PM2.5 exposure and mortality. The first study, 

Janes et al., 2007, In addition to the commenters mentioned above, all of the authors of 
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the publications by Janes et al. (2007) and Greven et al. (2011) (i.e., Francesca Dominici, 

Scott Zeger, Holly Janes, and Sonja Greven) submitted a joint comment to the public 

docket in order to clarify specific points regarding these two studies (Dominici et al., 

2012).

The first study, Janes et al. (2007), was evaluated in the Integrated Science 

Assessment (U.S. EPA, 2009a, p. 7-88). The second study, Greven et al., . (2011,), an 

extension of the Janes et al. (2007) study adding three more years of data, is a “new” 

study discussed in the Provisional Science Assessment (U.S. EPA, 2012) and the 

Response to Comments document.). Both studies used nationwide Medicare mortality 

data to examine the association between monthly average PM2.5 concentrations over the 

preceding 12 months and monthly mortality rates in 113 U.S. counties. and examined 

whether community-specific trends in monthly PM2.5 concentrations and mortality 

declined at the same rate as the national rate. The investigators decomposedexamined this 

by decomposing the association between PM2.5 and mortality into two components: (1) 

“national trends”national trends, defined as the association between the national average 

trend in monthly PM2.5 concentrations averaged over the previous 12 months and the 

national average trend in monthly mortality rates, and (2) “local trends,”, defined as 

county-specific deviations in monthly PM2.5 concentrations and monthly mortality rates

from national trends. 

The EPA does not question the results of the national trends analyses conducted 

by Janes et al. (2007) and Greven et al. (2011). These analyses demonstrated associations 

similar in magnitude and precision to other studies of long-term PM2.5 exposure and 

mortality. However, the authors hypothesized that the results from the “local” and 

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“national” analyses should be similar in magnitude, and that any difference between the 

two results would be due to unmeasured confounding.60 The authors provided no 

references to support their assumptions that associations between PM2.5 and mortality 

should be similar across temporal and spatial scales nor why any differences in effect 

estimate size would be due to unmeasured confounding.61 Both Janes et al. (2007) and 

Greven et al. (2011) observed positive and statistically significant associations between 

long-term exposure to PM2.5 and mortality in their national analyses. However, Janes et 

al. (2007) and Greven et al. (2011) eliminated all of the spatial variation in air pollution 

and mortality in their data set when estimating the national effect, focusing instead on 

both chronic (yearly) and sub-chronic (monthly) temporal differences in the data 

(Dominici et al. 2012). Janes et al. (2007) (Table 1) highlighted that over 90 percent of 

the variance in the data set used for the analyses conducted by both Janes et al. (2007) 

and Greven et al. (2011) was attributable to spatial variability, which the authors chose to 

discard. As noted above, the focus of the analyses by Janes et al. (2007) and Greven et al. 

(2011) was on two components: (1) a temporal or time component, i.e., the “national” 

trends analysis, which examined the association between the national average trend in 

monthly PM2.5 concentrations averaged over the previous 12 months and the national 

average trend in monthly mortality rates and (2) a space-by-time component, i.e., the 

“local” trends analysis, which examined county-specific deviations in monthly PM2.5

 

60 Unmeasured confounders need to be factors that are associated with both the exposure 

and outcome of interest, but in the case of the Janes and Greven studies this would also 

need to hold true over multiple dimensions of time and space as stated by Pope and 

Burnett, 2007.

61 In its evaluation of Janes et al. (2007) in the Integrated Science Assessment, the EPA 

did not identify limitations in the statistical methods used per se (U.S. EPA, 2009a, p. 

7-88) and included the results of the national-scale analyses in that study in the body of 

evidence that supported the determination that there is a causal relationship between 

long-term PM2.5 exposure and mortality.
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concentrations and monthly mortality rates from national trends. These two components 

combined comprised less than 10 percent of the variance in the data set. The authors 

included a focus on the space-by-time component, which represented approximately 5 

percent of the variance in the data set, in an attempt to identify, absent confounding, if 

PM2.5 was associated with mortality at this unique exposure window. Thus, these studies 

are not directly comparable to other cohort studies investigating the relationship between 

long-term exposure to PM2.5 and mortality, which make use of spatial variability in air 

pollution and mortality data.62 This point was highlighted by the study authors who stated 

that “when one considers that this wealth of information is not accounted for in [Janes 

2007], it is not as surprising that... vastly different estimates of the PM2.5/mortality 

relationship [were observed] than in other studies that do exploit that variability”

(Dominici et al., 2012, p. 2).

ConsistentThe EPA notes that the results of the local trends analyses conducted 

by Janes et al. (2007) and Greven et al. (2011) are limited by the monthly timescale used 

in these analyses. This view is consistent with comments on the Janes et al. (2007) study 

articulated in Pope and Burnett (2007), the EPA notes63 which noted that an important 

limitation of the local scale analysis conducted by Janes et al. (2007) and subsequently by 

 

62 Though not directly comparable, the national effect estimates for mortality reported by 

Janes et al. (2007) and Greven et al. (2011) are coincidentally similar in magnitude to 

those previously reported. It is important to note that previous cohort studies have 

focused on identifying spatial differences in PM2.5 concentrations between cities, while 

Janes et al. (2007) and Greven et al. (2011) focus primarily on temporal differences in 

PM2.5 concentrations. In fact, Greven et al. (2011) state “We do not focus here on a third 

type [of statistical approach] used in cohort studies, measuring the association between 

average PM2.5 levels and average age-adjusted mortality rates across cities (purely spatial 

or cross-sectional association).”

63 Some commenters argued that there were flaws in the criticisms offered by Pope and 

Burnett (2007) on the paper by Janes et al. (2007) (UARG, 2012, Attachment A, pp. 19 to 

23). The EPA responds to each of these specific comments in the Response to Comments 

document.
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Greven et al. (2011) iswas the subchronic exposure window considered in these analyses. 

Both studies used annual average PM2.5 concentrations to characterize long-term national 

trends which iswas consistent with exposure windows considered in other studies of longterm exposure to PM2.5 and mortality studies..

64 However, the local scale analyses used 

monthly average PM2.5 concentrations to characterize county-specific deviations from 

national trends (the “local” scale). The use of monthly average data likely does not 

provide enough exposure contrast to observe temporal changes in mortality at the local 

scale. It also represents a different exposure window than considered in the large body of 

evidence of health effects related to short-term (from less than one day to up to several

days) and chronic (one or more years) measures of PM2.5.

As discussed by Pope and Burnett (2007) and highlighted in the Integrated 

Science Assessment, the conclusions of Janes et al.Furthermore, the EPA disagrees with 

commenters that studies by Janes et al. (2007) and Greven et al. (2011) provide evidence 

that other studies of long-term exposure to PM2.5 and mortality are affected by 

unmeasured confounding. As noted above, the design of the studies conducted by Janes 

et al (2007) and Greven et al (2011) are fundamentally different than those used in other 

studies of long-term exposure to PM2.5 and mortality, including the ACS cohort and the 

Harvard Six Cities study. Studies, such as the ACS and Harvard Six Cities studies, used 

the spatial variation between cities to measure the effect of long-term (annual) exposures 

to PM2.5 on mortality risk, and did not conduct any analyses relying on the temporal 

variation in PM2.5. The opposite is true of the Janes et al. (2007) and Greven et al. (2011) 

studies which first removed the spatial variability in PM2.5 and then examined the 

 

64 As noted above, however, Janes et al. (2007) and Greven et al. (2011) focused on 

temporal variability and other studies of long-term exposure to PM2.5 and mortality focus 

on spatial variability.
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temporal variation at both the national and local scale to measure the effects of temporal 

differences in PM2.5 on mortality risk. Janes et al. (2007) and Greven et al. (2011) focus 

on changes in PM2.5 concentrations over time and, therefore, control for confounders 

would be based on including variables that vary over time rather than over space. As a 

result, any evidence of potential confounding of the PM2.5-mortality risk relationship 

derived from Janes et al. (2007) and Greven et al. (2011) cannot be extrapolated to draw 

conclusions related to potential spatial confounding in studies based on the spatial 

variation in PM2.5 concentrations.

As detailed in the Integrated Science Assessment (U.S. EPA, 2009a, section 7.6), 

and recognized by the authors of Janes et al. (2007) and Greven et al. (2011), the cohort 

studies that informed the causality determination for long-term PM2.5 exposure and 

mortality “have developed approaches to adjust for measured and unmeasured 

confounders” (Dominici et al., 2012, p. 2). These approaches were specifically designed 

to adjust for spatial confounding. The hypothesis that the authors of Janes et al. (2007) 

and Greven et al. (2011) chose to examine was that differences in the local and national 

effects indicated unmeasured temporal confounding in either the local or national effect 

estimate. This hypothesis was specific to these two studies that examined temporal 

variability in exposure to air pollution and did not include known potential confounders 

at either the national or local scale as time-varying covariates in the statistical model. The 

authors acknowledged that the interpretation of either the national or local estimates 

needs to occur with an appreciation of the potential confounding effects of national and 

local scale covariates that were omitted from the model (Dominici et al., 2012).

It is important to recognize that because Janes et al. (2007) and Greven et al. Formatted: Don't adjust space between Latin

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(2011) focused on variations in PM2.5 over time and not space, the results from these two 

studies do not provide any indication that other studies of long-term exposure to PM2.5

and mortality exhibit spatial confounding, or that PM2.5 does not cause mortality.65 The 

authors of Janes et al. (2007) and Greven et al. (2011) recognized that “it is entirely 

possible that these papers are looking for an association at a timescale for which no 

association truly exists” (Dominici et al.,. 2012, p. 3). Furthermore, as highlighted in the 

Integrated Science Assessment and discussed by Pope and Burnett (2007), the 

conclusions of Janes et al. (2007) “are overstated. . . [T]heir analysis tells us little or 

nothing about unmeasured confounding in those and related studies because the 

methodology of Janes et al. largely excludes the sources of variability that are exploited 

in those other studies. By using monthly mortality counts and lagged 12-month average 

pollution concentrations, the authors eliminate the opportunity to exploit short-term or 

day-to-day variability.” Some commenters argued that there were flaws in the criticisms 

offered by Pope and Burnett (2007) on the paper by Janes et al. (2007) (UARG, 2012, 

Attachment A, pp. 19 to 23). The EPA responds to each of these specific comments in the 

Response to Comments document. In its evaluation of Janes et al. (2007), the EPA did 

not identify limitations in the statistical methods used per se (U.S. EPA, 2009a, p. 7-88) 

and the results of the national-scale analyses were included in the body of evidence that 

supported the determination that there is a causal relationship between long-term PM2.5

exposure and mortality

In conclusion, the EPA believesinterprets the results of the analyses conducted by 

Janes et al. (2007) and Greven et al. (2011) areas being consistent with prior knowledge 

 

65 Further, the EPA notes that Janes et al. (2007) and Greven et al. (2011) provide no 

information relevant to examining confounding in studies of short-term exposure to 

PM2.5.
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of examining associations with long-term exposure to PM2.5 at the national scale using 

long-term average PM2.5 concentrations. However, forFor the reasons presented above 

and discussed in more detail in the Response to Comments document, the Agency 

disagrees with the commenters’ assumption that their “national” and “local” scale 

analyses should produce similar the results and their conclusion that differing results is 

evidence of Janes et al. (2007) and Greven et al. (2011) indicate unmeasured confounding. in 

the results of other cohort studies of long-term exposure to PM2.5 and mortality. 

Therefore, the EPA concludes that these studies do not bring into questioninvalidate the 

large body of epidemiological evidence indicatingthat supports the EPA’s determination 

that a causal associationrelationship exists between long-term PM2.5 exposure and 

mortality. 66

(b) Heterogeneity in Risk Estimates

IndustrySome commenters argued that the heterogeneity in risk estimates 

observed in multicitymulti-city epidemiological studies and the lack of statistical 

significance in many regional or seasonal estimates highlights a potential bias associated 

with combined multi-city epidemiological study results (e.g., API, 2012, Attachment 1, 

pp. 15 to 19). These commenters further argued that more refined intra-urban exposure 

estimates conducted for two of the largest cities included in the ACS study, Los Angeles 

and New York City, based on land-use regression models and/or kriging methods 

(Krewski et al., 2009) “underscore the importance of considering city-specific health 

estimates, which may account for heterogeneity in PM2.5 concentrations or other 

 

66 The EPA notes that the EPA’s conclusion with regard to interpretation of the results 

from Janes et al. (2007) and Greven et al. (2012) is supported by the study authors’ 

conclusion that “[o]ur results do not invalidate previous epidemiological studies”

(Dominici, 2012, p. 1 (emphasis original)).

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differences among cities, rather than relying on pooled nationwide results from multi-city 

studies” (API, 2012, Attachment 1, p. 17).

With respect to understanding the nature and magnitude of PM2.5-related risks, the 

EPA agrees that epidemiological studies evaluating health effects associated with longand short-term PM2.5 exposures have reported heterogeneity in responses between cities 

and effect estimates across geographic regions of the U.S. (U.S. EPA, 2009a, sections 

6.2.12.1, 6.3.8.1, 6.5.2, and 7.6.1; U.S. EPA, 2011a, p. 2-25). For example, when 

focusing on short-term PM2.5 exposure, the Integrated Science Assessment found that 

multi-city studies that examined associations with mortality and cardiovascular and 

respiratory hospital admissions and emergency department visits demonstrated greater 

cardiovascular effects in the eastern versus the western U.S. (Dominici, et al., 

20062006a; Bell et al., 2008; Franklin et al. (2007, 2008)). 

In addition, the Integrated Science Assessment evaluated studies that provided 

some evidence for seasonal differences in PM2.5 risk estimates, specifically in the 

northeast. The PM ISAIntegrated Science Assessment found evidence indicating that 

individuals may be at greater risk of dying from higher exposures to PM2.5 in the warmer 

months, and at greater risk of PM2.5 associated hospitalization for cardiovascular and 

respiratory diseases during colder months of the year. The limited influence of 

seasonality on PM risk estimates in other regions of the U.S. may be due to a number of 

factors including varying PM composition by season, exposure misclassification due to 

regional tendencies to spend more or less time outdoors and air conditioning usage, and 

the prevalence of infectious diseases during the winter months (U.S. EPA, 2009a, p. 3-

182). 

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Overall, the EPA took note in the proposal that uncertainties still remain 

regarding various factors that contribute to heterogeneity observed in epidemiological 

studies (77 FR 38909/3). Nonetheless, the EPA recognizes that this heterogeneity could 

be attributed, at least in part, to differences in PM2.5 composition across the U.S., as well 

as to exposure differences that vary regionally such as personal activity patterns, 

microenvironmental characteristics, and the spatial variability of PM2.5 concentrations in 

urban areas (U.S. EPA, 2009a, section 2.3.2; 77 FR 38910). 

As recognized in the Policy Assessment, the current epidemiological evidence 

and the limited amount of city-specific speciated PM2.5 data do not allow conclusions to

be drawn that specifically differentiate effects of PM2.5 in different locations (U.S. EPA, 

2011a, p. 2-25). Furthermore, the Integrated Science Assessment concluded “that many 

constituents of PM2.5 can be linked with multiple health effects, and the evidence is not 

yet sufficient to allow differentiation of those constituents or sources that are more 

closely related to specific health outcomes” (U.S. EPA, 2009a, p. 2-17). CASAC 

thoroughly reviewed the EPA’s presentation of the scientific evidence indicating 

heterogeneity in PM2.5 effect estimates in epidemiological studies and concurred with the 

overall conclusions presented in the Integrated Science Assessment.

(c) Exposure Measurement Error 

IndustrySome commenters argued that the EPA did not adequately consider 

exposure measurement error, which they asserted is an important source of bias in 

epidemiological studies that can bias effect estimates in either direction (e.g., API, 2012, 

Attachment 1, pp. 19 to 20). 

The EPA agrees that exposure measurement error is an important source of 

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uncertainty and that the variability in risk estimates observed in multi-city studies could 

be attributed, in part, to exposure error due to measurement-related issues (77 FR 38910). 

However, the Agency disagrees with the commenters’ assertion that exposure 

measurement error was not adequately considered in this review. The Integrated Science 

Assessment included an extensive discussion that addresses issues of exposure 

measurement error (U.S. EPA, 2009a, sections 2.3.2. and 3.8.6). Exposure measurement 

error may lead to bias in effect estimates in epidemiological studies. A number of studies 

evaluated in the last review (U.S. EPA, 2004, section 8.4.5) and in the current review 

(U.S. EPA, 2009a, section 3.8.6) have discussed the direction and magnitude of bias 

resulting from specified patterns of exposure measurement error (Armstrong 1998; 

Thomas et al. 1993; Carroll et al. 1995) and have generally concluded “classical” (i.e., 

random, within-person) exposure measurement error can bias effect estimates towards the 

null. Therefore, consistent with conclusions reached in the last review, the Integrated 

Science Assessment concluded “in most circumstances, exposure error tends to bias a 

health effect estimate downward” (U.S. EPA, 2009a, sections 2.3.2 and 3.8.6) (emphasis 

added). Thus, the EPA has both considered and accounted for the possibility of exposure 

measurement error, and the possible bias would make it more difficult to detect true 

associations, not less difficult. 

(d) Model Specification 

Commenters contendcontended that the EPA did not account for the fact that 

“selecting an appropriate statistical model for epidemiologic studies of air pollution 

involves several choices that involve much ambiguity, scant biological evidence, and a 

profound impact on analytic results, given that many estimated associations are weak” 

(ACC, 2012, p. 5) For short-term exposure studies, the EPA recognizes, as summarized 

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in the HEI review panel commentary that selecting a level of control to adjust for timevarying factors, such as temperature, in time-series epidemiologicepidemiological studies 

involves a trade-off (HEI, 2003). For example, if the model does not sufficiently adjust 

for the relationship between the health outcome and temperature, some effects of 

temperature could be falsely ascribed to the pollution variable. Conversely, if an overly 

aggressive approach is used to control for temperature, the result would possibly 

underestimate the pollution-related effect and compromise the ability to detect a small but 

true pollution effect (U.S. EPA, 2004, p. 8-236; HEI, 2003, p. 266). The selection of 

approaches to address such variables depends in part on prior knowledge and judgments 

made by the investigators, for example, about weather patterns in the study area and 

expected relationships between weather and other time-varying factors and health 

outcomes considered in the study. As demonstrated in Sectionsection 6.5 of the 

Integrated Science Assessment, the EPA thoroughly considered each of these issues and 

the overall effect of different model specifications on the association between short-term 

PM2.5 exposure and mortality. Regardless of the model employed, consistent positive 

associations were observed across studies that controlled for the potential confounding 

effects of time and weather using different approaches (U.S. EPA 20092009a, Figure 6-

27). The EPA also considered the influence of model specification in the examination of 

long-term PM2.5 exposure studies. For example, in Section 7.6section 7.6 of the 

Integrated Science Assessment, Figures 7-6 and 7-7 summarize the collective evidence 

that evaluated the association between long-term PM2.5 exposure and mortality. 

Regardless of the model used, these studies collectively found evidence of consistent 

positive associations between long-term PM2.5 exposure and mortality. 
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The EPA, therefore, disagrees with commenters that model specification was not 

considered when evaluating the epidemiologicepidemiological evidence used to form 

causality determinations. The EPA specifically points out that the process of assessing 

the scientific quality and relevance of epidemiologicepidemiological studies includes 

examining “important methodological issues (e.g., lag or time period between exposure 

and effects, model specifications, thresholds, mortality displacement) related to 

interpretation of the health evidence (U.S. EPA, 2009, p. 1-9).” Consistent with the 

conclusions of the 2004 PM AQCDAir Quality Criteria Document, the EPA recognizes 

that there is still no clear consensus at this time as to what constitutes appropriate control 

of weather and temporal trends in short-term exposure studies, and that no single 

statistical modeling approach is likely to be most appropriate in all cases (U.S. EPA, 

2004, p. 8-238). However, the EPA believes that the available evidence interpreted in 

light of these remaining uncertainties does provide increased confidence relative to the 

last review in the reported associations between short- and long-term PM2.5 exposures 

and mortality and morbidity effects, alone and in combination with other pollutants.

(e) Concentration-response Relationship

Additionally, commenters questioned the interpretation of the shape of the 

concentration-response relationship, specifically stating that multiple studies have 

demonstrated that there is a threshold in the PM-health effect relationship and that the 

log-linear model is not biologically plausible (API, 2012, Attachment 9; ACC, 2012, 

Appendix A, pp. 7 to 8). The EPA disagrees with this assertion due to the number of 

studies evaluated in the Integrated Science Assessment that continue to support the use of 

a no-threshold, log-linear model to most appropriately represent the PM concentration-
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response relationship (U.S. EPA, 2009a, section 2.4.3). While recognizing that 

uncertainties remain, the EPA believes that our understanding of this issue for both longand short-term exposure studies has advanced since the last review. As discussed in the 

Integrated Science Assessment, both long- and short-term exposure studies have 

employed a variety of statistical approaches to examine the shape of the concentrationresponse function and whether a threshold exists. While the EPA recognizes that there 

likely are individual biologicbiological thresholds for specific health responses, the 

Integrated Science Assessment concluded the overall evidence from existing 

epidemiological studies does not support the existence of thresholds at the population 

level, for effects associated with either long-term or short-term PM exposures within the 

ranges of air quality observed in these studies (U.S. EPA, 2009a, section 2.4.3).

67 The 

Integrated Science Assessment concluded that this evidence collectively supported the 

conclusion that a no-threshold, log-linear model is most appropriate (U.S. EPA, 2009a, 

sections 6.2.10.10, 6.5.2.7, and 7.6.4). CASAC likewise advised that “[a]lthough there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (Samet, 2010d, 

p. ii).

The EPA recognizes that some short-term exposure studies have examined the 

PMPM2.5 concentration-response relationship in individual cities or on a city-to-city basis 

and observed heterogeneity in the shape of the concentration-response curve across cities. 

As discussed in (b) above, these findings are a source of uncertainty that the EPA agrees 

requires further investigation. Nonetheless, the Integrated Science Assessment concluded 

 

67 While epidemiological analyses have not identified a population threshold in the range 

of air quality concentrations evaluated in these studies, the EPA recognizes that it is 

possible that such thresholds exist towards the lower end of these ranges (or below these 

ranges).
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that “the studies evaluated further support the use of a no-threshold, log-linear model, but 

additional issues such as the influence of heterogeneity in estimates between cities and 

the effects of seasonal and regional differences in PM on the concentration-responserelationship still require further investigation” (U.S. EPA, 2009a, p. 2-25). 

(f) Relative Toxicity of PM2.5 Components 

Some commenters highlighted uncertainties in understanding the role of 

individual constituents withwithin the mix of fine particles. These commenters asserted 

that a mass-based standard may not be appropriate due to the growing body of evidence 

indicating that certain PM2.5 components may be more closely related to specific health 

outcomes (e.g., EC and OC) (EPRI, 2012, p. 2). 

With regard to questions about the role of individual constituents within the mix 

of fine particles, as a general matter, the EPA recognizes that although new research 

directed toward this question has been conducted since the last review, important 

questions remain and the issue remains an important element in the Agency’s ongoing 

research program. At the time of the last review, the Agency determined that it was 

appropriate to continue to control fine particles as a group, as opposed to singling out any 

particular component or class of fine particles (71 FR 61162 to 61164, October 17, 2006). 

This distinction was based largely on epidemiological evidence of health effects using 

various indicators of fine particles in a large number of areas that had significant 

contributions of differing components or sources of fine particles, together with some 

limited experimental studies that provided some evidence suggestive of health effects 

associated with high concentrations of numerous fine particle components. 

In this review, as discussed in the proposal (77 FR 38922 to 38923) and in section 
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III.E.1 below, while most epidemiological studies continue to be indexed by PM2.5 mass, 

several recent epidemiological studies included in the Integrated Science Assessment 

have used PM2.5 speciation data to evaluate health effects associated with fine particle 

exposures. In the Integrated Science Assessment, the EPA thoroughly evaluated the 

scientific evidence that examined the effect of different PM2.5 components and sources on 

a variety of health outcomes (U.S. EPA, 2009a, section 6.6) and observed that the 

available information continues to suggest that many different chemical components of 

fine particles and a variety of different types of source categories are all associated with, 

and probably contribute to, effects associated with PM2.5. The Integrated Science 

Assessment concluded that the current body of scientific evidence indicated that “many 

constituents of PM can be linked with differing health effects and the evidence is not yet 

sufficient to allow differentiation of those constituents or sources that are more closely 

related to specific health outcomes” (U.S. EPA, 2009a, p. 2-26 and 6-212). Furthermore, 

the Policy Assessment concluded that the evidence is not sufficient to support eliminating 

any component or group of components associated with any specific source categories 

from the mix of fine particles included in the PM2.5 indicator (U.S. EPA, 2009a, p. 2-56). 

CASAC agreed that it was reasonable to retain PM2.5 as an indicator for fine particles in 

this review as “[t]here was insufficient peer-reviewed literature to support any other 

indicator at this time” (Samet, 2010c, p. 12).

This information is relevant to the Agency’s decision to retain PM2.5 as the 

indicator for fine particles as discussed in section III.E.1 below. The EPA also believes 

that it is relevant to the Agency’s conclusion as to whether revision of the suite of 

primary PM2.5 standards is appropriate. While there remain uncertainties about the role 
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and relative toxicity of various components of fine PM, the current evidence continues to 

support the view that fine particles should be addressed as a group for purposes of public 

health protection. 

In summary, in considering the above issues related to uncertainties in the 

underlying health science, on balance, the EPA believes that the available evidence 

interpreted in light of these remaining uncertainties does provide increased confidence 

relative to the last review in the reported associations between long- and short- term 

PM2.5 exposures and mortality and morbidity effects, alone and in combination with other 

pollutants, and supports stronger inferences as to the causal nature of the associations. 

The EPA also believes that this increased confidence, when taken in context of the entire 

body of available health effects evidence and in light of the evidence from 

epidemiological studies of associations observed in areas meeting the current primary 

PM2.5 standards, specifically in areas meeting the current primary annual PM2.5 standard, 

adds support to its conclusion that the current suite of PM2.5 standards needs to be revised 

to provide increased public health protection. 

(4) In asserting that there is no evidence of greater risk since the 2006 review to 

justify lowering the current annual PM2.5 standard, some commenters argued that, “if the 

current primary PM2.5 annual standard of 15 μg/m3 was considered to be adequately 

protective of public health in 2006, given relative risk estimates that EPA was using at 

that time, then that standard would surely still be adequately protective of the public 

health if relative risk estimates remain at the same level (or lower)” (UARG, 2012, 

Attachment 1, p. 24). These commenters compared risk coefficients used for mortality in 

the EPA’s risk assessment done in the last review with those from the Agency’s core risk 
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assessment done as part of this review, and they concluded that “the entire range of the 

core relative risk for long-term mortality is lower now than it was in the prior review” 

(UARG, 2012, Attachment 1, p. 24). These commenters used this conclusion as the basis 

for a claim that there is no reason to revise the current annual PM2.5 standard.

The EPA believes that this claim is fundamentally flawed as it fails to take into 

account that exposure to a pollutant causally associated at a population-level with

premature mortality and serious illness has significant public health consequences,

virtually regardless of the relative risk.. In comparing the scientific understanding of the 

risk presented by exposure to PM2.5 between the last and current reviews, one must 

examine not only the quantitative estimate of risk from those exposures (e.g., the 

numbers of premature deaths or increased hospital admissions at various concentrations), 

but also the degree of confidence that the Agency has that the observed health effects are 

causally linked to PM2.5 exposure at those concentrations. As documented in the 

Integrated Science Assessment and in the recommendations and conclusions of CASAC, 

the EPA recognizes significant advances in our understanding of the health effects of 

PM2.5, based on evidence that is stronger than in the last review because of its breadth 

and the substantiation of previously observed health effects.. As a result of these 

advances, the EPA is now more certain that fine particles, alone or in combination with 

other pollutants, present a significant risk to public health at concentrations allowed by 

the current primary PM2.5 standards. From this more comprehensive perspective, since 

the risks presented by PM2.5 are more certain, similar or even somewhat lower relative 

risk estimates would not be a basis to conclude that no revision to the suite of PM2.5

standards is “requisite” to protect public health with an adequate margin of safety. This 
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also ignores that the relative risk estimate is only one factor considered by the 

Administrator, e.g. it ignores that epidemiological studies since the last review indicate 

associations between PM2.5 and mortality and morbidity in areas meeting the current 

annual standard.

In any case, the commenters’ reliance on the flawed 2006 review is misplaced. As 

discussed in section III.A.2 above, the D.C. Circuit remanded Administrator Johnson’s 

2006 decision to retain the primary annual PM2.5 standard because the Agency failed to 

adequately explain why the annual standard provided the requisite protection from both 

short- and long-term exposure to fine particles including protection for at-risk 

populations. The 2006 standard was also at sharp odds with CASAC advice and 

recommendations as to the requisite level of protection (Henderson, 2006a,b). In other 

words, the judgment of the then-Administrator that the 2006 primary annual PM2.5

standard was requisite to protect public health with an adequate margin of safety is not an 

appropriate benchmark for comparison. 

(5) Some of these commenters also identified “new” as well as older studies that 

had been included in prior reviews as providing additional evidence that the causality 

determinations presented in the Integrated Science Assessment did not consider the 

totality of the scientific literature, further supporting their view that a revision of the 

PM2.5 is unwarranted. As discussed in section II.B.3 above, the EPA notes that, as in past 

NAAQS reviews, the Agency is basing the final decisions in this review on the studies 

and related information included in the PM ISAIntegrated Science Assessment that have 

undergone CASAC and public review, and will consider newly published studies for 

purposes of decision making in the next PM NAAQS review. In provisionally evaluating 
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commenters’ arguments (see Response to Comments document), the EPA notes that its 

provisional assessment of “new” science found that such studies did not materially 

change the conclusions reached in the Integrated Science Assessment (U.S. EPA, 2012b). 

3. Administrator’s Final Conclusions Concerning the Adequacy of the Current 

Primary PM2.5 Standards

Having carefully considered the public comments, as discussed above, the 

Administrator believes the fundamental scientific conclusions on the effects of PM2.5

reached in the Integrated Science Assessment, and discussed in the Policy Assessment, 

are valid. In considering whether the suite of primary PM2.5 standards should be revised, 

the Administrator places primary consideration on the evidence obtained from the 

epidemiological studies. The Administrator believes that this literature, combined with 

the other scientific evidence discussed in the ISAIntegrated Science Assessment, 

collectively represents a strong and generally robust body of evidence of serious health 

effects associated with both long- and short-term exposures to PM2.5. As discussed in the 

Integrated Science Assessment and Policy Assessment, the EPA believes that much 

progress has been made since the last review in reducing some of the major uncertainties 

that were important considerations in establishing the current suite of PM2.5 standards. In 

that context, the Administrator finds the evidence of serious health effects reported in 

exposure studies conducted in areas with long-term meansmean concentrations ranging 

from approximately at or above the level of the annual standard to long-term meansmean

concentrations significantly below the level of the annual standard to be compelling, 

especially in light of the extent to which such studies are part of an overall pattern of 

positive and frequently statistically significant associations across a broad range of 

studies. The information in the quantitative risk assessment lends support to this 
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conclusion.

There has been extensive critical review of this body of evidence, the quantitative 

risk assessment, and related uncertainties, including review by CASAC and the public. 

The public comments on the basis for the EPA’s proposed decision to revise the suite of 

primary PM2.5 standards have identified a number of issues about which different parties 

disagree including issues for which additional research is warranted. Having weighed all 

comments and the advice of CASAC, the Administrator believes that since the last 

review the overall uncertainty about the public health risks associated with both long- and 

short-term exposure to PM2.5 has been diminished to an important degree. The remaining 

uncertainties in the available evidence do not diminish confidence in the associations 

between exposure to fine particles and mortality and serious morbidity effects. Based on 

her increased confidence in the association between exposure to PM2.5 and serious public 

health effects, combined with evidence of such an association in areas that would meet 

the current standards , the Administrator agrees with CASAC that revision of the current 

suite of PM2.5 standards to provide increased public health protection is necessary. Based 

on these considerations, the Administrator concludes that the current suite of primary 

PM2.5 standards is not sufficient, and thus not requisite, to protect public health with an 

adequate margin of safety, and that revision is needed to increase public health 

protection. 

It is important to note that this conclusion, and the reasoning on which it is based, 

do not resolve the question of what specific revisions are appropriate. That requires 

looking specifically at the current 24-hour and annual PM2.5 standards, including their 

indicator, averaging times, forms, and levels, and evaluating the scientific evidence and 
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other information relevant to determining the appropriate revision of the suite of 

standards.

E. Conclusions on the Elements of the Primary Fine Particle Standards

1. Indicator

In initially setting standards for fine particles in 1997, the EPA concluded it was 

appropriate to control fine particles as a group, rather than singling out any particular 

component or class of fine particles. The EPA noted that community health studies had 

found significant associations between various indicators of fine particles, and that health 

effects in a large number of areas had significant mass contributions of differing 

components or sources of fine particles. In addition, a number of toxicological and 

controlled human exposure studies had reported health effects associations with high 

concentrations of numerous fine particle components. It was also not possible to rule out 

any component within the mix of fine particles as not contributing to the fine particle 

effects found in the epidemiologic studies (62 FR 38667, July 18, 1977). In establishing a 

size-based indicator in 1977 to distinguish fine particles from particles in the coarse 

mode, the EPA noted that the available epidemiological studies of fine particles were 

based largely on PM2.5 and also considered monitoring technology that was generally 

available. The selection of a 2.5 μm size cut reflected the regulatory importance of 

defining an indicator that would more completely capture fine particles under all 

conditions likely to be encountered across the U.S., especially when fine particle 

concentrations and humidity are likely to be high, while recognizing that some small 

coarse particles would also be captured by current methods to monitor PM2.5 (62 FR 

38666 to 38668, July 18, 1997). In the last review, based on the same considerations, the 
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EPA again recognized that the available information supported retaining the PM2.5

indicator and remained too limited to support a distinct standard for any specific PM2.5

component or group of components associated with any source categories of fine 

particles (71 FR 61162 to 61164, October 17, 2006).

In this current review, the same considerations continue to apply for selection of 

an appropriate indicator for fine particles. As an initial matter, the Policy Assessment 

recognizes that the available epidemiological studies linking mortality and morbidity 

effects with long- and short-term exposures to fine particles continue to be largely 

indexed by PM2.5. For the same reasons discussed in the last two reviews, the Policy 

Assessment concluded that it was appropriate to consider retaining a PM2.5 indicator to 

provide protection from effects associated with long- and short-term fine particle 

exposures (U.S. EPA, 20112011a, p. 2-50).

The Policy Assessment also considered the expanded body of evidence available 

in this review to consider whether there was sufficient evidence to support a separate 

standard for ultrafine particles68 or whether there was sufficient evidence to establish 

distinct standards focused on regulating specific PM2.5 components or a group of 

components associated with any source categories of fine particles (U.S. EPA, 2011a, 

section 2.3.1).

A number of studies available in this review have evaluated potential health 

effects associated with short-term exposures to ultrafine particles. As noted in the 

Integrated Science Assessment, the enormous number and larger, collective surface area 

of ultrafine particles are important considerations for focusing on this particle size 

 

68 Ultrafine particles, generally including particles with a mobility diameter less than or 

equal to 0.1 μm, are emitted directly to the atmosphere or are formed by nucleation of 

gaseous constituents in the atmosphere (U.S. EPA, 2009a, p. 3-3).
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fraction in assessing potential public health impacts (U.S. EPA, 2009a, p. 6-83). Per unit 

mass, ultrafine particles may have more opportunity to interact with cell surfaces due to 

their greater surface area and their greater particle number compared with larger particles 

(U.S. EPA, 2009a, p. 5-3). Greater surface area also increases the potential for soluble 

components (e.g., transition metals, organics) to adsorb to ultrafine particles and 

potentially cross cell membranes and epithelial barriers (U.S. EPA, 2009a, p. 6-83). In 

addition, evidence available in this review suggests that the ability of particles to enhance 

allergic sensitization is associated more strongly with particle number and surface area 

than with particle mass (U.S. EPA, 2009a, p. 6-127). 

New evidence, primarily from controlled human exposure and toxicological 

studies, expands our understanding of cardiovascular and respiratory effects related to 

short-term ultrafine particle exposures. However, the Policy Assessment concluded that 

this evidence was still very limited and largely focused on exposure to diesel exhaust, for 

which the Integrated Science Assessment concluded it was unclear whether the effects 

observed are due to ultrafine particles, larger particles within the PM2.5 mixture, or the 

gaseous components of diesel exhaust (U.S. EPA, 2009a, p. 2-22). In addition, the 

Integrated Science Assessment noted uncertainties associated with the controlled human 

exposure studies using concentrated ambient particle systems which have been shown to 

modify the composition of ultrafine particles (U.S. EPA, 2009a, p. 2-22, see also section 

1.5.3). 

The Policy Assessment recognized that there are relatively few epidemiological 

studies that have examined potential cardiovascular and respiratory effects associated 

with short-term exposures to ultrafine particles (U.S. EPA, 2011a, p. 2-51). These studies 
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have reported inconsistent and mixed results (U.S. EPA, 2009a, section 2.3.5).

Collectively, in considering the body of scientific evidence available in this 

review, the Integrated Science Assessment concluded that the currently available 

evidence was suggestive of a causal relationship between short-term exposures to 

ultrafine particles and cardiovascular and respiratory effects. Furthermore, the Integrated 

Science Assessment concluded that evidence was inadequate to infer a causal relationship 

between short-term exposure to ultrafine particles and mortality as well as long-term 

exposure to ultrafine particles and all outcomes evaluated (U.S. EPA, 2009a, sections 

2.3.5, 6.2.12.3, 6.3.10.3, 6.5.3.3, 7.2.11.3, 7.3.9, 7.4.3.3, 7.5.4.3, and 7.6.5.3; Table 2-6). 

With respect to our understanding of ambient ultrafine particle concentrations, at 

present, there is no national network of ultrafine particle samplers; thus, only episodic 

and/or site-specific data sets exist (U.S. EPA, 2009a, p. 2-2). Therefore, the Policy 

Assessment recognized a national characterization of concentrations, temporal and spatial 

patterns, and trends was not possible at this time, and the availability of ambient ultrafine 

measurements to support health studies was extremely limited (U.S. EPA, 2011a, p. 2-

51). In general, measurements of ultrafine particles are highly dependent on monitor 

location and, therefore, more subject to exposure error than accumulation mode particles 

(U.S. EPA, 2009a, p. 2-22). Furthermore, the number of ultrafine particles generally 

decreases sharply downwind from sources, as ultrafine particles may grow into the 

accumulation mode by coagulation or condensation (U.S. EPA, 2009a, p. 3-89). Limited 

studies of ambient ultrafine particle measurements have suggested that these particles 

exhibit a high degree of spatial and temporal heterogeneity driven primarily by 

differences in nearby source characteristics (U.S. EPA, 2009a, p. 3-84). Internal 
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combustion engines and, therefore, roadways are a notable source of ultrafine particles, 

so concentrations of these particles near roadways are generally expected to be elevated 

(U.S. EPA, 2009a, p. 2-3). Concentrations of ultrafine particles have been reported to 

drop off much more quickly with distance from roadways than fine particles (U.S. EPA, 

2009a, p. 3-84).

In considering both the currently available health effects evidence and the air 

quality data, the Policy Assessment concluded that this information was still too limited 

to provide support for consideration of a distinct PM standard for ultrafine particles (U.S.

EPA, 2011a, p. 2-52).

In addressing the issue of particle composition, the Integrated Science Assessment 

concluded that, “[f]rom a mechanistic perspective, it is highly plausible that the chemical 

composition of PM would be a better predictor of health effects than particle size” (U.S.

EPA, 2009a, p. 6-202). Heterogeneity of ambient concentrations of PM2.5 constituents 

(e.g., elemental carbon, organic carbon, sulfates, nitrates) observed in different 

geographical regions as well as regional heterogeneity in PM2.5-related health effects 

reported in a number of epidemiological studies are consistent with this hypothesis (U.S.

EPA, 2009a, section 6.6). 

With respect to the availability of ambient measurement data for fine particle 

components in this review, the Policy Assessment noted that there were now more 

extensive ambient PM2.5 speciation measurement data available through the Chemical 

Speciation Network (CSN) than in previous reviews (U.S. EPA, 2011a, section 1.3.2 and 

Appendix B, section B.1.3). The Integrated Science Assessment observed that data from 

the CSN provided further evidence of spatial and seasonal variation in both PM2.5 mass 
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and composition among cities and geographic regions (U.S. EPA, 2009a, pp. 3-50 to 3-

60; Figures 3-12 to 3-18; Figure 3-47). Some of this variation may be related to regional 

differences in meteorology, sources, and topography (U.S. EPA, 2009a, p. 2-3). 

The currently available epidemiological, toxicological, and controlled human 

exposure studies evaluated in the Integrated Science Assessment on the health effects 

associated with ambient PM2.5 constituents and categories of fine particle sources used a 

variety of quantitative methods applied to a broad set of PM2.5 constituents, rather than 

selecting a few constituents a priori (U.S. EPA, 2009a, p. 2-26). Epidemiological studies 

have used measured ambient PM2.5 speciation data, including monitoring data from the 

CSN, while all of the controlled human exposure and most of the toxicological studies 

have used concentrated ambient particles and analyzed the constituents therein (U.S.

EPA, 2009a, p. 6-203).69 The CSN provides PM2.5 speciation measurements generally on 

a one-in-three or one-in-six day sampling schedule and, thus, does not capture data every 

day at most sites.70

The Policy Assessment recognized that several new multi-city studies evaluating 

short-term exposures to fine particle constituents are now available. These studies 

continued to show an association between mortality and cardiovascular and/or respiratory 

 

69 Most studies considered between 7 to 20 ambient PM2.5 constituents, with elemental 

carbon, organic carbon, sulfates, nitrates, and metals most commonly measured. Many of 

the studies grouped the constituents with various factorization or source apportionment 

techniques to examine the relationship between the grouped constituents and various 

health effects. However, not all studies labeled the constituent groupings according to 

their presumed source and a small number of controlled human exposure and 

toxicological studies did not use any constituent grouping. These differences across 

studies substantially limit any integrative interpretation of these studies (U.S. EPA, 

2009a, p. 6-203). 

70 To expand our understanding of the role of specific PM2.5 components and sources 

with respect to the observed health effects, researchers have expressed a strong interest in 

having access to PM2.5 speciation measurements collected more frequently (U.S. EPA, 

2011a, p. 2-53, including footnote 47). 
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morbidity effects and short-term exposures to various PM2.5 components including 

nickel, vanadium, elemental carbon, organic carbon, nitrates, and sulfates (U.S. EPA, 

2011a, section 2.3.1; U.S. EPA, 2009a, sections 6.5.2.5 and 6.6). 

Limited evidence is available to evaluate the health effects associated with longterm exposures to PM2.5 components (U.S. EPA, 2009a, section 7.6.2). The Policy 

Assessment noted the most significant new evidence was provided by a study that 

evaluated multiple PM2.5 components and an indicator of traffic density in an assessment 

of health effects related to long-term exposure to PM2.5 (Lipfert et al., 20062006a). Using 

health data from a cohort of U.S. military veterans and PM2.5 measurement data from the 

CSN, Lipfert et al. (20062006a) reported positive associations between mortality and 

long-term exposures to nitrates, elemental carbon, nickel, and vanadium as well as traffic 

density and peak ozone concentrations (U.S. EPA, 2011a, p. 2-54; U.S. EPA, 2009a, pp. 

7-89 to 7-90). 

With respect to source categories of fine particles potentially associated with a 

range of health endpoints, the Integrated Science Assessment reported that the currently 

available evidence suggests associations between cardiovascular effects and a number of 

specific PM2.5–related source categories, specificallyincluding oil combustion, wood or 

biomass burning, motor vehicle emissions, and crustal or road dust sources (U.S. EPA, 

2009a, section 6.6; Table 6-18). In addition, a few studies have evaluated associations 

between PM2.5-related source categories and mortality. OneFor example, one study

reported an association between mortality and a PM2.5 coal combustion factor (Laden et 

al., 2000), while other studies linked mortality to a secondary sulfate long-range transport 

PM2.5 source (Ito et al., 2006; Mar et al., 2006) (U.S. EPA, 2009a, section 6.6.2.1). Other 
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studies have looked at different components of particulate matter. There was less 

consistency in associations observed between selected sources of fine particles and 

respiratory health effectsendpoints, which may be partially due to the fact that fewer 

studies have evaluated respiratory-related outcomes and measures. However, there was 

some evidence for PM2.5-related associations with secondary sulfate and decrements in 

lung function in asthmatic and healthy adults (U.S. EPA, 2009a, p. 6-211; Gong et al., 

2005; Lanki et al., 2006). Respiratory effects relating toA couple of studies have 

observed an association between respiratory endpoints in children and adults with asthma 

and surrogates for the crustal/soil/road dust and traffic sources of PM have been observed 

in asthmatic children and adults (U.S. EPA, 2009a, p. 6-205; Gent et al., 2009; Penttinen 

et al., 2006). 

Recent studies have shown that source apportionment methods have the potential 

to add useful insights into which sources and/or PM constituents may contribute to 

different health effects. Of particular interest are several epidemiological studies that 

compared source apportionment methods and reported consistent results across research 

groups (U.S. EPA, 2009a, p. 6-211; Hopke et al., 2006; Ito et al., 2006; Mar et al., 2006; 

Thurston et al., 2005). These studies reported associations between total mortality and 

secondary sulfate in two cities for two different lag times. The sulfate effect was stronger 

for total mortality in Washington D.C. and for cardiovascular-related mortality in 

Phoenix (U.S. EPA, 2009a, p. 6-204). These studies also found some evidence for 

associations with mortality and a number of source categories (e.g., biomass/wood 

combustion, traffic, copper smelter, coal combustion, sea salt) at various lag times (U.S.

EPA, 2009a, p. 6-204). Sarnat et al. (2008) compared three different source 
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apportionment methods and reported consistent associations between emergency 

department visits for cardiovascular diseases with mobile sources and biomass 

combustion as well as increased respiratory-related emergency department visits 

associated with secondary sulfate (U.S. EPA, 2009a, pp. 6-204 and 6-211).

Collectively, in considering the currently available evidence for health effects 

associated with specific PM2.5 components or groups of components associated with any 

source categories of fine particles as presented in the Integrated Science Assessment, the 

Policy Assessment concluded that additional information available in this review 

continues to provide evidence that many different constituents of the fine particle mixture 

as well as groups of components associated with specific source categories of fine 

particles are linked to adverse health effects (U.S. EPA, 2011a, p. 2-55). However, as 

noted in the Integrated Science Assessment, while “[t]here is some evidence for trends 

and patterns that link particular ambient PM constituents or sources with specific health 

outcomes...there is insufficient evidence to determine whether these patterns are 

consistent or robust” (U.S. EPA, 2009a, p. 6-210). Assessing this information, the 

Integrated Science Assessment concluded that “the evidence is not yet sufficient to allow 

differentiation of those constituents or sources that are more closely related to specific 

health outcomes” (U.S. EPA, 2009a, pp. 2-26 and 6-212). Therefore, the Policy 

Assessment concluded that the currently available evidence is not sufficient to support 

consideration of a separate indicator for a specific PM2.5 component or group of 

components associated with any source category of fine particles. Furthermore, the 

Policy Assessment concluded that the evidence is not sufficient to support eliminating 

any component or group of components associated with any source categories of fine 
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particles from the mix of fine particles included in the PM2.5 indicator (U.S. EPA, 2011a, 

p. 2-56). 

The CASAC agreed with the EPA staff conclusions presented in the Policy 

Assessment and concluded that it is appropriate to consider retaining PM2.5 as the 

indicator for fine particles and further asserted, “There [is] insufficient peer-reviewed 

literature to support any other indicator at this time” (Samet, 2010c, p. 12). CASAC 

expressed a strong desire for the EPA to “look ahead to future review cycles and 

reinvigorate support for the development of evidence that might lead to newer indicators 

that may correlate better with the health effects associated with ambient air 

concentrations of PM ...” (Samet, 2010c, p 2). 

Consistent with the staff conclusions presented in the Policy Assessment and 

CASAC advice, the Administrator proposed to retain PM2.5 as the indicator for fine 

particles. Further, the Administrator provisionally concluded that currently available 

scientific information does not provide a sufficient basis for supplementing mass-based, 

primary fine particle standards with standards using a separate indicator for ultrafine 

particles or a separate indicator for a specific PM2.5 component or group of components 

associated with any source categories of fine particles. In addition, the Administrator also 

provisionally concluded that the currently available scientific information did not provide 

a sufficient basis for eliminating any individual component or group of components 

associated with any source categories from the mix of fine particles included in the PM2.5

mass-based indicator. 

The EPA received comparatively few public comments on issues related to the 
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indicator for fine particles.71 Some commenters emphasized the need to conduct 

additional research to more fully understand the effect of specific PM2.5 components 

and/or sources on public health. These commenters expressed views about the importance 

of evaluating health effect associations with various fine particle components and types 

of source categories as a basis for focusing ongoing and future research to reduce 

uncertainties in this area and for considering whether alternative indicator(s) may be 

appropriate to consider in future PM NAAQS reviews for standards intended to protect 

against the array of health effects that have been associated with fine particles as indexed 

by PM2.5. For example, the PSR encouraged more research and monitoring related to 

PM2.5 components and noted the importance of components associated with coal 

combustion (PSR, 2012, pp. 5 to 6). EPRI asserted that “new” studies support focusing 

on EC and OC and encouraged the EPA to seriously consider the mass-based approach 

(EPRI, 2012, p. 2). Likewise, Georgia Mining Association supported additional 

monitoring and research efforts related to PM2.5 composition and specifically encouraged 

the evaluation of using particle number (e.g., particle count) (GMA, 2012, pp. 2 to 3). 

The Administrator agrees with CASAC as well as these commenters that the 

results of additional research and monitoring efforts will be helpful for informing future 

PM NAAQS reviews. Information from such studies could also help inform the 

development of strategies that emphasize control of specific types of emission sources so 

as to address particles of greatest concern to public health. However, based upon the 

scientific information considered in the Integrated Science Assessment as well as the 

public comments summarized above, the Administrator continues to take note there is 

 

71 No public comments were submitted regarding the use of a different size cut for fine 

particles.
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evidence that many different constituents of the fine particle mixture as well as groups of 

components associated with specific sources of fine particles are linked to adverse health 

effects. Furthermore, she recognizes that the evidence is not yet sufficient to differentiate 

those constituents or sources that are most closely related to specific health outcomes nor 

to exclude any PM2.5 components or sources of fine particles from the mix of particles 

included in the PM2.5 indicator. 

Having considered the public comments on this issue, the Administrator concurs 

with the Policy Assessment conclusions and CASAC recommendations and concludes 

that it is appropriate to retain PM2.5 as the indicator for fine particles. 

2. Averaging Time

In 1997, the EPA initially set both an annual standard, to provide protection from 

health effects associated with both long- and short-term exposures to PM2.5, and a 24-

hour standard to supplement the protection afforded by the annual standard (62 FR 38667 

to 38668, July, 18, 1997). In the last review, the EPA retained both annual and 24-hour 

averaging times (71 FR 61164, October 17, 2006). These decisions were based, in part, 

on evidence of health effects related to both long-term (from a year to several years) and 

short-term (from less than one day to up to several days) measures of PM2.5.

The overwhelming majority of studies conducted since the last review continue to 

utilize annual (or multi-year) and 24-hour averaging times, reflecting the averaging times 

of the current PM2.5 standards. These studies continue to provide evidence that health 

effects are associated with annual and 24-hour averaging times. Therefore, the Policy 

Assessment concluded it is appropriate to retain the current annual and 24-hour averaging 

times to provide protection from effects associated with both long- and short-term PM2.5
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exposures (U.S. EPA, 2011a, p. 2-57).

In considering whether the information available in this review supports 

consideration of different averaging times for PM2.5 standards specifically with regard to 

considering a standard with an averaging time less than 24 hours to address health effects 

associated with sub-daily PM2.5 exposures, the Policy Assessment noted there continues 

to be a growing body of studies that provide additional evidence of effects associated 

with exposure periods less than 24-hours (U.S. EPA, 2011a, p. 2-57). Relative to 

information available in the last review, recent studies provide additional evidence for 

cardiovascular effects associated with sub-daily (e.g., one to several hours) exposure to 

PM, especially effects related to cardiac ischemia, vasomotor function, and more subtle 

changes in markers of systemic inflammation, hemostasis, thrombosis and coagulation 

(U.S. EPA, 2009a, section 6.2). Because these studies have used different indicators (e.g., 

PM2.5, PM10, PM10-2.5, ultrafine particles), averaging times (e.g., 1, 2, and 4 hours), and 

health outcomes, it is difficult to draw conclusions about cardiovascular effects 

associated specifically with sub-daily exposures to PM2.5. 

With regard to respiratory effects associated with sub-daily PM2.5 exposures, the 

currently available evidence was much sparser than for cardiovascular effects and 

continues to be very limited. The Integrated Science Assessment concluded that for 

several studies of hospital admissions or medical visits for respiratory diseases, the 

strongest associations were observed with 24-hour average or longer exposures, not with 

less than 24-hour exposures (U.S. EPA, 2009a, section 6.3). 

Collectively, the Policy Assessment concluded that this information, when viewed 

as a whole, is too unclear, with respect to the indicator, averaging time and health 
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outcome, to serve as a basis for consideration of establishing a primary PM2.5 standard 

with an averaging time shorter than 24-hours at this time (U.S. EPA, 2011a, p. 2-57). 

With regard to health effects associated with PM2.5 exposure across varying 

seasons in this review, Bell et al. (2008) reported higher PM2.5 risk estimates for 

hospitalization for cardiovascular and respiratory diseases in the winter compared to 

other seasons. In comparison to the winter season, smaller statistically significant 

associations were also reported between PM2.5 and cardiovascular morbidity for spring 

and autumn, and a positive, but statistically non-significant association was observed for 

the summer months. In the case of mortality, Zanobetti and Schwartz (2009) reported a 

4-fold higher effect estimate for PM2.5-associated mortality for the spring as compared to 

the winter. Taken together, these results provided emerging but limited evidence that 

individuals may be at greater risk of dying from higher exposures to PM2.5 in the warmer 

months and may be at greater risk of PM2.5-associated hospitalization for cardiovascular 

and respiratory diseases during colder months of the year (U.S. EPA, 2011a, p. 2-58). 

Overall, the Policy Assessment observed that there are few studies presently 

available to deduce a general pattern in PM2.5-related risk across seasons. In addition, 

these studies utilized 24-hour exposure periods within each season to assess the 

PM2.5-associated health effects and do not provide information on health effects 

associated with a season-long exposure to PM2.5. Due to these limitations in the currently 

available evidence, the Policy Assessment concluded that there was no basis to consider a 

seasonal averaging time separate from a 24-hour averaging time.

Based on the above considerations, the Policy Assessment concluded that the 

currently available information provided strong support for consideration of retaining the 
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current annual and 24-hour averaging times but does not provide support for considering 

alternative averaging times (U.S. EPA, 2011a, p. 2-58). In addition, CASAC considered 

it appropriate to retain the current annual and 24-hour averaging times for the primary 

PM2.5 standards (Samet, 2010c, pp. 2 to 3). At the time of the proposal, the Administrator 

concurred with the staff conclusions and CASAC advice and proposed that the averaging 

times for the primary PM2.5 standards should continue to include annual and 24-hour 

averages to protect against health effects associated with long- and short-term exposures. 

Furthermore, the Administrator provisionally concluded, consistent with conclusions 

reached in the Policy Assessment and by CASAC, that the currently available 

information was too limited to support consideration of alternative averaging times to 

establish a national standard with a shorter-than 24-hour averaging time or with a 

seasonal averaging time. 

The EPA received no significant public comments on the issue of averaging time 

for the PM2.5 primary standards. The Administrator concurs with recommendations made 

by CASAC and the staff conclusions presented in the Policy Assessment and concludes, 

as proposed, that it is appropriate to retain the current annual and 24-hour averaging 

times for the primary PM2.5 standards to protect against health effects associated with 

long- and short-term exposure periods.

3. Form

The “form” of a standard defines the air quality statistic that is to be compared to 

the level of the standard in determining whether an area attains the standard. In this 

review, the EPA considers whether currently available information supports retaining or 

revising the forms for the annual or 24-hour PM2.5 standards. 
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a. Annual Standard

In 1997, the EPA established the form of the annual PM2.5 standard as an annual 

arithmetic mean, averaged over 3 years, from single or multiple community-oriented 

monitors. This form was intended to represent a relatively stable measure of air quality 

and to characterize longer-term area-wide PM2.5 concentrations, in conjunction with a 

24-hour standard designed to provide adequate protection against localized peak or 

seasonal PM2.5 concentrations. The level of the standard was to be compared to 

measurements made at each community-oriented monitoring site, or, if specific criteria 

were met, measurements from multiple community-oriented monitoring sites could be 

averaged (i.e., spatial averaging)72 (62 FR 38671 to 38672, July 18, 1997). The 

constraints were intended to ensure that spatial averaging would not result in inequities in 

the level of protection provided by the standard (62 FR 38672, July 18, 1997). This 

approach was consistent with the epidemiological studies on which the PM2.5 standard 

was primarily based, in which air quality data were generally averaged across multiple 

monitors in an area or were taken from a single monitor that was selected to represent 

community-wide exposures. 

In the last review, the EPA tightened the criteria for use of spatial averaging to 

provide increased protection for vulnerable populations exposed to PM2.5. This change 

was based in part on an analysis of the potential for disproportionate impacts on 

potentially at-risk populations, which found that the highest concentrations in an area 

tend to be measured at monitors located in areas where the surrounding population is 

more likely to have lower education and income levels and higher percentages of 

 

72 Spatial averaging as part of the form of the annual PM2.5 standard is unique to this 

standard and is not used with other PM standards nor with other NAAQS.
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minority populations (71 FR 61166/2, October 17, 2006; U.S. EPA, 2005, section 

5.3.6.1).

In this review, as outlined in section III.B above and discussed more fully in 

section III.B.3 of the proposal, there now exist more health data such that the Integrated 

Science Assessment has identified persons from lower socioeconomic strata as an at-risk 

population (U.S. EPA, 2009a, section 8.1.7; U.S. EPA, 2011a, section 2.2.1). Moreover, 

there now exist more years of PM2.5 air quality data than were available in the last 

review. Consideration in the Policy Assessment of the spatial variability across urban 

areas that was revealed by this expanded data base has raised questions as to whether an 

annual standard that allows for spatial averaging, even within specified constraints as 

narrowed in 2006 (71 FR 61165 to 61167, October 17, 2006), would provide appropriate 

public health protection. 

In considering the potential for disproportionate impacts on at-risk populations, 

the Policy Assessment considered an update of an air quality analysis conducted for the 

last review (U.S. EPA, 2011a, pp. 2-59 to 60; Schmidt, 2011a2011, Analysis A). This 

analysis focused on determining whether the spatial averaging provisions, as modified in 

2006, could introduce inequities in protection for at-risk populations exposed to PM2.5. 

Specifically, the Policy Assessment considered whether persons of lower socioeconomic 

status, minority groups, or different age groups (i.e., children or older adults) are more 

likely than the general population to live in areas in which the monitors recording the 

highest air quality values in an area are located. Data used in this analysis included 

demographic parameters measured at the Census Block or Census Block Group level, 

including percent minority population, percent minority subgroup population, percent of 
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persons living below the poverty level, percent of persons 18 years of age or older, and 

percent of persons 65 years of age and older. In each candidate geographic area, data 

from the Census Block(s) or Census Block Group(s) surrounding the location of the 

monitoring site (as delineated by radii buffers of 0.5, 1.0, 2.0, and 3.0 miles) in which the 

highest air quality value was monitored were compared to the average of monitored 

values in the area. This analysis looked beyond areas that would meet the current spatial 

averaging criteria and considered all urban areas (i.e., Core Based Statistical Areas or 

CBSAs) with at least two valid annual design value monitors (Schmidt, 2011, Analysis 

A). Recognizing the limitations of such cross-sectional analyses, the Policy Assessment 

observed that the highest concentrations in an area tend to be measured at monitors 

located in areas where the surrounding populations are more likely to live below the 

poverty line and to have higher percentage of minorities (U.S. EPA, 2011a, p. 2-60). 

Based upon the analysis described above, the Policy Assessment concluded that 

the existing constraints on spatial averaging, as modified in 2006, may be inadequate to 

avoid substantially greater exposures in some areas, potentially resulting in 

disproportionate impacts on at-risk populations of persons with lower SES levels as well 

as minorities. Therefore, the Policy Assessment concluded that it was appropriate to 

consider revising the form of the annual PM2.5 standard such that it did not allow for the 

use of spatial averaging across monitors. In doing so, the level of the annual PM2.5

standard would be compared to measurements made at the monitoring site that represents 

area-wide air quality recording the highest PM2.5 concentrations73 (U.S. EPA, 2011a, p. 

2-60). 

 

73 As discussed in section VIII.B.1 below, the EPA is revising several terms associated 

with PM2.5 monitor placement. Specifically, the EPA is revoking the term “communityoriented” and replacing it with the term “area-wide” monitoring. 
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The CASAC agreed with staff conclusions that it was “reasonable” for the EPA to 

eliminate the spatial averaging provisions (Samet, 2010d, p. 2). Further, in CASAC’s 

comments on the first draft Policy Assessment, it noted, “Given mounting evidence 

showing that persons with lower SES levels are a susceptible group for PM-related health 

risks, CASAC recommends that the provisions that allow for spatial averaging across 

monitors be eliminated for the reasons cited in the (first draft) Policy Assessment” 

(Samet, 2010c, p. 13). In its review of the second draft Policy Assessment, CASAC 

recognized “although much of the epidemiological research has been conducted using 

community-wide averages, several key studies reference the nearest measurement site, so 

that some risk estimates are not necessarily biased by the averaging process. Further, the 

number of such studies is likely to expand in the future” (Samet, 2010d, pp. 1 to 2).

Only two areas in the country used the initial spatial averaging provisions for 

demonstrating attainment with the primary annual PM2.5 standard set in 1997 (70 FR 

19847, April 14, 2005; U.S. EPA, 2006c). Since these provisions were tightened in 2006,

no area has used spatial averaging to demonstrate attainment. No areas in the country are 

currently using the spatial averaging provisions to demonstrate attainment with the 

current primary annual PM2.5 standard.

In considering the Policy Assessment’s conclusions based on the results of the 

analysis discussed above and concern over the evidence of potential disproportionate 

impacts on at-risk populations as well as CASAC advice, the Administrator proposed to 

revise the form of the annual PM2.5 standard to eliminate the use of spatial averaging. 

Thus, the Administrator proposed revising the form of the annual PM2.5 standard to 

compare the level of the standard with measurements from each “appropriate” monitor in 
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an area

74 with no allowance for spatial averaging. Thus, for an area with multiple 

monitors, the appropriate reporting monitor with the highest design value would 

determine the attainment status for that area.

Of the commenters noted in section III.D.2 above who supported a more stringent 

annual PM2.5 standard, those who commented on the form of the annual PM2.5 standard 

supported the EPA’s proposal to eliminate the spatial averaging provisions. These 

commenters contended that the EPA’s analyses of the potential impacts of spatial 

averaging, discussed above and in the proposal, (77 FR 38924), demonstrated that the 

current form results in uneven public health protection leading to disproportionate 

impacts on at-risk populations. Specifically, the ALA and other environmental and public 

health commenters contended that “spatial averaging allows exposure of people to 

unhealthy levels of pollution at specific locales even within an area meeting the standard” 

(ALA et al., 2012, p. 23). These commenters particularly focused on the importance for 

low-income and minority populations of eliminating the spatial averaging provisions. 

They concluded that spatial averaging “is an environmental justice concern because poor 

people are more likely to live near roads, depots, factories, ports, and other pollution 

sources.” Id. p. 24.

Other commenters (e.g., AAM, 2012; Dow, 2012) also supported the elimination 

of spatial averaging in order to “avoid potential disproportionate impacts on at-risk 

populations” and to maximize “the benefits to public health of reducing the annual PM2.5

standard.” However, these groups expressed concern that the elimination of spatial 

 

74 As discussed in section VIII.B.2.b below, the EPA concludes that PM2.5 monitoring 

sites at micro- and middle-scale locations should beare comparable to the annual standard 

unlessif the monitoring site has been approved by the Regional Administrator as a 

“relatively unique micro-scale, or localized hot-spot, or unique middle-scale 

site.”representing an area-wide location.
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averaging, in combination with the requirement for near road monitors (as discussed in 

section VIII.B.3.b.i of the proposal), would effectively and inappropriately increase the 

stringency of the annual PM2.5 standard. 

This concern was also shared by other commenters who disagreed with the 

elimination of spatial averaging. For example, the Class of ’85 RRG emphasized 

concerns about increasing the stringency of the standard while providing few health 

benefits if spatial averaging is eliminated, particularly in combination with the 

requirement for near-road monitors. These commenters contended that “[b]ecause EPA 

proposes to use the readings from the highest single worst case monitor (rather than the 

average of all community area monitors), and since roadway monitoring locations will 

likely be worst case monitors, the proposed NAAQS will become more stringent without 

targeting the PM2.5 species most harmful to human health” (Class of ’85 RRG, 2012, p. 

6).

Several commenters also maintained that because spatial averaging is consistent 

with how air quality data are considered in the underlying epidemiological studies, such 

averaging should not be eliminated. Specifically, commenters including NAM et al., 

AFPM, and ACC pointed out that PM2.5 epidemiological studies use spatially averaged 

multi-monitor concentrations, rather than the single highest monitor, when evaluating 

health effects. Therefore, these commenters contended that allowing spatial averaging 

would make the PM2.5 standard more consistent with the approaches used in the

epidemiological studies upon which the standard is based. In addition, some commenters

also contended that the EPA failed to consider whether modifying, rather than 

eliminating, the constraints on spatial averaging would have been sufficient to protect the 
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public health. If so, these commenters argued that “elimination of spatial averaging 

would go beyond what is requisite to protect the public health” (NAM et al., 2012, p. 

20)).

In considering the public comments on the form of the annual standard, the EPA 

recognizes a number of commenters agreed with the basis for the EPA’s proposal to 

eliminate spatial averaging. While other commenters expressed disagreement or concern 

with the proposed decision to eliminate the spatial averaging provisions, the Agency 

notes that these commenters did not challenge the analyses or considerations that 

provided the fundamental basis for the Administrator’s proposed decision. Rather, these 

commenters generally raised concerns that eliminating the option for spatial averaging 

would increase the stringency of the standard, especially in light of additional monitoring 

sites in near-road environments (as discussed in section VIII.B.3.b.1 below). In

The EPA does not agree with the comment that siting some monitors in near 

roadway environments makes the standard more stringent or impermissibly more 

stringent. As discussed in section VIII.B.3.b.i below, a significant fraction of the 

population lives in proximity to major roads, and these exposures occur in locations that 

represent ambient air. Monitoring in such areas does not make the standard more 

stringent than warranted, but rather affords the intended protection to the exposed 

populations, among them at-risk populations, exposed to fine particles in these areas. 

Thus, in cases where monitors in near roadway environments are deemed to be 

representative of area-wide air quality they would be compared to the annual standard (as 

discussed more fully in section VIII below). The 24-hour and annual NAAQS are 

designed to protect the public with an adequate margin of safety, and this siting provision 
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is fully consistent with providing the protection the standard is designed to provide and 

does not make the standard more stringent or more stringent than necessary. 

Monitors that are representative of area-wide air quality may be compared to the 

annual standard. This is consistent with the use of monitoring data in the epidemiological 

studies that provide the primary basis for determining the level of the annual standard. In 

addition, the EPA notes that the annual standard is designed to protect against both longand short-term exposures through controlling the broad distribution of air quality across 

an area over time.75 It is fully consistent with the protection the standard is designed to 

provide for near road monitors to be compared to the annual standard if the monitor is 

representative of area-wide air quality. This does not make the standard either more 

stringent or impermissibly more stringent.

In further considering these comments, the EPA notes that the stringency or level 

of protection provided by each NAAQS is not based solely on the form of the standard; 

rather, the four elements of the standard that together serve to define each standard (i.e., 

indicator, averaging time, form, and level) must be considered collectively in evaluating 

the protection afforded by each standard. Therefore, the EPA considers these comments 

are morealso appropriate to discuss collectively with other issues related to the 

appropriate level for annual standard, and are discussed below in sectionsections

III.E.4.c-d. 

In reaching a final decision on the form of the annual standard, the Administrator 

considers the available analyses, CASAC advice, and public comments on form as 

 

75 This is in contrast to the 24-hour standard which is designed to provide supplemental protection, 

addressing peak exposures that might not otherwise be addressed by the annual standard. Consistent with 

this, monitors are not required to be representative of area-wide air quality to be compared to the 24-hour 

standard.

Formatted: Default, Tab stops: Not at 0" +

0.5" + 1" + 1.5" + 2" + 2.5" + 3" + 3.5" +

4" + 4.5" + 5" + 5.5"
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discussed above. She also considers related issues in the public comments on the level of 

the annual standard as discussed in section III.E.4.c below. She notes that even when the 

annual PM2.5 standard was first set in 1997, the spatial averaging provisions included 

constraints intended to ensure that inequities in the level of protection would not result. 

These constraints on spatial averaging were tightened in the last review, based on an 

analysis showing the potential for spatial averaging to allow higher PM2.5 concentrations 

in locations where subgroups within the general population were potentially 

disproportionately exposed and hence, at disproportionate risk (e.g., low income and 

minority communities). The Administrator notes that in proposing to eliminate spatial 

averaging altogether in this review, she has relied on further analyses in the current 

review (Schmidt, 2011a2011, Analysis A). As discussed above and in the proposal (77 

FR 38924), these analyses showed that the current constraints on spatial averaging may 

be inadequate in some areas to avoid substantially greater exposures for people living 

near monitors recording the highest PM2.5 concentrations. Such exposures could result in 

disproportionate impacts to at-risk populations, including low-income populations as well 

as minority groups. 

On this basis, the Administrator concludes that public health would not be 

protected with an adequate margin of safety in all locations, as required by law, if 

disproportionately higher exposure concentrations in at-risk populations such as low 

income communities as well as minority communities were averaged together with lower 

concentrations measured at other sites in a large urban area. See ALA v. EPA, 134 F. 3d 

388, 389 (D.C. Cir., 1998) (“this court has held that ‘NAAQS must protect not only 

average healthy individuals, but also sensitive citizens such as children,’ and ‘if a 
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pollutant adversely affects the health of these sensitive individuals, EPA must strengthen 

the entire national standard’”) and Coalition of Battery Recyclers Association v. EPA, 604 

F 3d. 613, 617 (D.C. Cir., 2010) (“Petitioners’ assertion that the revised lead NAAQS is 

overprotective because it is more stringent than necessary to protect the entire population 

of young U.S. children ignores that the Clean Air Act allows protection of sensitive 

subpopulations.”) In reaching this conclusion, the Administrator further notes that her 

concern over possible disproportionate PM2.5-related health impacts in at-risk populations 

extends to populations living near important sources of PM2.5, including the large 

populations that live near major roadways.76

In light of all of the above considerations, including consideration of available 

analyses, CASAC advice, and public comments, the Administrator concludes that the 

current form of the annual PM2.5 standard should be revised to eliminate spatial averaging 

provisions. Thus, the level of the revised annual PM2.5 standard established with this 

notice will be compared with measurements from each appropriate monitor in an area, 

with no allowance for spatial averaging. The Administrator’s conclusions with regard to 

the appropriate level of the annual PM2.5 standard to set in conjunction with this form are 

discussed below in section III.E.4.d.

b. 24-Hour Standard

In 1997, the EPA established the form of the 24-hour PM2.5 standard as the 98th

percentile of 24-hour concentrations at each population-oriented monitor within an area, 

averaged over three years (62 FR at 38671 to 38674, July 18, 1997). The Agency selected

the 98th percentile as an appropriate balance between adequately limiting the occurrence 

 

76Section VIII.B.3.b.i below discusses public comments specifically related to the 

proposed requirement for near-road monitors. 
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of peak concentrations and providing increased stability which, when averaged over 3 

years, facilitated effective health protection through the development of more stable 

implementation programs. By basing the form of the standard on concentrations 

measured at population-oriented monitoring sites, the EPA intended to provide protection 

for people residing in or near localized areas of elevated concentrations. In the last 

review, in conjunction with lowering the level of the 24-hour standard, the EPA retained 

this form based in part on a comparison with the 99th percentile form.77

In revisiting the stability of a 98th versus 99th percentile form for a 24-hour 

standard intended to provide supplemental protection for a generally controlling annual 

standard, an analysis presented in the Policy Assessment considered air quality data 

reported in 2000 to 2008 to update our understanding of the ratio between peak-to-mean 

PM2.5 concentrations. This analysis provided evidence that the 98th percentile value was a 

more stable metric than the 99th percentile (U.S. EPA, 2011a, Figure 2-2, p. 2-62). 

At the time of the proposal, the Agency recognized that the selection of the 

appropriate form of the 24-hour standard includes maintaining adequate protection 

against peak 24-hour concentrations while also providing a stable target for risk 

management programs, which serves to provide for the most effective public health 

protection in the long run.78 As in previous reviews, the EPA recognized that a 

 

77 In reaching this final decision, the EPA recognized a technical problem associated with 

a potential bias in the method used to calculate the 98thpercentile concentration for this 

form. The EPA adjusted the sampling frequency requirement in order to reduce this bias. 

Accordingly, the Agency modified the final monitoring requirements such that areas that 

are within 5 percent of the standards are required to increase the sampling frequency to 

every day (71 FR 61164 to 61165, October 17, 2006).

78 See ATA III, 283 F.3d at 374-376 which concludes that it is legitimate for the EPA to 

consider overall stability of the standard and its resulting promotion of overall 

effectiveness of NAAQS control programs in setting a standard that is requisite to protect 

the public health. 
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concentration-based form, compared to an exceedance-based form, was more reflective 

of the health risks posed by elevated pollutant concentrations because such a form gives 

proportionally greater weight to days when concentrations are well above the level of the 

standard than to days when the concentrations are just above the level of the standard. 

Further, the Agency provisionally concluded that a concentration-based form, when 

averaged over three years, provided an appropriate balance between limiting peak 

pollutant concentrations and providing a stable regulatory target, thus facilitating the 

development of more stable implementation programs. 

In considering the information provided in the Policy Assessment and recognizing 

that the degree of public health protection likely to be afforded by a standard is a result of 

the combination of the form and the level of the standard, the Administrator proposed to 

retain the 98th percentile form of the 24-hour standard. The Administrator provisionally 

concluded that the 98th percentile form represents an appropriate balance between 

adequately limiting the occurrence of peak concentrations and providing increased 

stability relative to an alternative 99th percentile form. 

Few public commenters commented specifically on the form of the 24-hour 

standard. None of the public commenters raised objections to continuing the use of a 

concentration-based form for the 24-hour standard. Many of the individuals and groups 

who supported a more stringent 24-hour PM2.5 standard noted in section III.D.2 above, 

however, recommended a more restrictive concentration-based percentile form, 

specifically a 99th percentile form. The limited number of these commenters who 

provided a specific rationale for this recommendation generally expressed their concern 

that the 98th percentile form could allow too many days where concentrations exceeded 
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the level of the standard, and thus fail to adequately protect public health. Other public 

commenters representing state and local air agencies and industry groups generally 

supported retaining the current 98th percentile form. In most cases, these groups 

expressed the overall view that the current 24-hour PM2.5 standard, including the form of 

the current standard, should be retained. 

The EPA notes that the viewpoints represented in this review are similar to 

comments submitted in the last review and through various NAAQS reviews. The EPA 

recognizes that the selection of the appropriate form includes maintaining adequate 

protection against peak 24-hour values while also providing a stable target for risk 

management programs, which serves to provide for the most effective public health 

protection in the long run.79 Nothing in the commenters’ views has provided a reason to 

change the Administrator’s previous conclusion regarding the appropriate balance 

represented in the proposed form of the 24-hour PM2.5 standard. Therefore, the 

Administrator concurs with staff conclusions presented in the Policy Assessment and 

CASAC recommendations and concludes that it is appropriate to retain the 98th percentile 

form for the 24-hour PM2.5 standard.

4. Level

In the last review, the EPA selected levels for the annual and the 24-hour PM2.5

standards using evidence of effects associated with periods of exposure that were most 

closely matched to the averaging time of each standard. Thus, as discussed in section 

 

79 As just noted above, it is legitimate for the EPA to consider promotion of overall 

effectiveness of risk management programs designed to attain the NAAQS, including 

their overall stability, in setting a standard that is requisite to protect the public health. 

The context for the court’s discussion in ATA III is identical to that here; whether to adopt 

a 98th percentile form for a 24-hour standard intended to provide supplemental protection 

for a generally controlling annual standard.
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III.A.1, the EPA relied upon evidence from long-term exposure studies as the principal 

basis for selecting the level of the annual PM2.5 standard that would protect against 

effects associated with long-term exposures. The EPA relied upon evidence from the 

short-term exposure studies as the principal basis for selecting the level of the 24-hour 

PM2.5 standard that would protect against effects associated with short-term exposures. 

As summarized in section III.A.2 above, the 2006 decision to retain the level of the 

annual PM2.5 standard at 15 μg/m3 80 was challenged and on judicial review, the D.C. 

Circuit remanded the primary annual PM2.5 standard to the EPA, finding that EPA’s 

explanation for its approach to setting the level of the annual standard was inadequate. 

a. General Approach for Considering Standard Levels

Building upon the lessons learned in the previous PM NAAQS reviews, in 

considering alternative standard levels supported by the currently available scientific 

information, the Policy Assessment used an approach that integrated evidence-based and 

risk-based considerations, took into account CASAC advice, and considered the issues 

raised by the court in remanding the primary annual PM2.5 standard. Following the 

general approach outlined in section III.A.3 above, for the reasons discussed below, the 

Policy Assessment concluded it was appropriate to consider the protection afforded by 

the annual and 24-hour standards taken together against mortality and morbidity effects 

associated with both long- and short-term PM2.5 exposures. This was consistent with the 

approach taken in the review completed in 1997 rather than considering each standard 

separately, as was done in the review completed in 2006. 

 

80 Throughout this section, the annual standard levels are denoted as integer values for 

simplicity, although, as noted above in section II.B.1, Table 1, the annual standard level 

is defined to one decimal place, such that the current annual standard level is 15.0 μg/m3. 

Alternative annual standard levels discussed in this section are similarly defined to one 

decimal place.
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Beyond looking directly at the relevant epidemiologic evidence, the Policy 

Assessment considered the extent to which specific alternative PM2.5 standard levels were 

likely to reduce the nature and magnitude of both long-term exposure-related mortality 

risk and short-term exposure-related mortality and morbidity risk (U.S. EPA, 2011a, 

section 2.3.4.2; U.S.EPA, 2010a, section 4.2.2). As noted in section III.C.3 above, 

patterns of increasing estimated risk reductions were generally observed as either the 

annual or 24-hour standard, or both, were reduced below the level of the current 

standards (U.S. 2011a, Figures 2-11 and 2-12; U.S. EPA, 2010a, sections 4.2.2, 5.2.2, and 

5.2.3). 

Based on the quantitative risk assessment, the Policy Assessment observed, as 

discussed in section III.A.3, that analyses conducted for this and previous reviews 

demonstrated that much, if not most, of the aggregate risk associated with short-term 

exposures results from the large number of days during which the 24-hour average 

concentrations are in the low-to mid-range, below the peak 24-hour concentrations (U.S.

EPA, 2011a, p. 2-9). Furthermore, as discussed in section III.C above and in section 

III.C.3 of the proposal, the Risk Assessment observed that alternative annual standard 

levels, when controlling, resulted in more consistent risk reductions across urban study 

areas, thereby potentially providing a more consistent degree of public health protection 

(U.S. EPA, 2010a, pp. 5-15 to 5-16). In contrast, the Risk Assessment noted that the 

results of simulating alternative suites of PM2.5 standards including different 

combinations of alternative annual and 24-hour standard levels suggested that an 

alternative 24-hour standard level can produce additional estimated risk reductions 

beyond that provided by an alternative annual standard alone. However, the degree of 
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estimated risk reduction provided by alternative 24-hour standard levels was highly 

variable, in part due to the choice of rollback approached used (U.S. EPA, 2010a, p. 5-

17). 

Based on its review of the second draft Policy Assessment, CASAC agreed with 

the EPA staff’s general approach for translating the available epidemiological evidence, 

risk information, and air quality information into the basis for reaching conclusions on 

alternative standards for consideration. Furthermore, CASAC agreed “that it is 

appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” and 

“considers it appropriate to place the greatest emphasis” on health effects judged to have 

evidence supportive of a causal or likely causal relationship as presented in the Integrated 

Science Assessment (Samet, 2010d, p. 1). 

Therefore, the Policy Assessment concluded, consistent with specific CASAC 

advice, that it was appropriate to set a “generally controlling” annual standard that will 

lower a wide range of ambient 24-hour concentrations. The Policy Assessment concluded 

this approach would likely reduce aggregate risks associated with both long- and shortterm exposures with more consistency than a generally controlling 24-hour standard and 

would be the most effective and efficient way to reduce total PM2.5-related population 

risk and so provide appropriate protection. The staff believed this approach, in contrast to 

one focusing on a generally controlling 24-hour standard, would likely reduce aggregate 

risks associated with both long- and short-term exposures with more consistency and 

would likely avoid setting national standards that could result in relatively uneven 
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protection across the country due to setting standards that were either more or less 

stringent than necessary in different geographical areas. 

The Policy Assessment recognized that an annual standard intended to serve as 

the primary means for providing protection against effects associated with both long- and 

short-term PM2.5 exposures cannot be expected to offer an adequate margin of safety 

against the effects of all short-term PM2.5 exposures. As a result, in conjunction with a 

generally controlling annual standard, the Policy Assessment concluded it was 

appropriate to consider setting a 24-hour standard to provide supplemental protection, 

particularly for areas with high peak-to-mean ratios possibly associated with strong local 

or seasonal sources, or PM2.5-related effects that may be associated with shorter-thandaily exposure periods.

At the time of the proposal, the Administrator agreed with the approach discussed 

in the Policy Assessment as summarized in section III.A.3 above, and supported by 

CASAC, of considering the protection afforded by the annual and 24-hour standards 

taken together for mortality and morbidity effects associated with both long- and shortterm exposures to PM2.5. Furthermore, based on the evidence and quantitative risk 

assessment, the Administrator provisionally concluded it was appropriate to set a 

“generally controlling” annual standard that will lower a wide range of ambient 24-hour 

concentrations, with a 24-hour standard focused on providing supplemental protection, 

particularly for areas with high peak-to-mean ratios possibly associated with strong local 

or seasonal sources, or PM2.5-related effects that may be associated with shorter-than 

daily exposure periods. The Administrator provisionally concluded this approach would 

likely reduce aggregate risks associated with both long- and short-term exposures more 
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consistently than a generally controlling 24-hour standard and would be the most 

effective and efficient way to reduce total PM2-5-related population risk.

The Administrator is mindful that considering what standards are requisite to 

protect public health with an adequate margin of safety requires public health policy 

judgments that neither overstate nor understate the strength and limitations of the 

evidence or the appropriate inferences to be drawn from the evidence. At the time of the 

proposal, in considering how to translate the available information into appropriate 

standard levels, the Administrator weighed the available scientific information and 

associated uncertainties and limitations. For the purpose of determining what standard 

levels were appropriate to propose, the Administrator recognized, as did the EPA staff in 

the Policy Assessment, that there was no single factor or criterion that comprised the 

“correct” approach to weighing the various types of available evidence and information, 

but rather there were various approaches that were appropriate to consider. The 

Administrator further recognized that different evaluations of the evidence and other 

information before the Administrator could reflect placing different weight on the relative 

strengths and limitations of the scientific information, and different judgments could be 

made as to how such information should appropriately be used in making public health 

policy decisions on standard levels. This recognition led the Administrator to consider 

various approaches to weighing the evidence so as to identify appropriate standard levels 

to propose. In so doing, the Administrator encouraged extensive public comment on 

alternative approaches to weighing the evidence and other information so as to inform her 

public health policy judgments before reaching final decisions on appropriate standard 

levels.
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b. Proposed Decisions on Standard Levels

i. Consideration of the Alternative Standard Levels in the Policy Assessment

In recognizing the absence of a discernible population threshold below which 

effects would not occur, the Policy Assessment’s general approach for identifying 

alternative annual standard levels that were appropriate to consider focused on 

characterizing the part of the distribution of PM2.5 concentrations in which we had the 

most confidence in the associations reported in the epidemiological studies and 

conversely where our confidence in the association became appreciably lower. The most 

direct approach to address this issue, consistent with CASAC advice (Samet, 2010c, 

p.10), was to consider epidemiological studies reporting confidence intervals around 

concentration-response relationships (U.S. EPA, 2011a, p. 2-63). Based on a thorough 

search of the available evidence, the Policy Assessment identified only one study 

(Schwartz et al., 2008) that conducted a multi-model analysis to characterize confidence 

intervals around the estimated concentration-response relationship. The Policy 

Assessment concluded that this single relevant analysis was too limited to serve as the 

principal basis for identifying alternative standard levels in this review (U.S. EPA, 2011a, 

p. 2-70).

The Policy Assessment explored other approaches that considered different 

statistical metrics to identifyto characterize the part of the distributions of long-term mean 

PM2.5 concentrations that were most influential in generating health effect estimates in 

long- and short-term epidemiological studies, and placed greatest weight on those studies 

that reported positive and statistically significant associations (U.S. EPA, 2011a, p. 2-63). 

First, as discussed in section III.A.3 above, the Policy Assessment considered the 
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statistical metric used in previous reviews. This approach recognized the EPA’s views

that the strongest evidence of associations occurs at concentrations around the long-term 

mean concentration. Thus, in earlier reviews, the EPA focused on identifying standard 

levels that were somewhat below the long-term mean concentrations reported in PM2.5

exposureepidemiological studies. The long-term mean concentrations represented air 

quality data typically used in epidemiological analyses and provided a direct link between 

PM2.5 concentrations and the observed health effects. Further, these data were available 

for all long- and short-term exposure studies analyzed and, therefore, represented the data 

set available for the broadest set of epidemiological studies. 

However, consistent with CASAC’s comments on the second draft Policy 

Assessment81 (Samet, 2010d, p. 2), in preparing the final Policy Assessment, the EPA 

staff explored ways to take into account additional information from epidemiological 

studies, when available (Rajan et al., 2011). These analyses focused on evaluating 

different statistical metrics, beyond the long-term mean concentration, to characterize the 

part of the distribution of PM2.5 concentrations in which staff continued to have 

confidence in the associations observed in epidemiological studies and below which there 

was a comparative lack of data such that the staff’s confidence in the relationship was 

appreciably less. This would also be the part of the distribution of PM2.5 concentrations 

which had the most influence on generating the health effect estimates reported in 

epidemiological studies. As discussed in section III.A.3 above, the Policy Assessment 

recognized there was no one percentile value within a given distribution that was the 

 

81 While CASAC expressed the view that it would be most desirable to have information 

on concentration-response relationships, they recognized that it would also be “preferable 

to have information on the concentrations that were most influential in generating the 

health effect estimates in individual studies” (Samet, 2010d, p. 2). 
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most appropriate or “correct” way to characterize where our confidence in the 

associations becomes appreciably lower. The Policy Assessment concluded that focusing 

on concentrations within the lower quartile of a distribution, such as the range from the 

25th to the 10th percentile, was reasonable to consider as a region within which we begin 

to have appreciably less confidence in the associations observed in epidemiological 

studies.82 In the EPA staff’s view, considering lower PM2.5 concentrations, down to the 

lowest concentration observed in a study, would be a highly uncertain basis for selecting 

alternative standard levels (U.S. EPA, 2009a, p. 2-71).

As outlined in section III.A.3 above, the Policy Assessment recognized that there 

were two types of population-level information to consider in identifying the range of 

PM2.5 concentrations which have the most influence on generating the health effect 

estimates reported in epidemiological studies. The most relevant information to consider 

was the number of health events (e.g., deaths, hospitalizations) occurring within a study 

population in relation to the distribution of PM2.5 concentrations likely experienced by 

study participants. However, in recognizing that access to health event data may be 

restricted, and consistent with advice from CASAC (Samet 2010d, p.2), EPA staff also 

considered the number of participants within each study area, in relation to the 

 

82 In the last review, staff believed it was appropriate to consider a level for an annual 

PM2.5 standard that was somewhat below the averages of the long-term concentrations 

across the cities in each of the key long-term exposures studies, recognizing that the 

evidence of an association in any such study was strongest at and around the long-term 

average where the data in the study are most concentrated. For example, the interquartile 

range of long-term average concentrations within a study and a range within one standard 

deviation around the study mean were considered reasonable approaches for 

characterizing the range over which the evidence of association is strongest (U.S. EPA, 

2005, pp. 5-22 to 5-23). In this review, the Policy Assessment noted the interrelatedness 

of the distributional statistics and a range of one standard deviation around the mean 

which contains approximately 68 percent of normally distributed data, in that one 

standard deviation below the mean falls between the 25th and 10th percentiles (U.S. EPA, 

2011a, p. 2-71). 
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distribution of PM2.5 concentrations (i.e., study population data), as a surrogate for health 

event data. 

In applying this approach, the Policy Assessment focused on identifying the part 

of the distribution of PM2.5 concentrations which had the most influence on generating 

health effect estimates in epidemiological studies, as discussed in section III.A.3 above. 

As discussed below, in working with study investigators, the EPA staff was able to obtain 

health event data for three large multi-city studies (Krewski et al., 2009; Zanobetti and 

Schwartz, 2009; Bell et al., 2008) and population data for the same three studies and one 

additional long-term exposure study (Miller et al., 2007), as documented in a staff 

memorandum (Rajan et al., 2011).83 For the three studies for which both health event and 

study population data were available, the EPA staff analyzed the reliability of using study 

population data as a surrogate for health event data. Based on these analyses, the EPA 

staff recognized that the 10th and 25th percentiles of the health event and study population 

distributions are nearly identical and concluded that the distribution of population data 

can be a useful surrogate for event data, providing support for consideration of the study 

population data for Miller et al. (2007), for which health event data were not available 

(Rajan et al., 2011, Analysis 1 and Analysis 2, in particular, Table 1 and Figures 1 and 2).

With regard to the long-term mean PM2.5 concentrations which are relevant to the 

first approach, Figures 1 through 3 (U.S. EPA, 2011a, Figures 2-4, 2-5, 2-6, and 2-8) 

summarize data available for multi-city, long- and short-term exposure studies that 

evaluated endpoints classified in the Integrated Science Assessment as having evidence 

 

83 The distributional statistical analysis of population-level data built upon an earlier 

analysis that evaluated the distributions of air quality and associated population data for 

three long-term exposure studies and three short-term exposure studies (Schmidt et al., 

2010, Analysis 2). 
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of a causal or likely causal relationship or evidence suggestive of a causal relationship, 

showing the studies with long-term mean PM2.5 concentrations below 17 μg/m3.

84 As 

discussed in more detail in section III.E.4.b of the proposal, Figures 1 and 3 summarize 

the health outcomes evaluated, relative risk estimates, air quality data, and geographic 

scope for long- and short-term exposure studies, respectively, that evaluated mortality 

(evidence of a causal relationship); cardiovascular effects (evidence of a causal 

relationship); and respiratory effects (evidence of a likely causal relationship) in the 

general population, as well as in older adults, an at-risk population. Figure 2 provides this 

same summary information for long-term exposure studies that evaluated respiratory 

effects (evidence of a likely causal relationship) in children, an at-risk population, as well 

as developmental effects (evidence suggestive of a causal relationship). 

 

84 Additional studies presented and assessed in the Integrated Science Assessment report 

effects at higher long-term mean PM2.5 concentrations (e.g., U.S. EPA, 2009a, Figures 2-

1, 2-2, 7-6, and 7-7).
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Figure 1. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Long-term PM2.5 Exposure 

Studies of the General Population and Older Adults85, 

86

 

85 The air quality datalong-term mean PM2.5 concentrations reported by the study authors for the Miller et al. (2007) study isand Lipfert et al. 

(2006a) studies are discussed more fully in the Response to Comments document. (U.S. EPA, 2012a).

86

 The Goss et al. (2004) study patients over the age of 6 years enrolled in the Cystic Fibrosis Foundation National Patient Registry in 1999 

and 2000. The average age of the study participants was 18.4 (SD 10).

Formatted: Font: 12 pt
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Source: U.S. EPA, 2011a, Figure 2-4

Formatted: Font: 12 pt
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Figure 2. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Long-term PM2.5 Exposure 

Studies of Children

Source: U.S. EPA, 2011a, Figure 2-5
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Figure 3. Summary of Effect Estimates (per 10 μg/m3) and Air Quality Distributions for Multi-City, Short-term PM2.5 Exposure 

Studies of the General Population and Older Adults

Source: U.S. EPA, 2011a, Figure 2-6
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With regard to consideration of additional information from epidemiological studies which was 

relevant to the second approach, the EPA staff compiled a summary of the range of PM2.5 concentrations 

corresponding with the 25th to 10th percentiles of health event or study population data from the four 

multi-city studies, for which distributional statistics are available87 (U.S. EPA, 2011a, Figure 2-7; Rajan 

et al., 2011, Table 1). By considering this approach, one could focus on the range of PM2.5

concentrations below the long-term mean ambient concentrations over which we continue to have 

confidence in the associations observed in epidemiological studies (e.g., above the 25th percentile) where 

commensurate public health protection could be obtained for PM2.5-related effects and, conversely, 

identify the range in the distribution below which our confidence in the associations is appreciably less, 

to identify alternative annual standard levels. 

The mean PM2.5 concentrations associated with the studies summarized in Figures 1, 2, and 3 

and with the distributional statistics analyses (Rajan et al., 2011) are based on concentrations averaged 

across ambient monitors within each area included in a given study and then averaged across study areas 

to calculate an overall study mean concentration, as discussed above. Figure 4, discussed in more detail 

in section III.E.4.a of the proposal, summarizes statistical metrics for those key studies88 included in

 

87 The EPA staff obtained health event data (e.g., number of deaths, hospitalizations) occurring in a 

study population for three multi-city studies (Krewski et al., 2009; Zanobetti and Schwartz, 2009; Bell et 

al., 2008) and study population data were obtained for the same three studies and one additional study 

(Miller et al., 2007) (U.S. EPA, 2011a, p.2-71). If health event or study population data were available 

for additional studies, the EPA could employ distributional statistics to identify the broader range of 

PM2.5 concentrations that were most influential in generating health effect estimates in those studies. 

88 Long- and short-term exposure studies considered “key” studies for consideration are summarized in 

Figure 4 and include those studies observing effects for which the evidence supported a causal or likely 

causal association. This figure represents the subset of multi-city studies included in Figures 1 through 3 

that provided evidence of positive and generally statistically significant effects associated in whole or in 

part with more recent air quality data, generally representing health effects associated with lower PM2.5

concentrations than had previously been considered in the last review. The EPA notes that many of these 

studies evaluated multiple health endpoints, and not all of the effects evaluated provided evidence of 

positive and statistically significant effects. For purposes of informing the Administrator’s decision on 

the appropriate standard levels, the Agency considers the full body of scientific evidence and focuses on 

those aspects of the key studies that provided evidence of positive and generally statistically significant 

effects. 
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Figure 4. Translating Epidemiological Evidence from Multi-City Exposure Studies into an Annual PM2.5 Standard89

 

89 The air quality datalong-term mean PM2.5 concentrations reported by the study authors for the Miller et al. (2007) study isand Lipfert et al. 

(2006a) studies are discussed more fully in the Response to Comments document. (U.S. EPA, 2012a).

Formatted: Font: 12 pt
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Source: U.S. EPA, 2011a, Figure 2-
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Figures 1, 2, and 3 that provide evidence of positive and generally statistically significant 

PM2.5-related effects, which are relevant to the two approaches for translating 

epidemiological evidence into potential standard levels as discussed above. The top of 

Figure 4 includes information for long-term exposure studies evaluating health outcomes 

classified as having evidence of a causal or likely causal relationship with PM2.5

exposures (long-term mean PM2.5 concentrations indicated by diamond symbols). The 

middle of Figure 4 includes information for short-term exposure studies evaluating health 

outcomes classified as having evidence of a causal or likely causal relationship with 

PM2.5 exposures (long-term mean PM2.5 concentrations indicated by triangle symbols). 

The bottom of Figure 4 includes information for long-term exposures studies evaluating 

health outcomes classified as having evidence suggestive of a causal relationship (longterm mean PM2.5 concentrations indicated by square symbols). Figure 4 also summarizes 

the range of PM2.5 concentrations corresponding with the 25th (indicated by solid circles) 

to 10th (indicated by open circles) percentiles of the health event or study population data 

from the four multi-city studies (highlighted in bold text) for which distributional 

statistics are available.

In considering the evidence, the Policy Assessment recognized that NAAQS are 

standards set so as to provide requisite protection, neither more nor less stringent than 

necessary to protect public health, with an adequate margin of safety. This judgment, 

ultimately made by the Administrator, involves weighing the strength of the evidence and 

the inherent uncertainties and limitations of that evidence. Therefore, depending on the 

weight placed on different aspects of the evidence and inherent uncertainties, 

consideration of different alternative standard levels could be supported. 
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Given the currently available evidence discussed in more detail in section 

III.E.4.b of the proposal and considering the various approaches discussed above, the 

Policy Assessment concluded it was appropriate to focus on an annual standard level 

within a range of about 12 to 11 μg/m3(U.S. EPA, 2011a, pp. 2-82, 2-101, and 2-106). 

As illustrated in Figure 4, the Policy Assessment recognized that a standard level of 12 

μg/m3, at the upper end of this range, was somewhat below the long-term mean PM2.5

concentrations reported in all the multi-city, long- and short-term exposure studies that 

provided evidence of positive and statistically significant associations with health effects 

classified as having evidence of a causal or likely causal relationship, including 

premature mortality and hospitalizations and emergency department visits for 

cardiovascular and respiratory effects as well as respiratory effects in children. Further, a 

level of 12 μg/m3 would reflect consideration of additional population-level information 

from such epidemiological studies in that it generally corresponded with approximately 

the 25th percentile of the available distributions of health events data in the studies for 

which population-level information was available. In addition, a level of 12 μg/m3 would 

reflect some consideration of studies that provided more limited evidence of reproductive 

and developmental effects, which were suggestive of a causal relationship, in that it was 

about at the same level as the lowest long-term mean PM2.5 concentrations reported in 

such studies (see Figure 4). 

Alternatively the Policy Assessment recognized that an annual standard level of 

11 μg/m3, at the lower end of this range, was well below the lowest long-term mean 

PM2.5 concentrations reported in all multi-city long- and short-term exposure studies that 

provide evidence of positive and statistically significant associations with health effects 
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classified as having evidence of a causal or likely causal relationship. A level of 11 μg/m3

would reflect placing more weight on the distributions of health event and population 

data, in that this level was within the range of PM2.5 concentrations corresponding to the 

25th and 10th percentiles of all the available distributions of such data. In addition, a level 

of 11 μg/m3 was somewhat below the lowest long-term mean PM2.5 concentrations 

reported in reproductive and developmental effects studies that are suggestive of a causal 

relationship. Thus, a level of 11 μg/m3 would reflect an approach to translating the 

available evidence that places relatively more emphasis on margin of safety 

considerations and less certain causal relationships than would a standard set at a higher 

level. Such a policy approach would tend to weigh uncertainties in the evidence in such a 

way as to avoid potentially underestimating PM2.5-related risks to public health. Further, 

recognizing the uncertainties inherent in identifying any particular point at which our 

confidence in reported associations becomes appreciably less, the Policy Assessment 

concluded that the available evidence did not provide a sufficient basis to consider 

alternative annual standard levels below 11 μg/m3(U.S. EPA, 2011a, p. 2-81). 

The Policy Assessment also considered the extent to which the available evidence 

provided a basis for considering alternative annual standard levels above 12 μg/m3. As 

discussed below, the Policy Assessment concluded that it could be reasonable to consider 

a standard level up to 13 μg/m3based on a policy approach that weighed uncertainties in 

the evidence in such a way as to avoid potentially overestimating PM2.5-related risks to 

public health, especially to the extent that primary emphasis was placed on long-term 

exposure studies as a basis for an annual standard level. A level of 13 μg/m3 was 

somewhat below the long-term mean PM2.5 concentrations reported in all but one of the 
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long-term exposure studies providing evidence of positive and statistically significant 

associations with PM2.5-related health effects classified as having a causal or likely causal 

relationship. As shown in Figure 4, the one long-term exposure study with a long-term 

mean PM2.5 concentration just below 13 μg/m3 was the Miller et al.,(., (2007) study. 

However, as noted in section III.D.1.a of the proposal and discussed in more detail in the 

Response to Comments document, the Policy Assessment observed that in comparison to 

other long-term exposure studies, the Miller et al. study was more limited in that it was 

based on only one year of air quality data and the one year was after the health outcomes 

were reported (U.S. EPA, 2011a, pp. 2-81 to 2-82). Thus, to the extent that less weight 

was placed on the Miller et al. study than on other long-term exposure studies with more 

robust air quality data, a level of 13 μg/m3could be considered as being protective of 

long-term exposure related effects classified as having a causal or likely causal 

relationship. In also considering short-term exposure studies, however, the Policy 

Assessment noted that a level of 13 μg/m3 was below the long-term mean PM2.5

concentrations reported in most but not all such studies. In particular, two studies --

Burnett et al. (2004) and Bell et al. (2008) -- reported long-term mean PM2.5

concentrations of 12.8 and 12.9 μg/m3, respectively. In considering these studies, the 

Policy Assessment found no basis to conclude that these two studies were any more 

limited or uncertain than the other short-term exposure studies shown in Figures 3 and 4 

(U.S. EPA, 2011a, p. 2-82). On this basis, as discussed below, the Policy Assessment 

concluded that consideration of an annual standard level of 13 μg/m3 would have 

implications for the degree of protection that would need to be provided by the 24-hour 

standard, in order that the suite of PM2.5 standards, taken together, would provide 
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appropriate protection from effects on public health related to short-term exposure to 

PM2.5 (U.S. EPA, 2011a, p. 2-82).

The Policy Assessment also noted that a standard level of 13 μg/m3 would reflect 

a judgment that the uncertainties in the epidemiological evidence as summarized in 

section III.B above and discussed in more detail in section III.B.2 of the proposal, 

including uncertainties related to the heterogeneity observed in the epidemiological 

studies in the eastern versus western parts of the U.S., the relative toxicity of PM2.5

components, and the potential role of co-pollutants, are too great to warrant placing any 

weight on the distributions of health event and population data that extend down below 

the long-term mean concentrations into the lower quartile of the data. This level would 

also reflect a judgment that the evidence from reproductive and developmental effects 

studies that is suggestive of a causal relationship was too uncertain to support 

consideration of any lower level.

Beyond evidence-based considerations, the Policy Assessment also considered the 

extent to which the quantitative risk assessment supported consideration of these 

alternative standard levels or provided support for lower levels. In considering 

simulations of just meeting alternative annual standard levels within the range of 13 to 11 

μg/m3(in conjunction with the current 24-hour standard level of 35 μg/m3), the Policy 

Assessment concluded that important public health improvements are associated with risk 

reductions estimated for standard levels of 13 and 12 μg/m3and noted that the level of 11 

μg/m3 was not included in the quantitative risk assessment. The Policy Assessment noted 

that the overall confidence in the quantitative risk estimates varied for the different 

alternative standard levels evaluated and was stronger for the higher levels and 
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substantially lower for the lowest level evaluated (i.e., 10 μg/m3). Based on the above 

considerations, the Policy Assessment concluded that the quantitative risk assessment 

provided support for considering alternative annual standard levels within a range of 13 

to 11 μg/m3, but did not provide strong support for considering lower alternative standard 

levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that consideration of alternative 

annual standard levels in the range of 13 to 11 μg/m3 may be appropriate. Furthermore, 

the Policy Assessment concluded that the currently available evidence most strongly 

supported consideration of an alternative annual standard level in the range of 12 to 11 

μg/m3(U.S. EPA, 2011a, p. 2-82). The Policy Assessment concluded that an alternative 

level within the range of 12 to 11 μg/m3 would more fully take into consideration the 

available information from all long- and short-term PM2.5 exposure studies, including 

studies of at-risk populations, than would a higher level. This range also reflected placing 

weight on information from studies that helped to characterize the range of PM2.5

concentrations over which we continue to have confidence in the associations observed in 

epidemiological studies, as well as the extent to which our confidence in the associations 

was appreciably less at lower concentrations. 

As recognized in sections III.A.3 and III.E.4.a above, an annual standard intended 

to serve as the primary means for providing protection from effects associated with both 

long- and short-term PM2.5 exposures is not expected to provide appropriate protection 

against the effects of all short-term PM2.5 exposures (unless established at a level so low 

as to undoubtedly provide more protection than necessary for long-term exposures). Of 

particular concern are areas with high peak-to-mean ratios possibly associated with 
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strong local or seasonal sources, or PM2.5-related effects that may be associated with 

shorter-than-daily exposure periods. As a result, the Policy Assessment concluded that it 

was appropriate to consider alternative 24-hour PM2.5 standard levels that would 

supplement the protection provided by an annual standard. 

As outlined in section III.A.3 above, the Policy Assessment considered the 

available evidence from short-term PM2.5 exposure studies, as well as the uncertainties 

and limitations in that evidence, to assess the degree to which alternative annual and 24-

hour PM2.5 standards can be expected to reduce the estimated risks attributed to shortterm fine particle exposures. In considering the available epidemiological evidence, the 

Policy Assessment took into account information from multi-city studies as well as 

single-city studies. The Policy Assessment considered the distributions of 24-hour PM2.5

concentrations reported in short-term exposure studies, focusing on the 98th percentile 

concentrations to match the form of the 24-hour standard as discussed in section III.E.3.b 

above. In recognizing that the annual and 24-hour standards work together to provide 

protection from effects associated with short-term PM2.5 exposures, the Policy 

Assessment also considered information on the long-term mean PM2.5 concentrations 

from these studies. 

In addition to considering the epidemiological evidence, the Policy Assessment 

considered air quality information, specifically peak-to-mean ratios using county-level 

24-hour and annual design values, to characterize air quality patterns in areas possibly 

associated with strong local or seasonal sources. These patterns helped in understanding 

the extent to which different combinations of annual and 24-hour standards would be 

consistent with the policy goal of setting a generally controlling annual standard with a 
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24-hour standard that provides supplemental protection especially for areas with high 

peak-to-mean ratios (U.S. EPA, 2011a, p. 2-14). 

In considering the information provided by the short-term exposure studies, the 

Policy Assessment recognized that to the extent these studies were conducted in areas 

that likely did not meet one or both of the current standards, such studies did not help 

inform the characterization of the potential public health improvements of alternative 

standards set at lower levels. Therefore, in considering the short-term exposure studies to 

inform staff conclusions regarding levels of the 24-hour standard that are appropriate to 

consider, the Policy Assessment placed greatest weight on studies conducted in areas that 

likely met both the current annual and 24-hour standards.

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, as summarized in Figure 3 above and discussed in more detail in section 

III.E.4.c of the proposal, the Policy Assessment noted that, to the extent air quality 

distributions were reduced to reflect just meeting the current 24-hour standard, additional 

protection would be anticipated for the effects observed in the three multi-city studies 

with 98th percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett and 

Goldberg, 2003; Franklin et al., 2008). In the three additional studies with 98th percentile 

values below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 

μg/m3, the Policy Assessment noted that these studies reported long-term mean PM2.5

concentrations of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; Zanobetti 

and Schwartz, 2009; Dominici et al., 2006a). To the extent that consideration was given 

to revising the level of the annual standard, as discussed in section III.E.4.b of the 

proposal, the Policy Assessment recognized that potential changes associated with 
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meeting such an alternative annual standard would result in lowering risks associated 

with both long- and short-term PM2.5 exposures. Consequently, in considering a 24-hour 

standard that would operate in conjunction with an annual standard to provide appropriate 

public health protection, the Policy Assessment noted that to the extent that the level of 

the annual standard was revised to within a range of 13 to 11 μg/m3, in particular in the 

range of 12 to 11 μg/m

3

, additional protection would be provided for the long-term 

effects observed in these multi-city studies (U.S. EPA, 2011a, p. 2-84). 

Based on this information, the Policy Assessment concluded that the multi-city, 

short-term exposure studies generally provided support for retaining the 24-hour standard 

level at 35 μg/m3so long as the standard is in conjunction with an annual standard level 

revised to within a range of 12 to 11 μg/m3(U.S. EPA, 2011a, p. 2-84). Alternatively, in 

conjunction with an annual standard level of 13 μg/m3, the Policy Assessment concluded 

that the multi-city studies provided limited support for revising the 24-hour standard level 

somewhat below 35 μg/m3, such as down to 30 μg/m3, based on one study (Bell et al., 

2008) that reported positive and statistically significant effects with an overall 98th

percentile value below the level of the current 24-hour standard and an overall long-term 

mean concentration slightly less than 13 μg/m3(Figure 3; U.S. EPA, 2011a, p. 2-84). 

In reaching staff conclusions regarding alternative 24-hour standard levels that 

were appropriate to consider, the Policy Assessment also took into account relevant 

information from single-city studies that evaluated effects associated with short-term 

PM2.5 exposures. The Policy Assessment recognized that these studies may provide 

additional insights regarding impacts on at-risk populations and/or on areas with isolated 

peak concentrations. 
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As discussed in more detail in section III.E.4.c of the proposal, although a number 

of single-city studies reported effects at appreciably lower PM2.5 concentrations than 

multi-city short-term exposure studies, the uncertainties and limitations associated with 

the single-city studies were considerably greater than those associated with the multi-city 

studies and, thus, the Policy Assessment concluded there was less confidence in using 

these studies as a basis for setting the level of a standard. Therefore, the Policy 

Assessment concluded that the multi-city short-term exposure studies provided the 

strongest evidence to inform decisions on the level of the 24-hour standard, and the 

single-city studies did not warrant consideration of 24-hour standard levels different from 

those supported by the multi-city studies (U.S. EPA, 2011a, p. 2-88). 

In addition to considering the epidemiological evidence, the Policy Assessment 

took into account air quality information based on county-level 24-hour and annual 

design values to understand the public health implications of the alternative standard 

levels supported by the currently available scientific evidence, as discussed in section 

III.E.4.b above. Consistent with the general approach discussed in section III.A.3 above, 

the Policy Assessment considered the extent to which different combinations of 

alternative annual and 24-hour standard levels based on the evidence would support the 

policy goal of lowering annual and 24-hour air quality distributions by using the annual 

standard to be the “generally controlling” standard in conjunction with setting the 24-

hour standard to provide supplemental protection (U.S. EPA, 2011a, pp 2-88 to 2-91, 

Figure 2-10). 

Using information on the relationship of the 24-hour and annual design values, 

the Policy Assessment examined the implications of three alternative suites of PM2.5
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standards identified as appropriate to consider based on the currently available scientific 

evidence, as discussed above. The Policy Assessment concluded that an alternative suite 

of PM2.5 standards that would include an annual standard level of 11 or 12 μg/m3and a 

24-hour standard with a level of 35 μg/m3(i.e., 11/35 or 12/35) would result in the annual 

standard being the generally controlling standard in most areas although the 24-hour 

standard would continue to be the generally controlling standard in the Northwest (U.S.

EPA, 2011a, pp. 2-89 to 2-91 and Figure 2-10). These Northwest counties generally 

represented areas where the annual mean PM2.5 concentrations have historically been low 

but where relatively high 24-hour concentrations occur, often related to seasonal wood 

smoke emissions. Alternatively, combining an alternative annual standard of 13 μg/m3

with a 24-hour standard of 30 μg/m3 would result in many more areas across the country 

in which the 24-hour standard would likely become the controlling standard (the standard 

driving air quality distributions lower) than if an alternative annual standard of 12 or 11 

μg/m3 were paired with the current level of the 24-hour standard (i.e., 35 μg/m3). 

The Policy Assessment concluded that consideration of retaining the 24-hour 

standard level at 35 μg/m3 would reflect placing greatest weight on evidence from multicity studies that reported positive and statistically significant associations with health 

effects classified as having a causal or likely causal relationship. In conjunction with 

lowering the annual standard level, especially within a range of 12 to 11 μg/m3, this 

alternative recognized additional public health protection against effects associated with 

short-term PM2.5 exposures which would be provided by lowering the annual standard 

such that revision to the 24-hour standard would not be warranted (U.S. EPA, 2011a, p. 

2-91). 
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Beyond evidence-based considerations, the Policy Assessment also considered the 

extent to which the quantitative risk assessment supported consideration of retaining the 

current 24-hour standard level or provided support for lower standard levels. In 

considering simulations of just meeting the current 24-hour standard level of 35 μg/m3or 

alternative levels of 30 or 25 μg/m3(in conjunction with alternative annual standard 

levels within a range of 13 to 11 μg/m3), the Policy Assessment noted that the overall 

confidence in the quantitative risk estimates varied for the different standard levels 

evaluated and was stronger for the higher levels and substantially lower for the lowest 

level evaluated (i.e., 25 μg/m3). Based on this information, the Policy Assessment 

concluded that the quantitative risk assessment provided support for considering a 24-

hour standard level of 35 or 30 μg/m3(in conjunction with an alternative standard level 

within a range of 13 to 11 μg/m3) but did not provide strong support for considering 

lower alternative 24-hour standard levels (U.S. EPA, 2011a, pp. 2-102 to 2-103).

Taken together, the Policy Assessment concluded that while it was appropriate to 

consider an alternative 24-hour standard level within a range of 35 to 30 μg/m3, the 

currently available evidence most strongly supported consideration for retaining the 

current 24-hour standard level at 35 μg/m3in conjunction with lowering the level of the 

annual standard within a range of 12 to 11 μg/m3(U.S. EPA, 2011a, p. 2-92).

ii. CASAC Advice

Based on its review of the second draft Policy Assessment, CASAC agreed with 

the general approach for translating the available epidemiological evidence, risk 

information, and air quality information into the basis for reaching conclusions on 

alternative standards for consideration. Furthermore, CASAC agreed “that it is 
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appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” and 

“considers it appropriate to place the greatest emphasis” on health effects judged to have 

evidence supportive of a causal or likely causal relationship as presented in the Integrated 

Science Assessment (Samet, 2010d, p. 1). 

CASAC concluded that the range of levels presented in the second draft Policy 

Assessment (i.e., alternative annual standard levels within a range of 13 to 11 μg/m3and 

alternative 24-hour standard levels within a range of 35 to 30 μg/m3) “are supported by 

the epidemiological and toxicological evidence, as well as by the risk and air quality 

information compiled” in the Integrated Science Assessment, Risk Assessment, and 

second draft Policy Assessment. CASAC further noted that “[a]lthough there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level 

below which there is no risk for adverse health effects)” (Samet, 2010d, p. ii). 

Although CASAC supported the alternative standard level ranges presented in the 

second draft Policy Assessment, it did not express support for any specific levels or 

combinations of standards. Rather, CASAC encouraged the EPA to develop a clearer 

rationale in the final Policy Assessment for staff conclusions regarding annual and 24-

hour standards that were appropriate to consider, including consideration of the 

combination of these standards supported by the available information (Samet, 2010d, p. 

ii). Specifically, in commenting on a distributional statistical analysis of air quality and 

associated population data presented in the second draft Policy Assessment, CASAC 

encouraged staff to focus on information related to the concentrations that were most 
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influential in generating the health effect estimates in individual studies to inform 

alternative standard levels. CASAC urged that the EPA redo that analysis using health 

event or study population data (Samet, 2010d, p. 2). CASAC also commented that the 

approach presented in the second draft Policy Assessment to identify alternative 24-hour 

standard levels which focused on peak-to-mean ratios was not relevant for informing the 

actual level (Samet 2010d, p. 4). Further, they expressed the concern that the 

combinations of annual and 24-hour standard levels discussed in the second draft Policy 

Assessment (i.e., in the range of 13 to 11 μg/m3for the annual standard, in conjunction 

with retaining the current 24-hour PM2.5 standard level of 35 μg/m3; alternatively, 

revising the level of the 24-hour standard to 30 μg/m3in conjunction with an annual 

standard level of 11 μg/m3) “may not be adequately inclusive” and “[i]t was not clear 

why, for example a daily standard of 30 μg/m3should only be considered in combination 

with an annual level of 11 μg/m3” (Samet, 2010d, p. ii). CASAC encouraged the EPA to 

more clearly explain its rationale for identifying the 24-hour/annual combinations that are 

appropriate for consideration (Samet 2010d, p. ii). 

In considering CASAC’s advice as well as public comment on the second draft 

Policy Assessment, the EPA staff conducted additional analyses and modified their 

conclusions regarding alternative standard levels that were appropriate to consider. The 

staff conclusions in the final Policy Assessment (U.S. EPA, 2011a, section 2.3.4.4) 

differed somewhat from the alternative standard levels discussed in the second draft 

Policy Assessment (U.S. EPA, 2010f, section 2.3.4.3), upon which CASAC based its 

advice. Changes made in the final Policy Assessment were primarily focused on 

improving and clarifying the approach for translating the epidemiological evidence into a 
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basis for staff conclusions on the broadest range of alternative standard levels supported 

by the available scientific information and more clearly articulating the rationale for the 

staff’s conclusions (Wegman, 2011, pp. 1 to 2). Consistent with CASAC’s advice to 

consider more information from epidemiological studies, as discussed in section 

III.E.4.b.1 above, the EPA analyzed additional population-level data obtained from 

several study authors (Rajan et al., 2011). In transmitting the final Policy Assessment to 

CASAC, the Agency notified CASAC that the final staff conclusions reflected 

consideration of CASAC’s advice and that those staff conclusions were based, in part, on 

the specific distributional analysis that CASAC had urged the EPA to conduct (Wegman, 

2011, p.2). Thus, CASAC had an opportunity to comment on the final Policy 

Assessment, but chose not to provide any additional comments or advice after receiving 

it. 

iii. Administrator’s Proposed Decisions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate alternative standard levels to 

consider, the Administrator considered the epidemiological and other scientific evidence, 

estimates of risk reductions associated with just meeting alternative annual and/or 24-

hour standards, air quality analyses, related limitations and uncertainties, staff 

conclusions as presented in the Policy Assessment, and the advice of CASAC. As an 

initial matter, the Administrator agreed with the general approach discussed in the Policy 

Assessment as summarized in sections III.A.3 and III.E.4.a above, and supported by 

CASAC, of considering the protection afforded by the annual and 24-hour standards 

taken together for mortality and morbidity effects associated with both long- and shortterm exposures to PM2.5 (77 FR 38939). Furthermore, based on the evidence and 
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quantitative risk assessment, the Administrator provisionally concluded it is appropriate 

to set a “generally controlling” annual standard that will lower a wide range of ambient 

24-hour concentrations, with a 24-hour standard focused on providing supplemental 

protection, particularly for areas with high peak-to-mean ratios possibly associated with 

strong local or seasonal sources, or PM2.5-related effects that may be associated with 

shorter-than daily exposure periods. The Administrator provisionally concluded this 

approach would likely reduce aggregate risks associated with both long- and short-term 

exposures more consistently than a generally controlling 24-hour standard and would be 

the most effective and efficient way to reduce total PM2-5-related population risk. Id.

In reaching decisions on alternative standard levels to propose, the Administrator 

judged that it was most appropriate to examine where the evidence of associations 

observed in the epidemiological studies was strongest and, conversely, where she had 

appreciably less confidence in the associations observed in the epidemiological studies. 

Based on the characterization and assessment of the epidemiological and other studies 

presented and assessed in the Integrated Science Assessment and the Policy Assessment, 

the Administrator recognized the substantial increase in the number and diversity of 

studies available in this review including extended analyses of the seminal studies of 

long-term PM2.5 exposures (i.e., ACS and Harvard Six Cities studies) as well as important 

new long-term exposure studies (as summarized in Figures 1 and 2). Collectively, the 

Administrator noted that these studies, along with evidence available in the last review, 

provided consistent and stronger evidence of an association with premature mortality, 

with the strongest evidence related to cardiovascular-related mortality, at lower ambient 

concentrations than previously observed. The Administrator also recognized the 
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availability of stronger evidence of morbidity effects associated with long-term PM2.5

exposures, including evidence of cardiovascular effects from the WHI study and 

respiratory effects, including decreased lung function growth, from the extended analyses 

for the Southern California Children’s Health Study. Furthermore, the Administrator 

recognized new U.S. multi-city studies that greatly expanded and reinforced our 

understanding of mortality and morbidity effects associated with short-term PM2.5

exposures, providing stronger evidence of associations at ambient concentrations similar 

to those previously observed (as summarized in Figure 3). Id. at 38939-40.

The newly available scientific evidence built upon the previous scientific data 

base to provide evidence of generally robust associations and to provide a basis for 

greater confidence in the reported associations than in the last review. The Administrator 

recognized that the weight of evidence, as evaluated in the Integrated Science 

Assessment, was strongest for health endpoints classified as having evidence of a causal 

relationship. These relationships included those between long- and short-term PM2.5

exposures and mortality and cardiovascular effects. She recognized that the weight of 

evidence was also strong for health endpoints classified as having evidence of a likely 

causal relationship, which included those between long- and short-term PM2.5 exposures 

and respiratory effects. In addition, the Administrator made note of the much more 

limited evidence for health endpoints classified as having evidence suggestive of a causal 

relationship, including developmental, reproductive and carcinogenic effects. Id .at 

38940.

Based on information discussed and presented in the Integrated Science 

Assessment, the Administrator recognized that health effects may occur over the full 
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range of concentrations observed in the long- and short-term epidemiological studies and 

that no discernible threshold for any effects can be identified based on the currently 

available evidence (U.S. EPA, 2009a, section 2.4.3). She also recognized, in taking note 

of CASAC advice and the distributional statistics analysis discussed in section III.E.4.b.i 

above and in the Policy Assessment, that there was significantly greater confidence in 

observed associations over certain parts of the air quality distributions in the studies, and 

conversely, that there was significantly diminished confidence in ascribing effects to 

concentrations toward the lower part of the distributions.

Consistent with the general approach summarized in section III.A.3 above, and 

supported by CASAC as discussed in section III.E.4.a above, the Administrator generally 

agreed that it was appropriate to consider a level for an annual standard that was 

somewhat below the long-term mean PM2.5 concentrations reported in long- and shortterm exposure studies. In recognizing that the evidence of an association in any such 

study was strongest at and around the long-term average where the data in the study are 

most concentrated, she understood that this approach did not provide a bright line for 

reaching decisions about appropriate standard levels. The Administrator noted that longterm mean PM2.5 concentrations were available for each study considered and, therefore, 

represented the most robust data set to inform her decisions on appropriate annual 

standard levels. She also noted that the overall study mean PM2.5 concentrations were 

generally calculated based on monitored concentrations averaged across monitors in each 

study area with multiple monitors, referred to as a composite monitor concentration, in 

contrast to the highest concentration monitored in each study area, referred to as a 

maximum monitor concentration, which are used to determine whether an area meets a 
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given standard. In considering such long-term mean concentrations, the Administrator 

understood that it was appropriate to consider the weight of evidence for the health 

endpoints evaluated in such studies in giving weight to this information. Id.

Based on the information summarized in Figure 4 above and presented in more 

detail in the Policy Assessment (U.S. EPA, 2011a, chapter 2) for effects classified in the 

Integrated Science Assessment as having a causal or likely causal relationship with PM2.5

exposures, the Administrator observed an overall pattern of statistically significant 

associations reported in studies of long-term PM2.5 exposures with long-term mean 

concentrations ranging from somewhat above the current standard level of 15 μg/m3

down to the lowest mean concentration in such studies of 12.9 μg/m3(in Miller et al., 

2007).90 She observed a similar pattern of statistically significant associations in studies 

of short-term PM2.5 exposures with long-term mean concentrations ranging from around 

15 μg/m3down to 12.8 μg/m3(in Burnett et al., 2004). With regard to effects classified as 

providing evidence suggestive of a causal relationship, the Administrator observed a 

small number of long-term exposure studies related to developmental and reproductive 

effects that reported statistically significant associations with overall study mean PM2.5

 

90 The EPA notes that the Miller et al., (2007) study provides strong evidence of 

cardiovascular related effects associated with long-term PM2.5 exposures. At the time of 

the proposal, the EPA recognized the limited nature of the air quality data considered in 

this study (77 FR 38918, fn. 62). The EPA has reviewed those limitations, in conjunction 

with consideration of public comments received on the proposal as discussed in section 

III.E.4.c, in conjunction with reaching a final decision on the level of the annual standard. Formatted: Font: 12 pt
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concentrations down to 11.9 μg/m3(in Bell et al., 2007).91 Id.

The Administrator also considered additional information from epidemiological 

studies, consistent with CASAC advice, to take into account the broader distribution of 

PM2.5 concentrations and the degree of confidence in the observed associations over the 

broader air quality distribution. In considering this additional information, she understood 

that the Policy Assessment presented information on the 25th and 10th percentiles of the 

distributions of PM2.5 concentrations available from four multi-city studies to provide a 

general frame of reference as to the part of the distribution in which the data become 

appreciably more sparse and, thus, where her confidence in the associations observed in 

epidemiological studies would become appreciably less. 

As summarized in Figure 4 above, the Administrator took note of additional 

population-level data that were available for four studies (Krewski et al., 2009; Miller et 

al., 2007; Bell et al., 2008; Zanobetti and Schwartz, 2009), each of which reported 

statistically significant associations with health endpoints classified as having evidence of 

a causal relationship. In considering the long-term PM2.5 concentrations associated with 

the 25th percentile values of the population-level data for these four studies, she observed 

that these values ranged from somewhat above to somewhat below 12 μg/m3. The 

Administrator recognized that these four studies representedinclude some of the strongest 

evidence available within the overall body of scientific evidence and noted that three of 

these studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and Schwartz, 2009) were 

 

91 With respect to suggestive evidence related to cancer, mutagenic, and genotoxic 

effects, the PM2.5 concentrations reported in studies generally included ambient 

concentrations that are equal to or greater than ambient concentrations observed in 

studies that reported mortality and cardiovascular and respiratory effects (U.S. EPA, 

2009a, section 7.5), such that in selecting alternative standard levels that provide 

protection from mortality and cardiovascular and respiratory effects, it is reasonable to 

anticipate that protection will also be provided for carcinogenic effects.
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used as the basis for concentration-response functions used in the quantitative risk 

assessment (U.S. EPA, 2010a, section 3.3.3). 

In considering this information, the Administrator noted that CASAC advised that 

information about the long-term PM2.5 concentrations that were most influential in 

generating the health effect estimates in epidemiological studies can help to inform 

selection of an appropriate annual standard level. However, the Administrator also 

recognized that additional population-level data were available for only these four studies 

and, therefore, she believed that these studies comprised a more limited data set than one 

based on long-term mean PM2.5 concentrations for which data were available for all 

studies considered, as discussed above. 

The Administrator recognized, as summarized in section III.B.2 above, that 

important uncertainties remain in the evidence and information considered in this review 

of the primary fine particle standards. These uncertainties are generally related to 

understanding the relative toxicity of the different components in the fine particle 

mixture, the role of PM2.5 in the complex ambient mixture, exposure measurement errors 

inherent in epidemiological studies based on concentrations measured at fixed monitor 

sites, and the nature, magnitude, and confidence in estimated risks related to increasingly 

lower ambient PM2.5 concentrations. Furthermore, the Administrator noted that 

epidemiological studies have reported heterogeneity in responses both within and 

between cities and geographic regions across the U.S. She recognized that this 

heterogeneity may be attributed, in part, to differences in fine particle composition in 

different regions and cities. The Administrator also recognized that there are additional

limitations associated with evidence for reproductive and developmental effects, 
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identified as being suggestive of a causal relationship with long-term PM2.5 exposures,

including: the limited number of studies evaluating such effects; uncertainties related to 

identifying the relevant exposure time periods of concern; and limited toxicological 

evidence providing little information on the mode of action(s) or biological plausibility 

for an association between long-term PM2.5 exposures and adverse birth outcomes. Id. at 

38941.

The Administrator was mindful that considering what standards were requisite to 

protect public health with an adequate margin of safety required public health policy 

judgments that neither overstated nor understated the strength and limitations of the 

evidence or the appropriate inferences to be drawn from the evidence. In considering how 

to translate the available information into appropriate standard levels, the Administrator 

weighed the available scientific information and associated uncertainties and limitations. 

For the purpose of determining what standard levels were appropriate to propose, the 

Administrator recognized, as did EPA staff in the Policy Assessment, that there was no 

single factor or criterion that comprised the sole “correct” approach to weighing the 

various types of available evidence and information, but rather there were various 

approaches that are appropriate to consider. The Administrator further recognized that 

different evaluations of the evidence and other information before the Administrator 

could reflect placing different weight on the relative strengths and limitations of the 

scientific information, and different judgments could be made as to how such information 

should appropriately be used in making public health policy decisions on standard levels. 

This recognition led the Administrator to consider various approaches to weighing the 

evidence so as to identify appropriate standard levels to propose. In so doing, the 
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Administrator encouraged extensive public comment on alternative approaches to 

weighing the evidence and other information so as to inform her public health policy 

judgments before reaching final decisions on appropriate standard levels.

In considering the available information, the Administrator noted the advice of 

CASAC that the currently available scientific information, including epidemiological and 

toxicological evidence as well as risk and air quality information, provided support for 

considering an annual standard level within a range of 13 to 11 μg/m3and a 24-hour 

standard level within a range of 35 to 30 μg/m3. In addition, the Administrator recognized 

that the Policy Assessment concluded that the available evidence and risk-based 

information support consideration of annual standard levels in the range of 13 to 11 

μg/m3, and that the Policy Assessment also concluded that the evidence most strongly 

supported consideration of an annual standard level in the range of 12 to 11 μg/m3. In 

considering how the annual and 24-hour standards work together to provide appropriate 

public health protection, the Administrator observed that CASAC did not express support 

for any specific levels or combinations of standards within these ranges. Nor did CASAC 

choose to comment on additional information and analyses presented in the final Policy 

Assessment prepared in response to CASAC’s recommendations on the second draft 

Policy Assessment (Wegman, 2011). 

In considering the extent to which the currently available evidence and 

information provided support for specific standard levels within the ranges identified by 

CASAC and the Policy Assessment as appropriate for consideration, the Administrator 

initially considered standard levels within the range of 13 to 11 μg/m3for the annual 

standard. In so doing, the Administrator first considered the long-term mean PM2.5
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concentrations reported in studies of effects classified as having evidence of a causal or 

likely causal relationship, as summarized in Figure 4 above and discussed more broadly 

above. She noted that a level at the upper end of this range would be below most but not 

all the overall study mean concentrations from the multi-city studies of long- and shortterm exposures, whereas somewhat lower levels within this range would be below all 

such overall study mean concentrations. In considering the appropriate weight to place on 

this information, the Administrator again noted that the evidence of an association in any 

such study was strongest at and around the long-term average where the data in the study 

are most concentrated, and that long-term mean PM2.5 concentrations were available for 

each study considered and, therefore, represented the most robust data set to inform her 

decisions on appropriate annual standard levels. Further, she was mindful that this 

approach did not provide a bright line for reaching decisions about appropriate standard 

levels. Id.

In considering the long-term mean PM2.5 concentrations reported in studies of 

effects classified as having evidence suggestive of a causal relationship, as summarized 

in Figure 4 for reproductive and developmental effects, the Administrator noted that a 

level at the upper end of this range would be below the overall study mean concentration 

in one of the three studies, while levels in the mid- to lower part of this range would be 

below the overall study mean concentrations in two or three of these studies. In 

considering the appropriate weight to place on this information, the Administrator noted 

the very limited nature of this evidence of such effects and the additional uncertainties in 

these epidemiological studies relative to the studies that provide evidence of causal or 

likely causal relationships.
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The Administrator also considered the distributional analyses of population-level 

information that were available from four of the epidemiological studies that provide 

evidence of effects identified as having a causal relationship with long- or short-term 

PM2.5 concentrations for annual standard levels within the same range of 13 to 11 μg/m3. 

In so doing, the Administrator first noted that a level in the mid-part of this range 

generally corresponds with approximately the 25th percentile of the distributions of health 

events data available in three of these studies. The Administrator also noted that standard 

levels toward the upper part of this range would reflect placing substantially less weight 

on this information, whereas standard levels toward the lower part of this range would 

reflect placing substantially more weight on this information. In considering this 

information, the Administrator noted that there was no bright line that delineates the part 

of the distribution of PM2.5 concentrations within which the data become appreciably 

more sparse and, thus, where her confidence in the associations observed in 

epidemiological studies became appreciably less.

In considering mean PM2.5 concentrations and distributional analyses from the 

various sets of epidemiological studies noted above, the Administrator was mindful, as 

noted above, that such studies typically report concentrations based on composite monitor 

distributions, in which concentrations may be averaged across multiple ambient monitors 

that may be present within each area included in a given study. Thus, a policy approach 

that used data based on composite monitors to identify potential alternative standard 

levels would inherently build in a margin of safety of some degree relative to an 

alternative standard level based on measurements at the monitor within an area that 

records the highest concentration, or the maximum monitor, since once a standard was 
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set, concentrations at appropriate maximum monitors within an area were generally used 

to determine whether an area meets a given standard.

The Administrator also recognized that judgments about the appropriate weight to 

place on any of the factors discussed above should reflect consideration not only of the 

relative strength of the evidence but also on the important uncertainties that remained in 

the evidence and information being considered in this review. The Administrator noted 

that the extent to which these uncertainties influenced judgments about appropriate 

annual standard levels within the range of 13 to 11 μg/m3 would likely be greater for 

standard levels in the lower part of this range which would necessarily be based on fewer 

available studies than would higher levels within this range.

Based on the above considerations, the Administrator concluded that it was 

appropriate to propose to set a level for the primary annual PM2.5 standard within the 

range of 12 to 13 μg/m3. The Administrator provisionally concluded that a standard set 

within this range would reflect alternative approaches to appropriately placing the most 

weight on the strongest available evidence, while placing less weight on much more 

limited evidence and on more uncertain analyses of information available from a 

relatively small number of studies. Further, she provisionally concluded that a standard 

level within this range would reflect alternative approaches to appropriately providing an 

adequate margin of safety for the populations at risk for the serious health effects 

classified as having evidence of a causal or likely causal relationship, depending in part 

on the emphasis placed on margin of safety considerations. The Administrator recognized 

that setting an annual standard level at the lower end of this range would reflect an 

approach that placed more emphasis on the entire body of the evidence, including the 
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analysis of the distribution of air quality concentrations most influential in generating 

health effect estimates in the studies, and on margin of safety considerations, than would 

setting a level at the upper end of the range. Conversely, an approach that would support 

a level at the upper end of this range would place more emphasis on the remaining 

uncertainties in the evidence to avoid potentially overestimating public health 

improvements, and would generally support a view that the uncertainties remaining in the 

evidence are too great tosuch that the evidence does not warrant setting a lower annual 

standard level. Id. at 38942.

At the time of the proposal, while the Administrator recognized that CASAC 

advised, and the Policy Assessment concluded, that the available scientific information 

provided support for considering a range that extended down to 11 μg/m3, she concluded 

that proposing such an extended range would reflect a public health policy approach that 

placed more weight on relatively limited evidence and more uncertain information and 

analyses than she considered appropriate at this time. Nonetheless, the Administrator 

solicited comment on a level down to 11 μg/m3as well as on approaches for translating 

scientific evidence and rationales that would support such a level. Such an approach 

might reflect a view that the uncertainties associated with the available scientific 

information warrant a highly precautionary public health policy response that would 

incorporate a large margin of safety.

The Administrator recognized that potential air quality changes associated with 

meeting an annual standard set at a level within the range of 12 to 13 μg/m3 will result in 

lowering risks associated with both long- and short-term PM2.5 exposures. However, the 

Administrator recognized that such an annual standard intended to serve as the primary 
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means for providing protection from effects associated with both long- and short-term 

PM2.5 exposures would not by itself be expected to offer requisite protection with an 

adequate margin of safety against the effects of all short-term PM2.5 exposures. As a 

result, in conjunction with proposing an annual standard level in the range of 12 to 13 

μg/m3, the Administrator provisionally concluded that it was appropriate to continue to 

provide supplemental protection by means of a 24-hour standard set at the appropriate 

level, particularly for areas with high peak-to-mean ratios possibly associated with strong 

local or seasonal sources, or for PM2.5-related effects that may be associated with shorterthan-daily exposure periods. 

Based on the approach discussed in section III.A.3 above, at the time of the 

proposal the Administrator relied upon evidence from the short-term exposure studies as 

the principal basis for selecting the level of the 24-hour standard. In considering these 

studies as a basis for the level of a 24-hour standard, and having selected a 98th percentile 

form for the standard, the Administrator agreed with the focus in the Policy Assessment 

of looking at the 98th percentile values, as well as at the long-term mean PM2.5

concentrations in these studies.

In considering the information provided by the short-term exposure studies, the 

Administrator recognized that to the extent these studies were conducted in areas that 

likely did not meet one or both of the current standards, such studies did not help inform 

the characterization of the potential public health improvements of alternative standards 

set at lower levels. By reducing the PM2.5 concentrations in such areas to just meet the 

current standards, the Administrator anticipated that additional public health protection 

would occur. Therefore, the Administrator focused on studies that reported positive and 
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statistically significant associations in areas that would likely have met both the current 

24-hour and annual standards. She also considered whether or not these studies were 

conducted in areas that would likely have met an annual standard level of 12 to 13 μg/m3

to inform her decision regarding an appropriate 24-hour standard level. As discussed in 

section III.E.4.a, consistent with the Policy Assessment, the Administrator concluded that 

multi-city, short-term exposure studies provided the strongest data set for informing her 

decisions on appropriate 24-hour standard levels. The Administrator viewed the singlecity, short-term exposure studies as a much more limited data set providing mixed results 

and, therefore, she had less confidence in using those studies as a basis for setting the 

level of a 24-hour standard. With regard to the limited number of single-city studies that 

reported positive and statistically significant associations for a range of health endpoints 

related to short-term PM2.5 concentrations in areas that would likely have met the current 

suite of PM2.5 standards, the Administrator recognized that many of those studies had

significant limitations (e.g., limited statistical power, limited exposure data) or equivocal 

results (mixed results within the same study area) that made them unsuitable to form the 

basis for setting the level of a 24-hour standard. 

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, the Administrator observed an overall pattern of positive and 

statistically significant associations in studies with 98th percentile values averaged across 

study areas in the range of 45.8 to 34.2 μg/m3(Burnett et al., 2004; Zanobetti and 

Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and Goldberg, 2003; 

Franklin et al., 2008). The Administrator noted that, to the extent air quality distributions 

were reduced to reflect just meeting the current 24-hour standard, additional protection 
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would be anticipated for the effects observed in the three multi-city studies with 98th

percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett and Goldberg, 

2003; Franklin et al., 2008). In the three additional studies with 98th percentile values 

below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 34.3, and 34.8 μg/m3, 

the Administrator noted that these studies reported long-term mean PM2.5 concentrations 

of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; Zanobetti and Schwartz, 

2009; Dominici et al., 2006a).

In proposing to revise the level of the annual standard to within the range of 12 to 

13 μg/m3, as discussed above, the Administrator recognized that additional protection 

would be provided for the short-term effects observed in these multi-city studies in 

conjunction with an annual standard level of 12 μg/m3, and in two of these three studies 

in conjunction with an annual standard level of 13 μg/m3. She noted that the study-wide 

mean concentrations were based on averaging across monitors within study areas and that 

compliance with the standard would be based on concentrations measured at the monitor 

reporting the highest concentration within each area. The Administrator believed it would 

be reasonable to conclude that revision to the 24-hour standard would not be appropriate 

in conjunction with an annual standard within this range. Based on the above 

considerations related to the epidemiological evidence, the Administrator provisionally 

concluded that it was appropriate to retain the level of the 24-hour standard at 35 μg/m3, 

in conjunction with a revised annual standard level in the proposed range of 12 to 13 

μg/m3.

In addition to considering the epidemiological evidence, the Administrator also 

took into account air quality information based on county-level 24-hour and annual 
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design values to understand the public health implications of retaining the 24-hour 

standard level at 35 μg/m3in conjunction with an annual standard level within the 

proposed range of 12 to 13 μg/m3. She considered whether these suites of standards 

would meet a public health policy goal which included setting the annual standard to be 

the “generally controlling” standard in conjunction with setting the 24-hour standard to 

provide supplemental protection to the extent that additional protection is warranted. As 

discussed above, the Administrator provisionally concluded that this approach was the 

most effective and efficient way to reduce total population risk associated with both longand short-term PM2.5 exposures, resulting in more uniform protection across the U.S. than 

the alternative of setting the 24-hour standard to be the controlling standard. 

In considering the air quality information, the Administrator first recognized that 

there was no annual standard within the proposed range of levels, when combined with a 

24-hour standard at the proposed level of 35 μg/m3, for which the annual standard would 

be the generally controlling standard in all areas of the country. She further observed that 

such a suite of PM2.5 standards with an annual standard level of 12 μg/m3 would result in 

the annual standard as the generally controlling standard in most regions across the 

country, except for certain areas in the Northwest, where the annual mean PM2.5

concentrations have historically been low but where relatively high 24-hour 

concentrations occur, often related to seasonal wood smoke emissions (U.S. EPA, 2011a, 

pp. 2-89 to 2-91, Figure 2-10). Although not explicitly delineated on Figure 2-10 in the 

Policy Assessment, an annual standard of 13 μg/m3 would be somewhat less likely to be 

the generally controlling standard in some regions of the U.S. outside the Northwest in 

conjunction with a 24-hour standard level of 35 μg/m3.
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Taking the above considerations into account, the Administrator proposed to 

revise the level of the primary annual PM2.5 standard from 15.0 μg/m3to within the range 

of 12.0 to 13.0 μg/m3and to retain the 24-hour standard level at 35 μg/m3. In the 

Administrator’s judgment, such a suite of primary PM2.5 standards and the rationale 

supporting such levels could reasonably be judged to reflect alternative approaches to the 

appropriate consideration of the strength of the available evidence and other information 

and their associated uncertainties and the advice of CASAC.

The Administrator recognized that the final suite of standards selected from 

within the proposed range of annual standard levels, or the broader range of annual 

standard levels on which public comment was solicited, must be clearly responsive to the 

issues raised by the D.C. Circuit’s remand of the 2006 primary annual PM2.5 standard. 

Furthermore, at the time of the proposal, she recognized that the final suite of standards 

will reflect her ultimate judgment in the final rulemaking as to the suite of primary PM2.5

standards that would be requisite to protect the public health with an adequate margin of 

safety from effects associated with fine particle exposures. The final judgment to be made 

by the Administrator will appropriately consider the requirement for a standard that is 

neither more nor less stringent than necessary and will recognize that the CAA does not 

require that primary standards be set at a zero-risk level, but rather at a level that reduces 

risk sufficiently so as to protect public health with an adequate margin of safety. 

At the time of the proposal, having reached her provisional judgment to propose 

revising the annual standard level from 15.0 to within a range of 12.0 to 13.0 μg/m3and 

to propose retaining the 24-hour standard level at 35 μg/m3, the Administrator solicited 

public comment on this range of levels and on approaches to considering the available 
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evidence and information that would support the choice of levels within this range. The 

Administrator also solicited public comment on alternative annual standard levels down 

to 11 μg/m3and on the combination of annual and 24-hour standards that commenters 

may believe is appropriate, along with the approaches and rationales used to support such 

levels. In addition, given the importance the evidence from epidemiologic studies played 

in considering the appropriate annual and 24-hour levels, the Administrator solicited 

public comment on issues related to translating epidemiological evidence into standards, 

including approaches for addressing the uncertainties and limitations associated with this 

evidence.

c. Comments on Standard Levels

This section addresses comments that relate to consideration of the appropriate 

levels of the primary annual and 24-hour PM2.5 standards, including comments on the 

general approach used by the EPA to translate the available scientific information into 

standard levels and how specific PM2.5 exposure studies should be considered as a basis 

for the standard levels. These comments on standard levels expand upon the more general

comments that either supported or opposed any change to the current suite of primary 

PM2.5 standards, which are addressed above in section III.D.2.92 As explained there, one 

group of commenters generally opposed any change to the current primary PM2.5

standards and more specifically disagreed with the basis for the EPA’s proposal to revise 

the annual standard level. Another group of commenters supported revising the current 

suite of primary PM2.5 standards to provide increased public health protection. Some 

commenters in this second group argued that both the annual and 24-hour standard levels 

 

92 Specific comments on the forms of the annual and 24-hour standards are addressed in 

section III.E.3.a and III.E.3.b, respectively.
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should be lowered while other commenters in this group agreed with the EPA’s proposal 

to retain the level of the 24-hour standard in conjunction with revising the level of the 

annual standard. While generally supporting the EPA’s proposal to lower the level of the 

annual standard, many commenters in this group disagreed that a level within the EPA’s 

proposed range was adequately protective and supported a level of 11 μg/m3or below.

i. Annual Standard Level

The group of commenters opposed to any change to the current suite of primary 

PM2.5 standards generally raised questions regarding the underlying scientific evidence, 

including the causal determinations reached in the Integrated Science Assessment, and 

focused strongly on the uncertainties they saw in the scientific evidence as a basis for 

their conclusion that no changes to the current standard levels were warranted. In 

commenting on the proposed standard levels, these commenters typically relied on the 

arguments summarized and addressed above in section III.D.2 as to why they believed it 

was inappropriate for the EPA to make any revisions to the suite of primary PM2.5

standards. That is, they asserted that the EPA’s causal determinations were not 

adequately supported by the underlying scientific information; the biological plausibility 

of health effects observed in epidemiological studies has not been demonstrated in 

controlled human exposure and toxicological studies; uncertainties in the underlying 

health science are as great or greater than in 2006; there is no evidence of greater risk 

since the last review to justify tightening the current annual PM2.5 standard; and “new” 

studies not included in the Integrated Science Assessment continue to increase 

uncertainty about possible health risks associated with exposure to PM2.5.

With regard to the level of the annual standard, these commenters strongly 
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disagreed with the Agency’s proposed decision to revise the level to within a range of 12 

to 13 μg/m3and argued that the current standard level of 15 μg/m3should be retained. 

For example, UARG, API, and other commenters in this group raised a number of issues 

that they asserted called into question the EPA’s interpretation of the epidemiological 

evidence to support revising the annual standard level. These commenters raised specific 

questions related to the general approach used by the EPA to translate the air quality and 

other information from specific epidemiological studies into standard levels, including:

(1) the EPA’s approach for using composite monitor air quality distributions reported in 

epidemiological studies to select a standard level that would be compared to 

measurements at the monitor recording the highest value in an area to determine 

compliance with the standard; (2) the appropriate exposure period for effects observed in 

long-term exposure mortality studies; and (3) the use of the EPA’s analysis of 

distributions of underlying population-level data (i.e., health event and study population 

data) for those epidemiological studies for which such information was available. These 

commenters also raised questions regarding the EPA’s consideration of specific scientific 

evidence as a basis for setting a standard level, including: (4) evidence of respiratory 

morbidity effects in long-term exposure studies and (5) more limited evidence of health 

effects which have been categorized in the Integrated Science Assessment as suggestive 

of a causal relationship (i.e., developmental and reproductive outcomes). These 

comments are discussed in turn below.

(1) Some commenters in this group argued that one reason why they believe there 

is no basis for setting a standard level below 15 μg/m3is that the air quality metric from 

epidemiological studies that the EPA relied on in the proposal is not the same metric that 
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will be compared to the level of the standard to determine compliance with the standard. 

That is, commenters noted that the long-term mean PM2.5 concentrations that the EPA 

considered, shown in Figure 4 above, are composite monitor mean concentrations (i.e., 

concentrations averaged across multiple monitors within areas with more than one 

monitor), whereas the PM2.5 concentrations that will be compared to the level of the 

standard are maximum monitor concentrations (i.e., the concentration measured by the 

monitor within an area reporting the highest concentration). This comment was presented 

most specifically in UARG’s comments (UARG, 2012, Attachment 1, pp. 2 to 6), which 

raised two overarching issues as discussed below.

First, the commenter noted that the EPA’s approach of considering composite 

monitor mean PM2.5 concentrations in selecting a standard level, and then comparing the 

maximum monitor mean PM2.5 concentration in each area to the standard level when the 

standard is implemented, was characterized in the proposal as inherently having the 

potential to build in a margin of safety (UARG, 2012, Attachment 1, p. 4, citing 77 FR 

38905). The commenter asserted that the Administrator is ignoring this distinction 

between composite and maximum monitor concentrations, and that this approach creates 

an unwarranted case for lowering the standard level, since in the commenter’s view, it 

would result in a margin of safety that would be arbitrary, not based on evidence, and 

unquantified (UARG, 2012, Attachment 1, p. 4). In support of this view, the commenter 

asserted that there is a significant difference between composite monitor mean PM2.5

concentrations and maximum monitor mean PM2.5 concentrations. The commenter 

asserted that the maximum monitor value will always be higher than the composite 

monitor value (except in areas that contain only a single monitor), such that when an area 
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just attains the NAAQS, that area’s composite monitor long-term mean PM2.5

concentration will be lower than the level of the standard (UARG, 2012, Attachment 1, p. 

3).

Second, the commenter asserted that a more “reasoned and consistent approach 

would be to decide on a mean composite monitor PM2.5 level that should be achieved and 

then identify the maximum monitor level that would result in that composite value” 

(UARG, 2012, Attachment 1, p. 4). The commenter conducted an analysis of maximum 

monitor versus composite monitor annual mean PM2.5 concentrations using monitoring 

data93 from 2006 to 2008 and presented results averaged across areas within two groups 

(i.e., those with design values94 above the current standard level and those with design 

values just below the current standard level) to illustrate their suggested alternative 

approach. The commenter interpreted this analysis as showing that the composite monitor 

long-term mean PM2.5 concentrations from the subset of the epidemiological studies 

shown in Figure 4 (of the proposal and above) that the commenter considered to be an 

appropriate focus for this analysis would be achieved across the U.S. if the current annual 

NAAQS of 15 μg/m3is retained and attained. The commenter considered the subset of 

epidemiological studies that included only long-term exposures studies of effects for 

which the evidence is categorized as causal or likely causal, but did not consider shortterm exposure studies. On this basis, the commenter asserted that attaining the current 

annual PM2.5 standard would result in composite monitor long-term mean concentrations 

in all areas that would be generally within or below the range of the composite monitor 

 

93 The commenter indicated that this analysis was based on monitoring data for every 

core based statistical area (CBSA) in the EPA’s Air Quality System (AQS) database.

94 The design value is the air quality statistic that is compared to the level of the NAAQS 

to determine the attainment status of a given area.

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long-term mean concentrations from such studies and, as a result, there is no reason to 

lower the level of the current annual NAAQS.

In considering the first issue related to the EPA’s approach, the EPA notes that in 

proposing to revise both the form and level of the annual standard, the Administrator 

clearly took into account the distinction between the composite monitor long-term mean 

PM2.5 concentrations from the epidemiological studies, considered as a basis for selecting 

an annual standard level, and maximum monitor long-term mean PM2.5 concentrations. In 

deciding to focus on the composite monitor long-term mean concentrations in selecting 

the standard level, and on the maximum monitor concentrations in selecting the form of 

the standard (i.e., consistent with proposing to eliminate the option for spatial averaging 

across monitors within an area when implementing the standard95), the Administrator 

reasonably considered the distinction between these metrics in a manner that was 

consistent with advice from CASAC (Samet et al., 2010d, pp. 2 to 3).

As noted above in section III.A.3, the EPA recognizes that a statistical metric 

(e.g., the mean of a distribution) based on maximum monitor concentrations may be 

identical to or above the same statistical metric based on composite monitor 

concentrations. More specifically, many areas have only one monitor, in which case the 

composite and maximum monitor concentrations are identical. Based on the most recent 

data from the EPA’s AQS from 2009 to 2011 in the 331 CBSAs in which valid PM2.5

data are available, as discussed in Frank (2012a, Table 5), there were 208 such areas 

(with design values ranging up to about 15 μg/m3). Frank (2012a) also observed that 

other areas have multiple monitors with composite and maximum monitor mean PM2.5

concentrations that were the same or relatively close, with 57 areas in which the 

 

95 As discussed above in section III.E.3.a.

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maximum monitor mean concentration was no more than 0.5 μg/m3higher than the 

composite monitor mean concentration and 56 areas in which the difference was between 

0.6 and 2 μg/m3. Further, there were only a few other areas in which the maximum 

monitor mean concentration was appreciably higher than the composite monitor mean 

concentration, such as areas in which some monitors may be separately impacted by local 

sources. There were only 10 such areas in the country in which the maximum monitor 

mean concentration was between 2 to 6 μg/m3higher than the composite monitor 

concentration (Frank, 2012a, Table 4).96 Thus, the EPA does not agree that there is a 

significant difference between composite monitor mean PM2.5 concentrations and 

maximum monitor mean PM2.5 concentrations in the large majority of areas across the 

country.

In proposing to revise the form of the annual PM2.5 standard, as discussed above 

in section III.E.3.a, the EPA noted that when an annual PM2.5 standard was first set in 

1997, the form of the standard included the option for averaging across measurements at 

appropriate monitoring sites within an area, generally consistent with the composite 

monitor approach used in epidemiological studies, with some constraints intended to 

ensure that spatial averaging would not result in inequities in the level of protection for 

communities within large metropolitan areas. In the last review the EPA tightened the 

constraints on spatial averaging, and in this review has eliminated the option altogether, 

on the basis of analyses in each review that showed that such constraints may be 

inadequate to avoid substantially greater exposures for people living in locations around 

 

96 The average difference between the maximum and composite design value among the 

123 CBSAs with two or more monitors is 0.8 μg/m3and the median difference is 0.6 

μg/m3. The 25th and 75th percentiles are 0.3 and 1.0 μg/m3, respectively (Frank, 2012a, p. 

4). 
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the monitors recording the highest PM2.5 concentrations in some areas, potentially 

resulting in disproportionate impacts on at-risk populations of persons with lower SES 

levels as well as minorities. In light of these analyses, and consistent with the 

Administrator’s decision to revise the form of the annual PM2.5 standard by eliminating 

the option for spatial averaging, the EPA continues to conclude that a standard level 

based on consideration of long-term mean concentrations from composite monitors, and 

applied at each monitor within an area including the monitor measuring the highest 

concentration, is the appropriate approach to use in setting a standard that will protect 

public health, including the health of at-risk populations, with an adequate margin of

safety, as required by the CAA.

The EPA acknowledges that at proposal, the Agency characterized the approach 

of using maximum monitor concentrations to determine compliance with the standard, 

while selecting the standard level based on consideration of composite monitor 

concentrations, as one that inherently had the potential to build in a margin of safety (77 

FR 38905), and CASAC reiterated that view in supporting the EPA’s approach (Samet, 

2010d, p. 3). Nonetheless, in light of the discussion above, the EPA more specifically 

recognizes that this approach does not build in any margin of safety in the large number 

of areas across the country with only one monitor. Further, based on the analyses done to 

inform consideration of the form of the standard (Schmidt, 2011, Analysis A), the EPA 

concludes that this approach does not provide a margin of safety for the at-risk 

populations that live around the monitor measuring the highest concentration, such as in 

those few areas in which the maximum monitor concentration is appreciably higher than 

the composite monitor concentration. Rather, this approach properly treats those at-risk 
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populations the same way it does the broader populations that live in areas with only one 

monitor, by providing the same degree of protection for those at-risk populations that 

would otherwise be disproportionately impacted as it does for the broader populations in 

other areas, While the EPA recognizes that this approach can result in some additional 

margin of safety for the subset of areas with multiple monitors in which at-risk 

populations may not be disproportionately represented in areas around the maximum 

monitor, which may be the case in areas with relatively small differences between the 

maximum and composite monitor concentrations, the EPA notes that this margin would 

be relatively small in such areas.

Based on the above considerations, the EPA does not agree that the Agency’s 

approach of using maximum monitor concentrations to determine compliance with the 

standard, while selecting the standard level based on consideration of composite monitor 

concentrations creates an unwarranted case for lowering the standard level based on a 

margin of safety that would be arbitrary, not based on evidence, or lack quantification. 

The EPA recognizes that setting a standard to protect public health, including the health 

of at-risk populations, with an adequate margin of safety, depends upon selecting a 

standard level sufficiently below where there is clearthe EPA has found the strongest

evidence of health effects so as to provide such protection, and that the EPA’s approach 

regarding consideration of composite and maximum monitor concentrations is intended 

to, and does, serve to address this requirement as part of and not separate from the 

selection of an appropriate standard level based on the health effects evidence.

In considering the second issue related to the commenter’s suggested alternative 

approach, the EPA strongly disagrees with the commenter’s view that a more “reasoned 
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and consistent approach would be to decide on a mean composite monitor PM2.5 level 

that should be achieved and then identify the maximum monitor level that would result in 

that composite value” (UARG, 2012, Attachment 1, p. 4). As discussed above, the EPA 

notes that for areas with only one monitor, or with multiple monitors that measure 

concentrations that are very close in magnitude, the maximum monitor level that would 

limit the composite monitor PM2.5 level to be no greater than the level that should be 

achieved to protect public health with an adequate margin of safety, would essentially be 

the same as that composite monitor level. Further, as discussed above, even for areas in

which the maximum monitor concentration is appreciably higher than other monitor 

concentrations within the same area, public health would not be protected with an 

adequate margin of safety if the disproportionately higher exposures of at-risk, 

susceptible populations around the monitor measuring the highest concentration were in 

essence averaged away with measurements from monitors in other locations within large 

urban areas. Further, the commenter’s suggested approach would be based on annual 

average PM2.5 concentrations that have been measured over some past time period. Such 

an approach would reflect the air quality that existed in the past, but it would not 

necessarily provide appropriate constraints on the range of concentrations that would be 

allowed by such a standard in the future, when relationships between maximum and 

composite monitor concentrations in areas across the country may be different. For these 

reasons, the EPA fundamentally rejects the commenter’s suggested approach because it 

results in a standard that demonstrablyin the EPA’s views it would not protect public 

health, including providing protection for at-risk populations, with an adequate margin of 

safety in areas across the country.
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More specifically, in further considering the commenter’s analysis of design 

values based on maximum versus composite monitor annual mean PM2.5 concentrations 

using monitoring data from 2006 to 2008 which they assert supports retaining the current 

standard level of 15 μg/m3, the EPA finds flaws with the numerical results and the scope 

of the analysis, as well as flaws in the commenter’s translation of the analysis results into 

the basis for selecting an annual standard level.

In considering the commenter’s analysis, the EPA notes that the analysis 

compared maximum versus composite monitor annual mean PM2.5 concentrations, 

averaged over 3 years, for two groups of areas: (1) areas with design values that exceed 

the current annual standard level (i.e., greater than 15.0 μg/m3) and (2) areas with design 

values that are just attaining the current annual standard (i.e., between 14.5 and 15.0 

μg/m3).97 The commenter indicated that they used the full body of PM2.5 monitoring data 

from the EPA’s AQS database (UARG, 2012, Attachment 1, p. 4), In attempting to 

reproduce the commenter’s results, the EPA repeated the calculations using only valid air 

quality data (i.e., data that meet data completeness and monitor siting criteria) from the 

AQS database for the same time period (Frank, 2012a).98 Based on this corrected 

analysis, the EPA finds that the composite monitor concentrations averaged across the 

areas within each group are somewhat higher than those calculated by the commenter, 

 

97For the first group of areas (which included 33 areas), this analysis calculated an 

average across the areas of maximum monitor annual mean PM2.5 concentrations, 

averaged over 3 years, of 17.2 μg/m3compared to an average of composite monitor 

concentrations of 14.3 μg/m3. For the second group of areas (which included 11 areas), 

this analysis calculated an average across the areas of maximum monitor annual mean 

concentrations, averaged over 3 years, of 14.8 μg/m3compared to an average of 

composite monitor concentrations of 13.6 μg/m3(UARG, 2012, Attachment 1, Table 1).

98 The EPA notes that the Frank (2012a) analysis is similar to an earlier EPA staff 

analysis (Hassett-Sipple et al., 2010), which used air quality data from EPA’s AQS 

database to compare maximum versus composite monitor long-term mean PM2.5

concentrations across the study areas in six selected multi-city epidemiological studies.

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and the average differences between the maximum and composite monitor concentrations 

are somewhat smaller (Frank, 2012a, Table 3).99 Notably, the difference between the 

maximum and composite monitor average concentrations for the second group of areas is 

substantially reduced in the corrected analysis, such that the difference (averaged across 

the 10 areas with valid data in the second group) is approximately 0.5 μg/m3, not 1.2 

μg/m3as in the commenter’s analysis. In addition, the commenter’s analysis compared 

the average of the composite monitors to the average of the maximum monitors for each 

subset of areas. This comparison of averages across all the areas in each subset masks the 

fact that the large majority of areas across the country have only one monitor, with the 

composite monitor and maximum monitor values the same for such areas, and many 

other areas have a maximum monitor value that is close to the composite monitor value. 

As discussed above, these circumstances have a major impact on the protection that 

would be achieved by the approach suggested by the commenter.

With regard to the scope of the commenter’s analysis, the EPA finds that by 

limiting the scope to a small subset of areas with design values above or just below the 

current annual standard level of 15 μg/m3, the analysis ignores the large number of areas 

across the country with lower design values that are relevant to consider in light of the 

epidemiological evidence of serious health effects at lower concentrations, well below the 

level of the current standard.

 

99 The EPA’s analysis was intended to repeat the commenter’s analysis, but using only 

valid air quality data (from 2006 to 2008). For the first group of areas (which included 21 

areas with valid data), the EPA’s analysis calculated an average across the areas of 

maximum monitor annual mean concentrations, averaged of 3 years, of 16.8 μg/m3

compared to an average of composite monitor concentrations of 14.8 μg/m3. For the 

second group of areas (which included 10 areas with valid data), the EPA’s analysis 

calculated an average across the areas of maximum monitor annual mean concentrations, 

averaged over 3 years, of 14.8 μg/m3compared to an average of composite monitor 

concentrations of 14.2 μg/m

3

(Frank, 2012a, Table 3).

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In translating the analysis results into the basis for selecting an annual standard 

level, the commenter’s translation is premised on the view that the “natural focal point” 

for setting an annual PM2.5 standard level should be somewhere within the range of the 

long-term mean PM2.5 concentrations from the subset of epidemiological studies that 

included only long-term exposure studies of effects for which the evidence is categorized 

as causal or likely causal, but not for effects categorized as suggestive of causality, nor 

did it include short-term exposure studies (which are included in Figure 4 of the proposal 

notice and above). Such a view is not consistent with setting a standard that would 

provide sufficient protection from the serious health effects reported even in the limited 

subset of studies considered by the commenter, including protecting public health with an 

adequate margin of safety. As discussed below, the EPA does not agree with the 

commenter’s view as to the appropriate focal point for selecting the level of an annual 

PM2.5 standard, or with the limited set of studies considered by the commenter as a basis 

for selecting the level of the annual PM2.5 standard.

Regarding an appropriate focal point for selecting the level of the annual standard, 

as discussed in the proposal and as advised by CASAC, the EPA has focused on PM2.5

concentrations somewhat below the lowest long-term mean concentrations from each of 

the key studies of both long- and short-term exposures of effects for which the evidence 

is causal or likely causal, as considered by the EPA (i.e., the first two sets of studies 

shown in Figure 4). If the level of the annual standard was set just somewhere within the 

range of the long-term mean concentrations from the various long-term exposure studies, 

then one or more of the studies would have a long-term mean concentration below the 

selected level of the standard. Absent some reason to ignore or discount these studies, 
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which the commenter does not provide (and of which the EPA is unaware), setting such a 

standard would allow that level of air quality, where the evidence of health effects is 

strongest, and its associated risk of PM2.5-related mortality and/or morbidity effects to 

continue. Selecting such a standard level that would explicitly allow such projected 

PM2.5-related mortality and morbidity effects to continue to occur could not be 

considered sufficient to protect the public health with an adequate margin of safety.

Further, focusing on just the long-term mean PM2.5 concentrations in the key 

epidemiological studies – even the lowest long-term mean concentration from the set of 

key studies -- is not appropriate. Concentrations at and around the long-term mean

concentrations represent the part of the air quality distribution where the data in any 

given study are most concentrated and, thus, where the confidence in the magnitude and 

significance of an association in such study is strongest. However, the evidence of an 

association with adverse health effects in the studies is not limited to the PM2.5

concentrations just at and around the long-term mean, but rather extends more broadly to 

a lower part of the distribution, recognizing that no discernible population-level threshold 

for any such effects can be identified based on the available evidence. This broader 

region of the distribution of PM2.5 concentrations should be considered to the extent 

relevant information is available, recognizing that the degree of confidence in the 

association identified in a study would changebecome lower as one moves away 

frombelow concentrations at and around the long-term mean concentration in any given 

study. The commenter’s approach ignores this fundamental consideration.

Regarding the set of studies that is appropriate to inform the selection of the level 

of the annual PM2.5 standard, the EPA finds that limiting consideration only to the long-
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term exposure studies, as this commenter suggests, would be tantamount to ignoring the 

short-term exposure studies,

100 which provide some of the strongest evidence from the 

entire body of epidemiological studies. Thus, selecting an annual standard level using the 

limited set of studies suggested by the commenter would fail to provide a degree of 

protection that would be sufficient to protect public health with an adequate margin of 

safety.

For all the reasons discussed above, the EPA finds the commenter’s concerns with 

the EPA’s approach to considering composite and maximum monitor PM2.5

concentrations in selecting the level of the annual PM2.5 standard to be without merit. 

Further, the EPA finds no support in the commenter’s analysis for their suggested 

alternative approach.

(2) With respect to the appropriate exposure period for mortality effects observed 

in long-term exposure studies, some commenters in this group generally expressed views 

consistent with comments from UARG that argued that these studies “are most likely 

detecting health risk from earlier, higher PM2.5 levels and misattributing those risks to 

more recent, lower PM2.5 levels” (UARG, 2012, Attachment 1, p 7). Further, this 

commenter asserted that “there is no knowledge or evidence indicating whether 

 

100 The commenter suggests that the EPA should not place significant reliance on the 

long-term mean concentrations from short-term exposure studies because “[T]he 

short‐term studies did not use the annual average of PM2.5 to develop their associations; 

they used the daily 24‐hour averages of PM2.5. Thus, short‐term studies do not provide a 

natural indicator for the appropriate level of an annual standard....” (UARG, 2012, 

Attachment 1, p. 3). The EPA finds this argument unpersuasive. Quite simply, effects 

were observed in these studies with an air quality distribution that can meaningfully be 

characterized by these long-term mean concentrations. Indeed, in remanding the 2006 

standard, the D.C. Circuit discussed at length the interrelationship of the long- and shortterm standards and studies, and remanded the 2006 standard to the EPA, in part, for 

ignoring those relationships without adequate explanation. American Farm Bureau 

Federation v. EPA. 559 F. 3d at 522-24.
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premature deaths are the result of PM2.5 exposures in the most recent year; or due to 

physical damages incurred from PM2.5 exposures much earlier in life (with the impact on 

lifespan only emerging later in life); or due to total accumulated PM2.5 exposure over 

many years.” Id. FurtherIn addition, the commenter asserted that the long-term exposure 

studies of mortality are central to the EPA’s basis for proposing to set a lower annual 

standard level, since most of the estimated benefits associated with a lower annual PM2.5

standard are based on reductions in mortality related to long-term exposures to PM2.5.

As an initial matter, the EPA has recognized the challenge in distinguishing 

between PM2.5-associated effects due to past and recent long-term exposures, and in

identifying the relevant latency period for long-term exposure to PM and resultant health 

effects (U.S. EPA, 2009a, section 7.6.4; 77 FR 38941/1). While the EPA has 

acknowledged that there remain important uncertainties related to characterizing the most 

relevant exposure periods in long-term exposure studies, the assertion that there is “no 

knowledge or evidence” that helps to inform this issue is not correct, as discussed below.

Both in the last review and in the current review, the EPA has assessed studies 

that used different air quality periods for estimating long-term exposure and tested 

associations with mortality for the different exposure periods (U.S. EPA, 2004, section 

8.2.3.5; U.S. EPA 2009a, section 7.6.4). In this review, the Integrated Science 

Assessment discussed studies available since the last review that have assessed the 

relationship between long-term exposure to PM2.5 and mortality to explore the issue of 

the latency period between exposure to PM2.5 and death (U.S. EPA, 2009a, section 7.6.4).

Notably, in a recent analysis of the extended Harvard Six Cities Study, Schwartz 

et al. (2008) used model averaging (i.e., multiple models were averaged and weighted by 
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probability of accuracy) to assess exposure periods prospectively (77 FR 38907/1-2). The 

exposure periods were estimated across a range of unconstrained distributed lag models 

(i.e., same year, one year prior, two years prior to death). In comparing lags, the authors 

reported that the effects of changes in exposure to PM2.5 on mortality were strongest 

within a 2-year period prior to death (U.S. EPA, 2009a, p. 7-92, Figure 7-9). Similarly, a 

large multi-city study of the elderly found that the mortality risk associated with longterm exposure to PM10 reported cumulative effects that extended over the years that 

deaths were observed in the study population (i.e., the follow-up period) and for the 3-

year period prior to death (Zanobetti et al., 2008). 

Further, in a study of two locations that experienced an abrupt decline in PM2.5

concentrations (i.e., Utah Steel Strike, coal ban in Ireland), Röösli et al. (2005) reported 

that approximately 75 percent of health benefits were observed in the first 5 years (U.S.

EPA, 2009a, Table 7-9). Schwartz et al. (2008) and Puett et al. (2008) found, in a 

comparison of exposure periods ranging from 1 month to 48 months prior to death, that 

exposure to PM10 24 months prior to death exhibited the strongest association, and the 

weakest association was reported for exposure in the time period of 1 month prior to 

death. 

Overall, the EPA notes that the available evidence for determining the exposure 

period that is causally related to the mortality effects of long-term PM2.5 exposures, as 

discussed above, cannot specifically disentangle the effects observed in long-term 

exposure studies associated with more recent air quality measurements from effects that 

may have been associated with earlier, and most likely higher, PM2.5 exposures. While 

the evidence suggests that a latency period of up to five years would account for the 
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majority of deaths, it does not provide a basis for concluding that it is solely recent PM2.5

concentrations that account for the mortality risk observed in such studies. Nonetheless, 

the more recent air quality data does well at explaining the relationships observed 

between long-term exposures to PM2.5 and mortality, with the strongest association

observed in the two years prior to death. Further, the EPA recognizes that there is no 

discernible population-level threshold below which effects would not occur, such that it 

is reasonable to consider that health effects may occur over the full range of 

concentrations observed in the epidemiological studies, including the lower 

concentrations in the latter years.

In light of this evidence and these considerations, the EPA concludes that it is 

appropriate to consider air quality concentrations that are generally contemporaneous 

with the collection of health event data (i.e., collected over the same time period) as being 

causally associated with at least some proportion of the deaths assessed in a long-term 

exposure study. This would include long-term mean PM2.5 concentrations from most of 

the key long-term exposure studies of effects with causal or likely causal evidence shown 

in Figure 4 above, which reported long-term mean PM2.5 concentrations ranging from 

13.6 μg/m3to 14.3 μg/m3. These studies include studies of mortality by Eftim et al. 

(2008), which separately analyzed the ACS and Harvard Six City sites, Zeger et al. 

(2008), and Lipfert et al. (20062006a), as well as studies of morbidity endpoints by Goss 

et al. (2004), McConnell et al. (2003) and Gauderman et al. (2004), and Dockery et al. 

(1996) and Razienne et al. (1996). The EPA acknowledges that uncertainty in the 

relevant exposure period is most notable in two other long-term exposure studies of 

mortality. The Miller et al. (2007) reported a long-term mean PM2.5 concentration for a 1-
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year exposure period that post-dated the follow-up period in which health event data were 

collected by two years. Also, the Krewski et al. (2009) study reported a long-term mean 

PM2.5 concentration for an exposure period that included only the last two years of the 

18-year follow-up period. Based on these considerations, the EPA does not now consider 

it appropriate to put weight on the reported long-term mean concentrations from these 

two studies for the purpose of translating the information from the long-term mortality 

studies into a basis for selecting the level of the annual PM2.5 standard.101

In addition, the EPA acknowledges that exposure periods that extend at least a 

couple years prior to the follow-up period in which health event data were collected 

would likely more fully capture the PM-related deaths in such studies. To explore how 

much higher the long-term mean PM2.5 concentrations would likely have been had air 

quality data prior to the follow-up years of the studies been included, the EPA conducted 

a sensitivity analysis of long-term mean PM2.5 concentrations (Schmidt, 20122012a) 

particularly considering studies that only included deaths from a relatively recent followup period. As examples of such studies, this analysis considered the Eftim et al. (2008) 

study of mortality in 51the ACS sites and the Harvard Six Cities sites, as well as sites in 

the eastern region in the Zeger et al. (2008) study. Using data from the EPA’s AQS 

database, the analysis added the two years of air quality data just prior to the follow-up 

period in each study, which was 2000 to 2002 in Eftim et al. (2008) and 2000 to 2005 in 

Zeger et al. (2008). The analysis then calculated the extended long-term mean PM2.5

concentration for each study. As discussed in Schmidt (20122012a), in each case the 

 

101 Nonetheless, the EPA notes that the Krewski et al. (2009) and Miller et al. (2007) 

studies provide strong evidence of mortality and cardiovascular-related effects associated 

with long-term PM2.5 exposures to inform causality determinations reached in the 

Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6).
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long-term mean PM2.5 concentration averaged over the extended exposure period was 

less than 0.34 μg/m3higher than the long-term mean PM2.5 concentration averaged over 

the follow-up period. The EPA finds it reasonable to conclude that such a relatively small 

difference in long-term mean PM2.5 concentrations would likely apply for other long-term 

exposure studies that used similarly recent follow-up periods as well (e.g., Goss et al., 

2004; Lipfert et al., 20062006a).

Based on the above considerations, the EPA concludes that it is appropriate to 

consider the available air quality information from the long-term exposure studies, while 

taking into account the uncertainties in the relevant long-term exposure periods in 

weighing the information from these studies. The EPA recognizes that considering such 

information in selecting an appropriate annual standard level has the potential to build in 

some margin of safety. The EPA further concludes that it is appropriate to consider the 

air quality information from the set of long-term exposure studies discussed above in the 

context of the broader array of epidemiological studies that inform the EPA’s 

consideration of the level of the annual PM2.5 standard.

The EPA also notes that while the long-term exposure studies are an important 

component of the epidemiological evidence that informs the Agency’s consideration of 

the level of the annual standard, they do not provide the only relevant information, nor 

are they the set of studies for which the relevant long-term mean PM2.5 concentrations are 

the lowest. As discussed in the proposal, the EPA also considers the long-term mean 

PM2.5 concentrations from the short-term mortality and morbidity studies as providing 

important information in considering the level of the annual standard. As discussed

above, a large proportion of the aggregate risk associated with short-term exposures 
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results from the large number of days during which the 24-hour average concentrations 

are in the low- to mid-range of the concentrations observed in the studies. Thus, setting 

the level of the annual standard based on long-term mean concentrations, as well as the 

distribution of concentrations below the mean, in the short-term exposure studies is the 

most effective and efficient way to reduce total PM2.5-related risk from the broad array of 

mortality and morbidity effects associated with short-term exposures.

Further, the EPA notes that the relevant exposure period for the short-term 

exposure studies is the period contemporaneous with the collection of health event data, 

and that this exposure period is not subject to the uncertainties discussed above related to 

the long-term exposure studies. Recognizing that the long-term mean PM2.5

concentrations from several of the multi-city short-term exposure studies shown in Figure 

4 are below the long-term mean PM2.5 concentrations from the long-term exposure 

studies (with the exception of Miller et al., 2007).102 itIt is reasonable that in selecting the 

level of the annual standard primary consideration should be given to the information 

from this set of short-term exposure studies. There is no reasonable basis to discount the

long-term mean concentrations of the short-term exposure studies for purposes of setting 

the level of the annual standard. Thus, the commenter is incorrect in asserting that the 

long-term exposure studies, not the short-term exposure studies, would be central in the 

Administrator’s decision on the level of the annual standard. The standard is ultimately 

intended to protect not just against the single type of effect that contributes the most to 

quantitative estimates of risk to public health, but rather to the broad array of effects, 

 

102 As noted above, the EPA is not placing weight on the reported long-term mean 

concentrations from the Miller et al. (2007) study for the purpose of translating the 

information from the long-term mortality studies into a basis for selecting the level of the 

annual PM2.5 standard.

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including mortality and morbidity effects from long- and short-term exposures across the 

range of at-risk populations impacted by PM2.5-related effects.

(3) With regard to the EPA’s analysis of distributions of underlying populationlevel data (i.e., health event and study population data) and corresponding air quality data 

from each study area in certain key multi-city epidemiological studies (Rajan et al., 

2011), some commenters in this group raised a number of issues related to this analysis.

(API, 2012, Attachment 1 pp. 5 to 6; McClellan, 2012, pp.2 to 4). Some commenters 

noted the limited number of studies for which health event and study population data 

were available, and questioned whether these distributions would apply to other studies. 

Commenters expressed concerns that this analysis had not been formally reviewed by 

CASAC and was not published in the peer-review literature. Based on such concerns, 

some commenters asserted that the EPA should not consider this information as a basis 

for selecting a standard level.

As an initial matter, as discussed in section III.E.4.b above, the EPA agrees with 

CASAC’s advice that it is appropriate to consider additional data beyond the mean PM2.5

concentrations in key multi-city studies to help inform selection of the level of the annual 

PM2.5 standard. As both the EPA and CASAC recognize, in the absence of a discernible 

threshold, health effects may occur over the full range of concentrations observed in the 

epidemiological studies,. Nonetheless, the EPA recognizes that confidence in the 

magnitude and theresignificance of an association is a high degree of confidence that the 

observed effects are associated with concentrations that extend belowhighest at and 

around the long-term mean PM2.5 concentrations reported in the studies and the degree of 

confidence becomes lower at lower concentrations within any given study. Following 
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CASAC’s advice (Samet, 2010d, p.2), the EPA used additional population-level and air 

quality data made available by study authors to conduct an analysis of the distributions of 

such data, to help provide information oninform consideration of how the degree of 

confidence in the magnitude and significance of observed associations varies across the 

range of long-term mean PM2.5 concentrations in study areas within key multi-city 

epidemiological studies. In the EPA’s view, such consideration is important in selecting 

a level for an annual standard that will protect public health with an adequate margin of 

safety.

With regard to the number of multi-city studies for which an analysis of the 

distributions of population-level data across the study areas and the corresponding annual 

mean PM2.5 concentrations was done, the EPA noted at proposal that data for such an 

analysis were made available from study authors for four studies, including two longterm exposure studies and two short-term exposure studies.103 The EPA recognized that 

access to health event data can be restricted due to confidentiality issues, such that it is 

not reasonable to expect that such information could be made available from all studies. 

In considering the information from these four studies, the EPA has further taken into 

consideration uncertainties discussed in response to the above comment related to the 

appropriate exposure period for long-term exposure studies. Based on these 

considerations, as noted above, the EPA concludes that such uncertainties are an 

important factor in evaluating the usefulness of the air quality information from the two 

long-term exposure studies in this analysis (Krewski et al., 2009; Miller et al., 2007) and 

 

103 Health event data and study population data were available from two short-term 

exposure studies (Bell et al. 2008; Zanobetti and Schwartz, 2009) and one long-term 

exposure study (Krewski et al., 2009). Only study population data were available from 

another long-term exposure study (Miller et al., 2007).
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that it would not be appropriate to place weight on the distributional analysis of health 

event and air quality data from these two studies specifically for the purpose of 

translating the information from the long-term mortality studies into a basis for selecting 

the level of the annual PM2.5 standard. Such uncertainties are not relevant to the shortterm exposure studies, and thus, the Agency focuses on the two short-term exposure 

studies in this analysis (Bell et al., 2008; Zanobetti and Schwartz, (2009).

In focusing on these two short-term exposure studies, the EPA first notes that 

these studies are key multi-city studies that reported positive and statistically significant 

associations between mortality and cardiovascular-related hospital admissions across a 

large number of areas throughout the U.S. (112 U.S. cities in Zanobetti and Schwartz, 

2009; 202 U.S. counties in Bell et al., 2008) using relatively recent air quality and health 

event data (i.e., 1999 through 2005 in both studies). The EPA considers this to be a 

modest but important data set to use for this distributional analysis to help inform 

consideration of how much below the long-term mean PM2.5 concentrations in key multicity long- and short-term exposure studies the annual PM2.5 standard level should be set. 

While the EPA acknowledges that having such data available from more studies would 

have been useful, the Agency finds the information from this limited set of studies to be 

an important consideration in selecting an annual standard level, consistent with CASAC 

advice to consider such information.

The results of this distributional analysis are shown below in Figure 5 (adapted 

from Figure 3 in Rajan et al., 2011). For each study, this figure shows the cumulative 

frequency of the number of health events in each study area and the corresponding longterm mean PM2.5 concentrations in each area. Consistent with CASAC advice (Samet, 
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2010d, p.2), such an analysis helps to inform the EPA’s understanding of the long-term 

mean PM2.5 concentrations that were most influential in generating the health effect 

estimates in individual studies. In particular, the EPA recognizes that there is 

significantly greater confidence in the magnitude and significance of observed 

associations in that part of the air quality distribution corresponding to where the bulk of 

the health events evaluated in each study have been observed. With regard to the part of 

the distribution in which confidence in the magnitude and significance of associations 

observed may become appreciably lowerIn considering the results of this distributional 

analysis, as discussed more fully in the Response to Comments document, the EPA 

considers PM2.5 concentrations between the 25th and 10th percentiles of the distribution of 

health events to be a reasonable range for providing a general frame of reference for that 

part of the distribution in which confidence may becomein the magnitude and 

significance of the association may be appreciably lower. As highlighted in Figure 5, than 

confidence at and around the long-term mean PM2.5 concentrations corresponding with 

study areas contributing to the 25th and 10th percentiles of the distribution of deaths and 

cardiovascular-related hospitalizations in concentration. For the two short-term exposure 

studies included in this analysis, the EPA notes that the PM2.5 concentrations 

corresponding to the 25th percentiles of the distributions of health events were 12.5 μg/m3

and 10.311.5 μg/m3, respectively, for Zanobetti and Schwartz (2009),) and 11.5 μg/m3

and 9.8 μg/m3, respectively, for Bell et al. (2008). The EPA also), with the 10th

percentiles being lower by approximately 2 μg/m3in each study (Rajan et al., 2011, Table 

1). In considering this information, the EPA recognizes, however, that there is no clear 

dividing line or single percentile within a given distribution (including both above and 

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below the 25th percentile) provided by the scientific evidence that is most appropriate or 

‘correct’ to use to characterize where the degree of confidence in the associations 

warrants setting the annual standard level. The decision as to the appropriate standard 

level below the long-term mean concentrations of the key studies is largely a public 

health policy judgment to be made by the Administrator, taking into account all of the 

evidence and its related uncertainties, as discussed in section III.E.4.d below.

In response to concerns that this analysis was not reviewed by CASAC nor 

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published in the peer-reviewed literature, the EPA notes that this analysis was conducted 

to directly respond to advice from CASAC, as discussed in section III.E.4.b.1 above, in 

conjunction with their review of the Policy Assessment. The EPA notes that the same 

type of distributional analysis was presented in the second draft Policy Assessment based 

on air quality data, as well as population-weighted air quality data, rather than health 

event or study population data. In considering that distributional information, CASAC 

urged that the EPA redo the analysis using health event or study population data, which is 

exactly what the EPA did and presented in the final Policy Assessment. The EPA 

provided CASAC with the final Policy Assessment and communicated how the final staff 

conclusions reflected consideration of its advice and that those staff conclusions were 

based in part on the specific distributional analysis that CASAC had urged the EPA to 

conduct (Wegman, 2011, Attachment p. 2). CASAC did not choose to provide any 

additional comments or advice after receiving the final Policy Assessment. The EPA 

considers this distributional analysis to be the product of the peer review conducted by 

CASAC of the Policy Assessment, and thus does not agree with commenters’ 

characterization that the analysis lacked appropriate peer review. The EPA’s final 

analysis was based on the comments provided by CASAC, the peer review committee 

established pursuant to the CAA, on the draft analysis, such that the final analysis stems 

directly from CASAC’s advice and the EPA’s response to its comments.

Based on the above considerations, the EPA continues to conclude that its 

analysis of distributions of health event and air quality data from two key multi-city 

epidemiological studies provides important information related to understanding the 

associations between health events observed in each city (e.g., deaths, hospitalizations) 
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and the corresponding long-term mean PM2.5 concentrations observed in the studies. 

While recognizing that this is a relatively modest data set, the EPA further concludes that 

such information can appropriately help to inform the selection of the level of an annual 

standard that will protect public health with an adequate margin of safety from these 

types of health effects which are causally related to long- and short-term exposures to 

PM2.5.

(4) Some commenters in this group asserted there were limitations in the longterm exposure studies of morbidity, including studies evaluating respiratory effects in 

children. For example, one commenter (UARG, 2012, p. 12, Attachment 1, pp. 14 to 16) 

asserted there were serious limitations in the long-term exposure studies of respiratory 

morbidity in each of the studies considered by the EPA (including McConnell et al., 

2003; Gauderman et al., 2004; Dockery et al., 1996; Raizenne et al., 1996; and Goss et 

al., 2004) and argued that this evidence provides only a “weak association” with PM2.5

exposures. This commenter asserted that many of these long-term exposure studies

evaluating respiratory effects were considered at the time the EPA reaffirmed the current 

annual standard level of 15 μg/m3in 2006, that the Administrator in the last review 

determined that the information they provided “was too limited to serve as the basis for 

setting a level of a national standard,” and that they should be given little weight in 

setting the level of the annual standard in this review (UARG, 2012, Attachment 1, p. 

14). 

More specifically, this commenter asserted that the McConnell et al. (2003) and 

Gauderman et al. (2004) studies reported mixed results for associations with PM2.5 and 

stronger associations with NO2 (API, 2012, Attachment 1, pp. 14 to 15). Similarly, this 
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commenter argued that the Dockery et al. (1996) and Raizenne et al. (1996) studies 

showed stronger associations with acidity than with fine particles (measured as PM2.1). 

Id. pp. 15 to 16. With regard to the cystic fibrosis study, this commenter noted that the 

association between pulmonary exacerbations and PM2.5 in this study was no longer 

statistically significant when the model adjusted for each individual’s baseline lung 

function. The commenters referred to the data on lung function as an “important 

explanatory variable,” and suggested that the EPA should rely on results from the model 

that included individual baseline lung function information. Id. p. 16. For the reasons 

discussed below and in more detail in the Response to Comments document, the EPA 

disagrees with the commenters’ interpretation of these studies.

As an initial matter, the EPA notes that three of these studies (McConnell et al., 

2003; Dockery et al., 1996; Raizenne et al., 1996) as well as the initial studies from the 

Southern California Children’s Health Study (Peters et al., 1999; McConnell et al., 

1999’1999; Gauderman et al., 2000, 2002; Avol et al., 2001) were discussed and 

considered in the 2004 Air Quality Criteria Document (U.S. EPA, 2004) and, thus,

considered within the air quality criteria supporting the EPA’s final decisions in the 

review completed in 2006. Two additional studies (Gauderman et al., 2004; Goss et al., 

2004) were discussed and considered in the provisional science assessment conducted for 

the last review (U.S. EPA, 2006a). The EPA concluded that “new” studies considered in 

the provisional assessment completed in 2006 did not materially change any of the broad 

scientific conclusions regarding the health effects of PM exposure made in the Criteria 

Document (71 FR 61148 to 61149, October 17, 2006). All of these studies were 

considered in the Integrated Science Assessment that informs the current review (U.S.
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EPA, 2009a).

With regard to the Southern California Children’s Health Study, extended 

analyses considered in the Integrated Science Assessment provided evidence that 

clinically important deficits in lung function104 associated with long-term exposure to 

PM2.5 persist into early adulthood (U.S. EPA, 2009a, p. 7-27; Gauderman et al., 2004). 

These effects remained positive in copollutant models.105 Additional analyses of the 

Southern California Children’s Health Study cohort reported an association between 

long-term PM2.5 exposure and bronchitic symptoms (U.S. EPA, 2009a, p. 7-23 to 7-24; 

McConnell et al., 2003, long-term mean concentration of 13.8 μg/m3) that remained 

positive in co-pollutant models, with the PM2.5 effect estimates increasing in magnitude 

in some models and decreasing in others, and a strong modifying effect of PM2.5 on the 

association between lung function and asthma incidence (U.S. EPA, 2009a, 7-24; Islam et 

al., 2007). The outcomes observed in the more recent reports from the Southern 

California Children’s Health Study, including evaluation of a broader range of endpoints 

and longer follow-up periods, were larger in magnitude and more precise than reported in 

the initial version of the study. Supporting these results were new longitudinal cohort 

studies conducted by other researchers in varying locations using different methods (U.S.

EPA, 2009a, section 7.3.9.1). The EPA, therefore, disagrees with the commenters that the 

 

104 Clinical significance was defined as an FEV1 below 80 percent of the predicted value, 

a criterion commonly used in clinical settings to identify persons at increased risk for 

adverse respiratory conditions (U.S. EPA, 2009a, p. 7-29 to 7-30). The primary NAAQS 

for sulfur dioxide (SO2) also included this interpretation for FEV1 (75 FR 35525, June 22, 

2010).

105 Gauderman et al. (2004) clearly stated throughout their analysis that NO2 was one 

component of a highly correlated mixture that contains PM2.5. Gauderman et al. (2004) 

did not present the results from copollutants models but stated “two-pollutant models for 

any pair of pollutants did not provide a significantly better fit to the data than the 

corresponding single-pollutant models.”
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studies by McConnell et al. (2003) and Gauderman et al. (2004) are flawed and should 

not be used in the PM NAAQS review process.

The 24-City study106 by Dockery et al. (1996) (long-term mean concentration of 

14.5 μg/m3) was considered in the current as well as two previous reviews (U.S. EPA, 

2009a; U.S. EPA, 2004; U.S. EPA, 1996). This study observed that PM, specifically 

“particle strong acidity” and sulfate particles (indicators of fine particles), were 

associated with reports of bronchitis in the previous year. Similarly, the magnitude of the 

associations between bronchitis and PM10 and PM2.1 were similar to those for acidic 

aerosols and sulfate particles, though the confidence intervals for the PM10 and PM2.1

associations were slightly wider and the associations were not statistically significant.

Acid aerosols, sulfate, and fine particles are formed in secondary reactions of the 

emissions from incomplete combustion and these pollutants have similar regional and 

temporal distributions. As noted by the study authors, “the strong correlations of several 

pollutants in this study, especially particle strong acidity with sulfate (r=0.90) and PM2.1

(r=0.82), make it difficult to distinguish the agent of interest” (Dockery et al., 1996, p. 

505). Overall, Dockery et al. (1996) (and, similarly, Raizenne et al., 1996) observed 

similar associations between respiratory health effects and acid aerosols, sulfate, PM10

and PM2.1 concentrations. The commenters noted that the associations with particle 

acidity were sensitive to the inclusion of the six Canadian sites. The EPA notes that none 

of these Canadian cities were in the “sulfate belt” where particle strong acidity was

highest. Thus, the change in the effect estimate when the six Canadian cities were 

excluded from the analysis is likely due to the lower prevalence of bronchitis and the 

 

106 The 24-City study conducted by Dockery et al. (1996) included 18 sites in the U.S. 

and 6 sites in Canada. The Raizenne et al. (1996) study considered 22 of these 24 study 

areas. Athens, OH and South Brunswick, NJ were not included in this study. 
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lower concentrations of acid aerosols in these cities, and not due to some difference in 

susceptibility to bronchitis between the U.S. and Canadian populations that is not due to 

air pollution, as suggested by the commenters (UARG, 2012, Attachment 1, p. 15). In 

fact, contrary to the statements made by the commenters, the authors did not observe any 

subgroups that appeared to be markedly more susceptible to the risk of bronchitis. 

The Goss et al. (2004) study considered a U.S. cohort of cystic fibrosis patients 

and provided evidence of association between long-term PM2.5 exposures and 

exacerbations of respiratory symptoms resulting in hospital admissions or use of home 

intravenous antibiotics (U.S. EPA, 2009a, p. 7-25; long-term mean concentration of 13.7 

μg/m3). The commenters noted that the association between pulmonary exacerbations and 

PM2.5 in this study was no longer statistically significant when the model adjusted for 

each individual’s baseline lung function. The commenters referred to the data on lung 

function as an “important explanatory variable,” and suggested that the EPA should rely 

on results from the model that included individual baseline lung function information. 

The EPA disagrees with the commenters’ interpretation of this study. The Agency 

concludes it is unlikely that lung function is a potential confounder or an important 

explanatory variable in this study. In fact, the authors noted that “it is more likely that 

lung function decline may be intimately associated with chronic exposure to air pollutants 

and may be part of the causal pathway in worsening prognosis in CF [cystic fibrosis]; in 

support of this explanation, we found both cross-sectional and longitudinal strong inverse 

relationships between FEV1 and PM levels” (Goss et al., 2004, p. 819). The EPA notes 

that adjusting for a variable that is on the causal pathway can lead to overadjustment bias, 

which is likely to attenuate the association (Schisterman et al. 2009); this is likely what 
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was observed by the authors. Thus, the EPA continues to believe it is appropriate to focus 

on the results reported in Goss et al. (2004) that did not include individual baseline lung 

function in the model.

In addition, the EPA disagrees with commenters’ reliance solely on statistical 

significance when interpreting the study results from individual study results and the 

collective evidence across studies. As discussed in section III.D.2 above, statistical 

significance of individual study findings has played an important role in the EPA’s 

evaluation of the study’s results and the EPA has placed greater emphasis on studies 

reporting statistically significant results. However, in the broader evaluation of the 

evidence from many epidemiological studies, and subsequently during the process of 

forming causality determinations in the Integrated Science Assessment by integrating 

evidence from across epidemiological, controlled human exposure, and toxicological 

studies, the EPA has emphasized the pattern of results across epidemiological studies and 

whether the effects observed were coherent across the scientific disciplines for drawing 

conclusions on the relationship between PM2.5 and different health outcomes. 

As noted in section III.B.1.a of the proposal, with regard to respiratory effects, the 

Integrated Science Assessment concluded that extended analyses of studies available in 

the last review as well as new epidemiological studies conducted in the U.S. and abroad 

provided stronger evidence of respiratory-related morbidity associated with long-term 

PM2.5 exposure (77 FR 38918). The strongest evidence for respiratory-related effects 

available in this review was from epidemiological studies that evaluated decrements in 

lung function growth in children and increased respiratory symptoms and disease 
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incidence in adults (U.S. EPA, 2009a, sections 2.3.1.2, 7.3.1.1, and 7.3.2.1).107

In considering the collective evidence from epidemiological, toxicological, and 

controlled human exposure studies, including the studies discussed above, the EPA 

recognizes that the Integrated Science Assessment concluded that a causal relationship is 

likely to exist between long-term PM2.5 exposures and respiratory effects (U.S. EPA, 

2009a, p. 2-12, pp. 7-42 to 7-43). CASAC concurred with this causality determination 

(Samet, 2009f, p.9).

The commenter’s assertion that the EPA should adhere to its assessment of these 

studies as it did in the review completed in 2006 is significantly mistaken. Most 

obviously, the EPA’s final decision in the last review was held to be deficient by the D.C. 

Circuit in remanding the 2006 primary annual PM2.5 standard. As discussed in section 

III.A.2 above, the D.C. Circuit specifically held that the EPA did not provide a 

reasonable explanation of why certain morbidity studies, including an earlier study from 

the Southern California Children’s Health Study (Gauderman et al., 2000, long-term 

mean PM2.5 concentration approximately 15 μg/m3) and the 24-Cities Study (Raizenne et 

al., 1996, long-term mean concentrations approximately 14.5 μg/m3) did not warrant a 

more stringent annual PM2.5 standard when the long-term mean PM2.5 concentrations

reported in those studies were at or lower than the level of the annual standard. American 

Farm Bureau Federation v. EPA. 559 F. 3d at 525. Indeed, the court found that, viewed 

together, the Gauderman et al. (2000) and Raizenne et al., (1996) studies “are related and 

together indicate a significant public health risk...On this record, therefore, it appears the 

EPA too hastily discounted the Gauderman and 24-Cities studies as lacking in 

 

107 Supporting evidence comes from studies “that observed associations between longterm exposure to PM10 and an increase in respiratory symptoms and reductions in lung 

function growth in areas where PM10 is dominated by PM2.5” (U.S. EPA, 2009a, p. 2-12). 
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significance.” Id.

In this review, the EPA recognizes a significant amount of evidence beyond these 

two studies that expands our understanding of respiratory effects associated with longterm PM2.5 exposures. This body of scientific evidence includes an extended and new 

analyses from the Southern California Children’s Health Study (Gauderman et al., 2004; 

Islam et al., 2007; Stanojevic et al., 2008) as well as additional studies that examined 

these health effects (Kim et al., 2004; Goss et al., 2004). Thus, even more so than in the 

last review, the evidence indicates a “significant public health risk” to children from 

long-term PM2.5 exposures at concentrations below the level of the current annual 

standard. A standard that does not reflect appropriate consideration of this evidence 

would not be requisite to protect public health with an adequate margin of safety.

(5) With regard to the use of studies of health effects for which the EPA finds the 

evidence to be “suggestive” of a causal relationship, some commenters argued that such 

studies “do not merit any weight in the setting of the annual NAAQS” (e.g., UARG, 

2012, Appendix 1, p. 3).

The EPA disagrees with the commenter’s view that studies of health effects for 

which the evidence is suggestive of a causal relationship, rather than studies of health 

effects for which the evidence supports a causal or likely causal relationship, merit no 

weight at all in setting the NAAQS. To place no weight at all on such evidence would in 

essence treat such evidence as though it had been categorized as “not likely to be a causal 

relationship.” To do so would ignore the important distinctions in the nature of the 

evidence supporting these different causality determinations in the Integrated Science 

Assessment. It would also ignore the CAA requirement that primary standards are to be 
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set to provide protection with an adequate margin of safety, including providing 

protection for at-risk populations. Thus, ignoring this information in making decisions on 

the appropriate standard level would not be appropriate.108 Nonetheless, in considering 

studies of health effects for which the evidence is suggestive of a causal relationship, the 

EPA does believe that it is appropriate to place less weight on such studies than on 

studies of health effects for which there is evidence of a causal or likely causal 

relationship.

A second group of commenters supported revising the suite of primary PM2.5

standards to provide increased public health protection. These commenters found the 

available scientific information and technical analyses to be stronger and more 

compelling than in the last review. These commenters generally placed substantial weight 

on CASAC advice and on the EPA staff analyses presented in the final Policy 

Assessment, which concluded that the evidence most strongly supported an annual 

standard level within a range of 11 to 12 μg/m3(U.S. EPA, 2011a, p. 2-206). While some 

of these commenters felt that the level should be set within the proposed range (12 to 13 

μg/m3), most of these commenters advocated a level of 11 μg/m3.

109 For example, ALA 

et al., asserted:

The EPA’s proposed PM2.5 standards, while a step in the right direction are 

insufficient to protect public health, including the health of susceptible 

populations, with an adequate margin of safety as required by the Clean Air 

Act...we will discuss the enormous gap in public health protection afforded by an 

 

108 As discussed in section II.A above, the requirement that primary standards provide an 

adequate margin of safety was intended to address uncertainties associated with 

inconclusive scientific and technical information available at the time of standard setting. 

I was also intended to provide a reasonable degree of protection against hazards that 

research has not yet identified. This certainly encompasses consideration of effects for 

which there is evidence suggestive of a causal relationship.

109 As discussed in section III.E.4.c.ii, many of these commenters also supported 

lowering the level of the primary 24-hour PM2.5 standard.
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annual standard of 13 μg/m3, at the upper end of the proposed range, compared to 

the more protective 11 μg/m3, as advocated by our organizations (ALA et al., 

2012, p. 6). 

In general, these commenters expressed the view that given the strength of the 

available scientific evidence, the serious nature of the health effects associated with PM2.5

exposures, the large size of the at-risk populations, the risks associated with long- and 

short-term PM2.5 exposures, and the important uncertainties inherently present in the 

evidence, the EPA should follow a highly precautionary policy response by selecting an 

annual standard level that incorporates a large margin of safety. 

More specifically, these commenters offered a range of comments related to the 

general approach used by the EPA to select standard levels, including: (1) the EPA’s 

approach for setting a generally controlling annual standard; (2) the importance of the 

greatly expanded and stronger overall scientific data base; (3) consideration of the 

distributional statistical analysis conducted by the EPA and other approaches for 

translating the air quality information from specific epidemiological studies into standard 

levels; and (4) the significance of the PM2.5-related public health impacts, especially 

potential impacts on at-risk populations, including children, in reaching judgments on 

setting standards that provide protection with an adequate margin of safety. These 

comments are discussed in turn below.

(1) Some of these commenters disagreed with the EPA’s approach for setting a 

“generally controlling” annual standard in conjunction with a 24-hour standard providing 

supplemental protection particularly for areas with high peak-to-mean ratios. These 

commenters argued this approach would lead to “regional inequities” as demonstrated in 

the EPA’s analyses contained in Appendix C of the Policy Assessment (ALA et al., pp. 

26 to 27). Specifically, these commenters argued:
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There is no basis in the Clean Air Act for such a determination. The Clean Air 

Act requires only that the NAAQS achieve public health protection with an 

adequate margin of safety. It is well-documented that both long- and short-term 

exposures to PM2.5 have serious and sometimes irreversible health impacts. There 

is no health protection reason to argue that one standard should be “controlling” 

as a matter of policy without regard to the health consequences of such a policy. 

To adopt such a policy ignores the obligation to provide equal protection under 

the law to all Americans because it would result in uneven protection from air 

pollution in different localities and regions of the country (ALA et al., 2012, p. 

26).

The EPA believes these commenters misunderstood the basis for the EPA’s 

policy goal of setting a “generally controlling” annual standard. This approach relates 

exclusively to setting standards that will provide requisite protection against effects 

associated with both long- and short-term PM2.5 exposures. It does so by lowering the 

overall air quality distributions across an area, recognizing that changes in PM2.5 air 

quality designed to meet an annual standard would likely result not only in lower annual 

mean PM2.5 concentrations but also in fewer and lower peak 24-hour PM2.5

concentrations. As discussed in section III.A.3 in the proposal and above, the EPA 

recognizes that there are various ways to combine the two primary PM2.5 standards to 

achieve an appropriate degree of public health protection. Furthermore, the extent to 

which these two standards are interrelated in any given area depends in large part on the 

relative levels of the standards, the peak-to-mean ratios that characterize air quality 

patterns in an area, and whether changes in air quality designed to meet a given suite of 

standards are likely to be of a more regional or more localized nature. 

In focusing on an approach of setting a generally controlling annual standard, the 

EPA’s intent is in fact to avoid the potential “regional inequities” that are of concern to 

the commenters. The EPA judges that the most appropriate way to set standards that 

provide more consistent public health protection is by using the approach of setting a 
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generally controlling annual standard. This judgment builds upon information presented 

in the Policy Assessment as discussed in section III.A.3 above. More specifically, the 

Policy Assessment recognized that the short-term exposure studies primarily evaluated 

daily variations in health effects with monitor(s) that measured the variation in daily 

PM2.5 concentrations over the course of several years. The strength of the associations 

observed in these epidemiological studies was demonstrably in the numerous “typical” 

days within the air quality distribution, not in the peak days (U.S. EPA, 2011a, p. 2-9). In 

addition, the quantitative risk assessments conducted for this and previous reviews 

demonstrated the same point, that is, much, if not most, of the aggregate risk associated 

with short-term PM2.5 exposures results from the large number of days during which the 

24-hour average concentrations are in the low-to mid-range, below the peak 24-hour 

concentrations (U.S. EPA, 2011a, section 2.2.2; U.S. EPA, 2010a, section 3.1.2.2). In 

addition, there was no evidence suggesting that risks associated with long-term exposures 

were likely to be disproportionately driven by peak 24-hour concentrations.110

For these reasons, the Policy Assessment concluded that strategies that focused 

primarily on reducing peak days were less likely to achieve reductions in the PM2.5

concentrations that were most strongly associated with the observed health effects. 

Furthermore, the Policy Assessment concluded that an approach that focused on reducing 

peak exposures would most likely result in more uneven public health protection across 

the U.S. by either providing inadequate protection in some areas or overprotecting in 

other areas (U.S. EPA, 2011a, p. 2-9; U.S. EPA, 2010a, section 5.2.3). This is because 

 

110 In confirmation, a number of studies have presented analyses excluding higher PM 

concentration days and reported a limited effect on the magnitude of the effect estimates 

or statistical significance of the association (e.g., Dominici, 2006b; Schwartz et al., 1996; 

Pope and Dockery, 1992).
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reductions based on control of peak days are less likely to control the bulk of the air 

quality distribution. 

As a result, the EPA believes an approach that focuses on a generally controlling 

annual standard would likely reduce aggregate risks associated with both long- and shortterm exposures more consistently than a generally controlling 24-hour standard and, 

therefore, would be the most effective and efficient way to reduce total PM2-5-related 

population risk. The CASAC agreed with this approach and considered it was 

“appropriate to return to the strategy used in 1997 that considers the annual and the shortterm standards together, with the annual standard as the controlling standard, and the 

short-term standard supplementing the protection afforded by the annual standard” 

(Samet, 2010d, p. 1). For the reasons discussed above, the EPA disagrees with the 

comments that this approach will result in the concerns raised by the commenters; rather 

the EPA concludes that this approach will help to address these concerns.

(2) Many of these commenters asserted that the currently available scientific 

information is greatly expanded and stronger compared to the last review. Some of these 

commenters highlighted the availability of multiple, multi-city long- and short-term 

exposure studies providing “repeated, consistent evidence of effects below the current

annual standard level” (ALA et al., 2012, pp. 39 to 49) and, more specifically, 

“significant evidence of harm with strong confidence well below EPA’s proposed annual 

standard range of 12-13 μg/m3” (AHA et al., 2012, pp. 10 to 12). 

The EPA recognizes that in setting standards that are requisite to protect public 

health with an adequate margin of safety, the Administrator must weigh the various types 

of available scientific information in reaching public health policy judgments that neither 
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overstate nor understate the strength and limitations of this information or the appropriate 

inferences to be drawn from the available science. 

In general, the EPA agrees with these commenters’ views that the currently 

available scientific evidence is stronger “because of its breadth and the substantiation of 

previously observed health effects” (77 FR 38906/2) and provides “greater confidence in 

the reported associations than in the last review” (77 FR 38940/1). The EPA also agrees 

with the commenters’ position that it is appropriate to consider the regions within the 

broader air quality distributions where we have the strongest confidence in the 

associations reported in epidemiological studies in setting the level of the annual 

standard. However, as discussed in section III.E.4.d below, in weighing the available 

evidence and technical analyses, as well as the associated uncertainties and limitations in 

that information, the EPA disagrees with the commenters’ views regarding the extent to 

which the available scientific information provides support for considering an annual 

standard level below the proposed range (i.e., below 12 to 13 μg/m3). In particular, the 

EPA disagrees with the degree to which these commenters place more weight on the 

relatively more uncertain evidence that is suggestive of a causal relationship (e.g., low 

birth weight). Consistent with CASAC advice (Samet, 2010d, p. 1), the Agency 

concludes it is appropriate and reasonable to place the greatest emphasis on health effects 

for which the Integrated Science Assessment concluded there is evidence of a causal or 

likely causal relationship and to place less weight on the health effects that provide 

evidence that is only suggestive of a causal relationship...

(3) With regard to using the air quality information from epidemiological studies 

to inform decisions on standard levels, commenters in this group generally supported the 
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EPA’s efforts to explore different statistical metrics from epidemiological studies to 

inform the Administrator’s decisions. These commenters argued that by considering 

different analytic measures -- either concentrations one standard deviation below the 

long-term means reported in the epidemiological studies or the EPA’s distributional 

statistical analysis of population-level data that extends the approach used in previous PM 

NAAQS reviews to consider information beyond a single statistical metric -- “the annual 

standard must be significantly lower than EPA has proposed” (ALA et al., 2012, pp. 50 to 

61). Furthermore, with regard to characterizing the PM2.5 air quality at which associations 

have been observed, some of these commenters highlighted CASAC’s recommendation 

that “[f]urther consideration should be given to using the 10th percentile as a level for 

assessing various scenarios of levels for the PM NAAQS” (Samet, 2010c, p. 11) (ALA et 

al., 2012, p. 55). Other commenters urged that the EPA extend the distributional analysis 

to include additional studies. For example, CHPAC urged the EPA to also conduct 

distributional analysis for children’s health studies to better inform standards that would 

protect both children and adults from adverse health outcomes (CHPAC, 2012, p. 3)).

The EPA agrees with these commenters’ views that it is appropriate to take into 

account different statistical metrics from epidemiological studies to inform the decisions 

on standard levels that are appropriate to consider. in setting a standard that will protect 

public health with an adequate margin of safety. In the development of the Policy 

Assessment, the EPA staff explored various approaches for using information from 

epidemiological studies in setting the standards. The general approach used in the final 

Policy Assessment, discussed in sections III.A.3 and III.E.4.a above, reflects 

consideration of CASAC advice (Samet, 2010c, d) and public comments on multiple 
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drafts of the Policy Assessment.

With regard to using the distributional statistical analysis to characterize the 

confidence in the associations, the EPA emphasizes that there is no clear dividing line 

provided by the scientific evidence, and that choosing how far below the long-term mean 

concentrations from the epidemiological studies is appropriate to identify a standard level 

that will provide protection for the public health with an adequate margin of safety is 

largely a public health policy judgment. The EPA considers the region from 

approximately the 25th to 10th percentiles to be a reasonable range for providing a general 

frame of reference as to the part of the distribution over which our confidence in the 

magnitude and significance of the associations observed in epidemiological studies 

becomesis appreciably lower. However Based on these considerations, the EPA 

concludes that it is not appropriate to place as much confidence in the magnitude and 

significance of the associations over the lower percentiles of the distributions in each 

study as at and around the long-term mean concentrations. Thus, the EPA disagrees with 

the commenters’ views that the available informationthis analysis compels placing more 

emphasis on the lower part of this range in selecting a level for an annual standard that 

will protect public health with an adequate margin of safety. The EPA recognizes that 

this information comes primarily from two short-term exposure studies, a relatively 

modest data set. In light of the limited nature of this information, and in recognition of

more general uncertainties inherent in the epidemiological evidence, the Administrator

deems it reasonable not to place more emphasis on concentrations in the lower part of 

this range, as discussed below in section III.E.4.d.

With regard to the scope of the distributional statistical analysis, the EPA 
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requested additional population-level data from the study authors for a group of six multicity studies for which previous air quality analyses had been conducted (Hassett-Sipple et 

al., 2010; Schmidt et al., 2010, Analysis 2). These six studies were originally selected 

because they considered multiple locations representing varying geographic regions 

across multiple years. Thus, these studies provided evidence on the influence of different 

particle mixtures on health effects associated with long- and short-term PM2.5 exposures. 

In addition, these multi-city studies considered relatively more recent health events and 

air quality conditions (1999 to 2005). As discussed in section III.E.4.b.i above, the EPA 

received and analyzed population-level data for four of the six studies (Rajan et al., 

2011). Three of these four studies (Krewski et al., 2009; Bell et al., 2008; Zanobetti and 

Schwartz, 2009) served as the basis for the concentration-response functions used to 

develop the core risk estimates (U.S. EPA, 2010a, section 3.3.3). While, the EPA agrees 

that it would be useful to have such data from more studies, the Agency believes that the 

additional data that was requested and received from study authors provide useful 

information to help inform the Administrator’s selection of the annual standard level. 

(4) Many commenters in this group highlighted PM2.5-related impacts on at-risk 

populations, including potential impacts on children, older adults, persons with preexisting heart and lung disease, and low-income populations, to support their views that 

the annual standard should be revised to a level of 11 μg/m3or lower (CHPAC, 2012; 

AHA et al., 2012; ALA, 2012, pp. 29 to 38; Rom et al., 2012; Air Alliance Houston, et al, 

2012). These commenters urged the EPA to adopt a policy approach that placed less 

weight on the remaining uncertainties and limitations in the evidence and placed more 

emphasis on margin of safety considerations, including providing protection against 
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effects for which there is more limited scientific evidence. For example, CHPAC urged 

the EPA “to place the same weight on studies examining impacts on children’s health as 

that of adult studies. ... The fact that there may be stronger evidence from adult studies 

does not mean that standards based on adult studies will be protective for children and 

consequently will meet the standard requisite to protect public health with an adequate 

margin of safety” (CHPAC, 2012 p. 3). Furthermore, with regard to the EPA’s approach 

for weighing uncertainties, some of these commenters stated that “we find no justification 

in the preamble for an annual standard level as high as 13 μg/m3, other than the vague 

assertion that uncertainties increase at lower concentrations. Further, the final proposal 

completely failed to address the Policy Assessment recommendations that if 13 μg/m3

was proposed, the 24-hour standard should be strengthened as well” (ALA et al., p. 7).

The EPA has carefully evaluated and considered evidence of effects in at-risk 

populations. With regard to effects classified as having evidence of a causal or likely 

causal relationship with long- or short-term PM2.5 exposures (i.e., premature mortality, 

cardiovascular effects, and respiratory effects), the Agency takes note that it considered 

the full range of studies evaluating these effects, including studies of at-risk populations, 

to inform its review of the primary PM2.5 standards. Specific multi-city studies 

summarized in Figures 1, 2, and 3 above highlight evidence of effects observed in two 

different lifestages – children and older adults – that have been identified as at-risk 

populations. Thus, the EPA places as much weight on studies that explored effects in 

children for which the evidence is causal or likely causal in nature as on studies of such 

effects in adults, including older adults. .As discussed above in responses to commenters 

supporting the retention of the current standards, in setting the standard, the EPA has 
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focused on considering PM2.5 concentrations somewhat below the lowest long-term mean 

concentrations from each of the key studies of both long- and short-term exposures of 

effects for which the evidence supports a causal or likely causal relationship (i.e., the first 

two sets of studies shown in Figure 4). Absent some reason to ignore or discount these 

studies, which the commenter does not provide (and of which the EPA is unaware), the 

EPA considers the available evidence of effects in children as well as other at-risk 

populations.

With respect to the EPA’s consideration of more limited studies providing 

evidence suggestive of a causal relationship (e.g., developmental and reproductive 

effects), as noted above in responding to comments from the first group of commenters, 

the Agency agrees that it is important to place some weight on this body of evidence in 

setting standards that provide protection for at-risk populations, as required by the CAA. 

However, the Agency does not agree that the same weight must be placed on this 

information as on the body of scientific information for which there is evidence of a 

causal or likely causal relationship. To do so would ignore the difference in the breadth 

and strength of the evidence supporting the different causality determinations reached in 

the Integrated Science Assessment. 

With regard to weighing the uncertainties and limitations remaining in the 

evidence and technical analyses, as discussed in section II.A above, the EPA recognizes 

that in setting a primary NAAQS that provides an adequate margin of safety, the 

Administrator must consider a number of factors including the nature and severity of the 

health effects involved, the size of sensitive population(s) at risk, and the kind and degree 

of the uncertainties that remain. As discussed in section III.E.4.d below, the Agency 
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agrees with these commenters that, in weighing the available evidence and technical 

analyses including the uncertainties and limitations in this scientific information, there is 

no justification for setting a primary PM2.5 annual standard level as high as 13 μg/m3.

Finally, some commenters in both groups also identified “new” studies that were 

not included in the Integrated Science Assessment as providing further support for their 

views on the level of the annual standard. As discussed in section II.B.3 above, the EPA 

completed a provisional review and assessment of “new” studies published since the 

close of the Integrated Science Assessment, including “new” studies submitted by 

commenters (U.S. EPA, 2012a2012b). The provisional assessment found that the “new” 

studies expand the scientific information considered in the Integrated Science Assessment 

and provide important insights on the relationship between PM2.5 exposure and health 

effects of PM (U.S. EPA, 2012b). However, the EPA notes that the provisional 

assessment found that the “new” science did not materially change the conclusions 

reached in the Integrated Science Assessment. The EPA notes that, as in past NAAQS 

reviews, the Agency is basing the final decisions in this review on the studies and related 

information included in the Integrated Science Assessment that have undergone CASAC 

and public review, and will consider newly published studies for purposes of decision 

making in the next PM NAAQS review.

ii. 24-Hour Standard Level

With respect to the level of the 24-hour standard, the EPA received comments on 

the proposal from two distinct groups of commenters. One group that included virtually 

all commenters representing industry associations, businesses, and many States agreed 

with the Agency’s proposed decision to retain the level of the 24-hour PM2.5 standard. 
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The other group of commenters included many medical groups, numerous physicians and 

academic researchers, many public health organizations, some State and local agencies, 

five State Attorneys General, and a large number of individual commenters. These 

commenters disagreed with the Agency’s proposed decision and argued that EPA should 

lower the level of the 24-hour standard to 30 or 25 μg/m3. Comments from these groups 

on the level of the 24-hour PM2.5 standard are addressed below and in the Response to 

Comments Document.

As noted above, of the public commenters who addressed the level of the 24-hour 

PM2.5 standard, all industry commenters and most State and local commenters supported 

the proposed decision to retain the current level of 35 g/m3. In many cases, these groups 

agreed with the rationale supporting the Administrator’s proposed decision to retain the 

current 24-hour PM2.5 standard, including her emphasis on the annual standard as the 

generally controlling standard with the 24-hour standard providing supplementary 

protection, and her conclusion that multi-city, short-term exposure studies provide the 

strongest data set for informing decisions on the appropriate 24-hour standard level. 

Many of these commenters agreed with the Administrator’s view that the single-city, 

short-term studies provided a much more limited data set (e.g., limited statistical power, 

limited exposure data) and more equivocal results (e.g., mixed results within the same 

study area), making them an unsuitable basis for setting the level of the 24-hour standard. 

While these commenters agreed with the EPA’s proposed decision to retain the 

current 24-hour PM2.5 standard, some did not agree with the EPA’s approach to 

considering the evidence from short-term multi-city studies. For example, a commenter 

representing UARG pointed out that the 98th percentile concentrations reported in the 
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proposal for multi-city studies reflect the averages of 98th percentile concentrations across 

the cities included in those studies (UARG, 2012; Attachment 1; p. 25). This commenter 

contended that such averaged 98th percentile PM2.5 concentrations do not provide 

information that can appropriately inform a decision on the adequacy of the public health 

protection provided by the current or alternative 24-hour standards. 

While the EPA agrees that there is uncertainty in linking effects reported in multicity studies to specific air quality concentrations (U.S. EPA, 20112011a, section 2.3.4.1), 

the EPA disagrees with this commenter’s view that such uncertainty precludes the use of 

averaged 98th percentile PM2.5 concentrations to inform a decision on the appropriateness 

of the protection provided by the 24-hour PM2.5 standard. In particular, the EPA notes 

that averaged 98th percentile concentrations do provide information on the extent to 

which study cities contributing to reported associations would likely have met or violated 

the current 24-hour PM2.5 standard during the study period. As evidence of this, the EPA 

notes the three multi-city studies specifically highlighted by this commenter as having 

averaged 98th percentile 24-hour PM2.5 concentrations below 35 g/m3(Dominici et al., 

20062006a; Bell et al., 2008; Zanobetti and Schwartz, 2009). Based on the 98th

percentiles of 24-hour PM2.5 concentrations in the individual cities evaluated in these 

studies, the EPA notes that the majority of these study cities would likely have met the 

current standard during the study periods (Hassett-Sipple et al., 2010). Therefore, 

regardless of whether the averaged 98th percentile concentrations or the 98th percentile 

concentrations in each city are considered, these studies provide evidence for associations 

between short-term PM2.5 and mortality or morbidity across a large number of U.S. cities, 

the majority of which would likely have met the current 24-hour PM2.5 standard during 
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study periods. In their review of the PM Policy Assessment, CASAC endorsed the 

conclusions drawn from analyses of averaged 98th percentile 24-hour PM2.5

concentrations, and the EPA continues to conclude that this type of information can 

appropriately inform the Administrator’s decision on the current 24-hour PM2.5

standard.111

Another group of commenters argued that the 24-hour standard level should be 

lowered. Many of these commenters supported setting the level of the 24-hour PM2.5

standard at either 25 or 30 g/m3. In support of their position, the ALA et. al., AHA et al.,

five state Attorneys General, and a number of additional groups pointed to 98th percentile 

PM2.5 concentrations in locations of multi-city and single-city epidemiological studies. 

For example, the ALA and others pointed to multi-city studies by Dominici et al. 

(20062006a), Zanobetti and Schwartz (2009), Burnett et al. (2000), and Bell et al. (2008) 

as providing evidence for associations with mortality and morbidity in study locations 

with averaged (i.e., averaged across cities) 98th percentile 24-hour PM2.5 concentrations 

below 35 g/m3. These commenters also pointed to several single-city and panel studies 

reporting associations between short-term PM2.5 and mortality or morbidity in locations 

with relatively low 24-hour PM2.5 concentrations. Because some of these multi- and 

single-city studies have reported associations with health effects in locations with 98th

percentile PM2.5 concentrations below 35 g/m3, commenters maintained that the current 

 

111This is not to say that the EPA’s decision on whether to revise the 24-hour PM2.5

standard should be based on or only be informed by considerations of whether studies 

reported associations with mortality or morbidity in areas with averaged 98th percentile 

PM2.5 concentrations less than 35 mg/m3. As discussed below, in reaching a decision in 

this final notice on the most appropriate approach to strengthen the suite of PM2.5

standards, the Administrator considers the degree of public health protection provided by 

the combination of the annual and 24-hour standards together. 
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24-hour PM2.5 standard (i.e., with its level of 35 g/m3) does not provide an appropriate 

degree of protection in all areas.

In further support of their position that the level of the current 24-hour standard 

should be lowered, these commenters pointed out the variability across the U.S. in ratios 

of 24-hour to annual PM2.5 concentrations. They noted that some locations, including 

parts of the northwestern U.S., experience relatively low annual PM2.5 concentrations but 

can experience relatively high 24-hour concentrations at certain times of the year. In 

order to provide protection against effects associated with short-term PM2.5 exposures, 

especially in locations with high ratios of 24-hour to annual PM2.5 concentrations, these 

commenters advocated setting a lower level for the 24-hour standard. 

The EPA agrees with these commenters that it is appropriate to maintain a 24-

hour PM2.5 standard in order to supplement the protection provided by the revised annual 

standard, particularly in locations with relatively high ratios of 24-hour to annual PM2.5

concentrations. However, in highlighting 98th percentile PM2.5 concentrations in study 

locations without also considering the impact of a revised annual standard on short-term 

concentrations, these commenters ignore the fact that many areas would be expected to 

experience decreasing short- and long-term PM2.5 concentrations in response to a revised 

annual standard. 

In considering the specific multi-city studies highlighted by public commenters 

who advocated a more stringent 24-hour standard, the EPA notes that such studies have 

reported consistently positive and statistically significant associations with short-term 

PM2.5 exposures in locations with averaged 98th percentile PM2.5 concentrations ranging

from 45.8 to 34.2 μg/m3and long-term mean PM2.5 concentrations ranging from 13.4 to 
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12.9 (Burnett and Goldberg, 2003; Burnett et al., 2004; Dominici et al., 2006a; Bell et al., 

2008; Franklin et al., 2008; Zanobetti and Schwartz, 2009).112 The EPA notes that to the 

extent air quality distributions are reduced to meet the current 24-hour standard with its 

level of 35 g/m3and/or the revised annual standard with its level of 12 g/m3, additional 

protection would be anticipated against the effects reported in these short-term, multi-city 

studies. Put another way, to attain an annual standard with a level below the long-term 

means in the locations of these short-term studies (as EPA is adopting here), the overall 

air quality distributions in the majority of study cities will necessarily be reduced, 

resulting in lower daily PM2.5 ambient concentrations. We therefore expect that the 

revised annual standard will result in 98th percentile PM2.5 concentrations in these cities 

that are lower than those measured in the studies, and that the overall distributions of 

PM2.5 concentrations will be lower than those reported to be associated with health 

effects. Thus, even for effects reported in multi-city studies with averaged 98th percentile 

concentrations below 35 μg/m3, additional protection from the risks associated with 

short-term exposures is anticipated from the revised annual standard, without revising the 

24-hour standard, because long-term average PM2.5 concentrations in multi-city study 

 

112Commenters also highlighted associations with short-term PM2.5 concentrations 

reported in sub-analyses restricted to days with 24-hour concentrations at or below 35 

gμg/m3(Dominici, 2006b). These sub-analyses were not included in the original 

publication by Dominici et al. (2006a). Authors provided results of sub-analyses for the 

Administrator’s consideration in a letter to the docket following publication of the 

proposed rule in January 2006 (personal communication with Dr. Francesca Dominici, 

2006b). As noted in section III.A.3, these sub-analyses are part of the basis for the 

conclusion that there is no evidence suggesting that risks associated with long-term 

exposures are likely to be disproportionately driven by peak 24-hour concentrations. 

Because the sub-analyses did not present long-term average PM2.5 concentrations, it is 

not clear whether they reflected PM2.5 air quality that would have been allowed by the 

revised annual PM2.5 standard being established in this notice. 
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locations were above the level of the revised annual standard (i.e., 12 μg/m3).113 As 

discussed above, reducing the annual standard is the most efficient way to reduce the 

risks from short-term exposures identified in these studies, as the bulk of the risk comes 

from the large number of days across the bulk of the air quality distribution, not the 

relatively small number of days with peak concentrations. 

In considering the single-city studies highlighted by public commenters who 

advocated a more stringent 24-hour standard, the EPA first notes that, overall, these 

single-city studies reported mixed results. Specifically, some studies reported positive 

and statistically significant associations with PM2.5, some studies reported positive but 

non-significant associations, and several studies reported negative associations or a mix 

of positive and negative associations with PM2.5. In light of these inconsistent results, the 

proposal noted that the overall body of evidence from single-city studies is mixed, 

particularly in locations with 98th percentiles of 24-hour concentrations below 35 μg/m3. 

Therefore, although some single-city studies reported effects at appreciably lower PM2.5

concentrations than short-term multi-city studies, the uncertainties and limitations 

associated with the single-city studies were noted to be greater. In light of these greater 

uncertainties and limitations, the Administrator concluded in the proposal that she had 

less confidence in using these studies as a basis for setting the level of the standard (77 

FR 38943). 

Given the considerations and conclusions noted above, in the proposal the 

 

113It is also the case that additional protection is anticipated in locations with 98th

percentile 24-hour PM2.5 concentrations above 35 μg/m3, even if long-term 

concentrations are below 12 μg/m3. As noted in the proposal and in the Policy 

Assessment (U.S. EPA, 20112011a, Figure 2-10), parts of the northwestern U.S. are more 

likely than other parts of the country to violate the 24-hour standard and meet the revised 

annual standard. 
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Administrator concluded that the short-term multi-city studies provide the strongest 

evidence to inform decisions on the level of the 24-hour standard. Further, she viewed 

single-city, short-term exposure studies as a much more limited data set providing mixed 

results, and she had less confidence in using these studies as a basis for setting the level 

of a 24-hour standard (77 FR 38942).. In highlighting specific single-city studies, public 

health, environmental, and State and local commenters appear to have selectively focused 

on studies reporting associations with PM2.5 and to have overlooked studies that reported 

more equivocal results (e.g., Ostro et al., 2003; Rabinovitch et al., 2004; Slaughter et al., 

2005; Villeneuve et al., 2006) (U.S. EPA, 2011, Figure 2-9). As such, these commenters 

have not presented new information that causes the EPA to reconsider its decision to 

emphasize multi-city studies over single-city studies when identifying the appropriate 

level of the 24-hour PM2.5 standard.

In further considering the single-city studies highlighted by public commenters, 

the EPA notes that some commenters advocating for a lower level for the 24-hour PM2.5

standard also discussed short-term studies that have been published since the close of the 

Integrated Science Assessment. These recent studies were conducted in single cities or in 

small panels of volunteers. As in prior NAAQS reviews and as discussed above in more 

detail (section II.B.3), the EPA is basing its decisions in this review on studies and related 

information assessed in the Integrated Science Assessment. The studies assessed in the 

Integrated Science Assessment, and the conclusions based on those studies, have 

undergone extensive critical review by the EPA, CASAC, and the public. The rigor of 

that review makes the studies assessed in the Integrated Science Assessment, and the 

conclusions based on those studies, the most reliable source of scientific information on 
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which to base decisions on the NAAQS. 

However, as discussed above (section II.B.3), the EPA recognizes that ‘‘new 

studies” may sometimes be of such significance that it is appropriate to delay a decision 

on revision of a NAAQS and to supplement the pertinent air quality criteria so the studies 

can be taken into account. In the present case, the EPA’s provisional consideration of 

‘‘new studies” concludes that, taken in context, the “new” information and findings do 

not materially change any of the broad scientific conclusions made in the air quality 

criteria regarding the health effects of PM2.5 (U.S. EPA, 2012a2012b). 

For this reason, reopening the air quality criteria review would not be warranted, 

even if there were time to do so under the court order governing the schedule for 

completing this review. Accordingly, the EPA is basing its final decisions in this review 

on the studies and related information included in the PM Integrated Science Assessment 

(i.e., the air quality criteria) that has undergone CASAC and public review. The EPA will 

consider the “new studies” in the next periodic review of the PM NAAQS, which will 

provide an opportunity to fully assess these studies through a more rigorous review 

process involving the EPA, CASAC, and the public. 

Some public health, medical, and environmental commenters also criticized the 

EPA’s interpretation of PM2.5 risk results. These commenters presented risk estimates for 

combinations of annual and 24-hour standards using more recent air quality data than that 

used in the EPA’s Risk Assessment (U.S. EPA, 2010a). Based on these additional risk 

analyses, the ALA and other commenters contended that public health benefits could 

continue to increase as annual and 24-hour standard levels decrease below 13 g/m3and 

35 g/m3, respectively. 
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The EPA agrees with commenters that important public health benefits are 

expected as a result of revising the level of the annual standard to 12 g/m3, as is done in 

this notice, rather than 13 g/m3. The Agency also acknowledges that estimated PM2.5-

associated health risks continue to decrease with annual standard levels below 12 g/m3

and/or with 24-hour standard levels below 35 g/m3. However, the EPA disagrees with 

the commenters’ views regarding the extent to which risk estimates support setting 

standard levels below 12 μg/m3(annual standard) and 35 g/m3(24-hour standard).114

The CAA charges the Administrator with setting NAAQS that are “requisite” 

(i.e., neither more nor less stringent than necessary) to protect public health with an 

adequate margin of safety. In setting such standards the Administrator must weigh the 

available scientific evidence and information, including associated uncertainties and 

limitations. As described above, in reaching her proposed decisions on the PM2.5

standards that would provide “requisite” protection, the Administrator carefully 

considered the available scientific evidence and risk information, making public health 

policy judgments that, in her view, neither overstated nor understated the strengths and 

limitations of that evidence and information. In contrast, as discussed more fully above, 

public health, medical, and environmental commenters who recommended levels below 

35 g/m3for the 24-hour PM2.5 standard have not provided new information or analyses 

to suggest that such standard levels are appropriate, given the uncertainties and 

limitations in the available health evidence, particularly uncertainties in studies 

conducted in locations with 98th percentile 24-hour PM2.5 concentrations below 35 g/m3

and long-term average concentrations below 12 g/m3. 

 

114This section focuses on the 24-hour standard. Section III.E.4.c.i above also discusses 

these commenters’ recommendations within the context of the annual PM2.5 standard. 
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d. Administrator’s Final Conclusions on the Primary PM2.5 Standard Levels

In reaching her conclusions regarding appropriate standard levels, the 

Administrator has considered the epidemiological and other scientific evidence, estimates 

of risk reductions associated with just meeting alternative annual and/or 24-hour 

standards, air quality analyses, related limitations and uncertainties, the advice of 

CASAC, and extensive public comments on the proposal. After careful consideration of 

all of these, the Administrator has decided to revise the level of the primary annual PM2.5

standard from 15.0 μg/m3to 12.0 μg/m3and to retain the level of the primary 24-hour 

standard at 35 μg/m3.

As an initial matter, the Administrator agrees with the approach supported by 

CASAC and discussed in the Policy Assessment as summarized in sections III.A.3 and 

III.E.4.a above, of considering the annual and 24-hour standards together in determining 

the protection afforded against mortality and morbidity effects associated with both longand short-term exposures to PM2.5. This approach is consistent with the approach taken in 

the review completed in 1997, in contrast to the approach used in the review completed in 

2006 where each standard was considered independently of the other (i.e., only data from 

long-term exposure studies were used to inform the level of the annual standard and only 

data from short-term exposure studies were used to inform the level of the 24-hour 

standard).115

Based on the evidence and quantitative risk assessment, the Administrator 

concludes that it is appropriate to set an annual standard that is generally controlling, 

which will lower the broad distribution of 24-hour average concentrations in an area as 

well as the annual average concentration, so as to provide protection from both long- and 

 

115

 See 71 FR 61148 and 61168, October 17, 2006.
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short-term PM2.5 exposures. In conjunction with this, it is appropriate to set a 24-hour 

standard focused on providing supplemental protection, particularly for areas with high 

peak-to-mean ratios of 24-hour concentrations, possibly associated with strong local or 

seasonal sources, and for PM2.5-related effects that may be associated with shorter-than 

daily exposure periods. The Administrator concludes this approach will reduce aggregate 

risks associated with both long- and short-term exposures more consistently than a 

generally controlling 24-hour standard and is the most effective and efficient way to 

reduce total PM2-5-related population risk and so provide appropriate protectionto protect 

public health with an adequate margin of safety. 

BasedIn selecting the level of the annual PM2.5 standard, based on the 

characterization and assessment of the epidemiological and other studies presented and 

assessed in the Integrated Science Assessment and the Policy Assessment, the 

Administrator recognizes the substantial increase in the number and diversity of studies 

available in this review, including. This expanded body of evidence includes extended 

analyses of the seminal studies of long-term PM2.5 exposures (i.e., ACS and Harvard Six 

Cities studies) as well as important new long-term exposure studies (as summarized in 

Figures 1 and 2). Collectively, the Administrator notes that these studies, along with 

evidence available in the last review, provide consistent and stronger evidence than 

previously observed of an association between long-term PM2.5 exposures and premature 

mortality in areas with lower long-term ambient concentrations than previously observed, 

with the strongest evidence related to cardiovascular-related mortality. The Administrator 

also recognizes the availability of stronger evidence of morbidity effects associated with 

long-term PM2.5 exposures, including evidence of respiratory effects such as decreased 
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lung function growth, from the extended analyses for the Southern California Children’s 

Health Study and evidence of cardiovascular effects from the WHI study. Furthermore, 

the Administrator recognizes new U.S. multi-city studies that greatly expand and 

reinforce our understanding of mortality and morbidity effects associated with short-term 

PM2.5 exposures, providing stronger evidence of associations in areas with ambient 

concentrations similar to those previously observed in short-term exposure studies 

considered in the previous review (as summarized in Figure 3). 

The Administrator recognizes the strength of the scientific evidence for evaluating 

health effects associated with fine particles, noting that the newly available scientific 

evidence builds upon the previous scientific data base to provide evidence of generally 

robust associations and a basis for greater confidence in the reported associations than in 

the last review. She notes the conclusion of the Integrated Science Assessment that this 

body of evidence supports a causal relationship between long- and short-term PM2.5

exposures and mortality and cardiovascular effects and a likely causal relationship 

between long- and short-term PM2.5 exposures and respiratory effects. In addition, the 

Administrator notes additional, but more limited evidence, for a broader range of health 

endpoints including evidence suggestive of a causal relationship for developmental and 

reproductive effects as well as for carcinogenic effects.

Based on information discussed and presented in the Integrated Science 

Assessment, the Administrator recognizes that health effects may occur over the full 

range of concentrations observed in the epidemiological studies of both long-term and 

short-term exposures, since no discernible population-level threshold for any such effects 

can be identified based on the currently available evidence (U.S. EPA, 2009a, section 
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2.4.3). To inform her decisions on an appropriate level for the annual standard levelsthat 

will protect public health with an adequate margin of safety, in the absence of any 

discernible population-level thresholds, the Administrator judges that it is appropriate to 

consider the relative degree to which the evidence of an associationconfidence in the 

magnitude and significance of the associations observed in an epidemiological study is 

supportedstudies across the range of long-term PM2.5 concentrations in the study, and 

therefore the degree to which the study provides confidence in the effects across this 

range of concentrationssuch studies. Further, she recognizes, in taking note of CASAC 

advice and the distributional statistics analysis discussed in the Policy Assessment and in 

section III.E.4.a above, that there is significantly greater confidence in the magnitude and 

significance of observed associations for certain partsthe part of the air quality 

distribution corresponding to where the bulk of the health events evaluated in each study 

have been observed., generally at and around the long-term mean concentrations.

Conversely, she also recognizes that there is significantly diminished confidence in the 

magnitude and significance of observed associations in the lowestlower part of the air 

quality distribution corresponding to where a relatively small proportion of the health 

events were observed. Based on the above considerations, the Administrator takes into 

account information that characterizes the part of the air quality distributions that 

corresponds to the large proportion of health event data that was most influential in 

defining the association between PM2.5 and the health endpoints evaluated in each study. 

In so doing Further, the Administrator recognizes that the long-term mean 

concentrations, or any other specific point in the air quality distribution of each study, do 

not represent a “bright line” at and above which effects have been observed and below 
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which effects have not been observed. As just noted, no discernible population-level 

threshold for any such effects can be identified based on the currently available evidence. 

The Administrator notes that the concentrations at and around the long-term mean 

concentrations, for both the long- and short-term exposure studies, represents a part of the 

air quality distribution in which the health event data in any given study are generally 

most concentrated and, thus, where these data have the most influence on the magnitude 

and significance of the association observed in such study. The Administrator notes that 

long-term mean PM2.5 concentrations are generally available for each published study 

and, therefore, represent the most comprehensive data set to inform her decisions on the 

appropriate annual standard level.

Nonetheless, inIn considering the long-term mean concentrations reported in 

epidemiological studies, the Administrator recognizes that in setting an selecting a level 

of the annual standard levelthat will protect public health with an adequate margin of

safety, it is not sufficient to focus on a concentration generally somewhere within the 

range of long-term mean concentrations from the key long-term and short-term exposure 

studies that reported lower concentrations than had been observed in earlier reviews. 

These key studies provide information for various types of serious health endpoints 

(including mortality and morbidity effects), different study populations (which may 

include at-risk populations such as children and older adults), and different air quality 

distributions that are specific to each study. A level somewhere within the range of longterm mean concentrations of the full set of key studies would be higher than the longterm mean of at least one of the studies being considered and therefore would fail to 

protectnot provide a sufficient degree of protection against the health effects associated 
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with the air quality distributionobserved in that study. Absent some reasoned basis to 

place less weight on the evidence in the epidemiological study with the lowest long-term 

mean concentration among these key studies, this approach would fail to provide 

sufficient protectionnot be consistent with the requirement to set a standard that will 

protect public health with an adequate margin of safety.116 Thus, the Administrator 

recognizes it is important to protect against the serious effects observed in each of these 

studies. so as to protect public health with an adequate margin of safety. In so doing, she 

looks to identify the study with the lowest long-term mean concentration within the full

set of key studies to help inform her decision of the appropriate standard level which will 

provide protection for the broad array of health outcomes observed in all of the studies, 

including effects observed in at-risk populations.

Further, consistent with the general approach summarized in section III.E.4.a 

above and supported by CASAC as discussed in section III.E.4.b.ii above, the 

Administrator recognizes that it is appropriate to consider a level for an annual standard 

that is not just at but rather is somewhat below the long-term mean PM2.5 concentrations 

reported in each of the key long- and short-term exposure studies. In so doing, she 

focuses especially on multi-city studies that evaluated health endpoints for which the 

associations are causal or likely causal (i.e., mortality and cardiovascular and respiratory 

effects associated with both long- and short-term PM2.5 exposures). As discussed above, 

the importance of considering a level somewhat below the lowest long-term mean 

concentrations in this set of key studies is to establish a standard that would be protective 

against the observed effects in all of the studies, and that takes into account the relative 

 

116 See American Farm Bureau Federation v. EPA, 559 F. 3d 512, 525-26 (D.C. Cir. 

2009).

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degree of confidence in the magnitude and significance of observed associations across 

the air quality distributions in these studies.

The Administrator recognizes that there is no clear way to identify how much 

below the long-term mean concentrations of key studies to set a standard that would 

provide requisite protection with an adequate margin of safety. She therefore must use 

her judgment to weigh the available scientific and technical information, and associated 

uncertainties, to reach a final decision on the appropriate standard level. In considering 

the information in Figures 1 - 4 for effects classified as having evidence of a causal or 

likely causal relationship with long- or short-term PM2.5 exposures, she observes a cluster 

of short-term exposure studies with long-term mean concentrations within a range of 13.4 

μg/m3down to 12.8 μg/m3(Dominici et al., 20062006a; Burnett and Goldberg, 2003; 

Zanobetti and Schwartz, 2009; Bell et al., 2008; Burnett et al., 2004). She also observes a 

cluster of long-term exposure studies with long-term mean concentrations within a range 

of 14.5 μg/m3to 13.6 μg/m3(Dockery et al., 1996; Lipfert et al., 20062006a; Zeger et al., 

2008; McConnell et al., 2003; Goss et al., 2004; Eftim et al., 2008). For the reasons 

discussed in response to public comments in section III.E.4.c above, the Administrator is 

less influenced by the long-term mean PM2.5 concentrations from the Miller et al. (2007) 

and Krewski et al. (2009) studies with reported long-term mean PM2.5 concentrations of 

12.9 and 14.0 μg/m3, respectively. In each case, the most relevant exposure periods would 

likely have had higher mean PM2.5 concentrations than those reported in the studies.117

Thus, the Administrator considers the long-term mean PM2.5 concentrations from these 

 

117 In the case of Miller et al. (2007), the mean concentration is based on a single year of 

air quality data which post-dated by two years the period for which the health events data 

were collected. In the case of Krewski et al. (2009),the air quality data were based on the 

last two years of the 18-year period for which the health event data were collected. 

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two studies to be a highly uncertain basis for informing her selection of the annual 

standard level.118

To help guide her judgment of the appropriate level below the long-term mean 

concentrations in the epidemiological studies at which to set the standard, the 

Administrator considered additional information from epidemiological studies 

concerning the broader distribution of PM2.5 concentrations which correspond to the 

health events observed in these studies (e.g., deaths, hospitalizations). The Administrator 

observes that the development and use of this information in considering standard levels 

is consistent with CASAC’s advice, as discussed in section III.E.4.b.ii above, to focus on 

understanding the concentrations that were most influential in generating the health effect 

estimates in individual studies (Samet, 2010d, p. 2). 

In considering this additional population-level information, the Administrator 

recognizes that, in general, the confidence in the magnitude and significance of an 

association identified in a study is strongest at and around the long-term mean 

concentration for the air quality distribution, as this represents the part of the distribution 

in which the data in any given study are generally most concentrated. She also recognizes 

that the degree of confidence decreases as one moves towards the lower part of the 

distribution. Consistent with the approach used in the Policy Assessment, the 

Administrator believes that the range from approximately the 25th to 10th percentiles is a 

reasonable range for providing a general frame of reference as to the part of the 

distribution in which her confidence in the associations observed in epidemiological 

 

118 Nonetheless, as noted above, the EPA notes that the Krewski et al. (2009) and Miller 

et al. (2007) studies provide strong evidence of mortality and cardiovascular-related 

effects associated with long-term PM2.5 exposures to inform causality determinations 

reached in the Integrated Science Assessment (U.S. EPA, 2009a, sections 7.2.11 and 7.6). 
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studies becomesis appreciably lower. However, as noted above, it is important to 

emphasize that there is no clear dividing line or single percentile within a given 

distribution provided by the scientific evidence that is most appropriate or ‘correct’ to use 

to characterize where the degree of confidence in the associations warrants setting the 

annual standard level. The decision of the appropriate standard level below the long-term 

mean concentrations of the key studies, which in conjunction with the other elements of 

the standard would protect public health with an adequate margin of safety, is largely a 

public health policy judgment, taking into account all of the evidence and its related 

uncertainties. 

As discussed in section III.E.4.b, the Administrator takes note of additional 

population-level data that were made available to the EPA by study authors.119 In 

considering this information, the Administrator particularly focuses on the analysis of the 

distributions of the health event data for each area within these studies and the 

corresponding air quality data for the two short-term exposure studies (Zanobetti and 

Schwartz, 2009; Bell et al., 2008). These short-term exposure studies evaluate the 

relationship between daily changes (one or more days) in PM2.5 concentrations and daily 

changes in health events (e.g., deaths, hospitalizations), such that the air quality 

concentrations that comprise the most relevant exposure periods in these studies are 

 

119 As summarized in section III.E.4.a, population-level data were provided to the EPA 

for four studies. These four studies represent some of the strongest evidence showing 

associations between health effects and PM2.5 within the overall body of scientific 

evidence and include three studies (Krewski et al., 2009; Bell et al., 2008; and Zanobetti 

and Schwartz, 2009) that were used as the basis for concentration-response functions in 

the quantitative risk assessment (U.S. EPA, 2010a, section 3.3.3). The Administrator 

recognizes that the additional population-level data available for these four multi-city 

studies represents a more limited data set compared to the set of long-term mean PM2.5

concentrations which were available in the published literature for all studies considered 

in the Integrated Science Assessment. 

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contemporaneous with the health event data. In addition, these studies considered more 

recent air quality data, representing generally lower PM2.5 concentrations, in a large 

number of study areas across the U.S. Thus, such studies provide the most useful 

evidence for an analysis evaluating the distribution of health event data and the 

corresponding long-term mean PM2.5 concentrations across the areas included in each 

multi-city study. 

The Administrator also considered the additional population-level data that were 

made available to EPA for two long-term exposure studies (Krewski et al., 2009; Miller 

et al., 2007). She recognizes that in long-term exposure studies investigators follow a 

specific group of study participants (i.e., cohort) over time and across urban study areas, 

and evaluate how PM2.5 concentrations averaged over a period of years are associated 

with specific health endpoints (e.g., deaths) across cities. As discussed in response to 

public comments in section III.E.4.c, disentangling the effects observed in long-term 

exposure studies associated with more recent air quality measurements from effects that 

may have been associated with earlier, and most likely higher, PM2.5 exposures 

introduces some uncertainty with regard to understanding the appropriate exposure 

window associated with the observed effects. This is in contrast to the short-term 

exposure studies where the relevant exposure period is contemporaneous to the period for 

which the health data were collected. In light of these considerations, as noted above, the 

Administrator considers the analysis of air quality concentrations that correspond to the 

distribution of population-level data in these two studies to be a highly uncertain basis for 

informing her selection of the annual standard level. 

Based on the above considerations, the Administrator views the additional 
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population-level data for the two short-term exposure studies as appropriate to help 

inform her judgment of how much below the long-term mean concentrations to set the 

level of the annual standard. The Administrator notes that the long-term mean PM2.5

concentrations corresponding with study areas contributing to the 25th and 10th percentiles

of the distribution of deaths and cardiovascular-related hospitalizations in these two 

short-term exposure studies were 12.5 μg/m3and 10.311.5 μg/m3, respectively, for 

Zanobetti and Schwartz (2009),) and 11.5 μg/m3and 9.8 μg/m3, respectively, for Bell et 

al. (2008) (Figure 5 above; Rajan et al., 2011, Table 1).), with the 10th percentiles being 

lower by approximately 2 μg/m3in each study..

The Administrator recognizes, as summarized in section III.B above and 

discussed more fully in section III.B.2 of the proposal, that important uncertainties 

remain in the evidence and information considered in this review of the primary fine 

particle standards. These uncertainties are generally related to understanding the relative 

toxicity of the different components in the fine particle mixture, the role of PM2.5 in the 

complex ambient mixture, exposure measurement errors, and the nature and magnitude of 

estimated risks related to increasingly lower ambient PM2.5 concentrations. Furthermore, 

the Administrator notes that epidemiological studies have reported heterogeneity in 

responses both within and between cities and geographic regions across the U.S. She 

recognizes that this heterogeneity may be attributed, in part, to differences in fine particle 

composition in different regions and cities.120

 

120 Nonetheless, as explained in section III.E.1, the currently available evidence is not 

sufficient to support replacing or supplementing the PM2.5 indicator with any other 

indicator defined in terms of a specific fine particle component or group of components 

associated with any source categories of fine particles. Furthermore, the evidence is not 

sufficient to support eliminating any component or group of components associated with 

any source categories of fine particles from the mix of fine particles included in the PM2.5

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With regard to evidence for reproductive and developmental effects identified as 

being suggestive of a causal relationship with long-term PM2.5 exposures, the 

Administrator recognizes that there are a number of limitations associated with this body 

of evidence including: the limited number of studies evaluating such effects; uncertainties 

related to identifying the relevant exposure time periods of concern; and limited 

toxicological evidence providing little information on the mode of action(s) or biological 

plausibility for an association between long-term PM2.5 exposures and adverse birth 

outcomes. Nonetheless, the Administrator believes that this more limited body of 

evidence provides some support for considering that serious effects may be occurring in a 

susceptible population at concentrations lower than those associated with effects 

classified as having a causal or likely causal relationship with long-term PM2.5 exposures 

(i.e., mortality, cardiovascular, and respiratory effects).

Overall, the Administrator believes that the available evidence interpreted in light 

of the remaining uncertainties, as summarized above and discussed more fully in the 

Integrated Science Assessment and the Policy Assessment, provides increased confidence 

relative to information available in the last review and provides a strong basis for 

informing her final decisions in the current review. The Administrator is mindful that 

considering what standards are requisite to protect public health with an adequate margin 

of safety requires public health policy judgments that neither overstate nor understate the 

strength and limitations of the evidence or the appropriate inferences to be drawn from 

the evidence. In considering how to translate the available information into appropriate 

standard levels, the Administrator weighs the available scientific information and 

associated uncertainties and limitations. For the purpose of determining what annual 

 

indicator. 
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standard level is appropriate the Administrator recognizes that there is no single factor or 

criterion that comprises the ‘‘correct’’ approach to weighing the various types of 

available evidence and information.

In considering this information, the Administrator notes the advice of CASAC 

that “there are significant public health consequences at the current levels of the 

standards that justify consideration of lowering the PM2.5 NAAQS further” (Samet, 

2010c, p. 12). In addition, she recognizes that CASAC concluded, “although there is 

increasing uncertainty at lower levels, there is no evidence of a threshold (i.e., a level 

below which there is no risk for adverse effects)” (Samet, 20102010d, p.ii) and that the 

final decisions on standard levels must reflect a judgment of the available scientific 

information with respect to her interpretation of the CAA’s requirement to set primary 

standards that provide requisite protection to public health with an adequate margin of 

safety (Samet, 2010d, p. 4). The Administrator recognizes CASAC’s advice that the 

currently available scientific information provided support for considering an annual 

standard level within a range of 13 to 11 μg/m3and a 24-hour standard level within a 

range of 35 to 30 μg/m3. In considering how the annual and 24-hour standards work 

together to provide appropriate public health protection, the Administrator observes that 

CASAC did not express support for any specific levels or combinations of standards 

within these ranges. She also notes that CASAC encouraged the EPA staff to consider 

additional data from epidemiological studies to help quantify the characterization of the 

PM2.5 concentrations that were most influential in generating the health effect estimates 

in these studies (Samet, 2010d, p. 2). 

In response to CASAC’s advice, the Administrator recognizes that the EPA staff 
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acquired additional data from authors of key epidemiological studies and analyzed these 

data to characterize the distribution of PM2.5 concentrations in relation to health events 

data to better understand the degree of confidence in the associations observed in the 

studies as discussed above. The Administrator recognizes that the final Policy 

Assessment included consideration of these additional analyses in reaching final staff 

conclusions with regard to the broadest range of alternative standard levels supported by 

the science. She takes note that the final Policy Assessment concluded that while 

alternative standard levels within the range of 13 to 11 μg/m3 were appropriate to 

consider, the evidence most strongly supported consideration of an annual standard level 

in the range of 12 to 11 μg/m3. The Administrator is aware that, in transmitting the final 

Policy Assessment to CASAC, the Agency notified CASAC that the final staff 

conclusions reflected consideration of CASAC’s advice and that those staff conclusions 

were based, in part, on the specific distributional analysis that CASAC had urged the 

EPA to conduct (Wegman, 2011). Thus, CASAC had an opportunity to comment on the 

final Policy Assessment, but chose not to provide any additional comments or advice 

after receiving it. 

In selecting the annual standard level, the Administrator has considered many 

factors including the nature and severity of the health effects involved, the strength of the 

overall body of scientific evidence as considered in reaching causality determinations, the 

size of the at-risk populations, and the estimated public health impacts. She has also 

considered the kind and degree of the uncertainties that remain in the available scientific 

information. She recognizes that the association between PM2.5 and serious health effects 

is well established, including at concentrations below those allowed by the current 
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standard. Further, she recognizes the CAA requirement that requires primary standards to 

provide an adequate margin of safety was intended to address uncertainties associated 

with inconclusive scientific and technical information as well as to provide a reasonable 

degree of protection against hazards that research has not yet identified. In considering 

the currently available evidence, as summarized and discussed more broadly above, the 

information on risk, CASAC advice, the conclusions of the Policy Assessment, and 

public comments on the proposal, the Administrator strongly believes that a lower annual 

standard level is needed to protect public health with an adequate margin of safety. In this 

instance, the association between PM2.5 and serious health effects is well established, 

including at concentrations below those allowed by the current standard. 

In reaching her final decision on the appropriate annual standard level to set, the 

Administrator is mindful that the CAA does not require that primary standards be set at a 

zero-risk level, but rather at a level that reduces risk sufficiently so as to protect public 

health, including the health of at-risk populations, with an adequate margin of safety. On 

balance, the Administrator concludes that an annual standard level of 12 μg/m3 would be 

requisite to protect the public health with an adequate margin of safety from effects 

associated with long- and short-term PM2.5 exposures, while still recognizing that 

uncertainties remain in the scientific information. 

In the Administrator’s judgment, an annual standard of 12 μg/m3appropriately 

reflects placing greatest weight on evidence of effects for which the Integrated Science 

Assessment determined there is a causal or likely causal relationship with long- and 

short-term PM2.5 exposures. An annual standard level of 12 μg/m3is below the long-term 

mean PM2.5 concentrations reported in each of the key multi-city, long- and short-term 
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exposures studies providing evidence of an array of serious health effects (e.g., premature 

mortality, increased hospitalization for cardiovascular and respiratory effects). As noted 

above, the importance of considering a level somewhat below the lowest long-term mean 

concentration in the full set of studies considered is to set a standard that would be 

protectiveprovide appropriate protection against the observed effects in all such studies. 

In reaching her decision, the Administrator has taken into account that at and 

around the mean PM2.5 concentration in any given study represents a part of the air 

quality distribution in which the health event data in that study are generally most 

concentrated. Furthermore, in identifying an appropriate annual standard level below the 

long-term mean concentrations, she recognizes that there is no evidence to support the 

existence of any discernible threshold, and, therefore, she has a high degree of confidence 

that the observed effects are associated with concentrations not just at but extending 

somewhat below the long-term mean concentration. To further inform her judgment in 

setting the annual standard level so as to protect public health with an adequate margin of 

safety, the Administrator has placed weight on additional population-level information 

available from a subset of these epidemiological studies, consistent with CASAC advice. 

In particular, she has drawn from two short-term exposure studies, which provide the 

most relevant information for evaluating the distribution of health events and 

corresponding long-term PM2.5 concentrations. As explained above, this provides 

information onhelps inform her judgment as to the degree of confidence in the observed 

associations in the epidemiological studies. In this regard, the Administrator generally 

judges the region around the 25th percentile as a reasonable part of the distribution to help 

guide her decision on the appropriate standard level. Since this evidence comes primarily 
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from two studies, a relatively modest data set, the Administrator deems it reasonable not 

to draw further inferences from air quality and health event data in the lower part of the 

distribution for the purpose of setting a standard level. The Administrator notes that the 

long-term mean PM2.5 concentrations around the 25th percentile of the distributions of 

deaths and cardiovascular-related hospitalizations were approximately around 12.5 μg/m3

and 11.5 μg/m3in Zanobetti and Schwartz (2009) and in Bell et al. (2008), respectively 

(Figure 5; Rajan et al., 2011, Table 1).these two studies. The Administrator views this 

information as helpful in guiding her determination as to where she continues to have 

sufficienther confidence in the magnitude and significance of the associations to warrant 

setting an annualis reduced to such a degree that a standard set at a lower level would not 

be warranted to provide requisite protection that is neither more nor less than needed to 

provide an adequate margin of safety.

The Administrator also recognizes that a level of 12 μg/m3places some weight on 

studies which provide evidence of reproductive and developmental effects (e.g., infant 

mortality, low birth weight). These studies were identified in the Integrated Science 

Assessment as having evidence suggestive of a causal relationship with long-term PM2.5

concentrations. A level of 12 μg/m3is approximately the same level as the lowest longterm mean concentration reported in such studies (Figures 2 and 4; 11.9 μg/m3

for Bell et 

al., 2007).121 While the Administrator acknowledges that this evidence is limited, she 

 

121 With respect to cancer, mutagenic, and genotoxic effects, the Administrator observes 

that the PM2.5 concentrations reported in studies evaluating these effects generally 

included ambient concentrations that are equal to or greater than ambient concentrations 

observed in studies that reported mortality and cardiovascular and respiratory effects 

(U.S. EPA, 2009a, section 7.5). Therefore, the Administrator concludes that in selecting 

alternative standard levels that provide protection from mortality and cardiovascular and 

respiratory effects, it is reasonable to anticipate that protection will also be provided for 

carcinogenic effects.

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believes it is appropriate to place some weight on these studies in order to set a standard 

that provides protection with an adequate margin of safety, including providing 

protection for at-risk populations, as required by the CAA. Due to the limited nature of 

this evidence, she has determined it is not necessary to set a standard below the lowest 

long-term mean concentration in these studies. 

In reflecting on extensive public comments received on the proposal as discussed 

in section III.E.4.c above, the Administrator recognizes that some commenters have 

offered different evaluations of the evidence and other information available in this 

review and would make different judgments about the weight to place on the relative 

strengths and limitations of the scientific information and about how such information 

could be used in making public health policy decisions on the annual standard level. One 

group of such commenters who supported a higher annual standard level (e.g., above 13 

μg/m3) would place greater weight on the remaining uncertainties in the evidence as a 

basis for discounting the overall strength of the evidencesupporting a higher standard 

level than the Administrator judges to be appropriate. Such an approach is based on these 

commenters’ judgment that the uncertainties remaining in the evidence are too great to 

warrant setting a loweran annual standard below the current level. The Administrator 

does not agree. 

As an initial matter, an annual standard level of 13 μg/m3or higher would be 

above the long-term mean concentrations reported in two well-conducted, multi-city 

short-term exposure studies reporting positive and statistically significant associations of 

serious effects (Burnett et al., 2004 and Bell et al., 2008). These important studies are 

fully consistent with the pattern of evidence presented by the large body of evidence in 
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this review. As the Administrator recognized in the proposal, and as advised by CASAC, 

the appropriate focus for selecting the level of the annual PM2.5 standard is on 

concentrations somewhat below the lowest long-term mean concentrations from the set of 

key studies of both long-term and short-term PM2.5 exposures considered by the EPA 

(i.e., as shown in Figure 4). Thus, a standard level set at 13 μg/m3or higher would clearly 

not provide protection for the effects observed in the full set epidemiological studies and, 

therefore, this standard level could not be judged to be requisite with an adequate margin 

of safety.122

In addition, as noted above, in recognizing that there is no evidence to support the 

existence of a discernible threshold below which an effect would not occur, the 

Administrator is mindful that effects occur around and below the long-term mean 

concentrations reported in both the short-term and long-term the epidemiological studies. 

A standard level of 13 μg/m3or higher would place essentially no weight onnot 

appropriately take into account evidence from the two well-conducted, multi-city, shortterm exposure studies reporting serious effects with long-term mean concentrations 

below 13 μg/m3noted above (Burnett et al, 2004; Bell et al., 2008). Such a standard level 

would also place essentially no weight onnot appropriately take into account additional 

population-level data from a limited number of epidemiological studies. This approach 

would ignore CASAC’s advice to consider such information in order to better understand 

 

122 The Administrator is mindful that, in reviewing the 2006 final PM NAAQS decisions, 

the D.C. Circuit Court of Appeals concluded that the EPA failed to adequately explain 

why that annual standard provided requisite protection from effects associated with both 

long- and short-term exposures or from morbidity effects in children and other at-risk 

populations when long-term means of important short-term studies were below the level 

the Administrator selected for the annual standard. See American Farm Bureau v. EPA.

559 F. 3d 512, 524-26. There is no reasonable basis to discount these two studies for 

purposes of setting the level of the annual standard.

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the concentrations that were most influential in generatingover which there is a high 

degree of confidence regarding the health effect estimatesmagnitude and significance of 

the associations observed in individual epidemiological studies and where there is 

appreciably less confidence. 

Furthermore, a standard level of 13 μg/m3or higher would place no weight onnot 

appropriately take into account the more limited evidence of effects in some at-risk 

populations (e.g., low birth weight). In the Administrator’s view, placing no weight ona 

standard set at this informationlevel would fail tonot provide protection with an adequate 

margin of safety, including providing protection for at-risk populations. The 

Administrator is mindful that the CAA requirement that primary standards provide an 

adequate margin of safety, discussed in section II.A above, was intended to address 

uncertainties associated with inconclusive scientific and technical information available 

at the time of standard setting as well as to provide a reasonable degree of protection 

against hazards that research has not yet identified. 

In light of the entire body of evidence as discussed above, the Administrator 

judges that an annual standard level set above 12 μg/m3 would not be sufficient to protect 

public health with an adequate margin of safety from the serious health effects associated 

with long- and short-term exposure to PM2.5.

The Administrator also recognizes that a second group of commenters supported a 

lower annual standard level (e.g., no higher than 11 μg/m3). Such a standard level would 

reflect placing essentially as much weight on the relatively more limited data providing 

evidence suggestive of a causal relationship for effects observed in some at-risk 

populations (e.g., low birth weight) as on more certain evidence of effects classified as 
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having a causal or likely causal relationship with PM2.5 exposures. In the Administrator’s 

view, while it is important to place some weight on such suggestive evidence, it would 

not be appropriate to place as much weight on it as the commenters would do. 

An annual standard level of 11 μg/m3 would also reflect these commenters’ 

judgment that it is appropriate to focus on a lower part of the distributions of health event 

data from the small number of epidemiological studies for which this information was 

made available than the Administrator believes is warranted. In the Administrator’s view, 

using this type of information to set a standard level of 11 μg/m3or below would assume 

too high a degree of confidence in the magnitude and significance of the associations 

observed in the lower part of the distributions of health events observed in these studies. 

Given the uncertainties in the evidence and the limited set of studies for which the EPA 

has information on the distribution of health event data and corresponding air quality 

data, the Administrator believes it is not appropriate to focus on the lower part of the 

distributions of health events data. 

On balance, the Administrator finds that the available evidence interpreted in light 

of the remaining uncertainties does not justify a standard level set below 12 μg/m3as 

necessary to protect public health with an adequate margin of safety. 

After carefully considering the above considerations and the public comments 

summarized in section III.E.4.c above, the Administrator has decided to set the level of 

the primary annual PM2.5 standard at 12 μg/m3. In her judgment, a standard set at this 

level provides the requisite degree of public health protection, including the health of atrisk populations, with an adequate margin of safety and is neither more nor less stringent 

than necessary for this purpose.
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As discussed above, the Administrator concludes that an approach that focuses on 

setting a generally controlling annual standard is the most effective and efficient way to 

reduce total population risk associated with both long- and short-term PM2.5 exposures. 

Such an approach would result in more uniform protection across the U.S. than the 

alternative of setting the levels of the 24-hour and annual standard such that the 24-hour 

standard would generally be the controlling standard in areas across the country (see 

section III.A.3). 

The Administrator recognizes that potential air quality changes associated with 

meeting an annual standard level of 12.μg/m3 will result in lowering risks associated with 

both long- and short-term PM2.5 exposures by lowering the overall air quality 

distribution. However, the Administrator recognizes that such an annual standard alone 

would not be expected to offer sufficient protection with an adequate margin of safety 

against the effects of all short-term PM2.5 exposures. in all parts of the country. As a 

result, in conjunction with an annual standard level of 12 μg/m3, the Administrator 

concludes that it is appropriate to continue to provide supplemental protection by means 

of a 24-hour standard set at the appropriate level, particularly for areas with high peak-tomean ratios possibly associated with strong local or seasonal sources and for areas with 

PM2.5-related effects that may be associated with shorter-than-daily exposure periods. 

In selecting the level of a 24-hour standard meant to provide such supplemental 

protection, the Administrator relies upon evidence and air quality information from key 

short-term exposure studies. In considering these studies, the Administrator notes that to 

the extent air quality distributions in the study areas considered are reduced to meet the 

current 24-hour standard (at a level of 35 μg/m3) or to meet the revised annual standard 
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discussed above (at a level of 12 μg/m3), additional protection would be anticipated 

against the effects observed in these studies. In light of this, when selecting the 

appropriate level for the 24-hour standard, the Administrator considers both the 98th

percentiles of 24-hour PM2.5 concentrations and the long-term mean PM2.5 concentrations 

in the locations of the short-term exposure studies. She notes that such consideration of 

both short- and long-term PM2.5 concentrations can inform her decision on the extent to 

which a given 24-hour standard, in combination with the revised annual standard 

established in this notice, would provide protection against the health effects reported in 

short-term studies. 

As discussed in section III.E.4.a above, the Administrator concludes that multicity short-term exposure studies provide the strongest data set for informing her decisions 

on appropriate 24-hour standard levels. With regard to the limited number of single-city 

studies that reported positive and statistically significant associations for a range of health 

endpoints related to short-term PM2.5 concentrations in areas that would likely have met 

the current suite of PM2.5 standards, the Administrator recognizes that many of these 

studies had significant limitations (e.g., limited statistical power, limited exposure data) 

or equivocal results (mixed results within the same study area) that make them unsuitable 

to form the basis for setting the level of a 24-hour standard. 

With regard to multi-city studies that evaluated effects associated with short-term 

PM2.5 exposures, the Administrator observes an overall pattern of positive and 

statistically significant associations in studies with 98th percentile 24-hour values 

averaged across study areas within the range of 45.8 to 34.2 μg/m3(Burnett et al., 2004; 

Zanobetti and Schwartz, 2009; Bell et al., 2008; Dominici et al., 2006a, Burnett and 
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Goldberg, 2003; Franklin et al., 2008). The Administrator notes that, to the extent air 

quality distributions are reduced to reflect just meeting the current 24-hour standard, 

additional protection would be provided for the effects observed in the three multi-city 

studies with 98th percentile values greater than 35 μg/m3(Burnett et al., 2004; Burnett 

and Goldberg, 2003; Franklin et al., 2008). In the three additional multi-city studies with 

98th percentile values below 35 μg/m3, specifically 98th percentile concentrations of 34.2, 

34.3, and 34.8 μg/m3, the Administrator notes that these studies reported long-term mean 

PM2.5 concentrations of 12.9, 13.2, and 13.4 μg/m3, respectively (Bell et al., 2008; 

Zanobetti and Schwartz, 2009; Dominici et al., 2006a). In revising the level of the annual 

standard to 12 μg/m3, as discussed above, the Administrator recognizes that additional 

protection would be provided for the short-term effects observed in these multi-city 

studies such that revision to the 24-hour standard would not be warranted. That is, by 

lowering the level of the annual standard to 12 μg/m3, the 98th percentile of the 

distribution would be lowered as well such that additional protection from effects 

associated with short-term exposures would be afforded. Therefore, the epidemiological 

evidence supports a conclusion that it is appropriate to retain the level of the 24-hour 

standard at 35 μg/m3, in conjunction with a revised annual standard level of 12 μg/m3. 

In addition to considering the epidemiological evidence, the Administrator also 

has taken into account air quality information based on county-level 24-hour and annual 

design values to understand the implications of revising the annual standard level from 15 

to 12 μg/m3in conjunction with retaining the 24-hour standard level at 35 μg/m3. She has 

considered this information to evaluate the public health protection provided by the two 

standards in combination and to evaluate the most appropriate means of developing a 
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suite of standards providing requisite public health protection with an adequate margin of 

safety. 

In considering the air quality information, the Administrator observes that a suite 

of PM2.5 standards that includes an annual standard level of 12 μg/m3and a 24-hour 

standard level of 35 μg/m3 would result in the annual standard as the generally 

controlling standard in most regions across the country, except for certain areas in the 

Northwest, where the annual mean PM2.5 concentrations have historically been low but 

where relatively high 24-hour concentrations occur, often related to seasonal wood smoke 

emissions (U.S. EPA, 2011a, pp. 2-89 to 2-91, Figure 2-10). In fact, these are the type of 

areas for which the supplemental protection afforded by the 24-hour standard is intended, 

such that the two standards together provide the requisite degree of protection. The 

Administrator concludes the current 24-hour standard at a level of 35 μg/m3, in 

conjunction with a revised annual standard level of 12 μg/m3, will provide appropriate 

protection from effects observed in studies in such areas in which the long-term mean 

concentrations were below 12 μg/m3and the 98th percentile 24-hour concentrations were 

above 35 μg/m3(e.g., areas in the Northwest U.S.). 

After carefully taking the public comments and above considerations into 

account, the Administrator has decided to retain the current level of the primary PM2.5

24-hour standard at 35 μg/m3in conjunction with revising the annual standard level from 

15.0 μg/m3to 12.0 μg/m3.

123 In the Administrator’s judgment, this suite of primary PM2.5

standards and the rationale supporting these levels appropriately reflects consideration of 

the strength of the available evidence and other information and its associated 

 

123As noted in section II.B.1, Table 1 and section III.E.4.a above, the annual standard 

level is defined to one decimal place. Throughout this section, the annual standard levels 

discussed have been denoted as integer values (e.g., 12 μg/m3) for simplicity.

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uncertainties as well as the advice of CASAC and consideration of public comments.In 

the Administrator’s judgment, this suite of primary PM2.5 standards is sufficient but not 

more protective than necessary to protect the public health, including at-risk populations, 

with an adequate margin of safety from effects associated with long- and short-term 

exposures to fine particles. This suite of standards will provide significant protection 

from serious health effects including premature mortality and cardiovascular and 

respiratory morbidity effects that are causally or likely causally related to long- and shortterm PM2.5 exposures. These standards will also provide an appropriate degree of 

protection against other health effects for which there is more limited evidence of effects 

and causality, such as reproductive and developmental effects. This judgment by the 

Administrator appropriately considers the requirement for a standard that is requisite to 

protect public health but is neither more nor less stringent than necessary.124

D. Administrator’s Final Decisions on Primary PM2.5 Standards

For the reasons discussed above, and taking into account the information and 

assessments presented in the Integrated Science Assessment, Risk Assessment, and 

Policy Assessment, the advice and recommendations of CASAC, and public comments to 

date, the Administrator revises the current suite of primary PM2.5 standards. Specifically, 

the Administrator revises: (1) the level of the primary annual PM2.5 standard to 12.0 

μg/m3and (2) the form of the primary annual PM2.5 standard to one based on the highest 

appropriate area-wide monitor in an area, with no option for spatial averaging. In 

conjunction with revising the primary annual PM2.5 standard to provide protection from 

effects associated with long- and short-term PM2.5 exposures, the Administrator retains 

 

124 The Administrator also judges that this suite of standards addresses the issues raised 

by the D.C. Circuit’s remand of the 2006 primary annual PM2.5 standard by appropriately 

revising that standard.

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the level of 35 μg/m3and the 98th percentile form of the primary 24-hour PM2.5 standard 

to continue to provide supplemental protection for areas with high peak PM2.5

concentrations. The Administrator is not revising the current PM2.5 indicator or the 

annual and 24-hour averaging times for the primary PM2.5 standards. The Administrator 

concludes that this suite of standards would be requisite to protect public health with an 

adequate margin of safety against health effects potentially associated with long- and 

short-term PM2.5 exposures.

IV. Rationale for Final Decision on Primary PM10 Standard

This section presents the rationale for the Administrator’s final decision to retain 

the current 24-hour primary PM10 standard in order to continue to provide public health

protection against short-term exposures to inhalable particles in the size range of 2.5 to 

10 μm (i.e., PM10-2.5 or thoracic coarse particles). These are particles capable of reaching 

the most sensitive areas of the lung, including the trachea, bronchi, and deep lungs. The 

current standard uses PM10 as the indicator for thoracic coarse particles, and thus is 

referred to as a PM10 standard.125

As discussed more fully in the proposal and below, this rationale is based on a 

thorough review of the latest scientific evidence, published through mid-2009 and 

assessed in the Integrated Science Assessment (U.S. EPA, 2009a), evaluating human 

health effects associated with long- and short-term exposures to thoracic coarse particles. 

The Administrator’s final decision also takes into account: (1) the EPA staff analyses of 

air quality information and health evidence and staff conclusions regarding the current 

and potential alternative standards, as presented in the Policy Assessment for the PM 

 

125Throughout this section of the preamble, we are using the terms ‘thoracic coarse 

particles’, ‘inhalable coarse particles’, and ‘PM10-2.5’ synonymously.
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NAAQS (U.S. EPA, 20112011a); (2) CASAC advice and recommendations, as reflected 

in discussions at public meetings of drafts of the Integrated Science Assessment and 

Policy Assessment, and in CASAC’s letters to the Administrator; (3) the multiple rounds 

of public comments received during the development of the Integrated Science 

Assessment and Policy Assessment, both in connection with CASAC meetings and

separately; and (4) public comments (including testimony at the public hearings) received 

on the proposal. 

In presenting the rationale for the final decision to retain the current primary PM10

standard, this section discusses the EPA’s past reviews of the PM NAAQS and the 

general approach taken to review the current standard (section IV.A), the health effects 

associated with exposures to ambient PM10-2.5 (section IV.B), the consideration of the 

current and potential alternative standards in the Policy Assessment (section IV.C), 

CASAC recommendations regarding the current and potential alternative standards 

(section IV.D), the Administrator’s proposed decision to retain the current primary PM10

standard (section IV.E), public comments received in response to the Administrator’s 

proposed decision (section IV.F), and the Administrator’s final decision to retain the 

current primary PM10 standard (section IV.G). 

A. Background

The following sections discuss previous reviews of the PM NAAQS (section 

IV.A.1), the litigation of the EPA’s 2006 decision on the PM10 standards (section 

IV.A.2), and the general approach taken to review the primary PM10 standard in the 

current review (section IV.A.3). 

1. Previous Reviews of the PM NAAQS
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a. Reviews Completed in 1987 and 1997

The PM NAAQS have always included some type of a primary standard to 

protect against effects associated with exposures to thoracic coarse particles. In 1987, 

when the EPA first revised the PM NAAQS, the EPA changed the indicator for PM from 

TSP to focus on inhalable particles, those which can penetrate into the trachea, bronchi, 

and deep lungs (52 FR 24634, July 1, 1987). In that review, the EPA changed the PM 

indicator to PM10 based on evidence that the risk of adverse health effects associated with 

particles with a nominal mean aerodynamic diameter less than or equal to 10 μm was 

significantly greater than risks associated with larger particles (52 FR 24639, July 1, 

1987). 

In the 1997 review, in conjunction with establishing new fine particle (i.e., PM2.5)

standards (discussed above in sections II.B.1 and III.A.1), the EPA concluded that 

continued protection was warranted against potential effects associated with thoracic 

coarse particles in the size range of 2.5 to 10 μm. This conclusion was based on particle 

dosimetry, toxicological information, and on limited epidemiological evidence from 

studies that measured PM10 in areas where the coarse fraction was likely to dominate 

PM10 mass (62 FR 38677, July 18, 1997). The EPA concluded there that a PM10 standard 

could provide requisite protection against effects associated with particles in the size 

range of 2.5 to 10 μm.126 Although the EPA considered a more narrowly defined 

indicator for thoracic coarse particles in that review (i.e., PM10-2.5), the EPA concluded 

that it was more appropriate, based on existing evidence, to continue to use PM10 as the 

indicator. This decision was based, in part, on the recognition that the only studies of 

 

126With regard to the 24-hour PM10 standard, the EPA retained the indicator, averaging 

time, and level (150 μg/m3), but revised the form (i.e., from one-expected-exceedance to 

the 99th percentile). 
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clear quantitative relevance to health effects most likely associated with thoracic coarse 

particles used PM10. These were two studies conducted in areas where the coarse fraction 

was the dominant fraction of PM10, and which substantially exceeded the 24-hour PM10

standard (62 FR 38679). In addition, there were only very limited ambient air quality data 

then available specifically for PM10-2.5, in contrast to the extensive monitoring network 

already in place for PM10. Therefore, the EPA considered it more administratively 

feasible to use PM10 as an indicator. The EPA also stated that the PM10 standards would 

work in conjunction with the PM2.5 standards by regulating the portion of particulate 

pollution not regulated by the then newly adopted PM2.5 standards. 

In May 1998, a three-judge panel of the U.S. Court of Appeals for the District of 

Columbia Circuit found "ample support" for the EPA's decision to regulate coarse 

particle pollution, but vacated the 1997 PM10 standards, concluding that the EPA had 

failed to adequately explain its choice of PM10 as the indicator for thoracic coarse 

particles American Trucking Associations v. EPA, 175 F. 3d 1027, 1054-56 (D.C. Cir. 

1999). In particular, the court held that the EPA had not explained the use of an indicator 

under which the allowable level of coarse particles varied according to the amount of 

PM2.5 present, and which, moreover, potentially double regulated PM2.5. The court also 

rejected considerations of administrative feasibility as justification for use of PM10 as the 

indicator for thoracic coarse PM, since NAAQS (and their elements) are to be based 

exclusively on health and welfare considerations. Id. at 1054. Pursuant to the court’s 

decision, the EPA removed the vacated 1997 PM10 standards from the CFR (69 FR 

45592, July 30, 2004) and deleted the regulatory provision (at 40 CFR 50.6(d)) that 

controlled the transition from the pre-existing 1987 PM10 standards to the 1997 PM10
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standards (65 FR 80776, December 22, 2000). The pre-existing 1987 PM10 standards thus 

remained in place. Id. at 80777. 

b. Review Completed in 2006

In the review of the PM NAAQS that concluded in 2006, the EPA considered the 

growing, but still limited, body of evidence supporting associations between health 

effects and thoracic coarse particles measured as PM10-2.5.

127 The new studies available in 

the 2006 review included epidemiological studies that reported associations with health 

effects using direct measurements of PM10-2.5, as well as dosimetric and toxicological 

studies. In considering this growing body of PM10-2.5 evidence, as well as evidence from 

studies that measured PM10 in locations where the majority of PM10 was in the PM10-2.5

fraction (U.S. EPA, 2005, section 5.4.1), staff concluded that the level of protection 

afforded by the existing 1987 PM10 standard remained appropriate (U.S. EPA, 2005, p. 5-

67) but recommended that the indicator for the standard be revised. Specifically, staff 

recommended replacing the PM10 indicator with an indicator of urban thoracic coarse 

particles in the size range of 10-2.5 μm (U.S. EPA, 2005, pp. 5-70 to 5-71). The agency 

proposed to retain a standard for a subset of thoracic coarse particles, proposing a 

qualified PM10-2.5 indicator to focus on the mix of thoracic coarse particles generally 

present in urban environments. More specifically, the proposed revised thoracic coarse 

particle standard would have applied only to an ambient mix of PM10-2.5 dominated by 

resuspended dust from high-density traffic on paved roads and/or by industrial and 

construction sources. The proposed revised standard would not have applied to any 

 

127The PM Staff Paper (U.S. EPA, 2005) also presented results of a quantitative 

assessment of health risks for PM10-2.5. However, staff concluded that the nature and 

magnitude of the uncertainties and concerns associated with this risk assessment weighed 

against its use as a basis for recommending specific levels for a thoracic coarse particle 

standard (U.S. EPA, 2005, p. 5-69). 
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ambient mix of PM10-2.5 dominated by rural windblown dust and soils. In addition, 

agricultural sources, mining sources, and other similar sources of crustal material would 

not have been subject to control in meeting the standard (71 FR 2667 to 2668, January 

17, 2006). 

The Agency received a large number of comments overwhelmingly and 

persuasively opposed to the proposed qualified PM10-2.5 indicator (71 FR 61188 to 61197, 

October 17, 2006). After careful consideration of the scientific evidence and the 

recommendations contained in the 2005 Staff Paper, the advice and recommendations 

from CASAC, and the public comments received regarding the appropriate indicator for 

coarse particles, and after extensive evaluation of the alternatives available to the 

Agency, the Administrator decided it would not be appropriate to adopt the proposed 

qualified PM10-2.5 indicator, or any qualified indicator. Underlying this determination was 

the Administrator’s decision that it was requisite to provide protection from exposure to 

all thoracic coarse PM, regardless of its origin. The Administrator thus rejected 

arguments that there are no health effects from community-level exposures to coarse PM 

in non-urban areas (71 FR 61189). The EPA concluded that dosimetric, toxicological, 

occupational and epidemiological evidence supported retention of a primary standard for 

short-term exposures that included all thoracic coarse particles (i.e., particles of both 

urban and non-urban origin), consistent with the Act’s requirement that primary NAAQS 

must be requisite to protect the public health and provide an adequate margin of safety. 

At the same time, the Agency concluded that the standard should target protection toward 

urban areas, where the evidence of health effects from exposure to PM10-2.5 was strongest 

(71 FR at 61193, 61197). The proposed indicator was not suitable for that purpose. Not 
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only did it inappropriately provide no protection at all to many areas, but it failed to 

identify many areas where the ambient particle mix was dominated by coarse particles 

contaminated with urban/industrial types of coarse particles for which evidence of health 

effects was strongest (71 FR 61193). 

The Agency ultimately concluded that the existing indicator, PM10, was most 

consistent with the evidence. Although PM10 includes both coarse and fine PM, the 

Agency concluded that it remained an appropriate indicator for thoracic coarse particles 

because, as discussed in the PM Staff Paper (U.S. EPA, 2005, p. 2–54, Figures 2–23 and 

2– 24), fine particle levels are generally higher in urban areas and, therefore, a PM10

standard set at a single unvarying level will generally result in lower allowable 

concentrations of thoracic coarse particles in urban areas than in non-urban areas (71 FR 

61195 96). The EPA considered this to be an appropriate targeting of protection given 

that the strongest evidence for effects associated with thoracic coarse particles came from 

epidemiological studies conducted in urban areas and that elevated fine particle 

concentrations in urban areas could result in increased contamination of coarse fraction 

particles by PM2.5, potentially increasing the toxicity of thoracic coarse particles in urban 

areas (id.). Given the evidence that the existing (i.e. 1987) PM10 standard was established 

at a level and form which afforded requisite protection with an adequate margin of safety, 

the Agency retained the level and form of the 24-hour PM10 standard.128

The Agency also revoked the annual PM10 standard, in light of the conclusion in 

 

128Thus, the standard is met when a 24-hour average PM10 concentration of 150 μg/m3

is 

not exceeded more than one day per year, on average over a three-year period. As noted 

above, the 1987 PM10 standard was not adopted solely to control thoracic coarse 

particles. However, when reviewing this standard in the 2006 review, EPA determined 

that the level and form of the standard being reviewed (i.e., the 1987 PM10 standard) 

provided requisite protection with an adequate margin of safety from short-term 

exposures to thoracic coarse particles. 
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the PM Criteria Document (U.S. EPA, 2004, p. 9-79) that the available evidence does not 

suggest an association with long-term exposure to PM10-2.5 and the conclusion in the Staff 

Paper (U.S. EPA, 2005, p. 5-61) that there is no quantitative evidence that directly 

supports retention of an annual standard. This decision was consistent with CASAC 

advice and recommendations (Henderson, 20052005a,b). 

In the same rulemaking, the EPA also included a new FRM for the measurement 

of PM10-2.5 in the ambient air (71 FR 61212 to 61213, October 17, 2006). Although the 

standard for thoracic coarse particles does not use a PM10-2.5 indicator, the new FRM for 

PM10-2.5 was established to provide a basis for approving FEMs and to promote the 

gathering of scientific data to support future reviews of the PM NAAQS (71 FR 61202/3, 

October 17, 2006).129

2. Litigation Related to the 2006 Primary PM10 Standards

A number of groups filed suit in response to the final decisions made in the 2006 

review. See American Farm Bureau Federation v. EPA, 559 F. 3d 512 (DC Cir. 2009). 

Among the petitions for review were challenges from industry groups on the decision to 

retain the PM10 indicator and the level of the PM10 standard and from environmental and 

public health groups on the decision to revoke the annual PM10 standard. The court 

upheld both the decision to retain the 24-hour PM10 standard and the decision to revoke 

the annual standard. 

First, the court upheld the EPA’s decision for a standard to encompass all thoracic 

coarse PM, both of urban and non-urban origin. The court rejected arguments that the 

 

129As noted below, however, with this notice the EPA is revoking the requirement for 

PM10-2.5 speciation at NCore monitoring sites due to technical issues related to the 

development of appropriate monitoring methods (section VIII.B.3.c). The requirement 

for PM10-2.5 mass measurements at NCore sites is being retained.
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evidence showed there are no risks from exposure to non-urban coarse PM. The court 

further found that the EPA had a reasonable basis not to set separate standards for urban 

and non-urban coarse PM, namely the inability to reasonably define what ambient mixes 

would be included under either ‘urban’ or ‘non-urban;’ and the evidence in the record 

that supported the EPA’s appropriately cautious decision to provide “some protection 

from exposure to thoracic coarse particles... in all areas.” 559 F. 3d at 532-33. 

Specifically, the court stated, 

Although the evidence of danger from coarse PM is, as EPA recognizes, 

“inconclusive,” (71 FR 61193, October 17, 2006), the agency need not wait for 

conclusive findings before regulating a pollutant it reasonably believes may pose 

a significant risk to public health. The evidence in the record supports the EPA’s 

cautious decision that “some protection from exposure to thoracic coarse particles 

is warranted in all areas.” Id. As the court has consistently reaffirmed, the CAA 

permits the Administrator to “err on the side of caution” in setting NAAQS. 559 

F. 3d at 533.

The court also upheld the EPA’s decision to retain the level of the standard at 150 

μg/m3 and to use PM10 as the indicator for thoracic coarse particles. In upholding the 

level of the standard, the court referred to the conclusion in the Staff Paper that there is 

“little basis for concluding that the degree of protection afforded by the current PM10

standards in urban areas is greater than warranted, since potential mortality effects have 

been associated with air quality levels not allowed by the current 24-hour standard, but 

have not been associated with air quality levels that would generally meet that standard, 

and morbidity effects have been associated with air quality levels that exceeded the 

current 24-hour standard only a few times.” 559 F. 3d at 534. The court also rejected 

arguments that a PM10 standard established at an unvarying level will result in arbitrarily 

varying levels of protection given that the level of coarse PM would vary based on the 

amount of fine PM present. The court agreed that the variation in allowable coarse PM 
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was in accord with the strength of the evidence: typically less coarse PM would be 

allowed in urban areas (where levels of fine PM are typically higher), in accord with the 

strongest evidence of health effects from coarse particles. 559 F. 3d at 535-36. In 

addition, such regulation would not impermissibly double regulate fine particles, since 

any additional control of fine particles (beyond that afforded by the primary PM2.5

standard) would be for a different purpose: to prevent contamination of coarse particles 

by fine particles. 559 F. 3d at 535, 536. These same explanations justified the choice of 

PM10 as an indicator and provided the reasoned explanation for that choice lacking in the 

record for the 1997 standard. 559 F. 3d at 536. 

With regard to the challenge from environmental and public health groups, the 

court upheld the EPA’s decision to revoke the annual PM10 standard. The court rejected 

the argument that the EPA is required by law to have an annual PM10 standard, holding 

that section 109 (d)(1) of the Act allows the EPA to revoke a standard no longer 

warranted by the current scientific understanding. 559 F. 3d at 538. The court further held 

that the EPA’s decision to revoke the annual standard was supported by the science: 

The EPA reasonably decided that an annual coarse PM standard is not necessary 

because, as the Criteria Document and the Staff Paper make clear, the latest 

scientific data do not indicate that long-term exposure to coarse particles poses a 

health risk. The CASAC also agreed that an annual coarse PM standard is 

unnecessary. 559 F. 3d at 538-39.

3. General Approach Used in the Current Review 

The approach taken to considering the existing and potential alternative primary 

PM10 standards in the current review builds upon the approaches used in previous PM 

NAAQS reviews. This approach is based most fundamentally on using information from 

epidemiological studies and air quality analyses to inform the identification of a range of 
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policy options for consideration by the Administrator. The Administrator considers the 

appropriateness of the current and potential alternative standards, taking into account the 

four elements of the NAAQS: indicator, averaging time, form, and level.

Evidence-based approaches to using information from epidemiological studies to 

inform decisions on PM standards are complicated by the recognition that no population 

threshold, below which it can be concluded with confidence that PM-related effects do 

not occur, can be discerned from the available evidence (U.S. EPA, 2009a, sections 2.4.3

and 6.5.2.7).

130 As a result, any approach to reaching decisions on what standards are 

appropriate requires judgments about how to translate the information available from the 

epidemiological studies into a basis for appropriate standards, which includes 

consideration of how to weigh the uncertainties in reported associations across the 

distributions of PM concentrations in the studies. The approach taken to informing these 

decisions in the current review recognizes that the available health effects evidence 

reflects a continuum consisting of ambient levels at which scientists generally agree that 

health effects are likely to occur through lower levels at which the likelihood and 

magnitude of the response become increasingly uncertain. Such an approach is consistent 

with setting standards that are neither more nor less stringent than necessary, recognizing 

that a zero-risk standard is not required by the CAA.

Because the purpose of the PM10 standard is to protect against exposures to 

PM10-2.5, it is most appropriate to focus on PM10-2.5 health studies when considering the 

degree of public health protection provided by the current PM10 standard. Compared to 

health studies of PM10, studies that evaluate associations with PM10-2.5 provide clearer 

 

130Studies that have characterized the concentration-response relationships for PM 

exposures have evaluated PM10, which includes both coarse and fine particles, and PM2.5

(U.S. EPA, 2009a, sections 2.4.3 and 6.5.2.7). 
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evidence for health effects following exposures to thoracic coarse particles. In contrast, it 

is difficult to interpret PM10 studies within the context of a standard meant to protect 

against exposures to PM10-2.5 because PM10 is comprised of both fine and coarse particles, 

even in locations with the highest concentrations of PM10-2.5 (U.S. EPA, 2011a, Figure 3-

4). Therefore, the extent to which PM10 effect estimates reflect associations with PM10-2.5

versus PM2.5 can be highly uncertain. In light of this uncertainty, it is preferable to 

consider PM10-2.5 studies when such studies are available. Given the availability in this 

review of a number of studies that evaluated associations with PM10-2.5, and given that the 

Integrated Science Assessment weight-of-evidence conclusions for thoracic coarse 

particles were based on studies of PM10-2.5, in this review the EPA focuses primarily on 

studies that have specifically evaluated PM10-2.5.

131

 

As discussed in more detail in the Risk Assessment (U.S. EPA, 2010a, Appendix 

H), the EPA did not conduct a quantitative assessment of health risks associated with 

PM10-2.5. The Risk Assessment concluded that limitations in the monitoring network and 

in the health studies that rely on that monitoring network, which would be the basis for 

estimating PM10-2.5 health risks, would introduce significant uncertainty into a PM10-2.5

risk assessment such that the risk estimates generated would be of limited value in 

informing review of the standard. Therefore, it was judged that a quantitative assessment 

of PM10-2.5 risks is not supportable at this time (U.S. EPA, 2010a, p. 2-6). This decision 

does not indicate that health effects are not associated with exposure to thoracic coarse 

particles. Rather, as noted above, it reflects the conclusion that limitations in the 

available health studies and air quality information would introduce significant 

 

131It should also be noted that CASAC endorsed the approach adopted in the Integrated 

Science Assessment, which draws weight-of-evidence conclusions for PM2.5 and PM10-2.5, 

but not for PM10 (Samet, 2009f). 
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uncertainty into a quantitative assessment of PM10-2.5 risks such that the risk estimates 

generated would be of limited value in informing review of the standard. 

B. Health Effects Related to Exposure to Thoracic Coarse Particles

This section briefly outlines the key information presented in section IV.B of the 

proposal (77 FR 38947 to 38951, June 29, 2012), and discussed more fully in the 

Integrated Science Assessment (U.S. EPA, 2009a, Chapters 2, 4, 5, 6, 7, and 8) and the 

Policy Assessment (U.S. EPA, 20112011a, Chapter 3), related to health effects associated 

with thoracic coarse particle exposures. In looking across the new scientific evidence 

available in this review, our overall understanding of health effects associated with 

thoracic coarse particle exposures has been expanded, though important uncertainties 

remain. Some highlights of the key policy-relevant scientific evidence available in this 

review include the following:

(1) A number of multi-city and single-city epidemiological studies have evaluated 

associations between short-term PM10-2.5 and mortality, cardiovascular effects 

(e.g., including hospital admissions and emergency department visits), and/or 

respiratory effects. Despite differences in the approaches used to estimate ambient 

PM10-2.5 concentrations, the majority of these studies have reported positive, 

though often not statistically significant, associations with short-term PM10-2.5

concentrations. Most PM10-2.5 effect estimates remained positive in co-pollutant 

models that included either gaseous or particulate co-pollutants. In U.S. study 

locations likely to have met the current PM10 standard during the study period, a 

few PM10-2.5 effect estimates were statistically significant and remained so in copollutant models.

132

(2) A small number of controlled human exposure studies have reported alterations in 

heart rate variability or increased pulmonary inflammation following short-term 

exposure to PM10-2.5, providing some support for the associations reported in 

 

132The statistical significance of effect estimates provides important information on their 

statistical precision. However, when a group of studies report effect estimates that are 

similar in direction and magnitude, such a pattern of results warrants consideration of 

those studies even if not all reported statistically significant associations in single- or copollutant models (section III.D.2, above). In considering the PM10-2.5 epidemiologic 

studies below, the Administrator considers both the pattern of results across studies and 

the statistical significance of those results. 
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epidemiological studies. Toxicological studies that have examined the effects of 

PM10-2.5 have used intratracheal instillation and, because these studies do not 

directly mirror any real-world mode of exposure, they provide only limited 

evidence for the biological plausibility of PM10-2.5-induced effects.

(3) Using a more formal framework for reaching causal determinations than used in 

previous reviews, the Integrated Science Assessment concluded that the existing 

evidence is “suggestive” of a causal relationship between short-term PM10-2.5 

exposures and mortality, cardiovascular effects, and respiratory effects (U.S. 

EPA, 2009a, section 2.3.3).133 In contrast, the Integrated Science Assessment 

concluded that available evidence is “inadequate” to infer a causal relationship 

between long-term PM10-2.5 exposures and various health effects.

(4) There are several at-risk populations that may be especially susceptible or 

vulnerable to PM-related effects, including effects associated with exposures to 

coarse particles. These groups include those with preexisting heart and lung 

diseases, specific genetic differences, and lower socioeconomic status as well as 

the lifestages of childhood and older adulthood. Evidence for PM-related effects 

in these at-risk populations has expanded and is stronger than previously 

observed. There is emerging, though still limited, evidence for additional 

potentially at-risk populations, such as those with diabetes, people who are obese, 

pregnant women, and the developing fetus. 

(5) The Integrated Science Assessment concludes that currently available evidence is 

insufficient to draw distinctions in particle toxicity based on composition and 

notes that recent studies have reported that PM (both PM2.5 and PM10-2.5) from a 

variety of sources, including sources likely to be present in urban and non-urban 

locations, is associated with adverse health effects. 

Although new PM10-2.5 scientific studies have become available since the last 

review and have expanded our understanding of the association between PM10-2.5 and 

adverse health effects (see above and U.S. EPA, 2009a, Chapter 6), important 

 

133The causal framework draws upon the assessment and integration of evidence from 

across epidemiological, controlled human exposure, and toxicological studies, and the 

related uncertainties that ultimately influence our understanding of the evidence. This 

framework employs a five-level hierarchy that classifies the overall weight-of-evidence 

using the following categorizations: causal relationship, likely to be causal relationship, 

suggestive of a causal relationship, inadequate to infer a causal relationship, and not 

likely to be a causal relationship (U.S. EPA, 2009a, Table 1-3). In the case of a 

“suggestive” determination, “the evidence is suggestive of a causal relationship with 

relevant pollutant exposures, but is limited because chance, bias and confounding cannot 

be ruled out. For example, at least one high-quality epidemiologic study shows an 

association with a given health outcome but the results of other studies are inconsistent” 

(U.S. EPA, 2009a, Table 1-3). 
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uncertainties remain. These uncertainties, and their implications for interpreting the 

scientific evidence, include the following: 

(1) The potential for confounding by co-occurring pollutants, especially PM2.5, has 

been addressed with co-pollutant models in only a relatively small number of 

PM10-2.5 epidemiological studies (U.S. EPA, 2009a, section 2.3.3). This is a 

particularly important limitation given the relatively small body of experimental 

evidence (i.e., controlled human exposure and animal toxicological studies) 

available to support the associations between PM10-2.5 and adverse health effects. 

The net impact of such limitations is to increase uncertainty in characterizations 

of the extent to which PM10-2.5 itself, rather than one or more co-occurring 

pollutants, is responsible for the mortality and morbidity effects reported in 

epidemiological studies.

(2) There is greater spatial variability in PM10-2.5 concentrations than PM2.5

concentrations, resulting in increased exposure error for PM10-2.5 (U.S. EPA, 

2009a, p. 2-8). Available measurements do not provide sufficient information to 

adequately characterize the spatial distribution of PM10-2.5 concentrations (U.S. 

EPA, 2009a, section 3.5.1.1). The net effect of these uncertainties on PM10-2.5

epidemiological studies is to bias the results of such studies toward the null 

hypothesis. That is, as noted in the Integrated Science Assessment, these 

limitations in estimates of ambient PM10-2.5 concentrations “would tend to 

increase uncertainty and make it more difficult to detect effects of PM10-2.5 in 

epidemiologic studies” (U.S. EPA, 2009a, p. 2-21).

(3) Only a relatively small number of PM10-2.5 monitoring sites are currently 

operating and such sites have been in operation for a relatively short period of 

time, limiting the spatial and temporal coverage for routine measurement of PM10-

2.5 concentrations. Given these limitations in routine monitoring, epidemiological 

studies have employed different approaches for estimating PM10-2.5

concentrations. Given the relatively small number of PM10-2.5 monitoring sites, the 

relatively large spatial variability in ambient PM10-2.5 concentrations (see above), 

the use of different approaches to estimating ambient PM10-2.5 concentrations 

across epidemiological studies, and the limitations inherent in such estimates, the 

distributions of thoracic coarse particle concentrations over which reported health 

outcomes occur remain highly uncertain (U.S. EPA, 2009a, sections 2.2.3, 2.3.3, 

2.3.4, and 3.5.1.1). 

(4) There is relatively little information on the chemical and biological composition 

of PM10-2.5 and the effects associated with the various components (U.S. EPA, 

2009a, section 2.3.4). Without more information on the chemical speciation of 

PM10-2.5, the apparent variability in associations with health effects across 

locations is difficult to characterize (U.S. EPA, 2009a, section 6.5.2.3). 
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(5) One of the implications of the uncertainties and limitations discussed above is that 

the Risk Assessment concluded it would not be appropriate to conduct a 

quantitative assessment of health risks associated with PM10-2.5. The lack of a 

quantitative PM10-2.5 risk assessment in the current review adds to the uncertainty 

in any conclusions about the extent to which revision of the current PM10 standard 

would be expected to improve the protection of public health, beyond the 

protection provided by the current standard.134

C. Consideration of the Current and Potential Alternative Standards in the Policy 

Assessment

The following sections discuss the Policy Assessment’s consideration of the 

current and potential alternative standards to protect against exposures to thoracic coarse 

particles (U.S. EPA, 2011a, chapter 3). Section IV.C.1 discusses the consideration of the 

current standard while section IV.C.2 discusses the consideration of potential alternative 

standards in terms of the basic elements of a standard: indicator, averaging time, form, 

and level. 

1. Consideration of the Current Standard in the Policy Assessment

As discussed above the 24-hour PM10 standard is meant to protect the public 

health against exposures to thoracic coarse particles (i.e., PM10-2.5). In considering the 

adequacy of the current PM10 standard, the Policy Assessment considered the health 

effects evidence linking short-term PM10-2.5 exposures with mortality and morbidity (U.S. 

EPA, 2009a, chapters 2 and 6), the ambient PM10 concentrations in PM10-2.5 study 

locations (U.S. EPA, 2011a, section 3.2.1), the uncertainties and limitations associated 

with this health evidence (U.S. EPA, 2011a, section 3.2.1), and the consideration of these 

uncertainties and limitations as part of the weight of evidence conclusions in the 

Integrated Science Assessment (U.S. EPA, 2009a). 

 

134As noted above, the EPA’s decision not to conduct a quantitative risk assessment 

reflects uncertainty regarding the value of such an assessment, but does not indicate that 

health effects are not associated with exposure to thoracic coarse particles. 
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In considering the health evidence, air quality information, and associated 

uncertainties as they relate to the current PM10 standard, the Policy Assessment noted that 

a decision on the adequacy of the public health protection provided by that standard is a 

public health policy judgment in which the Administrator weighs the evidence and 

information, as well as its uncertainties. Therefore, depending on the emphasis placed on 

different aspects of the evidence, information, and uncertainties, consideration of 

different conclusions on the adequacy of the current standard could be supported. For 

example, the Policy Assessment noted that one approach to considering the evidence, 

information, and its associated uncertainties would be to place emphasis on the following 

(U.S. EPA, 2011a, section 3.2.3): 

(1) While most of PM10-2.5 effect estimates reported for mortality and morbidity were 

positive, many were not statistically significant, even in single-pollutant models. 

This includes effect estimates reported in study locations with PM10

concentrations above those allowed by the current 24-hour PM10 standard. 

(2) The number of epidemiological studies that have employed co-pollutant models to 

address the potential for confounding, particularly by PM2.5, remains limited. 

Therefore, the extent to which PM10-2.5 itself, rather than one or more copollutants, contributes to reported health effects remains uncertain. 

(3) Only a limited number of experimental studies provide support for the 

associations reported in epidemiological studies, resulting in further uncertainty 

regarding the plausibility of a causal link between PM10-2.5 and mortality and 

morbidity. 

(4) Limitations in PM10-2.5 monitoring and the different approaches used to estimate 

PM10-2.5 concentrations across epidemiological studies result in uncertainty as to 

the ambient PM10-2.5 concentrations at which the reported effects occur. 

(5) The chemical and biological composition of PM10-2.5, and the effects associated 

with the various components, remains uncertain. Without more information on the 

chemical speciation of PM10-2.5, the apparent variability in associations across 

locations is difficult to interpret. 

(6) In considering the available evidence and its associated uncertainties, the 

Integrated Science Assessment concluded that the evidence is “suggestive” of a 

causal relationship between short-term PM10-2.5 exposures and mortality, 

cardiovascular effects, and respiratory effects. These weight-of-evidence 

conclusions contrast with those for the relationships between PM2.5 exposures and 
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adverse health effects, which were judged in the Integrated Science Assessment to 

be either “causal” or “likely causal” for mortality, cardiovascular effects, and 

respiratory effects. 

The Policy Assessment concluded that, to the extent a decision on the adequacy of 

the current 24-hour PM10 standard were to place emphasis on the considerations noted 

above, it could be judged that, although it remains appropriate to maintain a standard to 

protect against short-term exposures to thoracic coarse particles, the available evidence 

suggests that the current 24-hour PM10 standard appropriately protects public health and 

provides an adequate margin of safety against effects that have been associated with 

PM10-2.5 exposures. Although such an approach to considering the adequacy of the current 

standard would recognize the positive, and in some cases statistically significant, 

associations between all types of PM10-2.5 and mortality and morbidity, it would place 

relatively greater emphasis on the limitations and uncertainties noted above, which tend 

to complicate the interpretation of that evidence. 

In addition, the Policy Assessment noted the judgment that, given the 

uncertainties and limitations in the PM10-2.5 health evidence and air quality information, it 

would not have been appropriate to conduct a quantitative assessment of health risks 

associated with PM10-2.5 (U.S. EPA, 2011a, p. 3-6; U.S. EPA, 2010a, pp. 2-6 to 2-7, 

Appendix H). As discussed above, the lack of a quantitative PM10-2.5 risk assessment adds 

to the uncertainty associated with any characterization of potential public health 

improvements that would be realized with a revised standard. 

The Policy Assessment also noted an alternative approach to considering the 

evidence and its uncertainties would place emphasis on the following (U.S. EPA, 2011a, 

section 3.2.3): 
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(1) Several multi-city epidemiological studies conducted in the U.S., Canada, and 

Europe, as well as a number of single-city studies, have reported generally 

positive, and in some cases statistically significant, associations between shortterm PM10-2.5 concentrations and adverse health endpoints including mortality and 

cardiovascular-related and respiratory-related hospital admissions and emergency 

department visits. 

(2) Both single-city and multi-city analyses, using different approaches to estimate 

ambient PM10-2.5 concentrations, have reported positive PM10-2.5 effect estimates 

in locations that would likely have met the current 24-hour PM10 standard. In a 

few cases, these PM10-2.5 effect estimates were statistically significant. 

(3) While limited in number, studies that have evaluated co-pollutant models have 

generally reported that PM10-2.5 effect estimates remain positive, and in a few 

cases statistically significant, when these models include gaseous pollutants or 

fine particles. 

(4) Support for the plausibility of the associations reported in epidemiological studies 

is provided by a small number of controlled human exposure studies reporting 

that short-term (i.e., 2-hour) exposures to PM10-2.5 decrease heart rate variability 

and increase markers of pulmonary inflammation. 

This approach to considering the health evidence, air quality information, and the 

associated uncertainties would place substantial weight on the generally positive PM10-2.5

effect estimates that have been reported for mortality and morbidity, even those effect 

estimates that are not statistically significant. The Policy Assessment concluded that this 

could be judged appropriate given that consistent results have been reported across 

multiple studies using different approaches to estimate ambient PM10-2.5 concentrations 

and that exposure measurement error, which is likely to be larger for PM10-2.5 than for 

PM2.5, tends to bias the results of epidemiological studies toward the null hypothesis, 

making it less likely that associations will be detected. Such an approach would place less 

weight on the uncertainties and limitations in the evidence that resulted in the Integrated 

Science Assessment conclusions that the evidence is only suggestive of a causal 

relationship. 
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Given all of the above, the Policy Assessment concluded that it would be 

appropriate to consider either retaining or revising the current 24-hour PM10 standard, 

depending on the approach taken to considering the available evidence, air quality 

information, and the uncertainties and limitations associated with that evidence and 

information (U.S. EPA, 2011a, section 3.2.3). 

2. Consideration of Potential Alternative Standards in the Policy Assessment

Given the conclusion that it would be appropriate to consider either retaining or 

revising the current PM10 standard, the Policy Assessment also considered what potential 

alternative standards, if any, could be supported by the available scientific evidence in 

order to increase public health protection against exposures to PM10-2.5. The Policy 

Assessment considered such potential alternative standards defined in terms of the 

elements of a standard (i.e., indicator, averaging time, form, and level). Key conclusions 

from the Policy Assessment regarding indicator, averaging time, and form included the 

following: 

(1) A PM10 indicator would continue to appropriately target protection against 

thoracic coarse particle exposures to those locations where the evidence is 

strongest for associations with adverse health effects (i.e., urban areas). 

(2) The available evidence supports the importance of maintaining a standard that 

protects against short-term exposures to all thoracic coarse particles. Given that 

the majority of this evidence is based on 24-hour average thoracic coarse particle 

concentrations, consideration of a 24-hour averaging time remains appropriate. 

(3) Given the limited body of evidence supporting PM10-2.5-related effects following 

long-term exposures, which resulted in the Integrated Science Assessment 

conclusion that the available evidence is “inadequate” to infer a causal 

relationship between long-term PM10-2.5 exposures and a variety of health effects, 

consideration of an annual thoracic coarse particle standard is not supported at 

this time. 

(4) To the extent it is judged appropriate to revise the current 24-hour PM10 standard, 

it would be appropriate to consider revising the form to the 3-year average of the 
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98th percentile of the annual distribution of 24-hour PM10 concentrations. 

In considering the available evidence and air quality information within the 

context of identifying potential alternative standard levels for consideration (assuming a 

decision were made that it is appropriate to amend the standard), the Policy Assessment 

first noted that a standard level as high as about 85 μg/m3, for a 24-hour PM10 standard 

with a 98th percentile form, could be supported. Based on considering air quality 

concentrations in study locations, the Policy Assessment noted that such a standard level 

would be expected to maintain PM10 and PM10-2.5 concentrations below those present in 

U.S. locations of single-city studies where PM10-2.5 effect estimates have been reported to 

be positive and statistically significant and below those present in some locations where 

single-city studies reported PM10-2.5 effect estimates that were positive, but not 

statistically significant. These include some locations likely to have met the current PM10

standard during the study periods (U.S. EPA, 2011a, section 3.3.4). 

The Policy Assessment also noted that, based on analysis of the number of people 

living in counties that could violate the current and potential alternative PM10 standards, a 

24-hour PM10 standard with a 98th percentile form and a level between 75 and 80 μg/m3

would provide a level of public health protection that is generally equivalent, across the 

U.S., to that provided by the current standard. Given this, the Policy Assessment 

concluded that it would be appropriate to consider standard levels in the range of 

approximately 75 to 80 μg/m3(with a 98th percentile form) , to the extent population 

counts were emphasized in comparing the public health protection provided by the 

current and potential alternative standards and to the extent it was judged appropriate to 

set a revised standard providing at least the level of public health protection that is 
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provided by the current standard, based on such population counts (U.S. EPA, 2011a, 

section 3.3.4). 

The Policy Assessment also concluded that alternative approaches to considering 

the evidence could lead to consideration of standard levels below 75 μg/m3for a standard 

with a 98th percentile form. For example, a number of single-city epidemiological studies 

have reported positive, though not statistically significant, PM10-2.5 effect estimates in 

locations with 98th percentile PM10 concentrations below 75 μg/m3. Given that exposure 

error is particularly important for PM10-2.5 epidemiological studies and can bias the results 

of these studies toward the null hypothesis (see section IV.B above), the Policy 

Assessment noted that it could be judged appropriate to place more weight on positive 

associations reported in these epidemiological studies, even when those associations are 

not statistically significant. In addition, the Policy Assessment noted that multi-city 

averages of 98th percentile PM10 concentrations in the locations evaluated by U.S. multicity studies of thoracic coarse particles (Zanobetti and Schwartz, 2009; Peng et al., 2008) 

were near or below 75 ppb. Despite uncertainties in the extent to which effects reported 

in multi-city studies are associated with the short-term air quality in any particular 

location, the Policy Assessment noted that emphasis could be placed on these multi-city 

averaged concentrations. The Policy Assessment concluded that, to the extent more 

weight is placed on single-city studies reporting positive, but not statistically significant, 

PM10-2.5 effect estimates and on multi-city studies, it could be appropriate to consider 

standard levels as low as 65 μg/m3 with a 98th percentile form (U.S. EPA, 2011a, section 

3.3.4). 

In considering potential alternative standard levels below 65 μg/m3, the Policy 
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Assessment noted that the overall body of PM10-2.5 health evidence is relatively uncertain, 

with somewhat stronger support in U.S. studies for associations with PM10-2.5 in locations 

with 98th percentile PM10 concentrations above 85 μg/m3than in locations with 98th

percentile PM10 concentrations below 65 μg/m3. In light of the limitations in the evidence 

for a relationship between PM10-2.5 and adverse health effects in locations with relatively 

low PM10 concentrations, along with the overall uncertainties in the body of PM10-2.5 

health evidence as described above and in the Integrated Science Assessment, the Policy 

Assessment concluded that consideration of standard levels below 65 μg/m3 was not 

appropriate (U.S. EPA, 2011a, section 3.3.4). 

D. CASAC Advice

Following their review of the first and second draft Policy Assessments, CASAC 

provided advice and recommendations regarding the current and potential alternative 

standards for thoracic coarse particles (Samet, 2010c,d). With regard to the existing PM10

standard, CASAC concluded that “the current data, while limited, is sufficient to call into 

question the level of protection afforded the American people by the current standard” 

(Samet, 2010d, p. 7). In drawing this conclusion, CASAC noted the positive associations 

in multi-city and single-city studies, including in locations with PM10 concentrations 

below those allowed by the current standard. In addition, CASAC gave “significant 

weight to studies that have generally reported that PM10-2.5 effect estimates remain 

positive when evaluated in co-pollutant models” and concluded that “controlled human 

exposure PM10-2.5 studies showing decreases in heart rate variability and increases in 

markers of pulmonary inflammation are deemed adequate to support the plausibility of 
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the associations reported in epidemiologic studies” (Samet, 2010d, p. 7).

135 Given all of 

the above conclusions CASAC recommended that “the primary standard for PM10 should 

be revised” (Samet, 2010d, p. ii and p. 7). In discussing potential revisions, while 

CASAC noted that the scientific evidence supports adoption of a standard at least as 

stringent as the current standard, they recommended revising the current standard in order 

to increase public health protection. In considering potential alternative standards, 

CASAC drew conclusions and made recommendations in terms of the major elements of 

a standard: indicator, averaging time, form, and level. 

The CASAC agreed with the EPA staff’s conclusions that the available evidence 

supports consideration in the current review of retaining the current PM10 indicator and 

the current 24-hour averaging time (Samet, 2010c, Samet, 2010d). Specifically, with 

regard to indicator, CASAC concluded that “[w]hile it would be preferable to use an 

indicator that reflects the coarse PM directly linked to health risks (PM10-2.5), CASAC 

recognizes that there is not yet sufficient data to permit a change in the indicator from 

PM10 to one that directly measures thoracic coarse particles” (Samet, 2010d, p. ii). In 

addition, CASAC “vigorously recommends the implementation of plans for the 

deployment of a network of PM10-2.5 sampling systems so that future epidemiological 

studies will be able to more thoroughly explore the use of PM10-2.5 as a more appropriate 

indicator for thoracic coarse particles” (Samet, 2010d, p. 7).

The CASAC also agreed that the evidence supports consideration of a potential 

alternative form. Specifically, CASAC “felt strongly that it is appropriate to change the 

 

135Nonetheless, CASAC endorsed the Integrated Science Assessment weight of evidence 

conclusions for PM10-2.5 (i.e., that the evidence is only “suggestive” of a causal 

relationship between short-term exposures and mortality, respiratory effects, and 

cardiovascular effects) (Samet, 2009e; Samet, 2009f).
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statistical form of the PM10 standard to a 98th percentile” (Samet, 2010d, p.7). In reaching 

this conclusion, CASAC noted that “[p]ublished work has shown that the percentile form 

has greater power to identify non-attainment and a smaller probability of 

misclassification relative to the expected exceedance form of the standard” (Samet, 

2010d. p. 7). 

With regard to standard level, in conjunction with a 98th percentile form, CASAC 

concluded that “alternative standard levels of 85 and 65 μg/m3(based on consideration of 

98th percentile PM10 concentration) could be justified” (Samet, 2010d, p.8). However, in 

considering the evidence and uncertainties, CASAC recommended a standard level from 

the lower part of the range discussed in the Policy Assessment, recommending a level 

“somewhere in the range of 75 to 65 μg/m3” (Samet, 2010d, p. ii). 

In making this recommendation, CASAC noted that the number of people living 

in counties with air quality not meeting the current standard is approximately equal to the 

number living in counties that would not meet a 98th percentile standard with a level 

between 75 and 80 μg/m3. CASAC used this information as the basis for their conclusion 

that a 98th percentile standard between 75 and 80 μg/m3 would be “comparable to the 

degree of protection afforded to the current PM10 standard” (Samet, 2010d, p. ii). Given 

this conclusion regarding the comparability of the current and potential alternative 

standards, as well as their conclusion on the public health protection provided by the 

current standard (i.e., that available evidence is sufficient to call it into question), 

CASAC recommended a level within a range of 75 to 65 μg/m3in order to increase 

public health protection, relative to that provided by the current standard (Samet 2010d, 

p. ii). 
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E. Administrator’s Proposed Conclusions Concerning the Adequacy of the Current 

Primary PM10 Standard

In considering the evidence and information as they relate to the adequacy of the 

current 24-hour PM10 standard, the Administrator first noted in the proposal that this 

standard is meant to protect the public health against effects associated with short-term 

exposures to PM10-2.5. In the last review, it was judged appropriate to maintain such a 

standard given the “growing body of evidence suggesting causal associations between 

short-term exposure to thoracic coarse particles and morbidity effects, such as respiratory 

symptoms and hospital admissions for respiratory diseases, and possibly mortality” (71 

FR 61185, October 17, 2006). Given the continued expansion in the body of scientific 

evidence linking short-term PM10-2.5 to health outcomes such as premature death and 

hospital visits, discussed in detail in the Integrated Science Assessment (U.S. EPA, 

2009a, Chapter 6) and summarized in the proposal, the Administrator provisionally 

concluded that the available evidence continued to support the appropriateness of 

maintaining a standard to protect the public health against effects associated with shortterm (e.g., 24-hour) exposures to all PM10-2.5. In drawing provisional conclusions in the 

proposal as to whether the current PM10 standard remains requisite (i.e., neither more nor 

less stringent than necessary) to protect public health with an adequate margin of safety 

against such exposures, the Administrator considered the following:

(1) The extent to which it is appropriate to maintain a standard that provides some 

measure of protection against all PM10-2.5, regardless of composition or source of 

origin;

(2) The extent to which it is appropriate to retain a PM10 indicator for a standard 

meant to protect against exposures to ambient PM10-2.5; and

(3) The extent to which the current PM10 standard provides an appropriate degree of 

public health protection.
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With regard to the first point, the proposal noted the conclusion from the last 

review that dosimetric, toxicological, occupational, and epidemiological evidence 

supported retention of a primary standard to provide some measure of protection against 

short-term exposures to all thoracic coarse particles, regardless of their source of origin or 

location, consistent with the Act’s requirement that primary NAAQS provide requisite 

protection with an adequate margin of safety (71 FR 61197). In that review, the EPA 

concluded that PM from a number of source types, including motor vehicle emissions, 

coal combustion, oil burning, and vegetative burning, are associated with health effects 

(U.S. EPA, 2004). This information formed part of the basis for the D.C. Circuit’s 

holding that it was appropriate for the thoracic coarse particle standard to provide “some 

protection from exposure to thoracic coarse particles...in all areas” (American Farm 

Bureau Federation v. EPA, 559 F. 3d at 532-33). 

In considering this issue in the proposal, the Administrator judged that the 

expanded body of scientific evidence in this review provides even more support for a 

standard that protects against exposures to all thoracic coarse particles, regardless of their 

location or source of origin. Specifically, the Administrator noted that epidemiological 

studies have reported positive associations between PM10-2.5 and mortality or morbidity in 

a large number of cities across North America, Europe, and Asia, encompassing a variety 

of environments where PM10-2.5 sources and composition are expected to vary widely. 

See 77 FR 38959. In considering this evidence, the Integrated Science Assessment 

concluded that “many constituents of PM can be linked with differing health effects” 

(U.S. EPA, 2009a, p. 2-26). While PM10-2.5 in most of these study areas is of largely 

urban origin, the Administrator noted that some recent studies have also linked mortality 
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and morbidity with relatively high ambient concentrations of thoracic coarse particles of 

non-urban crustal origin. In considering these studies, she noted the Integrated Science 

Assessment’s conclusion that “PM (both PM2.5 and PM10-2.5) from crustal, soil or road 

dust sources or PM tracers linked to these sources are associated with cardiovascular 

effects” (U.S. EPA, 2009a, p. 2-26). 

In light of this body of available evidence reporting PM10-2.5-associated health 

effects across different locations with a variety of sources, as well as the Integrated 

Science Assessment’s conclusions regarding the links between adverse health effects and 

PM sources and composition, the Administrator provisionally concluded in the proposal

that it is appropriate to maintain a standard that provides some measure of protection 

against exposures to all thoracic coarse particles, regardless of their location, source of 

origin, or composition (77 FR 38959-60).

With regard to the second point, in considering the appropriateness of a PM10

indicator for a standard meant to provide such public health protection, the Administrator 

noted that the rationale used in the last review to support the unqualified PM10 indicator 

(see above) remains relevant in the current review. Specifically, as an initial 

consideration, she noted that PM10 mass includes both coarse PM (PM10-2.5) and fine PM 

(PM2.5). As a result, the concentration of PM10-2.5 allowed by a PM10 standard set at a 

single level declines as the concentration of PM2.5 increases. At the same time, the 

Administrator noted that PM2.5 concentrations tend to be higher in urban areas than in 

rural areas (U.S. EPA, 2005, p. 2–54, and Figures 2–23 and 2– 24) and, therefore, a PM10

standard will generally allow lower PM10-2.5 concentrations in urban areas than in rural 

areas. 77 FR 38960.
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In considering the appropriateness of this variation in allowable PM10-2.5

concentrations, the Administrator considered the relative strength of the evidence for 

health effects associated with PM10-2.5 of urban origin versus non-urban origin. She 

specifically noted that, as described above and similar to the scientific evidence available 

in the last review, the large majority of the available evidence for thoracic coarse particle 

health effects comes from studies conducted in locations with sources more typical of 

urban and industrial areas than of rural areas. Although as just noted, associations with 

adverse health effects have been reported in some study locations where PM10-2.5 is 

largely non-urban in origin (i.e., in dust storm studies), particle concentrations in these 

study areas are typically much higher than reported in study locations where the PM10-2.5

is of urban origin. Therefore, the Administrator noted that the strongest evidence for a 

link between PM10-2.5 and adverse health impacts, particularly for such a link at relatively 

low particle concentrations, comes from studies where exposure is to PM10-2.5 of urban or 

industrial origin. 77 FR 38960.

The Administrator also noted that chemical constituents present at higher levels in 

urban or industrial areas, including byproducts of incomplete combustion (e.g. polycyclic 

aromatic hydrocarbons) emitted as PM2.5 from motor vehicles as well as metals and other 

contaminants emitted from anthropogenic sources, can contaminate PM10-2.5 (U.S. EPA, 

2004, p. 8–344; 71 FR 2665). While the Administrator acknowledged the uncertainty 

expressed in the Integrated Science Assessment regarding the extent to which, based on 

available evidence, particle composition can be linked to health outcomes, she also 

considered the possibility that PM10-2.5 contaminants typical of urban or industrial areas 

could increase the toxicity of thoracic coarse particles in urban locations (77 FR 38960).
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Given that the large majority of the evidence for PM10-2.5 toxicity, particularly at 

relatively low particle concentrations, comes from study locations where thoracic coarse 

particles are of urban origin, and given the possibility that PM10-2.5 contaminants in urban 

areas could increase particle toxicity, the Administrator provisionally concluded in the 

proposal that it remains appropriate to maintain a standard that targets public health 

protection to urban locations. Specifically, she concluded at proposal that it is appropriate 

to maintain a standard that allows lower ambient concentrations of PM10-2.5 in urban 

areas, where the evidence is strongest that thoracic coarse particles are linked to mortality 

and morbidity, and higher concentrations in non-urban areas, where the public health 

concerns are less certain. Id.

Given all of the above considerations and conclusions, the Administrator judged

that the available evidence supported retaining a PM10 indicator for a standard that is 

meant to protect against exposure to thoracic coarse particles. In reaching this initial 

judgment, she noted that, to the extent a PM10 indicator results in lower allowable 

concentrations of thoracic coarse particles in some areas compared to others, lower 

concentrations will be allowed in those locations (i.e., urban or industrial areas) where the 

science has shown the strongest evidence of adverse health effects associated with 

exposure to thoracic coarse particles and where we have the most concern regarding 

PM10-2.5 toxicity. Therefore, the Administrator provisionally concluded that the varying 

amounts of coarse particles that are allowed in urban vs. non-urban areas under the 24-

hour PM10 standard, based on the varying levels of PM2.5 present, appropriately reflect 

the differences in the strength of evidence regarding coarse particle effects in urban and 

non-urban areas (77 FR 38960).
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In reaching this provisional conclusion, the Administrator also noted that, in their 

review of the second draft Policy Assessment, CASAC concluded that “[w]hile it would 

be preferable to use an indicator that reflects the coarse PM directly linked to health risks 

(PM10-2.5), CASAC recognizes that there is not yet sufficient data to permit a change in 

the indicator from PM10 to one that directly measures thoracic coarse particles” (Samet, 

2010d, p. ii). In addition, CASAC “vigorously recommends the implementation of plans 

for the deployment of a network of PM10-2.5 sampling systems so that future 

epidemiological studies will be able to more thoroughly explore the use of PM10-2.5 as a 

more appropriate indicator for thoracic coarse particles” (Samet, 2010d, p. 7). Given this 

recommendation, the Administrator further judged that, although current evidence is not 

sufficient to identify a standard based on an alternative indicator that would be requisite 

to protect public health with an adequate margin of safety across the United States, 

consideration of alternative indicators (e.g., PM10-2.5) in future reviews is desirable and 

could be informed by additional research, as described in the Policy Assessment (U.S. 

EPA, 2011a, section 3.5). 

With regard to the third point, in evaluating the degree of public health protection 

provided by the current PM10 standard, the Administrator noted that the Policy 

Assessment discussed two different approaches to considering the scientific evidence and 

air quality information (U.S. EPA, 2011a, section 3.2.3). These different approaches, 

which are described above (section IV.C.1), lead to different conclusions regarding the 

appropriateness of the degree of public health protection provided by the current PM10

standard. The Administrator further noted that the primary difference between the two 

approaches lies in the extent to which weight is placed on the following (U.S. EPA, 
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2011a, section 3.2.3): 

(1) The PM10-2.5 weight-of-evidence classifications presented in the Integrated 

Science Assessment concluding that the existing evidence is suggestive of a 

causal relationship between short-term PM10-2.5 exposures and mortality, 

cardiovascular effects, and respiratory effects (a classification supported by 

CASAC);

(2) Individual PM10-2.5 epidemiological studies reporting associations in locations that 

meet the current PM10 standard, including associations that are not statistically 

significant;

(3) The limited number of PM10-2.5 epidemiological studies that have evaluated copollutant models;

(4) The limited number of PM10-2.5 controlled human exposure studies;

(5) Uncertainties in the PM10-2.5 air quality concentrations reported in 

epidemiological studies, given limitations in PM10-2.5 monitoring data and the 

different approaches used across studies to estimate ambient PM10-2.5

concentrations; and

(6) Uncertainties and limitations in the evidence that tend to call into question the 

presence of a causal relationship between PM10-2.5 exposures and 

mortality/morbidity.

In evaluating the different possible approaches to considering the public health 

protection provided by the current PM10 standard, the Administrator first noted that when

the available PM10-2.5 scientific evidence and its associated uncertainties are considered, 

the Integrated Science Assessment concluded that the evidence is suggestive of a causal 

relationship between short-term PM10-2.5 exposures and mortality, cardiovascular effects, 

and respiratory effects. As discussed in section IV.B.1 above and in more detail in the 

Integrated Science Assessment (U.S. EPA, 2009a, section 1.5), a suggestive 

determination is made when the “[e]vidence is suggestive of a causal relationship with 

relevant pollutant exposures, but is limited because chance, bias and confounding cannot 

be ruled out.” In contrast, the Administrator noted that she proposed to strengthen the 

annual fine particle standard based on a body of scientific evidence judged sufficient to 
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conclude that a causal relationship exists (i.e., mortality, cardiovascular effects) or is 

likely to exist (i.e., respiratory effects) (section III.B). 77 FR 38961. The suggestive 

judgment for PM10-2.5 reflects the greater degree of uncertainty associated with this body 

of evidence, as discussed above (sections IV.B and IV.C) and summarized below. 

In the proposal (77 FR 38961), the Administrator noted that the important 

uncertainties and limitations associated with the scientific evidence and air quality 

information raise questions as to whether public health benefits would be achieved by 

revising the existing PM10 standard. Such uncertainties and limitations include the 

following: 

(1) While PM10-2.5 effect estimates reported for mortality and morbidity were 

generally positive, most were not statistically significant, even in single-pollutant 

models. This includes effect estimates reported in some study locations with PM10

concentrations above those allowed by the current 24-hour PM10 standard. 

(2) The number of epidemiological studies that have employed co-pollutant models to 

address the potential for confounding, particularly by PM2.5, remains limited. 

Therefore, the extent to which PM10-2.5 itself, rather than one or more copollutants, contributes to reported health effects is less certain. 

(3) Only a limited number of experimental studies (i.e., controlled human exposure 

and animal toxicological) provide support for the associations reported in 

epidemiological studies, resulting in further uncertainty regarding the plausibility 

of the associations between PM10-2.5 and mortality and morbidity reported in 

epidemiological studies. 

(4) Limitations in PM10-2.5 monitoring data and the different approaches used by 

epidemiological study researchers to estimate PM10-2.5 concentrations across 

epidemiological studies result in uncertainty in the ambient PM10-2.5

concentrations at which the reported effects occur, increasing uncertainty in 

estimates of the extent to which changes in ambient PM10-2.5 concentrations would 

likely impact public health. 

(5) The lack of a quantitative PM10-2.5 risk assessment further contributes to 

uncertainty regarding the extent to which any revisions to the current PM10

standard would be expected to improve the protection of public health, beyond the 

protection provided by the current standard (see section III.B.5 above). 
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(6) The chemical and biological composition of PM10-2.5, and the effects associated 

with the various components, remains uncertain. Without more information on the 

chemical speciation of PM10-2.5, the apparent variability in associations across 

locations is difficult to interpret. 

In considering these uncertainties and limitations, the Administrator noted in 

particular the considerable degree of uncertainty in the extent to which health effects 

reported in epidemiological studies are due to PM10-2.5 itself, as opposed to one or more 

co-occurring pollutants. As discussed above, this uncertainty reflects the fact that there 

are a relatively small number of PM10-2.5 studies that have utilized co-pollutant models, 

particularly co-pollutant models that have included PM2.5, and a very limited body of 

controlled human exposure evidence supporting the biological plausibility of a causal 

relationship between PM10-2.5 and mortality and morbidity at ambient concentrations. The 

Administrator noted that these important limitations in the overall body of health 

evidence introduce uncertainty into the interpretation of individual epidemiological 

studies, particularly those studies reporting associations with PM10-2.5 that are not 

statistically significant. Given this, the Administrator reached the provisional conclusion 

in the proposal that it is appropriate to place relatively little weight on epidemiological 

studies reporting associations with PM10-2.5 that are not statistically significant in singlepollutant and/or co-pollutant models. Id.

With regard to this provisional conclusion, the Administrator noted that, for 

single-city mortality studies conducted in the United States where ambient PM10

concentration data were available for comparison to the current standard, positive and 

statistically significant PM10-2.5 effect estimates were only reported in study locations that 

would likely have violated the current PM10 standard during the study period (U.S. EPA, 

2011a, Figure 3-2). In U.S. study locations that would likely have met the current 
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standard, PM10-2.5 effect estimates for mortality were positive, but not statistically 

significant (U.S. EPA, 2011a, Figure 3-2). In considering U.S. study locations where 

single-city morbidity studies were conducted, and which would likely have met the 

current PM10 standard during the study period, the Administrator noted that PM10-2.5

effect estimates were both positive and negative, with most not statistically significant 

(U.S. EPA, 2011a, Figure 3-3).

In addition, in considering single-city analyses for the locations evaluated in a 

large U.S. multi-city mortality study (Zanobetti and Schwartz, 2009), the Administrator 

noted that associations in most of the study locations were not statistically significant and 

that this was the only study to estimate ambient PM10-2.5 concentrations as the difference 

between county-wide PM10 and PM2.5 mass. As discussed in the Policy Assessment and 

in the proposal, it is not clear how computed PM10-2.5 measurements, such as those used 

by Zanobetti and Schwartz (2009), compare with the PM10-2.5 concentrations obtained in 

other studies either by direct measurement or by calculating the difference using colocated samplers (U.S. EPA, 2009a, section 6.5.2.3). For these reasons, in the proposal 

the Administrator noted that “there is considerable uncertainty in interpreting the 

associations in these single-city analyses” (77 FR 38961-62).

The Administrator acknowledged that an approach to considering the available 

scientific evidence and air quality information that emphasizes the above considerations 

differs from the approach taken by CASAC. Specifically, in its review of the draft Policy 

Assessment CASAC placed a substantial amount of weight on individual studies, 

particularly those reporting positive health effects associations for PM10-2.5 in locations 

that met the current PM10 standard during the study period. In emphasizing these studies, 
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as well as the limited number of supporting studies that have evaluated co-pollutant 

models and the small number of supporting experimental studies, CASAC concluded that 

“the current data, while limited, is sufficient to call into question the level of protection 

afforded the American people by the current standard” (Samet, 2010d, p. 7) and 

recommended revising the current PM10 standard (Samet, 2010d).

The Administrator carefully considered CASAC’s advice and recommendations. 

She noted that in making its recommendation on the current PM10 standard, CASAC did 

not discuss its approach to considering the important uncertainties and limitations in the 

health evidence, and did not discuss how these uncertainties and limitations were 

reflected in its recommendation. Nor did CASAC discuss uncertainties in the reported 

concentrations of PM10-2.5 in the epidemiological studies, or how reported concentrations 

in the various studies relate to one another when differing measurement methodologies 

are used. As discussed above, such uncertainties and limitations contributed to the 

conclusions in the Integrated Science Assessment that the PM10-2.5 evidence is only 

suggestive of a causal relationship, a conclusion that CASAC endorsed (Samet, 2009e,f). 

Given the importance of these uncertainties and limitations to the interpretation of the 

evidence, as reflected in the weight of evidence conclusions in the Integrated Science 

Assessment and as discussed above, the Administrator judged it appropriate to consider 

and account for them when drawing conclusions about the potential implications of 

individual PM10-2.5 health studies for the current standard. 

In light of the above approach to considering the scientific evidence, air quality 

information, and associated uncertainties, the Administrator reached the following 

provisional conclusions in the proposal:
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(1) When viewed as a whole the available evidence and information suggests that the 

degree of public health protection provided against short-term exposures to 

PM10-2.5 does not need to be increased beyond that provided by the current PM10

standard. This provisional conclusion noted the important uncertainties and 

limitations associated with the overall body of health evidence and air quality 

information for PM10-2.5, as discussed above and as reflected in the Integrated 

Science Assessment weight-of-evidence conclusions; that PM10-2.5 effect 

estimates for the most serious health effect, mortality, were not statistically 

significant in U.S. locations that met the current PM10 standard and where coarse 

particle concentrations were either directly measured or estimated based on colocated samplers; and that PM10-2.5 effect estimates for morbidity endpoints were 

both positive and negative in locations that met the current standard, with most 

not statistically significant.

(2) The degree of public health protection provided by the current standard is not 

greater than warranted. This provisional conclusion noted that positive and 

statistically significant associations with mortality were reported in single-city 

U.S. study locations likely to have violated the current PM10 standard.

136

In reaching these provisional conclusions, the Administrator noted that the Policy 

Assessment also discussed the potential for a revised PM10 standard (i.e., with a revised 

form and level) to be “generally equivalent” to the current standard, but to better target 

public health protection to locations where there is greater concern regarding PM10-2.5-

associated health effects (U.S. EPA, 2011a, sections 3.3.3 and 3.3.4). In considering such 

a potential revised standard, the Policy Assessment discussed the large amount of 

variability in PM10 air quality correlations across monitoring locations and over time 

(U.S. EPA, 2011a, Figure 3-7) and the regional variability in the relative degree of public 

 

136There are similarities with the conclusions drawn by the Administrator in the last 

review. There, the Administrator concluded that there was no basis for concluding that 

the degree of protection afforded by the current PM10 standards in urban areas is greater 

than warranted, since potential mortality effects have been associated with air quality 

levels not allowed by the current 24-hour standard, but have not been associated with air 

quality levels that would generally meet that standard, and morbidity effects have been 

associated with air quality levels that exceeded the current 24-hour standard only a few 

times (71 FR 61202). In addition, the Administrator concluded that there was a high 

degree of uncertainty in the relevant population exposures implied by the morbidity 

studies suggesting that there is little basis for concluding that a greater degree of 

protection is warranted. Id. The D.C. Circuit in American Farm Bureau Federation v 

EPA explicitly endorsed this reasoning. 559 F. 3d at 534.
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health protection that could be provided by the current and potential alternative standards 

(U.S. EPA, 2011a, Table 3-2). In light of this variability, the Administrator noted the 

Policy Assessment conclusion that no single revised PM10 standard (i.e., with a revised 

form and level) would provide public health protection equivalent to that provided by the 

current standard, consistently over time and across locations (U.S. EPA, 2011a, section 

3.3.4). That is, a revised standard, even one that is meant to be “generally equivalent” to 

the current PM10 standard, could increase protection in some locations while decreasing 

protection in others (77 FR 38962).

In considering the appropriateness of revising the current PM10 standard in this 

way, the Administrator noted the following: 

(1) Positive PM10-2.5 effect estimates for mortality were not statistically significant in 

U.S. locations that met the current PM10 standard and where coarse particle 

concentrations were either directly measured or estimated based on co-located 

samplers, while positive and statistically significant associations with mortality 

were reported in locations likely to have violated the current PM10 standard. 

(2) Effect estimates for morbidity endpoints in locations that met the current standard 

were both positive and negative, with most not statistically significant. 

(3) Important uncertainties and limitations associated with the overall body of health 

evidence and air quality information for PM10-2.5, as discussed above and as 

reflected in the Integrated Science Assessment weight-of-evidence conclusions, 

call into question the extent to which the type of quantified and refined targeting 

of public health protection envisioned under a revised standard could be reliably 

accomplished. 

Given all of the above considerations, the Administrator noted that there is a large 

amount of uncertainty in the extent to which public health would be improved by 

changing the locations to which the PM10 standard targets protection. Therefore, she 

reached the provisional conclusion that the current PM10 standard should not be revised 

in order to change that targeting of protection. 

In considering all of the above, including the scientific evidence, the air quality 
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information, the associated uncertainties, and CASAC’s advice, the Administrator 

reached the provisional conclusion that the current 24-hour PM10 standard is requisite 

(i.e., neither more protective nor less protective than necessary) to protect public health 

with an adequate margin of safety against effects that have been associated with PM10-2.5.

In light of this provisional conclusion, the Administrator proposed to retain the current 

PM10 standard in order to protect against health effects associated with short-term 

exposures to PM10-2.5 (77 FR 38963).

The Administrator recognized that her proposed conclusions and decision to 

retain the current PM10 standard differed from CASAC’s recommendations, stemming 

from the differences in how the Administrator and CASAC considered and accounted for 

the evidence and its limitations and uncertainties. In light of CASAC’s views and 

recommendation to revise the current PM10 standard, the Administrator welcomed the 

public’s views on these different approaches to considering and accounting for the 

evidence and its limitations and uncertainties, as well as on the appropriateness of 

revising the primary PM10 standard, including revising the form and level of the standard. 

In doing so, the Administrator solicited comment on all aspects of the proposed decision, 

including her rationale for reaching the provisional conclusion that the current PM10

standard is requisite to protect public health with an adequate margin of safety and the 

provisional conclusion that it is not appropriate to revise the current PM10 standard by 

setting a “generally equivalent” standard with the goal of better targeting public health 

protection. 

F. Public Comments on the Administrator’s Proposed Decision to Retain the 

Primary PM10 Standard

This section discusses the major public comments received on the Administrator’s 

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proposed decision to retain the primary PM10 standard. Additional comments are 

addressed in the Response to Comments Document (U.S. EPA, 20122012a). 

Many public commenters agreed with the Administrator’s proposed decision to 

retain the current 24-hour primary PM10 standard. Among those expressing a position on 

this proposed decision, industry groups and most State and Local commenters endorsed 

the Administrator’s proposed rationale for retaining the current primary PM10 standard, 

including her consideration of the available scientific evidence and associated 

uncertainties and her consideration of CASAC recommendations. 

Although industry commenters generally agreed with the Administrator’s 

proposed decision to retain the current primary PM10 standard, a few alsosome also 

contended that the current standard is “excessively precautionary” (NMA and NCBA, 

2012, p. 4) and a few expressed support for a less stringent standard for coarse particles 

that are comprised largely of crustal material. For example, the Coarse Particulate Matter 

Coalition (CPMC) (2012) and several other industry commenters recommended that the

final decision allow application of a 98th percentile form for the current standard (i.e. with 

its level of 150 ug/m3) in cases where coarse particles consist primarily of crustal 

material. Such an approach would allow more yearly exceedances of the existing 

standard level than are allowed with the current one-expected-exceedance form. These 

industry commenters contended that a 98th percentile form applied in this way would 

provide appropriate regulatory relief for areas where the evidence for coarse particlerelated health effects is relatively uncertain. 

In reaching her conclusion that the current primary PM10 standard is requisite to 

protect public health with an adequate margin of safety, the Administrator considered the 
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degree of public health protection provided by the current standard as a whole, including 

all elements of that standard (i.e., indicator, averaging time, form, level). As discussed 

above and in the following section, this conclusion reflects the Administrator’s judgments 

that (1) the current standard appropriately provides some measure of protection against 

exposures to all thoracic coarse particles, regardless of their location, source of origin, or 

composition and (2) the current standard appropriately allows lower ambient 

concentrations of PM10-2.5 in urban areas, where the evidence is strongest that thoracic 

coarse particles are linked to mortality and morbidity, and higher concentrations in nonurban areas, where the public health concerns are less certain. 

Because the considerations that led to these judgments, and to the conclusion that 

the current primary PM10 standard is requisite to protect public health, took into account 

the degree of public health protection provided by the standard as a whole, it would not 

be appropriate to consider revising one element of the standard (e.g., the form, as 

suggested by commenters in this case) without considering the extent to which the other 

elements of the standard should also be revised. The change in form requested by 

industry commenters, without also lowering the level of the standard, would markedly 

reduce the public health protection provided against exposures to thoracic coarse 

particles.137 However, industry commenters have not presented new evidence or analyses 

 

137Based on regression analyses presented in the PA (U.S. EPA, 2011a, Figures 3-7 and 

3-8), PM10 one-expected-exceedance concentration-equivalent design values were 

between approximately 175 and 300 g/m3at monitoring locations recording 3-year 

averages of 98th percentile 24-hour PM10 concentrations around 150 g/m3(i.e., the level 

of the current standard). This suggests that, depending on the location, a 24-hour PM10

standard with a 98th percentile form in conjunction with the current level (i.e., as 

recommended by these commenters) could be “generally equivalent” to a 24-hour PM10

standard with a one-expected-exceedance form and a level as high as approximately 300 

g/m3. Based on this analysis, a 24-hour PM10 standard with a 98th percentile form and a 

level of 150 g/m3 would be markedly less health protective than the current standard. 

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to support their conclusion that an appropriate degree of public health protection could be 

achieved by allowing the use of an alternative form (i.e., 98th percentile) for some coarse 

particles, while retaining the other elements of the current standard. Nor have these 

commenters presented new evidence or analyses challenging the basis for the conclusion 

in the proposal that the varying amounts of coarse particles allowed in urban versus nonurban areas under the current 24-hour PM10 standard, based on the varying levels of 

PM2.5 present, appropriately reflect the differences in the strength of evidence regarding 

coarse particle effects in urban and non-urban areas. In light of this, EPA does not believe 

that a reduction in public health protection, such as that requested by industry 

commenters, is warranted. 

In further considering these comments, it is to be remembered that epidemiologic 

studies have not demonstrated that coarse particles of non-urban origin do not cause 

health effects, and commenters have not provided additional evidence on this point. 

While there are fewer studies of non-urban coarse particles than of urban coarse particles, 

several studies have reported positive and statistically significant associations between 

coarse particles of crustal, non-urban origin and mortality or morbidity (Ostro et al., 

2003; Bell et al., 2008; Chan et al., 2008; Middleton et al., 2008; Perez et al., 2008). 

These studies formed part of the basis for the PM Integrated Science Assessment 

conclusion that “recent studies have suggested that PM (both PM2.5 and PM10-2.5) from 

crustal, soil or road dust sources or PM tracers linked to these sources are associated with 

cardiovascular effects” (U.S. EPA, 20092009a, p. 2-26). Moreover, crustal coarse 

particles may be contaminated with toxic trace elements and other components from 

previously deposited fine PM from ubiquitous sources such as mobile source engine 
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exhaust, as well as by toxic metals from smelters or other industrial activities, animal 

waste, or pesticides (U.S. EPA, 2004, p. 8-344). In the proposal, the Administrator 

acknowledged the potential for this type of contamination to increase the toxicity of 

coarse particles of crustal, non-urban origin (77 FR 38960; see also 71 FR 61190).

In suggesting a change in the form of the current standard, industry commenters 

also did not address the manifold difficulties noted above, and in the last review, 

associated with developing an indicator that could reliably identify ambient mixes 

dominated by particular types of sources of coarse particles. See above and 71 FR 61193. 

Yet such an indicator would be a prerequisite of the type of standard these commenters 

request. 

For all of the reasons discussed above, the EPA does not agree with industry 

commenters who recommended allowing the application of a 98th percentile form for the 

current standard in cases where coarse particles consist primarily of crustal material. 

Some industry commenters contended that the uncertainties and limitations that 

precluded a quantitative risk assessment also preclude revising the PM10 standard. 

Although the EPA agrees that there are important uncertainties and limitations in the 

extent to which the quantitative relationships between ambient PM10-2.5 and health 

outcomes can be characterized in risk models, the Agency does not agree that such 

limitations alone preclude the option of revising a NAAQS. As noted above, the lack of a 

quantitative PM10-2.5 risk assessment in the current review adds uncertainty to conclusions 

about the extent to which revision of the current PM10 standard would be expected to 

improve the protection of public health, beyond the protection provided by the current 

standard. However, the EPA does not agree that such uncertainties necessarily preclude 
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revision of a NAAQS. Indeed, with respect to thoracic coarse particles, the D.C. Circuit 

noted that “[a]lthough the evidence of danger from coarse PM is, as the EPA recognizes, 

‘inconclusive’, the agency need not wait for conclusive findings before regulating a 

pollutant it reasonably believes may pose a significant risk to public health.” 559 F. 3d at 

533. Thus, the Administrator’s conclusion that the current 24-hour PM10 standard 

provides requisite protection of public health relies on her consideration of the broad 

body of evidence, rather than solely on the uncertainties that led to the decision not to 

conduct a quantitative assessment of PM10-2.5 health risks. 

Commenters representing a number of environmental groups and medical 

organizations disagreed with the Administrator’s proposal to retain the current primary PM10

standard. These commenters generally requested that the EPA revise the PM10 standard to 

increase public health protection, consistent with the recommendations from CASAC. 

As discussed above and in the proposal, in reaching provisional conclusions in the 

proposal regarding the current standard, the Administrator carefully considered CASAC’s 

advice and recommendations. She specifically noted that in making its recommendation 

on the current PM10 standard, CASAC did not discuss its approach to considering the 

important uncertainties and limitations in the health evidence, and did not discuss how 

these uncertainties and limitations were reflected in its recommendations. Such 

uncertainties and limitations contributed to the conclusions in the Integrated Science 

Assessment that the PM10-2.5 evidence is only suggestive of a causal relationship, a 

conclusion that CASAC endorsed (Samet, 2009e,f). These commenters also did not 

address the important uncertainties in the epidemiologic studies on which their comments 

are based. Given the importance of these uncertainties and limitations to the 

interpretation of the evidence, as reflected in the weight of evidence conclusions in the 
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Integrated Science Assessment and as discussed in the proposal, the Administrator judges

that it is appropriate to consider and account for them when drawing conclusions about 

the implications of individual PM10-2.5 health studies for the current standard.

Commenters have not provided new information that would change the Administrator’s 

views on the evidence and uncertainties. 

In recommending that the PM10 standard be revised, some commenters supported 

their conclusions by referencing studies that evaluated PM10, rather than PM10-2.5. These 

commenters contended that “[t]he most relevant studies to the setting of a PM10 standard 

are the thousands of studies that have reported adverse effects associated with PM10

pollution” (ALA et al., 2012). 

As discussed in the Policy Assessment, the proposal, and above, since the 

establishment of the primary PM2.5 standards, the purpose of the primary PM10 standard 

has been to protect against health effects associated with exposures to PM10-2.5. PM10 is 

the indicator, not the target pollutant. With regard to the appropriateness of considering 

PM10 health studies for the purpose of reaching conclusions on a standard meant to 

protect against exposures to PM10-2.5, the proposal noted that PM10 includes both fine and 

coarse particles, even in locations with the highest concentrations of PM10-2.5. Therefore,

the extent to which PM10 effect estimates reflect associations with PM10-2.5 versus PM2.5

can be highly uncertain and it is often unclear how PM10 health studies should be 

interpreted when considering a standard meant to protect against exposures to PM10-2.5. 

Given this uncertainty and the availability of a number of PM10-2.5 health studies in this 

review, the Integrated Science Assessment considered PM10-2.5 studies, but not PM10
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studies, when drawing weight-of-evidence conclusions regarding the coarse fraction.138

In light of the uncertainty in ascribing PM10-related health effects to the coarse or fine 

fractions, indicating that the best evidence for effects associated with exposures to 

PM10-2.5 comes from studies evaluating PM10-2.5 itself, and given CASAC’s support for 

the approach adopted in the Integrated Science Assessment, which draws weight-ofevidence conclusions for PM2.5 and PM10-2.5 but not for PM10 (Samet, 2009f), the EPA 

continues to conclude that it is appropriate to focus on PM10-2.5 health studies when 

considering the degree of public health protection provided by the current primary PM10

standard, a standard intended exclusively to provide protection against exposures to 

PM10-2.5.

G. Administrator’s Final Decision on the Primary PM10 Standard

In reaching a final decision on the primary PM10 standard, the Administrator takes 

into account the available scientific evidence, and the assessment of that evidence, in the 

Integrated Science Assessment; the analyses and staff conclusions presented in the Policy 

Assessment; the advice and recommendations of CASAC; and public comments on the 

proposal. In particular, as in the proposal, the Administrator places emphasis on her 

consideration of the following issues:

(1) The extent to which it is appropriate to maintain a standard that provides some 

measure of protection against all PM10-2.5, regardless of composition or source of 

origin;

(2) The extent to which it is appropriate to retain a PM10 indicator for a standard 

meant to protect against exposures to ambient PM10-2.5; and

 

138Although EPA relied in the 1997 review on evidence from PM10 studies, EPA did so 

out of necessity (i.e., there were as yet no reliable studies measuring PM10-2.5). In the 

2006 review, EPA placed primary reliance on epidemiologic studies measuring or 

estimating PM10-2.5, although there were comparatively few such studies. In this review, a 

larger body of PM10-2.5 studies are available. EPA regards these studies as the evidence to 

be given principal weight in reviewing the adequacy of the PM10 standard. 
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(3) The extent to which the current PM10 standard provides an appropriate degree of 

public health protection.

Each of these issues is discussed below. 

With regard to the first issue, as in the proposal the Administrator judges that the 

expanded body of scientific evidence available in this review provides ample support for 

a standard that protects against exposures to all thoracic coarse particles, regardless of 

their location or source of origin. There was already ample evidence for this position in 

the previous review,139and that evidence has since increased. Specifically, the 

Administrator notes that epidemiological studies have reported positive associations 

between PM10-2.5 and mortality or morbidity in a large number of cities across North 

America, Europe, and Asia, encompassing a variety of environments where PM10-2.5

sources and composition are expected to vary widely. In considering this evidence, the 

Integrated Science Assessment concludes that “many constituents of PM can be linked 

with differing health effects” (U.S. EPA, 2009a, p. 2-26). Although PM10-2.5 in most of 

these study areas is of largely urban origin, the Administrator notes that some recent 

studies have also linked mortality and morbidity with relatively high ambient 

concentrations of particles of non-urban crustal origin. In considering these studies, she 

notes the Integrated Science Assessment’s conclusion that “PM (both PM2.5 and PM10-2.5) 

from crustal, soil or road dust sources or PM tracers linked to these sources are associated 

with cardiovascular effects” (U.S. EPA, 2009a, p. 2-26). The Administrator likewise 

notes CASAC’s emphatic advice that a standard remains needed for all types of thoracic 

 

139 The D.C. Circuit agreed. See 559 F. 3d at 532-33.
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coarse PM.140 In light of this body of available evidence reporting PM10-2.5-associated 

health effects across different locations with a variety of sources, the Integrated Science 

Assessment’s conclusions regarding the links between adverse health effects and PM 

sources and composition, and CASAC’s advice, the Administrator concludes in the 

current review that it is appropriate to maintain a standard that provides some measure of 

protection against exposures to all thoracic coarse particles, regardless of their location, 

source of origin, or composition. 

With regard to the second issue, in considering the appropriateness of a PM10

indicator for a standard meant to provide such public health protection, the Administrator 

notes that the rationale used in the last review to support the unqualified PM10 indicator 

remains relevant in the current review. Specifically, as an initial consideration, she notes 

that PM10 mass includes both coarse PM (PM10-2.5) and fine PM (PM2.5). As a result, the 

concentration of PM10-2.5 allowed by a PM10 standard set at a single level declines as the 

concentration of PM2.5 increases. At the same time, the Administrator notes that PM2.5

concentrations tend to be higher in urban areas than rural areas (U.S. EPA, 2005, p. 2–54, 

and Figures 2–23 and 2– 24) and, therefore, a PM10 standard will generally allow lower 

PM10-2.5 concentrations in urban areas than in rural areas. 

In considering the appropriateness of this variation in allowable PM10-2.5

concentrations, the Administrator considers the relative strength of the evidence for 

health effects associated with PM10-2.5 of urban origin versus non-urban origin. She 

specifically notes that, as discussed in the proposal, the large majority of the available 

evidence for thoracic coarse particle health effects comes from studies conducted in 

 

140Indeed, CASAC recommended making the standard for all types of thoracic coarse PM 

more stringent (Samet, 2010d). 
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locations with sources more typical of urban and industrial areas than rural areas. While 

associations with adverse health effects have been reported in some study locations where 

PM10-2.5 is largely non-urban in origin (i.e., in dust storm studies), particle concentrations 

in these study areas are typically much higher than reported in study locations where the 

PM is of urban origin. Therefore, the Administrator notes that the strongest evidence for a 

link between PM10-2.5 and adverse health impacts, particularly for such a link at relatively 

low particle concentrations, comes from studies of urban or industrial PM10-2.5. 

The Administrator also notes that chemical constituents present at higher levels in 

urban or industrial areas, including byproducts of incomplete combustion (e.g. polycyclic 

aromatic hydrocarbons) emitted as PM2.5 from motor vehicles as well as metals and other 

contaminants emitted from anthropogenic sources, can contaminate PM10-2.5 (U.S. EPA, 

2004, p. 8–344; 71 FR 2665, January 17, 2006). While the Administrator acknowledges 

the uncertainty expressed in the Integrated Science Assessment regarding the extent to 

which particle composition can be linked to health outcomes based on available evidence, 

she also considers the possibility that PM10-2.5 contaminants typical of urban or industrial 

areas could increase the toxicity of thoracic coarse particles in urban locations. 

Given that the large majority of the evidence for PM10-2.5 toxicity, particularly at 

relatively low particle concentrations, comes from study locations where thoracic coarse 

particles are of urban origin, and given the possibility that PM10-2.5 contaminants in urban 

areas could increase particle toxicity, the Administrator concludes that it remains 

appropriate to maintain a standard that provides some protection in all areas but targets 

public health protection to urban locations. Specifically, she concludes that it is 

appropriate to maintain a standard that allows lower ambient concentrations of PM10-2.5 in 
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urban areas, where the evidence is strongest that thoracic coarse particles are linked to 

mortality and morbidity, and higher concentrations in non-urban areas, where the public 

health concerns are less certain. 

Given all of the above considerations and conclusions, the Administrator judges 

that the available evidence supports retaining a PM10 indicator for a standard that is 

meant to protect against exposures to thoracic coarse particles. In reaching this judgment, 

she notes that, to the extent a PM10 indicator results in lower allowable concentrations of 

thoracic coarse particles in some areas compared to others, lower concentrations will be 

allowed in those locations (i.e., urban or industrial areas) where the science has shown 

the strongest evidence of adverse health effects associated with exposure to thoracic 

coarse particles and where we have the most concern regarding PM10-2.5 toxicity. 

Therefore, the Administrator concludes that the varying amounts of coarse particles that 

are allowed in urban vs. non-urban areas under the 24-hour PM10 standard, based on the 

varying levels of PM2.5 present, appropriately reflect the differences in the strength of 

evidence regarding coarse particle effects in urban and non-urban areas.141, 142

In reaching this conclusion, the Administrator also notes that, in their review of 

 

141As discussed in the proposal, the Administrator recognizes that this relationship is 

qualitative. That is, the varying coarse particle concentrations allowed under the PM10

standard do not precisely correspond to the variable toxicity of thoracic coarse particles 

in different areas (insofar as that variability is understood). Although currently available 

information does not allow any more precise adjustment for relative toxicity, the 

Administrator believes the standard will generally ensure that the coarse particle levels 

allowed will be lower in urban areas and higher in non-urban areas. Addressing this 

qualitative relationship, the D.C. Circuit held that “[i]t is true that the EPA relies on a 

qualitative analysis to describe the protection the coarse PM NAAQS will provide. But 

the fact that the EPA’s analysis is qualitative rather than quantitative does not undermine 

its validity as an acceptable rationale for the EPA’s decision.” 559 F. 3d at 535. 

142The D.C. Circuit agreed with similar conclusions in the last review and held that this 

rationale reasonably supported use of an unqualified PM10 indicator for thoracic coarse 

particles. American Farm Bureau Federation v. EPA, 559 F. 3d at 535-36.
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the second draft Policy Assessment, CASAC concluded that “[w]hile it would be 

preferable to use an indicator that reflects the coarse PM directly linked to health risks 

(PM10-2.5), CASAC recognizes that there is not yet sufficient data to permit a change in 

the indicator from PM10 to one that directly measures thoracic coarse particles” (Samet, 

2010d, p. ii). Thus, consistent the considerations presented above and with CASAC 

advice, the Administrator concludes that it is appropriate to retain PM10 as the indicator 

for thoracic coarse particles.143

With regard to the third issue, in evaluating the degree of public health protection 

provided by the current PM10 standard, the Administrator first notes that when the 

available PM10-2.5 scientific evidence and its associated uncertainties were considered, the 

Integrated Science Assessment concluded that the evidence is suggestive of a causal 

relationship between short-term PM10-2.5 exposures and mortality, cardiovascular effects, 

and respiratory effects. As discussed above and in more detail in the Integrated Science 

Assessment (U.S. EPA, 2009a, section 1.5), a suggestive determination is made when the 

“[e]vidence is suggestive of a causal relationship with relevant pollutant exposures, but is 

limited because chance, bias and confounding cannot be ruled out.” In contrast, the 

Administrator notes that she is strengthening the annual fine particle standard based on a 

body of scientific evidence judged sufficient to conclude that a causal relationship exists 

(i.e., mortality, cardiovascular effects) or is likely to exist (i.e., respiratory effects). The 

suggestive judgment for PM10-2.5 reflects the greater degree of uncertainty associated with 

 

143In addition, CASAC “vigorously recommends the implementation of plans for the 

deployment of a network of PM10-2.5 sampling systems so that future epidemiological 

studies will be able to more thoroughly explore the use of PM10-2.5 as a more appropriate 

indicator for thoracic coarse particles” (Samet, 2010d, p. 7). Consideration of alternative 

indicators (e.g., PM10-2.5) in future reviews could be informed by additional research, as 

described in the Policy Assessment (U.S. EPA, 2011a, section 3.5).
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this body of evidence, as discussed above and in more detail in the proposal, and as 

summarized below. 

The Administrator notes that the important uncertainties and limitations 

associated with the scientific evidence and air quality information raise questions as to 

whether public health benefits would be achieved by revising the existing PM10 standard. 

Such uncertainties and limitations include the following: 

(1) While PM10-2.5 effect estimates reported for mortality and morbidity were 

generally positive, most were not statistically significant, even in single-pollutant 

models. This includes effect estimates reported in some study locations with PM10

concentrations above those allowed by the current 24-hour PM10 standard. 

(2) The number of epidemiological studies that have employed co-pollutant models to 

address the potential for confounding, particularly by PM2.5, remains limited. 

Therefore, the extent to which PM10-2.5 itself, rather than one or more copollutants, contributes to reported health effects remains uncertain. 

(3) Only a limited number of experimental studies provide support for the 

associations reported in epidemiological studies, resulting in further uncertainty 

regarding the plausibility of the associations between PM10-2.5 and mortality and 

morbidity reported in epidemiological studies. 

(4) Limitations in PM10-2.5 monitoring data and the different approaches used to 

estimate PM10-2.5 concentrations across epidemiological studies result in 

uncertainty in the ambient PM10-2.5 concentrations at which the reported effects 

occur, increasing uncertainty in estimates of the extent to which changes in 

ambient PM10-2.5 concentrations would likely impact public health. 

(5) The lack of a quantitative PM10-2.5 risk assessment further contributes to 

uncertainty regarding the extent to which any revisions to the current PM10

standard would be expected to improve the protection of public health, beyond the 

protection provided by the current standard (see section III.B.5 above). 

(6) The chemical and biological composition of PM10-2.5, and the effects associated 

with the various components, remains uncertain. Without more information on the 

chemical speciation of PM10-2.5, the apparent variability in associations across 

locations is difficult to characterize. 

In considering these uncertainties and limitations, the Administrator notes in 

particular the considerable degree of uncertainty in the extent to which health effects 
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reported in epidemiological studies are due to PM10-2.5 itself, as opposed to one or more 

co-occurring pollutants. As discussed above, this uncertainty reflects the fact that there 

are a relatively small number of PM10-2.5 studies that have evaluated co-pollutant models, 

particularly co-pollutant models that have included PM2.5, and a very limited body of 

controlled human exposure evidence supporting the plausibility of a causal relationship 

between PM10-2.5 and mortality and morbidity at ambient concentrations. The 

Administrator notes that these important limitations in the overall body of health

evidence introduce uncertainty into the interpretation of individual epidemiological 

studies, particularly those studies reporting associations with PM10-2.5 that are not 

statistically significant. Given this, the Administrator reaches the conclusion that it is 

appropriate to place relatively little weight on epidemiological studies reporting 

associations with PM10-2.5 that are not statistically significant in single-pollutant and/or 

co-pollutant models.144

With regard to this conclusion, the Administrator notes that, for single-city 

 

144The Administrator acknowledges that this approach to interpreting the evidence differs 

in emphasis from the approach she has adopted for the evidence relating to PM2.5. As 

discussed above in section III.E.4, for fine particles the Administrator has considered not 

only whether study results are statistically significant (or remain so after application of 

co-pollutant models), but she also places emphasis on the overall pattern of results across 

the epidemiological literature. This includes giving some credence to studies that 

reported statistically non-significant associations. This difference in emphasis stems from 

the much stronger overall body of evidence available for fine particles, compared to 

coarse particles. As discussed above, when the available PM2.5 scientific evidence and its 

associated uncertainties were considered, the Integrated Science Assessment concluded 

that the evidence was sufficient to conclude that causal relationships exist with mortality 

and cardiovascular effects, and that a causal relationship is likely to exist with respiratory 

effects. In contrast, the Integrated Science Assessment concluded that the evidence is 

suggestive of a causal relationship between short-term PM10-2.5 exposures and mortality, 

cardiovascular effects, and respiratory effects. A suggestive determination is made when 

the “[e]vidence is suggestive of a causal relationship with relevant pollutant exposures, 

but is limited because chance, bias and confounding cannot be ruled out” (U.S. EPA, 

2009a, section 1.5). The suggestive judgment for PM10-2.5 reflects the greater degree of 

uncertainty associated with this body of evidence.
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mortality studies conducted in the United States where ambient PM10 concentration data 

were available for comparison to the current standard, positive and statistically significant 

PM10-2.5 effect estimates were only reported in study locations that would likely have 

violated the current PM10 standard during the study period (U.S. EPA, 2011a, Figure 3-

2). In U.S. study locations that would likely have met the current standard, PM10-2.5 effect 

estimates for mortality were positive, but not statistically significant (U.S. EPA, 2011a, 

Figure 3-2). In considering U.S. study locations where single-city morbidity studies were 

conducted, and which would likely have met the current PM10 standard during the study 

period, the Administrator notes that PM10-2.5 effect estimates were both positive and 

negative, with most not statistically significant (U.S. EPA, 2011a, Figure 3-3).

In addition, in considering single-city analyses for the locations evaluated in a 

large U.S. multi-city mortality study (Zanobetti and Schwartz, 2009), the Administrator 

notes that associations in most of the study locations were not statistically significant and 

that this was the only study to estimate ambient PM10-2.5 concentrations as the difference 

between county-wide PM10 and PM2.5 mass. As discussed in the proposal, the 

Administrator notes that it is not clear how computed PM10-2.5 measurements, such as 

those used by Zanobetti and Schwartz (2009), compare with the PM10-2.5 concentrations 

obtained in other studies either by direct measurement by calculating the difference using 

co-located samplers (U.S. EPA, 2009a, section 6.5.2.3). For these reasons, as in the 

proposal, the Administrator notes that there is considerable uncertainty in interpreting the 

associations, and especially the concentrations at which such associations may have 

occurred, in these single-city analyses. 

The Administrator acknowledges that an approach to considering the available 
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scientific evidence and air quality information that emphasizes the above considerations 

differs from the approach taken by CASAC. Specifically, CASAC placed a substantial 

amount of weight on individual studies, particularly those reporting positive health 

effects associations in locations that met the current PM10 standard during the study 

period. In emphasizing these studies, as well as the limited number of supporting studies 

that have evaluated co-pollutant models and the small number of supporting experimental 

studies, CASAC concluded that “the current data, while limited, is sufficient to call into 

question the level of protection afforded the American people by the current standard” 

(Samet, 2010d, p. 7) and recommended revising the current PM10 standard (Samet, 

2010d).

The Administrator has carefully considered CASAC’s advice and 

recommendations. She notes that in making its recommendation on the current PM10

standard, CASAC did not discuss its approach to considering the important uncertainties 

and limitations in the health evidence, and did not discuss how these uncertainties and 

limitations are reflected in its recommendation. As discussed above, such uncertainties 

and limitations contributed to the conclusions in the Integrated Science Assessment that 

the PM10-2.5 evidence is only suggestive of a causal relationship, a conclusion that 

CASAC endorsed (Samet, 2009e,f). Given the importance of these uncertainties and 

limitations to the interpretation of the evidence, as reflected in the weight of evidence 

conclusions in the Integrated Science Assessment and as discussed above, the 

Administrator judges that it is appropriate to consider and account for them when 

drawing conclusions about the potential implications of individual PM10-2.5 health studies 

for the current standard. 
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In light of the above approach to considering the scientific evidence, air quality 

information, and associated uncertainties, the Administrator reaches the following 

conclusions:

(1) When viewed as a whole the available evidence and information suggests that the 

degree of public health protection provided against short-term exposures to 

PM10-2.5 should be maintained but does not need to be increased beyond that 

provided by the current PM10 standard. This conclusion emphasizes the important 

uncertainties and limitations associated with the overall body of health evidence 

and air quality information for PM10-2.5, as discussed above and as reflected in the 

Integrated Science Assessment weight-of-evidence conclusions; that PM10-2.5

effect estimates for the most serious health effect, mortality, were not statistically 

significant in U.S. locations that met the current PM10 standard and where coarse 

particle concentrations were either directly measured or estimated based on colocated samplers; and that PM10-2.5 effect estimates for morbidity endpoints were 

both positive and negative in locations that met the current standard, with most 

not statistically significant.

145

(2) The degree of public health protection provided by the current standard is not 

greater than warranted. This conclusion notes that positive and statistically 

significant associations with mortality were reported in single-city U.S. study 

locations likely to have violated the current PM10 standard.

146

 

145 This is not to say that the EPA could not adopt or revise a standard for a pollutant for 

which the evidence is suggestive of a causal relationship. Indeed, with respect to thoracic 

coarse particles itself, the D.C. Circuit noted that “[a]lthough the evidence of danger from 

coarse PM is, as the EPA recognizes, ‘inconclusive’, the agency need not wait for 

conclusive findings before regulating a pollutant it reasonably believes may pose a 

significant risk to public health.” American Farm Bureau Federation v EPA 559 F. 3d at 

533. As explained in the text above, it is the Administrator’s judgment that significant 

uncertainties presented by the evidence and information before her in this review, both as 

to causality and as to concentrations at which effects may be occurring, best support a 

decision to retain rather than revise the current primary 24-hour PM10 standard.

146There are similarities with the conclusions drawn by the Administrator in the last 

review. There, the Administrator concluded that there was no basis for concluding that 

the degree of protection afforded by the current PM10 standards in urban areas is greater 

than warranted, since potential mortality effects have been associated with air quality 

levels not allowed by the current 24-hour standard, but have not been associated with air 

quality levels that would generally meet that standard, and morbidity effects have been 

associated with air quality levels that exceeded the current 24-hour standard only a few 

times. 71 FR 61202. In addition, the Administrator concluded that there was a high 

degree of uncertainty in the relevant population exposures implied by the morbidity 

studies suggesting that there is little basis for concluding that a greater degree of 
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In reaching these conclusions, the Administrator notes that the Policy Assessment 

also discussed the potential for a revised PM10 standard (i.e., with a revised form and 

level) to be “generally equivalent” to the current standard, but to better target public 

health protection to locations where there is greater concern regarding PM10-2.5-associated 

health effects (U.S. EPA, 2011a, sections 3.3.3 and 3.3.4).147 In considering such a 

potential revised standard, the Policy Assessment discusses the large amount of 

variability in PM10 air quality correlations across monitoring locations and over time 

(U.S. EPA, 2011a, Figure 3-7) and the regional variability in the relative degree of public 

health protection that could be provided by the current and potential alternative standards 

(U.S. EPA, 2011a, Table 3-2). In light of this variability, the Administrator notes the 

Policy Assessment conclusion that no single revised PM10 standard (i.e., with a revised 

form and level) would provide public health protection equivalent to that provided by the 

current standard, consistently over time and across locations (U.S. EPA, 2011a, section 

3.3.4). That is, a revised standard, even one that is meant to be “generally equivalent” to 

the current PM10 standard, could increase protection in some locations while decreasing 

protection in other locations. 

In considering the appropriateness of revising the current PM10 standard in this 

way, the Administrator notes the following: 

 

protection is warranted. Id. The D.C. Circuit in American Farm Bureau Federation v 

EPA explicitly endorsed this reasoning. 559 F. 3d at 534.

147As discussed in detail above (section IV.C.2.d) and in the Policy Assessment (U.S. 

EPA, 2011a, sections 3.3.3 and 3.3.4), a revised standard that is generally equivalent to 

the current PM10 standard could provide a degree of public health protection that is 

similar to the degree of protection provided by the current standard, across the United 

States as a whole. However, compared to the current PM10 standard, such a generally 

equivalent standard would change the degree of public health protection provided in 

some specific areas, providing increased protection in some locations and decreased 

protection in other locations. 
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(1) As discussed above, positive PM10-2.5 effect estimates for mortality were not 

statistically significant in U.S. locations that met the current PM10 standard and 

where coarse particle concentrations were either directly measured or estimated 

based on co-located samplers, while positive and statistically significant 

associations with mortality were reported in locations likely to have violated the 

current PM10 standard. 

(2) Also as discussed above, effect estimates for morbidity endpoints in locations that 

met the current standard were both positive and negative, with most not 

statistically significant. 

(3) Important uncertainties and limitations associated with the overall body of health 

evidence and air quality information for PM10-2.5, as discussed above and as 

reflected in the Integrated Science Assessment weight-of-evidence conclusions, 

call into question the extent to which the type of quantified and refined targeting 

of public health protection envisioned under a revised standard could be reliably 

accomplished. 

Given all of the above considerations, the Administrator notes that there is a large 

amount of uncertainty in the extent to which public health would be improved by 

changing the locations to which the PM10 standard targets protection. Therefore, she 

reaches the conclusion that the current PM10 standard should not be revised in order to 

change that targeting of protection. 

In considering all of the above, including the scientific evidence, the air quality 

information, the associated uncertainties, CASAC’s advice, and public comments 

received on the proposed rule, the Administrator reaches the conclusion in the current 

review that the existing 24-hour PM10 standard, with its one-expected exceedance form 

and a level of 150 g/m3, is requisite (i.e., neither more protective nor less protective than 

necessary) to protect public health with an adequate margin of safety against effects that 

have been associated with PM10-2.5. In light of this conclusion, with this notice the 

Administrator retains the current PM10 standard.

V. Communication of Public Health Information

Sections 319(a)(1) and (3) of the CAA require the EPA to establish a uniform air 
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quality index for reporting of air quality. These sections specifically direct the 

Administrator to ‘‘promulgate regulations establishing an air quality monitoring system 

throughout the United States which utilizes uniform air quality monitoring criteria and 

methodology and measures such air quality according to a uniform air quality index’’ and 

‘‘provides for daily analysis and reporting of air quality based upon such uniform air 

quality index...’’ In 1979, the EPA established requirements for index reporting (44 FR 

27598, May 10, 1979). The requirement for State and local agencies to report the AQI 

appears in 40 CFR 58.50, and the specific requirements (e.g., what to report, how to 

report, reporting frequency, calculations) are in appendix G to 40 CFR part 58.

Information on the public health implications of ambient concentrations of criteria 

pollutants is currently made available primarily by AQI reporting through EPA’s 

AIRNow website.148 The current AQI has been in use since its inception in 1999.149 It 

provides accurate, timely, and easily understandable information about daily levels of 

pollution (40 CFR 58.50). The AQI establishes a nationally uniform system of indexing 

pollution levels for ozone, carbon monoxide, nitrogen dioxide, PM, and sulfur dioxide. 

The AQI is also recognized internationally as a proven tool to effectively communicate 

air quality information to the public. 

The AQI converts pollutant concentrations in a community’s air to a number on a 

scale from 0 to 500. Reported AQI values enable the public to know whether air pollution 

levels in a particular location are characterized as good (0–50), moderate (51–100), 

unhealthy for sensitive groups (101– 150), unhealthy (151–200), very unhealthy (201–

 

148 See http://www.airnow.gov/ .

149 In 1976, the EPA established a nationally uniform air quality index, then called the 

Pollutant Standard Index (PSI), for use by State and local agencies on a voluntary basis 

(41 FR 37660, September 7, 1976). In August 1999, the EPA adopted revisions to this air 

quality index (64 FR 42530, August 4, 1999) and renamed the index the AQI. 

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300), or hazardous (301–500). The AQI index value of 100 typically corresponds to the 

level of the short-term (e.g., daily or hourly standard) NAAQS for each pollutant. Below 

an index value of 100, an intermediate value of 50 was defined either as the level of the 

annual standard if an annual standard has been established (e.g., PM2.5, nitrogen dioxide), 

or as a concentration equal to one-half the value of the short-term standard used to define 

an index value of 100 (e.g., carbon monoxide). An AQI value greater than 100 means that 

a pollutant is in one of the unhealthy categories (i.e., unhealthy for sensitive groups, 

unhealthy, very unhealthy, or hazardous) on a given day. An AQI value at or below 100 

means that a pollutant concentration is in one of the satisfactory categories (i.e., moderate 

or good). The underlying health information that supports the NAAQS review also 

supports the selection of the AQI “breakpoints” – the ambient concentrations that 

delineate the various AQI categories for each pollutant.

Historically, state and local agencies have primarily used the AQI to provide 

general information to the public about air quality and its relationship to public health. 

For more than a decade, many states and local agencies, as well as the EPA and other 

Federal agencies, have been developing new and innovative programs and initiatives to 

provide more information to the public in a more timely way. These initiatives, including 

air quality forecasting, real-time data reporting through the AirNow website, and state 

and local air quality action day programs, can serve to provide useful, up-to-date, and 

timely information to the public about air pollution and its effects. Such information will 

help individuals take actions to avoid or to reduce exposures to ambient pollution at 

levels of concern to them. Thus, these programs have significantly broadened the ways in 

which state and local agencies can meet the nationally uniform AQI reporting 
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requirements and contribute to state and local efforts to provide community health 

protection.

With respect to an AQI value of 50, the historical approach is to set it at the same 

level of the annual primary standard, if there is one. This is consistent with the previous

AQI sub-index for PM2.5, in which the AQI value of 50 was set at 15 μg/m3in 1999, 

consistent with the level of the annual PM2.5 standard at that time. In recognition of the 

proposed change to the annual PM2.5 standard summarized in section III.F of the 

proposal, the EPA proposed a conforming change to the PM2.5 sub-index of the AQI to be 

consistent with the proposed change to the annual standard. As discussed below, no state 

or local agencies, or their organizations (e.g., NACAA), that commented on the proposed 

changes to the AQI disagreed with our proposed approach. Based on these comments, the 

EPA continues to see no basis for deviating from this approach in this review. Thus, the 

EPA is taking final action to set an AQI value of 50 at 12.0 μg/m3, 24-hour average, 

consistent with the final decision on the annual PM2.5 standard level (section III.F). 

With respect to an AQI value of 100, which is the basis for advisories to 

individuals in sensitive groups, in the proposal we described two general approaches that 

could be used to select the associated PM2.5 level. By far the most common approach, 

which has been used with all of the other sub-indices, is to set an AQI value of 100 at the 

same level as the short-term standard. In the proposal, the EPA recognized that some 

state and local air quality agencies have expressed a strong preference that the Agency set 

an AQI value of 100 equal to any short-term standard (77 FR 38964). These agencies 

typically express the view that this linkage is useful for the purpose of communicating 

with the public about the standard, as well as providing consistent messages about the 
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health impacts associated with daily air quality. The EPA proposed to use this approach 

to set the AQI value of 100 at 35 μg/m3, 24-hour average, consistent with the proposed 

decision to retain the current 24-hour PM2.5 standard. Id.

An alternative approach discussed in the proposal (77 FR 38964), was to directly 

evaluate the health effects evidence to select the level for an AQI value of 100. This was 

the approach used in the 1999 rulemaking to set the AQI value of 100 at a level of 40 

μg/m3, 24-hour average,150 when the 24-hour standard level was 65 μg/m3. This 

alternative approach was used in the case of the PM2.5 sub-index, because the annual and 

24-hour PM2.5 standards set in 1997 were designed to work together, and the intended 

degree of health protection against short-term risks was not defined by the 24-hour 

standard alone, but rather by the combination of the two standards working in concert. 

Indeed, at that time, the 24-hour standard was set to provide supplemental protection 

relative to the principal protection provided by the annual standard. In the proposal, the 

EPA solicited comment on this alternative approach in recognition that, as proposed, the 

24-hour PM2.5 standard is intended to continue to provide supplemental protection against 

effects associated with short-term exposures of PM2.5 by working in conjunction with the 

annual standard to reduce 24-hour exposures to PM2.5. The EPA recognized that in the 

past, some state and local air quality agencies have expressed support for this alternative 

approach. Using this alternative approach could have resulted in consideration of a lower 

level for an AQI value of 100, based on the discussion of the health information 

pertaining to the level of the 24-hour standard in section III.E.4 of the proposal. The EPA 

 

150 Currently, we are cautioning members of sensitive groups at the AQI value of 100 at 

35 μg/m3, 24-hour average, consistent with more recent guidance from the EPA with 

regard to the development of State emergency episode contingency plans (Harnett, 2009, 

Attachment B). 

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Subscript
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encouraged state and local air quality agencies to comment on both the approach and the 

level at which to set an AQI value of 100 together with any supporting rationale. Of the 

state or local agencies, or their organizations (e.g., NACAA), that commented on the 

proposed changes to the AQI, only one organization, NESCAUM, expressed some 

support for this approach. In its comments, NESCAUM expressed support for a 24-hour 

standard set at 30 μg/m3, 24-hour average. NESCAUM also expressed the view that EPA 

should carefully consider how to set the breakpoint for an AQI value of 100. NESCAUM 

expressed the view that if the EPA were to keep the 24-hour PM2.5 standard at 35 ug/m3, 

the annual standard would be controlling, and a 24-hour breakpoint at that level (35 

ug/m3) would not be very effective for the purposes of public health messaging.

However, other agencies, such as Georgia Department of Natural Resources (Georgia 

DNR), expressed the view that linkage between the short-term standard and the AQI of 

100 is useful for the purpose of communicating with the public about the standard as well 

as providing consistent messages about the health impacts associated with the daily air 

quality. Based on these comments, the EPA sees no basis for deviating from the approach

proposed in this review. Thus, the EPA is taking final action to set an AQI value of 100 at 

35 μg/m3, 24-hour average, consistent with the final decision on the 24-hour PM2.5

standard level (section III.F).

With respect to an AQI value of 150, this level is based upon the same health 

effects information that informs the selection of the level of the 24-hour standard and the 

AQI value of 100. The AQI value of 150 was set in the 1999 rulemaking at a level of 65

μg/m3, 24-hour average. In considering what level to propose for an AQI value of 150, 

we stated the view that the health effects evidence indicates that the level of 55 μg/m3, 
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24-hour average, is appropriate to use151 in conjunction with an AQI value of 100 set at

the level of 35 μg/m3. The Agency’s approach to selecting the levels at which to set the 

AQI values of 100 and 150 inherently recognizes that the epidemiological evidence upon 

which these decisions are based provides no evidence of discernible thresholds, below 

which effects do not occur in either sensitive groups or in the general population, at 

which to set these two breakpoints. Therefore, the EPA concluded the use of a 

proportional adjustment would be appropriate. Commenters did not comment on this 

proposed approach to revising the AQI value of 150; thus, the EPA is taking final action 

to set an AQI value of 150 at 55 μg/m3, 24-hour average.

Based on the air quality and health considerations discussed in section V of the 

proposal, the EPA concluded that it was appropriate to propose to retain the current level 

of 500 μg/m3, 24-hour average, for the AQI value of 500. In addition, the EPA solicited 

comment on alternative levels and approaches to setting a level for the AQI value of 500, 

as well as supporting information and rationales for such alternative levels. The EPA also 

solicited any additional information, data, research or analyses that may be useful to 

inform a final decision on the appropriate level to set the AQI value of 500. Receiving no

information with which to inform alternative approaches to setting an AQI value of 500, 

the EPA is taking final action to retain the current level of 500 μg/m3, 24-hour average, 

for the AQI value of 500.

For the intermediate breakpoints in the AQI between the values of 150 and 500, 

the EPA proposed PM2.5 concentrations that generally reflected a linear relationship 

between increasing index values and increasing PM2.5 values (77 FR 38965). The 

 

151 We note that this level is consistent with the level recommended in the more recent 

EPA guidance (Harnett, 2009, Attachment B), which is in use by many State and local 

agencies.
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available scientific evidence of health effects related to population exposures to PM2.5

concentrations between the level of the 24-hour standard and an AQI value of 500 

suggested a continuum of effects in this range, with increasing PM2.5 concentrations 

being associated with increasingly larger numbers of people likely to experience such 

effects. The generally linear relationship between AQI values and PM2.5 concentrations in 

this range is consistent with the health evidence. This also is consistent with the Agency’s 

practice of setting breakpoints in symmetrical fashion where health effects information 

does not suggest particular levels.

Table 2 below summarizes the finalized breakpoints for the PM2.5 sub-index.152

Table 2 shows the intermediate breakpoints for AQI values of 200, 300 and 400 based on 

a linear interpolation between the proposed levels for AQI values of 150 and 500. If a 

different level were to be set for an AQI value of 150 or 500, intermediate levels would 

be calculated based on a linear relationship between the selected levels for AQI values of 

150 and 500.

Table 2. Breakpoints for PM2.5 Sub-Index

AQI Category Index Values Proposed Breakpoints

(μg/m3, 24-hour average)

Good 0 - 50 0.0 – (12.0)

Moderate 51 - 100 (12.1) – 35.4

Unhealthy for 

Sensitive Groups 101 - 150 35.5 – 55.4

Unhealthy 151 - 200 55.5 - 150.4

Very Unhealthy 201 - 300 150.5 - 250.4

Hazardous 301 - 400 250.5 - 350.4

 

152 As discussed in section VII.C below, the EPA is also updating the data handling 

procedures for reporting the AQI and corresponding updates for other AQI-sub-indices 

presented in Table 2 of appendix G of 40 CFR part 58.
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401 - 500 350.5 - 500.4

In retaining the 500 level for the AQI as described above, we note that the EPA is 

not establishing a Significant Harm Level (SHL) for PM2.5. The SHL is an important part 

of air pollution Emergency Episode Plans, which are required for certain areas by CAA 

section 110(a)(2)(G) and associated regulations at 40 CFR 51.150, under the Prevention 

of Air Pollution Emergency Episodes program. The Agency believes that air quality 

responses established through an Emergency Episode Plan should be developed through a 

collaborative process working with State and Tribal air quality, forestry and agricultural 

agencies, Federal land management agencies, private land managers and the public. 

Therefore, if in future rulemaking the EPA proposes revisions to the Prevention of Air 

Pollution Emergency Episodes program, the proposal will include a SHL for PM2.5 that is 

developed in collaboration with these organizations. As discussed in the 1999 Air Quality 

Index Reporting Rule (64 FR 42530), if a future rulemaking results in a SHL that is 

different from the 500 value of the AQI for PM2.5, the AQI will be revised accordingly. 

The EPA also received more general comments on AQI reporting, comments that 

did not pertain to setting specific breakpoints. One set of commenters (e.g., API and 

UARG), expressed the view that changes to the AQI are not appropriate. They noted that 

air quality is getting better, and in fact is better than when EPA established the AQI. 

These commenters stated that the proposed changes to the annual standard and the AQI 

would mean that the public would hear less often that air quality is good, and thereby 

would receive apparently inconsistent or misleading messages that air quality is worse. 

The AQI has been revised several times in conjunction with revisions to the standards. 

State and local air quality agencies and organizations are proficient at communicating 
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with the public about the reasons for changes to the AQI. Therefore, the EPA strongly 

disagrees with these commenters that the public will receive inconsistent or misleading 

messages. Recognizing the importance of the AQI as a communication tool that allows 

the public to take exposure reduction measures when air quality may pose health risks, 

the EPA agrees with state and local air quality agencies and organizations that favored 

revising the AQI at the same time as the primary standard.

A few state and local air quality agencies and organizations recommended against 

using near-roadway PM2.5 monitors for AQI reporting. In support of this comment, they 

expressed the following views, that near-roadway monitors are source-oriented, represent 

micro-scale conditions, and the agencies don’t have experience using them for AQI 

reporting. The EPA disagrees with the comment in that these monitors will be sited at 

existing near-road stations sited to be representative of area-wide PM2.5 concentrations 

indicative of general population exposure. Accordingly, data from these near-road 

monitors should be included in the AQI since they provide information about PM2.5

levels that millions of people, who work, live and go to school near busy roadways, are 

exposed to. The stations are representative of somewhat elevated concentrations in nearroad environments, but since these stations represent many such locations throughout a 

metropolitan area, they are appropriate for characterizing exposure in typical portions of 

major urban areas. The EPA is committed to helping air quality agencies develop 

appropriate ways to report PM2.5 levels from these monitors using the AQI. 

VI. Rationale for Final Decisions on the Secondary PM Standards

This section presents the Administrator’s final decisions regarding the need to 

revise the current suite of secondary PM2.5 and PM10 standards to address visibility 
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impairment and other welfare effects considered in this review. As discussed more fully 

below,Specifically, this section describes the rationale for the Administrator’s final 

decision to retain the current suite of secondary PM standards to address PM-related 

visibility impairment as well as other PM-related welfare effects, including ecological

effects, effects on materials, and climate impacts. This suite of standards includes an 

annual PM2.5 standard of 15 μg/m3, a 24-hour PM2.5 standard of 35 μg/m3, and a 24-hour 

PM10 standard of 150 μg/m3. The Administrator is revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. Contrary to what was proposed, 

the Administrator has decided not to establish a distinct standard to address PM-related 

visibility impairment. The rationale for this decision is presented below.

The Administrator’s final decisions on the secondary standards are based on a 

thorough review, in the Integrated Science Assessment, of the latest scientific 

information, published through mid-2009, on welfare effects associated with fine and 

coarse particles in the ambient air, as presented in the Integrated Science Assessment. 

The final decisions also take into account: (1) staff assessments of the most policyrelevant information presented and assessed in the Integrated Science Assessment and 

staff analyses of air quality and visibility effects presented in the Visibility Assessment 

and the Policy Assessment, upon which staff conclusions regarding appropriate 

considerations in this review are based; (2) CASAC advice and recommendations, as 

reflected in discussions of drafts of the Integrated Science Assessment, Visibility 

Assessment, and Policy Assessment at public meetings, in separate written comments, 

and in CASAC’s letters to the Administrator; (3) the multiple rounds of public comments 
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received during the development of these documents, eitherboth in connection with 

CASAC meetings orand separately; and (4) public comments received on the proposal.

In particular, this section presents background information on the EPA’s previous 

and current reviews of the secondary PM standards (section VI.A), a summary of the 

proposed decisions regarding the secondary PM standards (section VI.B), a 

summarydiscussion of significant public comments received on those proposed decisions 

(section VI.C), and the Administrator’s final decisions on the secondary PM standards 

(section VI.D).

A. Background

The current suite of secondary PM standards is identical to the current suite of 

primary PM standards set in 2006, including 24-hour and annual PM2.5 standards and a 

24-hour PM10 standard. The current secondary PM2.5 standards are intended to provide 

protection from PM-related visibility impairment, whereas the entire suite of secondary 

PM standards is intended to provide protection from other PM-related effects on public 

welfare, including effects on sensitive ecosystems, materials damage and soiling, and 

climatic and radiative processes.

The approach used for reviewing the current suite of secondary PM standards 

built upon and broadened the approaches used in previous PM NAAQS reviews. The 

following discussion focuses particularly on the current secondary PM2.5 standards 

related to visibility impairment and provides a summary of the approaches used to review 

and establish secondary PM2.5 standards in the last two reviews (section VI.A.1); judicial 

review of the 2006 standards that resulted in the remand of the secondary annual and 24-

hour PM2.5 NAAQS to the EPA (section VI.A.2); and the approach used in this review 
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for evaluating the secondary PM2.5 standards used in this review (section VI.A.3).

1. Approaches Used in Previous Reviews

The original secondary PM2.5 standards were established in 1997, and a revision 

to the 24-hour standard was made in 2006. The approaches used in making final 

decisions on secondary standards in those reviews, as well as the current review, utilized 

different ways to consider the underlying body of scientific evidence. They also reflected 

an evolution in EPA’s understanding of the nature of the effect on public welfare from 

PM-related visibility impairment, from an approach focusingthat focused only on Federal 

Class I area visibility impacts to a more multifaceted approach that also 

considersconsidered PM-related impacts on visibility in non-Federal Class I area 

visibilityareas, such as in urban areas. This evolution occurred in conjunction with the 

expansion of available PM data and information from associatedvisibility-related studies 

of public perception, valuation, and personal comfort and well-being. 

In 1997, the EPA revised the PM NAAQS in part by establishing new identical 

primary and secondary PM2.5 standards. In revising the secondary standards, the EPA 

recognized that PM produces adverse effects on visibility and that impairment of 

visibility was being experienced throughout the U.S., in multi-state regions, urban areas, 

and remote mandatory Federal Class I areas alike. However, in considering an 

appropriate level for a secondary standard to address adverse effects of PM2.5 on 

visibility, the EPA concluded that the determination of a single national level was 

complicated by important regional differences influenced by factors such as background 

and current levels of PM2.5, composition of PM2.5, and average relative humidity. 

Variations in these factors across regions could thus result in situations where attaining 
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an appropriately protective concentration of fine particles in one region might or might 

not provide adequate protection in a different region. The EPA also determined that there 

was insufficient information at that time to establish a level for a national secondary 

standard that would represent a threshold above which visibility conditions would always 

be adverse and below which visibility conditions would always be acceptable.

Based on an assessment of the potential visibility improvements that would result 

from reaching attainment with the new primary standards for PM2.5, the EPA concluded 

that attainment of the annual and 24-hour PM2.5 primary standards would lead to 

visibility improvements in the eastern U.S. at both urban and regional scales, but little or 

no change in the western U.S., except in and near certain urban areas.

The EPA also considered the potential effectiveness of a regional haze program, 

required by sections 169A and 169B of the CAA153 to address those effects of PM on 

visibility that would not be addressed through attainment of the primary PM2.5 standards. 

The regional haze program would be designed to address the widespread, regionally 

uniform type of haze caused by a multitude of sources. The structure and requirements of 

sections 169A and 169B of the CAA provide for visibility protection programs that can 

be more responsive to the factors contributing to regional differences in visibility than 

can programs addressing the kinds of nationally applicable secondary NAAQS 

considered in the 1997 review. The regional haze visibility goal is more protective than a 

secondary NAAQS since the goal is to eliminate any anthropogenic impairment rather 

than to provide a level of protection from visibility impairment that is requisite to protect 

 

153 In 1977, Congress established as a national goal ‘‘the prevention of any future, and the 

remedying of any existing, impairment of visibility in mandatory Federal Class I areas 

which impairment results from manmade air pollution,” section 169A(a)(1) of the CAA. 

The EPA is required by section 169A(a)(4) of the CAA to promulgate regulations to 

ensure that ‘‘reasonable progress’’ is achieved toward meeting the national goal.
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the public welfare. Thus, an important factor considered in the 1997 review was whether 

a regional haze program, in conjunction with secondary standards set identical to the suite 

of PM2.5 primary standards, would provide appropriate protection for visibility in nonFederal Class I areas. The EPA concluded that the two programs and associated control 

strategies should provide such protection due to the regional approaches needed to 

manage emissions of pollutants that impair visibility in many of these areas.

For these reasons, in 1997 the EPA concluded that a national regional haze 

program, combined with a nationally applicable level of protection achieved through 

secondary PM2.5 standards set identical to the primary PM2.5 standards, would be more 

effective for addressing regional variations in the adverse effects of PM2.5 on visibility 

than would be national secondary standards for PM with levels lower than the primary 

PM2.5 standards. The EPA further recognized that people living in certain urban areas 

may place a high value on unique scenic resources in or near these areas and as a result 

might experience visibility problems attributable to sources that would not necessarily be 

addressed by the combined effects of a regional haze program and PM2.5 secondary 

standards. The EPA concluded that in such cases, state or local regulatory approaches, 

such as past action in Colorado to establish a local visibility standard for the City of 

Denver, would be more appropriate and effective in addressing these special situations 

because of the localized and unique characteristics of the problems involved. Visibility in 

an urban area located near a mandatory Federal Class I area could also be improved 

through state implementation of the then-current visibility regulations, by which emission 

limitations can be imposed on a source or group of sources found to be contributing to 

‘‘reasonably attributable’’ impairment in the mandatory Federal Class I area.
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Based on these considerations, in 1997 the EPA set secondary PM2.5 standards 

identical to the primary PM2.5 standards, that would work in conjunction with a regional 

haze programthe Regional Haze Program to be established under sections 169A and 

169B of the CAA, as the most appropriate and effective means of addressing the public 

welfare effects associated with visibility impairment. Together, the two programs and 

associated control strategies were expected to provide appropriate protection against PMrelated visibility impairment and enable all regions of the country to make reasonable 

progress toward the national visibility goal.

In 2006, the EPA revised the suite of secondary PM2.5 standards to address 

visibility impairment by making the suite of secondary standards identical to the revised 

suite of primary PM2.5 standards. The EPA’s decision regarding the need to revise the 

suite of secondary PM2.5 standards reflected a number of new developments that had 

occurred and sources of information that had become available following the 1997 

review. First, the EPA promulgated a Regional Haze Program in 1999 (65 FR 35713, 

July 1, 1999) which required states to establish goals for improving visibility in Federal 

Class I areas and to adopt control strategies to achieve these goals. Second, extensive new 

information from visibility and fine particle monitoring networks had become available, 

allowing for updated characterizations of visibility trends and PM concentrations in urban 

areas, as well as Federal Class I areas. These new data allowed the EPA to better 

characterize visibility impairment in urban areas and the relationship between visibility 

and PM2.5 concentrations. Finally, additional studies in the U.S. and abroad provided the 

basis for the establishment of standards and programs to address specific visibility 

concerns in a number of local areas. These studies (Denver, Phoenix, and British 
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Columbia) utilized photographic representations of visibility impairment and produced 

reasonably consistent results in terms of the visual ranges found to be generally 

acceptable by study participants. The EPA considered the information generated by these 

studies useful in characterizing the nature of particle-induced haze and for informing 

judgments about the acceptability of various levels of visual air quality in urban areas 

across the U.S. Based largely on this information, the Administrator concluded that it was 

appropriate to revise the secondary PM2.5 standards to provide increased protection from 

visibility impairment principally in urban areas, in conjunction with the regional haze 

program for protection of visual air quality in Federal Class I areas. 

In so doing, the Administrator recognized that PM-related visibility impairment is 

principally related to fine particle concentrations and that perception of visibility 

impairment is most directly related to short-term, nearly instantaneous levels of visual air 

quality. Thus, in considering whether the then-current suite of secondary standards would 

provide the appropriate degree of protection, he concluded that it was appropriate to 

focus on just the 24-hour secondary PM2.5 standard to provide requisite protection.

The Administrator then considered whether PM2.5 mass remained the appropriate 

indicator for a secondary standard to protect visibility, primarily in urban areas. The 

Administrator noted that PM-related visibility impairment is principally related to fine 

particle levels. Hygroscopic components of fine particles, in particular sulfates and 

nitrates, contribute disproportionately to visibility impairment under high humidity 

conditions. Particles in the coarse mode generally contribute only marginally to visibility 

impairment in urban areas. With the substantial addition to the air quality and visibility 

data made possible by the national urban PM2.5 monitoring networks, an analysis 
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conducted for the 2006 review found that, in urban areas, visibility levels showed far less 

difference between eastern and western regions on a 24-hour or shorter time basis than 

implied by the largely non-urban data available in the 1997 review. In analyzing how 

well PM2.5 concentrations correlated with visibility in urban locations across the U.S., the 

2005 Staff Paper concluded that clear correlations existed between 24-hour average PM2.5

concentrations and calculated (i.e., reconstructed) light extinction, which is directly 

related to visual range (U.S. EPA, 2005, p. 7-6). These correlations were similar in the 

eastern and western regions of the U.S. These correlations were less influenced by 

relative humidity and more consistent across regions when PM2.5 concentrations were 

averaged over shorter, daylight time periods (e.g., 4 to 8 hours) when relative humidity in 

eastern urban areas was generally lower and thus more similar to relative humidity in 

western urban areas. The 2005 Staff Paper noted that a standard set at any specific PM2.5

concentration would necessarily result in visual ranges that vary somewhat in urban areas 

across the country, reflecting the variability in the correlations between PM2.5

concentrations and light extinction. The 2005 Staff Paper concluded that it was 

appropriate to use PM2.5 as an indicator for standards to address visibility impairment in 

urban areas, especially when the indicator is defined for a relatively short period (e.g., 4 

to 8 hours) of daylight hours (U.S. EPA, 2005, p. 7-6). Based on their review of the Staff 

Paper, most CASAC Panel members also endorsed such a PM2.5 indicator for a secondary 

standard to address visibility impairment (Henderson, 2005a, p. 9). Based on the above 

considerations, the Administrator concluded that PM2.5 should be retained as the indicator 

for fine particles as part of a secondary standard to address visibility protection, in 

conjunction with averaging times from 4 to 24 hours.
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In considering what level of protection against PM-related visibility impairment 

would be appropriate, the Administrator took into account the results of the public 

perception and attitude surveys regarding the acceptability of various degrees of visibility 

impairment in the U.S. and Canada, state and local visibility standards within the U.S., 

and visual inspection of photographic representations of several urban areas across the 

U.S. In the Administrator’s judgment, these sources provided useful but still quite limited

information on the range of levels appropriate for consideration in setting a national 

visibility standard primarily for urban areas, given the generally subjective nature of the 

public welfare effect involved. Based on photographic representations of varying levels 

of visual air quality, public perception studies, and local and state visibility standards, the 

2005 Staff Paper had concluded that 30 to 20 μg/m3PM2.5 represented a reasonable range 

for a national visibility standard primarily for urban areas, based on a sub-daily averaging 

time (U.S. EPA, 2005, p. 7-13). The upper end of this range was below the levels at 

which illustrative scenic views are significantly obscured, and the lower end was around 

the level at which visual air quality generally appeared to be good based on observation 

of the illustrative views. This concentration range generally corresponded to median 

visual ranges in urban areas within regions across the U.S. of approximately 25 to 35 km, 

a range that was bounded above by the visual range targets selected in specific areas 

where state or local agencies placed particular emphasis on protecting visual air quality. 

In considering a reasonable range of forms for a PM2.5 standard within this range of 

levels, the 2005 Staff Paper had concluded that a concentration-based percentile form 

was appropriate, and that the upper end of the range of concentration percentiles for 

consideration should be consistent with the 98th percentile used for the primary standard 
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and that the lower end of the range should be the 92nd percentile, which represented the 

mean of the distribution of the 20 percent most impaired days, as targeted in the regional 

haze program (U.S. EPA, 2005 pp. 7-11 to 7-13). While recognizing that it was difficult 

to select any specific level and form based on then-currently available information

(Henderson, 2005a, p. 9), the CASAC Panel was generally in agreement with the ranges 

of levels and forms presented in the 2005 Staff Paper. 

The Administrator also considered the level of protection that would be afforded 

by the proposed suite of primary PM2.5 standards (71 FR 2681, January 17, 2006), on the 

basis that although significantly more information was available than in the 1997 review 

concerning the relationship between fine PM levels and visibility across the country, 

there was still little available information for use in making the relatively subjective value 

judgment needed in selecting the appropriate degree of protection to be afforded by such 

a standard. In so doing, the Administrator compared the extent to which the proposed 

suite of primary standards would require areas across the country to improve visual air 

quality with the extent of increased protection likely to be afforded by a standard based 

on a sub-daily averaging time. Based on such an analysis, the Administrator observed 

that the predicted percent of counties with monitors not likely to meet the proposed suite 

of primary PM2.5 standards was actually somewhat greater than the predicted percent of 

counties with monitors not likely to meet a sub-daily secondary standard with an 

averaging time of 4 daylight hours, a level toward the upper end of the range 

recommended in the 2005 Staff Paper, and a form within the recommended range. Based 

on this comparison, the Administrator tentatively concluded that revising the secondary 

24-hour PM2.5 standard to be identical to the proposed revised primary PM2.5 standard 
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(and retaining the then-current annual secondary PM2.5 standard) was a reasonable policy 

approach to addressing visibility protection primarily in urban areas. In proposing this 

approach, the Administrator also solicited comment on a sub-daily (4- to 8-hour 

averaging time) secondary PM2.5 standard (71 FR 2675 to 2781, January 17, 2006). 

In commenting on the proposed decision, the CASAC requested that a sub-daily 

standard to protect visibility “be favorably reconsidered” (Henderson, 2006a, p.6). The 

CASAC noted three cautions regarding the proposed reliance on a secondary PM2.5

standard identical to the proposed 24-hour primary PM2.5 standard: (1) PM2.5 mass 

measurement is a better indicator of visibility impairment during daylight hours, when 

relative humidity is generally low; the sub-daily standard more clearly matches the nature 

of visibility impairment, whose adverse effects are most evident during the daylight 

hours; using a 24-hour PM2.5 standard as a proxy introduces error and uncertainty in 

protecting visibility; and sub-daily standards are used for other NAAQS and should be 

the focus for visibility; (2) CASAC and its monitoring subcommittees had repeatedly 

commended EPA’s initiatives promoting the introduction of continuous and nearcontinuous PM monitoring and recognized that an expanded deployment of continuous 

PM2.5 monitors would be consistent with setting a sub-daily standard to protect visibility; 

and (3) the analysis showing a similarity between percentages of counties not likely to 

meet what the CASAC Panel considered to be a lenient 4- to 8-hour secondary standard 

and a secondary standard identical to the proposed 24-hour primary standard was a 

numerical coincidence that was not indicative of any fundamental relationship between 

visibility and health. The CASAC Panel further stated that “visual air quality is 

substantially impaired at PM2.5 concentrations of 35 μg/m3” and that “[i]t is not 
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reasonable to have the visibility standard tied to the health standard, which may change in 

ways that make it even less appropriate for visibility concerns” (Henderson, 2006a, pp. 5 

to 6). 

In reaching a final decision, the Administrator focused on the relative protection 

provided by the proposed primary standards based on the above-mentioned similarities in 

percentages of counties meeting alternative standards and on the limitations in the 

information available concerning studies of public perception and attitudes regarding the 

acceptability of various degrees of visibility impairment in urban areas, as well as on the 

subjective nature of the judgment required. In so doing, the Administrator concluded that 

caution was warranted in establishing a distinct secondary standard for visibility 

impairment and that the available information did not warrant adopting a secondary 

standard that would provide either more or less protection against visibility impairment in 

urban areas than would be provided by secondary standards set equal to the proposed 

primary PM2.5 standards.

2. Remand of 2006 Secondary PM2.5 Standards 

As noted above in section II.B.2 above, several parties filed petitions for review 

challenging EPA’s decision to set the secondary NAAQS for fine PM identical to the 

primary NAAQS. On judicial review, the D.C. Circuit remanded to the EPA for 

reconsideration the secondary NAAQS for fine PM because the Agency’s decision was 

unreasonable and contrary to the requirements of section 109(b)(2). American Farm 

Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir., 2009).

The petitioners argued that the EPA’s decision lacked a reasoned basis. First, they 

asserted that the EPA never determined what level of visibility was “requisite to protect 
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the public welfare.” They argued that the EPA unreasonably rejected the target level of 

protection recommended by its staff, while failing to provide a target level of its own. 

The court agreed, stating that “the EPA’s failure to identify such a level when deciding 

where to set the level of air quality required by the revised secondary fine PM NAAQS is 

contrary to the statute and therefore unlawful. Furthermore, the failure to set any target 

level of visibility protection deprived the EPA’s decision-making of a reasoned basis.” 

559 F. 3d at 530.

Second, the petitioners challenged EPA’s method of comparing the protection 

expected from potential standards. They contended that the EPA relied on a meaningless 

numerical comparison, ignored the effect of humidity on the usefulness of a standard 

using a daily averaging time, and unreasonably concluded that the primary standards 

would achieve a level of visibility roughly equivalent to the level the EPA staff and 

CASAC deemed “requisite to protect the public welfare.” The court found that the EPA’s 

equivalency analysis based on the percentages of counties exceeding alternative standards 

“failed on its own terms.” The same table showing the percentages of counties exceeding 

alternative secondary standards, used for comparison to the percentages of counties 

exceeding alternative primary standards to show equivalency, also included six other 

alternative secondary standards within the recommended CASAC range that would be 

more “protective” under EPA’s definition than the adopted primary standards. Two-thirds 

of the potential secondary standards within the CASAC’s recommended range would be 

substantially more protective than the adopted primary standards. The court found that 

the EPA failed to explain why it looked only at one of the few potential secondary 

standards that would be less protective, and only slightly less so, than the primary 
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standards. More fundamentally, however, the court found that the EPA’s equivalency 

analysis based on percentages of counties demonstrated nothing about the relative 

protection offered by the different standards, and that the tables offered no valid 

information about the relative visibility protection provided by the standards. 559 F. 3d at 

530-31.

Finally, the Staff Paper had made clear that a visibility standard using PM2.5 mass 

as the indicator in conjunction with a daily averaging time would be confounded by 

regional differences in humidity. The court noted that the EPA acknowledged this 

problem, yet did not address this issue in concluding that the primary standards would be 

sufficiently protective of visibility. 559 F. 3d at 530. Therefore, the court granted the 

petition for review and remanded for reconsideration the secondary PM2.5 NAAQS.

3. General Approach Used in the Policy Assessment for the Current Review

The approach used in this review broadened the general approaches used in the 

last two PM NAAQS reviews by utilizing, to the extent available, enhanced tools, 

methods, and data to more comprehensively characterize visibility impacts. As such, the 

EPA took into account considerations based on both the scientific evidence (“evidencebased”) and a quantitative analysis of PM-related impacts on visibility (“impact-based”) 

to inform conclusions related to the adequacy of the current secondary PM2.5 standards 

and alternative standards that were appropriate for consideration in this review. As in past 

reviews, the EPA also considered that the secondary NAAQS should address PM-related 

visibility impairment in conjunction with the Regional Haze Program, such that the 

secondary NAAQS would focus on protection from visibility impairment principally in 

urban areas in conjunction with the Regional Haze Program that is focused on improving 
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visibility in Federal Class I areas. The EPA again recognized that such an approach 

remains the most appropriate and effective means of addressing the public welfare effects 

associated with visibility impairment in areas across the country.

The Policy Assessment drew from the qualitative evaluation of all studies 

discussed in the Integrated Science Assessment (U.S. EPA, 2009a). Specifically, the 

Policy Assessment considered the extensive new air quality and source apportionment 

information available from the regional planning organizations, long-standing evidence 

of PM effects on visibility, and limited public preference studiesstudy information from 

four urban areas (U.S. EPA, 2009a, chapter 9), as well as the integration of evidence 

across disciplines (U.S. EPA, 2009a, chapter 2). In addition, limited information that had 

become available regarding the characterization of public preferences in urban areas 

provided some new perspectives on the usefulness of this information in informing the 

selection of target levels of urban visibility protection. On these bases, the Policy 

Assessment again focused assessments on visibility conditions in urban areas.

The conclusions in the Policy Assessment reflected EPA staff’s understanding of 

both evidence-based and impact-based considerations to inform two overarching 

questions related to (1) the adequacy of the current suite of PM2.5 standards and (2) what 

potential alternative standards, if any, should be considered in this review to provide 

appropriate protection from PM-related visibility impairment. In addressing these broad 

questions, the discussions in the Policy Assessment were organized around a series of 

more specific questions reflecting different aspects of each overarching question (U.S. 

EPA, 2011a, Figure 4-1). When evaluating the visibility protection afforded by the 

current or any alternative standards considered, the Policy Assessment took into account 
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the four basic elements of the NAAQS: indicator, averaging time, level, and form.

B. Proposed Decisions on Secondary PM Standards

At the time of proposal, the Administrator proposed to revise the suite of 

secondary PM standards by adding a distinct standard for PM2.5 to address PM-related 

visibility impairment, focused primarily on visibility in urban areas. This proposed 

standard was to be defined in terms of a PM2.5 visibility index, which would use 

measured PM2.5 mass concentration, in combination with speciation and relative humidity 

data, to calculate PM2.5 light extinction, translated into the deciview (dv) scale; a 24-hour 

averaging time; a 90th percentile form, averaged over 3 years; and a level of 28-30 dv. To 

address other non-visibility welfare effects, the Administrator proposed to retain the 

current suite of secondary PM standards generally, while revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this proposed change to the primary annual PM2.5 standard. Each of these proposed 

decisions is described in more detail in the proposal and below.

1. PM-Related Visibility Impairment

As discussed in Section VI.B. of the proposal, the Administrator’s proposed 

decision regarding a distinct secondary standard to provide protection from visibility 

impairment reflected careful consideration of the following: (1) the latest scientific 

information on visibility effects associated with PM as described in the Integrated 

Science Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of 

correlations between ambient PM2.5 and visibility impairment prepared by EPA staff in 

the Visibility Assessment (U.S. EPA, 2010b); and (3) specific conclusions regarding the 

need for revisions to the current standards (i.e., indicator, averaging time, form, and 
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level) that, taken together, would be requisite to protect the public welfare from adverse 

effects on visual air quality. This section summarizes key information from the proposal 

regarding the nature of visibility impairment, including the relationship between ambient 

PM and visibility, temporal variations in light extinction, periods during the day of 

interest for assessing visibility conditions, and exposure durations of interest (section 

VI.B.1.a); limited public perceptions and attitudes about visibility impairment and the 

impacts of visibility impairment on public welfare (section VI.B.1.b); CASAC advice 

regarding the need for, and design of, secondary standards to protect visibility (section 

VI.B.1.c); and the Administrator’s proposed conclusions regarding setting a distinct 

standard to address visibility impairment (section VI.B.1.d).

a. Nature of PM-related Visibility Impairment

As noted at the time of proposal, the fundamental science characterizing the 

contribution of PM, especially fine particles, to visibility impairment is well understood. 

This science provides the basis for the Integrated Science Assessment designation of the 

relationship between PM and visibility impairment as causal. New research available in 

this review, discussed in chapter 9 of the Integrated Science Assessment, continues to 

support and refine EPA’s understanding of the effect of PM on visibility and the source 

contributions to that effect in rural and remote locations. This research provides new 

insights regarding the regional source contributions to urban visibility impairment and 

better characterization of the increment in PM concentrations and visibility impairment 

that occur in many cities (i.e., the urban excess) relative to conditions in the surrounding 

rural areas (i.e., regional background). Ongoing urban PM2.5 speciated and aggregated 

mass monitoring has produced new information that has allowed for updated 
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characterization of current visibility levels in urban areas.

i. Relationship between Ambient PM and Visibility

Visibility impairment is caused by the scattering and absorption of light by 

suspended particles and gases in the atmosphere. When PM is present in the air, its 

contribution to light extinction typically greatly exceeds that of gases. The combined 

effect of light scattering and absorption by both particles and gases is characterized as 

light extinction, i.e., the fraction of light that is scattered or absorbed in the atmosphere. 

Light extinction can be quantified by a light extinction coefficient with units of 

1/distance, which is often expressed as 1/(1 million meters) or inverse megameters 

(abbreviated Mm-1) or in terms of an alternative scale known as the deciview scale, 

defined by the following equation:154

Deciview (dv) = 10 ln (bext / 10 Mm-1) 

The deciview scale is frequently used in the scientific literature on visibility, as well as in 

the Regional Haze Program. In particular, the deciview scale is used in the public 

perception studies that were considered in the past and current reviews to inform 

judgments about an appropriate degree of protection to be provided by a secondary 

NAAQS.

The amount of light extinction contributed by PM depends on the particle 

concentration as well as on the particle size distribution and composition and also on the 

relative humidity. As described in detail in section VI.B.1.a of the proposal, visibility 

 

154 As used in the Regional Haze Program, the term bext refers to light extinction due to 

PM2.5, PM10-2.5, and “clean” atmospheric gases. In the Policy Assessment, in focusing on 

light extinction due to PM2.5, the deciview values include only the effects of PM2.5 and 

the gases. The “Rayleigh” term associated with clean atmospheric gases is represented by 

the constant value of 10 Mm-1. Omission of the Rayleigh term would create the 

possibility of negative deciview values when the PM2.5 concentration is very low.
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scientists have developed an algorithm, known as the IMPROVE algorithm,155 to 

estimate light extinction using routinely monitored fine particle (PM2.5) speciation and 

coarse particle mass (PM10–2.5) data, as well as data on relative humidity. There is both an 

original and a revised version of the IMPROVE algorithm (Pitchford et al., 2007). The 

revised version was developed to address observed biases in the predictions using the 

original algorithm under very low and very high light extinction conditions.156 These 

IMPROVE algorithms are routinely used to calculate light extinction levels on a 24-hour 

basis in Federal Class I areas under the Regional Haze Program.

In either version of the IMPROVE algorithm, the concentration of each of the 

major aerosol components is multiplied by a dry extinction efficiency value and, for the 

hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also 

multiplied by an additional factor to account for the water growth to estimate these 

components’ contribution to light extinction. Summing the contribution of each 

component gives the estimate of total light extinction per unit distance denoted as the 

light extinction coefficient (bext), as shown below for the original IMPROVE algorithm. 

bext ≈ 3 x f(RH) x [Sulfate]

+ 3 x f(RH) x [Nitrate]

+ 4 x [Organic Mass]

+ 10 x [Elemental Carbon]

+ 1 x [Fine Soil]

+ 0.6 x [Coarse Mass]

+ 10 

 

155 The algorithm is referred to as the IMPROVE algorithm because it was developed 

specifically to use the aerosol monitoring data generated at network sites and with 

equipment specifically designed to support the IMPROVE program and was evaluated 

using IMPROVE optical measurements at the subset of sites that make those 

measurements (Malm et al., 1994).

156 These biases were detected by comparing light extinction estimates generated from the 

IMPROVE algorithm to direct optical measurements in a number of rural Federal Class I 

areas.
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Light extinction (bext) is in units of Mm-1, the mass concentrations of the 

components indicated in brackets are in units of μg/m3, and f(RH) is the unitless water 

growth term that depends on relative humidity. The final term of 10 Mm-1 is known as the 

Rayleigh scattering term and accounts for light scattering by the natural gases in 

unpolluted air. Despite the simplicity of this algorithm, it performs reasonably well and 

permits the contributions to light extinction from each of the major components 

(including the water associated with the sulfate and nitrate compounds) to be separately 

approximated. Inspection of the PM component-specific terms in the simple original 

IMPROVE algorithm shows that most of the PM2.5 components contribute 5 times or 

more light extinction than a similar concentration of PM10–2.5. 

The f(RH) term in the original algorithm reflects the increase in light scattering 

caused by particulate sulfate and nitrate under conditions of high relative humidity. 

Particles with hygroscopic components (e.g., particulate sulfate and nitrate) contribute 

more light extinction at higher relative humidity than at lower relative humidity because 

they change size in the atmosphere in response to ambient relative humidity conditions. 

For relative humidity below 40 percent the f(RH) value is 1, but it increases to 2 at 

approximately 66 percent, 3 at approximately 83 percent, 4 at approximately 90 percent, 

5 at approximately 93 percent, and 6 at approximately 95 percent relative humidity. The 

result is that both particulate sulfate and nitrate are more efficient per unit mass in light 

extinction than any other aerosol component for relative humidity above approximately 

85 percent where their total light extinction efficiency exceeds the 10 m2/g associated 

with elemental carbon (EC). PM containing elemental or black carbon (BC) absorbs light 

as well as scattering it, making it the component with the greatest light extinction 
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contributions per unit of mass concentration, except for the hygroscopic components 

under these high relative humidity conditions.157

SubsequentAs noted above, subsequent to the development of the original 

IMPROVE algorithm, an alternative algorithm (variously referred to as the “revised 

algorithm” or the “new algorithm” in the literature) has been developed. Itwas developed. 

The revised IMPROVE algorithm is different from the original algorithm in several 

important ways. First, the revised algorithm employs a more complex split-component 

mass extinction efficiency to correct biases believed to be related to particle size 

distributions, a sea salt term that can be important for remote coastal areas,.

158

Specifically, the revised algorithm incorporates terms to account for particles 

representing the different dry extinction and water uptake from two size modes of sulfate, 

nitrate and organic mass.

159 Second, the revised algorithm uses a different multiplier for 

 

157 The IMPROVE algorithm does not explicitly separate the light-scattering and lightabsorbing effects of elemental carbon.

158 In either version of the IMPROVE algorithm, the concentration of each of the major 

aerosol components is multiplied by a dry extinction efficiency value and, for the 

hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also 

multiplied by an additional factor to account for the water growth to estimate these 

components’ contribution to light extinction. Both the dry extinction efficiency and water 

growth terms have been developed by a combination of empirical assessment and 

theoretical calculation using typical particle size distributions associated with each of the 

major aerosol components.

159 The relative contributions of sulfate, nitrate and organic mass concentrations to 

visibility impairment with the revised algorithm are different than with the original 

algorithm due to the combination of the dry extinction coefficient and f(RH) functions for 

derived concentrations of small and large particles. The apportionment of the total fine 

particle concentration of each of the three PM2.5 components into the concentrations of 

the small and large size fractions was empirically developed for remote areas. The 

fraction of the fine particle component that is in the large mode is estimated by dividing 

the total concentration of the component by 20 ug/m

3

. If the total concentration of a 

component exceeds 20 ug/m3, all of it is assumed to be in the large mode. 
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organic carbon for purposes of estimating organic carbonaceous material,

160 to better 

represent aged aerosol found in remote areas.

161 In addition, the revised algorithm 

includes a term for hygroscopic sea salt that can be important for remote coastal areas,

and site-specific Rayleigh light scattering terms in place of a universal Rayleigh light 

scattering value. As noted in section VI.B.1.a of the proposal, the revised IMPROVE 

algorithm can yield higher estimates of current light extinction levels in urban areas on 

days with relatively poor visibility as compared to the original algorithm (Pitchford, 

2010). This difference is primarily attributable to the split-component mass extinction 

efficiency treatment in the revised algorithm. This revised algorithm was evaluated at 21 

remote locations and is generally used by RPOs and States for implementation of the 

Regional Haze Rule.

ii. Temporal Variations of Light Extinction

Particulate matter concentrations and light extinction in urban environments vary 

from hour to hour throughout the 24-hour day due to a combination of diurnal changes in 

meteorological conditions and systematic changes in emissions activity (e.g., rush hour 

traffic). Various factors combine to make early morning the most likely time for peak 

urban light extinction; although the net effects of the systematic urban- and larger-scale 

variations mean that peak daytime PM light extinction levels can occur any time of day, 

in many areas they occur most often in early morning hours (U.S. EPA, 2010b, sections 

3.4.2 and 3.4.3; Figures 3-9, 3-10, and 3-12). This temporal pattern in urban areas 

contrasts with the general lack of a strong diurnal pattern in PM concentrations and light 

 

160 The revised IMPROVE algorithm uses a multiplier of 1.8 for rural areas instead of 1.4 

as used in the original algorithm for the mean ratio of organic mass to organic carbon.

161 The revised IMPROVE algorithm uses a multiplier of 1.8 for rural areas instead of 1.4 

as used in the original algorithm for the mean ratio of organic mass to organic carbon.
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extinction in most Federal Class I areas, reflective of a relative lack of local sources as 

compared to urban areas. The use in the Regional Haze Program of 24-hour average 

concentrations in the IMPROVE algorithm is consistent with this general lack of a strong 

diurnal pattern in Federal Class I areas. 

iii. Periods during the Day of Interest for Assessment of Visibility

As noted in sections VI.B.1.b and VI.B.1.c of the proposal, daytime visibility has 

dominated the attention of those who have studied the visibility effects of air pollution, 

particularly in urban areas. The EPA recognizes, however, that physically PM light 

extinction behaves the same at night as during the day and can contribute to nighttime 

visibility effects by enhancing the scattering of anthropogenic light, contributing to the 

“skyglow” within and over populated areas, adding to the total sky brightness, and 

contributing to the reduction in contrast of stars against the background. However, little 

research has been conducted on nighttime visibility, and the state of the science is not 

comparable to that associated with daytime visibility impairment, particularly in terms of 

the impact on human welfare. The Policy Assessment notes that the science is not 

available at this time to support adequate characterization specifically of nighttime PM 

light extinction conditions and the related effects on public welfare (U.S. EPA, 2011a, p. 

4-18). Therefore the EPA has focused its assessments of PM visibility impacts in urban 

areas on daylight hours during this review. 

iv. Exposure Durations of Interest

As noted in section VI.B.1.d of the proposal, the roles that exposure duration and 

variations in visual air quality within any given exposure period play in determining the 

acceptability or unacceptability of a given level of visual air quality have not been 
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investigated via preference studies. In the preference studies available for this review, 

subjects were simply asked to rate the acceptability or unacceptability of each image of a 

haze-obscured scene, without being provided any suggestion of assumed duration or of 

assumed conditions before or after the occurrence of the scene presented. Preference 

and/or valuation studies show that atmospheric visibility conditions can be quickly 

assessed and preferences determined. The EPA is unaware of any studies that 

characterize the extent to which different frequencies and durations of exposure to 

visibility conditions contribute to the degree of public welfare impact that occurs. 

The Policy Assessment considered a variety of circumstances that are commonly 

expected to occur in evaluating the potential impact of visibility impairment on the public 

welfare based on available information (U.S. EPA, 2011a, pp. 4-19 to 4-20). In some 

circumstances, such as infrequent visits to scenic vistas in natural or urban environments, 

people are motivated specifically to take the opportunity to view a valued scene and are 

likely to do so for many minutes to hours to appreciate various aspects of the vista they 

choose to view. However, the public has many more opportunities to notice visibility 

conditions on a daily basis in settings associated with performing daily routines (e.g., 

during commutes and while working, exercising, or recreating outdoors). As noted in the 

Policy Assessment, information regarding the fraction of the public that has only one or a 

few opportunities to experience visibility during the day, or on the role the duration of the 

observed visibility conditions has on wellbeing effects associated with those visibility 

conditions, is not available (U.S. EPA, 2011a, p. 4-20). However, it is possible that 

people with limited opportunities to experience visibility conditions on a daily basis 

would receive the entire impact of the day’s visual air quality based on the visibility 
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conditions that occur during the short time period when they can see it. Since this group 

could be affected on the basis of observing visual air quality conditions for periods as 

short as one hour or less, and because during each daylight hour there are some people 

outdoors, commuting, or near windows, the Policy Assessment judged that it would be 

appropriate to use the maximum hourly value of PM light extinction during daylight 

hours for each day for purposes of evaluating the adequacy of the current suite of 

secondary standards. Other observers may have access to visibility conditions throughout 

the day. For this group, it might be that an hour with poor or “unacceptable” visibility can 

be offset by one or more other hours with clearer conditions. Therefore, the proposal 

acknowledged that it might also be appropriate to consider a multi-hour daylight 

exposure period.

v. Periods of Fog and Rain

As discussed in section VI.C of the proposal, the EPA also recognized that it is 

appropriate to give special treatment to periods of fog and rain when considering whether 

current PM2.5 standards adequately protect public welfare from PM-related visibility 

impairment. Visibility impairment occurs during periods with fog or precipitation 

irrespective of the presence or absence of PM. Therefore, it is logical that periods with 

naturally impaired visibility due to fog or precipitation should not be treated as having 

PM-impaired visibility. There are multiple ways to adjust visibility data to reduce the 

effects of fog and precipitation. In the Visibility Assessment, following the advice of 

CASAC, the EPA evaluated the effect of excluding daylight hours for which relative 

humidity was greater than 90 percent from analyses in order to avoid precipitation and 

fog confounding estimates of PM visibility impairment. For the 15 urban areas included 
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in the Visibility Assessment, the EPA found that a 90 percent relative humidity cutoff 

criterion was effective in that on average less than 6 percent of the daylight hours were 

removed from consideration, yet those hours had on average ten times the likelihood of 

rain, six times the likelihood of snow/sleet, and 34 times the likelihood of fog compared 

with hours with 90 percent or lower relative humidity. In the Regional Haze program, the 

EPA utilizes monthly average relative humidity values based on 10 years of 

climatological data to reduce the effect of fog and precipitation. This approach focuses on 

longer-term averages for each monitoring site and thereby eliminates the effect of very 

high humidity conditions on visibility at those locations.

b. Public Perception of Visibility Impairment

As described in section VI.B.2 of the proposal, there are two main types of studies 

that evaluate the public perception of urban visibility impairment: urban visibility 

preference studies and urban visibility valuation studies. As noted in the Integrated 

Science Assessment, “[b]oth types of studies are designed to evaluate individuals’ desire 

(or demand) for good visual air quality (VAQ) where they live, using different metrics to 

evaluate demand. Urban visibility preference studies examine individuals’ demand by 

investigating what amount of visibility degradation is unacceptable while economic 

studies examine demand by investigating how much one would be willing to pay to 

improve visibility” (U.S. EPA, 2009a, p. 9-66). Because of the limited number of new 

studies on urban visibility valuation, the Integrated Science Assessment cites to the 

discussion in the 2004 Criteria Document of the various methods one can use to 

determine the economic valuation of changes in visibility, which include hedonic 

valuation, contingent valuation and contingent choice, and travel cost.
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Contingent valuation studies are a type of stated preference study that measures 

the strength of preferences and expresses that preference in dollar values. Contingent 

valuation studies often include payment vehicles that require respondents to consider 

implementation costs and their ability to pay for visibility improvements in their 

responses. This study design aspect is critical because the EPA cannot consider

implementations costs in setting either primary or secondary NAAQS. Therefore in 

considering the information available to help inform the standard-setting process, the 

EPA has focused on the public perception studies that do not embed consideration of 

implementation costs. Nonetheless, the EPA recognizes that valuation studies do provide 

additional evidence that the public is experiencing losses in welfare due to visibility 

impairment.162 The public perception studies are described in detail below.

In order to identify levels of visibility impairment appropriate for consideration in 

setting secondary PM NAAQS to protect the public welfare, the Visibility Assessment 

comprehensively examined information that was available in this review regarding 

people’s stated preferences regarding acceptable and unacceptable visual air quality.

Light extinction is an atmospheric property that by itself does not directly 

translate into a public welfare effect. Instead, light extinction becomes meaningful in the 

context of the impact of differences in visibility on the human observer. This has been 

 

162 In the regulatory impact analysis (RIA) accompanying this rulemaking, the EPA 

describes a revised approach to estimate urban residential visibility benefits that applies 

the results of several contingent valuation studies. The EPA is unable to apply the public 

perception studies to estimate benefits because they do not provide sufficient information 

on which to develop monetized benefits estimates. Specifically, the public perception 

studies do not provide preferences expressed in dollar values, even though they do 

provide additional evidence that the benefits associated with improving residential 

visibility are not zero. As previously noted in this preamble, the RIA is done for 

informational purposes only, and the proposed decisions on the NAAQS in this 

rulemaking are not in any way based on consideration of the information or analyses in 

the RIA.
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studied in terms of the acceptability or unacceptability expressed for the visibility impact 

of a given level of light extinction by a human observer. The perception of the visibility 

impact of a given level of light extinction occurs in conjunction with the associated 

characteristics and lighting conditions of the viewed scene.163 Thus, a given level of light 

extinction may be perceived differently by observers looking at different scenes or the 

same scene with different lighting characteristics. Likewise, different observers looking 

at the same scene with the same lighting may have different preferences regarding the 

associated visual air quality. When scene and lighting characteristics are held constant, 

the perceived appearance of a scene (i.e., how well the scenic features can be seen and 

the amount of visible haze) depends only on changes in light extinction. This has been 

demonstrated using the WinHaze model (Molenar et al., 1994) that uses image processing 

technology to apply user-specified changes in light extinction values to the same base 

photograph with set scene and lighting characteristics. 

Much of what is known about the acceptability of levels of visibility comes from 

survey studies in which participants were asked questions about their preference or the 

value they place on various visibility levels as displayed to them in scenic photographs 

and/or WinHaze images with a range of known light extinction levels. Urban visibility 

preference studies for four urban areas were reviewed in theThe Visibility Assessment 

(U.S. EPA, 2010b, chapter 2) reviewed the limited number of urban visibility preference 

studies currently available (i.e., four studies) to assess the light extinction levels judged 

 

163 By “characteristics of the scene” the EPA means the distance(s) between the viewer 

and the object(s) of interest, the shapes and colors of the objects, the contrast between 

objects and the sky or other background, and the inherent interest of the objects to the 

viewer. Distance is particularly important because at a given value of light extinction, 

which is a property of air at a given point(s) in space, more light is actually absorbed and 

scattered when light passes through more air between the object and the viewer.
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by the participant to have acceptable visibility for those particular scenes.

The reanalysis of urban preference studies conducted in the Visibility Assessment 

for this review included three completed western urban visibility preference survey 

studies plus a pair of smaller focus studies designed to explore and further develop urban 

visibility survey instruments. The three western studies included one in Denver, Colorado 

(Ely et al., 1991), one in the lower Fraser River valley near Vancouver, British Columbia 

(BC), Canada (Pryor, 1996), and one in Phoenix, Arizona (BBC Research & Consulting, 

2003). A pilot focus group study was also conducted for Washington, DC (Abt 

Associates Inc., 2001). In response to an EPA request for public comment on the Scope 

and Methods Plan (74 FR 11580, March 18, 2009), comments were received (Smith, 

2009) about the results of a new focus group study of scenes from Washington, DC, that 

had been conducted on subjects from both Houston, Texas, and Washington, DC, using 

scenes, methods and approaches similar to the method and approach employed in the 

EPA pilot study (Smith and Howell, 2009). When taken together, these studies from the 

four different urban areas included a total of 852 individuals, with each individual 

responding to a series of questions while viewing a set of images of various urban visual 

air quality conditions. 

The approaches used in the four studies were similar and were all derived from 

the method first developed for the Denver urban visibility study. In particular, the studies 

all used a similar group interview type of survey to investigate the level of visibility 

impairment that participants described as “acceptable.” In each preference study, 

participants were initially given a set of “warm up” exercises to familiarize them with 

how the scene in the photograph or image appears under different VAQ conditions. The 
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participants next were shown 25 randomly ordered photographs (images), and asked to 

rate each one based on a scale of 1 (poor) to 7 (excellent). They were then shown the 

same photographs or images again, in the same order, and asked to judge whether each of 

the photographs (images) would violate what they would consider to be an appropriate 

urban visibility standard (i.e. whether the level of impairment was “acceptable” or 

“unacceptable”). The term “acceptable” was not defined, so that each person’s response 

was based on his/her own values and preferences for VAQ. However, when answering 

this question, participants were instructed to consider the following three factors: (1) The 

standard would be for their own urban area, not a pristine national park area where the 

standards might be stricter; (2) The level of an urban visibility standard violation should 

be set at a VAQ level considered to be unreasonable, objectionable, and unacceptable 

visually; and (3) Judgments of standards violations should be based on visibility only, not 

on health effects. While the results differed among the four urban areas, results from a 

rating exercise show that within each preference study, individual survey participants 

consistently distinguish between photos or images representing different levels of light 

extinction, and that more participants rate as acceptable images representing lower levels 

of light extinction than they do images representing higher levels. 

Given the similarities in the approaches used, it isthe EPA staff concluded that it 

was reasonable to compare the results to identify overall trends in the study findings and 

to conclude that this comparison can usefully inform the selection of a range of levels for 

use in further analyses. However, the staff also noted that variations in the specific 

materials and methods used in each study introduce uncertainties that should also be 

considered when interpreting the results of these comparisons. Key differences between 
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the studies include the following: 1) scene characteristics; 2) image presentation methods 

(e.g., projected slides of actual photos, projected images generated using WinHaze (a 

significant technical advance in the method of presenting visual air quality conditions), or 

use of a computer monitor screen; 3) number of participants in each study; 4) participant 

representativeness of the general population of the relevant metropolitan area; and 5)

specific wording used to frame the questions used in the group interview process. 

In the Visibility Assessment, each study was evaluated separately and figures 

developed to display the percentage of participants that rated the visual air quality 

depicted in each photograph as “acceptable.” Ely et al. (1991) introduced a “50% 

acceptability” criterion analysis of the Denver preference study results. The 50 percent 

acceptability criterion is designed to identify the visual air quality level (defined in terms

of deciviews or light extinction) that best divides the photographs into two groups: those 

with a visual air quality rated as acceptable by the majority of the participants, and those 

rated not acceptable by the majority of participants. The Visibility Assessment adopted 

this criterion as a useful index for comparison between studies. The results of each 

analysis were then combined graphically to allow for visual comparison. This 

information was then carried forward into the Policy Assessment. Figure 65 presents the 

graphical summary of the results of the studies in the four cities and draws on results 

previously presented in Figures 2-3, 2-5, 2-7, and 2-11 of chapter 2 in the Visibility 

Assessment. Figure 65 also contains lines at 20 dv and 30 dv that generally identify a 

range where the 50 percent acceptance criteria occur across all four of the urban 

preference studies (U.S. EPA, 2011a, p. 4-24). Out of the 114 data points shown in 

Figure 65, only one photograph (or image) with a visual air quality below 20 dv was 
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rated as acceptable by less than 50 percent of the participants who rated that 

photograph.164 Similarly, only one image with a visual air quality above 30 dv was rated 

acceptable by more than 50 percent of the participants who viewed it.165

Figure 65. Summary of Results of Urban Visibility Studies in Four Cities, Showing the 

Identified Range of the 50% Acceptance Criteria166

Source: U.S. EPA, 2011a, Figure 4-2; U.S. EPA 2010b, Figure 2-16

As Figure 65 above shows, each urban area has a separate and unique response 

 

164 Only 47 percent of the British Columbia participants rated a 19.2 dv photograph as 

acceptable.

165 In the 2001 Washington, DC study, a 30.9 dv image was used as a repeated slide. The 

first time it was shown 56 percent of the participants rated it as acceptable, but only 11 

percent rated it as acceptable the second time it was shown. The same visual air quality 

level was rated as acceptable by 4 percent of the participants in the 2009 study (Test 1). 

All three points are shown in Figure X5.

166 Top scale shows light extinction in inverse megameter units; bottom scale in 

deciviews. Logit analysis estimated response functions are shown as the color-coded 

curved lines for each of the four urban areas.
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curve that appears to indicate that it is distinct from the others.167 These curves are the 

result of a logistical regression analysis using a logit model of the greater than 19,000 

ratings of haze images as acceptable or unacceptable. The model results can be used to 

estimate the visual air quality in terms of dv values where the estimated response 

functions cross the 50 percent acceptability level, as well as any alternative criteria levels. 

Selected examples of these are shown in Table 4-1 of the Policy Assessment (U.S. EPA, 

2011a; U.S. EPA, 2010b, Table 2-4). This table shows that the logit model results also 

support the upper and lower ends of the range of 50th percentile acceptability values (e.g., 

near 20 dv for Denver and near 30 dv for Washington, DC) already identified in Figure 

65. 

Based on the composite results and the effective range of 50th percentile 

acceptability across the four urban preference studies shown in Figure 65 and Table 4-1 

of the Policy Assessment, benchmark levels of (total) light extinction were selected in a 

range from 20 dv to 30 dv (75 to 200 Mm-1)

168 for the purpose of provisionally assessing 

whether visibility conditions would be considered acceptable (i.e., less than the low end 

of the range), unacceptable (i.e., greater than the high end of the range), or potentially 

acceptable (within the range).) based on the very limited public preference information. A 

 

167 At present, data is only available for four urban areas, as presented in Figure 5 and 

discussed throughout this section. Additional research could help inform whether the 

range identified by combining the results of the studies depicted in Figure 5 is more 

broadly representative.

168 These values were rounded from 74 Mm-1

and 201 Mm-1to avoid an implication of 

greater precision than is warranted. Note that the middle value of 25 dv when converted 

to light extinction is 122 Mm-1is rounded to 120 Mm-1for the same reason. Assessments 

conducted for the Visibility Assessment and the first and second drafts of the Policy 

Assessment used the unrounded values. The Policy Assessment considered the results of 

assessment using unrounded values to be sufficiently representative of what would result 

if the rounded values were used that it was unnecessary to redo the assessments. That is 

why some tables and figures in the Policy Assessment reflected the unrounded values.
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midpoint of 25 dv (120 Mm-1) was also selected for use in the assessment. This level is 

also very near to the 50th percentile criterion value from the Phoenix study (i.e., 24.2 dv), 

which is by far the best of the four studies in terms of the fit of the data to the response 

curve and the representativeness of study participants. Based on the currently available 

information, the Policy Assessment concluded that the use of 25 dv to represent the 

middle of the distribution of results seemed well supported (U.S. EPA, 2011a, p. 4-25).

These three benchmark values provide a low, middle, and high set of light 

extinction conditions that are used to provisionally define daylight hours with urban haze 

conditions that have been judged unacceptable by at least 50 percent of the participants in 

one or more of these preference studies. As discussed above, PM light extinction is taken 

to be (total) light extinction minus the Rayleigh scatter,169 such that the low, middle, and 

high levels correspond to PM light extinction levels of about 65 Mm-1, 110 Mm-1, and 

190 Mm-1. In the Visibility Assessment, these three light extinction levels were called 

Candidate Protection Levels (CPLs). This term was also used in the Policy Assessment 

and in the proposal notice. It is important to note, however, that the degree of protection 

provided by a secondary NAAQS is not determined solely by any one component of the 

standard but by all the components (i.e., indicator, averaging time, form, and level) being 

applied together. Therefore, the Policy Assessment noted that the term CPL is meant only 

to indicate target levels of visibility within a range that the EPA staff felt appropriate for 

consideration that could, in conjunction with other elements of the standard, including 

indicator, averaging time, and form, potentially provide an appropriate degree of 

visibility protection.

 

169 Rayleigh scatter is light scattering by atmospheric gases which is on average about 10 

Mm-1.
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In characterizing the Policy Assessment’s confidence in each CPL and across the 

range, a number of issues were considered (U.S. EPA, 2011a, p. 4-26). Looking first at 

the two studies that define the upper and lower bounds of the range, the Policy 

Assessment considered whether they represent a true regional distinction in preferences 

for urban visibility conditions between western and eastern U.S. There was little 

information available to help evaluate the possibility of a regional distinction especially 

given that there have been preference studies in only one eastern urban area. Smith and 

Howell (2009) found little difference in preference response to Washington, DC, haze 

photographs between the study participants from Washington, DC, and those from 

Houston, Texas.170 This provides some limited evidence that the value judgment of the 

public in different areas of the country may not be an important factor in explaining the 

differences in these study results.

In further considering what factors could explain the observed differences in 

preferences across the four urban areas, the Policy Assessment noted that the urban 

scenes used in each study had different characteristics (U.S. EPA, 2011a, p. 4-26). For 

example, each of the western urban visibility preference study scenes included mountains 

in the background while the single eastern urban study did not. It is also true that each of 

the western scenes included objects at greater distances from the camera location than in 

the eastern study. There is no question that objects at a greater distance have a greater 

sensitivity to perceived visibility changes as light extinction is changed compared to 

otherwise similar scenes with objects at a shorter range. This alone might explain the 

 

170 The first preference study using WinHaze images of a scenic vista from Washington, 

DC was conducted in 2001 using subjects who were residents of Washington, DC. More 

recently, Smith and Howell (2009) interviewed additional subjects using the same images 

and interview procedure. The additional subjects included some residents of the 

Washington, DC area and some residents of the Houston, Texas area. 
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difference between the results of the eastern study and those from the western urban 

studies. Having scenes with the object of greatest intrinsic value nearer and hence less 

sensitive in the eastern urban area compared with more distant objects of greatest 

intrinsic value in the western urban areas could further explain the difference in 

preference results. 

Another question considered was whether the high CPL value that is based on the 

eastern preference results is likely to be generally representative of urban areas that do 

not have associated mountains or other valued objects visible in the distant background. 

Such areas would include the middle of the country, many areas in the eastern U.S., and 

possibly some areas in the western U.S. as well.171 Based on the currently available 

information, the Policy Assessment concluded that the high end of the CPL range (30 dv) 

is an appropriate level to consider (U.S. EPA, 2011a, p. 4-27).

With respect to the low end of the range, the Policy Assessment considered 

factors that might further refine its understanding of the robustness of this level. The 

Policy Assessment concluded that additional urban preference studies, especially with a 

greater variety in types of scenes, could help evaluate whether the lower CPL value of 20 

dv is generally supportable (U.S. EPA, 2011a, p. 4-27). Further, the reason for the 

noisiness in data points around the curves apparent in both the Denver and British 

Columbia results compared to the smoother curve fit of Phoenix study results could be 

explored. One possible explanation discussed in the Policy Assessment is that these older 

 

171 In order to examine this issue, an effort would have to be made to see if scenes in such 

areas could be found that would be generally comparable to the western scenes (e.g., 

scenes that contain valued scenic elements at more sensitive distances than that used in 

the eastern study). This is only one of a family of issues concerning how exposure to 

urban scenes of varying sensitivity affects public perception for which no preference 

study information is currently available.
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studies use photographs taken at different times of day and on different days to capture 

the range of light extinction levels needed for the preference studies. In contrast, the use 

of WinHaze in the Phoenix (and Washington, DC) study reduced variations that affect 

scene appearance preference rating and avoided the uncertainty inherent in using ambient 

measurements to represent sight path-averaged light extinction values. Reducing these 

sources of noisiness and uncertainty in the results of future studies of sensitive urban 

scenes could provide more confidence in the selection of a low CPL value.

Based on the above considerations, and recognizing the limitations in the 

currently available information, the Policy Assessment concluded that it is reasonable to 

consider a range of CPL values including a high value of 30 dv, a mid-range value of 25 

dv, and a low value of 20 dv (U.S. EPA, 2011a, p. 4-27). Based on its review of the 

second draft Policy Assessment, CASAC also supported this set of CPLs for 

consideration by the EPA in this review. CASAC noted that these CPL values were based 

on all available visibility preference data and that they bound the study results as 

represented by the 50 percent acceptability criteria. While recommending that further 

visibility preference studies be conducted to reduce remaining uncertainties,172 CASAC 

concluded that this range of levels iswas “adequately supported by the evidence 

presented” (Samet, 2010d, p. iii).

 

172 “CASAC has also identified needs for the next review cycle in terms of further 

research on a number of topics related to urban visibility; .... In particular, there is a need 

for the Agency to conduct additional urban visibility preference studies over a broad 

range of urban areas and viewing conditions, to further evaluate and refine the range of 

visibility levels considered to be acceptable in the current assessment.” (Samet, 2010a)
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c. Summary of Proposed Conclusions

i. Adequacy of the Current Standards for PM-Related Visibility Impairment

At the time of proposal, the Administrator provisionally concluded that the 

current suite of secondary PM standards areis not sufficiently protective of visual air 

quality, and that consideration should be given to an alternative secondary standard that 

would provide additional protection against PM-related visibility impairment, with a 

focus primarily in urban areas. This proposed conclusion was based on the information 

presented in the proposal with regard to the nature of PM-related visibility impairment, 

the results of public perception surveys on the acceptability of varying degrees of 

visibility impairment in urban areas, analyses of the number of days that are estimated to 

exceed a range of candidate protection levels under conditions simulated to just meet the 

current standards, and the advice of CASAC. This section summarizes key points from 

section VI.C of the proposal regarding visibility under current conditions, the degree of 

protection afforded by the current standards, and CASAC’s advice regarding the 

adequacy of the current standards.

As discussed in section VI.C.1 of the proposal, to evaluate visibility under current 

conditions the Visibility Assessment and Policy Assessment estimated PM-related light 

extinction173 levels for 15 urban areas174 in the United States. Consistent with the 

emphasis in this review on the hourly or multi-hour time periods that might reasonably 

 

173 PM-related light extinction is used here to refer to the light extinction caused by PM 

regardless of particle size; PM10 light extinction refers to the contribution by particles 

sampled through an inlet with a particle size 50 percent cutpoint of 10 μm diameter; and 

PM2.5 light extinction refers to the contribution by particles sampled through an inlet with 

a particle size 50 percent cutpoint of 2.5 μm diameter.

174 The 15 urban areas are Tacoma, Fresno, Los Angeles, Phoenix, Salt Lake City, Dallas, 

Houston, St. Louis, Birmingham, Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, 

and New York. 
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characterize the visibility effects experienced by various segments of the population, 

these analyses focused on using maximum 1-hour ofand 4-hour values of PM light 

extinction during daylight hours for purposes of evaluating the degree of visibility

impairment. Hourly average PM-related light extinction was analyzed in terms of both 

PM10 and PM2.5 light extinction. For reasons discussed above, hours with relative 

humidity greater than 90 percent were excluded from consideration. Recent visibility 

conditions in these urban areas were then compared to the CPLs identified above. The 

Visibility Assessment, which focused on PM10 light extinction in 14 of the 15 urban areas 

during the 2005 to 2007 time period,175 found that all 14 areas had daily maximum hourly 

PM10 light extinction values estimated to exceed even the highest CPL some of the days. 

Except for the two Texas areas and the non-California western urban areas, all of the 

other urban areas were estimated to have maximum hourly PM10 concentrations that 

exceeded the high CPL on about 20 percent to over 60 percent of the days. All 14 of the 

urban areas were estimated to have maximum hourly PM10 concentrations that exceeded 

the low CPL on about 40 percent to over 90 percent of the days. In general, areas in the 

East and in California tend to have a higher frequency of hourly visibility conditions 

estimated to be above the high CPL compared with those in the western U.S. 

The Policy Assessment repeated the Visibility Assessment-type modeling based 

on PM2.5 light extinction and data from the more recent 2007 to 2009 time period for the 

 

175 Comments on the second draft Visibility Assessment from those familiar with the 

monitoring sites in St. Louis indicated that the site selected to provide continuous PM10

monitoring, although less than a mile from the site of the PM2.5 data, was not 

representative of the urban area and resulted in unrealistically large PM10-2.5 values. The 

EPA staff considered these comments credible and set aside the St. Louis assessment 

results for PM10 light extinction. Thus, results and statements in the Policy Assessment 

regarding PM10 light extinction applied to only the other 14 areas. However, results 

regarding PM2.5 light extinction in most cases applied to all 15 study areas because the St. 

Louis estimates for PM2.5 light extinction were not affected by the PM10 monitoring issue.
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same 15 study areas (including St. Louis). While the estimates of the percentage of daily 

maximum hourly PM2.5 light extinction values exceeding the CPLs were somewhat lower 

than for PM10 light extinction, the patterns of these estimates across the study areas was 

found to be similar. More specifically, except for the two Texas and the non-California 

western urban areas, all of the other urban areas were estimated to have maximum hourly 

PM2.5 concentrations that exceeded the high CPL on about 10 percent up to about 50 

percent of the days based on PM2.5 light extinction, while all 15 areas were estimated to 

have maximum hourly PM2.5 concentrations that exceeded the low CPL on over 10 

percent to over 90 percent of the days. 

To evaluate how PM-related visibility would be affected by just meeting the 

current suite of PM2.5 secondary standards, the Policy Assessment applied the 

proportional rollback approach described in section VI.C.2 of the proposal to all the 

PM2.5 monitoring sites in each study area.176 After adjusting for composition, the Policy 

Assessment applied the original IMPROVE algorithm to calculate the PM10 light 

extinction, using “rolled back” PM2.5 component concentrations, the current conditions 

PM10-2.5 concentration for the day and hour, and relative humidity for the day and hour.

In these analyses, the Policy Assessment estimated both PM2.5 and PM10 light 

extinction in terms of both daily maximum 1-hour average values and multi-hour (i.e., 4-

hour) average values for daylight hours. Figure 4-7 and Table 4-6 of the Policy 

Assessment displayed the results of the rollback procedures as a box and whisker plot of 

daily maximum daylight 1-hour PM2.5 light extinction and the percentage of daily 

maximum hourly PM2.5 light extinction values estimated to exceed the CPLs when just 

 

176 Phoenix and Salt Lake City met the current PM2.5 NAAQS under current conditions 

and required no reduction.
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meeting the current suite of PM2.5 secondary standards for all 15 areas considered in the 

Visibility Assessment (including St. Louis) (excluding hours with relative humidity 

greater than 90 percent). These displays showed that the daily maximum 1-hour average 

PM2.5 light extinction values in all of the study areas other than the three western nonCalifornia areas were estimated to exceed the high CPL on about 8 percent up to over 30 

percent of the days and to exceed the middle CPL on about 30 percent up to about 70 

percent of the days, while all areas except Phoenix were estimated to have daily 

maximum 1-hour average PM2.5 light extinction values that exceeded the low CPL on 

over 15 percent to about 90 percent of the days. Figure 4-8 and Table 4-7 of the Policy 

Assessment present results based on daily maximum 4-hour average values. These 

displays show that the daily maximum 4-hour average PM2.5 light extinction values in all 

of the study areas other than the three western non-California areas and the two areas in 

Texas were estimated to exceed the high CPL on about 4 percent up to over 15 percent of 

the days and to exceed the middle CPL on about 15 percent up to about 45 percent of the 

days, while all areas except Phoenix were estimated to have daily maximum 4-hour 

average PM2.5 light extinction values that exceeded the low CPL on over 10 percent to 

about 75 percent of the days. A similar set of figures and tables were developed in terms 

of PM10 light extinction (U.S. EPA, 2011a, Figures 4-5 and 4-6, Tables 4-4 and 4-5). 

Taking the results of these analyses focusing on 1-hour and 4-hour maximum 

light extinction values into account, the Policy Assessment concluded that the available 

information in this review clearly called into question the adequacy of the current suite of 

PM2.5 standards in the context of public welfare protection from visibility impairment, 

primarily in urban areas, and supported consideration of alternative standards to provide 
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appropriate protection (U.S. EPA, 2011a, p. 4-39). This conclusion was based in part on 

the large percentage of days, in many urban areas, that were estimated to have maximum 

1-hour or 4-hour light extinction values that exceed the range of CPLs identified for 

consideration under simulations of conditions that would just meet the current suite of 

PM2.5 secondary standards. In particular, for air quality that was simulated to just meet 

the current PM2.5 standards, greater than 10 percent of the days were estimated to have 

peak light extinction values that exceed the highest, least protective CPL of 30 dv in 

terms of PM2.5 light extinction for 9 of the 15 urban areas, based on 1-hour average 

values, and would thus likely fail to meet a 90th percentile-based standard at that level. 

For these areas, the percent of days estimated to have maximum 1-hour values that 

exceed the highest CPL ranged from over 10 percent to over 30 percent. Similarly, when 

the middle CPL of 25 dv was considered, greater than 30 percent up to approximately 70 

percent of the days were estimated to have peak light extinction that exceeded that CPL 

in terms of PM2.5 light extinction, for 11 of the 15 urban areas, based on 1-hour average 

values. Based on a 4-hour averaging time, 5 of the areas were estimated to have at least 

10 percent of the days with peak light extinction exceeding the highest CPL in terms of 

PM2.5 light extinction, and 8 of the areas were estimated to have at least 30 percent of the 

days with peak light extinction exceeding the middle CPL in terms of PM2.5 light 

extinction. For the lowest CPL of 20 dv, the percentages of days with 4-hour maximum 

light extinction estimated to exceed that CPL are even higher for all cases considered. 

Based on all of the above, the Policy Assessment concluded that PM light extinction 

estimated to be associated with just meeting the current suite of PM2.5 secondary 

standards in many areas across the country exceeded levels and percentages of days that 
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could reasonably be considered to be important from a public welfare perspective (U.S. 

EPA, 2011a, p. 4-40). 

Further, the Policy Assessment concluded that use of the current indicator of 

PM2.5 mass, in conjunction with the current 24-hour and annual averaging times, is 

clearly called into question for a national standard intended to protect public welfare from 

PM-related visibility impairment (U.S. EPA, 2011a, p. 4-40). This is because such a 

standard is inherently variable in the degree of protection provided because of regional 

differences in relative humidity and species composition of PM2.5, which are critical 

factors in the relationship between the mix of fine particles in the ambient air and the 

associated impairment of visibility. The Policy Assessment noted that this concern was 

one of the important elements in the court’s decision to remand the PM2.5 secondary 

standards set in 2006 to the Agency. 

Thus, in addition to concluding that the available information clearly calls into 

question the adequacy of the protection against PM-related visibility impairment afforded 

by the current suite of PM2.5 standards, the Policy Assessment also concluded that it 

clearly calls into question the appropriateness of each of the current standard elements: 

indicator, averaging time, form, and level (U.S. EPA, 2011a, p. 4-40).

After reviewing the information and analysis in the second draft Policy 

Assessment, CASAC concluded that the “currently available information clearly calls 

into question the adequacy of the current standards and that consideration should be 

given to revising the suite of standards to provide increased public welfare protection” 

(Samet, 2010d, p. iii). CASAC noted that the detailed estimates of hourly PM light 

extinction associated with just meeting the current standards “clearly demonstrate that 
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current standards do not protect against levels of visual air quality which have been 

judged to be unacceptable in all of the available urban visibility preference studies.” 

Further, CASAC stated, with respect to the current suite of secondary PM2.5 standards, 

that “[T]he levels are too high, the averaging times are too long, and the PM2.5 mass 

indicator could be improved to correspond more closely to the light scattering and 

absorption properties of suspended particles in the ambient air” (Samet, 2010d, p. 9).

After considering the available evidence and the advice of CASAC, the 

Administrator concluded at the time of proposal that such information did provide an 

appropriate basis to inform a conclusion as to whether the current standards afford 

adequate protection against PM-related visibility impairment in urban areas. The 

Administrator took into account the information discussed above with regard to the 

nature of PM-related visibility impairment, the results of public perception surveys on the 

acceptability of varying degrees of visibility impairment in urban areas, analyses of the 

number of days on which peak 1-hour or 4-hour light extinction values are estimated to 

exceed a range of candidate protection levels under conditions simulated to just meet the 

current standards, and the advice of CASAC. She noted the clear causal relationship 

between PM in the ambient air and impairment of visibility, the evidence from the 

visibility preference studies, and the rationale for determining a range of candidate 

protection levels based on those studies. She also noted the relatively large number of 

days when maximum 1-hour or 4-hour light extinction values were estimated to exceed 

the three candidate protection levels, including the highest level of 30 dv, under the 

current standards. While recognizing the limitations in the available information on 

public perceptions of the acceptability of varying degree of visibility impairment and the 
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information on the number of days estimated to exceed the CPLs, she concluded that 

such information provided an appropriate basis to inform a conclusion as to whether the 

current standards provide adequate protection against PM-related visibility impairment in 

urban areas. Based on these considerations, and placing great importance on the advice of 

CASAC, the Administrator provisionally concluded that the current standards are not 

sufficiently protective of visual air quality, and that consideration should be given to an 

alternative secondary standard that would provide additional protection against PMrelated visibility impairment, with a focus primarily in urban areas.

Having reached this conclusion, the Administrator also stated at the time of 

proposal that the current indicator of PM2.5 mass, in conjunction with the current 24-hour 

and annual averaging times, is not well suited for a national standard intended to protect 

public welfare from PM-related visibility impairment. As noted in the proposal, the 

current standards do not incorporate information on the concentrations of various species 

within the mix of ambient particles, nor do they incorporate information on relative 

humidity, both of which play a central role in determining the relationship between the 

mix of PM in the ambient air and impairment of visibility. Such considerations were 

reflected both in CASAC’s advice to set a distinct secondary standard that would more 

directly reflect the relationship between ambient PM and visibility impairment and in the 

court’s remand of the current secondary PM2.5 standards. Based on the above 

considerations, at the time of proposal the Administrator provisionally concluded that the 

current secondary PM2.5 standards, taken together, are neither sufficiently protective nor 

suitably structured to provide an appropriate degree of public welfare protection from 

PM-related visibility impairment, primarily in urban areas. This led the EPA to consider 
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alternative standards by looking at each of the elements of the standards—indicator, 

averaging time, form, and level—as discussed below. 

ii. Indicator

At the time of proposal, the EPA considered three alternative indicators for a

PM2.5 standard designed to protect against visibility impairment: the current PM2.5 mass 

indicator; directly measured PM2.5 light extinction; and calculated PM2.5 light extinction. 

Directly measured PM2.5 light extinction is a measurement (or combination of 

measurements) of the light absorption and scattering caused by PM2.5 under ambient 

conditions. Calculated PM2.5 light extinction uses the IMPROVE algorithm to calculate 

PM2.5 light extinction using measured PM2.5 mass, speciated PM2.5 mass, and measured 

relative humidity. The Policy Assessment evaluated each of these alternatives, finally 

concluding that consideration should be given to establishing a new calculated PM2.5 light 

extinction indicator (U.S. EPA, 2011a, p. 4-51).

As discussed in section VI.D.1 of the proposal, the Policy Assessment concluded 

that consideration of the use of either directly measured PM2.5 light extinction or 

calculated PM2.5 light extinction as an indicator is justified because light extinction is a 

physically meaningful measure of the characteristic of ambient PM2.5 that is most 

relevant and directly related to PM-related visibility effects (U.S. EPA, 2011a, p. 4-41). 

Further, as noted above, PM2.5 is the component of PM responsible for most of the 

visibility impairment in most urban areas. In these areas, the contribution of PM10–2.5 is a 

minor contributor to visibility impairment most of the time. The Policy Assessment also 

indicated that the available evidence demonstrated a strong correspondence between 

calculated PM2.5 light extinction and PM-related visibility impairment, as well as the 
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significant degree of variability in visibility protection across the U.S. allowed by a PM2.5

mass indicator. The Policy Assessment recognized that while in the future it would be 

appropriate to consider a direct measurement of PM2.5 light extinction it was not an 

appropriate option in this review because a suitable specification of the equipment and 

associated performance verification procedures cannot be developed in the time frame for 

this review.

(a) PM2.5 Mass 

In terms of utilizing a PM2.5 mass indicator, the proposal noted that PM2.5 mass 

monitoring methods are in widespread use, including the FRM involving the collection of 

periodic (usually 1-day-in-6 or 1-day-in-3) 24-hour filter samples. However, these routine 

monitoring activities do not include measurement of the full water content of the ambient 

PM2.5 that contributes, often significantly, to visibility impacts.177 Further, the PM2.5 mass 

concentration monitors do not provide information on the composition of the ambient 

PM2.5, which plays a central role in the relationship between PM-related visibility 

impairment and ambient PM2.5 mass concentrations.178 Additional analyses discussed in 

the proposal that looked at the contribution of PM2.5 to total PM-related light extinction 

(defined in terms of hourly PM10 calculated light extinction) indicate that there is a poor 

correlation between hourly PM10 light extinction and hourly PM2.5 mass principally due 

to the impact of the water content of the particles on light extinction, which depends on 

both the composition of the PM2.5 and the ambient relative humidity. Both composition 

and especially relative humidity vary during a single day, as well as from day-to-day, at 

any site and time of year. Also, there are systematic regional and seasonal differences in 

 
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the distribution of ambient humidity and PM2.5 composition conditions that make it 

impossible to select a PM2.5 concentration that generally would correspond to the same 

PM-related light extinction levels across all areas of the nation. Analyses discussed in the 

proposal quantify the projected uneven protection that would result from the use of 1-

hour average PM2.5 mass as the indicator. 

(b) Directly Measured PM2.5 Light Extinction

PM light extinction has a nearly one-to-one relationship to light extinction, unlike 

PM2.5 mass concentration. As explained above, PM2.5 is the component responsible for 

the large majority of PM light extinction in most places and times. PM2.5 light extinction 

can be directly measured using several instrumental methods, some of which have been 

used for decades to routinely monitor the two components of PM2.5 light extinction (light 

scattering and absorption) or to jointly measure both as total light extinction (from which 

Rayleigh scattering is subtracted to get PM2.5 light extinction). As noted at the time of 

proposal, there are a number of advantages to direct measurements of light extinction for 

use in a secondary standard relative to estimates of PM2.5 light extinction calculated using 

PM2.5 mass and speciation data. These include greater accuracy of direct measurements 

with shorter averaging times and overall greater simplicity when compared to the need 

for measurements of multiple parameters to calculate PM light extinction.

In evaluating whether direct measurement of PM2.5 or PM10 light extinction is 

appropriate to consider in the context of this PM NAAQS review, the EPA solicited 

comment from the Ambient Air Monitoring and Methods Subcommittee (AAMMS) of 

CASAC. The CASAC AAMMS recommended that consideration of direct measurement 

should be limited to PM2.5 light extinction, and that although instruments suitable for this 
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purpose are commercially available at present, research is expected to produce even 

better instruments in the near term. The CASAC AAMMS advised against choosing any 

currently available commercial instrument, or even a general measurement approach, as 

an FRM because to do so could discourage development of other potentially superior 

approaches. Instead, the CASAC AAMMS recommended that the EPA develop 

performance-based approval criteria for direct measurement methods in order to put all 

approaches on a level playing field.

At the present time, the EPA has not undertaken to develop and test such 

performance-base approval criteria. The EPA anticipates that if an effort were begun it 

would take at least several years before such criteria would be ready for regulatory use. 

Thus, the Policy Assessment concluded that while in the future it would be appropriate to 

consider a direct measurement of PM2.5 light extinction, or the sum of separate 

measurements of light scattering and light absorption, as the indicator for the secondary 

PM2.5 standard, this is not an appropriate option in this review because a suitable 

specification of the equipment or appropriate performance-based verification procedures 

cannot be developed in the time frame for this review (U.S. EPA, 2011a, p. 4-51, -52).

(c) Calculated PM2.5 Light Extinction 

For the reasons discussed above, the Policy Assessment concluded that a 

calculated PM2.5 light extinction indicator would be the preferred approach. PM2.5 light 

extinction can be calculated from PM2.5 mass, combined with speciated PM2.5 mass 

concentration data plus relative humidity data, as is presently routinely done on a 24-hour 

average basis under the Regional Haze Program using data from the rural IMPROVE 

monitoring network. This same calculation procedure, using a 24-hour average basis, 
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could be used for a NAAQS focused on protecting against PM-related visibility 

impairment primarily in urban areas. This approach would use the type of data that is 

routinely collected from the urban CSN179 in combination with monthly average relative 

humidity data based on long-term climatological means as used in the Regional Haze 

Program (U.S. EPA, 2011a, Appendix G, section G.2). The proposal discussed the 

complex approach utilized in the Visibility Assessment for calculating hourly PM2.5 light 

extinction180 and discussed various simplified approaches for calculating these hourly 

values that were analyzed in the Policy Assessment. The Policy Assessment concluded 

that each of these simplified approaches provided reasonably good estimates of PM2.5

light extinction and each would be appropriate to consider as the indicator for a distinct 

hourly or multi-hour secondary standard (U.S. EPA, 2011a, p. 4-48). The proposal also 

recognized that the Policy Assessment identified a number of variations on these 

simplified approaches that it would be appropriate to consider, including:

(1) the use of the split-component mass extinction efficiency approach from the 

revised IMPROVE algorithm181

 

179 About 200 sites in the CSN routinely measure 24-hour average PM2.5 chemical 

components using filter-based samplers and chemical analysis in a laboratory, on either a 

1-day-in-3 or 1-day-in-6 schedule (U.S. EPA, 2011a, Appendix B, section B.1.3).

180 As noted at the time of proposal, the sheer size of the ambient air quality, 

meteorological, and chemical transport modeling data files involved with the Visibility 

Assessment approach would make it very difficult for state agencies or any interested 

party to consistently apply such an approach on a routine basis for the purpose of 

implementing a national standard defined in terms of the Visibility Assessment approach.

181 If the revised IMPROVE algorithm were used to define the calculated PM2.5 massbased indicator, it would not be possible to algebraically reduce the revised algorithm to a 

two-factor version as described above and in Appendix F of the Policy Assessment for 

the simplified approaches. Instead, five component fractions would be determined from 

each day of speciated sampling, and then either applied to hourly measurements of PM2.5

mass on the same day or averaged across a month and then applied to measurements of 

PM2.5 mass on each day of the month. 
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(2) the use of more refined value(s) for the organic carbon multiplier182

(3) the use of the reconstructed 24-hour PM2.5 mass (i.e., the sum of the five PM2.5

components from speciated monitoring) as a normalization value for the 

hourly measurements from the PM2.5 instrument as a way of better reflecting 

ambient nitrate concentrations

(4) the use of historical monthly or seasonal, or regional, speciation averages

Overall, the analyses conducted for the Visibility Assessment and Policy 

Assessment indicateindicated that the use of a calculated PM2.5 light extinction indicator 

would provide a much higher degree of uniformity in terms of the degree of protection 

from visibility impairment across the country than a PM2.5 mass indicator, because a 

calculated PM2.5 light extinction indicator would directly incorporate the effects of 

humidity and PM2.5 composition differences between various regions. Further, the 

proposal noted that the Policy Assessment concluded that consideration could be given to 

defining a calculated PM2.5 light extinction indicator on either a 24-hour or a sub-daily 

basis (U.S. EPA, 2011a, p. 4-52). However, the Policy Assessment noted that approval of 

continuous FEM monitors has been based only on 24-hour average, not hourly, PM2.5

mass. In addition, there are mixed results of data quality assessments on a 24-hour basis 

for these monitors, as well as the near absence of performance data for sub-daily 

averaging periods. Thus, while it is possible to utilize data from PM2.5 continuous FEMs 

on a 1-hour or multi-hour (e.g., 4-hour) basis, these factors increase the uncertainty of 

utilizing continuous methods to support 1-hour or 4-hour PM2.5 mass measurements as an 

input to the light extinction calculation. Therefore, as noted at the time of proposal, until 

issues regarding the comparability of 24-hour PM2.5 mass values derived from continuous 

 

182 An organic carbon (OC)-to-organic mass (OM) multiplier of 1.6 was used for the 

assessment, which was found to produce a value of OM comparable to the one derived 

with the original, albeit more complex, Visibility Assessment method.
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FEMs and filter-based FRMs183 are resolved, there is reason to be cautious about relying 

on a calculation procedure that uses hourly PM2.5 mass values reported by continuous 

FEMs in combination with speciated PM2.5 mass values from 24-hour filter-based 

samplers.

(d) CASAC Advice

In reviewing the second draft Policy Assessment, CASAC stated that it 

“overwhelmingly ... would prefer the direct measurement of light extinction,” 

recognizing it as the property of the atmosphere that most directly relates to visibility 

effects (Samet, 2010d, p. iii). CASAC noted that “[I]t has the advantage of relating 

directly to the demonstrated harmful welfare effect of ambient PM on human visual 

perception.” However, CASAC also concluded that the calculated PM2.5 light extinction 

indicator “appears to be a reasonable approach for estimating hourly light extinction” 

(Samet, 2010d, p. 11). Further, based on CASAC’s understanding of the time that would 

be required to develop an FRM for this indicator, CASAC agreed with the staff 

preference presented in the second draft Policy Assessment for a calculated PM2.5 light 

extinction indicator. CASAC noted that “[I]ts reliance on procedures that have already 

been implemented in the CSN and routinely collected continuous PM2.5 data suggest that 

it could be implemented much sooner than a directly measured indicator” (Samet, 2010d, 

p. iii).184

(e) Administrator's Proposed Conclusions on Indicator

 

183 Filter-based FRMs are designed to adequately quantify the amount of PM2.5 collected 

over 24-hours. They cannot be presumed to be appropriate for quantifying average 

concentrations over 1-hour or 4-hour periods.

184 In commenting on the second draft Policy Assessment, CASAC did not have an 

opportunity to review the assessment of continuous PM2.5 FEMs compared to collocated 

FRMs (Hanley and Reff, 2011) as presented and discussed in the final Policy Assessment 

(U.S. EPA, 2011a, p. 4-50).
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At the time of proposal, while agreeing with CASAC that a directly measured PM 

light extinction indicator would provide the most direct link between PM in the ambient 

air and PM-related light extinction, the Administrator provisionally concluded that this 

was not an appropriate option in this review because a suitable specification of currently 

available equipment or performance-based verification procedures cannot be developed 

in the time frame of this review. Taking all of the above considerations and CASAC 

advice into account, the Administrator provisionally concluded that a new calculated 

PM2.5 light extinction indicator, similar to that used in the Regional Haze Program (i.e., 

using an IMPROVE algorithm as translated into the deciview scale), was the appropriate 

indicator to replace the current PM2.5 mass indicator. Such an indicator, referred to as a 

PM2.5 visibility index, would appropriately reflect the relationship between ambient PM 

and PM-related light extinction, based on the analyses discussed in the proposal and 

incorporation of factors based on measured PM2.5 speciation concentrations and relative 

humidity data. In addition, selection of this type of indicator would address, in part, the 

issues raised in the court’s remand of the 2006 PM2.5 standards. The Administrator also 

noted that such a PM2.5 visibility index would afford a relatively high degree of 

uniformity of visual air quality protection in areas across the country by virtue of directly 

incorporating the effects of differences in PM2.5 composition and relative humidity across 

the country. 

Based on these above considerations, the Administrator proposed to set a distinct 

secondary standard for PM2.5 defined in terms of a PM2.5 visibility index (i.e., a 

calculated PM2.5 light extinction indicator, translated into the deciview scale) to protect 

against PM-related visibility impairment primarily in urban areas. The Administrator 
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proposed that such an index be based on the original IMPROVE algorithm in conjunction 

with monthly average relative humidity data based on long-term climatological means as 

used in the Regional Haze Program. The EPA solicited comment on all aspects of the 

proposed indicator, especially:

(1) the proposed use of a PM2.5 visibility index rather than a PM10 visibility index 

which would include an additional term for coarse particles;

(2) using the revised IMPROVE algorithm rather than the original IMPROVE 

algorithm;

(3) the use of alternative values for the organic carbon multiplier in conjunction with 

either the original or revised IMPROVE algorithm;

(4) the use of historical monthly, seasonal, or regional speciation averages;

(5) alternative approaches to determining relative humidity; and

(6) simplified approaches to generating hourly PM2.5 light extinction values for 

purposes of calculating an hourly or multi-hour indicator.

iii. Averaging Times

In this review, as discussed in section VI.D.2 of the proposal, consideration of 

appropriate averaging times for use in conjunction with a PM2.5 visibility index was 

informed by information related to the nature of PM visibility effects and the nature of 

inputs to the calculation of PM2.5 light extinction, as discussed above. The EPA 

considered both sub-daily (1- and 4-hour averaging times) and 24-hour averaging times. 

In considering sub-daily averaging times, the EPA has also considered what diurnal 

periods and ambient relative humidity conditions would be appropriate to consider in 

conjunction with such an averaging time.

As an initial matter, the Policy Assessment considered sub-daily averaging times. 

Taking into account what is known from available studies concerning how quickly people 

experience and judge visibility conditions, the possibility that some fraction of the public 
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experiences infrequent or short periods of exposure to ambient visibility conditions, and 

the typical rate of change of the path-averaged PM light extinction over urban areas, the 

initial analyses conducted as part of the Visibility Assessment focused on a 1-hour 

averaging time. In its review of the first draft Policy Assessment, CASAC agreed that a 

1-hour averaging time would be appropriate to consider, noting that PM effects on 

visibility can vary widely and rapidly over the course of a day and such changes are 

almost instantaneously perceptible to human observers (Samet, 2010c, p. 19). The Policy 

Assessment noted that this view related specifically to a standard defined in terms of a 

directly measured PM light extinction indicator, in that CASAC also noted that a 1-hour 

averaging time is well within the instrument response times of the various currently 

available and developing optical monitoring methods. 

However, CASAC also advised that if a PM2.5 mass indicator were to be used, it 

would be appropriate to consider “somewhat longer averaging times – 2 to 4 hours – to 

assure a more stable instrumental response” (Samet, 2010c, p. 19). In considering this 

advice, the Policy Assessment concluded that since a calculated PM2.5 light extinction 

indicator relies in part on measured PM2.5 mass, it would be appropriate to consider a 

multi-hour averaging time on the order of a few hours (e.g. 4-hours). A multi-hour 

averaging time might reasonably characterize the visibility effects experienced by the 

segment of the population who have access to visibility conditions often or continuously 

throughout the day. For this segment of the population, it may be that their perception of 

visual air quality reflects some degree of offsetting an hour with poor visual air quality 

with one or more hours of clearer visual conditions. Further, the Policy Assessment 

recognized that a multi-hour averaging time would have the effect of averaging away 
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peak hourly visibility impairment, which can change significantly from one hour to the 

next (U.S. EPA, 2011a, p. 4-53; U.S. EPA, 2010b, Figure 3-12). 

In considering either 1-hour or multi-hour averaging times, the Policy Assessment 

recognized that no data are available with regard to how the duration and variation of 

time a person spends outdoors during the daytime impacts his or her judgment of the 

acceptability of different degrees of visibility impairment. As a consequence, it is not 

clear to what degree, if at all, the protection levels found to be acceptable in the public 

preference studies would change for a multi-hour averaging time as compared to a 1-hour 

averaging time. Thus, the Policy Assessment concluded that it is appropriate to consider a 

1-hour or multi-hour (e.g., 4-hour) averaging time as the basis for a sub-daily standard 

defined in terms of a calculated PM2.5 light extinction indicator (U.S. EPA, 2011a, p. 4-

53).

In addition, as discussed above, some data quality uncertainties have been 

observed with regard to hourly data collected by FEMs. Specifically, as part of the review 

of data from all continuous FEM PM2.5 instruments operating at state/local monitoring 

sites, the Policy Assessment noted that the occurrence of questionable outliers in 1-hour 

data submitted to AQS from continuous FEM PM2.5 instruments had been observed at 

some of these sites (Evangelista, 2011). Some of these outliers were questionable simply 

by virtue of their extreme magnitude, as high as 985 μg/m3, whereas other values were 

questionable because they were isolated to single hours with much lower values before 

and after, a pattern that is much less plausible than if the high concentrations were more 

sustained.185 The Policy Assessment noted that any current data quality problems might 

 

185 Similarly questionable hourly data were not observed in the 2005 to 2007 continuous 

PM2.5 data used in the Visibility Assessment, all of which came from early-generation 
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be resolved in the normal course of monitoring program evolution as operators become 

more adept at instrument operation and maintenance and data validation or by improving 

the approval criteria and testing requirements for continuous instruments. Regardless, the 

Policy Assessment noted that multi-hour averaging of FEM data could serve to reduce 

the effects of such outliers relative to the use of a 1-hour averaging time. 

The Policy Assessment noted that there are significant reasons to consider using 

PM2.5 light extinction calculated on a 24-hour basis to reduce the various data quality 

concerns described above with respect to relying on continuous PM2.5 monitoring data. 

However, the Policy Assessment recognized that 24 hours is far longer than the hourly or 

multi-hour time periods that might reasonably characterize the visibility effects 

experienced by various segments of the population, including both those who do and do 

not have access to visibility conditions often or continuously throughout the day. Thus, 

the Policy Assessment concluded that the appropriateness of considering a 24-hour 

averaging time would depend upon the extent to which PM-related light extinction 

calculated on a 24-hour average basis would be a reasonable and appropriate surrogate 

for PM-related light extinction calculated on a sub-daily basis. 

To examine this relationship, the EPA conducted comparative analyses of 24-hour 

and 4-hour averaging times in conjunction with a calculated PM2.5 indicator. For these 

analyses, 4-hour average PM2.5 light extinction was calculated based on using the 

Visibility Assessment approach. The 24-hour average PM2.5 light extinction was 

calculated using the original IMPROVE algorithm and long-term relative humidity 

 

continuous instruments that had not been approved as FEMs. However, only 15 sites and 

instruments were involved in the Visibility Assessment analyses, versus about 180 

currently operating FEM instruments submitting data to AQS. Therefore, there were 

more opportunities for very infrequent measurement errors to be observed in the larger 

FEM data set. 
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conditions to calculate PM2.5 light extinction. Based on these analyses,186 which are 

presented and discussed in Appendix G of the Policy Assessment, scatter plots comparing 

24-hour and 4-hour calculated PM2.5 light extinction were constructed for each of the 15 

cities included in the Visibility Assessment and for all 15 cities pooled together (U.S. 

EPA, 2011a, Figures G-4 and G-5). Though there was some scatter around the regression 

line for each city because the calculated 4-hour light extinction values included dayspecific and hour-specific influences that are not captured by the simpler 24-hour 

approach, these analyses generally showed good correlation between 24-hour and 4-hour 

average PM2.5 light extinction, as evidenced by reasonably high city-specific and pooled 

R

2

values, generally in the range of over 0.6 to over 0.8.187 This suggested that PM2.5 light 

extinction calculated on a 24-hour basis is a reasonable and appropriate surrogate to 

PM2.5 light extinction calculated on a sub-daily basis. 

Taking the above considerations and CASAC’s advice into account, the Policy 

Assessment concluded that it would be appropriate to consider a 24-hour averaging time, 

in conjunction with a calculated PM2.5 light extinction indicator and an appropriately 

specified standard level, as discussed below. By using site-specific daily data on PM2.5

composition and site-specific long-term relative humidity conditions, this 24-hour 

average indicator would provide more consistent protection from PM2.5-related visibility 

impairment than would a secondary PM2.5 NAAQS based only on 24-hour or annual 

average PM2.5 mass. In particular, this approach would account for the systematic 

difference in humidity conditions between most eastern states and most western states. 

 

186 These analyses are also based on the use of a 90th percentile form, averaged over 3 

years, as discussed below in section VI.D.3 and in section 4.3.3 of the Policy Assessment 

(U.S. EPA, 2011a).

187 The EPA staff noted that the R2

value (0.44) for Houston was notably lower than for 

the other cities.
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The Policy Assessment also concluded that it would also be appropriate to consider a 

multi-hour, sub-daily averaging time, for example a period of 4 hours, in conjunction 

with a calculated PM2.5 light extinction indicator and with further consideration of the 

data quality issues discussed above. Such an averaging time, to the extent that data 

quality issues can be appropriately addressed, would be more directly related to the shortterm nature of the perception of visibility impairment, short-term variability in PMrelated visual air quality, and the short-term nature (hourly to multiple hours) of relevant 

exposure periods for segments of the viewing public. Such an averaging time would still 

result in an indicator that is less sensitive than a 1-hour averaging time to short-term 

instrument variability with respect to PM2.5 mass measurement. In conjunction with 

consideration of a multi-hour, sub-daily averaging time, the Policy Assessment concluded 

that consideration should be given to including daylight hours only and to applying a 

relative humidity screen of approximately 90 percent to remove hours in which fog or 

precipitation is much more likely to contribute to the observed visibility impairment (U.S. 

EPA, 2011a, p. 4-58). Recognizing that a 1-hour averaging time would be even more 

sensitive to data quality issues, including short-term variability in hourly data from 

currently available continuous monitoring methods, the Policy Assessment concluded 

that it would not be appropriate to consider a 1-hour averaging time in conjunction with a 

calculated PM2.5 light extinction indicator in this review (U.S. EPA, 2011a, p. 4-58). 

As noted above, in its review of the first draft Policy Assessment, CASAC 

concluded that PM effects on visibility can vary widely and rapidly over the course of a 

day and such changes are almost instantaneously perceptible to human observers (Samet, 

2010c, p. 19). Based in part on this consideration, CASAC agreed that a 1-hour averaging 
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time would be appropriate to consider in conjunction with a directly measured PM light 

extinction indicator, noting that a 1-hour averaging time is well within the instrument 

response times of the various currently available and developing optical monitoring 

methods. At that time, CASAC also advised that if a PM2.5 mass indicator were to be 

used, it would be appropriate to consider “somewhat longer averaging times – 2- to 4-

hours – to assure a more stable instrumental response” (Samet, 2010c, p. 19). Thus, 

CASAC’s advice on averaging times that would be appropriate for consideration was 

predicated in part on the capabilities of monitoring methods that were available for the 

alternative indicators discussed in the draft Policy Assessment. CASAC’s views on a

multi-hour averaging time would also apply to the calculated PM2.5 light extinction 

indicator since hourly PM2.5 mass measurements are also required for this indicator when 

calculated on a sub-daily basis.

It is important to note that at the time it provided advice on suitable averaging 

times, CASAC did not have the benefit of EPA’s subsequent assessment of the data 

quality issues associated with the use of continuous FEMs as the basis for hourly PM2.5

mass measurements. Furthermore, since CASAC only commented on the first and second 

drafts of the Policy Assessment, neither of which included discussion of a calculated 

PM2.5 indicator based on a 24-hour averaging time, CASAC did not have a basis to offer 

advice regarding a 24-hour averaging time. In addition, the 24-hour averaging time is not 

based on consideration of 24-hours as a relevant exposure period, but rather as a 

surrogate for a sub-daily period of 4 hours, which is consistent with CASAC’s advice 

concerning an averaging time associated with the use of a PM2.5 mass indicator.

Taking into account the information discussed above with regard to analyses and 
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conclusions presented in the final Policy Assessment the Administrator recognized that 

hourly or sub-daily, multi-hour averaging times, within daylight hours and excluding 

hours with relative humidity above approximately 90 percent, are more directly related 

than a 24-hour averaging time to the short-term nature of the perception of PM-related 

visibility impairment and the relevant exposure periods for segments of the viewing 

public. On the other hand, she recognized that data quality uncertainties have recently 

been associated with currently available instruments that would be used to provide the 

hourly PM2.5 mass measurements that would be needed in conjunction with an averaging 

time shorter than 24-hours. As a result, while the Administrator recognized the 

desirability of a sub-daily averaging time, she had strong reservations about proposing to 

set a standard at this time in terms of a sub-daily averaging time.

In considering the information and analyses related to consideration of a 24-hour 

averaging time, the Administrator recognized that the Policy Assessment concluded that 

PM2.5 light extinction calculated on a 24-hour averaging basis is a reasonable and 

appropriate surrogate for sub-daily PM2.5 light extinction calculated on a 4-hour average 

basis. In light of this finding and the views of CASAC based on its reviews of the first 

and second drafts of the Policy Assessment, the Administrator proposed to set a distinct 

secondary standard with a 24-hour averaging time in conjunction with a PM2.5 visibility 

index. 

iv. Form

As discussed in section VI.D.3 of the proposal, the “form” of a standard defines 

the air quality statistic that is to be compared to the level of the standard in determining 

whether the standard is achieved. The form of the current 24-hour PM2.5 NAAQS is such 
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that the level of the standard is compared to the 3-year average of the annual 98th

percentile value of the measured indicator. The purpose in averaging for three years is to 

provide stability from the occasional effects of inter-annual meteorological variability 

that can result in unusually high pollution levels for a particular year. The use of a multiyear percentile form, among other things, makes the standard less subject to the 

possibility of transient violations caused by statistically unusual indicator values, thereby 

providing more stability to the air quality management process that may enhance the 

practical effectiveness of efforts to implement the NAAQS. Also, a percentile form can 

be used to take into account the number of times an exposure might occur as part of the 

judgment on protectiveness in setting a NAAQS. For all of these reasons, the Policy 

Assessment concluded it would be appropriate to consider defining the form of a distinct 

secondary standard in terms of a 3-year average of a specified percentile air quality 

statistic (U.S. EPA, 2011a, p. 4-58).

The urban visibility preference studies that provided results leading to the range 

of CPLs being considered in this review offer no information that addresses the 

frequency of time that visibility levels should be below those values. Given this lack of 

information, and recognizing that the nature of the public welfare effect is one of 

aesthetics and/or feelings of well-being, the Policy Assessment concluded that it would 

not be appropriate to consider eliminating all exposures above the level of the standard 

and that allowing some number of hours/days with reduced visibility can reasonably be 

considered (U.S. EPA, 2011a, p. 4-59). In the Visibility Assessment, 90th, 95th, and 98th

percentile forms were assessed for alternative PM light extinction standards (U.S. EPA, 

2010b, section 4.3.3). In considering these alternative percentiles, the Policy Assessment 
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noted that the Regional Haze Program targets the 20 percent most impaired days for 

improvements in visual air quality in Federal Class I areas. If improvement in the 20 

percent most impaired days were similarly judged to be appropriate for protecting visual 

air quality in urban areas, a percentile well above the 80th percentile would be appropriate 

to increase the likelihood that all days in this range would be improved by control 

strategies intended to attain the standard. A focus on improving the 20 percent most 

impaired days suggests that the 90th percentile, which represents the median of the 

distribution of the 20 percent worst days, would be an appropriate form to consider. 

Strategies that are implemented so that 90 percent of days have visual air quality that is at 

or below the level of the standard would reasonably be expected to lead to improvements 

in visual air quality for the 20 percent most impaired days. Higher percentile values 

within the range assessed could have the effect of limiting the occurrence of days with 

peak PM-related light extinction in urban areas to a greater degree. In considering the 

limited information available from the public preference studies, the Policy Assessment 

found no basis to conclude that it would be appropriate to consider limiting the 

occurrence of days with peak PM-related light extinction in urban areas to a greater 

degree.

Another aspect of the form discussed in the proposal for a sub-daily averaging 

time was whether to include all daylight hours or only the maximum daily daylight 

hour(s). The maximum daily daylight 1-hour or multi-hour form would be most directly 

protective of the welfare of people who have limited, infrequent or intermittent exposure 

to visibility during the day (e.g., during commutes), but spend most of their time without 

an outdoor view. For such people a view of poor visibility during their morning commute 
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may represent their perception of the day’s visibility conditions until the next time they 

venture outside during daylight, which may be hours later or perhaps the next day. Other 

people have exposure to visibility conditions throughout the day. For those people, it 

might be more appropriate to include every daylight hour in assessing compliance with a 

standard, since it is more likely that each daylight hour could affect their welfare. 

The Policy Assessment did not have information regarding the fraction of the 

public that has only one or a few opportunities to experience visibility during the day, nor 

did it have information on the role the duration of the observed visibility conditions has 

on wellbeing effects associated with those visibility conditions. However, it is logical to 

conclude that people with limited opportunities to experience visibility conditions on a 

daily basis would experience the entire impact associated with visibility based on their 

short-term exposure. The impact of visibility for those who have access to visibility 

conditions often or continuously during the day may be based on varying conditions 

throughout the day. 

In light of these considerations, the analyses conducted as part of the Visibility 

Assessment analyses included both the maximum daily hour and the all daylight hours 

forms. The Policy Assessment noted that there is a close correspondence between the 

level of protection afforded for all 15 urban areas by a maximum daily daylight 1-hour 

approach using the 90th percentile form and an all daylight hours approach combined with 

the 98th percentile form (U.S. EPA, 2010b, section 4.1.4). This suggestssuggested that 

reductions in visibility impairment required to meet either form of the standard would 

provide protection to both fractions of the public (i.e., those with limited opportunities 

and those with greater opportunities to view PM-related visibility conditions). CASAC 
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generally supported consideration of both types of forms without expressing a preference 

based on its review of information presented in the second draft Policy Assessment 

(Samet, 2010d, p. 11).

In conjunction with a calculated PM2.5 light extinction indicator and alternative 

24-hour or sub-daily (e.g., 4-hour) averaging times, based on the above considerations, 

and given the lack of information on and the high degree of uncertainty over the impact 

on public welfare of the number of days with visibility impairment over a year, the Policy 

Assessment concluded that it would be appropriate to give primary consideration to a 90th

percentile form, averaged over three years (U.S. EPA, 2011a, p. 4-60). Further, in the 

case of a multi-hour, sub-daily alternative standard, the Policy Assessment concluded that 

it would be appropriate to give primary consideration to a form based on the maximum 

daily multi-hour period in conjunction with the 90th percentile form (U.S. EPA, 2011a, p. 

4-60). This sub-daily form would be expected to provide appropriate protection for 

various segments of the population, including those with limited opportunities during a 

day and those with more extended opportunities over the daylight hours to experience 

PM-related visual air quality.

Though CASAC did not provide advice as to a specific form that would be 

appropriate, it took note of the alternative forms considered in that document and 

encouraged further analyses in the final Policy Assessment that might help to clarify a 

basis for selecting from within the range of forms identified. In considering the available 

information and the conclusions in the final Policy Assessment in light of CASAC’s 

comments, at the time of proposal the Administrator provisionally concluded that a 90th

percentile form, averaged over 3 years, is appropriate, and proposed such a form in 
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conjunction with a PM2.5 visibility index and a 24-hour averaging time.

v. Level

As discussed in section VI.D.4 of the proposal, in considering appropriate levels 

for a 24-hour standard defined in terms of a PM2.5 visibility index and an 90th percentile 

form, averaged over 3 years, the Policy Assessment took into account the evidence- and 

impact-based considerations discussed above, with a focus on the results of public 

perception and attitude surveys related to the acceptability of various levels of visual air 

quality and on the important limitations in the design and scope of such available studies. 

The Policy Assessment considered a variety of approaches for identifying appropriate 

levels for such a standard, including utilizing both adjusted and unadjusted CPLs derived 

from the visibility preference studies. 

The Policy Assessment interpreted the results from the visibility preferences 

studies conducted in four urban areas to define a range of low, middle, and high CPLs for 

a sub-daily standard (e.g., 1- to 4-hour averaging time) of 20, 25, and 30 dv, which are 

approximately equivalent to PM2.5 light extinction of values of 65, 110, and 190 

Mm-1.The Policy Assessment noted that CASAC agreed that this was an appropriate 

range of levels to consider for such a standard The CASAC generally supported this 

approach, noting that the “EPA staff’s approach for translating and presenting the technical 

evidence and assessment results is logically conceived and clearly presented. The 20-30 

deciview range of levels chosen by EPA staff as “Candidate Protection Levels” is adequately 
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supported by the evidence presented” (Samet, 2010d, p. 11).188 The Policy Assessment also 

recognized that to define a range of alternative levels that would be appropriate to 

consider for a 24-hour calculated PM2.5 light extinction standard, it would be appropriate 

to consider whether some adjustment to these CPLs is warranted since these preference 

studies cannot be directly interpreted as applying to a 24-hour exposure period (as noted 

above and in Policy Assessment section 4.3.1). Considerations related to such 

adjustments are more specifically discussed below.

In considering alternative levels for a sub-daily standard based directly on the four 

preference study results, the Policy Assessment noted that the individual low and high 

CPLs are in fact generally reflective of the results from the Denver and Washington, DC 

studies respectively, and the middle CPL is very near to the 50th percentile criteria result 

from the Phoenix study, which was by far the best of the studies, providing somewhat 

more support for the middle CPL.

In considering the results from the four visibility preference studies, the Policy 

Assessment recognized that currently available studies are limited in that they were 

conducted in only four areas, three in the U.S. and one in Canada. Further, the Policy 

Assessment recognized that available studies provide no information on how the duration 

and variation of time a person spends outdoors during the daytime may impact their 

judgment of the acceptability of different degrees of visibility impairment. As such, there 

is a relatively high degree of uncertainty associated with using the results of these studies 

 

188 In 2009, the D.C. Circuit remanded the secondary PM2.5 standards to the EPA in part 

because the Agency failed to identify a target level of protection, even though EPA staff 

and CASAC had identified a range of target levels of protection that were appropriate for

consideration. The court determined that the Agency’s failure to identify a target level of 

protection as part of its final decision was contrary to the statute and therefore unlawful, 

and that it deprived EPA’s decision-making of a reasoned basis. See 559F. 3d at 528-31; 

see also section VI.A.2 above and the Policy Assessment, section 4.1.2.
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to inform consideration of a national standard for any specific averaging time. 

Nonetheless, the Policy Assessment concluded, as did CASAC, that these studies are 

appropriate to use for this purpose (U.S. EPA, 2011a, p. 4-61).

Using approaches described in section VI.C.4 of the proposal, the Policy 

Assessment explored various approaches to adjusting the CPLs derived from the 

preference studies to inform alternative levels for a 24-hour standard. These various 

approaches, based on analyses of 2007 – 2009 data from the 15 urban areas assessed in 

the Visibility Assessment, focused on estimating CPLs for a 24-hour standard that would 

provide generally equivalent protection as that provided by a 4-hour standard with CPLs 

of 20, 25, and 30 dv. In conducting these analyses, staff initially expected that the values 

of 24-hour average PM2.5 light extinction and daily maximum daylight 4-hour average 

PM2.5 light extinction would differ on any given day, with the shorter term peak value 

generally being larger. This would mean that, in concept, the level of a 24-hour standard 

should include a downward adjustment compared to the level of a 4-hour standard to 

provide generally equivalent protection. As discussed more fully in section G.5 of 

Appendix G and summarized below, this initial expectation was not found to be the case 

across the range of CPLs considered. In fact, as shown in TableTables G-67 and G-8 of 

Appendix G, and in the corrected version of Table G-6 found in Frank et al. (2012b),

189

in considering estimates aggregated or averaged over all 15 cities as well as the range of 

 

189Note that the city-specific ranges shown in Table G-6, Appendix G of the Policy 

Assessment are incorrectly stated for Approaches C and E. Drawing from the more 

detailed and correct results for Approaches C and E presented in Tables G-7 and G-8, 

respectively, the city-specific ranges in Table G-6 for Approach C should be 17 – 21 dv 

for the CPL of 20 dv; 21 – 25 dv for the CPL of 25 dv; and 24 – 30 dv for the CPL of 30 

dv; the city-specific ranges in Table G-6 for Approach E should be 17 – 21 dv for the 

CPL of 20 dv; 21 – 26 dv for the CPL of 25 dv; and 25 – 31 dv for the CPL of 30 dv. In 

the EPA’s reanalysis comparing 4- vs. 24-hour values, Frank et al. (2012b) recreated 

Table G-6 using the correct values from Tables G-7 and G-8.
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city-specific estimates for the various approaches considered, these analyses indicated 

that the generally equivalent 24-hour levels ranged from somewhat below the 4-hour 

level to just above the 4-hour level for each of the CPLs.190 In all cases, the range of cityspecific estimates of generally equivalent 24-hour levels included the 4-hour level for 

each of the CPLs of 20, 25, and 30 dv. As noted in the proposal, looking more broadly at 

these results could support consideration of using the same CPL for a 24-hour standard as 

for a 4-hour standard, recognizing that there is no one approach that can most closely 

identify a generally equivalent 24-hour standard level in each urban area for each CPL. 

The use of such an unadjusted CPL for a 24-hour standard would place more emphasis on 

the relatively high degree of spatial and temporal variability in relative humidity and fine 

particle composition observed in urban areas across the country, so as to reduce the 

potential of setting a 24-hour standard level that would require more than the intended 

degree of protection in some areas.

In considering the appropriate level of a secondary standard focused on protection 

from PM-related urban visibility impairment based on either a 24-hour or a multi-hour, 

sub-daily (e.g., 4-hour) averaging time, the EPA has been mindful of the important 

limitations in the available evidence from public preference studies. These uncertainties 

and limitations are due in part to the small number of stated preference studies available 

for this review; the relatively small number of study participants and the extent to which 

 

190As discussed in more detail in Appendix G of the Policy Assessment, some days have 

higher values for 24-hour average light extinction than for daily maximum 4-hour 

daylight light extinction, and consequently an adjusted "equivalent" 24-hour CPL can be 

greater than the original 4-hour CPL. This can happen for two reasons. First, the use of 

monthly average historical RH data will lead to cases in which the f(RH) values used for 

the calculation of 24-hour average light extinction are higher than all or some of the four 

hourly values of f(RH) used to determine daily maximum 4-hour daylight light extinction 

on the same day. Second, PM2.5 concentrations may be greater during non-daylight 

periods than during daylight hours.
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the study participants may not be representative of the broader study area population in 

some of the studies; and the variations in the specific materials and methods used in each 

study such as scene characteristics, the range of VAQ levels presented to study 

participants, image presentation methods and specific wording used to frame the 

questions used in the group interviews. In addition the EPA has noted that the scenic 

vistas available on a daily basis in many urban areas across the country generally may not 

have the inherent visual interest or the distance between viewer and object of greatest 

intrinsic value as in the Denver and Phoenix preference studies, and that there is the 

possibility that there could be regional differences in individual preferences for VAQ.

It is also important to note that as in past reviews, the EPA is considering a 

national visibility standard in conjunction with the Regional Haze Program as a means of 

achieving appropriate levels of protection against PM-related visibility impairment in 

urban, non-urban, and Federal Class I areas across the country. The EPA recognizes that 

programs implemented to meet a national standard focused primarily on the visibility 

problems in urban areas can be expected to improve visual air quality in surrounding nonurban areas as well, as would programs now being developed to address the requirements 

of the Regional Haze Program established for protection of visual air quality in Federal 

Class I areas. The EPA also believes that the development of local programs, such as 

those in Denver and Phoenix, can continue to be an effective and appropriate approach to 

provide additional protection, beyond that afforded by a national standard, for unique 

scenic resources in and around certain urban areas that are particularly highly valued by 

people living in those areas.

The Policy Assessment concluded that it is appropriate to give primary 
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consideration to alternative standard levels toward the upper end of the ranges identified 

above for 24-hour and sub-daily standards, respectively (U.S. EPA, 2011a, p. 4-63). 

Thus, the Policy Assessment concluded it is appropriate to consider the following 

alternative levels: a level of 28 dv or somewhat below, down to 25 dv, for a standard 

defined in terms of a calculated PM2.5 light extinction indicator, a 90th percentile form, 

and a 24-hour averaging time; and a standard level of 30 dv or somewhat below, down to 

25 dv, for a similar standard but with a 4-hour averaging time (U.S. EPA, 2011a, p. 4-

63). The Policy Assessment judged that such standards would provide appropriate 

protection against PM-related visibility impairment primarily in urban areas. The Policy 

Assessment noted that CASAC generally supported consideration of the 20 – 30 dv range 

as CPLs and, more specifically, that support for consideration of the upper part of the 

range of the CPLs derived from the public preference studies was expressed by some 

CASAC Panel members during the public meeting on the second draft Policy 

Assessment. The Policy Assessment concluded that such a standard would be appropriate 

in conjunction with the Regional Haze Program to achieve appropriate levels of 

protection against PM-related visibility impairment in areas across the country (U.S. 

EPA, 2011a, p. 4-63).

Based on the considerations discussed above and in section VI.D.4 of the 

proposal, and taking into account the advice of CASAC, at the time of proposal the 

Administrator provisionally concluded that it would be appropriate to establish a target 

level of protection – for a standard defined in terms of a PM2.5 visibility index; a 90th

percentile form averaged over 3 years; and a 24-hour averaging time – equivalent to the 

protection afforded by such a sub-daily (i.e., 4-hour) standard at a level of 30 dv, which is 
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the upper end of the range of CPLs identified in the Policy Assessment and generally 

supported by CASAC. More specifically, the Administrator provisionally concluded that 

a 24-hour level of either 30 dv or 28 dv could be construed as providing such a degree of 

protection, and that either level was supported by the available information and was 

generally consistent with the advice of CASAC. Thus, the EPA proposed two options for 

the level of a new 24-hour standard (defined in terms of a PM2.5 visibility index and a 90th

percentile form, averaged over 3 years) to provide appropriate protection from PMrelated visibility impairment: either 30 dv or 28 dv. As noted in the proposal, the option 

of setting such a 24-hour standard at a level of 30 dv would reflect recognition that there 

is considerable spatial and temporal variability in the key factors that determine the value 

of the PM2.5 visibility index in any given urban area, such that there is a relatively high 

degree of uncertainty as to the most appropriate approach to use in selecting a 24-hour 

standard level that would be generally equivalent to a specific 4-hour standard level. 

Selecting a 24-hour standard level of 30 dv would reflect a judgment that such substantial 

degrees of variability and uncertainty should be reflected in a higher standard level than 

would be appropriate if the underlying information were more consistent and certain. 

Alternatively, the option of setting such a 24-hour standard at a level of 28 dv would 

reflect placing more weight on statistical analyses of aggregated data from across the 

study cities and not placing as much emphasis on the city-to-city variability as a basis for 

determining an appropriate degree of protection on a national scale.

The information available for the Administrator to consider when setting the 

secondary PM standard raises a number of uncertainties. While CASAC supported 

moving forward with a new standard on the basis of the available information, CASAC 
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also recognized these uncertainties, referencing the discussion of key uncertainties and 

areas for future research in the second draft of the Policy Assessment. In discussing areas 

of future research, CASAC stated that: “The range of 50% acceptability values discussed 

as possible standards are based on just four studies (Figure 4-2), which, given the large 

spread in values, provide only limited confidence that the benchmark candidate 

protection levels cover the appropriate range of preference values. Studies using a range 

of urban scenes (including, but not limited to, iconic scenes – “valued scenic elements” 

such as those in the Washington, DC study), should also be considered” (Samet, 2010d, 

p. 12). The EPA solicited comment on how the Administrator should weigh those 

uncertainties as well as any additional comments and information to inform her 

consideration of these uncertainties. 

In addition, the EPA solicited comment on a number of other issues related to the 

level of the standard, including: 

(1) Both of the proposed levels and the various approaches to identifying generally 

equivalent levels upon which the alternative proposed levels are based.

(2) A broader range of levels down to 25 dv in conjunction with a 24-hour averaging 

time. 

(3) A range of alternative levels from 30 to 25 dv in conjunction with a sub-daily 

(e.g., 4-hour) averaging time.

(4) The strengths and limitations associated with the public preference studies and the 

use of these studies to inform the selection of a range of levels that could be used 

to provide an appropriate degree of public welfare protection when combined 

with the other elements of the standard (i.e. indicator, form and averaging time).

(5) Specific aspects of the public preference studies, including the extent to which the 

50 percent acceptability criterion is an appropriate basis for establishing target 

protection levels in the context of establishing a distinct secondary NAAQS to 

address PM-related visibility impairment in urban areas; how the variability 

among preference studies in the extent to which study participants may be 

representative of the broader study area population should be weighed in the 

context of considering these studies in reaching proposed conclusions on a 
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distinct secondary NAAQS; and the extent to which the ranges of VAQ levels 

presented to participants in each of the studies may have influenced study results 

and on how this aspect of the study designs should appropriately be weighed in 

the context of considering these studies in the context of this review.

vi. Administrator’s Proposed Conclusions Regarding PM Standards to Protect 

Visibility 

At the time of proposal, based on the considerations described above, the 

Administrator proposed to revise the suite of secondary PM standards by adding a 

distinct standard for PM2.5 to address PM-related visibility impairment, focused primarily 

on visibility in urban areas. This proposed visibility standard was to be defined in terms 

of a PM2.5 visibility index, which would use measured PM2.5 mass, combined with PM2.5

speciation data and relative humidity data, to calculate PM2.5 light extinction, translated 

into the deciview (dv) scale; a 24-hour averaging time; a 90th percentile form, averaged 

over 3 years; and a level of 28-30 dv. 

vii. Related Technical Analysis

At the time of proposal, the EPA conducted a two-pronged technical analysis of 

the relationships between the proposed PM2.5 visibility index standard and the current 24-

hour PM2.5 mass-based standard (Kelly, et al., 2012a). This analysis was designed to 

provide technical information to inform key issues related to implementing a distinct 

secondary standard for visibility as proposed. Specifically, the EPA recognized that 

significant technical issues were likely to arise for new or modified emissions sources 

conducting air quality analyses for purposes of demonstrating that they would not cause 

or contribute to a violation of the visibility standard under the Prevention of Significant 

Deterioration (PSD) program. Such a demonstration for the proposed secondary PM2.5

visibility index NAAQSstandard could require each PSD applicant to predict, via air 
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quality modeling, the increase in visibility impairment, in terms of the proposed PM2.5

visibility index, that would result from the proposed source's emissions in conjunction 

with an assessment of existing air quality (visibility impairment) conditions in terms of 

the proposed PM2.5 visibility index. The EPA noted that if this demonstration were to be 

attempted using the six-step procedure that the EPA proposed to use for calculating PM2.5

visibility index design values from monitored air concentrations of PM2.5 components, 

significant technical issues with the modeling procedures could arise.

To address these technical issues, the EPA sought to explore whether sources that 

met the requirements pertaining to the 24-hour mass-based standard of 35 μg/m3 would 

also meet the requirements pertaining to the proposed visibility index standard. As 

described in Kelly et al. (2012a), the first prong of the analysis addressed aspects of a 

PSD significant impact analysis by evaluating whether an individual source’s impact 

resulting in a small increase in the ambient PM2.5 concentration would produce a 

comparably small increase in visibility impairment. This analysis included estimates of 

PM2.5 speciation profiles based on direct PM2.5 emission profiles for a broad range of 

source categories and for theoretical upper and lower bound scenarios.

The second prong of the analysis addressed aspects of a PSD cumulative impact 

analysis by exploring the relationship between the three-year design values for the 

existing 24-hour PM2.5 NAAQSstandard and coincident design values for the proposed 

PM2.5 visibility index NAAQSstandard based on recent air quality data. This aspect of the 

analysis indicated that increases in 24-hour PM2.5 design values generally correspond to 

increases in visibility index design values, and vice-versa. The analysis further explored 

the appropriateness of using a demonstration that a source does not cause or contribute to 
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a violation of the 24-hour PM2.5 NAAQSstandard as a surrogate for a demonstration that 

a source does not cause or contribute to a violation of the proposed secondary PM2.5

visibility index NAAQSstandard. This analysis iswas based on 2008 to 2010 air quality 

data, and comparescompared the proposed level of 35 μg/m3for the 24-hour PM2.5

standard and for illustrative purposes an alternative standard level of 12 μg/m3for the 

annual PM2.5 standard with the proposed levels of 28 or 30 dv for the secondary PM2.5

visibility index standard in conjunction with a 24-hour averaging time and a 90th 

percentile form. The results indicateindicated that all (for the 30 dv level) or nearly all 

(for the 28 dv level) areas in attainment of the 24-hour PM2.5 standard would also behave 

been in attainment of the proposed secondary PM2.5 visibility index standard.

Based on this technical analysis, the EPA proposed that there is sufficient 

evidence that a demonstration that a source does not cause or contribute to a violation of 

the mass-based 24-hour PM2.5 NAAQSstandard serves as a suitable surrogate for 

demonstrating that a source does not cause or contribute to a violation of the proposed 

secondary 24-hour PM2.5 visibility index NAAQSstandard under the PSD program. As 

such, the EPA proposed to conclude that many or all sources undergoing PSD review for 

PM2.5 could rely upon their analysis for demonstrating that they do not cause or 

contribute to a violation of the mass-based 24-hour PM2.5 NAAQSstandard to also show 

that they do not cause or contribute to a violation of the proposed secondary PM2.5

visibility index NAAQSstandard, if a distinct visibility standard were finalized. 

Although this proposed “surrogacy policy” was designed to address an 

implementation-related issue, the second prong of the technical analysis addresses the 

broader technical question of the relationship between the existing 24-hour PM2.5
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NAAQSstandard and the proposed PM2.5 visibility index standard in terms of the degree 

of protection likely to be afforded by each standard. Specifically, the analysis indicated 

that depending on the level of the proposed PM2.5 visibility index standard, the existing 

24-hour PM2.5 mass-based standard would be as protective or in some areas more

protective of visibility than a distinct secondary standard set within the range of levels 

proposed. Commenters on the proposed PM2.5 visibility index explored the implications 

of this analysis at length, as discussed further below in section VI.C.1.f. For this reason, 

the analysis is described in some detail here.

Kelly et al. (2012a) noted that the relationship between design values for the 24-

hour PM2.5 NAAQSstandard and the proposed secondary visibility index standard is not 

obvious a priori because of differences in design value calculations for the standards. 

However, closer examination of this relationship indicated that increases or decreases in 

24-hour PM2.5 design values correspond, respectively, to increases or decreases in 

visibility index values. Specifically, based on measurements from 102 sites with 

complete data from 2008-2010, Kelly et al. (2012a) found linear correlations between the 

24-hour PM2.5 design values and the visibility index design values with r2values ranging 

from 0.65 to 0.98 across these sites, with an average r2value of 0.75 across all U.S. sites. 

Moreover, the data indicated that no design value existed where the visibility index 

design value exceeded 30 dv, but the 24-hour PM2.5 NAAQSstandard level of 35 μg/m3

was attained. Visibility index design values for certain sites in the Industrial Midwest 

were shown to exceed 28 dv despite the fact that the 24-hour PM2.5 design values for 

these sites were below 35 μg/m3. This was attributed to the combination of high nitrate 

and sulfate fractions, substantial RH adjustment factors, and PM2.5 distribution 
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characteristics that led to relatively high visibility index design values for a given 24-hour 

PM2.5 design value for counties in the Industrial Midwest.191 Kelly et al. (2012a) 

concluded that the “overall, design values based on 2008-2010 data suggest that counties 

that attain 24-hour PM2.5 NAAQS level of 35 μg/m3 would attain the proposed secondary 

PM2.5 visibility index NAAQS level of 30 dv and generally attain the level of 28 dv” (pp. 

17-18). In addition, the Kelly et al. analysis indicated that at sites that violated both the 

24-hour PM2.5 level and the secondaryproposed visibility index 30 dv level, the 

secondary standardproposed level of 30 dv would likely be attained if PM2.5

concentrations were reduced such that the 24-hour PM2.5 level of 35 μg/m3 was attained.

(Kelly et al.,2012a, p.15).

192 A key implication of this analysis, therefore, was that within 

the range of levels proposed by the EPA for thea visibility index NAAQSstandard (28-30 

dv), the 24-hour PM2.5 standard of 35 μg/m3 would be controlling in almost all (at 28 dv) 

or all (at 30 dv) instances.

2. Other (Non-Visibility) PM-related Welfare Effects

In the 2006 review, the EPA concluded that there was insufficient information to 

consider a distinct secondary standard based on PM-related impacts to ecosystems, 

materials damage and soiling, and climatic and radiative processes (71 FR 61144, 

October 17, 2006). Specifically, there was a lack of evidence linking various nonvisibility welfare effects to specific levels of ambient PM. In that review, to provide a 

 

191 Kelly et al. (2012a) also noted that “Regional reductions in sulfate PM2.5 due to 

emission controls planned as part of national rules as well as emission reductions 

associated with potential annual standard violations are expected to improve visibility in 

this region” (p. 17).

192 Kelly et al. (2012a), p.15.The analysis also showed that attaining the 24-hour PM2.5

NAAQSstandard level of 35 μg/m3 would result in achieving a lower PM2.5 visibility 

index level in certain areas of the country, largely western areas, than would be achieved 

in other areas of the country. This is due to differences in the composition of ambient 

PM2.5 and the lower relative humidity in those areas.
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level of protection for these welfare-related effects, the secondary standards were set 

equal to the revised primary standards to directionally improve the level of protection 

afforded vegetation, ecosystems, and materials (71 FR 61210, October 17, 2006).

This section briefly outlines key conclusions discussed more fully in section VI.E 

of the proposal regarding the non-visibility welfare effects of PM. These conclusions 

relate to the climate, ecological (including effects on plants, soil and nutrient cycling, 

wildlife and water) and materials damage effects of PM. For all of these effects, the 

Policy Assessment concluded that there is insufficient information at this time to revise 

the current suite of secondary standards. It is important to note that the Policy 

Assessment explicitly excluded discussion of the effects associated with deposited 

particulate matter components of NOx and SOx and their transformation products which 

are addressed fully in the joint review of the secondary NO2 and SO2 NAAQS.

a. Evidence of Other Welfare Effects Related to PM

With regard to the role of PM in climate, the proposal noted that there is 

considerable ongoing research focused on understanding aerosol contributions to changes 

in global mean temperature and precipitation patterns. The Integrated Science 

Assessment concluded “that a causal relationship exists between PM and effects on 

climate, including both direct effects on radiative forcing and indirect effects that involve 

cloud feedbacks that influence precipitation formation and cloud lifetimes” (U.S. EPA, 

2009a, section 9.3.10). These effects are discussed in more detail in section VI.E.1 of the 

proposal, which provides information on the major aerosol components of interest for 

climate processes, including black carbon (BC), organic carbon (OC), sulfates, nitrates, 

and mineral dusts, and the nature, magnitude, and direction (e.g., cooling vs. warming) of 
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various aerosol impacts on climate.193 The Policy Assessment concluded that aerosols 

alter climate processes directly through radiative forcing and by indirect effects on cloud 

brightness, changes in precipitation, and possible changes in cloud lifetimes (U.S. EPA, 

2011a, p. 5-10). Further, the Policy Assessment noted that the major aerosol components 

that contribute to climate processes (i.e. BC, OC, sulfate, nitrate and mineral dusts) vary 

in their reflectivity, forcing efficiencies and even in the direction of climate forcing, 

though there is an overall net climate cooling associated with aerosols in the global 

atmosphere (U.S. EPA, 2009a, section 9.2.10). The Policy Assessment concluded that the 

current mass-based PM2.5 and PM10 secondary standards were not an appropriate or 

effective means of focusing protection against PM-associated climate effects due to these 

differences in components (U.S. EPA, 2011a, p. 5-11). In addition, in light of the 

significant uncertainties in current scientific information and the lack of sufficient data, 

the Policy Assessment concluded it is not currently feasible to conduct a quantitative 

analysis for the purpose of informing revisions of the current secondary PM standards 

based on climate (U.S. EPA, 2011a, p. 5-11). Overall the Policy Assessment concluded 

that there is insufficient information at this time to base a national ambient standard on 

climate impacts associated with current ambient concentrations of PM or its constituents 

(U.S. EPA, 2011a, p. 5-11, -12).194

With regard to ecological effects, the proposal noted that several ecosystem 

components (e.g., plants, soils and nutrient cycling, wildlife and water) are impacted by 

PM air pollution, which may alter the services provided by affected ecosystems. 

 

193 Atmospheric PM is referred to as aerosols in the remainder of this section to be 

consistent with the Integrated Science Assessment.

194 This conclusion would apply for both the secondary (welfare-based) and the primary 

(health-based) standards. 
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Ecological effects include both direct effects due to deposition (e.g., wet, dry or occult) to 

vegetation surfaces and indirect effects occurring via deposition to ecosystem soils or 

surface waters where the deposited constituents of PM then interact with biological 

organisms. Some of the ecological effects considered in this review include direct effects 

to metabolic processes of plant foliage; contribution to total metal loading resulting in 

alteration of soil biogeochemistry and microbiology, and plant and animal growth and 

reproduction; and contribution to total organics loading resulting in bioaccumulation and 

biomagnification across trophic levels. Section VI.E.2 of the proposal summarizes key 

findings related to:

(1) Impacts on plants and the ecosystem services they provide due to deposition of 

PM to vegetative surfaces, which alters the radiation received by the plant, and 

uptake of deposited PM components by plants from soil or foliage, which can lead 

to stress and decreased photosynthesis;

(2) Impacts on ecosystem support services such as nutrient cycling, products such as 

crops and the regulation of flooding and water quality;

(3) Impacts on wildlife, especially due to biomagnification of heavy metals 

(especially Hg) up the food chain and bioconcentration of POPs and PBDEs; and

(4) Impacts of deposited PM, especially metals and organics, on the ecosystem 

services provided by water bodies, including primary production, provision of 

fresh water, regulation of climate and floods, recreational fishing and water 

purification.

The proposal noted that the Integrated Science Assessment had concluded that 

ecological evidence is sufficient to conclude that a causal relationship is likely to exist 

between deposition of PM and a variety of effects on individual organisms and 

ecosystems (U.S. EPA, 2009a, sections 2.5.3 and 9.4.7), and also noted that vegetation 

and other ecosystem components are affected more by particulate chemistry than size 

fraction. However, the proposal also pointed to the Integrated Science Assessment 

conclusion that it is generally difficult to characterize the nature and magnitude of effects 
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and to quantify relationships between ambient concentrations of PM and ecosystem 

response due to significant data gaps and uncertainties as well as considerable variability 

that exists in the components of PM and their various ecological effects. There are few 

studies that link ambient PM concentrations to observed effect. Most direct ecosystem 

effects associated with particulate pollution occur in severely polluted areas near 

industrial point sources (quarries, cement kilns, metal smelting) (U.S. EPA, 2009a, 

sections 9.4.3 and 9.4.5.7).

Based on the evidence available at this time, the proposal noted the following key 

conclusions in the Policy Assessment:

(1) A number of significant environmental effects that either have already occurred or 

are currently occurring are linked to deposition of chemical constituents found in 

ambient PM.

(2) Ecosystem services can be adversely impacted by PM in the environment, 

including supporting, provisioning, regulating and cultural services.

(3) The lack of sufficient information to relate specific ambient concentrations of 

particulate metals and organics to a degree of impairment of a specific ecological 

endpoint hinders the identification of a range of appropriate indicators, levels, 

forms and averaging times of a distinct secondary standard to protect against 

associated effects.

(4) Data from regionally-based ecological studies can be used to establish probable 

local, regional and/or global sources of deposited PM components and their 

concurrent effects on ecological receptors.

The proposal noted that the Policy Assessment had concluded that the currently 

available information is insufficient for purposes of assessing the adequacy of the 

protection for ecosystems afforded by the current suite of PM secondary standards or 

establishing a distinct national standard for ambient PM based on ecosystem effects of 

particulates not addressed in the NOx/SOx secondary review (e.g., metals, organics) (U.S. 

EPA, 2011a, p. 5-24). Furthermore, the Policy Assessment had concluded that in the 
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absence of information providing a basis for specific standards in terms of particle 

composition, the observations continue to support retaining an appropriate degree of 

control on both fine and coarse particles to help address effects to ecosystems and 

ecosystem components associated with PM (U.S. EPA, 2011a, p. 5-24).

With regard to materials damage, the proposal discussed effects associated with 

deposition of PM, including both physical damage (materials damage effects) and 

impaired aesthetic qualities (soiling effects). As with the other categories of welfare 

effects discussed above, the Integrated Science Assessment concluded that evidence is 

sufficient to support a causal relationship between PM and effects on materials (U.S. 

EPA, 2009a, sections 2.5.4 and 9.5.4). The deposition of PM can physically affect 

materials, adding to the effects of natural weathering processes, by potentially promoting 

or accelerating the corrosion of metals, by degrading paints and by deteriorating building 

materials such as stone, concrete and marble (U.S. EPA, 2009a, section 9.5). In addition, 

the deposition of ambient PM can reduce the aesthetic appeal of buildings and objects 

through soiling. The Policy Assessment made the following observations:

(1) Materials damage and soiling that occur through natural weathering processes are 

enhanced by exposure to atmospheric pollutants, most notably sulfur dioxide and 

particulate sulfates. 

(2) While ambient particles play a role in the corrosion of metals and in the 

weathering of materials, no quantitative relationships between ambient particle 

concentrations and rates of damage have been established.

(3) While soiling associated with fine and course particles can result in increased 

cleaning frequency and repainting of surfaces, no quantitative relationships 

between particle characteristics and the frequency of cleaning or repainting have 

been established.

(4) Limited new data on the role of microbial colonizers in biodeterioration processes 

and contributions of black crust to soiling are not sufficient for quantitative 

analysis.
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(5) While several studies in the PM Integrated Science Assessment and NOx/SOx

Integrated Science Assessment suggest that particles can promote corrosion of 

metals there remains insufficient evidence to relate corrosive effects to specific 

particulate levels or to establish a quantitative relationship between ambient PM 

and metal degradation. With respect to damage to calcareous stone, numerous 

studies suggest that wet or dry deposition of particles and dry deposition of 

gypsum particles can enhance natural weathering processes.

The Policy Assessment concluded that none of the new evidence in this review 

called into question the adequacy of the current standards for protecting against material 

damage effects, that such effects could play no quantitative role in determining whether 

revisions to the secondary PM NAAQS are appropriate at this time, and that observations 

continue to support retaining an appropriate degree of control on both fine and coarse 

particles to help address materials damage and soiling associated with PM (U.S. EPA, 

2011a, p. 5-29).

b. CASAC Advice

In advising the EPA regarding the non-visibility welfare effects, CASAC stated 

that it “concurs with the Policy Assessment’s conclusions that while these effects are 

important, and should be the focus of future research efforts, there is not currently a 

strong technical basis to support revisions of the current standards to protect against these 

other welfare effects” (Samet, 2010c). More specifically, with regard to climate impacts, 

CASAC concluded that while there is insufficient information on which to base a 

national standard, the causal relationship is established and the risk of impacts is high, so 

further research on a regional basis is urgently needed (Samet, 2010c, p. 5). CASAC also 

noted that reducing certain aerosol components could lead to increased radiative forcing 

and regional climate warming while having a beneficial effect on PM-related visibility. 

As a consequence, CASAC noted that a secondary standard directed toward reducing 

PM-related visibility impairment has the potential to be accompanied by regional 
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warming if light scattering aerosols are preferentially targeted.

With regard to ecological effects, CASAC concluded that the published literature 

is insufficient to support a national standard for PM effects on ecosystem services 

(Samet, 2010c, p.23). CASAC noted that the best-established effects are related to 

particles containing nitrogen and sulfur, which are being considered in the EPA’s 

ongoing review of the secondary NAAQS for NOx/SOx. With regard to PM-related 

effects on materials, CASAC concluded that the published literature, including literature 

published since the last review, is insufficient either to call into question the current level 

of the standard or to support any specific national standard for PM effects on materials 

(Samet, 2010c, p.23). Nonetheless, with regard to both types of effects, CASAC noted 

the importance of maintaining an appropriate degree of control of both fine and coarse 

particles to address such effects, even in the current absence of sufficient information to 

develop a standard.

c. Summary of Proposed Decisions Regarding Other Welfare Effects

Based on the above considerations and the advice of CASAC, at the time of 

proposal the Administrator provisionally concluded that it would not be appropriate to 

establish any distinct secondary PM standards to address other non-visibility PM-related 

welfare effects, including ecological effects, effects on materials, and climate impacts. 

Nonetheless, the Administrator concurred with the conclusions of the Policy Assessment 

and CASAC advice that it is important to maintain an appropriate degree of control of 

both fine and coarse particles to address such effects. Noting that there is an absence of 

information that would support any different standards, the Administrator proposed 
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generally to retain the current suite of secondary PM standards195 to address non-visibility 

welfare effects. Specifically, the Administrator proposed to retain all aspects of the 

current secondary 24-hour PM2.5 and PM10 standards. With regard to the secondary 

annual PM2.5 standard, the Administrator proposed to retain the level of the current 

standard and to revise the form of the standard by removing the option for spatial 

averaging consistent with this change to the primary annual PM2.5 standard.

C. Public Comments on Proposed Decisions regarding Secondary PM Standards

The EPA received a large number of comments on its proposed decisions with 

regard to secondary PM standards, with the large majority of those comments focusing on 

the proposal to set a distinct standard to protect against visibility impairment, discussed 

below in section VI.C.1. Very few commenters addressed the proposal to retain the 

existing secondary standards for non-visibility welfare effects, discussed below in section 

VI.C.2. As discussed in section VI.D. below, the Administrator has decided to retain the 

current suite of secondary PM standards generally, while revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. The Administrator has also 

decided, contrary to what was proposed, not to establish a distinct secondary standard to 

address PM-related visibility impairment. This section discusses EPA’s responses to the 

comments EPA received on its proposal, and the rationale behind the Administrator’s 

final decisions is discussed in section VI.D. below. 

1. Comments on Proposed Secondary Standard for Visibility Protection 

a. Overview of Comments

 

195 As summarized in section VI.A and Table 1 above, the current suite of secondary PM 

standards includes annual and 24-hour PM2.5 standards and a 24-hour PM10 standard. 
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Among those commenting on the proposal to set a distinct secondary PM2.5

visibility index standard, a large majority of commenters, including more than 25 state 

and local agencies; regional organizations such as NACAA, NESCAUM, and WESTAR; 

and industry commenters, such as ACC, API, BP, EPRI, NCBA, NEDA-CAP, NMA, 

NSSGA, and UARG, opposed setting a distinct secondary standard for visibility at this 

time. Many commenters in this group expressed the view that such a standard was not 

needed, primarily on the basis that adequate protection was provided by the existing 24-

hour secondary PM2.5 standard. Some of these commenters also expressed legal concerns 

with the nature of the proposed standard. Other commenters in this group supported a 

distinct secondary standard for visibility in concept, but expressed the view that it was 

premature to set such a standard pending collection of additional visibility preference 

study data and the resolution of a number of key technical issues. Support for setting such 

a distinct secondary standard for visibility at this time came from a second group of 

commenters, including the Department of the Interior (National Park Service), several 

states, the Mid-Atlantic/Northeast Visibility Union (MANE-VU), the National Tribal Air 

Association (NTAA), environmental organizations such as the Appalachian Mountain 

Club, National Parks Conservation Association, Earthjustice (AMC, et al.) and the 

League of Women Voters of Texas. These commenters argued that the existing 

secondary standards are not sufficiently protective of visual air quality, and that a distinct 

secondary standard similar to the proposed visibility index standard is both necessary and 

appropriate to ensure adequate protection of visibility.

Commenters in both groups expressed concerns about various aspects of the 

proposed distinct secondary standard, including the indicator, averaging time, level, and 
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form. In addition, a large number of commenters, including commenters from both 

groups, expressed concern and/or confusion over the relationship between the Regional 

Haze Program and the proposed distinct secondary standard for visibility, raising issues 

such as analytical differences in methods between the programs, monitoring issues, and 

other implementation challenges.

A discussion of the significant comments outlined above, including EPA’s 

responses to the comments, is presented here, with more detailed discussion in the 

Response to Comments document. Comments relating to the specific elements of the 

proposed standard – indicator, averaging time, form and level– are discussed in sections 

VI.C.1.b-e, respectively. Comments related to the need for a distinct secondary standard 

at this time are discussed in section VI.C.f. Legal issues raised by commenters opposed to 

setting a secondary standard based on the proposed visibility index are discussed in 

section VI.C.g. Finally, comments related to the relationship between a distinct secondary 

standard and the Regional Haze Program are discussed in section VI.C.h.196 While the 

EPA concludes in section VI.D below to retain the current suite of secondary PM2.5

standards, the appropriateness of the protection that would be provided by the proposed 

PM2.5 visibility index standard, and the relationship between this degree of protection and 

that provided by the current secondary 24-hour secondary PM2.5 standard, are key 

elements in the Administrator's decision, and are discussed below. 

b. Indicator

Numerous commenters, both those supporting a distinct secondary standard and 

 

196 Comments pertaining to implementation issues, which the Administrator may not 

consider in making decisions about setting national ambient air quality standards, are 

discussed in the Response to Comments document, as are comments regarding 

monitoring issues related to the proposed distinct visibility index standard.
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those opposed to setting such a standard, expressed views on the suitability of utilizing a 

PM2.5 calculated light extinction indicator for the standard as proposed. While these 

groups of commenters differed in terms of their views on the appropriateness of using 

calculated PM2.5 light extinction as the basis for the indicator rather than relying on direct 

measurements of PM2.5 light extinction, commenters from both groups expressed concern 

over specific elements of the proposed method of calculating PM2.5 light extinction. In 

particular, commenters expressed differing views on which IMPROVE algorithm should 

be utilized; whether it is appropriate to exclude coarse particles from the indicator; and 

whether the proposed protocols for incorporating data on relative humidity and PM2.5

species are appropriate.197

i. Comments on Calculated vs. Directly Measured Light Extinction

The majority of commenters in both groups noted the uncertainties associated 

with relying on a calculated light extinction indicator and stated a preference for utilizing 

direct light extinction measurements. However, recognizing the limitations on applying 

direct measurements at present, commenters supporting the proposal to set a distinct 

standard argued that relying on “calculated light extinction is a reasonable first approach” 

(DOI, p. 2). These commenters pointed to the advice of CASAC, which had 

acknowledged that it was not possible for the EPA to develop an FRM for direct 

measurement of light extinction within the time frame of this review and had concluded 

that relying on a calculated PM2.5 light extinction indicator represented a reasonable 

approach that could be implemented sooner than a directly measured indicator. These 

commenters generally supported the proposal to adopt a calculated PM2.5 light extinction 

 

197 Some commenters expressed concern about the omission of other contributors to 

visibility impairment from the visibility index, as discussed in the Response to Comments 

document.
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indicator, at least as an interim approach.

Commenters opposed to setting a distinct standard generally argued that it was 

inappropriate to rely on a calculated light extinction indicator rather than direct 

measurements. Some of these commenters argued that the proposed calculated light 

extinction indictor is ill suited for a bright line standard because the method uses average 

humidity and a reconstructed visibility measurement calculated from PM2.5 speciation 

filter analysis, rather than measuring what is actually observed by individuals. A number 

of commenters advocated postponing setting a distinct standard until an approach based 

on direct light extinction measurements can be adopted. Many of these commenters 

stated that relying on direct light extinction measurements would enable a standard to be 

based on a shorter averaging time, either 1-hour or sub-daily (4 to 6 hours), consistent 

with the more instantaneous nature of perceptions of visual air quality and the advice of 

CASAC in this review.

The EPA generally agrees with commenters that an indicator based on directly 

measured light extinction would provide the most direct link between PM in the ambient 

air and PM-related light extinction. However, as noted at the time of proposal and in 

accordance with the advice of CASAC, the EPA has concluded that this is not an 

appropriate option in this review because a suitable specification of currently available 

equipment or performance-based verification procedures could not be developed in the 

time frame of this review. Moreover, CASAC concluded that relying on a calculated 

PM2.5 light extinction indicator based on PM2.5 chemical speciation and relative humidity

data represented a reasonable approach. The inputs that are necessary include 

measurements that are available through existing monitoring networks and approved 
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protocols. Thus, the EPA remains confident that the available evidence demonstrates that 

a strong correspondence exists between calculated PM2.5 light extinction and PM-related 

visibility impairment. Furthermore, CASAC agreed, noting that the proposed calculated 

PM2.5 light extinction indicator based on the original IMPROVE algorithm “appears to be 

a reasonable approach for estimating hourly light extinction” (Samet, 2010d, p. 11) and 

“its reliance on procedures that have already been implemented in the CSN and routinely 

collected continuous PM2.5 data suggest that it could be implemented much sooner than a 

directly measured indicator” (Samet, 2010d, p. iii). Thus it would not be appropriate to 

postpone setting a distinct secondary standard until an approach based on direct light 

extinction measurements could be adopted.

ii. Comments on Specific Aspects of Calculated Light Extinction Indicator 

Some commenters, even those supporting the adoption of a calculated light 

extinction indicator, also expressed concern over specific aspects of the proposed 

indicator. First, a number of commenters expressed concern over the proposal to use the 

original IMPROVE algorithm as the basis for the calculated light extinction indicator. 

These commenters noted that the original IMPROVE algorithm has been shown to have 

consistent biases at both low and high levels of light extinction. In particular, these 

commenters expressed concern with the algorithm’s bias at higher levels of light 

extinction, which they pointed out were the conditions that might be encountered on 

hazier days in urban areas. 

Some commenters supported use of the new (revised) IMPROVE algorithm. 

These commenters noted that the revised equation has been through a peer review which 

confirmed that it is based on the best science and corrects the biases inherent in the 
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original algorithm. Commenters also noted that this revised algorithm has been widely 

incorporated into Regional Haze plans, and urged the EPA to use this same equation in 

the visibility index for the sake of consistency: “EPA approved this approach for regional 

haze and does not dispute its greater accuracy. Therefore, a national secondary ambient 

air quality standard based on criteria that accurately reflect the latest scientific knowledge 

logically should not revert to the original IMPROVE algorithm” (Oklahoma DEQ, p. 2). 

Other commenters noted that both the original and the revised IMPROVE algorithms 

were designed in support of the Regional Haze Program which is focused on largely rural 

Class I areas, and that neither algorithm is necessarily suitable for urban areas. Noting 

that the EPA has not thoroughly evaluated the applicability of either IMPROVE 

algorithm in urban areas, these commenters urged additional research to evaluate the 

suitability of either algorithm (or an alternative approach) in urban areas.

198

Second, a number of commenters argued that exclusion of coarse PM from the 

calculated light extinction indicator was inappropriate. These commenters noted that 

coarse particulate matter is an important contributor to visibility impairment in many 

areas, particularly in the western U.S., and that the levels of “acceptable” visual air 

quality derived from the visibility preference studies reflected total light extinction due to 

the full mix of particles (including coarse PM) in ambient air. A few commenters noted 

 

198 The new algorithm incorporates terms to account for particles representing the 

different dry extinction and water uptake from two size modes of sulfate, nitrate and 

organic mass; adjustments for the calculation of organic mass (OM) as 1.8*organic 

carbon (OC) to better represent aged aerosol found in remote areas; and a term for 

hygroscopic sea salt. While sulfate and nitrate contribute the same to visibility 

impairment as with the old formula, the effect of their relative concentrations is different 

due to the combination of the dry extinction coefficient and f(RH) functions for derived 

concentration of small and large particles. The new algorithm was evaluated at 21 remote

locations and it is generally used by RPOs and States for implementation of the Regional 

Haze Rule. 
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that due to the exclusion of coarse particles, a “deciview” calculated for purposes of the 

proposed PM2.5 visibility index is inconsistent with the unit as conventionally defined 

under the Regional Haze Program. Other commenters, however, supported the proposal 

to exclude coarse PM from the calculated light extinction indicator, noting the important 

role that PM2.5 plays in urban visibility and arguing it would be more difficult to control 

the contribution of coarse particle sources such as wind-blown dust to urban visibility 

impairment. 

Third, some commenters questioned why the EPA was proposing to rely on 

monthly average relative humidity (f(RH)) values when hourly humidity data are widely 

available, particularly in urban areas. One commenter argued that the EPA’s proposed 

approach involves “guessing relative humidity” rather than relying on accurate, readily 

available measurements (Oklahoma DEQ, p. 1). The commenter stated that since relative 

humidity is highly variable and weather dependent, the proposed approach “effectively 

undermines the capacity of the prescribed monitoring regime to identify periods when 

PM2.5 adversely affects visibility.” Other commenters supported this view, noting that 

relative humidity can vary substantially even within a 24-hour period, and that light 

extinction can be very sensitive to these changes. These commenters recommended that 

hourly or daily humidity measurements should be utilized in place of the proposed 

monthly average f(RH) values. 

Some commenters also recommended that the EPA should utilize a 90 percent 

relative humidity screen rather than 95 percent cap for purposes of eliminating periods in 

which visibility impairment is due to rain or fog. These commenters claimed that under a 

95 percent cap, both the average f(RH) values and the PM2.5 visibility index values could 
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be inflated in locations frequently affected by fog and/or precipitation. These commenters 

preferred the approach of excluding hours with relative humidity above 90 percent on the 

grounds that this approach would eliminate foggy/rainy hours irrespective of the 

frequency of occurrence.

The EPA does not agree with commenters who advocated using the revised 

IMPROVE algorithm. Both the original and the revised IMPROVE algorithms199 have 

been evaluated by comparing the calculated estimates of light extinction with coincident 

optical measurements. The As discussed above in section VI.B.1.a.i, the revised 

algorithm was developed to address observed biases in the predictions using the original 

algorithm under very low and very high light extinction conditions.

200 The new 

IMPROVE algorithm (Pitchford et al., 1997) incorporates additional terms to account for 

particles representing the different dry extinction and water uptake (f(RH)) from two size 

modes of sulfate, nitrate and organic mass, as well as adding a term for hygroscopic sea 

salt. There are also adjustments for the calculation of OM as 1.8*OC compared to 

1.4*OC in the original algorithm to better account for the more aged PM organic 

components found in remote areas, with further modifications and additions to better 

 

199 In either version of the IMPROVE algorithm, the concentration of each of the major 

aerosol components is multiplied by a dry extinction efficiency value and, for the 

hygroscopic components (i.e., ammoniated sulfate and ammonium nitrate), also 

multiplied by an additional factor to account for the water growth to estimate these 

components’ contribution to light extinction. Both the dry extinction efficiency and water 

growth terms have been developed by a combination of empirical assessment and 

theoretical calculation using typical particle size distributions associated with each of the 

major aerosol components.

200 The original algorithm is referred to as the IMPROVE algorithm because it was 

developed specifically to use the aerosol monitoring data generated at network sites and 

with equipment specifically designed to support the IMPROVE program and was 

evaluated using IMPROVE optical measurements at the subset of sites that make those 

measurements (Malm et al., 1994). The biases in this algorithm were detected by 

comparing light extinction estimates generated from the original IMPROVE algorithm to 

direct optical measurements in a number of rural Class I areas.
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account for differences in particle composition and aging in remote areas.

201 However, 

the EPA does not believe that these same modifications and additions would necessarily

be appropriate for calculating light extinction in urban areas. Instead, the EPA considers 

the original algorithm to be suitable for purposes of calculating urban light-extinction, 

although some adjustments may be appropriate for urban environments as well. The 

reasons why the original algorithm is suited to urban environments are discussed further 

below, along with adjustments that the EPA believes are likely appropriate based on the 

current (limited) state of knowledge.

First, the EPA considers that the multiplier of 1.8 used to convert OC to OM in 

the revised IMPROVE algorithm is too high for urban environments. The EPA is aware 

that there has been considerable debate within the research community about the 

appropriate multiplier to use to best represent urban environments. As discussed in 

Appendix F of the Policy Assessment (U.S. EPA, 2011a), the EPA used the SANDWICH 

mass closure approach (Frank, 2006) in the Urban Focused Visibility Assessment (U.S. 

EPA, 2010b) for purposes of calculating maximum daylight hourly PM2.5 light extinction 

and evaluated which multiplier would produce 24-hour results most similar to the 

SANDWICH approach using 24-hour PM2.5 organic carbon derived from the new 

Chemical Speciation Network (CSN) carbon monitoring protocol established in 2007.202

 

201 Specifically, the revised IMPROVE algorithm incorporates additional terms to 

account for particles representing the different dry extinction and water uptake (f(RH)) 

from two size modes of sulfate, nitrate and organic mass, as well as adding a term for 

hygroscopic sea salt. There are also adjustments for the calculation of OM as 1.8*OC 

compared to 1.4*OC in the original algorithm to better account for the more aged PM 

organic components found in remote areas.

202 Starting in 2007, the CSN adopted the IMPROVE monitoring protocol for the 

measurement of organic and elemental carbon using the IMPROVE analytical method 

and an IMPROVE-like sampler. The transition was completed in 2009. (See
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Analyses presented in Appendix F of the Policy Assessment indicate that a multiplier of 

1.6 is most appropriate for purposes of comparing the hourly PM2.5 light extinction with 

calculated 24-hour extinction (see Appendix F, section F.6 for a full explanation). The 

EPA also considers this higher multiplier to be a better approach for urban CSN

monitoring sites where the new measurements of organic carbon tend to be lower than 

those produced by the older NIOSH-type monitoring protocol (Malm, 2011). A multiplier 

of 1.6 is now used to calculate OM from OC measurements at CSN sites.

At the time of proposal, the EPA proposed to use the original IMPROVE 

algorithm with its 1.4 multiplier for converting OC to OM, but requested comment on 

whether this value was appropriate. Comments received by the Agency generally indicate 

that the OC-to-OM multiplier of 1.4 used in the original IMPROVE algorithm is too low

for urban areas. Based on the analyses presented in Appendix F of the Policy Assessment, 

the EPA agrees with these commenters. However, the EPA also believes that it would be 

inappropriate to use a multiplier as high as 1.8 to convert OC to OM in urban areas. As 

noted by commenters, the organic mass contribution to visibility impairment can be large, 

and generally OM is significantly larger in urban areas compared to surrounding rural 

areas.

203 Because a large portion of the organic component of urban PM results from 

nearby emissions sources, the total OM mass is generally closer to the measured organic 

carbon (OC) from which it is derived. This means it is appropriate to use a smaller 

multiplier to convert OC to OM in urban areas as compared to the value of 1.8 used in the 

revised algorithm, which is tailored to remote areas. The CASAC noted that urban OM 

 

“Modification of Carbon Procedures in the Speciation Network,”

http://www.epa.gov/ttn/amtic/files/ambient/pm25/spec/faqcarbon.pdf.)

203 The difference between higher PM2.5 mass in urban areas compared to surrounding 

regions, known as the urban excess, is largely attributed to organic mass (U.S. EPA

Particle Pollution Report, 2004)., 2004b).

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includes fresh emissions and the EPA concluded in the Visibility Assessment that “the 

original version is considered more representative of urban situations when emissions are 

still fresh rather than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3-19). 

Although the newrevised algorithm represents the best science of estimating extinction in 

remote areas with its aged aerosol, the commenters did not address how the EPA should 

modify the newrevised algorithm to best represent the more complex and different urban 

aerosol, particularly for OM. However, recognizing In light of all of these considerations, 

in particular the analyses the EPA conducted for Appendix F of the Policy Assessment

and the fact that the monitoring method for organic carbon has recently changed in the 

CSN network, the EPA agrees with commentersjudges that a multiplier of 1.4 is likely 

too low, and 6 for urban areas would be most appropriate for purposes of calculating 

PM2.5 light extinction in urban areas.204 In formulating this judgment, the EPA recognizes 

that a somewhat higher neither the original nor the revised IMPROVE algorithm has been 

tested for suitability in urban areas and that additional research is necessary to reduce the 

uncertainties about the most appropriate value would be appropriate. Based on the 

analyses the EPA conducted for Appendix F of the Policy Assessment, afor the OC to 

OM multiplier of 1.6 for urban areas is judged to in urban environments. With regard to 

other changes between the original and revised IMPROVE algorithms, the EPA also does 

not believe that it would be appropriate for data produced with the new CSN carbon 

monitoring protocol.205to include a term for hygroscopic sea salt for urban light 

 

204 The implications of this shift to a 1.6 multiplier for OC in urban areas for decisions 

about averaging time, level, and need for a distinct secondary standard are discussed 

further below in sections VI.C.1.c, VI.C.1.e, and VI.C.1.f, respectively.

205 The implications of this shift to a 1.6 multiplier for OC in urban areas for decisions 

about averaging time, level, and need for a distinct secondary standard are discussed 

further below in sections VI.C.1.c, VI.C.1.e, and VI.C.1.f, respectively.
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extinction, or to differentiate between different size modes of sulfate, nitrate, and organic 

mass as empirically defined by the revised IMPROVE algorithm. Unlike in some remote 

coastal locations, sea salt is not major contributor to light extinction in urban areas. 

Moreover, urban sources of salt include sanding of roads during the winter and those reentrained particles are mostly in the coarse size range. 

Like in remote areas, small and large size modes of sulfate, nitrate and organic 

mass would exist in the urban environment. However, the apportionment of the total fine 

particle concentration of each of the three PM2.5 components into the concentrations of 

the small and large size fractions would likely need a different approach than that used 

for remote areas. This is because of the closer proximity of urban sources to their 

emissions. This is a particular concern not only for organic mass, which as explained 

previously has a large contribution from nearby urban emission sources, but also for 

PM2.5 nitrate whose concentrations are also higher in urban areas compared to the 

surrounding regions. Thus, a higher portion of the total urban concentration may be in the 

small mode compared to remote areas and thus a different apportionment algorithm 

would be needed.

Finally, the EPA does not consider it necessary to employ site-specific Rayleigh 

light scattering terms in place of a universal Rayleigh light scattering value for purposes 

of calculating light extinction in urban areas for purposes of calculating the 90th

percentile values. The site-specific Rayleigh value is most important to accurately 

estimate extinction on the best visibility days which is an essential metric for the regional 

haze program.

For all of these reasons, the EPA considers the original IMPROVE algorithm 

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better suited to the task of calculating urban light extinction than the revised IMPROVE 

algorithm. However, the EPA does consider it appropriate to make certain adjustments to 

the original algorithm for purposes of calculating urban light extinction. As discussed 

above, the EPA believes it is appropriate to use a 1.6 multiplier to convert OC to OM in 

urban areas. In addition, the EPA believes it is appropriate to exclude the term for coarse 

particles from the equation. The EPA does not agree with commenters who suggested 

that coarse particles should be included in the calculated light extinction indicator. As 

noted in the proposal, PM2.5 is the component of PM responsible for most of the visibility 

impairment in most urban areas. Currently available data suggest that PM10–2.5 is a minor 

contributor to visibility impairment most of the time, although at some locations (U.S. 

EPA, 2010b, Figure 3-13 for Phoenix) PM10–2.5 can be a major contributor to urban 

visibility effects. While it is reasonable to assume that other urban areas in the desert 

southwestern region of the country may have conditions similar to the conditions shown 

for Phoenix, in fact few urban areas conduct continuous PM10-2.5 monitoring. This 

significantly increases the difficulty of assessing the role of coarse particles in urban 

visibility impairment. For example, among the 15 urban areas assessed in this review, 

only four areas had collocated continuous PM10 data allowing calculation of hourly PM10-

2.5 data for 2005 to 2007. In addition, PM10-2.5 is generally less homogenous in urban 

areas than PM2.5: in that coarse particlesparticle concentrations exhibit greater temporal 

variability and a steeper gradient across urban areas than fine particles (U.S. EPA, 2009a, 

p. 3-72). This makes it more challenging to select sites that would adequately represent 

urban visibility conditions. Thus, while it would be possible to include a PM10–2.5 light 

extinction term in a calculated light extinction indicator, as was done in the Visibility 
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Assessment, there is insufficient information available at this time to assess the impact 

and effectiveness of such a refinement in providing public welfare protection in areas 

across the country (U.S. EPA, 2011a, pp. 4-41 to 4-42). Therefore, the EPA concludes 

that it is not appropriate to set a standard based on a calculated light extinction indicator 

that includes coarse particles at this time, and the calculated indicator should be based on 

PM2.5 light extinction.

With regard to the suggestion by some commenters that the calculated light 

extinction indicator should be calculated using hourly humidity data, the EPA disagrees 

that concurrent humidity measurements should be used. The use of longer-term averages 

for each monitoring site adequately captures the seasonal variability of relative humidity 

and its effects of visibility impairment, and this approach focuses more on the underlying 

aerosol contributions to visibility impairment and less on the day-to-day variations in 

humidity. This provides a more stable indicator for comparison to the NAAQS and one 

that is more directly related to the underlying emissions that contribute to visibility 

impairment. 

With regard to the comments advocating the use of a 90 percent humidity screen 

as opposed to a 95 percent humidity cap, the EPA believes that relying on monthly 

average relative humidity values based on 10 years of climatological data appropriately 

reduces the effect of fog and precipitation. Although the approach of using a 95 percent 

humidity cap, as in the Regional Haze Program, includes some hours with relative 

humidity between 90-95 percent, the general approach of using a longer-term average for 

each monitoring site effectively eliminates the effect of very high humidity conditions on 

visibility at those locations.
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Therefore, taking all of the above considerations and CASAC advice into account, 

the EPA continues to conclude that a calculated PM2.5 light extinction indicator, similar 

to that used in the Regional Haze Program (i.e., using an IMPROVE algorithm as 

translated into the deciview scale), would be the most appropriate indicator to replace the 

current PM2.5 mass indicator for a distinct secondary standard. Moreover, the EPA 

continues to conclude that this calculated indicator should based on the original 

IMPROVE algorithm, adjusted to use a 1.6 OC multiplier and exclude the term for coarse 

particles, in conjunction with monthly average relative humidity data (i.e., f(RH) values) 

based on long-term climatological means as used in the Regional Haze Program. A PM2.5

visibility index defined in this way would appropriately reflect the relationship between 

ambient PM and PM-related light extinction, based on the analyses discussed in the 

proposal and reflecting the aerosol and relative humidity contributions to visibility 

impairment by incorporation of factors based on measured PM2.5 speciation 

concentrations and climatological average relative humidity data. In addition, this type of 

indicator would address, in part, the issues raised in the court’s remand of the 2006 PM2.5

standards. Such a PM2.5 visibility index would afford a relatively high degree of 

uniformity of visual air quality protection in areas across the country by virtue of directly 

incorporating the effects of differences in PM2.5 composition and relative humidity across 

the country. 

c. Averaging time

Few commenters specifically addressed the issue of averaging time. Those who 

did generally expressed the view that an hourly or sub-daily averaging time would be the 

most appropriate approach, as supported by CASAC and the EPA’s own analyses in this 
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review. These comments were generally consistent with the emphasis among all 

commenter groupscommenters on the desirability of adopting a directly measured light 

extinction indicator that could be measured on an hourly or sub-daily time scale. Some 

commenters noted that a standard based on a 4-6 hour averaging time would better 

capture peak daily light extinction while allowing stable signal quality; others urged EPA 

to adopt a 1-hour averaging time in conjunction with direct measurements. Commenters 

pointed to significant limitations associated with using a 24-hour averaging time, 

including the uncertainties in translating hourly or sub-daily visibility index values into 

24-hour equivalent values. Some commenters criticized the analysis presented in the 

Policy Assessment comparing the 24-hour calculated light extinction values to the 

maximum daylight 4-hour calculated light extinction values. These commenters stated 

that the scatter plots and regressions presented in the Policy Assessment indicate there is 

considerable variation in the 24-hour vs. 4-hour relationship, and interpreted this to mean 

that 24-hour light extinction values are a poor surrogate for 4-hour values. For example, 

several industry commenters cited an analysis which noted that the correlation coefficient 

between the 24-hour and 4-hour values was as low as r2= 0.42 in Houston, and stated 

that the EPA was being overly “optimistic” in concluding that city-specific and pooled r2

values in the range of 0.6 to 0.8 showed good correlation (UARG, Attachment 2, p. 27).

In addition, some commenters expressed concern over potential bias and greater 

uncertainty introduced by the inclusion of nighttime hours, noting that because relative 

humidity tends to be higher at night, inclusion of these hours could cause areas to “record 

NAAQS exceedances that have no corresponding visibility impairment value” (UARG, p. 

36). Commenters also emphasized the poor fit of a 24-hour averaging time with the near 
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instantaneous judgments about visibility impairment reflected in the visibility preference 

studies. Commenters also noted that there is greater hourly variation in PM 

concentrations and resulting visibility conditions in urban areas than in Class I areas; 

thus, while the Regional Haze Program uses 24-hour IMPROVE data, the commenters 

stated that a shorter averaging time is needed for an urban-focused PM2.5 visibility 

standard. Some commenters objected to a 24-hour averaging time as unsupported by the 

record in this review: “Because the science the Administrator relies on for the other 

elements of the proposed visibility standard is tied to short-term exposures to visibility 

impairment, the EPA has no basis for promulgating a standard that uses a 24-hour 

averaging time” (API, p. 43). These commenters claimed that while the EPA may not 

have the information or infrastructure in place to allow the Agency to set a standard 

based on a 1-hour or other sub-daily averaging time, this does not justify moving to a 24-

hour averaging time. 

Among commenters supporting the proposed distinct secondary standard for 

visibility, many commenters recognized the limitations on monitoring methods and 

currently available data that led to the EPA’s proposal to adopt a standard based on a 24-

hour averaging time. Most of these commenters acknowledged that the lack of reliable 

hourly speciation data means that a 24-hour averaging time is the only workable 

approach for a standard based on calculated light extinction. Commenters advocating a 

distinct secondary standard for visibility therefore generally supported the proposal to 

adopt a 24-hour averaging time, at least as an interim approach until a directly measured 

light extinction indicator could be adopted in the future. This approach was also 

supported by a few industry commenters who noted that since a visibility index standard 
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would be based on data from the IMPROVE and CSN monitors, which operate on a 24-

hour basis with 1-in-3 (or 1-in-6) day sampling, “it is imperative that EPA retain a 24-

hour averaging time if a secondary visibility standard is promulgated” (API, Attachment 

2, p. 9).

In response to comments supporting a 1-hour or sub-daily (4- to 6- hour) 

averaging time in conjunction with a direct light extinction measurements, the EPA notes 

that, as discussed above in the response to comments on indicator, the Agency has 

concluded that a directly measured light extinction indicator is not an appropriate option 

in this review, independent of the decision on averaging time. Having reached the 

conclusion that a calculated PM2.5 light extinction indicator would be most appropriate, 

the EPA has next considered what averaging time would be most desirable for such an 

indicator. As noted in the proposal, the EPA has recognized that hourly or sub-daily (4-

to 6-hour) averaging times, within daylight hours and excluding hours with high relative 

humidity, are more directly related than a 24-hour averaging time to the short-term nature 

of the perception of PM-related visibility impairment and the relevant exposure periods 

for segments of the viewing public. Thus, the Agency agrees with commenters’ general 

point that, as a starting premise, a sub-daily averaging time would generally be 

preferable. 

However, as noted at the time of proposal and discussed above in section 

VI.B.1.c, important data quality uncertainties have recently been identified in association 

with currently available instruments that would be used to provide the hourly PM2.5 mass 

measurements that would be needed in conjunction with an averaging time shorter than 

24 hours. As a result, at this time the Agency has strong technical reservations about a 
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secondary standard that would be defined in terms of a sub-daily averaging time. The 

data quality issues which have been identified, including short-term variability in hourly 

data from currently available continuous monitoring methods, effectively preclude 

adoption of a 1-hour averaging time in this review, given the sensitivity of a 1-hour 

averaging time to these data quality limitations. Even with regard to multi-hour averaging 

times, the EPA continues to conclude that the data quality concerns preclude adoption of 

a sub-daily averaging time.

Moreover, analyses conducted for the Policy Assessment indicate that PM2.5 light 

extinction calculated on a 24-hour average basis would be a reasonable and appropriate 

surrogate for PM2.5 light extinction calculated on a 4-hour basis. The scatter plots 

comparing 24-hour and 4-hour calculated PM2.5 light extinction in the Policy Assessment 

(U.S. EPA, 2011a, Figures G-4 and G-5) do show some scatter around the regression line 

for each city. This was to be expected, since the calculated 4-hour light extinction 

includes day-specific and hour-specific influences that are not captured by the simpler 

24-hour approach. Overall, however, in the EPA’s view, both the city-specific and pooled 

15-city 24-hour vs. 4-hour comparisons show strong correlation between the two 

averaging times. Moreover, the 90th percentile design values calculated for 4-hour vs. 

24-hour light extinction are much more closely correlated than are the values for 

individual days in particular urban areas calculated using these two approaches. Thus, 

while the EPA agrees with commenters who pointed out the relatively low correlation 

between 4- and 24-hour values in cities such as Houston, the Agency points out that the 

correlations of 90th percentile values are much higher, particularly when one considers 

the average values across urban areas. In general, the 90th percentile values line up better 
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and demonstrate closer to a one-to-one relationship. 

The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships 

between estimated 24-hour and 4-hour visibility impairment based on the variety of 

metrics discussed in Appendix G of the Policy Assessment that further supports this 

finding. The reanalysis more appropriately considered the uncertainty of the calculated 4-

hour values. It also considered the effect of changing the OC to OM multiplier used in 

urban areas with the new CSN monitoring protocol from 1.4 to 1.6. The revised analysis 

shows that, for all cases considered, the 24-hour values are generally closer to the 4-hour 

values than originally estimated. 

Since conclusions in the proposal about the relationship between 4-hour and 24-

hour values were drawn not just on the basis of the city-specific results but also on the 

more robust 90th percentile pooled values, the EPA disagrees with commenters who state 

that the Agency was overly optimistic in considering 24-hour values an appropriate 

surrogate for 4-hour values. Also, it is appropriate to focus on the 90th percentile design 

value comparison since the design values would determine attainment status and the 

degree of improvement in air quality that could be expected in areas instituting controls 

to meet the NAAQS. Therefore the EPA does not agree with commenters who state that a 

24-hour averaging time cannot serve as an appropriate surrogate for sub-daily periods of 

visibility impairment. On the contrary, the EPA continues to conclude, on the basis of 

this analysis, that PM2.5 light extinction calculated on a 24-hour basis is a reasonable and 

appropriate surrogate for sub-daily PM2.5 light extinction calculated on a 4-hour basis.

The EPA recognizes that the effect of adopting a 24-hour averaging time may be 

to smooth out some of the hour-by-hour variability in visibility index values. (Indeed, this 
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is true if we compare a 4-hour averaging time to a 1-hour averaging time as well.) Hourspecific influences which would be evident if an hourly or sub-daily averaging time were 

to be used will be masked to some extent when those hours are averaged together with 

other hours. This means, in part, that a 24-hour averaging time may effectively reduce 

peak values by means of averaging them together with other hours, which may have 

lower values. However, given the well documented variability in hourly visibility 

conditions, especially in urban areas, as noted by commenters, it is reasonable to assume 

that in some cases peak hours may be significantly influenced by atypical conditions, 

making it appropriate to adopt an averaging time that is sufficiently long to ensure that 

hour-specific influences are balanced against more typical conditions. Perhaps even more 

important is the concern that many peak hourly measurements may be significantly 

influenced by atypical instrument performance; this reinforces the conclusion that it is 

appropriate to adopt a longer averaging time, to ensure that hour-specific uncertainties 

are balanced against more robust measurements.

Thus, in agreement with commenters who supported a daily averaging time, the 

EPA concludes that a 24-hour averaging time would be appropriate for a distinct 

secondary standard based on a calculated PM2.5 light extinction indicator. 

d. Form

The EPA received very few comments with regard to the proposal to adopt a 90th

percentile form, averaged over 3-years, in conjunction with a PM2.5 visibility index and a 

24-hour averaging time. One commenter stated that it was inappropriate to use a 90th

percentile form, noting that this would result in the exclusion of a minimum of 36 days of 

data annually. The commenter expressed particular concern that this proposed approach, 
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in combination with a 24-hour standard based on an unadjusted CPL, would not capture 

the worst visibility impairment and that this would undermine “the intent of setting a 

meaningful secondary visibility standard” (AMC, et al., p. 2). Another commenter argued 

that the EPA had provided no scientific basis for why the 90th percentile form was 

suitable, and claimed that the Agency was making “a somewhat arbitrary judgment that 

people’s welfare would be affected only if adverse urban visibility were to occur more 

than 10 percent of the time” (API, Attachment 2, p. 4). 

On other hand, a few commenters who appeared to generally support the proposal 

to use a 90th percentile form advocated averaging the 90th percentile values over longer 

time periods, arguing that averaging over only 3 years would not provide a stable 

assessment of visual air quality in the West because this time period is insufficient to 

properly account for western drought and fire cycles. These commenters pointed to the 

approach in the Regional Haze Program of averaging visibility impairment over 5 years, 

and noted that even within this longer time period data can be significantly influenced by 

high emissions during significant fire years.

The EPA disagrees with all of these comments, noting first that a multi-year 

percentile form offers greater stability to the air quality management process by reducing 

the possibility that statistically unusual indicator values will lead to transient violations of 

the standard. Furthermore, this approach is consistent with the approach taken in other 

NAAQS, including the current 24-hour PM2.5 NAAQS, which has a 98th percentile form 

averaged over 3 years.The EPA disagrees with all of these comments. With regard to the 

comment opposing the 90th percentile form as inappropriately excluding the worst 

visibility days, the EPA notes that there is a significant lack of information on, and a high 
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degree of uncertainty regarding, the impact on public welfare of the number of days with 

visibility impairment over the course of a year. For example, the visibility preference 

studies used to derive the range of CPLs considered in this review offered no information 

regarding the frequency of time that visibility levels should be below those values. Based 

on this limitation, the EPA concluded in the Policy Assessment that it would not be 

appropriate to consider eliminating all exposures above the level of the standard and that 

it was reasonable to consider allowing some number of days with reduced visibility. 

Recognizing that the Regional Haze Program focuses attention on the 20 percent worst 

visibility days (i.e., those at or above the 80th percentile of visibility impairment), the 

EPA continues to believe, as noted in the proposal, that a percentile well above the 80th

percentile would be appropriate to increase the likelihood that all days in this range 

would be improved by control strategies intended to help areas attain the standard.

Focusing on the 90th percentile, which represents the median of the distribution of the 20 

percent worst visibility days, could be reasonably expected to lead to improvements in 

visual air quality on the 20 percent most impaired days. Thus, the EPA has made a 

reasoned judgment based on a full consideration of the upper end of the distribution of 

visibility impairment conditions and continues to conclude that it is appropriate to focus 

on the 90th percentile of visibility impairment values.

With regard to comments requesting the EPA adopt a longer multi-year averaging 

period for the 90th percentile values, the EPA disagrees that it would be appropriate to 

average the 90th percentile values over periods longer than 3 years. The EPA recognizes 

that a multi-year percentile form offers greater stability to the air quality management 

process by reducing the possibility that statistically unusual indicator values will lead to 
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transient violations of the standard. Utilizing a 3-year average form provides stability 

from the occasional effects of inter-annual meteorological variability that can result in 

unusually high pollution levels for a particular year. The Agency has adopted this

approach in other NAAQS, including the current secondary 24-hour PM2.5 NAAQS, 

which has a 98th percentile form averaged over 3 years. However, adopting a longer 

multi-year averaging period longer than 3 years would increase even more the number of 

days with visibility impairment above the target level of protection and would therefore 

reduce the protectiveness of a visibilitythe standard. Based on this the EPA does not 

believe it would be appropriate to average 90th percentile values over a period as long as 

five years. Therefore, the EPA continues to conclude that a 90th percentile form, averaged 

over 3 years, would be appropriate, in conjunction with a calculated PM2.5 visibility 

indexlight extinction indicator and a 24-hour averaging time. 

e. Level

With regard to level, commenters focused on two main themes. First, a large 

number of commenters addressed the information available from the public preference 

studies with regard to the acceptability of various levels of visual air quality. These 

comments, which are discussed in subsection VI.C.1.e.i below, address the EPA’s use of 

visibility preference studies as the basis for the selection of a range of appropriate levels 

for the Administrator to consider. Many commenters challenged the use of these studies 

as the basis for setting a distinct secondary standard, arguing that limitations in these 

studies rendered them an unsuitable and insufficient basis on which to establish such a 

standard. Second, commenters expressed different views as to what level(s) of a distinct 

secondary standard would be appropriate, if the EPA were to set such a standard. These 
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comments reflected consideration of the results of the public preference studies as well as 

analyses conducted in the Visibility Assessment and the Policy Assessment, as discussed 

in the proposal. Comments addressing the appropriateness of specific levels are discussed 

in subsection VI.C.1.e.ii below. 

i. Comments on Visibility Preference Studies 

A majority of commenters expressed the view that the existing preference studies 

provide an insufficient basis for selection by the Administrator of an appropriate level of 

public welfare visibility protection for a national standard. These commenters highlighted 

a number of limitations and uncertainties (enumerated below) and uncertainties 

associated with these studies as support for this view. In contrast, other commenters felt 

that despite certain limitations, these studies do provide a sufficient basis on which the 

Administrator can select an appropriate level of a standard to provide national public 

welfare visibility protection. The remainder of this section organizes and discusses these 

comments under four broad topic areas, including: (a) limitations and uncertainties 

associated with the visibility preference studies; (b) preference study methods and design; 

(c) use of preference study results for determining adversity; (d) the appropriateness of 

using regionally varying preference study results to select a single level for a national 

standard. 

(a) Preference Study Limitations and Uncertainties

A large and diverse number of limitations and uncertainties associated with the 

visibility preference studies have been identified and discussed in the public comments. 

Many of these same limitations and uncertainties were also identified and discussed by 

the EPA in the various documents developed throughout this review such as the 

Integrated Science Assessment, Visibility Assessment, and Policy Assessment, as well as 
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in the proposed rule (see section VI.b.2 at 77 FR 38975 and also at 38990).. The most 

important and fundamental limitations and uncertainties will be discussed here in the 

preamble, while more specific, unique or detailed comments will be addressed in the 

Response to Comments document. 

The primary or most frequent limitation(s) cited by many commenters 

relaterelates to the small number of preference studies that are available in this review. In 

particular, some commenters note that these preference studies cover just four locations, 

only three of which occur in the U.S., that the two studies conducted in Washington, DC 

were pilot studies, not full preference studies, and/or that three of the preference studies 

were conducted in the West, while only one was conducted in the East, providing only 

limited geographic coverage. Typically, these same commenters also pointed out that 

taken together, these limited studies only included a total of 852 participants, which they 

claimed was too small a sample size and unrepresentative nationally. These commenters 

thus concluded that there is insufficient information, both geographically and 

demographically, upon which to select a national level of a visibility index for purposes 

of visibility protection. 

In contrast, several commenters stated support for using the preference studies, 

concluding they provide an adequate basis, in spite of their limited nature. In particular, 

AMC et al. state:

We believe that these studies provide sufficient results to inform setting a national 

visibility standard. While the number of studies is small, they do incorporate 

spatial variation and, in the case of Denver and Phoenix, varied populations.... 

EPA should have confidence, rather than uncertainty, in the fact that these studies 

used different methods and respondents and yield a range of 20 – 24 dv, with one 

outlier of 29. (AMC, et al., pp. 6-7)

In responseRegarding the first group of commenters, the EPA notes that it is well 
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aware of the limited nature of the information, which it has described in great detail in the 

Integrated Science Assessment, Visibility Assessment, and Policy Assessment, as well as 

in section VI.B.2 of the proposed rule (77 FR 3897538973). The EPA further notes, 

however, that limited information does not preclude the Administrator from making 

judgments based on the best available science, taking into account the existing 

uncertainties and limitations associated with that available science. Thus, in reaching 

judgments based on the science, the Administrator appropriately weighs the associated 

uncertainties. In this review of the PM standards, the EPA notes that the CASACThe 

CASAC supported this view and concluded that the available information provided a 

sufficient basis on which the Administrator could form a judgment about requisite PMrelated public welfare visibility protection. Specifically, CASAC stated “[t]he 20-30 

deciview range of levels chosen by EPA staff as ‘Candidate Protection Levels’ is 

adequately supported by the evidence presented” (Samet, 2010b, p. iii). As discussed 

below, in determining an appropriate degree of protection for visibility,in the proposed 

rule (77 FR 38990), the Administrator has recognized and explicitly takentook into 

account the uncertainties and limitations in the science in determining an appropriate 

degree of protection when she proposed a level at the upper end of the recommended 

range. As discussed below, the Administrator continues to be mindful of these 

uncertainties and limitations in reaching her final determination regarding what 

constitutes an appropriate degree of protection with respect to PM-related visibility 

impairment. 

With respect to the comments of AMC et al., the EPA agrees that these studies 

provide a sufficient basis to inform the Administrator’s judgments regarding an 
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appropriate level of protection from PM-related visibility impairment, but she recognizes 

that these studies, which are the only studies before her, are a limited source of 

information. However, the EPA does not agree that the Washington, DC, results represent 

an outlier, and thus the EPA believes these results are appropriately included in the range 

identified for the Administrator to consider. 

Some commenters made the point that the EPA relied on much of this same 

evidence to reach the conclusion in 2006 that the information was too limited to allow 

selection of a national standard. For example, API stated: 

[T]he bulk of the VAQ preference studies were available during the previous PM 

NAAQS review and were considered by the Agency in its establishment of the 

2006 PM secondary NAAQS.... The Proposed Rule does not mention this fact 

and does not explain why many of these same studies now compel EPA to 

propose this new secondary NAAQS....The Proposed Rule notes in passing that, 

since the last review of the PM NAAQS, ‘limited information that has become 

available regarding the characterization of public preferences in urban areas has 

provided some new perspectives on the usefulness of this information in 

informing the selection of target levels of urban visibility protection.’ 77 Fed. 

Reg. at 38969/2. It is a serious oversight that the Proposed Rule makes no attempt 

to explain what that information is or how it affects the interpretation of the VAQ 

preference studies. This ‘limited information’ is an apparent reference to 

information provided by Dr. Anne Smith. (API, p. 37)

The EPA disagrees with these commenters. First, the EPA disagrees that it failed 

to distinguish between studies that were available in the previous review and the current 

review. The discussion in section IVVI.A.1 of the proposal specifically identifies the 

studies from Denver, Phoenix and British Columbia (77 FR 38967/2) as being considered 

in the last review. The EPA further disagrees with the implication that it is being 

circumspect about identifying the “limited information that has become available 

regarding the characterization of public preferences in urban areas.” Beginning in 

section IVVI.A.3 of the proposed rule (77 FR 38969), the EPA was clear about what 

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information, both preexisting and new, it relied upon in this review to inform its views 

and provide the basis for its proposal. In section VI.B.2, the EPA elaborates on the 

specific information, tools, methods and data which are considered in relation to the 

public preference studies, including the new information available since the last review. 

As noted above and in the proposal, in addition to the substantial PM urban air 

quality information and analyses new to this review, there are three other noteworthy 

sources of information that have specifically “provided some new perspectives on the 

usefulness of” the preference studies “in informing the selection of target levels of urban 

visibility protection.”” (77 FR 38969). They include: (1) results from additional urban 

visibility preference study experiments conducted for Washington, DC by Smith and 

Howell (2009) which added to the preference data for that location, provided proof that 

the 2001 study could be replicated, and shed light on the role of location in preference 

responses

206; (2) an extensivea review and reanalysis (Stratus Consulting, 2009) of the 

urban visibility public preference studies from the four urban areas, including the newly 

available Smith and Howell (2009) experiments which examined the similarities and 

differences between the studies and evaluated the potential significance of those 

differences on the study results207; and (3) additional analyses conducted in response to 

questions raised by the CASAC,, including most importantly a logit analysis presented in 

the (Deck and Lawson, 2010, as discussed in Chapter 2 and Appendix J of the Visibility 

Assessment (see Appendix J and discussion in Chapter 2)), which was requested and 

 

206 Smith, AE; Howell S (2009). An assessment of the robustness of visual air quality 

preference study results. CRA International. Washington, DC. 

http://yosemite.epa.gov/sab/sabproduct.nsf/B55911DF9796E5E385257592006FB737/$F

ile/CRA+VAQ+Pref+Robustness+Study+3+30+09+final.pdf.

207 Stratus Consulting, 2009. Review of Urban Visibility Public Preference Studies: Final 

Report. EPA-HQ-OAR-2007-0492-0042.
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reviewed by CASAC, which showed that each city’s responses represented unique and 

statistically different curves. 208 Taken together, these sources contributed to an increase 

in the EPA’s current knowledge and understanding of the robustness of each survey 

studystudy’s results, the appropriateness of comparing each study’s results to the others, 

and the key uncertainties relevant to data interpretation. In addition, in the last review the 

decision to not adopt a distinct secondary standard was remanded as contrary to law and 

failing to provide a reasoned explanation for the decision. As such it is not appropriate for 

purposes of comparison with the Administrator's judgment and reasoning in this review. 

(b) Preference Study Methods and Design 

In addition to the limitations and uncertainties noted above, many comments also 

asserted the methodologies used in the preference studies are fundamentally flawed. 

Many commenters cited some of the same issues that have already been identified by the 

EPA as sources of uncertainty and potential factors in producing the statistically different 

study results (see section VI.B.1.b above). As noted above, the EPA is well aware of the 

issues raised regarding the adequacy of the preference studies to serve as a basis for a 

secondary NAAQS (see 77 FR 38975) and solicited comment on how these uncertainties 

should be considered (see 77 FR 38990). Most of these same commenters also pointed to 

an assessment of the preference studies methodology provided by Smith and Howell 

(2009) and others as the basis for their views, as indicated by the following comments:

Smith and Howell (2009) show that VAQ preference study outcomes are 

malleable and depend entirely on the design of the study. Accordingly, such 

studies do not identify any meaningful threshold of acceptable visibility 

conditions. Despite Smith and Howell’s conclusions, EPA continues to assert that 

the VAQ preference studies can be used to identify minimally acceptable 

visibility conditions even though the Agency has never provided any valid 

 

208 Deck L., Lawson M. 2010. Statistical Analysis of Existing Urban Visibility 

Preference Studies. EPA-HQ-OAR-2007-0492-0089.
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scientific basis for discounting the Smith and Howell (2009) results. (API, p. 38)

Well-controlled preference studies discussed by Anne Smith of Charles River 

Associates at the March 2010 CASAC meeting demonstrated that the judgment of 

panel members was affected by the order in which photographs were presented 

and tendency to identify the middle of the range of visibility degredation as a 

threshold of acceptability. This points to a potential flaw in these studies and that 

artifacts caused by these tendencies may have influenced study results. 

Dismissing these inherent flaws in the existing preference studies and then using 

these studies to set a secondary NAAQS is arbitrary and capricious. (API, 

Attachment 2, p. 12) 

EPA also fails to acknowledge that the only study conducted since the last review 

rebuts the validity of the VAQ preference studies previously conducted. (UARG, 

Attachment 2, p. 28)

As is explained in a more detailed discussion in the Response to Comments 

document, the EPA disagrees that the analysisstudy conducted by Smith and Howell 

(2009) supports the conclusion that the preference study methodologies were 

fundamentally flawed.; however, the EPA notes that their experiments do identify areas 

where additional research would be useful to further inform our limited understanding of 

public preferences in urban areas. The EPA views the Smith and Howell experiments as 

increasing the EPA’s knowledge and understanding of the findings of the 2001 

Washington, DC focus group pilot study (Abt, 2001) in several important ways., although 

this information still remains limited overall. Specifically, the Smith and Howell results 

suggest: 1) the 2001 s results, while based on a small sample size of 9, may bewere 

consistent with results from a fair representation for the preferences forlarger sample of

the general Washington, DC population; 2) an individual’s preferences for visibility in 

one location may not depend on whether they live in that location; and 3) presentation 

method (i.e., changing from slide projection to computer monitor) doesdid not appear to 

affect the reported preferences. The EPA provides detailed responses to specific 
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comments on the design of the Smith and Howell experiments in the Response to 

Comments document.

(c) Preference Study Results and Adversity

A number of comments were received regarding the EPA’s use of preference 

study results to make the determination that adverse PM2.5-related visibility effects on the 

public welfare are occurring. In this context, several commenters questioned whether the 

EPA had made the case that unacceptable levels of visual air quality based on preference 

study results alone can be equated with an adverse public welfare effect. These 

commenters suggested that unless preference study information is linked to personal 

comfort and well-being or other associated welfare effects, it cannot form the basis of a 

determination of adversity. For example, Kennecott Utah Copper LLC stated that: 

Thus, EPA seemingly was building the foundation for a determination of what 

constitutes an adverse effect on visibility in the context of public welfare. 

However ... EPA subsequently veered toward an oversimplified focus on public 

acceptance of visibility conditions.... EPA’s discussion of visibility in the Policy 

Assessment and its proposed rule in the Federal Register focuses entirely on 

“acceptable” and “unacceptable” visual air quality and make no mention of an 

“adverse effect” in the context of visibility. EPA’s reliance on only 3 urban 

PSpreference studies represents a paucity of data and a wholesale abandonment of 

any effort to seek a scientifically measurable adverse effect. (Kennecott Utah 

Copper LLC, p. 26)

In response, the EPA first notes that the definition of effects on welfare included 

in section 302(h) of the CAA identifies both visibility and the broader category of effects 

on personal comfort and well-being as effects on welfare. In setting a secondary standard 

to address visibility impairment, the EPA considers the effect on the public from 

impairment of visibility as a separate and distinct welfare effect in its own right. The EPA 

is not required to translate this into terms of personal comfort and well-being, as visibility 

impairment is designated explicitly by Congress as an effect on welfare. While there may 
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be a large degree of overlap among these different welfare effects, the EPA properly 

focuses on evaluating all of the information before the Agency on the effect visibility 

impairment has on the public, whether or not this impairment would also be categorized 

as having an adverse effect on personal comfort and well-being. It is in the context of all 

of this information that the EPA makes the judgment as to the appropriate degree of 

protection from known and anticipated adverse effects on the public from visibility 

impairment. The EPA recognizes that there is uncertainty about the degree of adversity to 

the public welfare associated with PM-related visibility impairment. However a 

secondary standard is designed to provide protection from "known or anticipated" 

adverse effects, and a bright line determination of adversity is not required in judging the 

requisite degree of protection under section 109(b)(2).Furthermore, the EPA disagrees 

that it has abandoned its consideration of visibility-related impacts on the welfare effect 

of personal comfort and well-being, as is made clear in the following quote:

Research has demonstrated that people are emotionally affected by low visual air 

quality, that perception of pollution is correlated with stress, annoyance, and 

symptoms of depression, and that visual air quality is deeply intertwined with a 

‘‘sense of place,’’ affecting people’s sense of the desirability of a neighborhood 

(U.S. EPA, 2009a, section 9.2.4). Though it is not known to what extent these 

emotional effects are linked to different periods of exposure to poor visual air 

quality, providing additional protection against short-term exposures to levels of 

visual air quality considered unacceptable by subjects in the context of the 

preference studies would be expected to provide some degree of protection 

against the risk of loss in the public’s ‘‘sense of well-being.’’ (77 FR 38973/1, 

emphasis added)

The approach taken to address such qualitative, but policy-relevant, information 

in this review is the same as in other NAAQS reviews. The review is initiated with a 

comprehensive assessment of all possible public health and welfare effects associated 

with PM in the Integrated Science Assessment. Then policy-relevant effects for which 
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there is sufficient quantitative information to allow a determination of the change in risks 

associated with incremental changes in air quality are assessed (in this review, in the 

Visibility Assessment) and used to provide a quantitative basis to inform the selection of 

an appropriate range of levels for further consideration in the Policy Assessment. In the 

Policy Assessment, the EPA considers all important policy-relevant evidence and 

information, both quantitative and qualitative, in making recommendations regarding the 

range of policy options appropriate for the Administrator to consider. It is in the context 

of all of this information that the Administrator makes her final judgment as to the 

appropriate degree of protection from known and anticipated adverse effects on the 

public from visibility impairment. The EPA recognizes that there is uncertainty about the 

degree of adversity to the public welfare associated with impairment of VAQ. However a 

secondary standard is designed to provide protection from "known or anticipated" 

adverse effects, and a bright line determination of adversity is not required in judging the 

requisite degree of protection under section 109(b)(2).

Another issue raised in the comments regarding adversity is the EPA’s decision to 

use the 50 percent acceptability criterion from the public preference studies in 

determining candidate protection levels of visibility impairment. In particular, some of 

these commenters questioned the appropriateness of using the preference study 50 

percent acceptability criteria results as a basis for the selection of a national level of 

visibility protection. For example, AMC et al. recommended “a 75% acceptability 

criterion as a target that is in line with protecting the broader public from the negative 

effects of visibility impairment” (AMC, et al., p. 9).

In the Visibility Assessment, the EPA noted that the use of the 50 percent 
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acceptance level for urban visibility was first presented in Ely et al. (1991) (U.S. EPA, 

2010b, p. 2-5). Ely discussed the use of the 50 percent acceptability criterion as a 

reasonable basis for setting an urban visibility standard.

The standard was determined based on a 50% acceptability criterion, that is, the 

standard was set at the level of extinction that would divide the slides into two 

groups: those judged acceptable and those judged unacceptable by a majority of 

the people in the study. The criterion is politically reasonable because it defines 

the point where a majority of the study participants begin to judge slides as 

representing unacceptable visibility. It is also consistent with psychological 

scaling theory209 which indicates that a “true score” exceeds a standard when 

more than 50% of the “observed scores” exceed that standard. (Ely et al., 1991, p. 

11)

As Ely described, the 50 percent acceptability criterion and the preference study 

conducted by Ely were used as the basis for setting the level of the Denver Visibility 

Standard in 1990. TheThat same criterion was usedjudged appropriate and selected for 

use in the Phoenix preference study (BBC research, 2003) and was also used as the basis 

for setting the level of the Phoenix Visibility Standard in 2003. Most recently, the 50 

percent acceptability criterion has been recommended by the British Columbia Visibility 

Coordinating Committee as the basis for the visibility standard currently under 

consideration by British Columbia, Canada. ThusFurthermore, CASAC supported this 

approach, while recognizing the uncertainty associated with this issue. Specifically, 

CASAC agreed that “the 50th percentile for the acceptability criteria is logical, given the 

noted similarities in methodologies employed in the 4 study areas. ... In terms of choosing a 

specific percentile from the preference studies, we note that there may not be a “preferred” 

one, but in assessing preference studies to propose a PM secondary NAAQS, the 50th 

percentile is sufficient, as it is the basis for existing visibility indexes used in the 

Denver/Colorado Front Range and Phoenix metropolitan areas” (Samet, 2009c, pp. 8-9).

 

209 J.C. Nunnally, Psychometric Theory, 2nd ed., MacGraw-Hill, New York, 1978.
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Therefore, after considering the information that served as the original basis for its 

selection as described in Ely et al., 1991, and given its acceptance and use in existing 

visibility programs and the absence of information to support a different acceptability 

criterion, the EPA continues to conclude, consistent with the advice of CASAC, that it is 

reasonable to use the 50 percent acceptability criterion in determining target levels of 

protection from visibility impairment. 

(d) Appropriateness of using regionally varying preference study results to select 

a single level for a national standard. 

A number of commenters raised concerns regarding the bases for and implications 

of the differences observed in the preference study results, concluding that these results 

were due to regionally varying factors and thus could not be used to set a national 

standard. In particularFor example, some commenters asserted that because the 

confidence intervals around the four 50 percent acceptability levels do not overlap at all, 

and because there are variations in preference study designs and inherent differences ofin

the visual setting among cities and panels, the four preference curves and their associated 

50 percent dv values are city-specific and statistically different. The commenters 

assertedconcluded, therefore, that it was inappropriate to aggregate the 50th percentile dv 

values from multiple studies and concluded that they should instead be evaluated 

individually. 

Other commenters expressed the related view that the preference study results 

cannot be used to set a national standard for visibility impairment because the results 

show that visibility preferences vary regionally. For example, API stated that: 

The ‘one-size-fits-all’ approach...is not viable because it does not account for 

regional and city-specific factors that have been made evident in the disparity of 

preference study data.... It is well known, for example, that the level of light 

extinction to which people in different areas of the country are accustomed, as 
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well as the urban setting, are the primary factors that affect a person’s visual 

perception of an urban vista. Thus, the degree to which extinction threshold can 

be related to human welfare is inevitably regionally-dependent. (API, Attachment 

2, p. 4)

Some of these commenters argued that because acceptable visual air quality is regionally 

dependent, it would be more appropriate to develop distinct visibility standards at the 

state or local level. Others pointed out that areas lackingwhich lack “important visibility 

vistas” might not need standards at all, since flat areas without significant terrain have a 

limited maximum visual range (NEDA/CAP, p. 3).

Other commenters stated that due to regionally varying factors, such as relative 

humidity, it is not possible to select a single level for a national standard to protect 

visibility across the United States. In particular, these commenters pointed to differences 

between Eastern and Western areas, arguing that a single national standard could not 

offer the appropriate degree of protection in locations with distinct characteristics. For 

example:

[T]he proposed method falls short because it is not temporally or geographically 

representative enough to have any meaning.... The uncertainty evidenced in these 

studies and the non-uniformity between the western and eastern vistas makes it 

impossible at this time to set an acceptable light extinction value that would 

appropriately address visibility concerns in non-Class I areas. (New York 

DOH/DEC, pp. 5-6)

The EPA agrees that the preference curves and the 50 percent dv levels are 

separate and distinct data points representing four different VAQ preference curves for 

four unique urban scenes. However, the EPA does not agree that the study results 

necessarily support a conclusion that preferences are regionally dependent, nor does the 

EPA consider the fact that the four curves are distinct as a weakness of the approach or a 

reason that the results cannot be compared. In addition, the EPA does not agree that the 

study results necessarily support a conclusion that preferences are regionally dependent.
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In particular, the EPA notes that the results of Smith and Howell (2009) which show that 

participants in Houston and Washington, DC did not have significantly different views on 

acceptable air quality in Washington, DC, provide limited support for the conclusion that 

people’s preferences differ less because of where they live and more because of the scene 

they are viewing. Indeed, these preference study results seem to substantiate what is 

already known,

On the other hand, the existing literature indicates that people’s preferences for 

VAQ depend in large part on the characteristics and sensitivity of the scene being 

viewed. It is to be expected that people looking at different scenes would have different 

preferences, measured using the 50% acceptability criterion, depending on how changes 

in VAQ impact the various scenes. Therefore, it might also be the case that if the same 

scene was shown to participants in different regions of the country, a single preference 

curve could be drawn. The preference study results showing that people find less air 

quality degredation acceptable for sensitive scenes than for less sensitive scenes could be 

interpreted as validating the appropriateness of the approach for eliciting public 

preferences. It is the EPA’s view that the differences in the preference study results 

appropriately reflect the different sensitivities to visibility impairment among the four 

urban scenes and thus provide useful information about how people’s preferences for 

visual air quality would change when viewing urban scenes of differing sensitivities. 

The EPA understands there is a wide variety or range of urban scenes within the 

United States. These sensitive urban scenes include those with natural vistas such as the 

Colorado Rocky Mountains as well as those with iconic man-made urban structures like 

the Washington Monument. The EPA believes that the scenes presented in the four urban 
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areas include important types of sensitive valued urban scenes and therefore, when 

considered together, can inform the selection of a level of acceptable urban VAQ at the 

national scale, taking into account the variation across the country evidenced in the 

studies. This is discussed further in the next section, below.

The EPA agreesdoes agree with commenters that there are factors that vary 

regionally varying factors that are important to take into account when setting a national 

standard for visibility protection. Section VI.A above regarding the history of the 

secondary PM NAAQS review discusses the evolution of the EPA’s understanding 

regarding the regional differences in PM concentrations, relative humidity and other 

factors. As a result, the current review has gone to great lengths to address these factors, 

leading to the EPA’s proposal to use the IMPROVE algorithm to calculate light 

extinction in order to take into account the varying effects of relative humidity and 

speciated PM. While this approach does not result in a uniform level of ambient PM2.5, it 

does ensure a nationally uniform level of visibility protection. The EPA refers the reader 

to other sections of the final rule, including sections VI.B.1.a, VI.B.1.c, VI.C.1.b and 

VI.C.1.f, and the Response to Comments document for a more detailed response as to 

how it is taking these variables into account. 

ii. Specific Comments on Level

The EPA received relatively few comments endorsing a specific level for a 

distinct secondary standard for visibility. In general, commenters who opposed setting a 

distinct secondary standard at this time did not address the question of what level would 

be appropriate if the EPA were to set a distinct secondary standard for visibility; 

similarly, commenters who supported adopting a distinct secondary standard at this time 
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generally did not recommend a specific level. However, a few commenters did provide 

comments in support of a specific level or range of levels, with some commenters 

advocating standards at the upper end of the range of proposed levels (i.e., 30 dv), while 

others supported levels below the lower end of the proposed range (i.e., below 28 dv). 

As discussed above, a large number of commenters argued that the currently 

available data are insufficient to determine what constitutes a standard that would be 

neither more nor less protective than necessary and that no standard should be set at this 

time. These commenters pointed to the limitations and uncertainties in the preference 

studies discussed above as the basis for this claim. These commenters pointed to 

significant variation in the results of the preference studies in support of their arguments 

that the studies should not be used to derive a level for a distinct secondary standard for 

visibility. For example, one consultant cited by several industry commenters argued that 

the proposed level of 28 or 30 dv did not reflect the substantial difference in visibility 

preferences between the East and the West reflected in the preference studies (UARG, 

Attachment 2, p. 11), and that it did not reflect the full range of preferences (i.e., potential 

50 percent acceptability levels) likely to exist nationwide (UARG, Attachment 2, p. 19). 

This commenter further objected to the EPA’s proposal for a level of 28 or 30 dv on the 

grounds that the EPA had inaccurately adjusted 4-hour values into 24-hour values. Based 

on his analysis, the consultant concluded that “a range of adjusted values from 28 to 32 

dv is needed” to account for the majority of the spread between the 4-hour vs. 24-hour 

equivalent values at the upper end of the distribution of values. 

A number of commenters questioned whether the proposed range of levels was 

appropriate. One industry commenter claimed that the EPA had not explicitly justified 
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why a standard within the proposed range was requisite, stating that “EPA makes no 

attempt to explain how the proposed level of the standard is neither lower nor higher than 

necessary to protect public welfare” (NSSGA, p. 15). Arizona DEQ noted that since the 

proposed calculated light extinction indicator excluded coarse particles and Rayleigh 

scattering, the proposed levels of 28 or 30 dv were inconsistent with the visibility 

preference studies, which considered total light extinction. Noting these perceived 

problems with the proposed range of levels, a few commenters noted that if the EPA were 

to set a distinct secondary standard, the level should be set no lower than 30 dv, “to 

account for inconsistent value judgments, a great deal of spatial and temporal variability, 

and a very high level of uncertainty” (Texas CEQ, p. 7). 

In contrast, some commenters supporting the EPA’s proposal for a distinct 

secondary standard for visibility stated that the proposed range of levels from 28-30 dv 

was insufficiently protective based on a 24-hour averaging time, and recommended a 

lower level for the visibility index standard. These commenters expressed the view that 

the proposed levels of 28 or 30 dv represented neither adequate surrogates for equivalent 

4-hour values, as the EPA claimed, nor sufficiently protective levels based on recent air 

quality data. Several commenters stated that the EPA’s own analyses suggested that a 

standard set at a level of 28 or 30 dv was insufficiently protective based on a 24-hour 

averaging time. One commenter emphasized that the Policy Assessment had indicated a 

level between 25-28 dv was appropriate for a standard calculated on a 24-hour average, 

and encouraged the EPA to adopt a standard level of 25 dv. Several environmental 

groups provided comments stating that a 24-hour average would underestimate a 4-hour 

value by 13-42 percent and certain areas of the country—particularly the Northeast—
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would be affected disproportionately. These commenters suggested that a 24-hour PM2.5

visibility index standard should be set at a level of 18.6-20 dv. The Department of the 

Interior pointed to recent air quality data indicating that visibility on the 20% worst days 

in several large metropolitan areas, including Birmingham, Fresno, New York City, 

Phoenix, and Washington, DC was below 29 dv. While noting that these calculations 

were based on IMPROVE calculations which include contributions from coarse PM 

mass, DOI expressed the view that the proposed level of 28 to 30 dv would not provide 

adequate visibility protection compared to the current 24-hour PM2.5 standard of 35 

μg/m3and recommended that the standard be set at a level of 25 dv consistent with the 

results of the Phoenix visibility preference study. 

In contrast, the states of Arizona and Colorado submitted comments arguing that 

the visibility preference studies conducted in Phoenix and Denver, respectively, were 

designed to address a specific local problem and that the results of these studies were not 

an appropriate basis for selecting the level of a national standard. For example, Arizona 

DEQ noted: 

The cited studies were conducted considering total light extinction; including 

extinction resulting from particulate matter and Rayleigh scattering. Visibility 

impairment due to coarse particulate matter can be an important contributor in 

Arizona, specifically in the Phoenix area where ongoing measurements have been 

made. Therefore, ADEQ believes that the proposed levels of the secondary 

visibility standard are inconsistent with applicable urban studies. (Arizona DEQ, 

p. 2)

Similarly, the Colorado Department of Public Health and the Environment noted that the 

Denver visibility standard was designed to address “brown clouds”, i.e., strong inversions 

that occur in the Denver metropolitan area, and that this standard “is based on a specific 

view of Denver” associated with particular sight paths and direct measurement methods. 

The commenter stated that this standard “is applicable only to this location,” and that 
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these limitations make it potentially unsuitable for application as “a national secondary 

standard, particularly a proposed standard that does not use a direct measurement 

method” (Colorado DPHE, p. 2). 

While acknowledging the uncertainties and limitations associated with the 

visibility preference studies as discussed above, the EPA continues to conclude, as did 

CASAC, that the preference studies are appropriate to use as the basis for selecting a 

target level of protection from visibility impairment. However, the EPA agrees with 

commenters who emphasize the high degree of variability in visibility conditions and the 

potential variability in visibility preferences across different parts of the country. In light 

of the associated uncertainty, as noted in the proposal, the Administrator judged it 

appropriate to establish a target level of protection equivalent to the upper end of the 

range of Candidate Protection Levels (CPLs) identified in the Policy Assessment and 

generally supported by CASAC. Thus, the EPA proposed to set a 24-hour visibility index 

standard that would provide protection equivalent to the protection afforded by a 4-hour 

standard set at a level of 30 dv. In light of the comments received on the proposal, in 

particular comments emphasizing the uncertainty and variability in the results of the 

public preference studies, the EPA continues to conclude that this approach is warranted, 

and that it is appropriate to set a target level of protection equivalent to the protection that 

would be afforded by a 4-hour, 30 dv visibility index standard.

Moreover, the EPA disagrees with commenters who argued that the EPA’s 

approach for translating 4-hour CPLs into equivalent 24-hour values was inappropriate. 

In adjusting 4-hour values for purposes of defining an appropriate level for a 24-hour 

standard, the EPA noted at the time of proposal that there were multiple approaches for 
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estimating generally equivalent levels on a city-specific or national basis, and that both of 

. While expressing the view that it was appropriate to consider the two preferred 

approaches identifiedwith the highest r2values (Approaches A and B in Appendix G of 

the Policy Assessment),210 which used regressions of 90th percentile light extinction 

values, the EPA determined it would also be appropriate to consider the city-specific 

estimates resulting from Approaches C and E which showed greater variability than the 

aggregated estimates. Approaches C and E generated a range of city-specific estimates of 

generally equivalent 24-hour levels that encompassed the range of levels considered 

appropriate for 4-hour CPLs, including the CPL of 30 dv at the upper end of that range.

This led to the conclusion that it would be appropriate to use This information provided 

support for using the same CPL for a 24-hour standard as for a 4-hour standard, since no 

single approach could generate an equivalent 24-hour standard level in each urban area 

for each CPL. The EPA continues to conclude, as it did at the time of proposal, that using

an unadjusted 4-hour CPL for purposes of establishing a target level of protection for a 

24-hour standard. The EPA disagrees is appropriate because this approach places more 

emphasis on the relatively high degree of spatial and temporal variability in relative 

humidity and fine particle composition observed in urban areas across the country, 

consistent with commenters who argued that the EPA’s approach for translating 4-hour 

CPLs into equivalent 24-hour values was inappropriate. reanalysis discussed below. 

The EPA has conducted a reanalysis (Frank et al., 2012b) of the relationships 

 

210 In particular, EPA staff expressed a preference for Approach B in the Policy 

Assessment. However, in light of the additional information provided by the other 

approaches explored in Appendix G of the Policy Assessment and the reanalysis in 

Frank, et al. (2012b), the EPA judges it more appropriate to consider the range of values 

resulting from all five analytical approaches for purposes of informing decisions about 

the equivalent level of a 24-hour standard.
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between estimated 24-hour and 4-hour visibility impairment based on the variety of 

metrics discussed in Appendix G of the Policy Assessment. The reanalysis has more 

appropriately considered the uncertainty of the calculated 4-hour values. The revised 

analysis shows that, for all cases considered, the 24-hour equivalent level is 

evengenerally closer to the 4-hour values value at the upper end of the range of CPLs

than originally estimated, demonstratingas can be seen in the results for Approaches B, C, 

and D.211 For example, the reanalysis indicates that Approach B yields an adjusted 24-

hour CPL of 29 dv212 as generally being equivalent to a 4-hour CPL of 30 dv, while 

Approach C yields a 24-hour equivalent CPL of 29 dv averaged across cities and a range 

of city-specific values from 25-36 dv.

213,214 Not only are the 90th percentile and pooled 

average values closer to the 4-hour CPL of 30 dv, the range of city-specific results shows 

a wider spread that clearly encompasses the unadjusted 4-hour value of 30 dv near the 

midpoint of the city-specific range. This provides support for concluding that the EPA’s 

approach to translating of 4-hour CPLs into equivalent 24-hour values was appropriate, 

and that it is appropriate to use unadjusted 4-hour values for purposes of selecting a level 

 

211 Approach E as presented in the Policy Assessment is based on the median values for 

each city; these results are not affected by the regression analyses. Therefore, Approach E 

was not included in the reanalysis, and the results remain unchanged from those reported 

in the corrected Table G-6 as reported in Frank, et al., 2012b.

212 In Appendix G of the Policy Assessment, a 24-hour adjusted CPL of 28 dv was 

estimated to be equivalent to a 4-hour value of 30 dv under Approach B (annual 90th

percentile values regression).

213 In Appendix G of the Policy Assessment, under Approach C (all-days city-specific 

regression), a 24-hour adjusted CPL of 27 dv was estimated to be equivalent to a 4-hour 

CPL of 30 dv when averaged across cities, while city-specific values were estimated to 

range from 24-30 dv.

214 In the reanalysis, Approach D (all days pooled regression) generated results of 28 dv 

for the 24-hour CPL equivalent to a 4-hour value of 30 dv as compared to a value of 27 

dv in the original analysis described in Appendix G.
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for a standard based on a 24-hour averaging time.

215

Moreover, the EPA disagrees with commenters who argue that the currently

available evidence is sufficient to justify establishing a target level of protection at 25 dv 

or below. The EPA recognizes that 25 dv represents the middle of the range of 50 percent 

acceptability levels from the 4 cities studied, and represents the 50 percent acceptability 

level from the Phoenix study, which the Agency has acknowledged as the best of the four 

studies in terms of having the least noise in the preference study results and the most 

representative selection of participants... The EPA also notes the caveats discussed in the 

proposal regarding whether it would be appropriate to interpret results from the western 

studies as generally representative of a broader range of scenic vistas in urban areas 

across the country. The Policy Assessment noted significant differences in the 

characteristics of the urban scenes used in each study, with western urban visibility 

preference study scenes including mountains in the background and objects at greater 

distances, while scenes in the eastern study did not. Since objects at a greater distance 

have a greater sensitivity to perceived visibility changes as light extinction changes 

compared to otherwise similar scenes with objects at a shorter range, this likely explains 

 

215 The analysis in Appendix G of the Policy Assessment used the 4-hour light extinction 

value treated as the independent (x-axis) variable in an ordinary least squares regression. 

ThisThe EPA now concludes that this regression approach was inappropriatenot the most 

appropriate approach because that variable has error and in fact may be more uncertain 

than the calculated 24-hour extinction values. The Frank et al. (2012b) reanalysis uses an 

orthogonal regression instead of ordinary least squares regression and results in slopes 

closer to the 1:1 line for all the results, particularly for Dallas, TX. Furthermore, 

consistent with the EPA’s conclusion that a higher multiplier for converting OC to OM 

would be appropriate (see section VI.C.1.b.ii above), the reanalysis substitutes a 1.6 

multiplier for converting OC to OM in the calculation of 24-hour values instead of the 

value of 1.4 that was used in calculating 24-hour values for Appendix G. The higher 

multiplier is more consistent with the SANDWICH approach used to calculate the 4-hour 

values found in Appendix G. See Frank et al. (20122012b) for a more detailed 

explanation.
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part of the difference between the results of the eastern study and results of the western 

studies. In the proposal, the EPA noted that the scenic vistas available on a daily basis in 

many urban areas across the country generally do not have the inherent visual interest or 

the distance between viewer and object of greatest intrinsic value as in the Denver and 

Phoenix preference studies. Also, the Agency takes note of the caution expressed by 

Colorado and Arizona about using the results of the Denver and Phoenix preference 

studies, which were aimed at addressing specific local visibility problems, to inform the 

choice of level for a national standard. Therefore, the Agency considers it reasonable to 

conclude, especially in light of the significant uncertainties, that it is appropriate to place 

less weight on the western preference results and that the high CPL value (30 dv) that is 

based on the eastern preference results is likely to be more representative of urban areas 

that do not have associated mountains or other valued objects visible in the distant 

background. These areas would include the middle of the country and many areas in the 

eastern U.S., as well as some western areas. As a result, the EPA concludes that it is more 

appropriate to establish a target level of protection at the upper end of the range of 24-

hour CPLs considered, recognizing that no one level will be "correct" for every urban 

area in the country.

In considering the upper end of this range, the EPA must identify a target level of 

protection that is considered requisite to protect public welfare from a national 

perspective, recognizing that the same target level would apply in all locations. Making 

this judgment requires a balancing of the risks to the public welfare and the substantial 

uncertainties surrounding appropriate levels of visibility protection. As acknowledged in 

the proposal, the EPA recognizes that setting a target level of protection for a 24-hour 
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standard at 30 dv would reflect a judgment that the current substantial degrees of 

variability and uncertainty inherent in the public preference studies should be reflected in 

a higher target protection level than would be appropriate if the underlying information 

were more consistent and certain. Also, a 24-hour visibility index at a level of 30 dv 

would reflect recognition that there is considerable spatial and temporal variability in the 

key factors that determine the value of the PM2.5 visibility index in any given urban area, 

such that there is a relatively high degree of uncertainty as to the most appropriate 

approach to use in selecting a 24-hour standard level that would be generally equivalent 

to a specific 4-hour standard level. In light of these uncertainties, the EPA continues to 

believe that it is appropriate to establish a target level of protection for visual air quality 

of 30 dv, averaged over 24-hours, with a form as discussed above.

In reaching this conclusion, the EPA notes that any national ambient air quality 

standard for visibility would be designed to work in conjunction with the Regional Haze 

Program as a means of achieving appropriate levels of protection against PM-related 

visibility impairment in all areas of the country, including urban, non-urban, and Federal 

Class I areas. While the Regional Haze Program is focused on improving visibility in 

Federal Class I areas and a secondary visibility index NAAQS would focus on protecting 

visual air quality principally in urban areas, both programs could be expected to provide 

benefits in surrounding areas. In addition, the development of local programs, such as 

those in Denver and Phoenix, can continue to be an effective and appropriate approach to 

provide additional protection, beyond that afforded by a national standard, for unique 

scenic resources in and around certain urban areas that are particularly highly valued by 

people living in those areas. With regard to comments from the Department of Interior 
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noting that many large metropolitan areas have 24-hour IMPROVE values below 30 dv 

on the worst 20 percent of days already, the EPA notes that the purpose of establishing 

NAAQS is to ensure adequate protection of public welfare everywhere, not to mandate 

continuous improvements in areas that may already be relatively clean. In fact, the 

evidence from the IMPROVE program that many urban areas have total 24-hour PMrelated light extinction below 29 dv on the 20 percent worst visibility days suggests that 

many areas have relatively good visual air quality already.

f. Need for a Distinct Secondary Standard to Protect Visibility

Numerous commenters questioned whether a distinct secondary standard for 

visibility is necessary in light of the analysis described in section VI.B.1.c.vii above 

(Kelly et al., 2012a) which indicated that a 24-hour mass-based PM2.5 standard of 35 

μg/m3 would protect against visibility impacts exceeding the range of levels considered 

in the proposal (28-30 dv). While this analysis was conducted in support of proposed 

implementation requirements for a distinct secondary standard (specifically, the modeling 

demonstrations that would be required under the PSD program), the second prong of the 

analysis showed that within the range of levels proposed by the EPA for the visibility 

index NAAQS (28-30 dv), the 24-hour PM2.5 standard of 35 μg/m3 would generally be 

controlling. Kelly et al. (2012a) concluded that the “overall, design values based on 2008-

2010 data suggest that counties that attain 24-hour PM2.5 NAAQS level of 35 μg/m3

would attain the proposed secondary PM2.5 visibility index NAAQS level of 30 dv and 

generally attain the level of 28 dv” (pp. 17-18). 

Citing this conclusion, many state and local agencies and industry commenters 

argued that a visibility index standard in the range proposed (28-30 dv) would provide no 
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additional protection beyond that afforded by the existing secondary 24-hour PM2.5

NAAQS, and therefore no distinct visibility standard was necessary. These commenters 

advocated retaining the current 24-hour PM2.5 mass-based standard to protect against 

visibility effects. “Since the 24-hour PM2.5 standard already protects the welfare the 24-

hour PM2.5 visibility standard is designed to protect, the new standard is duplicative and 

unnecessary” (South Dakota DENR, p. 2). Furthermore, a number of state commenters 

objected to the additional resource burden associated with implementing a standard 

which had, in their view, no practical effect: “If the 24-hour PM2.5 mass standard has the 

same effect as the visibility standard, crafting complex regulations to implement another 

standard seems redundant” (South Carolina DHEC, p. 3). Other states agreed: “A PM2.5-

related Visibility Index appears redundant since the benefits achieved from the current 

primary and secondary annual and 24-hour PM2.5 standards already provide reductions 

that would improve visibility. Establishing a new PM2.5 secondary standard for visibility 

would be an additional complication and burden to the states that is not warranted” 

(Indiana DEM, p. 5). 

In addition, several commenters submitted additional analyses supporting their 

position that a 35 μg/m324-hour PM2.5 standard would provide at least equivalent 

protection to a distinct 24-hour visibility standard within the range of levels proposed 

(API, Attachment 2, p. 8 and Attachment 3, p. 1). 

In responding to these comments stating that a distinct visibility standard is not 

needed, the EPA notes as an initial matter that the Administrator provisionally concluded 

at the time of proposal that the current PM standards were not sufficiently protective of 

visual air quality, and that consideration should be given to an alternative secondary 
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standard that would provide additional protection against PM-related visibility 

impairment, especially in urban areas. This provisional conclusion was based on the 

results of public preference surveys on the acceptability of varying degrees of visibility 

impairment in urban areas, analyses of the number of days on which peak 1-hour or 4-

hour light extinction values were estimated to exceed a range of CPLs under conditions 

simulated to just meet the current standards, and the advice of CASAC. The 

Administrator also noted that the current indicator of PM2.5 mass, in conjunction with the 

current 24-hour and annual averaging times, was not well suited for purposes of 

protecting visibility, since it does not incorporate species composition or relative 

humidity, both of which play a central role in determining the impact of ambient PM on 

visibility. Taking into account the advice of CASAC and the court’s remand of the 

current secondary PM2.5 standards, the Administrator provisionally concluded that the 

current secondary standards were neither sufficiently protective nor suitably structured to 

provide an appropriate degree of public welfare protection from PM-related visibility 

impairment. As a result, the EPA proposed a new, distinct secondary standard that was 

designed to address these deficiencies.

The EPA notes that in critiquing the proposed secondary standard, commenters 

generally did not advocate that the form of the existing mass-based PM2.5 standards 

werewas better suited scientifically to the task of protecting fromagainst visibility 

impairment. Rather, the commenters’ position that a distinct secondary standard was not 

needed for purposes of protecting visibility was based almost entirely on the relative 

degree of protection likely to be afforded by the existing standards (in particular, the 

existing 24-hour PM2.5 standard) as compared to the proposed visibility index, along with 
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the relatively large uncertainties associated with the latter. Thus, for all the reasons 

discussed in the proposal with regard to the scientific appropriateness of an indicator that 

takes into account both species composition and relative humidity, the EPA continues to 

conclude that the proposed standard based on a visibility index would be appropriate 

scientifically to provide targeted protection of visibility, since it would provide a measure 

of PM-related light extinction that directly takes into account the factors (i.e., species 

composition and relative humidity) that influence the relationship between PM2.5 in the 

ambient air and PM-related visibility impairment. 

Furthermore, the EPA disagrees with commenters who stated that implementation 

concerns, in particular the additional resource burden associated with implementing a 

distinct secondary standard, should alter the Agency’s decision making with regard to a 

standard to protect visibility. The EPA may not take the costs of implementation into 

account in setting or revising the NAAQS. 

However, in light of the results of the Kelly et al. (2012a) analysis and the views 

expressed by commenters on the implications of this analysis for conclusions regarding 

the adequacy of the current secondary 24-hour PM2.5 standard, the EPA has reconsidered 

some of the conclusions drawn at the time of proposal, in particular with regard to the 

degree of protection that would be provided by the current secondary standard. Based on 

a review of comments related to indicator, averaging time, form and level, the Agency 

has concluded that (as described in sections VI.C.1b-e above) a standard defined in terms 

of a PM2.5 visibility index (based on speciated PM2.5 mass concentrations and relative 

humidity data to calculate PM2.5 light extinction), a 24-hour averaging time, and a 90th

percentile form, averaged over 3 years, and a level of 30 dv, would provide sufficient but 
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not more than necessary protection of the public welfare with regard to visual air quality. 

Having identified this target level of protection, the EPA is now in a position to compare 

it specifically to the existing secondary 24-hour PM2.5 standard of 35 μg/m3for purposes 

of determining whether it would provide more, the same, or less protection from visibility 

impairment. The EPA must consider both whether the existing secondary 24-hour PM2.5

standard of 35 μg/m3is sufficient (i.e. not under-protective) and whether it is more 

stringent than necessary (i.e. over-protective).

With regard to the degree to which the existing secondary 24-hour PM2.5 standard 

provides sufficient but not more than necessary protection for visibility, the EPA first 

notes that the kind of area-specific analysis conducted in Kelly et al. (2012a) is essential 

for addressing the court remand of the 2006 secondary standards. In the case of the 2006 

secondary standards, the EPA had argued that the 35 μg/m324-hour PM2.5 standard was 

requisite because one part of the proposed range for a distinct secondary standard the 

Agency had considered would affect the attainment status of a somewhat fewer counties 

than the 35 μg/m324-hour PM2.5 standard. The court rejected this kind of rough 

balancing, finding that the EPA’s equivalency analysis based on percentages of counties 

demonstrated nothing about the relative protection offered by the different standards. 

Based on this, an area-by-area evaluation of the relative degree of protection offered by 

different standards should be conducted to the extent air quality data is available. 

Kelly et al. (2012a) performed such an evaluation. Based on 2008-2010 data, 

there are no areas that would have exceeded a 30 dv, 24-hour visibility index standard 

that would not also have exceeded a 24-hour PM2.5 standard of 35 μg/m3.

216 Stated 

 

216 The Kelly at al. (2012a) analysis used a 1.4 multiplier to convert OC to OM at those 

monitors not using the new CSN monitoring protocol.
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another way, all areas that met the 24-hour PM2.5 standard of 35 μg/m3 would have had 

visual air quality at least as good as 30 dv (24-hour average, based on 90th percentile form 

averaged over 3 years). The Kelly (2012a) analysis also showed that for some areas, 

particularly in the West, areas that would have met a 24-hour PM2.5 standard of 35 μg/m3

would have had visual air quality better than 30 dv for the PM2.5 visibility index standard, 

and that at sites that violated both the 24-hour level and the secondary visibility index 30 

dv level, the secondary standardvisibility index level of 30 dv would likely be attained if 

PM2.5 concentrations were reduced such that the 24-hour PM2.5 level of 35 μg/m3 was 

attained. 

The EPA has conducted a reanalysis (Kelly et al., 2012b) to update the area-byarea analysis in the original Kelly et al. (2012a) analysis in three respects. FirstFirst, 

noting that the original Kelly at al. (2012a) analysis used a 1.4 multiplier to convert OC 

to OM at those monitors not using the new CSN monitoring protocol, the EPA 

recalculated the visibility index design values for 2008-2010 using a higher multiplier for 

converting OC to OM at monitors not already using the new CSN monitoring protocol 

SANDWICH approach, consistent with the Agency’s view that it is more appropriate to 

use a multiplier of 1.6 at such monitors as compared to 1.4, as described in section 

VI.C.1.a.ii, above.217 The recomputed visibility design index values for 2008-2010 show 

the same overall relationship to 24-hour PM2.5 design values as presented in Kelly et al., 

2012a. 

Second, the EPA repeated the calculations comparing visibility index design

index values with 24-hour PM2.5 design values using 2009-2011 data, the most recent 

 

217 Some of the OC measurements were produced with CSN’s newer monitoring protocol 

and did not require a change in the computed OM.
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three years of air quality information currently available.218 Third, the EPA modified the 

area-by-area evaluation to ensure consistency with the data completeness criteria of 40 

CFR part 50, Appendix N, including the removal of data approved by EPA as exceptional 

events, for the current 24–hour PM2.5 NAAQSstandard and the proposed visibility index 

NAAQSstandard.

The results of this reanalysis, as presented in Kelly et al. (2012b), show a similar 

pattern to that described in the original Kelly memo. Specifically, the analysis indicates 

that there arewere no areas with visibility impairment above 30 dv that dodid not also 

exceed the 24-hour PM2.5 standard of 35 μg/m3. The updated memo concludes that the 

results for 2009-2011 corroborate the findings for 2008-2010.

Based on these analyses (Kelly et al., 2012a; 2012b), the EPA concludes with a 

high degree of confidence that having air quality that meets the 24-hour PM2.5 standard of 

35 μg/m3 would be sufficient to ensure areas would not exceed 30 dv. The EPA notes that 

this conclusion from Kelly et al. (2012a) is supported by two analyses submitted by 

industry commenters (API, Attachments 2 and 3).

At the time of proposal, the EPA had reached a different conclusion, specifically 

that the 35 μg/m324-hour PM2.5 standard was not sufficiently protective. This conclusion 

was based, in part, on the analyses conducted for the Visibility Assessment and Policy 

Assessment regarding 1- to 4-hour peak light extinction values based on 2007-2009 data. 

For the reasons outlined above in sections VI.B.1.c and VI.C.1.c, the EPA originally 

focused on hourly or sub-daily timeframes for evaluating visibility conditions. However, 

data quality concerns effectively precluded adoption of a 1-hour or sub-daily averaging 

time in this review, and ultimately the EPA has concluded that a 24-hour averaging time 

 

218 The 2011 air quality data were not yet available at the time of proposal.
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can serve as an appropriate surrogate. In reaching this conclusion, the EPA has 

recognized that adopting a 24-hour averaging time will likely smooth out some of the 

hour-by-hour variability in visibility index values, and will effectively reduce peak values 

by averaging them together with other hours. In concluding it is appropriate to adopt a 

24-hour averaging time, which limits the impact of hour-specific influences, the Agency 

is now placing less weight on the results of the 1-hour and 4-hour analyses presented in 

the Visibility Assessment and the Policy Assessment which focused on identifying the 

percent of days with peak hourly light extinction above various CPLs. In light of the 

Agency’s conclusion that a 24-hour averaging time would be appropriate, the Agency has 

determined to place more weight on analyses of visibility conditions over a 24-hour time 

period, especially the results in Kelly et al. (2012a and 2012b). In addition, the EPA notes 

that the Kelly et al. analyses reflects updated air quality information from more recent 

years of data (2008-2010 for Kelly et al., 2012a; 2009-2011 for Kelly et al. 2012b) as 

compared to the air quality information used in the Visibility Assessment and Policy 

Assessment. 

In light of all of these considerations, including the results of the Kelly et al. 

(2012a; 2012b) analyses, and the supporting comments provided by a broad range of 

public commenters, the EPA is persuadednow concludes that the 24-hour PM2.5 standard 

of 35 μg/m3provides sufficient protection in all areas against the effects of visibility 

impairment. The EPA concludes that the existing 24-hour PM2.5 standard provideswould 

provide at least the target level of protection for visual air quality defined by a visibility 

index set at 30 dv, as described above, which the EPA judges appropriate.

However, the EPA also recognizes that it is important to evaluate whether such a 
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standard would be over-protective (i.e. more stringent than necessary to protect public 

welfare). The analyses presented in Kelly et al. (2012a; 2012b) indicates that the 24-hour 

PM2.5 standard of 35 μg/m3 would achieve more than the target level of protection of 

visual air quality (30 dv) in some areas. That is, when meeting a mass-based standard of 

35 μg/m3, some areas would have levels of PM-related visibility impairment far below 30 

dv. Thus, when considered by itself and without consideration of the secondary standards 

adopted for purposes of non-visibility welfare effects, the 24-hour PM2.5 standard of 35 

μg/m3 would be over-protective of visibility in some areas. However, it is important to 

note that as long as the current secondary 24-hour PM2.5 standard of 35 μg/m3remains in 

effect, this overprotection for visibility would occur, regardless of whether a distinct 

secondary standard based on a visibility index set at 30 dv were adopted. These issues are 

discussed more fully in section VI.D, which outlines the Administrator’s final 

conclusions on the secondary PM standards, below.

g. Legal Issues

Some commenters opposed the proposal to establish a distinct secondary 

standard that would be defined in terms of a PM2.5 visibility index. The proposed 

standard would use measured PM2.5 mass concentration, in combination with speciated 

PM2.5 mass concentration and relative humidity data, to calculate PM2.5 light extinction, 

translated to the deciview (dv) scale. The standard would also be defined in terms of a 

specified averaging time and form, and a level for the PM2.5 visibility index set at one of 

two options—either 30 dv or 28 dv. The commenters argued that the entire approach 

proposed by the EPA is inconsistent with the requirements of CAA section 109(b). They 

pointed to a number of different aspects of the proposal which in their view made it 
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incompatible with the CAA. For example, the Utility Air Resources Group (UARG) 

stated:

In the past, EPA has always used a measure of PM mass as the indicator for both 

primary and secondary PM NAAQS. Such a standard is, as a general matter, 

consistent with the directive in the CAA that the NAAQS “specify a level of air 

quality” and targets for control the listed criteria air pollutant. CAA § 109(b)(2). 

The standard contained in EPA’s proposed rule does neither of these things. 

Instead, it would (1) regulate relative humidity, which is not a criteria pollutant; 

(2) fail to “specify a level of air quality” as required by section 109(b)(2) of the 

CAA; and (3) result in a standard necessitating nationally variable PM 

concentrations instead of a standard establishing a nationally uniform, minimally 

acceptable PM concentration. (UARG, p. 22-23)

Other commenters raised similar or related issues, arguing that the EPA 

improperly set a visibility standard, and not a PM2.5 standard, and that NAAQS can only 

be set in terms of a level or concentration of the air pollutant. Commenters also argued 

that an endangerment finding and air quality criteria would be needed before the EPA 

could set a standard based on PM components. Each of these comments is discussed 

below.

As an initial matter, the commenters argued that the proposed standard is 

unlawful because it is “not a PM2.5 standard at all, but rather a visibility standard, and 

visibility is neither an air pollutant nor a criteria pollutant for which a NAAQS may be 

promulgated” (NMA/NCBA, p. 21). According to these commenters, the CAA requires 

that NAAQS be established as limits on the concentration of an air pollutant in ambient 

air, not limits on the “identifiable effects” caused by that air pollutant. These commenters 

claimed that reduced visibility due to light extinction is not an air pollutant but instead is 

an effect, noting that “the Act’s definition of ‘air pollutant’ speaks in terms of specific 

substances or matter in the ambient air” (NSSGA, p. 8). The commenters pointed to the 

use of the term “air pollutant” in sections 109(a)(1)(A) and (b)(2) as support for their 
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argument, as these provisions refer to setting standards for the “air pollutant” to address 

the effects associated with the presence of the air pollutant in the ambient air. They 

likewise pointed to section 108(a)(2)’s reference to the presence of the air pollutant in the 

ambient air. Since reduced visibility is not an air pollutant, they argue the EPA cannot set 

a NAAQS that is a standard for visibility. They argue that the proposed secondary 

standard it is not a PM2.5 standard as it does not limit the concentration of PM2.5 or any 

other fraction of particulate matter in the ambient air and therefore is not an “ambient air 

quality standard” for any pollutant.

One commenter argued that the EPA is required to “specify a level of air quality” 

under section 109(b)(2), which Congress intended as an acceptable concentration level of 

the air pollutant in the ambient air, noting that specification of acceptable visibility 

conditions is not the same as an acceptable air pollution concentration level. Citing 

American Farm Bureau v. EPA, 559 F.3d at 516, one commenter claimed that the court 

had affirmed that “the NAAQS—whether primary or secondary—is a mass-based 

standard” (Nevada DEP, p. 5). Commenters also refer to the legislative history of the 

1970 amendments, referring to NAAQS as setting the “maximum permissible ambient air 

level” for an air pollutant. The commenters argue that the proposed standard is improper 

because it does not limit the concentration of PM2.5 or any fraction of PM in ambient air, 

but improperly sets a limit on visibility effects.

With regard to humidity, these commenters argued that the proposed standard 

improperly regulates relative humidity because it is included in the calculation to 

determine the value of the visibility index. According to these commenters, the CAA 

allows the EPA to control criteria air pollutants through the NAAQS program, but not 
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other various substances. The commenters stated that the EPA recognized this in the last 

review, treating humidity as a confounding factor and considering addressing it by 

measuring PM2.5 mass-based concentration over the midday hours, when humidity would 

have the least effect. This would target the effects caused by PM, and not by humidity. 

Referring to American Farm Bureau v. EPA, 559 F.3d 512, 528 (D.C. Cir. 2009) and 77 

Fed. Reg. at 38979 n.153. UARG contested the proposed calculated visibility index as it 

does not approach relative humidity as a confounding factor but instead “embraces it and 

treats it as if it were a PM effect” (UARG p. 24).

The commenters also stated that the use of a calculated visibility index, and the 

failure to exclude the effects of humidity, would result in acceptable PM concentrations 

that vary across the nation. These commenters claimed that such a standard is 

inconsistent with the requirements of the CAA because the proposed approach fails to 

establish a nationally uniform PM concentration standard. For example, API argued that 

the proposed visibility index approach is “essentially specifying levels—not a level—of 

air quality” (API, p. 29). UARG agreed, and stated that the Act “requires that criteria 

pollutant concentrations throughout the nation reach, at the least, a single, specified 

ambient concentration level” (UARG, p. 25, emphasis in original). The commenters 

argue that a PM2.5 visibility index standard cannot provide equal protection nationwide 

due to geographic variation in key factors such as relative humidity that affect level of 

particles allowed in different areas. The commenters noted that establishing a single 

national level for the PM2.5 visibility index would necessarily result in unequal acceptable 

PM2.5 levels in different areas of the country, with lowest allowable PM2.5 levels in urban 

areas in the Southeast and highest allowable levels in the arid West. UARG recognized 
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that under section 108 the air quality criteria are to “address those variable factors 

(including atmospheric conditions) which of themselves or in combination with other

factors may alter the effects on public health or welfare of such air pollutant,” but stated 

that while section 108 “allows” this, it has no bearing on this issue. Instead, the 

commenter stated that the EPA may take such information into account in setting a 

permissible concentration of the pollutant that is uniform and national (UARG, p. 25). 

In addition, some commenters opposed to the proposed distinct secondary 

standard argued that in order to base a standard on measured levels of several speciated 

substances, the EPA must first make an endangerment finding and issue air quality 

criteria for each of the speciated substances included in the calculation of PM2.5 light 

extinction. According to these commenters, “EPA cannot use NAAQS to indirectly 

regulate multiple substances which are not criteria pollutants under the guise of 

establishing a visibility standard” (NMA/NCBA, p. 21). Noting that air quality criteria 

for particulate matter were issued in 1969, NMA/NCBA argued that the 1969 Criteria 

Document “did not establish air quality criteria for individual constituents that occur in 

particle form, instead it established criteria for particulate matter as a whole” (p. 27). In 

light of the fact that criteria have never been issued for “individual speciated components 

of particulate matter,” these commenters argued, “if EPA wishes to promulgate a rule 

such as its secondary visibility NAAQS, it first must make a finding that the speciated 

components listed in Appendix N endanger public health or welfare and then issue an air 

quality criteria document for those components” (NMA/NCBA, p. 29). According to 

these commenters, the approach the EPA adopted in promulgating a NAAQS for lead 

supports this view: 
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When EPA promulgated a NAAQS for lead, an individual substance in particle 

form, it did not assert that an endangerment finding or criteria document for lead 

was unnecessary because lead was already covered by the PM Criteria Document. 

Instead, EPA complied with the Section 108 and 109 NAAQS prerequisites for 

lead, just as it must do for Appendix N substances if it intends to promulgate a 

NAAQS for those substances. ... [In 1976], EPA listed lead as an air pollutant 

that adversely affected public health or welfare, issued an air quality criteria 

document for lead, and promulgated a NAAQS for lead. 43 Fed. Reg. 46246 (Oct. 

5, 1976). (NMA/NCBA, p. 29)

Finally, UARG argued that the EPA has in the past recognized that the secondary 

NAAQS is an inappropriate vehicle for regulating PM-related visibility, referring to 62 

Fed. Reg. at 38680, including fn 49. UARG claimed the same situation continues, and the 

EPA has not provided a valid basis for changing this conclusion. 

The EPA disagrees with the points raised by these commenters. While the EPA is 

not adopting the proposed secondary standard, as explained below, this decision is not 

based on concern over the EPA’s authority to adopt a secondary standard such as the one 

proposed. 

The proposed distinct secondary standard is a standard for PM2.5, and is not a 

“visibility standard.” The proposed secondary standard is based on the mass 

concentration of PM2.5 in the ambient air. The standard is defined in terms of calculated 

PM2.5 light extinction which is based on the measurement of the mass concentration of 

ambient PM2.5 over a 24-hour period. The measured mass concentration is adjusted based 

on information on the speciated mass components of the PM2.5 and the relative humidity, 

resulting in a calculated visibility index. The level of the visibility index, combined with 

the form of the standard and averaging time, identifies whether a level of ambient mass 

concentration of PM2.5 achieves the standard or not. Given any specific mass 

concentration of ambient PM2.5, combined with information on speciation and relative 

humidity, it can be determined whether the specific mass concentration of ambient PM2.5
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achieved the NAAQS. Hence, the proposed secondary NAAQS specifies acceptable 

levels of ambient mass concentration of PM2.5.

The combination of indicator, averaging time, form, and level of the proposed 

NAAQS is designed to provide the appropriate degree of protection from visibility 

impairment caused by ambient levels of PM2.5. It does this by calculating the light 

extinction associated with ambient concentrations of PM2.5 and specifying the level of 

acceptable PM2.5 mass concentration in terms of this calculation. However this does not 

change the fact that the standard is for the air pollutant PM2.5, and defines acceptable 

ambient levels of this air pollutant. It does not transform the standard into a “visibility 

standard” and not a standard for PM2.5. While the commenters had additional concerns 

over the use of relative humidity in the calculation, and the variation around the country 

of acceptable mass concentrations, those issues are separate and do not change the fact 

that the proposed standard defined in terms of calculated PM2.5 light extinction is based

on measurement of PM2.5 concentration in the ambient air, and is a NAAQS for PM2.5. 

With regard to the contention that section 109(b) limits the EPA to setting a 

standard that is based on the concentration of the pollutant in the ambient air, we note 

that the term “concentration” typically means some measure of relative content. For 

example, this would include relative measures such as mass per unit of volume or parts 

per million. The EPA has often used such metrics to define the NAAQS, largely because 

the scientific evidence of health or welfare effects supporting the NAAQS typically use 

such metrics in air pollution studies. For example, the current secondary standards for 

PM are defined in terms of the concentration of PM2.5 and PM10 in the ambient air,

measured as the dried mass of the particulate matter per unit of air. However section 
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109(b) does not require that a NAAQS be defined this way. 

Sections 109(a) and (b) both use the general term “air quality” when discussing 

the EPA’s obligation to set NAAQS. The NAAQS are clearly national ambient “air 

quality” standards under section 109(b), which specifies that the primary NAAQS “shall 

be ambient air quality standards” and the secondary NAAQS “shall specify a level of air 

quality.” Both the primary and secondary NAAQS are to be based on the “air quality 

criteria,” which are to accurately reflect the latest scientific knowledge on the effects on 

public health and welfare associated with “the presence of such air pollutant in the 

ambient air, in varying quantities.” Section 109(b), 108(a)(2). Congress spoke in broad 

terms, tasking the EPA with assessing the latest scientific knowledge about the public 

health and welfare associated with the presence of the pollutant in the air, without 

limiting this to consideration of only those effects associated with one or more measures 

of concentration of the air pollutant. Congress referred to any and all effects associated 

with the presence of the pollutant in the ambient air, not just the effects associated with 

the concentration of the pollutant in the ambient air. Based on this knowledge, the EPA is 

required to set standards for the quality of the air that will provide the appropriate degree 

of protection from these health and welfare effects, without limitation on how to measure 

or define air quality. While concentration in the air has typically been an appropriate way 

to set a standard to achieve these requirements, the more general terms used in section 

108(a) and 109(b) do not limit the EPA to using concentration as the only way to measure 

air quality for purposes of setting a NAAQS. The EPA is charged with setting air quality 

standards, and has the discretion under section 109(b) to choose the metric for defining 

air quality that is appropriate to address the health or welfare effect at issue.
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Congress did refer to “concentration” in certain situations. In section 109(c) 

Congress required the EPA to set a primary NAAQS for NO2 concentration over 3 hours. 

This addressed Congress’ concern over whether the then current NO2 standard, which 

used concentration as a metric, provided adequate protection. Congress also called on 

CASAC to advise the Administrator on the relative contribution to “air pollution 

concentrations” of natural and anthropogenic sources, under section 109(d)(2)(C)(iii). 

This information is in addition to the advice CASAC is required to provide concerning 

appropriate revisions to the “air quality criteria” and to the NAAQS under section 

109(d)(2)(B).219 While these provisions refer to ambient concentrations of pollutants, this 

reflects the EPA’s standard practice to date in setting NAAQS, and none of them change 

or limit the range of discretion provided under section 109(b) in setting NAAQS. They do 

not change the fact that the EPA is to set “air quality” standards, and is not limited to “air 

concentration” standards. The reference in the legislative history to a maximum 

permissible ambient air level for the pollutant also does not limit the EPA to a level of air 

pollutant concentration, as compared to a different metric for specifying the level of air 

quality, if that is judged to be appropriate. 

The text of sections 108 and 109 does not support the limited interpretation 

commenters suggest. Instead these provisions provide the EPA with significant discretion 

in determining the metric for air quality that is appropriate to achieve the required degree 

of protection of public welfare. The commenters’ interpretation would improperly limit 

 

219 In a provision that is not part of the CAA, in 1990 Congress required EPA to request a 

report from the National Academy of Sciences on the role of secondary national ambient 

air quality standards, including information on the “effects on welfare and the 

environment which are caused by ambient concentrations of pollutants” listed under 

section 108, and the “ambient concentrations of each such pollutant which would be 

adequate to protect welfare and the environment from such effects.” Section 817(a) of the 

CAA Amendments of 1990, P.L. 101-549.
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this discretion, interfering with achieving the goals of section 109(b).

For example, in this review the EPA considered whether it would be appropriate 

to base a secondary NAAQS on direct measurement of the light extinction caused by 

PM2.5. See 77 FR 38890, 38980-1 (June 29, 2012). There are several instrumental 

methods that directly measure PM2.5 light extinction – the amount of light extinction 

caused by the presence of PM2.5 in the ambient air. This is not a measure of the 

concentration of PM2.5 in the air, but a measure of the light extinction caused by PM2.5. 

This is clearly an effect associated with the presence of PM2.5 in the ambient air, and this 

atmospheric property is directly related to visibility effects. Unlike PM2.5 mass 

concentration, there is a close scientific relationship between directly measured PM2.5

light extinction and visibility effects.

It would appear straightforward to say that PM2.5 light extinction is a quality of 

the ambient air, and a secondary NAAQS that specified an acceptable level of PM2.5

based on directly measured PM2.5 light extinction would be an “ambient air quality 

standard” for the air pollutant that specifies a “level of air quality” designed to provide 

protection against visibility impairment. Unlike directly measured PM2.5 light extinction, 

the mass concentration of PM2.5 does not have the same direct relationship to light 

extinction, and specifying an acceptable level of mass concentration of PM2.5 would be a 

more indirect and less effective way to provide protection from visibility impairment 

caused by the presence of PM2.5 in the ambient air. Under the commenters’ interpretation, 

the EPA would be precluded from specifying a level of air quality in terms of directly 

measured PM2.5 light extinction, the more scientifically appropriate and direct measure of 

the effect PM2.5 has on visibility. Instead the EPA would be limited to the more indirect 
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and less effective specification of a level of concentration of PM2.5.

The commenters also objected to the inclusion of relative humidity as an 

adjustment factor in the calculation of PM2.5 light extinction. Contrary to the claims of 

these commenters, the use of calculated PM2.5 light extinction does not regulate relative 

humidity. The proposed secondary standard would define acceptable levels of ambient 

PM2.5, not acceptable levels of relative humidity. In addition, section 108 explicitly 

requires that the air quality criteria include information on the atmospheric conditions 

that can alter the effects of the air pollutant on public health or welfare, and relative 

humidity certainly has this kind of impact. Section 109(b) requires that the standard be 

based on the air quality criteria, indicating that this information can and should be taken 

into account in setting the standard. Including relative humidity as an adjustment factor in 

the calculation of PM2.5 light extinction is a reasonable and straightforward way to use 

the scientific information in the air quality criteria in establishing a standard to provide 

protection from visibility impairment.220

Some commenters pointed to the EPA’s position in the last review, stating that the 

EPA properly treated relative humidity as a confounding factor, and in this review 

improperly moves away from that position. See 77 FR at 38979, 71 FR 61144, 61205 

(October 17, 2006). In the last review the EPA considered a distinct PM2.5 mass-based 

secondary standard. In that context, limiting the measurement of PM2.5 mass 

concentration to the mid-day hours when relative humidity had the least impact would 

promote the correlation between measured PM2.5 mass concentration and light extinction, 

which would promote achievement of a relatively consistent degree of visibility 

 

220 UARG recognizes these provisions, but argues, as above, that this is limited by the 

requirement that the EPA set a NAAQS based solely on ambient concentration. 
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protection across the country. However in this rulemaking the proposed calculated PM2.5

light extinction standard achieves a consistent degree of visibility protection by directly 

accounting for humidity, in a scientifically defensible manner. The goal has not changed 

– achieving the desired degree of protection across the country. What has changed is that 

calculated PM2.5 light extinction is a more direct and scientifically appropriate way to 

achieve that result.

Finally, it should be made clear that water is not a separate compound from PM2.5

that confounds the impact PM2.5 has on light extinction. As described in the Integrated 

Science Assessment, “PM is the generic term for a broad class of chemically and 

physically diverse substances that exist as discrete particles (liquid droplets or solids) 

over a wide range of sizes” (U.S. EPA, 2009a, p. 1-4). “Particles composed of water 

soluble inorganic salts (i.e., ammoniated sulfate, ammonium nitrate, sodium chloride, 

etc.) are hygroscopic in that they absorb water as a function of relative humidity to form a 

liquid solution droplet. Aside from the chemical consequences of this water growth, the 

droplets become larger when relative humidity increases, resulting in increased light 

scattering. Hence, the same PM dry concentration produces more haze” (U.S. EPA, 

2009a, p. 9-6). Thus water is not a compound that is separate and apart from the particle 

that acts as an extraneous confounding factor.221 The effect of relative humidity occurs 

after the water becomes part of the particle. Certain water soluble salts absorb water and 

the resulting particle is larger in size and scatters more light, increasing the visibility 

impact of the particle. But the particle is still a PM2.5 particle. The fact that the PM 

 

221 According to the Integrated Science Assessment, “Confounding is ‘...a confusion of 

effects. Specifically, the apparent effect of the exposure of interest is distorted because 

the effect of an extraneous factor is mistaken for or mixed with the actual exposure effect 

(which may be null)’ (Rothman and Greenland, 1998, 086599)” (U.S. EPA, 2009a, p. 1-

16).
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NAAQS traditionally uses a measurement of the dried mass of the particles as the metric 

for the standard does not change the fact that the particles in the air include liquid 

droplets and particles that have increased in size because of absorption of water. These 

ambient PM2.5 particles are what is in the air and impacting visibility, not just the dried 

mass of PM2.5 that is measured in the laboratory and is currently used as the indicator for 

the PM NAAQS. Thus the commenters improperly claimed that the proposed secondary 

standard regulates water or relative humidity, and not PM2.5, when in fact the proposed 

secondary standard accounts in a scientific manner for the fact that some PM2.5 particles 

are larger in size and have a greater impact on light extinction when the relative humidity 

increases. 

The commenters also raised concerns that a standard based on calculated PM2.5

light extinction, compared to a standard based on just PM2.5 mass concentration, 

improperly results in variable levels of acceptable PM2.5 mass concentrations across the 

country. This stems from the adjustments in the calculation for speciated components of 

PM2.5 and relative humidity. According to commenters, this is improper as section 109(b) 

requires that the NAAQS set a single, specified ambient concentration that is nationally 

uniform across the country.

As discussed above, the text of section 109(b) does not specify this limitation of a 

single national acceptable concentration. Instead the secondary NAAQS is to specify a 

level of air quality that achieves the appropriate degree of protection. The proposed 

secondary standard would do just that – specify a level of air quality, defined in terms of 

calculated PM2.5 light extinction, that would achieve the desired degree of protection. The 

fact that this results in varying allowable levels of PM2.5 mass concentrations is not 
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inconsistent with the Act. The D.C. Circuit recently approved such a result. In the last 

review of the PM10 primary NAAQS, the court approved the EPA’s choice of an 

indicator that was designed to allow varying levels of acceptable coarse PM. The court 

stated that:

The industry petitioners next argue that the 150 μg/m3 standard for PM10 will 

result in arbitrarily varying levels of coarse PM, and that the agency should 

instead have used a PM10-2.5 indicator. The EPA does not dispute that using the 

PM10 indicator will result in coarse PM levels that vary within the limit of 150 

μg/m3. As the EPA explains: "Because the PM10 indicator includes both coarse 

PM (PM10-2.5) and fine PM (PM2.5), the concentration of PM10-2.5 allowed by a 

PM10 standard set at a single level declines as the concentration of PM2.5

increases. Thus, the level of coarse particles allowed varies depending on the level 

of fine particles present." Id. at 61,195.

Although the EPA acknowledges that a PM10 indicator will result in varying 

coarse PM levels, it does not agree that the variance will be arbitrary. The EPA 

agrees with the industry petitioners that protection from coarse particles should be 

targeted at urban areas, where coarse particles have been shown to pose the 

greatest danger. Id. at 61,194. But the agency argues that targeting of urban areas 

is effectively accomplished by using an indicator that permits the varying levels 

that the industry petitioners challenge. ... Id. at 61,195-96 (citations omitted). In 

other words: "The varying levels of coarse particles allowed by a PM10 indicator 

will therefore target protection in urban and industrial areas where the evidence of 

adverse health effects associated with exposure to coarse particles is strongest." 

Id.

The EPA also offers a further rationale for tying the stringency of coarse PM 

regulation to increases in the level of PM2.5. ... EPA argues that it is "logical to 

allow lower levels of coarse particles when fine particle concentrations are high. .

. . [I]nclusion of PM2.5 in the PM10 indicator for purposes of coarse particle 

protection would appropriately reflect the contribution that contaminants emitted 

in fine particle form can make to the overall health risk posed by coarse particles." 

Id. 

In sum, we find that the EPA has provided a reasonable explanation for its 

decision[ ] ... to utilize a standard that allows targeted variance in coarse PM 

levels in an inverse relationship to the amount of fine PM in the air. American 

Farm Bureau v. EPA, 559 F.3d 512, 534-5 (D.C. Cir. 2009). 

A similar result applies here. Under the proposed secondary standard there would 

be a single level of air quality specified for the NAAQS. The standard would apply across 
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the nation; it would not be a regional standard. The proposed standard would be the same 

standard everywhere—the acceptable level of mass concentration of PM2.5 would be 

defined the same way across the nation, using the same method of calculating the 

allowable concentration of PM2.5. The same degree of protection from visibility 

impairment would apply across the country. While the allowable amount of PM2.5 could 

vary, this would be a reasoned way to achieve the desired degree of protection from 

visibility impairment. The requirements of section 109(b) would be satisfied.

Commenters also objected that the EPA could not set a NAAQS for the separate 

components of PM2.5 without listing the components of PM2.5 under section 108, based 

on an endangerment finding, and issuing air quality criteria for these components. They 

argued that the issuance of air quality criteria for particulate matter starting in 1969 did 

not provide a lawful basis for a proposed secondary standard that is based on components 

of PM, as the 1969 air quality was for particulate matter “as a whole,” defining PM as 

particles smaller than 500 micrometers (NMA/NCBA, p. 27). However, as discussed 

above, the proposed standard sets the allowable limit on ambient concentrations of PM2.5. 

Information on both the speciated components of PM2.5 and the relative humidity affect 

how much light extinction is associated with any specific level of PM2.5, but the standard 

is for PM2.5. The D.C. Circuit has made it clear that PM2.5, just like PM10 and TSP before 

that, is an appropriate subset of PM for the EPA to focus on in setting the NAAQS based 

on the scientific evidence before the EPA. This focus of the NAAQS does not make the 

subset a new pollutant that requires listing and new air quality criteria under section 108 

before setting a NAAQS. American Trucking Association et al. v. EPA, 175 F.3d 1027, 

1055 (D.C. Cir. 1999). Commenters’ interpretation would apply to PM2.5 as well as to 
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components of PM2.5, and is inconsistent with the ATA decision. In addition, it is clear 

that the current air quality criteria do address the scientific basis for calculating PM2.5

light extinction as the EPA proposed (U.S. EPA, 2009a, pp. 9-5 to 9-8). 

Finally, at least one commenter argued that the EPA has concluded in prior 

reviews that the secondary NAAQS program is an inappropriate vehicle for regulating 

PM related visibility impairment (UARG, p. 26). UARG mischaracterized the EPA’s past 

decision-making. In past reviews the EPA has been clear that the EPA should take into 

account the existence of the visibility program under section 169A, the regional haze 

program, when considering a secondary NAAQS and should not treat the secondary 

NAAQS as the sole mechanism to address visibility impairment across the country. That 

is the approach the EPA has taken in this and prior reviews. See 77 FR at 38990. 

h. Relationship with Regional Haze Program

A large number of commenters expressed confusion and concern over differences 

between the proposed visibility index standard and the Regional Haze Program. This 

included commenters who supported setting a distinct secondary standard to protect 

visibility as well as those opposed to setting such a standard. A number of these 

commenters noted that visibility impairment would be assessed differently under the two 

approaches due to differences in the way light extinction is calculated, including different 

IMPROVE equations and differences in the inclusion and weighting of specific species 

and components. The commenters argued it would be inappropriate to have two different 

regimes for managing visibility impairment in the exact same location. These 

commenters claimed that since data from the IMPROVE monitoring network would 

inform nonattainment designations, as well as an area’s obligations under the Regional 
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Haze Program, there could be considerable confusion over how to draw nonattainment 

boundaries and what requirements would affect large sources in rural areas. These 

commenters also noted the resource burden associated with maintaining two different 

programs aimed at protecting visibility in the same geographic area. Some commenters 

argued that a visibility NAAQS should not apply to rural areas. The Department of the 

Interior requested that the EPA clearly define the geographic area to which the visibility 

index standard would be applicable, and suggested that Class I and Class II areas should 

generally be excluded from the standard. As discussed above, commenters questioned the 

need for a distinct visibility standard, arguing that the existing primary PM standards 

combined with the Regional Haze Program ensured adequate protection of visibility, 

even in urban areas.

In response to these comments relating to the overlap between the Regional Haze 

program and a distinct secondary standard designed to protect visibility principally in 

urban areas, the EPA notes that the objectives of each program are distinct. While the 

Regional Haze program is designed to eliminate man-made impairment of visibility in 

Federal Class I areas over the course of several decades, a distinct secondary standard for 

PM-related visibility impairment would be focused on providing a nationally applicable 

level of protection for all areas, particularly urban areas which do not receive targeted 

protection under the Regional Haze Program. Moreover, the metric used to assess 

visibility impairment differs between the two programs precisely because each program 

is aimed at a different aspect of the problem. Recognizing the importance of fresh 

emissions for urban visibility, the Visibility Assessment focused on visibility impairment 

as measured by the original IMPROVE equation because “the original version is 
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considered more representative of urban situations when emissions are still fresh rather 

than aged as at remote IMPROVE sites” (U.S. EPA, 2010b, p. 3-19). The Regional Haze 

Program, on the other hand, has shifted to a revised IMPROVE algorithm more suited to 

remote locations. While this difference is discussed in more detail in section VI.C.1.b 

above, the result is that each program would appropriately measure those aspects of 

visibility impairment most closely related to the problem the program is trying to prevent. 

Since the same data can be used to calculate both visibility impairment under the 

Regional Haze approach and the proposed visibility index, the additional calculation 

burden for state and local agencies would be light. Also, to the extent that there is any 

difference in terms of the emissions control obligations the two different programs would 

impose upon state and local areas, this is likely appropriate given the extent and nature of 

visibility impairment in those areas. The EPA notes that in general, there is likely to be 

substantial overlap in the control strategies a state or local area would pursue under either 

program. Thus, the EPA disagrees with commenters who stated that a distinct visibility 

standard as proposed would inherently conflict with the Regional Haze Program or that it 

would be appropriate to draw geographical distinctions that would explicitly exclude 

some areas (e.g., Class I areas) from the NAAQS. The EPA notes that the CAA requires 

that NAAQS be national in scope, and that the specific requirements laid out in the 

proposal for the distinct secondary standard would ensure that the protection it afforded 

would be appropriately targeted toward urban areas so that it could work in conjunction 

with—not be in conflict with—the Regional Haze Program under sections 169A and 

169B of the CAA.

2. Comments on the Proposed Decision Regarding Non-Visibility Welfare Effects
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Relatively few commenters addressed the proposal to retain the existing suite of 

secondary PM standards to address non-visibility welfare effects. A couple of states, 

including Mississippi and South Dakota, offered brief endorsements of the proposal, but 

the vast majority of commenters, even those who offered extensive comments on the 

proposed visibility index, did not comment on the proposal to retain the existing 

secondary standards.. A few other commenters offered more extensive comments on the 

proposal to retain the existing secondary standards, and these commenters opposed this 

aspect of the proposal for one of two reasons. First, some commenters opposed the 

proposal to retain the current secondary annual PM2.5 standard of 15 μg/m3in light of the 

proposal to revise the level of the primary annual PM2.5 standard to a level between 12-13 

μg/m3. Expressing concern over the implications of this decision for the air quality 

planning obligations of states, these commenters argued that the EPA should revise the 

secondary PM2.5 standards to be equivalent in all respects to the primary PM2.5 standards. 

For example, the American Association of State Highway and Transportation Officials 

(AASHTO) supported “retaining secondary standards that are consistent with the primary 

standards in order to reduce the complexity of the transportation and air quality planning 

processes, as well as the transportation conformity process” (AASHTO, p. 3). Thus, if the 

EPA were to adopt a lower level for the primary annual PM2.5 standard, the commenters 

recommended that the EPA adopt this same lower level for the primary secondary PM2.5

standard as well. 

In response to these comments, the EPA notes that the Agency lacks an 

appropriate scientific basis for revising the level of the secondary annual PM2.5 standard. 

As noted above in section VI.B.2, there is an absence of information that would support 
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any different secondary standards for PM. Comments related to the implementation 

challenges associated with distinct primary and secondary standards are not relevant to 

the Administrator’s final decisions regarding what standards are requisite to protect the 

public welfare. Therefore, the EPA continues to conclude that it would be appropriate to 

retain the current suite of secondary PM standards222 to address non-visibility welfare 

effects, while revising only the form of the secondary annual PM2.5 standard to remove 

the option for spatial averaging consistent with this change to the primary annual PM2.5

standard, as proposed.

Other commenters focused on the impacts of particulate matter on climate. One 

commenter cited a number of recent studies that considered mobile source black carbon 

emissions and associated climate impacts, and urged the EPA to protect the public 

welfare by setting “higher standards for gasoline quality” (Urban Air Initiative, p. 4). 

This commenter did not, however, advocate specific secondary NAAQS to address 

climate impacts of PM. More extensive comments on this same subject were provided by 

the Center for Biological Diversity (CBD), which urged the EPA to “set a separate limit 

for black carbon within the overall PM2.5 standard” to ensure that public welfare is fully 

protected “from the serious climate impacts of black carbon” (CBD, p. 2). This 

commenter argued that “[p]recaution is required for secondary NAAQS,” citing 

American Trucking Associations, Inc. v. EPA, 283 F.3d 355, 369 (D.C. Cir. 2002):

[N]othing in the Clean Air Act requires EPA to wait until it has perfect 

information before adopting a protective secondary NAAQS. Rather, the Act 

mandates promulgation of secondary standards requisite to protect public welfare 

from any “anticipated adverse effects associated with” regulated pollutants, 42 

U.S.C. § 7409(b)(2) (emphasis added), suggesting that EPA must act as soon as it 

has enough information (even if crude) to “anticipate[]” such effects[.]

 

222 As summarized in section VI.A and Table 1 above, the current suite of secondary PM 

standards includes annual and 24-hour PM2.5 standards and a 24-hour PM10 standard. 
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The commenter stressed the growing scientific evidence regarding the impacts of black 

carbon on climate, and argued that the EPA’s proposal ignores important research studies 

published within the last five years which provide improved estimates of the radiative 

forcing associated with black carbon, and the effects of black carbon on snow and ice, the 

Arctic climate, water availability and climate “tipping points.” The commenter also noted 

that reductions in cooling aerosol species, particularly sulfate, due to pollution control 

programs are leading to an “unmasking” of the true extent of warming due to the 

accumulation of greenhouse gases in the atmosphere. The commenter argued that this 

unmasking effect can be offset by ensuring “that sufficient black carbon reductions 

accompany reductions in overall aerosol pollution” (CBD, p. 10). The commenter also 

argued that the EPA did not consider the negative impacts of climate change on public 

health adequately in the proposal.

The commenter stated that the EPA had an obligation to address the impacts of 

black carbon in the PM NAAQS, despite the remaining uncertainties. The commenter 

pointed to the U.S. EPA’s report to Congress on Black Carbon (U.S. EPA, 2012c), stating 

that the “report shows that EPA is aware of the climate science and public health 

information that point to the importance of addressing black carbon pollution. EPA must 

use this information in its relevant decisionmaking” (CBD, p. 13). The commenter also 

noted that the United StatesU.S. participates in a number of international forums that 

have recognized the need to take action on black carbon, and argued that the U.S. has “an 

obligation under the Gothenburg Protocol to address black carbon pollution.” The 

commenter challenged the uncertainties cited by EPA with regard to the climate impacts 

of aerosols generally, arguing that they “do not apply to the regulation of black carbon” 
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(CBD, p. 14). Specifically, the commenter stated that “there are significant anthropogenic 

sources of black carbon that contribute a large proportion of total black carbon 

emissions”; that “there is enough information related to black carbon’s impact to know 

that global temperatures will rise due to black carbon emissions”; that spatial and 

temporal heterogeneity in black carbon emissions do not matter for estimating likely 

climate effects; that “[b]lack carbon’s negative climate impacts do not depend upon 

details of cloud interactions with aerosols”; and that the EPA does not need to be able to 

quantify the health or climate benefits precisely to know that it is appropriate to control 

black carbon as a specific component of PM under the CAA (CBD, pp. 14-15). 

As a result, the commenter concluded that the current size-based PM mass 

standard “is insufficient to fully protect health and welfare,” and that the EPA was 

obligated to establish a specific limit on black carbon as a component of PM. The 

commenter argued that “Black carbon must be regulated separately and in addition to 

PM2.5 because absent separate standards sulfates and nitrates may be more likely to be 

mitigated than the black carbon component of PM” (CBD, p. 17). To support this point, 

the commenter cited the conclusion in the Policy Assessment that:

The current standards that are defined in terms of aggregate size mass cannot be 

expected to appropriately target controls on components of fine and coarse 

particles that are related to climate forcing effects. Thus, the current mass-based 

PM2.5 and PM10 secondary standards are not an appropriate or effective means of 

focusing protection against PM-associated climate effects due to these differences 

in components. (U.S. EPA, 2011a, p. 5-11)

The commenter also noted that existing regulations on diesel engines, which are 

the largest source of black carbon in the United States, do not affect existing engines and 

vehicles, and stated that “The NAAQS program is one of the few opportunities to reduce 

Formatted: Indent: First line: 0.5"
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black carbon from existing engines, industrial and biofuel sources within the United 

States and rapidly reduce emissions from this pollutant” (CBD, p. 18).

The EPA agrees with the commenters’ assertion that the scientific information 

about the impacts of aerosol species on climate is developing rapidly, and that 

understanding of the magnitude of aerosol effects on climate and the contribution of 

individual aerosol components to those effects has improved substantially over the past 

decade. The EPA also agrees that certain species, in particular black carbon, play a 

significant role in multiple aspects of climate. The Policy Assessment recognized that 

“Aerosols can impact glaciers, snowpack, regional water supplies, precipitation and 

climate patterns,” and may contribute to the melting of ice and snow, a decrease in 

surface albedo, and climate impacts in the Arctic and other locations (U.S. EPA, 2011a, 

p. 5-9). The contribution of black carbon to these effects is discussed in detail in the 

EPA’s recent Report to Congress on Black Carbon (U.S. EPA, 2012c). In particular, 

black carbon plays an important role in heating the lower atmosphere by absorbing 

incoming solar radiation and outgoing terrestrial radiation, i.e. via “direct” radiative 

forcing.

However, the EPA disagrees that there is sufficient information available at this 

time to establish a NAAQS to protect against the climate impacts associated with current 

ambient concentrations of black carbon or other PM constituents. While the Integrated 

Science Assessment concluded that “a causal relationship exists between PM and effects 

on climate, including both direct effects on radiative forcing and indirect effects that 

involve cloud feedbacks that influence precipitation formation and cloud lifetime” (U.S. 

EPA, 2009a, section 9.3.10), it also identified substantial remaining uncertainties with 
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regard to the contribution of individual aerosol species to these climate effects. The 

contribution of individual aerosol components to total aerosol direct radiative forcing is 

more uncertain than the global average (U.S. EPA, 2009a, section 9.3.6.6), and the 

indirect effects of aerosols and aerosol components remain highly uncertain, in particular 

with regard to their complex interactions with clouds.

With regard to black carbon, for example, the EPA disagrees with CBD’s claims 

that “black carbon’s negative climate impacts do not depend upon details of cloud 

interactions with aerosols” and that the uncertainties associated with climate impacts of 

aerosols generally do not apply to black carbon. In fact, the EPA has pointed to cloud 

interactions as the area of greatest uncertainty with regard to black carbon: recognizing 

that black carbon affects cloud reflectivity (albedo), lifetime, and stability as well as 

precipitation, the Report to Congress on Black Carbon noted that “few quantitative 

estimates of these effects are available, and significant uncertainty remains. Due to all of 

the remaining gaps in scientific knowledge, it is difficult to place quantitative bounds on 

the forcing attributable to [black carbon] impacts on clouds at present” (U.S. EPA, 2012c, 

p. 4). The Report acknowledged that “most estimates of the forcing from aerosol indirect 

effects are based on all aerosol species (e.g. total PM) and are not estimated for 

individual species (e.g, BC alone)” (U.S. EPA, 2012c, p. 40). The Report concluded that 

it remains unclear the extent to which black carbon contributes to the overall aerosol 

indirect effect, and did not assign any central estimate or even a range of possible values 

to the role of black carbon in the overall aerosol indirect effect. With regard to black 

carbon’s net contribution to climate, therefore, the Report concluded:

The direct and snow/ice albedo effects of BC are widely understood to lead to 

climate warming. However, the globally averaged net climate effect of BC also 
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includes the effects associated with cloud interactions, which are not well 

quantified and may cause either warming or cooling. Therefore, though most 

estimates indicate that BC has a net warming influence, a net cooling effect 

cannot be ruled out. It is also important to note that the net radiative effect of all 

aerosols combined (including sulfates, nitrates, BC and OC) is widely understood 

to be negative (cooling) on a global average basis. (U.S. EPA, 2012c, p. 3)

Given the remaining uncertainties about the impact of aerosols on climate, there is even 

greater uncertainty with regard to how aerosol-induced climate change will affect public 

health. At this time, it is not possible to estimate the extent to which aerosols in general, 

let alone particular aerosol components, contribute to the occurrence or exacerbation of 

adverse health outcomes due to climate change. The EPA therefore disagrees with CBD’s 

claim that the EPA should pursue black carbon reductions for purposes of reducing the 

impacts of climate change on public health.

The Report to Congress on Black Carbon also stressed the importance of 

considering co-emitted PM species, such as SO2 and NOx, in evaluating the benefits of 

black carbon mitigation options. Noting that many of these co-emitted particles and gases 

have a cooling influence on climate, the Report noted the difficulty of estimating the net 

effect of various mitigation measures on net radiative forcing or other climate variables. 

The EPA concluded that the location and timing of emissions reductions would be 

critically important for achieving climate benefits, and that “more research is needed on 

the benefits of individual control measures in specific locations to support policy 

decisions made at the national level” (U.S. EPA, 2012c, p. 140). Thus, the EPA disagrees 

with CBD’s claim that spatial and temporal heterogeneity in black carbon emissions do 

not matter for estimating likely climate effects, and continues to believe that being able to 

quantify the climate impacts of various aerosol species, alone and in combination, is 
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essential for informing any possible revisions to the current secondary PM standards 

based on climate.

Furthermore, while the EPA agrees with the commenter that a large percentage of 

black carbon emissions come from anthropogenic sources, including diesel engines and 

vehicles, the EPA notes that existing regulations on mobile diesel engines are already 

reducing these emissions substantially. Between 1990 and 2005, new engine 

requirements resulted in a 32 percent reduction in black carbon emissions from mobile 

sources, and a further 86 percent reduction from 2005 levels is projected to occur by 2030 

as vehicles and engines meeting existing regulations are phased into the fleet (U.S. EPA, 

2012c, p. 175). Long-term historic data indicate that there has been a dramatic overall 

decline in black carbon emissions over the past century, due to changes in fuel use, more 

efficient combustion practices, and implementation of PM controls. Therefore, the EPA 

disagrees with CBD’s claim that a distinct black carbon NAAQS is necessary to achieve 

reductions in black carbon emissions. Clearly, U.S. emissions of black carbon are already 

declining substantially, suggesting that the existing mass-based PM standards, though not 

targeting black carbon specifically, have been effective in achieving black carbon 

emissions reductions in practice. As acknowledged in the Report to Congress on Black 

Carbon, “While [black carbon] is not the direct target of existing programs, it has been 

reduced through controls aimed at reducing ambient PM2.5 concentrations and/or direct 

particle emissions” (U.S. EPA, 2012c, p. 161). The EPA has acknowledged the need to 

encourage PM mitigation strategies that focus on reducing directly emitted PM2.5 for 

purposes of reducing black carbon, and this is reflected in U.S. commitments under the 

Gothenburg Protocol: the new provisions in the Protocol pertaining to PM encourage 
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parties to develop national inventories and projections for black carbon, and to “give 

priority” to black carbon when implementing measures to control PM. However, the EPA 

notes that the U.S. has not yet ratified the PM amendments to the Gothenburg Protocol, 

and furthermore, these amendments do not require action specifically to reduce black 

carbon, but rather encourage countries to take such actions voluntarily within the context 

of their broader PM reduction strategies. Thus the EPA disagrees with the commenter 

that the U.S. has an “obligation” to reduce black carbon under the Gothenburg Protocol, 

or that it has “agree[d] to choose mitigation options for particulate matter that focus on 

black carbon reductions” under the Protocol (CBD, p. 13).

In sum, the EPA notes the substantial remaining the uncertainties and gaps with 

regard to the climate impacts of PM components, including black carbon. These include 

the uncertainties associated with the spatial and temporal heterogeneity of PM 

components that contribute to climate forcing; the uncertainties associated with 

measurement of aerosol components; the inadequate consideration of aerosol impacts in 

climate modeling; and the currently insufficient data on local and regional microclimate 

variations and the heterogeneity of cloud formations. As a result, the EPA continues to 

conclude that it is not currently feasible to conduct a quantitative analysis for the purpose 

of informing revisions of the current secondary PM standards based on climate, and that 

there is insufficient information at this time to base a national ambient standard on 

climate impacts associated with current ambient concentrations of PM or any of its 

constituents.223

D. Conclusions on Secondary PM Standards

 

223 This conclusion applies for both the secondary (welfare-based) and the primary 

(health-based) standards. 
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This section describes the Administrator’s conclusions regarding the secondary 

PM standards and the rationale leading to the Administrator’s final decision to retain the 

current suite of secondary PM standards, including an annual PM2.5 standard of 15 μg/m3

a 24-hour PM2.5 standard of 35 μg/m3, and a 24-hour PM10 standard of 150 μg/m3, to 

address PM-related welfare effects, including visibility impairment as well as other PMrelated welfare effects, including ecological effects, effects on materials, and climate 

impacts. Specifically, this section explains the Administrator’s decision, consistent with 

the proposal, to retain the current suite of secondary PM standards generally, while 

revising only the form of the secondary annual PM2.5 standard to remove the option for 

spatial averaging consistent with this change to the primary annual PM2.5 standard. It also 

explains the Administrator’s decision, contrary to what was proposed, not to establish a 

distinct standard to address PM-related visibility impairment. 

In reaching conclusions regarding the need to revise the secondary PM standards 

for both visibility and non-visibility welfare effects, the Administrator has taken into 

account several key factors, including: (1) The latest scientific information on both 

visibility and non-visibility welfare effects associated with PM, as previously described; 

2) the advice of CASAC; and 3) the comments received during the public comment 

period, as discussed above. Based on this information, the Administrator has reached 

final conclusions about the secondary PM standards and made final decisions about those 

standards, as outlined below. Because the Administrator’s final conclusions with regard 

to the need to establish a distinct secondary standard to protect against visibility 

impairment reflect, in part, her conclusions on the secondary standardPM standards for 

non-visibility welfare effects, section VI.D.1 first outlines her conclusions regarding the 
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secondary PM standards to address non-visibility welfare effects. This is followed by 

section VI.D.2 which outlines her conclusions regarding a secondary PM standard to 

protectaddress PM-related visibility impairment. Finally, section VI.D.3 summarizes the 

Administrator’s final decisions with regard to the secondary PM standards for both 

visibility and non-visibility welfare effects.

1. Conclusions Regarding Secondary PM Standards to Address Non-Visibility Welfare 

Effects

With regard to the secondary PM standards to address non-visibility welfare 

effects, the Administrator concludes that it is generally appropriate to retain the existing 

secondary standards and that it is not appropriate to establish any distinct secondary PM 

standards to address non-visibility PM-related welfare effects. This conclusion is based 

on the considerations discussed above in section VI.B.2, including the latest scientific 

information and the advice of CASAC, and the public comments received on the 

proposal, as discussed above in section VI.C.2. The Administrator concurs with the 

advice of CASAC and the conclusions expressed at the time of proposal that it is 

important to maintain an appropriate degree of control of both fine and coarse particles to 

address non-visibility welfare effects, including ecological effects, effects on materials, 

and climate impacts. In the absence of information that would support any different 

standards the Administrator concludes that it is appropriate to retain the existing suite of 

secondary standards to address non-visibility welfare effects, as proposed. More 

specifically, the Administrator concludes it is appropriate to retain all aspects of the 

current 24-hour PM2.5 and PM10 standards. With regard to the secondary annual PM2.5

standard, the Administrator concludes that it is appropriate to retain a level of 15.0 μg/m3
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for this standard while revising only the form of the secondary annual PM2.5 standard to 

remove the option for spatial averaging consistent with this change to the primary annual 

PM2.5 standard. In reaching this conclusion, the Administrator notes that no areas in the 

country are currently using the option for spatial averaging to demonstrate attainment 

with the secondary annual PM2.5 standard.

2. Conclusions Regarding Secondary PM Standards for Visibility Protection

Having reached the conclusion that it is generally appropriate to retain the 

existing secondary standards to protect against non-visibility welfare effects, the 

Administrator next considered the target level of protection that would be requisite to 

protect public welfare with regard to visual air quality. The Administrator then 

determined whether to adopt a distinct secondary standard to achieve this target level of 

protection. In making this decision, the Administrator compared the degree of protection 

for visibility that would be provided by such a distinct secondary standard to the degree 

of protection provided by the existing secondary standards, focusing specifically on the 

secondary 24-hour PM2.5 standard of 35 μg/m3.

224

Based on the considerations discussed above in section VI.B and VI.C, the 

Administrator first concludes that a target level of protection for a secondary standard is 

most appropriately defined in terms of a PM2.5 visibility index as proposed, since it would 

provide a measure of PM-related light extinction that directly takes into account the 

factors (i.e., species composition and relative humidity) that influence the relationship 

between PM2.5 in the ambient air and PM-related visibility impairment. Such a PM2.5

 

224 This focus on the 24-hour PM2.5 standard reflects the Administrator’s judgments that 

PM-related visibility impairment is principally related to fine particle concentrations and 

that perception of visibility impairment is most directly related to short-term levels of 

visual air quality. 
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visibility index standard would afford a relatively high degree of uniformity of visual air 

quality protection in areas across the country by virtue of directly incorporating the 

effects of differences in PM2.5 composition and relative humidity across the country. 

In defining a target level of protection based on a PM2.5 visibility index, the 

Administrator has considered specific aspects of such an index, including the appropriate 

indicator, averaging time, level, and form. First, with regard to indicator, the 

Administrator notes the conclusion of CASAC that relying on a calculated PM2.5 light 

extinction indicator based on PM2.5 chemical speciation and relative humidity data 

represented a reasonable approach. Based on the analyses conducted in support of this 

rulemaking, as described above, as well as the advice of CASAC, the Administrator 

concludes that a calculated PM2.5 light extinction indicator that utilizes the original 

IMPROVE algorithm, adjusted to use a 1.6 OC multiplier and exclude the term for coarse 

particles, in conjunction with monthly average relative humidity data (i.e., f(RH) values) 

based on long-term climatological means would be the most appropriate indicator for a 

PM2.5 visibility index standard. 

With regard to averaging time, the Administrator notes that both CASAC and 

EPA staff have concluded that hourly or sub-daily (4- to 6-hour) averaging times, within 

daylight hours and excluding hours with high relative humidity, are more directly related 

than a 24-hour averaging time to the short-term nature of the perception of PM-related 

visibility impairment and the relevant exposure periods for segments of the viewing 

public. However, in light of the important data quality uncertainties that have recently 

been identified in association with currently available instruments that would be used to 

provide the hourly PM2.5 mass measurements that would be needed in conjunction with 
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an averaging time shorter than 24 hours, the Administrator concludes it would not be 

appropriate at this time to set a standard based on a sub-daily averaging time. Moreover, 

the Administrator notes that analyses conducted by the EPA during this review clearly 

indicate that PM2.5 light extinction calculated on a 24-hour average basis would be a 

reasonable and appropriate surrogate for PM2.5 light extinction calculated on a 4-hour 

basis. Thus, the Administrator concludes that a 24-hour averaging time would be 

appropriate for a PM2.5 visibility index. The Administrator recognizes that a 24-hour 

averaging time would effectively reduce the influence of peak hours of visibility 

impairment on visibility index values, but concludes that in light of the concern that peak 

hourly measurements may be significantly influenced by atypical conditions and/or 

atypical instrument performance, it is appropriate to adopt a longer averaging time to 

ensure that hour-specific influences and uncertainties are balanced against more robust 

measurements. 

With regard to form, the Administrator notes that consistent with the approach 

taken in other NAAQS, including the current 24-hour PM2.5 NAAQS, a multi-year 

percentile form offers greater stability to the air quality management process by reducing 

the possibility that statistically unusual indicator values will lead to transient violations of 

the standard. Utilizing a three-year average form provides stability from the occasional 

effects of inter-annual meteorological variability that can result in unusually high 

pollution levels for a particular year. Moreover, considering the lack of information on 

and the high degree of uncertainty regarding the impact on public welfare of the number 

of days with visibility impairment over the course of a year, the Administrator considers 

it reasonable to focus on the 90th percentile, which represents the median of the 
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distribution of the 20 percent worst visibility days, a key focus of the Regional Haze 

program. The Administrator concludes that ensuring that 90 percent of days have visual 

air quality that is at or below the target level of protection could be reasonably expected 

to lead to improvements in visual air quality on the 20 percent most impaired days, and 

that the limited information available in this review provides no basis for adopting a 

different form which would limit the occurrence of days with peak PM-related light 

extinction in urban areas to a greater degree. Therefore, the Administrator concludes that 

a 90th percentile form, averaged over 3 years, is appropriate, for purposes of establishing 

a target level of protection in terms of a 24-hour PM2.5 visibility index.

With regard to level, the Administrator concludes that in light of the uncertainty 

associated with the high degree of variability in visibility conditions and the potential 

variability in visibility preferences across different parts of the country, it is appropriate 

to establish a target level of protection based on the upper end of the range of Candidate 

Protection Levels (CPLs) identified in the Policy Assessment (i.e., 20-30 dv) and 

generally supported by CASAC. Thus, the Administrator concludes that it would be 

appropriate to set a target level of protection in terms of a PM2.5 visibility index with a 

24-hour averaging time that would provide protection equivalent to the protection 

afforded by a 4-hour PM2.5 visibility index with a level of 30 dv. Furthermore, the 

Administrator notes that for purposes of estimatingthe approaches used to estimate

generally equivalent levels for a 24-hour PM2.5 visibility index, both of the EPA’s two 

preferred approaches generated 90th percentile 24-hour values similar to the 4-hour 

values and a range of city-specific estimates of generally equivalent 24-hour levels that 

encompassed the range of levels considered appropriate for 4-hour CPLs, including the 
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CPL of 30 dv at the upper end of that range. The Administrator thus concludes that it 

would be appropriate to use an unadjusted 4-hour CPL for purposes of establishing a 

target level of protection in terms of a 24-hour PM2.5 visibility index. 

In considering the alternative levels proposed for a 24-hour standard, either 28 dv 

or 30 dv, the Administrator concludes that the current substantial degrees of variability 

and uncertainty inherent in the public preference studies should be reflected in a higher 

target protection level than would be appropriate if the underlying information were more 

consistent and certain. In addition, she concludes that, in light of the significant 

uncertainties, it is appropriate to place less weight on the results of western visibility 

preference studies and that the CPL value (30 dv) that is based on the eastern preference 

study results is likely to be more representative of urban areas that do not have associated 

mountains or other valued objects visible in the distant background For all of these 

reasons, the Administrator concludes that it is appropriate to set a target level of 

protection in terms of a 24-hour PM2.5 visibility index at 30 dv. 

In summary, in light of all the information available in this review, the 

Administrator concludes that the protection provided by a standard defined in terms of a 

PM2.5 visibility index (based on speciated PM2.5 mass concentrations and relative 

humidity data to calculate PM2.5 light extinction), a 24-hour averaging time, and a 90th

percentile form, averaged over 3 years, set at a level of 30 dv, would be requisite to 

protect public welfare with regard to visual air quality. 

In reaching this conclusion, the Administrator notes that any national ambient air 

quality standard forto address PM-related visibility impairment would be designed to 

work in conjunction with the Regional Haze Program as a means of achieving 
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appropriate levels of protection against PM-related visibility impairment in all areas of 

the country, including urban, non-urban, and Federal Class I areas. While the Regional 

Haze Program is focused on improving visibility in Federal Class I areas and a secondary 

NAAQS to address PM-related visibility NAAQSimpairment would focus on protecting 

visual air quality principally in urban areas, both programs could be expected to provide 

benefits in surrounding areas. In addition, the development of local programs, such as 

those in Denver and Phoenix, could continue to be an effective and appropriate approach 

to provide additional protection, beyond that afforded by a national standard, for unique 

scenic resources in and around certain urban areas that are particularly highly valued by 

people living in those areas.

Having concluded that the protection provided by a standard defined in terms of a 

PM2.5 visibility index, with a 24-hour averaging time, and a 90th percentile form, 

averaged over 3 years, set at a level of 30 dv, would be requisite to protect public welfare 

with regard to visual air quality, the Administrator next has to determine whether to adopt 

such a visibility index as a distinct secondary standard. This determination requires 

considering such a secondary standard not in isolation but in the context of the full suite 

of secondary standards. As discussed above, the Administrator has determined to retain 

the current suite of secondary PM standards to address non-visibility welfare effects 

(except for the form of the annual standard). A distinct secondary standard to address 

visibility impairment is properly considered in a context where there is also a 24-hour 

PM2.5 standard of 35 μg/m3. 

In this context, the Administrator has considered the degree of protection from 

visibility impairment afforded by the existing secondary PM2.5 standards. The 
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Administrator has considered both whether the existing 24-hour PM2.5 standard of 35 

μg/m3is sufficient (i.e. not under-protective) and whether it is not more stringent than 

necessary (i.e. not over-protective). 

As discussed above in section VI.C.1.f, the results of the Kelly et al. (2012a; 

2012b) analyses indicate that based on 2008-2010 and 2009-2011 data, all areas meeting 

the 24-hour PM2.5 standard of 35 μg/m3had visual air quality at least as good as 30 dv 

(24-hour average, based on 90th percentile form averaged over 3 years). This means that 

it is highly likely that the secondary 24-hour PM2.5 standard of 35 μg/m3 would be 

controlling relative to a 24-hour standard based on a PM2.5 visibility index set at a level of 

30 dv, and highly unlikely that areas would exceed the target level of protection for 

visibility of 30 dv without also exceeding the existing secondary 24-hour standard. On 

the basis of this evidence, and the supporting public comments, the Administrator judges 

that the 24-hour PM2.5 standard of 35 μg/m3provides sufficient protection in all areas 

against the effects of visibility impairment—i.e., that the existing 24-hour PM2.5 standard 

would provide at least the target level of protection for visual air quality of 30 dv which 

the Administrator judges appropriate.

The Administrator also recognizes that the analyses presented in Kelly et al. 

(2012a; 2012b) indicate that the 24-hour PM2.5 standard of 35 μg/m3also would likely 

achieve more than the target level of protection of visual air quality (30 dv) in some 

areas. That is, when meeting a mass-based standard of 35 μg/m3, some areas would have 

levels of PM-related visibility impairment below 30 dv. Thus, the 24-hour PM2.5 standard 

of 35 μg/m3 would be over-protective in some areas (i.e. more stringent than necessary) 

relative to the target level of protection for visibility. This is not surprising, as the current 
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mass-based standard does not account for variation in particle species and relative 

humidity. The 24-hour PM2.5 standard of 35 μg/m3 would provide more than the 

necessary protection in the areas where this would be expected, for example western 

areas with lower relative humidity. 

In light of the Administrator’s conclusion that it is appropriate to retain the 

current secondary 24-hour PM2.5 standard of 35 μg/m3for non-visibility welfare effects, 

the Administrator notes that this standard will remain in place regardless of whether she 

elects to set a distinct secondary standard in terms of a PM2.5 visibility index. The issue is 

not whether to adopt a PM2.5 visibility index standard when viewed in isolation, but 

whether such a distinct secondary standard should be adopted in addition to the current 

secondary 24-hour PM2.5 standard of 35 μg/m3. The EPA notes that adoption of such a 

distinct secondary standard is not needed to provide sufficient protection from visibility 

impairment with respect to the target level of protection determined above. In addition, 

adoption of such a distinct secondary standard would not change the fact that the current

secondary 24-hour PM2.5 standard of 35 μg/m3 would result in over-protection from 

visibility impairment in certain areas of the country. Such over-protection will occur 

whether or not such a distinct secondary standard is adopted. In effect, adopting such a 

distinct secondary standard would have no impact on the degree of protection provided 

from visibility impairment. Since sufficient protection from visibility impairment would 

be provided for all areas of the country without adoption of a distinct secondary standard, 

and adoption of a distinct secondary standard will not change the degree of overprotection provided for some areas of the country, the Administrator judges that adoption 

of such a distinct secondary standard is not needed to provide requisite protection for 
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both visibility and non-visibility related welfare effects. 

It is important to note that this conclusion is based on the specific target level of 

protection determined above, and the specific set of current secondary standards. The 

Administrator’s conclusion with regard to the sufficiency of the protection provided by 

the current suite of secondary standards is based on comparing the a 30 dv target level of 

protection for a PM2.5 visibility index standard against the degree of protection provided 

by the current secondary 24-hour PM2.5 standard of 35 μg/m3. It is the combination of the 

specific target level of protection and the current suite of secondary standards that is the 

basis for the decision not to adopt a distinct secondary standard in terms of a PM2.5

visibility index at this time. 

The EPA recognizes that, as in the last review, the final decision is to not adopt a 

distinct secondary standard to address visibility impairment. While the D.C. Circuit 

remanded the decision on a secondary standard in the last review, the EPA’s decision in 

this review fully comports withhas addressed the issues raised in the court’s remand. 

Here the EPA has clearly identified the target degree of protection (defined in terms of a 

PM2.5 visibility index at a level of 30 dv based on a 24-hour averaging time, and a 90th

percentile form, averaged over 3 years) that would be requisite to protect public welfare 

with regard to visual air quality. The EPA has carefully compared this degree of 

protection with that provided by the current secondary 24-hour PM2.5 secondary standard 

of 35 μg/m3, based on an area-specific analysis of recent air quality data and concluded 

that the degree of protection from visibility impairment provided by the current 

secondary standard is sufficient to protect public welfare consistent with section 

109(b)(2). This provides a clear basis for judging that the current secondary 24-hour 
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PM2.5 secondary standard of 35 μg/m3 would provide sufficient protection. The analysis 

also shows that the current 24-hour secondary 24-hour PM2.5 standard would provide 

more protection than is needed in some areas, largely because it does not take into 

account variable factors such as relative humidity. However, the EPA has recognized that 

adoption of a distinct secondary standard to address visibility, in addition to retaining the 

current secondary standard, would not change this result. The EPA has therefore 

concluded that adoption of such a distinct secondary standard, in addition to the current 

suite of secondary PM standards, is not needed to provide requisite protection for both 

visibility and non-visibility related welfare effects. Thus the EPA’s decision has carefully 

considered and accounted for the views of the court in the remand of the 2006 NAAQS. 

E. Administrator’s Final Decisions on Secondary PM Standards

To address PM-related welfare effects, including ecological effects, effects on 

materials, climate impacts, and visibility impairment, the Administrator is retaining the 

current suite of secondary PM standards, except for a change to the form of the annual 

standard. Specifically, the EPA is retaining the current secondary 24-hour PM2.5 and 

PM10 standard to address PM-related non-visibility welfare effects including ecological 

effects, effects on materials, and climate impacts, the EPA is retaining the current 

secondary 24-hour PM2.5 and PM10 standard and is revising only the form of the 

secondary annual PM2.5 standard to remove the option for spatial averaging consistent 

with this change to the primary annual PM2.5 standard. With respect to PM-related 

visibility impairment, the Administrator has identified a target degree of protection, 

defined in terms of a PM2.5 visibility index (based on speciated PM2.5 mass 

concentrations and relative humidity data to calculate PM2.5 light extinction), a 24-hour 
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averaging time, and a 90th percentile form, averaged over 3 years, and a level of 30 

deciviews (dv), which she judges to be requisite to protect public welfare with regard to 

visual air quality. The EPA’s analysis of monitoring data provides the basis for 

concluding that the current secondary 24-hour PM2.5 standard would provide sufficient 

protection, and in some areas greater protection, relative to this target protection level. 

Adding a distinct secondary standard to address PM-related visibility impairment would 

not affect this protection. Since sufficient protection from visibility impairment will be 

provided for all areas of the country without adoption of a distinct secondary standard, 

and adoption of a distinct secondary standard will not change the degree of overprotection of visual air quality provided for some areas of the country by the secondary 

24-hour PM2.5 standard, the Administrator judges that adoption of a distinct secondary 

standard, in addition to the current suite of secondary standards, is not needed to provide 

requisite protection for both visibility and non-visibility related welfare effects.

VII. Interpretation of the NAAQS for PM

This section discusses the EPA Administrator’s final decisions on the revisions 

proposed to the data handling procedures for the primary and secondary PM2.5 standards.

Appendix N to 40 CFR part 50 describes the computations necessary for determining

when the PM2.5 standards are met and also addresses which measurement data are

appropriate for comparison to the standards; as well, it specifies associated data reporting 

protocols, data completeness criteria, and rounding conventions. The EPA is modifying 

appendix N to conform to the revised PM2.5 standards; most notably, the EPA is 

simplifying the annual standard form and associatedamending the appendix N procedures 

by removing the option for spatial averaging. Spatial averaging had been a component of 

the annual standard form since the original PM2.5 NAAQS promulgation of 1997. In 
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addition to making changes to appendix N that correspond to the changechanges in the 

annual standard form and the revised primary annual standard level, the EPA is also 

finalizing additional proposed revisions to the appendix in order to codify existing 

practices currently included in guidance documents or implemented as the EPA standard 

operating procedures; better align appendix N language and requirements with changes in 

PM2.5 ambient monitoring and reporting requirements; provide greater clarity and

transparency in the provisions; and enhance consistency with data handling protocols 

utilized for other pollutants.

A. Revised Amendments to Appendix N: Interpretation of the NAAQS for PM2.5

As discussed in sections III and VI above, the EPA Administrator has decided to: 

(1) revise the form and level of the primary annual PM2.5 standard, and retain the current 

primary 24-hour PM2.5 standard (section III.F) and (2) retain the current secondary 24--

hour PM2.5 standard, and revise the form and retain the level of the secondary annual 

PM2.5 standard (for visibility and non-visibility-related welfare protection) (section VI.E). 

Appendix N is being revised to conform to those changes to the standards. In the 

proposal, the EPA recommended to add additional data handling procedures to appendix 

N for the proposed distinct secondary standard to address PM2.5-related visibility 

impairment. However, as discussed in section VI.D.3E, the Administrator has decided 

not to establish the proposed distinct secondary standard to address visibility impairment, 

and therefore, the associated proposed data handling procedures related to that proposed 

standard are not included in the final revised appendix N. 

In addition to the changes to appendix N necessitated by the annual NAAQS form 

and level revisions (discussed in depth in sections III and VI above), the EPA is also 
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finalizing additional revisions to appendix N in order to: (1) better align appendix N 

language and requirements with proposed changes in the PM2.5 ambient monitoring and 

reporting requirements as discussed in section VIII below; (2) enhance consistency with 

recently codified changes in data handling procedures for other criteria pollutants; (3) 

codify existing practices currently included in guidance documents or implemented as the 

EPA standard operating procedures; and (4) provide enhanced clarity and consistency in 

the articulation and application of appendix N provisions. Key elements of the finalized 

revisions to appendix N are summarized in sections VIVII.A.1 through VIVII.A.54

below, where the first four of these preamble sections which correspond to the similarly 

numbered sectionsections in appendix N with the exception of section 5. The proposed 

potential new fifth section of appendix N dealt with the proposed distinct PM2.5-related 

visibility secondary standard that was not finalized by the Administrator and thus the 

proposed appendix N section 5 wasis not included in the final appendix N. Furthermore, 

proposed changes to sections 1 through 4 of appendix N that also dealt with the proposed 

secondary visibility index standard (e.g., term definitions, rounding conventions, etc.) 

wereare also omitted from the final revised appendix.

1. General 

As proposed, the EPA is finalizing modifications to section 1.0 of appendix N to 

provide additional clarity regarding the scope and interpretation of the PM2.5 NAAQS. 

This appendix section now references the finalized revisions of the primary annual PM2.5

standard (40 CFR 50.18) and the retained secondary PM2.5 NAAQS. With regard to the 

appendix N term definitions which are delineated in this initial section, the EPA has 

added, modified, and eliminated term definitions, as appropriate, in accordance with the 
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proposedfinal data handling rule revisions such as the modification of terms that 

referenced spatial averaging. Additional term definitions were also added to reference 

otherwise unchanged appendix N content in an effort to streamline the appendix text, 

enhance clarity and thus improve readability and understanding. In particular, the 

definition of data substitution tests was shortened, and a definition for “test design value" 

(TDV) was added for completeness and for further clarity. This term was previously part 

of the data substitution definition and now it is more explicitly defined. The EPA notes 

that there were no substantive public comments received with regardsregard to this 

section.

2. Monitoring Considerations; Spatial Averaging.

As proposed, the EPA has finalized revisions to section 2.0 of appendix N 

consistent with the concurrent modification of the form of the primary annual PM2.5

standard that removes the option for spatial averaging. As described in more detail in 

section III.E.3.a above, the EPA decided to remove this option as part of the form of the 

primary annual PM2.5 standard in light of analysis that indicates that the existing 

constraints on spatial averaging, as modified in 2006, may be inadequate to avoid 

substantially greater exposures in some areas, potentially resulting in disproportionate 

impacts on susceptible populations (Schmidt 20112011a, Analysis A).

With respect to the form of the secondary annual PM2.5 standard, as discussed in 

section VVI.E.5 above, the EPA has decided to retain the current secondary annual PM2.5

standard to provide protection for welfare effects. However, as proposed, since In the 

proposal, the EPA believesbelieved it would be reasonable and appropriate to align the 

data handling procedures for the primary and secondary annual PM2.5 standards, the EPA 
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has removed and remove the option for spatial averaging for the secondary annual PM2.5

standard to be consistent with the revised change in the form of the primary annual PM2.5

standard. (FR 77 39000, June 29, 2012). The EPA notesnoted that no areas in the country 

are currently using the option for spatial averaging to demonstrate attainment with the 

secondary annual PM2.5 standard. There were no comments on the proposed change and 

the EPA has therefore concluded it appropriate to remove the option for spatial averaging 

for the secondary annual PM2.5 standard from Appendix N.

Consistent with the revised form of the primary and secondary annual PM2.5

standards, the levels of both standards will be compared to measurements from each 

appropriate (i.e., “eligible”) monitoring site in an area, as specified in 40 CFR 58.30, with 

no allowance for spatial averaging. Thus, for an area with multiple eligible monitoring 

sites, the site with the highest design value would determine the attainment status for that 

area. As a result of the decision to eliminate the spatial averaging option for both the 

primary and secondary annual standards, the EPA omitted all references to the spatial 

averaging option in the finalized version of appendix N. See section IIIII.E.3.a above for 

a discussion of EPA’s response to received public comment on the issue of removal of 

the spatial averaging option. 

3. Requirements for Data Use and Reporting for Comparisons with the NAAQS for 

PM2.5

In the proposal, the EPA suggested changes to section 3.0 of appendix N to 

correspond to the proposed new secondary standard to address PM-related visibility 

impairment. Since the EPA is not finalizing the proposed distinct secondary standard to 

address visibility impairment, none of these are proposed changes are necessary and are 

not being made. The EPA is, however, finalizing proposed changes to improve 
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consistency with procedures used for other NAAQS as well as to improve consistency 

with current standard operating procedures. Specifically, the EPA proposed revisions to 

this section regarding: (1) clarification of monitoring data appropriate to compare to the 

PM2.5 NAAQS; (2) clarification of procedures for combining monitoring data from 

collocated instruments into a single “combined site” record; and (3) codification of the 

current standard operating procedure whereby the EPA uses data for which the 

certification deadline has passed but the monitoring agency has not requested 

certification of the data to determine compliance with the PM2.5 NAAQS provided the 

data are complete and accurate (thus making appendix N consistent with data handling 

appendices for other criteria pollutants). In the final revision to appendix N, the EPA is 

incorporating all the above noted proposed modifications to section 3 of appendix N. 

Additional details describing the incorporated modifications are provided below.

With regardsregard to clarification of which monitoring data are appropriate for 

comparison to the PM2.5 NAAQS, the proposal acknowledged important data quality 

concerns associated with the PM2.5 measurements collected by continuous PM2.5 FEMs 

and referenced a subsequent preamble proposal section that discussed the issue in more 

depth and put forward a solution to mitigate the data quality concerns. The revised 

monitoring rule, promulgated today in conjunction with the PM NAAQS revision, 

includes, as proposed, language allowing monitoring agencies to identify PM2.5 FEMs 

that are not providing data of sufficient comparability to the FRM and, with EPA 

approval, to allow such data to be deemed ineligible for comparisons with the PM2.5

NAAQS225; see detailed discussion of this decision in section VIII.A.1 below. Rule 

 

225 The EPA also allows use of alternative methods where explicitly stated in the 

monitoring methodology requirements (appendix C of 40 CFR part 58), such as PM2.5

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language for the definition of “suitable monitors” in section 31.0 of the finalized revised 

appendix N accommodates and references thethis monitoring rule revision codified in 40 

CFR 58.11. 

With respect to the procedures for combining monitored data from collocated 

instruments into a single “combined site” data record, the EPA had proposed to revise the 

current methodology in situations where an FRM monitor operating on a non-daily 

schedule is collocated with a continuous FEM monitor (that has acceptable comparability 

with an FRM). As noted in the proposal, the EPA was not advocating a change to the 

actual procedures for constructing a combined site record but rather a modification to the 

subsequent evaluation of whether the specific measurements were considered 

“creditable” or “extra” samples226..

227

 The language clarification proposed has already 

 

Approved Regional Methods (ARMs) which can be used to determine compliance with 

the NAAQS. Monitoring agencies identifying ARMs that are not providing data of 

sufficient quality will also be allowed to exclude these data in making comparisons to the 

PM2.5 NAAQS. Currently, there are no designated ARMs for PM2.5.

226 Data for a combined site record originates by default from the designated “primary” 

monitor at the site location and is then augmented with data from collocated FRM or 

FEM monitors whenever valid data are not generated by the primary monitor. Samples in 

the combined site record are deemed “creditable” or “extra” according to the required 

sampling frequency for a specific monitoring site (i.e., “site-level sampling frequency”) 

which, by default, is defined to be the same as the sampling frequency required of the 

primary monitor. Samples in the combined site data record that correspond to scheduled 

days according to the site-level sampling frequency are deemed “creditable” and, thus, 

are considered for determining whether or not a specific monitoring site meets data 

completeness requirements. These samples also determine which daily value in the 

ranked list of daily values for a year represents the annual 98th percentile concentration. 

Samples that are not deemed “creditable” are classified as “extra” samples. These 

samples do not count towards data completeness requirements and do not affect which 

daily values represent the annual 98th percentile concentration; “extra” samples, however, 

are candidates for selection as the 98th percentile. 

227 Data for a combined site record originates by default from the designated “primary” 

monitor at the site location and is then augmented with data from collocated FRM or 

FEM monitors whenever valid data are not generated by the primary monitor. Samples in 

the combined site record are deemed “creditable” or “extra” according to the required 

sampling frequency for a specific monitoring site (i.e., “site-level sampling frequency”) 
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been implemented asis currently standard operating procedure in Agency design value 

computations so the language modification in appendix N merely codifiedproposed to 

modify actual practices228..

229 The revised appendix N finalized in today’s action 

 

which, by default, is defined to be the same as the sampling frequency required of the 

primary monitor. Samples in the combined site data record that correspond to scheduled 

days according to the site-level sampling frequency are deemed “creditable” and, thus, 

are considered for determining whether or not a specific monitoring site meets data 

completeness requirements. These samples also determine which daily value in the 

ranked list of daily values for a year represents the annual 98th percentile concentration. 

Samples that are not deemed “creditable” are classified as “extra” samples. These 

samples do not count towards data completeness requirements and do not affect which 

daily values represent the annual 98th percentile concentration; “extra” samples, however, 

are candidates for selection as the 98th percentile. 

228 Before the introduction of continuous FEMs, when two or more samplers were 

collocated at the same site, monitoring agencies typically identified the sampler that 

operated on the more frequent sampling schedule as the “primary” monitor for 

developing a single site record. However, due to concerns regarding the comparability of 

FEMs to FRMs operated in some monitoring agency networks, and as briefly discussed 

above and in more detail in section VII.A.1 below, many monitoring agencies have kept 

the FRM as the “primary” monitor and delegated the continuous FEM (which samples 

more frequently, except in cases where the FRM operates on an “every day” schedule) to 

be the “supplemental” (non-primary) collocated monitor. In such cases, FEM 

measurements reported on the FRM “off” days were technically considered “extra.” In 

light of this practice, the EPA modified standing operating procedures whereby 

supplemental collocated FEM samples reported on the FRM “off” days would be 

considered “scheduled" and “creditable.” Thus, collocated FEM samples would count 

towards data capture rates (actually, increasing both the numerator and the denominator 

in the capture rate equation), and also would count towards identifying annual 98th

percentile concentrations. Further, if data from a supplemental collocated FEM are 

missing on an FRM “off” day (and no unscheduled FRM data are reported that day), the 

EPA proposed not to identify these as “scheduled” days consistent with current practice, 

and thus, reported data generated from the supplemental collocated continuous FEMs can 

only help increase data capture rates (77 FR 39001).
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incorporates the modification as proposed. The EPA notes that there were no substantive 

public comments received regarding this change.

4. Comparisons with the PM2.5 NAAQS

Section 4.0 of appendix N specifies the procedures for comparing monitored data 

to the PM2.5 standards. The EPA proposed revisions to section 4.0 of appendix N to: (1) 

provide consistency with the proposed primary and secondary annual PM2.5 standards; (2) 

expand the data completeness assessments to be consistent with current guidance and 

standard operating procedures; and (3) simplify the procedure for calculating annual 98th

percentile concentrations when using an approved seasonal sampling schedule.

Consistent with the proposed decisions to revise the level of the primary annual 

PM2.5 standard (section III.FE.4.b.iii) and to retain the current level of the secondary 

annual PM2.5 standard (section VI.EB.1.c.vi), the EPA proposed to modify section 4.1(a) 

of appendix N to separately list the levels of the primary and secondary annual PM2.5

 

229 Before the introduction of continuous FEMs, when two or more samplers were 

collocated at the same site, monitoring agencies typically identified the sampler that 

operated on the more frequent sampling schedule as the “primary” monitor for 

developing a single site record. However, due to concerns regarding the comparability of 

FEMs to FRMs operated in some monitoring agency networks, and as briefly discussed 

above and in more detail in section VIII.B.3.b.iii below, many monitoring agencies have 

kept the FRM as the “primary” monitor and delegated the continuous FEM (which 

samples more frequently, except in cases where the FRM operates on an “every day” 

schedule) to be the “supplemental” (non-primary) collocated monitor. In such cases, 

FEM measurements reported on the FRM “off” days were technically considered “extra.” 

In light of this practice, EPA modified standing operating procedures whereby 

supplemental collocated FEM samples reported on the FRM “off” days would be 

considered “scheduled" and “creditable.” Thus, collocated FEM samples would count 

towards data capture rates (actually, increasing both the numerator and the denominator 

in the capture rate equation), and also would count towards identifying annual 98th

percentile concentrations. Further, if data from a supplemental collocated FEM are 

missing on an FRM “off” day (and no unscheduled FRM data are reported that day), the 

EPA proposed not to identify these as “scheduled” days consistent with current practice, 

and thus, reported data generated from the supplemental collocated continuous FEMs can 

only help increase data capture rates (77 FR 39001, June 29, 2012)).

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standards. The final revised appendix N incorporates this proposed change; this appendix 

N section now references the revised primary annual standard level of 12.0 μg/m3and the 

retained secondary annual standard level of 15.0 μg/m3. However, as discussed above 

with respect to the final decisionsdecision to not implementestablish a distinct secondary 

standard to provide protection for visibility impairment, the final appendix N now 

explicitly references all PM2.5 secondary standard protection (that is, protection from 

visibility impairment and non-visibility-related welfare effects) to be provided by the 

revised annual standard with retained level of 15.0 μg/m3and the retained 24-hour 

standard with retained level of 35 μg/m3. Consistent with the final decisions to remove 

the option for spatial averaging for the primary annual PM2.5 standard (section III.F)), as 

well as for the secondary annual PM2.5 standard (section VII.A.2), the EPA amended

section 4.4 of appendix N to remove equations and associated instructions relating to 

spatial averaging. 

With regard to assessments of data completeness, the EPA proposal included the 

incorporation of two additional data substitution tests230 (making a total of three data 

substitution tests) into appendix N for validating annual and 24-hour PM2.5 design values 

otherwise deemed incomplete (via the 75 percent and 11 creditable sample minimum 

quarterly data completeness requirements). The EPA proposed to add these tests in order 

to codify existing practices currently included in guidance documents (U.S.US EPA, 

1999) and implemented as the EPA standard operating procedures, and further, to make 

the data handling procedures for PM2.5 more consistent with the procedures used for other 

NAAQS. While the need for data substitution will lessen as more continuous 

 

230 Data substitution tests are supplemental data completeness assessments that use 

estimates of 24-hour average concentrations to fill in for missing data (i.e., “data 

substitution”). 

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PM2.5 monitors continue to be deployed in PM2.5 networks, the EPA believes that these 

substitution procedures are important to ensure that available data, if incomplete, can be 

confidently used to make comparisons to the NAAQS. As noted in the EPA proposal, 

data substitution tests are diagnostic in nature; that is; they are only used in an illustrative 

manner to show that the NAAQS status based on incomplete data is reasonable. If an

“incomplete”As codified in section 4 of Appendix N, data are substituted for missing data 

to produce a “test design value using substituted data ” which is compared to the level of 

the NAAQS. If the test design value passes the diagnostic test, thisthe “incomplete” 

design value (without the data substitutions) is then considered the true actual “complete

anda valid” design value. If an incomplete design value . If an “incomplete” design value 

does not pass any stipulated data substitution test, then the original design value is still 

considered incomplete (and not valid for NAAQS comparisons). Previously, section 

4.1(c) of appendix N specified only one data substitution test for validating an otherwise 

incomplete design value. That diagnostic test only applied to the primary and secondary 

annual PM2.5 standard and only applies in instances of a violation; this test is referred to 

as the “over NAAQS” minimum quarterly value” test and is used to determine if the 

NAAQS has not been met. The two proposed additional data substitution tests were to be 

applicable for making comparisons to the primary and secondary annual and 24-hour 

PM2.5 standards, specifically to show that the NAAQS had been met. One of these

proposed tests uses collocated PM10 data to fill in “slightly incomplete”231 data records, 

and the other uses quarter-specific maximum values to fill in slightly incomplete data 

records; these two test are referred to as the “under NAAQS collocated PM10 test” and 

 

231 Slightly incomplete is defined as less than 75 percent but at least 50 percent quarterly 

data capture.

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the “under NAAQS maximum quarterly value test”, respectively. Both tests are designed 

to confirm that the PM2.5 design value is valid and is less than the level of the NAAQS.

The EPA received several comments on the proposed addition of the two “under 

NAAQS” data substitution tests; to determine that the NAAQS was met. The majority of 

comments generally supported the proposed addition of thedata substitution tests. 

However, one commenter questioned the general philosophy of all appendix N data 

substitution tests (i.e., the existing “over NAAQS” onetest and the two proposed “under 

NAAQS” onestests) by suggesting that there were more appropriate techniques for filling 

in for missing data that would result in better estimates of true design value level. The 

EPA believes that the three data substitution tests provided in the finalized appendix N 

are all very conservative approaches to verifyingverify that the NAAQS standards are 

either met or not met, and that theythe test design values are not (and were never

intended) to be used as the best estimators of the actual expected design value level. 

concentration.

232

Another commenter questioned, and argued against, the use of collocated PM10

data in PM2.5 data substitution tests. The commenter stressed was that this type of test is 

not consistent with those established for other pollutants. The commenter further argued 

that while PM10 and PM2.5 are both measurements of particulate matter, they are 

essentially different pollutants with different sources and different dispersion 

characteristics, and further, that the ratio of PM2.5 to PM10 varies spatially and temporally. 

In general, the commenter claimed that the EPA had offered no explanation in the 

 

232 Appendix N states that when the data substitution tests are satisfied, then the NAAQS 

design values derived from reported PM2.5 data which otherwise would be considered to 

be incomplete shall be considered valid for comparisons to the PM2.5 NAAQS.
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proposal preamble of why the use of PM10 data were valid for a PM2.5 data substitution 

test. The EPA believes that PM10 data are appropriate for a PM2.5 data substitution test. 

Because PM2.5 data are a subset of PM10 data, a PM2.5 24-hour average concentration 

should always be less than or equal to a collocated PM10 24-hour average concentration. 

Even though the ratio of PM2.5 to PM10 may vary both temporally and spatially, that ratio 

should never exceed one. Because of the unique situation where one criteria pollutant 

(i.e., PM2.5) is a theoretical subset of another pollutant (i.e., PM10), the EPA therefore 

believes it appropriate to incorporate a unique data substitution test that uses collocated 

PM10 data as a conservative upper bound estimate for the missing PM2.5 data. Therefore, 

the EPA believes it appropriate to incorporate a data substitution test that uses collocated 

PM10 data as a conservative upper bound estimate for missing PM2.5 data. The EPA 

further acknowledges that due to current inconsistencies in the reference and equivalent 

methods for the two related pollutants and also because of usual collocation siting 

characteristics, there may be occasional a situations whereby a 24-hour average PM2.5

concentration actually exceeds a collocated 24-hour average PM10 concentration. 

However, the EPA notes that this situation is infrequent; in the recent 3-year timeframe 

of 2009-2011, this situation only occurred in eight percent of the collocated PM2.5 and 

PM10 data pairsIn general, the commenter claimed that the EPA had offered no 

explanation of why PM10 data were valid for a PM2.5 data substitution test. At the time of 

proposal, the EPA believed that PM10 data would be appropriate for a PM2.5 data 

substitution test. After consideration of public comments and additional air quality 

analyses, the EPA has decided that a collocated PM10 test is largely redundant with the 

maximum quarterly value test and thus not necessary to include it in Appendix N. The 
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EPA has analyzed the most recent three years of PM2.5 and PM10 data (2009-2011) and 

assessed the separate benefit of the PM10 substitution routine compared to the maximum 

quarterly value test (Schmidt, 2012b). In this assessment of 2009-2011 PM2.5 design 

values which did not meet the nominal data completeness requirements, the EPA found 

that the collocated PM10 test was almost entirely redundant with the maximum quarterly 

value test. It was also very infrequently needed as a separate test. For the annual 

NAAQS, the maximum quarter value test in 100 cases resulted in a test design value 

(TDVmax) less than or equal to 12.0 μg/m3. There were only two additional cases (i.e. 2 

percent) when TDVmax was greater than 12.0 μg/m3but the TDV associated with the 

collocated PM10 test was less than 12.0 μg/m3. Similarly for the 24-hour NAAQS, the 

maximum quarter value test in 116 cases resulted in a test design value (TDVmax) less 

than or equal to 35 μg/m3and again only 2 additional sites (less than 2 percent) passed 

the collocated PM10 test but not the maximum quarterly value test. Furthermore, the 

maximum quarterly value tests allowed the annual and 24-hour design value to be 

validated approximately 5 times more often than through the use of the collocated PM10

test. Accordingly, the EPA has decided to not include the collocated PM10 data 

substitution tests in Appendix N, and thereby further simplify the data handling 

procedures for making comparisons to the annual and daily NAAQS.

With regard to identifying annual 98th percentile concentrations for comparison to 

the primary and secondary 24-hour PM2.5 standards, the EPA suggested in the proposal to 

simplify the procedures used with an approved seasonal sampling schedule. Specifically, 

the EPA proposed to eliminate the use of a special formula for calculating annual 98th

percentile concentrations with a seasonal sampling schedule and thereby proposed to use 

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only one method for calculating annual 98th percentile concentrations for all sites (77 FR

39002, June 29, 2012). 

The proposal explained that with an approved seasonal sampling schedule, a site 

is typically samples as required to sample during periods of the year when the highest 

concentrations are expected to occur, but less frequently during periods of the year when 

lower concentrations are expected to occur (77 FR 39002, June 29, 2012). This type of 

sampling schedule generally leads to an unbalanced data record; that is, a data record 

with proportionally more ambient measurements (with respect to the total number of days 

in the sampling period) in the “high” season and proportionally fewer ambient 

measurements in the “low” season. In the last review, the EPA revised section 4.5 of 

appendix N to include a special formula for computing annual 98th percentile values 

when a site operates on an approved seasonal sampling schedule. This special formula 

accounted for an unbalanced data record and was consistent with guidance 

documentation (U.S.US EPA, 1999), and, where appropriate, with official OAQPS 

design value calculations (71 FR 61211, October 17, 2006). In cases where there is a 

balanced233 (or near-balanced) data record, the special formula yields the same result as 

the regular procedure for calculating annual 98th percentile concentrations.

To qualify for a seasonal sampling schedule, monitoring agencies are required to

collocateco-locate a continuous PM2.5 instrument with the seasonal sampling FRM. Since 

the last review, there has been considerable deployment of continuous PM2.5 FEM 

monitors. In situations where a PM2.5 FRM monitor operating on a non-daily periodic 

schedule (such as a 1-day-in-3 or a 1-day-in-6 schedule) is collocated with a continuous 

 

233 A balanced data record has the same proportion of ambient measurements (with 

respect to the total number of days in the sampling period) in the “high” season as in the 

“low” season.

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PM2.5 FEM monitor, data are combined based on procedures stated in section 3.0 of 

appendix N as modified, as discussed in section VIVII.A.3 above. The end result of 

combining Combining collocated FRM and FEM data is effectively an every- dayresults 

in a site-based sampling frequency, hence resulting which samples everyday and results

in a balanced data record. In such a case, if a site used a seasonal sampling schedule 

regime for the FRM monitor, these data would be balanced by the every-day FEM data 

and there would be no need for the special formula for calculating annual 98th percentile 

concentrations on the combined site data. 

InAs EPA noted in the proposal, the EPA noted that there were at that timeare

very few PM2.5 FRM monitors that actually operated on an approved seasonal sampling 

schedule (only 15 sites out of approximately 1,000 total sites in 2010) and that for almost 

half of those sites had a, the collocated continuous instrument was a PM2.5 FEM monitor

(77 FR 39002, June 29, 2012). The proposal stated that for the 3-year period 2008 to 

2010, the annual 98th percentile concentrations calculated with the special formula at 

those 15 sites were approximately five percent lower than if the regular procedure was 

used. The EPA also noted in the proposal that, in the last review, the Agency modified 

the monitoring requirements for areas with an FRM operating on a non-daily schedule 

such that, ifwhen the design values were within five percent of the 24-hour PM2.5

NAAQS, those areas would be required to increase the frequency of sampling to every 

day (40 CFR 58.12(d)(1); 71 FR 61165, October 17, 2006; 71 FR 61249, October 17, 

2006). The In consideration of these facts, the EPA proposed to simplify the data 

handling procedures for sites operating on a seasonal sampling schedule by eliminating 

the special formula and all references to it for the following reasons: (1) the small 
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difference between 98th percentile concentrations calculated using the special formula 

versus the regular procedure and the small number of sites currently using the special 

formula; (2) the EPA requires every day sampling in areas with design values that are 

within five percent of the 24-hour PM2.5 NAAQS; and (3) FRMs operating on an 

approved seasonal sampling schedule are required to be collocated with a continuous 

PM2.5 instrument (and if that instrument were an FEM, the resulting combined site record 

would tend to be balanced over the year and thus the special formula would be 

superfluous.) (77 FR 39002, June 29, 2012). Thus, the EPA proposal included only one 

method for calculating annual 98th percentile concentrations, the “regular” table look-up 

method specified in section 4.5(a)(1) of appendix N. 

In light of the rationale provided above and because there wereEPA received no 

significant negative comments received againstregarding the EPA proposal, the EPA has 

eliminated concludes it is appropriate to eliminate the special seasonal sampling 98th

percentile identification procedure from appendix N. The final revised appendix N 

specifies only one method for identifying annual 98th percentile concentrations; the table 

look-up method is now the only permitted technique for identifying annual 98th percentile 

concentrations.

B. Exceptional Events

The EPA is finalizing primary annual PM2.5-specific deadlines in 40 CFR 50.14 

by which air agencies234 must flag ambient air quality data that they believe have been 

affected by exceptional events and submit initial descriptions of those events. The EPA is 

also finalizing the deadlines by which air agencies must submit detailed exceptional 

 

234 References to “air agencies” are meant to include state, local, and tribal air agencies 

responsible for implementing the Exceptional Events Rule.

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events documentation to support the exclusion of those data from the EPA’s monitoringbased determinations of attainment or nonattainment with the revised primary annual 

PM2.5 NAAQS. The final exceptional events-related schedule is aligned with the 

designations schedule, discussed in greater detail in section IX, and is promulgated as 

proposed and as supported by multiple commenters. Without revisions to 40 CFR 50.14, 

an air agency may not be able to flag and submit documentation for some relevant data 

either because the generic deadlines may have already passed by the time the new or 

revised NAAQS is promulgated or because the generic deadlines require documentation 

submission at least 12 months prior to the date that the EPA must make a regulatory 

decision.

The EPA acknowledges the concern raised by a few commenters that numerous 

wildfires occurred between 2010 and 2012 that air agencies may determine influenced 

ambient air quality concentrations potentially affecting compliance with the revised 

primary annual PM2.5 NAAQS, and that air agencies may want to submit detailed 

exceptional events documentation associated with thesemultiple wildfires. Commenters 

further noted that 1 year to provide documentation of these potential exceptional events 

may not be sufficient. The EPA believes that the promulgated schedules provide 

sufficient time for air agencies to submit information related to the annual standard and 

for the EPA to fully consider and act on the submitted information during the initial area 

designation process. RecentlyThe EPA recently released draft exceptional events 

guidance that clarifies key provisions of the 2007 Exceptional Events Rule, provides 

examples of best practices, and streamlines the documentation development process, will 

help air agencies submit information in a timely manner.. The guidance provides 
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approaches that are broadly applicable to all event/pollutant combinations and would 

apply to many PM events, including wildfire/PM combinations. Additionally, the EPA 

has posted several concurred upon wildfire/PM exceptional event demonstration 

packages on its website at: http://www.epa.gov/ttn/analysis/exevents.htm. Considered 

together, the EPA believes this guidance will help air agencies submit information in a 

timely manner.235 The EPA notes that under the promulgated schedule, except for events 

that occur in December 2012, air agencies will have more than 1 year to provide 

documentation for these potential events. The EPA intends to work with potentially 

affected areas to identify, screen, and prioritize events potentially influencing compliance 

with the primary annual PM2.5 NAAQS and associated area designations. 

Also in response to comments, the EPA is clarifying that this preamble language 

and the associated promulgated exceptional events schedules apply only to the NAAQS 

that the EPA is newly promulgating or revising in this action, that is, the revised primary 

annual PM2.5 NAAQS. The promulgated exceptional event schedule revisions do not 

apply to the retained PM standards (i.e., secondary PM standards, primary 24-hour PM10, 

primary 24-hour PM2.5). Further, the revised/extended exceptional event schedules apply 

only to those data the EPA will use to establish initial area designations for the revised 

primary annual PM2.5 NAAQS.

The “Treatment of Data Influenced by Exceptional Events; Final Rule” (72 FR 

13560, March 22, 2007), known as the Exceptional Events Rule and codified at 40 CFR 

50.14, contains generic deadlines for an air agency to submit to the EPA specified 

information about exceptional events and associated air pollutant concentration data. As 

 

235 The EPA released draft exceptional events guidance documents (U.S. EPA, 2012e) for 

public comment via a Notice of Availability in the Federal Register on July 6, 2012 (77 

FR 39959). 
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discussed in the proposal, without revisions to 40 CFR 50.14, an air agency may not be 

able to flag and submit documentation for some relevant data because the generic 

deadlines may have already passed by the time the new or revised NAAQS is 

promulgated. Similarly, revisions to 40 CFR 50.14 are needed because air agencies may 

not be able to flag and submit documentation for events that occurred in December of 

2013 by 1 year before the designations are made in 2014 as is required by the existing 

generic schedule requires. 

To support appropriate consideration of exceptional event data influencing 

ambient air quality concentrations potentially affecting compliance with the revised 

primary annual PM2.5 NAAQS, the EPA is adopting revisions to 40 CFR 50.14 to change 

the submission dates for claimed exceptional events information affecting PM2.5 data 

considered during the initial area designations process under the promulgated revised 

primary annual PM2.5 NAAQS. As proposed, for air quality data collected in 2010 or 

2011, the EPA is extending to July 1, 2013 the otherwise applicable generic deadlines of 

July 1, 2011 and July 1, 2012, respectively, for flagging data and providing an initial 

description of an event (40 CFR 50.14(c)(2)(iii)). The EPA is retaining the existing 

generic deadline in the Exceptional Events Rule of July 1, 2013 for flagging data and 

providing an initial description of events occurring in 2012. Similarly, the EPA is 

revising to December 12, 2013, the deadline for submitting documentation to justify 

exceptional events occurring in 2010 through 2012 and potentially influencing

compliance with the revised primary annual PM2.5 NAAQS. The EPA believes these 

revisions/extensions will provide adequate time for air agencies to review potential PM2.5

exceptional events influencing compliance with the revised primary annual PM2.5
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NAAQS from 2010 to 2012, to notify the EPA by flagging the relevant data and 

providing an initial description in AQS, and to submit documentation to support claims 

for exceptional events. These schedule revisions will also allow the EPA to fully consider 

and act on the submitted information during the initial area designation process.

If an air agency intends the EPA to consider in the revised primary annual PM2.5

designations decisions whether PM2.5 data collected during 2013 influence compliance 

with the primary annual PM2.5 NAAQS, then the air agency must flag these data by the 

generic Exceptional Event Rule deadline of July 1, 2014. The EPA is finalizing August 1, 

2014, as the deadline for submitting documentation to justify PM2.5-related exceptional 

events occurring in 2013 and potentially influencing compliance with the revised primary 

annual PM2.5 NAAQS. The EPA believes that these deadlines provide air agencies with 

adequate time to review and identify potential exceptional events that occur in calendar 

year 2013 and for the EPA to fully consider and act on the submitted information during 

the initial area designation process.

While the EPA will make every effort to designate areas for the primary annual 

PM2.5 NAAQS on a 2 year schedule, the EPA recognizes that it may need up to an 

additional year for the designations process to ensure that states/tribes and the EPA base 

designations decisions on complete and sufficient information. If the EPA announces at a 

later date that it is extending the designations schedule beyond 2 years based on 

unavailability of data, the EPA will consider extending the 2013 exceptional event 

documentation submission schedule by promulgating additional revisions to 40 CFR 

50.14. 
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Therefore, using the authority provided in CAA section 319(b)(2) and in the 

Exceptional Events Rule at 40 CFR 50.14 (c)(2)(vi), the EPA is finalizing the schedule 

for data flagging and submission of demonstrations for PM2.5 exceptional events data 

potentially influencing compliance with the revised primary annual PM2.5 NAAQS

considered for initial area designations under the promulgated primary annual PM2.5

NAAQS as presented in Table 3. 
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Table 3. Revised Schedule for Exceptional Event Flagging and Documentation 

Submission for Data to be Used in Initial Area Designations for the 2012 PM2.5

NAAQS

NAAQS Pollutant/

Standard/(Level)/

Promulgation Date

Air Quality 

Data Collected 

for Calendar 

Year

Event Flagging & 

Initial Description 

Deadline

Detailed 

Documentation 

Submission Deadline

PM2.5/

Primary Annual 

Standard

(12.0 μg/m3) 

Promulgated 

December 14, 2012

2010 and 2011 July 1, 2013 December 12, 2013 

2012 July 1, 2013a December 12, 2013

2013 July 1, 2014a August 1, 2014

a

This date is the same as the general schedule in 40 CFR 50.14.

Note: The table of revised deadlines only applies to data the EPA will use to establish the 

initial area designations for the revised primary annual PM2.5 NAAQS. The general 

schedule applies for all other purposes, most notably, for data used by the EPA for 

redesignations to attainment.

C. Updates for Data Handling Procedures for Reporting the Air Quality Index

There were no comments regarding the proposed updates for data handling 

procedures for reporting the AQI. However, two table footnotes that were part of the 

existing rule were inadvertently omitted from the proposal. The inadvertently dropped 

footnotes were footnotes 3 and 4 of Table 2 (“Breakpoints for the AQI”) of appendix G 

(“Uniform Air Quality Index (AQI) and Daily Reporting”) to Part 58. Since the footnotes 

are still applicable, the EPA has included them in the final rule. The final rule also 

codifies all changes identified in the EPA proposal regarding data handling procedures 

for the AQI.

VIII. Amendments to Ambient Monitoring and Reporting Requirements

The EPA is finalizing a number of changes to the ambient air monitoring, 

reporting, and network design requirements associated with the PM NAAQS. Ambient 

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PM monitoring data are used to meet a variety of monitoring objectives including 

determining whether an area is in violation of the PM NAAQS. Ambient PM monitoring 

data are collected by state, local, and tribal monitoring agencies (“monitoring agencies”) 

in accordance with the monitoring requirements contained in 40 CFR parts 50, 53, and 

58. This section discusses the monitoring changes that the EPA is finalizing to support 

the revised PM NAAQS summarized in sections III.F, IV.F, and VI.F above.

The monitoring changes being finalized primarily relate to the revised primary 

PM2.5 NAAQS. Several monitoring changes were proposed specifically in support of a 

potential distinct secondary PM2.5 visibility index standard; however, as explained in 

Section VI, EPA is not finalizing a distinct secondary NAAQSstandard using a visibility 

index and therefore is not finalizing the monitoring changes that would have been 

necessary to support it. The EPA did not propose any monitoring changes associated with 

the PM10 NAAQS and accordingly none are finalizedis not adopting any in this final rule.

A. Issues Related to 40 CFR Part 53 (Reference and Equivalent Methods)

To be used in a determination of compliance with the PM NAAQS, PM data are 

typically collected using samplers or monitors employing an FRM or FEM. The EPA also 

allows use of alternative methods where explicitly stated in the monitoring methodology 

requirements (appendix C of 40 CFR part 58), such as PM2.5 ARMs which can be used to 

determine compliance with the NAAQS. The EPA prescribes testing and approval criteria 

for FRM and FEM methods in 40 CFR part 53.

1. PM2.5 and PM10-2.5 Federal Equivalent Methods 

As described in the proposal, the EPA continues to believe that an effective PM2.5

monitoring strategy includes the use of both filter-based FRM samplers and wellperforming continuous PM2.5 monitors. Well-performing continuous PM2.5 monitors 

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would include both non-designated continuous PM2.5 monitors and designated Class III236

continuous FEMs that meet the performance criteria described in table C-4 of 40 CFR 

part 53 when comparing to a collocated FRM operated by the monitoring agency. Only 

designated methods (i.e., FRMs, FEMs, and ARMs) are approved to be used in 

comparison to the NAAQS; however, non-designated methods may be useful to meet 

other monitoring objectives (e.g., reporting the AQI). The use of Class III continuous 

FEMs at SLAMS is described in more detail in section VIII.B.3.b.ii below. Monitoring 

agencies are encouraged to evaluate the quality of data being generated by FEMs and, 

where appropriate, to reduce the use of manual, filter-based samplers to improve 

operational efficiency and to lower overall operating costs. To encourage such a strategy, 

the EPA is working with numerous stakeholders including the monitoring committee of 

NACAA, instrument manufacturers, and monitoring agencies to support national data 

analyses of continuous PM2.5 FEM performance, and where such performance does not 

meet data quality objectives, to develop and institute a program of best practices to 

improve the quality and consistency of resulting data.

The EPA believes that progress is being made to implement well performing 

PM2.5 continuous FEMs across the nation. As noted in the proposal, the first few steps 

involved the EPA developing and approving the testing and performance criteria which 

were finalized in 2006, followed by instrument companies performing field testing and 

submitting applications to the EPA, and the EPA review and approval, as appropriate, of 

Class III FEMs. In the current step, monitoring agencies are testing and assessing the data 

 

236 Class III refers to those methods for PM2.5 or PM10-2.5 that are employed to provide 

PM2.5 or PM10-2.5 ambient air measurements representative of one-hour or less integrated 

PM2.5 or PM10-2.5 concentrations, as well as 24-hour measurements determined as, or 

equivalent to, the mean of 24 one-hour consecutive measurements.
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comparability from continuous PM2.5 FEMs.

While EPA did not propose any changes to the performance or testing criteria in 

40 CFR part 53 used to approve PM2.5 continuous FEMs, the EPA did propose an 

administrative change to part 53.9 – “Conditions of designations.” See 77 FR 39006. 

This section describes a number of conditions that must be met by a manufacturer as a 

condition of maintaining designation of an FRM or FEM. Subsection (c) of this section 

reads, “Any analyzer, PM10 sampler, PM2.5 sampler, or PM10-2.5 sampler offered for sale 

as part of a FRM or FEM shall function within the limits of the performance 

specifications referred to in 40 CFR 53.20(a), 53.30(a), 53.50, or 53.60, as applicable, for 

at least 1 year after delivery and acceptance when maintained and operated in accordance 

with the manual referred to in 40 CFR 53.4(b)(3).” The EPA’s intent in this requirement 

is to ensure that monitoring methods work within performance criteria, which includes 

methods for PM2.5 and PM10-2.5; however, there was no specific reference to performance 

criteria for Class II237 and III PM2.5 and PM10-2.5 methods. The EPA proposed to link the 

performance criteria referred to in 40 CFR part 53.35 associated with Class II and III 

PM2.5 and PM10-2.5 methods with this requirement for maintaining designation of 

approved FEMs. The specific performance criteria identified in 40 CFR 53.35 for PM2.5

and PM10-2.5 methods are available in table C-4 to subpart C of 40 CFR part 53.

All comments received on this proposed change were supportive and EPA is 

finalizing this change. The implication of this change is that instrument manufacturers 

and air agencies operating the equipment will have a shared responsibility for approved 

 

237 Class II refers to those methods for PM2.5 or PM10-2.5 in which integrated samples are 

taken by filtration and subjected to a subsequent filter conditioning process followed by a 

gravimetric mass determination, but which is not a Class I equivalent methods because of 

substantial deviations from the design specification of the sampler specified for reference 

methods in appendix L or O (as applicable) of part 50 of the CFR. 
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FEMs to meet required performance criteria for at least the first 12 months of operation, 

which is the typical warranty period for an instrument. By having a shared responsibility 

for an FEM to meet the performance criteria, instrument companies and air agencies will 

both be motivated to ensure the best practices for installing, operating, and servicing an 

instrument are carried out according to the instrument company’s operating manual and 

other readily available materials238 in support of each method.

2. Use of Chemical Speciation Network (CSN) Methods to Support the Proposed 

New Secondary PM2.5 Visibility Index NAAQS

The EPA had proposed to use CSN methods to support the proposed new 

secondary PM2.5 visibility index NAAQS; however, as explained in Section VI of this 

final rule, EPA is not finalizing the new secondary PM2.5 visibility index NAAQS and 

therefore has no need to finalize the CSN methods to support such a standard.

Despite our decision not to finalize formal requirements for CSN methods, this 

network remains a critical component in our PM monitoring program. The EPA, 

monitoring agencies, and external scientists and policy makers use PM2.5 data from the 

CSN to support several important monitoring objectives such as: development of 

modeling tools and the application of source apportionment modeling for control strategy 

development to implement the NAAQS; health effects and exposure research studies; 

assessment of the effectiveness of emission reductions strategies through the 

characterization of air quality; and development of SIPs. The use of the CSN to support 

all of these objectives will continue.

B. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)

 

238 At the recent National Air Quality Conference in May of 2012, a training session on 

“Best Practices for Operating PM2.5 Continuous FEMs” was conducted. Presentations 

from this session are publically available on EPA’s web site at: 

http://www.epa.gov/ttn/amtic/2012present.html.
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1. Terminology Changes

The EPA proposed to revise several terms associated with PM2.5 monitor 

placement to ensure consistency with other NAAQS and to conform with long-standing 

practices in siting of equipment by monitoring agencies. (77 FR 39007).

The EPA proposed to revoke the term “community-oriented” and replace it with 

the term “area-wide.” The term “community-oriented,” while used within the description 

of the design criteria for PM2.5, is not defined and has not been used in the design criteria 

for other NAAQS pollutants. Appendix D to 40 CFR part 58 presents a functional usage 

of the term where sites at the neighborhood and urban scale area are considered to be 

“community-oriented.” In addition, population-oriented, micro-or middle-scale PM2.5

monitoring may also be considered “community-oriented” when determined by the 

Regional Administrator to represent many such locations throughout a metropolitan area. 

The EPA proposed to replace this usage of “community-oriented” with the term “areawide” in the text of the PM2.5 network design criteria and to define it in 40 CFR 58.1 to 

provide a more consistent usage of this concept throughout appendix D of 40 CFR part 

58. The Specifically, the EPA proposed that the terminology would read – “Area-wide 

means all monitors sited at neighborhood, urban, and regional scales, as well as those 

monitors sited at either micro-or middle-scale that are representative of many such 

locations in the same CBSA.”

The EPA proposed to revoke the term “Community Monitoring Zone” (CMZ) and 

to remove references to it in 40 CFR part 58. Community monitoring zone is currently 

defined as “an optional averaging area with established, well defined boundaries, such as 
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county or census block, within an MPA239 that has relatively uniform concentrations of 

annual PM2.5 as defined by appendix N of 40 CFR part 50 of this chapter. Two or more 

community oriented state and local air monitoring stations (SLAMS) monitors within a 

CMZ that meet certain requirements as set forth in appendix N of 40 CFR part 50 may be 

averaged for making comparisons to the annual PM2.5 NAAQS.” The EPA proposed to 

revoke this term and references to it since, as discussed in section VII.A.2 above, the 

EPA proposed to eliminate all references to the now-revoked spatial averaging option

throughout appendix N.

Comments received on these terminologyThe one comment directly addressing 

the proposed rule changes were supportive (from the onea state air agency that 

commented on it. ) supported the proposal. A few industry commenters noted the change 

in the context of how monitoring data are used to compare to the NAAQS, but did not 

provide either supportive or negative comments on these twoaddress the proposed

specific terminology changes. However, as explained in section III.E.3.a, several industry 

commenters did provide comments critical of EPA’s proposal to revoke spatial averaging 

which is related to revoking the term “Community Monitoring Zone”. 

For the reasons explained above, the EPA is finalizing its proposed change to 

revoke the term “community-oriented” and to replace it with the term “area-wide.” The

EPA is also finalizing its proposal to revoke the term “Community Monitoring Zone” 

(CMZ) and references to it in 40 CFR part 58. 

2. Special Considerations for Comparability of PM2.5 Ambient Air Monitoring Data 

to the NAAQS

 

239 Monitoring Planning Area (MPA) means a contiguous geographic area with well 

established, well defined boundaries, such as a CBSA, county or State, having a common 

areas that is used for planning monitoring locations for PM2.5.
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In general, ambient monitors must meet a basic set of requirements before the 

resulting data can be used for comparison to the NAAQS. These requirements include 

the presence and implementation of an approved quality assurance project plan; the use 

of methods that are reference, equivalent, or other approved method as described in 

appendix C to 40 CFR part 58; and compliance with the probe and siting path criteria as 

described in appendix E to 40 CFR part 58. While these 40 CFR part 58 requirements 

apply to any monitor that provides data for comparison to the NAAQS, only in the PM2.5

monitoring requirementsthere are certain additional restrictions prescribed within thethat 

apply only to PM2.5 monitoring rules.

240 These additional restrictions provide that sites 

must be “population-oriented” for comparison to either the 24-hour or annual NAAQS, 

and specifically for comparison to the annual NAAQS, sites must additionally be sited to 

represent area-wide locations. There is a related provision that provides for comparing 

sites at micro- or middle-scales to the annual PM2.5 NAAQS when the site is determined 

by the Regional Administrator to represent a larger region of localized high ambient 

PM2.5 concentration. 

The inclusion of theseThese provisions have been in the PM2.5 monitoring 

requirementsregulations since the 1997 promulgationinception of the PM2.5 NAAQS . 

Nonetheless, these provisions and associated the fact that such monitoring requirements 

has resultedare not found in confusion whenthe requirements for all other criteria 

pollutants have created areas of uncertainty for the EPA and state, local, and tribal 

agencies consider the design of PM2.5that base implementation decisions on monitoring 

networksrequirements through programs such as well as how unmonitored locations 

 

240 These are referencedfound in 40 CFR 58.30 (Special considerations for data 

comparisons to the NAAQS). 

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should be treated in dispersion modeling exercises such as PSD, SIP planning, and the 

calculation of transportation conformity budgets. For example, in developing modeling 

guidance to support near-road transportation conformity modeling, the EPA struggled to 

determine how the identification of acceptable PM2.5 receptor locations can be reconciled 

with the PM2.5 monitoring regulations that reference potentially acceptable (or 

unacceptable) monitoring locations that may, or may not, be considered unique for 

purposes of comparing to the annual PM2.5 NAAQS. Accordingly, the EPA proposed to 

revise these particular PM2.5 requirements for consistency with long-standing practices in 

all other NAAQS pollutant monitoring networks, and to ensure that interpretation of the 

monitoring rules does not cause ambiguity in consideringimplementation examples that 

also include the treatment of unmonitored areas. (see 77 FR 39007-009). Each of these 

topics is described below.

a. Revoking Use of Population-Oriented as a Condition for Comparability of PM2.5

Monitoring Sites to the NAAQS

a. Eliminating the term “population oriented” from section 58.30

The EPA proposed to revoke theremove the term “population oriented” from 

section 58.30 so that there would no longer be an explicit requirement that PM2.5

monitoring sites be “population-oriented” for comparison to the PM2.5 NAAQS. The EPA 

concludednoted that this requirement is inconsistentnot entirely consistent with the 

definition of “ambient” used in the NAAQS. The EPA’s definition of ambient air is 

specified in 40 CFR 50.1 – “Ambient air means that portion of the atmosphere, external 

to buildings, to which the general public has access.” The EPA’s definition of 

“population-oriented” is provided in 40 CFR 58.1 - “Population-oriented monitoring (or 
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sites) means residential areas, commercial areas, recreational areas, industrial areas where 

workers from more than one company are located, and other areas where a substantial 

number of people may spend a significant fraction of their day.” The EPA’s intention in 

proposing to revoke the requirement that PM2.5 monitoring sites be “population-oriented” 

for comparison to the The NAAQS was to ensure that the monitoring rules do not have a 

different definition than the definition of are standards for concentrations “in the ambient 

air in 40 CFR 50.1 itself. We note that revoking this term in no way changes the 

requirements in the PM2.5 network design criteria, ”

241—i.e., air to which will continue to 

focus on sites representing “area-wide” locations; thus continuing to represent locations 

members of the public could be exposed— and all monitors used for NAAQS regulatory 

purposes must be representative of ambient air concentrations.242 Consistent with this 

requirement and the long-standing practice of monitoring agencies locating ambient 

monitors, the EPA’s experience is that PM2.5 monitors are placed in areas that are 

representative of population exposures. There are no PM2.5 monitors currently operating 

as “non-population exposure. While the use of the term “population-oriented” has little 

effect on how data from existing sites are treated (as explained below there are no 

remaining sites designated as not being “population-oriented”), the inclusion of this 

oriented” and the EPA does not believe that the requirement in the monitoring rules for 

near-road monitoring (discussed in detail further below) will result in monitors that are 

not representative of population exposures. At the same time, the specification that 

certain PM2.5 monitors must be “population-oriented” in the rules has created substantial 

confusion in how to treat potential locations of exposure for NAAQS-related regulatory 

 

241 See 40 CFR part 50.

242 See, e.g., 40 CFR 58.1 (defining “federal reference method” as “a method for 

sampling and analyzing the ambient air for an air pollutant...”)
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requirements other than monitoring network design, such as in applying modeling 

acrossas part of a PSD or SIP exercise.

243

 The EPA’s intention in proposing to remove the term “population oriented” from

section 58.30 was to remove a potential source of inconsistency in the monitoring rules as 

they apply for all the NAAQS. As noted earlier, the NAAQS provide protection for the 

public health and welfare in areas where the public can be exposed. For all other criteria 

pollutants, the monitoring requirements have no such restriction on the comparability of a 

monitor. In the case of PM2.5 however, the additional restriction of monitors being 

required to be “population-oriented” for comparability to the NAAQS has existed. The 

term “population oriented” has lacked a quantitative definition (e.g., the interpretation of 

“substantial number” in the definition of “population-oriented”), therefore monitoring 

agencies and those stakeholders who based implementation strategies and decisions on 

monitoring regulations have been uncertain about which locations would meet 

requirements described in §58.30, which do not exist for any other NAAQS. Monitoring 

agencies are also not in a position to precisely forecast where future residential, 

commercial, or recreational development may occur, therefore requiring that PM2.5

monitors that are to be compared to the NAAQS can only be located where “substantial 

numbers of people” live, work, or play (i.e., in the present tense) represents an 

area.unwise limitation on the flexibility of monitoring agencies to revise their PM2.5

networks to account for anticipated changes in demographics or development as well as a 

contradiction with the inherent applicability of the NAAQS in ambient air locations 

where the public has access (e.g., in any location outside the perimeter of a industrial 

 

243 Examples include dispersion modeling to support NAAQS attainment planning, 

associated SIP development, and the calculation of transportation conformity budgets.
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facility). From an operational standpoint, we note that revoking this term would not

change the requirements in the PM2.5 network design criteria. To the extent that the 

phrase “population-oriented” served to emphasize the need for micro- or middle-scale 

monitors to be representative of locations with population exposure to be comparable to 

the annual NAAQS, the definition of ambient air, together with the requirement in 

revised section 58.30 that such sites must be “area-wide” to be comparable to the annual 

NAAQS, adequately serves the same purpose. By revising the PM2.5 monitoring rules to

ensure consistency with the long-standing definition of ambient air applied to the other 

NAAQS pollutants, the EPA will be able to more clearly define how to treat potential 

exposure receptors for other NAAQS regulatory requirements, regardless of whether 

monitoring exists or not.

In reviewing the impact that this change might have on the nation’s PM2.5

monitoring network, the EPA notes that there are no remaining sites operating 

affirmatively as “non population-oriented.” The last known non population-oriented site 

at Sun Metro in El Paso Texas (AQS ID: 48-141-0053), was shut down in October 2010 

and is in the process of being moved to a nearby neighborhood. 

There were two commenters from air agencies on the issue of revoking population 

oriented as a condition for comparability of PM2.5 monitoring sites to the NAAQS. One 

was fully supportive of the proposed change and the other stated outPublic comments on 

this issue were supported by air agencies and public health and environmental groups. 

Two commenters from state agencies supported the proposed change, with one noting 

further that regardless of a change it is still the air agency’s responsibility to plan a 

network with sites that are appropriate for comparison to the NAAQS.
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 Most industry commenters did not support revoking “population-oriented” as a 

condition for comparison to the NAAQS. Most comments raised concerns with using 

data from an area where potentially no one is exposed.

Several public health and environmental groups supported revoking “population 

oriented” as a condition for comparability of PM2.5 monitoring sites to the NAAQS

stating that retaining such a policy is inconsistent with the text, purpose and intent of the 

Clean Air Act. Most industry commenters did not support revoking “populationoriented” as a condition for comparison to the NAAQS. Most of these comments raised

concerns with using data from an area where potentially no one is exposed.

In considering the publicthese comments on this aspect of the proposed rule,, the 

EPA agrees that it is appropriate for individual air agencies to provide a recommendation 

in the annual monitoring network plan regarding whether any site may or may not be 

appropriate for comparison to athe PM2.5 (or any) NAAQS. The roles of the air agency 

and the EPA in this process of identifying whether a site is, or is not, consistent with the 

network plan requirements for a NAAQS are specified in the previously alreadyestablished monitoring requirements of §58.10. In this approval process, the air agency 

initiates the recommendations and the EPA has the responsibility to approve, as 

appropriate, any plans that provide for changes to the network. 

Regarding comments from industry commenters that monitors may be sited in 

areas that do not represent exposure of the public to PM2.5, if we revoke the requirement 

for sites to be population oriented to be compared to the NAAQS, the EPA maintains that 

all existing sites are already representative of population exposure. The EPA also asserts 

that any monitor an air agency deploys meeting our definition of ambient air will be 
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located where the public is (or may be) exposed. 

The EPA notes the supportive comments from the one air agency and several 

public health and environmental groups on revoking population oriented as a condition 

for comparability of PM2.5 monitoring sites to the NAAQS.

EPA disagrees with the industry comments. As noted above, monitors (including 

those for PM2.5) must already meet the test of being representative of ambient air to be 

compared to the NAAQS, and thus such monitors meeting this test will be sited in 

locations where people are already located, or where they could be exposed, whether or 

not the term “population oriented” appears in section 58.30. Moreover, as discussed 

below, comparisons to the annual PM2.5 NAAQS can be only be from monitors “that are 

representative of area-wide air quality.” “Area-wide” monitors are those at the 

neighborhood scale or larger, or at smaller scales if they are representative of many such 

locations in the same CBSA. The EPA anticipates that a monitor that is sited as 

representative of ambient air at the neighborhood scale or larger (or of ambient air at 

many smaller areas) will be representative of population exposure. This conclusion is 

further supported by the fact that all current monitors used for comparison with the PM2.5

NAAQS are designated as “population-oriented.”244 

After consideration of the public comments, the EPA is finalizing its decision to 

revoke use of “population-oriented” as a condition for comparability of PM2.5 monitoring 

sites to the NAAQS. The EPA is making this decision to provide for more consistency in 

the monitoring rules, to ensure the rules for comparability of PM2.5 monitoring sites to the 

NAAQS are consistent with the definition of ambient air, and to remove an unnecessary 

 

244 The last known non population-oriented site at Sun Metro in El Paso Texas (AQS ID: 

48-141-0053), was shut down in October 2010 and is in the process of being moved to a 

nearby neighborhood.
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requirement that was causing confusion in the monitoring regulations The EPA concludes 

that the “population-oriented” language is unnecessary and inconsistent with other 

monitoring rules, and should therefore be removed.

b. Applicability of Micro- and Middle-scale Monitoring Sites to the Annual PM2.5

NAAQS

The EPA proposed clarifying language usedin 40 CFR section 58.30 to 

determineclarify when data from PM2.5 monitoring sites at micro- and middle-scale 

locations are comparablecan be compared to the annual PM2.5 NAAQS. The EPA’s 

intent in clarifying this language iswas to provide consistency and predictability in the 

interpretation of the monitoring regulations which will minimize the burden on state 

monitoring programs as they plan and implement their monitoring programs. he. The

EPA’s current rules, as specified in 40 CFR 58.30, state that “PM2.5 data that are 

representative, not of area-wide but rather, of relatively unique population-oriented 

micro-scale, or localized hot spot, or unique population-oriented middle-scale impact 

sites are only eligible for comparison to the 24-hour PM2.5 NAAQS. For example, if the 

PM2.5 monitoring site is adjacent to a unique dominating local PM2.5 source or can be 

shown to have average 24-hour concentrations representative of a smaller than 

neighborhood spatial scale, then data from a monitor at the site would only be eligible for 

comparison to the 24-hour PM2.5 NAAQS.” The EPA isWe proposed clarifying language 

to explicitly state that measuring PM2.5 in micro- and middle-scale environments near 

emissions of mobile sources, such as a highway, does not constitute being impacted by a 

“unique” source. Mobile and so could be compared to the annual PM2.5 NAAQS. We 

explained that mobile sources are rather ubiquitous and there are many locations 

throughout an urban area where elevated exposures attributable to such sources could 
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occur. Therefore, we proposed that in most cases the potential location for a PM2.5

monitoring site, including micro- and middle-scale sites near roadways, would be eligible 

for comparison to the annual NAAQS. The EPA’s existing definition of middle-scale for 

PM2.5, as specified in appendix D to 40 CFR part 58, already states, “(2) Middle scale—

People moving through downtown areas, or living near major roadways, encounter 

particle concentrations that would be adequately characterized by this spatial scale. Thus, 

measurements of this type would be appropriate for the evaluation of possible short-term 

exposure public health effects of particulate matter pollution. In many situations, 

monitoring sites that are representative of micro- or middle-scale impacts are not unique 

and are representative of many similar situations. This can occur along traffic corridors or 

other locations in a residential district. In this case, one location is representative of a 

number of small scale sites and is appropriate for evaluation of long-term or chronic 

effects. This scale also includes the characteristic concentrations for other areas with 

dimensions of a few hundred meters such as the parking lot and feeder streets associated 

with shopping centers, stadia, and office buildings.” With the reference to “traffic 

corridors” and related text, the EPA emphasizes that this type of location, which is 

referred to as near-road, should not be considered “unique.” We further noted that the 

existing definition of “middle scale” in appendix D to part 58 already indicates that traffic 

corridors can be middle scale, and hence not unique, and therefore comparable to the 

annual PM2.5 NAAQS (as well as to the 24-hour PM2.5 NAAQS) (77 FR 39008). 

The EPA and monitoring agencies already have a process for approving PM2.5

monitoring sites as described in the Annual Monitoring Network Plan due to the 

applicable EPA Regional Office by July 1 of each year (described in 40 CFR 58.10). 
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This existing process provides for identification of sites that are suitable and sites that are 

not suitable for comparison against the annual PM2.5 NAAQS (§58.10 (b)(7)). This 

clarifying language will provide consistency between the PM2.5 design criteria described 

in appendix D to 40 CFR part 58 and the example provided in the special considerations 

for data comparisons to the NAAQS network design (§58.30). This clarifying language 

will help to ensure a more consistent identification and approval of sites, and therefore 

provide a reduction in burden to monitoring agencies and EPA as annual monitoring 

network plans are prepared, reviewed, public comments are considered, plans are 

approved and implemented, and data are ultimately used. 

Air agencies that commented on this aspectpart of the proposed rule offered a 

variety of positions. One air agency stated that sites at these smaller scales should not be 

compared to the annual NAAQS. Another air agency stated that these sites should be 

considered for comparison with the annual PM2.5 NAAQS only when the air agency 

initiates a decision that such sites at these smaller scales are area-wide, should they be 

compared to the annual NAAQS. Alternatively, a. A different air agency offered that all 

micro- and middle-scale sites should be compared to the annual NAAQS since the 

wording of the provision is problematic and will be difficult for agencies to implement.

Industry commenters who offered comments on this aspect of the rule were 

largely against finalizing such a provision. The major concern raised was that from their 

perspective such a provision combined with other related provisions represented an 

unwarranted tightening of the NAAQS. Some industry commenters pointed out that 

there are examples of unique locations in near road environments and as such EPA 

should not presume that PM2.5 monitors in these locations should be applicable to the 
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annual PM2.5 NAAQS.

In considering comments on this part of the rule, and as described in the proposal, 

EPA notes that wethere are ensuring consistency in our applicabilityalready examples of 

treatingwhere the States and EPA have determined certain micro- and middle-scale 

monitoring sites which will reduce the ambiguity of the monitoring regulations. This 

consistency is accomplished by ensuring locations as applicable to the annual NAAQS 

and others where they were determined as not applicable to the regulatory language cited 

inannual PM2.5 NAAQS. These cases exist where a State proposed and the section Special 

Considerations for Data Comparisons to the NAAQS Network Design (§58.30) is written 

in a mannerRegional Administrator determined that is consistent with the longstanding 

definitions for either the micro- andscale or middle-scale insite did or did not represent 

many similar areas in a CBSA (40 CFR 58.30 and section 4.7 to Appendix D to Part, part

58.). The EPA also notes that the existing descriptions of the types of micro- and middlescale sites which are unique and cited in §58.30 will remain in placeare not being 

amended and that data from these types of sites would remain as not comparable to the 

annual PM2.5 NAAQS. Accordingly, PM2.5 data that are representative, not of area-wide 

but rather, of relatively unique population-oriented microscale, or localized hot spots, or 

unique middle scale impact sites will only be eligible for comparison to the 24-hour 

NAAQS.

The EPA notes the supportive comments from several public health and 

environmental groups on the applicability of area-wide Our proposal was to clarify

language to explicitly state that measuring PM2.5 in micro- and middle-scale monitoring 

sitesenvironments near emissions of mobile sources, such as a highway, does not 
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constitute being impacted by a “unique” source and so the site could be compared to the 

annual PM2.5 NAAQS. 

After a review of the comments, the However, in light of public comments 

pointing out that there are cases where near-road environments can be considered a 

unique location; EPA is not finalizing its decision on the applicabilitythis part of areawide micro-the rule language. Examples of such locations that are considered unique and 

middle-scale monitoring sitesshould therefore not be considered applicable to the annual 

PM2.5 NAAQS. As are explained later in section III.E.3.a on the form of the annual 

NAAQS, EPA notes that it took the variability in PM2.5 concentrations from area-wide 

micro- and middle-scale sites into account in setting the standard. Thus, the EPA 

disagrees with those comments indicating that this change would represent a tightening of 

the NAAQS. We are adding an example on the treatment of near road monitoring sites, 

indicating that they are generally representative of many such locations, and that 

consistent with our existing policy where micro- and middle-scale sites represent many 

such locations, these types of sites are comparable to the annual NAAQS. A key aspect of 

this decision also maintains the longstanding policy in siting and use of data from those 

PM2.5 monitoring sites that are unique and therefore not comparable to the annual 

NAAQS. As described above,VIII.B.3.b.i. As noted in the preamble to the proposed rule 

(77 FR 39008-09), air agencies and the EPA will use the annual monitoring network plan 

described in 40 CFR 58.10 for identification and approval of sites that are suitable and 

sites that are not suitable for comparison againstwith the annual PM2.5 NAAQS.

The EPA disagrees with those comments that asserted that the proposed change 

would have represented a tightening of the NAAQS. As explained in section III.E.3.a on 

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the form of the annual NAAQS, the EPA carefully considered that areas such as traffic 

corridors were potential high exposure areas, since a significant fraction of the 

population, including at-risk populations, live in proximity to major roads and should be 

afforded the degree of protection intended by the revisions to the form and level of the 

annual PM2.5 standard being adopted. Monitoring in such areas as traffic corridors does 

not make the annual standard more stringent than intended, but rather affords the 

populations of such middle- and micro-scale areas (where determined to represent areawide air quality) the requisite level of protection from long-term exposure to PM2.5.

3. Changes to Monitoring for the National Ambient Air Monitoring System

a. Background

As described in appendix D to 40 CFR part 58, the ambient air monitoring 

networks must be designed to meet three basic monitoring objectives: 

(a) Provide air pollution data to the general public in a timely manner. Data can be 

presented to the public in a number of attractive ways including through air quality maps, 

newspapers, Internet sites, and as part of weather forecasts and public advisories. 

(b) Support compliance with ambient air quality standards and emissions strategy 

development. Data from FRM, FEM, and ARM monitors for NAAQS pollutants will be 

used for comparing an area’s air pollution levels against the NAAQS. Data from monitors 

of various types can be used in the development of attainment and maintenance plans. 

SLAMS, and especially National Core Monitoring Network (NCore)245 station data, will 

be used to evaluate the regional air quality models used in developing emission strategies 

 

245 NCore is a multi-pollutant network that integrates several advanced measurements for 

particles, gases and meteorology (U.S. EPA, 2011a, Appendix B, section B.4). 

Measurements required at NCore include PM2.5 mass and speciation, PM10-2.5 mass, 

ozone, CO, SO2, NO, NOy, and basic meteorology. 

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and to track trends in air pollution abatement control measures’ impact on improving air 

quality. In monitoring locations near major air pollution sources, source-oriented 

monitoring data can provide insight into how well industrial sources are controlling their 

pollutant emissions. 

(c) Support for air pollution research studies. Air pollution data from the NCore 

network can be used to supplement data collected by researchers working on health 

effects assessments and atmospheric processes or for monitoring methods development 

work. 

To support the air quality management work indicated in the three basic air 

monitoring objectives, a network must be designed with a variety of types of monitoring 

sites. Monitoring sites must be capable of informing managers about many things 

including the peak air pollution levels, typical levels in populated areas, air pollution 

transported into and outside of a city or region, and air pollution levels near specific 

sources. Following is a listing of six general site types: (a) sites located to determine the 

highest concentrations expected to occur in the area covered by the network (highest 

concentration); (b) sites located to measure typical concentrations in areas of high 

population density (population oriented); (c) sites located to determine the impact of 

significant sources or source categories on air quality (source impact or source oriented); 

(d) sites located to determine general background concentration levels (general 

background); and (e) sites located to determine the extent of regional pollutant transport 

among populated areas (regional transport); and in support of secondary standards

(welfare related impacts).

b. Primary PM2.5 NAAQS
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The EPA proposed to add a near-road component to the PM2.5 network design 

criteria and to clarify the use of approved PM2.5 continuous FEMs at SLAMS.

ii. Addition of a Near-road Component to the PM2.5 Monitoring Network

The EPA proposed to add a near-road component to the PM2.5 monitoring 

network. (77 FR 39009). The EPA believesexplained that there are gradients in nearroadway PM2.5 that are most likely to be associated with heavily travelled roads 

(particularly those with significant heavy-duty diesel activity), and that the largest 

numbers of impacted populations are located in the largest CBSAs in the country

(Ntziachristos et al., 2007; Ross et al., 2007; Yanosky et al., 20082009; Zwack et al., 

2011). The EPA believesfurther noted that by adding a modest number of PM2.5

monitoring sites that are leveraged with measurements of other pollutants in the near-road 

environment, a number of key monitoring objectives will be supported, including 

collection of NAAQS comparable data in the near-road environment, support for longterm health studies investigating adverse effects on people, providing a better 

understanding of pollutant gradients impacting neighborhoods that parallel major roads, 

availability of data to validate performance of models simulating near-road dispersion, 

characterization of areas with potentially elevated concentrations and/or poor air quality,

implementation of a multi-pollutant paradigm as stated in the NO2 NAAQS proposed rule 

(74 FR 34442, July 15, 2009), and monitoring goals consistent with existing objectives 

noted in the specific design criteria for PM2.5 described in appendix D, 4.7.1(b) to 40 

CFR part 58. 

The monitoring methods that are appropriate for this purpose are an FRM, FEM, 

or ARM. The EPA recognizesrecognized that there are limitations in the ability of some 
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of these methods to accurately measure PM2.5 mass due to the incomplete retention of 

semi-volatile material on the sampling medium (U.S. EPA, 2009a, section 3.4.1.1). This 

limitation is relevant to the near-road environment as well as to other environments 

where PM is expected to have semi-volatile components. The EPA also 

recognizesrecognized that continuous PM2.5 FEMs, which provide mass concentration 

data on an hourly basis, are better suited to accomplish the goals of near-road monitoring 

as they will complement the time resolution of the other air quality measurements and 

traffic data collected at the same sites. In this regard, particular PM2.5 FEMs are generally 

better suited for near-road monitoring than FRMs. However, filter-based FRMs do offer 

some advantages which may be highly desirable for near-road monitoring, such as readily 

available filters for later chemical analysis such as for elemental composition by x-ray 

fluorescence and black carbon (BC) by transmissometry. As a result of these tradeoffs, 

monitoring agencies are encouraged to select one or more PM2.5 methods for deployment 

at near–road monitoring stations that best meet their agencies monitoring objectives 

while ensuring that at least one of those methods is appropriate for comparison to the 

NAAQS (i.e., a FRM, FEM, or ARM). The EPA believes that by allowing monitoring 

agencies to choose the FRM, FEM, or ARM method(s) that best fits their needs, whether 

filter-based or continuous, the data will still be able to meet the objectives cited above 

while ensuring maximum flexibility for the monitoring agencies in the operation of their 

network. 

Additionally, the EPA recognizes that the near-road sites would provide a 

valuable platform for evaluating emerging monitoring technologies and for measuring 

other pollutants besides PM2.5 mass to enhance knowledge of exposure in the near road 
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environment and to support the characterization and comparison of specific method 

readings in an emission-rich environment. Further, in its response to the EPA on a 

“Review of the “Near-road Guidance Document – Outline” and “Near-road Monitoring 

Pilot Study Objectives and Approach” (U.S. EPA, 2010i), the CASAC AAMMS cited 

several other measurements that may be useful or potentially linked to health and welfare 

effects such as BC, ultrafine particles, and particle size distribution (Russell and Samet, 

2010b, pp. xi and xii). The EPA agrees with these recommendations and encourages 

monitoring agencies to include these measurements, and others cited in the Subcommittee 

letter, where possible, in addition to the PM2.5 mass measurement. The EPA also 

encourages monitoring agencies to explore partnerships with instrument manufacturers 

and researchers to use the sites to evaluate the performance of emerging PM2.5

monitoring methods in the near-road environment, especially potential or current FEMs 

able to provide temporally resolved data and capture the semi-volatile components of 

PM2.5. Such emerging PM2.5 methods could be operated as SPMs246 to provide 

comparisons to the EPA approved methods supporting compliance to advance the 

understanding of instrument performance in the near-road environment. Monitoring 

agencies are also encouraged to partner with instrument manufacturers and researchers to 

operate monitors able to measure other PM properties relevant for the near-road 

environment (e.g., ultrafine particles, BC) to provide additional information about 

exposure to PM in this environment. The EPA is interested in supporting monitoring 

agencies willing to operate and report the data from these supplemental monitors. EPA 

 

246 SPMs means a monitor included in an air agency’s monitoring network that the 

agency has designated a a special purpose monitor in its monitoring network plan and in 

the Air Quality System database, and which the air agency does not count toward when 

showing compliance with the minimum monitoring requirements in appendix D to Part 

58.
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notes that the implementation of additional measurements, while encouraged, is 

completely voluntary to ensure maximum flexibility for state monitoring programs. 

The EPA believes that requiring a modest network of near-road compliance PM2.5

monitors is necessary to provide characterization of concentrations in near-road 

environments including for comparison to the NAAQS. These long-term monitors will 

supplement shorter-term networks operated by researchers to support the tracking of 

long-term trends247 of near-road PM2.5 mass concentrations and other pollutants in nearroad environments where people are exposed. Therefore, the EPA proposed to require 

near-roadway monitoring of PM2.5 at one location within each CBSA with a population 

of one million persons or greater. The EPA believes that this network will be adequate to 

support the NAAQS since the largest CBSAs are likely to have greater numbers of 

exposed populations, a higher likelihood of elevated near-road PM2.5 concentrations, and

a wide range of diverse situations with regard to traffic volumes, traffic patterns, roadway 

designs, terrain/topography, meteorology, climate, surrounding land use and population 

characteristics. Given the latest population data available, the proposed requirement 

would result in approximately 52 required near-road PM2.5 monitors across the country. 

An indirect benefit of this network design is that monitoring agencies in these largest 

CBSAs are more likely to already have redundant monitors that could be relocated to the 

 

247 For example, the emissions used for the PM NAAQS RIA modeling show that 

nationwide on-road primary PM2.5 emissions are expected to be reduced by 63% between 

2007 and 2020. Additionally, the elemental carbon portion of the on-road emissions is 

expected to drop by 81 percent between 2007 and 2020. Therefore, we expect that 

measured near-road PM2.5 gradients will be much lower in the future as elemental carbon 

is a large fraction of the gradient, due to future impacts of existing mobile source 

controls. 
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near-road environment, reducing costs for equipment and ongoing operation.248 While 

only a single near-road PM2.5 monitor is required within each of the CBSAs, agencies 

may elect to add additional PM2.5 monitoring sites in near-road environments. 

While the EPA recognizesrecognized that the location of maximum concentration 

of PM2.5 exposure from roadway sources might differ from the maximum location of NO2

or other pollutants, the EPA proposed to require that near-road PM2.5 monitors be 

collocated with the planned NO2 monitors. The NO2 network design considers multiple 

factors that are also relevant for PM2.5 concentrations (i.e., average annual daily traffic,

fleet mix, roadway design, congestion patterns, terrain, and meteorology) and significant 

thought and review has already gone into its design, including pilot studies at five

locations, and the development of a technical assistance document in conjunction with 

the affected monitoring agencies and the CASAC AAMMS (Russell and Samet, 2010b) 

to support deployment. Further, this collocation will allow multiple pollutants to be 

tracked in the near-road environment. To the extent that air agencies are still determining 

the optimum location for their multi-pollutant249near-road monitoring stations, EPA 

encourages consideration of sites that best reflect measurement of maximum 

concentrations associated with exposure of people living in areas that parallel major 

roads, to maximize the value of the data for use later in health studies. Therefore, while 

compromises may be necessary when siting a multi-pollutant near road monitoring 

station, on balance, the EPA believes this is the most efficient and beneficial approach for 

deployment of this component of the network.

 

248 EPA Regional Administrator approval would be required prior to the discontinuation 

of SLAMS monitors, based on the criteria described in 40 CFR 58.14(c).

249 NO2, CO, and now PM2.5 measurements are all expected to be collocated at near-road 

monitoring stations.

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The EPA notes that the planned 52 near-road monitors represent a small number 

of the total approximate 900 operating PM2.5 monitoring stations across the country. The 

EPA could have considered proposingproposed more near-road sites, however, the 

addition of sites in lower population CBSAs is not expected to lead to much if any 

difference in characterization of air quality since the bump in PM2.5 concentration 

associated with near-road environments in lower population CBSAs, which typically 

have correspondingly less travelled roads, is expected to be very small. The EPA could 

also have considered proposingproposed multiple sites in larger CBSAs; however, State 

monitoring programs are already working towards representative near-road monitoring 

stations and there is a synergistic value in ensuring these measurements are collocated 

with multiple other measurements to serve the monitoring objectives noted above. Since 

EPA has already finalized requirement of CO monitoring at near-road stations in CBSA’s 

with a population of 1 million or more at sites that are collocated with NO2, there would 

be less value in requiring any more than 52 PM2.5 monitors as any more stations will not 

have CO for use in multi-pollutant monitoring objectives (e.g., health studies and model 

evaluation).

Ideally, near-road sites would be located at the elevation and distance from the 

road where maximum concentration of PM2.5 exposure occurs in this environment, and 

within reasonable proximity to another area-wide PM2.5 compliance monitoring site at 

which a similar PM monitor is used (i.e., for comparison purposes). However, 

specificPM2.5 levels occur in this environment, representing locations where populations 

are exposed; for example, in apartments and other housing; schools located along major 

roadways; industrial parks where workers exposed; and in recreational areas such as 
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greenways, bikeways, and other park facilities that are often developed along roads. 

Specific to probe and siting criteria for near-road PM2.5 monitors, which is explained later 

in this section, EPA did not set additional criteria on what the elevation and distance 

requirements should be, beyond what is already defined for PM2.5 or near-road NO2

monitors for reasons explained above. Also, the EPA did not propose that the near-road 

PM2.5 monitors be located within a specific distance of other area-wide sites; however, 

monitoring agencies are encouraged to consider that a near-road site selected in 

accordance with monitoring requirements and also located in proximity to a robust areawide site, such as an NCore station, would provide useful information in characterizing 

the near-road contribution to multiple pollutants, including PM2.5 and tracking the 

decreasing trend that is expected in the PM2.5 near-road gradient over time, due to future 

impacts of existing mobile source controls.

The timeline to implement the near-road PM2.5 monitors should be as minimally 

disruptive to on-going operations of monitoring agency programs as possible recognizing 

monitoring agenciesagency resource constraints, while still meeting the need to collect 

for near-road PM2.5 data in a timely fashion. Since the near-road PM2.5 monitors were

proposed to be collocated with the emerging near-road NO2 network that was scheduled 

to be operational by January 1, 2013250, the EPA believes it is appropriate to wait until 

after the near-road NO2 network is established before implementing the near-road PM2.5

monitors. Therefore, the EPA proposed that each PM2.5 monitor planned for collocation 

with a near-road NO2 monitoring site be implemented no later than January 1, 2015.

 

250 The EPA has proposed a revised timeline for deployment of the near-road NO2

monitors, where all CBSAs with one million or more people are to have their first nearroad NO2 station operational by January 1, 2014 (77 FR 64244, October 19, 2012).
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The EPA received comments from a number of air agencies, industrial groups, 

and environmental and public health organizations on its proposal to require PM2.5

monitoring in near-road environments.

Among comments from air agencies, several commenters did not support the 

addition of near road monitoring with comments onciting the challenges of siting these 

stations and the additional cost it would require to operate the monitors. Several air 

agencies recognized the value of adding monitors to provide better characterization of 

exposures in near-road environments, but recommended a slower deployment of the 

PM2.5 monitors so that it can be phased in over a multi-year period. Several air agencies 

recommended that the PM2.5 monitoring in the near-road environment be deployed on a 

phased-in schedule with the first such monitors being required no sooner than one year 

after deployment of the NO2 sites. These air agencies stated that the phasing in of the 

PM2.5 monitors in the near road environment would allow more time to learn and share 

information on what worked best in deploying the NO2 monitors at near-road monitoring 

stations, since NO2 is the first pollutant required to be monitored at near-road stations. A 

few air agencies identified a need to more clearly support or require the maintenance of 

as much of the existing network of neighborhood scale PM2.5 monitoring sites as possible 

in regulatory text. These neighborhood scale PM2.5 sites were identified by commenters 

as the most broadly representative sites for characterizing CBSA wide exposures that are 

supportive of a number of monitoring objectives. A few air agencies also identified a 

need for flexibility in the proposed network design requirement that PM2.5 near-road 

monitors must be collocated with the NO2 monitors in the near-road environment. The 

commenters suggested allowing flexibility for air agencies to meet the requirement for 

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PM2.5 in a near-road environment by siting at a different near-road location where PM2.5

concentrations are expected to be high.

Most industry commenters did not support the addition of near-road monitoring 

for PM2.5, citingagain arguing that use ofusing data from such data should be for research 

purposes and, if usedmonitors, for comparison to the NAAQS, combined with other 

changes (i.e., elimination of “population-oriented” as a criteria for comparison to the 

NAAQS and the elimination of spatial averaging) would represent, in their judgement, a 

collective tightening of the PM2.5 NAAQS. A few of these commenters asserted that 

monitoring in the near-road environment is not representative of ambient air exposures. A 

few industry comments noted that if the EPA required PM2.5 monitoring in the near-road 

environment, any data collected should not be used for comparison to the NAAQS. One 

commenter stated it had no problem with monitoring in the near-road environment, so 

long as any such monitoring used to compare to the PM2.5 annual NAAQS is population–

oriented. One commenter stated that the decision to co-locate with NO2 monitors was 

based on convenience and the intent of the NO2 near-road monitoring is to find the 

highest micro-scale concentrations within a few meters of the most heavily travelled 

expressways, representing a unique situation. 

Environmental and public health groups strongly support the addition of PM2.5 

monitoring to the near-road environment. Commenters cited the large number of people 

that live in proximity to major roadways251 in their support for adding these monitors, 

 

251 One study identified that 45 million Americans live within 300 feet of a major 

roadway. or other source of mobile emissions. The commenterscommenters’ information 

is based on the American Housing Survey, which is available on the web at: 

http://www.census.gov/housing/ahs/data/ahs2009.html. The survey provides an estimate 

of the county’s housing units in the U.S. that are located with 300 feet of a highway with 

four or more lanes, or a railroad, or an airport.

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that such protection of people in these environments in long overdue, and that such data 

therefore be used for comparison to the NAAQS.

Regarding comments from air agencies that the near-road monitors are 

challenging to site and that there is additional cost in operating these monitors, the EPA 

maintains that the major challenges in siting would already be accomplished by 

implementing the required NO2 monitoring stations in near-road environments since the 

EPA fully expects that the PM2.5 monitors will be placed at the NO2 near roadway 

stations and has revised the PM2.5 monitoring requirements consistent with that 

expectation. The EPA also points out that the requirements for the minimum number of 

PM2.5 monitors is unchanged and that in most cases the addition of near-road PM2.5

monitors can be accomplished by relocating an existing monitor, with no net increase in 

monitors. Thus, while we are requiring a new component of the PM2.5 monitoring 

network, the overall size of the network is expected to remain about the same, and we 

expect that air agencies can meet this requirement by relocating existing lower-priority 

monitors. In considering comments from air agencies on a schedule for implementing 

PM2.5 monitors at near road monitoring stations, the EPA is persuaded by commenters 

from air agencies who stated that a phased deployment of the PM2.5 monitors would be a 

better approach as it would allow agencies to learn from the deployment of the NO2

monitors and a first phase of PM2.5 monitors. Phasing in the deployment of monitors is 

also consistent with previous CASAC advice (Russell and Samet, 2010b) on a schedule 

for deployment of near-road NO2 monitors.

Regarding comments from air agencies on maintaining the neighborhood scale 

monitoring stations as the largest part of the network as these sites are the most broadly 

 

 

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representative of exposures across CBSAs, the EPA supports such a goal. Neighborhood 

scale monitoring sites remain the backbone of the PM2.5 monitoring network and they 

will continue to represent over two thirds of the operating network following the 

deployment of the near-road monitors. The EPA expects that each CBSA required to 

monitor for PM2.5 will maintain its existing highest concentration area-wide monitoring 

site (referred to as the design value site) and not attempt to move such sites to near-road 

environments. Maintaining the area-wide and largely neighborhood scale design value 

sites is critical to the long-standing goal of using data to support a variety of monitoring 

objectives. The EPA also recognizes that while every PM2.5 monitor has value in some 

capacity at its current location, air agencies are expected to recommend relocation of 

monitors that are relatively low in priority to meet the near-road requirement. 

Regarding comments from air agencies on the need for flexibility in the network 

design requirement that PM2.5 near-road monitors must be collocated with the NO2

monitors in the near-road environment, the EPA points out that it prefers to maintain this 

requirement so that the multi-pollutant data are available to support the monitoring 

objectives cited above. However, the EPA also recognizes there may be cases where an 

air agency recommends siting their near-road PM2.5 monitor in another high 

concentration near-road environment. The EPA believes such cases will be very limited, 

but that these situations can be supported in one of two ways. First, EPA and the air 

agency can use their discretion to site two near-road PM2.5 monitors in the area. Second, 

the EPA can use its discretion in approving a deviation from the PM2.5 monitoring 

requirements as already exists in the network design criteria. Such deviations are to be 

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approved by the Regional Administrator as described in section 4.7.1 of Appendix D to 

part 58.

In considering public comments on the addition of PM2.5 monitors to the nearroad environment, the EPA points out that a significant fraction of the population lives in 

proximity to major roads and agrees that theseRegarding the comment that PM2.5 

monitors in near-road environments were sited for convenience, which due to siting with 

NO2 monitors a few meters from the road presents a unique situation, the EPA disagrees 

that these monitors were sited solely for convenience or that they would represent a 

unique situation within an urban area. On the initial point, the EPA believes that the 

characterization of representative maximum PM2.5 concentrations due to on-road mobile 

sources and the appropriate location of such PM2.5 monitors will be the same approximate 

locations that are the focus of the near-road NO2 network. This is due to the fact that 

PM2.5, like NOX, is disproportionately influenced by heavy duty (HD) vehicles which are 

predominantly diesel fueled, when compared to light duty (LD) vehicles which are 

primarily gasoline fueled. Specifically, for both PM2.5 and NOx, HD vehicles emit more 

of these two pollutants and their precursors on a per vehicle basis than LD vehicles. The 

EPA recognized this fact in the near-road NO2 network by requiring states to consider the 

fleet mix of candidate road segments where near-road monitoring might occur. In the 

design of the NO2 near-road network where the PM2.5 monitors will be installed, states 

were instructed to place a higher priority on those highly trafficked roads which have 

more diesel fueled vehicles using a metric called the fleet equivalent average annual daily 

traffic.

252

 As such, the Agency believes it is appropriate that required near-road PM2.5

 

252 See the Near-road NO2 Monitoring Technical Assistance Document at: 

http://www.epa.gov/ttn/amtic/files/nearroad/NearRoadTAD.pdf
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monitors would be located with near-road NO2 monitors as they are similarly influenced 

not only by fleet mix but also by total traffic count, congestion patterns, roadway design, 

terrain, and meteorology. On the second point with regard to such sites representing a 

unique situation within an urban area, EPA points out that the determination of a nearroad micro- or middle-scale site being considered to represent “area-wide” air quality or 

“unique” will be made on a case by case basis with the monitoring agency providing such 

recommendations in their annual monitoring network plans described in §58.10. 

Examples of such “unique” micro- and middle-scale locations are provided later in this 

section.

We do not accept the comment that siting some monitors in near roadway 

environments makes the standard impermissibly more stringent. A significant fraction of 

the population lives in proximity to major roads. These exposures occur in locations that 

represent ambient air for which the agency has a responsibility to ensure the public is 

protected. Regarding comments that the addition of near-road PM2.5 monitoring sites, 

combined with the elimination of spatial averaging, and revoking “population-oriented” 

as a criteria for comparison to the NAAQS, are a collective tightening of the NAAQS, as 

explained in section III.E.3.a, the EPA took these changes in to account in setting the 

standard. with an adequate margin of safety. Ignoring monitoring results from such areas 

(or not monitoring at all) would abdicate this responsibility. Put another way, monitoring 

in such areas does not make the standard more stringent, but rather affords requisite 

protection to the populations, among them at-risk populations, exposed to fine particulate 

in these areas. Thus, the EPA has made a determination to protect all area-wide locations, 

including those locations with populations living near major roads that are representative 

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of many such locations throughout an area. As discussed above, EPA concludes that the 

requirement to locate monitors to represent ambient air, along with other siting 

requirements, will ensure that monitors represent PM2.5 concentrations in areas of 

potential public exposure.

We do recognize, however, the possibility that some near-road monitoring 

stations may be representative of relatively unique locations versus the more 

representative area-wide situation mentioned above. This could occur because an air 

agency made a siting decision based on NO2 criteria that resulted in the characterization 

of a microscale environment that is not considered area-wide for PM2.5; for example, due 

to proximity to a unique source like a tunnel entrance, nearby major point source, or other 

relatively unique microscale hot spot. In these types of scenarios, air agencies would 

identify the site as a unique monitor comparable only to the 24-hour PM2.5 NAAQS per 

the language in section 58.30, and not comparable to the annual NAAQS, through the 

Annual Monitoring Network Plan process described earlier. Although EPA expects most 

near-road PM2.5 monitors to be sited to represent area-wide conditions, since a vast 

majority of the near-road stations have yet to be installed, we believe that providing such 

clarity and flexibility in siting and NAAQS comparability is warranted.

After careful consideration of the public comments, the EPA is finalizing its 

decision to add PM2.5 monitors to the near-road monitoring stations. The EPA is 

finalizing this decision as the near-road environment is an area where significant public 

exposure can occur, recognizing that this is a gap in the current PM2.5 monitoring 

networks, and because these PM2.5 monitors will be collocated with NO2 monitors in the 

near-road environment, there will not be a significant additional burden on the air 

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agencies253. However, in recognition of the comments from air agencies above, EPA is 

finalizing a revised and phased schedule for deployment of the PM2.5 monitors at nearroad stations. A minimum of one PM2.5 monitor in each CBSA with a population ≥greater 

than or equal to 2.5 million is to be collocated at a near-road NO2 monitoring station and 

must to be operational by January 1, 2015. The remaining CBSAs (i.e., those CBSAs 

with populations ≥greater than or equal to 1M, but less than 2.5M) must operational by 

January 1, 2017. This schedule will ensure that air agencies have sufficient time to learn 

from deployment of the NO2 monitors in near-road environments, that the highest 

population CBSAs begin operating their PM2.5 monitors in near-road environments first, 

and that the remaining PM2.5 monitors are deployed on the same schedule as the CO 

monitors (also, required by January 1, 2017254). In consideration of the comments 

regarding maintaining neighborhood scale monitoring sites as the largest portion of the 

network, the EPA is adding revising the wording of a requirement to maintain a portion 

of the networkthat requires at neighborhood scale or larger least one site to be in thean

area-wide location of expected maximum concentration, to wording that states that such 

sites must be in an area-wide location of expected maximum concentration while also 

being at the neighborhood or larger scale of representation. 

iii. Use of PM2.5 Continuous FEMs at SLAMS

The EPA proposed that each agency specify its intention and rationale to use or 

not use data from continuous PM2.5 FEMs that are eligible for comparison to the NAAQS 

as part of its annual monitoring network plan due to the applicable EPA Region Office by 

July 1 each year. The proposal also provided that the EPA Regional Administrator would 

 

253 The incremental one-time cost of moving the 52 monitors required to be located in the 

near-road environment is described in section X.B – Paperwork reduction Act. 

254 76 FR 54294, August 31, 2011

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be responsible for approving annual monitoring network plans where agencies have 

provided a recommendation that certain PM2.5 FEMs be considered ineligible for 

comparison to the NAAQS. 

In 2006, the EPA finalized new performance criteria for approval of continuous 

PM2.5 monitors as either Class III FEMs or ARMs. At the time of proposal, the EPA had

already approved six PM2.5 continuous FEMs255 and there are nearly 200 of these 

monitors already operating in State, local, and Tribal networks. Monitoring agencies have 

been deploying and field-testing these units over the last couple of years and the EPA 

recently compiled an assessment of the FEM data in relationship to collocated FRMs 

(Hanley and Reff, 2011; U.S. EPA, 2011a, pp. 4-50 to 4-51). As described in section 

VI.D.1.a.iii above, the proposal (FR 38983),the EPA found that some sites with 

continuous PM2.5 FEMs have an acceptable degree of comparability with collocated 

FRMs, while others had poor data comparability that would not meet the performance 

criteria used to approve the FEMs (71 FR 61285-61286, Table C-4, October 17, 2006). 

The EPA is encouraging use of the FEM data from those sites with acceptable data 

comparability including for purposes of comparison to the NAAQS. For sites with 

unacceptable data comparability, the EPA is working closely with the monitoring 

committee of the NACAA, instrument manufacturers, and monitoring agencies to 

document best practices on these methods to improve the comparability and consistency 

of resulting data wherever possible. The EPA believes that the performance of many of 

these continuous PM2.5 FEMs at locations with poor data comparability can be improved 

to a point where the acceptance criteria noted above can be met.

 

255 The EPA maintains a list of approved Reference and Equivalent Methods on its web 

site at: http://www.epa.gov/ttn/amtic/criteria.html. 
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Given the varying data comparability of continuous PM2.5 FEMs noted above, we 

believe that a need exists for flexibility in the approaches for how such data are used, 

particularly for the objective of determining NAAQS compliance. Accordingly, we 

proposed that monitoring agencies address the use of data from PM2.5 continuous FEMs 

in their annual monitoring network plans due to the applicable EPA Regional Office by 

July 1 of each year for any cases where the agency believes that the data generated by 

PM2.5 continuous FEMs in their network should not to be compared to the NAAQS. The 

annual network plans would include assessments such as comparisons of continuous 

FEMs to collocated FRMs, and analyses of whether the resulting statistical performance 

would meet the established approval criteria. Based on these quantitative analyses, 

monitoring agencies would have the option of requesting that data from continuous FEMs 

be excluded from NAAQS comparison subject to EPA approval; however, these data 

could still be utilized for other objectives such as AQI reporting. 

The issue exists of whether such data use provisions should be prospective only 

(i.e., future NAAQS comparability excluded based on an analysis of recent past 

performance) or a combination of retrospective and prospective (i.e., the implications of 

unacceptable FEM performance impacting usage of previously collected data as well as 

future data). In the proposal, the EPA stated that in most cases, monitoring agencies 

should be restricted to addressing prospective data issues to provide stability and 

predictability in the long-term PM2.5 data sets used for supporting attainment decisions. 

However, in the first year after this proposed option would become effective, we 

indicated it would be appropriate to provide monitoring agencies with a one-time 

opportunity to review already reported continuous PM2.5 FEM data and request that data 
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with unacceptable performance be restricted (retrospectively) from NAAQS 

comparability. Accordingly, in the first year after this rule becomes effective, we 

proposed that monitoring agencies have the option of requesting in their annual 

monitoring network plans that a portion or all of the existing continuous PM2.5 FEM data, 

as applicable, as well as future data, be restricted from NAAQS comparability for the 

period of time that the plan covers.256 In the proposal we stated that annual monitoring 

network plans in subsequent years would only need to cover new data for the period of 

time that the plan covers. 

As noted above, in cases where an agency is operating a PM2.5 continuous FEM 

that is not meeting the expected performance criteria used to approve the FEMs (71 FR 

61285 to 61286, Table C-4, October 17, 2006) when compared to their collocated FRMs, 

an agency can recommend that the data not be used for comparison to the NAAQS. 

However, all required SLAMS would still be required to have an operating FRM (or 

other well performing FEM, as evidenced by a prior collocation with an FRM) to ensure 

a data record is available for comparison to the NAAQS. In cases where a PM2.5

continuous FEM was not meeting the expected performance criteria, and the Regional 

Administrator has approved a recommendation that the FEM data not be considered 

eligible for comparison to the NAAQS, the data would still be required to be loaded to 

AQS; however, these data would be identified distinctly from data used for comparison to 

the NAAQS. 

The goal of proposing to allow monitoring agencies the opportunity to 

recommend not having data from PM2.5 continuous FEMs as comparable to the NAAQS 

 

256 Data from any PM2.5 monitor being used to meet minimum monitoring requirements 

could not be restricted from NAAQS comparability.
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is to ensure that only high quality data (i.e., data from FRMs which are already well 

established and new continuous FEMs that meet the performance criteria used to approve 

FEMs when compared to collocated FRMs operated in each agencies network) are used 

when comparing data to the PM2.5 NAAQS. Under the current monitoring regulations, a 

monitoring agency can identify a PM2.5 continuous FEM as an SPM, which allows the 

monitor to be operated for up to 24 months without its data being used in comparison to 

the NAAQS. While 24 months should be sufficient time to operate the monitor across all 

seasons, assess the data quality, and in some cases resolve operational issues with the 

instrument, it may still leave some agencies with monitors whose data are not sufficiently 

comparable to data from their FRMs. In these cases there may be a disincentive to 

continue operating the PM2.5 continuous FEM, especially in networks where the 

monitoring data are near the level of the NAAQS. With the proposed provision, where a 

monitoring agency can recommend not having data from PM2.5 continuous FEMs be

comparable to the NAAQS, a monitoring agency can continue to operate their PM2.5

continuous FEM to support other monitoring objectives (e.g., diurnal characterization of

PM2.5, AQI forecasting and reporting), while working through options for improved data 

comparability while still providing data for comparison to the NAAQS from an FRM.

The EPA believes that an assessment of FEM performance should include several 

elements based on the original performance criteria. The Agency also believes that 

certain modifications to the performance criteria are appropriate in recognition of the 

differences between how monitoring agencies operate routine monitors and how 

instrument manufacturers conduct required FRM and FEM testing protocols. The details 

below summarize these issues. 
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The EPA proposed to use the performance criteria used to approve the FEMs (71 

FR 61285 to 61286, Table C-4, October 17, 2006) for those agencies that recommend not 

having data from PM2.5 continuous FEMs be comparable to the NAAQS. To 

accommodate how routine monitoring networks operate, the EPA proposed that agencies 

seeking to demonstrate insufficient data comparability in anbase their assessment base 

the analysis mainly on collocated data from FRMs and continuous FEMs at monitoring 

stations in their network. The EPA does not believe it is practical to utilize the 

requirement in table C-4 of 40 CFR part 53 for having multiple FRMs and FEMs at each 

site since such arrangements are not typically found in monitoring agency networks. 

Accordingly, the requirement for assessing intra-method replicate precision would be 

inapplicable. Another consideration is the range of 24-hour data concentrations, for 

instance, the performance criteria in table C-4 of 40 CFR part 53, provides for an 

acceptable concentration range of 3 to 200 μg/m3. However, the EPA notes that during an

evaluation of data quality from two FEMs (U.S. EPA, 2011a, p. 4-50), the Agency found 

that including low concentration data was helpful for understanding whether an intercept 

or slope was driving a potential bias in an instrument. Therefore, the EPA proposed that 

agencies may include low concentration data (i.e., below 3 μg/m3) for purposes of 

evaluating the data comparability of continuous FEMs. With regard to the minimum 

number of samples needed for the assessment, the EPA notes that a minimum of 23 

sample pairs are specified for each season in table C-4 of 40 CFR part 53. Having 23 

sample pairs per season should be easily obtainable within one year for sites with a FRM 

operating on at least a 1 in 3 day sample frequency and we proposed that this requirement 

be applicable to the assessments being discussed here. For sites on a one in 6 day 
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sampling frequency, two years of data may be necessary to meet this requirement. The 

EPA recognizes that it would be best to assess the data based on the most recently 

available information; however, having data across all seasons in multiple years will 

provide a more robust data set for use in the data comparability assessment; therefore, the 

EPA proposed that data quality assessments be permitted to utilize up to the last three 

years of data for purposes of recommending not having data from PM2.5 continuous 

FEMs be comparable to the NAAQS.

The EPA recognizes that only a portion of continuous PM2.5 FEMs will be 

collocated with FRMs, and it would be impractical to restrict the applicability of data 

comparability assessments to only those sites that had collocated FRM and FEM 

monitors. In these cases, the monitoring agency will be permitted to group the sites that 

are not collocated with an FRM with another similar site that is collocated with an FRM 

for purposes of recommending that the data are not eligible for use in comparison to the 

NAAQS. Monitoring agencies may recommend having PM2.5 continuous FEM data 

eligible for comparison to the NAAQS from locations where the method has been 

demonstrated to provide acceptable data comparability, while also recommending not 

having it eligible in other types of areas where the method has not been demonstrated to 

meet data comparability criteria. For example, a rural site may be more closely associated 

with aged particles where volatilization issues are minimized resulting in acceptable data 

comparability between filter-based and continuous methods, while a highly populated 

urban site with fresh emissions with higher volatility may result in higher readings on the 

PM2.5 continuous FEM that would not meet the expected performance criteria as 

compared to a collocated FRM. In all cases where a monitoring agency chose to group 
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sites for purposes of identifying a subset of PM2.5 continuous FEMs that would not be 

comparable to the NAAQS, the assessment submitted with the annual monitoring 

network plan would have to provide sufficient detail to support the identification of 

which combinations of method and sites would, and would not, be comparable to the 

NAAQS, as well as the rationale and quantitative basis for the grouping and 

recommendation.

Most comments received on this issue were from air agencies. All air agencies 

either supported the proposal or supported it with a recommendation to continue to allow 

for retrospective assessments to be used such that data would not be compared to the 

NAAQS, if such an assessment showed that the data were not of sufficient comparability 

to a collocated FRM such that the continuous FEM should not be compared to the 

NAAQS. One air agency supported the proposal, except though it had reservations about 

how to best group sites together when a particular PM2.5 continuous FEM is not 

collocated with a FRM. 

The EPA notes the support by air agencies to finalize this provision. EPA also 

notes that all commenters who offered input on the retrospective use of assessments were 

supportive of allowing continued retrospective assessments in annual monitoring 

networks plans so that data may be recommended as excluded from comparison to the 

NAAQS under certain provisions. However, the EPA has some reservations about how 

and under what circumstances such an allowance should be made. The EPA notes the 

concern expressed from one agency about how to best group sites together when 

considering an assessment.

On the issue of whether to allow data collected to be retrospectively excluded 
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from comparison to the NAAQS, the EPA notes there are a number of considerations, 

including that several air agencies support such a policy. The EPA has evaluated how this 

issue can be achieved and believes that some consideration should be allowed, but also 

wants to ensure there is a consistent and easily recognizable interpretation of such cases 

where air agencies recommend excluding already collected and reported data. To help 

illustrate the possible outcomes of how this could work consider the following examples. 

Example 1: An agency finds that the bias between a collocated PM2.5 continuous FEM 

and FRM are acceptable, but near the limit of that acceptability and then finds a year later 

that the assessment indicates that the bias is just outside the limit of that acceptability. 

Such relatively small changes where an assessment indicates flipping in or out of the 

acceptable bias are in themselves acceptable since the overall Data Quality Objectives 

(DQOs) can still be met. The overall DQOs can still be met since there are a number of 

other factors that feed into the DQOs such as precision, data completeness, and especially 

sample frequency, which when operating a continuous FEM is a daily sample. Daily 

sampling provides less uncertainty than sampling at the one-in-three day or one-in-six 

day sampling frequencies, which are routinely employed by filter-based FRM samplers. 

Therefore, in this example the existing data should still be compared to the NAAQS, but 

the air agency should thoughtfully consider whether to recommend257 and the EPA will 

consider whether to approve that any new data from PM2.5 continuous FEMs are used in 

comparison to the NAAQS. If an air agency recommends to not use a PM2.5 continuous 

FEM for comparison to the NAAQS, it would need to ensure another approved method 

(i.e,., a filter-based FRM/FEM or other continuous FEM which is performing within 

acceptable performance criteria) is operating at the site or sites of interest. This would be 

 

257 Through the annual monitoring network plan explained in §58.10.
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expected for all SLAMS, but at a minimum the design value monitoring station for the 

area of interest would be required to have another approved PM2.5 method (i.e., an FRM, 

other filter-based FEM, or other continuous FEM or ARM with acceptable data 

comparability) operating on the required sample frequency or more often for that 

location. Example 2: A PM2.5 continuous FEM operated by an air agency is found to have 

a significant bias compared to a collocated FRM. If the air agency finds cause to 

invalidate the data (e.g., a flow sensor is found to be outside of acceptable limits), then it 

should invalidate the relevant data (i.e., data from the period going back to the last 

successful flow check or audit or other information that points to a cause that the flow 

sensor is not meeting its performance criteria) for all data uses and there is no follow-up 

issue of retrospective analysis. A case of finding cause to invalidate data would be based 

on validation criteria found in an air agencies approved quality assurance project plan 

(QAPP). Example 3: A PM2.5 continuous FEM operated by an air agency and previously 

identified as appropriate to compare to the NAAQS, is found to have a significant and 

unacceptable bias compared to a collocated FRM and there is no other reason to 

invalidate the data. That is, all other information points to the data being valid; however, 

there has been a significant shift in the comparability of the PM2.5 continuous FEM 

compared to a collocated FRM (which itself is found to be operating correctly and data 

are valid). A significant shift in the comparability would be noticeable by comparing 

assessments for a site from one year to the next and seeing a significant and unacceptable 

change in one of the key statistical metrics used in the evaluation (i.e., additive or 

multiplicative bias). Such a case of retrospectively recommending not using PM2.5

continuous FEM data should also take into account all other available information that 
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can help inform approving such a recommendation as part of an annual monitoring 

network plan. For example, do data from the PM2.5 performance evaluation program data 

also suggest an unacceptable bias for a specific period of interest with this method as 

used in the air agencies network? Note: This type of assessment is often limited by the 

small number of samples taken in the PEP program relative to the large number of 

collocated samples expected when an FRM and PM2.5 continuous FEM are collocated. In 

this type of example, the air agency might want to recommend not using the continuous 

FEM data for comparison to the NAAQS; however, the continuous FEM data could be 

appropriate for use in reporting the Air Quality Index (AQI) or other data uses either as is 

or if statistically correlated258 and corrected back to the collocated FRM. So in this last 

example, the PM2.5 continuous FEM data would be stored separately in the EPA’s data 

system so that they are eligible for use in AQI calculations, but not used in comparison to 

the NAAQS, if approved by the EPA. Again, the air agency should thoughtfully consider 

and state its position and rationale in the annual monitoring network plan on whether any 

future data should be compared to the NAAQS. 

Another issue to consider is the transparent and consistent use of PM2.5

continuous FEM data from a method where one air agency recommends using the data 

for comparison to the NAAQS and another specifically recommends to not use it for 

comparison to the NAAQS. The use of the annual monitoring plans ensures that the 

process is transparent; however, it may not ensure a consistent approach if one agency 

recommends exclusion of data and another agency does not. For example, consider two 

 

258 The EPA has had a long-standing policy of allowing PM2.5 continuous data to be 

statistically correlated and corrected to use in AQI reporting. A report is available on this:

See “Data Quality Objectives (DQOs) for Relating Federal Reference Method (FRM) and 

Continuous PM2.5 Measurements to Report an Air Quality Index (AQI), EPA-454/B-02-

002, November 2002”.
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adjacent air agencies operating the same make and model of a PM2.5 continuous FEM, 

where one air agency recommends using data and the other air agency recommends not 

using it for comparison to the NAAQS. While on its face it may seem straightforward 

that a method with acceptable comparability to a collocated FRM should perform 

similarly in other air agency networks where they have similar aerosol composition and 

climate, in practice there are a number of other variables that affect data comparability. 

Such factors that lead to differences in comparability might include differences in 

installation, training, development of SOPs, control of shelter conditions, maintenance of 

the continuous FEM, and performance of the FRMs which are being used as the basis of 

comparison to the continuous FEM. Also, there may be cases where the concentration 

levels are so far away from the level of the NAAQS (either substantially higher or lower) 

that it would not matter if the data are excluded or not, the same NAAQS determination 

would result. The EPA has considered these issues and in general believes that it would 

still be acceptable for one agency to use data for comparison to the NAAQS, while 

another agency does not, even if it’s the same method used in adjacent air agency 

networks. 

On the issue of grouping sites for purposes of allowing monitors that are not 

collocated to be included when recommending a method should not be compared to the 

NAAQS, the EPA believes that it is not necessary to provide specific details on what 

criteria are necessary to group sites as air agencies are in the best position to determine a 

recommendation of when such sites should or should not be grouped. However, to 

illustrate examples of possible ways to group sites, the air agency could take into account 

factors such as whether the sites are all in either a rural or urban location, since urban 
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locations tend to be impacted more directly by fresh emissions which are known to be 

more volatile, or whether there is consistency in the climate for the sites of interest as 

might be the case for sites near a large water body or at a high altitude. The EPA will 

consider these issues when evaluating air agency requests for approval.

The EPA is finalizing its proposal to allow each air agency to specify its intention 

to use or not use data from continuous PM2.5 FEMs that are eligible for comparison to the 

NAAQS as part of their annual monitoring network plan due to the applicable EPA 

Region Office by July 1 each year where adequate FRM data are available. The EPA’s 

approval of an annual monitoring network plan259 as a whole, or in part, will constitute 

concurrence with an air agency’s recommendation to use or not use data from continuous 

PM2.5 FEMs as eligible for comparison to the NAAQS, unless otherwise noted in the 

approval of the plan. The absence of an air agency statement specifying a position on use 

of data from a continuous PM2.5 FEM for comparison to the NAAQS will be interpreted 

as meaning that theall such data are applicable for comparison to the NAAQS following 

the provisions in Part 50, Appendix N on data handling and Part 58 on the monitoring 

requirements. In finalizing this decision the EPA will ensure, as proposed, that air 

agencies can identify already collected data from PM2.5 continuous FEMs that should not 

be used for comparison to the NAAQS. After considering comments in support of 

allowing additional retrospective assessments, the EPA is also finalizing an approach of 

allowing for the continued use of retrospective assessments to inform when already 

collected data should not be compared to the NAAQS, if there has been a significant 

change in the assessment of that data from previous years. 

 

259 Approval of an annual monitoring network plan is subject to approval of the EPA 

Regional Administrator as provided for in §58.10(a)(2).
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c. Revoking PM10-2.5 Speciation Requirements at NCore Sites

The EPA proposed to revoke the requirement for PM10-2.5 speciation monitoring 

as part of the current suite of NCore monitoring requirements. The requirement to 

monitor for PM10-2.5 mass (total) at all NCore multi-pollutant sites remains. PM10-2.5 mass 

monitoring commenced on January 1, 2011 as part of the nationwide startup of the NCore 

network (U.S. EPA, 2011a, p. 1-15).

As part of the process to further define appropriate techniques for PM10-2.5 

speciation monitoring, a public consultation with the CASAC AAMMS on monitoring 

issues related to PM10-2.5 speciation was held in February 2009 (74 FR 4196, January 23, 

2009). The subcommittee noted the lack of consensus on appropriate sampling and 

analytical methods for PM10-2.5 speciation and expressed concern that the Agency’s 

commitment to launch the PM10-2.5 monitoring network without sufficient time to analyze 

the data from a planned pilot project was premature (Russell, 2009). Based on the noted 

lack of consensus on PM10-2.5 speciation monitoring techniques, the Agency did 

implement a small pilot monitoring project to evaluate the available monitoring and 

analytical technologies.

The EPA pilot monitoring project was completed in 2011, with plans to analyze 

the data and prepare a final report on findings and recommendations in 2013. At that 

time, the EPA will consider what PM10-2.5 speciation sampling techniques, analytical 

methodologies, and monitoring design strategies would be most appropriate as part of a 

potential nation-wide monitoring deployment. Such a deployment could be based on the 

NCore multi-pollutant framework or some other strategy that allows flexibility and 

targets measurements in areas with higher levels of coarse particles. 
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All comments received from air agencies and multi-state organizations were 

supportive of the removal of the PM10-2.5 speciation requirement. 

A few industry commenters raised concerns about the availability of PM10-2.5

speciation data for research purposes. One environmental group opposed revoking the 

PM10-2.5 speciation requirement and expressed the need for PM10-2.5 data to support health 

effects research and future regulatory efforts. 

The EPA has considered the comments from air agencies that were all supportive 

of revoking the requirement, as well as the industry concernand environmental groups 

concerns that PM10-2.5 speciation data will not be available for research. The In 

considering these comments, the EPA recognizes the importance of efforts to develop 

and evaluate speciation monitoring approaches for PM10-2.5 given that there is relatively 

little information available on the chemical and biological composition of PM10-2.5 and on 

the health effects associated with the various components (U.S. EPA, 2009a, section 

2.3.4). Without more information on the chemical speciation of PM10-2.5, the apparent 

variability in associations with health effects across locations is difficult to characterize 

(U.S. EPA, 2009a, section 6.5.2.3). However, the EPA believes that until a final report on 

the findings from the pilot study is completed in 2013 and the results of the study can be 

considered, PM10-2.5 speciation is not ready for nationwide deployment. Therefore, the 

EPA is finalizing its decision to revoke the PM10-2.5 speciation requirement at NCore 

stations. Given the continued importance of characterizing PM10-2.5 species, and given 

that ongoing and future research will likely further inform the development of speciation 

methods, the appropriateness of requiring speciation monitoring for PM10-2.5 will be 

revisited in future reviews.
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d. Measurements for the Proposed New PM2.5 Visibility Index NAAQS

The EPA proposed requirements for sampling of PM2.5 chemical speciation in 

states with large CBSAs. However, as explained in section VI, the EPA is not finalizing 

the proposed secondary PM2.5 visibility index NAAQS and therefore is not finalizing the 

proposed monitoring changes associated with that standard. 

4. Revisions to the Quality Assurance Requirements for SLAMS, SPMs, and PSD

a. Quality Assurance Weight of Evidence

The EPA proposed to use a weight-of-evidence approach for determining whether 

the quality of data is appropriate for regulatory decision-making purposes. While the 

EPA believes that it is essential to require a minimum set of checks and procedures in 

appendix A to support the successful implementation of a quality system, the success or 

failure of any one check or series of checks should not preclude the EPA from 

determining that data are of acceptable quality to be used for regulatory decision-making 

purposes. Accordingly, the EPA proposed to include additional wording in appendix A to 

clarify the role that appendix A generated data quality indicators have in the overall 

quality system that supports ambient air monitoring activities. 

The EPA received eight comments on the weight of evidence approach with the 

majority of commenters endorsing the approach. One commenter felt that the “paragraph, 

as written, undermines the importance of the quality control/quality assurance system 

dictated in Part 58.” Some that supported the approach also provided a word of caution 

that “while a common sense approach to the assessment of quality data is important, 

minimum requirements are necessary to ensure scientifically-defensible data is being 

used in decision making”. The EPA agrees with the commenter’s points that data should 

be subject to a minimum set of requirements for data collection, reporting and quality. In 
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developing the weight of evidence approach, the EPA is not attempting to diminish the 

requirements of appendix A but rather ensure that other elements of a quality system that 

air agencies implement and are documented in their QAPP can also be used when judging 

whether data are valid for a particular monitoring objective. While the EPA considers the 

appendix A requirements the minimum for reporting, it is not the only data that the EPA 

and the air agencies use to judge quality. Therefore, if an appendix A requirement for 

some reason is not complete, the EPA concludes that it should not necessarily be the sole 

reason to declare the data invalid or unusable. One commenter who felt that the approach 

may be appropriate also suggested that the language of the proposal was vague and may 

weaken the ability of air monitoring agencies to validate their own data and instead 

allows the EPA to make decisions regarding data validity. In the majority of cases when 

the quality of ambient air data is called into question, the EPA Regions and air agencies

work together and reach consensus on data usability. The EPA agrees that the air 

agencies know more about their data and it is the air agencies responsibility to certify the 

data as valid. In most cases, the EPA and the air agencies will be in agreement on the 

validity and usability of this data. However, since the EPA is responsible for making final 

regulatory decisions concerning the NAAQS, in rare cases it may ultimately have to 

make a validity decision that the air agencies may not agree with. After consideration of

the general support received, the EPA will finalize the language as proposed. For the 

reasons explained above, the EPA concludes that this will not undermine the quality 

assurance system, but rather strengthen it.

A few commenters, although supporting the weight of evidence approach, also 

commented that appendix A minimum requirements should not only apply to all air 
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quality data collected by state, local, and tribal agencies, but also to “secondary” data 

collected by other monitoring efforts. The EPA understands that this term is used by these 

commenters to either represent the Chemical Speciation and IMPROVE Network data 

being used to calculate light extinction for the secondary PM2.5 visibility index NAAQS, 

or for criteria pollutant data collected by entities other than the state, local or tribal 

monitoring organizations. The EPA agrees with the comments that the appendix A 

requirements must apply to the CSN and IMPROVE data, if the data were being used for 

comparison to the secondary NAAQS, and included the term “PM2.5 CSN” to refer to 

both networks. However, since as explained in Section VI, the secondary PM2.5 visibility 

index NAAQS is not being finalized, the EPA will be removing any text related to the 

CSN and IMPROVE requirements from appendix A. If the term is being used by 

commenters to refer to criteria pollutant data collected by entities other than the state, 

local or tribal monitoring organizations then the appendix A requirements, as has always 

been the case, apply to those monitors.

b. Quality Assurance Requirements for the Chemical Speciation Network

The EPA proposed to include requirements for flow rate verifications and flow 

rate audits for the PM2.5 CSN. Air agencies currently perform these audits even though 

they are not currently required. Thus, although they would be considered a new 

requirement, they are not new implementation activities. In addition, the CSN already 

includes six collocated sites which the EPA proposes to include in the 40 CFR part 58 

appendix A requirements. The EPA proposed that PSD sites would not be required to 

collocate a second set of instruments for speciated PM2.5 mass monitoring.

There were no comments that specifically addressed the addition of collocation 
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and flow rate requirements in appendix A for the chemical speciation network (CSN). 

Since these flow rates have historically been included in the Agencies’ CSN Network 

Quality Assurance Project Plan and implemented for many years, air agencies may not 

have considered them any additional burden on the program. However, as explained in 

Section VI, the secondary PM2.5 visibility index NAAQS is not being finalized; therefore,

the EPA will not include these QA requirements into appendix A since the networks will 

not produce data to be used for NAAQS decisions. 

c. Waivers for Maximum Allowable Separation of Collocated PM2.5 Samplers and 

Monitors

The EPA proposed to allow waivers, when approved by the EPA Regional 

Administrator, for collocation of PM2.5 samplers and monitors of up to 10 meters so long 

as the site is at a neighborhood scale or larger. The EPA proposed to allow waivers for 

the maximum allowable distance associated with collocated PM2.5 samplers and 

monitors. Ensuring PM2.5 continuous FEMs and PM2.5 FRMs meet collocation 

requirements (i.e., 1 to 4 meters for PM2.5 samplers with flow rates of less than 200 

liters/minute) can be challenging, since in some cases multiple instruments, FEMs

installed in the shelter and FRMs installed on a platform, are being sited at the same 

station. The EPA believes that instruments spaced farther apart could be maintained

within the operational precision of the instruments, especially at sites located at larger 

scales of representation (e.g., neighborhood scale and larger). 

All comments received responded in support of the requirement allowing up to 10 

meter horizontal spacing for sites at a neighborhood or larger scale of representation. The 

EPA received no negative comments on this part of the proposal. During stakeholder 

presentations of the proposal, the EPA received a verbal comment that air agencies were 
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also having difficulty meeting the one meter vertical criteria since PM2.5 FEMs are 

typically housed in shelters with inlets extending through shelter roofs while the 

collocated FRM monitors are placed outside, usually on platforms somewhat lower to the 

ground. After considering this comment, and further discussion with EPA Office of 

Research and Development on spacing requirements, the agency will amend the appendix

A requirements to allow for a 1-3 meter vertical spacing which may be approved by the 

Regional Administrator for sites at a neighborhood or larger scale of representation. In 

addition, the language will be amended to allow for waiver approvals during annual 

network plan approval processes. Alternatively, the existing waiver provision outlined in 

paragraph 10 of Appendix E may be used. 

5. Revisions to Probe and Monitoring Path Siting Criteria 

a. Near-road Component to the PM2.5 Monitoring Network

The EPA proposed that the probe and siting criteria for the near-road component 

of the PM2.5 monitoring network design follow the same probe and siting criteria as the 

NO2 near-road monitoring sites. These requirements would provide that the monitoring 

probe be sited “...as near as practicable to the outside nearest edge of the traffic lanes of 

the target road segments; but shall not be located at a distance greater than 50 meters, in 

the horizontal, from the outside nearest edge of the traffic lanes of the target road 

segment” (section 6.4 of appendix E to 40 CFR part 58). 

The EPA received comments from several stakeholders on the probe and siting 

criteria for PM2.5 monitors in the near-road environment. One public health group offered 

detailed comments on the probe and siting criteria for PM2.5 monitors in near-road 

environments. While the commenter offered support for collocating the PM2.5 monitors 
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with NO2 monitors in the near-road environment, there was concern expressed regarding 

allowing monitors at sites of more than 15 meters from the traffic corridor which is the 

source of the air quality concern. The commenter points out that the EPA’s existing rules 

for siting localized hot spot sites in areas of highest concentration require sites at 

microscale locations which provide for a distance of no more than 15 meters from a 

major roadway. Several air agencies offered consistent comments that the inlet of the 

PM2.5 monitors should be the same as that of the near-roadway NO2 monitors; however, 

one of the commenters suggested that the requirements for distance to the nearest vertical 

wall or obstruction should match the requirements for current micro and middle scale 

installations of PM2.5 monitors. The concern expressed is that a wall or obstruction may 

disrupt the normal downwind flow across a roadway. 

In reviewing comments on probe and monitoring path criteria for PM2.5 monitors 

in near road environments, and whether to make any changes, the EPA has several issues 

to consider. One of the most important things to consider is what the intended network 

design of these monitors should be. As stated in the proposal our goal is to “better 

understand the health impacts of these (near-road PM2.5) exposures,” that a number of 

monitoring objectives can be supported by having near-road PM2.5 monitors, and that 

while it might be that the location of maximum concentration of PM2.5 exposure from 

near-roadway sources might differ from the maximum location of NO2 or other 

pollutants, we proposed to require that the near-road PM2.5 monitors be collocated with 

the planned NO2 monitors. The EPA did not propose to change the distance from 

obstructions for PM2.5 monitors in its proposal. 

As we stated in the proposal, the planned NO2 monitors are using several relevant 
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factors that are also relevant for siting of PM2.5 (e.g., average annual daily traffic and fleet 

mix [accounting for heavy duty vehicles] by road segment) and that significant thought 

and review are going into the design of the near-road stations. Therefore, the EPA is not 

persuaded that we should provide any additional constraints to the siting of the station 

(i.e., the distance from the roadway) than is already provided for in the NO2 near-road 

monitor probe and monitoring path siting criteria. The EPA is concerned that additional 

constraints (i.e., to require sites within 15 meters of the road), might have some 

advantages, but also might unnecessarily eliminate otherwise useful near-road locations 

that on balance might be a better candidate location. 

The EPA recognizes that there may be cases where the physical location of a 

near-road monitoring station is farther than 15 meters, but no greater than 1550 meters 

from the roadway, but such cases are presumed to still be the most useful location for the 

siting of the NO2 monitors, which we then proposed to collocate with PM2.5. Regardless 

of the actual distance of the inlet for the PM2.5 monitor at the near-road monitoring 

station, so long as it is collocated with the approved near-road station for NO2 and meets 

existing criteria, the EPA will consider this site to be appropriate as a near-road PM2.5

monitoring station. As explained in the proposal, there are a number of reasons to collect 

multi-pollutant data in the near-road environment. The EPA believes that these sites will 

be sufficient as representative maximum concentration sites for NO2 and PM2.5 in the 

near-road environment and not necessarily the single maximum concentration location of 

each pollutant for an area. . As noted above, where an air agency believes a different 

location is a more appropriate site for a near-road PM2.5 monitor, the EPA can use its 

discretion in approving a deviation from the PM2.5 monitoring requirements as already 
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exists in the network design criteria. A deviation would be appropriate for consideration 

where, for example, a state provides quantitative evidence demonstrating that peak 

ambient PM2.5 concentrations would occur in a near-road location which meets siting 

criteria but is not a near-road NO2 monitoring site. Such deviations are to be approved by 

the Regional Administrator as described in section 4.7.1 of Appendix D to part 58.

While it is still desirable for the near-road stations to be as close to the road as 

practical, there may be differences in the scale of representation of the near-road PM2.5

monitor from one location to another, while the NO2 near-road monitors are at the same 

scale of representation (i.e., micro-scale) in different locations. This is a result of the 

scale of representation being based on the pollutant at a location and not the location 

alone. Therefore, in cases where the station is 20 meters from a major road, the NO2

measurement may still be micro-scale, while the PM2.5 measurement would be middlescale if the average daily traffic count were sufficiently large enough.260 If a site with 

both measurements were 10 meters from a major road they would both be expected to be 

micro-scale sites.

In considering the comment on distance from obstructions, the EPA notes that a 

monitoring station with multiple measurements is effectively considered collocated for 

those measurements, even though the actual location of the inlets is slightly different 

from each other within the station. For example, a gas monitor (e.g., for carbon 

monoxide) may be pulling ambient air from a manifold with an inlet located on one part 

of a station roof, while a PM monitor is pulling air directly from its inlet located a few 

meters away on the same roof. The EPA believes it is appropriate and consistent with the 

 

260 See Table E-1 in Appendix E to Part 58 for defining the scale of representation of a 

PM sampler based on its distance to the nearest traffic lane and average daily traffic 

count.
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public comment above on distance from obstructions to maintain the existing 

requirements for distance from obstructions on a pollutant by pollutant basis, even if they 

are different for PM2.5 and NO2 monitors that will be at the same station. Air agencies 

will need to consider these distances from obstructions for each pollutant inlet probe (i.e., 

>1 meter for NO2 monitors and >2 meters for PM2.5 monitors) in locating monitors at the 

station.

The EPA is maintaining the existing probe and siting criteria for PM2.5 monitors; 

however, we are finalizing the provision that the required near-road component of the 

PM2.5 monitoring network design shall be collocated with a required NO2 monitor at 

near-road monitoring station. These near-road NO2 monitoring stations are to be sited 

“...as near as practicable to the outside nearest edge of the traffic lanes of the target road 

segments; but shall not be located at a distance greater than 50 meters, in the horizontal, 

from the outside nearest edge of the traffic lanes of the target road segment” (section 6.4 

of appendix E to 40 CFR part 58). The EPA is retaining the existing requirement that 

PM2.5 inlets, including those at near road stations, must be >2 meters from obstructions.\.

b. CSN Network

As explained in Section VI, the EPA is not finalizing the proposed secondary 

standard based on a PM2.5 visibility index and therefore will not be finalizing probe and 

siting criteria associated with the use of CSN measurements. 

c. Reinsertion of Table E-1 to Appendix E

The EPA proposed to reinsert table E-1 to appendix E of 40 CFR part 58. This 

table presents the minimum separation distance between roadways and probes or 

monitoring paths for monitoring neighborhood and urban scale ozone (O3) and oxides of 
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nitrogen (NO, NO2, NOx, NOy). This table was inadvertently removed during a previous 

CFR revision process. 

The only comments received on this topic were supportive of the reinsertion of 

table E-1; therefore, the EPA is finalizing the reinsertion of this table, unchanged from its 

prior iteration, back into the CFR. 

6. Additional Ambient Air Monitoring Topics

a. Annual Monitoring Network Plan and Periodic Assessment

In October of 2006, the EPA finalized new requirements for each state, or where 

applicable, local agency to perform and submit to their EPA Regional Offices an 

Assessment of the Air Quality Surveillance System (40 CFR 58.10). This assessment is 

required every five years. The first required five year assessments were due to EPA 

Regional Offices by July 1, 2010. The assessments are intended to provide a 

comprehensive look at each monitoring agency’s ambient air monitoring network to 

ensure that the network is meeting the minimum monitoring objectives defined in 

appendix D to 40 CFR part 58, whether new sites are needed, whether existing sites are 

no longer needed and can be terminated, and whether new technologies are appropriate 

for incorporation into the ambient air monitoring network.261

Since each agency has completed its first required five-year assessment, and 

several monitoring rule requirements have either been added or changed since this 

requirement was added in 2006, the EPA thought it was appropriate to review this 

requirement and solicit comment on any possible changes the EPA should consider that 

may improve the usefulness of the assessments. Specifically, the EPA solicited comment 

 

261 The EPA provides a link to these assessments on EPA’s website at: 

http://www.epa.gov/ttn/amtic/plans.html. A detailed description of the requirements for 

the assessments is described in 40 CFR 58.10.
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on ways to either streamline or add additional criteria for future assessments. 

The EPA also proposed to remove references to “community monitoring zones” 

and “spatial averaging” in the annual monitoring network plans due to EPA Regional 

Offices by July 1 of each year. The Agency proposed to remove these references since, as 

discussed in section VII.A.2 above, the EPA proposed to remove all references to the 

spatial averaging option throughout 40 CFR part 50 appendix N. Consistent with these 

changes, the EPA also proposed to remove references to community monitoring zones 

under the annual monitoring network plans described in 40 CFR 58.10. 

The EPA received comments from several air agencies on the five year 

assessments. Most comments on the five year assessments focused on the type and 

usefulness of assessment tools made available to air agencies during the last review. Of 

specific note were concerns that assessment tools used to evaluate networks on a regional 

or national basis do not provide the spatial resolution necessary to adequately assess state 

networks on a scale most useful to air agencies. This is especially true when attempting to 

evaluate smaller scale monitoring or pollutant gradients associated with near-road and 

source oriented monitoring. Suggestions for improvement identified the need for the EPA 

to work closely with air agencies early in the next cycle of assessments (due in 2015) so 

that any tools developed can be of benefit to the questions air agencies need to address 

for their programs. The EPA did not receive any comments on removing references to 

community monitoring zones specifically as it pertains to their listing in the annual 

monitoring network plans described in 40 CFR 58.10.262

The EPA took comment on potential improvements to the five year assessments. 

 

262 Comments on the substantive question of whether to revoke references to community 

monitoring zones were addressed in section VIII.B.1.
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All the recommendations received focused on the types of assessments to perform and 

ensuring that the EPA works closely with air agencies so that assessments will be of 

benefit to the air agencies. No specific recommendations were made to add or remove 

any of the requirements of the five year assessments and consequently the EPA is not 

making any changes. The EPA intends to work with air agencies to ensure future tools 

are as helpful as practiblepracticable. 

Consistent with the decision to end the practice of spatial averaging, the EPA is 

finalizing the removal of language that references “community monitoring zones” and 

“spatial averaging” in the annual monitoring network plans due to EPA Regional Offices 

by July 1 of each year. 

b. Operating Schedules

The EPA generally requires PM2.5 SLAMS to operate on at least a 1-day-in-3 

sampling schedule, unless a reduced sampling frequency is approved such as might be the 

case with a site that has a collocated continuous operating PM2.5 monitor.263 However, in 

the 2006 monitoring rule amendments, the EPA finalized a new requirement for the 

operating schedule of PM2.5 SLAMS sites (40 CFR 58.12). The new requirement stated 

that sites with a design value within plus or minus five percent of the 24-hour PM2.5

NAAQS must have an FRM or FEM operating on a daily sampling schedule. This 

requirement was included to minimize any statistical error associated with the form of the 

24-hour PM2.5 NAAQS (i.e., the 98th percentile). In section III.F, the Administrator is 

finalizing revisions to the level of the primary annual PM2.5 NAAQS. Accordingly, 

possible changes to sampling frequency requirements were also considered. 

 

263 All NCore stations must operate on at least a one-in-three day sample frequency for 

filter-based PM sampling.
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The EPA had previously considered how sample frequency affects the Data 

Quality Objectives in a consultation with the CASAC AAMMS in September of 2005 (70 

FR 51353 to 51354, August 30, 2005). As a result of that consultation, the EPA proposed 

(71 FR 2710 to 2808, January 17, 2006) and finalized (71 FR 61236 to 61328, October 

17, 2006) changes to the sample frequency requirements as part of the monitoring rule 

changes in 2006. In that work, the EPA demonstrated that having a higher sample count 

is generally more useful to minimize uncertainty for a percentile standard than an annual 

average. Given the decision to strengthen the primary annual PM2.5 NAAQS and the 

known burden of performing daily sampling using the filter-based samplers that are still a 

mainstay in monitoring agency networks, the issue of needing daily sampling for sites 

that have design values close to the level of the 24-hour PM2.5 standard was reconsidered 

if the site already has a design value above the level of the primary annual PM2.5

NAAQS. 

In a related issue, since the EPA finalized the requirement for daily sampling at 

sites within 5 percent of the 24-hour PM2.5 NAAQS in 2006, there has been confusion 

over the procedures for adjusting sample frequencies, where necessary, to account for 

variations in year-to-year design values. Therefore, the EPA proposed to revise this 

requirement in the following ways: (1) The EPA proposed that monitors would only be 

required to operate on a daily schedule if their 24-hour design values were within five 

percent of the 24-hour PM2.5 NAAQS and the site had a design value that was not above 

the level of the annual PM2.5 NAAQS. (2) The EPA proposed that review of data for 

purposes of determining applicability of this requirement at a minimum be included in 

each agency’s annual monitoring network plan described in 40 CFR 58.10 based on the 
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three most recent years of ambient data that were certified as of the May 1 annual 

deadline. However, monitoring agencies may request changes to sample frequency at any 

time of the year by submitting such a request to their applicable EPA Regional Office. 

Changes in sampling frequency are expected to take place by January 1 of the following 

year. Increased sampling is expected to be conducted for at least three years, unless a 

reduction in sampling frequency has been approved in a subsequent annual monitoring 

network plan or otherwise approved by the Regional Administrator. 

Comments received on the sample frequency requirements for PM2.5 were from 

air agencies, who were generally supportive of the EPA’s proposed approach.

The EPA is finalizing its proposal to modify the sample frequency requirements 

for triggering daily sampling so that only those areas with 24-hour design values within 

five percent of the 24-hour PM2.5 NAAQS and where the design value site is not above 

the level of the annual PM2.5 NAAQS would be required to operate on a daily sample 

frequency. The EPA is also finalizing all other aspects of this part of the proposal.

c. Data Reporting and Certification for CSN and IMPROVE Data

The EPA is not finalizing its proposal on minor changes to reporting and 

certification of data associated with CSN and IMPROVE networks since as explained in 

Section VI, EPA is not finalizing a secondary standard to support visibility impairment 

that would have used CSN and IMPROVE data.

d. Requirements for Archiving Filters

The EPA proposed to extend the requirement for archival of PM2.5, PM10, and 

PM10-2.5 filters from manual low-volume samplers (samplers with a flow rate of less than 

200 liters/minute) at SLAMS from one year after data collection to five years after data 
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collection. The archive of low-volume PM filters is an important toolresource for ongoing research and development of emission control strategies and for use in health and 

epidemiology research. During a workshop on Ambient Air Quality Monitoring and 

Health Research in 2008, retaining filters for laboratory analysis was identified as a key

recommendation to provide daily measurements of metals and elements (U.S. EPA, 

2008d, pp. 17 to 21). The EPA’s previous requirement of oneyearone-year is not 

sufficiently long for retrospective analysis of important episodes and for use in long-term 

epidemiology research. Since firstinitially requiring filter archival of low-volume PM 

filters in 1997, the EPA has always recommended longer archiving of filters and most 

agencies are already doing so. However, a small number of agencies have reported 

discarding older filters, despite the minimal cost of storing these filters. Since cold 

storage of a large number of filters may be cost prohibitive and of little benefit in 

retaining key aerosol species in the x-ray fluorescence (XRF) analyses, the EPA proposed

to minimize the costs of retaining filters by only requiring cold storage during the first 

year after sample collection. 

All comments received on this issue were from air agencies, which were largely 

supportive of such a change to this requirement. One air agency did report that it would 

present a hardship to store filters for such a long period of time as they did not have the 

room to support such a requirement.

The EPA is finalizing the requirement for archival of PM2.5, PM10, and PM10-2.5

filters from manual low-volume samplers (samplers with a flow rate of less than 200 

liters/minute) at SLAMS for a minimum of five years after data collection. , with cold 

storage only required for the first 12 months of archiving. The EPA will work closely 
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with air agencies through its EPA Regional Offices and laboratories to support any air 

agency unable to store filters for the new five year requirement. 

IX. Clean Air Act Implementation Requirements for the PM NAAQS

This section of the preamble discusses the general approach for air agencies264 to 

meet certain CAA requirements for implementing the revised primary annual PM2.5

NAAQS as part of the revised suite of NAAQS for PM. In accordance with CAA section 

107(d), the PM NAAQS revisions trigger a process under which states must and tribes 

may make recommendations to the Administrator regarding area designations, and the 

EPA will take final action on those designations. Under section 110 of the CAA and 

related provisions, states are also required to submit, for EPAthe EPA’s approval, SIPs 

that provide for the attainment and maintenance of the revised NAAQS through control 

programs directed at sources of direct PM2.5 and precursor emissions. If a state fails to 

adopt and implement the required SIPs by the time periods provided in the CAA, the 

EPA has responsibility under the CAA to adopt a Federal Implementation Plan (FIP) to 

assure that areas attain the NAAQS in an expeditious manner. Additionally, emissions 

sources and air agencies must address the revised PM NAAQS in the context of 

preconstruction air permitting requirements and the transportation conformity and general 

conformity processes. 

In addition to today’s revisions to the primary annual PM2.5 NAAQS, the EPA is 

taking final action on a limited subset of implementation-related program elements, to 

provide a workable transition to implementing the revised standards under the PSD 

permitting program and to clarify the anticipated schedule for initial area 

 

264 This and all subsequent references to “air agency” are meant to include state, local and 

tribal agencies responsible for the implementation of a PM2.5 control program. 
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designations.PSD implementation provision. To facilitate timely implementation of the 

PSD requirements resulting from the revised NAAQS, which would otherwise become 

applicable to all PSD permit applications upon the effective date of this final PM 

NAAQS rule, the EPA is finalizing a grandfathering provision for pending permit 

applications as originally proposed.. This final rule incorporates revisions to the PSD 

regulations that provide for grandfathering of PSD permit applications that have been 

determined to be complete on or before December 14, 2012 or for which public notice of 

a draft permit or preliminary determination has been published as of the effective date of 

today’s revised PM2.5 NAAQS. Accordingly, for projects eligible under the 

grandfathering provision, sources must meet the requirements associated with the prior 

primary annual PM2.5 NAAQS rather than the revised primary annual PM2.5 NAAQS. 

The EPA also proposed to implement a surrogacy approach for addressing PSD 

requirements associated with the proposed distinct secondary visibility index NAAQS. 

As described in section VI, the EPA is not finalizing a distinct secondary visibility index 

standard at this time and therefore the proposed surrogacy approach for implementing 

such a standard under the PSD program is unnecessary. Additionally, as discussed in 

section IV, today’s final rule does not include any changes to the existing PM10 NAAQS. 

Accordingly, this section of the preamble does not include any discussion of 

implementation specifically related to the PM10 NAAQS.

Under the schedule in section 107(d)(1) of the CAA, as confirmed in this action, 

state Governors and tribes, if they choose, are required to submit their initial designation 

recommendations for the revised primary annual PM2.5 NAAQS to the EPA no later than 

1 year following promulgation of the revised NAAQS (i.e., by December 13, 2013). The 
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EPA will provide designation guidance to air agencies concurrent with, or very shortly 

after today’s final NAAQS rule to assist them in formulating their designation 

recommendations. The EPA intends to complete initial designations for the revised 

primary annual PM2.5 NAAQS by December 12, 2014 using available air quality data 

from the current PM2.5 monitoring networks.

In addition to describing the limited subset of NAAQS implementation 

elementsPSD grandfathering provision being finalized in today’s rule and responding to 

associated public comments, this section of the preamble describes the EPA’s future 

plans for addressing the remaining aspects of implementation, such as infrastructure SIP 

submittals and nonattainment area planning. In the proposed rule, the EPA solicited 

preliminary comment on some of the issues that the Agency anticipates will need to be 

addressed in future guidance or regulatory actions related to implementation of the 

revised PM2.5 NAAQS. The EPA received comments on a few of these issues and, as 

explained in greater detail later in this section, the EPA either has considered or will 

consider, as appropriate, all substantive comments received as future guidance and

regulationsproposed rules are developed.

A. Designation of Areas

1. Overview of Clean Air Act Designations Requirements

After the EPA establishes or revises a NAAQS, the CAA requires the EPA and

the states to take steps to ensure that the new or revised NAAQS is met. The first step, 

known as the initial area designations, involves identifying areas of the country that either 

meet or do not meet the new or revised NAAQS along with the nearby areas contributing 

to violations. Section 107(d)(1) of the CAA states that, “By such date as the 
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Administrator may reasonably require, but not later than 1 year after promulgation of a 

new or revised national ambient air quality standard for any pollutant under section 109, 

the Governor of each state shall ... submit to the Administrator a list of all areas (or 

portions thereof) in the State” that designates those areas as nonattainment, attainment, or 

unclassifiable.265 Section 107(d)(1)(B)(i) further provides, “Upon promulgation or 

revision of a NAAQS, the Administrator shall promulgate the designations of all areas (or 

portions thereof) ... as expeditiously as practicable, but in no case later than 2 years from 

the date of promulgation. Such period may be extended for up to one year in the event the 

Administrator has insufficient information to promulgate the designations.” The term 

“promulgation” has been interpreted by the courts with respect to the NAAQS to be 

signature and widespread dissemination of a rule. By no later than 120 days prior to 

promulgating designations, the EPA is required to notify states of any intended 

modifications to their recommendations, including area boundaries, that the EPA may 

deem necessary. States then have an opportunity to demonstrate why the EPA's intended 

modification is inappropriate. Whether or not a state provides a recommendation, the 

EPA must timely promulgate the designation that it deems appropriate. While section 107 

of the CAA specifically addresses states, the EPA intends to follow the same process for 

tribes that choose to make a recommendation to the extent practicable, pursuant to section 

301(d) of the CAA regarding tribal authority, and the Tribal Authority Rule (63 FR 

7254;, February 12, 1998). To provide clarity and consistency in doing so, the EPA 

issued a 2011 guidance memorandum on working with tribes during the designations 

 

265 While the CAA says “designating” with respect to the Governor’s list, in the full 

context of the CAA section it is clear that the Governor actually makes a 

recommendation to which the EPA must respond via a specified process if the EPA does 

not accept it.
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process (Page, 2011).

2. Proposed Designations Schedules

When the EPA proposed the new and revised PM NAAQS on June 29, 2012, the 

EPA indicated an intention to follow the standard 2 -year schedule for initial area 

designations for both the revised primary annual PM2.5 standard and the proposed 

secondary PM visibility index standard, noting that promulgating initial area designations 

for these standards on the same schedule would provide early regulatory certainty for 

states. Under this approach, the EPA intended to complete initial designations for both 

the revised primary annual PM2.5 NAAQS and the secondary PM visibility index 

NAAQS by December 2014 using available air quality data from the current PM2.5 and 

speciation monitoring networks using the most recent 3 consecutive years of certified air 

quality monitoring data (i.e., most likely data from 2011-2013). 

The EPA’s June 29, 2012, notice proposed new requirements for establishing 

near-road PM2.5 monitors in certain cities (section VIIVIII.B.3.b.i of the proposal) and 

new requirements for each state with a CBSA over 1 million in population to add or

relocate an existing CSN (or IMPROVE) monitoring site in at least one of its CBSAs to 

collect speciated PM2.5 data to support implementation of the proposed secondary 

standard to address visibility impairment (section VIIVIII.A.2 of the proposal). The EPA

anticipated that 3 consecutive years of air quality data from any near-road monitoring 

sites or newly placed CSN (or IMPROVE) PM2.5 speciated monitoring site would not be 

available until 2018. The timing for both of these proposed monitoring changes would

preclude the use of the collected data in initial area designations, and therefore, the EPA 

stated in the proposal that initial area designations would not take into account 
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monitoring data from any newly established near-road monitoring sites, nor from newly 

established speciation monitoring sites. 

3. Comments and Responses

The EPA received numerous comments on the proposed designations schedules 

from states, state organizations, local air pollution control agencies, regional 

organizations, industry, environmental organizations, and health-related organizations. 

Most commenters expressed support for a standard 2-year schedule for initial area 

designations for the primary annual standard. Several commenters also encouraged the 

EPA to consider an additional year for initial area designations associated with the 

proposed secondary PM visibility index standard due to the lag in obtaining data from 

speciation monitoring networks, the variability in monitored relative humidity data, and 

the “unique” nature of the proposed secondary standard. For the reasons stated in section 

VI.D.2, the Administrator has decided not to establish the proposed distinct secondary 

standard to address visibility impairment, and therefore, the EPA will not promulgate 

initial area designations for a secondary PM visibility index standard. Because data are 

currently available from numerous existing PM2.5 mass monitoring sites to determine 

compliance with the revised primary annual PM2.5 NAAQS, the EPA believes it is 

appropriate to pursue a standard 2-year schedule for initial area designations for the 

primary annual PM2.5 NAAQS.

The EPA also received numerous comments related to the use of data from the 

proposed new near-road monitors in the designations process. Several commenters asked 

the EPA to clarify whether these data will be used if available for initial area 

designations. Others asked the EPA to provide guidance related to establishing 
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boundaries for areas containing violating near-road monitors. One commenter suggested 

that the EPA conduct dispersion modeling around transportation facilities in accordance 

with the EPA’s transportation conformity hotspot modeling guidance and use 

concentrations to determine attainment status for designations process. This same 

commenter also supported using modeling for unmonitored areas, e.g., communities near 

roadways. 

As previously stated, sectionAs previously stated, the EPA does not believe that 

data from the new near-road monitors will be available for the EPA to consider within the 

timeframe for initial area designation provided by the CAA. Section 107(d)(1)(B) of the 

CAA requires the EPA to designate areas no later than 2 years following promulgation of 

a new or revised NAAQS, or by December 2014. (The CAA provides the Agency an 

additional third year from promulgation should there be insufficient information on 

which to make compliance determinations). For initial area designations for the primary 

annual PM2.5 NAAQS, the EPA relies exclusively on monitoring data to identify areas to 

be designated nonattainment due to violations of the standards and then uses other 

information to identify areas contributing to violations. in those areas. See Catawba 

County v. EPA, 571 F.3d 12-13 (D.C. Cir. 2009). As indicated in the notice for the 

proposed PM NAAQSproposal, the initial set of near-roadway PM2.5 monitors are not 

expected towill be fully deployed untilby January 2015, with the requisite 3 years of air 

quality data available in 2018.266 The EPA intends to proceed with initial area 

designations using 3 years of consecutive air quality data from the existing, area-wide 

FRM/FEM/ARM PM2.5 monitoring sites to complete designations by December 2014.

 

266 The remainder of the near-road monitors in CBSAs with populations between 1

million but less than 2.5 million will be deployed by January 1, 2017.
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The EPA intends to consider data from all PM2.5 monitors meeting these criteria and 

would include near-road data, if available. Consistent with previous area designations 

processes used in informing boundary decisions, the EPA would then useanalyze a 

variety of area-specific factor analysesinformation267 in determining which nearby areas 

contribute to a violation. As previously indicated, the EPA relies on monitoring data to 

identify areas to be designated nonattainment due to violations of the standards and does 

not intend to conduct or use dispersion modeling around transportation facilities or in 

unmonitored areas to determine whether an area is violating the primary annual PM2.5

NAAQS for purposes of establishing nonattainment areas as this is not required by the 

statute. See Catawba County v. EPA, 571 F.3d 12-13 (D.C. Cir. 2009). The EPA intends 

to address the factor analysisuse of area-specific information and the boundary setting 

process, including the presumptive starting area boundary, in the designation guidance to 

the states, expected to be available shortly after promulgation of the PM NAAQS.

4. Final Intended Designations Schedules

In this final rule, the EPA is setting a revised, more protective primary annual 

PM2.5 NAAQS. After considering the public comments and for the reasons discussed 

above, the EPA intends to designate areas for the primary annual PM2.5 NAAQS on a 2-

year schedule from signature of this final PM NAAQS rule, as prescribed in CAA section 

107.268 Under the schedule in section 107(d)(1) of the CAA, as confirmed in this action, 

 

267The EPA has used area-specific factor analysesinformation to support boundary 

determinations by evaluating factors such as air quality data, emissions and emissionsrelated data, meteorology, geography/topography, and existing jurisdictional boundaries. 

This may include, as appropriate, information from non-FRM/FEM/ARM monitors and 

air quality modeling, where available, to help define an appropriate boundary for areas 

contributing to FRM/FEM/ARM-based monitored violations. 

268While the EPA intends to make every effort to designate areas for the primary annual 

PM2.5 NAAQS on a 2 -year schedule, the EPA recognizes that new information may later 

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state Governors and tribes, if they choose, are required to submit their initial designation 

recommendations for the revised primary annual PM2.5 NAAQS to the EPA no later than 

1 year following promulgation of the revised NAAQS (i.e., by December 13, 2013). 

These recommendations should be based on air quality data from the years 2010 to 2012. 

If the EPA intends to make any modifications to a state's or tribe’s recommendations, the 

EPA is required to notify the state or tribe no later than 120 days prior to finalizing the 

designation; this would be no later than August 14, 2014. States and tribes will then have 

an opportunity to demonstrate why the EPA's intended modification is inappropriate 

before the EPA makes the final designation decisions. Prior to the EPA’s signing a final 

rule by December 12, 2014, promulgating the initial area designations for the 2012 

primary annual PM2.5 NAAQS, data from 2013 may be available. If so, the EPA’s 

designations decisions will be based on air quality data from the years 2011 to 2013. 

States and tribes may update their recommendations when these new data become 

available. 

In the proposal, the EPA stated its intention to provide technical information and 

guidance to states shortly after promulgation of the NAAQS to assist states and tribes in 

the development of their designation recommendations. The EPA understands that 

developing recommendations on appropriate nonattainment area boundaries is a 

significant effort for states, especially for states with little or no experience in PM2.5 air 

quality planning. Therefore, the EPA plans to assist states throughout the designations 

process on technical and policy-related issues through outreach efforts that will provide 

information and data sources relevant to making designations decisions. The EPA will 

 

arise that justifies the need for additional time, up to 1 additional year available based on 

insufficiency of data, to complete the process. Any subsequent change to the designations 

schedule would be announced.
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include such information for the revised primary annual PM2.5 NAAQS on the general 

PM2.5 designations website at http://www.epa.gov/pmdesignations. The EPA also 

encourages states and tribes to consult with their EPA regional office as they develop 

their area recommendations.

B. Section 110(a)(2) Infrastructure SIP Requirements

The proposal described the CAA requirements for air quality management 

infrastructure SIPs that states must submit to the EPA within 3 years after promulgation 

of a new or revised primary standard. As discussed in the proposal, while the CAA 

allows the EPA to set a shorter time for submission of these SIPs, the EPA does not 

currently intend to do so. In the proposal, the EPA solicited comment on infrastructure 

SIP submittal timing, in addition to “all aspects” of infrastructure SIPs, for the Agency to 

consider in developing future guidance. The EPA received comments recommending that 

the EPA provide states an additional 18 months to submit SIPs for any revised secondary 

standard, but because the Agency is not revising the secondary NAAQS in this rule, the 

issue of whether or not to allow states extra time to submit infrastructure SIPs for the 

secondary NAAQS is now moot. The EPA received several comments on other aspects of

infrastructure SIPs, which are being considered in the development of a forthcoming 

guidance document on section 110 infrastructure SIP requirements that will apply to all 

NAAQS, including the revised PM2.5 NAAQS. In addition, the EPA may issue 

supplemental infrastructure SIP guidance specific to the revised PM2.5 NAAQS if needed. 

C. Implementing the Revised Primary Annual PM2.5 NAAQS in Nonattainment Areas 

In the proposal, the EPA described the basic CAA requirements that govern SIP 

submittals for nonattainment areas (77 FR 39019-21;38890, June 29, 2012 at 39019-21). 

The Agency did not propose any particular approach for implementing any revised PM2.5
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standards, but rather indicated its intent to carry out a notice-and-comment rulemaking to 

propose and issue a final implementation rule that willwould spell out the implementation 

requirements for the revised primary annual PM2.5 NAAQS and the revised monitoring 

regulations. The EPA acknowledges that several states and industry groups commented 

on the need for the EPA to issue an implementation rule, either in proposed or final form, 

simultaneous with this final PM NAAQS rule. Other commenters commented that the 

EPA should consult with states and local air agencies to develop the future 

implementation rule and to do so expeditiously, while another state commenter requested 

that the EPA commit to firm deadlines for issuing the future implementation rule and 

guidance related to infrastructure SIPs, among other things. 

We acknowledgeThe EPA acknowledges states’ need for timely guidance on how 

to implement the revised NAAQS. However, due to the number of unique and complex 

issues associated with the PM NAAQS proposal and uncertainty about the outcome of the 

final NAAQS, the EPA is not able to propose an implementation rule or finalize any 

aspect of the implementation program beyond a limited subset of issues (related to the 

PSD permitting program and initial area designations)grandfathering provision discussed 

later in this section at this time. Because we agree that it is beneficial to engage with air 

agencies early in the rule development process, however, we have initiated such 

discussions to inform the upcoming proposed rule. The EPA intends to finalize the 

implementation rule around the time the initial area designations process is finalized. 

One particular implementation-related issue that the EPA sought preliminary 

comment on in the proposal was the concept of a transition period during which any 

changes in monitoring requirements would not affect attainment plans and maintenance 
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plans for the 1997 and 2006 PM2.5 NAAQS. The EPA received a range of comments both 

in support of and in opposition to such a concept. Upon further analysis of the potential 

effect of monitoring requirement changes, and in consideration of comments received, we 

believe that it will not be necessary to provide for such a transition period in the future 

implementation rule because the changes in monitoring requirements included in this 

final rule would not automatically affect attainment plans and maintenance plans for the 

1997 or 2006 PM2.5 NAAQS. Specifically, there are currently approximately ten PM2.5 air 

quality monitors that have been identified as not comparable to the annual 

standardstandards as part of the annual state monitoring plan revision process. If a state 

chooses to revise the status of one of these monitors in order to make it comparable to the 

annual standardstandards because it is determined to be representative of many other 

similar locations, it would propose a change in status for that monitor in the next revision 

of the state PM2.5 monitoring plan (state revisions are due in June of each year). The EPA 

would then review and take action on the state's proposed change. The EPA believes that 

the monitoring plan revision process provides adequate procedural steps for identifying 

which monitors are to be comparable to the annual PM2.5 standardstandards. Thus for this 

reason, there is no need to include any "transition period" in a future rule.

The EPA appreciates the input received from commenters on implementation 

issues and will take it into consideration as we continue to work with air agencies to

develop our proposed implementation rule. In developing the future implementation rule

proposal, the EPA also plans to address any potential impact of the monitoring 

requirement changes being finalized in this rule, particularly on attainment planning and 

development of attainment demonstrations by states, and in doing so, we will consider 
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the preliminary comments received on this topic.

D. Prevention of Significant Deterioration and Nonattainment New Source Review 

Programs for the Revised Primary Annual PM2.5 NAAQS

The CAA requires states to include SIP provisions that address the 

preconstruction review of new stationary sources and the modification of existing 

sources. The preconstruction review of each new and modified source generally applies 

on a pollutant-specific basis and the requirements for each pollutant vary depending on 

whether the area is designated attainment (or unclassifiable) or nonattainment for that 

pollutant. Parts C and D of title I of the CAA contain specific requirements for the 

preconstruction review and permitting of new major stationary sources and major 

modifications, referred to as the PSD program and the nonattainment new source review 

(NNSR) program, respectively. Collectively, those permit requirements are commonly 

referred to as the “major NSR program” because of their applicability to new major 

stationary sources and major modifications. 

Today’s final rule revising the primary annual PM2.5 NAAQS will affect PSD 

permitting requirements as of the effective date of today’s final rule, [INSERT DATE 60 

DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER], which 

is also the effective date of the revised PM2.5 NAAQS. In addition, certain NNSR 

permitting requirements related to the revised PM2.5 NAAQS will take effect on and after 

the effective date of any nonattainment area designation for PM2.5. In order to minimize 

potential delays for pending PSD permit applications and to provide a reasonable 

transition, the EPA is finalizing a grandfathering provision for PSD permit applications 

that have reached a specified milestone in the permitting process. This final rule 

incorporates revisions to the PSD regulations that provide for grandfathering of PSD 
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permit applications for which the reviewing authority has determined the application to 

be complete on or before December 14, 2012 or for which the reviewing authority has 

first published public notice that a draft permit or preliminary determination for the 

permit has been issued prior to the effective date of today’s revised PM NAAQS. 

Accordingly, projects eligible under the grandfathering provisionsprovision must meet 

the requirements associated with the prior primary annual PM2.5 NAAQS rather than the 

revised primary annual PM2.5 NAAQS. As discussed in more detail in the following 

sections, the EPA is not now making any changes to the PM2.5 increments, nor are we 

revising any of the screening tools that are now used to implement PSDthe major NSR 

program for PM2.5. These screening tools include the significant emission rate (“SER”), 

used as a threshold for determining whether a given project is subject to major NSR 

permitting requirements under both PSD and NNSR; the significant impact levels 

(“SILs”), used to determine the scope of the required air quality analysis that must be 

carried out in order to demonstrate that the source’s emissions will not cause or 

contribute to a violation of any NAAQS or increment under the PSD program; and the 

significant monitoring concentration (“SMC”), a screening tool used to determine 

whether it may be appropriate to exempt a proposed source from the requirement to 

collect preconstruction ambient monitoring data as part of the required air quality 

analysis. 

1. Prevention of Significant Deterioration 

The PSD requirements set forth under part C (sections 160 through 169) of the 

CAA apply to new major stationary sources and major modifications locating in areas 

designated as “attainment” or “unclassifiable” with respect to the NAAQS for a particular 
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pollutant. The EPA regulations addressing the statutory requirements under part C for a 

PSD permit program can be found at 40 CFR 51.166 (containing the PSD requirements 

for an approved SIP) and 40 CFR 52.21 (the federal PSD permit program). For PSD, a 

“major stationary source” is one with the potential to emit 250 tons per year (tpy) or more 

of any air pollutant, unless the source or modification is classified under a list of 28 

source categories contained in the statutory definition of “major emitting facility” in 

section 169(1) of the CAA. For those 28 listed source categories, a “major stationary 

source” is one with the potential to emit 100 tpy or more of any air pollutant. A “major 

modification” is a physical change or a change in the method of operation of an existing 

major stationary source that results in a significant emissions increase and a significant 

net emissions increase of a regulated NSR pollutant. Under PSD, new major sources and 

major modifications must apply best available control technology (BACT) for each 

applicable pollutant and conduct an air quality analysis to demonstrate that the proposed 

constructionsource or project will not cause or contribute to a violation of any NAAQS or 

PSD increments (see CAA section 165(a)(3); 40 CFR 51.166(k); 40 CFR 52.21(k)). PSD 

requirements also include in appropriate cases an analysis of potential adverse impacts on 

Class I areas (see sections 162 and 165 of the CAA). 

PSD permitting requirements generally first became applicable to PM2.5 in 1997, 

on the effective date of the NAAQS for PM2.5 (Seitz, 1997). The EPA’s regulations 

define the term “regulated NSR pollutant” to include any pollutant for which a NAAQS 

has been promulgated or that is otherwise identified as a constituent or precursor to a 

NAAQS pollutant (40 CFR 51.166(b)(49); 40 CFR 52.21(b)(50)).269 In addition, on May 

 

269 Under various provisions of the CAA, PSD requirements are applicable to each 

pollutant subject to regulation under the CAA, excluding hazardous air pollutants. The 
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16, 2008, the EPA amended its rulesregulations to identify certain PM2.5 precursors (SO2

and NOx) as regulated NSR pollutants and adopt other provisions, such as a significant 

emissions rate for PM2.5, to facilitate implementation of PSD and NNSR program 

requirements for PM2.5 (73 FR 28321).270 Air agencies were required to revise their SIPs 

by May 16, 2011, to incorporate the required elements of the 2008 final rule.

On October 20, 2010, the EPA again amended the PSD rulesregulations at 40 

CFR 51.166 and 52.21 to add PSD increments as well as two screening tools for PM2.5 –

SILs and SMC (75 FR 64864). The October 2010 final rule became effective on 

December 20, 2010. The EPA indicated that the SILs and SMC for PM2.5, while useful 

tools for program implementation, are not considered mandatory elements of an 

approvable SIP; thus, no schedule was imposed on states for addressing those screening 

tools in their PSD rules. For the portions of the rule that addressed the PSD increments 

for PM2.5, states were required to submit the necessary SIP revisions (at least as stringent 

as the PSD requirements at 40 CFR 51.166) to the EPA for approval within 21 months 

from the date on which the EPA promulgated the new PM2.5 increments – by July 20, 

2012. The schedule for developing and submitting the revisions specifically for the 

adoption of new PSD increments in state PSD programs is prescribed by the CAA section 

166(b). As of October 20, 2011, sources for which PSD permits have been issued 

pursuant to the federal PSD program at 40 CFR 52.21 have been required, where 

 

definition of “regulated NSR pollutant” also includes pollutants subject to any standard 

under section 111 of the CAA or any Class I or II substance subject to title VI of the 

CAA.

270 It should be noted that on October 25, 2012, the definition of “regulated NSR 

pollutant” was revised to remove the requirement that condensable PM be included when 

considering “particulate matter emissions.” Accordingly, the definition now requires 

condensable PM to be counted for PM10 emissions and PM2.5 emissions, and for 

“particulate matter emissions” only when required by the applicable New Source 

Performance Standard or SIP. (See 77 FR 65107.) Formatted: Font: Italic
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applicable, to determine their impact on the PM2.5 increments.

The PSD program currently regulates emissions of PM using several indicators of 

particles, including “particulate matter emissions” (as regulated under various new source 

performance standards under 40 CFR part 60), “PM10 emissions,” and “PM2.5 emissions.” 

The latter two emission indicators are designed to be consistent with the ambient air 

indicators for PM that the EPA currently uses to define the PM NAAQS. As already 

noted, the PSD program also limits PM2.5 concentrations by regulating emissions of 

gaseous pollutants that result in the secondary formation of particulate matter. Those 

pollutants, known as PM2.5 precursors, generally include SO2 and NOx.

In addition to the NAAQS revisions contained in today’s final rule, the EPA is 

finalizing certain clarifications to the existing monitoring regulations codified at 40 CFR 

58.30 (Special considerations for data comparisons to the NAAQS). These clarifications 

are presented in detail in section VIII.B.2 of this preamble. The monitoring regulations 

provide a basis for determining whether specific monitoring sites are comparable to 

specific NAAQS. By extension, the EPA has also used the principles for making these 

determinations for monitoring sites to guide permitting authorities in assessing the 

comparability of specific receptor locations involved in PSD air quality analyses. 

Receptors are used in PSD modeling analyses to predict potential air quality impacts in 

the vicinity of the proposed new or modified facility and in some cases also at more 

distant Class I areas. Since the EPA interprets the regulation at 40 CFR 58.30 to apply in 

this context, the EPA will continue to use the principles in the revised regulations in 

guiding PSD modeling analysis design. Accordingly, the EPA recommends that air 

agencies qualify or disqualify specific receptor locations used in PSD air quality analyses 

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are evaluated consistent with the final monitoring provisions adopted in this regulation, 

and we intend to do so ourselves when we are the permitting authorityregulations, as 

amended by today’s rule.

a. Transition Provision (Grandfathering)

i. Proposal

As discussed previously in this preamble, today’s final rule establishes a revised 

level of the primary annual PM2.5 NAAQS.271 Longstanding EPA policy interprets the 

CAA and EPA regulations at 40 CFR 52.21(k)(1) and 51.166(k)(1) to generally require 

that PSD permit applications include a demonstration that new major stationary sources 

and major modifications will not cause or contribute to a violation of any NAAQS that is 

in effect as of the date the PSD permit is issued (Page, 2010a; Seitz, 1997). Thus, as a 

result of today’s final rule, any proposed major new and modified sources with permits 

pending at the time the PM2.5 NAAQS changes take effect would be expected to 

demonstrate compliance with the revised standard, absent some type of transition 

provision exempting such applications from the new requirements. 

In order to provide for a reasonable transition into the new PSD permitting 

requirements that will result from the revision of the primary annual PM2.5 NAAQS 

(primarily the requirement to demonstrate that emissions will not cause or contribute to a 

violation of the revised NAAQS) and the changes to the monitoring requirements 

discussed earlier, the EPA proposed to add a grandfathering provision to the federal PSD 

program codified at 40 CFR 52.21 that would apply to certain PSD permit applications 

that are pending on the effective date of the revised PM2.5 NAAQS. Specifically, the EPA 

 

271 The EPA is also revising the form of the annual primary standard by removing the 

option for spatial averaging. However, this provision has played no role in PSD so its 

removal has no implications for PSD.

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proposed to amend the federal PSD regulations at 40 CFR 52.21 to grandfather pending 

permit applications for which the Administrator or delegated air agency has published a 

public notice on the draft permit prior to the effective date of the revised PM2.5 NAAQS. 

Qualifying applications could continue being processed in accordance with the PSD 

requirements applicable to the pre-existing suite of PM NAAQS at the time the public 

notice on the draft permit was first published. The EPA also proposed that air agencies 

that issue PSD permits under their own SIP-approved PSD permit program should have 

the discretion to “grandfather” proposed PSD permits in the same manner under these 

same circumstances. Thus, the EPA also proposed to revise section 40 CFR 51.166 to 

provide a comparable exemption applicable to SIP-approved PSD programs.

In the preamble to the proposal, the EPA provided a detailed rationale and legal 

basis for the proposed grandfathering provision, also citing examples in which the EPA 

previously recognized that the CAA provides discretion for the EPA to grandfather PSD 

permit applications from requirements that become applicable while the application is 

pending (45 FR 52683, Aug. 7, 1980; 52 FR 24672, July 1, 1987; U.S. EPA, 2011c, pp. 

54 to 61). In summary, when read in combination, sections 165(a)(3), 165(c) and 301272

of the CAA provide the EPA with the discretion to promulgate regulations to grandfather 

pending permit applications from having to address a revised NAAQS where necessary to 

achieve a balance between the CAA objectives in order to protect the NAAQS on the one 

hand, and to avoid delays in processing PSD permit applications on the other. The EPA 

 

272 Section 165(a)(3) of the CAA generally requires that no major emitting facility may be 

constructed unless the owner or operator demonstrates that emissions from construction 

or operation of such facility will not cause or contribute to a violation of any NAAQS or 

PSD increment. Section 165(c) of the CAA requires that the EPA grant or deny any 

completed permit application not later than one year after the date of filing of such 

complete application. Section 301 of the CAA authorizes the EPA to prescribe such 

regulations as are necessary to carry out the functions under the CAA. 
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has also construed section 160(3) of the CAA, which states that a purpose of the PSD 

program is to “insure that economic growth will occur in a manner consistent with the 

preservation of existing clean air resources,” to call for a balancing of economic growth 

and protection of air quality (70 FR 59587 to 5958859582, Oct. 12, 2005 at 59587 to 

59588). The reasoning of those prior EPA actions is also applicable to the promulgation 

of revised PM NAAQS. 

In developing the proposed grandfathering provision, the EPA considered whether 

such a provision should include a sunset clause. A sunset clause would add a time limit 

beyond which an otherwise eligible permit application would no longer be grandfathered 

from specified new PSD permitting requirements. Consistent with past grandfathering 

actions described above, the EPA did not propose to include a sunset clause for the 

proposed grandfathering provision. 

ii. Comments and Responses

The majority of commenters, including all industry and state agency 

representatives, supported the EPA’s proposedproposal to adopt a grandfathering 

provision based on the purpose and rationale described in the preamble to the proposal. 

These commenters agreed that grandfathering certain pending PSD permit applications 

was reasonable to balance the CAA objectives to protect the NAAQS on one hand, and to 

avoid delays in processing PSD permit applicationapplications on the other. They also 

agreed grandfathering provides a reasonable transition into the PSD requirements 

associated with the revised NAAQS. Industry commenters also indicated that such a 

provision was important to economic growth and recovery, and was consistent with the 

purposes of the PSD program, i.e., to ensure that economic growth will occur in a manner 
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consistent with preservation of air quality. Several state commenters pointed out that 

finalizing the revised PM2.5 NAAQS without a grandfathering provision would result in a 

significant additional resource burden on both permit applicants and air agencies, which 

would have to reopen pending permit applications that have reached advanced stages in 

processing to address the revised standard. The commenters further noted that there 

would likely be little if any environmental benefit afforded by such a process. One state 

agency commenter performed a preliminary review of recent PSD permitting actions and 

determined that in all cases, the proposed primary annual PM2.5 standard would not have 

led to tighter permit restrictions or reduced emissions, and that a re-noticing of the 

preliminary permit decisions would accomplish nothing more than to change the margins

of compliance. In other words, re-noticing would have led to project delays with no 

reduction in PM2.5 impacts.

Four environmental group commenters (one representing a coalition of a health 

advocacy group and several environmental groups) opposed the proposed grandfathering 

provision based either on concerns about further delay in implementation of the revised 

PM NAAQS or on a position that the proposed grandfathering provision exceeds the 

EPA’s statutory authority and is unlawful. Commenters challenging the EPA’s legal 

authority to implement the proposed grandfathering provision contended that CAA 

sections 165 and 301 do not confer any authority on the EPA to grandfather PSD permit 

applications. The commenters asserted that CAA section 165(a) forecloses the EPA’s 

proposed approach, specifically citing CAA section 165(a)(3)(B) which provides that no 

major emitting facility “may be constructed” unless the facility’s owner or operator 

demonstrates emissions from the facility will not cause or contribute to the violation of 
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“any . . . national ambient air quality standard in any air quality control region.” These 

commenters further claimed that because Congress limited the applicability of the new 

PSD requirements in several ways, including specific grandfathering relief for sources 

constructed before the enactment of the 1977 Amendments to the CAA, the EPA is not 

authorized to waive otherwise applicable statutory requirements (citing Andrus v. Glover 

Constr. Co., 446 U.S 608, 116-17 (1980)). 

A subset of commenters also stated that the EPA’s proposed grandfathering 

approach undermines the policy choices made by Congress in adopting the PSD program 

that 1) it is preferable to prevent air pollution from becoming a problem in the first place, 

and 2) controls should be installed when new sources are being constructed rather than as 

retrofits on existing sources. 

One commenter asserted that there is no conflict between CAA sections 165(a) 

and 165(c) as the EPA had implied; therefore, there is no need for the EPA to invoke the 

regulatory authority of CAA section 301. This commenter also concluded that the EPA’s 

rationale of balancing of economic growth and the protection of air quality pursuant to 

CAA section 160(3) was unlawful, and that the EPA had not adequately explained the 

considerations it sought to balance and how the proposal would achieve its goals. The 

same commenter questioned the EPA’s authority to leverage principles of equity and 

fairness in proposing the grandfathering provision. The commenter also objected to the 

EPA’s rationale for choosing the public notice date of a draft permit as the milestone 

triggering the grandfathering provision, stating that the approach was contrary to statute 

because it would deprive interested persons of their statutory right to comment on 

elements of the application related to the current NAAQS. 
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The EPA does not agree with the interpretations of the CAA offered by the 

commenters opposing the proposed grandfathering provision. The EPA has previously 

exercised this discretion to establish grandfathering provisions in regulations. Indeed, the 

EPA has done so where provisions of the CAA contradict each other, citing the authority 

under section 301(a)(1) “to set transitional rules which accommodate reasonably the 

purpose and concerns behind the two contradictory provisions” (45 FR 52676, August 7, 

1980 at 52683). Furthermore, the EPA has noted and continues to recognize that even in 

the absence of a conflict between sections of the Act, “EPA would have the authority 

under section 301(a)(1) to exempt those projects in order to phase-in new requirements 

on a reasonable schedule.” Id. at 52683 n. 5. 

There is a conflict or tension between certain provisions of the CAA that the EPA 

must reconcile in situations where the ability of air agencies to complete action on a 

permit application within the statutory one-year deadline is likely to be impeded if a new 

or revised NAAQS becomes applicable at a late stage induring the permit application 

review process. We do not agree with the commenters’ arguments to the contrary. The 

CAA does not provide clear direction concerning how the EPA should apply section 

165(a)(3) of the Act to NAAQS that become effective at suchin circumstances where 

efforts to update a late stage in the permit application review process such that requiring 

applicants to address the new or revised NAAQS would jeopardizebe time consuming 

and impede compliance with the CAA obligation to take action on the application within 

one year after the completeness determination. Since Congress has not precisely spoken 

to this issue, the EPA has the discretion to apply a permissible interpretation of the Act 

that balances the requirements in the Act to make a decision on a permit application 
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within one year and to ensure that new and modified sources will only be authorized to 

construct after showing they can meet the substantive permitting criteria. Chevron, 

U.S.A., Inc. v. Natural Res. Def. Council, Inc., 467 U.S. 837, 843-44 (1984).

Targeted grandfathering applicable to a specific NAAQS does not waive the 

statutory requirements in section 165(a)(3), as some commenters assert. Rather, the 

grandfathering provision makes clear which NAAQS are covered by this provision of the 

Act when it is applied to a permit application that has reached a latespecific stage in the 

review process (i.e., the date the application is determined to be complete or the first date 

of publication of a public notice on the draft permit or preliminary determination) before 

the effectivea specified date of the revised NAAQS. Grandfathering resolves the question 

of how the EPA and other permitting authorities should interpret and apply section 

165(a)(3) of the Act in the case of today’s PM NAAQS revisions considering the 

requirement of section 165(c) of the Act that reviewing authorities make a decision on a 

permit application within one year of the date the application was determined complete. 

This is not a question of whether section 165(a)(3) applies; it is a question of which 

NAAQS this requirement should cover in the case of a pending PSD permit.

The EPA agrees that as a general rule, sections 165(a)(3) applies to “any 

NAAQS” that is effective as of the date a final PSD permit is initially issued (before any 

administrative appeal proceeding commences). However, these provisions cannot be read 

in isolation and should be construed in the context of other provisions in section 165 of 

the Act, such as section 165(c). Since the EPA is required to give effect to all provisions 

of the Act, in those circumstances where a strict reading of sections 165(a)(3) would 

frustrate congressional intent that the EPA and other implementing air agencies act in a 
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timely manner, the Agency has the discretion to interpret the reach of section 165(a)(3) to 

be limited to particular NAAQS that were proposed or effective prior to 

specificsignificant milestones in the permitting process. 

Thus, the EPA does not agree with the view expressed by some commenters that 

section 165(a)(3) must be read strictly in all circumstances to apply to all NAAQS in 

effect on the date the EPA issues a final permit decision, regardless of other 

circumstances or other requirements of the CAA. Such a reading fails to acknowledge or 

give meaning to section 165(c) of the Act. Legislative history illustrates congressional 

intent to avoid delays in permit processing. S. Rep. No. 94-717, at 26 (1976) ("nothing 

could be more detrimental to the intent of this section and the integrity of this Act than to 

have the process encumbered by bureaucratic delay”). 

The EPA is also not persuaded that the presence of a grandfathering provision in 

section 168(b) precludes the EPA from establishing grandfathering exemptions in other 

circumstances. The commenter’s reference to the Supreme Court’s observation that when 

“Congress expressly enumerates certain exceptions to a general prohibition, additional 

exceptions are not to be implied in the absence of evidence of a contrary legislative 

intent,” Andrus, 446 U.S. at 616-17, is not persuasive here. The Court applied this 

principle in a circumstance where there was a provision of law “expressly relating to 

contracts of the sort at issue here.” Id. These are not the circumstances here. Section 

168(b) of the Act does not expressly relate to the application of PSD permitting 

requirements to an application pending at the time of the promulgation of a new or 

revised NAAQS. Section 168(b) exempted facilities that were subject to permitting 

requirements under an earlier version of the PSD program created solely by the EPA 
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regulation prior to the enactment of section 165 of the CAA and other provisions that 

expressly authorized and established the requirements of the PSD permitting program 

applicable today. This exemption operated to continue existing requirements for certain 

sources after a fundamental change in the statutory and regulatory regime under which 

such sources were required to obtain authorization to construct or modify major 

stationary sources of air pollutants. Such an exemption does not expressly relate to the 

incorporation of a new requirement into the PSD program, under existing statutory 

authority, when the EPA promulgates a regulation that creates such a requirement. In this 

case, the EPA is not grandfathering permit applications from the general prohibition in 

section 165(a) against commencing construction in the absence of a permit issued “in 

accordance with the requirements of this part.” The CAA does not contain any express 

exemptions to the phrase “the requirements of this part” or from section 165(a)(3) of the 

Act that apply when the EPA promulgates a new or revised NAAQS. Furthermore, 

section 168(b) applied to sources that had commenced construction before new 

provisions of the CAA were enacted, whereas the grandfathering that the EPA proposed 

for purposes of the revised PM NAAQS is applicable to changes in regulatory 

requirements prior to the issuance of a permit. Thus, the adoption of a one-time 

grandfather provision upon enactment of the statutory PSD program is clearly different 

from grandfathering when the EPA promulgates a new or revised NAAQS, which the Act 

does not address. The fact that Congress expressly enumerated an exemption in section 

168 intended to ease transition upon enactment of the PSD provisions in the Act does not 

constrain the Agency with respect to offering reasonable transitional exemption 

provisions when EPA regulations create new PSD program requirements under those 
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statutory provisions.

The EPA agrees that the PSD program is based on the goals of preventing air 

pollution and installing controls when new sources are being constructed, but 

Sectionsection 160(3) of the Act also states that a purpose of the PSD program is to 

“insure that economic growth will occur in a manner consistent with the preservation of 

existing clean air resources.” The EPA continues to construe this provision to call for a 

balancing of economic growth and protection of air quality. See 70 FR 59582, October 

12, 2005 at 59587-88. Legislative history illustrates Congressional intent to avoid a 

moratorium on construction and delays in permit processing. The House Committee 

report describes how “the committee went to extraordinary lengths to assure that this 

legislation and the time needed to develop and implement regulations would not cause 

current construction to be halted or clamp even a temporary moratorium on planned 

industrial and economic development.” H.R. Rep. No. 95-294, 95th Cong., 1st Sess., at 

171 (1977). As an illustration of the lengths to which the committee went, the report lists 

five elements of the legislation, including the following statement: “to prevent disruption 

of present or planned sources, the committee has authorized extensive 'grandfathering' of 

both existing and planned sources.” Id. Furthermore, the Senate Committee report 

specifically discusses concerns about delays in program implementation. S. Rep. No. 94-

717, at 26 (1976) (“nothing could be more detrimental to the intent of this section and the 

integrity of this Act than to have the process encumbered by bureaucratic delay”).

In the 1980 PSD regulation, the EPA sought to strike a balance between 

competing goals of the CAA (45 FR 52683). The EPA explained that delaying certain 

construction “by imposing new PSD requirements could frustrate economic 
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development” and noted that the grandfathered projects “have a relatively minor effect on 

air quality.” Id. As a result, the EPA adopted a grandfathering provision that “would 

strike a rough balance between the benefits and costs of applying PSD to those projects.” 

Id. Although the EPA used issuance of permits previously required under the SIP in that 

case to determine eligibility for grandfathering, this precedent does not preclude the EPA 

from using another milestone in the permit process to determine eligibility in order to 

strike the appropriate balance in a different situation. In 1980, the EPA concluded that 

determining eligibility for grandfathering based on a complete application might “exempt 

many more projects from review” in one particular situation addressed in 1980 and that 

this “would fail to give adequate expression to the interests behind section 165, especially 

the goal of protecting air quality.” Id. This precedent does not preclude the EPA from 

using the publication date of public notice of a draft permit or preliminary determination 

(or other milestone) in another circumstance where such a date would not exempt a large 

number of projects or have an adverse impact on air quality. The interests behind section 

165 include both protection of air quality and timely decision-making on pending permit 

applications. The EPA is seeking here to give effect to balance the requirements in the 

Act to make a decision on a permit application within one year and to ensure that new 

and modified sources will only be authorized to construct after showing they can meet the 

substantive permitting criteria.

Moreover, this action is not based on an assertion of equitable power to disregard 

or override law, but rather on an interpretation of our statutory authority. In so doing, the 

EPA has in this case determined which regulatory requirements are covered by the 

statutory requirements that apply to an application that has reached a specified milestone 
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when the regulatory requirement was established. The EPA does not dispute that 

administrative agencies only have the powers conferred by statute. However, the EPA 

may interpret the statutory requirements consistent with Congressional intent and 

exercise its discretion in a thoughtful way in doing so. Thus, while an administrative 

agency in the executive branch does not have the equitable powers of a court, this does 

not necessarily mean an administrative agency cannot interpret its statutory authority to 

achieve equitable outcomes consistent with Congressional intent.

Based on the foregoing, the EPA believes it has adequately explained its 

consideration of the CAA requirements related to both NAAQS protection and timely 

decision-making on permit applications in designing the proposed grandfathering 

provision. As described below, the EPA is finalizing a grandfathering provision that 

applies to two categories of PSD permit applications: (1) those that the reviewing 

authority has determined to be complete on or before December 14, 2012, or (2) those for 

which the reviewing authority has first published a public notice on thethat a draft permit 

or preliminary determination, consistent with had been prepared prior to the effective 

date of the revised PM NAAQS. In the proposal. The EPA selected this eligibility 

criterion because it represents a well-documented point in time and is the latest and 

therefore most restrictive reasonable milestone in the permitting process short of final 

permit issuance. As such, the EPA’s approachEPA proposed to balancinggrandfather 

only the requirements in CAA sections 160 and 165 as finalized in this rule strongly 

favors NAAQS protection over exempting more projects from review under the revised 

PM NAAQS. 

As described in the proposal, before latter category, based on publication of a 
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public notice on a draft permit or preliminary determination is issued for public comment, 

the applicant still has a reasonable opportunity to amend its permit application to address 

new or revised NAAQS that became effective while the reviewing authority’s 

preliminary review of the application was underway. Furthermore, the reviewing 

authority still has the opportunity to review additional material and revise its fact sheet or 

statement of basis before beginning the public comment period on such a permit. 

However, if the EPA and other reviewing authorities were to require permit applicants to 

demonstrate that they will not cause or contribute to a violation of the revised PM 

NAAQS after the public comment period has begun, this would unduly delay the 

processing of the permit application by potentially requiring an additional public 

comment period and increased demand on the limited resources of the reviewing 

authority at a time when it should be focusedby the effective date of the final PM 

NAAQS. However, as described later in this section, based on considering consideration 

of public comments and preparing a final permit decision in order to conclude its review 

of a permit application in a timely manner. These permit applicationsreceived on the 

proposal, the EPA decided to augment the grandfathering provision to include 

applications that had been determined to be complete on or before December 14, 2012, 

the date of signature of the final rule. Permit applications qualifying under the final 

grandfathering provision must demonstrate that a qualifying new or modified source will 

not cause or contribute to a violation of the PM2.5 NAAQS and increments in effect as of 

the date the permit application is determined to be complete by the reviewing authority or 

as of the date the reviewing authority first publishes public notice of the draft permit or 

preliminary determination, depending on which prong of the grandfathering provision is 
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applicable.

The grandfathering provision does not apply to any other applicable PSD 

requirements related to PM2.5. Sources with projects qualifying under the grandfathering 

provision will be required to install BACT for PM2.5 emissions, demonstrate that project 

emissions will not cause or contribute to a violation of the PSD increments for PM2.5 or 

the PM2.5 NAAQS in effect at the time the permit application is determined to be 

complete or the public notice is first published on the draft permit or preliminary 

determination, and address Class I and additional impacts in accordance with the PSD 

regulatory requirements. Accordingly, the EPA does not expect that such a limitedthe 

grandfathering provision being finalized in today’s rule will result in significantly 

different air quality impacts than would occur absent any type of grandfathering or 

transition provision. One commenter has submitted an analysis to support this conclusion. 

Finally,As described in the proposal and some of the comments received from 

state agencies, if the EPA disagrees that its rationale for choosing the publication date of 

the public notice on a draftand other reviewing authorities were to require permit 

applicants to demonstrate that they will not cause or preliminary determination for public 

comment as the milestone triggering thecontribute to a violation of the revised PM 

NAAQS after the public comment period has begun, this would unduly delay the 

processing of the permit application by potentially requiring an additional public 

comment period and increased demand on the limited resources of the reviewing 

authority. The EPA disagrees with commenters who contend that grandfathering 

provision is contrary to statute because it would preclude public comment on elements of 

the application related to the current NAAQS. This argument is essentially another means 
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of challenging the EPA’s authority to implement any kind of grandfathering provision for 

phasing in new or revised NAAQS under the PSD program. With respect to an 

application grandfathered under the new provisions provided by today’s rule, interested 

persons will have the opportunity to comment on all aspects of PSD review for PM2.5,

including the air quality impacts associated with the revised NAAQS that became 

effective after the application was determined to be complete or after a public notice was 

published on the draft permit or preliminary determination, depending on which prong of 

the grandfathering provision applies. Section 165(a)(2) of the CAA and section 

51.166(q)(2)(v) require an opportunity for the public to comment on “the air quality 

impact of the source” and “other appropriate considerations.” The grandfathering 

provision does not necessarily take away the ability of the public to comment on the 

impact the source may have on the revised NAAQS (including the standard proposed 

several months earlier) or the discretion of the permitting authority to consider these 

comments. However, as provided by the grandfathering provision established today in the 

EPA’s PSD regulations, a permit applicant is not required to complete an analysis after 

the startdate of the public comment periodapplicable grandfathering milestone to 

demonstrate that it will not cause or contribute to a violation of the NAAQS that became 

effective after that date to obtain a permit. Thus, consistent with CAA section 165(a)(2), 

“the required analysis” will have “been conducted in accordance with regulation 

promulgated by the Administrator” and made available for public comment.

Several of the commenters supporting the proposed grandfathering provision in 

general recommended that the EPA establish the grandfathering milestone as the date that 

a complete permit application is submitted (or that a submitted permit application is 
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deemed complete by the reviewing agency) rather than the publication date of public 

notice for a draft permit or preliminary determination as proposed. These commenters 

pointed out the significant level of effort, resources and time involved in preparing all of 

the information necessary for a complete permit application, including a BACT analysis, 

air quality analysis, additional impacts analyses, and a Class I area impact analysis. They 

claimed that it would be unfair to establish a grandfathering milestone past the complete 

application date because the processes and timeframes involved in generating the draft 

permit or preliminary determination materials and publishing the public notice are largely 

out of the control of the permit applicant and vary from agency to agency. They further 

stated that requiring reevaluation of a proposed project to assess impacts with respect to 

the revised NAAQS after a permit application has been deemed complete would result in 

significant additional cost and delay. Several suchOne industry commenter pointed out 

that the EPA’s proposed grandfathering approach could place considerable pressure on 

permit authorities to expedite review of publication of draft permits or decisions before 

adequate internal review was completed, which could result in subsequent withdrawal of 

the permit. Several commenters cited prior EPA grandfathering provisions that relied 

upon that milestone, including the 1987 PM10 NAAQS (52 FR 24672, July 1, 1987) and 

the 1988 NO2 increments (53 FR 40656, October 17, 1998), and contended that the EPA 

had not justified the use of an alternative date for purposes of the proposed revisions to 

the PM2.5 NAAQS. 

Some state commenters also indicated that the proposed draft permit public notice 

date milestone could result in additional resource burden on the agency to expedite 

completion of draft permit packages and process public notices. Other state commenters 
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supported the EPA’s proposed draft permit or preliminary determination public notice 

date as the appropriate grandfathering eligibility milestone, indicating that this approach 

would provide states and industry certainty on the NAAQS demonstration required 

during the PM2.5 NAAQS transition period. 

We understand that using the The EPA acknowledges the comments raising 

concerns about an approach based solely on the public notice milestone date, and agrees 

that they warrant consideration of a different milestone date. Further, we agree that an 

alternate milestone for grandfathering based on the date a permit application is 

determined to be complete versus the would address many of these concerns. Therefore, 

the EPA has modified its proposed approach to address these concerns. In particular, the 

EPA agrees with commenters that a substantial portion of the level of effort, resource 

investment, and time involved in the PSD permit process occurs during the process of 

preparing a PSD permit application and obtaining a completeness determination from the 

reviewing authority. Of particular importance is the issue of the time delay and the effect 

on permitting authorities to meet permit issuance deadlines, as previously noted. 

Commenters have persuaded the EPA that reevaluation of a proposed project to assess 

impacts with respect to the revised NAAQS after a permit application has been deemed 

complete would result in significant additional delay, thus frustrating the statutory 

requirement to complete action on a permit application within one year of the 

completeness date. 

We also agree with commenters that after the permit application completeness 

determination stage in the permitting process, the applicant must have completed all of 

the required technical demonstrations (including a BACT analysis, air quality analysis, 
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additional impacts analyses, and Class I area impact analyses), and that the final stages of 

the permitting process prior to public notice (i.e., developing the draft permit or 

preliminary determination, developing supporting materials and publishing the public 

notice) are under the control of the permitting authority. Given the variable practices and 

timelines of permitting authorities in processing these final steps between permit 

application completeness and publication date of a public notice ofon the draft permit or 

preliminary determination would likelypointed out by commenters, we agree that the 

proposed grandfathering approach could result in reducedinequitable and burdensome 

outcomes in some circumstances. 

The EPA has therefore concluded based on public comments that it should add an 

additional grandfathering milestone to avoid substantial additional burden for some PSD 

permit applicants and exempt a greater number of pending and delay for permit 

applications from review under the revised standard. We disagreethat have reached a 

stage in the review process by which significant resources have been expended to 

complete fundamental PSD analyses and demonstrations that would have to be redone. 

After a PSD permit application has been determined complete, it may be time consuming 

for the applicant to amend its permit application to address new or revised NAAQS 

promulgated after that date. The time required to both amend the application and review 

the amended application would impose unreasonable additional burden and delay upon 

the applicant and the reviewing authority. As a result, if the EPA and other reviewing 

authorities were to require permit applicants to demonstrate that they will not cause or 

contribute to a violation of the revised PM NAAQS after the permit application is 

determined to be complete, or any later stage in the permitting process, this would unduly 
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delay the processing of the permit application and place increased demand on the limited 

resources of the reviewing authority at a time when it should be focused on preparing the 

draft permit and supporting materials, preparing a public notice, considering public 

comments and preparing a final permit decision in order to conclude its review of a 

permit application in a timely manner.

The EPA also agrees with commenters’ concerns that the proposed grandfathering 

approach, based solely on the date of publication of a public notice on a draft permit or 

preliminary determination, could in some cases result in pressure on permitting 

authorities to expedite review of publication of draft permits, resulting in additional 

burden on such permitting authorities and other potential adverse consequences. We note 

that expediting review is consistent with the requirement of section 165(c) of the CAA to 

process permit applications in a timely manner. We also observe that using the milestone 

of a completeness determination to determine eligibility for grandfathering could simply 

shift this pressure back to the stage in which a permitting authority is reviewing an 

application to determine if it is complete. A significant distinction, however that we are 

compelled by prior actions or by statute to establish any particular grandfathering 

eligibility milestone or that the complete application date is necessarily the appropriate 

milestone for grandfathering of PSD permit applications from new requirements 

associated with today’s revised PM NAAQS. As , is that the one-year deadline for 

completing application on a permit does not begin to run until the date that a permit 

application is determined complete. 

Based on the comments received and the EPA’s consideration of those comments 

described previouslyabove, the EPA soughthas decided to strikemodify the proposed 
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grandfathering approach by adding a balance between competing goalssecond category of 

the CAA in several prior instances, includingapplications to the 1980 PSD regulations 

(45 FR 52676, August 7, 1980). Thereinproposed qualifying criteria. Specifically, the 

EPA adoptedis finalizing a grandfathering provision that used issuance of permits 

previously required under the SIP to determine eligibility based on the rationale that this 

approach “would strike a rough balance between the benefits and costs of applying PSD 

to those projects.” The EPA also concluded that determining eligibility for grandfathering 

based on a complete application might “exempt many more projects from review” in one 

particular situation addressed in 1980 and that this “would fail to give adequate 

expression to the interests behind section 165, especially the extends grandfathering to 

permit applications that the reviewing authority has determined, on or before December 

14, 2012 (the signature date of the final rule), to be complete. We are adding this 

category to our originally proposed category: permit applications for which the 

permitting authority has first published a public notice that the draft permit or preliminary 

determination has been prepared prior to the effective date of the revised PM NAAQS. 

We are adding eligibility criteria rather than wholly replacing what we proposed 

for two reasons. First, the EPA understands that there may be some permitting authorities 

that do not issue formal determinations that an application is complete. Applications in 

these jurisdictions that may in fact have been complete and far enough along in the 

review process that a public notice could be issued before the effective date of the revised 

NAAQS could be significantly delayed if the EPA removed the eligibility criteria based 

on the publication of the public notice. Second, given that the EPA proposed to establish 

eligibility for grandfathering based on the timing of the public notice, some permitting 
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authorities and applicants may have anticipated that they had more time to take action to 

qualify for grandfathering and may have not acted as promptly as they could have to 

submit additional information or make a completeness determination. Retaining the 

proposed eligibility criteria avoids prejudice to parties that may have relied on the 

proposed rule in such a manner. 

For the second eligibility criterion added in this final rule, the EPA chose to use 

the date an application is determined complete, as requested by several commenters. In 

several existing provisions in sections 51.166(i) and 52.21(i) of the EPA’s regulations, a 

pending application was able to quality for grandfathering if it was submitted before the 

applicable date but subsequently determined complete after that date. However, this 

historic approach can be cumbersome to implement and can lead to inconsistent 

implementation and potential abuse. These concerns stem from the fact that there is a 

time lag between submittal and the completeness determination during which there are 

typically additional data requests by the permitting authority and supplemental 

application material submittals by the applicant. Therefore, it can be difficult to 

determine the specific date that the submitted application actually became complete; 

since this date could range from the initial submittal date, through a number of 

supplemental submittal dates, to the date the permitting authority formally determines the 

application to be complete. The EPA has chosen to use the date an application is 

determined complete because this date is easier to identify and apply. For PSD permits 

issued under 40 CFR 52.21, the EPA’s regulations in 40 CFR part 124 define the 

effective date of an application as the date the permitting authority notifies the applicant 

that the application is complete. 40 CFR 124.3(f). 
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The EPA chose to base the second eligibility criterion on the date this rule has 

been signed by the Administrator to avoid creating pressure on permitting authorities to 

determine applications complete. Such pressure could lead to premature findings of 

completeness and grandfathering of a larger number of applications than is warranted to 

avoid undue delays, thus increasing the air quality impact of the grandfathering provision. 

Notably, the one-year deadline for completing application on a permit does not begin to 

run until the date that a permit application is determined complete. While Congress 

desired timely action on a permit application, the statute gives permitting authorities 

leeway to ensure they have all the necessary information to proceed expeditiously on a 

permit application before the clock starts running. The goal of protecting air quality.” Id.

As pointed out by commenters, can thus be fulfilled without compromising 

Congressional intent for timely action by conducting a careful review of an application to 

determine that it is complete. Applications that have not yet been determined complete 

may be supplemented to ensure the proposed source does not cause or contribute to a 

violation of the revised NAAQS without compromising compliance with the one-year 

deadline in section 165(c). The EPA thus selected the signature date of the final rule to 

ensure the integrity of completeness determinations issued after the rule is signed and to 

limit the number of additional sources eligible for grandfathering. 

The final grandfathering provision appropriately balances the objectives of CAA 

section 165 to protect of air quality and ensure timely decision-making on permit 

applications, while also addressing concerns about resource burdens raised by 

commenters. In addition, as pointed out by commenters, the final grandfathering 

provision also provides an approach that is more consistent with prior the EPA’s 
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grandfathering actions, e.g., in the 1987 PM10 NAAQS, wherein the EPA selected the 

date of application completeness for grandfathering projects from requirements 

associated with the new NAAQS. These and other precedents, however, do not preclude 

the EPA from using the publication date of public notice of a draft permit or preliminary 

determination (or any other milestone) in another circumstance where it would 

appropriately balance the interests of CAA section 165, including both protection of air 

quality and timely decision-making on pending permit applications. 

The EPA proposed to establish grandfathering based on the first publication date 

of the public notice on the draft permit or preliminary determination because, for the 

reasons explained above, that milestone strikes the appropriate balance between 

protection of the NAAQS and timely permit processing as required by the CAA. 

Additionally, there are a number of issues associated with using application completeness 

as the eligibility milestone for grandfathering that would complicate and potentially 

undermine the objectives of the grandfathering provision and the balance of these 

competing interests. First, the term “complete” application, although generally defined in 

the regulations (see, e.g., 40 CFR 51.166(b)(22); 40 CFR 52.21(b)(22)) has not been 

interpreted consistently among the many air agencies that implement the PSD program in 

state, local and tribal jurisdictions. Applications have been determined complete based on 

the submission of particular forms or documents alone (sometimes called “administrative 

completeness”) or based on a deeper assessment of whether those forms and documents 

contain adequate information to begin processing the application or propose a permit 

decision (sometimes described as “technical completeness”). The EPA has clarified that it 

reads the definition of “complete” to mean “technical completeness” (Page, 2012), but 
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this interpretation is not necessarily applied by all permitting authorities at this time. 

Furthermore, some air agencies do not have a formal application completeness process 

codified in their implementing regulations. Given these inconsistencies in the 

mechanisms and criteria for determining application completeness among reviewing 

agencies, the EPA does not believe that establishment of a grandfathering provision 

based on application completeness would provide a definitive, nationally-consistent 

approach. Additionally, while some commenters pointed to potential burden associated 

with permit applicants pressuring reviewing authorities to expedite draft development and 

public notice publication, the EPA believes an equal or greater burden and potential 

abuse would result if the application completeness date were used as the eligibility 

milestone. Sources with projects much earlier in the planning process could be 

encouraged to hurriedly prepare and submit permit applications and pressure reviewing 

agencies to determine application completeness prior to the effective date of the revised 

PM NAAQS. Furthermore, this approach could provide an incentive for reviewing 

authorities to determine completeness of otherwise inadequate or incomplete 

applications, based on the varying agency criteria described above. Under that scenario, 

some sources would avoid having to evaluate the revised PM NAAQS without having 

reached the same level of completeness, threatening to tip the balance away from 

protection of the NAAQS and in favor of purely administrative and economic efficiency, 

which is not the balance of CAA provisions the EPA seeks to achieve. The EPA agrees 

with commenters that felt that use of the first publication date of the public notice of the 

draft permit or preliminary determination will provide a more definitive and nationallyconsistent milestone for grandfathering.
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Regarding the need for a sunset clause for the grandfathering provision, the 

majority of commenters supported, as proposed, not including such a clause, and no 

commenters specifically recommended that a sunset clause be established. Commenters 

pointed out that permit applicants and reviewing authorities already have strong 

incentives to issue final permits in a timely manner following the public notice stage, and 

that a sunset clause would not add any meaningful incentive to expedite the permitting 

process, rather potentially causing additional delays. One commenter stated that 

permitting authorities have ample discretion, which they routinely use, to refuse to issue a 

draft permit if additional information is requested during a comment period or the agency 

itself wants additional information following publication of a draft permit or preliminary 

determination. The same commenter indicated that permitting authorities also have 

sufficient discretion to reopen permit proceedings if they consider information in an 

application to be stale.

The EPA agrees with commenters that the addition of a sunset clause to the 

proposed grandfathering provision would not add meaningful additional incentive for 

sources or permitting authorities to expedite permitting processes. The EPA also agrees 

that a sunset clause could in fact result in further delays for permit actions that qualify for 

the proposed grandfathering provision in circumstances where unrelated and not 

reasonably avoidable factors cause final permit issuance to lapse beyond the sunset date. 

In such cases, the already delayed permit action would necessarily be further delayed to 

address PSD permitting requirements associated with the revised PM2.5 NAAQS, 

potentially triggering a domino effect of newly applicable requirements. As such, the 

EPA believes a sunset clause would diminish the value of the grandfathering provision 
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and likely introduce additional complexities in relation to specific permit actions.

A few industry commenters suggested, as an alternative to our proposed 

approach, that the EPA should effectively grandfather PSD permit actions from meeting 

requirements associated with the revised PM NAAQS by extending the effective date of 

the NAAQS by one year. These commenters argued that such an approach is preferable 

because it would address potential concerns about the inability of state agencies to 

implement the proposed grandfathering provision prior to rule adoption and SIP approval. 

Several industry groups and representatives also commented that the EPA should not 

eliminate state discretion to grandfather individual permits even without an express 

exemption. 

The EPA disagrees with extending the effective date of the revised PM NAAQS 

by one year because this approach would entirely defer the important health benefits 

associated with the revised PM NAAQS. Further, as discussed in the proposal, the EPA 

does not anticipate any issues related to implementation of the grandfathering provision 

in SIP approved state/local jurisdictions. The EPA proposed and is finalizing a revision to 

40 CFR 51.166 to provide a comparable exemption applicable to SIP-approved PSD 

programs, and air agencies that issue PSD permits under an EPA-approved PSD permit 

program should have the discretion to “grandfather” proposed PSD permits consistent 

with these final rule provisions. Even absent an express grandfathering provision in state 

rules, states have the discretion to permit grandfathering consistent with the federal 

regulations if the particular state’s laws and regulations may be interpreted to provide 

such discretion.273 However, state SIPs may not be less stringent than federal 

 

273 In one extraordinary case where the EPA had not previously adopted a grandfathering 

provision in regulations and had significantly exceeded the deadline in section 165(c) of 
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requirements. Accordingly, the EPA believes that such discretion must be limited to 

applying grandfathering consistent with the federal rule provisions. 

iii. Final Action

For the reasons articulated above, the EPA is finalizing a grandfathering provision 

under the PSD regulations that provides that qualifying sources and modifications shall 

not be required to demonstrate that their proposed emissions will not cause or contribute 

to a violation of the revised primary annual PM2.5 NAAQS but instead shall demonstrate 

that such emissions will not cause or contribute to the PM2.5 NAAQS in effect on the date 

the public notice onreviewing authority determines the draft permit application to be 

complete or the date the public notice on the draft permit or preliminary determination is 

first published, depending on which prong of the grandfathering provision is applicable. 

Under the final grandfathering provision, qualifying sources and modifications are those 

for which the reviewing authority has determined that the permit application is complete 

on or before December 14, 2012 or the permitting authority has first published a public 

notice on thethat a draft permit or preliminary determination is first publishedhas been 

prepared prior to the effective date of today’s final revisions to the PM NAAQS. 274 The 

 

the CAA, the EPA has taken the position that it may grandfather a specific source 

through adjudication, thus interpreting its regulations, as well as other authorities, to 

allow grandfathering in that extraordinary circumstance (U.S. EPA, 2011c, pp. 67 to 71). 

Although grandfathering without a specific exemption in regulations was justified based 

on the particular facts in that specific instance, the preferred approach is to enable 

grandfathering through express regulatory exemptions of the type proposed in this action 

(U.S. EPA, 2011c, p. 68).

274 There may be application completeness determinations or draft permits or 

/preliminary determinations for which a public notice was issued prior to October 20, 

2011, which is the date that PM2.5 increments became applicable requirements for any 

newly issued federal PSD permits under 40 CFR 52.21. It is not the EPA’s intention that 

the final grandfathering provision should relieve such a permit from the requirement to 

demonstrate compliance with those new PM2.5 increments, for which the EPA did not 
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relevant public notice requirements for the EPA and delegated agency issued permits are 

those in 40 CFR 124.10(c)(2), and the corresponding provisions for implementation-plan 

approved agency permits are those in 40 CFR 51.166(q)(2)(iii). The grandfathering 

provision is being incorporated into the regulations at 40 CFR 52.21 and 51.166 to 

provide the same transition for the EPA, delegated jurisdictions, and implementation 

plan-approved jurisdictions. The EPA is not establishing a sunset date for this 

grandfathering provision. 

b. Modeling Tools and Guidance Applicable to the Revised Primary Annual PM2.5

NAAQS 

Today’s final rule revising the level of the primary annual PM2.5 NAAQS from 

15.0 μg/m3to 12.0 μg/m3generally will require proposed new major stationary sources 

and modifications to take these changes into account as part of the required air quality 

analysis to demonstrate that the proposed emissions increase will not cause or contribute 

to a violation of the PM NAAQS. Upon the effective date of today’s final revisions to the 

PM NAAQS, proposed new major stationary sources and major modifications that are 

not grandfathered from the new requirements (as described in section IX.F.1.a) will be 

required to demonstrate compliance with the suite of PM NAAQS, including the revised 

primary annual PM2.5 NAAQS. 

PSD applicants are currently required to demonstrate compliance with the existing 

primary and secondary annual and 24-hour PM2.5 NAAQS and will need to consider the 

impact of their proposed emissions increases on the revised primary annual PM2.5

NAAQS. To assist sources and permitting authorities in carrying out the required air 

quality analysis for PM2.5 under the existing standards, the EPA issued, on March 23, 

 

adopt any grandfathering provisions but deferred implementation in accordance with the 

requirements of the CAA.
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2010, a guidance memorandum that recommends certain interim procedures to address 

the fact that compliance with the 24-hour PM2.5 NAAQS is based on a particular 

statistical form, and that there are technical complications associated with the ability of 

existing models to estimate the impacts of secondarily formed PM2.5 resulting from 

emissions of PM2.5 precursors (Page, 2010b). For the latter issue, the EPA recommended 

that special attention be given to the evaluation of monitored background air quality data, 

since such data readily account for the contribution of both primary and secondarily 

formed PM2.5 from existing sources affecting the area. 

To provide more detail and to address potential issues associated with the 

modeling of direct and precursor emissions of PM2.5, the EPA is now developing 

additional permit modeling guidance that will recommend appropriate technical 

approaches for conducting a PM2.5 NAAQS compliance demonstration, which includes 

more adequate accounting for contributions from secondary formation of ambient PM2.5

resulting from a proposed new or modified source’s precursor emissions. To this end, the 

EPA discussed this draft guidance in March 2012 at the EPA’s 10th Modeling 

Conference.275 Based on its review of comments received through the conference and 

further technical analyses, the EPA intends to issue final guidance by the end of calendar 

year 2012, prior to the effective date of today’s final PM NAAQS revisions.

The EPA also received a number of industry and state comments on the PM2.5

NAAQS proposal related to PM2.5 air quality impact analyses and associated existing 

modeling tools and procedures. In general, commenters identified the lack of approved 

air quality modeling tools and procedures to predict the impacts of single source 

 

275 The presentation on this draft guidance was posted on the EPA website at: 

http://www.epa.gov/ttn/scram/10thmodconf.htm. 
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emissions on PM2.5 concentration in ambient air as well as limitations associated with 

existing PM2.5 modeling tools and guidance. Commenters recommended the EPA address 

these existing issues and provide updated guidance through an open stakeholder process 

and preferably through notice-and-comment rulemaking. As described above, the EPA 

intends to issue revised PM2.5 modeling guidance prior to the effective date of today’s 

revised PM NAAQS to assist permit applicants and reviewing authorities in performing 

required air quality impact analyses. The EPA expects that this revised guidance will 

address all or most of the remaining issues related to PM2.5 air quality impact 

demonstrations under the PSD program, at least on an interim basis, until the EPA takes 

additional steps to improve existing regulatory models and procedures. To that end, the 

EPA is also pursuing regulatory updates to the Guideline on Air Quality Models (40 CFR 

part 51 Appendix W) to formalize new models and techniques as appropriate. The EPA 

recently granted a petition for rulemaking to specifically evaluate whether to incorporate 

into the Guideline new analytical techniquetechniques or models for secondary PM2.5 

(McCarthy, 2012). The EPA anticipates that this rulemaking will be proposed by the end 

of calendar year 2014 or early in calendar year 2015. 

c. PSD Screening Tools: Significant Emissions Rates, Significant Impact Levels, 

and Significant ModelingMonitoring Concentration

The EPA has historically allowed the use of screening tools to help facilitate the 

implementation of the NSR program by reducing the permit applicant’s burden and 

streamlining the permitting process for circumstances where emissions or concentrations 

could be considered de minimis. These screening tools, which all provide de minimis

thresholds of some kind, include SERs, SILs, and a SMC. The EPA promulgated a SER 

for PM2.5 in the 2008 final rule on NSR implementation as part of the first phase of NSR 
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amendments to address PM2.5 (74 FR 28333, May 16, 2008). The PM2.5 SER is used to 

determine whether any proposed major stationary source or major modification will emit 

sufficient amounts of PM2.5 to require review under the PSD program.276 Under the terms 

of the existing EPAthe EPA’s regulations, the applicable SER for PM2.5 is 10 tpy of 

direct PM2.5 emissions (including condensable PM) and, for precursors, 40 tpy of SO2

and 40 tpy of NOx emissions. 40 CFR 51.166(b)(23); 40 CFR 52.21(b)(23). This SER 

applies to permitting requirements based on both the annual and 24-hour PM2.5 NAAQS. 

The SERs are pollutant-specific but not specific to the averaging time of any NAAQS for 

a particular pollutant. 

Once it is determined that emissions resulting from the proposed new source or 

modification are significant for PM2.5, the permit applicant must complete an air quality 

analysis. 40 CFR 51.166(m)(1)(i); 40 CFR 52.21(m)(1)(i). The SIL helps to determine 

the scope of the required air quality analysis that must be carried out in order to 

demonstrate that the source’s emissions will not cause or contribute to a violation of any 

NAAQS or increment. The EPA promulgated SILs for PM2.5 in 2010 under a final rule 

that established increments, SILs, and a SMC for PM2.5 (75 FR 64890 to 6489464864, 

October 20, 2010 at 64890 to 64894). 

Historically, the EPA and other permitting authorities have allowed permit 

applicants to determine the scope of analysis required to satisfy section 165(a)(3) of the 

CAA by modeling their proposed emissions increase to predict ambient air quality 

impacts associated with that emissions increase, and by comparing this predicted increase 

in ambient concentration of PM2.5 to the applicable SIL, which is also expressed as an 

 

276 The PSD rules provide that a source that would emit major amounts of any regulated 

NSR pollutant must undergo review for that pollutant as well as any other regulated NSR 

pollutant that the source would emit in significant amounts.
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ambient PM2.5 concentration over a prescribed averaging time consistent with the 

NAAQS and increments. The EPA notes that the current PM2.5 SILs are the subject of a 

petition that challenges the EPA’s legal authority under the CAA to develop and 

implement those SILs, and also alleges that the PM2.5 SILs established by the EPA have 

not been adequately demonstrated to represent de minimis values. Sierra Club v. EPA, 

No. 10-1413 (D.C. CircuitCir. filed DecemberDec. 17, 2010). In the course of this 

litigation, the EPA has recognized the need to correct the text addressing the use of the 

PM2.5 SILs in the PSD regulations (40 C.F.R. 51.166(k)(2); 40 C.F.R. 52.21(k)(2)), and 

the EPA has asked the court to vacate and remand those provisions so that the EPA may 

correct them. However, the EPA does not believe this corrective action would preclude 

appropriate use of the PM2.5 SILs in the interim. The EPA has not asked the court to 

vacate the SILs in section 51.165(b) of its regulations. Furthermore, SILs that are not 

reflected in rules may be used if the permitting record provides adequate support that the 

values reflect a de minimis impact on air quality, consistent with the principles described 

in EPA memoranda establishing interim SILs for the one-hour SO2 and NO2 NAAQS.277

The revisions to the primary annual PM2.5 NAAQS do not affect the continued used of 

the PM2.5 SILs. 

Finally, the SMC, also measured as an ambient pollutant concentration (μg/m3), is 

a screening tool used to determine whether it may be appropriate to exempt a proposed 

source from the requirement to collect pre-construction ambient monitoring data as part 

of the required air quality analysis for a particular pollutant. The EPA promulgated the 

 

277 Page, 2010c; Page, 2010d. The EPA provided similar advice before it finalized the 

proposed PM2.5 SILs (Page, 2010b). See also, In re Mississippi Lime Co., PSD Permit 

Appeal 11-01, Slip. Op. at 34-41 (EAB August 9, 2011) and U.S. EPA, 2012d.
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existing SMC for PM2.5 in 2010 on the basis of the defined minimum detection limit for 

PM2.5 and the current information at that time concerning the physical capabilities of the 

PM2.5 FRM samplers. In that rulemaking, the EPA addressed uncertainties introduced 

into the measurement of PM2.5 due to variability in the mechanical performance of the 

PM2.5 samplers and micro-gravimetric analytical balances that weigh filter samples. Like 

the PM2.5 SILs, the SMC was challenged by the Sierra Club in the same petition, and is 

currently under review by the Court. 

In the proposal, the EPA did not propose any changes to the existing PM2.5 SERs, 

SILs and SMC, but solicited preliminary comment on whether any such changes would 

be appropriate. The EPA also indicated that any changes to the PM2.5 screening values 

would be addressed in a subsequent rulemaking that would specifically address various 

PSD implementation issues.

The EPA received several comments from industry and state agencies regarding 

the existing PSD screening tools and the potential need to adjust associated values based 

on the revised primary annual PM2.5 NAAQS. The majority of these commenters 

supported retaining the existing SERs, SILs and SMC for PM2.5 (and PM2.5 precursors in 

the case of the SERs), indicating that there was no compelling technical reason for 

revision based on the proposed revision to the primary PM2.5 NAAQS. One industry 

commenter indicated that there might be a need to revise the annual PM2.5 SILs based on 

the approach used in establishing the current value. However, this commenter and others 

recommended that any revisions to the PSD screening levels for PM2.5 be accomplished 

through a separate notice-and-comment rulemaking. Several state commenters that 

supported retention of the current PM2.5 SILs also urged the EPA to provide guidance on 
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the use of those existing SILs. Two sets of collaborative comments, one from academic 

researchers and one from health and environmental advocacy groups, stated that the EPA 

should revise the current PM2.5 SERs, SILs and SMC to reflect the revised NAAQS and 

true de minimis levels. 

Consistent with the proposal, the EPA is not makingOne set of collaborative 

comments from health and environmental advocacy groups stated that the EPA’s 

proposal to leave in place the PSD screening tools adopted with the previous PM 

NAAQS had no rational basis and was contrary to statutory requirements. These

commenters claimed that the EPA has no statutory authority to establish SILs and SMC 

for PM2.5, which is the subject of current litigation in Sierra Club v. EPA, No. 10-1413 

(D.C. Cir. filed Dec. 17, 2010). The EPA’s argument in support of the existing PSD 

screening tools is contained in a brief filed in that case, which is included in the docket 

for the final rule. Id., Brief of Respondent at 26-56 (June 26, 2012). These same 

commenters and one additional collaborative comment letter from academic researchers 

also stated that the EPA should revise the current PM2.5 SERs, SILs and SMC to reflect 

the revised NAAQS and true de minimis levels. 

The EPA did not propose to make and is not finalizing any changes to the existing 

PM2.5 SERs, SILs and SMC as part of this final rule. The EPA intends to consider the 

need for any future changes to these values in light of today’s revision of the primary 

annual PM2.5 NAAQS and considering public comments received. The EPA will address 

any changes to the PM2.5 SERs, SILs and SMC in a subsequent PSD implementation 

rulemaking if deemed necessary or appropriate. The EPA will determine the need for, 

and develop such rulemaking expeditiously, and any such forthcoming rulemaking will 
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provide an additional opportunity for public comment on specific proposed revisions to 

the PSD screening tool values for PM2.5. Until any rulemaking to amend existing 

regulations is completed, permitting decisions should continue to be based on the SERs 

for PM2.5 (and its precursors) and the SILs and SMC for PM2.5 in existing regulations.

d. PSD Increments

Section 166(a) of the CAA requires the EPA to promulgate “regulations to 

prevent the significant deterioration of air quality” for pollutants covered by the NAAQS. 

Among other things, the EPA has implemented this requirement through promulgation of 

PSD increments. The EPA promulgated PM2.5 increments in 2010 to prevent significant 

air quality deterioration with regard to the primary and secondary annual and 24-hour 

PM2.5 NAAQS (75 FR 64864, October 20, 2010). The revision to the primary annual 

PM2.5 NAAQS raises the question of whether the EPA should consider revising the 

annual PM2.5 increments. The EPA does not interpret section 166(a) of the Act to require 

that the EPA revise existing increments whenever the EPA revises a NAAQS for the 

same pollutant and averaging time,278 but the Agency interprets the Act to afford the EPA 

the discretion to do so. In the proposal, the EPA did not propose to revise the PM2.5

increments. In the meantime, the current PM2.5 increments remain in effect, and PSD 

permitting should continue pursuant to the current increments, with a minimum of 

disruption to the permitting process when the revised NAAQS take effect.

The EPA received few comments on whether there was any need or justification 

to revise the existing PSD increments for PM2.5. Industry and state agency commenters 

 

278 A United States District Court has upheld the EPA’s interpretation. See Order Granting 

Defendant’s Motion to Dismiss Mandatory Duty Claim, Wildearth Guardians v. Jackson, 

Case No. 11-cv-5651-YGR (N.D. Cal. May 7, 2012). An appeal of this decision is now 

pending with the United States Court of Appeals for the Ninth Circuit. 
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generally supported retaining the existing increments. Commenters again recommended 

that any revisions to the PSD increments for PM2.5 be accomplished through a separate 

notice-and-comment rulemaking. 

Consistent with the proposal, theThe EPA did not propose to make and is not 

makingfinalizing any changes to the existing PSD increments for PM2.5 as part of this 

final rule. The EPA will consider whether it is appropriate to propose any revised PSD 

increments for PM2.5 in the future. Any such forthcoming rulemaking will provide an 

additional opportunity for public comment on specific proposed revisions to the PSD 

increments for PM2.5. Until any rulemaking to amend existing regulations is completed, 

permitting decisions should continue to be based on the PSD increments for PM2.5 in 

existing regulations.

e. Other PSD Transition Issues

Several industry commenters expressed concern that a permitting problem would 

result from the fact that, upon promulgation of the revised PM2.5 NAAQS, ambient air 

quality monitoring data would show that for some areas, PM2.5 concentrations exceed the 

revised NAAQS, although those areas would not be formally designated as 

“nonattainment” until a later date pursuant to the designation process provided by the 

CAA. The commenters noted that sources locating in such areas would be required to 

obtain a PSD permit in order to construct or modify, but could not do so because the 

requirement that the new or modified source must demonstrate that it will not cause or 

contribute to a NAAQS violation, even though the area would technically already be ain

nonattainment area. The commenters further noted that once the nonattainment 

designation is made, section 173 of the Act provides a nonattainment area permit 
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program that specifies conditions under which a permit will be issued, including 

obtaining offsetting reductions in emissions rather than demonstrating through modeling 

or other analysis that the source will not cause or contribute to a violation of the NAAQS 

as required in PSD. Thus, the commenters urged the EPA to offer an interim approach 

that would avoid the imposition of an effective construction ban on such areas until such 

time as the nonattainment area designations and the nonattainment NSR offset 

requirements are in place instead of the PSD requirements. Some of the commenters 

specifically requested that the EPA provide either a surrogacy approach based on 

showing compliance with the pre-existing annual PM2.5 NAAQS or a PSD offset 

approach to avoid a construction moratorium in such areas.

The commenters are correct in that areas already in violation of the revised annual 

PM2.5 NAAQS upon the effective date of such NAAQS may not be formally designated 

nonattainment for two years or potentially longer in accordance with the statutory 

procedures for promulgating such area designations. In addition, it is the EPA’s 

longstanding policy that new and revised NAAQS must be implemented through the 

permitting process as of the NAAQS effective date (except for earlier projects that would 

qualify for any EPA-authorized grandfathering). Accordingly, new major stationary 

sources and major modifications for which permits will be issued on or after the effective 

date of the revised annual PM2.5 NAAQS must comply with the PSD requirement to 

demonstrate compliance with that and any other applicable NAAQS. 

We disagree, however, with the commenters’ conclusion that such circumstances 

will result in “the imposition of an effective construction ban on such areas.” First, as 

already described, the EPA has promulgatedis promulgating a grandfathergrandfathering
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provision that allows certain proposed new and modified sources to proceed with the 

permit process based on the requirements that were in effect previously, provided the 

permitting authority either has determined on the date whenor before December 14, 2012 

that the permit wasapplication is complete or has proposed the permit (i.e., the date on 

which the draft permit or preliminary determination washas been noticed for public 

comment).) prior to the date the revised PM standards become effective, which is 60 days 

after publication in the Federal Register. The grandfathering provision thus will enable 

some sources to avoid issues associated with potential violations of the revised annual 

PM2.5 NAAQS. 

Second, for those sources that are not eligible to be grandfathered under the new 

provision, existingpermitting authorities have the discretion to consider offsetting 

emissions reductions at other sources as part of a demonstration that an individual source 

seeking a permit will not cause or contribute to violation of the NAAQS. See, Page 

(2010c). The EPA has historically recognized in regulations and through other actions 

that sources applying for PSD permit may utilize offsets as part of the required PSD 

demonstration, even though the PSD provisions of the Clean Air Act do not expressly 

reference offsets in the same manner as the nonattainment NSR provisions of the Act. 

See, In re Interpower of New York, Inc., 5 E.A.D. 130, 141 (EAB 1994) (describing an 

EPA Region 2 PSD permit that relied in part on offsets to demonstrate the source would 

not cause or contribute to a violation of the NAAQS).

Existing EPA regulations provide a procedure by which major stationary sources 

and major modifications locating in an area designated as attainment or unclassifiable for 

any NAAQS, and found to cause or contribute to a NAAQS violation in any area, may 
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utilize offsets to address such adverse impacts and ultimately be issued a permit. See 40 

CFR 51.165(b). Specifically, paragraph (b)(3) of those regulations provides that the 

required permit program may include a provision allowing a proposed major source or 

major modification to reduce the impact of its emissions on air quality by obtaining 

sufficient emissions reductions to, at a minimum, compensate for its adverse ambient 

impact where the source or modification would otherwise cause or contribute to a 

violation of any NAAQS. On October 20, 2010, the EPA amended the requirements at 40 

CFR 51.165(b) to define a significant impact with regard to the PM2.5 NAAQS. See 75 

FR 64864 at 64902. 

As noted by some of the commenters, the EPA addressed this same issue in 1987 

when it promulgated a new set of NAAQS for PM10 and revised 40 CFR 51.165(b) of the 

regulations. See 52 FR 24672 (July 1, 1987) at 24684, 24686-87, 24698. For PM10, the 

EPA made it clear that when a proposed PSD source was found to cause or contribute to 

violation of the PM10 NAAQS, the source would be required satisfy the requirements of 

40 CFR 51.165(b) “to obtain, at a minimum, sufficient PM10 emission offsets to 

compensate for the source’s ambient impact in the area of the violation.” Such offsets 

were considered to satisfy the “cause or contribute to” language under section 

165(a)(3)(B) of the CAA. Id. at 24698.279 In response to comments concerning the 

 

279 In 1980, the EPA had determined that statutory requirement under CAA section 

165(a)(3)(B), providing that a proposed new or modified PSD source must demonstrate 

that it will not cause or contribute to a violation of any NAAQS, taken together with the 

requirements of section 110(a)(2)(D) of the CAA required all major stationary sources 

locating outside a nonattainment area but causing or contributing to a NAAQS violation 

to reduce the impact on air quality so as to assure attainment and maintenance of the 

NAAQS. In a footnote, the EPA further indicated that this offset requirement must apply 

to sources causing or contributing to a newly discovered NAAQS violation until the area 

is designated nonattainment. See 45 FR 31307 (May 13, 1980) at 31310. In this 1980 

rule, EPA adopted section 51.18(k), which was later renumbered section 51.165(b). EPA 
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appropriate criteria for applying this offset requirement for PSD purposes, the EPA also 

stated that any emissions offsets used for PSD purposes must meet applicability criteria 

that are at least as stringent as the offset criteria set forth in the nonattainment NSR 

requirements for offsets under 40 CFR 51.165(a)(3). Id. at 24684. 

We continue to believe that reliance on the 40 CFR 51.165(a)(3) criteria isprovide 

the most appropriate and necessaryguide for determining the creditability of PSD offsets, 

including any offsets obtained to satisfy the PSD requirements for the revised PM2.5

NAAQS prior to any anticipated designation of any area as nonattainment with that 

NAAQS. Since the purpose for using offsets in PSD is to show that additional emissions 

from the proposed construction will not cause or contribute to a violation, the EPA has 

not codified a requirement that such offsets necessarily must meet the same criteria that 

apply to offsets under the nonattainment NSR program. In fact, the EPA has previously 

observed that, in the context of PSD, it may not be necessary for a permit applicant to 

fully offset the proposed emissions increase if an emissions reduction of lesser quantity 

will be sufficient to mitigate the proposed source’s adverse air quality impact on a 

modeled violation. Page (2010c); 44 FR 3274, January 16, 1979, at 3278. (“Although full 

emission offsets are not required, such a source must obtain emission offsets sufficient to 

compensate for its air quality impact where the violation occurs.”). This may be 

particularly true where anticipated reductions from existing air quality regulations may 

mitigate the impacts of a proposed source’s emissions by the time the source begins 

operating in an area that is expected to be designated nonattainment. This would need to 

 

revised 51.165(b) in 1987 to expressly authorize an offset program to meet the 

requirements of section 110(a)(2)(D)(i), but this provision may also be interpreted to 

apply to section 165(a)(3)(B) of the CAA, consistent with EPA's reading of section 

51.18(k) in 1980. 
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be evaluated on a case-by-case basis. To the extent that any permit applicants may 

experience difficulties making the NAAQS compliance showing required to obtain a PSD 

permit in areas and as set forth in the Memorandum noted above, the EPA is committed

to working with permitting authorities and applicants to identify ways to apply offsets 

under the PSD program as necessary to meet PSD requirements. 

2. Nonattainment New Source Review 

Part D of Title I of the CAA pertains to the preconstruction review and permitting 

requirements for new major stationary sources and major modifications locating in areas 

designated “nonattainment” for a particular pollutant. Those requirements are commonly 

referred to as the NNSR program. The EPA regulations for the NNSR program are 

contained at 40 CFR 51.165, 52.24 and part 51, appendix S. 

For NNSR, “major stationary source” is generally defined as a source with the 

potential to emit at least 100 tpy or more of a pollutant for which an area has been 

designated “nonattainment.” The NNSR program applies only to pollutants for which the 

EPA has promulgated NAAQS. Because the EPA has defined the PM NAAQS, and has 

established area designations for PM, in terms of two separate indicators – PM10 and 

PM2.5 – each indicator is regulated separately for purposes of NNSR applicability. That 

is, for PM10, a “major stationary source” for NNSR applicability generally is a source that 

is located in a PM10 nonattainment area and has the potential to emit at least 100 tpy of 

PM10 emissions.280 For PM2.5, a “major stationary source” for NNSR applicability is a 

source that is located in a PM2.5 nonattainment area and has the potential to emit at least 

 

280 In some cases, however, the CAA and the EPA’s regulations define “major stationary 

source” for nonattainment area NSR in terms of a lower emissions rate dependent on the 

pollutant. For PM10, for example, a source having the potential to emit at least 70 tpy of 

PM10 is considered “major” if the source is located in a nonattainment area classified as a 

“Serious Area.”
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100 tpy of direct PM2.5 (“PM2.5 emissions”) or any individual precursor of PM2.5.

For a major modification, the NNSR rulesregulations rely upon SERs described 

previously in the PSD discussion in section IX.D.1. For NNSR, a major modification is a 

physical change or a change in the method of operation of an existing stationary source 

that is major for the nonattainment pollutant and results in a significant emissions 

increase and a significant net emissions increase of that nonattainment pollutant or any 

individual precursor of that pollutant. As described earlier, the EPA will be evaluating the 

existing SERs for PM2.5 and PM2.5 precursors, and will determine whether there is any 

basis for proposing changes to any of the existing values. Any decision to propose 

changing the existing SERs in a future rulemaking would also apply to their use in the 

NNSR program requirements.

The EPA has designated nonattainment areas for the existing primary annual and 

24-hour PM2.5 NAAQS independently, and the EPA also approves redesignations to 

attainment separately for the two averaging periods. Thus, an area may be nonattainment 

for the annual standard and unclassifiable/attainment or attainment for the 24-hour 

standard. In the proposal, the EPA indicated that no formal policy has yet been developed 

to address this situation, but that the EPA presently believes that it is reasonable to 

require that only NNSR (and not PSD) applies for PM2.5 in any area that is nonattainment 

for either averaging period.281 The same situation would have existed with respect to the 

proposed secondary visibility index standard, had the EPA elected to finalize such a 

standard. Accordingly, the EPA indicated in the proposal that it intends to address this 

issue in a future NSR rulemaking, but invited preliminary comment on whether it is 

 

281 However, transportation conformity requirements discussed in section IX.F below are 

dependent upon the averaging period(s) for which an area is designated nonattainment.
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appropriate to apply the NNSR program requirements for any pollutant that is designated 

nonattainment for at least one averaging period or at least one primary or secondary 

NAAQS for a particular pollutant. 

New major stationary sources or major modifications that trigger NNSR based on 

PM2.5 emissions (or emissions of a PM2.5 precursor) in a PM2.5 nonattainment area must 

install technology that meets the lowest achievable emission rate (LAER); secure 

appropriate emissions reductions to offset the proposed emissions increases; and perform 

other analyses as required under section 173 of the CAA. Following the promulgation of 

any revised NAAQS for PM2.5, some new nonattainment areas for PM2.5 may result. 

Where a state does not have any NNSR program or the current NNSR program does not 

apply to PM2.5, that state will be required to submit the necessary SIP revisions to ensure 

that new major stationary sources and major modifications for PM2.5 undergo 

preconstruction review pursuant to the NNSR program. Under section 172(b) of the 

CAA, the Administrator may provide states up to 3 years from the effective date of 

nonattainment area designations to submit the necessary SIP revisions meeting the 

applicable NNSR requirements. Nevertheless, permits issued to sources in nonattainment 

areas must satisfy the applicable requirements for nonattainment areas as of the effective 

date of the specific nonattainment designation; therefore, states whose existing NNSR 

program requirements, if any, cannot be interpreted to apply to the revised primary 

annual PM2.5 NAAQS at that time will be allowed to issue the necessary permits in 

accordance with the applicable nonattainment permitting requirements contained in the 

Emissions Offset Interpretative Ruling at 40 CFR part 51, appendix S, which would 

apply to the revised PM2.5 NAAQS upon its effective date (see 73 FR 38321, May 16, 
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2008 at 28340). The EPA did not propose any type of PM2.5 grandfathering provision at 

this time for purposes of NNSR.

Several industry commenters recommended that the EPA establish a 

grandfathering provision for NNSR as was proposed under the PSD program. A subset of 

these commenters recommended that grandfathering be accomplished by establishing an 

effective date for designations one year after initial publication in the Federal Register. 

However, no commenters provided any rationale or supporting basis for such a 

grandfathering provision or the underlying need for a transition into NNSR permitting for 

the revised PM2.5 NAAQS. 

The EPA disagrees with commenters that recommended a grandfathering 

provision for NNSR requirements associated with the revised PM2.5 NAAQS. As 

described in the proposal, the timetable for adopting new provisions under the statea

state’s NNSR program will not apply with regard to the revised NAAQS for PM2.5 until 

such time that an area is designated nonattainment for a particular standard. Major NSR 

permits for PM2.5 issued in areas newly designated as nonattainment for the revised 

primary annual PM2.5 NAAQS must, as of the effective date of such designation, meet the 

applicable NNSR requirements for PM2.5 (Seitz, 1991). As such, there may be cases 

where applicants with PSD permit applications for PM2.5 in progress will be required to 

revise their applications to address NNSR requirements for a newly designated PM2.5

nonattainment area, and such revisions could result in additional resource burden and 

permit delays. However, the EPA believes at this time that such cases will be very 

limited, and in addition there is a substantial lead time between the effective date of the 

revised PM2.5 NAAQS and the effective date of any associated new nonattainment 
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designations for permit applicants and air agencies to anticipate when the NNSR 

requirements will apply. Therefore, the EPA is not inclined at this time to pursue a 

rulemaking to establish a grandfathering provision for the revised PM2.5 NAAQS under 

the NNSR program. The EPA will independently, and in consultation with other 

reviewing authorities, work with permit applicants on specific projects requiring 

additional measures to achieve a workable transition into NNSR permitting requirements. 

The EPA will also continue to consider whether a regulatory grandfathering may become

necessary for NNSR, and if determined to be, will undertake any such action as part of a 

subsequent NSR implementation rulemaking with additional opportunity for public 

comment. 

A few commenters (industry and state agency)commenters addressed the issue of 

potential dual review (applying NNSR and PSD simultaneously) based on distinct 

designations for separate averaging times of the PM2.5 NAAQS. These commenters 

generally agreed with the EPA’s conclusion that it was reasonable to apply only the 

NNSR permitting requirements to such situations and not PSD. Regarding the issue of 

potential dual review for multiple averaging times of the PM2.5 NAAQS, since the 

proposal, the EPA has determined that existing regulations resolve this issue in favor of 

the conclusion suggested in the proposed rule. Based on the express terms of existing 

regulations, only the NNSR permit requirements, and not PSD, apply for the pollutant 

PM2.5 in cases where the area is designated nonattainment for at least one averaging time 

of the PM2.5 NAAQS. The federal PSD regulations provide that the PSD requirements 

(the requirements of paragraphs (j) through (r) of each section) “do not apply to a major 

stationary source or major modification with respect to a particular pollutant if the owner 
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or operator demonstrates that, as to that pollutant, the source or modification is located in 

an area designated as nonattainment under section 107 of the Act.” 40 CFR 52.21(i)(2) 

and 40 CFR 51.166(i)(2) (emphasis added). Thus, this provision expressly excludes from 

PSD any pollutant for which an area is designated nonattainment, without reference to a 

particular averaging period. For a number of years, it was the EPA’s practice to establish 

a single designation in an area for a particular pollutant. Accordingly, if the area was not 

meeting the NAAQS for a particular averaging period, the area was designated 

nonattainment—even though the area was likely meeting the NAAQS for one or more 

averaging periods for the same pollutant. The EPA’s statement in the proposal that we 

had not yet established a policy on the dual review question for PM2.5 was based on the 

fact that we had only recently begun establishing designations for each averaging time in 

the case of the PM2.5 NAAQS. However, at the time of the proposal, the EPA had not 

closely examined the applicability of the language in sections 51.166(i)(2) and 

52.21(i)(2) in this context. After closer inspection prompted by the comments on this 

issue, we do not read these provisions to authorize application of PSD to a pollutant when 

an area may be designated nonattainment for a particular averaging time, while also 

designated attainment or unclassifiable for a different averaging time for the same 

pollutant. 

As proposed, the EPA is not finalizing any changes under the NNSR program 

regulations as part of this final NAAQS rule. The EPA will consider the need for any 

changes to the NNSR program provisions and will implement any such changes as part of 

a future NSR implementation rule and/or guidance.

E. Transportation Conformity Program

Transportation conformity is required under CAA section 176(c) to ensure that 
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transportation plans, transportation improvement programs (TIPs) and federally 

supported highway and transit projects will not cause new air quality violations, worsen 

existing violations, or delay timely attainment of the relevant NAAQS or interim 

reductions and milestones. Transportation conformity applies to areas that are designated 

nonattainment and maintenance for transportation-related criteria pollutants: carbon 

monoxide, ozone, NO2, and PM2.5, and PM10. Transportation conformity for any revised 

NAAQS for PM2.5 does not apply until 1 year after the effective date of the 

nonattainment designation for that revised NAAQS (see CAA section 176(c)(6) and 40 

CFR 93.102(d)). The EPA’s Transportation Conformity Rule (40 CFR part 51, subpart T, 

and 40 CFR part 93, subpart A) establishes the criteria and procedures for determining 

whether transportation activities conform to the SIP. The EPA is not making any changes 

to the transportation conformity rule in this rulemaking. The EPA notes that the 

transportation conformity rule already addresses the PM2.5 and PM10 NAAQS. The EPA 

will review whether there is a need to issue new or revised transportation conformity 

guidance in light of this final rule. In developing new or revised guidance the EPA will 

consider the comments related to implementation of the transportation conformity rule 

that were received in response to the proposal.

As discussed in section VIII above, the EPA finalized certain clarifying changes 

to PM2.5 air quality monitoring regulations. These changes are designed to align different 

elements of the monitoring regulations for consistency, which will help facilitate the 

interpretation of modeling results from quantitative PM2.5 conformity hot-spot analyses 

for the annual standards by clarifying which receptors are comparable to the NAAQS. 

Due to these changes to the monitoring regulations, the EPA will update its 
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guidance on conformity quantitative PM2.5 hot-spot analyses as appropriate to make it 

consistent with the revised monitoring requirements (U.S. EPA, 2010j). The EPA intends 

that the current quantitative PM2.5 hot -spot guidance continues to apply to any 

quantitative PM2.5 hot-spot analysis that was begun before the effective date of these 

revisions to the monitoring regulations. Revised guidance on receptors to be compared to 

the annual PM2.5 standards for quantitative PM2.5 hot-spot analyses would apply to any 

quantitative PM2.5 hot-spot analysis begun after the effective date of the revised 

monitoring regulations. Nonattainment and maintenance areas are encouraged to use their 

interagency consultation processes to determine whether an analysis for a given project 

was started before the effective date of changes to the monitoring requirements. Applying 

the current guidance regarding whether or not a receptor can be compared to the annual 

PM2.5 NAAQS to analyses that had begun before the effective date of changes to the 

monitoring regulations is consistent with how the conformity rule and guidance address 

the transitional period for new emissions factor models or local planning assumptions (40 

CFR 93.110(a) and 93.111(b) and (c)). In both of those cases, analyses begun before the 

new model or data became available can be completed using the data and/or model that 

were available when the analyses began. The EPA rules allow this in order to conserve 

state resources by not making transportation planning agencies redo analyses simply 

because a model has been revised, new data have become available, or in this case, the 

EPA has revised its regulations for PM2.5 monitoring.

F. General Conformity Program

General conformity is required by CAA section 176(c). This section requires that 

actions by federal agencies do not cause new air quality violations, worsen existing 

violations, or delay timely attainment of the relevant NAAQS or interim reductions and 
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milestones. General conformity applies to any federal action (e.g., funding, licensing, 

permitting, or approving), other than projects that are Federal Highway Administration 

(FHWA)/Federal Transit Administration (FTA) projects as defined in 40 CFR 93.101 

(which are covered under transportation conformity described above), if the action takes 

place in a nonattainment or maintenance area for ozone, PM, NO2, carbon monoxide, 

lead, or SO2. General conformity also applies to a federal highway and transit project if it 

does not involve either Title 23 or 49 funding, but does involve FHWA or FTA approval 

such as is required for a connection to an Interstate highway or for a deviation from 

applicable design standards per 40 CFR 93.101. (The FHWA and FTA actions described 

here as not subject to general conformity are subject to transportation conformity.) 

General conformity for the revised PM NAAQS will not apply until 1 year after the 

effective date of a nonattainment designation for that NAAQS. The EPA’s General 

Conformity Rule (40 CFR 93.150 to 93.165) establishes the criteria and procedures for 

determining if a federal action conforms to the SIP. With respect to the revised PM 

NAAQS, federal agencies are expected to continue to estimate emissions for conformity 

analyses in the same manner as they are estimated for conformity analyses for the 1997 

and 2006 PM NAAQS. The EPA’s existing general conformity regulations include the 

basic requirement that a federal agency’s general conformity analysis be based on the 

latest and most accurate emissionemissions estimation techniques available (40 CFR 

93.159(b)), and the EPA expects that this same principle will be followed for analyses 

needed for these revised PM NAAQS. When updated and improved emissions estimation 

techniques become available, the EPA expects the federal agency to use thesethose

techniques. With this final rule, the EPA is making no changes to the general conformity 
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rule as it already addresses the PM2.5 and PM10 NAAQS. As noted in the proposal, the 

EPA will review the need to issue guidance describing how the current conformity rule 

applies in nonattainment and maintenance areas for the final revised primary annual 

PM2.5 NAAQS.

VI.X. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 

13563: Improving Regulation and Regulatory Review

Under section 3(f)(1) of Executive Order 12866 (58 FR 51735, October 4, 1993), 

this action is an "economically significant regulatory action” because it is likely to have 

an annual effect on the economy of $100 million or more. The $100 million threshold can 

be triggered by either costs or benefits, or a combination of them. Accordingly, the EPA 

submitted this action to the Office of Management and Budget (OMB) for review under 

Executive Orders 12866 and 13563 (76 FR 3821, January 21, 2011), and any changes 

made in response to OMB recommendations have been documented in the docket for this 

action.

The EPA prepared an analysis of the potential costs and benefits associated with 

this action. This analysis is contained in Regulatory Impact Analysis for the Proposed 

Revisions to the National Ambient Air Quality Standards for Particulate Matter, EPA 

452/R-12-003. A copy of the analysis is available in Docket No. EPA-HQ-OAR-2010-

0955. 

The estimates in the RIA are associated with the revised standard and alternative 

standard levels (in μg/m3) of the primary annual PM2.5 standards including: 13, 12, and 

11. Table 4 provides a summary of the estimated costs, monetized benefits, and net 

benefits associated with full attainment of these alternative standards.

Formatted: Indent: Left: 0", Numbered +

Level: 1 + Numbering Style: I, II, III, ... + Start

at: 10 + Alignment: Left + Aligned at: 0.25" +

Indent at: 0.75"
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Table 4. Total Costs, Monetized Benefits and Net Benefits in 2020 (millions of 2010$)

Full Attainmenta

Alternative 

PM2.5

Annual

Standards

(μg/m3)

Total Costsb Monetized Benefitsd Net Benefits

3%

Discount Ratec

7%

Discount Rate

3% 

Discount Rate

7% 

Discount Rate

3% 

Discount Rated

7% 

Discount Rate

13 $11 to $100 $11 to $100 $1,300 to $2,900 $1,200 to $2,600 $1,300200 to 

$2,800900

$1,100 to 

$2,500600

12 $53 to $350 $53 to 350 $4,000 to $9,100 $3,600 to $8,200 $4,0003,700 to 

$8,8009,000

$3,600300 to 

$7,9008,100

11 $320 to $1,700 $320 to $1,700 $13,000 to 

$29,000

$12,000 to 

$26,000

$1311,000 to 

$2729,000

$1110,000 to 

$2526,000

a

These estimates reflect incremental emissions reductions from an analytical baseline that gives an “adjustment ” to the San 

Joaquin and South Coast areas in California for NOx emissions reductions expected to occur between 2020 and 2025, when 

those areas are expected to demonstrate attainment with the revised standards. Full benefits of the revised standards in those 

two areas will not be realized until 2025.

b

The two cost estimates do not represent lower and upper bound estimates, but represent estimates generated by two different 

methodologies. The lower estimate is generated using the fixed-cost methodology, which assumes that technological change 

and innovation will result in the availability of additional controls by 2020 that are similar in cost to the higher end of the 

cost range for current, known controls. The higher estimate is generated using the hybrid methodology, which assumes that 

while additional controls may become available by 2020, they become available at an increasing cost and the increasing cost 

varies by geographic area and by degree of difficulty associated with obtaining the needed emissions reductions. 

c Due to data limitations, we were unable to discount compliance costs for all sectors at 3%. See section 7.2.2 of the RIA for 

additional details on the data limitations. As a result, the net benefit calculations at 3% were computed by subtracting the 

costs at 7% from the monetized benefits at 3%.
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d

The reduction in premature deaths each year accounts for over 90% of total monetized benefits. Mortality risk valuation 

assumes discounting over the SAB-recommended 20-year segmented lag structure. Not all possible benefits or disbenefits 

are quantified and monetized in this analysis. B is the sum of all unquantified benefits. Data limitations prevented us from 

quantifying these endpoints, and as such, these benefits are inherently more uncertain than those benefits that we were able 

to quantify. The range of benefits reflects the range of the central estimates from two mortality cohort studies (i.e., Krewski 

et al. (2009) to Lepeule et al. (2012)).
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B. Paperwork Reduction Act

The information collection requirements in this final rule have been submitted for 

approval to the Office of Management and Budget (OMB)OMB under the Paperwork 

Reduction Act, 44 U.S.C. 3501 et seq. The information collection requirements are not 

enforceable until OMB approves them. The Information Collection Request (ICR) 

document prepared by the EPA for these revisions to part 58 has been assigned EPA ICR 

number 0940.26. The information collected under 40 CFR part 53 (e.g., test results, 

monitoring records, instruction manual, and other associated information) is needed to 

determine whether a candidate method intended for use in determining attainment of the 

NAAQS in 40 CFR part 50 will meet the design, performance, and/or comparability 

requirements for designation as an FRM or FEM. The EPA does not expect the number 

of FRM or FEM determinations to increase over the number that is currently used to 

estimate burden associated with PM10, PM2.5, or PM10-2.5 FRM/FEM determinations 

provided in the current ICR for 40 CFR part 53 (EPA ICR numbers 0940.24). As such, 

no change in the burden estimate for 40 CFR part 53 has been made as part of this 

rulemaking. 

The information collected and reported under 40 CFR part 58 is needed to 

determine compliance with the NAAQS, to characterize air quality and associated health 

impacts, to develop emissions control strategies, and to measure progress for the air 

pollution program. The amendments finalized in this rule will revise the network design 

requirements for PM2.5 monitoring sites, resulting in the movement of 21 monitors to 

established near-road monitoring stations by January 1, 2015. The incremental burden 

associated with moving these 21 monitors that are required in 40 CFR part 58 (this is a 

one-time cost of relocating the monitors) is $28,570. Burden is defined at 5 CFR 

Formatted: Font: Italic, Not Superscript/

Subscript
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1320.3(b). State, local, and Tribal entities are eligible for Statestate assistance grants 

provided by the Federalfederal government under the CAA which can be used for 

monitors and related activities. An agency may not conduct or sponsor, and a person is 

not required to respond to, a collection of information unless it displays a currently valid 

OMB control number. The OMB control numbers for the EPA’s regulations in 40 CFR 

are listed in 40 CFR part 9.

To comment on the Agency's need for this information, the accuracy of the 

provided burden estimates, and any suggested methods for minimizing respondent 

burden, the EPA has established a public docket for this rule, which includes this ICR, 

under Docket ID number EPA-HQ-OAR-2007-0492. Submit any comments related to the 

ICR to the EPA and OMB. Send comments to the EPA at the Air and Radiation Docket 

and Information Center Docket in the EPA Docket Center (EPA/DC), EPA West, Room 

3334, 1301 Constitution Ave., NW, Washington, DC. The EPA Docket Center Public 

Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding 

legal holidays. The telephone number for the Reading Room is (202) 566-1744, and the 

telephone number for the Air and Radiation Docket and Information Center Docket is 

(202) 566-1742. An electronic version of the public docket is available at 

www.regulations.gov. Send comments to OMB at the Office of Information and 

Regulatory Affairs, Office of Management and Budget, 725 17th Street, NW, 

Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is required to make 

a decision concerning the ICR between 30 and 60 days after [INSERT DATE OF 

PUBLICATION IN THE FEDERAL REGISTER], a comment to OMB is best 

assured of having its full effect if OMB receives it by [INSERT DATE 30 DAYS 
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AFTER PUBLICATION IN THE FEDERAL REGISTER].

C. Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) generally requires an agency to prepare a 

regulatory flexibility analysis of any rule subject to notice and comment rulemaking 

requirements under the Administrative Procedure Act or any other statute unless the 

agency certifies that the rule will not have a significant economic impact on a substantial 

number of small entities. Small entities include small businesses, small organizations, and 

small governmental jurisdictions.

For purposes of assessing the impacts of this rule on small entities, small entity is 

defined as: (1) a small business that is a small industrial entity as defined by the Small 

Business Administration’s (SBA) regulations at 13 CFR 121.201; (2) a small 

governmental jurisdiction that is a government of a city, county, town, school district or 

special district with a population of less than 50,000; and (3) a small organization that is 

any not-for-profit enterprise which is independently owned and operated and is not 

dominant in its field.

After considering the economic impacts of this final rule on small entities, I 

certify that this action will not have a significant economic impact on a substantial 

number of small entities. This final rule will not impose any requirements on small 

entities. Rather, this rule establishes national standards for allowable concentrations of 

particulate matter in ambient air as required by section 109 of the CAA. See also 

American Trucking Associations v. EPA. 175 F.3d at 1044-45 (NAAQS do not have 

significant impacts upon small entities because NAAQS themselves impose no 

regulations upon small entities). We continue to be interested in the potential impacts of 
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the proposed rule on small entities and welcome comments on issues related to such 

impacts.

D. Unfunded Mandates Reform Act

This action contains no Federal mandates under the provisions of Title II of the 

Unfunded Mandates Reform Act of 1995 (UMRA), 2 U.S.C. 1531-1538 for state, local, 

or tribal governments or the private sector. The action imposes no enforceable duty on 

any state, local or tribal governments or the private sector beyond those duties already 

established in the CAA. Therefore, this action is not subject to the requirements of 

sections 202 or 205 of the UMRA. 

This action is also not subject to the requirements section 205 of the UMRA 

because it contains no regulatory requirements that might significantly or uniquely affect 

small governments. This action imposes no new expenditure or enforceable duty on any 

state, local, or tribal governments or the private sector, and the EPA has determined that 

this rule contains no regulatory requirements that might significantly or uniquely affect 

small governments.

Furthermore, in setting a NAAQS, the EPA cannot consider the economic or 

technological feasibility of attaining ambient air quality standards although such factors 

may be considered to a degree in the development of state plans to implement the 

standards. See also American Trucking Associations v. EPA, 175 F. 3d at 1043 (noting 

that because the EPA is precluded from considering costs of implementation in 

establishing NAAQS, preparation of a Regulatory Impact Analysis pursuant to the 

Unfunded Mandates Reform Act would not furnish any information which the court 

could consider in reviewing the NAAQS). The EPA acknowledges, however, that any 

corresponding revisions to associated SIP requirements and air quality surveillance 
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requirements, 40 CFR part 51 and 40 CFR part 58, respectively, might result in such 

effects. Accordingly, the EPA will address, as appropriate, unfunded mandates if and 

when it proposes any revisions to 40 CFR parts 51 or 58. 

E. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial 

direct effects on the states, on the relationship between the national government and the 

states, or on the distribution of power and responsibilities among the various levels of 

government, as specified in Executive Order 13132. The rule does not alter the 

relationship between the Federal government and the states regarding the establishment 

and implementation of air quality improvement programs as codified in the CAA. Under 

section 109 of the CAA, the EPA is mandated to establish and review NAAQS; however, 

CAA section 116 preserves the rights of states to establish more stringent requirements if 

deemed necessary by a state. Furthermore, this final rule does not impact CAA section 

107 which establishes that the states have primary responsibility for implementation of 

the NAAQS. Finally, as noted in section D (above) on UMRA, this rule does not impose 

significant costs on state, local, or Tribal governments or the private sector. Thus, 

Executive Order 13132 does not apply to this action.

However, as also noted in section D (above) on UMRA, the EPA recognizes that 

states will have a substantial interest in this rule and any corresponding revisions to 

associated air quality surveillance requirements, 40 CFR part 58. 

F. Executive Order 13175: Consultation and Coordination with Indian Tribal 

Governments

Executive Order 13175, entitled ‘‘Consultation and Coordination with Indian 

Tribal Governments’’ (65 FR 67249, November 9, 2000), requires the EPA to develop an 
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accountable process to ensure ‘‘meaningful and timely input by tribal officials in the 

development of regulatory policies that have tribal implications.” This rule concerns the 

establishment of national standards to address the health and welfare effects of particulate 

matter. Historically, the EPA’s definition of “tribal implications” has been limited to 

situations in which it can be shown that a rule has impacts on the tribes’ ability to govern 

or implications for tribal sovereignty. Based on this historic definition, this action does 

not have Tribal implications, as specified in Executive Order 13175 (65 FR 67249, 

November 9, 2000), i.e. because it does not have a substantial direct effect on one or 

more Indian tribes, since tribes are not obligated to adopt or implement any NAAQS. 

Nevertheless, we were aware that many tribes would be interest in this rule and we

undertook a number of outreach activities to inform tribes about the PM NAAQS review

and offered to two consultations with tribes. 

Although Executive Order 13175 does not apply to this rule, the EPA undertook a 

consultation process including: prior to proposal on March 29,2012 we sent letters to 

tribal leadership inviting consultation on the rule and then sent a second round of letters 

offering consultation after the proposal was issued on June 29, 2012. No tribe requested a 

formal consultation with the EPA. We conducted outreach and information calls to tribal 

environmental staff on May 9, 2012; June 15, 2012; and August 1, 2012. We also

participated on the National Tribal Air Association call on June 28, 2012.

As a result we received comments from the National Tribal Air Association, the 

Southern Ute Mountain Ute Tribe, and the Navajo Nation EPA. In general, these tribal 

organizations were supportive of the EPA’s proposal. 

G. Executive Order 13045: Protection of Children from Environmental Health and 

Safety Risks
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This action is subject to Executive Order 13045 (62 FR 19885, April 23, 1997) 

because it is an economically significant regulatory action as defined by Executive Order 

12866, and the EPA believes that the environmental health or safety risk addressed by 

this action may have a disproportionate effect on children. Accordingly, we have 

evaluated the environmental health or safety effects of PM exposures on children. The 

protection offered by these standards is especially important for children because 

childhood represents a lifestage associated with increased susceptibility to PM-related 

health effects. Because children have been identified as an at-risk population, we have 

carefully evaluated the environmental health effects of exposure to PM pollution among 

children. Discussions of the results of the evaluation of the scientific evidence and policy 

considerations pertaining to children are contained in sections III.B, III.D, III.E, IV.B, 

and IV.C of this preamble. The revised primary PM2.5 NAAQS discussed above will 

provide greater public health protection, including increased protection for at-risk 

populations such as children. 

H. Executive Order 13211: Actions that Significantly Affect Energy Supply, 

Distribution or Use

This action is not a “significant energy action” as defined in Executive Order 

13211, (66 FR 28355, May 22, 2001) because it is not likely to have a significant adverse 

effect on the supply, distribution, or use of energy. The purpose of this action concerns 

the review of the NAAQS for PM. The action does not prescribe specific pollution 

control strategies by which these ambient standards will be met. Such strategies are 

developed by states on a case-by-case basis, and the EPA cannot predict whether the 

control options selected by states will include regulations on energy suppliers, 

distributors, or users. 
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I. National Technology Transfer and Advancement Act 

Section 12(d) of the National Technology Transfer and Advancement Act of 1995 

(NTTAA), Public Law 104–113, section 12(d) (15 U.S.C. 272 note) directs the EPA to 

use voluntary consensus standards in its regulatory activities unless to do so would be 

inconsistent with applicable law or otherwise impractical. Voluntary consensus standards 

are technical standards (e.g., materials specifications, test methods, sampling procedures, 

and business practices) that are developed or adopted by voluntary consensus standards 

bodies. The NTTAA directs the EPA to provide Congress, through OMB, explanations 

when the Agency decides not to use available and applicable voluntary consensus 

standards.

This final rulemaking involves technical standards for environmental monitoring 

and measurement. Specifically, the EPA proposes to retain the indicators for fine (PM2.5) 

and coarse (PM10) particles. The indicator for fine particles is measured using the 

Reference Method for the Determination of Fine Particulate Matter as PM2.5 in the 

Atmosphere (appendix L to 40 CFR part 50), which is known as the PM2.5 FRM, and the 

indicator for coarse particles is measured using the Reference Method for the 

Determination of Particulate Matter as PM10 in the Atmosphere (appendix J to 40 CFR 

part 50), which is known as the PM10 FRM. 

To the extent feasible, the EPA employs a Performance-Based Measurement 

System (PBMS), which does not require the use of specific, prescribed analytic methods. 

The PBMS is defined as a set of processes wherein the data quality needs, mandates or 

limitations of a program or project are specified, and serve as criteria for selecting 

appropriate methods to meet those needs in a cost-effective manner. It is intended to be 

more flexible and cost effective for the regulated community; it is also intended to 
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encourage innovation in analytical technology and improved data quality. Though the 

FRM defines the particular specifications for ambient monitors, there is some variability 

with regard to how monitors measure PM, depending on the type and size of PM and 

environmental conditions. Therefore, it is not practically possible to fully define the FRM 

in performance terms to account for this variability. Nevertheless, our approach in the 

past has resulted in multiple brands of monitors being approved as FRM for PM, and we 

expect this to continue. Also, the FRMs described in 40 CFR part 50 and the equivalency 

criteria described in 40 CFR part 53, constitute a performance-based measurement system 

for PM, since methods that meet the field testing and performance criteria can be 

approved as FEMs. Since finalized in 2006 (71 FR, 61236, October 17, 2006) the new 

field and performance criteria for approval of PM2.5 continuous FEMs has resulted in the 

approval of six approved FEMs.

282 In summary, for measurement of PM2.5 and PM10, the 

EPA relies on both FRMs and FEMs, with FEMs relying on a PBMS approach for their 

approval. The EPA is not precluding the use of any other method, whether it constitutes a 

voluntary consensus standard or not, as long as it meets the specified performance 

criteria.

J. Executive Order 12898: Federal Actions to Address Environmental Justice in 

Minority Populations and Low-Income Populations

Executive Order 12898 (59 FR 7629, February 16, 1994) establishes federal 

executive policy on environmental justice. Its main provision directs federal agencies, to 

the greatest extent practicable and permitted by law, to make environmental justice part 

of their mission by identifying and addressing, as appropriate, disproportionately high 

 

282 A list of designated reference and equivalent methods is available on EPA’s website 

at: http://www.epa.gov/ttn/amtic/criteria.html.
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and adverse human health or environmental effects of their programs, policies, and 

activities on minority populations and low-income populations in the United States. 

The EPA maintains an ongoing commitment to ensure environmental justice for 

all people, regardless of race, color, national origin, or income. Ensuring environmental 

justice means not only protecting human health and the environment for everyone, but 

also ensuring that all people are treated fairly and are given the opportunity to participate 

meaningfully in the development, implementation, and enforcement of environmental 

laws, regulations, and policies. We conducted an outreach and information call with 

environmental justice organizations on August 9, 2012.

The EPA has identified potential disproportionately high and adverse effects on 

minority and/or low-income populations related to PM2.5 exposures. In addition, the EPA 

has identified persons from lower socioeconomic strata as an at-risk population for PMrelated health effects. As a result, the EPA has carefully evaluated the potential impacts 

on low-income and minority populations as discussed in section III.E.3.a of this 

preamble. Based on this evaluation and consideration of public comments on the 

proposal, the EPA is eliminating the spatial averaging provisions as part of the form of 

the primary annual PM2.5 standard to avoid potential disproportionate impacts on at-risk 

populations. The Agency expects this final rule will lead to the establishment of uniform 

NAAQS for PM. The Integrated Science Assessment and Policy Assessment contain the 

evaluation of the scientific evidence and policy considerations that pertain to these 

populations. These documents are available as described in the Supplementary 

Information section of this preamble and copies of all documents have been placed in the 

public docket for this action. 
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K. Congressional Review Act 

The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the Small 

Business Regulatory Enforcement Fairness Act of 1996, generally provides that before a 

rule may take effect, the agency promulgating the rule must submit a rule report, which 

includes a copy of the rule, to each House of the Congress and to the Comptroller 

General of the United States. The EPA will submit a report containing this rule and other 

required information to the U.S. Senate, the U.S. House of Representatives, and the 

Comptroller General of the United States prior to publication of the rule in the Federal 

Register. A major rule cannot take effect until 60 days after it is published in the Federal 

Register. This action is a “major rule” as defined by 5 U.S.C. §804(2). This rule will be 

effective [INSERT DATE 60 DAYS AFTER DATE OF PUBLICATION IN THE 

FEDERAL REGISTER].

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Subscript

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Samet J (2009f). Letter from Dr. Jonathan M. Samet, Chair, Clean Air Scientific 

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M.D.; Shayne Avery, M.D.; and Norma Kreilein, M.D. Docket No. EPA-HQOAR-2007-0492-7555. 

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Subscript
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South Dakota DENR (2012). Comments of the South Dakota Department of Environment 

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Texas CEQ (2012). Comments of the Texas Commission on Environmental Quality on 

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Research Triangle Park, NC. Office of Air Quality Planning and Standards. 

November 2006, revised February 14, 2007. Available: 

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Field Code Changed
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EPA/600/R-08/047F, 2008. Available: 

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Triangle Park, NC. EPA-452/P-09-007. September 2009. Available: 

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(Final Report). U.S. Environmental Protection Agency, Research Triangle Park, 
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NC, EPA-452/R-09-008a. Available: 

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Protection Agency, Research Triangle Park, NC. EPA-452/R-10-005. June 2010. 

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Report. Office of Air Quality Planning and Standards, U.S. Environmental 

Protection Agency, Research Triangle Park, NC. EPA-452/R-10-004. July 2010. 

Available: http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html.

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Ambient Air Quality Standards - First External Review Draft. Office of Air 

Quality Planning and Standards, U.S. Environmental Protection Agency, 

Research Triangle Park, NC. EPA 452/P-10-003. March 2010. Available: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 

U.S. EPA (2010d). Quantitative Risk Assessment for Particulate Matter – Second 

External Review Draft. Office of Air Quality Planning and Standards, U.S. 

Environmental Protection Agency, Research Triangle Park, NC. EPA-452/P-10-

001. February 2010. Available: 

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U.S. EPA (2010e). Particulate Matter Urban-Focused Visibility Assessment – Second 

External Review Draft. Office of Air Quality Planning and Standards, U.S. 

Environmental Protection Agency, Research Triangle Park, NC. EPA-452/P-10-

002. January 2010. Available: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html.

U.S. EPA (2010f). Policy Assessment for the Review of the Particulate Matter National 

Ambient Air Quality Standards - Second External Review Draft. Office of Air 

Quality Planning and Standards, U.S. Environmental Protection Agency, 

Research Triangle Park, NC. EPA 452/P-10-007. June 2010. Available: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 

U.S. EPA (2010g). White Paper on PM Light Extinction Measurements. Office of Air 

Quality Planning and Standards, U.S. Environmental Protection Agency, 

Research Triangle Park, NC. January 2010. Available: 

http://yosemite.epa.gov/sab/sabproduct.nsf/264cb1227d55e02c85257402007446a

4/823a6c8842610e768525764900659b22!OpenDocument

U.S. EPA (2010h). Risk and Exposure Assessment for Review of the Secondary National 

Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur. 

Office of Air Quality Planning and Standards, U.S. Environmental Protection 

Agency, Research Triangle Park, NC. EPA 452/R-09-008a/b. September 2009. 

Available: http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_rea.html. 
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U.S. EPA (2010i). White Paper regarding Draft Near-road Guidance Document – Outline 

and Draft Near-road Monitoring Pilot Study Objectives & Approach. Office of 

Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

Research Triangle Park, NC. August 24. 1010. Available: 

http://yosemite.epa.gov/sab/sabproduct.nsf/0/9E0F3E9D727323C1852577890059

6432/$File/Review+Document+for+Sept.+29+-

+30,+2010+AAMMS+Meeting.pdf. 

U.S. EPA (2010j). Transportation Conformity Guidance for Quantitative Hot-spot 

Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas. U.S. EPA 

Office of Transportation and Air Quality, Transportation and Regional Programs 

Division. December 2010. EPA-420-B-10-040. Available: 

http://www.epa.gov/otaq/stateresources/transconf/policy/420b10040.pdf. 

U.S. EPA (2010k). Integrated Science Assessment for Carbon Monoxide (Final Report). 

U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-09/019F, 

January 2010. Available: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=218686. 

U.S. EPA (2011a). Policy Assessment for the Review of the Particulate Matter National 

Ambient Air Quality Standards. Office of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, Research Triangle Park, NC. EPA 452/R11-003. April 2011. Available: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 

U.S. EPA (2011b). Policy Assessment for the Review of the Secondary National 

Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur. 

Office of Air Quality Planning and Standards, U.S. Environmental Protection 

Agency, Research Triangle Park, NC, EPA-452/R-11-005a, b. February 2011.

Available: http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html. 

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Significant Deterioration Permit for the Avenal Energy Project. U.S. 

Environmental Protection Agency. May 2011.

U.S. EPA (2011d). Integrated Science Assessment of Ozone and Related Photochemical 

Oxidants (Second External Review Draft). U.S. Environmental Protection 

Agency, Washington, DC, EPA/600/R-10/076B, 2011. September 2011. 

Available: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_2008_isa.html. 

U.S. EPA (2012a). Responses to Significant Comments on the 2012 Proposed Rule on 

the National Ambient Air Quality Standards for Particulate Matter (June 29, 2012; 

77 FR 38890). Docket Number EPA-HQ-OAR-2007-0492. U.S. Environmental 

Protection Agency. December 2012. Available: 

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U.S. EPA (2012b). Provisional Assessment of Recent Studies on Health Effects of 

Particulate Matter Exposure. U.S. Environmental Protection Agency. Office of 
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Research and Development, National Center for Environmental Assessment. 

EPA/600/R-12/056A. December 2012.

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Office of Air Quality Planning and Standards, U.S. Environmental Protection 

Agency, Research Triangle Park, NC, EPA-450/R-12-001. March 2012. 

Available: http://www.epa.gov/blackcarbon/.Available: 

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Objection to Permit for CF&I Steel, L.P. (EVRAZ Rocky Mountain Steel). U. S. 

Environmental Protection Agency. May 31, 2012. Available: 

http://www.epa.gov/region07/air/title5/petitiondb/petitions/evraz_response2011.p

df. 

U.S. EPA (2012e). Draft Guidance to Implement Requirements for the Treatment of Air 

Quality Monitoring Data Influenced by Exceptional Events. U.S. Environmental 

Protection Agency. June 2012. Available: 

http://www.epa.gov/ttn/analysis/exevents.htm.

Vandenberg J (2009). CASAC Review of Integrated Science Assessment for Particulate 

Matter: Second External Review Draft. Memorandum from John Vandenberg, 

Director, National Center for Environmental Assessment, Research Triangle Park 

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Scientific Advisory Committee, EPA Science Advisory Board Staff Office. 

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limestone: A three year study in Oxford, England. Building Environ, 38: 1217-

1224. 

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Matter National Ambient Air Quality Standards – Final Document. Memorandum 

from Lydia N. Wegman, Director, Health and Environmental Impacts Division, 

Office of Air Quality Planning and Standards, U.S. EPA to Holly Stallworth, 

Designated Federal Officer, Clean Air Scientific Advisory Committee, EPA 

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Uncertainty in Exposure Assessment, Harmonization Project Document No. 6. 

Published under joint sponsorship of the World Health Organization, the 

International Labour Organization and the United Nations Environment 

Programme. WHO Press, World Health Organization, 20 Avenue Appia, 1211 

Geneva 27, Switzerland.

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mortality in the United States, 1999-2002. Environ Health Perspect, 116: 110-115.

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Particulate Exposures Using Spatiotemporal Models for the Northeastern and 

Midwestern United States. EHP, 117(4): 522-529.

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mortality: A national analysis. Environ Health Perspect, 117: 898-903. 

Zanobetti A. (2009). Personal communication with Dr. Antonella Zanobetti; email to 

Jason Sacks, U.S. EPA, NCEA. June 1, 2009. Docket No. EPA-HQ-ORD-2007-

0517-0064.

Zeger S; McDermott A; Dominici F; Samet J (2007). Mortality in the Medicare 

population and chronic exposure to fine particulate air pollution. Johns Hopkins 

University. Baltimore. http://www.bepress.com/jhubiostat/paper133. 

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Page 695715 of 738757 – PM NAAQS Proposed Rule

List of Subjects 

40 CFR Part 50

Environmental protection, Air pollution control, Carbon monoxide, Lead, Nitrogen 

dioxide, Ozone, Particulate matter, Sulfur oxides.

40 CFR Part 51

Environmental protection, Administrative practices and procedures, Air pollution control, 

Intergovernmental relations

40 CFR Part 52

Environmental protection, Administrative practices and procedures, Air pollution control, 

Intergovernmental relations

40 CFR Part 53

Environmental protection, Administrative practice and procedure, Air pollution control, 

Intergovernmental relations, Reporting and recordkeeping requirements.

40 CFR Part 58

Environmental protection, Administrative practice and procedure, Air pollution control, 

Intergovernmental relations, Reporting and recordkeeping requirements

________________________________

Dated:

_________________________________

Lisa P. Jackson,

Administrator Formatted: Line spacing: single,

Widow/Orphan control, Adjust space between

Latin and Asian text, Adjust space between

Asian text and numbers, Tab stops: Not at 0" +

0.5" + 1" + 1.5" + 2" + 2.5" + 3" + 3.5" +

4" + 4.5" + 5" + 5.5"

Formatted: Font: 10 pt, No underline
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For the reasons set forth in the preamble, chapter I of title 40 of the Code of Federal 

Regulations is amended as follows:

PART 50 – NATIONAL PRIMARY AND SECONDARY AMBIENT AIR 

QUALITY STANDARDS

1. The authority citation for part 50 continues to read as follows:

Authority: 42 U.S.C. 7401 et seq.

*****

2. Section 50.3 is revised to read as follows:

§ 50.3 Reference conditions.

All measurements of air quality that are expressed as mass per unit volume (e.g., 

micrograms per cubic meter) other than for particulate matter (PM2.5) standards contained 

in §§ 50.7, 50.13, and 50.18, and lead standards contained in §50.16 shall be corrected to 

a reference temperature of 25 (deg) C and a reference pressure of 760 millimeters of 

mercury (1,013.2 millibars). Measurements of PM2.5 for purposes of comparison to the 

standards contained in §§50.7, 50.13, and 50.18, and of lead for purposes of comparison 

to the standards contained in §50.16 shall be reported based on actual ambient air volume 

measured at the actual ambient temperature and pressure at the monitoring site during the 

measurement period.

2.3.Table 1 in §50.14(c)(2)(vi) is revised to read as follows:

§ 50.14 Treatment of air quality monitoring data influenced by exceptional events.

* * * * *

(c) ***

(2) ***

(vi) ***

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Table 1. Special Schedules for Exceptional Event Flagging and Documentation 

Submission for Data to be Used in Initial Designations for New or Revised NAAQS

NAAQS Pollutant/

Standard/(Level)/

Promulgation Date

Air Quality Data 

Collected for 

Calendar Year

Event Flagging & 

Initial Description 

Deadline

Detailed 

Documentation 

Submission 

Deadline

PM2.5/24-Hr 

Standard (35 μg/m3) 

Promulgated 

October 17, 2006

2004-2006 October 1, 2007 April 15, 2008

Ozone/8-Hr 

Standard

(0.075 ppm)

Promulgated March 

12, 2008

2005-2007 June 18, 2009 June 18, 2009

2008 June 18, 2009 June 18, 2009

2009 60 days after the end 

of the calendar 

quarter in which the 

event occurred or 

February 5, 2010, 

whichever date 

occurs first.

60 days after the end 

of the calendar 

quarter in which the 

event occurred or 

February 5, 2010, 

whichever date 

occurs first.

NO2/1-Hr Standard 

(100 ppb) 

Promulgated 

February 9, 2010

2008 July 1, 2010 January 22, 2011

2009 July 1, 2010aJanuary 22, 2011

2010 April 1, 2011 July 1, 2011

SO2/1-Hr Standard 

(75 ppb) 

Promulgated June 

22, 2010

2008 October 1, 2010 June 1, 2011

2009 October 1, 2010 June 1, 2011

2010 June 1, 2011 June 1, 2011

2011 60 days after the end 

of the calendar 

quarter in which the 

event occurred or 

March 31, 2012, 

whichever date 

occurs first.

60 days after the end 

of the calendar 

quarter in which the 

event occurred or 

March 31, 2012, 

whichever date 

occurs first.

PM2.5/

Primary Annual 

Standard

(12 μg/m3) 

Promulgated 

December 14, 2012

2010 and 2011 July 1, 2013 December 12, 2013 

2012 July 1, 2013a December 12, 2013

2013 July 1, 2014a August 1, 2014

a

This date is the same as the general schedule in 40 CFR 50.14.
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Note: The table of revised deadlines only applies to data EPA will use to establish the 

initial area designations for new or revised NAAQS. The general schedule applies for all 

other purposes, most notably, for data used by the EPA for redesignations to attainment.

*****

3.4.Add §50.18 to read as follows:

§50.18 National primary ambient air quality standards for PM2.5.

(a) The national primary ambient air quality standards for PM2.5 are 12.0 micrograms per 

cubic meter (μg/m3) annual arithmetic mean concentration and 35 μg/m324-hour average 

concentration measured in the ambient air as PM2.5 (particles with an aerodynamic 

diameter less than or equal to a nominal 2.5 micrometers) by either:

(1) A reference method based on appendix L of this part and designated in 

accordance with part 53 of this chapter; or

(2) An equivalent method designated in accordance with part 53 of this chapter.

(b) The primary annual PM2.5 standard is met when the annual arithmetic mean 

concentration, as determined in accordance with appendix N of this part, is less than or 

equal to 12.0 μg/m3.

(c) The primary 24-hour PM2.5 standard is met when the 98th percentile 24-hour 

concentration, as determined in accordance with appendix N of this part, is less than or 

equal to 35 μg/m3. 

4.5.Appendix N to part 50 is revised to read as follows:

Appendix N to Part 50—Interpretation of the National Ambient Air Quality 

Standards for PM2.5

1.0 General

(a) This appendix explains the data handling conventions and computations necessary for 

determining when the national ambient air quality standards (NAAQS) for PM2.5 are met, 

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specifically the primary and secondary annual and 24-hour PM2.5 NAAQS specified in 

§50.7, 50.13, and 50.18. PM2.5 is defined, in general terms, as particles with an 

aerodynamic diameter less than or equal to a nominal 2.5 micrometers. PM2.5 mass 

concentrations are measured in the ambient air by a Federal Reference Method (FRM) 

based on appendix L of this part, as applicable, and designated in accordance with part 53 

of this chapter; or by a Federal Equivalent Method (FEM) designated in accordance with 

part 53 of this chapter; or by an Approved Regional Method (ARM) designated in 

accordance with part 58 of this chapter. Only those FRM, FEM, and ARM measurements 

that are derived in accordance with part 58 of this chapter (i.e., that are deemed 

“suitable”) shall be used in comparisons with the PM2.5 NAAQS. The data handling and 

computation procedures to be used to construct annual and 24-hour NAAQS metrics from 

reported PM2.5 mass concentrations, and the associated instructions for comparing these 

calculated metrics to the levels of the PM2.5 NAAQS, are specified in sections 2.0, 3.0, 

and 4.0 of this appendix. 

(b) Decisions to exclude, retain, or make adjustments to the data affected by exceptional 

events, including natural events, are made according to the requirements and process 

deadlines specified in §§50.1, 50.14 and 51.930 of this chapter.

(c) The terms used in this appendix are defined as follows:

Annual mean refers to a weighted arithmetic mean, based on quarterly means, as defined 

in section 4.4 of this appendix. 

The Air Quality System (AQS) is EPA’s official repository of ambient air data.

Collocated monitors refers to two or more air measurement instruments for the same 

parameter (e.g., PM2.5 mass) operated at the same site location, and whose placement is 
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consistent with §53.1 of this chapter. For purposes of considering a combined site record 

in this appendix, when two or more monitors are operated at the same site, one monitor is 

designated as the “primary” monitor with any additional monitors designated as 

“collocated.” It is implicit in these appendix procedures that the primary monitor and 

collocated monitor(s) are all deemed suitable for the applicable NAAQS comparison; 

however, it is not a requirement that the primary and monitors utilize the same specific 

sampling and analysis method.

The collocated PM10 data substitution test substitutes reported same-day PM10 FRM/FEM 

daily values from the same site for missing scheduled PM2.5 samples in data capture 

deficient quarters. 

Combined site data record is the data set used for performing calculations in appendix N. 

It represents data for the primary monitors augmented with data from collocated monitors

according to the procedure specified in 3.0(d) of this appendix.

Creditable samples are daily values in the combined site record that are given credit for 

data completeness. The number of creditable samples (cn) for a given year also governs 

which value in the sorted series of daily values represents the 98th percentile for that year.

Creditable samples include daily values collected on scheduled sampling days and valid 

make-up samples taken for missed or invalidated samples on scheduled sampling days. 

Daily values refer to the 24-hour average concentrations of PM2.5 mass measured (or 

averaged from hourly measurements in AQS) from midnight to midnight (local standard 

time) from suitable monitors...

Data substitution tests are diagnostic evaluations performed on an annual PM2.5 NAAQS 

design value (DV) or a 24-hour PM2.5 NAAQS DV to determine if that metricthose 
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metrics, which is otherwiseare judged to be based on incomplete (via the applicable 75

percent or 11 creditable sample minimum quarterly data capture requirements), in 

accordance with 4.1(b) or 4.2(b) shall nevertheless be deemed complete and valid for 

NAAQS comparisons, or alternatively, shall still be considered incomplete and not valid 

for NAAQS comparisons. There are three data substitution tests, the “maximum quarterly 

value” test, the “minimum quarterly value” test, and the “collocated PM10” test. Only one 

of the three tests needs to “pass” in order to validate the DV in question. These tests 

substitute actual same-site extreme daily values for missing data in an incomplete year(s), 

calculate a revised “test DV” using the original plus substituted data, and, if the test DV 

relays the same NAAQS status (i.e., meets or not meets) as the original (otherwise 

incomplete) DV, the test is deemed to have “passed” and since only one passing test is 

needed, the original DV (without the diagnostic data substitutions) is then considered 

complete and valid for NAAQS comparisons. If the test DV relays a different NAAQS 

status as the original (otherwise incomplete) DV, the test is deemed to have “failed”, and 

if all applicable substitution tests are “failed” then the original DV will still be considered 

incomplete and not valid for NAAQS comparisons.

There are two data substitution tests, the “minimum quarterly value” test and the 

“maximum quarterly value” test. Design values (DVs) are the 3-year average NAAQS 

metrics that are compared to the NAAQS levels to determine when a monitoring site 

meets or does not meet the NAAQS, calculated as shown in section 4... There are two 

separate DVs specified in this appendix:

(1) The 3-year average of PM2.5 annual mean mass concentrations for each eligible 

monitoring site is referred to as the “annual PM2.5 NAAQS DV”. 
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(2) The 3-year average of annual 98th percentile 24-hour average PM2.5 mass 

concentration values recorded at each eligible monitoring site is referred to as the “24-

hour (or daily) PM2.5 NAAQS DV”.

Eligible sites are monitoring stations that meet the criteria specified in §58.11 and §58.30 

of this chapter, and thus are approved for comparison to the annual PM2.5 NAAQS. For 

the 24-hour PM2.5 NAAQS, all site locations that meet the criteria specified in §58.11 are 

approved (i.e., eligible) for NAAQS comparisons.

Extra samples are non-creditable samples. They are daily values that do not occur on 

scheduled sampling days and that cannot be used as make-up samples for missed or 

invalidated scheduled samples. Extra samples are used in mean calculations and are 

included in the series of all daily values subject to selection as a 98th percentile value, but 

are not used to determine which value in the sorted list represents the 98th percentile. 

Make-up samples are samples collected to take the place of missed or invalidated 

required scheduled samples. Make-up samples can be made by either the primary or the 

collocated monitor. Make-up samples are either taken before the next required sampling 

day or exactly one week after the missed (or voided) sampling day.

The maximum quarterly value data substitution test substitutes actual “high” reported 

daily PM2.5 values from the same site (specifically, the highest reported non-excluded 

quarterly value(s) (year non-specific) contained in the combined site record for the 

evaluated 3-year period) for missing daily values. 

The minimum quarterly value data substitution test substitutes actual “low” reported 

daily PM2.5 values from the same site (specifically, the lowest reported quarterly value(s)

(year non-specific) contained in the combined site record for the evaluated 3-year period) 
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for missing daily values. 

98th percentile is the smallest daily value out of a year of PM2.5 mass monitoring data 

below which no more than 98 percent of all daily values fall using the ranking and 

selection method specified in section 4.5(a) of this appendix.

Primary monitors are suitable monitors designated by a state or local agency in their 

annual network plan (and in AQS) to beas the default data source for creating a combined 

site record for purposes of NAAQS comparisons. If there is only one suitable monitor at a 

particular site location, then it is presumed to be a primary monitor.

Quarter refers to a calendar quarter (e.g., January through March).

Quarterly data capture rate is the percentage of scheduled samples in a calendar quarter 

that have corresponding valid reported sample values. Quarterly data capture rates are

specifically calculated as the number of creditable samples for the quarter divided by the 

number of scheduled samples for the quarter, the result then multiplied by 100 and 

rounded to the nearest integer. 

Scheduled PM2.5 samples refers to those reported daily values which are consistent with 

the required sampling frequency (per §58.12 of this chapter) for the primary monitor, or 

those that meet the special exception noted in 3.0(e).

Seasonal sampling is the practice of collecting data at a reduced frequency during a 

season of expected low concentrations.

Suitable monitors are instruments that use sampling and analysis methods approved for 

NAAQS comparisons. For the annual and 24-hour PM2.5 NAAQS, suitable monitors 

include all FRMs, and all FEMs/ARMs except those specific continuous FEMs/ARMs 

disqualified by a particular monitoring agency network in accordance with §58.10(b)(13) 
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and approved by the EPA Regional Administrator per §58.11(e) of this chapter.

Test design values (TDV) are numerical values that used in the data substitution tests 

described in 4.1(c)(i) ,4.1(c)(ii) and 4.2(c)(i) to determine if the PM2.5 NAAQS DV with 

incomplete data are judged to be valid for NAAQS comparisons. There are two TDVs: 

TDVmin to determine if the NAAQS is not met and is used in the “minimum quarterly 

value” data substitution test and TDVmax to determine if the NAAQS is met and is used in 

the “maximum quarterly value” data substitution test. These TDV’s are derived by 

substituting historically low or historically high daily concentration values for missing 

data in an incomplete year(s).

Year refers to a calendar year.

2.0 Monitoring Considerations

(a) Section 58.30 of this chapter provides special considerations for data comparisons to 

the annual PM2.5 NAAQS.

(b) Monitors meeting the network technical requirements detailed in §58.11 of this 

chapter are suitable for comparison with the NAAQS for PM2.5. 

(c) Section 58.12 of this chapter specifies the required minimum frequency of sampling 

for PM2.5. Exceptions to the specified sampling frequencies, such as seasonal sampling, 

are subject to the approval of the EPA Regional Administrator and must be documented 

in the state or local agency Annual Monitoring Network Plan as required in §58.10 of this 

chapter and also in AQS.

3.0 Requirements for Data Use and Data Reporting for Comparisons with the 

NAAQS for PM2.5

(a) Except as otherwise provided in this appendix, all valid FRM/FEM/ARM PM2.5 mass 

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concentration data produced by suitable monitors that are required to be submitted to 

AQS, or otherwise available to EPA, meeting the requirements of part 58 of this chapter 

including appendices A, C, and E shall be used in the DV calculations. Generally, EPA 

will only use such data if they have been certified by the reporting organization (as 

prescribed by §58.15 of this chapter); however, data not certified by the reporting 

organization can nevertheless be used, if the deadline for certification has passed and 

EPA judges the data to be complete and accurate. 

(b) PM2.5 mass concentration data (typically collected hourly for continuous instruments 

and daily for filter-based instruments) shall be reported to AQS in micrograms per cubic 

meter (μg/m3) to at least one decimal place. If concentrations are reported to one decimal 

place, additional digits to the right of the tenths decimal place shall be truncated. If 

concentrations are reported to AQS with more than one decimal place, AQS will truncate 

the value to one decimal place for NAAQS usage (i.e., for implementing the procedures 

in this appendix). In situations where suitable PM2.5 data are available to EPA but not 

reported to AQS, the same truncation protocol shall be applied to that data. In situations 

where PM2.5 mass data are submitted to AQS, or are otherwise available, with less 

precision than specified above, these data shall nevertheless still be deemed appropriate 

for NAAQS usage. If data 

(c) Twenty-four-hour average concentrations will be computed in AQS from submitted

hourly PM2.5 concentration data for each corresponding day of the year and the result will

be stored in the first, or start, hour (i.e., midnight, hour ‘0’) of the 24-hour period. A 24-

hour average concentration shall be considered valid if at least 75 percent of the hourly 

averages (i.e., 18 hourly values) for the 24-hour period are available. In the event that less 

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than all 24 hourly average concentrations are available (i.e., less than 24, but at least 18), 

the 24-hour average concentration shall be computed on the basis of the hours available 

using the number of available hours within the 24-hour period as the divisor (e.g., 19, if 

19 hourly values are available). Twenty-four-hour periods with seven or more missing 

hours shall also be considered valid if, after substituting zero for all missing hourly 

concentrations, the resulting 24-hour average daily value is greater than the level of the 

24-hour PM2.5 NAAQS (i.e., greater than or equal to 35.5 μg/m3). Twenty-four hour 

average PM2.5 mass concentrations that are averaged in AQS from hourly values will be

truncated to one decimal place, consistent with the data handling procedure for the 

reported hourly (and also 24-hour filter-based) data.

(d) All calculations shown in this appendix shall be implemented on a site-level basis. 

Site level concentration data shall be processed as follows:

(1) The default dataset for PM2.5 mass concentrations for a site shall consist of the 

measured concentrations recorded from the designated primary monitor(s). All daily 

values produced by the primary monitor are considered part of the site record; this 

includes all creditable samples and all extra samples.

(2) Data for the primary monitors shall be augmented as much as possible with data from 

collocated monitors. If a valid daily value is not produced by the primary monitor for a 

particular day (scheduled or otherwise), but a value is available from a collocated 

monitor, then that collocated value shall be considered part of the combined site data 

record. If more than one collocated daily value is available, the average of those valid 

collocated values shall be used as the daily value. The data record resulting from this 

procedure is referred to as the “combined site data record.”
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(e) All daily values in a combined site data record are used in the calculations specified in 

this appendix,; however, not all daily values are given credit towards data completeness 

requirements. Only creditable samples are given credit for data completeness. Creditable 

samples include daily values in the combined site record that are collected on scheduled

sampling days and valid make-up samples taken for missed or invalidated samples on 

scheduled sampling days. Days are considered scheduled according to the required 

sampling frequency of the designated primary monitor with one exception. The exception 

is, if a collocated continuous FEM/ARM monitor has a more intensive sampling 

frequency than the primary FRM monitor, then samples contributed to the combined site 

record from that continuous FEM/ARM monitor are always considered scheduled and,

hence, also creditable. Daily values in the combined site data record that are reported for 

nonscheduled days, but that are not valid make-up samples are referred to as extra 

samples. 

4.0 Comparisons with the Annual and 24-Hour PM2.5 NAAQS

4.1 Annual PM2.5 NAAQS

(a) The primary annual PM2.5 NAAQS is met when the annual PM2.5 NAAQS DV is less 

than or equal to 12.0μg/m3at each eligible monitoring site. The secondary annual PM2.5

NAAQS is met when the annual PM2.5 NAAQS DV is less than or equal to 15.0 μg/m3at 

each eligible monitoring site.

(b) Three years of valid annual means are required to produce a valid annual PM2.5

NAAQS DV. A year meets data completeness requirements when quarterly data capture 

rates for all four quarters are at least 75 percent. However, years with at least 11 

creditable samples in each quarter shall also be considered valid if the resulting annual 

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mean or resulting annual PM2.5 NAAQS DV (rounded according to the conventions of 

section 4.3 of this appendix) is greater than the level of the applicable primary or 

secondary annual PM2.5 NAAQS. Furthermore, where the explicit 75 percent data capture 

and/or 11 sample minimum requirements are not met, the 3-year annual PM2.5 NAAQS

DV shall still be considered valid and complete if it passes at least one of the threetwo

data substitution tests stipulated below. 

(c) In the case of one, two, or three years that do not meet the completeness requirements 

of section 4.1(b) of this appendix and thus would normally not be useable for the 

calculation of a valid annual PM2.5 NAAQS DV, the annual PM2.5 NAAQS DV shall 

nevertheless be considered valid and complete if at leastif one of the test conditions 

specified in 4.1(c)(i),) and 4.1(c)(ii), and 4.1(c)(iii) is met. 

(i) An annual PM2.5 NAAQS DV that is above the level of the NAAQS can be validated 

if it passes the minimum quarterly value data substitution test. This type of data 

substitution is permitted only if there are at least 30 days across the three matching 

quarters of the three years under consideration (e.g., collectively, quarter 1 of year 1, 

quarter 1 of year 2 and quarter 1 of year 3) from which to select the quarter-specific low

value. Data substitution will be performed in all quarter periods that have less than 11 

creditable samples.

Procedure: Identify for each deficient quarter (i.e., those with less than 11 creditable 

samples) the lowest reported daily value for that quarter, looking across those three 

months of all three years under consideration. If after substituting the lowest reported 

daily value for a quarter for (11- cn) daily values in the matching deficient quarter(s) (i.e., 

to bring the creditable number for those quarters up to 11), the procedure yields a 
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recalculated annual PM2.5 NAAQS test DV (TDVmin) that is greater than the level of the 

standard, then the annual PM2.5 NAAQS DV is deemed to have passed the diagnostic test 

and is valid, and the annual PM2.5 NAAQS is deemed to have been violated in that 3-year 

period. 

(ii) An annual PM2.5 NAAQS DV that is equal to or below the level of the NAAQS can 

be validated if it passes the maximum quarterly value data substitution test. This type of 

data substitution is permitted only if there areis at least 30 days across the three matching 

quarters50 percent data capture in each quarter that is deficient of 75 percent data capture 

in each of the three years under consideration from which to select the quarter-specific 

high value. Data substitution will be performed in all quarter periods that have less than 

75 percent data capture but at least 50 percent data capture. If any quarter has less than 50 

percent data capture then this substitution test cannot be used.

Procedure: Identify for each deficient quarter (i.e., those with less than 75 percent but at 

least 50 percent data capture) the highest reported daily value for that quarter, excluding 

state-flagged data affected by exceptional events which have been approved for exclusion 

by the Administrator, looking across those three quarters of all three years under 

consideration. If after substituting the highest reported daily PM2.5 value for a quarter for 

all missing daily data in the matching deficient quarter(s) (i.e., to make those quarters 100 

percent complete), the procedure yields a recalculated annual PM2.5 NAAQS test DV

(TDVmax)that is less than or equal to the level of the standard, then the annual PM2.5

NAAQS DV is deemed to have passed the diagnostic test and is valid, and the annual 

PM2.5 NAAQS is deemed to have been met in that 3-year period.

(iii) An annual PM2.5 NAAQS DV that is equal to or below the level of the NAAQS can 
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be validated if it passes the collocated PM10 data substitution test. Data substitution will 

be performed in all quarter periods that have less than 75 percent data capture but at least 

50 percent data capture. If any quarter has less than 50 percent data capture then this 

substitution test cannot be used.

Procedure: Identify for each deficient quarter (i.e., those with less than 75 percent but at 

least 50 percent data capture), available collocated FRM/FEM PM10 values reported for 

each PM2.5 scheduled day that is missing a valid daily PM2.5 value. If there is more than 

one collocated daily PM10 value present for a particular day (that is scheduled for 

measuring PM2.5 but does not have a corresponding valid daily PM2.5 value), then the 

highest of those multiple daily PM10 values will be used as the substituted value. If, after 

substituting the available collocated daily PM10 values for as many as possible missing 

daily PM2.5 values in the deficient quarter(s), the procedure yields recalculated data 

capture rates of 75 percent or more, and a recalculated annual PM2.5 NAAQS test DV less 

than or equal to the level of the standard, then the annual PM2.5 NAAQS DV is deemed to 

have passed the diagnostic test and is valid, and the annual PM2.5 NAAQS is deemed to 

have been met in that 3-year period.

(d) An annual PM2.5 NAAQS DV based on data that do not meet the completeness 

criteria stated in 4(b) and also do not satisfy the test conditions specified in section 4(c), 

may also be considered valid with the approval of, or at the initiative of, the EPA 

Administrator, who may consider factors such as monitoring site closures/moves, 

monitoring diligence, the consistency and levels of the daily values that are available, and 

nearby concentrations in determining whether to use such data. 

(e) The equations for calculating the annual PM2.5 NAAQS DVs are given in section 4.4 
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of this appendix.

4.2 Twenty-four-hour PM2.5 NAAQS

(a) The primary and secondary 24-hour PM2.5 NAAQS are met when the 24-hour PM2.5

NAAQS DV at each eligible monitoring site is less than or equal to 35 μg/m3. 

(b) Three years of valid annual PM2.5 98th percentile mass concentrations are required to 

produce a valid 24-hour PM2.5 NAAQS DV. A year meets data completeness 

requirements when quarterly data capture rates for all four quarters are at least 75

percent. However, years shall be considered valid, notwithstanding quarters with less 

than complete data (even quarters with less than 11 creditable samples, but at least one 

creditable sample must be present for the year), if the resulting annual 98th percentile 

value or resulting 24-hour NAAQS DV (rounded according to the conventions of section 

4.3 of this appendix) is greater than the level of the standard. Furthermore, where the 

explicit 75 percent quarterly data capture requirement is not met, the 24-hour PM2.5

NAAQS DV shall still be considered valid and complete if it passes at least one of two 

applicable data substitution tests (i.e., the maximum quarterly value or collocated PM10

data substitution tests).test.

(c) In the case of one, two, or three years that do not meet the completeness requirements 

of section 4.2(b) of this appendix and thus would normally not be useable for the 

calculation of a valid 24-hour PM2.5 NAAQS DV, the 24-hour PM2.5 NAAQS DV shall 

nevertheless be considered valid and complete if either ofif the test conditions specified 

in 4.2(c)(i) or 4.2(c)(ii) are met. 

(i) A PM2.5 24-hour mass NAAQS DV that is equal to or below the level of the NAAQS 

can be validated if it passes the maximum quarterly value data substitution test. This type 
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of data substitution is permitted only if there areis at least 30 days across the three 

matching quarters50 percent data capture in each quarter that is deficient of 75 percent 

data capture in each of the three years under consideration from which to select the 

quarter-specific high value. Data substitution will be performed in all quarters that have 

less than 75 percent data capture but at least 50 percent data capture. If any quarter has 

less than 50 percent data capture then this substitution test cannot be used.

Procedure: Identify for each deficient quarter (i.e., those with less than 75 percent but at 

least 50 percent data capture) the highest reported daily PM2.5 value for that quarter, 

excluding state-flagged data affected by exceptional events which have been approved 

for exclusion by the Regional Administrator, looking across those three quarters of all 

three years under consideration. If, after substituting the highest reported daily maximum 

PM2.5 value for a quarter for all missing daily data in the matching deficient quarter(s) 

(i.e., to make those quarters 100 percent complete), the procedure yields a recalculated 3-

year 24-hour NAAQS test DV (TDVmax)less than or equal to the level of the standard, 

then the 24-hour PM2.5 NAAQS DV is deemed to have passed the diagnostic test and is 

valid, and the 24-hour PM2.5 NAAQS is deemed to have been met in that 3-year period. 

(ii) A 24-hour PM2.5 NAAQS DV that is equal to or below the level of the NAAQS can 

be validated if it passes the collocated PM10 data substitution test. Data substitution will 

be performed in all quarter periods that have less than 75 percent data capture but at least 

50 percent data capture. If any quarter has less than 50 percent data capture then this 

substitution test cannot be used.

Procedure: Identify for each deficient quarter (i.e., those with less than 75 percent but at 

least 50 percent data capture), available collocated FRM/FEM daily PM10 values reported 
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for each PM2.5 scheduled day that is missing a valid daily PM2.5 value. If there is more 

than one collocated daily PM10 value present for a particular day (that is scheduled for 

measuring PM2.5 but doesn’t have a corresponding valid daily PM2.5 value), then the 

highest of those daily PM10 values will be used as the substituted daily PM2.5 value. Daily 

PM10 values shall be truncated to integers if decimal places are reported. If, after 

substituting the available collocated daily PM10 values for as many as possible missing 

daily PM2.5 values in the deficient quarter(s), the procedure yields recalculated data 

capture rates of 75 percent or more, and a recalculated 24-hour PM2.5 NAAQS test DV

less than or equal to the level of the standard, then the 24-hour PM2.5 NAAQS DV is 

deemed to have passed the diagnostic test and is valid, and the 24-hour PM2.5 NAAQS is 

deemed to have been met in that 3-year period.

(d) A 24-hour PM2.5 NAAQS DV based on data that do not meet the completeness 

criteria stated in 4(b) and also do not satisfy the test conditions specified in section 4(c), 

may also be considered valid with the approval of, or at the initiative of, the EPA 

Administrator, who may consider factors such as monitoring site closures/moves, 

monitoring diligence, the consistency and levels of the daily values that are available, and 

nearby concentrations in determining whether to use such data. 

(e) The procedures and equations for calculating the 24-hour PM2.5 NAAQS DVs are 

given in section 4.5 of this appendix.

4.3 Rounding Conventions. For the purposes of comparing calculated PM2.5 NAAQS 

DVs to the applicable level of the standard, it is necessary to round the final results of the 

calculations described in sections 4.4 and 4.5 of this appendix. Results for all 

intermediate calculations shall not be rounded.
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(a) Annual PM2.5 NAAQS DVs shall be rounded to the nearest tenth of a μg/m3(decimals 

x.x5 and greater are rounded up to the next tenth, and any decimal lower than x.x5 is 

rounded down to the nearest tenth).

(b) Twenty-four-hour PM2.5 NAAQS DVs shall be rounded to the nearest 1 μg/m3

(decimals 0.5 and greater are rounded up to the nearest whole number, and any decimal 

lower than 0.5 is rounded down to the nearest whole number).

4.4 Equations for the Annual PM2.5 NAAQS.

(a) An annual mean value for PM2.5 is determined by first averaging the daily values of a 

calendar quarter using equation 1 of this appendix:

Equation 1

Where:

X

–

q,y

= the mean for quarter q of the year y;

nq = the number of daily values in the quarter; and

xi q,y = the ith value in quarter q for year y.

(b) Equation 2 of this appendix is then used to calculate the site annual mean:

Equation 2

Where:

X y = the annual mean concentration for year y (y = 1, 2, or 3); and

X q,y = the mean for quarter q of year y (result of equation 1).

(c) The annual PM2.5 NAAQS DV is calculated using equation 3 of this appendix.. 

nq

i

i q y

q

q y X

n

X

1

, ,

,

1

4

1

,

4

1

q

X y X q y
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Equation 3

Where:

X = the annual PM2.5 NAAQS DV; and

X y = the annual mean for year y (result of equation 2)

(d) The annual PM2.5 NAAQS DV is rounded according to the conventions in section 4.3 

of this appendix before comparisons with the levels of the primary and secondary annual 

PM2.5 NAAQS are made.

4.5 Procedures and Equations for the 24-Hour PM2.5 NAAQS

(a) When the data for a particular site and year meet the data completeness requirements 

in section 4.2 of this appendix, calculation of the 98th percentile is accomplished by the 

steps provided in this subsection. Table 1 of this appendix shall be used to identify annual 

98th percentile values.

Identification of annual 98th percentile values using the Table 1 procedure will be based 

on the creditable number of samples (as described below), rather than on the actual 

number of samples. Credit will not be granted for extra (non-creditable) samples. Extra 

samples, however, are candidates for selection as the annual 98th percentile. [The 

creditable number of samples will determine how deep to go into the data distribution, 

but all samples (creditable and extra) will be considered when making the percentile 

assignment.] The annual creditable number of samples is the sum of the four quarterly 

creditable number of samples.

Procedure: Sort all the daily values from a particular site and year by descending value. 

(For example: (x[1], x[2], x[3], * * *, x[n]). In this case, x[1] is the largest number and 

3

3 1

1

y

X X y
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x[n] is the smallest value.) The 98th percentile value is determined from this sorted series 

of daily values which is ordered from the highest to the lowest number. Using the left 

column of Table 1, determine the appropriate range for the annual creditable number of 

samples for year y (cny) (e.g., for 120 creditable samples per year, the appropriate range 

would be 101 to 150). The corresponding “n” value in the right column identifies the rank 

of the annual 98th percentile value in the descending sorted list of site specific daily 

values for year y (e.g., for the range of 101 to 150, n would be 3). Thus, P0.98, y = the nth

largest value (e.g., for the range of 101 to 150, the 98th percentile value would be the third 

highest value in the sorted series of daily values.

Table 1

Annual number of creditable

samples for year y (cny)

The 98th percentile for year y 

(P0.98,y),is the nth maximum 24-

hour average value for the year 

where n is the listed number

1 to 50 1

51 to 100 2

101 to 150 3

151 to 200 4

201 to 250 5

251 to 300 6

301 to 350 7

351 to 366 8

(b) The 24-hour PM2.5 NAAQS DV is then calculated by averaging the annual 98th

percentiles using equation 4 of this appendix: P0.98,y

Equation 4

Where

3

1

98 0.98, 0.

3

1

y

P P y
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P 0.98 = the 24-hour PM2.5 NAAQS DV; and

P0.98, y = the annual 98th percentile for year y

(c) The 24-hour PM2.5 NAAQS DV is rounded according to the conventions in section 

4.3 of this appendix before a comparison with the level of the primary and secondary 24-

hour NAAQS are made.

Part 51—[AMENDED]

PART 51—REQUIREMENTS FOR PREPARATION, ADOPTION, AND 

SUBMITTAL OF IMPLEMENTATION PLANS

5.1.The authority citation for part 51 continues to read as follows:

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.

Subpart I – [Amended]

6.2.In §51.166, add paragraph (i)(10) to read as follows:

§51.166 Prevention of significant deterioration of air quality

* * * * *

(i) Exemptions. (1) * * *

* * * * *

(10) The plan may provide that the requirements of paragraph (k)(1) of this section shall 

not apply to a stationary source or modification with respect to the national ambient air 

quality standards for PM2.5 in effect on [INSERT DATE 60 DAYS AFTER DATE OF 

PUBLICATION IN THE FEDERAL REGISTER] if:

(i) The reviewing authority has determined a permit application subject to this 

section to be complete on or before December 14, 2012. Instead, the requirements

in paragraph (k)(1) shall apply with respect to the national ambient air quality 

standards for PM2.5 in effect at the time the reviewing authority determined the 

permit application to be complete; or

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(ii) The reviewing authority has first published before that date[INSERT DATE 

60 DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL 

REGISTER] a public notice of a preliminary determination for the permit 

application subject to this section. Instead, the requirements in paragraph (k)(1) 

shall apply with respect to the national ambient air quality standards for PM2.5 in 

effect at the time of first publication of a public notice on the preliminary 

determination. 

* * * * *

PART 52—AMENDEDAPPROVAL AND PROMULGATION OF 

IMPLEMENTATION PLANS

7.1.The authority citation for part 52 continues to read as follows:

Authority: 42 U.S.C. 7401, et seq.

8.2.In §52.21, add paragraph (i)(11) to read as follows:

§52.21 Prevention of significant deterioration of air quality.

* * * * *

(i) * * *

* * * * *

(11) The requirements of paragraph (k)(1) of this section shall not apply to a stationary 

source or modification with respect to the national ambient air quality standards for PM2.5

in effect on [INSERT DATE 60 DAYS AFTER DATE OF PUBLICATION IN THE 

FEDERAL REGISTER] if the Administrator has first published before that date: 

(i) The Administrator has determined a permit application subject to this section 

to be complete on or before December 14, 2012. Instead, the requirements in 

paragraph (k)(1) shall apply with respect to the national ambient air quality 

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standards for PM2.5 in effect at the time the Administrator determined the permit 

application to be complete; or

(ii) The Administrator has first published before [INSERT DATE 60 DAYS 

AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER] a 

public notice that a draft permit subject to this section has been prepared. Instead, 

the requirements in paragraph (k)(1) shall apply with respect to the national 

ambient air quality standards for PM2.5 in effect on the date the Administrator first 

published a public notice that a draft permit has been prepared.

* * * * *

PART 53—[AMENDED]

PART 53—AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT 

METHODS

9.6.The authority citation for part 53 continues to read as follows:

Authority: Section 301(a) of the CAA (42 U.S.C. sec. 1857 g(a)), as amended by sec. 15 

(c)(2) of Pub. L. 91-604, 84 Stat. 1713, unless otherwise noted.

10.7. In §53.9, revise paragraph (c) to read as follows:

§53.9 Conditions of designation.

* * * * *

(c) Any analyzer, PM10 sampler, PM2.5 sampler, or PM10-2.5 sampler offered for sale as 

part of an FRM or FEM shall function within the limits of the performance specifications 

referred to in §53.20(a), §53.30(a), §53.35, §53.50, or §53.60, as applicable, for at least 1 

year after delivery and acceptance when maintained and operated in accordance with the 

manual referred to in §53.4(b)(3).

* * * * *

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PART 58—AMBIENT AIR QUALITY SURVEILLANCE

11.8. The authority citation of part 58 continues to read as follows:

Authority: 42 U.S.C. 7403, 7405, 7410, 7414, 7601, 7611, 7614, and 7619. 

12.9. Section 58.1 is amended by adding in alphabetical order a definition for 

“Area-wide” and by removing the definition for “Community monitoring zone 

(CMZ)” to read as follows:

§58.1 Definitions.

* * * * *

Area-wide means all monitors sited at neighborhood, urban, and regional scales, as well 

as those monitors sited at either micro-or middle scale that are representative of many 

such locations in the same CBSA.

* * * * *

13.10. Section 58.10 is amended as follows:

a. By revising paragraph (a)(2). 

b. By adding paragraph (a)(8).

c. By adding paragraph (b)(13).

d. By revising paragraph (c).

e. By revising paragraph (d).

§58.10 Annual monitoring network plan and periodic network assessment.

(a) * * * 

(2) Any annual monitoring network plan that proposes SLAMS network modifications

(including new monitoring sites, new determinations that data are not of sufficient quality 

to be compared to the NAAQS, and changes in identification of monitors as suitable or 

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not suitable for comparison against the annual PM2.5 NAAQS) is subject to the approval

of the EPA Regional Administrator, who shall provide opportunity for public comment 

and shall approve or disapprove the plan and schedule within 120 days. If the State or 

local agency has already provided a public comment opportunity on its plan and has 

made no changes subsequent to that comment opportunity, and has submitted the 

received comments together with the plan, the Regional Administrator is not required to 

provide a separate opportunity for comment.

***

(8)(i) A plan for establishing near-road PM2.5 monitoring sites in CBSAs having 2.5 

million or more persons, in accordance with the requirements of appendix D to this part,

shall be submitted as part of the annual monitoring network plan to the EPA Regional 

Administrator by July 1, 2014. The plan shall provide for these required monitoring 

stations to be operational by January 1, 2015. 

(ii) A plan for establishing near-road PM2.5 monitoring sites in CBSAs having 1 million 

or more persons, but less than 2.5 million persons, in accordance with the requirements of 

appendix D to this part, shall be submitted as part of the annual monitoring network plan

to the EPA Regional Administrator by July 1, 2016. The plan shall provide for these

required monitoring stations to be operational by January 1, 2017.

(b) * * * 

(13) The identification of any PM2.5 FEMs and/or ARMs used in the monitoring agency’s 

network where the data are not of sufficient quality such that data are not to be compared 

to the NAAQS. For required SLAMS where the agency identifies that the PM2.5 Class III 

FEM or ARM does not produce data of sufficient quality for comparison to the NAAQS, 
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the monitoring agency must ensure that an operating FRM or filter-based FEM meeting 

the sample frequency requirements described in §58.12 or other Class III PM2.5 FEM or 

ARM with data of sufficient quality is operating and reporting data to meet the network 

design criteria described in appendix D to this part. 

(c) The annual monitoring network plan must document how state and local agencies 

provide for the review of changes to a PM2.5 monitoring network that impact the location 

of a violating PM2.5 monitor. The affected state or local agency must document the

process for obtaining public comment and include any comments received through the 

public notification process within their submitted plan.

(d) The state, or where applicable local, agency shall perform and submit to the EPA 

Regional Administrator an assessment of the air quality surveillance system every 5 years 

to determine, at a minimum, if the network meets the monitoring objectives defined in 

appendix D to this part, whether new sites are needed, whether existing sites are no 

longer needed and can be terminated, and whether new technologies are appropriate for 

incorporation into the ambient air monitoring network. The network assessment must 

consider the ability of existing and proposed sites to support air quality characterization 

for areas with relatively high populations of susceptible individuals (e.g., children with 

asthma), and, for any sites that are being proposed for discontinuance, the effect on data 

users other than the agency itself, such as nearby states and tribes or health effects 

studies. The state, or where applicable local, agency must submit a copy of this 5-year 

assessment, along with a revised annual network plan, to the Regional Administrator. The 

assessments are due every five years beginning July 1, 2010.

* * * * *
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14.11. Section 58.11 is amended by adding paragraph (e) to read as follows:

§58.11 Network technical requirements.

* * * * *

(e) State and local governments must assess data from Class III PM2.5 FEM and ARM 

monitors operated within their network using the performance criteria described in table 

C-4 to subpart C of part 53, for any casecases where the data are identified as not of 

sufficient comparability to a collocated FRM, suchand the monitoring agency requests

that the FEM or ARM data should not be used in comparison to the NAAQS. These 

assessments are required in the monitoring agency’s annual monitoring network plan 

described in §58.10(b) for any casecases where the FEM or ARM is identified as not of 

sufficient comparability to a collocated FRM. For these collocated PM2.5 monitors the 

performance criteria apply with the following additional provisions:

(1) The acceptable concentration range (Rj), μg/m3 may include values down to 0 μg/m3.

(2) The minimum number of test sites shall be at least one; however, the number of test

sites will generally include all locations within an agency’s network with collocated 

FRMs and FEMs or ARMs.

(3) The minimum number of methods shall include at least one FRM and at least one 

FEM or ARM.

(4) Since multiple FRMs and FEMs may not be present at each site; the precision statistic 

requirement does not apply, even if precision data are available.

(5) All seasons must be covered with no more than thirty six consecutive months of data

in total aggregated together.

(6) The key statistical metric to include in an assessment is the bias (both additive and 

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multiplicative) of the PM2.5 continuous FEM(s) compared to a collocated FRM(s). 

Correlation is required to be reported in the assessment, but failure to meet the correlation 

criteria, by itself, is not cause to exclude data from a continuous FEM monitor. 

15.12. Section 58.12 is amended by revising paragraph (d)(1)(iii) and by 

removing and reserving paragraph (f)(2) to read as follows:

§ 58.12 Operating schedules.

* * * * *

(d) * * * 

(1) ***

(iii) Required SLAMS stations whose measurements determine the 24-hour design value 

for their area and whose data are within plus or minus 5 percent of the level of the 24-

hour PM2.5 NAAQS must have an FRM or FEM operate on a daily schedule if thethat 

area’s design value for the annual NAAQS is less than the level of the annual PM2.5

standard. A continuously operating FEM or ARM PM2.5 monitor satisfies this 

requirement unless it is identified in the monitoring agency’s annual monitoring network 

plan as not appropriate for comparison to the NAAQS.

(f) ***

(2) Reserved

* * * * *

16.13. Section 58.13 is amended by adding paragraph (f) to read as follows:

§58.13 Monitoring network completion.

* * * * *

(f) PM2.5 monitors required in near-road environments as described in appendix D to this 

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part, must be physically established and operating under all of the requirements of this 

part, including the requirements of appendices A, C, D, and E to this part, no later than:

(1) January 1, 2015 for PM2.5 monitors in CBSAs having 2.5 million persons or more; or

(2) January 1, 2017 for PM2.5 monitors in CBSAs having 1 million or more, but less than 

2.5 million persons.

17.14. Section 58.16 is amended by revising paragraphs (a) and (f) to read as 

follows:

§58.16 Data submittal and archiving requirements.

(a) The state, or where appropriate, local agency, shall report to the Administrator, via 

AQS all ambient air quality data and associated quality assurance data for SO2; CO; O3; 

NO2; NO; NOy; NOx; Pb–TSP mass concentration; Pb–PM10 mass concentration; PM10

mass concentration; PM2.5 mass concentration; for filter-based PM2.5 FRM/FEM the field 

blank mass, sampler-generated average daily temperature, and sampler-generated average 

daily pressure; chemically speciated PM2.5 mass concentration data; PM10-2.5 mass 

concentration; meteorological data from NCore and PAMS sites; average daily 

temperature and average daily pressure for Pb sites if not already reported from sampler 

generated records; and metadata records and information specified by the AQS Data 

Coding Manual (http://www.epa.gov/ttn/airs/airsaqs/manuals/manuals.htm). The state, or 

where appropriate, local agency, may report site specific meteorological measurements 

generated by onsite equipment (meteorological instruments, or sampler generated) or 

measurements from the nearest airport reporting ambient pressure and temperature. Such 

air quality data and information must be submitted directly to the AQS via electronic 

transmission on the specified quarterly schedule described in paragraph (b) of this 

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section.

* * * * *

(f) The state, or where applicable, local agency shall archive all PM2.5, PM10, and 

PM10−2.5 filters from manual low-volume samplers (samplers having flow rates less than 

200 liters/minute) from all SLAMS sites for a minimum period of 5 years after collection. 

These filters shall be made available for supplemental analyses, including destructive 

analyses if necessary, at the request of EPA or to provide information to state and local 

agencies on particulate matter composition. Note: destructive chemical analysis may be 

necessary and are acceptable. Other Federal agencies may request access to filters for 

purposes of supporting air quality management or community health—such as biological 

assay—through the applicable EPA Regional Administrator. The filters shall be archived 

according to procedures approved by the Administrator, which shall include cold storage 

of filters after post-sampling laboratory analyses for at least 12 months following field 

sampling. The EPA recommends that particulate matter filters be archived for longer 

periods, especially for key sites in making NAAQS-related decisions or for supporting 

health-related air pollution studies.* * * * *

* * * * *

Subpart C—Special Purpose Monitors

18.15. Section 58.20 is amended by revising paragraph (c) to read as follows:

§58.20 Special purpose monitors (SPM).

* * * * *

(c) All data from an SPM using an FRM, FEM, or ARM which has operated for more 

than 24 months are eligible for comparison to the relevant NAAQS, subject to the 

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conditions of §58.11(e) and 58.30, unless the air monitoring agency demonstrates that the 

data came from a particular period during which the requirements of appendix A, 

appendix C, or appendix E to this part were not met, subject to review and EPA Regional 

Office approval as part of the annual monitoring network plan described in §58.10.

* * * * *

19.16. The heading for Subpart D is revised to read as follows:

Subpart D—Comparability of Ambient Data to the NAAQS

20.17. Section 58.30 is amended by revising paragraph (a) to read as follows:

§58.30 Special considerations for data comparisons to the NAAQS.

(a) Comparability of PM2.5 data. The primary and secondary annual and 24-hour PM2.5

NAAQS are described in part 50 of this chapter. Monitors that follow the network 

technical requirements specified in §58.11 are eligible for comparison to the NAAQS.

subject to the additional requirements of this section. PM2.5 measurement data from all 

eligible monitors are comparable to the 24-hour PM2.5 NAAQS;. PM2.5 measurement data 

from all eligible monitors that are representative of area-wide air quality are comparable

to the annual PM2.5 NAAQS. As specified inConsistent with appendix D to this part, 

section 4.7.1, when micro- or middle-scale PM2.5 monitoring sites collectively identify a 

larger region of localized high ambient PM2.5 concentrations; for example, a PM2.5

monitoring site located in a near-road environment where there are many other similar 

locations in the same CBSA, such sites would be considered representative of an areawide location and, therefore, eligible for comparison to the annual PM2.5 NAAQS. PM2.5

measurement data from monitors that are not representative of area-wide air quality but 

rather of relatively unique micro-scale, or localized hot spot, or unique middle-scale 

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impact sites are not eligible for comparison to the annual PM2.5 NAAQS. PM2.5

measurement data from these monitors are eligible for comparison to the 24-hour PM2.5

NAAQS. For example, if a micro- or middle-scale PM2.5 monitoring site is adjacent to a 

unique dominating local PM2.5 source, then the PM2.5 measurement data from such a site 

would only be eligible for comparison to the 24-hour PM2.5 NAAQS. Approval of sites 

that are suitable and sites that are not suitable for comparison with the annual PM2.5

NAAQS is provided for as part of the annual monitoring network plan described in

§58.10. 

* * * * *

21.18. Appendix A to part 58 is amended as follows:

a. By redesignating the existing introductory paragraph in section 1 as paragraph 

(b) in section 1.

b. By adding paragraph (a) to section 1.

c. By revising paragraphs 3.2.5.6, and 3.2.6.3.

d. By revising Table A-1.

The revisions and additions read as follows:

Appendix A to Part 58—Quality Assurance Requirements for SLAMS, SPMs and 

PSD Air Monitoring

1. * * *

(a) Each monitoring organization is required to implement a quality system that provides 

sufficient information to assess the quality of the monitoring data. The quality system 

must, at a minimum, include the specific requirements described in this appendix of this 

subpart. Failure to conduct or pass a required check or procedure, or a series of required 

checks or procedures, does not by itself invalidate data for regulatory decision making. 

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Rather, monitoring agencies and EPA shall use the checks and procedures required in this 

appendix in combination with other data quality information, reports, and similar 

documents showing overall compliance with part 58. Accordingly, EPA and monitoring 

agencies shall use a "weight of evidence" approach when determining the suitability of 

data for regulatory decisions. The EPA reserves the authority to use or not use monitoring 

data submitted by a monitoring organization when making regulatory decisions based on 

the EPA's assessment of the quality of the data. Generally, consensus built validation 

templates or validation criteria already approved in Quality Assurance Project Plans 

(QAPPs) should be used as the basis for the weight of evidence approach. 

(b) This appendix specifies the minimum quality system requirements applicable to 

SLAMS air monitoring data and PSD data for the pollutants SO2, NO2, O3, CO, Pb, 

PM2.5, PM10 and PM10-2.5 submitted to EPA. This appendix also applies to all SPM 

stations using FRM, FEM, or ARM methods which also meet the requirements of 

appendix E of this part, unless alternatives to this appendix for SPMs have been approved 

in accordance with §58.11(a)(2). Monitoring organizations are encouraged to develop and 

maintain quality systems more extensive than the required minimums. The permitgranting authority for PSD may require more frequent or more stringent requirements. 

Monitoring organizations may, based on their quality objectives, develop and maintain 

quality systems beyond the required minimum. Additional guidance for the requirements 

reflected in this appendix can be found in the “Quality Assurance Handbook for Air 

Pollution Measurement Systems”, volumeVolume II, part 1 (see reference 10 of this 

appendix) and at a national level in references 1, 2, and 3 of this appendix.

* * * * *
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3.2.5* * * 

3.2.5.6 The two collocated monitors must be within 4 meters of each other and at least 2 

meters apart for flow rates greater than 200 liters/min or at least 1 meter apart for 

samplers having flow rates less than 200 liters/min to preclude airflow interference. A 

waiver allowing up to 10 meters horizontal distance and up to 3 meters vertical distance 

(inlet to inlet) between a primary and collocated sampler may be approved by the 

Regional Administrator for sites at a neighborhood or larger scale of representation. This 

waiver may be approved during the annual network plan approval process. Calibration, 

sampling, and analysis must be the same for all the collocated samplers in each agency’s

network.

* * * * *

3.2.6* * * 

3.2.6.3 The two collocated monitors must be within 4 meters of each other and at least 2 

meters apart for flow rates greater than 200 liters/min or at least 1 meter apart for 

samplers having flow rates less than 200 liters/min to preclude airflow interference. A 

waiver allowing up to 10 meters horizontal distance and up to 3 meters vertical distance 

(inlet to inlet) between a primary and a collocated sampler may be approved by the 

Regional Administrator for sites at a neighborhood or larger scale of representation

taking into consideration safety, logistics, and space availability. This waiver may be 

approved during the annual network plan approval process. Calibration, sampling, and 

analysis must be the same for all the collocated samplers in each agency’s network.

* * * * *

and
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Table A–1 of Appendix A to Part 58—Difference and Similarities Between SLAMS 

and PSD Requirements

Topic SLAMS PSD

Requirements

1. The development, documentation, 

and implementation of an approved 

quality system

2. The assessment of data quality

3. The use of reference, equivalent, or 

approved methods

4. The use of calibration standards 

traceable to NIST or other primary 

standard

5. The participation in EPA 

performance evaluations and the 

permission for EPA to conduct 

system audits

Same as SLAMS

Monitoring and QA 

Responsibility

State/local agency via the “primary 

quality assurance organization” Source owner/operator.

Monitoring Duration Indefinitely Usually up to 12 months.

Annual Performance 

Evaluation (PE)

Standards and equipment different 

from those used for spanning, 

calibration, and verifications. Prefer 

different personnel

Personnel, standards and 

equipment different from 

those used for spanning, 

calibration, and verifications.

PE audit rate:

—

Automated 100% per year 100% per quarter.

—Manual Varies depending on pollutant. See 

Table A–2 of this appendix 100% per quarter.

Precision 

Assessment:

—

Automated

One-point QC check biweekly but 

data quality dependent

One point QC check 

biweekly.

—Manual Varies depending on pollutant. See 

Table A–2 of this appendix

One site: 1 every 6 days or 

every third day for daily 

monitoring (TSP and Pb).

Reporting

—

Automated

By site—EPA performs calculations 

annually

By site—source 

owner/operator performs 

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calculations each sampling 

quarter.

—Manual By reporting organization—EPA 

performs calculations annually

By site—source 

owner/operator performs 

calculations each sampling 

quarter.

* * * * *

22.19. Appendix D to part 58 is amended as follows:

a. By revising paragraphs 4.7.1(b) and 4.7.1(c)(1).

b. By removing paragraph 4.7.5

c. By removing and reserving paragraph 4.8.2

Appendix D to Part 58—Network Design Criteria for Ambient Air Quality 

Monitoring

* * * * *

4.7.1* * *

(b) Specific Design Criteria for PM2.5. The required monitoring stations or sites must be 

sited to represent area-wide air quality. These sites can include sites collocated at PAMS. 

These monitoring stations will typically be at neighborhood or urban-scale; however, 

micro-or middle-scale PM2.5 monitoring sites that represent many such locations 

throughout a metropolitan area are considered to represent area-wide air quality.

(1) At least one monitoring station is to be sited at neighborhood or larger scale in an area 

of expected maximum concentration.

(2) For CBSAs with a population overof 1,000,000 or more persons, at least one PM2.5

monitor is to be collocated at a near-road NO2 station required in section 4.3.2(a) of this 

appendix. 

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(3) For areas with additional required SLAMS, a monitoring station is to be sited in an 

area of poor air quality.

(4) Additional technical guidance for siting PM2.5 monitors is provided in references 6 

and 7 of this appendix.

(c) ***

(1) Micro-scale. This scale would typify areas such as downtown street canyons and 

traffic corridors where the general public would be exposed to maximum concentrations 

from mobile sources. In some circumstances, the micro-scale is appropriate for 

particulate sites. SLAMS sites measured at the micro-scale level should, however, be 

limited to urban sites that are representative of long-term human exposure and of many 

such microenvironments in the area. In general, micro-scale particulate matter sites 

should be located near inhabited buildings or locations where the general public can be 

expected to be exposed to the concentration measured. Emissions from stationary sources 

such as primary and secondary smelters, power plants, and other large industrial 

processes may, under certain plume conditions, likewise result in high ground level 

concentrations at the micro-scale. In the latter case, the micro-scale would represent an 

area impacted by the plume with dimensions extending up to approximately 100 meters. 

Data collected at micro-scale sites provide information for evaluating and developing hot 

spot control measures. 

* * * * *

4.7.5 [Removed]

* * * * *

4.8* * * 
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4.8.2[Reserved] 

* * * * *

23.20. Appendix E to part 58 is amended as follows:

a. By adding table E-1 to paragraph 6.

b. By revising table E-4.

Appendix E to Part 58—Probe and Monitoring Path Siting Criteria for Ambient Air 

Quality Monitoring

* * * * *

6. * * *

Table E-1 ofto Appendix E toof Part 58. Minimum Separation Distance Between 

Roadways and Probes or Monitoring Paths for Monitoring Neighborhood and 

Urban Scale Ozone (O3) and Oxides of Nitrogen (NO, NO2, NOx, NOy)

Roadway average daily traffic, 

vehicles per day

Minimum distance 1

(meters)

Minimum distance1,2

(meters)

< 1,000 ...............................

10,000 ................................

15,000 ................................

20,000 ................................

40,000 ................................

70,000 ................................

> 110,000 ...........................

10

10

20

30

50

100

250

10

20

30

40

60

100

250

1 Distance from the edge of the nearest traffic lane. The distance for intermediate traffic 

counts should be interpolated from the table values based on the actual traffic count.

2 Applicable for ozone monitors whose placement has not already been approved as of 

December 18, 2006.

* * * * *

11. * * * 

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Table E–4 of Appendix E to Part 58—Summary of Probe and Monitoring Path 

Siting Criteria

Pollutant

Scale 

(maximum 

monitoring 

path length, 

meters)

Height from 

ground to 

probe, inlet 

or 80% of 

monitoring 

path1

(meters)

Horizontal 

and vertical 

distance from 

supporting 

structures2to 

probe, inlet or 

90% of 

monitoring 

path1(meters)

Distance 

from trees to 

probe, inlet 

or 90% of 

monitoring 

path1

(meters)

Distance from 

roadways to 

probe, inlet or 

monitoring 

path1(meters)

SO2

3,4,5,6

Middle (300 m) 

Neighborhood 

Urban, and 

Regional (1 km)

2–15 >1 >10 N/A

CO4,5,7

Micro 

[downtown

or street canyon

sites], micro

[near-road

sites], middle

(300 m) and

Neighborhood

(1 km).

2.5–3.5; 2–7; 

2–

15.

>1 >10

2–10 for 

downtown

areas or

street canyon

microscale; ≤50

for near-road

microscale; see

Table E–2 of

this appendix

for middle and

neighborhood

scales.

O3

3,4,5

Middle (300 m) 

Neighborhood, 

Urban, and 

Regional (1 km)

2–15 >1 >10

See Table E–1 

of this appendix 

for all scales.

NO2

3,4,5

Micro (Nearroad [50–300

m])

2–7 (micro)); >1 >10

≤50 for nearroad microscale.

Middle (300 m) 2–15 (all 

other scales)

Neighborhood, 

Urban, and 

Regional (1 km)

See Table E–1 

of this appendix 

for all other 

scales

Ozone Neighborhood 2–15 >1 >10 See Table E–4 

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precursors 

(for 

PAMS)345

and Urban (1 

km)

of this appendix 

for all scales.

PM, 

Pb3,4,5,6,8

Micro, Middle, 

Neighborhood, 

Urban and 

Regional

2–7 (micro); 

2–7 (middle 

PM10-2.5); 2-7 

for near-road; 

2–15 (all 

other scales)

>2 (all scales, 

horizontal 

distance only)

>10 (all 

scales)

2–10 (micro); 

see Figure E–1 

of this appendix 

for all other 

scales. ≤50 for 

near-road

N/A—Not applicable.

1Monitoring path for open path analyzers is applicable only to middle or neighborhood 

scale CO monitoring, middle, neighborhood, urban, and regional scale NO2 monitoring, 

and all applicable scales for monitoring SO2,O3, and O3 precursors.

2When probe is located on a rooftop, this separation distance is in reference to walls, 

parapets, or penthouses located on roof.

3

Should be greater than 20 meters from the dripline of tree(s) and must be 10 meters from 

the dripline when the tree(s) act as an obstruction.

4Distance from sampler, probe, or 90 percent of monitoring path to obstacle, such as a 

building, must be at least twice the height the obstacle protrudes above the sampler, 

probe, or monitoring path. Sites not meeting this criterion may be classified as middle 

scale (see text).

5Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if 

the probe is on the side of a building or a wall.

6

The probe, sampler, or monitoring path should be away from minor sources, such as 

furnace or incineration flues. The separation distance is dependent on the height of the 

minor source's emission point (such as a flue), the type of fuel or waste burned, and the 

quality of the fuel (sulfur, ash, or lead content). This criterion is designed to avoid undue 

influences from minor sources.

7

For micro-scale CO monitoring sites, the probe must be >10 meters from a street 

intersection and preferably at a midblock location.

8Collocated monitors must be within 4 meters of each other and at least 2 meters apart for 

flow rates greater than 200 liters/min or at least 1 meter apart for samplers having flow 

rates less than 200 liters/min to preclude airflow interference, unless a waiver is in place 

as approved by the Regional Administrator pursuant to section 3 of Appendix A.

* * * * *

Appendix F to Part 58 [Reserved]

* * * * * 

* * * * *

Appendix F to Part 58 [Reserved]

* * * * * 
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24.21. Appendix G to Part 58 is amended as follows:

a. By revising section 9.

b. By revising section 10.

c. By revising section 12, including table 2.

d. By revising section 13.

Appendix G to Part 58 - Uniform Air Quality Index (AQI) and Daily Reporting

* * * * * 

9. How Does the AQI Relate to Air Pollution Levels?

For each pollutant, the AQI transforms ambient concentrations to a scale from 0 to 500. 

The AQI is keyed as appropriate to the national ambient air quality standards (NAAQS) 

for each pollutant. In most cases, the index value of 100 is associated with the numerical 

level of the short-term standard (i.e., averaging time of 24-hours or less) for each 

pollutant. The index value of 50 is associated with the numerical level of the annual 

standard for a pollutant, if there is one, at one-half the level of the short-term standard for 

the pollutant, or at the level at which it is appropriate to begin to provide guidance on 

cautionary language. Higher categories of the index are based on increasingly serious 

health effects and increasing proportions of the population that are likely to be affected. 

The index is related to other air pollution concentrations through linear interpolation 

based on these levels. The AQI is equal to the highest of the numbers corresponding to 

each pollutant. For the purposes of reporting the AQI, the sub-indexes for PM10 and 

PM2.5 are to be considered separately. The pollutant responsible for the highest index 

value (the reported AQI) is called the “critical” pollutant.

10. What Monitors Should I Use to Get the Pollutant Concentrations For 

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Calculating the AQI?

You must use concentration data from State/Local Air Monitoring Station (SLAMS) or 

parts of the SLAMS required by 40 CFR 58.10 for each pollutant except PM. For PM, 

calculate and report the AQI on days for which you have measured air quality data (e.g., 

from continuous PM2.5 monitors required in Appendix D to this part). You may use PM 

measurements from monitors that are not reference or equivalent methods (for example, 

continuous PM10 or PM2.5 monitors). Detailed guidance for relating non-approved 

measurements to approved methods by statistical linear regression is referenced in 

section 13 below.

* * * * *

12. How do I calculate the AQI?

i. The AQI is the highest value calculated for each pollutant as follows:

a. Identify the highest concentration among all of the monitors within each reporting area 

and truncate as follows:

(1) Ozone – truncate to 3 decimal places

PM2.5 – truncate to 1 decimal place

PM10 – truncate to integer

CO – truncate to 1 decimal place

SO2 – truncate to integer

NO2 – truncate to integer

***
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TABLE 2.—BREAKPOINTS FOR THE AQI

These breakpoints Equal these AQI’s

O3 (ppm)

8-hour

O3 (ppm)

1-hour1

PM2.5

(μg/m3)

24-hour

PM10

(μg/m3)

24-hour

CO 

(ppm)

8-hour

SO2

(ppb)

1-hour

NO2

(ppb)

1-hour

AQI Category

0.000-

0.059 .................. 0.0 – 12.0 0-54 0.0-4.4 0-35 0-53 0-50 Good

0.060-

0.075 .................. 12.1 – 35.4 55-154 4.5-9.4 36-75 54-100 51-100 Moderate

0.076-

0.095

0.125-

0.164 35.5 – 55.4 155-254 9.5-12.4 76-185 101-360

101-

150

Unhealthy for 

Sensitive 

Groups

0.096-

0.115

0.165-

0.204

3

55.5 - 150.4 255-354 12.5-

15.4

4

186-304 361-649

151-

200 Unhealthy

0.116-

0.374

0.205-

0.404

3

150.5 - 250.4 355-424 15.5-

30.4

4

305-604 650-1249

201-

300

Very 

Unhealthy

(

2

)............... 0.405-

0.504

3

250.5 - 350.4 425-504 30.5-

40.4

4

605-804 1250-

1649

301-

400

Hazardous

(

2

)............... 0.505-

0.604

3

350.5 - 500.4 505-604 40.5-

50.4

4

805-1004 1650-

2049

401-

500

1 Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas 

where an AQI based on 1-hour ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour 

ozone index value, the 1-hour ozone index value may be calculated, and the maximum of the two values reported.

2

8-hour O3 values do not define higher AQI values ( 301). AQI values of 301 or greater are calculated with 1-hour O3

concentrations.

3

If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.

4 1-hr SO2 values do not define higher AQI values ( 200). AQI values of 200 or greater are calculated with 24-hour SO2

concentrations.
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13. What Additional Information Should I Know?

The EPA has developed a computer program to calculate the AQI for you. The program 

prompts for inputs, and it displays all the pertinent information for the AQI (the index 

value, color, category, sensitive group, health effects, and cautionary language). The 

EPA has also prepared a brochure on the AQI that explains the index in detail (The Air 

Quality Index), Reporting Guidance (Technical Assistance Document for the Reporting 

of Daily Air Quality –the Air Quality Index (AQI)) that provides associated health 

effects and cautionary statements, and Forecasting Guidance (Guideline for Developing 

an Ozone Forecasting Program) that explains the steps necessary to start an air pollution 

forecasting program. You can download the program and the guidance documents at 

www.airnow.gov. Reference for relating non-approved PM measurements to approved 

methods (Eberly, S., T. Fitz-Simons, T. Hanley, L. Weinstock., T. Tamanini, G. 

Denniston, B. Lambeth, E. Michel, S. Bortnick. Data Quality Objectives (DQOs) For 

Relating Federal Reference Method (FRM) and Continuous PM2.5 Measurements to 

Report an Air Quality Index (AQI). U.S. Environmental Protection Agency, Research 

Triangle Park, NC. EPA-454/B-02-002, November 2002) can be found on the Ambient 

Monitoring Technology Information Center (AMTIC) website, 

http://www.epa.gov/ttnamti1/.