Vol. 77 Friday, 

No. 126 June 29, 2012 

Part II 

Environmental Protection Agency 

40 CFR Parts 50, 51, 52, et al. 

National Ambient Air Quality Standards for Particulate Matter; Proposed 

Rule 

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38890 Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / Proposed Rules 

ENVIRONMENTAL PROTECTION 

AGENCY 

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

[EPA–HQ–OAR–2007–0492; FRL–9682–9] 

RIN 2060–AO47 

National Ambient Air Quality 

Standards for Particulate Matter 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Proposed 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 

proposes to make revisions to the 

primary and secondary NAAQS for PM 

to provide requisite protection of public 

health and welfare, respectively, and to 

make corresponding revisions to the 

data handling conventions for PM and 

ambient air monitoring, reporting, and 

network design requirements. The EPA 

also proposes revisions to the 

prevention of significant deterioration 

(PSD) permitting program with respect 

to the proposed NAAQS revisions. With 

regard to primary standards for fine 

particles (generally referring to particles 

less than or equal to 2.5 micrometers 

(mm) in diameter, PM2.5), the EPA 

proposes to revise the annual PM2.5

standard by lowering the level to within 

a range of 12.0 to 13.0 micrograms per 

cubic meter (mg/m3), so as to provide 

increased protection against health 

effects associated with long- and shortterm 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. The EPA proposes changes to 

the Air Quality Index (AQI) for PM2.5 to 

be consistent with the proposed primary 

PM2.5 standards. With regard to the 

primary standard for particles generally 

less than or equal to 10 mm in diameter 

(PM10), the EPA proposes 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 

proposes to revise the suite of secondary 

PM standards by adding a distinct 

standard for PM2.5 to address PM-related 

visibility impairment and to retain the 

current standards generally to address 

non-visibility welfare effects. The 

proposed distinct secondary standard 

would 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 dv or 28 

dv. 

DATES: Comments must be received on 

or before August 31, 2012. 

Public Hearings: The EPA intends to 

hold public hearings on this proposed 

rule in July 2012. These will be 

announced in a separate Federal 

Register notice that provides details, 

including specific dates, times, 

addresses, and contact information for 

these hearings. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–HQ– 

OAR–2007–0492 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 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 email. The www.regulations.gov Web 

site 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. 

Docket: All documents in the docket 

are listed on the www.regulations.gov 

Web site. This includes documents in 

the rulemaking docket (Docket ID No. 

EPA–HQ–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 Web site. 

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 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: 

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Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / Proposed Rules 38891 

General Information 

What should I consider as I prepare my 

comments for EPA? 

1. Submitting CBI. Do not submit this 

information to the EPA through 

www.regulations.gov or email. Clearly 

mark the part or all of the information 

that you claim to be CBI. For CBI 

information in a disk or CD ROM that 

you mail to the EPA, mark the outside 

of the disk or CD ROM as CBI and then 

identify electronically within the disk or 

CD ROM the specific information that is 

claimed as CBI. In addition to one 

complete version of the comment that 

includes information claimed as CBI, a 

copy of the comment that does not 

contain the information claimed as CBI 

must be submitted for inclusion in the 

public docket. Information so marked 

will not be disclosed except in 

accordance with procedures set forth in 

40 CFR part 2. 

2. Tips for Preparing Your Comments. 

When submitting comments, remember 

to: 

• Identify the rulemaking by docket 

number and other identifying 

information (subject heading, Federal 

Register date and page number). 

• Follow directions—the agency may 

ask you to respond to specific questions 

or organize comments by referencing a 

Code of Federal Regulations (CFR) part 

or section number. 

• Explain why you agree or disagree, 

suggest alternatives, and substitute 

language for your requested changes. 

• Describe any assumptions and 

provide any technical information and/ 

or data that you used. 

• Provide specific examples to 

illustrate your concerns, and suggest 

alternatives. 

• Explain your views as clearly as 

possible, avoiding the use of profanity 

or personal threats. 

• Make sure to submit your 

comments by the comment period 

deadline identified. 

Availability of Related Information 

A number of the documents that are 

relevant to this rulemaking are available 

through EPA’s Office of Air Quality 

Planning and Standards (OAQPS) 

Technology Transfer Network (TTN) 

Web site 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 

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 

III. Rationale for Proposed 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. Health Effects Related to Exposure to 

Fine Particles 

1. Nature of Effects 

a. Health Effects Associated With Longterm PM2.5 Exposures 

b. Health Effects Associated With Shortterm PM2.5 Exposures 

c. Summary 

2. Limitations and Uncertainties 

Associated With the Currently Available 

Evidence 

3. At-Risk Populations 

4. Potential PM2.5-Related Impacts on 

Public Health 

C. Quantitative Characterization of Health 

Risks 

1. Overview 

2. Summary of Design Aspects 

3. Risk Estimates and Key Observations 

D. Conclusions on the Adequacy of the 

Current Primary PM2.5 Standards 

1. Evidence-Based Considerations in the 

Policy Assessment 

a. Associations With Long-term PM2.5

Exposures 

b. Associations With Short-term PM2.5

Exposures 

2. Summary of Risk-Based Considerations 

in the Policy Assessment 

3. CASAC Advice 

4. Administrator’s Proposed 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. Approach Used in the Policy Assessment 

b. Consideration of the Annual Standard in 

the Policy Assessment 

c. Consideration of the 24-Hour Standard 

in the Policy Assessment 

d. CASAC Advice 

e. Administrator’s Proposed Conclusions 

on the Primary PM2.5 Standard Levels 

F. Administrator’s Proposed Decisions on 

Primary PM2.5 Standards 

IV. Rationale for Proposed Decision on 

Primary PM10 Standard 

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 Policy 

Assessment for the Current Review 

B. Health Effects Related to Exposure to 

Thoracic Coarse Particles 

1. Nature of Effects 

a. Short-term PM10-2.5 Exposure and 

Mortality 

b. Short-term PM10-2.5 Exposure and 

Cardiovascular Effects 

c. Short-term PM10-2.5 Exposure and 

Respiratory Effects 

2. Potential Impacts of Sources and 

Composition on PM10-2.5 Toxicity 

3. Ambient PM10 Concentrations in PM10-2.5

Study Locations 

4. At-Risk Populations 

5. Limitations and Uncertainties 

Associated With the Currently Available 

Evidence 

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 

a. Indicator 

b. Averaging Time 

c. Form 

d. Level 

i. Evidence-Based Considerations in the 

Policy Assessment 

ii. Air Quality-Based Considerations in the 

Policy Assessment 

iii. Integration of Evidence-Based and Air 

Quality-Based Considerations in the 

Policy Assessment 

D. CASAC Advice 

E. Administrator’s Proposed Conclusions 

Concerning the Adequacy of the Current 

Primary PM10 Standard 

F. Administrator’s Proposed Decision on 

the Primary PM10 Standard 

V. Communication of Public Health 

Information 

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VI. Rationale for Proposed 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. PM-Related Visibility Impairment 

1. Nature of PM-Related Visibility 

Impairment 

a. Relationship Between Ambient PM and 

Visibility 

b. Temporal Variations of Light Extinction 

c. Periods During the Day of Interest for 

Assessment of Visibility 

d. Exposure Durations of Interest 

2. Public Perception of Visibility 

Impairment 

C. Adequacy of the Current Standards for 

PM-Related Visibility Impairment 

1. Visibility Under Current Conditions 

2. Protection Afforded by the Current 

Standards 

3. CASAC Advice 

4. Administrator’s Proposed Conclusions 

on the Adequacy of the Current 

Standards for PM-Related Visibility 

Impairment 

D. Consideration of Alternative Standards 

for Visibility Impairment 

1. Indicator 

a. Alternative Indicators Considered in the 

Policy Assessment 

i. PM2.5 Mass 

ii. Directly Measured PM2.5 Light 

Extinction 

iii. Calculated PM2.5 Light Extinction 

iv. Conclusions in the Policy Assessment 

b. CASAC Advice 

c. Administrator’s Proposed Conclusions 

on Indicator 

2. Averaging Times 

a. Alternative Averaging Times 

i. Sub-Daily 

ii. 24-Hour 

iii. Conclusions in the Policy Assessment 

b. CASAC Advice 

c. Administrator’s Proposed Conclusions 

on Averaging Time 

3. Form 

4. Level 

E. Other PM-Related Welfare Effects 

1. Climate 

2. Ecological Effects 

a. Plants 

b. Soil and Nutrient Cycling 

c. Wildlife 

d. Water 

e. Effects Associated With Ambient PM 

Concentrations 

f. Conclusions in the Policy Assessment 

3. Materials Damage 

4. CASAC Advice 

5. Administrator’s Proposed Conclusions 

on Secondary Standards for Other PMrelated Welfare Effects 

F. Administrator’s Proposed Decisions on 

Secondary PM Standards 

VII. Interpretation of the NAAQS for PM 

A. Proposed 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 Annual and 24- 

Hour PM2.5 NAAQS 

5. Data Handling Procedures for the 

Proposed New Secondary PM2.5

Visibility Index NAAQS 

B. Exceptional Events 

C. Proposed Updates for Data Handling 

Procedures for Reporting the Air Quality 

Index 

VIII. Proposed 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 CSN Methods to Support the 

Proposed New Secondary PM2.5

Visibility Index NAAQS 

B. Proposed Changes to 40 CFR Part 58 

(Ambient Air Quality Surveillance) 

1. Proposed 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 MiddleScale Monitoring Sites to the Annual 

PM2.5 NAAQS 

3. Proposed Changes to Monitoring for the 

National Ambient Air Monitoring 

System 

a. Background 

b. Primary PM2.5 NAAQS 

i. Proposed 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 Requirements at 

NCore Sites 

d. Measurements for the Proposed New 

PM2.5 Visibility Index NAAQS 

4. Proposed 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. Proposed 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 

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

Requirements 

C. Implementing the Proposed Revised 

Primary Annual PM2.5 NAAQS in 

Nonattainment Areas 

D. Implementing the Primary and 

Secondary PM10 NAAQS 

E. Implementing the Proposed New PM2.5

Visibility Index NAAQS in 

Nonattainment Areas 

F. Prevention of Significant Deterioration 

and Nonattainment New Source Review 

Programs for the Proposed Revised 

Primary Annual PM2.5 NAAQS and the 

Proposed New Secondary PM2.5

Visibility Index NAAQS 

1. Prevention of Significant Deterioration 

a. Grandfathering Provision 

b. Recent Guidance Applicable to the 

Proposed Revised Primary Annual PM2.5

NAAQS 

c. Surrogacy Approach for the Proposed 

New Secondary PM2.5 Visibility Index 

NAAQS 

d. PSD Screening Provisions: Significant 

Emissions Rates, Significant Impact 

Levels, and Significant Modeling 

Concentration 

e. PSD Increments 

2. Nonattainment New Source Review 

G. Transportation Conformity Program 

H. 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 

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 

themselves. This proposed rulemaking 

is being done pursuant to these statutory 

requirements. The schedule for this 

proposed rule is set out in a court order. 

In 2006, the EPA completed the 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 (mm) in 

diameter, PM2.5), at that time, the EPA 

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revised the level of the primary 24-hour 

PM2.5 standard from 65 to 35 mg/m3 and 

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 mm 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. (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 EPA because 

EPA failed to explain adequately why 

the standard provided the requisite 

protection from both short- and longterm 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 is responding to the court’s 

remands as part of the current review of 

the PM NAAQS. 

This review was initiated in June 

2007. Between 2007 and 2011, EPA 

prepared draft and final Integrated 

Science Assessments, Risk and 

Exposure Assessments, and Policy 

Assessments. Multiple drafts of all of 

these documents were subject to review 

by the public and peer reviewed by 

EPA’s Clean Air Scientific Advisory 

Committee (CASAC). This proposed 

rulemaking is the next step in the 

review process. 

In this rulemaking, the EPA proposes 

to make revisions to the suite of primary 

and secondary standards for PM to 

provide increased protection of public 

health and welfare. We also discuss 

EPA’s current perspectives on 

implementation issues related to the 

proposed revisions to the PM NAAQS. 

The EPA proposes revisions to the 

Prevention of Significant Deterioration 

(PSD) permitting regulations to address 

the proposed changes in the primary 

and secondary PM NAAQS. The EPA 

also proposes an approach for 

implementing the PSD program 

specifically for the proposed secondary 

standard. The EPA is also proposing to 

update the Air Quality Index (AQI) for 

PM2.5 and to make changes in the data 

handling conventions for PM and 

ambient air monitoring, reporting, and 

network design requirements to 

correspond with the proposed changes 

to the standards. 

B. Summary of Major Provisions 

With regard to the primary standards 

for fine particles, EPA proposes to revise 

the annual PM2.5 standard by lowering 

the level from 15.0 to within a range of 

12.0 to 13.0 mg/m3 so as to provide 

increased protection against health 

effects associated with long- and shortterm exposures. The EPA proposes to 

retain the level (35 mg/m3) and the form 

(98th percentile) of the 24-hour PM2.5

standard to provide supplemental 

protection against health effects 

associated with short-term exposures. 

This proposed action would provide 

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 proposes to eliminate spatial 

averaging provisions as part of the form 

of the annual standard to avoid 

potential disproportionate impacts on 

at-risk populations. 

The proposed changes to the primary 

annual PM2.5 standard are within the 

range that CASAC advised the Agency 

to consider. These changes 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 proposed 

changes also reflect 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 

proposes to conclude 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 the 

proposed revisions are warranted to 

provide the appropriate degree of 

increased public health protection. The 

EPA solicits comment on all aspects of 

the proposed primary PM2.5 standards. 

With regard to the primary standard 

for coarse particles, EPA proposes to 

retain the current 24-hour PM10

standard, with a level of 150 mg/m3 and 

a one-expected 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 proposes to 

conclude 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 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. The Administrator welcomes 

the public’s views on these approaches 

to considering and accounting for the 

evidence and its limitations and 

uncertainties. 

With regard to the secondary PM 

standards, the EPA proposes to revise 

the suite of secondary PM standards by 

adding a distinct standard for PM2.5 to 

address PM-related visibility 

impairment. More specifically, the EPA 

proposes to establish a secondary 

standard 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, 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 deciviews 

(dv) or 28 dv. The EPA also proposes to 

rely upon the existing Chemical 

Speciation Network (CSN) to provide 

appropriate monitoring data for 

calculating PM2.5 visibility index values. 

The proposed secondary standard is 

based on the long-standing science 

characterizing the contribution of PM, 

especially fine particles, to visibility 

impairment and on air quality analyses, 

with consideration also given to a 

reanalysis of public perception surveys 

regarding people’s stated preferences 

regarding acceptable and unacceptable 

visual air quality. Based on this 

information, the Administrator proposes 

to conclude that the current secondary 

PM2.5 standards are not sufficiently 

protective of the public welfare with 

respect to visual air quality. The EPA 

solicits comment on all aspects of the 

proposed secondary standard. 

To address other non-visibility 

welfare effects including ecological 

effects, effects on materials, and climate 

impacts, the EPA proposes to retain the 

current suite of secondary PM standards 

generally, while proposing to revise 

only the form of the secondary annual 

PM2.5 standard to remove the option for 

spatial averaging consistent with this 

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1The 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). 

2Welfare 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, manmade 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.’’ 

proposed change to the primary annual 

PM2.5 standard. 

The proposed revisions to the PM 

NAAQS would trigger 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 proposed 

new or revised NAAQS for PM2.5. States 

will also review, modify and 

supplement their existing state 

implementation plans. The proposed 

NAAQS revisions would affect the 

applicable air permitting requirements 

and the transportation conformity and 

general conformity processes. This 

notice provides background information 

for understanding the implications of 

the proposed NAAQS revisions for these 

implementation processes and describes 

and requests comment on EPA’s current 

perspectives on implementation issues. 

In addition, the EPA proposes to revise 

its PSD regulations to provide limited 

grandfathering from the requirements 

that result from the revised PM NAAQS 

for permit applications for which the 

public comment period has begun when 

the revised PM NAAQS take effect. The 

EPA also proposes to implement a 

surrogate approach that would provide 

a mechanism for permit applicants to 

demonstrate that they will not cause or 

contribute to a violation of the proposed 

secondary PM2.5 visibility index 

NAAQS. It is the EPA’s intention to 

finalize any time-sensitive revisions to 

its PSD regulations at the same time as 

any new or revised NAAQS are 

finalized. 

With regard to implementationrelated activities, the EPA intends to 

promulgate rules or develop guidance 

related to NAAQS implementation on a 

schedule that provides timely clarity to 

the states, tribes, and other parties 

responsible for NAAQS 

implementation. The EPA solicits 

comment on all implementation aspects 

during the public comment period for 

this notice and will consider these 

comments as it develops future 

rulemaking or guidance, as appropriate. 

On other topics, the EPA proposes 

changes to the Air Quality Index (AQI) 

for PM2.5 to be consistent with the 

proposed primary PM2.5 standards. The 

EPA also proposes revisions to the data 

handling procedures consistent with the 

proposed 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 proposes updates to several 

aspects of the monitoring regulations 

and specifically proposes 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 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 Supreme Court in Whitman v. 

American Trucking Associations, 531 

U.S. 457, 465–472, 475–76 (2001), as 

discussed 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 proposed 

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 $88 million to $220 

million (for 13.0 mg/m3) and from $2.3 

billion to $5.9 billion per year (for 12.0 

mg/m3) in 2020 and costs ranging from 

$2.9 million (for 13.0 mg/m3) to $69 

million (for 12.0 mg/m3) per year. 

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 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

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 

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3Lists of CASAC members and of members of the 

CASAC PM Review Panel are available at: http:// 

yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/ 

CommitteesandMembership?OpenDocument. 

4Particulate 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. 

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 sensitive population(s) at 

risk, 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 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), 

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 

Clean Air Scientific Advisory 

Committee (CASAC).3

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 indicator 4

and 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 mm (referred to as total suspended 

particles or TSP). The primary standards 

(measured by the indicator TSP) were 

260 mg/m3, 24-hour average, not to be 

exceeded more than once per year, and 

75 mg/m3, annual geometric mean. The 

secondary standard was 150 mg/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 mm, 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 mg/m3 with no more 

than one expected exceedance per year; 

and (2) replacing the annual TSP 

standard with a PM10 standard of 50 mg/ 

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). 

TABLE 1—SUMMARY OF NATIONAL AMBIENT AIR QUALITY STANDARDS PROMULGATED FOR PM 1971–2006 5

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.6

Annual .......... 15.0 μg/m3 ........................ Annual arithmetic mean, averaged over 3 years.7 8

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.6

Annual .......... 15.0 μg/m3 ........................ Annual arithmetic mean, averaged over 3 years.7 9

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5When not specified, primary and secondary 

standards are identical. 

6The 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 mg/m3 would 

round to 35 mg/m3 and thus meet the 24-hour 

standard and a 3-year average of 35.50 mg/m3 would 

round to 36 and, hence, violate the 24-hour 

standard (40 CFR part 50, appendix N). 

7The level of the annual standard is defined to 

one decimal place (i.e., 15.0 mg/m3) as determined 

by rounding. For example, a 3-year average annual 

mean of 15.04 mg/m3 would round to 15.0 mg/m3

and, thus, meet the annual standard and a 3-year 

average of 15.05 mg/m3 would round to 15.1 mg/m3

and, hence, violate the annual standard (40 CFR 

part 50, appendix N). 

8The 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 communitywide air quality monitoring sites (‘‘spatial 

averaging’’). 

9The 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). 

10See 40 CFR parts 50, 53, and 58 for more 

information on reference and equivalent methods 

for measuring PM in ambient air. 

11Monitoring stations sited to represent 

community-wide air quality would typically be at 

the neighborhood or urban-scale; however, where a 

population-oriented micro or middle-scale PM2.5

monitoring station represents many such locations 

throughout a metropolitan area, these smaller scales 

might also be considered to represent communitywide air quality [40 CFR part 58, appendix D, 

4.7.1(b)]. 

12Population-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). 

TABLE 1—SUMMARY OF NATIONAL AMBIENT AIR QUALITY STANDARDS PROMULGATED FOR PM 1971–2006 5—Continued 

Final rule Indicator Averaging 

time Level Form 

PM10 ............. 24-hour ......... 150 μg/m3 ......................... Not to be exceeded more than once per year on average over a 3-year period. 

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).10 The EPA established two 

new PM2.5 standards: an annual 

standard of 15 mg/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 11; and a 24- 

hour standard of 65 mg/m3, based on the 

3-year average of the 98th percentile of 

24-hour PM2.5 concentrations at each 

population-oriented monitor 12 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 EPA’s 

decision to establish fine particle 

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 (DC Cir. 1999), rehearing 

granted in part and denied in part, 195 

F. 3d 4 (DC 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 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 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 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 any determinate 

criterion for drawing lines’’ to 

determine where the standards should 

be set. Consistent with EPA’s longstanding interpretation and DC 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 (DC Cir. 1999). The 

full Court of Appeals denied 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 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 EPA’s discretion, 

affirming 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 

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13 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 mg/m3 in conjunction with a form within a range 

of the 92nd to 98th percentile (71 FR 2685, January 

17, 2006). 

14CASAC 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). 

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 EPA’s PM2.5

standards were reasonably supported by 

the administrative record and were not 

‘‘arbitrary and capricious.’’ American 

Trucking Associations v. EPA, 283 F. 3d 

355, 369–72 (DC 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 and public review of 

several drafts, 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 their 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 in 

the range of 35 to 30 mg/m3 with a 98th 

percentile form, in concert with revising 

the level of the primary annual PM2.5

standard in the range of 14 to 13 mg/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 mg/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-hour PM10 standard with a 

24-hour standard defined in terms of a 

new indicator, PM10-2.5, this proposed 

indicator 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 mg/m3 based 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.13

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 

promulgated 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 mg/m3, 

retained the level of the primary annual 

PM2.5 standard at 15 mg/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 that the 

standard remained at a level of 150 mg/ 

m3 with a one-expected exceedance 

form), 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 61212–13). 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 

‘‘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 mg/ 

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 Agency’’ (Henderson et al., 2006b, p. 

2).14 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 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 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). 

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15See workshop materials available at: http:// 

www.regulations.gov/search/Regs/home.html#home 

Docket ID numbers EPA–HQ–OAR–2007–0492–008; 

EPA–HQ–OAR–2007–0492–009; EPA–HQ–OAR– 

2007–0492–010; and EPA–HQ–OAR–2007–0492– 

012. 

16The 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, EPA 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, 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). 

17All 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. 

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 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 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. 3d 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. 3d 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, respectively (72 

FR 34003 to 34004, June 20, 2007).15

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 policyrelevant 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.16 17

A major element in the process for 

reviewing the NAAQS is the 

development of an Integrated Science 

Assessment. This document provides a 

concise evaluation and integration of 

the policy-relevant science, including 

key science judgments upon with 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). The first 

external review draft Integrated Science 

Assessment (U.S. EPA, 2008b; 73 FR 

77686, December 19, 2008) was 

reviewed by CASAC and the public 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 

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. 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) 

were 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 

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, 

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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, EPA staff prepared a 

Policy Assessment. The Policy 

Assessment is intended to help ‘‘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 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. 

CASAC’s comments on this preliminary 

draft were considered in developing a 

first draft PA (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. The second draft document 

was reviewed by CASAC at a meeting 

on July 26 and 27, 2010 (75 FR 32763, 

June 9, 2010). CASAC (Samet, 2010d) 

and public comments on the second 

draft Policy Assessment were 

considered by EPA staff 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 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 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. 

The court order, entered on June 2, 2012 

and amended on June 6, 2012, provides 

that EPA will sign, for publication, a 

notice of proposed rulemaking 

concerning its review of the PM NAAQS 

no later than June 14, 2012. 

The EPA is aware that a number of 

new scientific studies on the health 

effects of PM have been published since 

the mid-2009 cutoff date for inclusion in 

the Integrated Science Assessment. As 

in the last PM NAAQS review, the EPA 

intends to conduct a provisional review 

and assessment of any significant new 

studies published since the close of the 

Integrated Science Assessment, 

including studies that may be submitted 

during the public comment period on 

this proposed rule in order to ensure 

that, before making a final decision, the 

Administrator is fully aware of the new 

science that has developed since 2009. 

In this provisional assessment, the EPA 

will examine these new studies in light 

of the literature evaluated in the 

Integrated Science Assessment. This 

provisional assessment and a summary 

of the key conclusions will be placed in 

the rulemaking docket. 

Today’s action presents the 

Administrator’s proposed decisions on 

the current PM standards. Throughout 

this preamble there are a number of 

conclusions, findings, and 

determinations that are part of the 

rationales for the decisions proposed by 

the Administrator. They are referred to 

throughout as ‘‘provisional’’ 

conclusions, findings, and 

determinations to reflect that they are 

not intended to be final or conclusive 

but rather proposals for public 

comment. The EPA invites general, 

specific, and technical comments on all 

issues involved with this proposal, 

including all such proposed decisions 

and provisional conclusions, findings, 

and determinations. 

C. Related Control Programs To 

Implement PM Standards 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once the 

EPA has established them. Under 

section 110 of the CAA, and related 

provisions, states are to submit, for 

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 

program (CAA sections 160 to 169). In 

addition, Federal programs provide for 

nationwide reductions in emissions of 

PM and other air pollutants through the 

Federal motor vehicle and motor vehicle 

fuel control program under title II of the 

Act (CAA sections 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 for stationary 

sources under section 111 of the CAA. 

Currently, there are 55 areas in the 

U.S. (with a population of more than 

100 million) that are designated as 

nonattainment for either the annual or 

24-hour PM2.5 standards. Regarding the 

1997 PM2.5 standards, the EPA 

designated 39 nonattainment areas in 

2005. Regarding the 2006 24-hour PM2.5

standard, the EPA designated 31 areas 

in 2009 and added one area in 2010. 

Sixteen areas are currently designated as 

nonattainment for both the 1997 and 

2006 PM2.5 standards. With regard to the 

PM10 NAAQS, 45 areas (with a 

population of more than 25 million) are 

currently designated as nonattainment. 

Upon any revisions to the PM NAAQS, 

the EPA would work with the states to 

conduct a new area designation process. 

Upon designation of new nonattainment 

areas, certain states would then be 

required to develop SIPs to attain the 

standards. In developing their 

attainment plans, states would first take 

into account projected emission 

reductions from federal and state rules 

that have been already 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 the Transport Rule for 

electric generating units, 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 would 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 

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18Nonetheless, 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. 

19 In this proposal, the term ‘‘at-risk’’ is the 

broadly encompassing term used for groups with 

specific factors that increase the risk of PM-related 

health effects in a population. In the Integrated 

Science Assessment, as discussed in section III.B.3 

below, the term ‘‘susceptibility’’ was used broadly 

to recognize populations at greater risk. 

regulations as appropriate. Section IX 

includes additional discussion of 

designation and implementation issues 

associated with any revised PM 

NAAQS. 

III. Rationale for Proposed Decisions on 

the Primary PM2.5 Standards 

This section presents the rationale for 

the Administrator’s proposed decision 

to revise the level and form of the 

existing primary annual PM2.5 standard 

and to retain the existing primary 24- 

hour PM2.5 standard. As discussed more 

fully below, this rationale 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 shortterm exposures to fine particles in the 

ambient air. This proposal also takes 

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; and (3) public comments 

received during the development of 

these documents, either in connection 

with CASAC meetings or separately. 

In developing this proposal, the 

Administrator recognizes that the CAA 

requires her to reach a public health 

policy judgment as to what standards 

would be requisite 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 standards to propose, and 

subsequently 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, 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 studies using PM2.5

measurements, since studies conducted 

in other countries may well reflect 

different demographic and air pollution 

characteristics.18

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 proposed 

rule, the EPA has drawn upon an 

integrative synthesis of the entire body 

of evidence between 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 shortterm 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 in taking into account 

potential impacts on at-risk 

populations19 and in considering 

alternative standard levels that provide 

protection with an appropriate margin 

of safety. 

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 

opportunities for public review and 

comment. 

Although important uncertainties 

remain in the qualitative and 

quantitative characterizations of health 

effects attributable to ambient fine 

particles, the review of this information 

has been extensive and deliberate. This 

intensive evaluation of the scientific 

evidence and quantitative assessments 

has provided an 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 potential 

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 will 

draw 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 this 

proposal. 

In presenting the rationale for the 

proposed revisions of 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 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 II.A.3 describes EPA’s general 

approach used in the current review for 

considering the need to retain or revise 

the current suite of fine particle 

standards. Section III.B summarizes the 

body of scientific evidence supporting 

the rationale for the proposed decisions, 

including key health endpoints 

associated with long- and short-term 

exposures to ambient fine particles. This 

overview includes a discussion of atrisk populations and potential PM2.5- 

related impacts on public health. 

Section III.C outlines the approach 

taken by the EPA to assess health risks 

associated with exposure to ambient 

PM2.5, including a discussion of key 

uncertainties and limitations associated 

with these analyses. Section III.D 

discusses the scientific evidence, air 

quality, risk-based information; CASAC 

advice; and the Administrator’s 

proposed decisions related to the 

adequacy of the current standards. 

Section III.E discusses the scientific 

evidence, air quality, and risk-based 

information; CASAC advice; and the 

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20 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). 

Administrator’s proposed decisions 

related to alternative standards. Section 

III.F summarizes the Administrator’s 

proposed decisions with regard to the 

primary PM2.5 NAAQS. 

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 mg/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 mg/m3, based on the 3-year average of 

the 98th percentile of 24-hour PM2.5

concentrations at each populationoriented 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 PM 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, or in the form of 

concentration-response 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 not occur, can 

be discerned from the available 

evidence. 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. 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 used in the previous reviews 

focused on consideration of alternative 

standard levels that were somewhat 

below the long-term mean PM2.5

concentrations reported in 

epidemiological studies (U.S. EPA, 

2011a, section 2.1.1). This approach 

recognized that the strongest evidence 

of PM2.5-related associations occurs at 

concentrations near 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 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 PM2.5.20 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 a national annual standard 

(62 FR 38669/3). 

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 both short- and longterm exposure studies, 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 mg/m3 to protect 

against effects associated with shortterm 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 mg/m3 to 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). 

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 

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21By utilizing this approach, the Agency would 

also be responsive to the remand of the 2006 

standard. As noted in section III.A.2, the DC 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. 

any of the litigants and, thus, 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 shortand 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. 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 shortterm exposures to PM2.5.’’ American 

Farm Bureau Federation v. EPA, 559 F. 

3d at 520–24. 

With respect to protection from longterm 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 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 multi-city 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 short-term studies. See 

559 F. 3d at 522 (‘‘[e]ven if the longterm 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 id. 

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). 

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 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, 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 is broader and more integrative 

than in past reviews. Our general 

approach also reflects consideration of 

the issues raised by the court in its 

remand of the primary annual PM2.5

standard, 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 takes 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, EPA staff developed a final 

Policy Assessment (U.S. EPA, 2011a) 

which provides as broad an array of 

policy options as is supportable 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 types of information 

presented in this document. 

The Policy Assessment concludes it is 

most appropriate to consider the 

protection against PM2.5-related 

mortality and morbidity effects, 

associated with both long- and shortterm 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).21 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 recognizes 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 recognizes 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 effects associated with both shortand 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 

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22 In confirmation, a number of studies that have 

presented analyses excluding higher PM 

concentration days 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). 

23The 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 shortterm 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 

concludes that the studies evaluated support the 

use of a no-threshold, log-linear model but 

recognizes 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). 

recognizes that the short-term exposure 

studies are 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 

is 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 demonstrate 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 is no 

evidence suggesting that risks associated 

with long-term exposures are likely to 

be disproportionately driven by peak 

24-hour concentrations.22 For these 

reasons, strategies that focus primarily 

on reducing peak days are less likely to 

achieve reductions in the PM2.5

concentrations that are most strongly 

associated with the observed health 

effects. 

Furthermore, a policy approach that 

focuses 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, 

2010a, section 5.2.3). This is because 

reductions based on control of peak 

days are less likely to control the bulk 

of the air quality distribution, as 

discussed above. 

The Policy Assessment concludes 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, is 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 shortterm 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 

concludes, however, that an annual 

standard intended to serve as the 

primary means for providing protection 

from effects associated with both longand short-term PM2.5 exposures cannot 

be expected to offer an adequate margin 

of safety against the effects of all shortterm PM2.5 exposures. As a result, in 

conjunction with a generally controlling 

annual standard, the Policy Assessment 

concludes it is 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 shorterthan-daily exposure 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 focuses 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 

is 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 considers effects and 

estimated risks associated with shortterm exposures. In both cases, the Policy 

Assessment places greatest weight on 

health effects that have 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 focuses on 

specific at-risk populations. The Policy 

Assessment places relatively greater 

weight on statistically significant 

associations that yield relatively more 

precise effect estimates and that are 

judged to be robust to confounding by 

other air pollutants. In the case of shortterm exposure studies, the Policy 

Assessment places 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 

considers a number of factors (U.S. EPA, 

2011a, section 2.2). As an initial matter, 

the Policy Assessment considers 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 considers 

evidence on at-risk populations and 

potential impacts on such populations. 

Further, the Policy Assessment explores 

the extent to which PM2.5-related health 

effects have been observed in areas 

where air quality distributions extend to 

lower levels 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 

alternative standard levels for 

consideration (U.S. EPA, 2011a, sections 

2.1.3 and 2.3.4), the Policy Assessment 

first recognizes 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).23 In the absence of any 

discernible thresholds, the Agency’s 

general approach for identifying 

appropriate standard levels for 

consideration involves 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 recognizes 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 consider can help 

inform decisions to either retain or 

revise the standards. Final decisions 

will necessarily also take into account 

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24This 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. 

25Epidemiological studies typically report PM2.5

concentrations averaged across the available 

ambient monitors. For multi-city studies, this 

metric reflects concentrations averaged across one 

or more ambient monitors within each area 

included in a given study 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. 

26 In the PM NAAQS review completed in 2006, 

the Staff Paper 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. 

27The second draft Policy Assessment focused on 

the distributions of PM2.5 concentrations 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, EPA 

considered area-specific health event and areaspecific population data along with corresponding 

PM2.5 concentrations to generate a cumulative 

distribution of the population data relative to longterm mean PM2.5 concentrations to determine the 

most influential range (U.S. EPA, 2011a, Figure 2– 

7 and associated text). 

28Using 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, as noted above, the composite 

monitor concentration represents the average 

concentration across one or more monitors within 

each area included in a given study and then 

averaged across study areas for an overall study 

mean PM2.5 concentration. 

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 is 

appropriate to consider, the Policy 

Assessment focuses on identifying an 

annual standard that provides requisite 

protection from effects associated with 

both long- and short-term exposures. In 

so doing, the Policy Assessment 

explores different approaches for 

characterizing the range of PM2.5

concentrations over which our 

confidence in the nature of the 

associations for both long- and shortterm exposures is greatest, as well as the 

extent to which our confidence is 

reduced at lower PM2.5 concentrations. 

The approach that most directly 

addresses this issue considers studies 

that present confidence intervals around 

concentration-response relationships, 

and in particular, analyses that average 

across multiple concentration-response 

models rather than considering a single 

concentration-response model.24 The 

Policy Assessment explores the extent 

to which such analyses have 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 

concludes that identifying this area of 

uncertainty in the concentrationresponse relationship could be used to 

inform identification of alternative 

standard levels that are appropriate to 

consider. 

Further, the Policy Assessment 

explores other approaches that consider 

different statistical metrics from 

epidemiological studies. The Policy 

Assessment first takes 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.25 This 

approach recognizes that the strongest 

evidence of PM2.5-related associations 

occurs at concentrations near the longterm (i.e., annual) mean. In using this 

approach, the Policy Assessment places 

greatest weight on those long- and shortterm exposure studies that reported 

statistically significant associations with 

mortality and morbidity effects. 

In extending this approach, the Policy 

Assessment also considers information 

beyond a single statistical metric of 

PM2.5 concentrations (i.e., the mean) to 

the extent such information is available. 

In so doing, the Policy Assessment 

employs 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 

epidemiological studies. Thus, the 

Policy Assessment considers the range 

of PM2.5 concentrations where the data 

analyzed in the study (i.e., air quality 

and population-level data, as discussed 

below) are most concentrated, 

specifically, the range of PM2.5

concentrations around the long-term 

mean over which our confidence in the 

associations observed in the 

epidemiological studies is greatest. The 

Policy Assessment then focuses on the 

lower part of this range to characterize 

where in the distributions the data 

become appreciably more sparse and, 

thus, where our understanding of the 

associations correspondingly becomes 

more uncertain. The Policy Assessment 

recognizes there is no one percentile 

value within a given distribution that is 

the most appropriate or ‘‘correct’’ way to 

characterize where our confidence in 

the associations becomes appreciably 

lower. The Policy Assessment 

concludes that the range from the 25th 

to 10th percentiles is a reasonable range 

to consider as a region where we have 

appreciably less confidence in the 

associations observed in 

epidemiological studies.26

In considering distributional statistics 

from epidemiological studies, the final 

Policy Assessment focused on two types 

of population-level metrics that CASAC 

advices are most useful to consider in 

identifying the PM2.5 concentrations 

most influential in generating the health 

effect estimates reported in the 

epidemiological studies.27 Consistent 

with CASAC advice, the most relevant 

information is 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, as discussed in section 

III.E.4.b below, the Policy Assessment 

also considers the number of study 

participants within each study area as 

an appropriate surrogate for health 

event data. 

The Policy Assessment recognizes 

that an approach considering analyses 

of confidence intervals around 

concentration-response functions is 

intrinsically related to an approach that 

considers different distributional 

statistics. Both of these approaches 

could be employed to identify the range 

of PM2.5 concentrations over which we 

have the most confidence in the 

associations reported in epidemiological 

studies. 

In applying these approaches, the 

Policy Assessment considers PM2.5

concentrations from long- and shortterm PM2.5 exposure studies using 

composite monitor distributions.28 For 

multi-city studies, this distribution 

reflects concentrations averaged across 

one or more ambient monitors within 

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29The maximum monitor distribution is relevant 

because it is generally used to determine whether 

a given standard is met in an area and the extent 

to which ambient PM2.5 concentrations need to be 

reduced in order to bring an area into attainment 

with the standard. However, maximum monitor 

distributions represent a far less robust metric than 

composite monitor distributions for consideration 

of alternative annual standard levels in part because 

they are available for only a few epidemiological 

studies. 

30Statistical 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. 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 and the composite monitor distributions 

may be well below the maximum monitor 

distributions (U.S. EPA, 2011a, p. 2–14). 

31 Design values are the metrics (i.e., statistics) 

that are compared to the NAAQS levels to 

determine compliance. 

each area included in a given study and 

then averaged across study areas for an 

overall study mean PM2.5 concentration. 

Beyond considering air quality 

concentrations based on composite 

monitor distributions, the second draft 

Policy Assessment also considered 

PM2.5 concentrations based on 

measurements at the monitor within 

each area that records the highest 

concentration (i.e., maximum monitor) 

(U.S. EPA, 2010f, sections 2.1.3 and 

2.3.4.1).29 Although the second draft 

Policy Assessment discussed whether 

consideration of alternative annual 

standard levels should be based on 

composite or maximum monitor 

distributions, the final Policy 

Assessment, consistent with CASAC 

advice (Samet, 2010d, p. 3), concluded 

that it is most reasonable to place more 

weight on an approach based on 

composite monitor distributions, which 

represent the PM2.5 concentrations 

typically presented and used in 

epidemiological analyses and which 

provide a direct link between PM2.5

concentrations and the observed health 

effects reported in both long- and shortterm exposure studies (U.S. EPA, 2011a, 

p. 2–13). 

In reaching staff conclusions on 

alternative standard levels that are 

appropriate to consider, the Policy 

Assessment also includes a broader 

consideration of the uncertainties 

related to the concentration-response 

relationships from multi-city, long- and 

short-term exposure studies. Most 

notably, these uncertainties relate to our 

currently limited understanding of the 

heterogeneity of relative risk estimates 

in areas across the country. This 

heterogeneity may be attributed, in part, 

to the potential for different components 

within the mix of ambient fine particles 

to differentially contribute to health 

effects observed in the studies and to 

exposure-related factors (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 further 

discussed in section III.B.2 below. 

The Policy Assessment recognizes 

that the level of protection afforded by 

the NAAQS relies both on the level and 

the form of the standard. The Policy 

Assessment concludes that a policy 

approach that uses data based on 

composite monitor distributions to 

identify alternative standard levels, and 

then compares those levels to 

concentrations at maximum monitors to 

determine if an area meets a given 

standard, inherently has the potential to 

build in some margin of safety (U.S. 

EPA, 2011a, p. 2–14).30 This conclusion 

is 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 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 informs our understanding 

of the protection afforded by the suite of 

standards against effects associated with 

short-term exposures. In considering the 

short-term exposure studies, the Policy 

Assessment evaluates both the 

distributions of 24-hour PM2.5

concentrations, with a focus on the 98th 

percentile concentrations to match the 

form of the current 24-hour PM2.5

standard, to the extent such data were 

available, 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 considers air quality 

information based on county-level 24- 

hour and annual design values 31 to 

understand the policy implications of 

the alternative standard levels 

supported by the underlying science. In 

particular, the Policy Assessment 

considers 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. Based on the evidence-based 

considerations outlined above, the 

Policy Assessment develops integrated 

conclusions with regard to alternative 

suites of standards, including both 

annual and 24-hour standards that are 

appropriate to consider in this review 

based on the currently available 

evidence and air quality information. In 

so doing, the Policy Assessment 

discusses 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 considers the quantitative risk 

estimates and the key observations 

presented in the Risk Assessment. This 

assessment includes 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 

considers 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 characterizes 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 recognizes 

the uncertainties inherent in such risk 

estimates, and takes 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 considers 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 

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32The term ‘‘likely causal association’’ was used 

in the 2004 Criteria Document to summarize the 

strength of the available epidemiological 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). 

33Causal inferences, as discussed in the 

Integrated Science Assessment, 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). 

related to both long- and short-term 

PM2.5 exposures. 

The Policy Assessment conclusions 

related to the primary PM2.5 standards 

reflect 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) potential alternative 

standards, if any, that are appropriate to 

consider in this review to protect 

against effects associated with both 

long- and short-term exposures to fine 

particles. In addressing these broad 

questions, the discussions included in 

the Policy Assessment were organized 

around a series of more specific 

questions reflecting different aspects of 

each overarching question (U.S. EPA, 

2011a, chapter 2, Figure 2–1). When 

evaluating the health protection 

afforded by the current or any 

alternative suites of standards 

considered, the Policy Assessment takes 

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 

provides a comprehensive basis to help 

inform the judgments required of the 

Administrator in reaching decisions 

about the current and potential 

alternative primary fine particle 

standards and in responding to the 

remand of the 2006 primary annual 

PM2.5 standard. 

B. Health Effects Related to Exposure to 

Fine Particles 

This section outlines key information 

contained 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. As was 

true in the last review, evidence from 

epidemiologic studies plays a key role 

in the Integrated Science Assessment’s 

evaluation of the scientific evidence. 

The following sections discuss 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). 

1. Nature of Effects 

In considering the strength of the 

associations between long- and shortterm exposures to PM2.5 and health 

effects, the Policy Assessment notes that 

in the PM NAAQS review completed in 

2006 the Agency 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 exposures with cardiovascularrelated and respiratory-related mortality 

and morbidity (U.S. EPA, 2004, p. 9–46; 

U.S. EPA, 2005, p. 5–4). Overall, the 

epidemiological evidence 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).32

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 (U.S. EPA, 2009a, sections 

2.3.1 and 2.3.2). The currently available 

evidence is stronger in comparison to 

evidence available in the last review 

because of its breadth and the 

substantiation of previously observed 

health effects. A number of large multicity epidemiological studies have been 

conducted throughout the U.S., 

including extended analyses of studies 

that were important to inform decisionmaking in the last review. These studies 

have reported consistent increases in 

morbidity and/or mortality related to 

ambient PM2.5 concentrations, with the 

strongest evidence reported for 

cardiovascular-related effects. In 

addition, the findings of new 

toxicological and controlled human 

exposure studies greatly expand and 

provide stronger support for a number 

of potential biologic mechanisms or 

pathways for cardiovascular and 

respiratory effects associated with longand short-term PM exposures (U.S. EPA, 

2009a, p. 2–17; chapter 5; Figures 5–4 

and 5–5). 

With regard to causal inferences 

described in the Integrated Science 

Assessment, the Policy Assessment 

notes that since the last review, the 

Agency has developed a more formal 

framework for reaching causal 

determinations that 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 (U.S. 

EPA, 2011a, p. 2–18; U.S. EPA, 2009a, 

section 1.5). 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 a 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).33

Using this causal framework, the 

Integrated Science Assessment 

concludes that the collective evidence is 

largely consistent with past studies and 

substantially strengthens what was 

known about fine particles in the last 

review to reach the conclusion that a 

causal relationship exists between both 

long- and short-term exposures to PM2.5

and mortality and cardiovascular effects 

including cardiovascular-related 

mortality. The Integrated Science 

Assessment also concludes that the 

collective evidence continues to support 

a likely causal relationship between 

long- and short-term PM2.5 exposures 

and respiratory effects, including 

respiratory-related mortality. Further, 

the Integrated Science Assessment 

concludes that the currently available 

evidence is 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) (U.S. 

EPA, 2009a, sections 2.3.1 and 2.6; 

Table 2–6; U.S. EPA, 2011a, Table 2–1). 

a. Health Effects Associated With LongTerm PM2.5 Exposures 

With regard to mortality, the 

Integrated Science Assessment 

concludes that newly available evidence 

significantly strengthens the link 

between long-term exposure to PM2.5

and mortality, while providing 

indications that the magnitude of the 

PM2.5-mortality association may be 

larger than previously estimated (U.S. 

EPA, 2009a, sections 7.2.10, 7.2.11, and 

7.6.1; Figures 7–6 and 7–7). A number 

of large U.S. cohort studies have been 

published since the last review, 

including extended analyses of the 

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34Coronary and cerebrovascular events include 

myocardial infarction, coronary artery 

revascularization (e.g., bypass graft, angioplasty, 

stent, atherectomy), congestive heart failure and 

stroke. 

35Supporting evidence comes from studies ‘‘that 

observed associations between long-term 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). 

36Clinical 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 

includes this interpretation for FEV1 (75 FR 35525, 

June 22, 2010). 

American Cancer Society (ACS) and 

Harvard Six Cities studies (U.S. EPA, 

2009a, pp. 7–84 to 7–85; Figure 7–6; 

Krewski et al., 2009; Pope et al., 2004; 

Jerrett et al., 2005; Laden et al., 2006). 

In addition, new long-term PM2.5

exposure studies evaluating mortality 

impacts in additional cohorts are now 

available (U.S. EPA, 2009a, section 7.6). 

For example, the Women’s Health 

Initiative (WHI) Observational Study 

reported effects of PM2.5 on 

cardiovascular-related mortality in postmenopausal women with no previous 

history of cardiac disease (Miller et al., 

2007). The PM2.5 effect estimate in this 

study remained positive and statistically 

significant in a multi-pollutant model 

that included gaseous co-pollutants as 

well as coarse particles. In addition, 

multiple studies observed PM2.5- 

associated mortality among older adults 

using Medicare data (Eftim et al., 2008; 

Zeger et al., 2007, 2008). Collectively, 

these new studies, along with evidence 

available in the last review, provide 

consistent and stronger evidence of 

associations between long-term 

exposure to PM2.5 and mortality (U.S. 

EPA, 2009a, sections 2.3.1 and 7.6). 

The strength of the causal relationship 

between long-term PM2.5 exposure and 

mortality also builds upon new studies 

providing evidence of improvement in 

community health following reductions 

in ambient fine particles. Pope et al. 

(2009) documented the population 

health benefits of reducing ambient air 

pollution by correlating past reductions 

in ambient PM2.5 concentrations with 

increased life expectancy. These 

investigators reported that reductions in 

ambient fine particles during the 1980s 

and 1990s account for as much as 15 

percent of the overall improvement in 

life expectancy in 51 U.S. metropolitan 

areas, with the fine particle reductions 

reported to be associated with an 

estimated increase in mean life 

expectancy of approximately 5 to 9 

months (U.S. EPA, 2009a, p. 7–95; Pope 

et al., 2009). An extended analysis of the 

Harvard Six Cities study found that as 

cities cleaned up their air, locations 

with the largest reductions in PM2.5 saw 

the largest improvements in reduced 

mortality rates, while those with the 

smallest decreases in PM2.5

concentrations saw the smallest 

improvements (Laden et al., 2006). 

Another extended follow-up to the 

Harvard Six Cities study investigated 

the delay between changes in ambient 

PM2.5 concentrations and changes in 

mortality (Schwartz et al., 2008) and 

reported that the effects of changes in 

PM2.5 were seen within the 2 years prior 

to death (U.S. EPA, 2009a, p. 7–92; 

Figure 7–9). 

With regard to cardiovascular effects, 

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. The Integrated Science 

Assessment concludes that the strongest 

evidence comes from recent studies 

investigating cardiovascular-related 

mortality. This includes evidence from 

a number of large, multi-city U.S. longterm cohort studies including extended 

follow-up analyses of the ACS and 

Harvard Six Cities studies, as well as the 

WHI study (U.S. EPA, 2009a, sections 

7.2.10 and 7.6.1; Krewski et al., 2009; 

Pope et al., 2004; Laden et al., 2006; 

Miller et al., 2007). Pope et al. (2004) 

reported a positive association between 

mortality and long-term PM2.5 exposure 

for a number of specific cardiovascular 

diseases, including ischemic heart 

disease, dysrhythmia, heart failure, and 

cardiac arrest (U.S. EPA, 2009a, p. 7–84; 

Figure 7–7). Krewski et al. (2009) 

provides further evidence for mortality 

related to ischemic heart disease 

associated with long-term PM2.5

exposure (U.S. EPA, 2009a, p. 7–84, 

Figure 7–7). 

With regard to cardiovascular-related 

morbidity associated with long-term 

PM2.5 exposures, studies were not 

available in the last review. Recent 

studies, however, have provided new 

evidence linking long-term exposure to 

PM2.5 with cardiovascular outcomes that 

has ‘‘expanded upon the continuum of 

effects ranging from the more subtle 

subclinical measures to 

cardiopulmonary mortality’’ (U.S. EPA, 

2009a, p. 2–17). In the current review, 

studies are now available that evaluated 

a number of endpoints ranging from 

subtle indicators of cardiovascular 

health to serious clinical events 

associated with coronary heart disease 

and cardiovascular and cerebrovascular 

disease.34 The most important new 

evidence comes from the WHI study 

which provides evidence of nonfatal 

cardiovascular events including both 

coronary and cerebrovascular events 

(Miller et al., 2007; U.S. EPA, 2009a, 

sections 7.2.9 and 7.6.1). Toxicological 

studies provide supportive evidence 

that the cardiovascular morbidity effects 

observed in long-term exposure 

epidemiological studies are biologically 

plausible and coherent with studies of 

cardiovascular-related mortality as well 

as with studies of cardiovascular-related 

effects associated with short-term 

exposures to PM2.5, as described below 

(U.S. EPA, 2009a, p. 7–19). 

With regard to respiratory effects, the 

Integrated Science Assessment 

concludes that 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 associated 

with long-term PM2.5 exposure. The 

strongest evidence for respiratoryrelated effects available in this review is 

from studies that evaluated decrements 

in lung function growth, increased 

respiratory symptoms, and asthma 

development (U.S. EPA, 2009a, sections 

2.3.1.2, 7.3.1.1, and 7.3.2.1).35

Specifically, extended analyses of the 

Southern California Children’s Health 

Study provide evidence that clinically 

important deficits in lung function 36

associated with long-term exposure to 

PM2.5 persist into early adulthood (U.S. 

EPA, 2009a, p. 7–27; Gauderman et al., 

2004). 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 24; McConnell et al., 2003) 

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, 2009, 7– 

24; Islam et al., 2007). The outcomes 

observed in these 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 

previously reported. Supporting these 

results are new longitudinal cohort 

studies conducted by other researchers 

in varying locations using different 

methods (U.S. EPA, 2009a, section 

7.3.9.1). New evidence from a U.S. 

cohort of cystic fibrosis patients 

provided evidence of association 

between long-term PM2.5 exposures and 

exacerbations of respiratory symptoms 

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37Single-city Bayes-adjusted effect estimates for 

the 112 cities analyzed in Zanobetti and Schwartz 

(2009) were provided by the study author (personal 

communication with Dr. Antonella Zanobetti, 2009; 

see also U.S. EPA, 2009a, Figure 6–24). 

resulting in hospital admissions or use 

of home intravenous antibiotics (U.S. 

EPA, 2009a, p. 7–25; Goss et al., 2004). 

Toxicological studies provide 

coherence and biological plausibility for 

the respiratory effects observed in 

epidemiological studies (U.S. EPA, 

2009a, p. 7–42). For example, pre- and 

postnatal exposure to ambient levels of 

urban particles has been found to affect 

lung development in an animal model 

(U.S. EPA, 2009a, section 7.3.2.2; p. 7– 

43). This finding is important because 

impaired lung development is one 

mechanism by which PM exposure may 

decrease lung function growth in 

children (U.S. EPA, 2009a, p. 2–12; 

section 7.3). 

With regard to respiratory-related 

mortality associated with long-term 

PM2.5 exposure, the Integrated Science 

Assessment concludes that ‘‘when 

deaths due to respiratory causes are 

separated from all-cause (nonaccidental) 

and cardiopulmonary deaths, there is 

limited and inconclusive evidence for 

an effect of PM2.5 on respiratory 

mortality, with one large cohort study 

finding a reduction in deaths due to 

respiratory causes associated with 

reduced PM2.5 concentrations, and 

another large cohort study finding no 

PM2.5 associations with respiratory 

mortality’’ (U.S. EPA, 2009a, p. 7–41). 

The extended follow-up of the Harvard 

Six Cities study reported a positive but 

statistically non-significant association 

between long-term PM2.5 exposure and 

respiratory-related mortality (Laden et 

al., 2006), whereas Pope et al. (2004) 

found no association in the ACS cohort 

(U.S. EPA, 2009a, p. 7–84). There is 

emerging but limited evidence for an 

association between long-term PM2.5

exposure and respiratory mortality in 

post-neonatal infants where long-term 

exposure was considered as 

approximately one month to one year 

(U.S. EPA, 2009a, pp. 7–54 to 7–55). 

Emerging evidence of short- and longterm exposure to PM2.5 and respiratory 

morbidity and infant mortality provide 

some support for the weak respiratoryrelated mortality effects observed. 

Beyond effects considered to have 

causal or likely causal relationships 

with long-term PM2.5 exposure as 

discussed above, the following health 

outcomes are classified in the Integrated 

Science Assessment as having evidence 

suggestive of a causal relationship with 

long-term PM2.5 exposure: (1) 

Reproductive and developmental effects 

and (2) cancer, mutagenicity, and 

genotoxicity effects (U.S. EPA, 2009a, 

Table 2–6). With regard to reproductive 

and developmental effects, the 

Integrated Science Assessment notes, 

‘‘[e]vidence is accumulating for PM2.5- 

related effects on low birth weight and 

infant mortality, especially due to 

respiratory causes during the postneonatal period’’ (U.S. EPA, 2009a, p. 

2–13). New evidence available in this 

review reports significant associations 

between exposure to PM2.5 during 

pregnancy and lower birth weight and 

infant mortality, with less consistent 

evidence for pre-term birth and 

intrauterine growth restriction. (U.S. 

EPA, 2009a, section 7.4). The Integrated 

Science Assessment further notes that 

‘‘[i]nfants and fetal development 

processes may be particularly 

vulnerable to PM exposure, and 

although the physical mechanisms are 

not fully understood, several hypotheses 

have been proposed involving direct 

effects on fetal health, altered placenta 

function, or indirect effects on the 

mother’s health’’ (U.S. EPA, 2009a, 

section 7.4.1). Although toxicological 

studies provide some evidence that 

supports an association between longterm PM2.5 exposure and adverse 

reproductive and developmental 

outcomes, there is ‘‘little mechanistic 

information or biological plausibility for 

an association between long-term PM 

exposure and adverse birth outcomes 

(e.g., low birth weight, infant 

mortality)’’ (U.S. EPA, 2009a, p. 2–13). 

With regard to cancer, mutagenic and 

genotoxic 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 and sections 2.3.1.2 and 7.5; Table 

7–7; Figures 7–6 and 7–7). The extended 

follow-up to the ACS study reported an 

association between long-term PM2.5

exposure and lung cancer mortality 

(U.S. EPA, 2009a, p. 7–71; Krewski et 

al., 2009) as did the extended follow-up 

to the Harvard Six Cities study when 

considering the entire 25-year follow-up 

period (Laden et al., 2006). There is 

some evidence, primarily from in vitro 

studies, providing biological plausibility 

for the PM-lung cancer relationships 

observed in epidemiological studies 

(U.S. EPA, 2009a, p. 7–80), although in 

vivo toxicological studies of 

carcinogenicity generally reported 

mixed results (U.S. EPA, 2009a, section 

7.5). 

b. Health Effects Associated With ShortTerm PM2.5 Exposures 

In considering effects associated with 

short-term PM2.5 exposure, the body of 

currently available scientific evidence 

has been expanded greatly by the 

publication of a number of new multicity, time-series studies that have used 

uniform methodologies to investigate 

the effects of short-term fine particle 

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 climate and 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 (U.S. EPA 

2009a, chapter 6). 

With regard to mortality, new U.S. 

and Canadian multi-city and single-city 

PM2.5 exposure studies have found 

generally consistent positive 

associations between short-term PM2.5

exposures and cardiovascular- and 

respiratory-related mortality as well as 

all-cause (non-accidental) mortality 

(U.S. EPA, 2009a, sections 2.3.1.1, 

6.2.11 and 6.5.2.2; Figures 6–26, 6–27, 

and 6–28). In an analysis of the National 

Morbidity, Mortality, and Air Pollution 

Study (NMMAPS) data, Dominici et al. 

(2007) reported associations between 

fine particle exposures and all-cause 

and cardiopulmonary-related mortality 

(U.S. EPA, 2009a, p. 6–175, Figure 6– 

26). In a study of 112 U.S. cities, 

Zanobetti and Schwartz (2009) reported 

positive associations (in 99 percent of 

the cities) and frequently statistically 

significant associations (in 55 percent of 

the cities) between short-term PM2.5

exposure and total (non-accidental) 

mortality (U.S. EPA, 2009a, pp. 6–176 to 

6–179; Figures 6–23 and 6–24).37 A 

Canadian 12-city study (Burnett et al., 

2004) is generally consistent with an 

earlier Canadian 8-city study (Burnett 

and Goldberg, 2003). Both studies 

reported a positive and statistically 

significant association between shortterm PM2.5 exposure and mortality (U.S. 

EPA, 2009a, p. 6–182, Figure 2–1), 

although the influence of nitrogen 

dioxide (NO2) and limited PM2.5 data for 

several years during the study period 

somewhat diminished the findings 

reported in the 12-city study. In 

addition to these multi-city studies, 

evidence from available single-city 

studies suggests that gaseous 

copollutants do not confound the PM2.5- 

mortality association (U.S. EPA, 2009a, 

section 2.3.1.1). Collectively, these 

studies provide generally consistent and 

much stronger evidence for PM2.5- 

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38Seasonal differences in effects may be related 

to PM2.5 composition as well as regional differences 

in climate and infrastructure that may affect time 

spent outdoors or indoors, housing characteristics 

including air conditioning usage, and differences in 

baseline incidence rates (U.S. EPA, 2009a, p. 3– 

182). 

associated mortality than the evidence 

available in the last review (U.S. EPA, 

2011a, p. 2–24). 

With regard to cardiovascular effects, 

new multi-city as well as single-city 

short-term PM2.5 exposure studies 

conducted since the last review support 

a largely positive and frequently 

statistically significant association 

between short-term exposure to PM2.5

and cardiovascular-related morbidity 

and mortality, substantiating prior 

findings. For example, among a multicity cohort of older adults participating 

in the Medicare Air Pollution Study 

(MCAPS), investigators reported 

evidence of a positive association 

between short-term PM2.5 exposures and 

hospital admissions related to 

cardiovascular outcomes (U.S. EPA, 

2009a, pp. 6–57 to 58; Dominici et al, 

2006a; Bell et al, 2008). The strongest 

evidence for cardiovascular effects has 

been observed predominantly for 

hospital admissions and emergency 

department visits for ischemic heart 

disease and congestive heart failure, and 

cardiovascular-related mortality (U.S. 

EPA, 2009a, Figure 2–1, p. 6–79, 

sections 6.2.10.3, 6.2.10.5, and 6.2.11; 

Bell et al., 2008; Dominici et al., 2006a; 

Tolbert et al., 2007; Zanobetti and 

Schwartz, 2009). In studies that 

evaluated the potential for confounding 

using co-pollutant models, PM2.5 effect 

estimates for cardiovascular-related 

hospital admissions and emergency 

department visits generally remained 

positive, with the magnitude of PM2.5

effect estimates increasing in some 

models and decreasing in others (U.S. 

EPA, 2009a, Figure 6–5). Furthermore, 

these findings are supported by a recent 

study of a multi-city cohort of women 

participating in the WHI study that 

reported a positive but statistically 

nonsignificant association between 

short-term exposure to PM2.5 and 

electrocardiogram measures of 

myocardial ischemia (Zhang et al., 

2009). 

In focusing on respiratory effects, the 

strongest evidence from short-term 

PM2.5 exposure studies has been 

observed for respiratory-related 

emergency department visits and 

hospital admissions for chronic 

obstructive pulmonary disease (COPD) 

and respiratory infections (U.S. EPA, 

2009a, sections 2.3.1.1 and 6.3.8.3; 

Figures 2–1 and 6–13; Dominici et al., 

2006a). In studies that employed copollutant models to evaluate the 

potential for confounding, PM2.5 effect 

estimates for respiratory-related hospital 

admissions and emergency department 

visits generally remained positive, with 

the magnitude of PM2.5 effect estimates 

increasing in some models and 

decreasing in others (U.S. EPA, 2009a, 

Figure 6–15). Evidence for PM2.5-related 

respiratory effects has also been 

observed in panel studies, which 

indicate associations with respiratory 

symptoms, pulmonary function, and 

pulmonary inflammation among 

asthmatic children (U.S. EPA, 2009a, p. 

2–10). Although not consistently 

observed, some controlled human 

exposure studies have reported small 

decrements in various measures of 

pulmonary function following 

controlled exposures to PM2.5 (U.S. EPA, 

2009a, p. 2–10). Furthermore, the 

comparatively larger body of 

toxicological evidence since the last 

review is coherent with the evidence 

from epidemiological and controlled 

human exposure studies that examined 

short-term exposures to PM2.5 and 

respiratory effects (U.S. EPA, 2009a, 

section 6.3.10.1). 

c. Summary 

In considering the extent to which 

newly available scientific evidence 

strengthens or calls into question 

evidence of associations identified in 

the last review between ambient fine 

particle exposures and health effects, 

the Policy Assessment recognizes that 

much progress has been made in 

assessing some key uncertainties related 

to our understanding of health effects 

associated with long- and short-term 

exposure to PM2.5. As briefly discussed 

above as well as in the more complete 

discussion of the evidence as presented 

and assessed in the Integrated Science 

Assessment, the Policy Assessment 

notes that the newly available 

information combined with information 

available in the last review provides 

substantially stronger confidence in a 

causal relationship between long- and 

short-term exposures to PM2.5 and 

mortality and cardiovascular effects. In 

addition, the newly available evidence 

reinforces and expands the evidence 

supporting a likely causal relationship 

between long- and short-term exposure 

to PM2.5 and respiratory effects. The 

body of scientific evidence is somewhat 

expanded but is still limited with 

respect to associations between longterm PM2.5 exposures and 

developmental and reproductive effects 

as well as cancer, mutagenic, and 

genotoxic effects. The Integrated 

Science Assessment concludes that 

these data provide evidence that is 

suggestive of a causal relationship for 

these effects. Thus, the Policy 

Assessment concludes there is stronger 

and more consistent and coherent 

support for associations between longand 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. 

2. Limitations and Uncertainties 

Associated With the Currently Available 

Evidence 

With respect to understanding the 

nature and magnitude of PM2.5-related 

risks, the Policy Assessment recognizes 

that important uncertainties remain in 

the current review (U.S. EPA, 2011a, 

p. 2–25). Epidemiological studies 

evaluating health effects associated with 

long- and short-term PM2.5 exposures 

have reported heterogeneity in 

responses both within and between 

cities and geographic regions within the 

U.S. In particular, the Policy 

Assessment notes that there are 

challenges with interpreting differences 

in health effects observed in the eastern 

versus western parts of the U.S., 

including evaluating effects stratified by 

seasons.38 This heterogeneity may be 

attributed, in part, to differences in the 

fine particle composition or related to 

exposure measurement error. 

In considering the relationships 

between PM composition and health 

effects, the ISA notes that the scientific 

evidence continues to evolve and 

concludes that, while many constituents 

of PM 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 (U.S. EPA, 2009a, p. 2– 

17). In particular, based on assessing the 

body of available evidence, the ISA 

notes that (1) cardiovascular effects have 

been linked with elemental carbon as 

well as with PM2.5 from crustal sources, 

traffic, and wood smoke/vegetative 

burning; (2) respiratory effects have 

been linked with secondary sulfate 

PM2.5 as well as with PM2.5 from crustal/ 

soil/road dust and traffic sources; and 

(3) a few studies have reported 

associations between total mortality and 

secondary sulfate/long-range transport, 

traffic, and salt. While specific PM2.5

constituents have been linked with 

various PM2.5-related health effects in a 

small number of studies, research 

continues to focus on the identification 

of specific constituents or sources that 

may be most closely related to specific 

PM2.5-related health outcomes. 

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39A 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). 

40Although studies have primarily used 

exposures to PM10 or PM2.5, the available evidence 

suggests that the identified factors also increase risk 

from PM10-2.5 (U.S. EPA, 2009a, section 8.1.8). 

41The term ‘‘susceptible population’’ is defined 

in the Integrated Science Assessment as 

‘‘[P]opulations that have a greater likelihood of 

experiencing health effects related to exposure to an 

air pollutant (e.g., PM) due to a variety of factors 

including, but not limited to: Genetic or 

developmental factors, race, gender, lifestage, 

lifestyle (e.g., smoking status and nutrition) or 

preexisting disease; as well as population-level 

factors that can increase an individual’s exposure 

to an air pollutant (e.g., PM) such as socioeconomic 

status [SES], which encompasses reduced access to 

health care, low educational attainment, residential 

location, and other factors (U.S. EPA, 2009a, p. 8– 

1). 

Exposure measurement error is also 

an important source of uncertainty (U.S. 

EPA, 2009a, section 3.8.6). 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, climate, source 

characteristics, ventilation usage, 

personal activity patterns, 

photochemistry). As noted in the 

Integrated Science Assessment, 

exposure measurement error can 

introduce bias and increased 

uncertainty in associated health effect 

estimates (U.S. EPA, 2009a, p. 2–17). 

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).39 As noted above, 

although short-term studies of 

cardiovascular and respiratory hospital 

admissions and emergency department 

visits generally reported that PM2.5

effect estimates remained positive, the 

magnitude of those effect estimates 

increased in some models and 

decreased in others. In addition, 

although evidence from single-city 

studies available in the last review 

suggests that gaseous copollutants do 

not confound the PM2.5-related 

mortality association (U.S. EPA, 2004, 

section 8.4.3.3), a conclusion that is 

supported by studies that examined the 

PM10-mortality relationship (U.S. EPA, 

2009a, p. 6–182 and 6–201), many 

recent U.S. multi-city studies have not 

analyzed multipollutant models. 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 decision making in this review. 

3. At-Risk Populations 

In identifying population groups or 

lifestages at greatest risk for health risk 

from a specific pollutant, the terms 

susceptibility, vulnerability, sensitivity, 

and at-risk are commonly employed. 

The definition for these terms 

sometimes varies, but in most instances 

‘‘susceptibility’’ refers to biological or 

intrinsic factors (e.g., lifestage, gender, 

preexisting disease/conditions) while 

‘‘vulnerability’’ refers to nonbiological 

or extrinsic factors (e.g., socioeconomic 

factors). However, factors included in 

the terms ‘‘susceptibility’’ and 

‘‘vulnerability’’ are intertwined and are 

difficult to distinguish. In the Integrated 

Science Assessment, the term 

‘‘susceptibility’’ has been used broadly 

to recognize populations that have a 

greater likelihood of experiencing 

effects related to ambient PM 

exposure40, such that use of the term 

‘‘susceptible populations’’ in the 

Integrated Science Assessment is used 

as a term that encompasses factors 

related both to susceptibility and 

vulnerability.41 In the development of a 

more recent Integrated Science 

Assessment, the Agency is using the 

term ‘‘at-risk’’ groups to more broadly 

define the populations with 

characteristics that increase the risk of 

pollutant-related health effects (U.S. 

EPA, 2011d, p. 8–1). Therefore, in this 

proposal, the term ‘‘at-risk’’ is the 

broadly encompassing term used for 

groups with specific factors that 

increase the risk of PM-related health 

effects in a population. At-risk 

populations could exhibit a greater risk 

of PM-related health effects than the 

general population for a number of 

reasons including: being affected by 

lower concentrations of PM, 

experiencing a larger health impact at a 

given PM concentration or being 

exposed to higher PM concentrations 

than the general population. Given the 

heterogeneity of individual responses to 

PM exposures, the severity of the health 

effects experienced by an at-risk 

population may be much greater than 

that experienced by the population at 

large. 

As summarized below and presented 

in more detail in chapter 8 of the 

Integrated Science Assessment and 

section 2.2.1 of the Policy Assessment, 

the currently available epidemiological 

and controlled human exposure 

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) (U.S. EPA, 2009a, section 

2.4.1, Table 8–2). In addition, 

toxicological studies provide underlying 

support for the biological mechanisms 

that potentially lead to increased 

susceptibility to PM-related health 

effects (U.S. EPA, 2009a, sections 2.4.1 

and 8.1.8). 

Two different lifestages have been 

associated with increased risk to PMrelated health effects: childhood (i.e., 

less than 18 years of age) and older 

adulthood (i.e., 65 years of age and 

older). Childhood represents a lifestage 

where susceptibility to PM exposures 

may be related to the following 

observations: children spend more time 

outdoors; children have greater activity 

levels than adults; children have 

exposures resulting in higher doses per 

body weight and lung surface area; and 

the developing lung is prone to damage, 

including irreversible effects, from 

environmental pollutants as it continues 

to develop through adolescence (U.S. 

EPA, 2009a, section 8.1.1.2). Older 

adults represent a lifestage where 

susceptibility to PM-associated health 

effects may be related to the higher 

prevalence of pre-existing 

cardiovascular and respiratory diseases 

found in this age group compared to 

younger age groups as well as the 

gradual decline in physiological 

processes that occur as part of the aging 

process (U.S. EPA, 2009a, section 

8.1.1.1). In addition, accumulating 

evidence suggests that the developing 

fetus may also represent an additional 

lifestage that is at greater risk to PM 

exposures (U.S. EPA, 2009a, sections 

2.3.1.2 and 7.4). 

With regard to mortality, recent 

epidemiological studies have continued 

to find that older adults are at greater 

risk of all-cause (non-accidental) 

mortality associated with short-term 

exposure to both PM2.5 and PM10, 

providing consistent and stronger 

evidence of effects in this at-risk 

population compared to the last review 

(U.S. EPA, 2009a, Figure 7–7, section 

8.1.1.1, Zeger et al., 2008). Evidence is 

accumulating for PM2.5-related infant 

mortality, especially due to respiratory 

causes during the post-neonatal period 

(U.S. EPA, 2009a, sections 2.3.1.2 and 

7.4). 

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42Socioeconomic status is a composite measure 

that usually consists of economic status, measured 

by income; social status measured by education; 

and work status measured by occupation (U.S. EPA, 

2009a, p. 8–14). 

43For percentages, see http://www.cdc.gov/ 

ASTHMA/nhis/06/table4-1.htm. For population 

estimates, see http://www.cdc.gov/ASTHMA/nhis/ 

06/table3-1.htm. 

With regard to morbidity effects, 

currently available studies provide 

evidence that older adults have 

heightened responses, especially for 

cardiovascular-related effects, and 

children have heightened responses for 

respiratory-related effects (U.S. EPA, 

2009a, p. 2–23). In considering 

respiratory-related effects in children 

associated with long-term PM 

exposures, the Policy Assessment 

recognizes that our understanding of 

effects on lung development has been 

strengthened based on newly available 

evidence that is consistent and coherent 

across different study designs, locations, 

and research groups (U.S. EPA, 2011a, 

p. 2–28). The strongest evidence comes 

from the extended follow-up for the 

Southern California Children’s Health 

Study which includes several new 

studies that report positive associations 

between long-term exposure to PM2.5

and respiratory morbidity, particularly 

for such endpoints as lung function 

growth, respiratory symptoms (e.g., 

bronchitic symptoms), and respiratory 

disease incidence (U.S. EPA, 2009a, 

section 7.3; McConnell et al, 2003; 

Gauderman et al., 2004; Islam et al., 

2007). These analyses provide evidence 

that PM2.5-related effects persist into 

early adulthood and are more robust 

and larger in magnitude than previously 

reported. With regard to respiratory 

effects in children associated with shortterm exposures to PM2.5, currently 

available studies provide stronger 

evidence of respiratory-related 

hospitalizations with larger effect 

estimates observed among children. In 

addition, reductions in lung function 

(i.e., FEV1) and increases in respiratory 

symptoms and medication use 

associated with PM exposures have 

been reported among asthmatic children 

(U.S. EPA, 2009a, sections 6.3.1, 6.3.2.1, 

and 8.4.9). 

A number of health conditions have 

been found to put individuals at greater 

risk for adverse effects following 

exposure to PM. The currently available 

evidence confirms and strengthens 

evidence in the last review that 

individuals with underlying 

cardiovascular and respiratory diseases 

are more susceptible to PM exposures 

(U.S. EPA, 2009a, section 8.1.6; U.S. 

EPA, 2011a, section 2.2.1). There is also 

emerging evidence that people with 

diabetes, who are at risk for 

cardiovascular disease, as well as obese 

individuals, may have increased 

susceptibility to PM exposures (U.S. 

EPA, 2009a, section 8.1.6.4). As 

discussed in section 8.1.6 of the 

Integrated Science Assessment, this 

body of evidence includes findings from 

epidemiological and human clinical 

studies that associations with mortality 

or morbidity are greater in those with 

pre-existing conditions, and also 

includes evidence from toxicological 

studies using animal models of 

cardiopulmonary disease. 

Stronger evidence is available in this 

review than the last indicating that 

people from lower socioeconomic strata 

are an at-risk population relative to PM 

exposures (U.S. EPA, 2009a, section 

8.1.7; U.S. EPA, 2011a, section 2.2.1). 

Persons with lower socioeconomic 

status (SES) 42 have been generally 

found to have a higher prevalence of 

pre-existing diseases; limited access to 

medical treatment; and increased 

nutritional deficiencies, which can 

increase this population’s risk to PMrelated effects. 

Investigation of potential genetic 

susceptibility has provided evidence 

that individuals with specific genetic 

differences are more susceptible to PMrelated effects (U.S. EPA, 2009a, pp. 8– 

7 to 8–9). More research is needed to 

better understand the relationship 

between genetic effects and potential 

susceptibility to PM-related effects (U.S. 

EPA, 2011a, p. 2–109). 

In summary, there are several at-risk 

populations that may be especially 

susceptible or vulnerable to PM-related 

effects. 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. The available evidence does not 

generally allow distinctions to be drawn 

between the PM indicators in terms of 

whether populations are more at-risk to 

a particular size fraction (i.e., PM2.5 and 

PM10-2.5). 

4. Potential PM2.5-Related Impacts on 

Public Health 

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 as 

described in section III.B.3. 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. In addition, it 

appears that mortality risks are not 

limited to the very frail. Taken together, 

these results suggest that exposure to 

ambient PM2.5 concentrations can have 

substantial public health impacts. 

With regard to at-risk populations in 

the United States, approximately 7 

percent of adults (approximately 16 

million adults) and 9 percent of 

children (approximately 7 million 

children) have asthma (U.S. EPA 2009a, 

Table 8–3; CDC, 2008 43). In addition, 

approximately 4 percent of adults have 

been diagnosed with chronic bronchitis 

and approximately 2 percent with 

emphysema (U.S. EPA, 2009a, Table 8– 

3). Approximately 11 percent of adults 

have been diagnosed with heart disease, 

6 percent with coronary heart disease, 

23 percent with hypertension, and 8 

percent with diabetes (U.S. EPA, 2009a, 

Table 8–3). In addition, approximately 3 

percent of the U.S. adult population has 

suffered a stroke (U.S. EPA, 2009a, 

Table 8–3). Therefore, large portions of 

the United States population are in 

groups that may be at increased risk to 

health effects associated with exposures 

to ambient PM2.5. The size of the 

potentially at-risk population suggests 

that exposure to ambient PM2.5 has 

significant impact on public health in 

the United States. 

C. Quantitative Characterization of 

Health Risks 

1. Overview 

In this review, the quantitative risk 

assessment builds on the approach used 

and lessons learned in the last review 

and focuses 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 are largely the same as those 

articulated in the risk assessment 

conducted for the last review. These 

goals include: (1) To provide estimates 

of the potential magnitude of premature 

mortality and/or selected morbidity 

effects in the population associated with 

recent ambient level of PM2.5 and with 

simulating just meeting the current and 

alternative suites of PM2.5 standards in 

15 selected urban study areas, 

including, where data were available, 

consideration of impacts on at-risk 

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44The 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 have relatively low PM2.5 concentrations 

(e.g., urban areas in Florida) and, consequently, the 

Risk Assessment concludes failure to include these 

areas in the set of urban study areas is unlikely to 

exclude high PM2.5-risk locations (U.S. EPA, 2010a, 

section 4.4.1). 

45 Incidence rates express the occurrence of a 

disease or event (e.g., death, hospital admission) in 

a specific period of time, usually per year. Rates are 

expressed either as a value per population group 

(e.g., the number of cases in Philadelphia County) 

or a value per number of people (e.g., the number 

of cases per 10,000 residents in Philadelphia 

County), and may be age- and/or sex-specific. 

Incidence rates vary among geographic areas due to 

differences in populations characteristics (e.g., age 

distribution) and factors promoting illness (e.g., 

smoking rates, air pollution concentrations). The 

baseline incidence rate provides an estimate of the 

incidence rate (i.e., number of cases of the health 

effect per year, usually per 10,000 or 100,000 

general population) in the assessment location 

unrelated to changes in ambient PM2.5

concentrations in that location (U.S. EPA, 2010a, 

section 3.4). 

46The hybrid rollback approach involves a 

combination of an initial step of a more localized 

reduction in ambient PM2.5 concentrations at 

source-oriented monitors followed by a regional 

pattern of reduction across all monitors in a study 

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 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.B.3. 

2. Summary of Design Aspects 

Based on a review of the evidence 

presented and assessed in the Integrated 

Science Assessment and criteria for 

selecting specific health effect 

endpoints discussed in the Risk 

Assessment (U.S. EPA, 2010a, section 

3.3.1), the following broad categories of 

health endpoints were included in the 

quantitative risk assessment: (1) Allcause, ischemic heart disease-related, 

cardiopulmonary-related, and lung 

cancer-related mortality associated with 

long-term PM2.5 exposure; (2) nonaccidental, cardiovascular-related, and 

respiratory-related mortality associated 

with short-term PM2.5 exposure; and (3) 

cardiovascular-related and respiratoryrelated hospital admissions and asthmarelated emergency department visits 

associated with short-term PM2.5

exposure. 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 

(U.S. EPA, 2010a, p. 3–29). The 

quantitative risk assessment estimates 

risks for various health effects in 15 

urban study areas. The selection of 

urban study areas was based on a 

number of criteria including: (1) 

Consideration of urban study areas 

evaluated in the last PM risk 

assessment; (2) consideration of 

locations evaluated in key 

epidemiological studies; (3) preference 

for locations with relatively elevated 

annual and/or 24-hour PM2.5 monitored 

concentrations; and (4) preference for 

including locations from different 

regions across the country, reflecting 

potential differences in PM2.5 sources, 

composition, and potentially other 

factors which might impact PM2.5- 

related risk (U.S. EPA, 2010a, section 

3.3.2). Based on the results of several 

analyses examining the 

representativeness of these 15 urban 

study areas in the broader national 

context, the Risk 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

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).44

In order to estimate the incidence of 

a particular health effect associated with 

recent ambient conditions in a specific 

urban study area attributable to PM2.5

exposures, as well as the change in 

incidence corresponding to a given 

change in PM2.5 concentrations resulting 

from simulating just meeting current or 

alternative PM2.5 standards, three 

elements are required (U.S. EPA, 2010a, 

section 3.1.1, Figures 3–2 and 3–3). 

These elements are: (1) Air quality 

information (including recent air quality 

data for PM2.5 from ambient monitors for 

the selected location, estimates of 

background PM2.5 concentrations 

appropriate for that location, and a 

method for adjusting the recent data to 

reflect patterns of air quality estimated 

to occur when the area just meets a 

given set of PM2.5 standards); (2) relative 

risk-based concentration-response 

functions that provide an estimate of the 

relationship between the health 

endpoints of interest and ambient PM2.5

concentrations; and (3) baseline health 

effects incidence rates and population 

data, which are needed to provide an 

estimate of the incidence of health 

effects in an area before any changes in 

PM2.5 air quality.45

The Risk Assessment includes 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 provide information to 

evaluate and rank the potential impacts 

of key sources of uncertainty on the core 

risk estimates (U.S. EPA, 2010a, sections 

3.5 and 4.3, Table 4–3). In addition, the 

sensitivity analyses represent a set of 

reasonable alternatives to the core set of 

risk estimates that fall within an overall 

set of plausible risk estimates 

surrounding the core estimates (U.S. 

EPA, 2010a, section 4.3.2). 

Recent air quality was characterized 

for the 15 urban study areas based on 

24-hour PM2.5 concentrations measured 

for 3 years (i.e., 2005, 2006, and 2007) 

as described in section 3.2.1 of the Risk 

Assessment. Different methodologies 

were then used to simulate conditions 

for just meeting the current or 

alternative PM2.5 standards (U.S. EPA, 

2010a, section 3.2.3). This included 

using the single rollback approach used 

in the risk assessment conducted for the 

last review which reflects a uniform 

regional pattern of reductions in 

ambient PM2.5 concentrations across 

monitors (i.e., proportional rollback 

approach). The proportional rollback 

approach was used in generating the 

core risk estimates (U.S. EPA, 2010a, 

section 3.2.3.1). In sensitivity analyses, 

the Risk Assessment also applied two 

alternative rollback approaches (i.e., 

hybrid and locally-focused rollback 

approaches)46 to better characterize 

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area (U.S. EPA, 2010a, section 3.2.3.2). The locallyfocused rollback approach involves a focused 

reduction of concentrations only at those monitors 

exceeding the current or alternative 24-hour 

standard levels (U.S. EPA, 2010a, section 3.2.3.3). 

47The peak-to-mean ratio of ambient PM2.5

concentrations also has a direct bearing on whether 

the 24-hour or annual standard will be the generally 

controlling standard for a particular study area, 

with higher peak-to-mean ratios generally being 

associated with locations where the 24-hour 

standard is likely the controlling standard. 

48Policy-relevant background estimates used in 

the risk assessment model were based on 

information presented in the Integrated Science 

Assessment (U.S. EPA, 2009a, section 3.7, Table 3– 

23) and discussed in the Risk Assessment (U.S. 

EPA, 2010a, section 3.2.2). These values were 

generated based on a combination of Community 

Multiscale Air Quality model (CMAQ) and Goddard 

Earth Observing System (GEOS)-Chem modeling 

(U.S. EPA, 2009a, section 3.7.1.2; U.S. EPA, 2010a, 

section 3.2.2). 

49As noted in section 3.3.3 of the Risk 

Assessment, multi-city studies have a number of 

advantages over single-city studies including: 

increased statistical power providing effect 

estimates with relatively greater precision and 

reduced problems with publication bias (i.e., in 

which studies with statistically insignificant or 

negative results are less likely to get published than 

those with positive and/or statistically significant 

results). 

50As noted in the last review, the ACS study 

population has persons generally representative of 

a higher SES (e.g., higher educational status) 

relative to the Harvard Six Cities study population 

(12 percent versus 28 percent of the cohort had less 

than a high school education, respectively) (U.S. 

EPA, 2004, p. 8–118). The Policy Assessment 

concludes that the Harvard Six Cities study cohort 

may provide a more representative sample of the 

broader national population than the ACS study 

cohort (U.S. EPA, 2011a, p. 2–40). 

potential variability in the way air 

quality in urban areas responds to 

programs put in place to meet the 

current or alternative PM2.5 standards. 

In considering the three rollback 

approaches collectively, the 

proportional and locally-focused 

methods are approaches that are more 

likely to represent ‘‘bounding’’ scenarios 

related to the spatial pattern of future 

reductions in ambient PM2.5

concentrations. In contrast, the hybrid 

approach, in principle, reflects a more 

plausible or representative rollback 

strategy since it: (1) Reflects 

consideration for site-specific 

information regarding larger PM2.5

sources and their potential impact on 

source-oriented monitors and (2) 

combines elements of more locallyfocused and regionally-focused patterns 

of reductions (U.S. EPA, 2010a, section 

3.2.3). 

The peak-to-mean ratio of ambient 

PM2.5 concentrations within a study area 

informs the type of rollback approach 

used to simulate just meeting the 

current or alternative suites of standards 

to determine the magnitude of the 

reduction in annual mean PM2.5

concentrations for that study area and 

consequently the degree of risk 

reduction.47 For example, study areas 

with relatively high peak-to-mean ratios 

are likely to have greater estimated risk 

reductions for the current suite of 

standards (depending on the 

combination of 24-hour and annual 

design values), and such locations can 

be especially sensitive to the type of 

rollback approach used, with the 

proportional rollback approach resulting 

in notably greater estimated risk 

reduction compared with the locallyfocused rollback approach. In contrast, 

study areas with lower peak-to-mean 

ratios typically experience greater risk 

reductions when simulating just 

meeting the current or alternative 

annual-standard level than with 

simulating just meeting the current or 

alternative 24-hour standard level (again 

depending on the combination of 24- 

hour and annual design values). In 

addition, the type of rollback approach 

used will tend to have less of an impact 

on the magnitude of risk reductions for 

study areas with lower peak-to-mean 

ratios. Consideration of these two 

factors helps to inform an 

understanding of the nature and pattern 

of estimated risk reductions and risk 

remaining upon simulation of just 

meeting the current and alternative 

suites of standards across the urban 

study areas (U.S. EPA, 2010a, section 

5.2.1). 

The concentration-response functions 

used in the risk assessment were based 

on findings from epidemiological 

studies that have relied on fixed-site, 

population-oriented, ambient monitors 

as a surrogate for actual ambient PM2.5

exposures. The risk assessment 

addresses risks attributable to 

anthropogenic sources and activities 

(i.e., risk associated with concentrations 

above policy-relevant background).48

This approach of estimating risks in 

excess of background was judged to be 

more relevant to policy decisions 

regarding ambient air quality standards 

than risk estimates that include effects 

potentially attributable to PM2.5

concentrations that are not associated 

with North American anthropogenic 

emissions. 

In modeling risk associated with longand short-term PM2.5 exposures, the 

Risk Assessment initially focused on 

selecting concentration-response 

functions from multi-city studies.49

Concentration-response functions from 

two single-city studies provided 

coverage for additional health effect 

endpoints (i.e., emergency department 

visits for cardiovascular and/or 

respiratory effects) associated with 

short-term PM2.5 exposures (U.S. EPA, 

2010a, p. 3–37). 

With regard to modeling risks 

associated with long-term PM2.5

exposure, concentration-response 

functions used in the risk model are all 

based on cohort studies, in which a 

cohort of individuals is followed over 

time. In the core analysis, estimated 

premature mortality risk associated with 

long-term PM2.5 concentrations used 

concentration-response functions from 

the extended ACS study (Krewski et al., 

2009). This study had a number of 

advantages including: analyses that 

expanded upon previous publications 

presenting evaluations of the ACS longterm cohort study and extending the 

follow-up period to eighteen years; a 

rigorous examination of different model 

forms for estimating effect estimates; 

coverage for a range of ecological 

variables (e.g., social, economic, and 

demographic factors) which allowed for 

consideration of whether these factors 

confound or modify the relationship 

between PM2.5 exposure and mortality; 

and updated and expanded data sets on 

incidence and exposure (U.S. EPA, 

2010a, p 2–9 and 3–38). 

As discussed in section III.B.3, 

persons of lower socioeconomic status 

have been identified as an at-risk 

population. The ACS study cohort does 

not provide representative coverage for 

persons of lower-socioeconomic status 

and, thus, the Risk Assessment 

concludes that using the concentrationresponse functions from this study may 

result in risk estimates that are biased 

low (U.S. EPA, 2010a, p. 5–7). 

Therefore, concentration-response 

functions from a reanalysis of the 

Harvard Six Cities study (Krewski et al., 

2000) were used in a sensitivity analysis 

to better support generalizing the results 

of the risk assessment across the broader 

national population.50

While being mindful that the use of 

concentration-response functions from 

Krewski et al. (2009) introduces 

potential for low bias in the core risk 

estimates, the Policy Assessment also 

recognizes many strengths of this study 

and reasons for its continued use for 

generating the core risk estimates, 

including: consideration of a large 

number of metropolitan statistical areas, 

inclusion of two time periods for the air 

quality data which allowed us to 

consider different exposure windows, 

and analysis of a wide range of 

concentration-response function 

models. Therefore, the Risk Assessment 

concludes that concentration-response 

functions obtained from this study had 

the greatest overall support and were 

appropriate to incorporate in the core 

risk model (U.S. EPA, 2010a, p. 3–38). 

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51To provide consistency for the different 

concentration-response functions selected from the 

long-term exposure studies, and, in particular, to 

avoid the choice of lowest measured levels unduly 

influencing the results of the risk assessment, the 

Risk Assessment concluded it was appropriate to 

select a single lowest measured level—5.8 mg/m3

from the later exposure period evaluated in Krewski 

et al. (2009)—to use in estimating risks associated 

with long-term PM2.5 exposures (U.S. EPA, 2010a, 

p. 3–3). 

52 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). 

In the core analysis, for modeling 

health endpoints associated with longterm exposure, the Risk Assessment 

concluded that modeling risks down to 

policy-relevant background would 

require substantial extrapolation of the 

estimated concentration-response 

functions below the range of the data on 

which they were estimated (i.e., the 

lowest measured levels reported in the 

epidemiological studies were 

substantially above policy-relevant 

background). Therefore, the Risk 

Assessment concluded it was most 

appropriate in the core analysis to 

estimate risk only down to the lowest 

measured level to avoid introducing 

additional uncertainty into the analysis 

(U.S. EPA, 2010a, 3–1 to 3–3).51 A 

sensitivity analysis comparing the 

impact of estimated risks down to 

policy-relevant background rather than 

down to the lowest measured level (U.S. 

EPA, 2010a, section 3.5.4.1) used annual 

estimates of policy-relevant background 

values for specific geographic regions 

(U.S. EPA, 2010a, section 3.2.2, Table 3– 

2). 

With regard to modeling risks 

associated with short-term PM2.5

exposure, concentration-response 

functions from two time-series studies 

were selected as the primary studies to 

support the core analysis. 

Concentration-response functions from 

Zanobetti and Schwartz (2009) were 

used in estimating premature nonaccidental, cardiovascular-related, and 

respiratory-related mortality. 

Concentration-response functions from 

Bell et al. (2008) were used in 

estimating cardiovascular-related and 

respiratory-related hospital admissions. 

In addition, concentration-response 

functions from two single-city studies 

were used to estimate emergency 

department visits for cardiovascular 

and/or respiratory illnesses associated 

with short-term PM2.5 exposure (Tolbert 

et al., 2007; Ito et al., 2007; U.S. EPA, 

2010a, p. 3–37). 

For modeling health endpoints 

associated with short-term PM2.5

exposure, the Risk Assessment estimates 

risk down to policy-relevant background 

exclusively using quarterly values to 

represent the appropriate block of days 

within a simulated year (U.S. EPA, 

2010a, section 3.2.2, Table 3–2). 

To estimate the change in incidence 

of a health endpoint associated with a 

given change in PM2.5 concentrations, 

information on the baseline incidence of 

that endpoint is needed (U.S. EPA, 

2010a, section 3.4). In calculating a 

baseline incidence rate to be used with 

a concentration-response function from 

a given epidemiological study, the Risk 

Assessment matched the counties, age 

grouping, and International 

Classification of Diseases (ICD) codes 

used in that study (U.S. EPA, 2010a, 

section 3.4.2). 

An important component of a 

population health risk assessment is the 

characterization of both uncertainty and 

variability.52 The design of the risk 

assessment includes a number of 

elements to address these issues, 

including using guidance from the 

World Health Organization (WHO, 

2008) as a framework for developing the 

approach used for characterizing 

uncertainty in the analyses (U.S. EPA, 

2010a, section 3.5). 

The Risk Assessment considers key 

sources of variability that can impact 

the nature and magnitude of risks 

associated with simulating just meeting 

current and alternative standard levels 

across the urban study areas (U.S. EPA, 

2010a, section 3.5.2). These sources of 

variability include those that contribute 

to differences in risk across urban study 

areas, but do 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 focuses on 

factors that not only introduce 

variability into risk estimates across 

study areas, but also play 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 are likely to 

affect population risks and the degree to 

which they were (or were not) fully 

captured in the design of the risk 

assessment are discussed in section 

3.5.2 of the Risk Assessment. These 

sources include: PM2.5 composition; 

intra-urban variability in ambient PM2.5

concentrations; 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; 

factors related to demographic and 

socioeconomic status; behavioral 

differences across urban study areas 

(e.g., time spent outdoors); baseline 

incidence rates; and 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 (U.S. EPA, 2010a, pp. 3–67 to 

3–69). 

Single and multi-factor sensitivity 

analyses were combined with a 

qualitative analysis to assess the impact 

of potential sources of uncertainty on 

the core risk estimates (U.S. EPA, 2010a, 

sections 3.5.3 and 3.5.4). The 

quantitative sensitivity analyses 

informed our understanding of sources 

of uncertainty that may have a moderate 

to large impact on the core risk 

estimates including: (1) Characterizing 

intra-urban population exposure in the 

context of epidemiology studies linking 

PM2.5 to specific health effects; (2) 

statistical fit of the concentrationresponse 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 (U.S. EPA, 2011a, 

section 5.1.4). In addition to identifying 

sources of uncertainty with a moderate 

to large impact on the core risk 

estimates, the single and multi-element 

sensitivity analyses also produced a set 

of reasonable alternative risk estimates 

that allowed us to place the results of 

the core analysis in context with regard 

to uncertainty and potential bias (U.S. 

EPA, 2010a, section 5.1.4). The 

qualitative uncertainty analysis 

supplemented the quantitative 

sensitivity analyses by allowing 

coverage for sources of uncertainty that 

could not be readily included in the 

sensitivity analysis (U.S. EPA, 2010a, 

section 3.5.3). 

With respect to the long-term 

exposure-related mortality risk 

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53 Given increased emphasis placed in this 

analysis on long-term exposure-related mortality, 

the uncertainty analyses completed for this health 

endpoint category were more comprehensive than 

those conducted for analyses of short-term 

exposure-related mortality and morbidity. This 

reflects, to some extent, limitations in the 

epidemiological data available for addressing 

uncertainty in the latter categories (U.S. EPA, 

2010a, section 3.5.4.2). 

54As noted in section 3.2.1 of the Risk 

Assessment (U.S. EPA, 2010a), estimates of longterm exposure-related mortality are actually based 

on an annual mean PM2.5 concentration that is the 

average across monitors in a study area (i.e., based 

on the composite monitor distribution). Therefore, 

in considering changes in long-term exposurerelated mortality, it is most appropriate to compare 

composite monitor estimates generated for a study 

area under each alternative suite of standards 

considered. The annual mean at the highest 

reporting monitor (i.e., based on the maximum 

monitor distribution) for a study area is the annual 

design value. The annual design value is used to 

determine the percent reduction in PM2.5

concentrations required to meet a particular 

standard. Both types of air quality estimates are 

provided in Table 3–4 of the Risk Assessment (U.S. 

EPA, 2010a, pp. 3–25 to 3–27). 

55Estimates of short-term PM2.5 exposure-related 

mortality and morbidity are based on composite 

monitor 24-hour PM2.5 concentrations. However, 

similar to the case with long-term exposure-related 

mortality, under the current rules, it is the 98th 

percentile 24-hour concentration estimated at the 

maximum monitor (the 24-hour design value) that 

will determine the degree of reduction required to 

meet a given 24-hour standard level (U.S. EPA, 

2011a, p. 2–37). 

56Of the 15 urban study areas, only Dallas and 

Phoenix have both annual and 24-hour design 

values below the levels of the current standards 

based on 2005–2007 air quality data (U.S. EPA, 

2010a, Table 3–3). 

estimates,53 the most important sources 

of uncertainty identified in the 

quantitative sensitivity analyses 

included: selection of concentrationresponse functions; modeling risk down 

to policy-relevant background versus 

lowest measured level; and the choice of 

rollback approach used to simulate just 

meeting current or alternative standards 

(U.S. EPA, 2011a, p. 2–39). With regard 

to the qualitative analysis of 

uncertainty, the following sources were 

identified as potentially having a large 

impact on the core risk estimates for the 

long-term exposure-related mortality: 

characterization of intra-urban 

population exposures; impact of 

historical air quality; and potential 

variation in effect estimates reflecting 

differences in PM2.5 composition (U.S. 

EPA, 2011a, p. 2–39). 

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). Two 

additional analyses examined potential 

bias and overall confidence in the risk 

estimates. The first analysis explored 

potential bias in the core risk estimates 

by considering a set of alternative 

reasonable risk estimates generated as 

part of a sensitivity analysis. The second 

analysis compared the annual mean 

PM2.5 concentrations associated with 

simulating just meeting the current and 

alternative suites of standards with the 

air quality distribution used in deriving 

the concentration-response functions 

applied in modeling mortality risk. 

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 longterm mean PM2.5 concentration) (U.S. 

EPA, 2011a, p. 2–38). 

3. Risk Estimates and Key Observations 

As discussed below, three factors 

figure prominently in the interpretation 

of the risk estimates associated with 

simulating just meeting the current and 

alternative suites of standards, 

including: (1) The importance of 

changes in annual mean PM2.5

concentrations for a specific study area 

in estimating changes in risks related to 

both long- and short-term exposures 

associated with recent air quality 

conditions and air quality simulated to 

just meet the current and alternative 

suites of PM2.5 standards; (2) the ratio of 

peak- to-mean ambient PM2.5

concentrations in a study area; and (3) 

the spatial pattern of ambient PM2.5

reductions that result from using 

different approaches to simulate just 

meeting the current standard levels (i.e., 

rollback approaches). The latter two 

factors are interrelated and influence the 

degree of risk reduction estimated under 

the current suite of standards. 

The magnitude of both long- and 

short-term exposure-related risk 

estimated to remain upon just meeting 

the current suite of standards is strongly 

associated with the simulated change in 

annual mean PM2.5 concentrations. The 

role of annual mean PM2.5

concentrations in driving long-term 

exposure-related risk estimates is 

intuitive given that risks are modeled 

using the annual mean air quality 

metric.54 The fact that short-term 

exposure-related risk estimates are also 

driven by changes in long-term mean 

PM2.5 concentrations is less intuitive, 

since changes in mean 24-hour PM2.5

concentrations are used to estimate 

changes in risk for this time period.55

Analyses show that short-term 

exposure-related risks are 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 (U.S. EPA, 2010a, 

section 3.1.2.2). Consequently, the 

largest part of the estimates of shortterm exposure-related risk is related to 

the changes in the portion of the 

distribution of short term PM2.5

exposures that are well represented by 

changes in the annual mean. Therefore, 

the Policy Assessment focuses on 

changes in annual mean PM2.5

concentrations to inform our 

understanding of patterns of both longand short-term exposure-related risk 

estimates across the set of urban study 

areas evaluated in the quantitative risk 

assessment (U.S. EPA, 2011a, pp. 2–36 

to 2–37). 

In estimating PM2.5-related risks likely 

to remain upon simulation of just 

meeting the current annual and 24-hour 

standards in the 15 urban study areas, 

the Risk Assessment focuses on the 13 

areas that would likely not have met the 

current suite of PM2.5 standards based 

on recent air quality (2005 to 2007). 

These 13 areas have annual and/or 24- 

hour design values that are above the 

levels of the current standards (U.S. 

EPA, 2010a, Table 3–3).56 Based on the 

core risk estimates for these areas, using 

the proportional rollback approach, the 

Policy Assessment makes the following 

key observations regarding the 

magnitude of risk remaining upon 

simulation of just meeting the current 

suite of standards: 

(1) Long-term exposure-related mortality 

risk estimated to remain upon just meeting 

the current standards are significant: 

Premature mortality related to ischemic heart 

disease attributable to long-term PM2.5

exposure was estimated to range from less 

than 100 to approximately 2,000 cases per 

year across the urban study areas. The 

variability in these estimates reflects, to a 

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57Premature mortality for all causes attributed to 

PM2.5 exposure was estimated to be in a range 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). 

58Patterns of risk reduction across alternative 

annual standard levels, in terms of percent change 

relative to risk estimates upon simulating just 

meeting the current standards, are similar for all 

health endpoints modeled (i.e., all-cause, ischemic 

heart disease-related, and cardiopulmonary-related 

mortality). This similarity reflects the fact that the 

concentration-response functions used in the 

quantitative risk assessment are close to linear 

across the range of ambient PM2.5 concentrations 

evaluated. However, estimated incidence will vary 

by health endpoint (U.S. EPA, 2011a, pp. 2–93 to 

2–94, footnote 70). 

great extent, differences in the size of study 

area populations. These estimates represent 

from 4 to 17% of all mortality related to 

ischemic heart disease in a given year for the 

urban study areas evaluated, representing a 

measure of risk that takes into account 

differences in population size and baseline 

mortality rates (U.S. EPA, 2011a, p. 2–43, 

Table 2–2). These estimates of risk for 

mortality related to ischemic heart disease 

associated with long-term PM2.5 exposure 

would likely be in a range of thousands of 

deaths per year for the 15 urban study 

areas 57 (U.S. EPA, 2011a, pp. 2–46 to 2–47). 

Based on these risk estimates for premature 

mortality related to ischemic heart disease 

alone, the Policy Assessment concludes that 

risks estimated to remain upon simulation of 

just meeting the current suite of standards are 

important from a public health standpoint 

(U.S. EPA, 2011a, p. 2–47). The Risk 

Assessment also includes estimated risks for 

premature mortality related to 

cardiopulmonary effects and lung cancer, 

which increase the total annual incidence of 

mortality attributable to long-term PM2.5

exposure (see U.S. EPA, 2010a, section 4.2.1). 

(2) Short-term exposure-related mortality 

risk estimated to remain upon just meeting 

the current standards are much smaller than 

long-term exposure-related mortality risks: 

Cardiovascular-related mortality associated 

with short-term PM2.5 exposure was 

estimated to range from less than 10 to 500 

cases per year across the urban study areas. 

These estimates represent approximately 1 to 

2 percent of total cardiovascular-related 

mortality in a given year for the urban study 

areas evaluated (U.S. EPA, 2011a, p. 2–43, 

Table 2–3). 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 substantially 

lower, up to an order of magnitude smaller, 

for short-term exposure-related mortality 

(U.S. EPA, 2011a, p. 2–47). 

(3) Short-term exposure-related morbidity 

risk estimated to remain upon just meeting 

the current standards indicate 

hospitalizations are significantly larger for 

cardiovascular-related rather than 

respiratory-related events and emergency 

department visits for asthma-related events 

are significant: Cardiovascular-related 

hospitalizations were estimated to range from 

approximately 10 to 800 cases per year across 

the study areas, which are less than 1 percent 

of total cardiovascular-related 

hospitalizations (U.S. EPA, 2011a, p. 2–43, 

Table 2–3). Respiratory-related hospital 

admissions attributable to short-term PM2.5

exposure were significantly smaller than 

those related to cardiovascular events (U.S. 

EPA, 2010a, Tables E–102 and E–111). 

Cardiovascular- and respiratory-related 

hospital admissions together ranged up to 

approximately 1,000 admissions per year 

across the urban study areas. The estimated 

incidence of asthma-related emergency 

department visits is several times larger than 

the estimates of cardiovascular- and 

respiratory-related hospital admissions (U.S. 

EPA, 2011a, p. 2–47; U.S. EPA, 2010a, Tables 

E–118 to E–123 

(4) Substantial variability exists in the 

magnitude of risk remaining across urban 

study areas: Estimated risks remaining upon 

just meeting the current suite of standards 

vary substantially across study areas, even 

when considering risks normalized for 

differences in population size and baseline 

incidence rates. This variability is a 

consequence of the substantial differences in 

the annual mean PM2.5 concentrations across 

study areas that result from simulating just 

meeting the current standards. This is 

important because, as discussed above, 

annual mean concentrations are highly 

correlated with both long- and short-term 

exposure-related risk. The variability in 

annual mean PM2.5 concentrations occurred 

primarily in those study areas in which the 

24-hour standard was the generally 

controlling standard. In such areas, the 

variability in estimated risks across study 

areas was largest when regional patterns of 

reductions in PM2.5 concentrations were 

simulated, using the proportional rollback 

approach, as was done in the core analysis. 

Less variability was observed when more 

localized patterns of PM2.5 reductions were 

simulated using the locally-focused rollback 

approach, as was done in a sensitivity 

analysis. When simulations were done using 

the locally-focused rollback approach, 

estimated risks remaining upon just meeting 

the current suite of standards were 

appreciably larger than those estimated in the 

core analysis (U.S. EPA, 2011a, p. 2–46; U.S. 

EPA, 2010a, section 4.3.1.1). 

(5) Simulation of just meeting the current 

suite of standards results in annual mean 

PM2.5 concentrations well below the current 

standard for some study areas: In simulating 

just meeting the current suite of standards, 

the resulting composite monitor annual mean 

PM2.5 concentrations ranged from about 15 

mg/m3 (for those study areas in which the 

annual standard was controlling) down to as 

low as about 8 mg/m3 (for those study areas 

in which the 24-hour standard was the 

generally controlling standard or the annual 

mean concentration was well below 15 mg/m3

based on recent air quality) (U.S. EPA, 2011a, 

p. 2–46). 

Reductions in risk associated with 

simulating air quality to just meet 

alternative standard levels were also 

estimated in this review (U.S. EPA, 

2010a, sections 4.2.2, 5.2.2, and 5.2.3; 

U.S. EPA, 2011a, section 2.3.4.2). The 

estimated percent of risk reductions are 

depicted graphically in the Policy 

Assessment (US 2011a, Figures 2–11 

and 2–12), showing patterns of 

estimated risk reductions associated 

with alternative suites of standards.58

These figures also depict the level of 

confidence associated with the risk 

estimates generated for simulating just 

meeting the current standards as well as 

alternative standard levels considered. 

As would be expected, patterns of 

increasing estimated risk reductions are 

generally observed as either the annual 

or 24-hour standard, or both, are 

reduced over the ranges considered in 

the Risk Assessment. A number of the 

key observations regarding the 

magnitude of risk remaining upon 

simulation of just meeting the 

alternative suites of standards are 

analogous to the observations identified 

above for simulation of just meeting the 

current standards (U.S. EPA, 2011a, 

pp. 2–97 to 2–100). 

With regard to characterizing 

estimates of PM2.5-related risk 

associated with simulation of alternative 

standards, the Policy Assessment 

recognizes that greater overall 

confidence is associated with estimates 

of risk reduction than for estimates of 

absolute risk remaining (U.S. EPA, 

2011a, p. 2–94). Furthermore, the Policy 

Assessment recognizes 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 (U.S. EPA, 2011a, p. 2– 

97). In addition, the Policy Assessment 

observes that in considering the overall 

confidence associated with the 

quantitative analyses, the Risk 

Assessment recognizes that: (1) 

Substantial variability exists in the 

magnitude of risk remaining across 

urban study areas and (2) in general, 

higher confidence is associated with 

risk estimates based on PM2.5

concentrations near the mean PM2.5

concentrations in the underlying 

epidemiological studies providing the 

concentration-response functions. 

The variability in risk is a 

consequence of the substantial 

differences in the annual mean PM2.5

concentrations across urban study areas 

that result from simulating just meeting 

current or alternative standards. As 

PM2.5 concentrations decrease from the 

mean PM2.5 concentrations, the Risk 

Assessment concludes there is 

decreasing confidence in the risk 

estimates (U.S. EPA, 2010a, p. 5–16). As 

lower long-term mean PM2.5

concentrations are simulated (i.e., 

ambient concentrations further from 

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59Most of the alternative model specifications 

supported by the currently available scientific 

information produced risk estimates that are higher 

(by up to a factor of 2 to 3) than the core risk 

estimates (U.S. EPA, 2011a, pp. 2–40 and 2–41). 

recent air quality conditions), the 

potential variability in such factors as 

the spatial pattern of ambient PM2.5

reductions (i.e., rollback) increases, 

thereby introducing greater uncertainty 

into the simulation of composite 

monitor annual mean PM2.5

concentrations, and, consequently, in 

the risk estimates (U.S. EPA, 2010a, 

Appendix J). 

Based on consideration of the 

composite monitor annual mean PM2.5

concentrations involved in estimating 

long-term exposure-related mortality, 

the Risk Assessment has higher 

confidence in using those 

concentrations that generally fall well 

within the range of ambient PM2.5

concentrations considered in fitting the 

concentration-response functions used 

(i.e., within one standard deviation of 

the mean PM2.5 concentration reported 

in Krewski et al. (2009) for 1999–2000) 

as inputs to the risk model. For 

example, with the exception of one 

urban study area, those areas estimated 

to have risk reductions using alternative 

annual standard levels of 13 and 14 mg/ 

m3 had simulated composite monitor 

annual mean concentrations ranging 

from approximately 10.6 to 13.3 mg/m3. 

With lower alternative annual standard 

levels of 12 mg/m3 and 10 mg/m3, the 

composite monitor annual mean values 

ranged from approximately 9.0 to 11.4 

mg/m3 and 7.6 and 8.9 mg/m3, 

respectively. These concentrations are 

towards the lower end of the range of 

ACS data (in some cases approaching 

the lowest measured level) used in 

fitting the concentration-response 

functions, particularly for an annual 

standard level of 10 mg/m3, and, thus, 

the Policy Assessment concludes there 

is less confidence in the risk estimates 

associated with these levels compared 

with those for the higher alternative 

annual standard levels considered (U.S. 

EPA, 2011a, p. 2–99). Thus, while 

simulation of risks for an alternative 

annual standard level of 10 mg/m3

suggests that additional risk reductions 

could be expected with alternative 

annual standards below 12 mg/m3, the 

Policy Assessment recognizes that there 

is potentially greater uncertainty 

associated with these risk estimates 

compared with estimates generated for 

the higher alternative annual standard 

levels considered in the quantitative 

risk assessment, since these estimates 

required simulation of relatively greater 

reductions in ambient PM2.5

concentrations (U.S. EPA, 2011a, 

p. 2–98). 

The results of simulating alternative 

suites of PM2.5 standards including a 

combination of alternative annual and 

24-hour standard levels suggest 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 estimated risk 

reduction provided by the alternative 

24-hour standard is highly variable (U.S. 

EPA, 2010a, section 4.2.2). Thus, the 

Risk Assessment concludes more 

consistent reductions in estimated risk 

and consequently degrees of public 

health protection are estimated to result 

from simulating just meeting the 

alternative annual standard levels 

considered (U.S. EPA, 2010a, pp. 5–15 

to 5–16). Furthermore, the Policy 

Assessment concludes that the urban 

study areas with the greatest degree of 

estimated reduction associated with 

simulating just meeting alternative 24- 

hour standard levels of 30 and 25 mg/m3

also had the lowest estimated annual 

mean PM2.5 concentrations, and, 

therefore, there was substantially lower 

confidence in these risk estimates (U.S. 

EPA, 2011a, pp. 2–99 to 2–100). 

Based on the consideration of both the 

qualitative and quantitative assessments 

of uncertainty, the Risk Assessment 

concludes 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 Risk 

Assessment conclude 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 59 (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 recognizes that the 

currently available scientific 

information includes evidence for a 

broader range of health endpoints and 

at-risk populations beyond those 

included in the quantitative risk 

assessment, including lung function 

growth and respiratory symptoms in 

children and reproductive and 

developmental effects (U.S. EPA, 2011a, 

section 2.2.1). 

In considering the set of quantitative 

risk estimates and related uncertainties 

and limitations related to long- and 

short-term PM2.5 exposure discussed 

above together with consideration of the 

health endpoints which could not be 

quantified, the Policy Assessment 

concludes this information provides 

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 (U.S. EPA, 2011a, 

p. 2–47). Furthermore, while the 

alternative 24-hour standard levels 

considered (when controlling) did result 

in additional estimated risk reductions 

beyond those estimated for alternative 

annual standards alone, these additional 

estimated reductions are highly 

variable, in part due to different rollback 

approaches. Conversely, the Risk 

Assessment recognizes 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 

The initial issue to be addressed in 

the current review of the primary PM2.5

standards is whether, in view of the 

additional information now available, 

the existing standards should be 

retained or revised. In evaluating 

whether it is appropriate to retain or 

revise the current suite of standards, the 

Administrator considered the scientific 

information from the last review and the 

broader body of evidence and 

information now available. The 

Administrator has taken into account 

both evidence- and risk-based 

considerations in developing 

conclusions on the adequacy of the 

current primary PM2.5 standards. 

Evidence-based considerations (section 

III.D.1) include the assessment of 

epidemiological, toxicological, and 

controlled human exposure studies 

evaluating long- or short-term 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 policyrelevant considerations as discussed in 

section III.B above and in the Policy 

Assessment (U.S. EPA, 2011a, section 

2.2.1). The risk-based considerations 

(section III.D.2) 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). The 

advice received from CASAC is 

discussed in section III.D.3. Finally, the 

Administrator’s proposed conclusion on 

the adequacy of the current PM2.5

primary standards is provided in section 

III.D.4. 

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60The study periods referred to in the Policy 

Assessment (U.S. EPA, 2011a) and in this proposed 

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 status 

data that were included. 

61Aggregate mean concentration provided by 

study author (personal communication from Dr. 

Francine Laden, 2009). 

62Miller et al. (2007) studied postmenopausal 

women without previous cardiovascular disease in 

36 study areas from 1994 to 1998, with a median 

follow-up period of six years. The ambient PM2.5

monitor nearest to a study subject’s residence 

(within 30 miles or 48 kilometers) was identified 

and used to assign long-term mean PM2.5

concentrations to each subject. The annual average 

concentration in the year 2000 was the primary 

exposure measure because of the substantially 

increased network of monitors in that year, as 

compared with previous years. Miller et al. (2007) 

reported a long-term mean PM2.5 concentration 

across study areas of 13.5 mg/m3. This concentration 

was presented in the Integrated Science Assessment 

(U.S. EPA, 2009a, Figure 2–2, Table 7–8) and 

discussed in the second draft Policy Assessment 

(U.S. EPA, 2010f, Figure 2–4). In response to a 

request from the EPA for additional information on 

the air quality data used in selected epidemiological 

studies (Hassett-Sipple and Stanek, 2009), study 

investigators provided updated air quality data for 

the study period. The updated long-term mean 

PM2.5 concentration provided by the study authors 

was 12.9 mg/m3 (personal communication from 

Cynthia Curl, 2009; Stanek et al., 2010). The EPA 

notes that this updated long-term mean 

concentration matches the composite monitor 

approach annual mean calculated by staff for the 

year of air quality data (i.e., 2000) considered by the 

study investigators (Hassett-Sipple et al., 2010, 

Attachment A, p. 6). The updated air quality data 

for the Women’s Health Initiative study was 

presented and considered in the final Policy 

Assessment (U.S. EPA, 2011a, p. 2–32). The Policy 

Assessment notes that in comparison to other longterm exposure studies, the WHI study was more 

limited in that it was based on only one year of air 

quality data (U.S. EPA, 2011a, p. 2–82). 

63Zeger 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 mg/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). 

1. Evidence-Based Considerations in the 

Policy Assessment 

In light of the health evidence 

described above, specifically with 

regard to factors contributing to greater 

susceptibility to health effects 

associated with ambient PM2.5

exposures, the Policy Assessment 

considers the extent to which the 

currently available scientific evidence 

reports associations between fine 

particle exposures and health effects 

that extend to air quality concentrations 

that are lower than had previously been 

observed or that have been observed in 

areas that would likely meet the current 

suite of PM2.5 standards (U.S. EPA, 

2011a, section 2.2.1). As noted above, 

the Integrated Science Assessment 

concludes there is no evidence to 

support the existence of a discernible 

threshold below which effects would 

not occur (U.S. EPA, 2009a, section 

2.4.3). 

a. Associations With Long-term PM2.5

Exposures 

With regard to associations observed 

in long-term PM2.5 exposure studies, the 

Policy Assessment recognizes that 

extended follow-up analyses of the ACS 

and Harvard Six Cities studies provide 

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 mg/m3 across 50 metropolitan areas 

for 1979–1983 (Pope et al., 1995; 

Krewski et al., 2000).60 In extended 

analyses, positive and statistically 

significant effects of approximately 

similar magnitude were associated with 

declining PM2.5 concentrations, from an 

aggregate long-term mean in 58 

metropolitan areas of 21.2 mg/m3 in the 

original monitoring period (1979–1983) 

to 14.0 mg/m3 for 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 mg/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 mg/m3 for 

1980–1985 (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 mg/m3 for 

1980–1988 61 (Laden et al., 2006). In an 

additional analysis of the extended 

follow-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). 

New cohort studies provide 

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 greater in magnitude 

(U.S. EPA, 2011a, pp. 2–32 to 2–33). 

The WHI study reported positive and 

most often statistically significant 

associations between long-term PM2.5

exposure and cardiovascular-related 

mortality, with much larger relative risk 

estimates than in the ACS and Harvard 

Six Cities studies, as well as morbidity 

effects at an aggregate long-term mean 

PM2.5 concentration of 12.9 mg/m3 for 

2000 (Miller et al., 2007).62 Using the 

Medicare cohort, Eftim et al. (2008) 

reported somewhat higher effect 

estimates than in the ACS and Harvard 

Six Cities studies with aggregate longterm mean concentrations of 13.6 mg/m3

and 14.1 mg/m3, respectively, for 2000– 

2002. The MCAPS 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 mg/m3, 

although no association was reported for 

the western region with an aggregate 

long-term PM2.5 median concentration 

of 13.1 mg/m3 (U.S. EPA, 2009a, p. 7–88; 

Zeger et al., 2008).63 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 mg/m3 and 13.7 

mg/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 mg/m3 (Peters et al., 

1999). These results were found to be 

consistent with results of cross-sectional 

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 mg/m3. 

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 mg/m3 across 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 concludes evidence is 

suggestive of a causal relationship, the 

Policy Assessment notes a limited 

number of birth outcome studies that 

reported positive and statistically 

significant effects related to aggregate 

long-term mean PM2.5 concentrations 

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64Single-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). 

65This sub-analysis was not included in the 

original publication (Dominici et al., 2006a). 

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. 

down to approximately 12 mg/m3 (U.S. 

EPA, 2011a, p. 2–33). 

Collectively, the Policy Assessment 

concludes that currently available 

evidence provides support for 

associations between long-term PM2.5

exposure and mortality and morbidity 

effects that extend to air quality 

concentrations that are lower than had 

previously been observed, with 

aggregate long-term mean PM2.5

concentrations extending to well below 

the level of the current annual standard. 

These studies evaluated a broader range 

of health outcomes in the general 

population and in at-risk populations 

than were considered in the last review, 

and include extended follow-up for 

prospective epidemiological studies that 

were important in the last review as 

well as additional evidence in important 

new cohorts. 

b. Associations With Short-term PM2.5

Exposures 

In light of the mixed findings reported 

in single-city, short-term exposure 

studies, the Policy Assessment places 

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 recognizes that long-term 

mean concentrations reported in new 

multi-city U.S. and Canadian studies 

provide evidence of associations 

between short-term PM2.5 exposure and 

mortality at similar air quality 

distributions than had previously been 

observed in an 8-cities Canadian study 

(Burnett and Goldberg, 2003; aggregate 

long-term mean PM2.5 concentration of 

13.3 mg/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 shortterm PM2.5 exposure, in which the 

aggregate long-term mean PM2.5

concentration was 13.2 mg/m3 (U.S. 

EPA, 2009a, Figure 6–24). Furthermore, 

city-specific effect estimates indicate the 

association between short-term 

exposure to PM2.5 and total mortality 

and cardiovascular- and respiratoryrelated mortality is consistently positive 

for an overwhelming majority (99 

percent) of the 112 cities across a wide 

range of air quality concentrations (longterm mean concentrations ranging from 

6.6 mg/m3 to 24.7 mg/m3; U.S. EPA, 

2009a, Figure 6–24, p. 6–178 to 179). 

The EPA staff notes 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 mg/m3 to 18.7 mg/m3 and 24- 

hour PM2.5 city-mean 98th percentile 

concentrations ranging from 18.4 to 64.9 

mg/m3 (personal communication with 

Dr. Antonella Zanobetti, 2009).64

With regard to cardiovascular and 

respiratory morbidity effects, in the first 

analysis of the MCAPS 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 mg/m3. 

Furthermore, a sub-analysis restricted to 

days with 24-hour average 

concentrations of PM2.5 at or below 35 

mg/m3 indicated 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).65 In an extended 

analysis of the MCAPS study, 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 longterm mean PM2.5 concentration was 12.9 

mg/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 

mg/m3, not only indicate that effects are 

occurring in areas that would meet the 

current standards but also suggest 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 

concludes that the findings from shortterm PM2.5 exposure studies provide 

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 are 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 mg/m3 (Dominici, 2006b). 

In evaluating the currently available 

scientific evidence, as summarized in 

section III.B, the Policy Assessment first 

concludes that there is 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 addresses the 

question of whether the available 

evidence supports consideration of 

standards that are more protective than 

the current standards. In so doing, the 

Policy Assessment considers whether 

there is now evidence that health effect 

associations have been observed in areas 

that likely met the current suite of PM2.5

standards. As discussed above, longand short-term PM2.5 exposure studies 

provide 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 recognizes that 

important limitations and uncertainties 

associated with this expanded body of 

scientific evidence, noted above in 

section III.B.2, need 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 

concludes 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 shortterm exposures. Furthermore, the Policy 

Assessment concludes this evidence 

provides strong support for considering 

fine particle standards that would afford 

increased protection beyond that 

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66Based 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). 

67Premature 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). 

afforded by the current standards (U.S. 

EPA, 2011a, p. 2–35). 

2. Summary of Risk-Based 

Considerations in the Policy Assessment 

In addition to evidence-based 

consideration, the Policy Assessment 

also considers 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 notes 

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; 66

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 

concludes these risks are important 

from a public health standpoint (U.S. 

EPA, 2011a, p. 2–47). This conclusion 

reflects consideration of both the 

severity and the magnitude of the 

effects. For example, the risk assessment 

indicates the possibility that 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 

standards 67 (U.S. EPA, 2011a, pp. 2–46 

to 2–47). Moreover, additional risks are 

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 short-term 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 concludes that it is 

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 above provides 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). 

In considering the risks estimated to 

occur upon simulating just meeting the 

current PM2.5 standards, the Policy 

Assessment concludes that these 

estimated risks can reasonably be 

judged to be important from a public 

health perspective and provide strong 

support for consideration of alternative 

standards that would provide increased 

protection beyond that afforded by the 

current PM2.5 standards (U.S. EPA, 

2011a, p. 2–48). 

3. CASAC Advice 

CASAC, based on their review of 

drafts of the Integrated Science 

Assessment, the Risk Assessment, and 

the Policy Assessment, has 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 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 evidencebased 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). 

4. Administrator’s Proposed 

Conclusions Concerning the Adequacy 

of the Current Primary PM2.5 Standards 

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 staff 

conclusions and associated rationales 

presented in the Policy Assessment, 

views expressed by CASAC, and public 

comments. In particular, the 

Administrator recognizes that the 

Integrated Science Assessment 

concludes that the results of 

epidemiological and experimental 

studies form a plausible and coherent 

data set that supports a causal 

relationship between long- and shortterm PM2.5 exposures and mortality and 

cardiovascular effects, and a likely 

causal relationship between long- and 

short-term PM2.5 exposures and 

respiratory effects. Moreover, the 

Administrator reflects that these effects 

have 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. See 

American Trucking Associations v. 

EPA, 283 F. 3d at 369, 376 (revision of 

level of existing standards justified 

when effects are observed in areas that 

meet those standards). With regard to 

the results of the quantitative risk 

assessment, the Administrator notes that 

the Risk Assessment concludes that the 

risks estimated to remain upon 

simulation of just meeting the current 

standards are important from a public 

health standpoint in terms of both the 

severity and magnitude of the effects. 

Based on her consideration of these 

conclusions, as well as consideration of 

CASAC’s conclusion that the evidence 

and risk assessment clearly call into 

question the adequacy of the public 

health protection provided by the 

current PM2.5 NAAQS, the 

Administrator provisionally concludes 

that the current primary PM2.5

standards, taken together, are not 

requisite to protect public health with 

an adequate margin of safety and that 

revision is needed to provide increased 

public health protection. The 

Administrator provisionally concludes 

that the scientific evidence and 

information on risk provide strong 

support for consideration of alternative 

standards that would provide increased 

public health protection beyond that 

afforded by the current PM2.5 standards. 

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68Ultrafine particles, generally including 

particles with a mobility diameter less than or equal 

to 0.1 mm, are emitted directly to the atmosphere 

or are formed by nucleation of gaseous constituents 

in the atmosphere (U.S. EPA, 2009a, p. 3–3). 

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 sizebased 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 mm 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 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 concludes that it is 

appropriate to consider retaining a PM2.5

indicator to provide protection from 

effects associated with long- and shortterm fine particle exposures (U.S. EPA, 

2011, p. 2–50). 

The Policy Assessment also considers 

the expanded body of evidence 

available in this review to consider 

whether there is sufficient evidence to 

support a separate standard for ultrafine 

particles 68 or whether there is 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 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 concludes this 

evidence is still very limited and largely 

focused on exposure to diesel exhaust, 

for which the Integrated Science 

Assessment concludes it is unclear if 

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 notes 

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 recognizes 

that there are relatively few 

epidemiological studies that have 

examined potential cardiovascular and 

respiratory effects associated with shortterm exposures to ultrafine particles 

(U.S. EPA, 2011a, p. 2–51). These 

studies 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 concludes that the currently 

available evidence is suggestive of a 

causal relationship between short-term 

exposures to ultrafine particles and 

cardiovascular and respiratory effects. 

Furthermore, the Integrated Science 

Assessment concludes that evidence is 

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 recognizes a national 

characterization of concentrations, 

temporal and spatial patterns, and 

trends is not possible at this time, and 

the availability of ambient ultrafine 

measurements to support health studies 

is 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 

suggest 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 combustion engines 

and, therefore, roadways are a notable 

source of ultrafine particles, so 

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69Most 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). 

70To 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). 

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 

concludes that this information is 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 concludes 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, 

there are 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). Data from the CSN provide 

further evidence of spatial and seasonal 

variation in both PM2.5 mass 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-inthree or one-in-six day sampling 

schedule and, thus, do not capture data 

every day at most sites.70

The Policy Assessment recognizes 

that several new multi-city studies 

evaluating short-term exposures to fine 

particle constituents are now available. 

These studies continue to show an 

association between mortality and 

cardiovascular and/or respiratory 

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 long-term exposures to PM2.5

components (U.S. EPA, 2009a, section 

7.6.2). The Policy Assessment notes the 

most significant new evidence is 

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., 2006). Using health data from a 

cohort of U.S. military veterans and 

PM2.5 measurement data from the CSN, 

Lipfert et al. (2006) 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 associated with a range of 

health endpoints, the Integrated Science 

Assessment reports that the currently 

available evidence suggests associations 

between cardiovascular effects and a 

number of specific PM2.5–related source 

categories, specifically 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. These studies include a study 

that 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). There 

is less consistency in associations 

observed between sources of fine 

particles and respiratory health effects, 

which may be partially due to the fact 

that fewer studies have evaluated 

respiratory-related outcomes and 

measures. However, there is 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 to 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, DC and 

for cardiovascular-related morality 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 

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 

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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 concludes 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 

concludes 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 concludes 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 

concludes that 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 indicator 

(U.S. EPA, 2011a, p. 2–56). 

The CASAC 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 

proposes to retain PM2.5 as the indicator 

for fine particles. Further, the 

Administrator provisionally concludes 

that currently available scientific 

information does not provide a 

sufficient basis for supplementing massbased, 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. Furthermore, the 

Administrator also provisionally 

concludes that the currently available 

scientific information does 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. 

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 multiyear) 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 concludes 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

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 notes 

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 is much sparser than for 

cardiovascular effects and continues to 

be very limited. The Integrated Science 

Assessment concludes 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 

concludes that this information, when 

viewed as a whole, is too unclear, with 

respect to the indicator, averaging time 

and health 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 

provide 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 

observes that there are few studies 

presently available to deduce a general 

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71As discussed in section VIII.B.1 below, the EPA 

is proposing to revise several terms associated with 

PM2.5 monitor placement. Specifically, the EPA is 

proposing to revoke the term ‘‘communityoriented’’ and replace it with the term ‘‘area-wide’’ 

monitoring. 

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 concludes that there is no 

basis to consider a seasonal averaging 

time separate from a 24-hour averaging 

time. 

Based on the above considerations, 

the Policy Assessment concludes that 

the currently available information 

provides strong support for 

consideration of retaining current 

annual and 24-hour averaging timers but 

does not provide support for 

considering alternative averaging times 

(U.S. EPA, 2011a, p. 2–58). In addition, 

CASAC considers it appropriate to 

retain the current annual and 24-hour 

averaging times for the primary PM2.5

standards (Samet, 2010c, pp. 2 to 3). 

The Administrator concurs with the 

staff conclusions and CASAC advice 

and proposes 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 concludes, 

consistent with conclusions reached in 

the Policy Assessment and by CASAC, 

that the currently available information 

is 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. 

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, we consider 

whether currently available information 

supports consideration of alternative 

forms for the annual or 24-hour PM2.5

standards. 

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 areawide 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 (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 minority populations (71 

FR 61166/2, October 17, 2006; U.S. EPA, 

2005, section 5.3.6.1). 

In this review, as discussed in section 

III.B.3, 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 is 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, would provide appropriate public 

health protection. 

In considering the potential for 

disproportionate impacts on at-risk 

populations, the Policy Assessment 

recognizes an update of an air quality 

analysis conducted for the last review 

(U.S. EPA, 2011a, pp. 2–59 to 60; 

Schmidt, 2011a, Analysis A). This 

analysis focuses on determining if 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 

considers whether persons of lower 

socioeconomic status 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 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, 2011a, Analysis A). 

Recognizing the limitations of such 

cross-sectional analyses, the Policy 

Assessment observes 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 concludes 

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 

concludes that it is appropriate to 

consider revising the form of the annual 

PM2.5 standard such that it does 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 areawide air quality recording the highest 

PM2.5 concentrations 71 (U.S. EPA, 

2011a, p. 2–60). 

The CASAC agreed with staff 

conclusions that it is ‘‘reasonable’’ for 

the EPA to eliminate the spatial 

averaging provisions (Samet, 2010d, p. 

2). Further, in CASAC’s comments on 

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72As discussed in section VIII.B.2.b below, the 

EPA proposes that PM2.5 monitoring sites at microand middle-scale locations be comparable to the 

annual standard unless the monitoring site has been 

approved by the Regional Administrator as a 

‘‘relatively unique micro-scale, or localized hotspot, or unique middle-scale site.’’ 

73 In reaching this final decision, the EPA 

recognized a technical problem associated with a 

potential bias in the method used to calculate the 

98th percentile 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). 

74See 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. The context 

for the court’s discussion is identical to that here; 

whether to adopt a 98th percentile form for a 24- 

hour primary PM2.5 standard intended to provide 

supplemental protection for a generally controlling 

annual standard. 

75Throughout this section, the annual standard 

level is denoted as an integer value for simplicity, 

although, as noted above in section II.B.1, Table 1, 

the standard level is defined to one decimal place, 

such that the current standard level is 15.0 mg/m3. 

Alternative standard levels discussed in this section 

are similarly defined to one decimal place. 

the first draft Policy Assessment, they 

noted, ‘‘Given mounting evidence 

showing that persons with lower SES 

levels are a susceptible group for PMrelated 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 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 proposes to 

revise the form of the annual PM2.5

standard to eliminate the use of spatial 

averaging. Thus, the Administrator 

proposes revising the form of the annual 

PM2.5 standard to compare the level of 

the standard with measurements from 

each ‘‘appropriate’’ monitor in an area72

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. 

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 populationoriented 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 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.73

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 

considers air quality data reported in 

2000 to 2008 to update our 

understanding of the ratio between 

peak-to-mean PM2.5 concentrations. 

This analysis provides evidence that the 

98th percentile value is a more stable 

metric than the 99th percentile (U.S. 

EPA, 2011a, Figure 2–2, p. 2–62). 

The Agency recognizes 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.74 As 

in previous reviews, the EPA recognizes 

that a concentration-based form, 

compared to an exceedance-based form, 

is 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 concludes that a 

concentration-based form, when 

averaged over three years, provides 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 

proposes to retain the 98th percentile 

form of the 24-hour standard. The 

Administrator provisionally concludes 

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. 

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 

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 

exposures studies as the principal basis 

for selecting the level of the 24-hour 

PM2.5 standard that would protect 

against effects associated with shortterm exposures. As summarized in 

section III.A.2 above, the 2006 decision 

to retain the level of the annual PM2.5

standard at 15 mg/m3 75 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. Approach Used in the Policy 

Assessment 

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 uses an approach that 

integrates evidence-based and riskbased considerations, takes into account 

CASAC advice, and considers the issues 

raised by the court in remanding the 

primary annual PM2.5 standard. 

Following the general approach 

outlined in section III.A.3, for the 

reasons discussed below, the Policy 

Assessment concludes it is 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 is 

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. 

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76As discussed in section III.B.1 above, the Policy 

Assessment recognizes that single-city studies 

provide ancillary evidence to multi-city studies in 

support of calling into question the adequacy of the 

current suite of standards. However, in light of the 

mixed findings reported in single-city short-term 

PM2.5 exposure studies, and the likelihood that 

these results are influenced by localized events and 

not representative of air quality across the country, 

the Policy Assessment places comparatively greater 

weight on the results from multi-city studies in 

considering alternative annual and 24-hour 

standard levels (U.S. EPA, 2011a, p. 2–64). 

Beyond looking directly at the 

relevant epidemiologic evidence, the 

Policy Assessment considers the extent 

to which specific alternative PM2.5

standard levels are 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 are generally observed as 

either the annual or 24-hour standard, 

or both, are 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 

observes, as discussed in section III.A.3, 

that analyses conducted for this and 

previous reviews demonstrate 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 midrange, below the peak 24-hour 

concentrations (U.S. EPA, 2011a, p. 2– 

9). Furthermore, as discussed in section 

III.C.3, the Risk Assessment observes 

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 notes that while 

the results of simulating alternative 

suites of PM2.5 standards including 

different combinations of alternative 

annual and 24-hour standard levels 

suggest 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 

estimated risk reduction provided by 

alternative 24-hour standard levels is 

highly variable, in part due to the choice 

of rollback approached used (U.S. EPA, 

2010a, p. 5–17). 

Therefore, the Policy Assessment 

concludes, consistent with CASAC 

advice (Samet 2010c, p. 1), that it is 

appropriate to set a ‘‘generally 

controlling’’ annual standard that will 

lower a wide range of ambient 24-hour 

concentrations. The Policy Assessment 

concludes this approach would likely 

reduce aggregate risks associated with 

both long- and short-term 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 believes 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. 

The Policy Assessment recognizes 

that an annual standard intended to 

serve as the primary means for 

providing protection against effects 

associated with both long- and shortterm PM2.5 exposures cannot be 

expected to offer an adequate margin of 

safety against the effects of all shortterm PM2.5 exposures. As a result, in 

conjunction with a generally controlling 

annual standard, the Policy Assessment 

concludes it is 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 shorterthan-daily exposure periods. 

Based on the above considerations, 

the approach used in the Policy 

Assessment to identify alternative 

standard levels that are appropriate for 

consideration focuses on translating 

information from epidemiological 

studies into the basis for staff 

conclusions on levels. This approach is 

broader and more integrative than the 

general approach used by the EPA in 

previous reviews (see summary in 

section III.A.3 above; U.S. EPA, 2011a, 

sections 2.1.3 and 2.3.4.1) and reflects 

the more extensive and stronger body of 

scientific evidence now available on 

health effects related to long- and shortterm PM2.5 exposures, a more 

comprehensive quantitative risk 

assessment, and more extensive PM2.5

air quality data. In considering the 

currently available information, the 

Policy Assessment focuses on 

identifying levels for an annual standard 

and a 24-hour standard that, in 

combination, provide protection against 

health effects associated with both longand short-term PM2.5 exposures. The 

Policy Assessment also considers the 

extent to which various combinations of 

annual and 24-hour standards reflect 

setting a generally controlling annual 

standard with a 24-hour standard 

providing supplemental protection (U.S. 

EPA, 2011a, sections 2.1.3, 2.3.4.1). 

As discussed in the Policy 

Assessment, EPA staff recognizes that 

there is no single factor or criterion that 

comprises the ‘‘correct’’ approach for 

reaching conclusions on alternative 

standard levels for consideration, but 

rather there are various approaches that 

are reasonable to consider (U.S. EPA, 

2011a, section 2.3.4.1). In reaching 

conclusions in the Policy Assessment 

on the ranges of standard levels that are 

appropriate to consider, staff considered 

the relative weight to place on different 

evidence. The Policy Assessment 

initially focuses on long- and short-term 

PM2.5 exposure studies conducted in the 

U.S. and Canada and places the greatest 

weight on health outcomes judged in 

the Integrated Science Assessment as 

having evidence to support a causal or 

likely causal relationship. The Policy 

Assessment also considers the evidence 

for a broader range of health outcomes 

judged in the Integrated Science 

Assessment to have evidence suggestive 

of a causal relationship, specifically 

studies that focus on effects in 

susceptible populations, to evaluate 

whether this evidence provides support 

for considering lower alternative 

standard levels. 

Several factors were taken into 

account in placing relative weight on 

the body of available epidemiological 

studies, for example, study 

characteristics, including study design 

(e.g., time period of air quality 

monitoring, control for potential 

confounders); strength of the study (in 

terms of statistical significance and 

precision of results); and availability of 

population-level and air quality 

distribution data. As noted above in 

section III.A.3, the Policy Assessment 

places greatest weight on information 

from multi-city epidemiological studies 

to inform staff conclusions regarding 

alternative annual standard levels. 

These studies have a number of 

advantages compared to single-city 

studies 76 that include providing 

representation of ambient PM2.5

concentrations and potential health 

impacts across a range of diverse 

locations providing spatial coverage for 

different regions across the country, 

reflecting differences in PM2.5 sources, 

composition, and potentially other 

exposure-related factors which might 

impact PM2.5-related risks; lack of 

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77The EPA carefully analyzed the published 

evidence, but was unable to identify any short-term 

PM2.5 exposure studies that characterized 

confidence intervals around concentration-response 

relationships. Nor did CASAC or public comments 

on this issue, as addressed in their comments on the 

second draft Policy Assessment, identify any 

additional analyses. 

78While 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). 

79 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, 

Continued 

‘publication bias’ (U.S. EPA, 2004, p. 8– 

30); and consideration of larger study 

populations that afford the possibility of 

generalizing to the broader national 

population and provide higher 

statistical power than single-city studies 

to detect potentially statistically 

significant associations with relatively 

more precise effect estimates. 

In reaching conclusions in the Policy 

Assessment regarding alternative 24- 

hour standard levels that are 

appropriate to consider, staff also 

considers relevant information from 

single-city short-term PM2.5 exposure 

studies. Although, as discussed above, 

multi-city studies have greater power to 

detect associations and provide broader 

geographic coverage in comparison to 

single-city studies, the extent to which 

effects reported in multi-city short-term 

PM2.5 exposure studies are associated 

with the specific short-term air quality 

in any particular location is unclear, 

especially when considering short-term 

concentrations at the upper end of the 

air quality distribution (i.e., at the 98th 

percentile value) for a given study area. 

In contrast, single-city studies are more 

limited in terms of power and 

geographic coverage but the link 

between reported health effects and the 

air quality in a given study area is more 

straightforward. Therefore, the Policy 

Assessment considers the results of both 

multi-city and single-city short-term 

exposure studies to inform staff 

conclusions regarding alternative levels 

that are appropriate to consider for a 24- 

hour standard that is intended to 

provide supplemental protection in 

areas where the annual standard may 

not offer appropriate protection against 

the effects of all short-term exposures 

(U.S. EPA, 2011a, pp. 2–62 to 2–65). 

b. Consideration of the Annual Standard 

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 are appropriate to 

consider focuses on characterizing the 

range of PM2.5 concentrations over 

which we have the most confidence in 

the associations reported in the 

epidemiological studies, and conversely 

where our confidence in the association 

becomes appreciably lower. The most 

direct approach to address this issue, 

consistent with CASAC advice (Samet, 

2010c, p.10), is 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 three long-term PM2.5

exposure studies reporting confidence 

intervals around concentration-response 

functions (i.e., Schwartz et al., 2008; 

Pope et al., 2002; Miller et al., 2007; 

U.S. EPA, 2011a, pp. 2–65 to 2–70 and 

Figure 2–3).77 In its assessment of these 

studies, the Policy Assessment places 

greater weight on analyses that averaged 

across multiple concentration-response 

models since this approach represents a 

more robust examination of the 

underlying concentration-response 

relationship than analyses considering a 

single concentration-response model. 

Although these analyses of long-term 

exposure to PM2.5 provide information 

on the lack of any discernible 

population threshold, only Schwartz et 

al. (2008) conducted a multi-model 

analysis to characterize confidence 

intervals around the estimated 

concentration-response relationship that 

can help inform at what PM2.5

concentrations we have appreciably less 

confidence in the nature of the 

underlying concentration-response 

relationship. Although analyses of 

confidence intervals associated with 

concentration-response relationships 

can help inform consideration of 

alternative standard levels, the Policy 

Assessment concludes that the single 

relevant analysis now available is 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 explores other 

approaches that considered different 

statistical metrics to identify ranges of 

long-term mean PM2.5 concentrations 

that were most influential in generating 

health effect estimates in long- and 

short-term epidemiological studies, 

placing 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 statistical 

metric used in previous reviews. This 

approach recognizes 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

exposure studies. The long-term mean 

concentrations represent air quality data 

typically used in epidemiological 

analyses and provide a direct link 

between PM2.5 concentrations and the 

observed health effects. Further, these 

data are available for all long- and shortterm exposure studies analyzed and, 

therefore, represent the data set 

available for the broadest set of 

epidemiological studies. 

However, consistent with CASAC’s 

comments on the second draft Policy 

Assessment 78 (Samet, 2010d, p. 2), in 

preparing the final Policy Assessment, 

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 

range of PM2.5 concentrations down 

through which staff continued to have 

confidence in the associations observed 

in epidemiological studies and below 

which there is a comparative lack of 

data such that the staff’s confidence in 

the relationship was appreciably less. 

This would also be the range of PM2.5

concentrations which has the most 

influence on generating the health effect 

estimates reported in epidemiological 

studies. As discussed in section III.A.3 

above, the Policy Assessment recognizes 

there is no one percentile value within 

a given distribution that is the most 

appropriate or ‘‘correct’’ way to 

characterize where our confidence in 

the associations becomes appreciably 

lower. The Policy Assessment 

concludes that focusing on 

concentrations within the lower quartile 

of a distribution, such as the range from 

the 25th to the 10th percentile, is 

reasonable to consider as a region 

within which we begin to have 

appreciably less confidence in the 

associations observed in 

epidemiological studies.79 In staff’s 

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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). 

80Additional 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). 

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 recognizes that 

there are 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 is 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 distribution of PM2.5

concentrations (i.e., study population 

data), as an appropriate surrogate for 

health event data. 

In applying this approach, the Policy 

Assessment focuses on identifying the 

broader range 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, 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). For 

the three studies for which both health 

event and study population data were 

available, EPA staff analyzed the 

reliability of using study population 

data as a surrogate for health event data. 

Based on these analyses, 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 

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 mg/m3.80 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 atrisk population, as well as 

developmental effects (evidence 

suggestive of a causal relationship). By 

following the general approach used in 

previous PM NAAQS reviews, one 

could consider identifying alternative 

standard levels that are somewhat below 

the long-term mean PM2.5

concentrations reported in these 

epidemiological studies. 

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81Health event data (e.g., number of deaths, 

hospitalizations) occurring in a study population 

were obtained 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. 

With regard to consideration of 

additional information from 

epidemiological studies which is 

relevant to the second approach, EPA 

has 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 

available 81 (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. As 

noted above in section III.A.3 and 

discussed in the Policy Assessment, a 

policy approach that uses data based on 

composite monitor distributions to 

identify alternative standard levels, and 

then compares those levels to 

concentrations at appropriate maximum 

monitors to determine if an area meets 

a given standard, inherently has the 

potential to build in some margin of 

safety (U.S. EPA, 2011a, p. 2–14). In 

analyses conducted by EPA staff based 

on selected long- and short-term 

exposure studies, the Policy Assessment 

notes that the differences between the 

maximum and composite distributions 

were greater for studies with fewer years 

of air quality data (i.e., 1 to 3 years) and 

smaller numbers of study areas (i.e., 36 

to 51 study areas). The differences in the 

maximum and composite monitor 

distribution were much smaller (i.e., 

generally within five percent) for 

studies with more years of air quality 

data (i.e., up to 6 years) and larger 

numbers of study areas (i.e., 112 to 204 

study areas) (Hassett-Sipple et al., 2010; 

U.S. EPA, 2010f, section 2.3.4.1). 

Therefore, any margin of safety that may 

be provided by a policy approach that 

uses data based on composite monitor 

distributions to identify alternative 

standard levels, and then compares 

those levels to concentrations at 

appropriate maximum monitors to 

determine if an area meets a given 

standard, will vary depending upon the 

number of monitors and air quality 

distributions within a given area. See 

also, section III.A.3 above. 

Figure 4 summarizes statistical 

metrics for those studies included in 

Figures 1, 2, and 3 that provide 

evidence of statistically significant 

PM2.5-related effects, which are relevant 

to the two approaches for translating 

epidemiological evidence into standard 

levels discussed above. The top of 

Figure 4 includes information for longterm exposure studies evaluating health 

outcomes classified as having evidence 

of a casual or likely casual relationship 

with PM2.5 exposures (long-term mean 

PM2.5 concentrations indicated by 

diamond symbols). The middle of 

Figure 4 includes information for shortterm exposure studies evaluating health 

outcomes classified as having evidence 

of a casual or likely casual 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 

(long-term 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. 

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82As discussed in section III.D.1.a above, the 

lowest long-term mean PM2.5 concentration 

reported in the long-term exposure studies was 

based on updated air quality data for Miller et al. 

(2007). As noted in the Policy Assessment, these air 

quality data were based on only one year of ambient 

measurements (2000) and in comparison to other 

long-term exposure studies that considered 

multiple years of air quality data, were much more 

limited (U.S. EPA, 2011a, pp. 2–81 to 2–82). 

83As noted in section 7.4 of the Integrated 

Science Assessment, Parker et al. (2005) reported 

that over a 9-month exposure period (mean PM2.5

concentration of 15.4 mg/m3) a significant decrease 

in birth weight was associated with infants in the 

highest quartile of PM2.5 exposure as compared to 

infants exposed in the lowest quartile. 

In looking first at the long-term mean 

PM2.5 concentrations reported in the 

multi-city long-term exposure studies, 

as summarized at the top of Figure 4, 

the Policy Assessment observes positive 

and often statistically significant 

associations at long-term mean PM2.5

concentrations ranging from 16.4 to 12.9 

mg/m3 82 (Laden et al., 2006; Lipfert et 

al., 2006; Krewski et al., 2009; Goss et 

al., 2004; Miller et al.; 2007; Zeger et al., 

2008; Eftim et al., 2008; Dockery et al., 

1996; McConnell et al., 2003). In 

considering the one long-term PM2.5

exposure study for which health event 

data are available (Krewski et al., 2009), 

the Policy Assessment observes that the 

long-term mean PM2.5 concentrations 

corresponding with study areas 

contributing to the 25th and 10th 

percentiles of the distribution of 

mortality data are 12.0 mg/m3 and 10.2 

mg/m3, respectively (Figure 4; U.S. EPA, 

2011a, Figure 2–7; Rajan et al., 2011, 

Table 1). As identified above, although 

less directly relevant than event data, 

the number of participants within each 

study area can be used as a surrogate for 

health event data in relation to the 

distribution of PM2.5 concentrations. 

The long-term mean PM2.5

concentrations corresponding with 

study areas contributing to the 25th and 

10th percentiles of the distribution of 

study participants for Miller et al. (2007) 

were 11.2 mg/m3 and 9.7 mg/m3, 

respectively (Figure 4; U.S. EPA, 2011a, 

Figure 2–7; Rajan et al., 2011, Table 1). 

In then considering information from 

multi-city, short-term exposure studies 

reporting positive and statistically 

significant associations with these same 

broad health effect categories, as 

summarized in the middle of Figure 4, 

the Policy Assessment observes positive 

and statistically significant associations 

at long-term mean PM2.5 concentrations 

in a similar range of 15.6 to 12.8 mg/m3

(Franklin et al., 2007, 2008; Klemm and 

Mason, 2003; Burnett and Goldberg, 

2003; Zanobetti and Schwartz, 2009; 

Burnett et al., 2004; Bell et al., 2008; 

Dominici et al., 2006a; see Figure 3). In 

considering the two multi-city, shortterm PM2.5 exposure studies for which 

health event data are available, the 

Policy Assessment observes 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 are 12.5 mg/m3 and 10.3 

mg/m3, respectively, for Zanobetti and 

Schwartz (2009), and 11.5 mg/m3 and 9.8 

mg/m3, respectively, for Bell et al. (2008) 

(Figure 4; U.S. EPA, 2011a, Figure 2–7; 

Rajan et al., 2011, Table 1). 

Taking into consideration additional 

studies of specific at-risk populations 

(i.e., children), the Policy Assessment 

expands its evaluation of the long-term 

exposure studies to include a broader 

range of health outcomes judged in the 

Integrated Science Assessment to have 

evidence suggestive of a causal 

relationship. This evidence was taken 

into account to evaluate whether it 

provides support for considering lower 

alternative levels than if weight were 

only placed on studies for which health 

effects have been judged in the 

Integrated Science Assessment to have 

evidence supporting a causal or likely 

causal relationship. The Policy 

Assessment makes note of a limited 

number of studies that provide emerging 

evidence for PM2.5-related low birth 

weight and infant mortality, especially 

related to respiratory causes during the 

post-neonatal period. This more limited 

body of evidence, as summarized at the 

bottom of Figure 4, indicates positive 

and often statistically significant effects 

associated with long-term PM2.5 mean 

concentrations in the range of 14.9 to 

11.9 mg/m3 (Woodruff et al., 2008; Liu 

et al., 2007; Bell et al., 2007; see Figure 

2). As illustrated in Figure 2, although 

Parker and Woodruff (2008) did not 

observe an association between 

quarterly estimates of exposure to PM2.5

and low birth weight in a multi-city U.S. 

study, other U.S. and Canadian studies 

did report positive and statistically 

significant associations between PM2.5

and low birth weight at lower ambient 

concentrations (Bell et al., 2007; Liu et 

al., 2007).83 There remain significant 

limitations (e.g., identifying the 

etiologically relevant time period) in the 

evaluation of evidence on the 

relationship between PM2.5 exposures 

and birth outcomes (U.S. EPA, 2009a, 

pp. 7–48 and 7–56) which should be 

taken into consideration in reaching 

judgments about how to weigh these 

studies of potential impacts on specific 

susceptible populations in considering 

alternative standard levels that provide 

protection with an appropriate margin 

of safety. 

With respect to carcinogenicity, 

mutagenicity, and genotoxicity 

(evidence suggestive of a causal 

relationship), the strongest evidence 

currently available is from long-term 

prospective cohort studies that report 

positive associations between PM2.5 and 

lung cancer mortality. At this time, 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, in selecting 

alternative standard levels appropriate 

to consider, the Policy Assessment 

noted that, in providing protection 

against mortality and cardiovascular 

and respiratory effects it is reasonable to 

anticipate that protection will also be 

provided for carcinogenicity, 

mutagenicity, and genotoxicity effects 

(U.S. EPA, 2011a, p. 2–78). 

In summarizing the currently 

available evidence and air quality 

information within the context of 

identifying potential alternative annual 

standard levels for consideration, the 

Policy Assessment first notes that the 

Integrated Science Assessment 

concludes there is no evidence of a 

discernible population threshold below 

which effects would not occur. Thus, 

health effects may occur over the full 

range of concentrations observed in the 

epidemiological studies. In the absence 

of any discernible thresholds, the 

general approach used in the Policy 

Assessment for identifying alternative 

standard levels that would provide 

appropriate protection against effects 

observed in epidemiological studies has 

focused on the central question of 

identifying the range of PM2.5

concentrations below the long-term 

mean concentrations where we continue 

to have confidence in the associations 

observed in epidemiological studies. 

In considering the evidence, the 

Policy Assessment recognizes 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, 

considerations of different alternative 

standard levels could be supported. 

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84As outlined in section III.A.3, the Policy 

Assessment considers the 25th percentile to be the 

start of the range of PM2.5 concentrations below the 

mean within which the data become appreciably 

more sparse and, thus, where our confidence in the 

associations observed in epidemiological studies 

begins to become appreciably less. 

85As discussed in section III.A.3, the Policy 

Assessment identifies the range from the 25th to the 

10th percentiles as a reasonable range to consider, 

in that it is a range where we have appreciably less 

confidence in the associations observed in 

epidemiological studies (U.S. EPA, 2011a, p. 2–12). 

Given the currently available 

evidence and considering the various 

approaches discussed above, the Policy 

Assessment concludes it is appropriate 

to focus on an annual standard level 

within a range of about 12 to 11 mg/m3

(U.S. EPA, 2011a, pp. 2–82, 2–101, and 

2–106). As illustrated in Figure 4, a 

standard level of 12 mg/m3, at the upper 

end of this range, is somewhat below 

the long-term mean PM2.5

concentrations reported in all the multicity, long- and short-term exposure 

studies that provide 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 mg/m3 would 

reflect consideration of additional 

population-level information from such 

epidemiological studies in that it 

generally corresponds with 

approximately the 25th percentile of the 

available distributions of health events 

data in the studies for which 

population-level information was 

available.84 In addition, a level of 12 mg/ 

m3 would reflect some consideration of 

studies that provide more limited 

evidence of reproductive and 

developmental effects, which are 

suggestive of a causal relationship, in 

that it is about at the same level as the 

lowest long-term mean PM2.5

concentrations reported in such studies 

(see Figure 4). 

Alternatively, an annual standard 

level of 11 mg/m3, at the lower end of 

this range, is 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 classified as having evidence of 

a causal or likely causal relationship. A 

level of 11 mg/m3 would reflect placing 

more weight on the distributions of 

health event and population data, in 

that this level is within the range of 

PM2.5 concentrations corresponding to 

the 25th and 10th percentiles of all the 

available distributions of such data.85 In 

addition, a level of 11 mg/m3 is 

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 mg/m3

would reflect an approach to translating 

the available evidence that places 

relatively more emphasis on margin of 

safety considerations 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 

concludes that the available evidence 

does not provide a sufficient basis to 

consider alternative annual standard 

levels below 11 mg/m3 (U.S. EPA, 2011a, 

p. 2–81). 

The Policy Assessment also considers 

the extent to which the available 

evidence provides a basis for 

considering alternative annual standard 

levels above 12 mg/m3. As discussed 

below, the Policy Assessment concludes 

that it could be reasonable to consider 

a standard level up to 13 mg/m3 based 

on a policy approach that tends to 

weigh 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 is placed on 

long-term exposure studies as a basis for 

an annual standard level. A level of 13 

mg/m3 is somewhat below the long-term 

mean PM2.5 concentrations reported in 

all but one of the 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 mg/m3 is the 

WHI study (Miller et al., 2007). As noted 

in section III.D.1.a above, the Policy 

Assessment observes that in comparison 

to other long-term exposure studies, the 

WHI study was more limited in that it 

was based on only one year of air 

quality data (U.S. EPA, 2011a, pp. 2–81 

to 2–82). Thus, to the extent that less 

weight is placed on the WHI study than 

on other long-term exposure studies 

with more robust air quality data, a level 

of 13 mg/m3 could 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, the Policy Assessment notes 

that a level of 13 mg/m3 is below the 

long-term mean PM2.5 concentrations 

reported in most such studies, but is 

above the long-term means of 12.8 and 

12.9 mg/m3 reported in Burnett et al. 

(2004) and Bell et al. (2008), 

respectively. In considering these 

studies, the Policy Assessment finds no 

basis to conclude that these two studies 

are 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 

concludes that consideration of an 

annual standard level of 13 mg/m3

would have implications for the degree 

of protection that would need to be 

provided by the 24-hour standard, such 

that taken together the suite of PM2.5

standards would provide 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 notes that 

a standard level of 13 mg/m3 would 

reflect a judgment that the uncertainties 

in the epidemiological evidence as 

summarized in section III.B.2 above, 

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 is too uncertain to support 

consideration of any lower level. 

Beyond evidence-based 

considerations, the Policy Assessment 

also considered the extent to which 

quantitative risk assessment supports 

consideration of these alternative 

standard levels or provides support for 

lower levels. In considering simulations 

of just meeting alternative annual 

standard levels within the range of 13 to 

11 mg/m3 (in conjunction with the 

current 24-hour standard level of 35 mg/ 

m3), the Policy Assessment concluded 

that important public health 

improvements are associated with risk 

reductions estimated for standard levels 

of 13 and 12 mg/m3, noting that the level 

of 11 mg/m3 was not included in the 

quantitative risk assessment. The Policy 

Assessment noted that the overall 

confidence in the quantitative risk 

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estimates varied for the different 

alternative standard levels evaluated 

and was stronger for the higher levels 

and substantially lower for the lowest 

level evaluated (i.e., 10 mg/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 mg/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 

concludes that consideration of 

alternative annual standard levels in the 

range of 13 to 11 mg/m3 may be 

appropriate. Furthermore, the Policy 

Assessment concludes that the currently 

available evidence most strongly 

supports consideration of an alternative 

annual standard level in the range of 12 

to 11 mg/m3 (U.S. EPA, 2011a, p. 2–82). 

The Policy Assessment concludes that 

an alternative level within the range of 

12 to 11 mg/m3 would more fully take 

into consideration the available 

information from all long- and shortterm PM2.5 exposure studies, including 

studies of at-risk populations, than 

would a higher level. This range would 

also reflect placing weight on 

information from studies that help 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 is 

appreciably less at lower 

concentrations. 

c. Consideration of the 24-Hour 

Standard in the Policy Assessment 

As recognized in section III.A.3 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 strong local or seasonal 

sources, or PM2.5-related effects that 

may be associated with shorter-thandaily exposure periods. As a result, the 

Policy Assessment concludes that it is 

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 considers 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 short-term 

fine particle exposures. In considering 

the available epidemiological evidence, 

the Policy Assessment takes into 

account information from multi-city 

studies as well as single-city studies. 

The Policy Assessment considers 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 considers information 

on the long-term mean PM2.5

concentrations from these studies. 

In addition to considering the 

epidemiological evidence, the Policy 

Assessment also considers 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 help 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 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 

recognizes that to the extent these 

studies were conducted in areas that 

likely did not meet one or both of the 

current standards, such studies do 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 places 

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 shortterm PM2.5 exposures, as summarized in 

Figure 3, the Policy Assessment 

observes 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 mg/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 Policy Assessment notes 

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 mg/m3

(Burnett et al., 2004; Burnett and 

Goldberg, 2003; Franklin et al., 2008). In 

the three additional studies with 98th 

percentile values below 35 mg/m3, 

specifically 98th percentile 

concentrations of 34.2, 34.3, and 34.8 

mg/m3, the Policy Assessment notes that 

these studies reported long-term mean 

PM2.5 concentrations of 12.9, 13.2, and 

13.4 mg/m3, respectively (Bell et al., 

2008; Zanobetti and Schwartz, 2009; 

Dominici et al., 2006a). To the extent 

that consideration is given to revising 

the level of the annual standard, as 

discussed above in section III.E.4.b, the 

Policy Assessment recognizes that 

potential changes associated with 

meeting such an alternative annual 

standard would result in lowering risks 

associated with both long- and shortterm PM2.5 exposures. Consequently, in 

considering a 24-hour standard that 

would work in conjunction with an 

annual standard to provide appropriate 

public health protection, the Policy 

Assessment notes that to the extent that 

the level of the annual standard is 

revised to within a range of 13 to 11 mg/ 

m3, in particular in the range of 12 to 

11 mg/m3, additional protection would 

be provided for the effects observed in 

these multi-city studies (U.S. EPA, 

2011a, p. 2–84). 

In summary, the Policy Assessment 

concludes that the multi-city, short-term 

exposure studies generally provide 

support for retaining the 24-hour 

standard level at 35 mg/m3 in 

conjunction with an annual standard 

level revised to within a range of 12 to 

11 mg/m3 (U.S. EPA, 2011a, p. 2–84). 

Alternatively, in conjunction with an 

annual standard level of 13 mg/m3, the 

Policy Assessment concludes that the 

multi-city studies provide limited 

support for revising the 24-hour 

standard level somewhat below 35 mg/ 

m3, such as down to 30 mg/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 in conjunction 

with an overall long-term mean 

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concentration slightly less than 13 mg/ 

m3 (Figure 3; U.S. EPA, 2011a, p. 2–84). 

In reaching staff conclusions 

regarding alternative 24-hour standard 

levels that are appropriate to consider, 

the Policy Assessment also takes into 

account relevant information from 

single-city studies that evaluated effects 

associated with short-term PM2.5

exposures. The Policy Assessment 

recognizes that these studies may 

provide additional insights regarding 

impacts on susceptible populations and/ 

or on areas with isolated peak 

concentrations. Although, as discussed 

in section III.E.4.a above, multi-city 

studies have advantages over single-city 

studies in terms of statistical power to 

detect associations and broader 

geographic coverage as well as other 

factors such as less likelihood of 

publication bias, reflecting differences 

in PM2.5 sources, composition, and 

potentially other factors that could 

impact PM2.5-related effects, multi-city 

studies often present overall effect 

estimates rather than single-city effect 

estimates. Since short-term air quality 

can vary considerably across cities, the 

extent to which effects reported in 

multi-city studies are associated with 

short-term air quality in any particular 

location is uncertain, especially when 

considering short-term concentrations at 

the upper end of the distribution of 

daily PM2.5 concentrations (i.e., at the 

98th percentile value). In contrast, 

single-city studies are more limited in 

terms of power and geographic coverage 

but the link between reported health 

effects and the air quality in a given 

study area is more straightforward to 

establish. Therefore, the Policy 

Assessment also considers evidence 

from single-city, short-term exposure 

studies to inform staff conclusions 

regarding alternative levels that are 

appropriate to consider for a 24-hour 

standard that is intended to provide 

supplemental protection in areas where 

the annual standard may not provide an 

adequate margin of safety against the 

effects of all short-term PM2.5 exposures. 

As discussed above for the multi-city 

studies, the Policy Assessment takes 

into account both the 24-hour PM2.5

concentrations in the single-city studies, 

focusing on the 98th percentile air 

quality values, as well as the long-term 

mean PM2.5 concentrations. The Policy 

Assessment considers single-city studies 

conducted in areas that would likely 

have met the current suite of PM2.5

standards as most useful for informing 

staff conclusions related to the level of 

the 24-hour standard (U.S. EPA, 2011a, 

Figure 2–9). The Policy Assessment 

notes that additional single-city studies 

summarized in that Figure 2–9 were 

conducted in areas that would likely 

have met one but not both of the current 

PM2.5 standards. To the extent changes 

in air quality designed to just meet the 

current suite of PM2.5 standards are 

undertaken, one could reasonably 

anticipate additional public health 

protection will occur in these study 

areas. Therefore, the Policy Assessment 

concludes that these studies are not 

helpful to inform staff conclusions 

regarding alternative standard levels 

that are appropriate to consider (U.S. 

EPA, 2011a, p. 2–87). 

With regard to single-city studies that 

were conducted in areas that would 

likely have met both the current 24-hour 

and annual standards, the Policy 

Assessment first considers studies that 

reported positive and statistically 

significant associations. In considering 

this group of studies, the Policy 

Assessment notes Mar et al. (2003) 

reported a positive and statistically 

significant association for premature 

mortality in Phoenix with a long-term 

mean concentration of 13.5 mg/m3 in 

conjunction with a 98th percentile value 

of 32.2 mg/m3 (U.S. EPA, 2011a, Figure 

2–9). To the extent that consideration is 

given to revising the level of the annual 

standard, within a range of 13 to 11 mg/ 

m3, as discussed above, additional 

protection would be provided for the 

effects observed in this study (U.S. EPA, 

2011a, p. 2–87). 

Four additional studies reported 

positive and statistically significant 

associations with 98th percentile values 

within a range of 31.2 to 25.8 mg/m3 and 

long-term mean concentrations within a 

range of 12.1 to 8.5 mg/m3 (Delfino et al., 

1997; Peters et al., 2001; Stieb et al., 

2000; and Mar et al., 2004; U.S. EPA, 

2011a, Figure 2–9). Delfino et al. (1997) 

reported statistically significant 

associations between PM2.5 and 

respiratory emergency department visits 

for older adults (greater than 64 years 

old) but not young children (less than 2 

years old), in one part of the study 

period (summer 1993) but not the other 

(summer 1992). Peters et al. (2001) 

reported a positive and statistically 

significant association between shortterm exposure to PM2.5 (2-hour and 24- 

hour averaging times) and onset of acute 

myocardial infarction in Boston. Stieb et 

al. (2000) reported positive and 

statistically significant associations with 

cardiovascular- and respiratory-related 

emergency department visits in Saint 

John, Canada, in single pollutant models 

but not in multi-pollutant models (U.S. 

EPA, 2004, pp. 8–154 and 8–252 to 8– 

253). Mar et al. (2004) reported a 

positive and statistically significant 

association for short-term PM2.5

exposures in relation to respiratory 

symptoms among children but not 

adults in Spokane, however, this study 

had very limited statistical power 

because of the small number of children 

and adults evaluated. 

The Policy Assessment also considers 

short-term single-city PM2.5 exposure 

studies that reported positive but 

nonstatistically significant associations 

for cardiovascular and respiratory 

endpoints in areas that would likely 

have met both the current 24-hour and 

annual standards. The 98th percentile 

values reported in these studies ranged 

from 31.6 to 17.2 mg/m3 and the longterm mean concentrations ranged from 

13.0 to 7.0 mg/m3 (U.S. EPA, 2011a, 

Figure 2–9). These studies included 

consideration of cardiovascular-related 

mortality effects in Phoenix (Wilson et 

al., 2007), asthma medication use in 

children in Denver (Rabinovitch et al., 

2006), hospital admissions for 

hemorrhagic and ischemic stroke in 

Edmonton, Canada (Villeneuve et al., 

2006), and hospital admissions for 

ischemic stroke/transient ischemic 

attack in Nueces County, TX (Lisabeth 

et al., 2008). 

Lastly, the Policy Assessment 

considers single-city studies conducted 

in areas that would likely have met both 

the current 24-hour and annual 

standards that reported null findings. 

The 98th percentile values reported in 

these studies ranged from 29.6 to 24.0 

mg/m3 and the long-term mean 

concentrations ranged from 10.8 to 8.5 

mg/m3 (U.S. EPA, 2011a, Figure 2–9). 

These studies reported no associations 

with short-term PM2.5 exposures and 

cardiovascular-related hospital 

admissions and respiratory-related 

emergency department visits (Slaughter 

et al., 2005) and cardiovascular-related 

emergency department visits (Schreuder 

et al., 2006) in Spokane; asthma 

exacerbation in children in Denver 

(Rabinovitch et al., 2004); and hospital 

admissions for transient ischemic attack 

in Edmonton, Canada (Villeneuve et al., 

2006). 

Viewing the evidence as a whole, the 

Policy Assessment observes a 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. Many of these studies 

had significant limitations (e.g., limited 

statistical power, limited exposure data) 

or equivocal results (i.e., mixed results 

within the same study area) as briefly 

identified above and discussed in more 

detail in the Policy Assessment (U.S. 

EPA, 2011a, p. 2–88). Other studies 

reported positive but not statistically 

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significant results or null associations 

also in areas that would likely have met 

the current suite of PM2.5 standards. 

Overall, the entire body of results from 

these single-city studies is mixed, 

particularly as 24-hour 98th percentile 

concentrations go below 35 mg/m3. 

Although a number of single-city 

studies report effects at appreciably 

lower PM2.5 concentrations than multicity short-term exposure studies, the 

uncertainties and limitations associated 

with the single-city studies were greater 

and, thus, the Policy Assessment 

concludes there is less confidence in 

using these studies as a basis for setting 

the level of a standard. Therefore, the 

Policy Assessment concludes that the 

multi-city short-term exposure studies 

provide the strongest evidence to inform 

decisions on the level of the 24-hour 

standard, and the single-city studies do 

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 takes into account air 

quality information based on countylevel 24-hour and annual design values 

to understand the implications of the 

alternative standard levels supported by 

the currently available scientific 

evidence, as discussed in section 

III.E.4.b above. As discussed in section 

III.A.3 above, the Policy Assessment 

concludes that a policy goal which 

includes 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, is the most 

effective and efficient way to reduce 

total population risk associated with 

both long- and 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. 

Therefore, the Policy Assessment 

considers the extent to which different 

combinations of alternative annual and 

24-hour standard levels based on the 

evidence would support this policy goal 

(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 examines 

the implications of three alternative 

suites of PM2.5 standards identified as 

appropriate to consider based on the 

currently available scientific evidence, 

as discussed above. The Policy 

Assessment concludes that an 

alternative suite of PM2.5 standards that 

would include an annual standard level 

of 11 or 12 mg/m3 and a 24-hour 

standard with a level of 35 mg/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 

represent 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 mg/m3 with a 24- 

hour standard of 30 mg/m3 would result 

in many more areas across the country 

in which the 24-hour standard would 

likely become the controlling standard 

than if an alternative annual standard of 

12 or 11 mg/m3 were paired with the 

current level of the 24-hour standard 

(i.e., 35 mg/m3). 

The Policy Assessment concludes that 

consideration of retaining the 24-hour 

standard level at 35 mg/m3 would reflect 

placing greatest weight on evidence 

from multi-city 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 mg/ 

m3, this alternative would recognize 

additional public health protection 

against effects associated with shortterm 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). 

The Policy Assessment also 

recognizes an alternative approach to 

considering the evidence that provides 

some support for revising the level 

below 35 mg/m3, perhaps as low as 30 

mg/m3 (U.S. EPA, 2011a, p. 2–92). This 

alternative 24-hour standard level 

would be more compatible with an 

alternative annual standard of 13 mg/m3

based on placing greater weight on one 

multi-city short-term exposure study 

(Bell et al., 2008) that reported positive 

and statistically significant effects at a 

98th percentile value less than 35 mg/m3

(i.e., 34.2 mg/m3) in conjunction with a 

long-term mean concentration less than 

13 mg/m3 (i.e., 12.9 mg/m3). 

Beyond evidence-based 

considerations, the Policy Assessment 

also considered the extent to which the 

quantitative risk assessment supports 

consideration of retaining the current 

24-hour standard level or provides 

support for lower standard levels. In 

considering simulations of just meeting 

the current 24-hour standard level of 35 

mg/m3 or alternative levels of 30 or 25 

mg/m3 (in conjunction with alternative 

annual standard levels within a range of 

13 to 11 mg/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 mg/m3). Based on this 

information, the Policy Assessment 

concluded that the quantitative risk 

assessment provides support for 

considering a 24-hour standard level of 

35 or 30 mg/m3 (in conjunction with an 

alternative standard level within a range 

of 13 to 11 mg/m3) but does 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 

concludes that while it is appropriate to 

consider an alternative 24-hour standard 

level within a range of 35 to 30 mg/m3, 

the currently available evidence most 

strongly supports consideration for 

retaining the current 24-hour standard 

level at 35 mg/m3 in conjunction with 

lowering the level of the annual 

standard within a range of 12 to 11 mg/ 

m3 (U.S. EPA, 2011a, p. 2–92). 

d. 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 

appropriate to return to the strategy 

used in 1997 that considers the annual 

and the short-term 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 mg/m3 and alternative 24-hour 

standard levels within a range of 35 to 

30 mg/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 

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‘‘[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 are 

appropriate to consider, including 

consideration of the combination of 

these standards supported by the 

available information (Samet, 2010d, p. 

ii). Specifically, CASAC encouraged 

staff to focus on information related to 

the concentrations that were most 

influential in generating the health 

effect estimates in individual studies to 

inform alternative standard levels 

(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 mg/m3 for the annual 

standard, in conjunction with retaining 

the current 24-hour PM2.5 standard level 

of 35 mg/m3; alternatively, revising the 

level of the 24-hour standard to 30 mg/ 

m3 in conjunction with an annual 

standard level of 11 mg/m3) ‘‘may not be 

adequately inclusive’’ and ‘‘[i]t was not 

clear why, for example a daily standard 

of 30 mg/m3 should only be considered 

in combination with an annual level of 

11 mg/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, EPA staff 

conducted additional analyses and 

modified their conclusions regarding 

alternative standard levels that are 

appropriate to consider. The staff 

conclusions in the final Policy 

Assessment (U.S. EPA, 2011a, section 

2.3.4.4) differ 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 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, the EPA 

analyzed additional population-level 

data obtained from several study 

investigators. In commenting on draft 

staff conclusions in the second draft 

Policy Assessment, CASAC did not have 

an opportunity to review the staff 

analyses of distributional statistics to 

identify the broader range of PM2.5

concentrations that were most 

influential in generating health effect 

estimates in epidemiological studies 

(Rajan et al., 2011). In addition, CASAC 

was not aware of the revised long-term 

mean PM2.5 concentration in the WHI 

study as discussed in section III.D.1.a 

above or the staff’s inclusion of that 

value in its evaluation of the evidence 

(i.e., in Figures 1 and 4 above and 

related discussion). The WHI study is 

the only long-term cohort study that 

provides information regarding effects 

classified as having evidence of a causal 

or likely causal relationship associated 

with a long-term PM2.5 concentration 

below 13 mg/m3. Furthermore, CASAC 

did not have an opportunity to review 

the staff’s revised rationale for the 

combinations of alternative standards 

suggested in the final Policy 

Assessment. 

e. Administrator’s Proposed 

Conclusions on the Primary PM2.5

Standard Levels 

In reaching her conclusions regarding 

appropriate alternative standard levels 

to consider, 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 

and the advice of CASAC. As an initial 

matter, the Administrator agrees with 

the 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. This is 

consistent with the approach taken in 

the review completed in 1997, in 

contrast to considering each standard 

separately, as was done in the review 

completed in 2006. Furthermore, based 

on the evidence and quantitative risk 

assessment, the Administrator 

provisionally concludes 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 concludes 

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. 

In reaching decisions on alternative 

standard levels to propose, the 

Administrator judges that it is most 

appropriate to examine where the 

evidence of associations observed in the 

epidemiological studies is strongest and, 

conversely, where she has 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 recognizes 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 takes 

note that these studies, along with 

evidence available in the last review, 

provide consistent and stronger 

evidence of an association with 

premature mortality, with the strongest 

evidence related to cardiovascularrelated mortality, at lower ambient 

concentrations than previously 

observed. The Administrator also 

recognizes the 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 recognizes new U.S. 

multi-city studies that greatly expand 

and reinforce our understanding of 

mortality and morbidity effects 

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86With 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. 

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). 

The newly available scientific 

evidence builds 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 recognizes 

that the weight of evidence, as evaluated 

in the Integrated Science Assessment, is 

strongest for health endpoints classified 

as having evidence of a causal 

relationship. These relationships 

include those between long- and shortterm PM2.5 exposures and mortality and 

cardiovascular effects. She recognizes 

that the weight of evidence is also 

strong for health endpoints classified as 

having evidence of a likely causal 

relationship, which include those 

between long- and short-term PM2.5

exposures and respiratory effects. In 

addition, the Administrator makes 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. 

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 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 recognizes, in 

taking note of CASAC advice and the 

distributional statistics analysis 

discussed in section III.E.4.b above and 

in the Policy Assessment, that there is 

significantly greater confidence in 

observed associations over certain parts 

of the air quality distributions in the 

studies, and conversely, that there is 

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.d above, the 

Administrator generally agrees that it is 

appropriate to consider a level for an 

annual standard that is somewhat below 

the long-term mean PM2.5

concentrations reported in long- and 

short-term exposure studies. In 

recognizing that the evidence of an 

association in any such study is 

strongest at and around the long-term 

average where the data in the study are 

most concentrated, she understands that 

this approach does not provide a bright 

line for reaching decisions about 

appropriate standard levels. The 

Administrator notes that long-term 

mean PM2.5 concentrations are available 

for each study considered and, 

therefore, represent the most robust data 

set to inform her decisions on 

appropriate annual standard levels. She 

also notes that the overall study mean 

PM2.5 concentrations are 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 

study area, referred to as a maximum 

monitor concentration, which are used 

to determine whether an area meets a 

given standard. In considering such 

long-term mean concentrations, the 

Administrator understands that it is 

appropriate to consider the weight of 

evidence for the health endpoints 

evaluated in such studies in giving 

weight to this information. 

Based on the information summarized 

in Figure 4 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 observes an overall 

pattern of statistically significant 

associations reported in studies of longterm PM2.5 exposures with long-term 

mean concentrations ranging from 

somewhat above the current standard 

level of 15 mg/m3 down to the lowest 

mean concentration in such studies of 

12.9 mg/m3 (in Miller et al., 2007). She 

observes a similar pattern of statistically 

significant associations in studies of 

short-term PM2.5 exposures with longterm mean concentrations ranging from 

around 15 mg/m3 down to 12.8 mg/m3 (in 

Burnett et al., 2004). With regard to 

effects classified as providing evidence 

suggestive of a causal relationship, the 

Administrator observes 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 concentrations down to 11.9 mg/ 

m3 (in Bell et al., 2007).86

The Administrator also considers 

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 understands 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 within which the data 

become appreciably more sparse and, 

thus, where her confidence in the 

associations observed in 

epidemiological studies would become 

appreciably less. As discussed in 

section III.E.4.b above and summarized 

in Figure 4, the Administrator takes note 

of additional population-level data that 

are available for four studies (Krewski et 

al., 2009; Miller et al., 2007; Bell et al., 

2008; Zanobetti and Schwartz, 2009), 

each of which report 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 observes that 

these values range from somewhat 

above to somewhat below 12 mg/m3

(Figure 4). The Administrator recognizes 

that these four studies represent some of 

the strongest evidence available within 

the overall body of scientific evidence 

and notes that three of these studies 

(Krewski et al., 2009; Bell et al., 2008; 

Zanobetti and Schwartz, 2009) were 

used as the basis for concentrationresponse functions used in the 

quantitative risk assessment (U.S. EPA, 

2010a, section 3.3.3). However, the 

Administrator also recognizes that 

additional population-level data are 

available for only these four studies and, 

therefore, she believes that these studies 

comprise a more limited data set than 

one based on long-term mean PM2.5

concentrations for which data are 

available for all studies considered, as 

discussed above. In considering this 

information, the Administrator notes 

that CASAC advised that information 

about the long-term PM2.5

concentrations that were most 

influential in generating the health 

effect estimates in epidemiological 

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studies can help to inform selection of 

an appropriate annual standard level. 

The Administrator recognizes, 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 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. The Administrator 

also recognizes that there are additional 

limitations associated with evidence for 

reproductive and developmental effects, 

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. 

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 

standard levels are appropriate to 

propose, the Administrator recognizes, 

as did EPA staff in the Policy 

Assessment, that there is no single 

factor or criterion that comprises the 

‘‘correct’’ approach to weighing the 

various types of available evidence and 

information, but rather there are various 

approaches that are appropriate to 

consider. The Administrator further 

recognizes 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 leads the Administrator to 

consider various approaches to 

weighing the evidence so as to identify 

appropriate standard levels to propose. 

In so doing, the Administrator 

encourages 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 notes the 

advice of CASAC that the currently 

available scientific information, 

including epidemiological and 

toxicological evidence as well as risk 

and air quality information, provides 

support for considering an annual 

standard level within a range of 13 to 11 

mg/m3 and a 24-hour standard level 

within a range of 35 to 30 mg/m3. In 

addition, the Administrator recognizes 

that the Policy Assessment concludes 

that the available evidence and riskbased information support 

consideration of annual standard levels 

in the range of 13 to 11 mg/m3, and that 

the Policy Assessment also concludes 

that the evidence most strongly supports 

consideration of an annual standard 

level in the range of 12 to 11 mg/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 in these ranges, although she 

recognizes that CASAC did not have an 

opportunity to review additional 

information and analyses presented in 

the final Policy Assessment prepared in 

response to CASAC’s recommendations 

on the second draft Policy Assessment. 

Nor did CASAC have an opportunity to 

review the EPA staff’s revised rationale 

for the combinations of alternative 

standards presented in the final 

document. 

In considering the extent to which the 

currently available evidence and 

information provide support for specific 

standard levels within the ranges 

identified by CASAC and the Policy 

Assessment as appropriate for 

consideration, the Administrator 

initially considers standard levels 

within the range of 13 to 11 mg/m3 for 

the annual standard. In so doing, the 

Administrator first considers the longterm mean PM2.5 concentrations 

reported in studies of effects classified 

as having evidence of a causal or likely 

causal relationship, as summarized in 

Figure 4 and discussed more broadly 

above. She notes 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 short-term 

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 

notes that the evidence of an association 

in any such study is 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 are available for each 

study considered and, therefore, 

represent the most robust data set to 

inform her decisions on appropriate 

annual standard levels. Further, she is 

mindful that this approach does not 

provide a bright line for reaching 

decisions about appropriate standard 

levels. 

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 notes 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 notes 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. 

The Administrator also considers 

additional 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 shortterm PM2.5 concentrations for annual 

standard levels within the same range of 

13 to 11 mg/m3. In so doing, the 

Administrator first notes 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 

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these studies. The Administrator also 

notes 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 notes that there is 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 becomes 

appreciably less. 

In considering mean PM2.5

concentrations and distributional 

analyses from the various sets of 

epidemiological studies noted above, 

the Administrator is 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 uses 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 is set, concentrations at 

appropriate maximum monitors within 

an area are generally used to determine 

if an area meets a given standard. 

The Administrator also recognizes 

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 remain in 

the evidence and information being 

considered in this review. The 

Administrator notes that the extent to 

which these uncertainties influence 

judgments about appropriate annual 

standard levels within the range of 13 to 

11 mg/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 concludes that it is 

appropriate to propose to set a level for 

the primary annual PM2.5 standard 

within the range of 12 to 13 mg/m3. The 

Administrator provisionally concludes 

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 concludes 

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 recognizes that setting an 

annual standard level at the lower end 

of this range would reflect an approach 

that places more emphasis on the entire 

body of the evidence, including the 

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 

to warrant setting a lower annual 

standard level. 

While the Administrator recognizes 

that CASAC advised, and the Policy 

Assessment concluded, that the 

available scientific information provides 

support for considering a range that 

extended down to 11 mg/m3, she 

concludes that proposing such an 

extended range would reflect a public 

health policy approach that places more 

weight on relatively limited evidence 

and more uncertain information and 

analyses than she considers appropriate 

at this time. Nonetheless, the 

Administrator solicits comment on a 

level down to 11 mg/m3 as 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 recognizes that 

potential air quality changes associated 

with meeting an annual standard set at 

a level within the range of 12 to 13 mg/ 

m3 will result in lowering risks 

associated with both long- and shortterm PM2.5 exposures. However, the 

Administrator recognizes that such an 

annual standard intended to serve as the 

primary means for providing protection 

from effects associated with both longand 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 mg/m3, the Administrator 

provisionally 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-to-mean ratios possibly associated 

with strong local or seasonal sources, or 

for PM2.5-related effects that may be 

associated with shorter-than-daily 

exposure periods. 

Based on the approach discussed in 

section III.A.3 above, the Administrator 

has relied upon evidence from the shortterm 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 agrees 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 recognizes 

that to the extent these studies were 

conducted in areas that likely did not 

meet one or both of the current 

standards, such studies do 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 anticipates 

that additional public health protection 

will occur. Therefore, the Administrator 

has focused on studies that reported 

positive and statistically significant 

associations in areas that would likely 

have met both the current 24-hour and 

annual standards. She has also 

considered whether or not these studies 

were conducted in areas that would 

likely have met an annual standard level 

of 12 to 13 mg/m3 to inform her decision 

regarding an appropriate 24-hour 

standard level. As discussed in section 

III.E.4.a, the Administrator concludes 

that multi-city, short-term exposure 

studies provide the strongest data set for 

informing her decisions on appropriate 

24-hour standard levels. The 

Administrator views the single-city, 

short-term exposure studies as a much 

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more limited data set providing mixed 

results and, therefore, she has less 

confidence in using these 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 

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 shortterm PM2.5 exposures, the Administrator 

observes 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 mg/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 notes that, 

to the extent air quality distributions are 

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 mg/m3 (Burnett et al., 

2004; Burnett and Goldberg, 2003; 

Franklin et al., 2008). In the three 

additional studies with 98th percentile 

values below 35 mg/m3, specifically 98th 

percentile concentrations of 34.2, 34.3, 

and 34.8 mg/m3, the Administrator notes 

that these studies reported long-term 

mean PM2.5 concentrations of 12.9, 13.2, 

and 13.4 mg/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 mg/m3, as discussed above, the 

Administrator recognizes 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 mg/m3, 

and in two of these three studies in 

conjunction with an annual standard 

level of 13 mg/m3. She notes that the 

study-wide mean concentrations are 

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 

believes it would be reasonable to 

conclude that revision to the 24-hour 

standard would not be warranted in 

conjunction with an annual standard 

within this range. Based on the above 

considerations related to the 

epidemiological evidence, the 

Administrator provisionally concludes 

that it is appropriate to retain the level 

of the 24-hour standard at 35 mg/m3, in 

conjunction with a revised annual 

standard level in the proposed range of 

12 to 13 mg/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 

retaining the 24-hour standard level at 

35 mg/m3 in conjunction with an annual 

standard level within the proposed 

range of 12 to 13 mg/m3. She has 

considered whether this suite of 

standards would meet a public health 

policy goal which includes 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 concludes 

that this approach is the most effective 

and efficient way to reduce total 

population risk associated with both 

long- and 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 

recognizes that there is no annual 

standard within the proposed range of 

levels, when combined with a 24-hour 

standard at the proposed level of 35 mg/ 

m3, for which the annual standard 

would be the generally controlling 

standard in all areas of the country. She 

further observes that such a suite of 

PM2.5 standards with an annual 

standard level of 12 mg/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 mg/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 mg/m3. 

Taking the above considerations into 

account, the Administrator proposes to 

revise the level of the primary annual 

PM2.5 standard from 15.0 mg/m3 to 

within the range of 12.0 to 13.0 mg/m3

and to retain the 24-hour standard level 

at 35 mg/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 recognizes 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 is 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, the final suite of standards 

will reflect the Administrator’s 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 zerorisk level, but rather at a level that 

reduces risk sufficiently so as to protect 

public health with an adequate margin 

of safety. 

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 mg/m3 and to 

propose retaining the 24-hour standard 

level at 35 mg/m3, the Administrator 

solicits public comment on this range of 

levels and on approaches to considering 

the available evidence and information 

that would support the choice of levels 

within this range. The Administrator 

also solicits public comment on 

alternative annual standard levels down 

to 11 mg/m3 and 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 

plays in considering the appropriate 

annual and 24-hour levels, the 

Administrator solicits public comment 

on issues related to translating 

epidemiological evidence into 

standards, including approaches for 

addressing the uncertainties and 

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87With regard to the 24-hour PM10 standard, the 

EPA retained the indicator, averaging time, and 

level (150 mg/m3), but revised the form (i.e., from 

one-expected-exceedance to the 99th percentile). 

limitations associated with this 

evidence. 

F. Administrator’s Proposed 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 

proposes to revise the current primary 

PM2.5 standards. Specifically, the 

Administrator proposes to revise: (1) 

The level of the primary annual PM2.5

standard to a level within the range of 

12.0 to 13.0 mg/m3 and (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 

allowance 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 proposes to retain the 

level and form of the primary 24-hour 

PM2.5 standard to provide supplemental 

protection for areas with high peak 

PM2.5 concentrations. The 

Administrator provisionally concludes 

that such a revised suite of standards, 

including a revised annual standard 

together with the current 24-hour 

standard, could provide requisite 

protection against health effects 

potentially associated with long- and 

short-term PM2.5 exposures. The 

Administrator is not proposing any 

revisions to the current PM2.5 indicator 

and the annual and 24-hour averaging 

times for the primary PM2.5 standards. 

Data handling conventions are specified 

in proposed revisions to appendix N, as 

discussed in section VII below. The 

Administrator solicits comment on all 

aspects of this proposed decision. 

IV. Rationale for Proposed Decision on 

Primary PM10 Standard 

This section presents the rationale for 

the Administrator’s proposed decision 

to retain the current 24-hour PM10

standard to continue to provide public 

health protection against short-term 

exposures to thoracic coarse particles, 

that is inhalable particles which can 

penetrate into the trachea, bronchi, and 

deep lungs and which are in the size 

range of 2.5 to 10 mm (PM10-2.5). As 

discussed more fully below, this 

rationale 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 thoracic coarse 

particles in the ambient air. This 

proposal also takes 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 health evidence presented 

in 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 and Policy 

Assessment at public meetings, in 

separate written comments, and in 

CASAC’s letters to the Administrator; 

and (3) public comments received 

during the development of these 

documents, either in connection with 

CASAC meetings or separately. The EPA 

notes that the final decision for 

retaining or revising the current primary 

PM10 standard is a public health policy 

judgment made by the Administrator. 

The Administrator’s final decision will 

draw upon scientific information and 

analyses related to health effects; 

judgments about uncertainties that are 

inherent in the scientific evidence and 

analyses; CASAC advice; and comments 

received in response to this proposal. 

In presenting the rationale for the 

proposed decision to retain the current 

primary PM10 standard, this section 

begins with background information on 

EPA’s past reviews of the PM NAAQS 

and the general approach taken to 

review the current PM10 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), and the Administrator’s 

proposed conclusions regarding the 

adequacy of the current primary PM10

standard (section IV.E). Section IV.F 

summarizes the Administrator’s 

proposed decision with regard to the 

primary PM10 NAAQS. 

A. Background 

The following sections discuss 

previous reviews of the PM NAAQS 

(section IV.A.1), the litigation of the 

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 

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). 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 mm 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 mm. 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). Thus, the EPA 

concluded that a PM10 standard could 

provide requisite protection against 

effects associated with particles in the 

size range of 2.5 to 10 mm.87 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 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, it was judged 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 newly 

adopted PM2.5 standards. 

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88The 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). 

89Thus, the standard is met when a 24-hour 

average PM10 concentration of 150 mg/m3 is not 

exceeded more than one day per year, on average 

over a three-year period. 

In May 1998, a three-judge panel of 

the U.S. Court of Appeals for the District 

of Columbia Circuit found ‘‘ample 

support’’ for 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

standards (65 FR 80776, December 22, 

2000). The pre-existing 1987 PM10

standards 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 PM10-2.5.88

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 mm (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 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 decision that it 

was requisite to provide protection from 

exposure to all thoracic coarse PM, 

regardless of its origin, rejecting 

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 nonurban origin), consistent with the Act’s 

requirement that primary NAAQS 

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 only did it inappropriately 

provide no protection at all to many 

areas, but it failed to identify many 

areas where the ambient 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 nonurban areas (71 FR 61195 to 96, October 

17, 2006). 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 PM10 standard afforded 

requisite protection with an adequate 

margin of safety, the Agency retained 

the level and form of the 24-hour PM10

standard.89

The Agency also revoked the annual 

PM10 standard, in light of the 

conclusion in 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. 

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 

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NAAQS (71 FR 61202/3, October 17, 

2006). 

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 (D.C. 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 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 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 nonurban 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 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 EPA’s decision 

to retain the level of the standard at 150 

mg/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 

accorded 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 EPA’s decision 

to revoke the annual PM10 standard. 

Specifically, the court stated the 

following: 

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 Policy 

Assessment for 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 policy options for 

consideration by the Administrator. The 

Administrator considers the 

appropriateness of the current and 

potential alternative standards, taking 

into account the four basic elements of 

the NAAQS: Indicator, averaging time, 

form, and level. 

In contrast to previous reviews, where 

PM10 studies conducted in locations 

where PM10 is comprised 

predominantly of PM10-2.5 were 

considered (U.S. EPA, 2005, pp. 5–49 to 

5–50), the focus in the current review is 

on PM10-2.5 studies. 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). In light 

of the considerable uncertainty in the 

extent to which PM10 effect estimates 

reflect associations with PM10-2.5 versus 

PM2.5, together with the availability in 

this review of a number of studies that 

evaluated associations with PM10-2.5 and 

the fact that the Integrated Science 

Assessment weight of evidence 

conclusions for thoracic coarse particles 

were based on studies of PM10-2.5, the 

EPA focuses in this review on studies 

that have specifically evaluated PM10-2.5. 

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 PMrelated effects do not occur, can be 

discerned from the available evidence 

(U.S. EPA, 2009a, section 2.4.3). 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. 

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 

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90The Integrated Science Assessment discusses 

the framework for causality determinations (U.S. 

EPA, 2009a, section 1.5). 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). 

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). 

B. Health Effects Related to Exposure to 

Thoracic Coarse Particles 

The following sections discuss 

available information on the health 

effects associated with exposures to 

PM10-2.5, including the nature of such 

health effects (section IV.B.1), the 

impacts of sources and composition on 

particle toxicity (section IV.B.2), 

ambient PM10 concentrations in PM10-2.5

study locations (section IV.B.3), at-risk 

populations (section IV.B.4), and 

limitations and uncertainties (section 

IV.B.5). 

1. Nature of Effects 

Since the conclusion of the last 

review, the Agency has developed a 

more formal framework for reaching 

causal inferences from the body of 

scientific evidence. As discussed above 

in section III.B.1, this framework uses a 

five-level hierarchy that classifies the 

overall weight of evidence using the 

following categorizations: Causal 

relationship, likely to be a 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, 

section 1.5). Applying this framework to 

thoracic coarse particles, the Integrated 

Science Assessment concludes 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).90 In contrast, the Integrated 

Science Assessment concludes that 

available evidence is ‘‘inadequate’’ to 

infer a causal relationship between longterm PM10-2.5 exposures and various 

health effects (U.S. EPA, 2009a, sections 

7.2 to 7.6). Similar to the judgment 

made in the 2004 AQCD regarding longterm exposures (U.S. EPA, 2004, p. 9– 

79), the Integrated Science Assessment 

states, ‘‘To date, a sufficient amount of 

evidence does not exist in order to draw 

conclusions regarding the health effects 

and outcomes associated with long-term 

exposure to PM10-2.5’’ (U.S. EPA, 2009a, 

section 2.3.4). Given these weight of 

evidence conclusions in the Integrated 

Science Assessment, EPA’s 

consideration of the scientific evidence 

for PM10-2.5 focuses on effects that have 

been linked with short-term exposures. 

The evidence supporting a link between 

short-term thoracic coarse particle 

exposures and adverse health effects is 

discussed in detail in the Integrated 

Science Assessment (U.S. EPA, 2009a, 

Chapter 6) and is summarized briefly 

below for mortality (section IV.B.1.a), 

cardiovascular effects (section IV.B.1.b), 

and respiratory effects (section IV.B.1.c). 

a. Short-Term PM10-2.5 Exposure and 

Mortality 

The Integrated Science Assessment 

assesses a number of multi-city and 

single-city epidemiological studies that 

have evaluated associations between 

mortality and short-term PM10-2.5

concentrations (U.S. EPA, 2009a, Figure 

6–30 presents PM10-2.5 mortality studies 

assessed in the last review and the 

current review). Different studies have 

used different approaches to estimate 

ambient PM10-2.5. Some studies have 

used the difference between PM10 and 

PM2.5 mass, either measured at colocated monitors (e.g., Lipfert et al., 

2000; Mar et al., 2003; Ostro et al., 2003; 

Sheppard et al., 2003; Wilson et al., 

2007) or as the difference in countywide average concentrations (Zanobetti 

and Schwartz, 2009), while other 

studies have measured PM10-2.5 directly 

with dichotomous samplers (e.g., 

Burnett and Goldberg, 2003; Fairley et 

al., 2003; Burnett et al., 2004; Klemm et 

al., 2004). Despite differences in the 

approaches used to estimate ambient 

PM10-2.5 concentrations, the majority of 

multi- and single-city studies have 

reported positive associations between 

PM10-2.5 and mortality, though most of 

these associations were not statistically 

significant (U.S. EPA, 2009a, Figure 6– 

30). 

One important PM10-2.5 study 

conducted since the last review of the 

PM NAAQS is the U.S. multi-city study 

by Zanobetti and Schwartz (2009), 

which reported positive and statistically 

significant associations with PM10-2.5 for 

all-cause, cardiovascular-related, and 

respiratory-related mortality (U.S. EPA, 

2009a, section 6.5.2.3). In this study, 

effect estimates for all-cause and 

respiratory-related mortality remained 

statistically significant in co-pollutant 

models that included PM2.5, while the 

effect estimate for cardiovascular-related 

mortality remained positive but not 

statistically significant. Several other 

multi-city studies have reported 

positive, but not statistically significant, 

PM10-2.5 effect estimates for mortality 

(U.S. EPA, 2009a, Figure 6–30). 

When risk estimates in the study by 

Zanobetti and Schwartz (2009) were 

evaluated by climatic region (U.S. EPA, 

2009a, Figure 6–28), a mix of positive 

and negative PM10-2.5 effect estimates 

were reported in the regions that 

typically have the highest ambient 

PM10-2.5 concentrations (i.e., regions 

corresponding to the western and 

southwestern U.S.). Regional effect 

estimates from western regions of the 

United States were generally not 

statistically significant. Positive and 

statistically significant effect estimates 

were more often reported in regions that 

typically have lower PM10-2.5

concentrations (i.e., regions generally 

corresponding to the eastern half of the 

U.S.) (Schmidt and Jenkins, 2010 for 

PM10-2.5 concentrations). In addition, 

single-city empirical Bayes-adjusted 

effect estimates (calculated using the 

methods discussed in Le Tertre et al., 

2005) for the 47 cities evaluated by 

Zanobetti and Schwartz (2009) were 

generally positive, though typically not 

statistically significant (U.S. EPA, 

2009a, Figure 6–29). 

Of the available single-city PM10-2.5

mortality studies, most reported 

positive, but not statistically significant, 

PM10-2.5 effect estimates (U.S. EPA, 

2009a, Figure 6–30). Of the three studies 

that did report statistically significant 

effect estimates (Mar et al., 2003; Ostro 

et al., 2003; Wilson et al., 2007), Ostro 

et al. (2003) reported that PM10-2.5 effect 

estimates remained statistically 

significant in co-pollutant models that 

included either ozone or NO2. The 

single-city studies by Mar et al. (2003) 

and Wilson et al. (2007) did not utilize 

co-pollutant models. 

b. Short-Term PM10-2.5 Exposure and 

Cardiovascular Effects 

The Integrated Science Assessment 

assesses a number of studies that have 

evaluated the link between short-term 

ambient concentrations of thoracic 

coarse particles and cardiovascular 

effects. Single- and multi-city 

epidemiological studies generally report 

positive associations between short-term 

PM10-2.5 concentrations and hospital 

admissions or emergency department 

visits for cardiovascular causes (U.S. 

EPA, 2009a, sections 2.3.3 and 6.2.12.2). 

However, as is the case for the mortality 

studies, most of these positive 

associations are not statistically 

significant. In addition, most PM10-2.5

effect estimates remained positive, but 

not statistically significant, in copollutant models that included either 

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91PM10-2.5 controlled human exposure studies 

have not been conducted in children. 

gaseous or particulate co-pollutants 

(U.S. EPA, 2009a, Figure 6–5). 

An important cardiovascular 

morbidity study published since the last 

review of the PM NAAQS is the U.S. 

multi-city study by Peng et al. (2008). 

This study evaluates hospital 

admissions and emergency department 

visits for cardiovascular disease in 

Medicare patients (MCAPS, Peng et al., 

2008). The authors report a positive and 

statistically significant association 

between 24-hour PM10-2.5 concentrations 

and cardiovascular disease 

hospitalizations in a single pollutant 

model using air quality data for 108 U.S. 

counties with co-located PM10 and PM2.5

monitors. The magnitude of this effect 

estimate was larger in counties with 

higher degrees of urbanization and 

larger in the eastern U.S. than the 

western U.S., though this regional 

difference was not statistically 

significant (Peng et al., 2008). The 

PM10-2.5 effect estimate was reduced 

only slightly in a co-pollutant model 

that included PM2.5, but it was no longer 

statistically significant (U.S. EPA, 

2009a, sections 2.3.3, 6.2.10.9). 

In addition to this U.S. multi-city 

study, positive associations reported for 

short-term PM10-2.5 exposures and 

cardiovascular-related morbidity 

reached statistical significance in a 

multi-city study in France (Host et al., 

2007) and single-city studies in Detroit 

(Ito, 2003) and Toronto (Burnett et al., 

1999) (U.S. EPA, 2009a, Figures 6–2 and 

6–3). In contrast, associations were 

positive but not statistically significant 

in single-city studies conducted in 

Atlanta (Metzger et al., 2004; Tolbert et 

al., 2007) and Boston (Peters et al., 2001) 

(and for some endpoints in Detroit (Ito, 

2003)) (U.S. EPA, 2009a, Figures 6–1 to 

6–3, and 6–5). 

The plausibility of the positive 

associations reported for PM10-2.5 and 

cardiovascular-related hospital 

admissions and emergency department 

visits receives some measure of support 

from a small number of controlled 

human exposure studies that have 

reported alterations in heart rate 

variability following short-term 

exposure to PM10-2.5 (Gong et al., 2004; 

Graff et al., 2009); by short-term PM10-2.5

epidemiological studies reporting 

positive associations with 

cardiovascular-related mortality; by a 

small number of recent epidemiological 

studies that have examined dust storm 

events and reported increases in 

cardiovascular-related emergency 

department visits and hospital 

admissions (see below); and by 

associations with other cardiovascular 

effects including heart rhythm 

disturbances and changes in heart rate 

variability (U.S. EPA, 2009a, sections 

2.3.3 and 6.2.12.2). The few 

toxicological studies that examined the 

effect of PM10-2.5 on cardiovascular 

health effects used intratracheal 

instillation and, as a result, provide only 

limited evidence on the biological 

plausibility of PM10-2.5 induced 

cardiovascular effects (U.S. EPA, 2009a, 

sections 2.3.3 and 6.2.12.2). 

c. Short-Term PM10-2.5 Exposure and 

Respiratory Effects 

The Integrated Science Assessment 

also assesses a number of studies that 

have evaluated the link between shortterm ambient concentrations of thoracic 

coarse particles and respiratory effects. 

This includes recent studies conducted 

in the U.S., Canada, and France (U.S. 

EPA, 2009a, section 6.3.8), including the 

U.S. multi-city study of Medicare 

patients by Peng et al. (2008). As 

discussed above, Peng estimated 

PM10-2.5 concentrations as the difference 

between PM10 and PM2.5 concentrations 

measured by co-located monitors. The 

authors reported a positive, but not 

statistically significant, PM10-2.5 effect 

estimate for respiratory-related hospital 

admissions. Single-city studies have 

reported positive, and in some cases 

statistically significant, PM10-2.5 effect 

estimates for respiratory-related hospital 

admissions and emergency department 

visits (U.S. EPA, 2009a, Figures 6–10 to 

6–15). Some of these PM10-2.5 respiratory 

morbidity studies have reported positive 

and statistically significant PM10-2.5

effect estimates in co-pollutant models 

that included gaseous pollutants while 

others reported that PM10-2.5 effect 

estimates remain positive, but not 

statistically significant, in such copollutant models (U.S. EPA, 2009a, 

Figure 6–15). 

A limited number of epidemiological 

studies have focused on specific 

respiratory morbidity outcomes and 

reported both positive and negative, but 

generally not statistically significant, 

associations between PM10-2.5 and lower 

respiratory symptoms, wheeze, and 

medication use (U.S. EPA, 2009a, 

sections 2.3.3.1 and 6.3.1.1; Figures 6– 

7 to 6–9). Although controlled human 

exposure studies have not observed an 

effect on lung function or respiratory 

symptoms in healthy or asthmatic 

adults in response to short-term 

exposure to PM10-2.5, healthy volunteers 

have exhibited increases in markers of 

pulmonary inflammation.91

Toxicological studies using inhalation 

exposures are still lacking, but 

pulmonary injury and inflammation has 

been reported in animals after 

intratracheal instillation exposure (U.S. 

EPA, 2009a, section 6.3.5.3) and, in 

some cases, PM10-2.5 was found to be 

more potent than PM2.5. 

2. Potential Impacts of Sources and 

Composition on PM10-2.5 Toxicity 

In the absence of a systematic national 

effort to characterize PM10-2.5

components, relatively little information 

(e.g., compared to fine particles) is 

available in the current review to inform 

consideration of the potential for 

composition to impact PM10-2.5 toxicity. 

Given this, the Integrated Science 

Assessment concludes that currently 

available evidence is insufficient to 

draw distinctions in 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 is associated with adverse 

health effects (U.S. EPA, 2009a, section 

2.4.4). 

As discussed above, positive 

associations between short-term PM10-2.5

concentrations and mortality and 

morbidity have been reported in a 

number of urban locations in the U.S., 

Canada, and Europe. While little is 

known about how PM10-2.5 composition 

varies across these locations or about 

how that variation could affect particle 

toxicity (U.S. EPA, 2009a, sections 2.3.3, 

2.3.4, 2.4.4), a number of trace elements 

(e.g., chromium, cobalt, nickel, copper, 

zinc, arsenic, selenium, and lead) have 

been detected in PM10-2.5 from urban 

locations (U.S. EPA, 2004, section 

3.2.4). 

An indication of the sources of some 

of these trace elements (e.g., metals such 

as lead, copper, and zinc) in ambient 

PM10-2.5 samples has been obtained by 

examining urban runoff (U.S. EPA, 

2004, section 3.2.4). Wind-abrasion on 

building siding and roofs (coatings such 

as lead paint and building material such 

as brick, metal, and wood siding); brake 

wear (brake pads contain significant 

quantities of copper and zinc); tire wear 

(zinc is used as a filler in tire 

production); and burning engine oil 

could all produce particles containing 

metals (U.S. EPA, 2004, section 3.2.4). 

Once deposited on the ground, these 

elements can be resuspended with other 

material as PM10-2.5. In addition, 

resuspended crustal particles may 

become contaminated with trace 

elements and other components from 

previously deposited fine PM (e.g., 

metals from smelters or steel mills, 

PAHs from automobile exhaust, 

pesticides from agricultural lands) (U.S. 

EPA, 2004, section 8.5, p. 8–344). 

In considering the potential for 

PM10-2.5 composition to impact toxicity, 

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92As discussed in more detail in the Policy 

Assessment (U.S. EPA, 2011a), these analyses are 

based on comparison of the one-expectedexceedance concentration-equivalent design values 

in study locations to the level of the current 

standard. The one-expected-exceedance 

concentration-equivalent design value is used as a 

surrogate concentration for comparison to the 

standard level in order to gain insight into whether 

a particular area would likely have met or violated 

the current PM10 standard. Therefore, locations 

with one-expected-exceedance concentrationequivalent design values below the level of the 

current PM10 standard (i.e., 150 mg/m3) would likely 

meet that standard (U.S. EPA, 2011a, section 3.2.1). 

93Multi-city studies assess PM10-2.5-associated 

health effects among large study populations and 

provide enhanced power to detect PM10-2.5- 

associated health effects. In addition, multi-city 

studies often provide spatial coverage for different 

regions across the country, reflecting differences in 

PM10-2.5 sources, composition, and potentially other 

factors that could impact PM10-2.5-related effects. 

These factors make multi-city studies particularly 

important when drawing conclusions about health 

effect associations. 

94See a previous footnote above and the Policy 

Assessment (U.S. EPA, 2011a, section 3.2.1) for an 

explanation of how PM10 air quality in study 

locations was compared to the current PM10

standard. 

it is useful to consider studies 

conducted in locations where PM10-2.5

composition is expected to be very 

different from that in typical urban 

locations. Specifically, a small number 

of studies have examined the health 

impacts of dust storm events (U.S. EPA, 

2009a, sections 6.2.10.1 and 6.5.2.3). 

Although these studies do not link 

specific particle constituents to health 

effects, they do provide some 

information on the toxicity of particles 

of non-urban crustal origin. Several of 

these studies have reported positive and 

statistically significant associations 

between dust storm events and 

morbidity or mortality, including the 

following: 

(1) Middleton et al. (2008) reported that 

dust storms in Cyprus were associated with 

a statistically significant increase in risk of 

hospitalization for all causes and a nonsignificant increase in hospitalizations for 

cardiovascular disease. 

(2) Chan et al. (2008) studied the effects of 

Asian dust storms on cardiovascular-related 

hospital admissions in Taipei, Taiwan and 

reported a statistically significant increase 

associated with 39 Asian dust events. 

Evaluating the same data, Bell et al. (2008) 

also reported positive and statistically 

significant associations between 

hospitalization for ischemic heart disease 

and PM10-2.5. 

(3) Perez et al. (2008) tested the hypothesis 

that outbreaks of Saharan dust exacerbate the 

effects of PM10-2.5 on daily mortality in Spain. 

During Saharan dust days, the PM10-2.5 effect 

estimate was larger than on non-dust days 

and it became statistically significant, 

whereas it was not statistically significant on 

non-dust days. 

In addition, a study in Coachella Valley 

by Ostro et al. (2003) reported 

statistically significant associations in a 

location where thoracic coarse particles 

are expected to be largely due to 

windblown dust. 

In contrast to the studies noted above, 

some dust storm studies have reported 

associations that were not statistically 

significant. Specifically, Bennett et al. 

(2006) reported on a dust storm in the 

Gobi desert that transported PM across 

the Pacific Ocean, reaching western 

North America in the spring of 1998. 

The authors reported no excess risk of 

cardiovascular-related or respiratoryrelated hospital admissions associated 

with the dust storm in the population of 

British Columbia’s Lower Fraser Valley 

(Bennett et al., 2006). In addition, Yang 

et al. (2009) reported that 

hospitalizations for congestive heart 

failure were elevated during or 

immediately following 54 Asian dust 

storm events, though effect estimates 

were not statistically significant. 

3. Ambient PM10 Concentrations in 

PM10-2.5 Study Locations 

As discussed above, a 24-hour PM10

standard is in place to protect public 

health against exposures to PM10-2.5. 

Given this, the EPA considers ambient 

PM10 concentrations in locations where 

PM10-2.5 health studies have been 

conducted (U.S. EPA, 2011a, section 

3.2.1). Specifically, the Agency 

considers study locations for which 

ambient PM10 data are available for 

comparison to the current standard,92

including study locations evaluated in 

single-city U.S. studies, in Bayesadjusted single-city analyses of the U.S. 

locations assessed by Zanobetti and 

Schwartz (2009), in single-city studies 

conducted outside the U.S., and in 

recent U.S. multi-city studies (Peng et 

al., 2008; Zanobetti and Schwartz, 

2009). 

In considering 24-hour PM10

concentrations in locations of specific 

PM10-2.5 epidemiological studies, the 

EPA has focused primarily on U.S. 

study locations where single-city 

analyses have been conducted (U.S. 

EPA, 2011a, sections 3.2.1 and 3.3.4). 

While multi-city studies are particularly 

important when drawing conclusions 

about health effect associations,93 it can 

be difficult to use these studies to link 

air quality in a given location to health 

effects in that same location. Multi-city 

studies often present overall effect 

estimates rather than single-city effect 

estimates, while short-term air quality 

can vary considerably across cities. 

Therefore, the extent to which effects 

reported in multi-city studies are 

associated with the short-term air 

quality in any particular location is 

uncertain, especially when considering 

short-term concentrations at the upper 

end of the distribution of daily 

concentrations for pollutants with 

relatively heterogeneous spatial 

distributions such as PM10-2.5 and PM10

(U.S. EPA, 2009a, section 2.1.1.2). In 

contrast, single-city studies are more 

limited in terms of power and 

geographic coverage but the link 

between reported health effects and the 

short-term air quality in a given city is 

more straightforward to establish. As a 

result, in considering 24-hour PM10

concentrations in locations of 

epidemiological studies, the EPA has 

focused primarily on single-city studies 

and single-city analyses of the locations 

evaluated in the multi-city study by 

Zanobetti and Schwartz (2009) (U.S. 

EPA, 2011a, sections 3.2.1 and 3.3.4). 

Of the 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).94 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). Amongst 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, PM10-2.5 effect estimates were 

both positive and negative, with most 

not statistically significant (U.S. EPA, 

2011a, Figure 3–3). 

As discussed above, PM10-2.5 effect 

estimates for mortality were generally 

positive but not statistically significant 

in Bayes-adjusted single-city analyses in 

the locations evaluated by Zanobetti and 

Schwartz (U.S. EPA, 2009a, Figure 6– 

30). These effect estimates were 

generally similar in magnitude and 

precision, particularly for 

cardiovascular-related mortality, across 

a wide range of estimated PM10-2.5

concentrations (U.S. EPA, 2009a, Figure 

6–29). In most of the cities evaluated (37 

of the 45 for which appropriate PM10 air 

quality data were available for 

comparison to the current standard, as 

described in Schmidt and Jenkins (2010) 

and Jenkins (2011), PM10 concentrations 

were below those that would have been 

allowed by the current PM10 standard 

(U.S. EPA, 2011a, section 3.2.1). Of 

these 37 cities that would likely have 

met the current PM10 standard during 

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95This is the case because the maximum 

monitored 24-hour PM10 concentration (116 mg/m3) 

was below the level of the current PM10 standard 

(150 mg/m3). 

96The one-expected-exceedance concentrationequivalent design value is used as a surrogate 

concentration for comparison to the standard level 

in order to gain insight into whether a particular 

area would likely have met or violated the current 

PM10 standard. Therefore, locations with oneexpected-exceedance concentration-equivalent 

design values below the level of the current PM10

standard (i.e., 150 mg/m3) would likely meet that 

standard (U.S. EPA, 2011a, section 3.2.1). 

97Although the Integrated Science Assessment 

notes that in PM10-2.5 studies of respiratory-related 

hospital admissions and emergency department 

visits, ‘‘the strongest relationships were observed 

among children’’ (U.S. EPA, 2009a, section 2.3.3.1). 

As discussed above (section III.B.3), children may 

be more at increased risk for effects associated with 

ambient PM exposures because, compared to adults, 

children typically spend more time outdoors and at 

higher activity levels; they have exposures that 

result in higher doses per body weight and lung 

surface area; and there is the potential for 

irreversible effects on the developing lung (U.S. 

EPA, 2009a, section 8.1.1.2). 

98For percentages, see http://www.cdc.gov/ 

ASTHMA/nhis/06/table4-1.htm. For population 

estimates, see http://www.cdc.gov/ASTHMA/nhis/ 

06/table3-1.htm. 

the study period, positive and 

statistically significant PM10-2.5 effect 

estimates were reported in three 

locations (Chicago, Pittsburgh, 

Birmingham). Of the eight cities likely 

to have violated the current PM10

standard during the study period, 

PM10-2.5 effect estimates were positive 

and statistically significant in three 

(Detroit, St. Louis, Salt Lake City). 

In considering PM10-2.5

epidemiological studies conducted in 

Canada and elsewhere outside the U.S., 

the EPA notes that PM10 air quality 

information beyond that published by 

the study authors is generally not 

available. The available PM10

concentration data for these study areas 

is typically not appropriate for 

comparison to the current PM10

standard (i.e., concentrations are 

averaged across monitors, rather than 

from the highest monitor in the study 

area, and/or concentrations are reported 

as means or medians). However, in a 

small number of cases it is possible to 

draw conclusions based on available air 

quality information about whether a 

study area would likely have met or 

violated the current PM10 standard. 

For example, Lin et al. (2002) reported 

positive and statistically significant 

associations between PM10-2.5 and 

asthma hospital admissions in children 

in Toronto (U.S. EPA, 2009a; Figures 6– 

12 and 6–15). The authors reported a 

maximum PM10 concentration measured 

at a single monitor in the study area of 

116 mg/m3, indicating that the PM10 air 

quality in Toronto during this study 

would have been allowed by the current 

24-hour PM10 standard.95

In contrast Middleton et al. (2008), 

who reported that dust storms in Cyprus 

were associated with a statistically 

significant increase in risk of 

hospitalization for all causes and a nonsignificant increase in hospitalizations 

for cardiovascular diseases, reported a 

maximum 24-hour PM10 concentration 

of 1,371 mg/m3. Thus, the dust stormassociated increases in hospitalizations 

reported in this study occurred in an 

area with PM10 concentrations that were 

likely well above those allowed by the 

current standard. Other dust storm 

studies did not report maximum 24- 

hour PM10 concentrations from 

individual monitors, though the studies 

by Chan et al. (2008) and Bell et al. 

(2008), which reported positive and 

statistically significant associations 

between dust storm metrics and 

cardiovascular-related hospital 

admissions, reported that 24-hour PM10

concentrations, averaged across 

monitors, exceeded 200 mg/m3. It is 

likely that peak concentrations 

measured at individual monitors in 

these studies were much higher and, 

therefore, 24-hour PM10 concentrations 

in these study areas were likely above 

those allowed by the current standard. 

In addition to the single-city studies 

discussed above, multi-city averages of 

PM10 one-expected-exceedance 

concentration-equivalent design 

values 96 for recent U.S. multi-city 

studies were 110 mg/m3, for the 

locations evaluated by Zanobetti and 

Schwartz (2009), and 100 mg/m3, for the 

locations evaluated by Peng et al. (2008) 

(U.S. EPA, 2011a, section 3.2.1). As 

discussed above, the extent to which 

multi-city PM10-2.5 effect estimates are 

associated with the air quality in any 

particular location is uncertain. 

4. At-Risk Populations 

Specific groups within the general 

population are likely at increased risk 

for suffering adverse effects following 

PM10-2.5 exposures. As discussed in 

section III.B.3 above, in this proposal, 

the term ‘‘at-risk’’ is the all 

encompassing term used for groups with 

specific factors that increase the risk of 

PM-related health effects in a 

population. 

Although studies have primarily used 

exposures to PM10 or PM2.5 to 

investigate potential at-risk populations, 

the available evidence suggests that the 

identified factors also increase risk from 

PM10-2.5 97 (U.S. EPA, 2009a, section 

8.1.8). As discussed in section III.B.3 

above, at-risk populations include those 

with preexisting heart and lung diseases 

(e.g., asthma), 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 atrisk populations, such as those with 

diabetes, people who are obese, 

pregnant women, and the developing 

fetus (U.S. EPA, 2009a, section 2.4.1 and 

Table 8–2). 

Given the range of at-risk groups, the 

population potentially affected by 

PM10-2.5 is large. In the United States, 

approximately 7 percent of adults 

(approximately 16 million adults) and 9 

percent of children (approximately 7 

million children) have asthma (U.S. 

EPA, 2009a, Table 8–3; CDC, 2008 98). In 

addition, approximately 4 percent of 

adults have been diagnosed with 

chronic bronchitis and approximately 2 

percent with emphysema (U.S. EPA, 

2009a, Table 8–3). Approximately 11 

percent of adults have been diagnosed 

with heart disease, 6 percent with 

coronary heart disease, 23 percent with 

hypertension, and 8 percent with 

diabetes (U.S. EPA, 2009a, Table 8–3). 

In addition, approximately 3 percent of 

the U.S. adult population has suffered a 

stroke (U.S. EPA, 2009a, Table 8–3). 

Therefore, although exposures to 

ambient PM10-2.5 have not been well 

characterized on a national scale, the 

size of the potentially at-risk population 

suggests that ambient PM10-2.5 could 

have a significant impact on public 

health in the United States. 

5. Limitations and Uncertainties 

Associated With the Currently Available 

Evidence 

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 uncertainties 

remain. These uncertainties, and their 

implications for interpreting the 

scientific evidence, are discussed below. 

The Integrated Science Assessment 

concludes that an important uncertainty 

in interpreting PM10-2.5 epidemiological 

studies is the potential for confounding 

by co-occurring pollutants, particularly 

PM2.5. This issue 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 

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99The EPA has required PM10-2.5 mass 

monitoring, as part of the NCore network, beginning 

January 1, 2011 at approximately 80 stations. The 

NCore network is a multi-pollutant network that 

includes measurements of particles, gases, and 

meteorology (71 FR 61236, October 17, 2006). 

NCore monitoring stations are located away from 

direct emissions sources that could substantially 

impact the detection of area-wide concentrations. 

The network is comprised of stations in both urban 

and rural areas. Urban NCore stations are generally 

to be located at an urban or neighborhood scale to 

provide exposure concentrations that are expected 

to be representative of the metropolitan area. Rural 

NCore stations are to be located, to the maximum 

extent practicable, at a regional or larger scale away 

from any large local emission source, so that they 

represent ambient concentrations over an extensive 

area (U.S. EPA, 2011a, Appendix B, section B.4). 

100 In addition, several sources of uncertainty can 

be specifically associated with PM10-2.5

concentrations that are estimated based on colocated monitors. For example, the potential for 

differences among operational flow rates and 

temperatures for PM10 and PM2.5 monitors add to 

the potential for exposure misclassification. As 

discussed in Appendix B of the Policy Assessment 

(U.S. EPA, 2011a, sections B.2 and B.3), PM10 data 

are often reported at standard temperature and 

pressure (STP) while PM2.5 is reported at local 

conditions (LC). In these cases, the PM10 data 

should be adjusted to LC when estimating PM10-2.5

concentrations. In many of the epidemiological 

studies that estimated PM10-2.5 concentrations based 

on co-located monitors, it is not made explicitly 

clear whether this adjustment was made, adding to 

the overall uncertainty in the PM10-2.5

concentrations that are associated with health 

effects. 

experimental evidence (i.e., controlled 

human exposure and animal toxicology 

studies) available to support the 

plausibility of 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. 

Another important uncertainty is 

related to exposure error. The Integrated 

Science Assessment concludes that 

‘‘there is greater spatial variability in 

PM10-2.5 concentrations than PM2.5

concentrations, resulting in increased 

exposure error for the larger size 

fraction’’ (U.S. EPA, 2009a, p. 2–8) and 

that 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). 

In addition, there is uncertainty in the 

air quality estimates used in PM10-2.5

epidemiological studies (U.S. EPA, 

2009a, sections 2.3.3, 2.3.4) and, 

therefore, in the ambient PM10-2.5

concentrations that are associated with 

mortality and morbidity. 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.99 Given these 

limitations in routine monitoring, 

epidemiological studies have employed 

different approaches for estimating 

PM10-2.5 concentrations. For example, 

several of the studies discussed above, 

including the multi-city study by Peng 

et al. (2008), estimated PM10-2.5 by 

taking the difference between mass 

measured at co-located PM10 and PM2.5

monitors while the study by Zanobetti 

and Schwartz (2009) used the difference 

between county-wide average PM10 and 

PM2.5 concentrations. In addition, a 

small number of studies have directly 

measured PM10-2.5 concentrations with 

dichotomous samplers (e.g., Burnett et 

al., 2004; Villeneuve et al., 2003; Klemm 

et al., 2004). 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 with a dichotomous 

sampler or by calculating the difference 

using co-located samplers (U.S. EPA, 

2009a, section 6.5.2.3).100 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 

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). 

Another uncertainty results from the 

relative lack of 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). As discussed 

above, a few recent studies have 

evaluated associations between health 

effects and particles of non-urban, 

crustal origin by evaluating the health 

impacts of dust storm events. Though 

these studies provide some information 

on the health effects of ambient particles 

that likely differ in composition from 

the particles of urban origin that are 

typically studied, 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). 

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

(U.S. EPA, 2009b, Appendix H). The 

decision not to conduct a PM10-2.5 risk 

assessment for the current review was 

based on consideration of several key 

uncertainties, including the following: 

(1) Concerns that monitoring data that 

would be used in a PM10-2.5 risk assessment 

(i.e., for the period 2005 to 2007) would not 

match ambient monitoring data used in the 

underlying epidemiological studies 

providing concentration-response functions. 

(2) Uncertainty in the prediction of 

ambient levels under current and alternative 

standard levels. 

(3) Concerns that locations used in the risk 

assessment may not be representative of areas 

experiencing the most significant 24-hour 

peak PM10-2.5 concentrations (and 

consequently, may not capture locations with 

the highest risk). 

(4) Concerns about the relatively small (i.e., 

compared to PM2.5) health effects database 

that supplies the concentration-response 

relationships. 

When considered together, the 

limitations outlined above resulted in 

the conclusion that a quantitative 

PM10-2.5 risk assessment would not 

significantly enhance the review of the 

NAAQS for coarse-fraction PM. 

Specifically, these limitations would 

likely result in sufficient uncertainty in 

the resulting risk estimates to 

significantly limit their utility to inform 

policy-related questions, including the 

assessment of whether the current 

standard is protective of public health 

and characterization of the degree of 

additional public health protection 

potentially afforded by alternative 

standards. 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. 

C. Consideration of the Current and 

Potential Alternative Standards in the 

Policy Assessment 

The following sections discuss EPA’s 

consideration of whether to revise the 

current PM10 standard, as well as our 

consideration of potential alternative 

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standards, drawing from such 

considerations in the Policy Assessment 

(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 

(section IV.C.2.a), averaging time 

(section IV.C.2.b), form (section 

IV.C.2.c), and level (section IV.C.2.d). 

1. Consideration of the Current Standard 

in the Policy Assessment 

As discussed above, a 24-hour PM10

standard is in place 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 EPA considers the 

health effects evidence linking shortterm 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). 

In considering the health evidence, air 

quality information, and associated 

uncertainties as they relate to the 

current PM10 standard, the EPA notes 

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 notes 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 3.2.1): 

(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 co-pollutants, 

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 

in 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 characterize. 

(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 weightof-evidence conclusions contrast with those 

for the relationships between PM2.5

exposures and 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 concludes 

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. 

Although such an approach to 

considering the adequacy of the current 

standard would recognize the positive, 

and in some cases statistically 

significant, associations between 

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 

notes that, when considering the 

uncertainties and limitations in the 

PM10-2.5 health evidence and air quality 

information, the EPA judged that it 

would not be 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 notes an 

alternative approach to considering the 

evidence and its uncertainties would 

place emphasis on the following: 

(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 short-term PM10-2.5

concentrations and adverse health endpoints 

including mortality and cardiovascularrelated 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 

concludes 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. 

Given all of the above, the Policy 

Assessment concludes 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 

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101Other than the dust storm studies, we note 

that the study in Coachella Valley by Ostro et al. 

(2003) reported statistically significant associations 

in a location where thoracic coarse particles are 

expected to be largely due to windblown dust. 

Specifically, we note the CASAC conclusion in the 

last review that ‘‘studies from Ostro et al. showed 

significant adverse health effects, primarily 

involving exposures to coarse-mode particles 

arising from crustal sources’’ (Henderson, 2005b). In 

considering this study, we also note the relatively 

high PM10 concentrations in the study area (U.S. 

EPA, 2011a, Figure 3–2), which would not have met 

the current PM10 standard. 

evidence, air quality information, and 

the uncertainties and limitations 

associated with that evidence and 

information. 

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. These 

considerations are discussed below in 

terms of indicator, averaging time, form, 

and level. 

a. Indicator 

As noted above, PM10 includes both 

PM10-2.5 and PM2.5, with the relative 

contribution of each to PM10 mass 

varying across locations and over time. 

In the most recent review completed in 

2006, the EPA concluded that the PM10

indicator remained appropriate in large 

part because a PM10 standard would 

provide some measure of protection 

against exposures to all PM10-2.5

regardless of source or location, while 

also targeting protection to urban areas, 

where the evidence of effects from 

exposure to coarse PM is the strongest 

(71 FR at 61196, October 17, 2006). As 

noted above, the court explicitly 

endorsed this reasoning. 559 F. 3d at 

535–36. 

In considering the indicator in the 

current review, the Policy Assessment 

evaluated the extent to which PM10 is 

comprised of PM10-2.5 across locations 

and over time. Based on the air quality 

analyses in the Integrated Science 

Assessment (U.S. EPA, 2009a, section 

3.5.1.1) and Schmidt and Jenkins (2010), 

and based on the concentration 

estimates of Zanobetti and Schwartz 

(2009), the Policy Assessment notes that 

PM10-2.5 typically makes up a larger 

portion of PM10 mass in the western 

United States, with the southwest region 

having the highest ratios of PM10-2.5 to 

PM10. In addition, the ratios of PM10-2.5

to PM10 across the U.S. tended to be 

higher on days with relatively high 

PM10 concentrations than on days with 

more typical PM10 concentrations (i.e., 

comparing days with concentrations at 

or above the 95th percentile to all days) 

(U.S. EPA, 2011a, section 3.3.1, Figure 

3–4). Given this, the Policy Assessment 

concludes that high daily PM10

concentrations are driven, at least in 

part, by elevated PM10-2.5 mass and that 

a PM10 standard focusing on the upper 

end of the distribution of daily PM10

concentrations could effectively control 

ambient PM10-2.5 concentrations (U.S. 

EPA, 2011a, p. 3–28). 

The Policy Assessment also 

considered the appropriateness of a 

PM10 standard, given that such a 

standard allows lower PM10-2.5

concentrations in areas with higher fine 

particle concentrations (urban areas) 

than areas with lower fine particle 

concentrations (rural areas) (U.S. EPA, 

2011a, section 3.3.1). In considering this 

issue, the Policy Assessment notes that 

most of the evidence for positive 

associations between PM10-2.5 and 

morbidity and mortality, particularly 

evidence for these associations at 

relatively low concentrations of PM10-2.5, 

comes from a number of studies 

conducted in locations where the 

PM10-2.5 is expected to be largely of 

urban origin (U.S. EPA, 2009a, Chapter 

6). Although some studies have reported 

positive associations between relatively 

high concentrations of particles of nonurban origin (i.e., crustal material from 

windblown dust in non-urban areas, see 

above) and mortality and morbidity, the 

Policy Assessment notes that the extent 

to which these associations would 

remain at the lower particle 

concentrations more typical of U.S. and 

Canadian urban study locations remains 

uncertain.101

Given these considerations, and given 

the increased potential for coarse 

particles in urban areas to become 

contaminated by toxic components of 

fine particles from urban/industrial 

sources (U.S. EPA, 2004 at 8–344; 71 FR 

61196, October 17, 2006), the Policy 

Assessment concludes that it is 

reasonable to consider an indicator that 

targets control to areas with the types of 

ambient mixes generally present in 

urban areas. The Policy Assessment 

notes that such an indicator would 

focus control on areas with ambient 

mixes known with greater certainty to 

be associated with adverse health effects 

and, therefore, would provide public 

health benefits with the greatest degree 

of certainty. Therefore, as in the last 

review, the Policy Assessment reaches 

the conclusion that a PM10 indicator 

would appropriately target protection to 

those locations where the evidence is 

strongest for associations between 

adverse health effects and exposures to 

thoracic coarse particles (U.S. EPA, 

2011a, p. 3–29). 

In contrast, the Policy Assessment 

notes that a PM10-2.5 indicator, for a 

standard set at a single unvarying level, 

would not achieve this targeting, given 

that allowable thoracic coarse particle 

concentrations would be the same 

regardless of the location or the likely 

sources of PM. Therefore, given the 

currently available evidence, one 

possible result of using a PM10-2.5

indicator would be a standard that is 

overprotective in rural areas and/or 

underprotective in urban areas (Id.). 

Given all of the above considerations, 

the Policy Assessment concludes that 

the available evidence supports 

consideration in the current review of a 

PM10 indicator for a standard that 

protects against exposures to thoracic 

coarse particles. The Policy Assessment 

further concludes that consideration of 

alternative indicators (e.g., PM10-2.5) in 

future reviews is desirable and could be 

informed by additional research (U.S. 

EPA, 2011a, section 3.5). 

b. Averaging Time 

Based primarily on epidemiological 

studies that reported positive 

associations between short-term (24- 

hour) PM10-2.5 concentrations and 

mortality and morbidity, the 

Administrator concluded in the last 

review that the available evidence 

supported a 24-hour averaging time for 

a standard intended to protect against 

exposures to thoracic coarse particles. In 

contrast, given the relative lack of 

studies supporting a link between longterm exposures to thoracic coarse 

particles and morbidity or mortality 

(U.S. EPA, 2004, Chapter 9), the 

Administrator further concluded that an 

annual coarse particle standard was not 

warranted at that time (71 FR 61198– 

61199, October 17, 2006). 

In the current review, the Policy 

Assessment notes the conclusions from 

the Integrated Science Assessment 

regarding the weight of evidence for 

short-term and long-term PM10-2.5

exposures as well as the studies on 

which those conclusions are based. 

Specifically, as discussed above, the 

Integrated Science Assessment 

concludes 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). This conclusion is based 

largely on epidemiological studies 

which have primarily evaluated 

associations between 24-hour PM10-2.5

concentrations and morbidity and 

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102As noted above (section IV.A.1.a), in the 1997 

review the EPA revised the form of the 24-hour 

PM10 standard to the 99th percentile. However, the 

D.C. Circuit Court vacated the revised rule, based 

on EPA’s retention of the PM10 indicator, and the 

1987 standards remained in place (including the 

one-expected-exceedance form for the 24-hour 

standard). 

103With regard to this conclusion, the Policy 

Assessment also notes that PM10-2.5 is likely to make 

a larger contribution to PM10 mass on days with 

relatively high PM10 concentrations than on days 

with more typical PM10 concentrations (see above). 

104As noted in section III.E.3.b above, stability of 

implementation programs has been held to be a 

legitimate consideration in determining a NAAQS 

(American Trucking Associations v. EPA, 283 F. 3d 

at 374 to 75). 

105See also, ATA III, 283 F. 3d at 374–75 

(upholding 98th percentile form since ‘‘otherwise 

States would have to design their pollution control 

programs around single high exposure events that 

may be due to unusual meteorological conditions 

alone, rendering the programs less stable—and 

hence, we assume, less effective—than programs 

designed to address longer-term average 

conditions.’’). In contrast, in the recently completed 

review of the primary SO2 NAAQS, a 99th 

percentile form was adopted. However, in the case 

of SO2, the standard was intended to limit 5-minute 

exposures and a 99th percentile form was markedly 

more effective at doing so than a 98th percentile 

form (75 FR 35540 to 41, June 22, 2010). 

106Similar considerations are noted in section 

III.E.3.b above, with regard to the form of the 

primary 24-hour PM2.5 standard. 

mortality (e.g., U.S. EPA, 2009a, Figure 

2–3), though a small number of 

controlled human exposure studies have 

reported effects following shorter 

exposures (i.e., 2-hours) to PM10-2.5 (U.S. 

EPA, 2009a, sections 6.2.1.2 and 

6.3.3.2). In contrast, with respect to 

long-term exposures, the Integrated 

Science Assessment concludes that 

available evidence is inadequate to infer 

a causal relationship with all health 

outcomes evaluated (U.S. EPA, 2009a, 

section 2.3). Specifically, the Integrated 

Science Assessment states, ‘‘To date, a 

sufficient amount of evidence does not 

exist in order to draw conclusions 

regarding the health effects and 

outcomes associated with long-term 

exposure to PM10-2.5’’ (U.S. EPA, 2009a, 

section 2.3.4). 

In considering these weight-ofevidence determinations, the Policy 

Assessment concludes that, at a 

minimum, they suggest the importance 

of maintaining a standard that protects 

against short-term exposures to thoracic 

coarse particles. Given that the majority 

of the evidence supporting the link 

between short-term PM10-2.5 and 

morbidity and mortality is based on 24- 

hour average thoracic coarse particle 

concentrations, the Policy Assessment 

concludes that the evidence available in 

this review continues to support 

consideration of a 24-hour averaging 

time for a PM10 standard meant to 

protect against effects associated with 

short-term exposures to PM10-2.5 (U.S. 

EPA, 2011a, p. 3–31). 

The Policy Assessment further 

concludes that the available evidence 

does not support consideration of an 

annual thoracic coarse particle standard 

at this time. In reaching this conclusion, 

the Policy Assessment also notes that, to 

the extent a short-term standard requires 

areas to reduce their 24-hour ambient 

particle concentrations, long-term 

concentrations would also be expected 

to decrease (Id.). Therefore, a 24-hour 

standard meant to protect against shortterm exposures to thoracic coarse 

particles would also be expected to 

provide some protection against 

potential effects associated with longterm exposures to ambient 

concentrations. 

c. 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 that 

standard. As discussed above, in the last 

review the Administrator retained the 

one-expected exceedance form of the 

primary 24-hour PM10 standard. This 

decision was linked to the overall 

conclusion that ‘‘the level of protection 

from coarse particles provided by the 

current 24-hour PM10 standard remains 

requisite to protect public health with 

an adequate margin of safety’’ (71 FR 

61202, October 17, 2006). Because 

revising either the level or the form of 

the standard would have altered the 

protection provided, the Administrator 

concluded that such changes ‘‘would 

not be appropriate based on the 

scientific evidence available at this 

time’’ (71 FR 61202). Therefore, the 

decision in the last review to retain the 

one-expected-exceedance form was part 

of the broader decision that the existing 

24-hour standard provided requisite 

public health protection. 

In the current review, the Policy 

Assessment considers the form of the 

standard within the context of the 

overall decision on whether, and if so 

how, to revise the current 24-hour PM10

standard. Given the conclusions above 

regarding the appropriate indicator and 

averaging time for consideration for 

potential alternative standards, the 

Policy Assessment considers potential 

alternative forms for a 24-hour PM10

standard. 

Although the selection of a specific 

form must be made within the context 

of decisions on the other elements of the 

standard, the Policy Assessment notes 

that the EPA generally favors 

concentration-based forms for shortterm standards. In 1997, the EPA 

established a 98th percentile form for 

the 24-hour PM2.5 standard and, in 2010, 

the EPA established a 98th percentile 

form for the primary 1-hour NO2

standard (62 FR 38671, July 18, 1997; 75 

FR 6474, February 9, 2010) and a 99th 

percentile form for the primary 1-hour 

SO2 standard (75 FR 35541, June 22, 

2010).102 In making these decisions, the 

EPA noted that, compared to an 

exceedance-based form, a concentrationbased form is 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. In 

addition, when averaged over three 

years, these concentration-based forms 

were judged to provide an appropriate 

balance between limiting peak pollutant 

concentrations and providing a stable 

regulatory target, facilitating the 

development of stable implementation 

programs. 

These considerations are also relevant 

in the current review of the 24-hour 

PM10 standard. Specifically, the Policy 

Assessment concludes that it is 

appropriate to consider concentrationbased forms that would provide a 

balance between limiting peak pollutant 

concentrations and providing a stable 

regulatory target. To accomplish this, it 

would be appropriate to consider forms 

from the upper end of the annual 

distribution of 24-hour PM10

concentrations.103 However, given the 

potential for local sources to have 

important impacts on monitored PM10

concentrations (U.S. EPA, 2009a, 

section 2.1.1.2), the Policy Assessment 

also notes that it would be appropriate 

to consider forms that, when averaged 

over three years, would be expected to 

promote the stability of local 

implementation programs.104 In 

considering these issues in the most 

recent review of the primary NO2

NAAQS, the Policy Assessment notes 

that a 98th percentile form was adopted, 

rather than a 99th percentile form, due 

to the potential for ‘‘instability in the 

higher percentile concentrations’’ near 

local sources (75 FR 6493, February 9, 

2010).105 106

In considering the potential 

appropriateness of a 98th percentile 

form in the current review, the Policy 

Assessment notes that, compared to the 

current PM10 standard, attainment status 

for a PM10 standard with a 98th 

percentile form would be based on a 

more stable air quality statistic and 

would be expected to be less influenced 

by relatively rare events that can cause 

elevations in PM10 concentrations over 

short periods of time (Schmidt, 2011b). 

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107As noted above, local sources can have 

important impacts on monitored PM10

concentrations. In the recent review of the NO2

primary NAAQS, where this was also an important 

consideration, a 98th percentile form was adopted, 

rather than a 99th percentile form, due to the 

potential for ‘‘instability in the higher percentile 

concentrations’’ near local sources (75 FR 6493, 

February 9, 2010). A similar conclusion in the 

current review led the Policy Assessment to focus 

on the 98th percentile rather than the 99th 

percentile, in considering potential alternative 

forms for a PM10 standard. 

108Section 3.3.4 of the Policy Assessment (U.S. 

EPA, 2011a) discusses potential alternative 

standard levels that would be appropriate to 

consider in conjunction with a revised standard 

with a 98th percentile form. 

109The memo by Schmidt (2011b) identifies 

specific counties that are expected to meet, and 

counties that are not likely to meet the current 

standard and potential alternative standards with 

98th percentile forms. 

110This analysis considered a revised PM10

standard with a 98th percentile form and a level 

from the middle of the range discussed in section 

3.3.4 of the Policy Assessment (i.e., 75 mg/m3) (U.S. 

EPA, 2011a). 

111Most studies that have evaluated the potential 

for thresholds have focused on PM10 or PM2.5. 

However, there is no scientific basis for drawing 

different conclusions for PM10-2.5. 

Specifically, the Policy Assessment 

notes that in areas that monitor PM10

every six days, every three days, or 

every day the PM10 concentrations that 

are comparable to the current standard 

level are, respectively, the highest, 2nd 

highest, or 4th highest 24-hour PM10

concentrations measured during a three 

year period. In contrast, for the same 

monitoring frequencies, the PM10

concentrations that would be 

comparable to the level of a standard 

with a 98th percentile form would be 

the three-year average of the 2nd 

highest, 3rd highest, or 7th/8th highest 

24-hour PM10 concentrations measured 

during a single year (U.S. EPA, 2011a, 

p. 3–33). 

In further considering this issue the 

Policy Assessment notes that, compared 

to the current one-expected-exceedance 

form, a concentration-based form 

specified as a percentile of the annual 

distribution of PM10 concentrations 

(e.g., such as a 98th percentile form) 

would be expected to better compensate 

for missing data and less-than-daily 

monitoring. This is a particularly 

important consideration in the case of 

PM10 because, depending largely on 

ambient concentrations, the frequency 

of PM10 monitoring differs across 

locations (i.e., either daily, 1 in 2 days, 

1 in 3 days, or 1 in 6 days) (U.S. EPA, 

2011a, section 1.3 and Appendix B). As 

discussed in earlier reviews of the PM 

NAAQS (e.g., 62 FR 38671, July 18, 

1997), an area’s attainment status for a 

standard with a 98th percentile form 

would be based directly on monitoring 

data rather than on a calculated value 

adjusted for missing data or less-thanevery-day monitoring, as is the case 

with the current one-expectedexceedance form. 

In light of all of the above 

considerations, the Policy Assessment 

concludes that, 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 98th 

percentile of the annual distribution of 

24-hour PM10 concentrations (U.S. EPA, 

2011a, p. 3–34).107

In their review of the second draft 

Policy Assessment, CASAC noted that 

such a change in form ‘‘will lead to 

changes in levels of stringency across 

the country’’ and recommended that 

this issue be explored further (Samet, 

2010d). In considering this issue, the 

Policy Assessment acknowledges that, 

given differences in PM10 air quality 

distributions across locations (U.S. EPA, 

2009a, Table 3–10), a revised standard 

with a 98th percentile form would likely 

target public health protection to some 

different locations than does the current 

standard with its one-expectedexceedance form (U.S. EPA, 2011a, p. 3– 

34). The final Policy Assessment notes 

that a further consideration with regard 

to the appropriateness of revising the 

form of the current PM10 standard is the 

extent to which, when compared with 

the current standard, a revised standard 

with a 98th percentile form would be 

expected to target public health 

protection to areas where we have more 

confidence that ambient PM10-2.5 is 

associated with adverse health effects 

(Id., p. 3–34 to 3–35). 

In giving initial consideration to this 

issue, the Policy Assessment used 

recent PM10 air quality concentrations 

(i.e., from 2007–2009) to identify 

counties that would meet, and counties 

that would violate, the current PM10

standard as well as potential alternative 

standards with 98th percentile forms 

(Schmidt, 2011b).108 109 In some cases, 

counties that would violate the current 

standard do so because of a small 

number of ‘‘outlier’’ days (e.g., as few as 

one such day in three years) with PM10

concentrations well-above more typical 

concentrations (Schmidt, 2011b). Mean 

and 98th percentile PM10 and PM10-2.5

concentrations were higher in counties 

that would have violated a revised 

standard with a 98th percentile form but 

met the current standard 110 than in 

counties that violated the current 

standard, but would have met a revised 

standard with a 98th percentile form 

(Schmidt, 2011b). This analysis suggests 

that, to the extent a revised PM10

standard with a 98th percentile form 

could target public health protection to 

different areas than the current 

standard, those areas preferentially 

targeted by a revised standard generally 

have higher ambient concentrations of 

thoracic coarse particles. The issue of 

targeting public health protection is 

considered further in section 3.3.4 of the 

Policy Assessment (U.S. EPA, 2011a) 

and below, within the context of 

considering specific potential 

alternative standard levels for a 24-hour 

PM10 standard with a 98th percentile 

form. 

d. Level 

As noted above, the Policy 

Assessment concluded that, to the 

extent it is judged in the current review 

that the 24-hour PM10 standard does not 

provide adequate public health 

protection against exposures to thoracic 

coarse particles, potential alternative 

standards could be considered. The 

Policy Assessment considers potential 

alternative levels for a 24-hour PM10

standard with a 98th percentile form. To 

inform consideration of this issue, the 

Policy Assessment considers the 

available scientific evidence and air 

quality information (U.S. EPA, 2011a, 

section 3.3.4). 

i. Evidence-Based Considerations in the 

Policy Assessment 

As discussed above, in considering 

the evidence as it relates to potential 

alternative standard levels, the Policy 

Assessment first considers the relative 

weight to place on specific 

epidemiological studies, including the 

weight to place on the uncertainties 

associated with those studies. The 

Policy Assessment considers several 

factors in placing weight on specific 

epidemiological studies including the 

extent to which studies report 

statistically significant associations with 

PM10-2.5 and the extent to which the 

reported associations are robust to copollutant confounding, in particular 

confounding by PM2.5. In addition, the 

Policy Assessment considers the extent 

to which associations with PM10-2.5 can 

be linked to the air quality in a specific 

location. With regard to this, as noted 

above, the Policy Assessment places the 

greatest weight on information from 

single-city analyses. 

In considering PM air quality in study 

locations, the Policy Assessment also 

notes that the available evidence does 

not support the existence of thresholds, 

or lowest-observed-effects levels, in 

terms of 24-hour average concentrations 

(U.S. EPA, 2009a, section 2.4.3).111 In 

the absence of an apparent threshold, 

for purposes of identifying a range of 

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112See http://www.epa.gov/ttn/airs/airsaqs/. 

113As discussed above, the one-expectedexceedance concentration-equivalent design value 

is used as a surrogate concentration for comparison 

to the standard level in order to gain insight into 

whether a particular area would likely have met or 

violated the current PM10 standard. Therefore, 

locations with one-expected-exceedance 

concentration-equivalent design values below the 

level of the current PM10 standard (i.e., 150 mg/m3) 

would likely meet that standard (U.S. EPA, 2011a, 

section 3.2.1). 

114The ‘‘generally equivalent’’ concentration also 

differs depending on the years of monitoring data 

used. For example, when this analysis was 

restricted to only the most recent years available 

(i.e., 2007 to 2009), the ‘‘generally equivalent’’ 98th 

percentile PM10 concentration was 78 mg/m3. Given 

the temporal variability in the relationship between 

the current standard level and 98th percentile PM10

concentrations, and the potential for the ‘‘generally 

equivalent’’ 98th percentile concentration to vary 

year-to-year, staff concluded that it remains 

appropriate to consider the correlation analyses that 

use the broader range of available monitoring years 

(i.e., 1998–2008), as these analyses are likely to be 

more robust than analyses based on a shorter period 

of time. 

115These analyses are based on three years of air 

quality data in order to simulate the requirements 

for determining whether areas attain or violate the 

current PM10 standard, which requires 

consideration of 3 years of air quality data. 

standard levels potentially supported by 

the health evidence, the Policy 

Assessment focuses on the range of 

PM10 concentrations that have been 

measured in locations where U.S. 

epidemiological studies have reported 

associations with PM10-2.5 (U.S. EPA, 

2009a, Figures 6–1 to 6–30 for studies). 

In single-city mortality studies, as 

well as the single-city analyses of the 

locations evaluated by Zanobetti and 

Schwartz (2009), positive and 

statistically significant PM10-2.5 effect 

estimates were reported in some 

locations with 98th percentile PM10

concentrations ranging from 200 mg/m3

to 91 mg/m3 (U.S. EPA, 2011a, section 

3.3.4). Lower PM10 concentrations were 

present in locations where positive, but 

not statistically significant, effect 

estimates were reported and when 

averaged across locations evaluated in 

the multi-city study by Zanobetti and 

Schwartz (2009) (U.S. EPA, 2011a, 

section 3.3.4). 

Among U.S. morbidity studies, Ito 

(2003) reported a positive and 

statistically significant PM10-2.5 effect 

estimate for hospital admissions for 

ischemic heart disease in Detroit, where 

the 98th percentile PM10 concentration 

(102 mg/m3) was also within this range 

(U.S. EPA, 2011a, section 3.3.4 and 

Figure 3–6). PM10-2.5 effect estimates in 

this study remained positive, and in 

some cases statistically significant, in 

co-pollutant models with gaseous 

pollutants (U.S. EPA, 2009a, Figures 6– 

5 and 6–15). Lower PM10 concentrations 

were present in locations where 

positive, but not statistically significant, 

effect estimates were reported and when 

averaged across locations evaluated in 

the multi-city study by Peng et al. (2008) 

(U.S. EPA, 2011a, section 3.3.4). 

ii. Air Quality-based Considerations in 

the Policy Assessment 

In addition to the evidence-based 

considerations described above, the 

Policy Assessment estimated the level of 

a 24-hour PM10 standard with a 98th 

percentile form that would approximate 

the degree of protection, on average 

across the country, provided by the 

current 24-hour PM10 standard with its 

one-expected-exceedance form. The 

initial approach to estimating this 

‘‘generally equivalent’’ 98th percentile 

PM10 concentration was to use EPA’s 

Air Quality System (AQS)112 as the 

basis for evaluating correlations 

between 98th percentile PM10

concentrations and one-expectedexceedance concentration equivalent 

design values (Schmidt and Jenkins, 

2010).113 Based on these correlations, 

using monitoring data from 1988 to 

2008, a 98th percentile PM10

concentration of 87 mg/m3 is, on 

average, generally equivalent to the 

current standard level (U.S. EPA, 2011a, 

Figure 3–7). However, given the 

variability in the distributions of PM10

concentrations across locations (U.S. 

EPA, 2009a, Table 3–10; Schmidt and 

Jenkins, 2010), the range of equivalent 

concentrations varies considerably (95 

percent confidence interval ranges from 

63 to 111 mg/m3) (Schmidt and Jenkins, 

2010). As a consequence, the Policy 

Assessment notes that in some locations 

a 98th percentile standard with a level 

of 87 mg/m3 would likely be more 

protective than the current standard 

while in other locations it would likely 

be less protective than the current 

standard.114

The Policy Assessment also evaluates 

regional differences in the relationship 

between 98th percentile PM10

concentrations and one-expectedexceedance concentration equivalent 

design values (U.S. EPA, 2011a, Figure 

3–8), based on air quality data from 

1988 to 2008. The 98th percentile PM10

concentrations that are, on average, 

generally equivalent to the current 

standard level ranged from just below 

87 mg/m3 in the Southeast, Southwest, 

upper Midwest, and outlying areas (i.e., 

generally equivalent 98th percentile 

PM10 concentrations ranged from 82 to 

85 mg/m3 in these regions) to just above 

87 mg/m3 in the Northeast, industrial 

Midwest, and southern California (i.e., 

generally equivalent 98th percentile 

PM10 concentrations ranged from 88 to 

93 mg/m3 in these regions) (Schmidt, 

2011b). However, within each of these 

regions there is considerable variability 

in the ‘‘generally equivalent’’ 98th 

percentile PM10 concentration across 

monitoring sites (U.S. EPA, 2011a, 

Figure 3–8). 

To provide a broader perspective on 

the relationship between the current 

standard and potential alternative 

standards with 98th percentile forms, 

the Policy Assessment also compares 

the size of the populations living in 

counties with PM10 one-expectedexceedance concentration-equivalent 

design values greater than the current 

standard level to the size of the 

populations living in counties with 98th 

percentile PM10 concentrations above 

different potential alternative standard 

levels (based on air quality data from 

2007 to 2009 115). Such comparisons can 

be considered as surrogates for 

comparisons of the breadth of public 

health protection provided by the 

current and potential alternative 

standards. Based on these comparisons, 

a 98th percentile PM10 standard with a 

level between 75 and 80 mg/m3 would 

be most closely equivalent to the current 

standard. That is, compared to the 

number of people living in counties that 

would violate the current PM10

standard, a similar number live in 

counties that would violate a revised 24- 

hour PM10 standard with a 98th 

percentile form and a level between 75 

and 80 mg/m3 (U.S. EPA, 2011a, Table 

3–2). However, there is considerably 

more variability across regions in the 

potential alternative standard that, 

based on this analysis, would be 

generally equivalent to the current PM10

standard (U.S. EPA, 2011a, section 

3.3.4). 

Given the variability in the 

relationship between the current 

standard and potential alternative 

standards with 98th percentile forms, 

the Policy Assessment concludes that 

no single potential alternative standard 

level, for a revised standard with a 98th 

percentile form, would provide public 

health protection equivalent to that 

provided by the current standard, 

consistently over time and across 

locations. 

One consequence of this variability, 

as noted above in the discussion of the 

form of the standard, would be that a 

24-hour PM10 standard with a 98th 

percentile form and a revised level 

would likely target public health 

protection to some different locations 

than does the current standard. 

Therefore, in further considering the 

appropriateness of revising the form and 

level of the current PM10 standard, the 

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116Positive and statistically significant PM10-2.5

effect estimates for Birmingham, Chicago, and 

Pittsburgh are reported in the Integrated Science 

Assessment (U.S. EPA, 2009a, Figure 6–29; from 

cities evaluated by Zanobetti and Schwartz, 2009). 

Effect estimates for Detroit are reported by Ito et al. 

(2003). 

117Philadelphia (Lipfert et al., 2000), Detroit (Ito 

et al., 2003), Santa Clara (CA) (Fairley et al., 2003), 

Seattle (Sheppard et al., 2003), Atlanta (Klemm et 

al., 2004), Spokane (Slaughter et al., 2005), Bronx 

and Manhattan (NYS DOH, 2006), and 39 of the 

cities evaluated by Zanobetti and Schwartz (2009) 

(U.S. EPA, 2009a, Figure 6–29). 

118Pittsburgh (Chock et al., 2000), Coachella 

Valley (CA) (Ostro et al., 2003), Phoenix (Mar et al., 

2003; Wilson et al., 2007), and 6 of the cities 

evaluated by Zanobetti and Schwartz (2009) (U.S. 

EPA, 2009a, Figure 6–29). 

Policy Assessment considered the 

extent to which, when compared with 

the current standard, a revised PM10

standard would be expected to target 

public health protection to areas where 

we have more confidence that PM10-2.5

is associated with adverse health effects. 

To address this question, the Policy 

Assessment considered the potential 

impact of revising the form and level of 

the PM10 standard in locations where 

health studies have reported 

associations with PM10-2.5. 

The Policy Assessment initially 

considers U.S. study locations that 

would likely have met the current PM10

standard during the study period and 

where positive and statistically 

significant associations with PM10-2.5

were reported. Only Birmingham, 

Chicago, Pittsburgh, and Detroit 116 met 

these criteria. During study periods, 

none of these areas would likely have 

met a 98th percentile 24-hour PM10

standard with a level at or below 87 mg/ 

m3 (U.S. EPA, 2011a, section 3.3.4 and 

Table 3–3). 

The Policy Assessment also 

considered U.S. locations where health 

studies have reported positive 

associations (both statistically 

significant and non-significant) between 

PM10-2.5 and mortality or morbidity. 

Such positive associations were 

reported in 47 locations that would 

likely have met the current PM10

standard during the study period.117 Of 

these 47 locations, 13 would likely not 

have met a 98th percentile 24-hour PM10

standard with a level at 87 mg/m3, 20 

would likely not have met a 98th 

percentile 24-hour PM10 standard with a 

level of 75 mg/m3, and 31 would likely 

not have met a 98th percentile 24-hour 

PM10 standard with a level of 65 mg/m3

(U.S. EPA, 2011a, section 3.3.4). 

In addition to the above analyses, the 

Policy Assessment also considered 

locations where health studies reported 

positive associations with PM10-2.5 and 

where ambient PM10 concentrations 

were likely to have exceeded those 

allowed under the current PM10

standard during the study period. Nine 

locations met these criteria.118 Of these 

locations, all would also likely have 

exceeded a 98th percentile PM10

standard with a level at or below 87 mg/ 

m3 (U.S. EPA, 2011a, section 3.3.4). 

Therefore, among U.S. study locations 

where PM10-2.5-associated health effects 

have been reported, some areas met the 

current standard but would likely have 

violated a 98th percentile PM10 standard 

with a level at or below 87 mg/m3. In 

contrast, the locations that violated the 

current standard would also likely have 

violated a 98th percentile PM10 standard 

with a level at or below 87 mg/m3. Given 

this, the Policy Assessment concludes 

that, compared to the current PM10

standard, a 24-hour PM10 standard with 

a 98th percentile form could potentially 

better target public health protection to 

locations where we have more 

confidence that ambient PM10-2.5

concentrations are associated with 

mortality and/or morbidity (U.S. EPA, 

2011a, pp. 3–45 to 3–46). 

iii. Integration of Evidence-Based and 

Air Quality-Based Considerations in the 

Policy Assessment 

In considering the integration of the 

evidence and air quality information 

within the context of identifying 

potential alternative standard levels for 

consideration, the Policy Assessment 

first notes the following: 

(1) Analyses of air quality correlations 

suggest that a 98th percentile 24-hour PM10

concentration as high as 87 mg/m3 could be 

considered generally equivalent to the 

current PM10 standard, over time and across 

the country. 

(2) A 98th percentile 24-hour PM10

standard with a level at or below 87 mg/m3

would be expected to maintain PM10 and 

PM10-2.5 concentrations below those present 

in U.S. locations where single-city studies 

have reported PM10-2.5 effect estimates that 

are positive and statistically significant 

(lowest concentration in such a location was 

91 mg/m3). Although some single-city studies 

have reported positive PM10-2.5 effect 

estimates in locations with 98th percentile 

PM10 concentrations below 87 mg/m3, these 

effect estimates were not statistically 

significant. 

(3) Multi-city average 98th percentile PM10

concentrations were below 87 mg/m3 for 

recent U.S. multi-city studies, which have 

reported positive and statistically significant 

PM10-2.5 effect estimates. However, the extent 

to which effects reported in multi-city 

studies are associated with the short-term air 

quality in any particular location is highly 

uncertain. 

(4) Epidemiological studies have reported 

positive, and in a few instances statistically 

significant, associations with PM10-2.5 in 

some locations likely to have met the current 

PM10 standard but not a PM10 standard with 

a 98th percentile form and a level at or below 

87 mg/m.3

To the extent the above 

considerations are emphasized, the 

Policy Assessment notes that a standard 

level as high as about 85 mg/m3, for a 

24-hour PM10 standard with a 98th 

percentile form, could be supported. 

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 singlecity 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. Given this, when compared to 

the current standard, a 24-hour PM10

standard with a 98th percentile form 

and a level at or below 85 mg/m3 could 

have the effect of focusing public health 

protection on locations where there is 

more confidence that PM10-2.5 is 

associated with mortality and/or 

morbidity. 

Given the above, the Policy 

Assessment concludes that a 98th 

percentile standard with a level as high 

as 85 mg/m3 could be considered to the 

extent that more weight is placed on the 

appropriateness of focusing public 

health protection in areas where 

positive and statistically significant 

associations with PM10-2.5 have been 

reported, and to the extent less weight 

is placed on PM10-2.5 effect estimates 

that are not statistically significant and/ 

or that reflect estimates across multiple 

cities. The Policy Assessment notes that 

it could be judged appropriate to place 

less weight on PM10-2.5 effect estimates 

that are not statistically significant given 

the relatively large amount of 

uncertainty that is associated with the 

broader body of PM10-2.5 health 

evidence, including uncertainty in the 

extent to which health effects evaluated 

in epidemiological studies result from 

exposures to PM10-2.5 itself, rather than 

one or more co-occurring pollutants. 

This uncertainty, as well as other 

uncertainties discussed above, are 

reflected in the Integrated Science 

Assessment conclusions that the 

evidence is ‘‘suggestive’’ of a causal 

relationship (i.e., rather than ‘‘causal’’ or 

‘‘likely causal’’) between short-term 

PM10-2.5 and mortality, respiratory 

effects, and cardiovascular effects. In 

addition, the Policy Assessment 

concludes that it could be appropriate to 

place less weight on 98th percentile 

PM10 concentrations averaged across 

multiple cities, given the uncertainty in 

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119With regard to limitations and uncertainties in 

the evidence, CASAC endorsed the ISA 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). 

linking multi-city effect estimates with 

the air quality in any particular location. 

However, the Policy Assessment also 

notes that, overall across the U.S., based 

on recent air quality information (i.e., 

2007–2009), fewer people live in 

counties with 98th percentile 24-hour 

PM10 concentrations above 85 mg/m3

than in counties likely to exceed the 

current PM10 standard (U.S. EPA, 2011a, 

Table 3–2 and p. 3–48). These results 

could be interpreted to suggest that a 

98th percentile standard with a level of 

85 mg/m3 would decrease overall public 

health protection compared to the 

current standard. Based on this 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 mg/m3 would 

provide a level of public health 

protection that is generally equivalent, 

across the U.S., to that provided by the 

current standard. To the extent these 

population counts are emphasized in 

comparing the public health protection 

provided by the current and potential 

alternative standards, and to the extent 

it is judged appropriate to set a revised 

standard that provides at least the level 

of public health protection that is 

provided by the current standard based 

on such population counts, the Policy 

Assessment concludes that it would be 

appropriate to consider standard levels 

in the range of approximately 75 to 80 

mg/m3 (Id.). 

The Policy Assessment concludes that 

alternative approaches to considering 

the evidence could also lead to 

consideration of standard levels below 

75 mg/m3. 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 mg/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.5 above), 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 multi-city averages of 98th 

percentile PM10 concentrations in the 

locations evaluated by Zanobetti and 

Schwartz (2009) and Peng et al. (2008) 

were 77 and 68 mg/m3, respectively. 

Both of these multi-city studies reported 

positive and statistically significant 

PM10-2.5 effect estimates that remained 

positive in co-pollutant models that 

included PM2.5, though only Zanobetti 

and Schwartz (2009) reported PM10-2.5

effect estimates that remained 

statistically significant in such copollutant models. Despite uncertainties 

in the extent to which effects reported 

in these multi-city studies are associated 

with the short-term air quality in any 

particular location, emphasis could be 

placed on these multi-city associations. 

The Policy Assessment concludes 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 

mg/m3 (U.S. EPA, 2011a, p. 3–48). A 

standard level of 65 mg/m3 would be 

expected to provide a substantial margin 

of safety against health effects that have 

been associated with PM10-2.5 and, as 

discussed above, could better focus 

(compared to the current standard) 

public health protection on areas where 

health studies have reported 

associations with PM10-2.5. 

In considering potential alternative 

standard levels below 65 mg/m3, the 

Policy Assessment notes that, as 

discussed above, 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 

mg/m3 than in locations with 98th 

percentile PM10 concentrations below 

65 mg/m3. Specifically, the Policy 

Assessment notes the following (Id., 

p. 3–49): 

(1) Epidemiological studies, either singlecity or multi-city, have not reported positive 

and statistically significant PM10-2.5 effect 

estimates in locations with 98th percentile 

PM10 concentrations (multi-city average 98th 

percentile concentrations in the case of 

multi-city studies) at or below 65 mg/m3. 

(2) Although some single-city morbidity 

studies have reported positive, but not 

statistically significant, associations with 

PM10-2.5 in locations with 98th percentile 

PM10 concentrations below 65 mg/m3, the 

results of U.S. morbidity studies were 

generally less consistent than those of 

mortality studies, with some PM10-2.5 effect 

estimates being positive while others were 

negative (i.e., negative effect estimates were 

reported in several studies conducted in 

Atlanta, where the 98th percentile PM10

concentrations ranged from 67 mg/m3 to 

71 mg/m3). 

(3) Although Bayes-adjusted single-city 

PM10-2.5 effect estimates were positive, but 

not statistically significant, in some locations 

with PM10 concentrations below 65 mg/m3, 

these effect estimates were based on the 

difference between community-wide PM10

and PM2.5 concentrations. As discussed 

above, it is not clear how these estimates of 

PM10-2.5 concentrations compare to those 

more typically used in other studies to 

calculate PM10-2.5 effect estimates. At present, 

few corroborating studies are available that 

use other approaches (i.e., co-located 

monitors, dichotomous samplers) to 

estimate/measure PM10-2.5 in locations with 

98th percentile PM10 concentrations below 

65 mg/m3. 

In light of these 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 concludes that 

while it could be judged appropriate to 

consider standard levels as low as 65 

mg/m3, it is not appropriate, based on 

the currently available body of 

evidence, to consider standard levels 

below 65 mg/m3. 

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).119 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 copollutant 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 

the associations reported in 

epidemiologic studies’’ (Samet, 2010d, 

p. 7). 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 current 

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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 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 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 mg/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 mg/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 mg/m3. CASAC used 

this information as the basis for their 

conclusion that a 98th percentile 

standard between 75 and 80 mg/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 mg/m3 in order to 

increase public health protection, 

relative to that provided by the current 

standard (Samet 2010d, p. ii). 

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 notes 

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 above, the 

Administrator provisionally concludes 

that the available evidence continues to 

support the appropriateness of 

maintaining a standard to protect the 

public health against effects associated 

with short-term (e.g., 24-hour) 

exposures to PM10-2.5. In drawing 

conclusions as to whether the current 

PM10 standard is 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 has 

considered: 

(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. 

With regard to the first point, in the 

last review the EPA concluded 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 an adequate margin of safety (71 

FR 61197, October 17, 2006). In that 

review, the EPA concluded that 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). In litigation of 

the decisions from the last review, the 

D.C. Circuit affirmed the conclusion that 

it was appropriate 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 

current review, the Administrator 

judges that the expanded body of 

scientific evidence 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 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). While 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). 

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 

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120The 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. 

121The 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. 

Integrated Science Assessment’s 

conclusions regarding the links between 

adverse health effects and PM sources 

and composition, the Administrator 

provisionally 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 point, 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 

(see above) 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 nonurban origin. She specifically notes 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 rural areas. While 

associations with adverse health effects 

have been reported in some study 

locations where PM10-2.5 is largely nonurban 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 provisionally 

concludes that it remains appropriate to 

maintain a standard that 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 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 provisionally 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.120 121

In reaching this initial conclusion, the 

Administrator also notes 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 judges 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 notes that 

the Policy Assessment discusses 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 in detail 

(section IV.C.1), lead to different 

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conclusions regarding the 

appropriateness of the degree of public 

health protection provided by the 

current PM10 standard. The 

Administrator further notes that the 

primary difference between the two 

approaches lies in the extent to which 

weight is placed on the following (U.S. 

EPA, 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; 

(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 

co-pollutant models; 

(4) The limited number of PM10-2.5

controlled human exposure studies; 

(5) Uncertainties in the PM10-2.5 air quality 

concentrations used 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 notes that when the 

available PM10-2.5 scientific evidence 

and its associated uncertainties are 

considered, the Integrated Science 

Assessment concludes 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 notes that she is 

proposing to strengthen 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) (section III.B). The 

suggestive judgment for PM10-2.5 reflects 

the greater degree of uncertainty 

associated with this body of evidence, 

as discussed above in detail (sections 

IV.B.5 and IV.C.1) 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 co-pollutants, 

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 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 provisional 

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 singlepollutant and/or co-pollutant models. 

With regard to this provisional 

conclusion, the Administrator notes 

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 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 the singlecity analyses for the locations evaluated 

in the multi-city study by Zanobetti and 

Schwartz (2009), the Administrator 

notes that associations in most of these 

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 above, 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 with a dichotomous 

sampler or by calculating the difference 

using co-located samplers (U.S. EPA, 

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122As noted in section IV.B.5 above and in the 

Policy Assessment (U.S. EPA, 2011a, p. 3–16), there 

are also important uncertainties in estimates of 

ambient PM10-2.5 concentrations based on the 

difference between PM10 mass and PM2.5 mass, as 

measured at co-located monitors. 

123This 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 provisional 

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. 

124There 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 

at 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. 

125As 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. 

2009a, section 6.5.2.3).122 For these 

reasons, the Administrator notes that 

there is considerable uncertainty in 

interpreting the associations in these 

single-city analyses. 

The Administrator acknowledges 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, 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. 

In light of the above approach to 

considering the scientific evidence, air 

quality information, and associated 

uncertainties, the Administrator reaches 

the following provisional conclusions: 

(1) Given 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; 

given 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 co-located samplers; and given 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; 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.123

(2) Given 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, the degree of public health 

protection provided by the current standard 

is not greater than warranted.124

In reaching these provisional 

conclusions, the Administrator notes 

that the Policy Assessment also 

discusses 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).125 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: 

(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 colocated 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 weightof-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 

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126See http://www.airnow.gov/. 

127 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. 

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 

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 information, the associated 

uncertainties, and CASAC’s advice, the 

Administrator reaches 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 proposes 

to retain the current PM10 standard in 

order to protect against health effects 

associated with short-term exposures to 

PM10-2.5. 

The Administrator recognizes that her 

proposed conclusions and decision to 

retain the current PM10 standard differ 

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 welcomes 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. 

F. Administrator’s Proposed Decision on 

the Primary PM10 Standard 

For the reasons discussed above, and 

taking into account the information and 

assessments presented in the Integrated 

Science Assessment and the Policy 

Assessment and the advice and 

recommendations of CASAC, the 

Administrator proposes to retain the 

current primary PM10 standard. The 

Administrator solicits comment on all 

aspects of this 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. 

V. Communication of Public Health 

Information 

Sections 319(a)(1) and (3) of the CAA 

require the EPA to establish a uniform 

air 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 Web site.126 The 

current AQI has been in use since its 

inception in 1999.127 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. In fact, many countries have 

created similar indices based on the 

AQI. 

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 (201–300), 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). Decisions about the pollutant 

concentrations at which to set the 

various AQI breakpoints that delineate 

the various AQI categories for each 

pollutant specific sub-index within the 

AQI draw directly from the underlying 

health information that supports the 

NAAQS review. 

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, realtime data reporting through the AIRNow 

Web site, and 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 and can 

encourage the public to take actions that 

will reduce air pollution on days when 

levels are projected to be at levels of 

concern to local communities. Thus, 

these programs have significantly 

broadened the ways in which state and 

local agencies can meet the nationally 

uniform AQI reporting requirements, 

and are contributing to state and local 

efforts to provide community health 

protection and to attain or maintain 

compliance with the NAAQS. The EPA 

and state and local agencies recognize 

that these programs are interrelated with 

AQI reporting and with the information 

on the effects of air pollution on public 

health that is generated through the 

periodic review, and revision when 

appropriate, of the NAAQS. 

In recognition of the proposed change 

to the primary annual PM2.5 standard 

summarized in section III.F above, the 

EPA proposes a conforming change to 

the PM2.5 sub-index of the AQI to be 

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128Currently, we are cautioning members of 

sensitive groups at the AQI value of 100 at 35 mg/ 

m3, 24-hour average, consistent with more recent 

guidance from EPA with regard to the development 

of State emergency episode contingency plans 

(Harnett, 2009, Attachment B). 

129We 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. 

130We note that a level of 350 mg/m3 is 

recommended for an AQI value of 500 in the more 

recent EPA guidance (Harnett, 2009, Attachment B). 

consistent with the proposed change to 

the annual standard. The health effects 

information that supports the proposed 

decisions on the PM2.5 standards, as 

discussed in section III.B above, is also 

the basis for the proposed decisions on 

the AQI discussed below in this section. 

The EPA intends to finalize conforming 

changes to the AQI in conjunction with 

the Agency’s final decisions on the 

primary annual and 24-hour PM2.5

standards, if revisions to such standards 

are promulgated. 

With respect to an AQI value of 50, 

as discussed above, the historical 

approach is to set it at the same level of 

the annual standard, if there is one. This 

is consistent with the current AQI subindex for PM2.5, in which the current 

AQI value of 50 is set at 15 mg/m3, 

consistent with the level of the current 

primary annual PM2.5 standard. The 

EPA sees no basis for deviating from 

this approach in this review. Thus, the 

EPA proposes to set an AQI value of 50 

within a range of 12 to 13 mg/m3, 24- 

hour average, consistent with the 

proposed annual PM2.5 standard level 

(section III.F). The final AQI value of 50 

will be set at the level of the annual 

PM2.5 standard that is promulgated. 

With respect to an AQI value of 100, 

which is the basis for advisories to 

individuals in sensitive groups, there 

are 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 the 

other sub-indices as noted above, is to 

set an AQI value of 100 at the same level 

as the short-term standard. The EPA 

recognizes 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. 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 health 

impacts associated with daily air 

quality. The EPA proposes to use this 

approach to set the AQI value of 100 at 

35 mg/m3, 24-hour average, consistent 

with the proposal to retain the current 

24-hour PM2.5 standard (section III.F). If 

the 24-hour standard is set at a different 

level, the EPA proposes to set an AQI 

value of 100 at the level of the 24-hour 

PM2.5 standard that is promulgated. 

An alternative approach is 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 mg/m3, 24-hour 

average,128 when the 24-hour standard 

level was 65 mg/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 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. The EPA is soliciting 

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 recognizes that some 

state and local air quality agencies have 

expressed support for this alternative 

approach. Using this alternative 

approach could result 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 

above. The EPA encourages state and 

local air quality agencies that use the 

AQI to comment on both the approach 

and the level at which to set an AQI 

value of 100 together with any 

supporting rationale. 

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 mg/m3, 24- 

hour average. In considering what level 

to propose for an AQI value of 150, we 

believe that the health effects evidence 

indicates that the level of 55 mg/m3, 24- 

hour average, is appropriate to use 129 in 

conjunction with an AQI value of 100 

set at the proposed level of 35 mg/m3. 

Thus, if the EPA sets an AQI value of 

100 at the PM2.5 level of 35 mg/m3, 24- 

hour average, the Agency proposes to 

set an AQI value of 150 at the PM2.5

level of 55 mg/m3, 24-hour average. If, 

however, the EPA decides to set an AQI 

value of 100 at a lower level, then the 

EPA would adjust an AQI value of 150 

proportionally. 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, EPA concludes the use of a 

proportional adjustment would be 

appropriate. 

With respect to an AQI value of 500, 

a review of the history of the AQI value 

of 500 for PM10 and of the AQI value of 

500 for PM2.5 is useful background. The 

current AQI value of 500 for PM10 was 

set in 1987 at the level of 600 mg/m3, 24- 

hour average, on the basis of increased 

mortality associated with historical 

wintertime pollution episodes in 

London (52 FR 24687 to 24688, July 1, 

1987). Particle concentrations during 

these episodes, measured by the British 

Smoke method, were in the range of 500 

to 1000 mg/m3. In the 1987 rulemaking 

that established the upper bound index 

value for PM10, the EPA cited a 

generally held opinion that the British 

Smoke method measures PM with a 

cutpoint of approximately 4.5 microns 

(52 FR 24688, July 1, 1987). In 

establishing this value for PM10, the 

EPA assumed that concentrations of 

PM10, which includes both coarse and 

fine particles, during episodes of 

concern, would be about 100 mg/m3

higher than the PM concentration 

measured in terms of British Smoke (52 

FR 24688, July 1, 1987). The upper 

bound index value of 600 mg/m3 was 

developed by selecting the lower end of 

the range of harmful concentrations 

during the historical wintertime 

pollution episodes in London (500 mg/ 

m3) and adding a margin of 100 mg/m3

to account for this measurement 

difference. The current PM2.5

concentration corresponding to an AQI 

value of 500 set in the 1999 rulemaking 

is 500 mg/m3, 24-hour average.130

Because there were few PM2.5

monitoring data available at that time, 

the decision was based on the stated 

assumption that PM concentrations 

measured by the British Smoke method 

were approximately equivalent to PM2.5

concentrations. In considering whether 

it is appropriate to retain or revise the 

AQI value of 500 for PM2.5, the EPA 

notes that the 1999 rulemaking was 

based on an assumption of approximate 

equivalence between the British Smoke 

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131As discussed in section VII.C below, the EPA 

is also proposing to update 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. 

method and the current PM2.5 method. 

This assumption is not entirely 

consistent with the view cited in 1987 

that the British Smoke method has a 

size cutpoint of 4.5 microns (52 FR 

24688, July 1, 1987), such that it would 

be reasonable to expect based on 

considering size cutpoint alone that a 

level of 500 mg/m3 based on the British 

Smoke method would generally be 

equivalent to a somewhat lower level 

based on the current PM2.5 method. 

Nonetheless, more recent comparisons 

between British Smoke and PM2.5

measurement methods (Heal, et al., 

2005; Chaloulakou, et al., 2005) suggest 

that on average British Smoke can be 

less than or more than PM2.5, but 

generally represents a larger fraction in 

the seasons and locations when PM2.5

predominantly results from directly 

emitted carbonaceous particles such as 

from combustion sources. More 

generally, the EPA recognizes that 

extremely high PM concentrations that 

would most likely be associated with 

combustion sources (e.g., coal burning 

in historic the London event, wildfires 

in contemporary U.S. environments) are 

typically dominated by fine particles, 

such that there may be very little 

difference between these measurement 

methods at such high levels. 

Further, in considering the body of 

more recent health effects evidence 

available in this review, the EPA 

concludes that there is little information 

about more recent air pollution episodes 

similar to the wintertime pollution 

episodes in London and associated 

impacts on community health upon 

which to base a decision. Thus, the EPA 

concludes that it remains appropriate to 

use the historical wintertime pollution 

episodes in London as the basis for 

setting an AQI value of 500 for PM2.5 as 

described above because it is still the 

best available directly relevant 

information. Nonetheless, the EPA takes 

note of a limited number of more recent 

studies cited in the Integrated Science 

Assessment that evaluated wood smoke 

health impacts which found effects such 

as cardiovascular morbidity and 

mortality as well as respiratory effects, 

albeit at much lower levels (U.S. EPA, 

2009a, sections 6.2 and 6.6). These more 

recent health studies may provide some 

support for considering a lower PM2.5

level for an AQI value of 500. 

Based on the above considerations, 

the EPA concludes that it is appropriate 

to propose to retain the current level of 

500 mg/m3, 24-hour average, for the AQI 

value of 500. The EPA solicits comment 

on alternative approaches to setting a 

level for the AQI value of 500 and on 

alternative levels that commenters 

believe may be appropriate as well as 

supporting information and rationales 

for such alternative levels. The EPA also 

solicits 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. 

For the intermediate breakpoints in 

the AQI between the values of 150 and 

500, the EPA proposes PM2.5

concentrations that generally reflect a 

linear relationship between increasing 

index values and increasing PM2.5

values. The 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 

suggest 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 

proposed breakpoints for the PM2.5 subindex.131 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—PROPOSED BREAKPOINTS FOR PM2.5 SUB-INDEX

AQI category Index values 

Proposed breakpoints 

(μg/m3, 24-hour 

average) 

Good ........................................................................................................................................................ 0–50 0.0–(12.0–13.0) 

Moderate .................................................................................................................................................. 51–100 (12.1–13.1)–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 

401–500 

250.5–350.4 

350.5–500.4 

In proposing to retain the 500 level for 

the AQI as described above, we note 

that the EPA is not proposing to 

establish 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 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. 

VI. Rationale for Proposed Decisions on 

the Secondary PM Standards 

This section presents the rationale for 

the Administrator’s proposed decisions 

to revise the current suite of secondary 

PM standards by adding a distinct 

standard for PM2.5 to address PM-related 

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38966 Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / Proposed Rules 

132 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. 

visibility impairment while retaining 

the current secondary PM2.5 and PM10

standards to address the other welfare 

effects considered in this review. In 

particular, this section presents 

background information on EPA’s 

previous and current reviews of the 

secondary PM standards (section VI.A), 

information on visibility impairment 

(section VI.B), conclusions on the 

adequacy of the current secondary PM2.5

standards to protect against PM-related 

visibility impairment (section VI.C), 

conclusions on alternative standards to 

protect against PM-related visibility 

impairment (section VI.D), conclusions 

on secondary PM standards to address 

other PM-related welfare effects (section 

VI.E), and a summary of the 

Administrator’s proposed decisions on 

the secondary PM standards (section 

VI.F). 

A. Background 

The current suite of secondary PM 

standards is identical to the current 

suite of primary PM standards, 

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 

builds upon and broadens the 

approaches used in previous PM 

NAAQS reviews. The following 

discussion focuses particularly on the 

current 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 current approach for 

evaluating the secondary PM2.5

standards (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, utilize different 

ways to consider the underlying body of 

scientific evidence. They also reflect an 

evolution in EPA’s understanding of the 

nature of the effect on public welfare 

from visibility impairment, from an 

approach focusing only on Federal Class 

I area visibility impacts to a more 

multifaceted approach that also 

considers PM-related impacts on nonFederal Class I area visibility, such as in 

urban areas. This evolution has 

occurred in conjunction with the 

expansion of available PM data and 

information from associated studies of 

public perception, valuation, and 

personal comfort and well-being. 

In 1997, the EPA revised the identical 

primary and secondary 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 

regional differences. These differences 

included several factors that influence 

visibility 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 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 these considerations, the 

EPA assessed potential visibility 

improvements in urban areas and on a 

regional scale that would result from 

attainment of the new primary 

standards for PM2.5. The agency 

concluded that the spatially averaged 

form of the annual PM2.5 standard was 

well suited to the protection of 

visibility, which involves effects of 

PM2.5 throughout an extended viewing 

distance across an urban area. Based on 

air quality data available at that time, 

many urban areas in the Northeast, 

Midwest, and Southeast, as well as Los 

Angeles, were expected to see 

perceptible improvement in visibility if 

the annual PM2.5 primary standard were 

attained. The EPA also concluded that 

attainment of the 24-hour PM2.5

standard in some areas would be 

expected to reduce, to some degree, the 

number and intensity of ‘‘bad visibility’’ 

days, resulting in improvement in the 

20 percent of days having the greatest 

impairment over the course of a year. 

Having concluded that attainment of 

the annual and 24-hour PM2.5 primary 

standards would lead to visibility 

improvements in many eastern and 

some western urban areas, the EPA also 

considered whether these standards 

could provide potential improvements 

to visibility on a regional scale. Based 

on information available at the time, the 

EPA concluded that attainment of 

secondary PM2.5 standards set identical 

to the primary PM2.5 standards would be 

expected to result in 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 then considered the 

potential effectiveness of a regional haze 

program, required by sections 169A and 

169B of the CAA 132 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 a nationally 

applicable secondary NAAQS. The 

regional haze visibility goal is more 

protective than a secondary NAAQS 

since the goal addresses any 

anthropogenic impairment rather than 

just impairment at levels determined to 

be adverse to 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 

non-Federal 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 

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emissions of pollutants that impair 

visibility in many of these areas. 

For these reasons, 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. 

Based on these considerations, in 

1997 the EPA set secondary PM2.5

standards identical to the primary PM2.5

standards, in conjunction with a 

regional haze program 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 PM-related visibility 

impairment and enable all regions of the 

country to make reasonable progress 

toward the national visibility goal. 

In 2006, 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 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 particleinduced 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 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. 

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 

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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 mg/m3 PM2.5

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 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 (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 

near-continuous 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 

mg/m3’’ and that ‘‘[i]t is not 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 abovementioned 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 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 EPA’s 

decision lacked a reasoned basis. First, 

they asserted that EPA never 

determined what level of visibility was 

‘‘requisite to protect the public welfare.’’ 

They argued that EPA unreasonably 

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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 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 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 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 standards. More 

fundamentally, however, the court 

found that 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 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 

broadens 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 is 

taking into account considerations 

based on both the scientific evidence 

(‘‘evidence-based’’) 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 are 

appropriate for consideration in this 

review. As in past reviews, the EPA is 

also considering 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 

visibility in Federal Class I areas. The 

EPA again recognizes that such an 

approach is the most appropriate and 

effective means of addressing the public 

welfare effects associated with visibility 

impairment in areas across the country. 

The Policy Assessment draws from 

the qualitative evaluation of all studies 

discussed in the Integrated Science 

Assessment (U.S. EPA, 2009a). 

Specifically, the Policy Assessment 

considers the extensive new air quality 

and source apportionment information 

available from the regional planning 

organizations, long-standing evidence of 

PM effects on visibility, and public 

preference studies 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 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. On these bases, the Policy 

Assessment again focuses assessments 

on visibility conditions in urban areas. 

The conclusions in the Policy 

Assessment reflect 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 takes into account 

the four basic elements of the NAAQS: 

indicator, averaging time, level, and 

form. 

B. PM-Related Visibility Impairment 

As discussed below, the rationale for 

the Administrator’s proposed decision 

regarding secondary PM standards to 

protect against visibility impairment 

focuses on those considerations most 

influential in the Administrator’s 

proposed decisions, including 

consideration of: (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 level) that, taken together, would be 

requisite to protect the public welfare 

from adverse effects on visual air 

quality. 

This section outlines key information 

contained in the Integrated Science 

Assessment, the Visibility Assessment 

and the Policy Assessment on: (1) 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 and (2) public 

perceptions and attitudes about 

visibility impairment and the impacts of 

visibility impairment on public welfare. 

1. Nature of PM-Related Visibility 

Impairment 

New research conducted by regional 

planning organizations in support of the 

Regional Haze Rule, as 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 

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133The 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). 

134These 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. 

135To calculate ammonium sulfate, multiply the 

CSN measurement of the sulfate ion by 1.375. To 

calculate ammonium nitrate, multiply the CSN 

measurement of the nitrate ion by 1.29 (Lowenthal 

and Kumar, 2006). 

contributions to that effect in rural and 

remote locations. Additional byproducts of this research include 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 characterization of 

current visibility levels in urban areas. 

Information from both of these sources 

of PM data, while useful, has not 

however changed the fundamental and 

long understood science characterizing 

the contribution of PM, especially fine 

particles, to visibility impairment. This 

science, briefly summarized below, 

provides the basis for the Integrated 

Science Assessment designation of the 

relationship between PM and visibility 

impairment as causal. 

a. 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. 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 is 

quantified by a light extinction 

coefficient with units of 1/distance, 

which is often expressed in the 

technical literature as 1/(1 million 

meters) or inverse megameters 

(abbreviated Mm¥1). When PM is 

present in the air, its contribution to 

light extinction typically greatly exceeds 

that of gases. 

The amount of light extinction 

contributed by PM depends on the 

particle size distribution and 

composition, as well as its particle 

concentration. If details of the ambient 

particle size distribution and 

composition (including the mixing of 

components) are known, Mie theory can 

be used to accurately calculate PM light 

extinction (U.S. EPA, 2009a, chapter 9). 

However, routine monitoring rarely 

includes measurements of particle size 

and composition information with 

sufficient detail for such calculations. 

To make estimation of light extinction 

more practical, visibility scientists have 

developed a much simpler algorithm, 

known as the IMPROVE algorithm,133 to 

estimate light extinction using routinely 

monitored fine particle (PM2.5) 

speciation and coarse particle mass 

(PM10-2.5) data. In addition, relative 

humidity information is needed to 

estimate the contribution by liquid 

water that is in solution with 

hygroscopic PM components (U.S. EPA, 

2009a, section 9.2.2.2; U.S. EPA, 2010b, 

chapter 3). 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.134 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. 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. They have 

been evaluated by comparing the 

algorithm estimates of light extinction 

with coincident optical measurements. 

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 

Light extinction (bext) is in units of 

Mm¥1, the mass concentrations of the 

components indicated in brackets are in 

units of mg/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. The dry extinction 

efficiency for particulate organic mass is 

larger than those for particulate sulfate 

and nitrate principally because the 

density of the dry inorganic compounds 

is higher than that assumed for the PM 

organic mass components. 

For the first two terms, ‘‘sulfate’’ is 

defined in terms of ammonium sulfate 

and ‘‘nitrate’’ is defined in terms of 

ammonium nitrate. Since IMPROVE 

does not include ammonium ion 

monitoring, the assumption is made that 

all sulfate is fully neutralized 

ammonium sulfate and all nitrate is 

assumed to be ammonium nitrate.135

Though often reasonable, neither 

assumption is always true (see U.S. 

EPA, 2009a, section 9.2.3.1). In the 

eastern U.S. during the summer there is 

insufficient ammonia in the atmosphere 

to neutralize the sulfate fully. Fine 

particle nitrates can include sodium or 

calcium nitrate, which are the fine 

particle fraction of generally much 

coarser particles due to nitric acid 

interactions with sea salt at near-coastal 

areas (sodium nitrate) or nitric acid 

interactions with calcium carbonate in 

crustal aerosol (calcium nitrate). Despite 

the simplicity of the 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. 

The f(RH) term 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 

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136The IMPROVE algorithm does not explicitly 

separate the light-scattering and light-absorbing 

effects of elemental carbon. 

137Consistent with calculations used in the 

IMPROVE network and the Regional Haze Program, 

the fine soil component is calculated using the 

following formula: 

Fine Soil = 2.20 × [Al] + 2.49 × [Si] + 1.63 × [Ca] 

+ 2.42 × [Fe] + 1.94 × [Ti]. 

138The revised IMPROVE algorithm uses a 

multiplier of 1.8 instead of 1.4 as used in the 

original algorithm for the mean ratio of organic 

mass to organic carbon. 

139As 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 a negative deciview values when the 

PM2.5 concentration is very low. 

approximately 85 percent where their 

total light extinction efficiency exceeds 

the 10 m2/g associated with elemental 

carbon (EC). Based on this algorithm, 

particulate sulfate and nitrate are 

estimated to have comparable light 

extinction efficiencies (i.e., the same dry 

extinction efficiency and f(RH) water 

growth terms), so on a per unit mass 

concentration basis at any specific 

relative humidity they are treated as 

equally effective contributors to 

visibility effects. 

As noted above, 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. 

PM containing elemental or black 

carbon (BC) absorbs light as well as 

scattering it, making it the component 

with the greatest light extinction 

contributions per unit of mass 

concentration, except for the 

hygroscopic components under high 

relative humidity conditions.136

With regard to the fifth and sixth 

terms, the fine soil component is based 

on measurement of five elements: 

Aluminum (Al), silicon (Si), calcium 

(Ca), iron (Fe), and titanium (Ti).137

Inspection of the PM componentspecific 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. 

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. It 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, a 

different multiplier for organic carbon 

for purposes of estimating organic 

carbonaceous material,138 and sitespecific Rayleigh light scattering terms 

in place of a universal Rayleigh light 

scattering value. These features of the 

revised IMPROVE algorithm are 

described in section 9.2.3.1 of the 

Integrated Science Assessment, which 

also presents a comparison of the 

estimates produced by the two 

algorithms for rural areas. Compared to 

the original algorithm, the revised 

IMPROVE algorithm can yield higher 

estimates of current light extinction 

levels in urban areas on days with 

relatively poor visibility (Pitchford, 

2010). This difference is primarily 

attributable to the split-component mass 

extinction efficiency treatment in the 

revised algorithm rather than to the 

inclusion of a sea salt term or the use 

of site-specific Rayleigh scattering 

values. 

As mentioned above, particles are not 

the only contributor to ambient 

visibility conditions. Light scattering by 

gases also occurs in ambient air. Under 

pristine atmospheric conditions, 

naturally occurring gases such as 

elemental nitrogen and oxygen cause 

what is known as Rayleigh scattering. 

Rayleigh scattering depends on the 

density of air, which is a function 

primarily of the elevation above sea 

level, and can be treated as a sitedependent constant. The Rayleigh 

scattering contribution to light 

extinction is only significant under 

pristine conditions. The only other 

commonly occurring atmospheric gas to 

appreciably absorb light in the visible 

spectrum is nitrogen dioxide. Nitrogen 

dioxide forms in the atmosphere from 

nitrogen oxide emissions associated 

with combustion processes. These 

combustion processes also emit PM at 

levels that generally contribute much 

higher light extinction than the nitrogen 

dioxide (i.e., nitrogen dioxide 

absorption is generally less than 

approximately 5 percent of the light 

extinction, except where emission 

controls remove most of the PM prior to 

releasing the remaining gases to the 

atmosphere). The final term in the 

IMPROVE algorithm of 10 Mm¥1 is 

known as the Rayleigh scattering term 

and accounts for light scattering by the 

natural gases in unpolluted air. The 

remainder of this section focuses on the 

contribution of PM, which is typically 

much greater than that of gases, to 

ambient light extinction, unless 

otherwise specified. 

In the following discussions, visual 

air quality is characterized in terms of 

both light extinction, as discussed 

above, and an alternative scale for 

characterizing visibility—the deciview 

scale—that is defined directly in terms 

of light extinction (expressed in units of 

Mm¥1) by the following equation: 139

Deciview (dv) = 10 ln (bext/10 Mm¥1). 

The deciview scale is frequently used 

in the scientific and regulatory 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. 

b. 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). Generally, low mixing heights at 

night and during the early morning 

hours tend to trap locally produced 

emissions, which are diluted as the 

mixing height increases due to heating 

during the day. Low temperatures and 

high relative humidity at night are 

conducive to the presence of 

ammonium nitrate particles and water 

growth by hygroscopic particles 

compared with the generally higher 

temperatures and lower relative 

humidity later in the day. These 

combine to make early morning the 

most likely time for peak urban light 

extinction. Superimposed on such 

systematic time-of-day variations are the 

effects of synoptic meteorology (i.e., 

those associated with changing weather) 

and regional-scale air quality that can 

generate peak light extinction impacts 

any time of day. The net effects of the 

systematic urban- and larger-scale 

variations are that peak daytime PM 

light extinction levels can occur any 

time of day, although in many areas 

they most often occur 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 extinction in 

most Federal Class I areas, reflective of 

a relative lack of local sources as 

compared to urban areas. The use in the 

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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. 

c. Periods During the Day of Interest for 

Assessment of Visibility 

Visibility is typically associated with 

daytime periods because people are 

outside more during the day than at 

night and there are more viewable 

scenes at a distance during the day than 

at night. The Policy Assessment 

recognizes, however, that physically PM 

light extinction behaves the same at 

night as during the day, 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. These effects produce 

the visual result of a reduction in the 

number of visible stars and the 

disappearance of diffuse or subtle 

phenomena such as the Milky Way. The 

extinction of starlight is a secondary and 

minor effect also caused by increased 

PM scattering and absorption. 

However, there are significant and 

important differences between daytime 

and nighttime visual environments with 

regard to how light extinction per se 

relates to visual air quality (or visibility) 

and public welfare. First, daytime 

visibility has dominated the attention of 

those who have studied the visibility 

effects of air pollution, particularly in 

urban areas. As a result, little research 

has been conducted on nighttime 

visibility and the state of the science is 

not comparable to that associated with 

daytime visibility impairment. As noted 

in the Policy Assessment, no urbanfocused preference or valuation studies 

providing information on public 

preferences for nighttime visual air 

quality have been identified (U.S. EPA, 

2011a, p. 4–17). Second, in addition to 

air pollution, nighttime visibility is 

affected by the addition of light into the 

sight path from numerous sources, 

including anthropogenic light sources in 

urban environments such as artificial 

outdoor lighting, which varies 

dramatically across space, and natural 

sources including the moon, planets, 

and stars. Light sources and ambient 

light conditions are typically five to 

seven orders of magnitude dimmer at 

night than in sunlight. Moonlight, like 

sunlight, introduces light throughout an 

observer’s sight path at a constant angle. 

On the other hand, dim starlight 

emanates from all over the celestial 

hemisphere while artificial lights are 

concentrated in cities and illuminate the 

atmosphere from below. These different 

light sources will yield variable changes 

in visibility as compared to what has 

been established for the daytime 

scenario, in which a single source, the 

sun, is by far the brightest source of 

light. Third, the human psychophysical 

response (e.g., how the human eye sees 

and processes visual stimuli) at night is 

expected to differ (U.S. EPA, 2009a, 

section 9.2.2). 

Given the above, 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). Thus, the Policy Assessment 

focuses its assessments of PM visibility 

impacts in urban areas on daylight 

hours. For simplicity, and because 

perceptions and welfare effects from 

light extinction-related visual effects 

during the minutes of actual sunrise and 

sunset have not been explored, daylight 

hours are defined as those hours 

entirely after the local sunrise time and 

before the local sunset time. 

In so doing, the Policy Assessment 

notes that the 24-hour averaging time 

used in the Regional Haze Program 

includes nighttime conditions (U.S. 

EPA, 2011a, p. 4–18). It also notes, 

however, that the goal of the Regional 

Haze Program is to address any 

manmade impairment of visibility 

without regard to distinctions between 

daylight and nighttime conditions. 

Moreover, because of the lack of strong 

diurnal patterns in most Federal Class I 

areas, both nighttime and daylight 

visibility are strongly correlated with 

24-hour average visibility conditions, so 

a 24-hour averaging period is suitable 

for driving both daylight and nighttime 

visibility towards their natural 

conditions. Also, the focus on 24-hour 

average visibility allows the Regional 

Haze Program to make use of more 

practically obtained ambient speciated 

PM measurements of adequate accuracy 

than if a shorter averaging period were 

used, which is an important 

consideration especially given the 

remoteness of many Federal Class I area 

monitoring sites and given the low PM 

concentrations that must be measured 

accurately in such areas. 

In addition, when natural conditions 

such as fog and rain cause poor 

visibility, it can be reasonably assumed 

that the light extinction properties of the 

air that are attributable to air pollution 

are not important from a public welfare 

perspective. Thus, it is appropriate to 

give special treatment to such periods 

when considering whether current PM2.5

standards adequately protect public 

welfare from PM-related visibility 

impairment. In evaluating alternative 

sub-daily standards, the Policy 

Assessment addresses this issue by 

screening out hours with particularly 

high relative humidity. As discussed 

further below, the Policy Assessment 

uses a relative humidity screen of 90 

percent on the basis that it serves as a 

reasonable surrogate for excluding hours 

affected by fog and rain (U.S. EPA, 

2011a, p. 4–18). 

d. Exposure Durations of Interest 

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 has not been 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. A momentary glance at an 

image of a scene (i.e., less than a 

minute) is enough for study participants 

to judge the acceptability or 

unacceptability of the viewed visual air 

quality conditions. Moreover, 

individual participants in general 

consistently judge the acceptability of 

same-scene images that differed only 

with respect to light extinction levels 

when these images were presented 

repeatedly for such short periods. That 

is, individuals generally did not say that 

a higher-light extinction image was 

acceptable while saying a lower-light 

extinction, same-scene image was 

unacceptable, even though they could 

not compare images side-to-side. 

However, the Policy Assessment does 

not have information about what 

assumptions, if any, the participants 

may have made about the duration of 

exposure in determining the 

acceptability of the images and EPA 

staff 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. 

In the absence of such studies, the 

Policy Assessment considers 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 

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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. In such 

circumstances, the viewer may 

consciously evaluate how the visual air 

quality at that time either enhances or 

diminishes the experience or view. 

However, the public also 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). These scenes, whether iconic 

or generic, may not be consciously 

viewed for their scenic value and may 

not even be noticed for periods 

comparable to what would be the case 

during purposeful visits to scenic visits, 

but their visual air quality may still 

affect a person’s sense of wellbeing. 

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 wellbeing.’’ 

Some people have mostly intermittent 

opportunities on a daily basis (e.g., 

during morning and/or afternoon 

commutes) to experience ambient 

visibility conditions because they spend 

much of their time indoors without 

access to windows. For such people a 

view of poor visual air quality during 

their morning commute may provide 

their perception of the day’s visibility 

conditions until the next time they 

venture outside during daylight hours 

later or perhaps the next day. Other 

people have exposure to visibility 

conditions throughout the day, 

conditions that may differ from hour to 

hour. A day with multiple hours of 

visibility impairment would likely be 

judged as having a greater impact on 

their wellbeing than a day with just one 

such hour followed by clearer 

conditions. 

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 logical to 

conclude 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 

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 judges 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. This approach 

would recognize that at least some but 

not all of the population of an area will 

actually be exposed to this worst hour 

and that some of the people who are 

exposed to this worst hour may not have 

an opportunity to observe clearer 

conditions in other hours if they were 

to occur. Moreover, because visibility 

conditions and people’s daily activities 

on work/school days both tend to follow 

the same diurnal pattern day after day, 

those who are exposed only to the worst 

hour will tend to have this experience 

day after day. 

For another group of observers, those 

who have access to visibility conditions 

often or continuously throughout the 

day, the impact of the day’s visibility 

conditions on their welfare may be 

based on the varying visibility 

conditions they observe 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. 

Based on these considerations, the 

Policy Assessment judges that it would 

also be appropriate to use a maximum 

multi-hour daylight period for 

evaluating the adequacy of the current 

suite of secondary standards (U.S. EPA, 

2011a, p. 4–20). 

The above discussion is based on 

what people see, which is determined 

by the extinction of light along the paths 

between observers and the various 

objects they view. A related but separate 

issue is what measurement period is 

relevant, if what will be measured is the 

light extinction property or the PM 

concentration of the local air at a fixed 

site. Light extinction conditions at a 

fixed site can change quickly (i.e., in 

less than a minute). Sub-hourly 

variations in light extinction determined 

at any point in the atmosphere are likely 

the result of small-scale spatial 

pollution features (i.e., high 

concentration plumes that have just 

been generated in the immediate 

vicinity due to local sources or that have 

been transported by the wind across that 

point). These small-scale pockets of air 

causing short periods of higher light 

extinction at the fixed site likely do not 

determine the visual effect for scenes 

with longer sight paths. In contrast, 

atmospheric sight path-averaged light 

extinction which is pertinent to 

visibility impacts generally changes 

more slowly (i.e., tens of minutes 

generally), because a larger air mass 

must be affected by a broader set of 

emission sources or the larger air mass 

must be replaced by a cleaner or dirtier 

air mass due to the wind operating over 

time. At typical wind speeds found in 

U.S. cities, an hour corresponds to a few 

tens of kilometers of air flowing past a 

point, which is similar to sight path 

lengths of interest in urban areas. Based 

on the above considerations, the Policy 

Assessment concludes hourly average 

light extinction would generally be 

reasonably representative of the net 

visibility effect of the spatial pattern of 

light extinction levels, especially along 

site paths that generally align with the 

wind direction (U.S. EPA, 2011a, 

p. 4–21). 

2. Public Perception of Visibility 

Impairment 

As noted in the Integrated Science 

Assessment, 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 

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.’’ 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 

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140 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. 

141By ‘‘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. 

valuation of changes in visibility, which 

include hedonic valuation, contingent 

valuation and contingent choice, and 

travel cost. 

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.140

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 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.141 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 userspecified 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 the Visibility 

Assessment (U.S. EPA, 2010b, chapter 

2) to assess the light extinction levels 

judged 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 includes 

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 answered while viewing a set 

of images of various urban visual air 

quality conditions. 

The approaches used in the four 

studies are similar and are 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 

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 

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142Only 47 percent of the British Columbia 

participants rated a 19.2 dv photograph as 

acceptable. 

143 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 5. 

144Top 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. 

extinction than do images representing 

higher levels. 

Given the similarities in the 

approaches used, it is 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, 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 

the studies include: (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 the criterion as a useful index 

for comparison between studies. The 

results of each individual analysis were 

then combined graphically to allow for 

visual comparison. This information 

was then carried forward into the Policy 

Assessment. Figure 5 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 5 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 5, only 

one photograph (or image) with a visual 

air quality below 20 dv was rated as 

acceptable by less than 50 percent of the 

participants who rated that 

photograph.142 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.143

As Figure 5 above shows, each urban 

area has a separate and unique response 

curve that appears to indicate that it is 

distinct from the others. 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 


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145These values were rounded from 74 Mm¥1

and 201 Mm¥1 to 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¥1 is rounded to 120 Mm¥1 for 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 considers 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 

reflect the unrounded values. 

146Rayleigh scatter is light scattering by 

atmospheric gases which is on average about 10 

Mm¥1. 

147The 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. 

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 5. 

Based on the composite results and 

the effective range of 50th percentile 

acceptability across the four urban 

preference studies shown in Figure 5 

and Table 4–1 of the Policy Assessment, 

benchmark levels of (total) light 

extinction were selected by the Policy 

Assessment in a range from 20 dv to 30 

dv (75 to 200 Mm¥1) 145 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). A 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 least noisy 

preference results and the most 

representative selection of participants. 

Based on the currently available 

information, the Policy Assessment 

concludes 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% 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,146 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 continues to be 

used in this 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 notes that the 

term CPL is meant only to indicate 

target levels of visibility within a range 

that EPA staff feels is appropriate for 

consideration that could, in conjunction 

with other elements of the standard, 

including indicator, averaging time, and 

form, provide an appropriate degree of 

visibility protection. 

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 

considers whether they represent a true 

regional distinction in preferences for 

urban visibility conditions between 

western and eastern U.S. There is 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.147 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 notes 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 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 and many areas in the eastern 

U.S., and possibly some areas in the 

western U.S. as well. 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. Based on the 

currently available information, the 

Policy Assessment concludes 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 concludes 

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 

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148One example of an indicator and measurement 

approach for which indicator values could be 

higher than true PM light extinction as a result of 

fog would be a light extinction indicator measured 

in part by an unheated nephelometer, which is an 

optical instrument for measuring PM light 

scattering from an air sample as it flows through a 

measurement chamber. Raindrops would be 

removed by the initial size-selective inlet device, 

although some particles associated with fog may be 

small enough that they might pass through the inlet 

and enter the measurement chamber of the 

instrument. This would result in a reported 

scattering coefficient that does not correspond to 

true PM light extinction. Direct measurement of 

light extinction using an open-path instrument 

would be even more affected by both fog and 

precipitation. 

149PM-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% cutpoint of 10 mm 

diameter; and PM2.5 light extinction refers to the 

contribution by particles sampled through an inlet 

with a particle size 50% cutpoint of 2.5 mm 

diameter. 

150The 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. 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, is not 

representative of the urban area and resulted in 

unrealistically large PM10-2.5 values. The EPA staff 

considers these comments credible and has set 

aside the St. Louis assessment results for PM10 light 

extinction. Thus, results and statements in this 

Policy Assessment regarding PM10 light extinction 

apply to only the other 14 areas. However, results 

regarding PM2.5 light extinction in most cases apply 

to all 15 study areas because the St. Louis estimates 

for PM2.5 light extinction were not affected by the 

PM10 monitoring issue. 

explanation discussed in the Policy 

Assessment is that these older 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 concludes 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 

supports this set of CPLs for 

consideration by the EPA in this review. 

CASAC notes 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. CASAC 

concludes that this range of levels is 

‘‘adequately supported by the evidence 

presented’’ (Samet, 2010d, p. iii). 

C. Adequacy of the Current Standards 

for PM-Related Visibility Impairment 

As noted above, visibility impairment 

occurs during periods with fog or 

precipitation irrespective of the 

presence or absence of PM. While it is 

a popular notion that areas with many 

foggy or rainy days are ‘‘dreary’’ places 

to live compared to areas with more 

sunny days per year, the Policy 

Assessment has no basis for taking into 

account how the occurrence of such 

days might modify the effect of 

pollution-induced hazy days on public 

welfare. It is logical that periods with 

naturally impaired visibility due to fog 

or precipitation should not be treated as 

having PM-impaired visibility. 

Moreover, depending on the specific 

indicator, averaging time, and 

measurement approach used for the 

NAAQS, foggy conditions might result 

in measured or calculated indicator 

values that are higher than the light 

extinction actually caused by PM.148

Therefore, in order to avoid 

precipitation and fog confounding 

estimates of PM visibility impairment, 

and as advised by CASAC as part of its 

comments on the first draft Visibility 

Assessment, the assessment of visibility 

conditions was restricted to daylight 

hours with relative humidity less than 

or equal to 90 percent when evaluating 

sub-daily alternative standards (U.S. 

EPA, 2010b, section 3.3.5, Appendix G). 

The EPA recognizes that not all 

periods with relative humidity above 90 

percent have fog or precipitation. 

Removing those hours from 

consideration for a secondary PM 

standard would involve a tradeoff 

between the benefits of not including 

many of the hours with meteorological 

causes of visibility impacts and the loss 

of public welfare protection of not 

including some hours with high relative 

humidity without fog or precipitation, 

where the growth of hygroscopic PM 

into large solution droplets results in 

enhanced PM visibility impacts. For the 

15 urban areas included in the 

assessment for which meteorological 

data were obtained to allow an 

examination of the co-occurrence of 

high relative humidity and fog or 

precipitation, a 90 percent relative 

humidity cutoff criterion is effective in 

that on average less than 6 percent of 

the daylight hours are removed from 

consideration, yet those hours have 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. Based on these 

findings, the Policy Assessment 

concludes that it is appropriate that a 

sub-daily standard intended to protect 

against PM-related visibility impairment 

would be defined in such a way as to 

exclude hours with relative humidity 

greater than approximately 90 percent, 

regardless of measured values of light 

extinction or PM (U.S. EPA, 2011a, p. 4– 

29). 

1. Visibility Under Current Conditions 

Recent visibility conditions have been 

characterized in the Policy Assessment 

in terms of PM-related light 

extinction 149 levels for the 15 urban 

areas 150 that were selected for analysis 

in the Visibility Assessment. Hourly 

average PM-related light extinction was 

analyzed in terms of both PM10 and 

PM2.5 light extinction. These recent 

visibility conditions were then 

compared to the CPLs identified above. 

From Figure 4–3 and Table 4–2 in the 

Policy Assessment (U.S. EPA, 2010b, 

Figure 3–8 and Table 3–7, respectively) 

it can be seen that among these 14 urban 

areas, those in the East and in California 

tend to have a higher frequency of 

visibility conditions estimated to be 

above the high CPL compared with 

those in the western U.S. Both the figure 

and table are based on data from the 

2005 to 2007 time period and exclude 

hours with relative humidity greater 

than 90 percent. These displays indicate 

that all 14 urban areas have daily 

maximum hourly PM10 light extinction 

values that are 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 are estimated to 

exceed the high CPL from about 20 

percent to over 60 percent of the days. 

It is also noted that all 14 of the urban 

areas are estimated to exceed the low 

CPL from about 40 percent to over 90 

percent of the days. 

The Policy Assessment repeats the 

Visibility Assessment-type modeling 

based on PM2.5 light extinction and data 

from the more recent 2007 to 2009 time 

period for the same 15 study areas 

(including St. Louis), as described in 

Policy Assessment Appendix F. Figure 

4–4 and Table 4–3 in the Policy 

Assessment present the same type of 

information as do Figure 4–3 and Table 

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151The selection of the site used to assess 

visibility was driven by the need for several types 

of PM data, and for most study areas the site with 

the highest annual or 24-hour design value did not 

have the needed types of data. 

4–2, respectively. While the estimates of 

the percentage of daily maximum 

hourly PM2.5 light extinction values 

exceeding the CPLs are somewhat lower 

than for PM10 light extinction, the 

patterns of these estimates across the 

study areas are similar. More 

specifically, except for the two Texas 

and the non-California western urban 

areas, all of the other urban areas are 

estimated to exceed the high CPL from 

about 10 percent up to about 50 percent 

of the days based on PM2.5 light 

extinction, while all 15 areas are 

estimated to exceed the low CPL from 

over 10 percent to over 90 percent of the 

days. 

2. Protection Afforded by the Current 

Standards 

The Policy Assessment also 

conducted analyses to assess the 

likelihood that PM-related visibility 

impairment would exceed the various 

CPLs for a scenario based on simulating 

just meeting the current suite of PM2.5

secondary standards: 15 mg/m3 annual 

average PM2.5 concentration and 35 mg/ 

m3 24-hour average PM2.5 concentration 

with a 98th percentile form, averaged 

over three years. As described in the 

Visibility Assessment, the steps needed 

to model meeting the current NAAQS 

involve explicit consideration of 

changes in PM2.5 components. First, the 

Policy Assessment applied proportional 

rollback to all the PM2.5 monitoring sites 

in each study area, taking into account 

policy-relevant background PM2.5 mass, 

to ‘‘just meet’’ the current NAAQS 

scenario for the area as a whole, not just 

at the visibility assessment study site. 

The quantitative health risk assessment 

document (U.S. EPA, 2010a) describes 

this air quality roll-back procedure in 

detail. The degree of rollback (i.e., the 

percentage reduction in non-policyrelevant background PM2.5 mass) is 

controlled by the highest annual or 24- 

hour design value, which in most study 

areas is from a site other than the site 

used in this visibility assessment.151

The relevant result from this analysis is 

the percentage reduction in non-policyrelevant background PM2.5 mass needed 

to ‘‘just meet’’ the current NAAQS, for 

each study area. These percentage 

reductions are shown in Table 4–4 of 

the Visibility Assessment. It was noted 

that Phoenix and Salt Lake City meet 

the current PM2.5 NAAQS under current 

conditions and require no reduction. 

PM2.5 levels in these two cities were not 

‘‘rolled up.’’ Second, for each day and 

hour for each PM2.5 component, the 

Policy Assessment subtracted the 

policy-relevant background 

concentration from the current 

conditions concentration to determine 

the non- policy-relevant background 

portion of the current conditions 

concentration. Third, the Policy 

Assessment applied the same 

percentage reduction from the first step 

to the non- policy-relevant background 

portion of each of the five PM2.5

components and added back the policyrelevant background portion of the 

component. Finally, the Policy 

Assessment applied the original 

IMPROVE algorithm, using the reduced 

PM2.5 component concentrations, the 

current conditions PM10-2.5

concentration for the day and hour, and 

relative humidity for the day and hour 

to calculate the PM10 light extinction. 

In these analyses, the Policy 

Assessment has 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 display 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 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 show 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 are estimated to exceed 

the high CPL from about 8 percent up 

to over 30 percent of the days and the 

middle CPL from about 30 percent up to 

about 70 percent of the days, while all 

areas except Phoenix are estimated to 

exceed the low CPL from 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 nonCalifornia areas and the two areas in 

Texas are estimated to exceed the high 

CPL from about 4 percent up to over 15 

percent of the days and the middle CPL 

from about 15 percent up to about 45 

percent of the days, while all areas 

except Phoenix are estimated to exceed 

the low CPL from over 10 percent to 

about 75 percent of the days. A similar 

set of figures and tables have been 

developed in terms of PM10 light 

extinction (U.S. EPA, 2011a, Figures 4– 

5 and 4–6, Tables 4–4 and 4–5). 

Taking into account the above 

considerations, the Policy Assessment 

concludes that the available information 

in this review, as described above and 

in the Visibility Assessment and 

Integrated Science Assessment, clearly 

calls 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 supports consideration 

of alternative standards to provide 

appropriate protection (U.S. EPA, 

2011a, p. 4–39). 

This conclusion is based in part on 

the large percentage of days, in many 

urban areas, 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 is simulated to just meet 

the current PM2.5 standards, greater than 

10 percent of the days are estimated to 

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 exceed the highest CPL 

ranges from over 10 percent to over 30 

percent. Similarly, when the middle 

CPL of 25 dv is considered, greater than 

30 percent up to approximately 70 

percent of the days are estimated to 

exceed 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 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 exceeding the middle CPL in 

terms of PM2.5 light extinction. For the 

lowest CPL of 20 dv, the percentages of 

days estimated to exceed that CPL are 

even higher for all cases considered. 

Based on all of the above, the Policy 

Assessment concludes 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 exceeds levels 

and percentages of days that could 

reasonably be considered to be 

important from a public welfare 

perspective (U.S. EPA, 2011a, p. 4–40). 

Further, the Policy Assessment 

concludes that use of the current 

indicator of PM2.5 mass, in conjunction 

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152 In the second draft Policy Assessment, the 

calculated PM2.5 light extinction indicator was 

referred to as speciated PM2.5 mass calculated light 

extinction. 

153 In 2009, the D.C. Circuit remanded the 

secondary PM2.5 standards to the Agency in part 

because the EPA did not address the problem that 

a PM2.5 mass-based standard using a daily averaging 

time would be confounded by regional differences 

in relative humidity, although EPA had 

acknowledged this problem. The EPA notes that the 

light extinction indicators considered in the Policy 

Assessment explicitly took into account differences 

in relative humidity in areas across the country 

(U.S. EPA, 2011a, section 4.3.1). 

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 

confounded by 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 notes 

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, as 

discussed above in section 4.1.2. 

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 concludes 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). 

3. CASAC Advice 

Based on its review of the second 

draft Policy Assessment, CASAC 

concludes 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 notes that the detailed estimates 

of hourly PM light extinction associated 

with just meeting the current standards 

‘‘clearly demonstrate that 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 states, 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). 

4. Administrator’s Proposed 

Conclusions on the Adequacy of Current 

Standards for PM-Related Visibility 

Impairment 

In considering whether the current 

suite of secondary PM2.5 standards is 

requisite to protect the public welfare 

against PM-related visibility impairment 

primarily in urban areas, the 

Administrator has taken 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 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. As 

an initial matter, the Administrator 

recognizes the clear causal relationship 

between PM in the ambient air and 

impairment of visibility. She takes note 

of the evidence from the visibility 

preference studies, and the rationale for 

determining a range of candidate 

protection levels based on those studies. 

She notes the relatively large number of 

days 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 

information on the number of days 

estimated to exceed the CPLs, the 

Administrator concludes that such 

information provides 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 concludes 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 PM-related visibility 

impairment, with a focus primarily in 

urban areas. 

Having reached this conclusion, the 

Administrator also recognizes 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 PMrelated visibility impairment. She 

recognizes that 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 plays a central 

role in determining the relationship 

between the mix of PM in the ambient 

air and impairment of visibility. The 

Administrator notes that such 

considerations were reflected in 

CASAC’s advice to set a distinct 

secondary standard that would more 

directly reflect the relationship between 

ambient PM and visibility impairment. 

The Administrator also notes that such 

considerations were reflected in the 

court’s remand of the current secondary 

PM2.5 standards. Based on the above 

considerations, the Administrator 

provisionally concludes that the current 

secondary PM2.5 standards, taken 

together, are neither sufficiently 

protective nor are they suitably 

structured to provide an appropriate 

degree of public welfare protection from 

PM-related visibility impairment, 

primarily in urban areas. Thus, the 

Administrator has considered 

alternative standards by looking at each 

of the elements of the standards— 

indicator, averaging time, form, and 

level—as discussed below. 

D. Consideration of Alternative 

Standards for Visibility Impairment 

1. Indicator 

a. Alternative Indicators Considered in 

the Policy Assessment 

As described below, the Policy 

Assessment considers three indicators: 

The current PM2.5 mass indicator and 

two alternative indicators, including 

directly measured PM2.5 light extinction 

and calculated PM2.5 light extinction 

(U.S. EPA, 2011a, section 4.3.1.1).152

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 speciated 

PM2.5 mass and measured relative 

humidity.153

The Policy Assessment concludes 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 characteristic that is most relevant 

and directly related to PM-related 

visibility effects (U.S. EPA, 2011a, 

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154FRM filters are stabilized in a laboratory at 

fixed temperature and relative humidity levels, 

which alters whatever water content was present on 

the filter when removed from the sampler. FEM 

instruments are designed to meet performance 

criteria compared to FRM measurements, and 

accordingly typically manage temperature and/or 

humidity at the point of measurement to levels that 

are not the same as ambient conditions. 

155As discussed below, 24-hour average PM2.5

chemical component mass is measured at about 200 

CSN sites. 

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, 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. 

Few urban areas conduct continuous 

PM10-2.5 monitoring. 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 the absence of 

PM10-2.5 air quality information from a 

much larger number of urban areas 

across the country, it is not possible at 

this time to know in how many urban 

areas PM10-2.5 is a major contributor to 

urban visibility effects, though 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. PM10-2.5 is generally less 

homogenous in urban areas than PM2.5, 

making it more challenging to select 

sites that would adequately represent 

urban visibility conditions. 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 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). 

The basis for considering each of 

these three indicators is discussed 

below. The discussion also addresses 

monitoring data requirements for 

directly measured PM2.5 light extinction 

and for calculated PM2.5 light extinction. 

The following discussion also takes into 

consideration different averaging times 

since the combination of indicator and 

averaging time is relevant to 

understanding the monitoring data 

requirements. Consideration of 

alternative averaging times is addressed 

more specifically in section VI.D.2 on 

averaging time. 

i. PM2.5 Mass 

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. Blank and 

loaded filters are weighed to determine 

24-hour PM2.5 mass. Continuous PM2.5

monitoring produces hourly average 

mass concentrations and is conducted at 

about 900 locations. About 180 of these 

locations employ newer model 

continuous instruments that have been 

approved by EPA as FEMs, although the 

Policy Assessment notes that FEM 

approval has been based only on 

24-hour average, not hourly, PM2.5 mass. 

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.154 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.155

The overall performance of 1-hour 

average PM2.5 mass as a predictor of 

PM-related visibility impairment as 

indicated by PM10 calculated light 

extinction can be seen in scatter plots 

shown in Figure 4–9 of the Policy 

Assessment for two illustrative urban 

areas, Pittsburgh and Philadelphia 

(Similar plots for all 14 urban areas that 

have estimates of PM10 light extinction 

are in Appendix D, Figure D–2 of U.S. 

EPA, 2010b). These illustrative 

examples demonstrate the large 

variations in hourly PM10 light 

extinction corresponding to any specific 

level of hourly PM2.5 mass concentration 

as well as differences in the statistical 

average relationships (depicted as the 

best fit lines) between cities. This poor 

correlation between hourly PM10 light 

extinction and hourly PM2.5 mass is not 

due to any great extent to the 

contribution of PM10-2.5 to light 

extinction, but rather is 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. This 

contributes to the noisiness of the data 

on the relationship at any site and time 

of year. Also, there are systematic 

regional and seasonal differences in 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. 

As part of the Visibility Assessment, 

an assessment was conducted that 

estimated PM10 light extinction levels 

that may prevail if areas were simulated 

to just meet a range of alternative 

secondary standards based on hourly 

PM2.5 mass as the indicator. Appendix 

E of the Policy Assessment contains the 

results of this rollback-based 

assessment. This assessment quantifies 

the projected uneven protection, noted 

qualitatively above, that would result 

from the use of 1-hour average PM2.5

mass as the indicator. 

ii. Directly Measured PM2.5 Light 

Extinction 

PM light extinction is the major 

contributor to light extinction, which is 

the property of the atmosphere that is 

most directly related to visibility effects. 

It differs from light extinction by the 

nearly constant contributions for 

Rayleigh (or clean air) light scattering 

and the minor contributions by NO2

light absorption. The net result is that 

PM light extinction has a nearly one-toone 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. Direct 

measurement of PM2.5 light extinction 

can be accomplished 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). 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. 

As part of the Visibility Assessment, 

an assessment was conducted that 

estimated PM10 light extinction levels 

that may prevail in 14 urban study areas 

if the areas were simulated to just meet 

a secondary standard based on directly 

measured hourly PM10 light extinction 

as the indicator (U.S. EPA, 2010b, 

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156This assessment was conducted prior to staff’s 

decision to focus on PM2.5 light extinction 

indicators in the Policy Assessment. 

157About 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). 

158The original IMPROVE algorithm was selected 

for the described analysis in the Visibility 

Assessment because of its simplicity relative to the 

revised algorithm. 

159Sulfate, adjusted nitrate, derived water, 

inferred carbonaceous mass (SANDWICH) 

approach. 

160 Daily temperature data were also used as part 

of the SANDWICH method. 

161The 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. 

section 4.3).156 As would be expected, 

this assessment indicated that a 

secondary standard based on a directly 

measured PM10 light extinction 

indicator would provide the same 

percentage of days having values above 

the level of the standard in each of the 

areas, with the percentage being 

dependent on the statistical form of the 

standard. The Policy Assessment 

considers this assessment reasonably 

informative for a directly measured 

PM2.5 light extinction indicator as well, 

because in most of the assessment study 

areas PM10 light extinction is dominated 

by PM2.5 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 produced a White Paper on 

Particulate Matter (PM) Light Extinction 

Measurements (U.S. EPA, 2010g), and 

solicited comment on the White Paper 

from the Ambient Air Monitoring and 

Methods Subcommittee (AAMMS) of 

CASAC. In its review of the White Paper 

(Russell and Samet, 2010a), the CASAC 

AAMMS made the recommendation that 

consideration of direct measurement 

should be limited to PM2.5 light 

extinction as this can be accomplished 

by a number of commercially available 

instruments and because PM2.5 is 

generally responsible for most of the PM 

visibility impairment in urban areas. 

The CASAC AAMMS indicated that it is 

technically more challenging at this 

time to accurately measure the PM10-2.5

component of light extinction. 

The CASAC AAMMS also commented 

on the capabilities of currently available 

instruments, and expressed optimism 

regarding the near-term development of 

even better instruments for such 

measurement than are now 

commercially available. 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 

EPA develop performance-based 

approval criteria for direct measurement 

methods in order to put all approaches 

on a level playing field. Such criteria 

would necessarily include procedures 

and pass/fail requirements for 

demonstrating that the performance 

criteria have been met. For example, 

instruments might be required to 

demonstrate their performance in a 

wind tunnel, where the concentration of 

PM2.5 components, and thus of PM2.5

light extinction, could be controlled to 

known values. It might also be possible 

to devise approval testing procedures 

based on operation in ambient air, 

although knowing the true light 

extinction level (without in effect 

treating some particular instrument as if 

it were the FRM) would be more 

challenging. 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. 

iii. Calculated PM2.5 Light Extinction 

As discussed above in section VI.B.1 

above, PM2.5 light extinction can be 

calculated from 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, could also be used for a NAAQS 

focused on protecting against PMrelated visibility impairment primarily 

in urban areas. This could use the type 

of data that is routinely collected from 

the urban CSN 157 in combination with 

climatological relative humidity data as 

used in the Regional Haze Program (U.S. 

EPA, 2011a, Appendix G, section G.2). 

This calculation procedure, using the 

original IMPROVE light extinction 

equation presented above in section 

VI.B.1 on a 24-hour basis (or the revised 

IMPROVE equation), does not require 

PM2.5 mass concentration 

measurements. 

Alternatively, a conceptually similar 

approach could be applied in urban 

areas on an hourly or multi-hour basis. 

Applying this conceptual approach on a 

sub-daily basis would involve 

translating 24-hour speciation data into 

hourly estimates of species 

concentrations, and using 24-hour 

average species concentrations in 

conjunction with hourly PM2.5 mass 

concentrations. This translation can be 

made using more or less complex 

alternative approaches, as discussed 

below. 

The approach used to generate hourly 

PM10 light extinction for the Visibility 

Assessment was a relatively more 

complex method for implementing such 

a conceptual approach. It involved the 

use of the original IMPROVE 

algorithm 158 with estimates of hourly 

PM 2.5 components derived from dayspecific 24-hour and hourly 

measurements of PM 2.5 mass, 24-hour 

measurements of PM 2.5 composition, 

hourly measurements of PM 2.5 mass and 

(for some but not all study sites) hourly 

PM10-2.5 mass, along with hourly relative 

humidity information (U.S. EPA, 2010b, 

section 3.3). The Visibility Assessment 

approach also involved the use of 

output from a chemical transport 

modeling run to provide initial 

estimates of diurnal profiles for PM2.5

components at particular sites. The 

Visibility Assessment approach entailed 

numerous and complex data processing 

steps to generate hourly PM2.5

composition information from these less 

time-resolved data, including 

application of a mass-closure approach, 

referred to as the SANDWICH 

approach 159 (Frank, 2006), to adjust for 

nitrate retention differences between 

FRM and CSN filters, which is a 

required step for consistency with the 

IMPROVE algorithm and for estimating 

organic carbonaceous material via mass 

balance.160 The EPA staff employed 

complex custom software to do these 

data processing steps. 

While the complexity of the approach 

used in the Visibility Assessment was 

reasonable for assessment purposes at 

15 urban areas, the Policy Assessment 

recognizes that a relatively more simple 

approach would be more 

straightforward and have greater 

transparency, and thus should be 

considered for purposes of a national 

standard.161 Therefore, the Policy 

Assessment evaluated the degree to 

which simpler approaches would 

correlate with the results of the highly 

complex method used in the Visibility 

Assessment. This evaluation of two 

specific simpler approaches (described 

briefly below and in more detail in U.S. 

EPA, 2011a, Appendix F, especially 

Table F–1) demonstrated that the PM2.5

portions of the PM10 light extinction 

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162The original IMPROVE algorithm was the 

basis for the approaches considered in the Policy 

Assessment to maintain comparability to the 

estimates developed in the Visibility Assessment. 

This allowed the effects of other simplifications 

relative to the Visibility Assessment approach to be 

better discerned. 

163 If the revised IMPROVE algorithm were used 

to define the calculated PM2.5 mass-based 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. 

164An 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. 

165The degree of emission reduction needed to 

meet a standard is tightly tied to the degree to 

which the design value exceeds the level of the 

standard. 

166Several monitoring agencies utilize IMPROVE 

in urban areas to meet their chemical speciation 

monitoring needs. These sites are known as 

IMPROVE-protocol stations. 

values developed for the Visibility 

Assessment can be well approximated 

using the same IMPROVE algorithm 

applied to hourly PM2.5 composition 

values that were much more simply 

generated than with the method used in 

the Visibility Assessment. 

The simplified approaches examined 

were aimed at calculating hourly PM2.5

light extinction using the original 

IMPROVE algorithm (see section 

VI.B.1.a. above) excluding the Rayleigh 

term for light scattering by atmospheric 

gases and the term for PM10-2.5.162 These 

approaches, including a description of 

the sources of the data and steps 

required to determine calculated PM2.5

light extinction for these simplified 

approaches, are described in more detail 

in the Policy Assessment (U.S. EPA, 

2011a, pp. 4–46 to 48, Appendix F, 

Table F–2). Also, Table F–1 of 

Appendix F of the Policy Assessment 

compares and contrasts each of these 

approaches with the Visibility 

Assessment approach and with each 

other. 

The hourly PM2.5 light extinction 

values generated by using either 

simplified approach are comparable to 

those developed for use in the Visibility 

Assessment as indicated by the 

regression statistics for scatter plots of 

the paired data (i.e., the slopes of the 

regression equation and the R2 values 

are near 1 as shown in U.S. EPA, 2011a, 

Appendix F, Tables F–3 and F–4). 

Appendix F notes that both approaches 

underestimate PM2.5 light extinction on 

some days in a few study areas, which 

the Policy Assessment attributes to the 

occurrence of very high nitrate 

concentrations and the failure of the 

FRM-correlated/adjusted FEM 

instrument to report the entire nitrate 

mass. Nevertheless, the Policy 

Assessment concludes that each of these 

simplified approaches provides 

reasonably good estimates of PM2.5 light 

extinction and each is appropriate to 

consider as the indicator for a distinct 

hourly or multi-hour secondary 

standard (U.S. EPA, 2011a, p. 4–48). 

In addition, the Policy Assessment 

notes that there are variations of these 

simplified approaches that may also be 

appropriate to consider. For example, 

some variations that may improve the 

correlation with actual ambient light 

extinction in certain areas of the country 

include the use of the split-component 

mass extinction efficiency approach 

from the revised IMPROVE 

algorithm,163 the use of more refined 

value(s) for the organic carbon 

multiplier (see U.S. EPA, 2011a, 

Appendix F),164 and 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. Other 

variations may serve to simplify the 

calculation of PM2.5 light extinction 

values, such as those suggested by 

CASAC for consideration, including the 

use of historical monthly or seasonal 

speciation averages as well as speciation 

estimates on a regional basis (Samet, 

2010d, p. 11). Some of these variations 

would also be appropriate to consider in 

conjunction with a 24-hour average 

calculated PM2.5 light extinction 

indicator, including the use of the 

revised IMPROVE algorithm, the use of 

an alternative value for the organic 

carbon multiplier (e.g., 1.6), and the use 

of historical monthly or seasonal, or 

regional, speciation averages. 

As mentioned above, as part of the 

Visibility Assessment, an assessment 

was conducted of PM10 light extinction 

levels that would prevail if areas met a 

standard based on directly measured 

hourly PM10 light extinction as the 

indicator. This assessment indicated 

that a standard based on a directly 

measured PM10 light extinction 

indicator would provide the same 

percentage of days having indicator 

values above the level of the standard 

across areas, with the percentage being 

dependent on the statistical form of the 

standard. This assessment was based on 

the more complex Visibility Assessment 

approach to estimating PM10 light 

extinction, rather than the simpler 

approaches for estimating PM2.5 light 

extinction. Nevertheless, the generally 

close correspondence between design 

values for PM2.5 light extinction 

developed consistent with the Visibility 

Assessment approach and design values 

based on the simplified approaches 

(U.S. EPA, 2011a, Appendix F, Figure 

F–5) suggest that the findings regarding 

the protection offered by alternative 

PM10 light extinction standards using 

directly measured light extinction 

would also hold quite well for standards 

based on the simplified indicators.165

Thus, the Policy Assessment concludes 

that the use of a calculated PM2.5 light 

extinction indicator would provide a 

much higher degree of uniformity in 

terms of the visibility levels across the 

country than is possible using PM2.5

mass as the indicator (U.S. EPA, 2011a, 

p. 4–49). This is due to the fact that the 

PM2.5 mass indicator does not account 

for the effects of humidity and PM2.5

composition differences between 

various regions, while a calculated 

PM2.5 light extinction indicator directly 

incorporates those effects. 

The inputs that would be necessary to 

use either simplified approach to 

calculate a sub-daily PM2.5 light 

extinction indicator (e.g., 1- or 4-hour 

averaging time) include PM2.5 chemical 

speciation, relative humidity, and 

hourly PM2.5 mass measurements. In 

defining a standard in terms of 

calculated light extinction, the criteria 

for allowable protocols for these 

calculations would need to be specified. 

It would be appropriate to base these 

criteria on the protocols utilized in the 

IMPROVE 166 and CSN networks, as 

well as sampling and analysis protocols 

for ambient relative humidity sensors, 

and approved FEM mass monitors for 

PM2.5. Any approach to approving 

methods for use in calculating a light 

extinction indicator should take 

advantage of the existing inventory of 

monitoring and analysis methods. 

The CSN measurements have a strong 

history of being reviewed by CASAC 

technical committees, both during their 

initial deployment about ten years ago 

(Mauderly 1999a,b) and during the more 

recent transition to carbon sampling that 

is consistent with the IMPROVE 

protocols (Henderson, 2005c). Because 

the methods for the CSN are well 

documented in a nationally 

implemented Quality Assurance Project 

Plan (QAPP) and accompanying 

standard operating procedures (SOPs), 

are validated through independent 

performance testing, and are used to 

meet multiple data objectives (e.g., 

source apportionment, trends, and as an 

input to health studies), consideration 

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167For the purposes of using relative humidity 

measurements to derive multi-hour or 24-hour 

average PM2.5 calculated light extinction, the nonlinear f(RH) enhancement factor should be 

developed separately for each hour and then 

averaged over the desired multi-hour period. This 

averaging approach is consistent with derivation of 

climatological f(RH) factors used by the IMPROVE 

program and for the Regional Haze rule. 

168The EPA maintains a list of designated 

Reference and Equivalent Methods on its Web site 

at: http://www.epa.gov/ttn/amtic/files/ambient/ 

criteria/reference-equivalent-methods-list.pdf. 

169Filter-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. 

should be given to an approach that 

utilizes the existing methods as the 

basis for criteria for allowable sampling 

and analysis protocols for purposes of a 

calculated light extinction indicator. 

Such an approach of basing criteria on 

the current CSN and IMPROVE methods 

provides a nationally consistent way to 

provide the chemical species data used 

in the light extinction calculation, while 

preserving the opportunity for improved 

methods for measuring the chemical 

species. For relative humidity, in 

conjunction with either hourly, multihour, or 24-hour average calculated 

PM2.5 light extinction, consideration 

should be given to simply using criteria 

based on available relative humidity 

sensors such as already utilized by the 

National Oceanic and Atmospheric 

Administration (NOAA) at routine 

weather stations. These relative 

humidity sensors are already widely 

used by a number of monitoring 

agencies and can be easily compared to 

other relative humidity 

measurements.167 Finally, the 

simplified approaches for a sub-daily 

averaging period depend on having 

values of hourly PM2.5 mass, as 

discussed below. 

Since 2008, EPA has approved several 

PM2.5 continuous mass monitoring 

methods as FEMs.168 These methods 

have several advantages over filterbased FRMs, such as producing hourly 

data and the ability to report air quality 

information in near real-time. However, 

initial assessments of the data quality as 

operated by state and local monitoring 

agencies have had mixed results. A 

recent assessment of continuous FEMs 

and collocated FRMs conducted by EPA 

staff (Hanley and Reff, 2011) found 

some sites and continuous FEM 

instruments to have an acceptable 

degree of comparability of 24-hour 

average PM2.5 mass values derived from 

continuous FEMs and filter-based 

FRMs, while others had poor data 

quality that would not meet current data 

quality objectives. The EPA is working 

closely with the monitoring committee 

of the National Association of Clean Air 

Agencies (NACAA), instrument 

manufacturers, and monitoring agencies 

to document and communicate best 

practices on these methods to improve 

quality and consistency of resulting 

data. It should be noted that 

performance testing submitted to EPA 

for purposes of designating the PM2.5

continuous methods as FEMs, and the 

recent assessment of collocated FRMs 

and continuous FEMs, are both based on 

24-hour sample periods. Therefore, the 

EPA does not have similar performance 

data for continuous PM2.5 FEMs for 1- 

hour or 4-hour averaging periods, nor is 

there an accepted practice to generate 

performance standards for these time 

periods.169 Until issues regarding the 

comparability of 24-hour PM2.5 mass 

values derived from continuous FEMs 

and filter-based FRMs 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 and speciated 

PM2.5 mass values from 24-hour filterbased samplers. Section 4.3.2.1 of the 

Policy Assessment discusses another 

reason for such caution, based on a 

preliminary assessment of hourly data 

from continuous FEMs (U.S. EPA, 

2011a, pp. 4–52 to 4–54). 

This section has addressed the types 

of measurements that would be 

necessary to support a calculated PM2.5

light extinction indicator for either 24- 

hour or sub-daily (e.g., 1-hour and 4- 

hour) averaging periods. Considerations 

related specifically to each of these 

alternative averaging times, in 

conjunction with a standard defined in 

terms of a calculated PM2.5 light 

extinction indicator, are discussed 

further in section 4.3.2 of the Policy 

Assessment. 

iv. Conclusions in the Policy 

Assessment 

Taking the above considerations and 

CASAC’s advice into account, the Policy 

Assessment concludes that 

consideration should be given to 

establishing a new calculated PM2.5 light 

extinction indicator (U.S. EPA, 2011a, p. 

4–51). This conclusion takes into 

consideration the available evidence 

that demonstrates a strong 

correspondence between calculated 

PM2.5 light extinction and PM-related 

visibility impairment, as well as the 

significant degree of variability in 

visibility protection across the U.S. 

allowed by a PM2.5 mass indicator. 

While a secondary standard that uses a 

PM2.5 mass indicator could be set to 

provide additional protection from 

PM2.5-related visibility impairment, the 

Policy Assessment concludes that the 

advantages of using a calculated PM2.5

light extinction indicator make it the 

preferred choice (U.S. EPA, 2011a, p. 4– 

51). In addition, the Policy Assessment 

recognizes 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, it concludes 

that 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). 

Further, the Policy Assessment 

concludes 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). In either case, it would 

be appropriate to base criteria for 

allowable monitoring and analysis 

protocols to obtain PM2.5 speciation 

measurements on the protocols utilized 

in the IMPROVE and CSN networks. 

Further, in the case of a calculated PM2.5

light extinction indicator defined on a 

sub-daily basis, it would be appropriate 

to consider using the simplified 

approaches described, or some 

variations on these approaches. In 

reaching this conclusion, as discussed 

above, the Policy Assessment notes that 

while it is possible to utilize data from 

PM2.5 continuous FEMs on a 1-hour or 

multi-hour (e.g., 4-hour) basis, the 

mixed results of data quality 

assessments on a 24-hour basis, as well 

as the near absence of performance data 

for sub-daily averaging periods, 

increases 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. 

b. CASAC Advice 

Based on its review of 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 concludes 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 

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170 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). 

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).170

c. Administrator’s Proposed 

Conclusions on Indicator 

In reaching a proposed conclusion on 

the appropriate indicator for a standard 

intended to protect against PM-related 

visibility impairment, as an initial 

matter, the Administrator concurs 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. However, she also recognizes 

that while instruments currently exist 

that can directly measure PM2.5 light 

extinction, they are not an appropriate 

option in this review because a suitable 

specification of the equipment or 

performance-based verification 

procedures cannot be developed in the 

time frame of this review. 

Taking the above considerations and 

CASAC advice into account, the 

Administrator provisionally concludes 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), is an appropriate 

indicator to replace the current PM2.5

mass indicator. Such an indicator, 

referred to as a PM2.5 visibility index, 

appropriately reflects the relationship 

between ambient PM and PM-related 

light extinction, based on the analyses 

discussed above and incorporation of 

factors based on measured PM2.5

speciation concentrations and relative 

humidity data. In addition, this 

addresses, in part, the issues raised in 

the court’s remand of the 2006 PM2.5

standards. The Administrator also notes 

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 the above considerations, 

the Administrator proposes 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 PMrelated visibility impairment primarily 

in urban areas. The Administrator 

proposes that such an index be based on 

the original IMPROVE algorithm in 

conjunction with climatological relative 

humidity data as used in the Regional 

Haze Program. A more detailed 

discussion of the steps involved in the 

calculation of PM2.5 visibility index 

values is presented in section VII.A.5 

below. 

The Administrator solicits comment 

on all aspects of the proposed indicator. 

In particular, the Administrator solicits 

comment on the proposed use of a PM2.5

visibility index rather than a PM10

visibility index which would include an 

additional term for coarse particles. The 

Administrator also solicits comment on 

alternatively using the revised 

IMPROVE algorithm rather than the 

original IMPROVE algorithm the use of 

alternative values for the organic carbon 

multiplier in conjunction with either 

the original or revised IMPROVE 

algorithm; the use of historical monthly, 

seasonal, or regional speciation 

averages; and on alternative approaches 

to determining relative humidity, as 

discussed above. Further, in 

conjunction with an hourly or multihour indicator, comment is solicited on 

variations on the simplified approaches 

discussed above and on other 

approaches that may be appropriate to 

consider for such an indicator. 

2. Averaging Times 

a. Alternative Averaging Times 

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, as 

discussed above in section VI.B.1 and in 

section 4.2.1 of the Policy Assessment, 

and the nature of inputs to the 

calculation of PM2.5 light extinction, as 

discussed above in section VI.D.1 and in 

section 4.3.1 of the Policy Assessment. 

Based on this information, the Policy 

Assessment considered both sub-daily 

(1- and 4-hour averaging times) and 24- 

hour averaging times, as discussed 

below. In considering sub-daily 

averaging times, the Policy Assessment 

also addressed what diurnal periods and 

ambient relative humidity conditions 

would be appropriate to consider in 

conjunction with such an averaging 

time. 

i. Sub-daily 

As an initial matter, in considering 

sub-daily averaging times, the Policy 

Assessment took 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 experiences 

infrequent or short periods of exposure 

to ambient visibility conditions, and the 

typical rate of change of the pathaveraged PM light extinction over urban 

areas. While perception of change in 

visibility can occur in less than a 

minute, meaningful changes to pathaveraged light extinction occur more 

slowly. As discussed above and in 

section 4.2.1 of the Policy Assessment, 

one hour is a short enough averaging 

period to result in indicator values that 

are close to the maximum one- or fewminute visibility impact that an 

observer could be exposed to within the 

hour. Further, a 1-hour averaging time 

could reasonably characterize the 

visibility effects experienced by the 

segment of the population that 

experiences infrequent short-term 

exposures during peak visibility 

impairment periods in each area/site. 

Based on the above considerations, the 

initial analyses conducted in the Policy 

Assessment as part of the Visibility 

Assessment to support consideration of 

alternative standards 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 

notes 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 concludes that since 

a calculated PM2.5 light extinction 

indicator relies in part on measured 

PM2.5 mass, as discussed above and in 

section 4.3.1 of the Policy Assessment, 

it is also appropriate to consider a 

multi-hour averaging time in 

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171Similarly 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 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. 

172The 90 percent relative humidity cap 

assessment was conducted as part of the Visibility 

Assessment on all 15 of the urban areas, including 

St. Louis. 

conjunction with such an indicator 

(U.S. EPA, 2011a, p. 4–53). 

Thus, the Policy Assessment has 

considered multi-hour averaging times, 

on the order of a few hours as illustrated 

by a 4-hour averaging time. Such 

averaging times 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 

recognizes that a multi-hour averaging 

time would have the effect of averaging 

away 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 

recognizes 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 concludes 

that it is appropriate to consider a 1- 

hour or multi-hour (e.g., 4-hour) 

averaging time as the basis for a subdaily standard defined in terms of a 

calculated PM2.5 light extinction 

indicator (U.S. EPA, 2011a, p. 4–53). 

Additionally, as part of the review of 

data from all continuous FEM PM2.5

instruments operating at state/local 

monitoring sites, as discussed above, the 

Policy Assessment notes that the 

occurrence of questionable outliers in 1- 

hour data submitted to AQS from 

continuous FEM PM2.5 instruments has 

been observed at some of these sites 

(Evangelista, 2011). Some of these 

outliers are questionable simply by 

virtue of their extreme magnitude, as 

high as 985 mg/m3, whereas other values 

are questionable because they are 

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.171 The nature and frequency 

of questionable 1-hour FEM data 

collected in the past two years are being 

investigated. At this time, the Policy 

Assessment notes that any current data 

quality problems might 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 notes 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. 

In considering an appropriate diurnal 

period for use in conjunction with a 

sub-daily averaging time, the Policy 

Assessment recognizes that nighttime 

visibility impacts, described in the 

Integrated Science Assessment (U.S. 

EPA, 2009a, section 9.2.2) are 

significantly different from daytime 

impacts and are not sufficiently well 

understood to be included at this time. 

As a result, consistent with CASAC 

advice (Samet, 2010c, p. 4), the Policy 

Assessment concludes that it would be 

appropriate to define a sub-daily 

standard in terms of only daylight hours 

at this time (U.S. EPA, 2011a, p. 4–54). 

In the Visibility Assessment, daylight 

hours were defined to be those morning 

hours having no minutes prior to local 

sunrise and afternoon hours having no 

minutes after local sunset. This 

definition ensures the exclusion of 

periods of time where the sun is not the 

primary outdoor source of light to 

illuminate scenic features. 

In considering the well-known 

interaction of PM with ambient relative 

humidity conditions, the Policy 

Assessment recognizes that PM is not 

generally the primary source of 

visibility impairment during periods 

with fog or precipitation. In order to 

reduce the probability that hours with a 

high degree of visibility impairment 

caused by fog or precipitation are 

unintentionally used for purposes of 

determining compliance with a 

standard, the Policy Assessment 

determined that a relative humidity 

screen that excludes daylight hours with 

average relative humidity above 

approximately 90 percent is appropriate 

(U.S. EPA, 2001, pp. 4–54 to 4–55; see 

also U.S. EPA, 2010b, section 3.3.5, 

Appendix G). For example, for the 15 

urban areas 172 included in the Visibility 

Assessment, a 90 percent relative 

humidity cutoff criterion proved 

effective in that on average less than 6 

percent of the daylight hours were 

removed from consideration, yet those 

same hours had on average 10 times the 

likelihood of rain, 6 times the likelihood 

of snow/sleet, and 34 times the 

likelihood of fog compared with hours 

with 90 percent or lower relative 

humidity. However, not all periods with 

relative humidity above 90 percent have 

fog or precipitation. The Policy 

Assessment recognizes that removing 

those hours from consideration involves 

a tradeoff between the benefits of 

avoiding many of the hours with 

meteorological causes of visibility 

impacts and not counting some hours 

without fog or precipitation in which 

high humidity levels (e.g., greater than 

90 percent) lead to the growth of 

hygroscopic PM to large solution 

droplets resulting in larger PM visibility 

impacts. 

ii. 24–Hour 

As discussed in section 4.3.1 of the 

Policy Assessment and below, there are 

significant reasons to consider using 

PM2.5 light extinction calculated on a 

24-hour basis to reduce the various data 

quality concerns over relying on 

continuous PM2.5 monitoring data. 

However, the Policy Assessment 

recognizes 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, as discussed above 

and in section 4.3.2.1 of the Policy 

Assessment. Thus, consideration of a 

24-hour averaging time depends 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 subdaily basis, as discussed below in this 

section. Further, since a 24-hour 

averaging time combines daytime and 

nighttime periods, the Policy 

Assessment recognizes that the public 

preference studies do not directly 

provide a basis for identifying an 

appropriate level of protection, in terms 

of 24-hour average light extinction, 

based on judgments of acceptable 

daytime visual air quality obtained in 

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173These 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). 

174The EPA staff note that the R2 value (0.44) for 

Houston was notably lower than for the other cities. 

those studies. Thus, consideration of a 

24-hour averaging time also depends 

upon developing an approach to 

translate the candidate levels of 

protection derived from the public 

preference studies, which the Policy 

Assessment has interpreted on an 

hourly or multi-hour basis, to a 

candidate level of protection defined in 

terms of a 24-hour average calculated 

light extinction, as discussed in 

section.VI.D.4 below. 

To determine whether 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, the 

Policy Assessment performed 

comparative analyses of 24-hour and 4- 

hour averaging times in conjunction 

with a calculated PM2.5 indicator.173

These analyses are presented and 

discussed in Appendix G, section G.4 of 

the Policy Assessment. 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 calculations used the 

original IMPROVE algorithm and longterm (1988 to 1997) average relative 

humidity conditions, to calculate 

monthly average values of the relative 

humidity term in the IMPROVE 

algorithm, consistent with the approach 

used for the Regional Haze Program. 

Similar to the approach used to assess 

a sub-daily visibility index discussed in 

section VI.2.a.i above, these 1988–1997 

humidity data are similarly screened to 

remove the effect of high hourly relative 

humidity. In this case, any relative 

humidity value great than 95 percent 

was treated as 95 percent. Because 10- 

years of hourly data were used to 

produce a single humidity term for each 

month, the EPA believes that the 

resulting monthly average of the 

humidity term is sufficient and 

appropriate to reduce the effects of fog 

or precipitation. Based on these 

analyses, scatter plots comparing 24- 

hour and 4-hour calculated PM2.5 light 

extinction are shown 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). It can be seen, as expected, 

that there is some scatter around the 

regression line for each city, because the 

calculated 4-hour light extinction 

includes day-specific and hour-specific 

influences that are not captured by the 

simpler 24-hour approach. The Policy 

Assessment notes that this scatter could 

be reduced by the use of same-day 

hourly relative humidity data to 

calculate a 24-hour average value of the 

relative humidity term in the IMPROVE 

algorithm. In the Policy Assessment, 

scatter plots are also shown for the 

annual 90th percentile values, based on 

data from 2007 to 2009, for 4-hour and 

24-hour calculated PM2.5 light 

extinction across all 15 cities (U.S. EPA, 

2011a, Figure G–7) and for the 3-year 

design values across all 15 cities (U.S. 

EPA, 2011a, Figure G–8). These analyses 

showed good correlation between 24- 

hour and 4-hour average PM2.5 light 

extinction, as evidenced by reasonably 

high city-specific and pooled R2 values, 

generally in the range of over 0.6 to over 

0.8.174

iii. Conclusions in the Policy 

Assessment 

Taking the above considerations and 

CASAC’s advice into account, the Policy 

Assessment concludes that it is 

appropriate to consider in this review a 

24-hour averaging time, in conjunction 

with a calculated PM2.5 light extinction 

indicator and an appropriately specified 

standard level (U.S. EPA, 2011a, p. 4– 

57). This conclusion reflects the 

judgment 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 average basis. 

This conclusion is also predicated on 

consideration of a 24-hour average 

standard level, as discussed below and 

in section 4.3.4 of the Policy 

Assessment, that is appropriately 

translated from the CPLs derived from 

the public preference studies, which the 

Policy Assessment has interpreted as 

providing information on the 

acceptability of daytime visual air 

quality over an hourly or multi-hour 

exposure period. 

A 24-hour average calculated PM2.5

light extinction indicator would avoid 

data quality uncertainties that have 

recently been associated with currently 

available instruments for measurement 

of hourly PM2.5 mass. The particular 24- 

hour indicator considered by the Policy 

Assessment uses the original IMPROVE 

algorithm and long-term relative 

humidity conditions to calculate PM2.5

light extinction. 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. 

Further, the Policy Assessment 

concludes 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 that have been 

raised by the recent EPA study of 

continuous FEMs (U.S. EPA, 2011a, p. 

4–58). Such an averaging time, to the 

extent that data quality issues can be 

appropriately addressed, would be more 

directly related to the short-term nature 

of the perception of visibility 

impairment, short-term variability in 

PM-related 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 concludes 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 concludes 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). 

b. CASAC Advice 

As noted above, in its review of the 

first draft Policy Assessment, CASAC 

concludes 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 

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 

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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 subdaily basis. 

In considering this advice, the Policy 

Assessment first notes that CASAC did 

not have the benefit of EPA’s recent 

assessment of the data quality issues 

associated with the use of continuous 

FEMs as the basis for hourly PM2.5 mass 

measurements. The Policy Assessment 

also notes that since earlier drafts of this 

Policy Assessment did not include 

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. 

c. Administrator’s Proposed 

Conclusions on Averaging Time 

In reaching a proposed conclusion on 

the appropriate averaging time for a 

standard intended to protect against 

PM-related visibility impairment, the 

Administrator has taken into account 

the information discussed above with 

regard to analyses and conclusions 

presented in the final Policy Assessment 

as well as the views of CASAC based on 

its reviews of the first and second drafts 

of the Policy Assessment. As an initial 

matter, the Administrator recognizes 

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 shortterm nature of the perception of PMrelated visibility impairment and the 

relevant exposure periods for segments 

of the viewing public. On the other 

hand, she recognizes 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 recognizes the 

desirability of a sub-daily averaging 

time, she has 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 recognizes that the Policy 

Assessment concludes 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 (U.S. EPA, 2011a, 

p. 4–57). In light of this finding, the 

Administrator proposes to set a distinct 

secondary standard with a 24-hour 

averaging time in conjunction with a 

PM2.5 visibility index. 

Further, in light of the desirability of 

a sub-daily averaging time, the 

Administrator solicits comment on a 

sub-daily (e.g., 4-hour) averaging time 

and related data quality issues 

associated with currently available 

monitoring instrumentation. In so 

doing, the Administrator notes that 

CASAC’s advice on averaging times was 

predicated in part on the capabilities of 

available monitoring instrumentation as 

CASAC understood them when it 

provided its advice. 

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 the standard is 

achieved. The form of the current 24- 

hour PM2.5 NAAQS is such 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 multi-year 

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 concludes it is 

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 concludes 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 notes 

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 finds no 

basis to conclude that it would be 

appropriate to consider limiting the 

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175 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. 

related light extinction in urban areas to 

a greater degree. 

Another aspect of the form that was 

considered in the Visibility Assessment 

for a sub-daily (i.e., 1-hour) averaging 

time is whether to include all daylight 

hours or only the maximum daily 

daylight hour. This consideration would 

also be relevant for a multi-hour 

(e.g., 4-hour) averaging time, although 

such an analysis was not included in 

the Visibility Assessment. The 

maximum daily daylight 1-hour or 

multi-hour form is 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 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 does not have 

information regarding the fraction of the 

public that has only one or a few 

opportunities to experience visibility 

during the day, nor does 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 

Visibility Assessment analyses included 

both the maximum daily hour and the 

all daylight hours forms. The Policy 

Assessment observed a close 

correspondence between the level of 

protection afforded for all 15 urban 

areas in the assessment by the 

maximum daily daylight 1-hour 

approach using the 90th percentile form 

and the all daylight hours approach 

combined with the 98th percentile form 

(U.S. EPA, 2010b, section 4.1.4). On this 

basis, the Policy Assessment notes that 

the 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 PMrelated visibility conditions). The Policy 

Assessment also notes that CASAC 

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 concludes that it is 

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 

concludes that it is 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. 

Based on its review of the second 

draft Policy Assessment, CASAC did not 

provide advice as to a specific form that 

would be appropriate, but 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, the Administrator 

provisionally concludes that a 90th 

percentile form, averaged over 3 years, 

is appropriate, and proposes such a 

form in conjunction with a PM2.5

visibility index and a 24-hour averaging 

time. 

4. Level 

In considering alternative levels for a 

new standard that would provide 

requisite protection against PM-related 

visibility impairment primarily in urban 

areas, the Policy Assessment has taken 

into account the evidence- and impactbased considerations discussed above 

and in section 4.2.1 of the Policy 

Assessment, 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 this information in the 

context of a standard defined in terms 

of a calculated PM2.5 light extinction 

indicator, discussed above and in the 

Policy Assessment section 4.3.1; with 

alternative averaging times of 24-hours 

or multi-hour, sub-daily periods 

(e.g., 4-hours), discussed above and in 

Policy Assessment section 4.3.2; and a 

90th percentile-based form, discussed 

above and in section 4.3.3 of the Policy 

Assessment. 

As part of the Policy Assessment’s 

assessment of the adequacy of the 

current standards, summarized in 

section VI.B. above and in Policy 

Assessment section 4.2.1, it 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 notes 

that CASAC agreed that this was an 

appropriate range of levels to consider 

for such a standard (Samet, 2010d, p. 

11).175 The Policy Assessment also 

recognizes 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 is 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. 

As an initial matter, in considering 

alternative levels for a sub-daily 

standard based directly on the four 

preference study results, the Policy 

Assessment notes that the individual 

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176Note 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. 

177As 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 nondaylight periods than during daylight hours. 

178To provide some perspective in considering 

these results (U.S. EPA, 2011a, Appendix G, Table 

G–6), the Policy Assessment notes that 1 dv is about 

the amount that persons can distinguish when 

viewing scenic vistas, and that a difference of 1 dv 

is equivalent to about a 10 percent difference in 

light extinction expressed in Mm¥1. 

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. As discussed 

above and in section 4.2.1 of the Policy 

Assessment, the Phoenix study was by 

far the best of the studies, providing 

somewhat more support for the middle 

CPL. In considering the results from 

these studies, the Policy Assessment 

recognizes that the 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 recognizes 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 to inform consideration of 

a national standard for any specific 

averaging time. Nonetheless, the Policy 

Assessment concludes, as did CASAC, 

that these studies are appropriate to use 

for this purpose (U.S. EPA, 2011a, p. 4– 

61). 

In considering potential alternative 

levels for a 24-hour standard, the Policy 

Assessment explores various 

approaches to adjusting the CPLs 

derived directly from the preference 

studies, as presented and discussed in 

Appendix G of the Policy Assessment, 

especially section G–5. 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 so doing, 

staff recognized that there are multiple 

approaches for estimating generally 

equivalent levels on a city-specific or 

national basis, and that the inherent 

spatial and temporal variability in 

relative humidity and fine particle 

composition across cities leads to a set 

of alternative estimates of levels that 

may be construed as being generally 

equivalent on a national basis. 

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 Table 

G–6 of Appendix G,176 in considering 

estimates aggregated or averaged over all 

15 cities as well as the range of cityspecific estimates for the various 

approaches considered, 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.177

Some of the approaches used in these 

analyses focused on comparing 24-hour 

and 4-hour light extinction values in 

each of the 15 urban areas, whereas 

other approaches focused on 

comparisons based on using aggregated 

data across the urban areas. Two of 

these approaches, which used 

regressions of city-specific annual 90th 

percentile light extinction values or 3- 

year light extinction design values, gave 

nearly identical results and were 

considered by staff to be most 

appropriate for further consideration. 

These approaches (shown in U.S. EPA, 

2011a, Appendix G, Figures G–7 and G– 

8, referred to as Approaches A and B) 

were preferred by staff based on the 

high R2 values of the regressions and 

because the regressions were 

determined by data from days with 

PM2.5 light extinction conditions in the 

range of 20 to 40 dv. This contrasted 

with the other approaches that were 

influenced by PM2.5 light extinction 

conditions well below this range. Based 

on these analyses (presented in 

Appendix G of the Policy Assessment), 

the Policy Assessment notes that the 

single approach thought by staff to be 

more appropriate for further 

consideration (referred to as Approach B 

in Appendix G) yielded adjusted 24- 

hour CPLs of 21, 25, and 28 dv as being 

levels that are generally equivalent in an 

aggregate or central tendency sense to 4- 

hour CPLs of 20, 25, and 30 dv.178

Two of the approaches yielded not 

only estimates of generally equivalent 

levels on an aggregated basis but also 

city-specific estimates (referred to as 

Approaches C and E in Appendix G) 

that showed greater variability than the 

aggregated estimates. In all cases, the 

range of city-specific estimates of 

generally equivalent 24-hour levels 

included the 4-hour level for each of the 

CPLs of 20, 25, and 30 dv (as shown in 

Tables G–7 and G–8, Appendix G of the 

Policy Assessment, for Approaches C 

and E, respectively). 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 more broadly considering 

alternative standard levels that would 

be appropriate for a nationally 

applicable secondary standard focused 

on protection from PM-related urban 

visibility impairment based on either a 

24-hour or multi-hour, sub-daily (e.g., 4- 

hour) averaging time, the Policy 

Assessment was mindful of the 

important limitations in the available 

evidence from public preference 

studies. While the Policy Assessment 

concluded, consistent with CASAC 

advice, that it is appropriate to consider 

a distinct secondary PM2.5 standard to 

address PM-related visibility 

impairment focused primarily in urban 

areas based on the evidence from public 

preference studies, it also recognized 

that there are a number of uncertainties 

and limitations associated with the 

preference studies that have served as a 

basis for selecting an appropriate range 

of levels to consider, as discussed above 

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in section VI.B.2. 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 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 Policy Assessment was 

mindful 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, and 

that there is the possibility that there 

could be regional differences in 

individual preferences for VAQ. 

Given the uncertainties and 

limitations noted above, the EPA 

broadly solicits comment on the 

strengths and limitations associated 

with these 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). In particular, the EPA 

solicits comment on the following 

specific aspects of the public preference 

studies and on how these studies should 

appropriately be considered in this 

review. Recognizing that all of these 

studies evaluated a 50 percent 

acceptability criterion as the basis for 

reaching judgments in the context of 

each study, the EPA requests comment 

on the extent to which this 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. Recognizing 

that these studies vary in the extent to 

which the study participants may be 

representative of the broader study area 

population, the EPA requests comment 

on how this aspect of the study designs 

should appropriately be weighed in the 

context of considering these studies in 

reaching proposed conclusions on a 

distinct secondary NAAQS. The EPA 

also solicits comment on 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. 

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 non-urban 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. 

Based on the above considerations, 

the Policy Assessment concludes that it 

is appropriate to give primary 

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 concludes 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 judges that 

such standards would provide 

appropriate protection against PMrelated visibility impairment primarily 

in urban areas. The Policy Assessment 

notes 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 concludes 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 above considerations, 

taking into account the conclusions in 

the Policy Assessment and the extent to 

which those conclusions reflected 

consideration of CASAC advice during 

the development of the Policy 

Assessment, as an initial matter, the 

Administrator provisionally concludes 

that it is 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 the upper end of the range of CPLs 

identified in the Policy Assessment and 

generally supported by CASAC. More 

specifically, the Administrator 

provisionally concludes 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 is 

supported by the available information 

and is generally consistent with the 

advice of CASAC. 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. 

In light of these provisional 

conclusions, the Administrator proposes 

to set 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 PM-related visibility impairment 

based on one of two options. One option 

is to set the level of such a standard at 

30 dv and the other option is to set the 

level at 28 dv. In so doing, the 

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179For the purposes of this discussion, NOX and 

SOX refers to all oxides of nitrogen and all oxides 

of sulfur, respectively. 

180 In the sections of the Integrated Science 

Assessment included from IPCC AR4 and CCSP 

SAP2.3 (U.S. EPA, 2009a, section 9.3), the term 

‘‘aerosols’’ is more frequently used than ‘‘PM’’ and 

that word is retained in the Policy Assessment (U.S. 

EPA, 2011a, section 5.2) and in this section of the 

preamble. 

Administrator solicits comment on each 

of these levels and on the various 

approaches to identifying generally 

equivalent levels discussed above upon 

which the alternative proposed levels 

are based. Recognizing that there was 

some support for consideration of a 

broader range of levels, the 

Administrator also solicits comment on 

a range of levels down to 25 dv in 

conjunction with a 24-hour averaging 

time. Further, having solicited comment 

on a sub-daily (e.g., 4-hour) averaging 

time, the Administrator also solicits 

comment on a range of alternative levels 

from 30 to 25 dv in conjunction with 

such a sub-daily averaging time. 

Finally, as we have indicated, 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 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). We invite comment on 

how the Administrator should weigh 

those uncertainties as well as any 

additional comments and information to 

inform her consideration of these 

uncertainties. 

E. Other PM-Related Welfare Effects 

In the 2006 review, the Administrator 

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 

non-visibility welfare effects to specific 

levels of ambient PM. To provide a level 

of protection for 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). 

In that review, the 2004 AQCD 

concluded that regardless of size 

fraction, particles containing nitrates 

and sulfates have the greatest potential 

for widespread environmental 

significance (U.S. EPA, 2004, sections 

4.2.2 and 4.2.3.1). Considerable 

supporting evidence was available that 

indicated a significant role of oxides of 

nitrogen and sulfur, and their 

transformation products in acidification 

and nutrient enrichment of terrestrial 

and aquatic ecosystems (71 FR 61209, 

October 17, 2006). The recognition of 

these ecological effects, coupled with 

other considerations detailed below, led 

EPA to initiate a joint review of the 

secondary NO2 and SO2 NAAQS that is 

considering the gaseous and particulate 

species of oxides of nitrogen and sulfur 

with respect to the ecosystem-related 

welfare effects that result from the 

deposition of these pollutants and 

transformation products. 

This section presents the Policy 

Assessment’s conclusions with regard to 

the current suite of secondary PM 

standards to protect against nonvisibility PM-related welfare effects. 

Specifically, the Policy Assessment has 

assessed the relevant information 

related to effects of atmospheric PM on 

the environment, including effects on 

climate, ecological effects, and 

materials. Non-visibility welfare-based 

effects of oxides of nitrogen and sulfur 

are divided between two NAAQS 

reviews; (1) PM NAAQS review and, (2) 

the joint secondary NAAQS review for 

oxides of nitrogen (NOX) and oxides of 

sulfur (SOX).179 The scope of each 

document and the compounds of 

nitrogen and sulfur considered in each 

review are summarized in this section 

and in Table 5–1 of the Policy 

Assessment. 

In reviewing the current suite of 

secondary PM standards, the Policy 

Assessment considers all PM-related 

effects that are not being covered in the 

ongoing NOX/SOX review, including 

visibility impairment (U.S. EPA, 2011a, 

chapter 4), climate forcing effects (U.S. 

EPA, 2011a, section 5.2), ecological 

effects (U.S. EPA, 2011a, section 5.3), 

and materials damage (U.S. EPA, 2011a, 

section 5.4). By excluding the effects 

associated with deposited particulate 

matter components of NOX and SOX and 

their transformation products which are 

addressed fully in the NOX/SOX

secondary review, the discussion of 

ecological effects of PM has been 

narrowed to focus on effects associated 

with the deposition of metals and, to a 

lesser extent, organics (U.S. EPA, 2011a, 

section 5.3). With regard to the materials 

section, because the NOX/SOX review is 

not considering materials, the 

discussion includes particles and gases 

that are associated with the presence of 

ambient NOX and SOX, as well as 

reduced forms of nitrogen such as 

ammonia and ammonium ions for 

completeness. 

In contrast, the proposed rulemaking 

for the joint NOX/SOX secondary review 

(76 FR 46084, August 1, 2011) focuses 

on the welfare effects associated with 

exposures from deposited particulate 

and gaseous forms of oxides of nitrogen 

and sulfur and related nitrogen- and 

sulfur-containing compounds and 

transformation products on ecosystem 

receptors, including effects of acidifying 

deposition associated with particulate 

nitrogen and sulfur. In addition, the 

NOX/SOX secondary review includes 

evidence related to direct ecological 

effects of gas-phase NOX and SOX. 

1. Climate 

Information and conclusions about 

what is currently known about the role 

of PM in climate is summarized in 

Chapter 9 of the Integrated Science 

Assessment (U.S. EPA, 2009a). The 

Integrated Science Assessment 

concludes ‘‘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). The Policy Assessment 

summarizes and synthesizes the policyrelevant science in the Integrated 

Science Assessment for the purpose of 

helping to inform consideration of 

climate aspects in the review of the 

secondary PM NAAQS (U.S. EPA, 

2011a, section 5.2). This discussion is 

summarized below. 

Atmospheric PM (referred to as 

aerosols 180 in the remainder of this 

section to be consistent with the 

Integrated Science Assessment) affects 

multiple aspects of climate. These 

include absorbing and scattering of 

incoming solar radiation, alterations in 

terrestrial radiation, effects on the 

hydrological cycle, and changes in 

cloud properties (U.S. EPA, 2009a, 

section 9.3.1). Major aerosol 

components that contribute to climate 

processes include black carbon (BC), 

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organic carbon (OC), sulfates, nitrates, 

and mineral dusts. There is a 

considerable ongoing research effort 

focused on understanding aerosol 

contributions to changes in global mean 

temperature and precipitation patterns. 

The Climate Change Research Initiative 

identified research on atmospheric 

concentrations and effects of aerosols as 

a high research priority (National 

Research Council, 2001) and the IPCC 

2007 Summary for Policymakers states 

that anthropogenic contributions to 

aerosols remain the dominant 

uncertainty in radiative forcing (IPCC, 

2007). The current state of the science 

of climate alterations attributable to PM 

is in flux as a result of continually 

updated information. 

Global climate change has 

increasingly been the focus of intense 

international research endeavors. As 

discussed in chapter 5 of the Policy 

Assessment, major efforts are underway 

to understand the complexities inherent 

in atmospheric aerosol interactions and 

to decrease uncertainties associated 

with climate estimations. 

Aerosols have direct and indirect 

effects on climate processes. The direct 

effects of aerosols on climate result 

mainly from particles scattering light 

away from Earth into space, directly 

altering the radiative balance of the 

Earth-atmosphere system. This 

reflection of solar radiation back to 

space decreases the transmission of 

visible radiation to the surface of the 

Earth and results in a decrease in the 

heating rate of the surface and the lower 

atmosphere. At the same time, 

absorption of either incoming solar 

radiation or outgoing terrestrial 

radiation by particles, primarily BC, 

results in an increased heating rate in 

the lower atmosphere. Global estimates 

of aerosol direct radiative forcing (RF) 

were recently summarized using a 

combined model-based estimate (Forster 

et al., 2007). The overall, model-derived 

aerosol direct RF was estimated in the 

IPCC AR4 as ¥0.5 (¥0.9 to ¥0.1) watts 

per square meter (W/m2), with an 

overall level of scientific understanding 

of this effect as ‘‘medium low’’ (Forster 

et al., 2007), indicating a net cooling 

effect in contrast to greenhouse gases 

(GHGs) which have a warming effect. 

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). The direct effect of 

radiative scattering by atmospheric 

particles exerts an overall net cooling of 

the atmosphere, while particle 

absorption of solar radiation leads to 

warming. For example, the presence of 

OC and sulfates decrease warming from 

sunlight by scattering shortwave 

radiation back into space. Such a 

perturbation of incoming radiation by 

anthropogenic aerosols is designated as 

aerosol climate forcing, which is 

distinguished from the aerosol radiative 

effect of the total aerosol (natural plus 

anthropogenic). The aerosol climate 

forcing and radiative effect are 

characterized by large spatial and 

temporal heterogeneities due to the 

wide variety of aerosol sources, the 

spatial non-uniformity and 

intermittency of these sources, the short 

atmospheric lifetime of aerosols 

(relative to that of the greenhouse gases), 

and processing (chemical and 

microphysical) that occurs in the 

atmosphere. For example, OC can be 

warming (positive forcer) when 

deposited on or suspended over a highly 

reflective surface such as snow or ice 

but, on a global average, is a negative 

forcer in the atmosphere. 

More information has also become 

available on indirect effects of aerosols. 

Particles in the atmosphere indirectly 

affect both cloud albedo (reflectivity) 

and cloud lifetime by modifying the 

cloud amount, and microphysical and 

radiative properties (U.S. EPA, 2009a, 

section 9.3.6.4). The RF due to these 

indirect effects (cloud albedo effect) of 

aerosols is estimated in the IPCC AR4 to 

be ¥0.7 ( ¥1.8 to ¥0.3) W/m2 with the 

level of scientific understanding of this 

effect as ‘‘low’’ (Forster et al., 2007). 

Aerosols act as cloud condensation 

nuclei (CCN) for cloud formation. 

Increased particulates in the atmosphere 

available as CCN with no change in 

moisture content of the clouds have 

resulted in an increase in the number 

and decrease in the size of cloud 

droplets in certain clouds that can 

increase the albedo of the clouds (the 

Twomey effect). Smaller particles slow 

the onset of precipitation and prolong 

cloud lifetime. This effect, coupled with 

changes in cloud albedo, increases the 

reflection of solar radiation back into 

space. The altitude of the clouds also 

affects cloud radiative forcing. Low 

clouds reflect incoming sunlight back to 

space but do not effectively trap 

outgoing radiation, thus cooling the 

planet, while higher elevation clouds 

reflect some sunlight but more 

effectively can trap outgoing radiation 

and act to warm the planet (U.S. EPA, 

2009a, section 9.3.3.5). 

The total negative RF due to direct 

and indirect effects of aerosols 

computed from the top of the 

atmosphere, on a global average, is 

estimated at ¥1.3 (¥2.2 to ¥0.5) W/m2

in contrast to the positive RF of +2.9 

(+3.2 to +2.6) W/m2 for anthropogenic 

GHGs (IPCC 2007, p. 200). 

The understanding of the magnitude 

of aerosol effects on climate has 

increased substantially in the last 

decade. Data on the atmospheric 

transport and deposition of aerosols 

indicate a significant role for PM 

components in multiple aspects of 

climate. Aerosols can impact glaciers, 

snowpack, regional water supplies, 

precipitation, and climate patterns (U.S. 

EPA, 2009a, section 9.3.9). Aerosols 

deposited on ice or snow can lead to 

melting and subsequent decrease of 

surface albedo (U.S. EPA, 2009a, section 

9.3.9.2). Aerosols are potentially 

important agents of climate warming in 

the Arctic and other locations (U.S. 

EPA, 2009a, section 9.3.9). 

Carbonaceous aerosols emitted from 

intermittent fires can occur at large 

enough scales to affect hemispheric 

aerosol concentrations. In addition to 

incidental fires, routine biomass 

burning, usually associated with 

agriculture in eastern Europe, has also 

been shown to contribute to 

hemispheric concentrations of 

carbonaceous aerosols and is therefore 

recognized as having a significant 

impact on PM2.5 concentrations and 

climate forcing (U.S. EPA, 2009a, 

section 9.3.7). 

A series of studies available since the 

last review examines the role of aerosols 

on local and regional scale climate 

processes (U.S. EPA, 2009a, section 

9.3.9.3). Studies on the South Coast Air 

Basin (SCAB) in California indicate 

aerosols may reduce near-surface wind 

speeds, which, in turn reduce 

evaporation rates and increase cloud 

lifetimes. The overall impact can be a 

reduction in local precipitation 

(Jacobson and Kaufmann, 2006). 

Conditions in the SCAB impact 

ecologically sensitive areas including 

the Sierra Nevadas. Precipitation 

suppression due to aerosols in 

California (Givati and Rosenfield, 2004) 

and other similar studies in Utah and 

Colorado found that mountain 

precipitation decreased by 15 to 30 

percent downwind of pollution sources. 

Evidence of regional-scale impacts of 

aerosols on meteorological conditions in 

other regions of the U.S. is lacking. 

Advances in the understanding of 

aerosol components and how they 

contribute to climate change have 

enabled refined global forcing estimates 

of individual PM constituents. The 

global mean radiative effect from 

individual components of aerosols was 

estimated for the first time in the IPCC 

AR4 where they were reported to be (all 

in W/m2 units): ¥0.4 (+0.2) for sulfate, 

¥0.05 (+0.05) for fossil fuel-derived OC, 

+0.2 (+0.15) for fossil fuel derived BC, 

+0.03 (+0.12) for biomass burning, ¥0.1 

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181This conclusion would apply for both the 

secondary (welfare-based) and the primary (healthbased) standards. 

(+0.1) for nitrates, and ¥0.1 (+0.2) for 

mineral dust (U.S. EPA, 2009a, section 

9.3.10). Sulfate and fossil fuel-derived 

OC cause negative forcing whereas BC 

causes positive forcing because of its 

highly absorbing nature (U.S. EPA, 

2009a, 9.3.6.3). Although BC comprises 

only a small fraction of anthropogenic 

aerosol mass load and aerosol optical 

depth (AOD), its forcing efficiency (with 

respect to either AOD or mass) is an 

order of magnitude stronger than sulfate 

and particulate organic matter (POM), so 

its positive shortwave forcing largely 

offsets the negative direct forcing from 

sulfate and POM (IPCC, 2007; U.S. EPA, 

2009a, 9.3.6.3). Global loadings for 

nitrates and anthropogenic dust remain 

very difficult to estimate, making the 

radiative forcing estimates for these 

constituents particularly uncertain (U.S. 

EPA, 2009a, section 9.3.7). 

Improved estimates of anthropogenic 

emissions of some aerosols, especially 

BC and OC, have promoted the 

development of improved global 

emissions inventories and sourcespecific emissions factors useful in 

climate modeling (Bond et al. 2004). 

Recent data suggests that BC is one of 

the largest individual warming agents 

after carbon dioxide (CO2) and perhaps 

methane (CH4) (Jacobson 2000; Sato et 

al., 2003; Bond and Sun 2005). There 

are several studies modeling BC effects 

on climate and/or considering emission 

reduction measures on anthropogenic 

warming detailed in section 9.3.9 of the 

Integrated Science Assessment. In the 

U.S., most of the warming aerosols are 

emitted by biomass burning and internal 

engine combustion and much of the 

cooling aerosols are formed in the 

atmosphere by oxidation of SO2 or 

volatile organic compounds (VOCs) 

(U.S. EPA, 2009a, section 3.3). Fires 

release large amounts of BC, CO2, CH4

and OC (U.S. EPA, 2009a, section 9.3.7). 

Based on the above newly available 

scientific information on climate-aerosol 

relationships, the Policy Assessment 

concludes 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 notes 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). In light of this 

information, the Policy Assessment 

considered the appropriateness of the 

current secondary standards defined in 

terms of PM2.5 and PM10 indicators, for 

providing protection against potential 

climate effects of aerosols. 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 Policy Assessment 

concludes that the current mass-based 

PM2.5 and PM10 secondary standards are 

not an appropriate or effective means of 

focusing protection against PMassociated climate effects due to these 

differences in components (U.S. EPA, 

2011a, p. 5–11). 

Further, in light of the uncertainties 

associated with the spatial and temporal 

heterogeneity of PM components that 

contribute to climate forcing and the 

uncertainties associated with 

measurement of aerosol components, 

the inadequate consideration of aerosol 

impacts in climate modeling and the 

insufficient data on local and regional 

microclimate variations and the 

heterogeneity of cloud formations, the 

Policy Assessment concludes 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). 

Based on these considerations, the 

Policy Assessment concludes 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).181

2. Ecological Effects 

Information on what is currently 

known about ecological effects of PM is 

summarized in Chapter 9 of the 

Integrated Science Assessment (U.S. 

EPA, 2009a). Four main categories of 

ecological effects are identified in the 

Integrated Science Assessment: Direct 

effects, effects of PM-altered radiative 

flux, indirect effects of trace metals, and 

indirect effects of organics. Exposure to 

PM for direct effects occurs via 

deposition (e.g., wet, dry or occult) to 

vegetation surfaces, while indirect 

effects occur via deposition to 

ecosystem soils or surface waters where 

the deposited constituents of PM then 

interact with biological organisms. Both 

fine and coarse-mode particles may 

affect plants and other organisms; 

however, PM size classes do not 

necessarily relate to ecological effects 

(U.S. EPA, 1996). More often, the 

chemical constituents drive the 

ecosystem response to PM (Grantz et al., 

2003). The trace metal constituents of 

PM considered in the ecological effects 

section of the Integrated Science 

Assessment are cadmium (Cd), copper 

(Cu), chromium (Cr), mercury (Hg), 

nickel (Ni) and zinc (Zn). Ecological 

effects of lead (Pb) in particulate form 

are covered in the Air Quality Criteria 

Document for Lead (U.S. EPA, 2006). 

The organics included in the ecological 

effects section of the PM Integrated 

Science Assessment are persistent 

organic pollutants (POPs), polyaromatic 

hydrocarbons (PAHs), and 

polybromiated diphenyl ethers (PBDEs). 

Ecological effects of PM 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. 

The Integrated Science Assessment 

states that overall, 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 

based on information from the previous 

review and limited new findings in this 

review (U.S. EPA, 2009a, sections 2.5.3 

and 9.4.7). However the Integrated 

Science Assessment also finds, in many 

cases, it is difficult to characterize the 

nature and magnitude of effects 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. 

Ecological effects of PM must then be 

evaluated to determine if they are 

known or anticipated to have an adverse 

impact on public welfare. 

Characterizing a known or anticipated 

adverse effect to public welfare is an 

important component of developing any 

secondary NAAQS. The most recent 

secondary NAAQS reviews have 

assessed changes in ecosystem structure 

or processes using a weight-of-evidence 

approach that uses both quantitative 

and qualitative data. A paradigm useful 

in evaluating ecological adversity is the 

concept of ecosystem services. 

Ecosystem services consist of the varied 

and numerous ways that ecosystems are 

important to human welfare. 

Ecosystems provide many goods and 

services that are of vital importance for 

the functioning of the biosphere and 

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provide the basis for the delivery of 

tangible benefits to human society. An 

EPA initiative to consider how 

ecosystem structure and function can be 

interpreted through an ecosystem 

services approach has resulted in the 

inclusion of ecosystem services in the 

NOX/SOX Risk and Exposure 

Assessment (U.S. EPA, 2009h). The 

Millennium Ecosystem Assessment 

(MEA) defines these to include 

supporting, provisioning, regulating, 

and cultural services (Hassan et al., 

2005). 

An important consideration in 

evaluating biologically adverse effects of 

PM and linkages to ecosystem services 

is that many of the MEA categories 

overlap and any one pollutant may 

impact multiple services. For example, 

deposited PM may alter the composition 

of soil-associated microbial 

communities, which may affect 

supporting services such as nutrient 

cycling. Changes in available soil 

nutrients could result in alterations to 

provisioning services such as timber 

yield and regulating services such as 

climate regulation. If enough 

information is available, these 

alterations can be quantified based upon 

economic approaches for estimating the 

value of ecosystem services. Valuation 

may be important from a policy 

perspective because it can be used to 

compare the benefits of altering versus 

maintaining an ecosystem. Knowledge 

about the relationships linking ambient 

concentrations and ecosystem services 

can be used to inform a policy judgment 

on a known or anticipated adverse 

public welfare effect. 

The Policy Assessment seeks to build 

upon and focus this body of science 

using the concept of ecosystem services 

to qualitatively evaluate linkages 

between biologically adverse effects and 

particulate deposition. This approach is 

similar to that taken in the NOX/SOX

Risk and Exposure Assessment in which 

the relationship between air quality 

indicators, deposition of nitrogen and 

sulfur, ecologically relevant indicators, 

and effects on sensitive receptors are 

linked to changes in ecosystem structure 

and services (U.S. EPA, 2009h). This 

approach considers the benefits 

received from the resources and 

processes that are supplied by 

ecosystems. Several ecosystem 

components (e.g., plants, soils, water, 

and wildlife) are impacted by PM air 

pollution, which may alter the services 

provided by the ecosystems in question. 

Key scientific evidence regarding PM 

effects on plants, soil and nutrient 

cycling, wildlife, and water available 

since the last review is summarized 

below to evaluate how this information 

has improved understanding of 

ecosystem responses to PM. 

a. Plants 

As primary producers, plants play a 

pivotal role in energy flow through 

ecosystems. Ecosystem services derived 

from plants include all of the categories 

(supporting, provisioning, regulating, 

and cultural) identified in the MEA 

(Hassan et al., 2005). Vegetation 

supports other ecosystem processes by 

cycling nutrients through food webs and 

serving as a source of organic material 

for soil formation and enrichment. Trees 

and plants provide food, wood, fiber, 

and fuel for human consumption. Flora 

help to regulate climate by sequestering 

CO2, and control flooding by stabilizing 

soils and cycling water via uptake and 

evapotranspiration. Plants are 

significant in aesthetic, spiritual, and 

recreational aspects of human 

interactions. 

Particulate matter can adversely 

impact plants and ecosystem services 

provided by plants by deposition to 

vegetative surfaces (U.S. EPA, 2009a, 

section 9.4.3). Particulates deposited on 

the surfaces of leaves and needles can 

block light, altering the radiation 

received by the plant. PM deposition 

can obstruct stomata limiting gas 

exchange, damage leaf cuticles, and 

increase plant temperatures. This level 

of PM accumulation is typically 

observed near sources of heavy 

deposition such as smelters and mining 

operations (U.S. EPA, 2009a, section 

9.4.3). Plants growing on roadsides 

exhibit impact damage from near-road 

PM deposition, having higher levels of 

organics and heavy metals, and 

accumulate salt from road de-icing 

during winter months (U.S. EPA, 2009a, 

sections 9.4.3.1 and 9.4.5.7). 

In addition to damage to plant 

surfaces, deposited PM can be taken up 

by plants from soil or foliage. The 

ability of vegetation to take up heavy 

metals and organics is dependent upon 

the amount, solubility, and chemical 

composition of the deposited PM. 

Uptake of PM by plants from soils and 

vegetative surfaces can disrupt 

photosynthesis, alter pigments and 

mineral content, reduce plant vigor, 

decrease frost hardiness, and impair 

root development. The Integrated 

Science Assessment indicates that there 

are little or no effects on foliar processes 

at ambient levels of PM (U.S. EPA, 

2009a, sections 9.4.3 and 9.4.7). 

However, damage due to atmospheric 

pollution can occur near individual 

point sources or under conditions where 

plants are subjected to multiple 

stressors. 

Although all heavy metals can be 

directly toxic at sufficiently high 

concentrations, only Cu, Ni, and Zn 

have been documented as being 

frequently toxic to plants (U.S. EPA, 

2004), while toxicity due to Cd, Co, and 

Pb has been observed less frequently 

(Smith, 1990; U.S. EPA, 2009a, section 

9.4.5.3). In general, plant growth is 

negatively correlated with trace metal 

and heavy metal concentration in soils 

and plant tissue (Audet and Charest, 

2007). Trace metals, particularly heavy 

metals, can influence forest growth. 

Growth suppression of foliar microflora 

has been shown to result from iron (Fe), 

aluminum (Al), and Zn. These three 

metals can also inhibit fungal spore 

formation, as can Cd, Cr, magnesium 

(Mg), and Ni (see Smith, 1990). Metals 

cause stress and decreased 

photosynthesis (Kucera et al., 2008) and 

disrupt numerous enzymes and 

metabolic pathways (Strydom et al., 

2006). Excessive concentrations of 

metals result in phytotoxicity. 

New information since the last review 

provides additional evidence of plant 

uptake of organics (U.S. EPA, 2009a, 

section 9.4.6). An area of active study is 

the impact of PAHs on provisioning 

ecosystem services due to the potential 

for human and other animal exposure 

via food consumption (U.S. EPA, 2009a, 

section 9.4.6 page 9–190). The uptake of 

PAHs depends on the plant species, site 

of deposition, physical and chemical 

properties of the organic compound, 

and prevailing environmental 

conditions. It has been established that 

most bioaccumulation of PAHs by 

plants occurs via leaf uptake, and to a 

lesser extent, through roots. Differences 

between species in uptake of PAHs 

confound attempts to quantify impacts 

to ecosystem provisioning services. 

Plants as ecosystem regulators can 

serve as passive monitors of pollution 

(U.S. EPA, 2009a, section 9.4.2.3). 

Lichens and mosses are sensitive to 

pollutants associated with PM and have 

been used with limited success to show 

spatial and temporal patterns of 

atmospheric deposition of metals (U.S. 

EPA, 2009a, section 9.4.2.3). A 

limitation to employing mosses and 

lichens to detect for the presence of air 

pollutants is the difference in uptake 

efficiencies of metals between species. 

Thus, quantification of ecological effects 

is not possible due to the variability of 

species responses (U.S. EPA, 2009a, 

section 9.4.2.3). 

A potentially important regulating 

ecosystem service of plants is their 

capacity to sequester contaminants (U.S. 

EPA, 2009a, section 9.4.5.3). Ongoing 

research on the application of plants to 

environmental remediation efforts are 

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yielding some success in removing 

heavy metals and organics from 

contaminated sites (phytoremediation) 

with tolerant plants such as the willow 

tree (Salix spp.) and members of the 

family Brassicaceae (U.S. EPA, 2009a, 

section 9.4.5.3). Tree canopies can be 

used in urban locations to capture 

particulates and improve air quality 

(Freer-Smith et al., 2004). Plant foliage 

is a sink for Hg and other metals and 

this regulating ecosystem service may be 

impacted by atmospheric deposition of 

trace metals. 

An ecological endpoint 

(phytochelatin concentration) associated 

with presence of metals in the 

environment has been correlated with 

the ecological effect of tree mortality 

(Grantz et al., 2003). Metal stress may be 

contributing to tree injury and forest 

decline in the Northeastern U.S. where 

red spruce populations are declining 

with increasing elevation. Quantitative 

assessment of PM damage to forests 

potentially could be conducted by 

overlaying PM sampling data and 

elevated phytochelatin levels. However, 

limited data on phytochelatin levels in 

other species currently hinders use of 

this peptide as a general biomarker for 

PM. 

The presence of PM in the atmosphere 

affects ambient radiation as discussed in 

the Integrated Science Assessment 

which can impact the amount of 

sunlight received by plants (U.S. EPA, 

2009a, section 9.4.4). Atmospheric PM 

can change the radiation reaching leaf 

surfaces through attenuation and by 

converting direct radiation to diffuse 

radiation. Diffuse radiation is more 

uniformly distributed in a tree canopy, 

allowing radiation to reach lower leaves. 

The net effect of PM on photosynthesis 

depends on the reduction of 

photosynthetically active radiation 

(PAR) and the increase in the diffuse 

fraction of PAR. Decreases in crop 

yields (provisioning ecosystem service) 

have been attributed to regional scale air 

pollution, however, global models 

suggest that the diffuse light fraction of 

PAR can increase growth (U.S. EPA, 

2009a, section 9.4.4). 

b. Soil and Nutrient Cycling 

Many of the major indirect plant 

responses to PM deposition are chiefly 

soil-mediated and depend on the 

chemical composition of individual 

components of deposited PM. Major 

ecosystem services impacted by PM 

deposition to soils include support 

services such as nutrient cycling, 

products such as crops and regulating 

flooding and water quality. Upon 

entering the soil environment, PM 

pollutants can alter ecological processes 

of energy flow and nutrient cycling, 

inhibit nutrient uptake to plants, change 

microbial community structure and, 

affect biodiversity. Accumulation of 

heavy metals in soils depends on factors 

such as local soil characteristics, 

geologic origin of parent soils, and metal 

bioavailability. It can be difficult to 

assess the extent to which observed 

heavy metal concentrations in soil are of 

anthropogenic origin (U.S. EPA, 2009a, 

section 9.4.5.1). Trace element 

concentrations are higher in some soils 

that are remote from air pollution 

sources due to parent material and local 

geomorphology. 

Heavy metals such as Zn, Cu, and Cd 

and some pesticides can interfere with 

microorganisms that are responsible for 

decomposition of soil litter, an 

important regulating ecosystem service 

that serves as a source of soil nutrients 

(U.S. EPA, 2009a, sections 9.4.5.1 and 

9.4.5.2). Surface litter decomposition is 

reduced in soils having high metal 

concentrations. Soil communities have 

associated bacteria, fungi, and 

invertebrates that are essential to soil 

nutrient cycling processes. Changes to 

the relative species abundance and 

community composition can be 

quantified to measure impacts of 

deposited PM to soil biota. A 

mutualistic relationship exists in the 

rhizophere (plant root zone) between 

plant roots, fungi, and microbes. Fungi 

in association with plant roots form 

mycorrhizae that are essential for 

nutrient uptake by plants. The role of 

mychorrizal fungi in plant uptake of 

metals from soils and effects of 

deposited PM on soil microbes is 

discussed in section 9.4.5.2 of the 

Integrated Science Assessment. 

c. Wildlife 

Animals play a significant role in 

ecosystem function including nutrient 

cycling and crop production (supporting 

ecosystem service), and as a source of 

food (provisioning ecosystem service). 

Cultural ecosystem services provided by 

wildlife include bird and animal 

watching, hunting, and fishing. Impacts 

on these services are dependent upon 

the bioavailability of deposited metals 

and organics and their respective 

toxicities to ecosystem receptors. 

Pathways of PM exposure to fauna 

include ingestion, absorption and 

trophic transfer. Bioindicator species 

(known as sentinel organisms) can 

provide evidence of contamination due 

to atmospheric pollutants. Use of 

sentinel species can be of particular 

value because chemical constituents of 

deposited PM are difficult to 

characterize and have varying 

bioavailability (U.S. EPA, 2009a, section 

9.4.5.5). Snails readily bioaccumulate 

contaminants such as PAHs and trace 

metals. These organisms have been 

deployed as biomonitors for urban 

pollution and have quantifiable 

biomarkers of exposure including 

growth inhibition, impairment of 

reproduction, peroxidomal 

proliferation, and induction of metal 

detoxifying proteins (metallothioneins) 

(Gomet-de Vaufleury, 2002; Regoli, et. 

al, 2006). Earthworms have also been 

used as sensitive indicators of soil metal 

contamination. 

Evidence of deposited PM effects on 

animals is limited (U.S. EPA, 2009a, 

section 9.4.5.5). Trophic transfer of 

pollutants of atmospheric origin has 

been demonstrated in limited studies. 

PM may also be transferred between 

aquatic and terrestrial compartments. 

There is limited evidence for 

biomagnifications of heavy metals up 

the food chain except for Hg which is 

well known to move readily through 

environmental compartments (U.S. EPA, 

2009a, section 9.4.5.6). Bioconcentration 

of POPs and PBDEs in the Arctic and 

deep-water oceanic food webs indicates 

the global transport of particleassociated organics (U.S. EPA, 2009a, 

section 9.4.6). Salmon migrations are 

contributing to metal accumulation in 

inland aquatic systems, potentially 

impacting the provisioning and cultural 

ecosystem service of fishing (U.S. EPA, 

2009a, section 9.4.6). Stable isotope 

analysis can be applied to establish 

linkages between PM exposure and 

impacts to food webs however, the use 

of this evaluation tool is limited for this 

ecological endpoint due to the 

complexity of most trophic interactions 

(U.S. EPA, 2009a, section 9.4.5.6). 

Foraging cattle have been used to assess 

atmospheric deposition and subsequent 

bioaccumulation of Hg and trace metals 

and their impacts on provisioning 

services (U.S. EPA, 2009a, section 

9.4.2.3). 

d. Water 

New limited information on impacts 

of deposited PM on receiving water 

bodies indicate that the ecosystem 

services of primary production, 

provision of fresh water, regulation of 

climate and floods, recreational fishing 

and water purification are adversely 

impacted by atmospheric inputs of 

metals and organics (U.S. EPA, 2009a, 

sections 9.4.2.3 and 9.4.5.4). Deposition 

of PM to surfaces in urban settings 

increases the metal and organic 

component of storm water runoff (U.S. 

EPA, 2009a, sections 9.4.2.3). This 

atmospherically-associated pollutant 

burden can then be toxic to aquatic 

biota. 

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Atmospheric deposition can be the 

primary source of some organics and 

metals to watersheds. The contribution 

of atmospherically deposited PAHs to 

aquatic food webs was demonstrated in 

high elevation mountain lakes with no 

other anthropogenic contaminant 

sources (U.S. EPA, 2009a, section 9.4.6). 

Metals associated with PM deposition 

limit phytoplankton growth, impacting 

aquatic trophic structure. Long-range 

atmospheric transport of 47 pesticides 

and degradation products to the 

snowpack in seven national parks in the 

Western U.S. was recently quantified 

indicating PM-associated contaminant 

inputs to receiving waters during spring 

snowmelt (Hageman et al., 2006). 

The recently completed Western 

Airborne Contaminants Assessment 

Project (WACAP) is the most 

comprehensive database on 

contaminant transport and PM 

depositional effects on sensitive 

ecosystems in the U.S. In this project, 

the transport, fate, and ecological 

impacts of anthropogenic contaminants 

from atmospheric sources were assessed 

from 2002 to 2007 in seven ecosystem 

components (air, snow, water, sediment, 

lichen, conifer needles and fish) in eight 

core national parks (Landers et al., 

2008). The goals of the study were to 

identify where the pollutants were 

accumulating, identify ecological 

indicators for those pollutants causing 

ecological harm, and to determine the 

source of the air masses most likely to 

have transported the contaminants to 

the parks (U.S. EPA, 2009a, section 

9.4.6). The study concluded that 

bioaccumulation of semi-volatile 

organic compounds was observed 

throughout park ecosystems (Landers et 

al., 2008). Findings from this study 

included the observation of an 

elevational gradient in PM deposition 

with greater accumulation at higher 

altitude areas of the parks. Furthermore, 

specific ecological indicators were 

identified in the WACAP that can be 

useful in assessing contamination on 

larger spatial scales. 

In the WACAP study, 

bioaccumulation and biomagnification 

of airborne contaminants were 

demonstrated on a regional scale in 

remote ecosystems in the Western 

United States. Contaminants were 

shown to accumulate geographically 

based on proximity to individual 

sources or source areas, primarily 

agriculture and industry (Landers et al., 

2008). Although this assessment focuses 

on chemical species that are 

components of PM, it does not 

specifically assess the effects of 

particulates versus gas-phase forms; 

therefore, in most cases it is difficult to 

apply the results to this assessment 

based on particulate concentration and 

size fraction (U.S. EPA, 2009a, section 

9.4.6). There is a need for ecological 

modeling of PM components in different 

environmental compartments to further 

elucidate links between PM and 

ecological indicators. 

Europe and other countries are using 

the critical load approach to assess 

pollutant effects at the level of the 

ecosystem. This type of assessment 

requires site-specific data and 

information on individual species 

responses to PM. In respect to trace 

metals and organics, there are 

insufficient data for the vast majority of 

U.S. ecosystems to calculate critical 

loads. However, a methodology is being 

presented in the NOX/SOX Secondary 

Risk and Exposure Assessment (U.S. 

EPA, 2010h) to calculate atmospheric 

concentrations from deposition that may 

be applicable to other environmental 

contaminants. 

e. Effects Associated With Ambient PM 

Concentrations 

As reviewed above, there is 

considerable data on impacts of PM on 

ecological receptors, but few studies 

that link ambient PM concentrations to 

observed effect. This is due, in part, to 

the nature, deposition, transport and 

fate of PM in ecosystems. PM is not a 

single pollutant, but a heterogeneous 

mixture of particles differing in size, 

origin and chemical composition (U.S. 

EPA, 2009a, section 9.4.1). The 

heterogeneity of PM exists not only 

within individual particles or samples 

from individual sites, but to even a 

greater extent, between samples from 

different sites. Since vegetation and 

other ecosystem components are 

affected more by particulate chemistry 

than size fraction, exposure to a given 

mass concentration of airborne PM may 

lead to widely differing plant or 

ecosystem responses, depending on the 

particular mix of deposited particles. 

Many of the PM components 

bioaccumulate over time in organisms 

or plants making correlations to ambient 

concentrations of PM difficult. 

Bioindicator organisms demonstrated 

biological effects including growth 

inhibition, metallothionein induction 

and reproductive impairment when 

exposed to complex mixtures of ambient 

air pollutants (U.S. EPA, 2009a, section 

9.4.5.5). Other studies quantify uptake 

of metals and organics by plants or 

animals. However, due to the difficulty 

in correlating individual PM 

components to a specific physiological 

response, these studies are limited. 

Furthermore, there may be differences 

in uptake between species such as 

differing responses to metal uptake 

observed in mosses and lichens (U.S. 

EPA, 2009a, section 9.4.2.3). PM may 

also biomagnify across trophic levels 

confounding efforts to link atmospheric 

concentrations to physiological 

endpoints (U.S. EPA, 2009a, section 

9.4.5.6). 

Evidence of PM effects that are linked 

to a specific ecological endpoint can be 

observed when ambient levels are 

exceeded. 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). 

Extensive research on biota near point 

sources provide some of the best 

evidence of ecosystem function impacts 

and demonstrates that deposited PM has 

the potential to alter species 

composition over long time scales. The 

Integrated Science Assessment indicates 

at 4 km distance, species composition of 

vegetation, insects, birds, and soil 

microbiota changed, and within 1 km 

only the most resistant organisms were 

surviving (U.S. EPA, 2009a, section 

9.4.5.7). 

f. Conclusions in the Policy Assessment 

Based on the above discussions, the 

Policy Assessment made the following 

observations: 

(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. 

Taking into consideration the 

responses to specific questions 

regarding the adequacy of the current 

secondary PM standards for ecological 

effects, the Policy Assessment 

concludes that the available information 

is insufficient to assess the adequacy of 

the protection for ecosystems afforded 

by the current suite of PM secondary 

standards (U.S. EPA, 2011a, p. 5–24). 

Ecosystem effects linked to PM are 

difficult to determine because the 

changes may not be observed until 

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pollutant deposition has occurred for 

many decades. Because the high levels 

necessary to cause injury occur only 

near a few limited point sources and/or 

on a very local scale, protection against 

these effects alone may not provide 

sufficient basis for considering a 

separate secondary NAAQS based on 

the ecological effects of particulate 

metals and organics. Data on ecological 

responses clearly linked with 

atmospheric PM is not abundant enough 

to perform a quantitative analysis 

although the WACAP study may 

represent an opportunity for 

quantification at a regional scale. The 

Policy Assessment further concludes 

that available evidence is not sufficient 

for 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). 

The Policy Assessment considered the 

appropriateness of continuing to use the 

PM2.5 and PM10 size fractions as the 

indicators for protection of ecological 

effects of PM. The chemical constitution 

of individual particles can be strongly 

correlated with size, and the 

relationship between particle size and 

particle composition can be quite 

complex, making it difficult in most 

cases to use particle size as a surrogate 

for chemistry. At this time it remains to 

be determined as to what extent PM 

secondary standards focused on a given 

size fraction would result in reductions 

of the ecologically relevant constituents 

of PM for any given area. Nonetheless, 

in the absence of information that 

provides a basis for specific standards in 

terms of particle composition, the Policy 

Assessment concludes that 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). 

3. Materials Damage 

Welfare effects on materials 

associated with deposition of PM 

include both physical damage (materials 

damage effects) and impaired aesthetic 

qualities (soiling effects). Because the 

effects of PM are exacerbated by the 

presence of acidic gases and can be 

additive or synergistic due to the 

complex mixture of pollutants in the air 

and surface characteristics of the 

material, this discussion will also 

include those particles and gases that 

are associated with the presence of 

ambient oxides of nitrogen and oxides 

of sulfur, as well as reduced forms of 

nitrogen (such as ammonia and 

ammonium ions) for completeness. 

Building upon the information 

presented in the last PM Staff Paper 

(U.S. EPA, 2005), and including the 

limited new information presented in 

Chapter 9 of the PM Integrated Science 

Assessment (U.S. EPA, 2009a) and 

Annex E. Effects of NOY, NHX, and SOX

on Structures and Materials of the 

Integrated Science Assessment for 

Oxides of Nitrogen and SulfurEcological Criteria (U.S. EPA, 2008c) the 

following sections consider the policyrelevant aspects of physical damage and 

aesthetic soiling effects of PM on 

materials including metal and stone. 

The Integrated Science Assessment 

concludes 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). 

Particles contribute to these physical 

effects because of their electrolytic, 

hygroscopic and acidic properties, and 

their ability to sorb corrosive gases 

(principally sulfur dioxide). In addition, 

the deposition of ambient PM can 

reduce the aesthetic appeal of buildings 

and objects through soiling. Particles 

consisting primarily of carbonaceous 

compounds cause soiling of commonly 

used building materials and culturally 

important items such as statues and 

works of art. Soiling is the deposition of 

particles on surfaces by impingement, 

and the accumulation of particles on the 

surface of an exposed material that 

results in degradation of its appearance 

(U.S. EPA, 2009a, section 9.5). Soiling 

can be remedied by cleaning or 

washing, and depending on the soiled 

material, repainting. 

The majority of available new studies 

on materials effects of PM are from 

outside the U.S., however, they provide 

limited new data for consideration of 

the secondary standard. 

Metal and stone are also susceptible 

to damage by ambient PM. Considerable 

research has been conducted on the 

effects of air pollutants on metal 

surfaces due to the economic 

importance of these materials, 

especially steel, Zn, Al, and Cu. Chapter 

9 of the PM Integrated Science 

Assessment and Annex E of the NOX/ 

SOX Integrated Science Assessment 

summarize the results of a number of 

studies on the corrosion of metals (U.S. 

EPA, 2009a; U.S. EPA, 2008c). Moisture 

is the single greatest factor promoting 

metal corrosion, however, deposited PM 

can have additive, antagonistic or 

synergistic effects. In general, sulfur 

dioxide is more corrosive than oxides of 

nitrogen although mixtures of oxides of 

nitrogen, sulfur dioxide and other 

particulate matter corrode some metals 

at a faster rate than either pollutant 

alone (U.S. EPA, 2008c, Annex E.5.2). 

Information from both the PM Integrated 

Science Assessment and NOX/SOX

Integrated Science Assessment suggest 

that the extent of damage to metals due 

to ambient PM is variable and 

dependent upon the type of metal, 

prevailing environmental conditions, 

rate of natural weathering and presence 

or absence of other pollutants. 

The PM Integrated Science 

Assessment and NOX/SOX Integrated 

Science Assessment summarize the 

results of a number of studies on PM 

and stone surfaces. While it is clear 

from the available information that 

gaseous air pollutants, in particular 

sulfur dioxide, will promote the 

deterioration of some types of stones 

under specific conditions, carbonaceous 

particles (non-carbonate carbon) and 

particles containing metal oxides may 

help to promote the decay process. 

Studies on metal and stone summarized 

in the Integrated Science Assessment do 

not show an association between 

particle size, chemical composition and 

frequency of repair. 

A limited number of new studies 

available on materials damage effects of 

PM since the last review consider the 

relationship between pollutants and 

biodeterioration of structures associated 

with microbial communities that 

colonize monuments and buildings 

(U.S. EPA, 2009a, section 9.5). Presence 

of air pollutants may synergistically 

enhance microbial deterioration 

processes. The role of heterotrophic 

bacteria, fungi and cyanobacteria in 

biodeterioration varied by local 

meterological conditions and pollutant 

components. 

Particulate matter deposition onto 

surfaces such as metal, glass, stone and 

paint can lead to soiling. Soiling results 

when PM accumulates on an object and 

alters the optical characteristics 

(appearance). The reflectivity of a 

surface may be changed or presence of 

particulates may alter light 

transmission. These effects can impact 

the aesthetic value of a structure or 

result in reversible or irreversible 

damage to statues, artwork and 

architecturally or culturally significant 

buildings. Due to soiling of building 

surfaces by PM, the frequency and 

duration of cleaning may be increased. 

Soiling affects the aesthetic appeal of 

painted surfaces. In addition to natural 

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38998 Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / Proposed Rules 

182As 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. 

factors, exposure to PM may give 

painted surfaces a dirty appearance. 

Pigments in works of art can be 

degraded or discolored by atmospheric 

pollutants, especially sulfates (U.S. 

EPA, 2008c, Annex E–15). 

Formation of black crusts due to 

carbonaceous compounds and buildup 

of microbial biofilms results in 

discoloration of surfaces. Black crust 

includes a carbonate component derived 

from building material and OC and EC. 

In limited new studies quantifying the 

organic carbon and elemental 

contribution to soiling by black crust, 

organic carbon predominated over 

elemental carbon at almost all locations 

(Bonazza et al., 2005). Limited new 

studies suggest that traffic is the major 

source of carbon associated with black 

crust formation (Putaud et al., 2004) and 

that soiling of structures in Oxford, UK 

showed a relationship with traffic and 

nitrogen dioxide concentrations (Viles 

and Gorbushina, 2003). These findings 

attempt to link atmospheric 

concentrations of PM to observed 

damage. However, no data on rates of 

damage are available and all studies 

were conducted outside of the U.S. 

Based on the above discussion, the 

Policy Assessment makes 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. 

(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. 

Revisiting the overarching policy 

question as to whether the available 

scientific evidence supports or calls into 

question the adequacy of the protection 

for materials afforded by the current 

suite of secondary PM standards, the 

Policy Assessment concludes that no 

new evidence in this review calls into 

question the adequacy of the protection 

for materials afforded by the current 

standard (U.S. EPA, 2011a, p. 5–29). PM 

effects on materials can play no 

quantitative role in considering whether 

any revisions of the secondary PM 

NAAQS are appropriate at this time. 

Nonetheless, in the absence of 

information that provides a basis for 

establishing a different level of control, 

the Policy Assessment concludes 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). 

4. CASAC Advice 

Regarding the other 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 concludes 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 notes 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 notes that a 

secondary standard directed toward 

reducing PM-related visibility 

impairment has the potential to be 

accompanied by regional warming if 

light scattering aerosols are 

preferentially targeted. 

With regard to ecological effects, 

CASAC concludes that the published 

literature is insufficient to support a 

national standard for PM effects on 

ecosystem services (Samet, 2010c, p.23). 

CASAC notes 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 concludes 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 notes 

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. 

5. Administrator’s Proposed 

Conclusions on Secondary Standards for 

Other PM-related Welfare Effects 

Based on the above considerations 

and the advice of CASAC, the 

Administrator provisionally concludes 

that it is not appropriate to establish any 

distinct secondary PM standards to 

address other non-visibility PM-related 

welfare effects. Nonetheless, the 

Administrator concurs 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, including 

ecological effects, effects on materials, 

and climate impacts. In the absence of 

information that would support any 

different standards, the Administrator 

proposes generally to retain the current 

suite of secondary PM standards182 to 

address non-visibility welfare effects. 

These secondary standards were set 

identical to the primary PM standards in 

the last review. More specifically, the 

Administrator proposes 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 proposes to retain the 

level of the current standard and to 

revise the form of the standard by 

removing the option for spatial 

averaging for the reasons discussed 

below in section VII.A. 2. In so doing, 

she 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. 

F. Administrator’s Proposed Decisions 

on Secondary PM Standards 

With regard to the secondary PM 

standards, the Administrator proposes 

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 

distinct secondary standard is defined 

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183With regard to the PM10 NAAQS, as 

summarized in sections IV.F and VI.F, the EPA is 

proposing to retain the current primary and 

secondary PM10 standards. Data handling 

procedures for these PM10 standards would remain 

as presented in 40 CFR part 50, appendix K. 

in terms of a calculated PM2.5 light 

extinction indicator, translated into the 

deciview scale, which is referred to as 

a PM2.5 visibility index; a 24-hour 

averaging time; a 90th percentile form, 

averaged over 3 years; and a level set at 

one of two options—either 30 dv or 28 

dv. The Administrator solicits comment 

on a range of levels for such a standard 

down to 25 dv, as well as on alternative 

standards to address PM-related 

visibility impairment, including a subdaily averaging time (e.g., 4 hours) and 

associated alternative levels in the range 

of 30 to 25 dv. To address other nonvisibility welfare effects, the 

Administrator proposes to revise the 

form of the secondary annual PM2.5

standard to remove the option for 

spatial averaging and to retain all other 

elements of the current suite of 

secondary PM standards. 

VII. Interpretation of the NAAQS for 

PM 

With regard to the NAAQS for PM2.5, 

this section discusses EPA’s proposed 

revisions to the data handling 

procedures in 40 CFR part 50, appendix 

N, for the proposed primary and 

secondary annual and 24-hour 

standards for PM2.5 (referred to as PM2.5

standards) and for the proposed distinct 

secondary standard for PM2.5 to address 

PM-related visibility impairment 

(referred to as the PM2.5 visibility index 

standard).183 Appendix N describes the 

computations necessary for determining 

when these standards are met and also 

addresses which measurement data are 

appropriate for comparison to the 

proposed standards, as well as data 

reporting protocols, data completeness 

criteria, and rounding conventions. 

As discussed in sections III and VI 

above, the EPA is proposing 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); (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 

non-visibility-related welfare protection 

(section VI.F); and (3) establish a 

distinct secondary PM2.5 visibility index 

standard (section VI.F). The EPA 

proposes to revise appendix N to 

conform to the proposed revisions to the 

standards. The Agency also proposes to 

make additional changes in the 

appendix N data handling provisions to 

codify existing practices currently 

included in guidance documents or 

implemented as EPA standard operating 

procedures as well as to provide greater 

clarity and consistency in the 

application of these provisions. The 

proposed revisions to appendix N are 

discussed in section VII.A below. 

Section 1(b) of appendix N refers to 

special considerations that may be given 

to data resulting from exceptional 

events. An exceptional event is defined 

in 40 CFR 50.1 as an event that affects 

air quality, is not reasonably 

controllable or preventable, is an event 

caused by human activity that is 

unlikely to recur at a particular location 

or a natural event, and is determined by 

the Administrator in accordance with 40 

CFR 50.14 to be an exceptional event. 

Air quality data that are determined to 

have been affected by an exceptional 

event under the procedural steps, 

substantive criteria, and schedule 

specified in section 50.14 may be 

excluded from consideration when EPA 

makes a determination that an area is 

meeting or violating the associated 

NAAQS. Proposed revisions to the 

schedule specified in section 50.14 for 

data flagging and submission of 

demonstrations for exceptional events 

data considered for initial area 

designations under the proposed revised 

primary and secondary PM standards 

are discussed in section VII.B below. 

Several proposed updates and 

clarifications to the data handling 

provisions associated with AQI 

reporting in 40 CFR part 58, Appendix 

G are discussed in section VII.C below. 

These modifications reflect the 

proposed changes to the AQI sub-index 

for PM2.5 as discussed in section V 

above and harmonize reporting 

procedures for AQI sub-indices for other 

criteria pollutants. 

A. Proposed Amendments to Appendix 

N: Interpretation of the NAAQS for 

PM2.5

As discussed below, the proposed 

revisions to appendix N corresponding 

to proposed changes in the standards 

addressed in sections III and VI above 

are: (1) Modification of the level of the 

primary annual PM2.5 standard (sections 

VII.A.1 and VII.A.4); (2) modification of 

the form of the primary and secondary 

annual PM2.5 standards to remove the 

option for spatial averaging (sections 

VII.A.2 and VII.A.4); and (3) addition of 

data handling procedures that detail 

how to make comparisons to the 

proposed secondary standard for PM2.5

that addresses PM-related visibility 

impairment (section VII.A.5), as well as 

to summarize associated changes 

proposed in other sections of appendix 

N to accommodate this proposed 

standard (sections VII.A.1, VII.A.2, and 

VII.A.3). In addition to these three 

proposed appendix modifications that 

are discussed in depth in sections III 

and VI above, the EPA also proposes 

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 EPA standard operating 

procedures; and (4) provide enhanced 

clarity and consistency in the 

articulation and application of appendix 

N provisions. Key elements of the 

proposed revisions to appendix N are 

summarized in sections VII.A.1 through 

VII.A.5 below, where each of these 

preamble sections corresponds to the 

similarly numbered section in appendix 

N. 

1. General 

The EPA proposes to modify section 

1.0 of appendix N to provide additional 

clarity regarding the scope and 

interpretation of the NAAQS for PM2.5. 

This section would reference the 

proposed revisions to the primary 

annual PM2.5 standard and the proposed 

revision to the form of the secondary 

annual PM2.5 standard (40 CFR 50.18) 

and the proposed addition of a distinct 

secondary PM2.5 visibility index 

standard (40 CFR 50.19). As 

summarized in section VI.F, the 

proposed secondary standard is defined 

in terms of a calculated PM2.5 light 

extinction indicator, which would use 

24-hour average speciated PM2.5 mass 

concentration data, along with 

associated relative humidity 

information, to calculate light 

extinction, which would then be 

translated to the deciview scale (referred 

to as a PM2.5 visibility index); a 24-hour 

averaging time; a 90th percentile form 

averaged over 3 years; and a level of 

either 30 dv or 28 dv. The result (i.e., 

the PM2.5 visibility index design value) 

would be compared to the level of the 

standard. As noted earlier, the NAAQS 

indicator and proposed data handling 

procedures are similar to those of the 

Regional Haze Program. The EPA 

proposes to add to section 1.0 of 

appendix N, a reference to section 2.9 of 

appendix C to 40 CFR part 58 which 

identifies the acceptable methods for the 

speciated PM2.5 mass concentration 

data. With regard to the appendix N 

term definitions which are delineated in 

this initial section, the EPA proposes to 

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184As discussed in more detail in section 

VIII.B.2.b below, the EPA is proposing to change the 

current presumption in 40 CFR 58.30 that microand middle-scale monitoring sites are ‘‘unique’’ and 

are comparable only to the 24-hour PM2.5 standards, 

unless approved by the Regional Administrator to 

collectively identify a larger region of localized high 

ambient PM2.5 concentrations. Today’s proposal, if 

finalized, would change this presumption, such that 

micro- and middle-scale monitoring sites would not 

be presumed to be unique and, therefore, would be 

comparable to the annual PM2.5 standards as well 

as the 24-hour PM2.5 standards, unless the Regional 

Administrator determines that the micro- or 

middle-scale site is unique. 

185The 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 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 would also be allowed to exclude 

these data in making comparisons to the PM2.5 and 

PM2.5 visibility index NAAQS. Currently, there are 

no designated ARMs for PM2.5. 

add, modify, or eliminate term 

definitions, as appropriate, in 

accordance with the proposed data 

handling rule revisions such as the 

addition of terms associated with the 

proposed secondary PM2.5 visibility 

index standard and the modification of 

terms that referenced spatial averaging. 

Additional term definitions are also 

being added to reference otherwise 

unchanged appendix N logic in an effort 

to streamline the appendix text, 

enhance clarity and thus improve 

readability and understanding. 

2. Monitoring Considerations 

The EPA proposes revisions to section 

2.0 of appendix N consistent with the 

proposed modification of the form of the 

primary annual PM2.5 standard to 

remove the option for spatial averaging. 

As described in more detail in section 

III.E.3.a above, the EPA is proposing to 

remove this option as part of the form 

of the primary annual PM2.5 standard. 

This proposed change is based on an 

analysis that indicates 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 2011a, Analysis 

A). 

With respect to the form of the 

secondary annual PM2.5 standard, while, 

as discussed in section VI.E.5 above, the 

EPA is proposing to retain the current 

secondary annual PM2.5 standard to 

provide protection for non-visibility 

welfare effects, the EPA believes it 

would be reasonable and appropriate to 

align the data handling procedures for 

the primary and secondary annual PM2.5

standards. Therefore, the EPA proposes 

to remove the option for spatial 

averaging for the secondary annual 

PM2.5 standard consistent with the 

proposed change in the form of the 

primary annual PM2.5 standard. The 

EPA 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. 

Consistent with the proposed change 

to revise the forms of the primary and 

secondary annual PM2.5 standards, the 

levels of the standards would be 

compared to measurements from each 

appropriate (i.e., ‘‘eligible’’) monitoring 

site in an area operated in accordance 

with the network technical 

requirements specified in 40 CFR 58.11, 

the operating schedule described in 40 

CFR 58.12, and the special 

considerations for data comparisons to 

the NAAQS 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 this proposed change, the EPA 

proposes to remove all references to the 

spatial averaging option throughout 

appendix N. 

3. Requirements for Data Use and 

Reporting for Comparisons With the 

NAAQS for PM2.5

The EPA proposes to make changes to 

section 3.0 of appendix N to correspond 

with the proposed secondary PM2.5

visibility index standard, to improve 

consistency with procedures used for 

other NAAQS, and to improve 

consistency with current standard 

operating procedures. Specifically, the 

EPA proposes revisions to this section 

regarding: (1) Requirements for 

reporting monitored aggregated PM2.5

and speciated PM2.5 mass concentration 

data; (2) clarification of monitoring data 

appropriate to compare to the PM2.5 and 

PM2.5 visibility index NAAQS; (3) 

clarification of procedures for using 

hourly concentrations to calculate 

24-hour concentrations; and (4) 

clarification of procedures for 

combining monitoring data from 

collocated instruments into a single 

‘‘combined site’’ record. Further, the 

EPA proposes to codify, in this same 

section, modifications to the PM2.5 data 

handling provisions to make them 

consistent with recent changes made for 

other criteria pollutants. For example, 

data for which the certification deadline 

has passed, and the monitoring agency 

has not requested certification of the 

data, can nevertheless be used to 

determine compliance with the PM2.5

NAAQS and the PM2.5 visibility index 

NAAQS when EPA judges the data to be 

complete and accurate. 

With regard to the criteria for 

reporting PM2.5 concentrations, section 

3.0 of appendix N specifies that PM2.5

mass concentrations used for NAAQS 

comparisons shall be reported in units 

of mg/m3 with the values truncated (not 

rounded) to one digit to the right of the 

decimal point (i.e., truncated to one 

decimal place). Since, to date, appendix 

N has dealt only with PM2.5 mass 

concentrations, intrinsically these 

requirements have dealt only with that 

particular set of data. 

With regard to the proposed 

secondary PM2.5 visibility index 

standard, the EPA already has a 

requirement in 40 CFR 58.16 to report 

speciated PM2.5 mass concentration 

data. This includes the nine required 

speciated PM2.5 mass concentration 

inputs (i.e., sulfate, nitrate, OC (and 

related PM2.5 OC which is reported OC 

with an adjustment for the organic 

carbon artifact present on a filter), EC, 

Al, Si, Ca, Fe, and Ti) used to calculate 

PM2.5 visibility index values as 

described in section VII.A.5 below. 

Specifically, the EPA proposes to 

require that all nine parameters be used 

in the appendix N procedures in units 

of mg/m3 with the values rounded to 

four decimal places (or three significant 

digits if the value is 0.1 mg/m3 or larger). 

These rounding conventions are 

consistent with the AQS reporting 

protocols used in the CSN program, 

discussed in section VIII.A.2 below, 

which is proposed to be a major source 

of ambient data used in calculating 

PM2.5 visibility index design values to 

compare to the level of proposed 

secondary NAAQS. 

Monitoring sites eligible for 

comparison to the NAAQS for PM2.5

include those following the network 

technical requirements specified in 40 

CFR 58.11 as well as following the 

eligibility criteria specified in 40 CFR 

58.30.184 However, as discussed in 

section VIII.A.1 below, an analysis of 

the quality of data from two different 

methods used by FEMs has indicated 

that some sites with continuous PM2.5

FEMs have an acceptable degree of 

comparability with collocated FRMs, 

while other FEMs have less acceptable 

data comparability that would not meet 

the performance criteria originally used 

to approve the FEMs (Hanley and Reff, 

2011). Therefore, as explained in more 

detail in section VIII.B.3.b.ii below, the 

EPA is proposing to allow monitoring 

agencies to identify PM2.5 FEMs that are 

not providing data of sufficient 

comparability to the FRM and, with 

EPA approval, to exclude the use of 

these data in making comparisons to the 

NAAQS for PM2.5.185

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186 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. 

187 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’’). 

With regard to data handling 

procedures for using hourly mass 

concentrations to calculate 24-hour 

average mass concentrations, current 

procedures are specific for handling 

aggregated PM2.5 mass concentrations 

and are not currently relevant for 

handling the speciated PM2.5 mass 

concentrations that would be used for 

calculating PM2.5 visibility index design 

values for the proposed secondary 

standard. In considering data handling 

procedures for hourly speciated PM2.5

mass concentrations, the EPA notes that 

the vast majority of speciation data 

collected across the country are from 

filter-based sampling methods which 

typically operate on a 24-hour sampling 

period. There are several monitoring 

sites reporting hourly speciation data, 

but even in these cases the methods 

employed only provide for a small 

number of speciation parameters (e.g., 

EC, OC, sulfate) to be reported. 

However, in anticipation that such 

continuous methods might be more 

widely implemented for the speciated 

PM2.5 mass components in the future, 

the EPA proposes to add clarifying 

language to section 3.0(a) to indicate 

that the data handling procedures for 

using hourly concentration data to 

calculate 24-hour average concentration 

data would be applicable to both 

aggregated PM2.5 mass concentrations 

and speciated PM2.5 mass 

concentrations. 

With respect to the procedures for 

combining monitored data from 

collocated instruments into a single 

‘‘combined site’’ data record, the EPA 

proposes 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). 

The EPA is not proposing to change the 

procedures for calculating a combined 

site record 186 but rather the subsequent 

evaluation of whether the specific 

measurements are considered 

‘‘creditable’’ or ‘‘extra’’ samples. 

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. 

Before the introduction of continuous 

PM2.5 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 

continuous PM2.5 FEMs to FRMs 

operated in some monitoring agency 

networks, and as briefly discussed 

above and in more detail in section 

VIII.A.1 below, many monitoring 

agencies have kept the FRM as the 

‘‘primary’’ monitor while continuing to 

evaluate the continuous FEM monitor. 

In cases where the FRM either does not 

have a scheduled measurement or has a 

measurement that is invalidated and the 

continuous FEM data are available for 

use, and the continuous FEM data are 

not identified as not to be used (i.e., a 

special purpose monitor (SPM) in its 

first 24 months of operation) the FEM 

data will be substituted into the site 

record. In cases where the continuous 

FEM measurements are reported on the 

FRM ‘‘off’’ days, these data are 

technically considered ‘‘extra’’ samples. 

In light of this practice, the EPA 

modified standing operating procedures 

and now proposes a conforming 

revision to section 3.0(e) whereby 

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, 

consistent with current practices, if data 

from a collocated FEM are missing on 

an FRM ‘‘off’’ day (and no unscheduled 

FRM data are reported that day), the 

EPA proposes not to identify these as 

‘‘scheduled’’ samples. Thus, reported 

data generated from the collocated 

continuous FEMs can only help increase 

data capture rates. The EPA specifically 

solicits comment on whether ‘‘nonprimary’’ (i.e., collocated) FEM data 

should be combined with the primary 

data as part of the comparison to the 

NAAQS for PM2.5. 

The EPA proposes to utilize the same 

general procedures for combining 

speciated PM2.5 mass concentration data 

from collocated monitors into a single 

‘‘combined site’’ record as those 

specified for the PM2.5 mass 

measurements. 

4. Comparisons With the Annual and 

24-Hour PM2.5 NAAQS 

Section 4.0 of appendix N specifies 

the procedures for comparing monitored 

data to the annual and 24-hour PM2.5

standards. The EPA proposes 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.F) and to retain the current level of 

the secondary annual PM2.5 standard 

(section VI.F), the EPA proposes to 

modify section 4.1(a) of appendix N to 

separately list the levels of the primary 

and secondary annual PM2.5 standards. 

Additionally, consistent with the 

proposed decision 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 

proposes to amend section 4.4 of 

appendix N to remove equations and 

associated instructions that relate to 

spatial averaging. 

With regard to assessments of data 

completeness, the EPA proposes to 

include two additional data substitution 

tests 187 (making a total of three data 

substitution tests) 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 

checks). Data substitution tests are 

diagnostic in nature; that is; they are 

only used in an illustrative manner to 

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188 ‘‘Slightly incomplete’’ is defined as less than 

75 percent but greater than or equal to 50 percent 

data capture. 

189A 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. 

show that the NAAQS status based on 

incomplete data is reasonable. If an 

‘‘incomplete’’ design value using 

substituted data passes the diagnostic 

test, this ‘‘incomplete’’ design value 

(without the data substitutions) is then 

considered the true actual ‘‘complete’’ 

design value. If an incomplete design 

value does not pass any stipulated data 

substitution test, then the original 

design value is still considered 

incomplete. 

Currently, section 4.1(c) specifies one 

data substitution test for validating an 

otherwise incomplete design value. This 

diagnostic test is only applicable to the 

primary and secondary annual PM2.5

standard and only applies in instances 

of a violation. The EPA proposes to 

modify the data completeness 

requirements by adding two additional 

data substitution tests for handling 

incomplete data sets in order to make 

the data handling procedures for PM2.5

more consistent with the procedures 

used for other NAAQS pollutants and to 

codify existing practices currently 

included in guidance documents (U.S. 

EPA, 1999) and implemented as EPA 

standard operating procedures. The 

proposed additional data substitution 

tests would be applicable for making 

comparisons to the primary and 

secondary annual and 24-hour PM2.5

standards. One of these tests uses 

collocated PM10 data to fill in ‘‘slightly 

incomplete’’ 188 data records, and the 

other uses quarter-specific maximum 

values to fill in ‘‘slightly incomplete’’ 

data records. 

With regard to identifying annual 

98th percentile concentrations for 

comparison to the primary and 

secondary 24-hour PM2.5 standards, the 

EPA proposes to simplify the 

procedures used with an approved 

seasonal sampling schedule. 

Specifically, the EPA proposes to 

eliminate the use of a special formula 

for calculating annual 98th percentile 

concentrations with a seasonal sampling 

schedule and proposes to use only one 

method for calculating annual 98th 

percentile concentrations at all sites. 

Currently, with an approved seasonal 

sampling schedule, a site typically 

samples as required 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. 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. EPA, 

1999), and, where appropriate, with 

official OAQPS design value 

calculations (71 FR 61211, October 17, 

2006). In cases where there is a 

balanced 189 (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 collocate 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 PM2.5 FEM 

monitor, data are combined based on 

procedures stated in section 3.0 of 

appendix N as modified as discussed in 

section VII.A.3 above. The end result of 

combining collocated FRM and FEM 

data is effectively an ‘‘every day’’ sitebased sampling frequency, resulting 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. 

The EPA notes that currently there are 

very few PM2.5 FRM monitors that 

actually operate on an approved 

seasonal sampling schedule (only 15 

sites out of approximately 1,000 total 

sites in 2010) and that almost half of 

these sites have a collocated PM2.5 FEM 

monitor. For the most recent 3-year 

period (2008–2010), the annual 98th 

percentile concentrations calculated 

with the special formula at these 15 

sites were approximately five percent 

lower than if the regular procedure was 

used. The EPA also notes that, in the 

last review, the Agency modified the 

monitoring requirements for areas with 

an FRM operating on a non-daily 

schedule such that, if the design values 

were within five percent of the 24-hour 

PM2.5 NAAQS, such areas are 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). Thus, the EPA 

proposes to simplify the data handling 

procedures for sites operating on a 

seasonal sampling schedule by 

eliminating the special formula and all 

references to it based on: (1) The small 

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 requirements for every day 

sampling in areas with design values 

that are within five percent of the 24- 

hour PM2.5 NAAQS; and (3) the EPA 

requirement that FRMs operating on an 

approved seasonal sampling schedule 

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). Thus, the EPA 

proposes to use only one method for 

calculating annual 98th percentile 

concentrations for all sites, that being 

the ‘‘regular’’ table look-up method 

specified in section 4.5(a)(1) of 

appendix N. The EPA solicits comment 

on the proposal to eliminate the special 

formula for sites operating on a seasonal 

sampling schedule. 

5. Data Handling Procedures for the 

Proposed Secondary PM2.5 Visibility 

Index NAAQS 

As summarized in section VI.F above, 

the EPA is proposing to establish a 

distinct secondary standard for PM2.5 to 

address PM-related visibility 

impairment. The EPA is proposing to 

define this standard in terms of a PM2.5

visibility index (section VI.D.1.c), which 

would use 24-hour average speciated 

PM2.5 mass concentration and historic 

monthly average relative humidity data 

to calculate PM2.5 light extinction, 

translated into the deciview scale, 

similar to the Regional Haze Program. 

The EPA proposes to add a new 

section 5.0 to appendix N to detail the 

data handling procedures for calculating 

PM2.5 visibility index design values and 

comparing these design values to the 

level of the proposed PM2.5 visibility 

index NAAQS. These proposed 

procedures are drawn from and are 

generally consistent with the original 

approach used in the Regional Haze 

Program [U.S. EPA, 2003] and discussed 

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190 In the IMPROVE program, artifact adjusted OC 

(i.e., PM2.5 OC) is simply reported as OC. That is 

the value used to produce OM for haze calculations. 

For the CSN measurements, the OC artifact needed 

to convert measured OC into PM2.5 OC is estimated 

from sampler-specific network-wide field blanks 

(Frank, 2012). 

191Fine Soil = 2.2[Al] + 2.49[Si] + 1.63[Ca] + 

2.42[Fe] + 1.94[Ti] 

192To facilitate the use of relative humidity data, 

the EPA would make this ten-year climatological 

data base publically available on its Web site. 

in the Policy Assessment (U.S. EPA, 

2011a, chapter 4, Appendix G). 

As discussed in section VI.B.1.a 

above, visibility impairment is caused 

by the scattering and absorption of light 

by suspended particles and gases in the 

atmosphere. The combined effect of 

light scattering and absorption by both 

particles and gases is characterized as 

light extinction. The amount of light 

extinction contributed by PM depends 

on the particle size distribution and 

composition, as well as the 

concentrations of speciated components 

of ambient PM. To make estimation of 

light extinction more practical, visibility 

scientists have developed simple 

algorithms, referred to as the IMPROVE 

algorithms to relate speciated PM2.5

concentrations to light extinction. 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. 

The EPA proposes to define the PM2.5

visibility index using a PM2.5 light 

extinction indicator calculated on a 

24-hour basis using the original 

IMPROVE algorithm without the terms 

for coarse mass and Rayleigh scatter. As 

discussed in section VI.D.1.c above, 

using such an index appropriately 

reflects the relationship between 

ambient PM and PM-related light 

extinction. When converting PM2.5 light 

extinction values in Mm¥1 to the 

deciview scale, the Rayleigh scattering 

term must be included to avoid the 

possibility of negative values. 

Consistent with the analyses and 

terminology used in the Policy 

Assessment (U.S. EPA, 2011a, chapter 4, 

Appendix G), PM2.5 light extinction 

(PM2.5 bext) is defined as 

The above formula is implemented 

using 24-hr speciated PM2.5

concentration data together with 

monthly climatological relative 

humidity factors as outlined below. The 

six steps involved in the calculation of 

the PM2.5 visibility index values are as 

follows: 

(1) As discussed in Section VI.B.1.a above, 

‘‘sulfate’’ is defined as ammonium sulfate 

and ‘‘nitrate’’ is defined as ammonium 

nitrate. Multiply 24-hour average speciation 

measurements of sulfate and nitrate ions by 

factors 1.375 and 1.29, respectively, to 

convert the reported ion concentrations into 

sulfate and nitrate ammonium concentrations 

(appendix N, equations 5a and 5b). 

(2) Convert artifact adjusted measured OC, 

which is termed ‘‘PM2.5 OC’’, into an estimate 

of organic mass (OM). The PM2.5 OC is 

derived by subtracting the samplerdependent OC measurement artifact from the 

measured OC.190 The PM2.5 OC is then 

multiplied by 1.4 to account for the 

additional mass of hydrogen, oxygen and 

other elements associated with the carbon in 

measured OC (appendix N, equation 5c). 

(3) Calculate fine soil/crustal PM2.5 (FS) 

component based on measurements of five 

soil derived elements (i.e., Al, Si, Ca, Fe, and 

Ti) together with multipliers to account for 

their normal oxides 191 (appendix N, equation 

5d). 

(4) Determine a representative long-term 

monthly average of hourly relative humidity 

hygroscopic growth factors, referred to as 

f(RH) values, at the speciation monitoring 

site, for each month of the year. There will 

be 12 such values for any monitoring site. 

The EPA proposes that the f(RH) values be 

selected using historical data. A spatial 

interpolation of historical relative humidity 

data is available which presents a gridded 

field of f(RH) values across the U.S. at a 

resolution of 0.25 degrees (SAIC, 2001). As 

discussed in section VI.D.2.a.ii above, these 

monthly average values were developed to 

support the Regional Haze Program and are 

based on considering any hour with relative 

humidity greater than 95 percent as 95 

percent. Because 10 years of hourly data were 

used to produce a single humidity term for 

each month, the EPA believes that the 

resulting monthly average of the humidity 

term is sufficient and appropriate to reduce 

the effects of fog or precipitation. The EPA 

proposes that the 10-year climatological data 

base be used to specify the f(RH) value 

associated with the grid-point closest in 

distance to the speciation monitoring site.192

(5) Apply the original IMPROVE algorithm 

without the terms for coarse mass and 

Rayleigh scatter (appendix N, equation 6) to 

calculate a daily average PM2.5 light 

extinction (PM2.5 bext, in units of Mm¥1). 

(6) To translate PM2.5 light extinction to the 

deciview scale for making comparisons to the 

level of the proposed secondary PM2.5

visibility index standard, the following 

equation, which includes the term for 

Rayleigh scattering term, is used: 

The EPA solicits comment on all aspects 

of the calculation of the PM2.5 visibility 

index, PM2.5 bext. 

As discussed in section VI.D.3 above, 

the EPA is proposing a 90th percentile 

form, averaged over 3 years, for the 

proposed secondary PM2.5 visibility 

index standard. Thus, 3 years of valid 

24-hr speciated PM2.5 mass 

concentration data would be required to 

calculate PM2.5 visibility index design 

values. The proposed new section 5.0 

for appendix N addresses data 

completeness requirements for 

speciated PM2.5 mass concentrations 

(section 5.0(b)), specifically that PM2.5

visibility index values be present for at 

least 11 creditable days of each quarter, 

for each of the three consecutive years. 

The 11 sample minimum is consistent 

with criteria specified for the current 

and proposed primary and secondary 

annual PM2.5 standards (i.e., 40 CFR part 

50, appendix N 4.1(b)) and, furthermore, 

has been used extensively for various 

PM characterization exercises (e.g., U.S. 

EPA, 2009a; U.S. EPA, 2011a). In 

addition, the proposed new section 5.0 

outlines procedures for identifying 

annual 90th percentile PM2.5 visibility 

index values (section 5.0(d)(3)) similar 

to procedures used to identify annual 

98th percentile values for the primary 


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193 Design values calculated by the EPA are 

computed and published annually by EPA’s 

OAQPS and reviewed in conjunction with the EPA 

Regional Offices. These values are available at: 

http://www.epa.gov/airtrends/values.html. 

194References to ‘‘state’’ are meant to include 

state, local and tribal agencies responsible for 

implementing the Exceptional Events Rule. 

and secondary 24-hour PM2.5 standards. 

In situations where a year does not 

contain the minimum 11 creditable 

samples in each quarter, the EPA 

proposes (in section 5.0) to still consider 

the identified 90th percentile index 

value to be valid if it, or a 3-year average 

of 90th percentile index values (i.e., a 

visibility impairment design value) 

including it, exceeds the level of the 

NAAQS. The EPA is not proposing any 

data substitution tests for PM2.5

visibility index design values like those 

codified and proposed for the 

aggregated PM2.5 mass standard design 

values; however, the EPA solicits 

comment on the inclusion of such data 

substitution tests. 

With regard to rounding conventions, 

the EPA proposes that all decimal digits 

be retained in the intermediate steps of 

the calculation of the PM2.5 light 

extinction indicator and that the PM2.5

visibility index values be rounded to the 

nearest tenth deciview. Furthermore, 

the EPA proposes to round the 3-year 

average 90th percentile PM2.5 visibility 

index design values to the nearest 1 dv 

for comparison to the level of the 

proposed secondary standard. 

Consistent with current procedures 

for PM and the other criteria pollutants, 

the EPA plans to calculate design values 

for the proposed secondary PM2.5

visibility index NAAQS using the 

procedures described above. The EPA 

plans to post these design values on its 

Web site.193

B. Exceptional Events 

States 194 are responsible for 

identifying air quality data that they 

believe warrant special consideration, 

including data affected by exceptional 

events. States identify such data by 

flagging (making a notation in a 

designated field in the electronic data 

record) specific values in the AQS 

database. States must flag the data and 

submit supporting documentation 

showing that the data have been affected 

by exceptional events if they wish the 

EPA to consider excluding the data in 

regulatory decisions, including 

determining whether or not an area is 

attaining the proposed revised PM 

NAAQS. 

All states and areas of Indian country 

that include areas that could exceed the 

proposed PM NAAQS and could 

therefore be designated as 

nonattainment for the proposed PM 

NAAQS have the potential to be affected 

by this rulemaking. Therefore, this 

action would apply to all states; to local 

air quality agencies to which a state has 

delegated relevant responsibilities for 

air quality management including air 

quality monitoring and data analysis; 

and to tribal air quality agencies where 

appropriate. 

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 a state to submit to EPA 

specified information about exceptional 

events and associated air pollutant 

concentration data. A state must 

initially notify the EPA that data have 

been affected by an event by July 1 of 

the calendar year following the year in 

which the event occurred. This is done 

by flagging the data in AQS and 

providing an initial event description. 

The state must also, after notice and 

opportunity for public comment, submit 

a demonstration to justify any claim 

within three years after the quarter in 

which the data were collected. 

However, if a regulatory decision based 

on the data (for example, a designation 

action) is anticipated, the schedule to 

flag data in AQS and submit complete 

documentation to EPA for review may 

be shortened and all information must 

be submitted to the EPA no later than 

one year before the decision is to be 

made. 

These generic deadlines in the 

Exceptional Events Rule are suitable 

after initial designations have been 

made under a NAAQS or when an area 

is to be redesignated, either from 

attainment to nonattainment or from 

nonattainment to attainment, and the 

redesignation status may depend on the 

excluded data. However, these same 

generic deadlines may need to be 

adjusted to accommodate the initial area 

designation process and schedule under 

a newly revised NAAQS. Until the level 

and form of the NAAQS have been 

promulgated, a state does not know 

whether the criteria for excluding data 

(which are tied to the level and form of 

the NAAQS) were met for a given event. 

In some cases, the generic deadlines, 

especially the deadlines for flagging 

some relevant data, may have already 

passed by the time the new or revised 

NAAQS is promulgated. In addition, it 

may not be feasible for information on 

some exceptional events that may affect 

final designations decisions to be 

collected and submitted to EPA at least 

one year in advance of the final 

designation decision. This scheduling 

constraint could have the unintended 

consequence of the EPA designating an 

area nonattainment because of 

uncontrollable natural or other qualified 

exceptional events. 

The Exceptional Events Rule at 

section 50.14(c)(2)(vi) indicates ‘‘when 

EPA sets a NAAQS for a new pollutant 

or revises the NAAQS for an existing 

pollutant, it may revise or set a new 

schedule for flagging exceptional event 

data, providing initial data descriptions 

and providing detailed data 

documentation in AQS for the initial 

designations of areas for those NAAQS.’’ 

The EPA intends to promulgate the 

revised PM NAAQS in December 2012. 

State Governors (and tribes, if they 

choose) should submit designations 

recommendations by December 2013, 

based on air quality data from the years 

2010 to 2012 or 2011 to 2013, if there 

are sufficient data for these years. Initial 

designations under the revised NAAQS 

would be made by December 2014 based 

on air quality data from the years 2011 

to 2013. (See section IX.A for a more 

detailed discussion of the designation 

schedule.) Assuming this schedule, all 

events to be considered during the 

designations process would need to be 

flagged and fully documented by states 

one year prior to designations, or by 

December 2013, under the existing 

generic deadline in the Exceptional 

Events Rule. Without revision to 40 CFR 

50.14, a state would not be able to flag 

and submit documentation regarding 

events that occurred in December 2013 

by one year before designations are 

made in December 2014. The EPA 

believes this is not an appropriate 

restriction, and therefore is proposing 

revisions to 40 CFR 50.14. 

The EPA proposes revisions to 40 CFR 

50.14 only to change submission dates 

for information supporting claimed 

exceptional events affecting PM data for 

initial area designations under the 

proposed new and revised PM NAAQS. 

The proposed rule language at the end 

of this notice shows the changes that 

would apply assuming promulgation of 

the new and revised PM NAAQS in 

December 2012 and initial area 

designations by December 2014. For air 

quality data collected in 2010 or 2011, 

the EPA proposes 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 

proposes to retain 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 proposes to revise to December 12, 

2013 the deadline for submitting 

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documentation to justify PM-related 

exceptional events occurring in 2010 

through 2012. The EPA believes these 

revisions/extensions will provide 

adequate time for states to review the 

impact of exceptional events from 2010 

through 2012 on any revised standards, 

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. 

If a state intends the EPA to consider 

in the PM designations decisions 

whether PM data collected during 2013 

have been affected by exceptional 

events, the EPA proposes that these data 

must be flagged by the generic 

Exceptional Event Rule deadline of July 

1, 2014. The EPA proposes to revise to 

August 1, 2014 the deadline for 

submitting documentation to justify PMrelated exceptional events occurring in 

2013. The EPA believes that these 

deadlines provide states with adequate 

time to review and identify potential 

exceptional events that occur in 

calendar year 2013. 

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 proposes 

to modify the schedule for data flagging 

and submission of demonstrations for 

exceptional events data considered for 

initial area designations under the 

proposed PM primary and secondary 

NAAQS as presented in Table 3. If the 

promulgation date for a revised PM 

NAAQS occurs on a different date than 

in December 2012, the EPA will revise 

the final PM exceptional event flagging 

and documentation submission 

deadlines accordingly, consistent with 

the logic of this proposal, to provide 

states with reasonably adequate 

opportunity to review, identify, and 

document exceptional events that may 

affect an area designation under a 

revised NAAQS. The EPA invites 

comment on these proposed changes, 

shown in Table 3, to the exceptional 

event data flagging and documentation 

submission deadlines for the proposed 

revised PM NAAQS. 

TABLE 3—REVISED SCHEDULE FOR EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE

USED IN INITIAL AREA DESIGNATIONS FOR THE 2012 PM 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-Hour Standard (final level and promulgation date TBD) .......... 2010 to 2011 .............

2012 ..........................

2013 ..........................

July 1, 2013 ...............

a July 1, 2013 .............

a July 1, 2014 .............

December 12, 2013. 

December 12, 2013. 

August 1, 2014. 

PM2.5/Annual Standard (final level and promulgation date TBD) ............ 2010 to 2011 .............

2012 ..........................

2013 ..........................

July 1, 2013 ...............

a July 1, 2013 .............

a July 1, 2014 .............

December 12, 2013. 

December 12, 2013. 

August 1, 2014. 

Secondary PM (final level and promulgation date TBD) ......................... 2010 to 2011 .............

2012 ..........................

2013 ..........................

July 1, 2013 ...............

a July 1, 2013 .............

a July 1, 2014 .............

December 12, 2013. 

December 12, 2013. 

August 1, 2014. 

aThis 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 final initial area designations for revised NAAQS. The general schedule applies for all other purposes, most notably, for data used 

by the EPA for redesignations to attainment. TBD = to be determined. 

C. Proposed Updates for Data Handling 

Procedures for Reporting the Air Quality 

Index 

The EPA is proposing to update 

appendix G of 40 CFR part 58 to clarify 

units, breakpoint precision, and 

truncation methods for AQI sub-indices. 

These changes are intended to 

harmonize the AQI reporting 

requirements with data handling 

provisions expressed elsewhere in 40 

CFR part 50. Currently, the breakpoints 

for NO2 and SO2 in Table 2 of appendix 

G of 40 CFR part 58 are expressed in 

parts per million (ppm). The EPA 

proposes to change the sub-indices for 

NO2 and SO2 to be based on parts per 

billion (ppb) rather than ppm to be 

consistent with the units used for 

defining the current levels of the 

primary NO2 and SO2 NAAQS (75 FR 

6474, February 9, 2010; 75 FR 35520, 

June 22, 2010). In addition, in 

modifying the sub-index for NO2 to 

express the breakpoints in units of ppb, 

the EPA proposes to clarify the 

breakpoints for NO2 in the Very 

Unhealthy and Hazardous ranges to 

include four rather than three 

significant digits to increase precision. 

Finally, the EPA proposes to modify 

appendix G to explicitly identify 

truncation methods for using ambient 

measured concentrations in AQI 

calculations. 

VIII. Proposed Amendments to Ambient 

Monitoring and Reporting 

Requirements 

The EPA proposes changes to the 

ambient air monitoring, reporting, and 

network design requirements associated 

with the PM NAAQS. Ambient 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 proposing to 

support the proposed PM NAAQS 

summarized in sections III.F, IV.F, and 

VI.F above. 

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 

In 2006, the EPA finalized new testing 

and performance criteria for Class II and 

Class III FEMs (71 FR 61281 to 61289, 

October 17, 2006). Class II methods are 

equivalent methods for PM2.5 or PM10-2.5

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195The EPA consulted with the CASAC AAMMS 

on several PM monitoring topics in a public 

meeting on September 21 and 22, 2005. Materials 

from this meeting can be found on EPA’s Web site 

at: http://www.epa.gov/ttn/amtic/casacinf.html. 

196A list of designated Reference and Equivalent 

methods is available on EPA’s Web site at: http:// 

www.epa.gov/ttn/amtic/criteria.html. 

that utilize a PM2.5 sampler or PM10-2.5

sampler in which integrated PM2.5

samples or PM10-2.5 samples are 

obtained from the atmosphere by 

filtration and are then subjected to a 

filter conditioning process followed by 

gravimetric mass determination. Class II 

equivalent methods are different from 

Class I equivalent methods because of 

substantial deviations from the design 

specifications of the sampler specified 

for reference methods in appendix L or 

appendix O (as applicable) of 40 CFR 

part 50. 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 onehour 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. These new 

testing and performance criteria were 

developed by the EPA and reviewed 

through consultation with the CASAC 

AAMMS 195 and then through proposal 

(71 FR 2710 to 2808, January 17, 2006) 

and final rulemaking in 2006 (71 FR 

61236 to 61328, October 17, 2006). The 

performance criteria were designed to 

ensure enough stringency in testing that 

subsequently deployed monitors would 

provide data of expected quality (i.e., 

they would meet the data quality 

objectives), but not so stringent that 

instrument manufacturers would be 

discouraged from testing their 

instrument and seeking approval as a 

Class II or III equivalent method. At the 

time of this proposal, the EPA has 

approved two PM10-2.5 Class II manual 

methods, one Class III PM10-2.5

continuous method, and six Class III 

PM2.5 continuous methods.196

While the EPA has approved these 

PM2.5 Class III continuous FEMs, only 

two of those methods are deployed on 

a wide-enough basis across the country 

to support initial analyses of data 

quality and comparability to collocated 

FRM samplers. The Policy Assessment 

discusses an analysis of the quality of 

data from these two FEMs (U.S. EPA, 

2011a, p. 4–50). This initial analysis 

found that some sites with continuous 

PM2.5 FEMs have an acceptable degree 

of comparability with collocated FRMs, 

while others had less acceptable data 

comparability that would not meet the 

performance criteria used to approve the 

FEMs. 

The EPA continues to believe that an 

effective PM2.5 monitoring strategy 

includes the use of both filter-based 

FRM samplers and well-performing 

continuous PM2.5 monitors. Wellperforming continuous PM2.5 monitors 

would include both non-approved 

continuous PM2.5 monitors and 

approved Class III 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. 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, reduce the use of 

manual, filter-based samplers to 

improve operational efficiency and 

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 earlier, 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 EPA review and approval, 

as appropriate, of Class III FEMs. In the 

current step, monitoring agencies are 

testing and assessing the data 

comparability from continuous PM2.5

FEMs. While some agencies are 

achieving acceptable data comparability 

and others are not, the EPA wants to 

ensure that all monitoring agencies have 

the appropriate information to 

maximize data quality from their PM2.5

continuous FEMs before considering 

any changes to regulatory testing 

requirements intended to demonstrate 

equivalency of candidate Class III FEMs. 

Since we are still early in the process of 

learning the data comparability between 

approved PM2.5 continuous methods 

and collocated FRMs (assessments 

across the country are only available for 

two of the six methods), and some of the 

agencies operating those methods are 

achieving acceptable data 

comparability, the EPA does not believe 

it is appropriate at this time to propose 

any modifications to either the 

performance or testing criteria in 40 

CFR part 53 used to approve PM2.5

continuous FEMs. 

While EPA is not proposing any 

changes to the performance or testing 

criteria in 40 CFR part 53 used to 

approve PM2.5 continuous FEMs, the 

EPA proposes an administrative change 

to part 53.9—‘‘Conditions of 

designations.’’ 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 methods 

work within performance criteria, 

which includes methods for PM2.5 and 

PM10-2.5; however, there is no specific 

reference to performance criteria for 

Class II and III PM2.5 and PM10-2.5

methods. Therefore, the EPA proposes 

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. 

2. Use of CSN Methods To Support the 

Proposed New Secondary PM2.5

Visibility Index NAAQS 

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 initial CSN began with a pilot of 13 

sites in 2000 and grew rapidly over the 

next two years. Since 2006, the size of 

the CSN has remained relatively stable 

at approximately 200 stations. 

The methods employed in the CSN 

are well documented and uniformly 

implemented across the country. 

However, between May 2007 and 

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197 In the IMPROVE program, artifact adjusted OC 

(i.e., PM2.5 OC) is simply reported as OC. That is 

the value used to produce OM for haze calculations. 

For the CSN measurements, the OC artifact needed 

to convert measured OC into PM2.5 OC is estimated 

from sampler-specific network-wide field blanks 

(Frank, 2012). 

198Appendix C to 40 CFR part 58—Ambient Air 

Quality Monitoring Methodology is where EPA 

specifies the criteria pollutant monitoring methods 

which must be used at SLAMS and NCore, which 

are a subset of SLAMS. 

199CSN documents are available at: http:// 

www.epa.gov/ttn/amtic/speciepg.html; IMPROVE 

documents are available at: http:// 

vista.cira.colostate.edu/improve/Data/QA_QC/ 

qa_qc_Branch.htm). 

200SOP’s for the CSN program are available in 

Docket number EPA–HQ–OAR–2007–0492 and on 

EPA’s Web site at: http://www.epa.gov/ttn/amtic/ 

specsop.html. SOP’s for the IMPROVE program are 

available in Docket number EPA–HQ–OAR–2007– 

0492 and on the IMPROVE Web site at: http:// 

vista.cira.colostate.edu/improve/publications/ 

IMPROVE_SOPs.htm. 

201These are referenced in 40 CFR 58.30 (Special 

considerations for data comparisons to the 

NAAQS). 

October 2009, the CSN transitioned to a 

new method of sampling and analyses 

for carbon that is consistent with the 

IMPROVE network methodology.197 The 

CSN measurements have a strong 

history of being reviewed by CASAC 

technical committees, both during their 

initial deployment about ten years ago, 

and during the more recent transition to 

carbon sampling that is consistent with 

the IMPROVE protocols (Henderson, 

2005c). The CSN network is described 

in the Policy Assessment (U.S. EPA, 

2011a, Appendix B, section B.1.3). 

As noted in section VI.D.1.c above, 

the proposed new secondary standard 

for PM2.5 to address PM-related 

visibility impairment is defined in terms 

of a PM2.5 visibility index, which would 

use PM2.5 speciation measurement data. 

The EPA proposes that measurements 

using either the CSN or IMPROVE 

methods 198 be eligible for use to 

calculate PM2.5 visibility index values. 

The EPA believes this proposed 

approach is appropriate because the 

methods for CSN and IMPROVE are 

well documented 199 in nationally 

implemented Quality Assurance Project 

Plans (QAPPs) and accompanying 

Standard Operating Procedures (SOPs) 

are validated through independent 

performance testing, and because 

numerous state, local, and tribal 

agencies are already experienced in the 

use of these methods. 

With reference to CSN methods, the 

EPA is specifically not proposing to 

include testing or performance criteria 

for approval of CSN measurements as 

FRMs. The EPA believes that the 

proposed framework of using the 

current, well-documented set of CSN 

and IMPROVE methods provides a 

nationally consistent way to provide the 

chemical species data used in 

calculating PM2.5 visibility index values, 

while preserving the flexibility for 

timely improvements to methods for 

measuring chemical species. Monitoring 

programs wishing to establish methods 

for chemical speciation in support of the 

proposed PM2.5 visibility index would 

do so by following the methods and 

SOP’s publically available on both the 

IMPROVE or the EPA (for CSN) Web 

sites.200 The EPA solicits comment on 

this approach to include the CSN and 

IMPROVE measurements by reference 

and not require that such methods be 

approved as FRMs. 

As discussed in section VII.A.5 above, 

the calculation of the PM2.5 visibility 

index values would use historic 

monthly average relative humidity data 

based on a ten-year climatological data 

base. This data base would be based on 

measurements of relative humidity 

reported through NOAA at routine 

weather stations and not relative 

humidity measurements specific to the 

SLAMS stations. 

B. Proposed Changes to 40 CFR Part 58 

(Ambient Air Quality Surveillance) 

1. Proposed Terminology Changes 

The EPA proposes to revise several 

terms associated with PM2.5 monitor 

placement to ensure consistency with 

other NAAQS and to conform with longstanding practices in siting of 

equipment by monitoring agencies. 

The EPA proposes 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, microor middle-scale PM2.5 monitoring may 

also be considered ‘‘communityoriented’’ when determined by the 

Regional Administrator to represent 

many such locations throughout a 

metropolitan area. The EPA proposes to 

replace this functional usage of 

‘‘community-oriented’’ with the term 

‘‘area-wide’’ 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 EPA proposes 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 proposes to revoke the term 

‘‘Community Monitoring Zone’’ (CMZ) 

and 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 county or 

census block, within an MPA 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 

proposes to revoke this term and 

references to it since, as discussed in 

section VII.A.2 above, the EPA is 

proposing to eliminate all references to 

the spatial averaging option throughout 

appendix N. 

2. Special Considerations for 

Comparability of PM2.5 Ambient Air 

Monitoring Data to the NAAQS 

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 a monitor 

that provides data for comparison to the 

NAAQS, only in the PM2.5 monitoring 

requirements are additional restrictions 

prescribed within the monitoring 

rules.201 These additional restrictions 

provide that sites must be ‘‘populationoriented’’ 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 

smaller scales to the annual NAAQS 

when the (micro- or middle-scale) site 

collectively identifies a larger region of 

localized high ambient PM2.5

concentration. 

The inclusion of these provisions in 

the PM2.5 monitoring requirements since 

the 1997 promulgation of the PM2.5

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202Modeling can be associated with either PSD or 

transportation conformity as discussed in sections 

IX.F and IX.G, respectively, below. 

NAAQS and associated monitoring 

requirements has resulted in substantial 

ambiguity when the EPA and state, 

local, and tribal agencies consider the 

design of PM2.5 monitoring networks as 

NAAQS are revised as well as how 

unmonitored locations should be treated 

in modeling exercises.202 Accordingly, 

the EPA proposes to revise these 

particular PM2.5 requirements for 

consistency with long-standing 

practices in all other NAAQS pollutant 

monitoring networks, and to ensure 

interpretation of the monitoring rules 

does not cause ambiguity in considering 

treatment of unmonitored areas. Each of 

these topics and our proposal to revoke 

or modify the requirements is described 

below. 

a. Revoking Use of Population-Oriented 

as a Condition for Comparability of 

PM2.5 Monitoring Sites to the NAAQS 

The EPA proposes to revoke the 

requirement that PM2.5 monitoring sites 

be ‘‘population-oriented’’ for 

comparison to the NAAQS. This 

requirement is inconsistent with our 

definition of ambient air which the 

NAAQS employ. 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 ‘‘populationoriented’’ is provided in 40 CFR 58.1— 

‘‘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.’’ The EPA’s 

intention in proposing to revoke the 

requirement that PM2.5 monitoring sites 

be ‘‘population-oriented’’ for 

comparison to the NAAQS is to ensure 

that the monitoring rules do not create 

an ambiguity in the use of data by 

having a different definition from the 

definition of ambient air in 40 CFR 50.1 

itself. Also, EPA’s proposal to revoke 

this term in no way changes the 

requirements in the PM2.5 network 

design criteria, which will continue to 

focus on sites representing ‘‘area-wide’’ 

locations; thus continuing to represent 

locations with population exposure. 

While the use of the term ‘‘populationoriented’’ 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 requirement in the monitoring rules 

creates substantial ambiguity in how to 

treat potential locations of exposure 

such as in applying modeling across an 

area. By reverting to the long-standing 

definition of ambient air, the EPA will 

be able to more clearly define how to 

treat potential exposure receptors, 

regardless of whether monitoring exists 

or not. 

In reviewing the impact that this 

proposed 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. While 

a monitoring agency could still set up a 

new site in any area, including one in 

an area that does not meet the definition 

of population-oriented, which the EPA 

is proposing to revoke, there are other 

monitoring options that may provide 

more useful information and still result 

in data that are not comparable to the 

NAAQS; for instance, using a chemical 

speciation network sampler that 

provides chemical species information 

or continuous PM2.5 monitor that 

provides high time-resolution data, but 

is not approved as an FEM. Even if a 

monitoring agency wanted to use an 

FRM, an agency could still operate a 

monitor for up to 24 months as an SPM 

without any risk of data being used for 

comparison to the NAAQS. 

b. Applicability of Micro- and Middlescale Monitoring Sites to the Annual 

PM2.5 NAAQS 

The EPA is clarifying language used 

to determine when PM2.5 monitoring 

sites at micro- and middle-scale 

locations are comparable to the annual 

NAAQS. EPA’s intent in clarifying this 

language is to provide consistency and 

predictability in the interpretation of the 

monitoring regulations to minimize the 

burden on state monitoring programs as 

they plan and implement their 

monitoring programs. 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 is 

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 

sources are rather ubiquitous and, as 

such, there are many locations 

throughout an urban area where 

elevated exposures could occur. 

Therefore, any potential location for a 

PM2.5 monitoring site, even 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.’’ 

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). 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 

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203NCore 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. 

language will help to ensure a more 

consistent identification and approval of 

sites, and therefore 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. 

3. Proposed 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) 203 station 

data, will be used to evaluate the 

regional air quality models used in 

developing emission strategies and to 

track trends in air pollution abatement 

control measures’ impact on improving 

air quality. In monitoring locations near 

major air pollution sources, sourceoriented 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. To summarize some of 

these sites, here 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; (b) sites located to measure 

typical concentrations in areas of high 

population density; (c) sites located to 

determine the impact of significant 

sources or source categories on air 

quality; (d) sites located to determine 

general background concentration 

levels; and (e) sites located to determine 

the extent of regional pollutant transport 

among populated areas; and in support 

of secondary standards. 

b. Primary PM2.5 NAAQS 

In this section, the EPA proposes 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. 

i. Proposed Addition of a Near-road 

Component to the PM2.5 Monitoring 

Network 

The EPA believes that there are 

gradients in near-roadway PM2.5 that are 

most likely to be associated with heavily 

travelled roads, particularly those with 

significant heavy-duty diesel activity, 

with the largest numbers of impacted 

populations in the largest CBSAs in the 

country (Ntziachristos et al., 2007; Ross 

et al., 2007; Yanosky et al., 2008; Zwack 

et al., 2011). To better understand the 

potential health impacts of these 

exposures, the EPA proposes to add a 

near-road component to the compliance 

network design for PM2.5 monitoring. 

The EPA believes 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 long-term 

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 

recognizes that there are limitations in 

the ability of some of these PM methods 

to accurately measure PM2.5 mass due to 

the incomplete retention of semivolatile 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 recognizes 

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 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 

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). EPA believes that by allowing 

State monitoring agencies to choose the 

FRM, FEM, or ARM method(s) that best 

fits their needs, whether filter-based or 

continuous, that the data will still be 

able to meet the objectives cited above 

while ensuring maximum flexibility for 

the States 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 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, 

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204EPA Regional Administrator approval would 

be required prior to the discontinuation of SLAMS 

monitors, based on the criteria described in 

paragraph 58.14(c) to 40 CFR part 58. 

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 methods in the nearroad 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 SPMs to provide 

comparisons to the EPA approved 

methods supporting compliance to 

advance the understanding of 

instrument performance in the nearroad 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 notes that the 

implementation of additional 

measurements, while encouraged, is 

completely voluntary to ensure 

maximum flexibility for state 

monitoring programs. The EPA solicits 

comment on the best way to support 

such research efforts. 

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. These long-term 

monitors will supplement shorter-term 

networks operated by researchers to 

support the tracking of long-term trends 

of near-road PM2.5 mass concentrations 

and other pollutants in near-road 

environments. Therefore, the EPA 

proposes 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, this 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 have 

redundant monitors that could be 

relocated to the near-road environment, 

reducing costs for equipment and 

ongoing operation.204 While only a 

single 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 recognizes that the 

location of maximum concentration of 

PM2.5 from roadway sources might differ 

from the maximum location of NO2 or 

other pollutants, the EPA proposes 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 (e.g., 

average annual daily traffic and fleet 

mix by road segment) and significant 

thought and review has gone into its 

design, including pilot studies at two 

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. Therefore, while there 

may be limitations to collocating the 

proposed 52 near-road PM2.5 monitors 

with the NO2 stations that will also host 

CO monitors, on balance, EPA believes 

this is the most efficient and beneficial 

approach for deployment of this 

component of the network. ThU.S. EPA 

is seeking to maximize the utility of the 

network while also reducing the burden 

on monitoring agencies that have 

already put significant effort into 

designing their near-road stations for 

NO2 and CO. 

The EPA notes that the 52 proposed 

near-road monitors represent a small 

number of the total approximate 900 

operating PM2.5 monitoring stations 

across the country. The EPA could 

consider proposing 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 corresponding less travelled roads, 

is expected to be very small. The EPA 

could also consider proposing multiple 

sites in larger CBSAs; however, State 

monitoring programs are already 

working towards representative nearroad monitoring stations and there is a 

synergistic value in ensuring these 

measurements are collocated with 

multiple measurements to serve the 

monitoring objectives noted above. 

Since EPA has already finalized 

requirement of CO monitoring at nearroad stations in CBSAs 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). Also, EPA wants to ensure 

there is minimal disruption to the 

existing network and moving more than 

the proposed 52 PM2.5 monitors may 

lead to losing some valuable existing 

PM2.5 stations. Therefore, EPA believes 

the 52 proposed near road monitoring 

stations represent the least burdensome, 

but most useful number of near-road 

monitoring stations to meet the 

monitoring objectives cited above for 

deployment across the country. 

Ideally, near-road sites would be 

located at the elevation and distance 

from the road where maximum 

concentration of PM2.5 occurs in this 

environment, and within reasonable 

proximity to an area-wide PM2.5

compliance monitoring site at which a 

similar PM monitor is used (i.e., for 

comparison purposes). Although the 

EPA is not proposing that the near-road 

PM2.5 monitors be located within a 

specific distance of area-wide sites, 

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 area-wide site, 

such as an NCore station, would provide 

useful information in characterizing the 

near-road contribution to multiple 

pollutants, including PM2.5. 

The timeline to implement the 

proposed near-road PM2.5 monitors 

should be as minimally disruptive to 

on-going operations of monitoring 

agency programs as possible, while still 

meeting the need to collect for near-road 

PM2.5 data in a timely fashion. Since the 

near-road PM2.5 monitors are proposed 

to be collocated with the emerging nearroad NO2 network that is scheduled to 

be operational by January 1, 2013, the 

EPA believes it is appropriate to wait 

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205 Data from any PM2.5 monitor being used to 

meet minimum monitoring requirements could not 

be restricted from NAAQS comparability. 

until after the near-road NO2 network is 

established before implementing the 

near-road PM2.5 monitors. Therefore, the 

EPA proposes that each PM2.5 monitor 

planned for collocation with a near-road 

NO2 monitoring site be implemented no 

later than January 1, 2015. The EPA 

believes this proposed deadline 

provides an appropriate amount of time 

for monitoring agencies to select 

existing PM2.5 monitors suitable for 

relocation, receive EPA approval, and 

physically relocate the PM2.5 monitor to 

the near-road NO2 site. Based on this 

proposed timeline, complete data sets 

(i.e., 3-years representing 2015–2017), 

from PM2.5 monitors in the near-road 

environment would be available to 

calculate site-level design values in 

2018. 

In summary, the EPA proposes to 

specifically include a near-road 

component in the PM2.5 network design 

criteria for CBSA’s of 1 million persons 

or greater, with at least one PM2.5

monitor collocated with a near-road 

NO2 and CO monitors by January 1, 

2015. EPA believes that the 52 proposed 

PM2.5 monitors to be collocated with 

NO2 and CO monitors in the near-road 

environment represent the minimal 

number of sites needed to characterize 

PM2.5 in representative near road 

environments of large population 

CBSA’s. EPA believes that a number of 

PM2.5 monitors can be moved from 

single pollutant locations to multipollutant locations in the near-road 

environment, thus encouraging 

efficiencies in operation by monitoring 

agencies and reducing the burden of 

continuing to support some of the 

existing single pollutant PM2.5 stations. 

The EPA solicits comment on this 

approach, especially the proposed 

network design requirements; any 

alternative strategies that would provide 

comparable long-term characterization 

of PM2.5 in area-wide locations of 

maximum concentration in the absence 

of a specific near-road compliance 

requirement for monitoring of PM2.5; 

priorities for the collection of 

supplemental data at a small subset of 

near-road monitoring sites to enhance 

knowledge of particle exposure (e.g., 

non-compliance SPMs); and the interest 

of monitoring agencies (or other parties) 

in the collection of supplemental (e.g., 

non-compliance) measurements relevant 

for the near-road environment. 

ii. Use of PM2.5 Continuous FEMs at 

SLAMS 

The EPA proposes that each agency 

specify their 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. The proposal also provides 

that the EPA Regional Administrator 

would 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. The EPA has 

already approved six PM2.5 continuous 

FEMs and there are nearly 200 of these 

monitors already operating in State, 

local, and Tribal networks. Monitoring 

agencies have been deploying and fieldtesting 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 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. 

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 utilized, particularly for 

the objective of determining NAAQS 

compliance. Accordingly, we propose 

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; 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). The EPA believes 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 believe it is appropriate to 

provide monitoring agencies with a onetime opportunity to review already 

reported continuous PM2.5 FEM data 

and request that data with unacceptable 

performance be restricted 

(retrospectively) from NAAQS 

comparability. Accordingly, in the first 

year after this rule becomes effective, we 

propose 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.205 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 

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meeting the expected performance 

criteria, and the Regional Administrator 

has approved that the FEM data will 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 stored separately 

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 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 

method 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 method across all seasons, 

assess the data quality, and in some 

cases resolve operational issues with the 

instrument, it may still leave some 

agencies with methods 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 is 

near the level of the NAAQS. With the 

proposed provision where a monitoring 

agency can recommend not having data 

from PM2.5 continuous FEMs as 

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. 

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 versus how 

instrument manufacturers conduct 

required FRM and FEM testing 

protocols. The details below summarize 

these issues. The EPA proposes 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 as 

comparable to the NAAQS. To 

accommodate how routine monitoring 

networks operate, the EPA proposes that 

agencies seeking to demonstrate 

insufficient data comparability in an 

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 mg/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 were helpful for 

understanding whether an intercept or 

slope was driving a potential bias in an 

instrument. Therefore, the EPA 

proposes that agencies may include low 

concentration data (i.e., below 3 mg/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 propose that 

this requirement be applicable to the 

assessments being discussed here. For 

sites on a one in 6-day 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 proposes 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 as 

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 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 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. 

The EPA solicits comment on all 

aspects of this proposed approach of 

allowing monitoring agencies to 

recommend that PM2.5 continuous FEM 

data should not be compared to the 

NAAQS, when demonstrated to not 

meet the performance criteria used to 

approve FEMs based on data in their 

own network, and as appropriate, 

approved by the EPA Regional 

Administrators as ineligible for 

comparison to the NAAQS. 

c. Revoking PM10-2.5 Speciation 

Requirements at NCore Sites 

The EPA issued revisions to the 

Ambient Air Monitoring Regulations 

(40 CFR parts 53 and 58) on October 17, 

2006 (71 FR 61236). In the 2006 final 

rule, the EPA required that PM10-2.5

speciation be conducted at NCore multipollutant monitoring stations by January 

1, 2011. PM10-2.5 speciation at NCore 

was intended to support further 

research in the understanding of the 

chemical composition and sources of 

PM10, PM10-2.5 and PM2.5 at a variety of 

urban and non-urban NCore locations. 

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Subsequent to the completion of the 

2006 final monitoring rule, several 

technical issues were raised concerning 

the readiness of PM10-2.5 speciation 

monitoring methodologies to support 

such a nation-wide deployment strategy. 

Based on these issues and as explained 

in detail below, the EPA proposes 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. 

Monitoring was 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). At that 

time, 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 2006 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 plan and implement a small 

pilot monitoring project to evaluate the 

available monitoring and analytical 

technologies and supplement the 

PM10-2.5 speciation measurements that 

have mostly been done as part of other 

research efforts. The EPA expects that 

this field study will address the issues 

needed to develop a more robust, longterm PM10-2.5 speciation monitoring 

plan. 

The EPA pilot monitoring project will 

be completed in 2011, with plans to 

analyze the data and prepare a final 

report on findings and 

recommendations in 2012. At that time, 

the EPA will consider what PM10-2.5

speciation sampling techniques, 

analytical methodologies, and network 

design strategies would be most 

appropriate as part of a potential nationwide monitoring deployment. Such a 

deployment could be based on the 

NCore multi-pollutant framework, or 

some other strategy that targets such 

measurements in areas with higher 

levels of coarse particles. This latter 

type of strategy would be consistent 

with CASAC AAMMS members written 

comments that not all NCore sites 

would be adequate for PM10-2.5

speciation and that more flexibility in 

PM10-2.5 speciation network design 

would allow for a geographically 

diverse network to support health 

studies and research (Russell, 2009). 

The EPA may consider reintroducing 

some PM10-2.5 speciation monitoring 

requirements in a subsequent 

monitoring rulemaking or as part of a 

future review of the PM NAAQS. Until 

that time, the EPA believes it is 

appropriate to propose to revoke the 

current set of PM10-2.5 speciation 

monitoring requirements. The EPA 

solicits comment on this proposed 

revision to monitoring requirements. 

d. Measurements for the Proposed New 

PM2.5 Visibility Index NAAQS 

The EPA proposes requirements for 

sampling of PM2.5 chemical speciation 

in states with large CBSAs. The CSN has 

been operating for approximately 10 

years and as described earlier in this 

proposal already supports a number of 

important monitoring objectives. Since 

the CSN network is already well 

established in states with large CBSAs, 

the EPA believes that using the data 

from these existing sites as an input for 

calculating PM2.5 visibility index values 

will help ensure that the network can 

continue to support existing objectives, 

while also supporting the proposed new 

secondary standard. 

To ensure the CSN network can 

support its existing network objectives 

while also supporting the proposed new 

secondary PM2.5 visibility index 

standard (section VI.F), the EPA 

proposes that each state with a CBSA 

over 1 million have measurements 

based on the methods in CSN (or 

IMPROVE), as discussed in section 

VII.A.5 above, in at least one of its 

CBSAs. For states with urban or 

suburban NCore Stations, their existing 

CSN measurements at all NCore sites 

would be appropriate to meet this 

proposed requirement. For states with 

multiple high population CBSAs, the 

EPA proposes that each CBSA with a 

population over 2.5 million people be 

required to have CSN measurements. 

The EPA does not believe it would be 

appropriate to require multiple cities in 

the same state to have CSN 

measurements for purposes of 

supporting the proposed new secondary 

PM2.5 visibility index standard when 

these cities have relatively smaller 

populations (i.e., less than 2.5 million 

people) as the chemical species data 

may be similar across cities in the same 

state. The exception to this will be the 

most highly populated states and cities, 

which are either already covered by 

requirements for multiple NCore 

stations or the proposed population 

threshold of 2.5 million people. For 

example, the following high population 

states are already required to have 

multiple NCore stations: California, 

Florida, Illinois, Michigan, New York, 

North Carolina, Ohio, Pennsylvania, and 

Texas. The EPA also proposes that states 

be allowed to request alternative CBSAs 

to locate their CSN measurements, when 

the alternative location is better suited 

to support providing data for multiple 

monitoring objectives, including for the 

proposed new secondary PM2.5 visibility 

index standard. For example, in some 

cases a large CBSA with a marine 

influence may have relatively cleaner 

air than a smaller inland CBSA in the 

same state with a lower population. In 

these cases, states may request an 

alternative location for their CSN 

measurements. The EPA solicits 

comment of these proposed 

requirements and on alternative 

requirements for CSN measurements to 

support the proposed new secondary 

PM2.5 visibility index standard. 

The EPA proposes that the network 

design criteria for CSN measurements 

focus on area-wide locations that are 

generally representative of long 

distances throughout a CBSA. For most 

CBSAs, this will mean that the existing 

inventory of CSN measurements can be 

used where the location of the sampling 

equipment is at an NCore station or 

other station(s) sited at the 

neighborhood or urban scale of 

representation. The EPA points out that 

while the existing PM2.5 network design 

criteria established to support the 

primary PM2.5 NAAQS focuses on the 

area-wide locations of expected 

maximum concentration, there would 

not necessarily be the same focus for the 

proposed new secondary PM2.5 visibility 

index standard. One reason for this 

difference is that for urban visibility, we 

are interested in the impact of visibility 

degradation over as representative a 

location as possible as the impact of the 

aerosol is a function of an entire site 

path and not just one monitoring 

location within a CBSA. Also, the EPA 

is interested in leveraging as much of 

the existing inventory of CSN and 

IMPROVE measurements operating in 

CBSAs where they can support the 

proposed new secondary PM2.5 visibility 

index standard. 

The EPA considered the issue of 

siting measurements to support a new 

secondary standard to address PMrelated visibility impairment during a 

consultation with the CASAC AAMMS 

(75 FR 4069, January 26, 2010). In its 

letter to the EPA, the CASAC AAMMS 

stated that ‘‘the Subcommittee strongly 

favored collocation of extinction 

measurements with PM mass, PM 

speciation, and precursor gas 

measurements, identifying continuous 

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206See http://www.hazecam.net/. 

PM mass and speciation measurements 

as being of particular value. NCore 

multi-pollutant monitoring sites were 

identified as worth considering even 

though these would not necessarily 

capture maximum concentrations and 

visibility impairment in an urban area’’ 

(Russell and Samet, 2010a, p. 18). The 

EPA notes that the Subcommittee also 

identified that ‘‘[t]here was general 

support for making public 

communication an important 

consideration in network design, for 

example by selecting a monitoring site 

that can be associated with a vista that 

is recognized by a significant fraction of 

the local population’’ (Russell and 

Samet, 2010a, p. 18). While the EPA 

agrees that siting associated with a 

recognizable vista would be a useful 

consideration for establishing new sites, 

the EPA does not believe it would be 

appropriate to include such a 

requirement for cities with existing sites 

as this may disrupt the use of data to 

meet other important monitoring 

objectives. The EPA also notes existing 

long-standing public communication 

tools such as the ‘‘Haze-Cam’’ network 

are already well suited for public 

communications of important vistas.206

In addition to collocation with several 

important measurements at NCore as 

cited by the Subcommittee, the EPA is 

also encouraging monitoring agencies to 

add other important measurements such 

as commercially available technologies 

for light absorption and light scattering; 

however, the EPA does not believe these 

technologies should be specified by 

regulation. 

Since EPA’s proposal to require CSN 

(or IMPROVE) sampling is consistent 

with a network that is largely already in 

place, there is no expectation new sites 

will be needed. However, from time to 

time there is a disruption of sampling 

due to loss of a sites lease agreement or 

other circumstances. Therefore, for any 

state that does not have a minimally 

required CSN (or IMPROVE) set of 

measurements in place, the EPA 

proposes that these measurements be in 

place and sampling by January 1, 2015. 

4. Proposed Revisions to the Quality 

Assurance Requirements for SLAMS, 

SPMs, and PSD 

a. Quality Assurance Weight of 

Evidence 

The EPA believes that the process by 

which monitoring organizations and the 

EPA use the appendix A of 40 CFR part 

58 regarding quality assurance 

requirements in regulatory decision 

making needs to be articulated. Prior 

interpretations of appendix A have led 

to disqualification of data for 

noncompliance with a particular 

appendix A requirement. The proposed 

language described below, provides the 

interpretation the EPA would use 

moving forward. 

The appendix A to 40 CFR part 58 

requirements represent a portion of the 

quality control activities that are 

implemented by monitoring 

organizations to control data quality. 

The EPA believes that while 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 does not preclude the EPA from 

determining that data are of acceptable 

quality to be used for regulatory 

decision-making purposes. The EPA 

proposes to use a weight-of-evidence 

approach for determining whether the 

quality of data is appropriate for 

regulatory decision-making purposes. 

Furthermore, the suitability of data for 

any regulatory purpose also relies, in 

part, on several other quality-related 

requirements found elsewhere in 

40 CFR part 58. These requirements 

include air monitoring methodology 

(appendix C), network design criteria 

(appendix D) and network design plans 

for SLAMS, probe siting criteria 

(appendix E), the reporting of data to 

AQS, data completeness, and data 

certification by the reporting 

organization. This weight of evidence 

approach recognizes that all 

measurement systems have uncertainty 

and there are numerous factors that can 

affect data quality at a particular 

monitoring site. The specific appendix 

A criteria are designed to provide a 

quantification of this uncertainty, 

support a framework for assessing such 

uncertainty against known data quality 

goals and to support corrective actions 

when necessary to control uncertainty 

back to acceptable levels. Accordingly, 

the EPA proposes 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. 

b. Quality Assurance Requirements for 

the Chemical Speciation Network 

The EPA proposes to include 

requirements for flow rate verifications 

and flow rate audits for the PM2.5 CSN. 

These audits are currently being 

performed so, although they will 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 

proposes that PSD sites would not be 

required to collocate a second set of 

instruments for speciated PM2.5 mass 

monitoring. 

The EPA performed an assessment of 

measurement uncertainty from the 

collocated CSN and IMPROVE stations 

using the proposed visibility index 

(Papp, 2012) and concluded that the 

current data quality goals for the PM2.5

mass can be achieved for the proposed 

calculated light extinction indicator. 

c. Waivers for Maximum Allowable 

Separation of Collocated PM2.5 Samplers 

and Monitors 

The EPA proposes to allow waivers 

for the maximum allowable distance 

associated with collocated PM2.5

samplers and monitors. As described in 

section VIII.A.1 of this proposal, the 

EPA has already approved six Class III 

PM2.5 continuous FEMs. Several of these 

approved FEMs are required to be 

installed in a shelter with sufficient 

control of heating and air conditioning 

to ensure stable operation of the 

instrument. In many cases monitoring 

agencies are installing these approved 

continuous FEMs in shelters where they 

already have gas analyzers operating. 

Some agencies operate filter-based 

samplers (e.g., PM2.5 FRMs) on top of 

their shelter, while others operate 

platforms next to their shelter. In either 

case, 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, some installed in 

the shelter and some installed on a 

platform, are being sited at the same 

station. 

The EPA believes that maintaining the 

current requirement of 1 to 4 meters for 

PM2.5 samplers with flow rates of less 

than 200 liters/minute is useful since it 

ensures consistency with long-standing 

practices of collocation and ensures that 

any air drawn through collocated 

samplers is well within the operational 

precision of the instruments. However, 

the EPA also believes that instruments 

spaced farther apart could also be 

within the operational precision of the 

instruments, especially at sites located 

at larger scales of representation (e.g., 

neighborhood scale and larger). The 

EPA already defines a collocated scale 

in its document ‘‘Guidance for Network 

Design and Optimum Site Exposure for 

PM2.5 and PM10 (U.S. EPA, 1997). In this 

document, the EPA defines a collocated 

scale as 1 to 10 meters. The EPA 

believes that almost all agencies would 

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207The EPA provides a link to these assessments 

on EPA’s Web site 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. 

208All NCore stations must operate on at least a 

one-in-three day sample frequency for filter-based 

PM sampling. 

be able to site collocated PM samplers 

and monitors within 10 meters. 

Therefore, the EPA proposes 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 

solicits comment on this proposed 

change to allow waivers of the 

maximum allowable distance for 

collocated PM2.5 samplers and monitors. 

5. Proposed Probe and Monitoring Path 

Siting Criteria 

a. Near-Road Component to the PM2.5

Monitoring Network 

The EPA proposes that the probe and 

siting criteria for the near-road 

component to 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 solicits 

comment on this proposed probe and 

siting criteria for the proposed near-road 

component to the PM2.5 monitoring 

network design. 

b. CSN Network 

The EPA proposes to extend the 

existing probe and monitoring path 

siting criteria described in appendix E 

to 40 CFR part 58 for PM2.5 FRMs and 

FEMs to the CSN measurements. The 

EPA believes that monitoring agencies 

are already following the probe and 

siting criteria for PM2.5 when 

conducting CSN measurements; that is, 

at neighborhood, urban, and regional 

scale sites the probe height must be 2 to 

15 meters above ground level. All other 

aspects of the existing PM2.5 probe and 

siting criteria would also apply 

including minimum distances from 

horizontal supporting structures (i.e., 

greater than 2 meters) and minimum 

distance to the drip-line of a tree (i.e., 

greater than 10 meters). The IMPROVE 

program SOP (IMPROVE, 1996) on site 

selection already provides for meeting 

probe and siting criteria described in 

Appendix E. The EPA solicits comment 

on extending the existing probe and 

siting criteria for PM to the speciation 

measurements used to support the 

proposed new secondary PM2.5 visibility 

index standard. 

c. Reinsertion of Table E–1 to 

Appendix E 

The EPA is proposing 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 nitrogen 

(NO, NO2, NOX, NOY). This table was 

inadvertently removed during a 

previous CFR revision process. The EPA 

is utilizing this proposed rule to reinsert 

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 fiveyear assessments were submitted to EPA 

Regional Offices on or before July 1, 

2010. The assessments are intended to 

provide a comprehensive look at each 

monitoring agencies 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.207

Since each state has completed their 

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 thinks it is 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 solicits comment 

on ways to either streamline or add 

additional criteria for future 

assessments. Even if no changes to the 

requirements are recommended by any 

commenters, the EPA is especially 

interested in learning from monitoring 

agencies that may have ideas on how to 

improve future assessments. Such ideas 

may not necessarily have to be 

incorporated into regulation, but could 

be referred to in our guidance on 

network assessments (U.S. EPA, 2007b). 

The EPA proposes 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 proposes to remove 

these references since, as discussed in 

section VII.A.2 above, the EPA is 

proposing to remove all references to 

the spatial averaging option throughout 

40 CFR part 50 appendix N. Consistent 

with these changes, the EPA also 

proposes to remove references to 

community monitoring zones under the 

annual monitoring network plans 

described in 40 CFR 58.10. 

b. Operating Schedules 

The EPA generally requires PM2.5

SLAMS to operate on at least a 1-dayin-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.208 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 proposing to revise the 

level of the primary annual PM2.5

NAAQS. Accordingly, she is now 

considering whether this proposed 

change should result in any changes to 

sampling frequency requirements. 

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 proposed 

strengthening of the primary annual 

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209 Data certification requirements are described 

in 40 CFR 58.15. 

210This and all subsequent references to ‘‘state’’ 

are meant to include state, local and tribal agencies 

responsible for the implementation of a PM2.5

control program. 

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 should be 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 proposes to revise this 

requirement in the following ways: (1) 

The EPA proposes that monitors would 

only be required to operate on a daily 

schedule if their 24-hour design values 

are within five percent of the 24-hour 

PM2.5 NAAQS and the site has a design 

value that is not above the level of the 

annual PM2.5 NAAQS. (2) The EPA 

proposes 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 three most 

recent years of ambient data that were 

certified as of the May 1 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. The EPA solicits 

comment on these proposed changes to 

the required operating schedule for 

PM2.5 SLAMS. 

c. Data Reporting and Certification for 

CSN and IMPROVE Data 

The EPA solicits comment on minor 

changes to reporting and certification of 

data associated with CSN and IMPROVE 

data. The chemical analyses of filters 

associated with CSN measurements 

results in reporting of data that are 

usually within three months of the 

sample collection. This fits within the 

existing reporting requirements for most 

ambient air measurements that data be 

reported within 90 days past the end of 

the previous quarterly reporting period 

(40 CFR 58.15). However, some agencies 

also use IMPROVE or their own internal 

laboratory for processing of chemical 

analyses. IMPROVE is known to 

validate and report its data on a 

schedule that is approximately 12 to 18 

months after sample collection. At least 

one state laboratory continues to 

provide chemical analysis of filters 

associated with sites that are not NCore 

(Note: All NCore stations use either 

IMPROVE or the CSN National 

Laboratory contractor for their 

speciation laboratory analysis). 

Therefore, the EPA solicits comment on 

including the existing reporting 

requirements when reporting CSN 

measurements. In addition, the EPA also 

solicits comment on a longer reporting 

and certification 209 schedule 

specifically for CSN and IMPROVE that 

appropriately balances having sufficient 

time to analyze, validate, and report 

data with the need to have the data in 

sufficient time to use in assessments 

including calculating the proposed 

PM2.5 visibility index values discussed 

in section VII.A.5 above. Since 2010, the 

EPA has required states to certify their 

data by May 1 of each year. Since in 

some cases chemical speciation data 

may not be fully validated and 

submitted to EPA by May 1 of a given 

year, the EPA solicits comment on 

having data certification of these 

speciation measurements take place by 

May 1 of the following year. For 

example, if the fourth quarter chemical 

speciation data were not fully available 

to certify by May 1 of the following year, 

it would be certified another 12 months 

after that. The EPA solicits comment on 

the reporting and certification schedules 

for chemical speciation data. 

d. Requirements for Archiving Filters 

The EPA proposes to extend the 

requirement for archival of PM2.5, PM10, 

and PM10-2.5 filters from manual lowvolume 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 

collection. The archive of low-volume 

PM filters is an important tool 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 current requirement of one-year is 

not sufficiently long for retrospective 

analysis of important episodes and for 

use in long-term epidemiology research. 

Since first requiring filter archival of 

low-volume PM filters in 1997, the EPA 

has always recommended longer filters 

archives 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 proposes to minimize the costs of 

retaining filters by only requiring cold 

storage during the first year after sample 

collection. Therefore, the EPA solicits 

comment on this proposal to extend the 

filter archival requirement from one to 

five years, but only require cold storage 

during the first year. 

IX. Clean Air Act Implementation 

Requirements for the PM NAAQS 

The proposed revisions to the primary 

annual PM2.5 NAAQS and the proposed 

secondary PM2.5 visibility index 

NAAQS discussed in sections III.F and 

VI.F above, if finalized, would trigger a 

process under which states 210 will 

make recommendations to the 

Administrator regarding area 

designations, and the EPA will take 

final action on these designations. States 

will also be required to review, modify, 

and supplement their existing 

implementation plans. The proposed 

PM NAAQS revisions would also affect 

the applicable air permitting 

requirements and the transportation 

conformity and general conformity 

processes. This section provides 

background information for 

understanding the possible implications 

of the proposed NAAQS changes, and 

describes the EPA’s plans for providing 

states necessary guidance or rules in a 

timely manner to clarify how they are 

affected and to assist their 

implementation efforts. This section 

also describes existing EPA 

interpretations of CAA requirements 

and other EPA guidance relevant to 

implementation of new or revised 

NAAQS. Relevant CAA provisions that 

provide potential flexibility with regard 

to meeting implementation timelines are 

also discussed. 

This section also contains a 

discussion of several requirements of 

the stationary source construction 

permit programs under the CAA that 

may be affected by the proposed 

revisions of the PM NAAQS. These are 

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211While the CAA says ‘‘designating’’ with 

respect to the Governor’s letter, 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. 

212The EPA has used area-specific factor analyses 

to support boundary determinations by evaluating 

factors such as air quality data, emissions data, 

population density and degree of urbanization, 

traffic and commuting patterns, meteorology, and 

geography/topography. 

the PSD and Nonattainment New Source 

Review (NNSR) programs. To facilitate 

implementation of the PSD 

requirements, which would be the first 

of the implementation requirements to 

become applicable upon the effective 

date of the final NAAQS rule, the EPA 

proposes as part of this rulemaking to 

add a grandfathering provision to its 

regulations that would apply to certain 

PSD permit applications that are 

pending on the effective date of the 

revised PM NAAQS. If the proposed 

NAAQS revisions are finalized, this rule 

could be finalized at the same time as 

the revised NAAQS. This section also 

discusses other possible actions under 

consideration to facilitate 

implementation of the PSD and NNSR 

programs (see section IX.F). 

The EPA intends to propose 

additional appropriate regulations or 

issue guidance related to the 

implementation requirements for the 

revised PM NAAQS at a later date or 

dates. These may include additional 

revisions to both the PSD and NNSR 

regulations, as well as the promulgation 

of rules or development of guidance 

related to NAAQS implementation. 

These actions will be taken on a 

schedule that provides timely assistance 

to responsible states. Accordingly, in 

this section, the EPA solicits comment 

on several issues that the Agency 

anticipates will need to be addressed in 

future guidance or regulatory actions. 

Because these issues are not relevant to 

the establishment of the NAAQS, the 

EPA does not expect to respond, nor is 

the Agency required to respond, to these 

comments in the final action on this 

proposal, but the EPA expects these 

comments will be helpful as future 

guidance and regulations are developed. 

A. Designation of Areas 

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 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.211 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 boundaries as the 

EPA may deem necessary. States then 

have an opportunity to comment on the 

EPA’s tentative decision. 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 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 process (Page, 2011). 

Monitoring data are currently 

available from numerous existing PM2.5

mass and PM2.5 speciation sites to 

determine compliance with the 

proposed revised primary annual PM2.5

NAAQS and with the proposed PM2.5

visibility index NAAQS. As discussed 

in sections III and VI above, the EPA is 

proposing 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); 

(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 non-visibility-related 

welfare protection (section VI.F); and (3) 

establish a distinct secondary PM2.5

visibility index standard (section VI.F). 

The EPA’s examination of air quality 

monitoring data current at the time of 

this proposal indicates that, for the 

proposed levels for primary standards 

and the secondary PM2.5 visibility index 

standard, it is likely that the vast 

majority of monitors violating this 

secondary standard would overlap with 

monitors violating the primary 

standards. Since the same types of 

emissions sources contribute to 

concentrations affecting attainment 

status for both the proposed primary 

and secondary NAAQS, the EPA expects 

that the nonattainment area boundaries 

in locations with such overlap would be 

identical. The EPA will, consistent with 

previous area designations, use areaspecific factor analysis 212 to support 

area boundary decisions for both the 

primary and secondary standards. The 

EPA intends to more fully address 

issues affecting area designations in 

designations guidance that will be 

issued around the same time as any 

revised PM2.5 NAAQS are finalized. The 

EPA solicits comment related to 

establishing nonattainment area 

boundaries for the proposed revised 

primary annual PM2.5 NAAQS and the 

proposed secondary PM2.5 visibility 

index NAAQS, including any relevant 

technical information that should be 

considered by the EPA, and any input 

on the extent to which different 

considerations may be relevant to 

establishing boundaries for a secondary 

PM2.5 NAAQS. 

For the reasons stated above, upon 

promulgation of the revised NAAQS, 

the EPA currently intends to move 

forward on the same schedule with the 

initial area designations for both the 

revised primary annual PM2.5 standard 

and the secondary PM2.5 visibility index 

standard. The EPA notes that 

promulgating initial area designations 

for these standards on the same 

schedule will provide early regulatory 

certainty for states. The EPA intends to 

promulgate the revised PM NAAQS in 

December 2012 and complete initial 

designations for both the revised 

primary annual PM2.5 NAAQS and the 

secondary PM2.5 visibility index 

NAAQS by December 2014 using 

available air quality data from the 

current PM2.5 and speciation monitoring 

networks. These designations would 

follow the standard 2-year process 

described previously and would be 

based on 3 consecutive years of certified 

air quality monitoring data from the 

years 2010 to 2012, or 2011 to 2013. 

(Note, as discussed in sections IV.F and 

VI.F above, the EPA is proposing to 

retain the current primary 24-hour PM10

standard and to revise the form of the 

secondary annual PM2.5 standard to 

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213As discussed in section in VII.A.2 above, the 

EPA is proposing to remove the option for spatial 

averaging from the form of the secondary annual 

PM2.5 NAAQS consistent with the proposed change 

in the form of the primary annual PM2.5 standard. 

The EPA does not consider this change to trigger 

a new round of non-discretionary designations for 

this standard. 

remove the option for spatial averaging 

and to retain all other elements of the 

current suite of secondary PM standards 

to address non-visibility welfare effects. 

A new round of mandatory designations 

for these standards would occur only if 

these standards change.213) 

In today’s action, as discussed in 

section VIII.B.3.b.i above, the EPA is 

proposing to add requirements for 

establishing near-road PM2.5 monitors in 

certain cities. If these requirements are 

finalized, the EPA anticipates that it 

will take up to 3 years to establish new 

monitoring sites for PM2.5 mass, plus an 

additional 3 years of monitoring 

thereafter to determine compliance with 

the mass-based primary and secondary 

PM2.5 NAAQS based on these new 

monitors. This means that a complete 

set of air quality data for use in 

designations from any near-road 

monitoring sites would not be available 

until 2018. Also, as discussed in section 

VIII.B.3.d above, the EPA is proposing 

that each state with a CBSA over 1 

million in population would need to 

have a 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. This proposal may require 

the addition of new monitors, or the 

relocation of existing monitors, in some 

CBSAs. The EPA is also proposing in 

today’s action to extend the data 

certification period for speciation 

measurements by 12 months. Thus, 

even if EPA were to consider taking an 

additional year to complete the 

designations process (i.e., in December 

2015 instead of in December 2014), data 

from new PM2.5 near-road monitoring 

sites would not be available prior to the 

extended CAA designation deadline; 

and data from certain CSN (or 

IMPROVE) monitors also may not be 

available prior to the extended CAA 

designation deadline. For these reasons, 

the EPA does not currently intend to 

delay designations based on 

unavailability of data for either the 

revised primary or distinct secondary 

standards in order to be able to include 

data from these new monitors. Initial 

area designations would not take into 

account monitoring data from any 

newly established near-road monitoring 

sites, nor from newly established 

speciation monitoring sites. 

The EPA recognizes that the number 

of PM2.5 speciation monitoring sites 

available to support the state Governors’ 

designation recommendations and 

EPA’s decisions for the proposed 

secondary PM2.5 visibility index 

NAAQS will be much smaller than the 

number of PM2.5 FRM/FEM/ARM sites 

available to support designation 

recommendations and decisions for the 

revised annual primary PM2.5 NAAQS. 

Therefore, it may well be that more 

areas of the nation are designated 

unclassifiable (or unclassifiable/ 

attainment) for the proposed PM2.5

visibility index NAAQS than for the 

proposed revised primary annual PM2.5

NAAQS, if finalized. At this time the 

EPA does not believe that taking an 

additional year to complete designations 

for the secondary PM2.5 visibility index 

NAAQS would change this outlook. 

However, the EPA intends to remain 

flexible with regard to the designation 

schedule for the proposed revised PM2.5

NAAQS and will reassess the potential 

need for an extended schedule upon 

issuance of the final NAAQS rule and 

thereafter. 

In summary, the EPA intends to 

provide designation guidance to the 

states at the time of the promulgation of 

revised NAAQS or very shortly 

thereafter, to assist them in formulating 

these recommendations. In accordance 

with section 107(d)(4) of the CAA, the 

EPA currently believes that state 

Governors (and tribes, if they choose) 

should submit their initial designation 

recommendations for both the revised 

primary annual PM2.5 NAAQS and the 

distinct secondary PM2.5 visibility index 

NAAQS to the EPA no later than 1 year 

following promulgation of any revised 

NAAQS (e.g., in December 2013 

assuming promulgation of the revised 

PM NAAQS in December 2012). If the 

Administrator intends to modify any 

state area recommendation, the EPA 

would notify the appropriate state 

Governor no later than 120 days prior to 

making final designation decisions. A 

state that believes the Administrator’s 

modification is inappropriate would 

have an opportunity to demonstrate to 

EPA why it believes its original 

recommendation (or a revised 

recommendation) is more appropriate 

before designations are promulgated. 

The Administrator would take any 

additional input from the state into 

account in making final designation 

decisions. 

As previously stated, the EPA plans to 

issue guidance regarding designations 

for the revised PM2.5 NAAQS at or very 

shortly after the time of their final 

promulgation. The EPA invites 

preliminary comment on all aspects of 

the designation process at this time, 

which the Agency will consider in 

developing that guidance. 

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

Requirements 

The CAA directs states to address 

basic SIP requirements to implement, 

maintain, and enforce the standards. 

States are to develop and maintain an 

air quality management infrastructure 

that includes enforceable emission 

limitations, a permitting program, an 

ambient monitoring program, an 

enforcement program, air quality 

modeling capabilities, and adequate 

personnel, resources, and legal 

authority. Under CAA sections 110(a)(1) 

and 110(a)(2), states are to submit these 

SIPs within 3 years after promulgation 

of a new or revised primary standard. 

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. Section 110(b) of the CAA 

provides that the EPA may extend the 

deadline for the ‘‘infrastructure’’ SIP 

submission for a new secondary 

standard by up to 18 months beyond the 

initial 3 years. If both the revised 

primary annual PM2.5 NAAQS and the 

distinct secondary PM2.5 visibility index 

NAAQS are finalized, the EPA currently 

believes it would be more efficient for 

states and the EPA if each affected state 

submits a single section 110 

infrastructure SIP that addresses both 

standards at the same time (i.e., within 

3 years of promulgation of any revisions 

to the NAAQS for PM), because the EPA 

does not at present discern any need for 

there to be any substantive difference in 

the infrastructure SIPs for the two 

standards. However, the EPA also 

recognizes that states may prefer the 

flexibility to submit the secondary 

NAAQS infrastructure SIP at a later 

date. The EPA solicits comment on 

these infrastructure SIP submittal timing 

considerations. The EPA intends to 

provide guidance regarding the required 

date(s) for submission of infrastructure 

SIPs at the same time as or very shortly 

after promulgation of the revised 

NAAQS. 

Section 110(a)(2) of the CAA includes 

the following paragraphs describing 

specific requirements of infrastructure 

SIPs: (A) Emission limits and other 

control measures, (B) Ambient air 

quality monitoring/data system, (C) 

Programs for enforcement of control 

measures and for construction or 

modification of stationary sources, (D)(i) 

Interstate pollution transport and (D)(ii) 

Interstate and international pollution 

abatement, (E) Adequate resources and 

authority, conflict of interest, and 

oversight of local governments and 

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regional agencies, (F) Stationary source 

monitoring and reporting, (G) 

Emergency episodes, (H) SIP revisions, 

(I) Plan revisions for nonattainment 

areas, (J) Consultation with government 

officials, public notification, PSD and 

visibility protection, (K) Air quality 

modeling and submission of modeling 

data, (L) Permitting fees, and 

(M) Consultation and participation by 

affected local entities. 

The EPA interprets the CAA such that 

for two of the section 110(a)(2) 

elements, both of which pertain to 

nonattainment area requirements in part 

D, title I of the CAA, the required 

submittal date should not be governed 

by the 3-year submission deadline of 

section 110(a)(1). Therefore, for the 

reasons explained below, the following 

section 110(a)(2) elements are 

considered by EPA to be outside the 

scope of infrastructure SIP actions: 

(1) Section 110(a)(2)(C) to the extent it 

refers to permit programs (known as 

‘‘nonattainment new source review’’) 

under part D; and (2) section 110(a)(2)(I) 

(plan revisions for nonattainment areas) 

in its entirety. The EPA does not expect 

infrastructure SIP submittals to include 

regulations or emission limits 

developed specifically for attaining the 

relevant standard in areas designated 

nonattainment for the proposed revised 

PM2.5 NAAQS. Infrastructure SIPs for 

any final revised PM2.5 NAAQS will be 

due before PM2.5 SIPs are due to 

demonstrate attainment with the same 

NAAQS. (New emissions limitations 

and other control measures to attain a 

revised PM2.5 NAAQS will be due 3 

years from the effective date of 

nonattainment area designation as 

required under CAA section 172(c) and 

will be reviewed and acted upon 

through a separate process.) For this 

reason, the EPA does not expect 

infrastructure SIP submissions to 

identify new nonattainment area 

emissions controls. 

It is the responsibility of each state to 

review its air quality management 

program’s infrastructure SIP provisions 

in light of each revised NAAQS. Most 

states have revised and updated their 

infrastructure SIPs in recent years to 

address requirements associated with 

revised NAAQS. It may be the case that 

for a number of infrastructure elements, 

the state may believe it has adequate 

state regulations already adopted and 

approved into the SIP to address a 

particular requirement with respect to 

the revised PM NAAQS. For such 

portions of the state’s infrastructure SIP 

submittal, the state may provide a 

‘‘certification’’ specifying that certain 

existing provisions in the SIP are 

adequate. Although the term 

‘‘certification’’ does not appear in the 

CAA as a type of infrastructure SIP 

submittal, the EPA sometimes uses the 

term in the context of infrastructure 

SIPs, by policy and convention, to refer 

to a state’s minimal SIP submittal (e.g., 

in the form of a letter to the EPA from 

the state Governor or her/his designee). 

If a state determines that its existing 

SIP-approved provisions are adequate in 

light of the revised PM NAAQS with 

respect to a given infrastructure SIP 

element (or sub-element), then the state 

may make a ‘‘certification’’ that the 

existing SIP contains provisions that 

address those requirements of the 

specific section 110(a)(2) infrastructure 

elements. In the case of a certification, 

the submittal does not have to include 

a copy of the relevant provision (e.g., 

rule or statute) itself. Rather, the 

submittal may provide citations to the 

SIP-approved state statutes, regulations, 

or non-regulatory measures, as 

appropriate, which meet the relevant 

CAA requirement. Like any other SIP 

submittal, such certification can be 

made only after the state has provided 

reasonable notice and opportunity for 

public hearing. This ‘‘reasonable notice 

and opportunity for public hearing’’ 

requirement for infrastructure SIP 

submittals appears at section 110(a), and 

it comports with the more general SIP 

requirement at section 110(l) of the 

CAA. Under the EPA’s regulations at 40 

CFR part 51, if a public hearing is held, 

an infrastructure SIP submittal must 

include a certification by the state that 

the public hearing was held in 

accordance with the EPA’s procedural 

requirements for public hearings. See 40 

CFR part 51, appendix V, paragraph 

2.1(g), and 40 CFR 51.102. 

In consultation with its EPA Regional 

Office, a state should follow applicable 

EPA regulations governing 

infrastructure SIP submittals in 40 CFR 

part 51—e.g., subpart I (Review of New 

Sources and Modifications), subpart J 

(Ambient Air Quality Surveillance), 

subpart K (Source Surveillance), subpart 

L (Legal Authority), subpart M 

(Intergovernmental Consultation), 

subpart O (Miscellaneous Plan Content 

Requirements), subpart P (Protection of 

Visibility), and subpart Q (Reports). For 

the EPA’s general criteria for 

infrastructure SIP submittals, refer to 40 

CFR part 51, appendix V, Criteria for 

Determining the Completeness of Plan 

Submissions. A recent EPA guidance 

memorandum identifies a number of 

alternatives that are available to states to 

reduce the administrative burden, cost, 

and time required to complete the CAArequired steps that are part of 

submitting infrastructure and other SIP 

revisions to EPA (McCabe, 2011). The 

EPA also notes that many of the 

infrastructure SIP provisions are not 

NAAQS-specific, and therefore are 

likely to have been approved as part of 

SIP actions associated with other 

recently promulgated NAAQS (e.g., 

2006 PM2.5 and 2008 lead NAAQS). 

The EPA intends to issue a separate 

guidance document on section 110 

infrastructure SIP requirements for any 

revised PM NAAQS. The target date for 

issuing such guidance would be no later 

than 1 year after the revised PM NAAQS 

are finalized (2 years before state 

submittals are due). The EPA invites 

preliminary comment on all aspects of 

infrastructure SIPs at this time, which 

the Agency will consider in developing 

future guidance. 

C. Implementing the Proposed Revised 

Primary Annual PM2.5 NAAQS in 

Nonattainment Areas 

Part D of the CAA describes the 

various program requirements that 

apply to nonattainment areas for 

different NAAQS. Section 172 (found in 

subpart 1 of part D) includes the general 

SIP requirements that govern the PM2.5

program. Under section 172, states are 

required to submit SIPs within 3 years 

of the effective date of area designations 

by the EPA. These plans need to show 

how the nonattainment area will attain 

the primary PM2.5 standards ‘‘as 

expeditiously as practicable,’’ but 

presumptively no later than within 5 

years from the effective date of 

designations. However, in certain cases, 

the EPA can approve attainment dates 

up to 10 years from the effective date of 

designations, as appropriate, 

considering the severity of the air 

quality concentrations in the area, and 

the availability and feasibility of 

emission control measures per section 

172(a)(2)(C). 

Section 172(a)(1) of the CAA 

authorizes the EPA to establish 

classification categories for areas 

designated nonattainment for the 

primary or secondary PM NAAQS, but 

does not require the EPA to do so. The 

implementation program for the 1997 

and 2006 primary and secondary PM2.5

standards did not include a tiered 

classification system. This provided a 

relatively simple implementation 

structure and flexibility for states to 

implement control programs tailored to 

the specific nature of the problem and 

source mix in each area. For this same 

reason, the EPA also does not intend to 

establish classifications for 

nonattainment areas for the proposed 

revised primary annual PM2.5 standard 

(or for a revised primary 24-hour 

standard if one is promulgated). 

However, the EPA solicits comment on 

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whether a classification system would 

be appropriate and how a classification 

system could be designed. 

In April 2007, the EPA issued a 

detailed PM2.5 implementation rule 

(72 FR 20586; April 25, 2007) to provide 

guidance to states regarding 

development of SIPs to attain the 1997 

PM2.5 NAAQS. The EPA believes that 

the overall framework and policy 

approach of the implementation rule for 

the 1997 PM2.5 NAAQS provides 

effective and appropriate guidance on 

the general approach for states to follow 

in planning for attainment of the revised 

primary annual PM2.5 standard. The 

EPA intends to develop and propose a 

revised implementation rule that will 

address any new implementation 

requirements as a result of the proposed 

revised primary annual PM2.5 NAAQS 

and the proposed revised monitoring 

regulations. The EPA intends to propose 

this implementation rule within 1 year 

after the revised PM NAAQS are 

promulgated, and finalize the 

implementation rule by no later than the 

time the area designations process is 

finalized (approximately 1 year later). 

The EPA believes that for many issues, 

regulatory text similar to that of the 

existing implementation rule for the 

1997 PM2.5 NAAQS can be included in 

this new implementation rule. In the 

implementation rule for the 1997 PM2.5

NAAQS, there are a few specific 

references to the 1997 annual PM2.5

NAAQS or associated implementation 

dates; in a proposed implementation 

rule for any revised PM2.5 NAAQS, such 

references would be updated as 

appropriate. In addition, the EPA 

expects to consider options for 

potentially updating certain policies in 

the existing implementation rule based 

on new information or implementation 

experience. The EPA solicits 

preliminary comment on the 

implementation issues that the Agency 

should consider for updating. 

Under the approach outlined in the 

implementation rule for the 1997 PM2.5

NAAQS, the state begins the 

development of an attainment 

demonstration with the evaluation of 

the air quality improvements the 

nonattainment area can expect in the 

future due to ‘‘on the books’’ existing 

federal, state, and local emission 

reduction measures. The state then must 

conduct a further assessment of 

emission sources in the nonattainment 

area, and the additional reasonably 

available control measures (RACM) and 

reasonably available control technology 

(RACT) that can be implemented by 

these sources, in determining how soon 

the area can attain the standard. (Under 

the current implementation rule, the 

sources for consideration would be 

those emitting SO2, direct PM2.5, and 

presumptively NOX. Sources of the 

other PM2.5 precursors, VOC and 

ammonia, presumptively do not need to 

be evaluated for control measures unless 

demonstrated by the state or the EPA as 

significant contributors to PM2.5

concentrations in the relevant 

nonattainment area.) Under section 172 

of the CAA as interpreted by the EPA, 

attainment demonstrations must include 

a RACM analysis showing that no 

additional reasonably available 

measures could be adopted and 

implemented such that the SIP could 

specify an attainment date that is 1 or 

more years earlier. 

The evaluation of these potential 

emission reductions and associated air 

quality improvement is commonly 

performed with sophisticated air quality 

modeling tools. Given that fine particle 

concentrations are affected both by 

regionally-transported pollutants (e.g., 

SO2 and NOX emissions from power 

plants) and emissions of direct PM2.5

from local sources in the nonattainment 

area (e.g., steel mills, rail yards, and 

highway mobile sources), the EPA 

recommends the use of regional gridbased models (such as CMAQ and 

CAMx) in combination with sourceoriented dispersion models (such as 

AERMOD) to develop PM2.5 attainment 

strategies for the revised annual primary 

NAAQS. Although the EPA projects 

significant improvements in PM2.5

concentrations regionally from a 

number of recently promulgated rules 

such as the Cross State Air Pollution 

Rule (76 FR 48208, August 8, 2011) and 

the Mercury and Air Toxics Standards 

rule (77 FR 9304, February 16, 2012) 

that will result in SO2 and NOX

reductions from many geographically 

dispersed sources, local reductions of 

direct PM2.5 emissions also result in 

important health benefits. On a per ton 

basis, reductions of direct PM2.5

emissions are more effective in reducing 

PM2.5 concentrations than reductions of 

precursor emissions. Therefore, 

reductions of direct PM2.5 emissions 

should play a key role in attainment 

planning as well. 

Each nonattainment area needs to 

ensure that it will make ‘‘reasonable 

further progress’’ (RFP) in accordance 

with section 172(c)(2) of the CAA from 

the time of SIP submittal to its 

attainment date. Under the approach 

outlined in the implementation rule for 

the 1997 PM2.5 NAAQS, for an area that 

can demonstrate it will attain the 

standard within the presumptive 5-year 

period from designation, its attainment 

demonstration will be considered to 

meet the RFP requirement. The EPA 

believes it is appropriate to apply this 

same approach for the revised annual 

primary PM2.5 standard. The EPA 

believes there should be no additional 

RFP requirements for such an area 

because the SIP and attainment 

demonstration would be due 3 years 

after designations and its attainment 

date will be only 2 years after that date. 

An area that cannot demonstrate 

attainment within the presumptive 5- 

year period would be required to 

provide a separate RFP plan showing 

that the area will achieve emission 

reductions by certain interim milestone 

dates which provide for ‘‘generally 

linear’’ progress over the course of the 

implementation period. All PM2.5

attainment plans must also include 

contingency measures which would 

apply without significant delay in the 

event the area fails to attain by its 

attainment date. 

The EPA expects that the same 

general approach for determining 

attainment of the 1997 PM2.5 primary 

standard by the attainment deadline 

would be followed for determining 

attainment with any primary PM2.5

standard. Attainment would be 

evaluated based on the 3 most recent 

years of certified, complete, and qualityassured air quality data in the 

nonattainment area. The EPA also 

would expect to include similar 

flexibility provisions for an area to be 

able to obtain two 1-year attainment 

date extensions under certain 

circumstances. In the 1997 PM2.5

NAAQS implementation rule, an area 

whose design value based on the most 

recent 3 years of data exceeds the 

standard could receive a 1-year 

attainment date extension if the air 

quality concentration for the third year 

alone does not exceed the level of the 

standard. Similarly, an area that has 

received a 1-year extension could 

receive a second 1-year extension if the 

average of the area’s air quality 

concentration in the ‘‘extension year’’ 

and the previous year does not exceed 

the level of the standard. 

The EPA notes that in other sections 

of today’s proposal, the EPA describes 

new requirements for deploying nearroad monitors and clarifies certain 

existing monitoring provisions. As 

discussed in the designations section, 

the EPA would not expect that data 

from any new near-road PM2.5 monitors 

would be available in time to consider 

during the initial area designations 

process, and therefore such monitoring 

data would not be the basis for 

designating a new nonattainment area at 

the time of initial designations. The EPA 

plans to address any potential 

implications of the proposed monitoring 

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214For example, it may be possible that a new 

near-road monitoring site has collected 3 years of 

data and shown a violation before final EPA action 

has been taken on an attainment plan or 

maintenance plan for the 1997 or 2006 NAAQS. 

215This analysis was based on 2008 to 2010 air 

quality data and for illustrative purposes used an 

alternative standard level of 12 mg/m3 for the 

primary annual PM2.5 standard and the proposed 

level of 35 mg/m3 level for the primary 24-hour 

PM2.5 standard together with the proposed levels of 

30 and 28 dv in conjunction with a 24-hour 

averaging time and a 90th percentile form for the 

secondary PM2.5 visibility index standard. The 

relationships between design values as 

characterized here are dependent upon the specific 

level and form of each of the standards. 

changes on attainment planning and 

development of attainment 

demonstrations by states in the future 

implementation rule. The EPA requests 

comment on any specific attainment 

planning considerations for future SIPs 

that may be associated with today’s 

proposed changes to monitoring 

provisions. 

With regard to implementation of the 

pre-existing standards for PM2.5, the 

EPA’s current opinion is that the 

changes in the monitoring regulations, if 

finalized, should not result in any new 

requirements with respect to attainment 

plans or maintenance plans for the 1997 

or the 2006 PM2.5 NAAQS during some 

specified transition period.214 For 

example, if the proposed PM NAAQS 

revisions and revised monitoring 

regulations are finalized in December 

2012, many states will have recently 

submitted, or will be close to submitting 

their implementation plans to attain the 

2006 24-hour PM2.5 NAAQS (also due in 

December 2012). In addition, state and 

EPA actions are still under way with 

regard to adopting and approving 

certain attainment plans and 

maintenance plans for nonattainment 

areas under the 1997 PM2.5 standards. 

The EPA does not believe it would be 

reasonable for requirements applicable 

to such attainment plans and 

maintenance plans to change beginning 

immediately upon any revision of the 

monitoring regulations. It could be very 

burdensome on state air quality 

programs to revise SIPs that have 

already been submitted to EPA or that 

have been under development for some 

time and are about to be submitted. The 

EPA believes that a more reasonable 

approach would be to provide for a 

transition period before the revised 

monitoring network and data 

comparability provisions would affect 

implementation plan and maintenance 

plan requirements. The EPA believes it 

would be important for the transition 

period to provide enough time for the 

EPA to complete action on attainment 

and maintenance SIPs for the 1997 or 

2006 PM2.5 NAAQS that were initiated 

and completed (or that are close to 

completion) by states before finalization 

of the proposed changes to the 

monitoring regulations. The EPA 

believes that if a SIP for the 1997 or 

2006 PM2.5 NAAQS has been approved 

during the transition period, the state 

would not be under an obligation to 

revise it unless the EPA has made a SIP 

call. The EPA invites preliminary 

comment on this transition period 

concept, and on an appropriate date by 

which the transition period should be 

concluded. 

D. Implementing the Primary and 

Secondary PM10 NAAQS 

As summarized in sections IV.F and 

VI.F above, the EPA is proposing to 

retain the current primary and 

secondary 24-hour PM10 standards to 

protect against the health effects 

associated with short-term exposures to 

thoracic coarse particles and against 

welfare effects. If this approach is 

finalized, the EPA would retain the 

existing implementation strategy for 

meeting the CAA requirements for PM10. 

States and emission sources would 

continue to follow the existing guidance 

and regulations for implementing the 

current standards. 

E. Implementing the Proposed 

Secondary PM2.5 Visibility Index 

NAAQS in Nonattainment Areas 

In past actions, the EPA has set the 

secondary PM standards identical to the 

primary PM standards. In this action, as 

summarized in section VI.F above, the 

EPA is proposing a distinct secondary 

PM2.5 visibility index NAAQS. In 

addition, as also summarized in section 

VI.F above, the EPA is proposing to 

retain the current annual and 24-hour 

secondary PM2.5 standards to provide 

protection against non-visibility welfare 

effects. Although the proposed 

secondary PM2.5 visibility index 

NAAQS would differ from the primary 

PM2.5 NAAQS (and existing secondary 

PM2.5 NAAQS) with respect to 

indicator/index, statistical form, and 

level, attainment of this standard would, 

like the PM2.5 mass-based standards, 

depend on ambient measurements (i.e., 

specifically speciated PM2.5 mass 

concentrations). The EPA expects that 

implementation of emission reduction 

measures that will help to achieve the 

mass-based 1997 and 2006 primary and 

secondary PM2.5 standards and the 

proposed revised primary annual PM2.5

standard will also provide important 

improvements in visibility and 

substantial progress toward meeting the 

proposed secondary PM2.5 visibility 

index standard because these emission 

reduction measures will address the 

same sources and pollutants which also 

contribute to PM-related visibility 

impairment. In fact, as discussed below 

in section IX.F.1, an analysis of the 

relationships between recent design 

values for the proposed primary (annual 

and 24-hour) PM2.5 standards and 

coincident design values for the 

proposed PM2.5 visibility index standard 

indicates that all or nearly all areas in 

attainment of the proposed primary 

PM2.5 standards would also likely be in 

attainment of the proposed secondary 

PM2.5 visibility index standard (Kelly, et 

al. 2012).215

Section 172(a)(1) of the CAA 

authorizes the EPA to establish 

classification categories for areas 

designated nonattainment for the 

primary or secondary PM NAAQS, but 

does not require the EPA to do so. The 

implementation program for the 1997 

and 2006 primary and secondary PM2.5

standards did not include a tiered 

classification system. This provided a 

relatively simple implementation 

structure and flexibility for states to 

implement control programs tailored to 

the specific nature of the problem and 

source mix in each area. For this same 

reason, the EPA also does not intend to 

establish classifications for 

nonattainment areas for the proposed 

secondary PM2.5 visibility index 

standard. 

Section 172(a)(2) of the CAA provides 

the same statutory framework for 

implementing secondary standards in 

nonattainment areas as it does for 

primary standards, except that it 

provides different attainment date 

requirements for secondary standards. 

The attainment date for the proposed 

revised primary annual PM2.5 standard 

is as expeditiously as practicable, but 

presumptively within 5 years of the date 

of designation, with the possibility of an 

attainment date of up to 10 years for 

certain areas with more severe air 

quality problems. For secondary 

NAAQS, however, section 172(a)(2)(B) 

defines the attainment date for an area 

designated nonattainment as ‘‘the date 

by which attainment can be achieved as 

expeditiously as practicable’’ but with 

no maximum limitation. Thus, it is 

possible for the EPA to approve an 

implementation plan that provides for 

attainment of the secondary standards 

by a date more than 10 years after the 

date of designation with an appropriate 

demonstration. 

As noted in the above section on 

implementing the primary PM2.5

standard, the EPA expects that the same 

general approach for providing two 

possible 1-year extensions to the 

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attainment date would also apply to any 

revised secondary PM2.5 standard. 

Attainment would be evaluated based 

on the 3 most recent years of certified, 

complete, and quality-assured air 

quality data in the nonattainment area. 

The EPA also would expect to include 

similar flexibility provisions for an area 

to be able to obtain two 1-year 

attainment date extensions under 

certain circumstances. An area whose 

design value based on the most recent 

3 years of data exceeds the standard 

could receive a 1-year attainment date 

extension if the deciview index for the 

third year alone does not exceed the 

level of the standard. Similarly, an area 

that has received a 1-year extension 

could receive a second 1-year extension 

if the average of the area’s deciview 

index in the ‘‘extension year’’ and the 

previous year does not exceed the level 

of the standard. 

As noted previously, the EPA expects 

that implementation of control measures 

to achieve the 1997 and 2006 primary 

annual and 24-hour PM2.5 standards and 

the proposed revised primary annual 

PM2.5 standard will address the same 

sources and pollutants that contribute to 

PM-related visibility impairment, and, 

thus, great progress can be achieved 

toward attaining the proposed 

secondary PM2.5 visibility index 

standard as a result of clean air 

programs designed principally to 

improve public health by attaining the 

primary PM2.5 standards. However, 

because the proposed secondary PM2.5

standard is based on a visibility index 

rather than a mass concentration, 

implementation can be expected to 

present new challenges when 

developing part D SIPs. For example, 

while the proposed revision to the level 

and form of the primary annual PM2.5

standard does not pose any new issues 

with respect to air quality modeling 

methods, the speciated nature of the 

index for the proposed secondary PM2.5

visibility index standard does pose new 

modeling issues. For this reason, the 

EPA invites commenters to present 

information concerning air quality 

modeling and other issues that are 

expected to be unique to implementing 

the proposed secondary PM2.5 visibility 

index standard in nonattainment areas 

and that should be considered by EPA 

in the development of the future 

implementation rule and related 

guidance. The EPA particularly seeks 

input on how implementation planning 

for the proposed secondary PM2.5

visibility index standard can be 

integrated as much as possible with 

implementation planning for the 

proposed revised primary annual PM2.5

standard to increase the efficiency of the 

process and reduce administrative 

burden on state agencies and 

stakeholders. The EPA will consider 

these comments in developing a 

proposed implementation rule and 

related guidance for the revised 

standards. 

F. Prevention of Significant 

Deterioration and Nonattainment New 

Source Review Programs for the 

Proposed Revised Primary Annual PM2.5

NAAQS and the Proposed Secondary 

PM2.5 Visibility Index 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 

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 NNSR program, 

respectively. Collectively, those permit 

requirements are commonly referred to 

as the ‘‘major NSR program.’’ 

The proposed revised primary annual 

PM2.5 NAAQS and proposed secondary 

PM2.5 visibility index NAAQS, if 

finalized, would affect certain PSD 

permitting actions as of the effective 

date for those NAAQS and would affect 

certain NNSR permitting actions on and 

after the effective date of an area 

designation as ‘‘nonattainment’’ for 

PM2.5. In order to minimize the potential 

for disruption to NSR permitting, the 

EPA is proposing, in section IX.F.1.a of 

this preamble, a grandfathering 

provision for certain PSD permits that 

are already in process, and is also 

proposing, in section IX.F.1.c, a 

surrogacy approach for implementing 

PSD permitting requirements for the 

proposed secondary PM2.5 visibility 

index NAAQS. These provisions will 

assure that NSR permitting will be able 

to continue using provisions and 

processes virtually identical to those 

already in place for the existing PM2.5

NAAQS, except that, in evaluating 

whether a source causes or contributes 

to a NAAQS violation, an applicant 

would need to compare the source’s 

impacts to a different level and form of 

the primary annual standard, if finalized 

as proposed. As discussed in more 

detail in the following sections, the EPA 

is not now proposing to change the 

PM2.5 increments, nor are we proposing 

to revise screening tools that are now 

used to implement PSD for PM2.5, such 

as the significant emission rate, 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, 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; or the 

significant monitoring concentration, 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. 

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 

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 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 construction 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). 

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216Under various provisions of the CAA, PSD 

requirements are applicable to each pollutant 

subject to regulation under the CAA, excluding 

hazardous air pollutants. The 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. 

217The EPA is also proposing to revise 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. 

PSD permitting requirements first 

became applicable to PM2.5 in 1997 

when EPA established a NAAQS for 

PM2.5 (Seitz, 1997). The EPA’s 

regulations define the term ‘‘regulated 

NSR pollutant’’ to include ‘‘[a]ny 

pollutant for which a national ambient 

air quality standard has been 

promulgated and any pollutant 

identified [in EPA regulations] as a 

constituent or precursor to such 

pollutant’’ (40 CFR 51.166(b)(49); 40 

CFR 52.21(b)(50)).216 In addition, on 

May 16, 2008, the EPA amended its 

rules 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). States 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 rules at 40 CFR 

51.166 and 52.21 to add PSD increments 

as well as two screening tools for 

PM2.5—significant impact levels (SILs) 

and a significant monitoring 

concentration (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, 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 are 

required to submit the necessary SIP 

revisions (at least as stringent as the 

PSD requirements at 40 CFR 51.166) to 

EPA for approval within 21 months 

from the date on which the EPA 

promulgated the new PM2.5

increments—by July 20, 2012. This 

particular schedule is prescribed by the 

CAA specifically for the adoption of 

new PSD increments in state PSD 

programs. Sources for which PSD 

permits are issued pursuant to the 

federal PSD program at 40 CFR 52.21 

after October 20, 2011, must 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 in 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 

themselves, for which proposed and 

other possible implementation 

approaches are described further below, 

the EPA is proposing certain 

clarifications to the existing monitoring 

regulations codified at 40 CFR 58.30 

(Special considerations for data 

comparisons to the NAAQS). These 

proposed 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 used the principles for making 

these determinations for monitoring 

sites to also 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. The EPA 

will continue to use these principles in 

guiding PSD modeling analysis design. 

Accordingly, if the proposed PM 

NAAQS revisions and monitoring 

regulation clarifications described 

previously are finalized, the EPA will 

advise permitting agencies to qualify or 

disqualify specific receptor locations 

used in PSD air quality analyses 

consistent with those final provisions, 

and we will do so ourselves when we 

are the permitting authority. 

With regard to the specific revisions 

being proposed to the PM NAAQS, 

today’s action, if finalized as proposed, 

would affect sources applying for PSD 

permits in several ways. We first discuss 

the implications for PSD with respect to 

the proposed revised primary annual 

PM2.5 standard (some of which also 

apply to the proposed secondary PM2.5

visibility index standard), and then the 

unique implications for PSD with 

respect to the proposed secondary PM2.5

visibility index standard. 

a. Grandfathering Provision 

As discussed previously in this 

preamble, the EPA is proposing to revise 

the level of the primary annual PM2.5

NAAQS and establish a secondary PM2.5

visibility index NAAQS.217

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 

must include a demonstration that new 

sources and 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, if the proposed 

revision to the primary annual PM2.5

NAAQS and the proposed secondary 

PM2.5 visibility index NAAQS are 

promulgated, any proposed new and 

modified sources with permits pending 

at the time those PM2.5 NAAQS changes 

take effect would be expected to 

demonstrate compliance with them, 

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 

proposed revision of the primary annual 

NAAQS, the proposed addition of a 

distinct secondary NAAQS for visibility 

protection, and the changes to the 

monitoring requirements discussed 

earlier, the EPA proposes 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 PM 

NAAQS. The EPA proposes that the 

grandfathering provision would apply 

specifically to pending PSD permit 

applications for which the proposed 

permit (draft permit or preliminary 

determination) has been noticed for 

public comment before the effective 

date of the revised NAAQS. 

The proposed grandfathering 

provision would not be the first such 

grandfathering provision adopted by the 

EPA. The Agency 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). As discussed in 

more detail in these referenced actions, 

section 165(a)(3) of the CAA requires 

that a permit applicant demonstrate that 

its proposed project will not cause or 

contribute to a violation of any NAAQS. 

At the same time, section 165(c) of the 

CAA requires that a PSD permit be 

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218 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 the CAA, the EPA has 

taken the position that it may grandfather through 

adjudication respecting a specific source, 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 EPA generally believes the preferred 

approach is to enable grandfathering through 

express regulatory exemptions of the type proposed 

in this action (U.S. EPA, 2011c, p. 68). 

219There may be proposed permits 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 grandfathering 

provision proposed today should relieve such a 

permit from the requirement to demonstrate 

compliance with those new PM2.5 increments, for 

which the EPA did not adopt any grandfathering 

provisions but deferred implementation in 

accordance with the requirements of the CAA. 

granted or denied within 1 year after the 

permitting authority determines the 

application for such permit to be 

complete. In addition, section 301 of the 

CAA authorizes the Administrator ‘‘to 

prescribe such regulations as are 

necessary to carry out his functions 

under this chapter.’’ When read in 

combination, these three provisions 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 to 

protect the NAAQS on the one hand, 

and to avoid delays in processing PSD 

permit applications on the other. The 

EPA 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 

59588, Oct. 12, 2005). The reasoning of 

those prior EPA actions is also 

applicable to the promulgation of 

revised PM NAAQS.218

The CAA provides the EPA with 

discretion to establish the appropriate 

milestone within the permitting process 

for determining that a permit 

application is eligible for grandfathering 

(U.S. EPA, 2011c, p. 81). For example, 

in 1987, the EPA used the date of 

submittal of a complete permit 

application as the milestone upon 

which to base the grandfathering of a 

source from new permitting 

requirements associated with the 

revisions made to the PM NAAQS at 

that time (52 FR 24672, July 1, 1987 at 

24703). In the context of the 

implementation of the revisions to the 

PM NAAQS that are being proposed 

today, the EPA is proposing to use a 

different milestone to establish the date 

before which permits may be 

grandfathered. Accordingly, to avoid 

unreasonable delays in permit 

processing and issuance, and based on 

basic principles of fairness and equity, 

we believe that it is appropriate to allow 

pending permit applications that have 

reached the notice and comment period 

on a proposed permit (that is, a notice 

has been issued for public comment on 

the proposed permit action) by the 

effective date of the revised PM NAAQS 

to continue being processed in 

accordance with the PM NAAQS 

requirements in place as the time of the 

public notice on the proposed permit.219

Before a proposed permit is issued for 

public comment, the applicant still has 

a reasonable opportunity to amend its 

permit application to address new or 

revised NAAQS that become effective 

while the reviewing authority’s 

preliminary consideration of the 

application is underway. Furthermore, 

the reviewing authority 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 apply new 

permitting requirements based on 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 additional work by the 

reviewing authority at a time when it 

should be focused on considering public 

comments and preparing a final permit 

decision in order to conclude its review 

of a permit application in a timely 

manner. Through this proposal, the EPA 

is providing notice to current and future 

permit applicants that they may have to 

provide an analysis showing that their 

facility will not cause or contribute to a 

violation of the revised NAAQS for PM 

if a proposed permit is not issued for 

public comment before such NAAQS 

become effective. 

Accordingly, the EPA proposes to 

amend the federal PSD regulations at 40 

CFR 52.21 to provide a grandfathering 

provision to allow for the continued 

review of permits proposed before a 

revision to the 2006 p.m. NAAQS under 

the PM NAAQS that applied at the time 

of the public notice on the proposed 

permit. The EPA also proposes that 

states that issue PSD permits under a 

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 

proposes to revise section 40 CFR 

51.166 to provide a comparable 

exemption applicable to SIP-approved 

PSD programs. 

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 

action would no longer be grandfathered 

from PSD permitting requirements 

associated with a revised PM NAAQS. 

Consistent with past grandfathering 

actions described above, the EPA is not 

proposing to include a sunset clause for 

the proposed grandfathering provision. 

Permit applicants and reviewing 

authorities already have strong 

incentives to process applications and 

issue draft permits in a timely manner, 

and the EPA does not believe that the 

addition of a sunset clause to the 

proposed grandfathering provision 

would add meaningful additional 

incentive for sources or permitting 

authorities to expedite permitting 

processes. Furthermore, the EPA 

believes 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 draft 

permit issuance and public notice to 

lapse beyond the sunset date. In such 

cases, the already delayed permit action 

would be further delayed to address 

PSD permitting requirements associated 

with the revised PM 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 and likely 

introduce additional complexities in 

relation to specific permit actions. 

However, the EPA solicits comment on 

whether a sunset clause would be 

appropriate under certain 

circumstances, and if so, what time 

limits would be placed on the 

grandfathering period associated with 

the revised PM NAAQS. 

b. Recent Guidance Applicable to the 

Proposed Revised Primary Annual PM2.5

NAAQS 

Today’s proposal to revise the level of 

the primary annual PM2.5 NAAQS from 

15.0 mg/m3 to a level within the range 

of 12.0 and 13 mg/m3 and to establish a 

distinct secondary PM2.5 visibility index 

NAAQS generally will require proposed 

new major stationary sources and 

modifications to take these changes into 

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220The presentation on this draft guidance was 

posted on the EPA Web site at: http://www.epa.gov/ 

ttn/scram/10thmodconf.htm. 

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. If the PM NAAQS are 

revised as proposed, and when effective, 

proposed sources that are not 

grandfathered from the new 

requirements (as described in section 

IX.F.1.a) would be required to 

demonstrate compliance with the suite 

of PM NAAQS, including the revised 

primary annual PM2.5 NAAQS and the 

proposed secondary PM2.5 visibility 

index 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 their impact on the 

revised primary annual PM2.5 NAAQS, 

if finalized. 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, 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. 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 for the existing PM2.5

NAAQS, 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. 

(As discussed in the next section, these 

recommended approaches may be 

extended to the proposed secondary 

NAAQS as well under a surrogacy 

approach). To this end, the EPA 

discussed this draft guidance in March 

2012 at the EPA’s 10th Modeling 

Conference.220 Based on its review of 

public comments received and further 

technical analyses, the EPA intends to 

issue final guidance by the end of 

calendar year 2012. 

c. Surrogacy Approach for the Proposed 

Secondary PM2.5 Visibility Index 

NAAQS 

As summarized in section VI.F of this 

preamble, the EPA is proposing a 

distinct secondary NAAQS for PM2.5

that will provide protection against 

visibility impairment, measured in 

terms of a visibility index using a 

calculated PM2.5 light extinction 

indicator (see section VI.D.1 above). The 

PM2.5 visibility index values are 

determined using a six-step procedure 

involving 24-hour speciated PM2.5

concentration data together with 

climatological relative humidity factors. 

The EPA plans to calculate design 

values for the proposed secondary PM2.5

visibility index NAAQS using the 

procedures described in section VII.A.5 

above, relying upon ambient PM2.5

speciation measurement data available 

through the CSN or IMPROVE methods 

and spatial interpolation of historical 

relative humidity data. 

As explained above, the PSD program 

requires individual new or modified 

stationary sources to carry out an air 

quality analysis to demonstrate that 

their proposed emissions increases will 

not cause or contribute to a violation of 

any NAAQS. Such a demonstration for 

the proposed secondary PM2.5 visibility 

index NAAQS could require each PSD 

applicant to predict, via air quality 

modeling, the visibility impairment that 

will result from its proposed emissions 

in conjunction with an assessment of 

existing air quality (visibility 

impairment) conditions. Under 40 CFR 

51.166(l)(1) and 40 CFR 52.21(l)(1), all 

applications of air quality modeling for 

purposes of determining whether a new 

or modified source will cause or 

contribute to a NAAQS violation, 

including a violation of the proposed 

secondary visibility index NAAQS for 

PM2.5, must be based upon air quality 

models specified in appendix W to 40 

CFR part 51. Currently there are no air 

quality models identified in Appendix 

W that are recommended for regulatory 

applications (Appendix W to 40 CFR 

part 51, Section 3.1.1(b)) for addressing 

the atmospheric chemistry associated 

with secondary formation of PM2.5. 

Thus, if this demonstration were to be 

attempted using the six-step procedure 

that the EPA is proposing to use for 

calculating PM2.5 visibility index design 

values, significant technical issues with 

the modeling procedures could arise. 

Those technical difficulties include the 

current limitations on speciated sourcespecific emissions data for model input; 

the lack of an EPA-approved air quality 

model with the capability to address the 

atmospheric chemistry associated with 

secondary formation of PM2.5; and the 

lack of PSD screening tools for 

streamlining the air quality analysis 

process. In addition, due to the limited 

monitoring network for speciated PM2.5, 

some sources may not be able to rely on 

existing speciated monitoring data to 

adequately represent the background air 

quality and thereby satisfy 

preconstruction monitoring 

requirements. Consequently, those 

prospective PSD sources could be 

required to collect new data in order to 

determine the representative 

background concentrations of PM2.5

species (i.e., those required for 

calculating the PM2.5 visibility index 

values as described in section VII.A.5 

above). 

Recognizing these difficult technical 

issues, the EPA believes that there is an 

essential need to provide alternative 

approaches to enable prospective PSD 

sources to demonstrate that they will 

not cause or contribute to a violation of 

the secondary PM2.5 visibility index 

NAAQS, if finalized as proposed. To 

meet this need, the EPA believes that it 

is reasonable to allow the use of a 

surrogacy approach, as discussed below, 

for at least the interim period while 

technical issues are being resolved, but 

which could potentially be continued 

beyond such time if shown to be 

appropriate. The EPA is providing 

notice of its intent to follow such an 

approach and is asking for comments on 

the approach as discussed in the 

remainder of this section. The Agency 

believes that following this approach 

will facilitate the transition to a 

workable PSD permitting approach 

under the proposed secondary PM2.5

visibility index NAAQS. 

To support consideration of 

alternative approaches that could be 

used by prospective PSD sources, the 

EPA conducted a two-pronged technical 

analysis of the relationships between 

the proposed PM2.5 visibility index 

standard and the 24-hour PM2.5

standards (Kelly, et al., 2012). 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 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 analysis 

indicated that small increases in PM2.5

concentrations caused by individual 

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221As identified in section IX.E above, the 

relationships between design values characterized 

in the Kelly, et al. (2012) analysis and summarized 

here are dependent upon the specific level and form 

of each of these standards. 

222The 1997 PM10 Surrogate Policy formally 

ended on May 16, 2011. See 76 FR 28646 (May 18, 

2011). 

sources produce similarly small changes 

in visibility impairment for ambient 

conditions near the proposed standard 

level of either 30 dv or 28 dv. The 

second prong of the analysis addressed 

aspects of a PSD cumulative impact 

analysis by exploring the relationship 

between the 3-year design values for the 

primary and secondary 24-hour PM2.5

standards and coincident design values 

for the proposed PM2.5 visibility index 

standard based on recent air quality 

data. This analysis showed that 

visibility generally decreases when 

daily PM2.5 concentrations increase, and 

vice versa. This analysis further 

explored the appropriateness of using a 

demonstration that a source will not 

cause or contribute to a violation of the 

24-hour PM2.5 standards as a surrogate 

for a demonstration that a source will 

not cause or contribute to a violation of 

the proposed secondary PM2.5 visibility 

index standard. The Kelly, et al. (2012) 

analysis was based on 2008 to 2010 air 

quality data and on the proposed 

retention of the 24-hour PM2.5 standards 

with a level of 35 mg/m3 in conjunction 

with a 98th percentile form (sections 

III.F and IV.F) and the proposed 

secondary PM2.5 visibility index 

standard with a level of either 30 dv or 

28 dv in conjunction with 24-hour 

averaging time and a 90th percentile 

form (see section VI.F).221 This analysis 

indicated that all or nearly all areas in 

attainment of the 24-hour PM2.5

standards would also likely be in 

attainment of the proposed secondary 

PM2.5 visibility index standard. 

The EPA believes that this technical 

analysis is robust and will have broad 

national application. Based on this 

technical analysis, the EPA currently 

believes that there is sufficient evidence 

that, for the purposes of making a 

demonstration under the PSD program 

that a new or modified source will not 

cause or contribute to a violation of the 

proposed secondary 24-hour PM2.5

visibility index NAAQS, a 

demonstration that the source will not 

cause or contribute to a violation of the 

mass-based 24-hour PM2.5 NAAQS 

serves as a suitable surrogate. As such, 

many or all sources undergoing PSD 

review for PM2.5 would be able to rely 

upon their analysis demonstrating that 

they will not cause or contribute to a 

violation of the mass-based 24-hour 

PM2.5 NAAQS to also demonstrate that 

they will not cause or contribute to a 

violation of the proposed secondary 

PM2.5 visibility index NAAQS, if 

finalized. The described surrogate 

approach would thus serve to overcome 

the technical challenges discussed 

above and minimize otherwise 

burdensome and costly air quality 

analyses associated with individual 

sources being required to perform 

separate and distinct analyses with 

regard to the proposed secondary PM2.5

visibility index standard. The EPA 

believes this surrogacy approach is 

appropriate to fulfill PSD requirements 

for individual sources in PSD areas, 

which, by definition, will not have been 

designated as nonattainment for the 

PM2.5 visibility index NAAQS. 

However, our proposed surrogacy 

approach for PSD should not be 

construed as a proposal to use a 

surrogacy approach for designating 

nonattainment areas or for 

implementing programs to attain the 

visibility index NAAQS in those areas. 

The surrogacy approach is not 

intended to replace or otherwise 

undermine the validity of the analytical 

techniques employed for air quality 

related value (AQRV) assessments, 

including visibility, required under 40 

CFR 51.166(p) and 40 CFR 52.21(p). The 

federal land managers (FLM)—federal 

officials with direct responsibility for 

management of Federal Class I parks 

and wilderness areas—have an 

affirmative responsibility to protect the 

AQRVs of such lands, and to provide 

the appropriate procedures and analysis 

techniques for assessing AQRVs 

(Appendix W to 40 CFR part 51, 

Sections 6.1(b) and 6.2.3(a)). The FLMs 

have developed specific modeling 

approaches for AQRV assessments that 

are not specifically governed under the 

requirements set forth in 40 CFR 

51.166(l)(1) and 40 CFR 52.21(l)(1), thus 

the surrogacy approach is not applicable 

to the AQRV assessments under the PSD 

program. 

The surrogate approach could be 

incorporated into the PSD program in 

any of three alternative ways. First, the 

decision as to whether the surrogate 

approach is adequate could be handled 

on a case-by-case basis in consultation 

with the permitting authority, similar to 

the existing consultation process under 

the EPA’s Guideline on Air Quality 

Models for ozone and secondary PM2.5

impacts (40 CFR part 51, appendix W, 

section 5.2.1.c), with no presumption 

regarding its adequacy. Second, the EPA 

could establish a rebuttable 

presumption that the surrogate 

approach is applicable for all permits 

through either guidance or a notice-andcomment rulemaking. In either the first 

or second alternative, there would be a 

possibility that reliance on a surrogatebased demonstration could be subjected 

to challenge for any particular permit 

analysis. Third, the EPA could establish 

that the surrogate approach is applicable 

for all permits, also through a noticeand-comment rulemaking. The EPA 

seeks comment on all of the identified 

issues and proposed alternative 

implementation mechanisms associated 

with the proposed surrogate approach. It 

is the Agency’s intention to issue either 

guidance or new regulatory provisions 

as just described for a surrogacy 

approach by the time any final revisions 

to the PM NAAQS become effective, so 

that sources seeking permits will not be 

unnecessarily delayed. 

While noting the importance of the 

surrogacy approach as an essential 

initial strategy due to limitations on 

data and analytical tools, the EPA also 

notes that when a technically robust 

surrogate relationship exists there may 

not be a need to apply an end date for 

the use of a surrogacy approach. 

Without an end date, PSD applicants 

would always have the option of relying 

upon such a demonstration if they 

would so choose. This would offer longterm benefits in terms of simplification 

and resource savings for applicants and 

reviewing authorities. Accordingly, 

based on the technical analysis for the 

standards analyzed (Kelly, et al, 2012) 

which supports the surrogacy approach 

for demonstrating that a source will not 

cause or contribute to a violation of the 

proposed secondary PM2.5 visibility 

index NAAQS, the EPA may determine 

that it is not necessary to announce an 

end date for using it. The EPA invites 

comment on this aspect of the proposal 

as well. 

For context, the EPA notes that with 

regard to sources being required to 

demonstrate that they would not cause 

or contribute to a violation of the 1997 

PM2.5 NAAQS, the EPA has previously 

issued an interim policy (Seitz, 1997). 

Under the 1997 policy, which is no 

longer in effect,222 the EPA stated that 

demonstrating compliance with the NSR 

requirements for controlling PM10

emissions and for analyzing impacts on 

PM10 air quality could be used to 

demonstrate compliance with the PM2.5

NSR requirements. This approach was 

designed to control PM2.5 emissions and 

protect PM2.5 air quality until certain 

technical difficulties concerning PM2.5

were resolved. At that time, however, 

we did not support the policy with any 

technical analysis to show how a 

demonstration of compliance with the 

PM10 NAAQS would satisfy the PM2.5

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223The 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. 

requirements and support the issuance 

of a PSD permit. Consequently, the EPA 

later concluded that, in keeping with 

numerous court opinions regarding the 

use of surrogates, PSD applicants and 

reviewing authorities seeking to rely 

specifically on the 1997 PM10 Surrogate 

Policy should consider certain 

overarching legal principles, including 

that a surrogate may be used only after 

it has been shown to be reasonable 

(such as where the surrogate is a 

reasonable proxy for the pollutant or has 

a predictable correlation to the 

pollutant) and that the relationship 

between the regulated pollutant and the 

surrogate pollutant can be shown to 

apply in the specific instance where an 

applicant or reviewing authority seeks 

to rely upon it. In keeping with these 

principles, the Agency believes that the 

surrogate approach now being proposed 

for use in demonstrating that a source 

will not cause or contribute to a 

violation of the proposed secondary 

PM2.5 visibility index NAAQS is 

supported by a robust technical 

analysis. The EPA invites comment on 

this analysis, which is provided in the 

docket for this action. 

The EPA notes that the analysis 

supporting the surrogacy approach for 

the PSD program is distinct from and 

serves a different purpose than the 

analyses conducted to inform the 

Administrator’s proposed conclusion on 

the appropriate indicator for a standard 

intended to protect against PM-related 

visibility impairment. As discussed in 

section VI.A above, the EPA has long 

recognized that the determination of a 

single, appropriate national level for a 

secondary standard to address PMrelated visibility impairment is 

complicated by regional differences in 

several factors that influence visibility, 

such as background and current PM2.5

concentrations, PM2.5 composition, and 

average relative humidity. Variations in 

these factors across regions could thus 

result in situations where attaining an 

appropriately protective concentration 

of fine particles in one region might or 

might not provide the appropriate 

degree of protection in a different 

region. Although the analysis upon 

which the surrogacy approach is based 

(Kelly, et al., 2012) generally shows that 

daily PM2.5 visibility index values 

decrease when daily PM2.5 mass 

concentrations decrease, and vice versa, 

there is nonetheless considerable 

variability in that relationship across the 

range of ambient fine particle 

concentrations. As a result, as discussed 

in section VI.D.1.d above, the 

Administrator provisionally concludes 

that a calculated PM2.5 light extinction 

indicator is an appropriate indicator to 

replace the current PM2.5 mass indicator 

and that such an indicator 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. 

d. PSD Screening Provisions: Significant 

Emissions Rates, Significant Impact 

Levels, and Significant Modeling 

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 a 

significant emissions rate (SER), 

significant impact levels (SILs), and a 

significant monitoring concentration 

(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 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.223 Under the 

terms of the existing EPA 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. At 

this time, the EPA is not proposing any 

change to the existing PM2.5 SER as a 

result of the proposed revisions to the 

primary annual PM2.5 NAAQS and the 

proposed secondary PM2.5 visibility 

index NAAQS. However, the EPA 

intends to consider this issue in a 

subsequent rulemaking that will 

specifically address various PSD 

implementation issues that are being 

described herein. The EPA will solicit 

comment on any proposed changes to 

the SERs for PM2.5 and its precursors at 

that time, but also invites preliminary 

suggestions at this time that we may 

consider in developing that proposed 

rulemaking. 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 in existing 

regulations. 

Once it is determined that the 

proposed new source or modification is 

significant for PM2.5, the permit 

applicant must complete an air quality 

analysis. 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 SMC for PM2.5 (75 

FR 64890 to 64894, October 20, 2010). 

A separate PM2.5 SIL is defined for each 

averaging period for which PM2.5

NAAQS and increments currently exist, 

as well as for each of the three area 

classifications, i.e., Class I, II and III, 

that Congress established in the CAA for 

PSD purposes. 

Historically, sources have been 

allowed to model their proposed 

emissions increase to predict ambient 

impacts associated with that emissions 

increase, and to compare this predicted 

ambient concentration of PM2.5 to the 

applicable SIL, which is also expressed 

as an ambient PM2.5 concentration over 

a prescribed averaging time consistent 

with the NAAQS and increments. At 

this time, the EPA is not proposing to 

revise the annual SIL for PM2.5 as a 

result of the proposed revision to the 

primary annual PM2.5 NAAQS. 

However, the EPA intends to review this 

issue and will consider any potential 

need to revise the existing SIL in a 

separate rulemaking addressing PSD 

implementation issues. The EPA 

welcomes preliminary comments 

concerning this issue, but will also 

provide an additional opportunity for 

comments at a later date in the event 

that a subsequent proposal is made to 

revise the annual PM2.5 SIL. 

While the proposed secondary PM2.5

visibility index NAAQS is a 24-hour 

standard for which no PM2.5 SIL is 

currently defined, there is a question as 

to whether the existing 24-hour PM2.5

SIL, expressed on a PM2.5 mass basis 

(mg/m3), would be appropriate for this 

proposed secondary NAAQS, expressed 

in terms of a PM2.5 visibility index. As 

discussed in section IX.F.1.c above, the 

EPA conducted an analysis to evaluate 

whether an individual source’s impact 

resulting in a small increase in PM2.5

concentration would produce a 

comparably small increase in visibility 

impairment (Kelly et al., 2012). The 

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224The primary and secondary NAAQS for PM2.5

have been the same up until this time where EPA 

is proposing a distinct secondary NAAQS for PMrelated visibility impairment. 

225 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.’’ 

analysis indicates that small increases 

in PM2.5 concentrations caused by 

individual sources produce similarly 

small changes in visibility impairment 

for ambient conditions near the 

proposed standard level of either 30 dv 

or 28 dv. 

The EPA is not proposing any 

possible alternatives to the existing 24- 

hour PM2.5 SIL in this proposed rule, 

but instead intends to issue a separate 

rulemaking to assess this and other 

related PSD implementation issues. The 

EPA also wishes to note 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 existing PM2.5 SILs have not 

been adequately demonstrated to 

represent de minimis values. Sierra Club 

v. EPA, No. 10–1413 (D.C. Circuit filed 

December 17, 2010). In the course of 

this litigation, the EPA has recognized 

the need to correct the text of two PM2.5

SILs provisions in the regulations, and 

the EPA has asked the court to vacate 

those provisions so that the EPA may 

correct them. However, the EPA does 

not believe this corrective action would 

preclude use of the PM2.5 SILs in the 

interim, and the EPA intends to provide 

guidance on continued use of the PM2.5

SILs (in a manner consistent with 

principles articulated by the EPA in the 

rulemaking and litigation) pending this 

correction of the regulatory text. The 

proposed revised primary annual PM2.5

NAAQS and the proposed secondary 

PM2.5 visibility index NAAQS do not 

affect the continued used of the PM2.5

SILs in accordance with the forthcoming 

guidance described above. As a separate 

matter, the EPA intends to consider the 

need for a new SIL specifically for 

implementing any secondary PM2.5

visibility index NAAQS under the PSD 

program. In the event that we do 

proceed, the EPA now welcomes 

preliminary comments as to how such a 

SIL could be developed. The EPA will 

also provide an additional opportunity 

for comments at a later date in the event 

that a subsequent proposal is made to 

establish a separate SIL for the 

secondary PM2.5 visibility index 

NAAQS, if such a secondary NAAQS is 

finalized. 

Finally, the SMC, also measured as an 

ambient pollutant concentration (mg/ 

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 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. In a future NSR 

implementation rulemaking that will 

follow this rulemaking, the EPA intends 

to evaluate the types of additional 

ambient data, if any, that may need to 

be collected by a proposed source 

concerning the proposed secondary 

PM2.5 visibility index NAAQS, and the 

feasibility of individual sources being 

required to gather such additional 

information. The EPA welcomes 

preliminary comments concerning this 

issue, but will provide additional 

opportunity for comment when a 

subsequent NSR implementation 

rulemaking is proposed concerning the 

proposed revisions to the PM NAAQS. 

e. 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 224

(75 FR 64864, October 20, 2010). The 

proposed revision to the primary annual 

PM2.5 NAAQS raises the question of 

whether the EPA should consider 

revising the annual PM2.5 increments. 

Similarly, the EPA’s proposed action to 

establish a distinct secondary PM2.5

visibility index NAAQS raises the 

question of whether revisions to the 

PM2.5 increments are appropriate to 

address public welfare considerations 

protected by the proposed secondary 

standard. 

In this proposal, the EPA is not 

proposing to revise the PM2.5

increments. The EPA will consider 

whether it is appropriate to propose 

such an action in the future, and if so, 

would undertake the necessary 

rulemaking. The EPA invites 

preliminary comments at this time on 

such a need, and on issues we should 

consider if we undertake a rule 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. 

2. Nonattainment New Source Review 

The requirements under part D of the 

CAA pertain 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.’’ Thus, 

the NNSR program applies 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.225 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 100 tpy of 

direct PM2.5 (‘‘PM2.5 emissions’’) or a 

precursor of PM2.5. 

For a major modification, the NNSR 

rules rely upon SERs described 

previously in the PSD discussion in 

section IX.F.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 that 

results in a significant net emissions 

increase of that nonattainment 

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 the existing 

values. Any decision to propose 

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226However, transportation conformity 

requirements discussed in section IX.G below are 

dependent upon the averaging period(s) for which 

an area is designated nonattainment. 

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. 

While no formal policy has yet been 

developed to address this situation, 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.226 Looking forward, 

the EPA proposes that areas would be 

designated for a proposed secondary 

PM2.5 visibility index NAAQS 

independently of designations for the 

mass-based annual and 24-hour PM2.5

NAAQS. Accordingly, the EPA intends 

to address this issue in a future NSR 

rulemaking, but invites comments now 

on whether it is 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 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 NNSR 

requirements as of the effective date of 

the nonattainment designation; 

therefore states lacking the appropriate 

NNSR program requirements at that 

time will be allowed to issue such 

permits during the SIP revision period 

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 PM NAAQS upon their effective 

date. The EPA is not proposing any type 

of PM2.5 grandfathering provision at this 

time for purposes of NNSR. The 

timetable for adopting new provisions 

under the state 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. Further 

consideration of the need for a 

grandfathering provision for purposes of 

NNSR for the revised NAAQS for PM2.5

will be made and addressed in the 

future, as appropriate. 

G. Transportation Conformity Program 

Transportation conformity is required 

under CAA section 176(c) to ensure that 

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 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 proposing changes 

to the transportation conformity rule in 

this proposed rulemaking. The EPA 

notes that the transportation conformity 

rule already addresses the PM2.5 and 

PM10 NAAQS. However, in the future, 

the EPA will review the need to issue 

or revise guidance describing how the 

current conformity rule applies in 

nonattainment and maintenance areas 

for any revised primary or distinct 

secondary PM NAAQS, as needed. 

As discussed in section VIII above, 

the EPA is proposing certain clarifying 

changes to PM2.5 air quality monitoring 

regulations These proposed 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 hotspot analyses for the annual standards 

by clarifying which receptors are 

comparable to the NAAQS. 

If the EPA finalizes these changes to 

the monitoring regulations, the EPA will 

update its guidance on quantitative 

PM2.5 hot-spot analyses as appropriate 

to make it consistent with the revised 

monitoring requirements (U.S. EPA, 

2010j). If the proposed revisions to the 

monitoring requirements are finalized, 

the EPA intends that the current 

quantitative PM2.5 hot spot guidance 

would continue to apply to any 

quantitative PM2.5 hot-spot analysis that 

was begun before the effective date of 

these proposed 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 allows 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. 

H. General Conformity Program 

General conformity is required by 

CAA section 176(c). This section 

requires that federal agencies do not 

adopt, accept, approve, or fund 

activities that are not consistent with 

state air quality goals. General 

conformity applies to any federal action 

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39030 Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / Proposed Rules 

(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 any revised PM 

NAAQS would 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 any 

revision to the primary or secondary 

standards, a federal agency would be 

expected to continue to estimate 

emissions for conformity analyses in the 

same manner as they are estimated for 

conformity analyses for the current PM 

NAAQS. 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 emission 

estimation techniques available (40 CFR 

93.159(b)), and EPA would expect that 

this same principle would be followed 

for analyses needed with respect to any 

revised PM NAAQS. When updated and 

improved emissions estimation 

techniques become available, EPA 

would expect the federal agency to use 

these techniques. The EPA is not 

proposing changes to the general 

conformity rule in this proposed 

rulemaking. The general conformity rule 

already addresses the PM2.5 and PM10

NAAQS. 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 and 

secondary PM NAAQS. 

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. 

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. 

In addition, 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 alternative levels (in mg/ 

m3) of the primary annual/24-hour 

PM2.5 standards including: 13/35, 12/35, 

11/35, and 11/30. Table 4 provides a 

summary of the estimated costs, 

monetized benefits, and net benefits 

associated with full attainment of these 

alternative standards. 

TABLE 4—TOTAL COSTS, MONETIZED BENEFITS AND NET BENEFITS IN 2020 a (MILLIONS OF 2006$) b FULL ATTAINMENT

Alternate PM2.5

Standards 

(annual/24-hour, in 

μg/m3) 

Total costs Monetized benefits c Net benefits c

3% 

Discount 

rate 

7% 

Discount 

rate 

3% 

Discount rate 

7% 

Discount rate 

3% 

Discount rate d

7% 

Discount rate 

13/35 ............................... $2.9 $2.9 $88 to $220 $79 to $200 $85 to $220 $76 to $200 

12/35 ............................... 69 69 $2,300 to $5,900 $2,100 to $5,400 $2,300 to $5,900 $2,000 to $5,300 

11/35 ............................... 270 270 $9,200 to $23,000 $8,300 to $21,000 $8,900 to $2300 $8,000 to $21,000 

11/30 ............................... 390 390 $14,000 to $36,000 $13,000 to $33,000 $14,000 to $36,000 $13,000 to $33,000 

aValues are rounded to two significant figures. 

b Using a 2010$ year increases estimated costs and benefits by approximately 8%. 

cThe reduction in premature death each year accounts for over 90 percent 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. d Due to data limitations, we were unable to discount compliance costs for all sectors at 3%. As a result, the net benefit calculations at 3% 

were computed by subtracting the monetized benefits at 3% minus the costs at 7%. 

B. Paperwork Reduction Act 

This action does not impose an 

information collection burden under the 

provisions of the Paperwork Reduction 

Act, 44 U.S.S. 3501 et seq. Burden is 

defined at 5 CFR 1320.3(b). There are no 

information collection requirements 

directly associated with revisions to a 

NAAQS under section 109 of the CAA. 

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. 

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After considering the economic 

impacts of this proposed rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This proposed 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 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. 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 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 proposed 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. Therefore, in the spirit of 

Executive Order 13132, and consistent 

with EPA policy to promote 

communications between the EPA and 

state and local governments, the EPA 

specifically solicits comment on this 

proposed rule from state and local 

officials. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

The action does not have tribal 

implications, as specified in Executive 

Order 13175 (65 FR 67249, November 9, 

2000). 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. The 

Tribal Authority Rule gives Tribes the 

opportunity to develop and implement 

CAA programs such as the PM NAAQS, 

but it leaves to the discretion of the 

Tribe whether to develop these 

programs and which programs, or 

appropriate elements of a program, they 

will adopt. Thus, Executive Order 13175 

does not apply to this rule. 

Although Executive Order 13175 does 

not apply to this rule, the EPA 

consulted with tribal officials or other 

representatives of tribal governments in 

developing this action. 

The EPA specifically solicits 

additional comments on this proposed 

rule from tribal officials. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

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 may be especially 

important for children because 

childhood represents a lifestage 

associated with increased susceptibility 

to PM-related health effects. Because 

children have been identified as a 

susceptible 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, 

IV.B, and IV.C of this preamble. A 

listing of documents that contain the 

evaluation of scientific evidence and 

policy considerations that pertain to 

children is found in the section on 

Children’s Environmental Health in the 

Supplementary Information section of 

this preamble, and a copy of all 

documents have been placed in the 

public docket for this action. 

The public is invited to submit 

comments or identify peer-reviewed 

studies and data that assess effects of 

early life exposure to PM. 

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. 

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227A list of designated reference and equivalent 

methods is available on EPA’s Web site at: http:// 

www.epa.gov/ttn/amtic/criteria.html. 

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. 

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 proposed 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. The EPA 

also proposes to add a distinct 

secondary standard for PM2.5 defined in 

terms of a calculated PM2.5 light 

extinction indicator, which would use 

PM2.5 mass species and relative 

humidity data to calculate PM2.5 light 

extinction. 

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 

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.227 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. 

For the proposed secondary standard 

defined in terms of a calculated PM2.5

light extinction indicator, the EPA 

proposes to use existing monitoring 

technologies that are already deployed 

in the CSN and IMPROVE monitoring 

programs as well as relative humidity 

data from sensors already deployed at 

routine weather stations. The sampling 

and analysis protocols in use in the CSN 

program are the result of substantial 

input and recommendations from 

CASAC both during their initial 

deployment about ten years ago, and 

during the more recent transition to 

carbon sampling that is consistent with 

IMPROVE protocols (Henderson 2005c). 

Monitoring agencies also played a 

strong role in directing the sampling 

technologies used in the CSN. During 

the first few years of implementing the 

CSN there were up to four different 

sampling approaches used in the 

network. Over time as monitoring 

agencies shared their experiences and 

data with each other, several agencies 

shifted their network operations to the 

sampling technology used today. By 

2008, the EPA was working closely with 

all remaining monitoring agencies to 

transition to the current CSN sampling 

for ions and elements. All carbon 

sampling was fully transitioned to the 

current method by October of 2009 for 

consistency with the IMPROVE 

program. Therefore, while the current 

CSN sampling methods were not 

developed or adopted by a voluntary 

consensus standard body, they are the 

result of harmonizing the network by 

monitoring agency users and EPA. The 

CSN network and methods are 

described in more detail in the Policy 

Assessment (U.S. EPA, 2011a, Appendix 

B, section B.1.3). 

The EPA welcomes comments on this 

aspect of the proposed rulemaking and, 

specifically, invites the public to 

identify potentially applicable voluntary 

consensus standards for any of the 

proposed indicators with an explanation 

as to why such standards should be 

used in this regulation. 

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 

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. The EPA has 

identified potential disproportionately 

high and adverse effects on minority 

and/or low-income populations from 

this proposed rule. 

The EPA has identified persons from 

lower socioeconomic strata as a 

susceptible population for PM-related 

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. The Agency 

expects this proposed rule would lead 

to the establishment of uniform NAAQS 

for PM. The Integrated Science 

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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. 

The public is invited to submit 

comments or identify peer-reviewed 

studies and data that assess effects of 

PM on low-income populations and 

minority populations. 

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Opinions on Visibility Impairment Due 

to Air Pollution: Summary Report. 

Available: http://www.epa.gov/ttn/ 

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Abt Associates (2005). Particulate Matter 

Health Risk Assessment for Selected 

Urban Areas. Final Report. Bethesda, 

MD. Prepared for the Office of Air 

Quality Planning and Standards, U.S. 

Environmental Protection Agency, 

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JH, Raizenne M, Speizer FE (1996). 

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Sulfur. (Final Report). US Environmental 

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2010. Available: http://www.epa.gov/ttn/ 

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10–002. January 2010. Available: http:// 

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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: June 14, 2012. 

Lisa P. Jackson, 

Administrator. 

For the reasons set forth in the 

preamble, chapter I of title 40 of the 

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Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / Proposed Rules 39041 

Code of Federal Regulations is proposed 

to be 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. 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) * * * 

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 ....................................

2008 ..............................................

2009 ..............................................

June 18, 2009 ...............................

June 18, 2009 ...............................

60 days after the end of the calendar quarter in which the 

event occurred or February 5, 

2010, whichever date occurs 

first.

June 18, 2009 

June 18, 2009 

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 ..............................................

2009 ..............................................

2010 ..............................................

July 1, 2010 ..................................

July 1, 2010a ................................

April 1, 2011 .................................

January 22, 2011. 

January 22, 2011. 

July 1, 2011. 

SO2/1-Hr Standard (75 ppb) Promulgated June 22, 2010.2008 ..............................................

2009 ..............................................

2010 ..............................................

2011 ..............................................

October 1, 2010 ............................

October 1, 2010 ............................

June 1, 2011 .................................

60 days after the end of the calendar quarter in which the 

event occurred or March 31, 

2012, whichever date occurs 

first.

June 1, 2011. 

June 1, 2011. 

June 1, 2011. 

60 days after the end of the calendar quarter in which the 

event occurred or March 31, 

2012, whichever date occurs 

first. 

PM2.5/24-Hour Standard (final level 

and promulgation date TBD).

2010 to 2011 ................................

2012 ..............................................

2013 ..............................................

July 1, 2013 ..................................

July 1, 2013a ................................

July 1, 2014a ................................

December 12, 2013. 

December 12, 2013. 

August 1, 2014. 

PM2.5/Annual Standard (final level 

and promulgation date TBD).

2010 to 2011 ................................

2012 ..............................................

2013 ..............................................

July 1, 2013 ..................................

July 1, 2013a ................................

July 1, 2014a ................................

December 12, 2013. 

December 12, 2013. 

August 1, 2014. 

PM2.5 Visibility Index (final level 

and promulgation date TBD).

2010 to 2011 ................................

2012 ..............................................

2013 ..............................................

July 1, 2013 ..................................

July 1, 2013a ................................

July 1, 2014a ................................

December 12, 2013. 

December 12, 2013. 

August 1, 2014. 

aThis date is the same as the general schedule in 40 CFR 50.14. 

Note: The table of revised deadlines only applies to data EPA will use to establish the final initial area designations for new NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment. TBD = to be determined. 

* * * * * 

3. 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 to 

13.0] micrograms per cubic meter (mg/ 

m3) annual arithmetic mean 

concentration and 35 mg/m3 24-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 to 13.0] mg/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 mg/m3. 

4. Add § 50.19 to read as follows: 

§ 50.19 National secondary ambient air 

quality standard for PM2.5

(a) The following national secondary 

ambient air quality standard for PM is 

in addition to the national secondary 

ambient air quality standards for PM10

specified in § 50.6 and for PM2.5

specified in § 50.13. 

(1) [30 or 28] deciviews (dv), 24-hour 

average concentration, based on a 

calculated PM2.5 visibility index using 

methods based on appendix C of part 58 

of this chapter. 

(2) [Reserved]. 

(b) The 24-hour secondary PM2.5

visibility index standard is met when 

the 90th percentile 24-hour calculated 

PM2.5 visibility index, as determined in 

accordance with appendix N of this 

part, is less than or equal to [30 or 28] 

dv. 

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 

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39042 Federal Register / Vol. 77, No. 126 / Friday, June 29, 2012 / Proposed Rules 

necessary for determining when the national 

ambient air quality standards (NAAQS) for 

PM2.5 are met, including the primary and 

secondary annual and 24-hour PM2.5 NAAQS 

specified in § 50.7, 50.13, and 50.18, and the 

secondary PM2.5 visibility index NAAQS 

specified in § 50.19. 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. 

Chemically speciated PM2.5 mass 

concentrations are derived from ambient air 

measurements using the methods specified in 

appendix C of part 58 of this chapter. 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. The data handling and 

computation procedures to be used to 

construct the PM2.5 visibility index metric 

from reported speciated PM2.5 concentrations 

(and related climatological relative humidity 

hygroscopic growth factors), and the 

associated instructions for comparing these 

computed metrics to the level of the PM2.5

visibility index NAAQS, are specified in 

sections 2.0, 3.0, and 5.0 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 

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 or 90th 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 for the annual and 24-hour 

PM2.5 NAAQS 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. Daily values for 

the PM2.5 visibility index NAAQS refer to the 

24-hour average PM2.5 visibility index values 

derived from reported speciated PM2.5

measurements and corresponding f(RH) 

factors using the formulae specified in 

section 5.0 of this appendix. 

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 metric, 

which is otherwise judged incomplete (via 

the applicable 75 percent data capture or 11 

creditable samples per quarter minimum data 

completeness options), 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. 

Deciview is the unit of measure for the 

level of the secondary PM2.5 visibility index 

NAAQS. This metric describes changes in 

uniform light extinction that can be 

perceived by a human observer. One 

deciview represents the minimal perceptible 

change in visibility to the human eye. Daily 

calculated PM2.5 light extinction values in 

units of Mm¥1 are translated to PM2.5

visibility index values in terms of deciviews 

according to equation 7 in section 5(d)(3) of 

this appendix. 

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 sections 4.0 

and 5.0 of this appendix. There are three 

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 

PM 2.5 NAAQS DV.’’ 

(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.’’ 

(3) The 3-year average of annual 90th 

percentile 24-hour average PM2.5 visibility 

index values calculated for each eligible 

monitoring site is referred to as the ‘‘PM2.5

visibility index NAAQS DV.’’ 

Elemental carbon (EC) is the reported 

concentration of PM2.5 elemental carbon from 

the speciation methods identified in 

appendix C to part 58 of this chapter. 

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 and the PM2.5

visibility index 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 or 90th 

percentile value, but are not used to 

determine which value in the sorted list 

represents the 98th or 90th percentile. 

Fine soil (FS) is the calculated measure of 

PM2.5 crustal material. It is derived from the 

reported speciated PM2.5 concentrations of 

aluminum (Al), silicon (Si), calcium (Ca), 

iron (Fe), and titanium (Ti) using formula 5d 

in 5(d)(1) of this appendix. FS data is 

generated from the speciation methods 

identified in appendix C to part 58 of this 

chapter. 

f(RH) is a unitless water growth factor used 

to relate a given relative humidity (RH) to its 

impact on PM2.5 light-scattering. 

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 nonexcluded quarterly values (year non-specific) 

contained in the combined site record for the 

evaluated 3-year period) for missing daily 

values. 

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The minimum quarterly value data 

substitution test substitutes actual ‘‘low’’ 

reported daily PM2.5 values from the same 

site (specifically, the lowest reported 

quarterly values (year non-specific) 

contained in the combined site record for the 

evaluated 3-year period) for missing daily 

values. 

98th percentile [90th percentile] is the 

smallest daily value out of a year of PM2.5

mass monitoring data [PM2.5-related visibility 

indices] below which no more than 98 [90] 

percent of all daily values fall using the 

ranking and selection method specified in 

section 4.5(a) [5.0(d)(4)] of this appendix. 

Nitrate is the fully neutralized PM2.5 nitrate 

ion (NO3

 ̄) concentration. It is the reported 

concentration of NO3

 ̄ multiplied by a factor 

(1.29) to account for full neutralization with 

ammonium. See equation 5b in 5(d)(1) of this 

appendix. Nitrate data is generated from the 

speciation methods identified in appendix C 

to part 58 of this chapter. 

Organic mass (OM) is the concentration of 

PM2.5 organic carbon (PM2.5 OC) multiplied 

by a factor (1.4) to adjust the OC for other 

elements (e.g., hydrogen and oxygen) 

assumed to be associated with the PM2.5 OC. 

See equation 5c in 5(d)(1) of this appendix. 

Organic mass data is generated from the 

speciation methods identified in appendix C 

to part 58 of this chapter. 

PM2.5 bext is a calculated measure of the 

total fraction of light that is attenuated by 

PM2.5 particles per unit distance (e.g., per 

inverse megameter, Mm¥1). The estimate is 

derived from daily average speciated PM2.5

mass concentrations and climatological 

monthly average relative humidity data via 

equation 6 in 5(d)(2) of this appendix. 

PM2.5 organic carbon (PM2.5 OC) refers to 

the measured organic carbon with an 

adjustment for adsorbed organic vapors 

(known as the organic carbon artifact). PM2.5

organic carbon data is generated from the 

speciation methods identified in Appendix C 

to Part 58. 

PM2.5 visibility index is the indicator used 

for the secondary PM2.5 visibility index 

NAAQS. The index is computed on a 24-hour 

average basis from PM2.5 bext using equation 

7 in 5(d)(3) of this appendix. 

Primary monitors are suitable monitors 

designated by a state or local agency in their 

annual network plan (and in AQS) to be 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. 

Speciation methods refer to the PM2.5

chemical speciation methods identified in 

section 2.9.2 of appendix C to part 58 of this 

chapter which include those used by the 

Chemical Speciation Network (CSN) and the 

Interagency Monitoring of Protected Visual 

Environment (IMPROVE) network. 

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 per § 58.11 of this chapter. 

For the PM2.5 visibility index NAAQS, 

suitable monitors include the speciation 

methods specified in section 2.9.2 of 

appendix C of part 58 of this chapter which 

include those used by the CSN and the 

IMPROVE network. 

Sulfate is the fully neutralized PM2.5

sulfate ion (SO2¥4) concentration. It is the 

reported concentration of SO2¥4 multiplied 

by a factor (1.375) to account for full 

neutralization with ammonium. See equation 

5a in 5(d)(1) of this appendix. Sulfate data 

are generated from the speciation methods 

identified in appendix C to part 58 of this 

chapter. 

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. All speciation samplers 

using the speciation methods specified in 

section 2.9.2 of appendix C of part 58 of this 

chapter are deemed suitable for comparisons 

to the PM2.5 visibility index NAAQS. 

(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 concentration data and speciated PM2.5

mass 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 (mg/m3) to at 

least one decimal place, with additional 

digits to the right being 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 PM2.5 mass data are submitted to AQS 

with less precision than specified above, 

these data shall nevertheless still be deemed 

appropriate for NAAQS usage. For the 

purpose of calculating PM2.5 visibility index 

values, the speciated PM2.5 component 

concentrations of sulfate, nitrate, PM2.5 OC, 

EC, Al, Si, Ca, Fe, and Ti, the AQS will 

convert (if necessary) reported concentrations 

into units of mg/m3 rounded to four decimal 

places (0.xxxx5 rounds up), or three 

significant digits when the concentration 

value is 0.1 or more. In situations where 

fewer decimal places or significant digits 

than specified above are reported to AQS, 

such data shall nevertheless still be deemed 

appropriate for NAAQS usage. 

(c) Block 24-hour average concentrations 

will be computed in AQS from submitted 

hourly PM2.5 concentration data (mass or 

species) 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 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). For PM2.5 mass 

concentrations, 24-hour periods with seven 

or more missing hours shall 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 mg/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 filterbased) data; twenty-four-hour average PM2.5

speciated mass concentrations that are 

averaged in AQS from hourly values will be 

rounded to four decimal places (or three 

significant digits if the average is greater than 

0.1), consistent with the data handling 

procedures 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 and 

speciated concentrations for a site shall 

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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 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.’’ 

(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 for 

aggregated PM2.5 mass. The exception is, if a 

collocated continuous FEM 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 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. For the PM2.5 visibility index 

NAAQS, creditable samples are based on 

daily values of PM2.5 bext (which essentially 

require non-missing values for the nine 

required input speciated PM2.5 parameters, 

all reported on the same scheduled sampling 

days). Section 5.0 of this appendix specifies 

in further detail the procedure for calculating 

PM2.5 visibility index values and the ensuing 

determination of whether they are creditable 

or not. 

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 [12.0 to 13.0] mg/m3 at 

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 

mg/m3 at 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 

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 three 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 one (or more) of the test 

conditions specified in 4.1(c)(i), 4.1(c)(ii), 

and 4.1(c)(iii) is met. 

(1) 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 quarterspecific 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 recalculated annual PM2.5 NAAQS 

test DV 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 exceeded in that 

3-year period. 

(2) 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 are at least 30 days across the three 

matching quarters 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 

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 

months 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 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. 

(3) An annual 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 

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 

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 mg/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 

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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 data capture 

requirement is not met, the 24-hour PM2.5

NAAQS DV shall still be considered valid 

(and complete) if it passes one (or both) of 

two applicable data substitution tests (i.e., 

the maximum quarterly value or collocated 

PM10 data substitution tests). 

(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 

‘‘complete and valid’’ if either of the test 

conditions specified in 4.2(c)(i) or 4.2(c)(ii) 

are met. 

(1) 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 of data substitution is permitted only if 

there are at least 30 days across the three 

matching quarters of the three years under 

consideration from which to select the 

quarter-specific high value. 

Procedure: Identify for each deficient 

quarter (i.e., those with less than 75 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 

Administrator, looking across those three 

months 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 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. 

(2) 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, available collocated FRM/FEM daily 

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 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. 

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. 

(a) Annual PM2.5 NAAQS DVs shall be 

rounded to the nearest tenth of a mg/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 mg/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: 

Where: 

X ̄ q,y = the mean for quarter q of the year y; 

nq = the number of daily values in the 

quarter; and 

X ̄ i 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: 

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. 

Where: 

X ̄ = the annual PM2.5 NAAQS DV; and 

Xy = 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 

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). 


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TABLE 1 

Annual number of 

creditable samples 

for year y (cny) 

P 0.98, y is the 

nth maximum 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: 

Where: 

P0.98 = 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. 

5.0 Comparisons With the Secondary PM2.5

Visibility Index NAAQS 

(a) The secondary PM2.5 visibility index 

NAAQS is met when the PM2.5 visibility 

index NAAQS DV at each eligible monitoring 

site is less than or equal to [30 or 28] 

deciviews. 

(b) Three years of valid annual 90th 

percentile concentrations of 24-hour average 

PM2.5 visibility index values are required to 

produce a valid PM2.5 visibility index 

NAAQS DV. A year meets data completeness 

requirements when there are at least 11 

creditable daily values of PM2.5 visibility 

indices in each quarter (all four of the year); 

a daily value is defined as one that contains 

valid estimates for all five major speciation 

PM2.5 components: Sulfate, nitrate, OM, EC, 

and FS. In order to derive these five major 

components, 24-hour average concentrations 

are needed for the following nine parameters: 

EC, Al, Si, Ca, Fe, and Ti, and PM2.5 OC. 

Years with less than 11 creditable samples in 

each quarter shall still be considered 

complete and the corresponding identified 

90th percentile deemed valid, if the 90th 

percentile value for that year or a resulting 

3-year average 90th percentile value (i.e., a 

PM2.5 visibility index NAAQS DV) 

encompassing that annual value exceeds the 

NAAQS level (i.e., [30 or 28] deciviews). The 

use of less than complete data (i.e., data not 

meeting the criteria stated in this subsection) 

is subject to the approval of the EPA 

Administrator, who may consider factors 

such as monitoring site closures/moves, 

monitoring diligence, and nearby 

concentrations in determining whether to use 

such data. 

(c) Rounding Conventions: For the 

purposes of calculating PM2.5 visibility index 

NAAQS DVs to compare to the level of the 

standard, it is necessary to round the final 

results of the calculations described in 

sections 5(d) of this appendix as noted 

below. Results for all intermediate 

calculations shall not be rounded unless 

otherwise specified. 

(1) Daily deciview values shall be rounded 

to the nearest 0.1 deciview (decimals 0.x5 

and greater are rounded up to the next tenth, 

and any decimal lower than 0.x5 is rounded 

down to the stated tenth). 

(2) The PM2.5 visibility index NAAQS DV 

shall be rounded to the nearest 1 deciview 

(decimal values x.5 and greater are rounded 

up to the nearest whole number, and any 

decimal values lower than x.5 are rounded 

down to the nearest whole number). 

(d) Procedures and Equations for the 

Secondary PM2.5 Visibility Index NAAQS 

(1) The five major speciation components 

(Sulfate, Nitrate, OM, EC, and FS) are derived 

from reported concentrations of 

EC, Al, Si, Ca, Fe, and Ti, and reported/ 

adjusted concentrations of PM2.5 OC, 

according to the equations below: 


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Where: 

OMi = organic mass for day i; and 

PM2.5 OCi = measured organic carbon with an 

adjustment for adsorbed organic vapors 

Where: 

FSi = fine soil for day i; and 

Ali = the reported aluminum concentration 

for day i; and 

Sii = the reported silicon concentration for 

day i; and 

Cai = the reported calcium concentration for 

day i; and 

Fei = the reported iron concentration for day 

i; and 

Tii = the reported titanium concentration for 

day i 

(2) Daily estimates of PM2.5-related 

calculated light-extinction, PM2.5 bext

(expressed in units of inverse 

megameters (Mm¥1)), are derived by 

equation 6. The components sulfate, 

nitrate, OM, and FS are derived using 

formulae, 5a, 5b, 5c, and 5d. The 

component EC is the reported 

concentration of PM2.5 elemental 

carbon. The f(RH) value corresponding 

to each site-day shall be identified from 

the most recent 10-year average 

climatological database. This database 

contains spatially gridded monthly 

values of f(RH). The database record for 

the grid-point closest in distance to the 

monitoring site shall be selected for 

utilization in calculating PM2.5 bext. The 

monthly value identified from the 

database record for the selected grid 

location will be the one corresponding 

to the sample month of the reported 

input speciation concentrations. 

(3) Daily estimates of PM2.5 bext, in 

units of Mm¥1, are converted to PM2.5

visibility index values, in units of 

deciviews, according to equation 7. 

Where: 

PM2.5 _ visibility _ indexi = PM2.5 visibility 

index value (in deciview units) for day 

i; and 

PM2.5 _ Bext i = PM2.5-related light extinction 

(in Mm¥1 units) for day i 

(4) Identification of annual 90th 

percentile PM2.5 visibility index values 

is accomplished by the steps provided 

in this subsection. Table 2 of this 

appendix shall be used to identify 

annual 90th percentile values according 

to the creditable number of 24-hour 

PM2.5 visibility index values calculated 

for the year. 

Procedure: Sort all the daily PM2.5

visibility index 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 


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and x[n] is the smallest value.) The 90th 

percentile is determined from this 

sorted series of values which is ordered 

from the highest to the lowest number. 

Using the left column of Table 2, 

determine the appropriate range for the 

annual creditable number of samples for 

year y (ny) (e.g., for 35 creditable 

samples in a year, the appropriate range 

would be 31 to 40). The corresponding 

‘‘nth’’ value in the right column 

identifies the rank of the annual 90th 

percentile value in the descending 

sorted list of PM2.5 visibility index 

values for year y (e.g., for the range of 

31 to 40, n is equal to 4). Thus, P0.90, y

= the nth largest value (e.g., for the 

range of 31 to 40, the 90th percentile 

value would be the fourth highest value 

in the sorted series of PM2.5 visibility 

index values). 

(5) The PM2.5 visibility index NAAQS 

DV is then calculated by averaging the 

annual 90th percentile PM2.5 visibility 

index values for three consecutive years 

using equation 8 of this appendix: 

Where: 

P0.90 = the PM2.5 visibility index NAAQS DV; 

and 

P0.90.y = the annual 90th percentile PM2.5

visibility index value for year y 

TABLE 2 

Annual number of 

creditable samples 

for year ‘‘y’’ (cny) 

P 0.90, y is the nth maximum for the year where 

n is the listed number 

1 to 10 .................. 1 

11 to 20 ................ 2 

21 to 30 ................ 3 

31 to 40 ................ 4 

41 to 50 ................ 5 

51 to 60 ................ 6 

61 to 70 ................ 7 

71 to 80 ................ 8 

81 to 90 ................ 9 

91 to 100 .............. 10 

101 to 110 ............ 11 

111 to 120 ............ 12 

121 to 130 ............ 13 

131 to 140 ............ 14 

141 to 150 ............ 15 

151 to 160 ............ 16 

161 to 170 ............ 17 

171 to 180 ............ 18 

181 to 190 ............ 19 

191 to 200 ............ 20 

201 to 210 ............ 21 

211 to 220 ............ 22 

221 to 230 ............ 23 

231 to 240 ............ 24 

241 to 250 ............ 25 

251 to 260 ............ 26 

261 to 270 ............ 27 

271 to 280 ............ 28 

TABLE 2—Continued 

Annual number of 

creditable samples 

for year ‘‘y’’ (cny) 

P 0.90, y is the nth maximum for the year where 

n is the listed number 

281 to 290 ............ 29 

291 to 300 ............ 30 

301 to 310 ............ 31 

311 to 320 ............ 32 

321 to 330 ............ 33 

331 to 340 ............ 34 

341 to 350 ............ 35 

351 to 360 ............ 36 

361 to 366 ............ 37 

PART 51—REQUIREMENTS FOR 

PREPARATION, ADOPTION, AND 

SUBMITTAL OF IMPLEMENTATION 

PLANS 

6. 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] 

7. In § 51.166, add paragraph (i)(10) to 

read as follows: 

§ 51.166 Prevention of significant 

deterioration of air quality. 

* * * * * 

(i) Exemptions. * * * 

(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 as in effect on 

[EFFECTIVE DATE OF FINAL RULE] if 

the reviewing authority has first 

published before that date public notice 

that a preliminary determination for the 

permit subject to this section has been 

issued. Instead, the requirements in 

paragraph (k)(1) shall apply with respect 

to the national ambient air quality 

standards for PM2.5 as in effect at the 

time of the public notice on the 

proposed permit. 

* * * * * 

PART 52—APPROVAL AND 

PROMULGATIONS OF 

IMPLEMENTATION PLANS 

8. The authority citation for part 52 

continues to read as follows: 

Authority: 42 U.S.C. 7401, et seq. 

9. 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 as in effect 

on [EFFECTIVE DATE OF FINAL RULE] 

if the Administrator has first published 

before that date 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

as in effect on the date the 

Administrator first published a public 

notice that a draft permit has been 

prepared. 

* * * * * 

PART 53—AMBIENT AIR MONITORING 

REFERENCE AND EQUIVALENT 

METHODS 

10. The authority citation for part 53 

continues to read as follows: 

Authority: Section 301(a) of the Clean Air 

Act (42 U.S.C. 1857g(a)), as amended by sec. 

15(c)(2) of Pub. L. 91–604, 84 Stat. 1713, 

unless otherwise noted. 

11. 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). 

* * * * * 

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

12. 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. 

13. 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)’’. 

The addition reads 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. 

* * * * * 

14. Section 58.10 is amended as 

follows: 


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a. By adding paragraph (a)(8). 

b. By adding paragraph (b)(13). 

c. By revising paragraph (c). 

d. By revising paragraph (d). 

The additions and revisions read as 

follows: 

§ 58.10 Annual monitoring network plan 

and periodic network assessment. 

(a) * * * 

(8) A plan for establishing near-road 

PM2.5 monitoring sites in accordance 

with the requirements of appendix D to 

this part shall be submitted to the 

Regional Administrator by July 1, 2014. 

The plan shall provide for all required 

monitoring stations to be operational by 

January 1, 2015. 

(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 collected for the period of time 

that the plan covers (i.e., the next 18 

months or until a new plan is submitted 

addressing this issue) 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, the 

monitoring agency must ensure that an 

operating FRM or filter-based FEM 

meeting the sample frequency 

requirements described in § 58.10 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. 

* * * * * 

15. 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 case where the data are 

identified as not of sufficient 

comparability to a collocated FRM, such 

that the FEM or ARM 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)(13) 

for any case where the FEM or ARM is 

identified as not of sufficient 

comparability to a collocated FRM. The 

performance criteria apply with the 

following provisions to accommodate 

how monitoring agencies operate their 

collocated PM2.5 methods: 

(1) The acceptable concentration 

range (Rj), mg/m3 may include values 

down to 0 mg/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 apply; the precision statistic 

requirement does not apply, even if 

precision data are available. 

(5) All seasons must be covered with 

no more than three years in total 

aggregated together. 

16. Section 58.12 is amended by 

revising paragraph (d)(1)(iii) and by 

removing and reserving paragraph (f)(2). 

The revision reads as follows: 

§ 58.12 Operating schedules. 

* * * * * 

(d) * * * 

(1) * * * 

(iii) Required SLAMS stations whose 

measurements determine the design 

value for their area and that are within 

plus or minus 5 percent of the 24-hour 

PM2.5 NAAQS must have an FRM or 

FEM operate on a daily schedule if the 

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. 

* * * * * 

17. Section 58.13 is amended by 

adding paragraphs (f) and (g) to read as 

follows: 

§ 58.13 Monitoring network completion. 

* * * * * 

(f) PM2.5 monitors required in nearroad environments as described in 

appendix D to this part, must be 

physically established no later than 

January 1, 2015, and at that time, must 

be operating under all of the 

requirements of this part, including the 

requirements of appendices A, C, D, and 

E to this part. 

(g) CSN (or IMPROVE) monitoring 

stations required as described in 

appendix D to this part not already 

operational, must be physically 

established no later than January 1, 

2015, and at that time must be operating 

under all of the requirements of this 

part, including the requirements of 

appendices A, C, D, and E to this part. 

18. 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 filterbased 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 

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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 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 at the request of 

EPA or to provide information to state 

and local agencies on particulate matter 

composition. 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 

19. 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 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. 

* * * * * 

Subpart D—Comparability of Ambient 

Data to the NAAQS 

20. The heading for Subpart D is 

revised to read as set forth above. 

21. 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. 

(1) PM2.5 measurement data from all 

eligible monitors are compared to the 

24-hour PM2.5 NAAQS. 

(2) PM2.5 measurement data from all 

eligible monitors that are representative 

of area-wide air quality are compared to 

the annual PM2.5 NAAQS. 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. As specified in 

appendix D to this part, section 4.7.1, 

when micro- or middle-scale PM2.5

monitoring sites are presumed to 

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, these sites would be considered 

representative of an area-wide 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 

relatively unique middle-scale impact 

sites are not eligible for comparison to 

the annual PM2.5 NAAQS. As specified 

in § 58.30(a)(1), 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. 

* * * * * 

22. Appendix A to part 58 is amended 

as follows: 

a. By redesignating the existing 

introductory paragraph in section 1 as 

paragraph (c) in section 1 and revising 

it. 

b. By adding paragraph (a) to section 

1. 

c. By adding paragraph (b) to section 

1. 

d. By revising paragraph 1.1.3. 

e. By revising paragraphs 3.2.3, 3.2.4, 

3.2.5.6, and 3.2.6.3. 

f. By adding paragraph 3.2.9. 

g. By revising paragraphs 3.3.2 and 

3.3.3. 

h. By adding paragraph 3.3.9. 

i. By revising paragraphs (b) and (c) in 

section 4. 

j. By adding paragraph (c)(6) in 

section 4. 

k. By revising paragraph 4.3 and 4.3.1. 

l. By revising Tables A–1 and A–2. 

The revisions and additions read as 

follows: 

Appendix A to Part 58—Quality 

Assurance Requirements for SLAMS, 

SPMs and PSD Air Monitoring 

* * * * * 

1. * * * 

(a) For this Appendix, the term ‘‘PM2.5’’ 

refers to PM2.5 mass measurements used in 

determining whether areas meet the primary 

and secondary PM2.5 standards and ‘‘PM2.5

CSN’’ refers to the chemically speciated 

PM2.5 mass measurements used to calculate 

PM2.5 light extinction to determine if areas 

meet the secondary PM standard to address 

visibility impairment. 

(b) 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. Rather, the checks and 

procedures required in this appendix shall be 

used in combination with other data quality 

information, reports, and similar documents 

showing overall compliance with part 58 by 

the monitoring agencies and by EPA, and 

using 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. 

(c) 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, 

PM2.5 CSN, PM10 and PM10-2.5 submitted to 

EPA. This appendix also applies to all SPM 

stations using FRM, FEM, or ARM methods 

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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’’, volume II, part 1 (see reference 10 

of this appendix) and at a national level in 

references 1, 2, and 3 of this appendix. 

* * * * * 

1.1.3 The requirements for precision 

assessment for the automated methods are 

the same for both SLAMS and PSD. However, 

for manual methods, only one collocated site 

is required for PSD. PM2.5 CSN collocation is 

not required for PSD sites. 

* * * * * 

3. * * * 

3.2 * * * 

3.2.3 Flow Rate Verification for 

Particulate Matter. A one-point flow rate 

verification check must be performed at least 

once every month on each automated 

analyzer used to measure PM10, PM10-2.5, 

PM2.5, and PM2.5 CSN. The verification is 

made by checking the operational flow rate 

of the analyzer. If the verification is made in 

conjunction with a flow rate adjustment, it 

must be made prior to such flow rate 

adjustment. Randomization of the flow rate 

verification with respect to time of day, day 

of week, and routine service and adjustments 

is encouraged where possible. For the 

standard procedure, use a flow rate transfer 

standard certified in accordance with section 

2.6 of this appendix to check the analyzer’s 

normal flow rate. Care should be used in 

selecting and using the flow rate 

measurement device such that it does not 

alter the normal operating flow rate of the 

analyzer. Report the flow rate of the transfer 

standard and the corresponding flow rate 

measured by the analyzer. The percent 

differences between the audit and measured 

flow rates are used to assess the bias of the 

monitoring data as described in section 4.2.2 

of this appendix (using flow rates in lieu of 

concentrations). 

3.2.4 Semi-Annual Flow Rate Audit for 

Particulate Matter. Every 6 months, audit the 

flow rate of the PM10, PM10-2.5, PM2.5, and 

PM2.5 CSN particulate analyzers. Where 

possible, EPA strongly encourages more 

frequent auditing. The audit should 

(preferably) be conducted by a trained 

experienced technician other than the 

routine site operator. The audit is made by 

measuring the analyzer’s normal operating 

flow rate using a flow rate transfer standard 

certified in accordance with section 2.6 of 

this appendix. The flow rate standard used 

for auditing must not be the same flow rate 

standard used to calibrate the analyzer. 

However, both the calibration standard and 

the audit standard may be referenced to the 

same primary flow rate or volume standard. 

Great care must be used in auditing the flow 

rate to be certain that the flow measurement 

device does not alter the normal operating 

flow rate of the analyzer. Report the audit 

flow rate of the transfer standard and the 

corresponding flow rate measured (indicated) 

by the analyzer. The percent differences 

between these flow rates described in section 

4.2.3 of this appendix are used to validate the 

one-point flow rate verification checks 

described in section 4.2.2 of this appendix. 

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 of up to 10 meters between a primary 

and collocated sampler may be approved by 

the Regional Administrator for sites at a 

neighborhood or larger scale of 

representation. 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 of up to 10 meters 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. 

Calibration, sampling, and analysis must be 

the same for all the collocated samplers in 

each agency’s network. 

* * * * * 

3.2.9 Collocated Sampling Procedures for 

PM2.5 CSN. PM2.5 CSN Collocation is not 

required for PSD sites. A minimum of six 

collocated sites are required nationally for 

the CSN monitoring network. Sites selected 

for collocation should reflect spatial, 

temporal, and constituent variability of the 

chemical speciation network. Collocated sites 

may be rotated within the network at 3 year 

intervals. Decisions on rotations will be made 

by the Regional Administrator taking into 

consideration geographic coverage, chemical 

species, and capabilities of the monitoring 

agency. Data from the collocated sites will be 

used to estimate precision of the secondary 

PM standard to address visibility 

impairment. For each pair of collocated 

monitors, designate one sampler as the 

primary monitor whose concentrations will 

be used to report air quality for the site, and 

designate the other as the audit monitor. 

3.2.9.1 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. 

Calibration, sampling, and analysis must be 

the same for all the collocated samplers in 

each agency’s network. 

3.2.9.2 Sample the collocated audit 

monitor on a 12-day schedule. Report the 

measurements from both primary and 

collocated audit monitors at each collocated 

sampling site. The calculations for evaluating 

precision between the two collocated 

monitors are described in section 4.3.1 of this 

appendix. 

3.3 * * * 

3.3.2 Flow Rate Verification for 

Particulate Matter. Follow the same 

procedure as described in section 3.2.3 of 

this appendix for PM2.5, PM2.5 CSN, PM10

(low-volume instruments), and PM10-2.5. 

High-volume PM10 and TSP instruments can 

also follow the procedure in section 3.2.3 but 

the audits are required to be conducted 

quarterly. The percent differences between 

the audit and measured flow rates are used 

to assess the bias of the monitoring data as 

described in section 4.2.2 of this appendix. 

3.3.3 Semi-Annual Flow Rate Audit for 

Particulate Matter. Follow the same 

procedure as described in section 3.2.4 of 

this appendix for PM2.5, PM2.5 CSN, PM10, 

PM10-2.5 and TSP instruments. The percent 

differences between these flow rates 

described in section 4.2.3 of this appendix 

are used to validate the one-point flow rate 

verification checks described in section 4.2.2 

of this appendix. 

Great care must be used in auditing highvolume particulate matter samplers having 

flow regulators because the introduction of 

resistance plates in the audit flow standard 

device can cause abnormal flow patterns at 

the point of flow sensing. For this reason, the 

flow audit standard should be used with a 

normal filter in place and without resistance 

plates in auditing flow-regulated highvolume samplers, or other steps should be 

taken to assure that flow patterns are not 

perturbed at the point of flow sensing. 

* * * * * 

3.3.9 Collocated Sampling Procedures for 

PM2.5 CSN. PM2.5 CSN Collocation is not 

required for PSD sites. Follow the same 

procedure as described in Section 3.2.9 

4. * * * 

(b) The EPA will provide annual 

assessments of data quality aggregated by site 

and primary quality assurance organization 

for SO2, NO2, O3 and CO; by primary quality 

assurance organization for PM10, PM2.5, and 

Pb; and by primary quality assurance 

organization and nationally for PM10-2.5, Pb at 

NCore, and PM2.5 CSN. 

(c) At low concentrations, agreement 

between values (measurements or 

calculations) of collocated samplers, 

expressed as relative percent difference or 

percent difference, may be relatively poor. 

For this reason, collocated pairs are selected 

for use in the precision and bias calculations 

only when both values are equal to or above 

the following limits: 

* * * * * 

(6) PM2.5 CSN: 5 deciviews 

* * * * * 

4.3 Statistics for the Assessment of PM2.5, 

PM2.5 CSN, and PM10-2.5

4.3.1 Precision Estimate. Precision for 

collocated instruments for PM2.5, PM2.5 CSN, 

and PM10-2.5 may be estimated where both the 

primary and collocated instruments are the 

same method designation and when the 

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method designations are not similar. Follow 

the procedure described in section 4.2.1 of 

this appendix. In addition, one may want to 

perform an estimate of bias when the primary 

monitor is an FEM and the collocated 

monitor is an FRM. Follow the procedure 

described in section 4.1.3 of this appendix in 

order to provide an estimate of bias using the 

collocated data. 

* * * * * 

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.Same as SLAMS. 

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. 

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 calculations each sampling quarter. 

—Manual ..................................................... By reporting organization—EPA performs calculations annually.By site—source owner/operator performs calculations each sampling quarter. 

TABLE A–2 OF APPENDIX A TO PART 58—MINIMUM DATA ASSESSMENT REQUIREMENTS FOR SLAMS SITES

Method Assessment method Coverage Minimum frequency Parameters reported 

Automated Methods 

1-Point QC for SO2, NO2, O3, 

CO.

Response check at concentration 0.01–0.1 ppm SO2, NO2, 

O3, and 1–10 ppm CO.

Each analyzer .......................... Once per 2 weeks ................... Audit concentration 1 and 

measured concentration 2. 

Annual performance evaluation 

for SO2, NO2, O3, CO.

See section 3.2.2 of this appendix.Each analyzer .......................... Once per year .......................... Audit concentration 1 and 

measured concentration 2 for 

each level. 

Flow rate verification PM10,

PM2.5, PM2.5 CSN PM10-2.5.

Check of sampler flow rate ...... Each sampler ........................... Once every month ................... Audit flow rate and measured 

flow rate indicated by the 

sampler. 

Semi-annual flow rate audit 

PM10, PM2.5, PM2.5 CSN 

PM10-2.5.

Check of sampler flow rate 

using independent standard.

Each sampler ........................... Once every 6 months .............. Audit flow rate and measured 

flow rate indicated by the 

sampler. 

Collocated sampling PM2.5,

PM10-2.5.

Collocated samplers ................ 15% .......................................... Every 12 days .......................... Primary sampler concentration 

and duplicate sampler concentration. 

PM2.5 CSN ............................... Collocated samplers ................ 6 per national network ............. Every 12 days .......................... Primary sampler concentration 

and duplicate sampler concentration. 

Performance evaluation program PM2.5, PM10-2.5.

Collocated samplers ................ 1. 5 valid audits for primary QA 

orgs, with ≤5 sites.

2. 8 valid audits for primary QA 

orgs, with >5 sites.

3. All samplers in 6 years. 

Over all 4 quarters ................... Primary sampler concentration 

and performance evaluation 

sampler concentration. 

Manual Methods 

Collocated sampling PM10,

TSP, PM10-2.5, PM2.5, PbTSP, Pb-PM10.

Collocated samplers ................ 15% .......................................... Every 12 days PSD—every 6 

days.

Primary sampler concentration 

and duplicate sampler concentration. 

PM2.5 CSN ............................... Collocated samplers ................ 6 per network ........................... Every 12 days .......................... Primary sampler concentration 

and duplicate sampler concentration. 

Flow rate verification PM10

(low-vol), PM10-2.5, PM2.5,

PM2.5 CSN, Pb-PM10.

Check of sampler flow rate ...... Each sampler ........................... Once every month ................... Audit flow rate and measured 

flow rate indicated by the 

sampler. 

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TABLE A–2 OF APPENDIX A TO PART 58—MINIMUM DATA ASSESSMENT REQUIREMENTS FOR SLAMS SITES—Continued 

Method Assessment method Coverage Minimum frequency Parameters reported 

Flow rate verification PM10

(high-vol), TSP, Pb-TSP.

Check of sampler flow rate ...... Each sampler ........................... Once every quarter .................. Audit flow rate and measured 

flow rate indicated by the 

sampler. 

Semi-annual flow rate audit 

PM10, TSP, PM10-2.5, PM2.5,

PM2.5 CSN, Pb-TSP, PbPM10.

Check of sampler flow rate 

using independent standard.

Each sampler, all locations ...... Once every 6 months .............. Audit flow rate and measured 

flow rate indicated by the 

sampler. 

Pb audit strips Pb-TSP, PbPM10.

Check of analytical system 

with Pb audit strips.

Analytical .................................. Each quarter ............................ Actual concentration and audit 

concentration. 

Performance evaluation program PM2.5, PM10-2.5.

Collocated samplers ................ 1. 5 valid audits for primary QA 

orgs, with ≤5 sites.

2. 8 valid audits for primary QA 

orgs, with >5 sites.

3. All samplers in 6 years. 

Over all 4 quarters ................... Primary sampler concentration 

and performance evaluation 

sampler concentration. 

Performance evaluation program Pb-TSP, Pb-PM10.

Collocated samplers ................ 1. 1 valid audit and 4 collocated samples for primary 

QA orgs, with >5 sites.

2. 2 valid audits and 6 collocated samples for primary 

QA orgs, with >5 sites.

Over all 4 quarters ................... Primary sampler concentration 

and performance evaluation 

sampler concentration. Primary sampler concentration 

and duplicate sampler concentration. 

1Effective concentration for open path analyzers. 

2 Corrected concentration, if applicable, for open path analyzers. 

* * * * * 

23. Appendix C to part 58 is amended 

as follows: 

a. By revising paragraph 2.9. 

b. In section 6.0 by adding references 

8 through 13. 

Appendix C to Part 58—Ambient Air 

Quality Monitoring Methodology 

* * * * * 

2.9 Use of Chemical Speciation Methods at 

SLAMS 

PM2.5 chemical speciation network (CSN) 

stations include analysis for elements, 

selected anions and cations, and carbon. 

Descriptions of the CSN standard operating 

procedures and QAPP are available in 

references 10 and 11. Interagency Monitoring 

of Protected Visual Environments 

(IMPROVE) station methods also provide 

analysis for elements, selected anions and 

cations, and carbon, and in addition include 

a PM10 mass channel. Descriptions of the 

IMPROVE samplers and the data they collect 

are available in references 4, 5, and 6 of this 

appendix. The CSN Quality Assurance 

Project Plan (QAPP) (which include field 

SOPs), and laboratory SOPs are available in 

references 8 through 13. 

2.9.1 Use of IMPROVE Samplers at a 

SLAMS Site. IMPROVE samplers may be 

used in SLAMS for monitoring of regional 

background and regional transport 

concentrations of fine particulate matter. The 

IMPROVE samplers were developed for use 

in the IMPROVE network to characterize all 

of the major components and many trace 

constituents of the particulate matter that 

impair visibility in Federal Class I Areas. 

2.9.2 Use of CSN or IMPROVE sampling 

methods at a SLAMS site to provide chemical 

species data used in the PM2.5 light 

extinction calculation. Chemical species data 

resulting from CSN or IMPROVE sampling 

methods used at SLAMS are eligible for use 

in the PM2.5 light extinction calculation 

defined in Appendix N to 40 CFR Part 50. 

* * * * * 

6.0 References 

* * * * * 

8. Quality Assurance Project Plan: PM2.5

Chemical Speciation Sampling at Trends, 

NCore, Supplemental and Tribal Sites. Office 

of Air Quality Planning and Standards, 

Research Triangle Park, NC 27711. EPA–454/ 

B–12–003. June 2012. 

9. Standard Operating Procedure for the XRay Fluorescence Analysis of Particulate 

Matter Deposits on Teflon Filters, RTI 

International, Research Triangle Park, NC. 

August 19, 2009. 

10. Standard Operating Procedure for PM2.5

Cation Analysis, RTI International, Research 

Triangle Park, NC. August 25, 2009. 

11. Standard Operating Procedure for PM2.5

Anion Analysis, RTI International, Research 

Triangle Park, NC. August 26, 2009. 

12. Standard Operating Procedure for 

Cleaning Nylon Filters Used for the 

Collection of PM2.5 Material, RTI 

International, Research Triangle Park, NC. 

August 25, 2009. 

13. DRI Standard Operating Procedure #2– 

216r2—DRI Model 2001 Thermal/Optical 

Carbon Analysis (TOR/TOT) of Aerosol Filter 

Samples—Method IMPROVE_A, Reno, NC, 

Revised July 2008. 

24. Appendix D to part 58 is amended as 

follows: 

a. By revising paragraphs 4.7.1(b), 

4.7.1(c)(1), and 4.7.4 

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. * * * 

4.7 * * * 

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 in an area of expected maximum 

concentration. 

(2) For MSAs with a population over 

1,000,000, at least one PM2.5 FRM, FEM, or 

ARM is to be collocated at a near-road NO2

station described in section 4.3.2(a) of this 

appendix. 

(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. 

* * * * * 

4.7.4 PM2.5 Chemical Speciation Site 

Requirements. 

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(a) Each state shall continue to conduct 

chemical speciation monitoring and analysis 

at sites designated to be part of the PM2.5

Speciation Trends Network (STN). The 

selection and modification of these STN sites 

must be approved by the Administrator. The 

PM2.5 chemical speciation urban trends sites 

shall include analysis for elements, selected 

anions and cations, and carbon. Samples 

must be collected using the monitoring 

methods and the sampling schedules 

approved by the Administrator. Chemical 

speciation is encouraged at additional sites 

where the chemically resolved data would be 

useful in developing state implementation 

plans and supporting atmospheric or health 

effects related studies. 

(b) For purposes of supplying chemical 

species data for use in the calculated PM2.5

light extinction indicator, states shall be 

required to operate CSN or IMPROVE 

monitoring stations at SLAMS under the 

following provisions: 

(1) Operation of CSN or IMPROVE 

measurements is only required in states 

having at least one CBSA with a population 

of 1,000,000 or more people; however, 

multiple CBSAs with a population of 

1,000,000 or more people in the same state 

are not each required to have CSN or 

IMPROVE methods operating at SLAMS 

unless specified below. 

(2) The requirement to operate at least one 

CSN or IMPROVE monitoring station in a 

CBSA at a SLAMS shall be considered met 

by any approved NCore or STN station 

operating in a CBSA within the state. 

(3) All CBSAs with a population of 

2,500,000 or more people shall be required to 

have at least one CSN or IMPROVE 

monitoring station at a SLAMS within the 

CBSA; alternatively, the CSN or IMPROVE 

monitoring station may be sited in another 

CBSA adjacent to or downwind of the CBSA 

with a population of 2,500,000 or more 

people, when the alternative CBSA is 

expected to have a higher design value for 

the secondary PM NAAQS for visibility 

impairment. 

(4) When siting additional CSN or 

IMPROVE monitoring equipment at SLAMS, 

the location of the monitoring site can be 

either a representative area-wide location for 

the CBSA or in an area-wide location of 

expected maximum concentration. 

* * * * * 

25. Appendix E to part 58 is amended 

as follows: 

a. By adding paragraph (d) to section 

1. 

b. By adding table E–1 to section 6 

after paragraph (c) introductory text. 

c. By revising table E–4 in section 11. 

Appendix E to Part 58—Probe and 

Monitoring Path Siting Criteria for 

Ambient Air Quality Monitoring 

* * * * * 

1. * * * 

(d) PM2.5 CSN measurement equipment 

sited at SLAMS to provide data for use in the 

calculation for comparison to the secondary 

PM standard to address visibility impairment 

follow the same probe and siting criteria as 

prescribed for PM samplers in this appendix. 

* * * * * 

6. * * * 

TABLE E–1 TO APPENDIX E OF 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 

distance1

(meters) 

Minimum 

distance1 2

(meters) 

≤1,000 ................... 10 10 

10,000 ................... 10 20 

15,000 ................... 20 30 

20,000 ................... 30 40 

40,000 ................... 50 60 

70,000 ................... 100 100 

≥110,000 ............... 250 250 

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. 2Applicable for ozone monitors whose 

placement has not already been approved as 

of December 18, 2006. 

* * * * * 

11. * * * 

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 path 1

(meters) 

Horizontal and vertical 

distance from supporting 

structures 2 to probe, inlet 

or 90% of monitoring 

path1

(meters) 

Distance from trees to 

probe, inlet or 90% of 

monitoring path 1

(meters) 

Distance from roadways 

to probe, inlet or monitoring path 1

(meters) 

SO2 3456 ........................ Middle (300 m) Neighborhood Urban, and Regional (1 km).2–15 ................................ >1 .................................... >10 .................................. N/A. 

CO 457 ............................. Micro, middle (300 m), 

Neighborhood (1 km).

31⁄2: 2–15 ........................ >1 .................................... >10 .................................. 2–10; see Table E–2 of 

this appendix for middle and neighborhood 

scales. 

O3 345 .............................. Middle (300 m) Neighborhood, Urban, and Regional (1 km).2–15 ................................ >1 .................................... >10 .................................. See Table E–1 of this 

appendix for all scales. 

NO2 345 ............................ Micro (Near-road [50– 

300 m]).

2–7 (micro); .................... >1 .................................... >10 .................................. ≤50 meters for near-road 

micro-scale. 

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 precursors (for 

PAMS) 345.

Neighborhood and Urban 

(1 km).

2–15 ................................ >1 .................................... >10 .................................. See Table E–4 of this 

appendix for all scales. 

PM, Pb 34568 ................. Micro, Middle, Neighborhood, Urban and Regional.2–7 (micro); 2–7 (middle 

PM10-2.5); 2–7 for nearroad; 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. 

1 Monitoring 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.

3Should 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. 4 Distance 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). 5 Must 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. 6The 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. 7For micro-scale CO monitoring sites, the probe must be >10 meters from a street intersection and preferably at a midblock location. 

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8 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, unless a waiver is in place as approved by the Regional Administrator. 

26. Appendix G to Part 58 is 

amended: 

a. By revising sections 9 and 10. 

b. By revising paragraph 12.i.a and 

table 2 in 12.i.d. 

c. By revising section 13. 

The revisions read as follows: 

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 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 nonapproved measurements to approved 

methods by statistical linear regression is 

referenced in section 13 of this appendix. 

* * * * * 

12. * * * 

i. * * * 

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 

d. * * * 

TABLE 2—BREAKPOINTS FOR THE AQI 

These breakpoints Equal these AQI’s 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour 1

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—(12.0–13.0) 0–54 0.0–4.4 0–35 0–53 0–50 Good. 

0.060–0.075 ...................... (12.1–13.1)—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 55.5—150.4 255–354 12.5–15.4 186–304 361–649 151–200 Unhealthy. 

0.116–0.374 ...................... 0.205–0.404 150.5—250.4 355–424 15.5–30.4 305–604 650–1249 201–300 Very Unhealthy. 

(2) ...................................... 0.405–0.504 250.5—350.4 425–504 30.5–40.4 605–804 1250–1649 301–400 Hazardous. 

(2) ...................................... 0.505–0.604 350.5—500.4 505–604 40.5–50.4 805–1004 1650–2049 401–500 

1Areas 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. 

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. FitzSimons, 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) Web site, 

http://www.epa.gov/ttnamti1/. 

[FR Doc. 2012–15017 Filed 6–19–12; 4:15 pm] 

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