Tuesday, 

October 17, 2006 

Part II 

Environmental 

Protection Agency 

40 CFR Part 50 

National Ambient Air Quality Standards 

for Particulate Matter; Final Rule 


pwalker on PROD1PC61 with RULES2
61144 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 50 

[EPA–HQ–OAR–2001–0017; FRL–8225–3] 

RIN 2060–AI44 

National Ambient Air Quality 

Standards for Particulate Matter 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: Based on its review of the air 

quality criteria and national ambient air 

quality standards (NAAQS) for 

particulate matter (PM), EPA is making 

revisions to the primary and secondary 

NAAQS for PM to provide increased 

protection of public health and welfare, 

respectively. With regard to primary 

standards for fine particles (generally 

referring to particles less than or equal 

to 2.5 micrometers (μm) in diameter, 

PM2.5), EPA is revising the level of the 

24-hour PM2.5 standard to 35 

micrograms per cubic meter (μg/m3) and 

retaining the level of the annual PM2.5

standard at 15μg/m3. With regard to 

primary standards for particles generally 

less than or equal to 10μm in diameter 

(PM10), EPA is retaining the 24-hour 

PM10 and revoking the annual PM10

standard. With regard to secondary PM 

standards, EPA is making them identical 

in all respects to the primary PM 

standards, as revised. 

DATES: This final rule is effective on 

December 18, 2006. 

ADDRESSES: The EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2001–0017. All 

documents in the docket are listed on 

the www.regulations.gov Web site. 

Although listed in the index, some 

information is not publicly available, 

e.g. confidential business information or 

other information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

is not placed on the Internet and will be 

publicly available only in hard copy 

form. Publicly available docket 

materials are available either 

electronically through 

www.regulations.gov or in hard copy at 

the Air and Radiation Docket and 

Information Center, EPA/DC, EPA West, 

Room B102, 1301 Constitution Ave., 

NW., Washington, DC. This Docket 

Facility is open from 8:30 a.m. to 4:30 

p.m., Monday through Friday, excluding 

legal holidays. The Docket telephone 

number is 202–566–1741. The 

telephone number for the Public 

Reading Room is 202–566–1744. 

The EPA Docket Center suffered 

damage due to flooding during the last 

week of June 2006. The Docket Center 

is continuing to operate. However, 

during the cleanup, there will be 

temporary changes to Docket Center 

telephone numbers, addresses, and 

hours of operation for people who wish 

to visit the Public Reading Room to 

view documents. Consult EPA’s Federal 

Register notice at 71 FR 38147 (July 5, 

2006) or the EPA Web site at 

www.epa.gov/epahome/dockets.htm for 

current information on docket status, 

locations and telephone numbers. 

FOR FURTHER INFORMATION CONTACT: Ms. 

Beth M. Hassett-Sipple, Mail Code 

C504–06, Health and Environmental 

Impacts Division, Office of Air Quality 

Planning and Standards, U.S. 

Environmental Protection Agency, 

Research Triangle Park, North Carolina 

27711, telephone: (919) 541–4605, email: hassett-sipple.beth@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in 

today’s preamble: 

I. Background 

A. Summary of Revisions to the PM 

NAAQS 

B. Legislative Requirements 

C. Overview of Air Quality Criteria and 

Standards Review for PM 

D. Related Control Programs to Implement 

PM Standards 

E. Summary of Proposed Revisions to the 

PM NAAQS 

F. Organization and Approach to Final PM 

NAAQS Decisions 

II. Rationale for Final Decisions on Primary 

PM2.5 Standards 

A. Introduction 

1. Overview 

2. Overview of Health Effects Evidence 

3. Overview of Quantitative Risk 

Assessment 

B. Need for Revision of the Current 

Primary PM2.5 Standards 

1. Introduction 

2. Comments on the Need for Revision 

3. Conclusions Regarding the Need for 

Revision 

C. Indicator for Fine Particles 

D. Averaging Time of Primary PM2.5

Standards 

E. Form of Primary PM2.5 Standards 

1. 24-Hour PM2.5 Standard 

2. Annual PM2.5 Standard 

F. Level of Primary PM2.5 Standards 

1. 24-Hour PM2.5 Standard 

2. Annual PM2.5 Standard 

G. Final Decisions on Primary PM2.5

Standards 

III. Rationale for Final Decisions on Primary 

PM10 Standards 

A. Introduction 

1. Overview 

2. Overview of Health Effects Evidence 

3. Overview of Quantitative Risk 

Assessment 

B. Need for Revision of the Current 

Primary PM10 Standards 

1. Overview of the Proposal 

2. Comments on the Need for Revision 

C. Indicator for Thoracic Coarse Particles 

1. Introduction 

2. Comments on Indicator for Thoracic 

Coarse Particles 

3. Decision Not to Revise PM10 Indicator 

a. Unqualified PM10–2.5 Indicator 

b. PM10 Indicator 

c. Unqualified PM10 Indicator, with 

Adjustment to the PM2.5 Component 

4. Conclusions Regarding Indicator for 

Thoracic Coarse Particles 

D. Conclusions Regarding Averaging Time, 

Form, and Level of the Current PM10

Standards 

1. Averaging Time 

2. Level and Form of the 24-Hour PM10

Standard 

E. Final Decisions on Primary PM10

Standards 

IV. Rationale for Final Decisions on 

Secondary PM Standards 

A. Visibility Impairment 

1. Visibility Impairment Related to 

Ambient PM 

2. Need for Revision of the Current 

Secondary PM2.5 Standards to Protect 

Visibility 

3. Indicator of PM for Secondary Standard 

to Address Visibility Impairment 

4. Averaging Time of a Secondary PM2.5

Standard for Visibility Protection 

5. Final Decisions on Secondary PM2.5

Standards for Visibility Protection 

B. Other PM-Related Welfare Effects 

1. Evidence of Non-Visibility Welfare 

Effects Related to PM 

2. Need for Revision of the Current 

Secondary PM Standards to Address 

Other PM-Related Welfare Effects 

C. Final Decisions on Secondary PM 

Standards 

V. Interpretation of the NAAQS for PM 

A. Amendments to Appendix N— 

Interpretation of the National Ambient 

Air Quality Standards for PM2.5

1. General 

2. PM2.5 Monitoring and Data Reporting 

Considerations 

3. PM2.5 Computations and Data Handling 

Conventions 

4. Conforming Revisions 

B. Proposed Appendix P—Interpretation of 

the National Ambient Air Quality 

Standards for PM10–2.5

C. Amendments to Appendix K— 

Interpretation of the National Ambient 

Air Quality Standards for PM10

VI. Reference Methods for the Determination 

of Particulate Matter as PM10–2.5 and 

PM2.5

A. Appendix O to Part 50—Reference 

Method for the Determination of Coarse 

Particulate Matter as PM10–2.5 in the 

Atmosphere 

B. Amendments to Appendix L—Reference 

Method for the Determination of Fine 

Particulate Matter (as PM2.5) in the 

Atmosphere 

VII. Issues Related to Implementation of PM10

Standards 

A. Summary of Comments Received on 

Transition 


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Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 61145 

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.’’

B. Impact of Decision on PM10

Designations 

C. Impact of Decision on State 

Implementation Plans (SIPs) and Control 

Obligations 

D. Consideration of Fugitive Emissions for 

New Source Review (NSR) Purposes 

E. Handling of PM10 Exceedances Due to 

Exceptional Events 

VIII. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and 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 & 

Safety Risks 

H. Executive Order 13211: Actions that 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer 

Advancement Act 

J. Executive Order 12898: Federal Actions 

to Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

K. Congressional Review Act 

References 

I. Background 

A. Summary of Revisions to the PM 

NAAQS 

Based on its review of the air quality 

criteria and national ambient air quality 

standards (NAAQS) for particulate 

matter (PM), EPA is making revisions to 

the primary and secondary NAAQS for 

PM to provide increased protection of 

public health and welfare, respectively. 

With regard to primary standards for 

fine particles (generally referring to 

particles less than or equal to 2.5 

micrometers (μm) in diameter, PM2.5), 

EPA is revising the level of the 24-hour 

PM2.5 standard to 35 micrograms per 

cubic meter μg/m3), providing increased 

protection against health effects 

associated with short-term exposure 

(including premature mortality and 

increased hospital admissions and 

emergency room visits), and retaining 

the level of the annual PM2.5 standard at 

15 μg/m3, continuing protection against 

health effects associated with long-term 

exposure (including premature 

mortality and development of chronic 

respiratory disease). The EPA is revising 

the form of the annual PM2.5 standard 

with regard to the criteria for spatial 

averaging, such that averaging across 

monitoring sites is allowed if the annual 

mean concentration at each monitoring 

site is within 10 percent of the spatially 

averaged annual mean, and the daily 

values for each monitoring site pair 

yield a correlation coefficient of at least 

0.9 for each calendar quarter. 

With regard to primary standards for 

particles generally less than or equal to 

10μm in diameter (PM10), EPA is 

retaining the 24-hour PM10 standard to 

protect against the health effects 

associated with short-term exposure to 

coarse particles (including hospital 

admissions for cardiopulmonary 

diseases, increased respiratory 

symptoms and possibly premature 

mortality). Given that the available 

evidence does not suggest an association 

between long-term exposure to coarse 

particles at current ambient levels and 

health effects, EPA is revoking the 

annual PM10 standard. 

With regard to secondary PM 

standards, EPA is revising the current 

24-hour PM2.5 secondary standard by 

making it identical to the revised 24- 

hour PM2.5 primary standard, retaining 

the annual PM2.5 and 24-hour PM10

secondary standards, and revoking the 

annual PM10 secondary standard. This 

suite of secondary PM standards is 

intended to provide protection against 

PM-related public welfare effects, 

including visibility impairment, effects 

on vegetation and ecosystems, and 

materials damage and soiling. 

B. Legislative Requirements 

Two sections of the Clean Air Act 

(CAA) govern the establishment and 

revision of the NAAQS. Section 108 (42 

U.S.C. 7408) directs the Administrator 

to identify and list ‘‘air pollutants’’ that 

‘‘in his judgment, may reasonably be 

anticipated to endanger public health 

and welfare’’ and whose ‘‘presence 

* * * in the ambient air results from 

numerous or diverse mobile or 

stationary sources’’ and to issue air 

quality criteria for those that are listed. 

Air quality criteria are intended to 

‘‘accurately reflect the latest scientific 

knowledge useful in indicating the kind 

and extent of identifiable effects on 

public health or welfare which may be 

expected from the presence of [a] 

pollutant in ambient air * * * .’’

Section 109 (42 U.S.C. 7409) directs 

the Administrator to propose and 

promulgate ‘‘primary’’ and ‘‘secondary’’

NAAQS for pollutants listed under 

section 108. 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 include 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. Lead Industries 

Association v. EPA, 647 F.2d 1130, 1154 

(D.C. Cir 1980), cert. denied, 449 U.S. 

1042 (1980); American Petroleum 

Institute v. Costle, 665 F.2d 1176, 1186 

(D.C. Cir. 1981), cert. denied, 455 U.S. 

1034 (1982). 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 include 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 a background concentration level (see 

Lead Industries Association v. EPA, 

supra, 647 F.2d at 1156 n. 51), but 

rather at a level that reduces risk 

sufficiently so as to protect public 

health with an adequate margin of 

safety. 

In addressing the requirement for an 

adequate margin of safety, EPA 

considers such factors as the nature and 

severity of the health effects involved, 

the size of the 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. Lead 


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61146 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 

Industries Association v. EPA, supra, 

647 F.2d at 1161–62. 

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 

establishing primary and secondary 

standards, 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). 

Section 109(d)(1) of the CAA 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 in accordance with [the 

provisions in section 109(b) on primary 

and secondary standards].’’ This 

includes the authority to modify or 

revoke a standard or standards, as 

appropriate under these provisions. 

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 * * *.’’ This independent 

review function is performed by the 

Clean Air Scientific Advisory 

Committee (CASAC) of EPA’s Science 

Advisory Board. 

C. Overview of Air Quality Criteria and 

Standards Review for PM 

Particulate matter is the generic term 

for a broad class of chemically and 

physically diverse substances that exist 

as discrete particles (liquid droplets or 

solids) over a wide range of sizes. 

Particles originate from a variety of 

anthropogenic stationary and mobile 

sources as well as from natural sources. 

Particles may be emitted directly or 

formed in the atmosphere by 

transformations of gaseous emissions 

such as sulfur oxides (SOX), nitrogen 

oxides (NOX), and volatile organic 

compounds (VOC). The chemical and 

physical properties of PM vary greatly 

with time, region, meteorology, and 

source category, thus complicating the 

assessment of health and welfare effects. 

More specifically, the PM that is the 

subject of the air quality criteria and 

standards reviews includes both fine 

particles and thoracic coarse particles, 

which are considered as separate 

subclasses of PM pollution based in part 

on long-established information on 

differences in sources, properties, and 

atmospheric behavior between fine and 

coarse particles (EPA, 2005, section 2.2). 

Fine particles are produced chiefly by 

combustion processes and by 

atmospheric reactions of various 

gaseous pollutants, whereas thoracic 

coarse particles are generally emitted 

directly as particles as a result of 

mechanical processes that crush or 

grind larger particles or the 

resuspension of dusts. Sources of fine 

particles include, for example, motor 

vehicles, power generation, combustion 

sources at industrial facilities, and 

residential fuel burning. Sources of 

thoracic coarse particles include, for 

example, traffic-related emissions such 

as tire and brake lining materials, direct 

emissions from industrial operations, 

construction and demolition activities, 

and agricultural and mining operations. 

Fine particles can remain suspended in 

the atmosphere for days to weeks and 

can be transported thousands of 

kilometers, whereas thoracic coarse 

particles generally deposit rapidly on 

the ground or other surfaces and are not 

readily transported across urban or 

broader areas. 

The last review of PM air quality 

criteria and standards was completed in 

July 1997 with notice of a final decision 

to revise the existing standards (62 FR 

38652, July 18, 1997). In that decision, 

EPA revised the PM NAAQS in several 

respects. While EPA determined that the 

PM NAAQS should continue to focus on 

particles less than or equal to 10 μm in 

diameter (PM10), EPA also determined 

that the fine and coarse fractions of 

PM10 should be considered separately. 

The EPA added new standards, using 

PM2.5 as the indicator for fine particles 

(with PM2.5 referring to particles with a 

nominal aerodynamic diameter less 

than or equal to 2.5 μm), and using PM10

as the indicator for purposes of 

regulating the coarse fraction of PM10

(referred to as thoracic coarse particles 

or coarse-fraction particles; generally 

including particles with a nominal 

aerodynamic diameter greater than 2.5 

μm and less than or equal to 10 μm, or 

PM10–2.5). The EPA established two new 

PM2.5 standards: An annual standard of 

15 μg/m3, based on the 3-year average of 

annual arithmetic mean PM2.5

concentrations from single or multiple 

community-oriented monitors; and a 24- 

hour standard of 65 μg/m3, based on the 

3-year average of the 98th percentile of 

24-hour PM2.5 concentrations at each 

population-oriented monitor within an 

area. Also, EPA established a new 

reference method for the measurement 

of PM2.5 in the ambient air and adopted 

rules for determining attainment of the 

new standards. To continue to address 

thoracic coarse particles, EPA retained 

the annual PM10 standard, while 

revising the form, but not the level, of 

the 24-hour PM10 standard 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, petitions for review were 

filed by a large number of parties, 

addressing a broad range of issues. In 

May 1999, 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 (D.C. Cir. 1999) (‘‘ATA I’’) 

rehearing granted in part and denied in 

part, 195 F.3d 4 (D.C. Cir. 1999) (‘‘ATA 

II’’), 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 that EPA’s 

justification for the use of PM10 as an 

indicator for coarse particles was 

arbitrary. 175 F.3d at 1054–55. Pursuant 

to the court’s decision, EPA removed 

the vacated 1997 PM10 standards from 

the regulations (CFR) (69 FR 45592, July 

30, 2004) and deleted the regulatory 

provision (at 40 CFR 50.6(d)) that 

controlled the transition from the preexisting 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. 

More generally, the panel held (over 

one judge’s dissent) that EPA’s approach 

to establishing the level of the standards 

in 1997, both for PM and for 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 EPA, stating that when EPA 

considers these factors for potential 

non-threshold pollutants ‘‘what EPA 

lacks is any determinate criterion for 


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Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 61147 

3The HEI is a non-profit, independent research 

institute jointly and equally funded by EPA and 

multiple industries that conducts research on the 

health effects of air pollution. 

drawing lines’’ to determine where the 

standards should be set. Consistent with 

EPA’s long-standing interpretation and 

D.C. Circuit precedent, the panel also 

reaffirmed prior rulings holding that in 

setting NAAQS EPA is ‘‘not permitted to 

consider the cost of implementing those 

standards.’’ Id. at 1040–41. 

Both sides filed cross appeals on these 

issues to the United States Supreme 

Court, and the Court 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 (2001). 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 guided 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 

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 traditional 

standard of judicial 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 (D.C. Cir. 

2002) (‘‘ATA III’’). 

In October 1997, EPA published its 

plans for the current periodic review of 

the PM criteria and NAAQS (62 FR 

55201, October 23, 1997), including the 

1997 PM2.5 standards and the 1987 PM10

standards. The approach in this review 

continues to address fine and thoracic 

coarse particles separately. This 

approach has been reinforced by new 

information that has advanced our 

understanding of differences in human 

exposure relationships and dosimetric 

patterns characteristic of these two 

subclasses of PM pollution, as well as 

the apparent independence of health 

effects that have been associated with 

them in epidemiologic studies (EPA, 

2004a, section 3.2.3). See also ATA I, 

175 F. 3d at 1053–54, 1055–56 (EPA 

justified in establishing separate 

standards for fine and thoracic coarse 

particles). 

As part of the process of preparing an 

updated Air Quality Criteria Document 

for Particulate Matter (henceforth, the 

‘‘Criteria Document’’), EPA’s National 

Center for Environmental Assessment 

(NCEA) hosted a peer review workshop 

in April 1999 on drafts of key Criteria 

Document chapters. The first external 

review draft Criteria Document was 

reviewed by CASAC and the public at 

a meeting held in December 1999. Based 

on CASAC and public comment, NCEA 

revised the draft Criteria Document and 

released a second draft in March 2001 

for review by CASAC and the public at 

a meeting held in July 2001. A 

preliminary draft of a staff paper, 

Review of the National Ambient Air 

Quality Standards for Particulate Matter: 

Assessment of Scientific and Technical 

Information (henceforth, the ‘‘Staff 

Paper’’) prepared by EPA’s Office of Air 

Quality Planning and Standards 

(OAQPS) was released in June 2001 for 

public comment and for consultation 

with CASAC at the same public 

meeting. Taking into account CASAC 

and public comments, a third draft 

Criteria Document was released in May 

2002 for review at a meeting held in July 

2002. 

Shortly after the release of the third 

draft Criteria Document, the Health 

Effects Institute (HEI) 3 announced that 

researchers at Johns Hopkins University 

had discovered problems with 

applications of statistical software used 

in a number of important 

epidemiological studies that had been 

discussed in that draft Criteria 

Document. In response to this 

significant issue, EPA took steps in 

consultation with CASAC and the 

broader scientific community to 

encourage researchers to reanalyze 

affected studies and to submit them 

expeditiously for peer review by a 

special expert panel convened at EPA’s 

request by HEI. The results of this 

reanalysis and peer-review process were 

subsequently incorporated into a fourth 

draft Criteria Document, which was 

released in June 2003 and reviewed by 

CASAC and the public at a meeting held 

in August 2003. 

The first draft Staff Paper, based on 

the fourth draft Criteria Document, was 

released at the end of August 2003, and 

was reviewed by CASAC and the public 

at a meeting held in November 2003. 

During that meeting, EPA also consulted 

with CASAC on a new framework for 

the final chapter (integrative synthesis) 

of the Criteria Document and on 

ongoing revisions to other Criteria 

Document chapters to address previous 

CASAC comments. The EPA held 

additional consultations with CASAC at 

public meetings held in February, July, 

and September 2004, leading to 

publication of the final Criteria 

Document in October 2004 (EPA, 

2004a). The second draft Staff Paper, 

based on the final Criteria Document, 

was released at the end of January 2005, 

and was reviewed by CASAC and the 

public at a meeting held in April 2005. 

The CASAC’s advice and 

recommendations to the Administrator, 

based on its review of the second draft 

Staff Paper, were further discussed 

during a public teleconference held in 

May 2005 and are provided in a June 6, 

2005 letter to the Administrator 

(Henderson, 2005a). The final Staff 

Paper takes into account the advice and 

recommendations of CASAC and public 

comments received on the earlier drafts 

of this document. The Administrator 

subsequently received additional advice 

and recommendations from the CASAC, 

specifically on potential standards for 

thoracic coarse particles, in a 

teleconference on August 11, 2005, and 

in a letter to the Administrator dated 

September 15, 2005 (Henderson, 2005b). 

The final Staff Paper was reissued in 

December 2005 to add CASAC’s final 

letter as an attachment (EPA, 2005). 

The schedule for completion of this 

review is governed by a consent decree 

resolving a lawsuit filed in March 2003 

by a group of plaintiffs representing 

national environmental organizations. 

The lawsuit alleged that EPA had failed 

to perform its mandatory duty, under 

section 109(d)(1), of completing the 

current review within the period 

provided by statute. American Lung 

Association v. Whitman (No. 

1:03CV00778, D.D.C. 2003). An initial 

consent decree was entered by the court 

in July 2003 after an opportunity for 

public comment. The consent decree, as 

modified by the court, provides that 

EPA will sign for publication notices of 

proposed and final rulemaking 

concerning its review of the PM NAAQS 

no later than December 20, 2005 and 

September 27, 2006, respectively. 

On December 20, 2005, EPA issued its 

proposed decision to revise the NAAQS 

for PM (71 FR 2620, January 17, 2006) 

(henceforth ‘‘proposal’’). In the 

proposal, EPA identified proposed 

revisions to the standards, based on the 

air quality criteria for PM, and to related 

data handling conventions and federal 

reference methods for monitoring PM. 

The proposal solicited public comments 

on alternative primary and secondary 

standards and related matters. 

The EPA held several public hearings 

across the country to provide direct 

opportunities for public comment on 

the proposed revisions to the PM 

NAAQS. On March 8, 2006, EPA held 

three concurrent 12-hour public 

hearings in Philadelphia, PA; Chicago, 

IL; and San Francisco, CA. At these 

public hearings, EPA heard testimony 


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4For ease of reference, these studies will be 

referred to as ‘‘new’’ studies or ‘‘new’’ science, 

using quotation marks around the word new. 

Referring to studies that were published too 

recently to have been included in the 2004 Criteria 

Document as ‘‘new’’ studies is intended to clearly 

differentiate such studies from those that have been 

published since the last review and are included in 

the 2004 Criteria Document (these studies are 

sometimes referred to as new (without quotation 

marks) or more recent studies, to indicate that they 

were not included in the 1996 Criteria Document 

and thus are newly available in this review). 

from 280 individuals representing 

themselves or specific interested 

organizations. 

More than 120,000 comments were 

received from members of the public 

and various interested groups on the 

proposed revisions to the PM NAAQS 

by the close of the public comment 

period on April 17, 2006. CASAC 

provided additional advice to 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 standards for thoracic coarse particles 

(Henderson, 2006). Major issues raised 

in the public comments are discussed 

throughout the preamble of this final 

action. A comprehensive summary of all 

significant comments, along with EPA’s 

responses (henceforth ‘‘Response to 

Comments’’), can be found in the docket 

for this rulemaking (Docket No. EPA–

HQ–OAR–2001–0017). 

In the proposal, EPA recognized that 

there were a number of new scientific 

studies on the health effects of PM that 

had been published recently and 

therefore were not included in the 

Criteria Document.4 The EPA 

committed to conduct a review and 

assessment of any significant ‘‘new’’

studies, including studies submitted 

during the public comment period. The 

purpose of this review was to ensure 

that the Administrator was fully aware 

of the ‘‘new’’ science before making a 

final decision on whether to revise the 

current PM NAAQS. The EPA screened 

and surveyed the recent literature, 

including studies submitted during the 

public comment period, and conducted 

a provisional assessment (EPA, 2006a) 

that places the results of those studies 

of potentially greatest policy relevance 

in the context of the findings of the 

Criteria Document. 

The provisional assessment found 

that the ‘‘new’’ studies expand the 

scientific information and provide 

important insights on the relationship 

between PM exposure and health effects 

of PM. The provisional assessment also 

found that ‘‘new’’ studies generally 

strengthen the evidence that acute and 

chronic exposure to fine particles and 

acute exposure to thoracic coarse 

particles are associated with health 

effects; some of the ‘‘new’’

epidemiologic studies report effects in 

areas with lower concentrations of PM2.5

or PM10–2.5 than those in earlier reports; 

‘‘new’’ toxicology and epidemiologic 

studies link various health effects with 

a range of fine particle sources and 

components; and ‘‘new’’ toxicology 

studies report effects of thoracic coarse 

particles but do not provide evidence to 

support distinguishing effects from 

exposure to urban and rural particles. 

Further, the provisional assessment 

found that the results reported in the 

studies do not dramatically diverge from 

previous findings, and, taken in context 

with the findings of the Criteria 

Document, the new information and 

findings do not materially change any of 

the broad scientific conclusions 

regarding the health effects of PM 

exposure made in the Criteria 

Document. 

The EPA believes it was important to 

conduct a provisional assessment in this 

case, so that the Administrator would be 

aware of the science that developed too 

recently for inclusion in the Criteria 

Document. However it is also important 

to note that EPA’s review of that science 

to date has been limited to screening, 

surveying, and preparing a provisional 

assessment of these studies. Having 

performed this limited provisional 

assessment, EPA must decide whether 

to consider the newer studies in this 

review and take such steps as may be 

necessary to include them in the basis 

for the final decision, or to reserve such 

action for the next review of the PM 

NAAQS. 

As in prior NAAQS reviews, EPA is 

basing its decision in this review on 

studies and related information 

included in the Criteria Document and 

Staff Paper, which have undergone 

CASAC and public review. The studies 

assessed in the Criteria Document, and 

the integration of the scientific evidence 

presented in that document, have 

undergone extensive critical review by 

EPA, CASAC, and the public during the 

development of the Criteria Document. 

The rigor of that review makes these 

studies, and their integrative 

assessment, the most reliable source of 

scientific information on which to base 

decisions on the NAAQS, decisions that 

all parties recognize as of great import. 

NAAQS decisions can have profound 

impacts on public health and welfare, 

and NAAQS decisions should be based 

on studies that have been rigorously 

assessed in an integrative manner not 

only by EPA but also by the statutorily 

mandated independent advisory 

committee, as well as the public review 

that accompanies this process. As 

described above, the provisional 

assessment did not and could not 

provide that kind of in-depth critical 

review. 

This decision is consistent with EPA’s 

practice in prior NAAQS reviews. Since 

the 1970 amendments, the EPA has 

taken the view that NAAQS decisions 

are to be based on scientific studies and 

related information that have been 

assessed as a part of the pertinent air 

quality criteria. See e.g., 36 FR 8186 

(April 30, 1971) (EPA based original 

NAAQS for six pollutants on scientific 

studies discussed in air quality criteria 

documents and limited consideration of 

comments to those concerning validity 

of scientific basis); 38 FR 25678, 25679–

25680 (September 14, 1973) (EPA 

revised air quality criteria for sulfur 

oxides to provide basis for reevaluation 

of secondary NAAQS). This 

longstanding interpretation was 

strengthened by new legislative 

requirements enacted in 1977, which 

added section 109(d)(2) of the Act 

concerning CASAC review of air quality 

criteria. EPA has consistently followed 

this approach. 52 FR 24634, 24637 (July 

1, 1987) (after review by CASAC, EPA 

issued a post-proposal addendum to the 

PM Criteria Document, to address 

certain new scientific studies not 

included in the 1982 Criteria 

Document); 61 FR 25566, 25568 (May 

22, 1996) (after review by CASAC, EPA 

issued a post-proposal supplement to 

the 1982 Criteria Document to address 

certain new health studies not included 

in the 1982 Criteria Document or 1986 

Addendum). The EPA recently 

reaffirmed this approach in its decision 

not to revise the ozone NAAQS in 1993, 

as well as in its final decision on the PM 

NAAQS in the 1997 review. 58 FR 

13008, 13013–13014 (March 9, 1993) 

(ozone review); 62 FR 38652, 38662 

(July 18, 1997) (The EPA conducted a 

provisional assessment but based the 

final PM decision on studies and related 

information included in the air quality 

criteria that had been reviewed by 

CASAC). 

As discussed in EPA’s 1993 decision 

not to revise the NAAQS for ozone, new 

studies may sometimes be of such 

significance that it is appropriate to 

delay a decision on revision of NAAQS 

and to supplement the pertinent air 

quality criteria so the new studies can 

be taken into account (58 FR at 13013–

13014, March 9, 1993). In the present 

case, the provisional assessment of 

recent studies concludes that, taken in 

context, the new information and 

findings do not materially change any of 

the broad scientific conclusions 

regarding the health effects of PM 

exposure made in the Criteria 


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5The EPA has recently conducted a review of the 

process by which the Agency performs periodic 

NAAQS reviews to identify ways in which the 

process could be strengthened and streamlined 

(EPA, 2006b). The EPA intends to incorporate 

recommendations from the NAAQS process review 

into the next PM NAAQS review. 

Document. For this reason, reopening 

the air quality criteria review would not 

be warranted even if there were time to 

do so under the court order governing 

the schedule for this rulemaking. 

Accordingly, EPA is basing the final 

decisions in this review on the studies 

and related information included in the 

PM air quality criteria that have 

undergone CASAC and public review. 

The EPA will consider the newly 

published studies for purposes of 

decision making in the next periodic 

review of the PM NAAQS, which will 

provide the opportunity to fully assess 

them through a more rigorous review 

process involving EPA, CASAC, and the 

public. 

In order to facilitate a comprehensive 

and timely review of the newly 

available science, the Administrator has 

directed EPA staff to begin the next 

review of the PM NAAQS immediately.5

D. Related Control Programs To 

Implement PM Standards 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once EPA 

has established them. Under section 110 

of the CAA (42 U.S.C. 7410) and related 

provisions, States are to submit, for EPA 

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 EPA, also 

administer the prevention of significant 

deterioration (PSD) program under 

sections 160–169 of the CAA (42 U.S.C. 

7470–7479) for these pollutants. In 

addition, the Act provides for 

nationwide reductions in emissions of 

these and other air pollutants through 

related programs, such as the Federal 

Mobile Source Control Program under 

Title II of the CAA (42 U.S.C. 7521–

7574), which involves controls for 

automobile, truck, bus, motorcycle, 

nonroad and off-highway engines and 

aircraft emissions; the new source 

performance standards under section 

111 (42 U.S.C. 7411); and the national 

emission standards for hazardous air 

pollutants under section 112 (42 U.S.C. 

7412). 

As described in a recent EPA report, 

The Particle Pollution Report: Current 

Understanding of Air Quality and 

Emissions through 2003 (EPA, 2004b), 

State and Federal programs have made 

substantial progress in reducing ambient 

concentrations of PM10 and PM2.5. For 

example, PM10 concentrations have 

decreased 31 percent nationally since 

1988. Regionally, PM10 concentrations 

decreased most in areas with 

historically higher concentrations—the 

Northwest (39 percent decline), the 

Southwest (33 percent decline), and 

southern California (35 percent decline). 

Direct emissions of PM10 have decreased 

approximately 25 percent nationally 

since 1988. 

Programs aimed at reducing direct 

emissions of particles have played an 

important role in reducing PM10

concentrations, particularly in western 

areas. Some examples of PM10 controls 

include paving unpaved roads and 

using best management practices for 

agricultural sources of resuspended soil. 

Of the 87 areas that were designated 

nonattainment for PM10 in the early 

1990s, 64 now meet those standards. In 

cities that have not attained the PM10

standards, the number of days above the 

standards is down significantly. 

Nationally, PM2.5 concentrations have 

declined by 10 percent from 1999 to 

2003. Generally, PM2.5 concentrations 

have also declined the most in regions 

with the highest concentrations—the 

Southeast (20 percent decline), southern 

California (16 percent decline), and the 

Industrial Midwest (9 percent decline). 

With the exception of the Northeast, the 

remaining regions posted modest 

declines in PM2.5 concentrations from 

1999 to 2003. Direct emissions of PM2.5

have decreased by 5 percent nationally 

over the past 5 years. 

National programs that affect regional 

emissions have also contributed to 

lower sulfate concentrations and, 

consequently, to lower PM2.5

concentrations, particularly in the 

Industrial Midwest and Southeast. 

National ozone-reduction programs 

designed to reduce emissions of volatile 

organic compounds (VOCs) and 

nitrogen oxides (NOX) have also helped 

reduce carbon and nitrates, both of 

which are components of PM2.5. 

Additionally, EPA’s Acid Rain Program 

has substantially reduced sulfur dioxide 

(SO2) emissions from power plants since 

1995 in the eastern United States, 

contributing to lower PM 

concentrations. Nationally, SO2

emissions have declined 9 percent, NOX

emissions have declined 9 percent, and 

VOC emissions have declined by 12 

percent from 1999 to 2003. In eastern 

States affected by the Acid Rain 

Program, sulfates decreased 7 percent 

over the same period. 

Over the next 10 to 20 years, national 

and regional regulations will make 

major reductions in ambient PM2.5

levels. The Clean Air Interstate Rule 

(CAIR) and the NOX SIP Call will 

further reduce SO2 and NOX emissions 

from electric generating units and 

industrial boilers across the eastern half 

of the U.S.; regulations to implement the 

1997 ambient air quality standards for 

PM2.5 will require direct PM2.5 and 

PM2.5 precursor controls in 

nonattainment areas; and new national 

mobile source regulations affecting 

heavy-duty diesel engines, highway 

vehicles, and other mobile sources will 

reduce emissions of NOX, direct PM2.5, 

SO2, and VOCs. The EPA estimates that 

these regulations for stationary and 

mobile sources will cut SO2 emissions 

by 6 million tons annually in 2015 from 

2001 levels. Emissions of NOX will be 

cut by 9 million tons annually in 2015 

from 2001 levels. Emissions of VOCs 

will drop by 3 million tons, and direct 

PM2.5 emissions will be cut by 200,000 

tons in 2015, compared to 2001 levels. 

In 2005, 39 nonattainment areas were 

designated as not attaining the PM2.5

standards established in 1997. SIPs for 

these areas are due in April 2008. 

Nonattainment areas are required to 

attain the standards as ‘‘expeditiously as 

practicable’’ based on implementation 

of federal measures already in place and 

the adoption of other reasonable control 

strategies for sources located in the 

nonattainment area and state. The 

presumptive timeframe for attainment is 

within five years of designation, 

although EPA may approve extended 

attainment dates of an additional one to 

five years for areas with more serious 

problems. 

Modeling done by EPA indicates that 

by 2010, 18 of the 39 currently 

designated nonattainment areas are 

projected to come into attainment with 

those standards just based on regulatory 

programs already in place, including 

CAIR, the Clean Diesel Rules, and other 

Federal measures. Between 2010 and 

2015, further reductions in PM 

concentrations in the eastern U.S. are 

projected due to existing federal 

programs alone, on the order of 0.5 to 

1.5 μg/m3. All areas in the eastern U.S. 

will have lower PM2.5 concentrations in 

2015 relative to present-day conditions. 

In most cases, the predicted 

improvement in PM2.5 ranges from 10 

percent to 20 percent. 

E. Summary of Proposed Revisions to 

the PM NAAQS 

For reasons discussed in the proposal, 

the Administrator proposed to revise the 

current primary and secondary PM2.5

and PM10 standards. With regard to the 

primary PM2.5 standards, the 

Administrator proposed to revise the 

level of the 24-hour PM2.5 standard to 35 


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61150 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 

6The CASAC PM Review Panel is comprised of 

the seven members of the chartered CASAC, 

supplemented by fifteen subject-matter experts 

appointed by the Administrator to provide 

additional scientific expertise relevant to this 

review of the PM NAAQS. 

μg/m3, and to revise the form of the 

annual PM2.5 standard by changing the 

constraints on the optional use of spatial 

averaging to include the criterion that 

the minimum correlation coefficient 

between monitor pairs to be averaged be 

0.9 or greater, determined on a seasonal 

basis, and the criterion that differences 

between monitor values not exceed 10 

percent. Related revisions for PM2.5 data 

handling conventions and for the 

reference method for monitoring PM as 

PM2.5 were also proposed. 

With regard to the primary PM10

standards, the Administrator proposed 

to revise the current standards to 

provide more targeted protection from 

thoracic coarse particles that are of 

concern to public health. In part, the 

Administrator proposed to establish a 

new indicator for thoracic coarse 

particles in terms of PM10–2.5, the 

definition of which included 

qualifications that identified both the 

mix of such particles that were 

provisionally determined to be of 

concern to public health, and were thus 

included in the indicator, and those for 

which currently available information 

was provisionally determined to be 

insufficient as a basis from which to 

infer a public health concern, and were 

thus excluded. More specifically, the 

proposed PM10–2.5 indicator was 

qualified so as to include any ambient 

mix of PM10–2.5 that is dominated by 

resuspended dust from high-density 

traffic on paved roads and PM generated 

by industrial sources and construction 

sources, and to exclude any ambient 

mix of PM10–2.5 that is dominated by 

rural windblown dust and soils and PM 

generated by agricultural and mining 

sources. The Administrator also 

proposed that agricultural sources, 

mining sources, and other similar 

sources of crustal material shall not be 

subject to control in meeting the 

proposed standard. The Administrator 

proposed to replace the current primary 

24-hour PM10 standard with a 24-hour 

standard defined in terms of this new 

PM10–2.5 indicator. The proposed new 

standard would be met at an ambient air 

quality monitoring site when the 3-year 

average of the annual 98th percentile 

24-hour average PM10–2.5 concentration 

is less than or equal to 70 μg/m3, which 

would generally maintain the degree of 

public health protection afforded by the 

current PM10 standards from short-term 

exposure to thoracic coarse particles of 

concern. Requirements for monitoring 

sites that would be appropriate for 

determining compliance with this 

proposed PM10–2.5 standard were 

included as part of proposed revisions 

to EPA’s ambient air monitoring 

regulations (see 71 FR 2710, 2736–2728 

and 71 FR 2706–2707 (proposing to 

incorporate these requirements as part 

of the standard)). These proposed 

requirements included a five-part test 

for determining whether a potential 

monitoring site is suitable for 

comparison to the standard, all five 

parts of which had to be met. In 

summary, the suitability test included 

the following general provisions: a 

monitoring site must be within an 

urbanized area that has a population of 

at least 100,000 persons; the site must 

be within a block group with a 

population density greater than 500 

people per square mile; the site must be 

a ‘‘population-oriented’’ site; the site 

may not be adjacent to a large emissions 

source or otherwise within the microscale environment affected by a large 

source; and, if the first four provisions 

are met, a site-specific assessment must 

show that the ambient mix of PM10–2.5

sampled at the site would be dominated 

by resuspended dust from high-density 

traffic on paved roads and PM generated 

by industrial sources and construction 

sources, and would not be dominated by 

rural windblown dust and soils and PM 

generated by agricultural and mining 

sources. Related new PM10–2.5 data 

handling conventions and a new 

reference method for monitoring PM as 

PM10–2.5 were also proposed. The 

Administrator also proposed to revoke 

and not replace the annual PM10

standard. 

With regard to the secondary PM2.5

and PM10 standards, the Administrator 

proposed to revise the current standards 

by making them identical in all respects 

to the proposed primary PM2.5 and 

PM10–2.5 standards to address PM-related 

welfare effects including visibility 

impairment, effects on vegetation and 

ecosystems, materials damage and 

soiling, and effects on climate change. 

F. Organization and Approach to Final 

PM NAAQS Decisions 

This action presents the 

Administrator’s final decisions on the 

review of the current primary and 

secondary PM2.5 and PM10 standards. 

Primary standards for fine particles and 

for thoracic coarse particles are 

addressed below in sections II and III, 

respectively. Consistent with the 

decisions made by EPA in the last 

review and with the conclusions in the 

Criteria Document and Staff Paper, fine 

and thoracic coarse particles continue to 

be considered as separate subclasses of 

PM pollution. Secondary standards for 

fine and thoracic coarse particles are 

addressed below in section IV. Related 

data handling conventions and federal 

reference methods for monitoring PM 

are addressed below in sections V and 

VI, respectively. 

Today’s final decisions separately 

addressing fine and thoracic coarse 

particles are based on a thorough review 

in the Criteria Document of scientific 

information on known and potential 

human health and welfare effects 

associated with exposure to these 

subclasses of PM at levels typically 

found in the ambient air. These final 

decisions also take into account: (1) 

Staff assessments in the Staff Paper of 

the most policy-relevant information in 

the Criteria Document as well as a 

quantitative risk assessment based on 

that information; (2) CASAC advice and 

recommendations, as reflected in its 

letters to the Administrator, its 

discussions of drafts of the Criteria 

Document and Staff Paper at public 

meetings, and separate written 

comments prepared by individual 

members of the CASAC PM Review 

Panel 6 (henceforth, ‘‘CASAC Panel’’); 

(3) public comments received during the 

development of these documents, either 

in connection with CASAC meetings or 

separately; and (4) extensive public 

comments received on the proposed 

rulemaking. 

II. Rationale for Final Decisions on 

Primary PM2.5 Standards 

A. Introduction 

1. Overview 

This section presents the 

Administrator’s final decisions 

regarding the need to revise the current 

primary PM2.5 NAAQS, and, more 

specifically, regarding revisions to the 

level of the 24-hour standard and to the 

form of the annual standard. As 

discussed more fully below, the 

rationale for the final decision on 

appropriate revisions to the primary 

PM2.5 NAAQS includes consideration 

of: (1) Evidence of health effects related 

to short- and long-term exposures to fine 

particles; (2) insights gained from a 

quantitative risk assessment; and (3) 

specific conclusions regarding the need 

for revisions to the current standards 

and the elements of PM2.5 standards 

(i.e., indicator, averaging time, form, 

and level) that, taken together, are 

requisite to protect public health with 

an adequate margin of safety. 

In developing this rationale, EPA has 

drawn upon an integrative synthesis of 

the entire body of evidence on 

associations between exposure to 


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7 ‘‘Confounding’’ occurs when a health effect that 

is caused by one risk factor is attributed to another 

variable that is correlated with the causal risk 

factor; epidemiologic analyses attempt to adjust or 

control for potential confounders (EPA, 2004a, 

section 8.1.3.2; EPA, 2005, section 3.6.4). A 

‘‘threshold’’ is a concentration below which it is 

expected that effects are not observed (EPA, 2004a, 

section 8.4.7; EPA, 2005, section 3.6.6). ‘‘Gaseous 

co-pollutants’’ generally refer to other commonlyoccurring air pollutants, specifically O3, CO, SO2

and NO2. ‘‘Measurement error’’ refers to uncertainty 

in the air quality measurements, while ‘‘exposure 

misclassification’’ includes uncertainty in the use of 

ambient pollutant measurements in characterizing 

population exposures to PM (EPA, 2004a, section 

8.4.5; EPA, 2005, section 3.6.2) 

8 ‘‘Crustal’’ is used here to describe particles of 

geologic origin, which can be found in both fineand coarse-fraction PM. 

ambient fine particles and a broad range 

of health endpoints (EPA, 2004a, 

Chapter 9), focusing on those health 

endpoints for which the Criteria 

Document concluded that the 

associations are likely to be causal. This 

body of evidence includes hundreds of 

studies conducted in many countries 

around the world, using various 

indicators of fine particles. In its 

assessment of the evidence judged to be 

most relevant to decisions on elements 

of the primary PM2.5 standards, 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. 

As with virtually any policy-relevant 

scientific research, there is uncertainty 

in the characterization of health effects 

attributable to exposure to ambient fine 

particles, most generally with regard to 

whether observed associations are likely 

causal in nature and, if so, whether 

there are exposure levels below which 

such associations are no longer likely. 

As discussed below, an unprecedented 

amount of new research has been 

conducted since the last review, with 

important new information coming from 

epidemiologic, toxicologic, controlled 

human exposure, and dosimetric 

studies. Moreover, the newly available 

research studies evaluated in the 

Criteria Document have undergone 

intensive scrutiny through multiple 

layers of peer review, with extended 

opportunities for review and comment 

by CASAC and the public. While 

important uncertainties remain, the 

review of the health effects information 

has been extensive and deliberate. In the 

judgment of the Administrator, this 

intensive evaluation of the scientific 

evidence provides an adequate basis for 

regulatory decision making at this time. 

This review also provides important 

input to EPA’s research plan for 

improving our future understanding of 

the relationships between exposures to 

ambient fine particles and health effects. 

The health effects information and 

quantitative risk assessment were 

summarized in sections II.A and II.B of 

the proposal (71 FR 2626–2641) and are 

only briefly outlined below in sections 

II.A.2 and II.A.3. Subsequent sections of 

this preamble provide a more complete 

discussion of the Administrator’s 

rationale, in light of key issues raised in 

public comments, for concluding that it 

is appropriate to revise the current 

primary PM2.5 standards (section II.B), 

as well as a more complete discussion 

of the Administrator’s rationale for 

retaining or revising the specific 

elements of the primary PM2.5

standards, namely the indicator (section 

II.C); averaging time (section II.D); form 

(section II.E); and level (section II.F). A 

summary of the final decisions on 

revisions to the primary PM2.5 standards 

is presented in section II.G. 

2. Overview of Heath Effects Evidence 

This section briefly outlines the 

information presented in Section II.A of 

the proposal on the health effects 

associated with exposure to fine 

particles. As was true in the last review, 

evidence from epidemiologic studies 

plays a key role in the Criteria 

Document’s evaluation of the scientific 

evidence. Some highlights of the new 

epidemiologic evidence available since 

the last review include: 

(1) New multi-city studies that use 

uniform methodologies to investigate 

the effects of various indicators of PM 

on health with data from multiple 

locations with varying climate and air 

pollution mixes, contributing to 

increased understanding of the role of 

various potential confounders, 

including gaseous co-pollutants, on 

observed associations with fine 

particles. These studies provide more 

precise estimates of the magnitude of an 

effect of exposure to PM, including fine 

particles, than most smaller-scale 

individual city studies. 

(2) More studies of various health 

endpoints evaluating associations 

between effects and exposures to fine 

particles and thoracic coarse particles 

(discussed below in section III), as well 

as ultrafine particles or specific 

components (e.g., sulfates, nitrates, 

metals, organic compounds, and 

elemental carbon) of fine particles. 

(3) Numerous studies of 

cardiovascular endpoints, with 

particular emphasis on assessment of 

cardiovascular risk factors or 

physiological changes. 

(4) Studies relating population 

exposure to fine particles and other 

pollutants measured at centrally located 

monitors to estimates of exposure to 

ambient pollutants at the individual 

level. Such studies have led to a better 

understanding of the relationship 

between ambient fine particle levels and 

personal exposures to fine particles of 

ambient origin. 

(5) New statistical approaches to 

addressing issues related to potential 

confounding by gaseous co-pollutants, 

possible thresholds for effects, and 

measurement error and exposure 

misclassification.7

(6) Efforts to evaluate the effects of 

fine particles from different sources 

(e.g., motor vehicles, coal combustion, 

vegetative burning, crustal 8), using 

factor analysis or source apportionment 

methods with fine particle speciation 

data. 

(7) New ‘‘intervention studies’’

providing evidence for improvements in 

respiratory or cardiovascular health 

with reductions in ambient 

concentrations of particles and gaseous 

co-pollutants. 

In addition, the body of evidence on 

PM-related effects has greatly expanded 

since the last review with findings from 

studies of potential mechanisms or 

pathways by which particles may result 

in the effects identified in the 

epidemiologic studies. These studies 

include important new dosimetry, 

toxicologic and controlled human 

exposure studies, as highlighted below. 

(8) Animal and controlled human 

exposure studies using concentrated 

ambient particles (CAPs), new 

indicators of response (e.g., C-reactive 

protein and cytokine levels, heart rate 

variability), and animal models 

simulating sensitive human 

subpopulations. The results of these 

studies are relevant to evaluation of 

plausibility of the epidemiologic 

evidence and provide insights into 

potential mechanisms for PM-related 

effects. 

(9) Dosimetry studies using new 

modeling methods that provide 

increased understanding of the 

dosimetry of different particle size 

classes and in members of potentially 

sensitive subpopulations, such as 

people with chronic respiratory disease. 

Section II.A of the proposal provides 

a detailed summary of key information 

contained in the Criteria Document 

(EPA, 2004a, Chapters 6–9), and in the 

Staff Paper (EPA, 2005, Chapter 3), on 

the known and potential effects 

associated with exposure to fine 

particles including information on 

specific constituents and information on 

the effects of fine particles in 

combination with other pollutants that 

are routinely present in the ambient air 


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9The EPA continues to support the development 

and application of risk assessment methods with 

the goal of improving the characterization of risks 

and the communication of uncertainties in such 

risk estimates. 

10The risk assessment was discussed in the Staff 

Paper (EPA, 2005, chapter 4) and presented more 

fully in a technical support document, Particulate 

Matter Health Risk Assessment for Selected Urban 

Areas (Abt Associates, 2005). The assessment scope 

and methodology were developed with 

considerable input from the CASAC Panel and the 

public, with CASAC concluding that the general 

assessment methodology and framework were 

appropriate (Hopke, 2002). 

(71 FR 2626–2637). The information 

highlighted there summarizes: 

(1) Multiple biologic mechanisms that 

may be responsible for morbidity/ 

mortality effects associated with 

exposure to ambient fine particles, 

including potential mechanisms or 

pathways related to direct effects on the 

respiratory system, systemic effects that 

are secondary to effects in the 

respiratory system including 

cardiovascular effects, or direct 

cardiovascular effects. 

(2) The nature of the effects that have 

been reported to be associated with fine 

particle exposures including premature 

mortality, aggravation of respiratory and 

cardiovascular disease (as indicated by 

increased hospital admissions and 

emergency department visits), changes 

in lung function and increased 

respiratory symptoms, as well as new 

evidence for more subtle indicators of 

cardiovascular health. 

(3) An integrated evaluation of the 

health effects evidence, with emphasis 

on key issues raised in interpreting 

epidemiological studies, along with 

supporting evidence from experimental 

(e.g., dosimetric and toxicologic) 

studies. 

(4) Sensitive or vulnerable 

subpopulations that appear to be at 

greater risk to such effects, including 

individuals with pre-existing heart and 

lung diseases, older adults, and 

children. 

(5) Conclusions, based on the 

magnitude of these subpopulations and 

risks identified in health studies, that 

exposure to ambient fine particles can 

have substantial public health impacts. 

3. Overview of Quantitative Risk 

Assessment 

In addition to a comprehensive 

evaluation of the health effects evidence 

available in this review, EPA conducted 

a quantitative health risk assessment for 

selected health effects to provide 

additional information and insights that 

can help inform decision making on the 

NAAQS, while recognizing the 

limitations of such an assessment.9 As 

discussed in section II.B of the proposal, 

the approach used to develop 

quantitative risk estimates associated 

with exposures to PM2.5 was built upon 

the more limited risk assessment 

conducted during the last review (61 FR 

65650). The expanded and updated 

assessment conducted in this review 

included estimates of risks of mortality 

(total non-accidental, cardiovascular, 

and respiratory), morbidity (hospital 

admissions for cardiovascular and 

respiratory causes), and respiratory 

symptoms (not requiring 

hospitalization) associated with recent 

short-term (daily) ambient PM2.5 levels 

and risks of total, cardiopulmonary, and 

lung cancer mortality associated with 

long-term exposure to PM2.5 in a number 

of example urban areas.10

The EPA recognized that there were 

many sources of uncertainty and 

variability inherent in the inputs to this 

assessment and that there was a high 

degree of uncertainty in the resulting 

PM2.5 risk estimates. Such uncertainties 

generally relate to a lack of clear 

understanding of a number of important 

factors, including, for example, the 

shape of concentration-response 

functions, particularly when, as here, 

effect thresholds can neither be 

discerned nor determined not to exist; 

issues related to selection of appropriate 

statistical models for the analysis of the 

epidemiologic data; the role of 

potentially confounding and modifying 

factors in the concentration-response 

relationships; issues related to 

simulating how PM2.5 air quality 

distributions will likely change in any 

given area upon attaining a particular 

standard, since strategies to reduce 

emissions are not yet defined; and 

whether there would be differential 

reductions in the many components 

within PM2.5 and, if so, whether this 

would result in differential reductions 

in risk. While some of these 

uncertainties were addressed 

quantitatively in the form of estimated 

confidence ranges around central risk 

estimates, other uncertainties and the 

variability in key inputs were not 

reflected in these confidence ranges, but 

rather were addressed through separate 

sensitivity analyses or characterized 

qualitatively. 

The concentration-response 

relationships used in the assessment 

were based on findings from human 

epidemiological studies that relied on 

fixed-site, population-oriented, ambient 

monitors as a surrogate for actual 

ambient PM2.5 exposures. The risk 

assessment included a series of base 

case estimates that, for example, 

included various cutpoints intended as 

surrogates for alternative assumed 

population thresholds. In its review of 

the Staff Paper and risk assessment, the 

CASAC Panel commented that for the 

purpose of estimating public health 

impacts, it ‘‘favored the primary use of 

an assumed threshold of 10 μg/m3 ’’ and 

that ‘‘a major research need is for more 

work to determine the existence and 

level of any thresholds that may exist or 

the shape of nonlinear concentrationresponse curves at low levels of 

exposure that may exist’’ (Henderson, 

2005a). Other uncertainties were 

addressed in various sensitivity 

analyses (e.g., the use of single-versus 

multi-pollutant models, use of singleversus multi-city models, use of a 

distributed lag model) and had a more 

moderate and often variable impact on 

the risk estimates in some or all of the 

cities. 

Key observations and insights from 

the PM2.5 risk assessment, together with 

important caveats and limitations, were 

discussed in section II.B of the proposal. 

In general, estimated risk reductions 

associated with going from just meeting 

the current suite of PM2.5 standards to 

just meeting alternative suites of annual 

and 24-hour standards for all the 

various assumed cutpoints show 

patterns of increasing estimated risk 

reductions as either the annual or 24- 

hour standard, or both, were reduced 

over the range considered in this 

assessment, and the estimated 

percentage reductions in risk were 

strongly influenced by the assumed 

cutpoint level (see EPA, 2005, Figures 

5–1, 5–2, 5A–1, and 5A–2). In 

comparing the risk estimates for the 

only two specific locations that were 

included in both the prior and current 

assessments, the magnitude of the 

estimates associated with just meeting 

the current annual standard, in terms of 

percentage of total incidence, were very 

similar for mortality associated with 

long-term exposures. Current risk 

estimates for just meeting the current 

suite of PM2.5 standards were similar in 

one of the locations (Philadelphia) and 

somewhat lower in the other location 

(Los Angeles) for mortality associated 

with short-term exposures. 

B. Need for Revision of the Current 

Primary PM2.5 Standards 

1. Introduction 

The initial issue to be addressed in 

the current review of the primary PM2.5

standards is whether, in view of the 

advances in scientific knowledge 

reflected in the Criteria Document and 

Staff Paper, the existing standards 

should be revised. As discussed in 

section II.A of the proposal (71 FR 

2625–2637), the Staff Paper concluded, 

based on the information and 


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11 In addressing this question, the Criteria 

Document had recognized that although there are 

likely biologic threshold levels in individuals for 

specific health responses, the available 

epidemiologic evidence neither supports nor refutes 

the existence of thresholds at the population level 

for the effects of PM2.5 on mortality across the range 

of concentrations in the studies, for either long-term 

or short-term PM2.5 exposures (EPA, 2004a, section 

9.2.2.5). 

conclusions presented in the Criteria 

Document, that while important 

uncertainties and research questions 

remain, much progress has been made 

since the last review in reducing some 

key uncertainties related to our 

understanding of the scientific 

evidence. The newly available 

information generally reinforces and 

provides increased confidence in the 

likely causal nature of the associations 

between short- and long-term exposure 

to PM2.5 and mortality and morbidity 

effects observed in the last review, and 

provides additional information to 

inform judgments as to the extent to 

which such associations likely remain at 

lower exposure levels within the range 

of ambient air quality. 

The examination of short- and longterm exposures to specific components, 

properties, and sources of fine particles 

and mixtures of fine particles with 

gaseous co-pollutants that are linked 

with health effects, and the biological 

mechanisms underlying the observed 

linkages, remain important research 

needs. Other important research needs 

include better characterizing the shape 

of concentration-response functions, 

including identification of potential 

threshold levels, and methodological 

issues such as those associated with 

selecting appropriate statistical models 

in time-series studies to address timevarying factors (such as weather) and 

other factors (such as other pollution 

variables), and better characterizing 

population exposures. 

Nonetheless, important progress has 

been made in advancing our 

understanding of potential mechanisms 

by which ambient PM2.5, alone and in 

combination with other pollutants, is 

causally linked with cardiovascular, 

respiratory, and lung cancer 

associations observed in epidemiologic 

studies. Due to reanalyses and 

extensions of key long-term exposure 

studies, there is now greater confidence 

in the causal nature of associations with 

long-term exposures to fine particles 

than in the last review. There is also an 

increased understanding of the 

populations that are the most 

susceptible to PM2.5-related effects. In 

addition, health effect associations 

reported in epidemiologic studies have 

been found to be generally robust to 

confounding by co-pollutants, 

especially for the more numerous shortterm exposure studies. Further, while 

groups of commenters had differing 

views on the extent to which, if at all, 

newly available evidence increases 

confidence in associations between 

PM2.5 and mortality and morbidity 

effects, and on the extent of progress 

that has been made in reducing 

uncertainties since the last review, 

virtually no commenters argued for any 

relaxation of the current PM2.5

standards. Based on these 

considerations, EPA finds that overall 

the available evidence has increased the 

scientific basis supporting the health 

impacts of exposure to PM2.5, and not 

lessened it, providing clear support for 

fine particle standards that are at least 

as protective as the current PM2.5

standards. 

Having reached this initial 

conclusion, EPA addresses the question 

whether the available evidence supports 

consideration of standards that are more 

protective than the current PM2.5

standards. In considering this question, 

EPA first notes that the current 

standards were set as a suite that 

together would most effectively and 

efficiently protect the public against 

health effects related to both short- and 

long-term exposures to fine particles (62 

FR at 38669). In so doing, the Agency 

set the annual standard to be the 

‘‘generally controlling’’ standard for 

lowering both short- and long-term 

PM2.5 concentrations. In conjunction 

with such an annual standard, the 

current 24-hour standard was set to 

provide only supplemental protection 

against days with high peak PM2.5

concentrations, localized ‘‘hotspots,’’ or 

risks arising from seasonal emissions 

that might not be well controlled by a 

national annual standard. As discussed 

below in section II.F, in considering 

what evidence to use as the basis for the 

1997 annual standard, EPA placed 

greater emphasis on the short-term 

exposure studies, which were judged to 

be the strongest evidence at that time. 

The long-term exposure studies 

available at that time provided only 

supporting evidence for the annual 

standard, which was set primarily based 

on short-term exposure studies. 

In addressing the question whether 

the evidence now available in this 

review supports consideration of 

standards that are more protective than 

the current PM2.5 standards, the Staff 

Paper considered whether (1) 

statistically significant health effects 

associations with short-term exposures 

to fine particles occur in areas that 

would likely meet the current PM2.5

standards, or (2) associations with longterm exposures to fine particles extend 

down to lower air quality levels than 

had previously been observed.11

In considering the available 

epidemiologic evidence in this review 

to address the question of whether more 

protective standards should be 

considered, the Staff Paper took a 

broader approach than was used in the 

last review. This approach reflects the 

more extensive and stronger body of 

evidence now available on health effects 

related to both short- and long-term 

exposure to PM2.5, and places relatively 

greater emphasis on evidence from longterm exposure studies than was done in 

the last review. As discussed below in 

section II.F, this broader approach was 

used at the time of proposal to consider 

the much expanded body of evidence 

from short-term exposure studies as the 

principal basis for setting the 24-hour 

standard to protect against health effects 

associated with short-term exposures to 

PM2.5, and to consider the stronger and 

more robust body of evidence from longterm exposure PM2.5 studies as the 

principal basis for setting the annual 

standard to protect against health effects 

associated with long-term exposures to 

PM2.5. 

In first considering whether areas in 

which short-term exposure studies have 

been conducted would likely meet the 

current PM2.5 standards, the focus is 

principally on comparing the long-term 

average PM2.5 concentration in a study 

area with the level of the current 

‘‘generally controlling’’ annual PM2.5

standard. In considering the available 

epidemiologic evidence related to shortterm exposures, the Staff Paper focused 

on specific epidemiologic studies that 

show statistically significant 

associations between PM2.5 and health 

effects for which the Criteria Document 

judged associations with PM2.5 to be 

likely causal (EPA, 2005, section 

5.3.1.1). Many more U.S. and Canadian 

studies are now available that provide 

evidence of associations between shortterm exposure to PM2.5 and serious 

health effects in areas with air quality at 

and above the level of the current 

annual PM2.5 standard (15 μg/m3). 

Moreover, a few newly available shortterm exposure mortality studies provide 

evidence of statistically significant 

associations with PM2.5 in areas with air 

quality levels below the levels of the 

current PM2.5 standards. In considering 

these studies, the Staff Paper focused on 

those that include adequate gravimetric 

PM2.5 mass measurements, and noted 

where the associations are generally 

robust to alternative model specification 

and to the inclusion of potentially 

confounding co-pollutants. Three 


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12As noted in the Staff Paper, these studies were 

reanalyzed to address questions about the 

application of the statistical software used in the 

original analyses, and the study results from 

Phoenix and Santa Clara County were little changed 

in alternative models (Mar et al., 2003; Fairley, 

2003), although Burnett and Goldberg (2003) 

reported that their results were sensitive to using 

different temporal smoothing methods. Two of 

these studies also reported significant associations 

with gaseous pollutants (Mar et al., 2003; Fairley, 

2003), and one of these studies included multipollutant model results in reanalyses, reporting that 

associations with PM2.5 remained significant with 

gaseous pollutants (Fairley, 2003). The 98th 

percentile 24-hour concentrations were 

approximately 59 μg/m3 in Fairley et al. (2003), 39 

μg/m3 in Burnett and Goldberg (2003), and 32 μg/ 

m3 in Mar et al. (2003). 

13As noted in section II.B of the proposal, the 

reported linear or log-linear concentration-response 

functions were applied down to 7.5 μg/m3 in 

estimating risk associated with long-term exposure 

(i.e., the lowest measured level in the extended ACS 

study), and down to the estimated policy-relevant 

background level in estimating risk associated with 

short-term exposure (i.e., 3.5 μg/m3 for eastern 

urban areas and 2.5 μg/m3 for western urban areas). 

studies, conducted in Phoenix (Mar et 

al., 2003), Santa Clara County, CA 

(Fairley, 2003) and eight Canadian cities 

(Burnett and Goldberg, 2003), report 

statistically significant associations 

between short-term PM2.5 exposure and 

total or cardiovascular mortality in areas 

in which long-term average PM2.5

concentrations ranged between 13 and 

14 μg/m3 and 98th percentile 24-hour 

concentrations ranged between 32 and 

59 μg/m3.12

In also considering the new 

epidemiologic evidence available from 

U.S. and Canadian studies of long-term 

exposure to fine particles, the Criteria 

Document noted that new studies have 

built upon studies available in the last 

review and concluded that these studies 

have confirmed and strengthened the 

evidence of associations for both 

mortality and respiratory morbidity 

(EPA, 2004a, section 9.2.3). For 

mortality, the Criteria Document placed 

greatest weight on the reanalyses and 

extensions of the Six Cities and ACS 

studies, finding that these studies 

provide strong evidence for associations 

with fine particles (EPA, 2004a, p. 9–

34), notwithstanding the lack of 

consistent results in other long-term 

exposure studies. For morbidity, the 

Criteria Document found that new 

studies of a cohort of children in 

Southern California have built upon 

earlier limited evidence to provide fairly 

strong evidence that long-term exposure 

to fine particles is associated with 

development of chronic respiratory 

disease and reduced lung function 

growth (EPA, 2004a, pp. 9–33 to 9–34). 

In addition to strengthening the 

evidence of association, the new 

extended ACS mortality study (Pope et 

al., 2002) observed statistically 

significant associations with 

cardiorespiratory mortality (including 

lung cancer mortality) across a range of 

long-term mean PM2.5 concentrations 

that was lower than was reported in the 

original ACS study available in the last 

review. 

Beyond the epidemiologic studies 

using PM2.5 as an indicator of fine 

particles, a large body of newly 

available evidence from studies that 

used PM10 in areas where fine particles 

would likely dominate this 

measurement, as well as other 

indicators or components of fine 

particles (e.g., sulfates, combustionrelated components), provides 

additional support for the conclusions 

reached in the last review as to the 

likely causal role of ambient PM, and 

the likely importance of fine particles in 

contributing to observed health effects. 

Such studies notably include new 

multi-city studies, intervention studies 

(that relate reductions in ambient PM to 

observed improvements in respiratory 

or cardiovascular health), and sourceoriented studies (e.g., suggesting 

associations with combustion- and 

vehicle-related sources of fine particles). 

The Criteria Document also noted that 

new epidemiologic studies of asthmarelated increased physician visits and 

symptoms, as well as new studies of 

cardiac-related risk factors, suggest 

likely much larger public health impacts 

due to ambient fine particles than just 

those indexed by the mortality and 

morbidity effects considered in the last 

review (EPA, 2004a, p. 9–94). 

In reviewing this information, the 

Staff Paper recognized that important 

limitations and uncertainties associated 

with this expanded body of evidence for 

PM2.5 and other indicators or 

components of fine particles need to be 

carefully considered in determining the 

weight to be placed on the body of 

studies available in this review. For 

example, the Criteria Document noted 

that although PM-effects associations 

continue to be observed across most 

new studies, the newer findings do not 

fully resolve the extent to which the 

associations are properly attributed to 

PM acting alone or in combination with 

other gaseous co-pollutants or to the 

gaseous co-pollutants themselves. The 

Criteria Document concluded, however, 

that overall the newly available 

epidemiologic evidence, especially for 

the more numerous short-term exposure 

studies, substantiates that associations 

for various PM indicators with mortality 

and morbidity are robust to confounding 

by co-pollutants (EPA, 2004a, p. 9–37). 

While the limitations and 

uncertainties in the available evidence 

suggest caution in interpreting the 

epidemiologic studies at the lower 

levels of air quality observed in the 

studies, the Staff Paper concluded that 

the evidence now available provides 

strong support for considering fine 

particle standards that would provide 

increased protection beyond that 

afforded by the current PM2.5 standards. 

The Staff Paper noted that a more 

protective suite of PM2.5 standards 

would reflect the generally stronger and 

broader body of evidence of associations 

with mortality and morbidity now 

available in this review, both in shortterm exposue studies at levels below the 

current standards and in long-term 

exposure studies that extend to lower 

levels of air quality than in earlier 

studies, as well as increased 

understanding of possible underlying 

mechanisms. 

In addition to this evidence-based 

evaluation, the Staff Paper also 

considered the extent to which health 

risks estimated to occur upon 

attainment of 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, noted 

above in section II.A.3. In so doing, the 

Staff Paper first noted that the risk 

assessment addressed several key 

uncertainties through various base case 

analyses, as well as through sensitivity 

analyses, as noted above in section 

II.A.3 and discussed in section II.B of 

the proposal (71 FR 2637–2641). In 

considering the health risks estimated to 

occur upon attainment of the current 

PM2.5 standards, the Staff Paper focused 

in particular on a series of base case risk 

estimates, while recognizing that the 

confidence ranges in the selected base 

case estimates do not reflect all the 

identified uncertainties. These risks 

were estimated using not only the linear 

or log-linear concentration-response 

functions reported in the studies,13 but 

also using alternative modified linear 

functions as surrogates for assumed 

non-linear functions that would reflect 

the possibility that thresholds may exist 

in the reported associations within the 

range of air quality observed in the 

studies. Regardless of the relative 

weight placed on the risk estimates 

associated with the concentrationresponse functions reported in the 

studies or with the modified functions 

favored by CASAC (discussed above in 

section II.A.3), the risk assessment 

indicated the possibility that thousands 

of premature deaths per year would 

occur in urban areas across the U.S. 

upon attainment of the current PM2.5


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14The Staff Paper recognized how highly 

dependent any specific risk estimates are on the 

assumed shape of the underlying concentrationresponse functions, noting nonetheless that 

mortality risks are not completely eliminated when 

current PM2.5 standards are met in a number of 

example urban areas even using the highest 

assumed cutpoint levels considered in the risk 

assessment (EPA, 2005, p. 5–15). 

15Of the individual Panel members who 

submitted written comments expressing views on 

appropriate levels of the PM2.5 standards, only one 

did not support changes to either the 24-hour or 

annual standard to provide additional public health 

protection (Henderson, 2005a). 

16The EPA notes that this increased confidence 

in the long- and short-term associations generally 

reflects less uncertainty as to the likely causal 

nature of such associations, but does not address 

directly the question of the extent to which such 

associations remain toward the lower end of the 

range of ambient PM2.5 concentrations. This 

question is central to the Agency’s evaluation of the 

relevant evidence to determine appropriate 

standards levels, as discussed below in section II.F. 

standards.14 Beyond the estimated 

incidences of premature mortality, the 

Staff Paper also recognized that 

similarly substantial numbers of 

incidences of hospital admissions, 

emergency room visits, aggravation of 

asthma and other respiratory symptoms, 

and increased cardiac-related risk are 

also likely in many urban areas, based 

on risk assessment results (EPA, 2005, 

Chapter 4) and on the discussion related 

to this ‘‘pyramid of effects’’ in the 

Criteria Document (EPA, 2004a, section 

9.2.5). Based on these considerations, 

the Staff Paper concluded that the 

estimates of risks likely to remain upon 

attainment of the current PM2.5

standards are indicative of risks that can 

reasonably be judged to be important 

from a public health perspective (EPA, 

2005, section 5.3.1.). 

In considering available evidence, risk 

estimates, and related limitations and 

uncertainties, the Staff Paper concluded 

that the available information clearly 

calls into question the adequacy of the 

current suite of PM2.5 standards and 

provides strong support for revising the 

current suite of PM2.5 standards to 

provide increased public health 

protection. Also, taking into account 

these considerations, the CASAC 

advised the Administrator that a 

majority of CASAC Panel members were 

in agreement that the primary 24-hour 

and annual PM2.5 standards ‘‘should be 

modified to provide increased public 

health protection’’ (Henderson, 2005a). 

The CASAC further advised that 

changes to either the annual standard or 

the 24-hour standard, or both, could be 

recommended, and expressed reasons 

that formed the basis for the consensus 

among the Panel members for placing 

more emphasis on lowering the 24-hour 

standard (Henderson, 2005a).15

At the time of proposal, in 

considering whether the suite of PM2.5

standards should be revised to provide 

requisite public health protection, the 

Administrator carefully considered the 

rationale and recommendations 

contained in the Staff Paper, the advice 

and recommendations from CASAC, 

and public comments to date on this 

issue. In so doing, the Administrator 

placed primary consideration on the 

evidence obtained from the studies, and 

provisionally found the evidence of 

serious health effects reported in shortterm exposure studies conducted in 

areas that would attain the current 

standards to be compelling, especially 

in light of the extent to which such 

studies are part of an overall pattern of 

positive and frequently statistically 

significant associations across a broad 

range of studies that collectively 

represent a strong and robust body of 

evidence. As discussed in the Criteria 

Document and Staff Paper, the 

Administrator recognized that much 

progress has been made since the last 

review in addressing some of the key 

uncertainties that were important 

considerations in establishing the 

current suite of PM2.5 standards. For 

example, progress made since the last 

review provides increased confidence in 

the long-term exposure studies as a 

basis for considering whether any 

revision of the annual standard is 

appropriate and increased confidence in 

the short-term exposure studies as a 

basis for considering whether any 

revision of the 24-hour standard is 

appropriate.16 In considering the risk 

assessment presented in the Staff Paper, 

the Administrator noted that the 

assessment contained a sensitivity 

analysis but not a formal uncertainty 

analysis, making it difficult to use the 

risk assessment to form a judgment of 

the probability of various risk estimates. 

Instead, the Administrator viewed the 

risk assessment in light of his evaluation 

of the underlying studies. Seen in this 

light, the risk assessment informs the 

determination of the public health 

significance of risks to the extent that 

the evidence is judged to support an 

effect at a particular level of air quality. 

Based on these considerations, the 

Administrator provisionally concluded 

that the current 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. 

2. Comments on the Need for Revision 

General comments based on relevant 

factors that either support or oppose any 

change to the current suite of PM2.5

primary standards are addressed in this 

section. Comments on specific shortand long-term exposure studies that 

relate to consideration of the 

appropriate levels of the 24-hour and 

annual PM2.5 standards are addressed 

below in sections II.F.1 and II.F.2, 

respectively. General comments based 

on implementation-related factors that 

are not a permissible basis for 

considering the need to revise the 

current standards are addressed in the 

Response to Comments document. 

Many public comments received on 

the proposal asserted that the current 

PM2.5 standards are insufficient to 

protect public health with an adequate 

margin of safety and revisions to the 

standards are appropriate. Among those 

calling for revisions to the current 

standards are medical groups, including 

the American Medical Association, the 

American Thoracic Society, the 

American Academy of Pediatrics, and 

the American College of Cardiology, as 

well as medical doctors and academic 

researchers. For example, the American 

Medical Association stated that PM air 

pollution is ‘‘a national public health 

problem’’ and supported more stringent 

standards based on studies that provide 

evidence of associations between PM2.5

and serious health effects in areas with 

PM2.5 concentrations that are below the 

1997 standards. Other medical 

associations offered the following views 

in support of more protective standards: 

As professional organizations that represent 

physicians treating patients with diseases 

either caused by or exacerbated by air 

pollution, we are keenly aware of the impact 

air quality has on the individual health of our 

patients. As such we are committed to 

supporting a standard for PM that is 

protective of the health of vulnerable 

populations including children, seniors and 

patients with respiratory and cardiac 

conditions * * *. In short, a significant body 

of research has described potential 

mechanisms for and the range of health 

effects caused by PM air pollution. The 

undersigned physician organizations find the 

body of scientific evidence to be rigorous, 

comprehensive and compelling enough to 

justify a significant tightening of the existing 

NAAQS PM standards. [American Thoracic 

Society et al.] 

In a letter signed from environmental 

health researchers and physicians, 

similar conclusions were drawn: 

More than 2,000 peer-reviewed studies have 

been published since 1996 * * *. These 

studies, as discussed and interpreted in the 

2004 EPA Criteria Document, validate earlier 

epidemiologic studies linking both acute and 

chronic fine particle pollution with serious 

morbidity and mortality. The newer research 

has also expanded the list of health effects 

associated with PM, and has identified health 

effects at lower exposure levels than 


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previously reported. In fact, the science is 

now sufficiently strong that it is appropriate 

to conclude that PM2.5 is causally associated 

with numerous adverse health effects in 

humans, at exposure levels far below the 

current standards. [Schwartz et al., 2005] 

Similar conclusions were also reached 

in comments by many national, state, 

and local public health organizations, 

including, for example, the American 

Lung Association, the American Heart 

Association, the American Cancer 

Society, the American Public Health 

Association, and the National 

Association of Local Boards of Health, 

as well as in letters to the Administrator 

from EPA’s advisory panel on children’s 

environmental health (Children’s Health 

Protection Advisory Committee, 2005, 

2006). All of these medical and public 

health commenters stated that the 

current PM2.5 standards need to be 

revised, and that even more protective 

standards than those proposed by EPA 

are needed to protect the health of 

sensitive population groups. Many 

individual commenters also expressed 

such views. 

State and local air pollution control 

authorities who commented on the 

PM2.5 standards supported revision of 

the suite of current PM2.5 standards, as 

did the National Tribal Air Association. 

The State and Territorial Air Pollution 

Program Administrators and the 

Association of Local Air Pollution 

Control Officials (STAPPA/ALAPCO) 

urged that EPA revise the PM2.5

standards in accordance with the 

recommendations of CASAC. Each of 

the individual State environmental/ 

public health agencies that commented 

on the PM2.5 standards supported 

revisions to the current standards, with 

most supporting standards consistent 

with CASAC’s recommendations. The 

Northeast States for Coordinated Air 

Use Management (NESCAUM) argued 

for even more stringent revisions to the 

standards. 

The commenters noted above 

primarily based their views on the body 

of evidence assessed in the Criteria 

Document, finding it to be stronger and 

more compelling than in the last review. 

These commenters generally placed 

much weight on CASAC’s interpretation 

of the body of available evidence and 

the results of EPA’s risk assessment, 

both of which formed the basis for 

CASAC’s recommendation to revise the 

PM2.5 standards to provide increased 

public health protection was based. 

Some of these commenters 

specifically mentioned the independent 

reanalysis of the original ACS and Six 

Cities long-term exposure studies 

conducted by HEI (Krewski et al., 2000) 

that concluded that the original data 

were of high quality, the original results 

could be fully replicated, and the results 

were robust to alternative model 

specifications. Some also mentioned the 

ACS extended study (Pope et al., 2002) 

and the Southern California children’s 

cohort study (Gauderman et al., 2002) as 

providing evidence of mortality and 

morbidity effects associated with longterm exposures to PM2.5 at lower levels 

than had previously been studied. A 

number of short-term exposure studies 

were also cited by some of these 

commenters as providing evidence of 

mortality and morbidity effects at levels 

well below the level of the current 24- 

hour PM2.5 standard. In addition, many 

of these commenters generally 

concluded that progress had been made 

in reducing many of the uncertainties 

identified in the last review and in 

better understanding mechanisms by 

which PM2.5 may be causing the 

observed health effects. 

Some of these commenters also noted 

the results of EPA’s risk assessment, 

concluding that it showed that the risks 

estimated to remain when the current 

standards are met are large and 

important from a public health 

perspective and warrant increased 

protection. Some of these commenters 

expressed the view that PM2.5-related 

risks are likely larger than those 

estimated in EPA’s risk assessment, in 

part because EPA based its risk 

assessment on the ACS extended study 

which had greater exposure 

measurement error than other studies, 

leading to an underestimate of the 

relative risk, and because EPA 

incorporated an assumed ‘‘cutpoint’’ in 

its assessment that is not supported by 

studies that find no evidence of a 

threshold. 

In general, all of these commenters 

agreed on the importance of results from 

the large body of scientific studies 

reviewed in the Criteria Document and 

on the need to revise the suite of PM2.5

standards as articulated in EPA’s 

proposal, while generally differing with 

EPA’s proposed judgments about the 

extent to which the standards should be 

revised based on this evidence. The EPA 

generally agrees with these commenters’

conclusion regarding the need to revise 

the current suite of PM2.5 standards. The 

scientific evidence noted by these 

commenters was generally the same as 

that assessed in the Criteria Document 

and the Staff Paper, and EPA agrees that 

this evidence provides a basis for 

concluding that the current PM2.5

standards, taken together, are not 

adequately protective of public health. 

For reasons discussed below in section 

II.F, however, EPA disagrees with 

aspects of these commenters’ views on 

the level of protection that is 

appropriate and supported by the 

available scientific information. 

Some of these commenters also 

identified ‘‘new’’ studies that were not 

included in the Criteria Document as 

providing further support for the need 

to revise the PM2.5 standards. As 

discussed above in section I.C, EPA 

notes that, as in past NAAQS reviews, 

the Agency is basing the final decisions 

in this review on the studies and related 

information included in the PM air 

quality criteria that have undergone 

CASAC and public review, and will 

consider the newly published studies 

for purposes of decision making in the 

next PM NAAQS review. Nonetheless, 

in provisionally evaluating commenters’

arguments (see Response to Comments 

document), EPA notes that its 

provisional assessment of ‘‘new’’

science found that such studies did not 

materially change the conclusions in the 

Criteria Document. 

Another group of commenters 

representing industry associations and 

businesses opposed revising the current 

PM2.5 standards. These views are most 

extensively presented in comments from 

the Utility Air Regulatory Group 

(UARG), representing a group of electric 

generating companies and organizations 

and several national trade associations, 

and from Pillsbury, Winthrop, Shaw 

and Pittman (Pillsbury et al.) on behalf 

of 19 industry and business associations 

(including, for example, the Alliance of 

Automobile Manufacturers, the 

American Iron and Steel Institute, the 

National Association of Manufacturers, 

the American Petroleum Institute, and 

the U.S. Chamber of Commerce). 

These and other commenters in this 

group generally mentioned many of the 

same studies that were cited by the 

commenters who supported revising the 

standards, as well as other studies, but 

highlighted different aspects of these 

studies in reaching substantially 

different conclusions about their 

strength and the extent to which 

progress has been made in reducing 

uncertainties in the evidence since the 

last review. These commenters generally 

expressed the view that the current 

standards provide the requisite degree 

of public health protection. They then 

considered whether the evidence that 

has become available since the last 

review has established a more certain 

risk or a risk of effects that are 

significantly different in character to 

those that provided a basis for the 

current standards, or whether the 

evidence demonstrates that the risk to 

public health upon attainment of the 

current standards would be greater than 


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17As noted above, this increased confidence in 

the long- and short-term associations generally 

reflects less uncertainty as to the likely causal 

nature of such associations, but does not address 

directly the question of the extent to which such 

associations remain toward the lower end of the 

range of ambient PM2.5 concentrations. This 

question is central to the Agency’s evaluation of the 

relevant evidence to determine appropriate 

standards levels, as discussed below in section II.F. 

was understood when EPA established 

the current standards in 1997. 

In supporting their view that the 

present suite of primary PM2.5 standards 

continues to provide the requisite 

public health protection and should not 

be revised, UARG and others generally 

stated: (1) That the effects of concern 

have not changed significantly since 

1997; (2) that the uncertainties in the 

underlying health science are as great or 

greater than in 1997; (3) that the 

estimated risk upon attainment of the 

current PM2.5 standards has decreased 

since 1997; and (4) that ‘‘new’’ studies 

not included in the Criteria Document 

continue to increase uncertainty about 

possible health risks associated with 

exposure to PM2.5. These comments are 

discussed in turn below. 

(1) In asserting that effects of concern 

have not changed significantly since 

1997, some of these commenters stated 

that more subtle physiological changes 

in the cardiovascular system are the 

only type of new PM-related effect 

identified in this review. They stated 

that such subtle effects are far less 

serious than the cardiovascular effects 

such as aggravation of cardiovascular 

disease that had been considered in the 

last review. The EPA disagrees with the 

assertion that subtle changes in the 

cardiovascular system are the only type 

of new PM-related effect identified in 

this review. Further, EPA believes that 

evidence of physiological changes in the 

cardiovascular system is important in 

that it increases confidence in 

inferences about the causal nature of the 

associations between fine particles and 

cardiovascular-related mortality and 

hospital admissions. 

As discussed in the Criteria Document 

(EPA, 2004a, p. 9–75), epidemiologic 

studies published since the last review 

have expanded upon and extended the 

evidence examining possible links 

between long-term exposures to fine 

particles and increased risk of lung 

cancer incidence and mortality, which 

was considered to be insufficient to 

support such a linkage in the last 

review. In this review, however, the 

epidemiologic evidence now available 

‘‘support(s) an association between 

long-term exposure to fine particles and 

lung cancer mortality; and the new 

toxicological studies provide credible 

evidence for the biological plausibility 

of these associations’’ (EPA, 2004a, p. 9–

76). More specifically, the Criteria 

Document highlighted ‘‘the newer 

results of the extension of the ACS 

study analyses (that include more years 

of participant follow-up and address 

previous criticisms of the earlier ACS 

analyses), which indicate that long-term 

ambient PM exposures are associated 

with increased risk of lung cancer. That 

increased risk appears to be in about the 

same range as that seen for a nonsmoker 

residing with a smoker, with any 

consequent life-shortening due to lung 

cancer’’ (EPA 2004a, p. 9–94). 

In addition, as noted earlier, the 

Criteria Document identified increased 

nonhospital medical visits (physician 

visits) and aggravation of asthma 

associated with short-term exposure to 

PM2.5 as being newly identified effects 

since the last review, and concluded 

that findings of such effects ‘‘suggest 

likely much larger health impacts and 

costs to society due to ambient PM than 

just those indexed either by just hospital 

admissions/visits and/or mortality.’’ Id. 

Further, the Criteria Document (EPA, 

2004a, p. 9–79) noted that there may be 

PM-related health effects in infants and 

children, although only very limited 

evidence of such effects exists. 

(2) In asserting that the uncertainties 

in the underlying health science are as 

great or greater than in 1997, 

commenters in this group variously 

discussed a number of issues including: 

The lack of demonstrated mechanisms 

by which PM2.5 may be causing 

mortality and morbidity effects; 

uncertainty in the shape of the 

concentration-response functions; the 

potential for co-pollutant confounding; 

uncertainty in the role of individual 

constituents of fine particles; and the 

sensitivity of epidemiological results to 

statistical model specification. Each of 

these issues is addressed below. In 

summary, these commenters concluded 

that the substantial uncertainties 

present in the last review have not been 

resolved, that a previously unrecognized 

sensitivity to model specification has 

been newly identified, and/or that the 

uncertainty about the possible health 

risks associated with PM2.5 exposure has 

not diminished. As discussed below, 

although EPA agrees that important 

uncertainties remain, and that future 

research directed toward addressing 

these uncertainties is warranted, EPA 

believes that overall uncertainty about 

possible health risks associated with 

both short- and long-term PM2.5

exposure has diminished since the last 

review. As noted above, the greater 

confidence in short-term exposure 

studies supports the Administrator’s 

increased reliance on those studies as 

the basis for the 24-hour standard, and 

greater confidence in long-term 

exposure studies supports the 

Administrator’s increased reliance on 

those studies as the basis for the annual 

PM2.5.17

With regard to the issue of 

mechanisms, these commenters noted 

that although EPA recognizes that new 

evidence is now available on potential 

mechanisms and plausible biological 

pathways, the evidence still does not 

resolve all questions about how PM2.5 at 

ambient levels could produce the effects 

in question in this review. They further 

assert that even if more recent 

information has advanced our 

understanding of such mechanisms, it 

would not justify revision of the 

standard. The EPA notes that in the last 

review, the Agency considered the lack 

of demonstrated biologic mechanisms 

for the varying effects observed in 

epidemiologic studies to be an 

important caution in its integrated 

assessment of the health evidence, upon 

which the standards were based. Since 

the last review, there has been a great 

deal of research directed toward 

advancing our understanding of biologic 

mechanisms. While this research has 

not resolved all questions, and further 

research is warranted, it has provided 

important insights as discussed in 

section II.A.1 of the proposal (71 FR 

2626–2627). As noted there, the findings 

from this new research indicate that 

different health responses are linked 

with different particle characteristics 

and that both individual components 

and complex particle mixtures appear to 

be responsible for many biologic 

responses relevant to fine particle 

exposures. The Criteria Document (EPA, 

2004a, p. 7–206) concluded: ‘‘Thus, 

there appear to be multiple biologic 

mechanisms that may be responsible for 

observed morbidity/mortality due to 

exposure to ambient PM. It also appears 

that many biological responses are 

produced by PM whether it is composed 

of a single component or a complex 

mixture.’’ Further, EPA believes that 

progress made in gaining insights into 

potential mechanisms lends support to 

the biologic plausibility of results 

observed in epidemiologic studies (71 

FR 2636). The mechanistic evidence 

now available, taken together with 

newly available epidemiologic 

evidence, increases the Agency’s 

confidence that observed associations 

are causal in nature, such that the risks 

of health effects attributed to short- and 

long-term exposure to PM2.5, acting 

alone and/or in combination with 

gaseous co-pollutants, are now more 


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18 In assessing such uncertainties in this review 

relative to the last review, EPA notes that in the last 

review the level of uncertainty associated with 

long-term exposure studies was such that they were 

not relied on as the primary basis for the annual 

standard. In the last review, relative risk estimates 

from long-term exposure studies were deemed 

‘‘highly uncertain’’ (62 FR 38668) and health effects 

from long-term exposure were characterized as 

‘‘potentially independent’’ (Id.) from those 

associated with short-term exposure. 

certain than was understood in the last 

review. 

With regard to uncertainty in 

concentration-response functions, these 

commenters concluded that ‘‘because 

the actual shape of this function 

remains unknown, this uncertainty has 

not been reduced since 1997’’ (UARG, p. 

17). The EPA notes that, in contrast to 

the last review when few studies had 

quantitatively assessed the form of the 

concentration-response function or the 

potential for a threshold, several new 

studies available in this review have 

used different methods to examine this 

question, and most have been unable to 

detect threshold levels in time-series 

mortality studies. The Criteria 

Document (EPA, 2004a, p. 9–44) 

recognized that in multi-city and most 

single-city time-series studies, statistical 

tests comparing linear and various 

nonlinear or threshold models have not 

shown statistically significant 

distinctions between them; where 

potential threshold levels have been 

suggested in single-city studies, they are 

at fairly low levels (Id. at p. 9–45). 

Further, the shape of concentrationresponse functions for long-term 

exposure to PM2.5 was evaluated using 

data from the ACS cohort, with the HEI 

reanalysis finding near-linear increasing 

trends through the range of particle 

levels observed in this study, and the 

extended ACS study reporting that the 

various mortality associations were not 

significantly different from linear (71 FR 

2635).18 However, EPA agrees that 

uncertainties remain in our 

understanding of the shape of 

concentration-response functions, and, 

consistent with the conclusion in the 

Criteria Document, has concluded that 

the available evidence does not either 

support or refute the existence of 

population thresholds for effects 

associated with short- or long-term 

exposures to PM across the range of 

concentrations in the studies. Even 

while recognizing that uncertainties 

remain, EPA believes that our 

understanding of this issue for both 

short- and long-term exposure studies 

has been advanced since the last review. 

With regard to co-pollutant 

confounding, these commenters asserted 

that EPA has been ‘‘dismissive’’ of this 

issue in assessing the epidemiologic 

evidence of associations between PM 

and mortality and morbidity endpoints 

(UARG, p. 18). These commenters 

asserted that EPA has inappropriately 

concluded that PM-related mortality 

and morbidity associations are generally 

robust to confounding, which is one of 

the criteria considered in drawing 

inferences about the extent to which 

observed statistical associations are 

likely causal in nature. The commenters 

focused on an examination of the extent 

to which statistically significant PM2.5

associations based on one-pollutant 

models in a number of time-series 

studies, and in an analysis of 

associations with long-term exposures 

in the ACS cohort studies, often did not 

remain statistically significant in twopollutant models. 

In general, EPA does not believe that 

the examination of this issue put 

forward by these commenters reflects 

the complexities inherent in assessing 

the issue of co-pollutant confounding. 

As discussed in the proposal (71 FR 

2634) and more fully in the Criteria 

Document (EPA, 2004a, section 8.4.3; 

chapter 9, section 9.2.2.2.2), although 

multi-pollutant models may be useful 

tools for assessing whether gaseous copollutants may be potential 

confounders, such models cannot 

determine whether in fact they are. 

Interpretation of the results of multipollutant models is complicated by 

correlations that often exist among air 

pollutants, by the fact that some 

pollutants play a role in the atmospheric 

reactions that form other pollutants 

such as secondary fine particles, and by 

the inherent statistical power of the 

studies in question. While single-city 

multi-pollutant models have received a 

great deal of attention during this 

review, the Criteria Document also 

noted several other approaches to 

examining the question, including a 

more careful examination of personal 

exposures to PM and co-pollutants, the 

use of factor or principal component 

analyses, and the use of intervention 

studies (EPA, 2004a, pp. 8–245 to 8–

246). The Criteria Document also 

recognized that it is important to 

consider the issue of potential copollutant confounding in the context of 

the more recent evidence available 

about the biological plausibility of 

associations between the various 

pollutants and health outcomes, model 

specification, and exposure error (EPA, 

2004a, p. 8–254). 

An example of other approaches to 

examining potential co-pollutant 

confounding is the study of personal 

exposure to fine particles and copollutant gases done in Baltimore 

(Sarnat et al., 2001). This study found 

that day-to-day variations in monitored 

ambient gases were not associated with 

day-to-day changes in personal 

exposures to those gases, but they were 

associated with day-to-day changes in 

personal exposure to PM2.5. One 

reasonable interpretation of this study is 

that for cities like Baltimore, changes in 

model results when ambient gases are 

included in multi-pollutant models may 

stem from such gases being surrogates 

for exposures to particles and not 

confounders at all. 

The broader examination of this issue 

in the Criteria Document included a 

focus on evaluating the stability of the 

size of the effect estimates in time-series 

studies using single- and multipollutant models, as illustrated in 

Figures 8–16 through 8–19 (EPA, 2004a, 

pp. 8–248 to 8–251). This examination 

found that for most time-series studies, 

there was little change in effect 

estimates based on single- and multipollutant models, although recognizing 

that in some cases, the PM effect 

estimates were markedly reduced in size 

and lost statistical significance in 

models that included one or more 

gaseous pollutants. The Criteria 

Document also noted that PM and the 

gaseous co-pollutants were often highly 

correlated, and it is generally the case 

that high correlations existed between 

pollutants where PM effect estimates 

were reduced in size with the inclusion 

of gaseous co-pollutants. With regard to 

the analysis of multiple pollutants from 

the ACS cohort, it is important to note 

that the effects estimates for fine 

particles actually increased in two 

pollutant models that incorporated CO, 

NO2, and ozone, and were reduced only 

for models that incorporated SO2. The 

Criteria Document recognized, however, 

that SO2 is a precursor for fine particle 

sulfates, which complicates the 

interpretation of multi-pollutant model 

results, and that mortality may be 

associated with not only PM2.5 but also 

with other components of the mix of 

ambient pollutants in this long-term 

exposure study. 

Far from being dismissive, EPA has 

examined this issue in detail based on 

the much more extensive body of 

relevant evidence available in this 

review. This Criteria Document 

concluded that ‘‘the most consistent 

findings from amidst the diversity of 

multi-pollutant evaluation results for 

different sites is [sic] that the PM signal 

most often comes through most clearly.’’

(EPA, 2004a, p. 8–254.) While 

acknowledging that these analyses have 

not fully disentangled the relative role 

of co-pollutants, EPA believes that this 

examination provides greater 

confidence than in the last review that 


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19As discussed in section II.A.2.a of the proposal 

(71 FR 2629–2630, 2633), this body of studies 

includes those that did not use generalized additive 

models or were reanalyzed to address problems 

with applications of statistical software used in a 

number of important studies, as noted above in 

section I.C. 

observed effects can be attributed to 

short- and long-term exposures to PM2.5, 

alone and in combination with other 

pollutants, while recognizing that 

potential confounding by co-pollutants 

remains a very challenging issue to 

address, even with well-designed 

studies. 

With regard to questions about the 

role of individual constituents within 

the mix of fine particles, these 

commenters pointed out that EPA 

recognized this issue as an important 

uncertainty in the last review and did so 

again in this review. These commenters 

then expressed the view that such 

continued uncertainty provides no 

grounds for reconsidering the Agency’s 

1997 conclusion that the current PM2.5

standards provide the requisite 

protection. As a general matter, EPA 

agrees that although new research 

directed toward this question has been 

conducted since the last review, 

important questions remain and the 

issue remains an important element in 

the Agency’s ongoing research program. 

The EPA does not agree, however, that 

continued uncertainty with regard to the 

relative toxicity of components within 

the mix of fine particles, in and of itself, 

provides grounds for not revising the 

suite of PM2.5 standards. Rather, the full 

body of health effects evidence that has 

become available since the last review 

provides a basis for concluding that 

additional public health protection is 

warranted to protect against health 

effects that have been associated with 

exposure to fine particles measured as 

PM2.5 mass. 

At the time of the last review, the 

Agency determined that it was 

appropriate to control fine particles as a 

group, as opposed to singling out any 

particular component or class of fine 

particles. This distinction was based 

largely on epidemiologic evidence of 

health effects using various indicators of 

fine particles in a large number of areas 

that had significant contributions of 

differing components or sources of fine 

particles, together with some limited 

experimental studies that provided 

some evidence suggestive of health 

effects associated with high 

concentrations of numerous fine particle 

components. In this review, as 

discussed in section II.D of the proposal 

(71 FR 2643–2645) and below in section 

II.C, while most epidemiologic studies 

continue to be indexed by PM2.5, some 

epidemiologic studies also have 

continued to implicate various 

components within the mix of fine 

particles that have been more commonly 

studied (e.g., sulfates, nitrates, carbon, 

organic compounds, and metals) as 

being associated with adverse effects 

(EPA, 2004a, p. 9–31, Table 9–3). In 

addition, several recent epidemiologic 

studies included in the Criteria 

Document have used PM2.5 speciation 

data to evaluate associations between 

mortality and fine particles from 

different sources, and some toxicologic 

studies have provided evidence for 

effects associated with various fine 

particle components or sizedifferentiated subsets of fine particles. 

The available information continues 

to suggest that many different chemical 

components of fine particles and a 

variety of different types of source 

categories are all associated with, and 

probably contribute to, effects 

associated with PM2.5. Consequently, 

there continues to be no basis to 

conclude that any individual fine 

particle component cannot be associated 

with adverse health effects (EPA, 2005, 

p. 5–17). This information is relevant to 

the Agency’s decision to retain PM2.5 as 

the indicator for fine particles (as 

discussed below in section II.C). The 

EPA also believes that it is relevant to 

the Agency’s conclusion as to whether 

revision of the suite of PM2.5 standards 

is appropriate. Furthermore, while there 

remains uncertainty about the role and 

relative toxicity of various components 

of fine PM, the current evidence 

continues to support the view that fine 

particles should be addressed as a group 

for purposes of public health protection, 

and the remaining uncertainty does not 

call for delaying any increase in public 

health protection that other evidence 

indicates may be warranted. 

With regard to the sensitivity of 

epidemiologic associations to the use of 

different statistical models and different 

approaches to model specification used 

by researchers, these commenters 

identified this issue of model sensitivity 

as an area in which uncertainty in 

interpreting epidemiologic evidence has 

increased since the last review. 

Comments from UARG, Pillsbury et al., 

the Annapolis Center and others 

pointed to examples where individual 

study results are sensitive to the use of 

alternative models, and to reviews that 

recommend further exploration of this 

issue in future research, as a basis for 

asserting that current modeling 

approaches are too uncertain to use the 

available epidemiologic studies as a 

basis for revising the current PM2.5

standards. The EPA agrees that recent 

work on model sensitivity has raised 

new concerns and the Agency has given 

much attention to this issue. In so 

doing, EPA recognizes, as does the HEI 

and other researchers, that there is no 

clear consensus at this time as to what 

constitutes appropriate control of 

weather and temporal trends in timeseries studies, and that no single 

statistical modeling approach is likely to 

be most appropriate in all cases (EPA 

2004a, p. 8–238). 

While recognizing the need for further 

research on this issue, EPA believes that 

the body of time-series epidemiologic 

studies considered in this review 19

provides an appropriate basis for 

informing the Agency’s decisions on 

whether to revise the 24-hour PM2.5

standard, consistent with the conclusion 

of the HEI review panel (‘‘* * * the 

revised findings will continue to help 

inform regulatory decisions regarding 

PM.’’ HEI, 2003; EPA, 2004a, p. 8–237). 

More specifically, as discussed in the 

proposal (71 FR 2633–2634), the recent 

time-series epidemiologic studies 

evaluated in the Criteria Document have 

included some degree of control for 

variations in weather and seasonal 

variables. However, as summarized in 

the HEI review panel commentary, 

selecting a level of control to adjust for 

time-varying factors, such as 

temperature, in time-series 

epidemiologic studies involves a tradeoff. For example, if the model does not 

sufficiently adjust for the relationship 

between the health outcome and 

temperature, some effects of 

temperature could be falsely ascribed to 

the pollution variable. Conversely, if an 

overly aggressive approach is used to 

control for temperature, the result 

would possibly underestimate the 

pollution-related effect and compromise 

the ability to detect a small but true 

pollution effect (EPA, 2004a, p. 8–236; 

HEI, 2003, p. 266). The selection of 

approaches to address such variables 

depends in part on prior knowledge and 

judgments made by the investigators, for 

example, about weather patterns in the 

study area and expected relationships 

between weather and other time-varying 

factors and health outcomes considered 

in the study. 

The HEI commentary also reached 

several other relevant conclusions about 

the reanalysis of time-series studies: 

upon reanalysis, the PM effect persisted 

in the majority of studies; in some of the 

large number of studies in which the 

PM effect persisted, the estimates of PM 

effects were substantially reduced; in 

the few studies in which further 

sensitivity analyses were performed, 

some showed marked sensitivity of the 

PM effect estimate to the degree of 

smoothing and/or the specification of 


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20More specifically, in multivariate models, the 

association found between mortality and long-term 

PM2.5 exposure was little changed with addition of 

education level to the model (Krewski et al., 2000, 

p. 184). This indicates that education level was not 

a confounder in the relationship between fine 

particles and mortality, but the relationship 

between fine particles and mortality is larger in the 

population subsets with lower education in this 

study and not statistically significant in the 

population subset with the highest education (EPA, 

2004, p. 8–100). 

weather; and, in most studies, 

parametric smoothing approaches used 

to obtain correct standard errors of the 

PM effect estimates produced slightly 

larger standard errors than with the use 

of generalized additive models. 

However, the impact of these larger 

standard errors on the level of statistical 

significance of the PM effect was minor 

(EPA, 2004a, pp. 8–237 to 8–238). While 

recognizing the need for further 

exploration of alternative modeling 

approaches for time-series analyses, the 

Criteria Document found that the 

studies included in this part of the 

reanalysis, in general, continued to 

demonstrate associations between PM 

and mortality and morbidity beyond 

those attributable to weather variables 

alone (EPA, 2004a, pp. 8–340, 8–341). 

For long-term exposure to fine 

particles, the reanalysis and extended 

analyses of data from prospective cohort 

studies have shown that reported 

associations between mortality and 

long-term exposure to fine particles are 

robust to alternative modeling strategies 

(Krewski et al., 2000). As stated in the 

reanalysis report, ‘‘The risk estimates 

reported by the Original Investigators 

were remarkably robust to alternative 

specifications of the underlying risk 

models, thereby strengthening 

confidence in the original findings’

(Krewski et al., 2000, p. 232). In the 

extended analysis, Krewski et al. (2000) 

did identify model sensitivities related 

to education level and spatial patterns 

in the data (e.g., correlations in air 

pollutant concentrations between cities 

within a region of the country). 

However, these model sensitivities do 

not invalidate the findings of 

statistically significant associations 

between long-term exposure to PM2.5

and mortality. For example, while the 

association was stronger for the subset 

of the ACS cohort with the least 

education, there was an association with 

cardiorespiratory mortality in the entire 

population.20

In considering these issues related to 

uncertainties in the underlying health 

science, on balance, EPA believes that 

the available evidence interpreted in 

light of these remaining uncertainties 

does provide increased confidence 

relative to the last review in the 

reported associations between shortand long-term PM2.5 exposures and 

mortality and morbidity effects, alone 

and in combination with other 

pollutants, and generally supports 

stronger inferences as to the causal 

nature of the associations. The EPA also 

believes that this increased confidence, 

when taken in context of the entire body 

of available health effects evidence and 

in light of the evidence from short-term 

exposure studies of associations 

observed in areas meeting the current 

suite of PM2.5 standards, adds support to 

its conclusion that the current suite of 

PM2.5 standards needs to be revised to 

provide increased public health 

protection. This increased confidence 

also adds support to the Administrator’s 

decision to place greater reliance on the 

long-term exposure studies as the basis 

for the annual PM2.5 standard and to 

place greater reliance on the short-term 

exposure studies as the basis for the 24- 

hour PM2.5 standard. 

(3) In asserting that the estimated risk 

upon attainment of the current PM2.5

standards has decreased since 1997 

(UARG, p. 23), these commenters 

compared results of EPA’s risk 

assessment done in the last review with 

those from the Agency’s risk assessment 

done as part of this review, and they 

concluded that risks upon attainment of 

the current PM2.5 standards ‘‘are almost 

surely far below those that were 

predicted in 1997’’ (UARG, p. 25). These 

commenters used this conclusion as the 

basis for a claim that there is no reason 

to revise the current PM2.5 standards. In 

particular, UARG and other commenters 

claimed that based on this purported 

reduction in risk estimates EPA cannot 

reconcile a decision to provide a greater 

level of health protection now than that 

afforded by the current standards with 

the ‘‘not lower or higher than is 

necessary’’ standard articulated by the 

Supreme Court in Whitman. 

The EPA believes that this claim is 

fundamentally flawed for three reasons, 

as discussed in turn below: (i) It 

mischaracterizes the use of the 

quantitative risk assessment in the 1997 

rulemaking; (ii) it is factually incorrect 

in comparing the quantitative risks 

estimated in 1997 with those estimated 

in the current rulemaking; and (iii) it 

fails to take into account that with 

similar risks, increased certainty in the 

risks presented by PM2.5 implies greater 

concern than in the last review. 

First, this claim mischaracterizes 

EPA’s use of the risk assessment in 1997 

in part by not recognizing that the 

illustrative risk assessment conducted 

for portions of two cities (Philadelphia 

and Los Angeles) in the last review was 

only used qualitatively to assess the 

need to revise the then-current PM10

standards. The EPA used the 1997 risk 

assessment estimates to confirm the 

conclusions drawn primarily from the 

epidemiological studies that ambient 

PM2.5 levels allowed under the then 

current PM10 standards presented a 

serious public health problem. EPA did 

not use it as a basis for selecting the 

level of the 1997 PM standards. See 62 

FR at 38656, 65; ATA III, 283 F. 3d at 

373–74 (noting that EPA did not base 

the level of the standards on the 

numerical results of the risk 

assessment). In so doing, the 

Administrator concurred with CASAC’s 

judgment that the quantitative risk 

estimates at the time were too uncertain 

for EPA to rely on in deciding the 

appropriate levels for the PM2.5 NAAQS. 

Therefore, the final decision on the level 

of the NAAQS was not based on the 

absolute or relative risk reductions 

estimated in the quantitative risk 

assessment. Instead, the decision was 

based on a direct assessment of the 

available epidemiological studies and 

the concentration levels observed in 

urban areas examined in the studies 

where statistically significant effects 

had been observed. Since EPA did not 

rely on the 1997 quantitative risk 

estimates in setting the level of the 1997 

standards, the 1997 estimates associated 

with those levels do not represent a 

decision on a requisite level of 

quantified risk from PM exposure, and 

therefore do not support the argument 

that a lower estimated risk is more than 

is necessary to provide the requisite 

level of protection. As a result, the 

suggested quantitative comparison 

between the 1997 estimates and the 

current estimates of risks at the levels of 

the current standards is not an 

appropriate basis for determining 

whether the current suite of PM2.5

standards needs to be revised. 

Second, EPA relies on the current risk 

estimates associated with meeting the 

current standards in a qualitative 

manner, as in 1997, to inform the 

conclusions drawn primarily from the 

epidemiological studies on whether 

ambient PM2.5 levels allowed under the 

current suite of PM2.5 standards present 

a serious public health problem 

warranting revision of the suite of PM2.5

standards. The 1997 estimate of these 

risks, or any comparison of the 1997 risk 

estimates to the current estimates, are 

irrelevant for that purpose, as the 1997 

estimates reflect an outdated analysis 

that has been updated in this review to 

reflect the current science. 

Further, even if the 1997 and current 

risk assessments were legitimately 

comparable for decision-making 

purposes, it would still be factually 


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incorrect to conclude that EPA accepted 

significantly greater risk in 1997 than is 

now estimated to be associated with the 

1997 standards based on the most recent 

risk assessment. It is important to note 

that a very large proportion of the 

quantitative risks estimated in 1997 and 

today comes from long-term exposure 

mortality. The primary estimates from 

the current risk assessment (which 

assume a potential threshold of 10 μg/ 

m3, as recommended by CASAC) result 

in residual risks in terms of percent of 

total incidence that are about the same 

in the current review as they were in the 

last review for both Philadelphia and 

Los Angeles. 

Third, it is important to take into 

account EPA’s increased level of 

confidence in the associations between 

short- and long-term PM2.5 exposures 

and mortality and morbidity effects. In 

comparing the scientific understanding 

of the risk presented by exposure to 

PM2.5 between the last and current 

reviews, one must examine not only the 

quantitative estimate of risk from those 

exposures (e.g. the numbers of 

premature deaths or increased hospital 

admissions at various levels), but also 

the degree of confidence that the 

Agency has that the observed health 

effects are causally linked to PM2.5

exposure at those levels. As 

documented in the Criteria Document 

and the recommendations and 

conclusions of CASAC, EPA recognizes 

significant advances in our 

understanding of the health effects of 

PM2.5, based on reanalyses, extended 

analyses and new epidemiology studies, 

new human and animal studies 

documenting effects of concentrated 

ambient particles, new laboratory 

studies identifying and investigating 

biological mechanisms of PM toxicity, 

and new studies addressing the utility 

of using ambient monitors to assess 

population exposures to particles of 

outdoor origin. As a result of these 

advances, EPA is now more certain that 

fine particles, alone or in combination 

with other pollutants, present a 

significant risk to public health at levels 

at or above the range of levels that the 

Agency had considered for these 

standards in 1997. From this more 

comprehensive perspective, since the 

risks presented by PM2.5 are more 

certain and the overall current 

quantitative risk estimates are about the 

same as in 1997, PM2.5-related risks are 

now of greater concern than in the last 

review. 

In sum, quantitative risk estimates 

were not a basis for EPA’s decision in 

setting a level for the PM2.5 standards in 

1997, and they do not set any quantified 

‘‘benchmark’’ for the Agency’s decision 

to revise the PM2.5 standards at this 

time. In any case, there is not a 

significant difference in the risk 

estimates from 1997 to now. Finally, 

EPA believes that confidence in the 

causal relationships between short- and 

long-term exposures to fine particles 

and various health effects has increased 

markedly since 1997. Therefore, similar 

or even somewhat lower quantitative 

risk estimates today would not be a 

basis to conclude that no revision to the 

suite of PM2.5 standards is ‘‘requisite’’ to 

protect public health with an adequate 

margin of safety. 

(4) Some of these commenters also 

identified ‘‘new’’ studies that were not 

included in the Criteria Document as 

showing ‘‘continued erosion of the 

hypothesis that there is a causal 

connection between fine PM mass and 

health effects’’ and further supporting 

‘‘the conclusion that more stringent 

PM2.5 standards are not justified’’

(Pillsbury et al., p. 14). As discussed 

above in section I.C, EPA notes that, as 

in past NAAQS reviews, the Agency is 

basing the final decisions in this review 

on the studies and related information 

included in the PM air quality criteria 

that have undergone CASAC and public 

review, and will consider newly 

published studies for purposes of 

decision making in the next PM NAAQS 

review. Nonetheless, in provisionally 

evaluating commenters’ arguments (see 

Response to Comments document), EPA 

notes that its provisional assessment of 

‘‘new’’ science found that such studies 

did not materially change the 

conclusions in the Criteria Document. 

3. Conclusions Regarding the Need for 

Revision 

Having carefully considered the 

public comments, as discussed above, 

the Administrator believes the 

fundamental scientific conclusions on 

the effects of PM2.5 reached in the 

Criteria Document and Staff Paper, 

discussed above in section II.B.1, 

remain valid. In considering whether 

the suite of primary PM2.5 standards 

should be revised, the Administrator 

places primary consideration on the 

evidence obtained from the 

epidemiologic studies, and finds the 

evidence of serious health effects 

reported in short-term exposure studies 

conducted in areas that would meet the 

current suite of PM2.5 standards to be 

compelling, especially in light of the 

extent to which such studies are part of 

an overall pattern of positive and 

frequently statistically significant 

associations across a broad range of 

studies. The Administrator believes that 

this literature collectively represents a 

strong and generally robust body of 

evidence of serious health effects 

associated with both short- and longterm exposures to PM2.5. Further, the 

Administrator believes that the 

increased confidence in the evidence of 

health effects associated with long-term 

exposure to PM2.5 supports relying on 

long-term exposure studies as the basis 

for setting the annual standard in this 

review. This is in contrast to 1997 when 

EPA relied primarily on evidence from 

the then-available short-term exposure 

studies as the primary basis for setting 

the annual standard. As discussed in the 

Criteria Document and Staff Paper, the 

Administrator believes that much 

progress has been made since the last 

review in reducing some of the major 

uncertainties that were important 

considerations in establishing the 

current suite of PM2.5 standards. 

Extensive critical review of this body 

of evidence, the quantitative risk 

assessment, and related uncertainties 

during the criteria and standards review 

process, including review by CASAC 

and the public of the basis for EPA’s 

proposed decision to revise the suite of 

primary PM2.5 standards, has identified 

a number of issues about which 

different reviewers disagree and for 

which additional research is warranted. 

Nonetheless, on balance, the 

Administrator believes that the 

remaining uncertainties in the available 

evidence do not diminish confidence in 

the associations between serious 

mortality and morbidity effects and 

exposure to fine particles, in particular 

as reported in peer-reviewed short-term 

exposure studies at levels allowed by 

the current standards. In this regard, the 

Administrator agrees with CASAC and 

the majority of public commenters that 

revision of the current suite of PM2.5

standards to provide increased public 

health protection is both appropriate 

and necessary. Based on these 

considerations, the Administrator 

concludes that the current suite of 

primary PM2.5 standards, taken together, 

is not sufficient and thus not requisite 

to protect public health with an 

adequate margin of safety, and that 

revision is needed to provide increased 

public health protection. 

It is important to note that this 

conclusion, and the reasoning on which 

it is based, do not address the question 

of what specific revisions are 

appropriate. That requires looking 

specifically at the current indicator, 

averaging time, form, and level of the 

24-hour and annual PM2.5 standards, 

and evaluating the evidence relevant to 

determining whether any of those 

elements should be revised. The 

analyses discussed above concerning 

the need to revise the current standards 


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21Mar et al. (2000) noted that sulfate alone in a 

single-pollutant model was not associated with 

cardiovascular mortality, but that the sulfate 

‘‘factor,’’ which was so associated, contained 

elevated levels of lead and bromine. The authors 

state that the health association with the sulfate (S) 

factor ‘‘may be reflective of the contribution of Pb 

[lead] and Br [bromine] to the S factor.’’ Mar et al. 

(2003) did not provide information about singlepollutant analysis of sulfate or about contribution 

of Pb and Br to the S factor. 22More specifically, statistically significant 

associations were reported with factors representing 

fine particles from oil burning, industrial and 

sulfate aerosol sources in Newark and with particles 

from oil burning and motor vehicle sources in 

Camden, and no statistically significant associations 

were reported in Elizabeth. 

go no further than determining whether 

the evidence, taken as a whole, 

indicates that greater public health 

protection is needed than that provided 

by the current suite of PM2.5 standards. 

C. Indicator for Fine Particles 

In 1997, EPA established PM2.5 as the 

indicator for fine particles. In reaching 

this decision, the Agency first 

considered whether the indicator 

should be based on the mass of a sizedifferentiated sample of fine particles or 

on one or more components within the 

mix of fine particles. Second, in 

establishing a size-based indicator, a 

size cut needed to be selected that 

would appropriately distinguish fine 

particles from particles in the coarse 

mode. 

In addressing the first question in the 

last review, EPA determined that it was 

appropriate to control fine particles as a 

group, as opposed to singling out any 

particular component or class of fine 

particles. Community health studies had 

found significant associations between 

various indicators of fine particles 

(including PM2.5 or PM10 in areas 

dominated by fine particles) and health 

effects in a large number of areas that 

had significant mass contributions of 

differing components or sources of fine 

particles, including sulfates, wood 

smoke, nitrates, secondary organic 

compounds and acid sulfate aerosols. In 

addition, a number of animal 

toxicologic and controlled human 

exposure studies had reported health 

effects associations with high 

concentrations of numerous fine particle 

components (e.g., sulfates, nitrates, 

transition metals, organic compounds), 

although such associations were not 

consistently observed. It also was not 

possible to rule out any component 

within the mix of fine particles as not 

contributing to the fine particle effects 

found in epidemiologic studies. For 

these reasons, EPA concluded that total 

mass of fine particles was the most 

appropriate indicator for fine particle 

standards rather than an indicator based 

on PM composition (62 FR 38667). 

Having selected a size-based indicator 

for fine particles, the Agency then based 

its selection of a specific size cut on a 

number of considerations. In focusing 

on a size cut within the size range of 1 

to 3 μm (i.e., the intermodal range 

between fine and coarse mode 

particles), the Agency noted that the 

available epidemiologic studies of fine 

particles were based largely on PM2.5; 

only very limited use of PM1 monitors 

had been made. While it was recognized 

that using PM1 as an indicator of fine 

particles would exclude the tail of the 

coarse mode in some locations, in other 

locations it would miss a portion of the 

fine PM, especially under high humidity 

conditions, which would result in 

falsely low fine PM measurements on 

days with some of the highest fine PM 

concentrations. The selection of a 2.5 

μm size cut reflected the regulatory 

importance that was placed on defining 

an indicator for fine particle standards 

that would more completely capture 

fine particles under all conditions likely 

to be encountered across the U.S., 

especially when fine particle 

concentrations are likely to be high, 

while recognizing that some small 

coarse particles would also be captured 

by PM2.5 monitoring. Thus, EPA’s 

selection of 2.5 μm as the size cut for 

the fine particle indicator was based on 

considerations of consistency with the 

epidemiologic studies, the regulatory 

importance of more completely 

capturing fine particles under all 

conditions, and the potential for limited 

intrusion of coarse particles in some 

areas; it also took into account the 

general availability of monitoring 

technology (62 FR 38668). 

In this current review, the same 

considerations continue to apply for 

selection of an appropriate indicator for 

fine particles. As an initial matter, the 

available epidemiologic studies linking 

mortality and morbidity effects with 

short- and long-term exposures to fine 

particles continue to be largely indexed 

by PM2.5. Some epidemiologic studies 

also have continued to implicate various 

components within the mix of fine 

particles that have been more commonly 

studied (e.g., sulfates, nitrates, carbon, 

organic compounds, and metals) as 

being associated with adverse effects 

(EPA, 2004a p. 9–31, Table 9–3). In 

addition, several recent studies have 

used PM2.5 speciation data to evaluate 

the association between mortality and 

particles from different sources 

(Schwartz, 2003; Mar et al., 2003; Tsai 

et al., 2000; EPA, 2004a, section 8.2.2.5). 

Schwartz (2003) reported statistically 

significant associations for mortality 

with factors representing fine particles 

from traffic and residual oil combustion 

that were little changed in reanalysis to 

address statistical modeling issues, and 

also an association between mortality 

and coal combustion-related particles 

that was reduced in size and lost 

statistical significance in reanalysis. In 

Phoenix, significant associations were 

reported between mortality and fine 

particles from traffic emissions, 

vegetative burning, and regional sulfate 

sources that remained unchanged in 

reanalysis models (Mar et al., 2003).21

Finally, a small study in three New 

Jersey cities reported significant 

associations between mortality and fine 

particles from industrial, oil burning, 

motor vehicle and sulfate aerosol 

sources, though the results were 

somewhat inconsistent between cities 

(Tsai et al., 2000).22 No significant 

increase in mortality was reported with 

a source factor representing crustal 

material in fine particles (EPA, 2004a, p. 

8–85). Recognizing that these three 

studies represent a very preliminary 

effort to distinguish effects of fine 

particles from different sources, and that 

the results are not always consistent 

across the cities, the Criteria Document 

found that these studies indicate that 

exposure to fine particles from 

combustion sources, but not crustal 

material, is associated with mortality 

(EPA, 2004a, p. 8–77). Animal 

toxicologic and controlled human 

exposure studies have continued to link 

a variety of PM components or particle 

types (e.g., sulfates, notably primary 

metal sulfate emissions from residual oil 

burning, metals, organic constituents, 

bioaerosols, diesel particles) with health 

effects, though often at high 

concentrations (EPA, 2004a, section 

7.10.2). In addition, some recent studies 

have suggested that the ultrafine subset 

of fine particles (generally including 

particles with a nominal aerodynamic 

diameter less than 0.1 μm) may also be 

associated with adverse effects (EPA, 

2004a, pp. 8–67 to 8–68). 

The Criteria Document recognized 

that, for a given health response, some 

fine particle components are likely to be 

more closely linked with that response 

than others. The presumption that 

different PM constituents may have 

differing biological responses is 

toxicologically plausible and an 

important source of uncertainty in 

interpreting such epidemiologic 

evidence. For specific effects there may 

be stronger correlation with individual 

PM components than with aggregate 

particle mass. In addition, particles or 

particle-bound water can act as carriers 

to deliver other toxic agents into the 

respiratory tract, suggesting that 


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23No public comments were submitted regarding 

the use of a different size for fine particles. 

exposure to particles may elicit effects 

that are linked with a mixture of 

components more than with any 

individual PM component (EPA, 2004a, 

section 9.2.3.1.3). 

Thus, epidemiologic and toxicologic 

studies have provided evidence for 

effects associated with various fine 

particle components or sizedifferentiated subsets of fine particles. 

The Criteria Document concluded: 

‘‘These studies suggest that many 

different chemical components of fine 

particles and a variety of different types 

of source categories are all associated 

with, and probably contribute to, 

mortality, either independently or in 

combinations’’ (EPA, 2004a, p. 9–31). 

Conversely, the Criteria Document 

provided no basis to conclude that any 

individual fine particle component 

cannot be associated with adverse 

health effects (EPA, 2005, p. 5–17). In 

short, there is not sufficient evidence 

that would lead toward the selection of 

one or more PM components as being 

primarily responsible for effects 

associated with fine particles, nor is 

there sufficient evidence to suggest that 

any component should be eliminated 

from the indicator for fine particles. The 

Staff Paper continued to recognize the 

importance of an indicator that not only 

captures all of the most harmful 

components of fine particles (i.e., an 

effective indicator), but also emphasizes 

control of those constituents or 

fractions, including sulfates, transition 

metals, and organics that have been 

associated with health effects in 

epidemiologic and/or toxicologic 

studies, and is thus most likely to result 

in the largest risk reduction (i.e., an 

efficient indicator). Taking into account 

the above considerations, the Staff Paper 

concluded that it remains appropriate to 

control fine particles as a group; i.e., 

that total mass of fine particles is the 

most appropriate indicator for fine 

particle standards (EPA, 2005, p. 5–17). 

With regard to an appropriate size cut 

for a size-based indicator of total fine 

particle mass, the Criteria Document 

concluded that advances in our 

understanding of the characteristics of 

fine particles continue to support the 

use of particle size as an appropriate 

basis for distinguishing between these 

subclasses, and that a nominal size cut 

of 2.5 μm remains appropriate (EPA, 

2004a, p. 9–22). This conclusion 

followed from a recognition that within 

the intermodal range of 1 to 3 μm there 

is no unambiguous definition of an 

appropriate size cut for the separation of 

the overlapping fine and coarse particle 

modes. Within this range, the Staff 

Paper considered size cuts of both 1 μm 

and 2.5 μm. Consideration of these two 

size cuts took into account that there is 

generally very little mass in this 

intermodal range, although in some 

circumstances (e.g., windy, dusty areas) 

the coarse mode can extend down to 

and below 1 μm, whereas in other 

circumstances (e.g., high humidity 

conditions, usually associated with very 

high fine particle concentrations) the 

fine mode can extend up to and above 

2.5 μm. The same considerations that 

led to the selection of 2.5 μm size cut 

in the last review—that the 

epidemiologic evidence was largely 

based on PM2.5 and that it was more 

important from a regulatory perspective 

to capture fine particles more 

completely under all conditions likely 

to be encountered across the U.S. 

(especially when fine particle 

concentrations are likely to be high) 

than to avoid some coarse-mode 

intrusion into the fine fraction in some 

areas—led to the same recommendation 

in the Staff Paper (EPA, 2005, p. 5–18), 

which was endorsed by CASAC in its 

recommendations for PM2.5 standards 

(Henderson, 2005a, p. 6). In addition, 

the Staff Paper recognized that particles 

can act as carriers of water, oxidative 

compounds, and other components into 

the respiratory system, which adds to 

the importance of ensuring that larger 

accumulation-mode particles are 

included in the fine particle size cut 

(EPA, 2005, p. 5–18). 

Consistent with the Staff Paper and 

CASAC recommendations, the 

Administrator proposed to retain PM2.5

as the indicator for fine particles. 

Further, the Administrator provisionally 

concluded that currently available 

studies do not provide a sufficient basis 

for supplementing mass-based fine 

particle standards with standards for 

any specific fine particle component or 

subset of fine particles, or for 

eliminating any individual component 

or subset of components from fine 

particle mass standards. Addressing the 

current uncertainties in the evidence of 

effects associated with various fine 

particle components and types of source 

categories is an important element in 

EPA’s ongoing PM research program. 

In so doing, the Administrator also 

noted that some commenters had 

expressed views about the importance 

of evaluating health effect associations 

with various fine particle components 

and types of source categories as a basis 

for focusing ongoing and future research 

to reduce uncertainties in this area and 

for considering whether alternative 

indicator(s) are now or may be 

appropriate for standards intended to 

protect against the array of health effects 

that have been associated with fine 

particles as indexed by PM2.5. 

Information from such studies could 

also help inform the development of 

strategies that emphasize control of 

specific types of emission sources so as 

to address particles of greatest concern 

to public health. While recognizing that 

the studies evaluated in the Criteria 

Document provided some limited 

evidence of such associations that is 

helping to focus research activities, the 

Administrator solicited broad public 

comment on issues related to studies of 

fine particle components and types of 

source categories and their usefulness as 

a basis for consideration of alternative 

indicator(s) for fine particle standards. 

In general, comment was solicited on 

relevant new published research, 

recommendations for studies that would 

be appropriate for inclusion in future 

research activities, and approaches to 

assessing the available and future 

research results to determine whether 

alternative indicators for fine particles 

are warranted to provide effective 

protection of public health from effects 

associated with long- and short-term 

exposure to ambient fine particles (71 

FR at 2645). More specifically, the 

proposal solicited comment on a 

number of related issues, including the 

extent to which reducing particular 

types of PM (differentiated by either size 

or chemistry) might alter the size and 

toxicity of remaining particles; the 

extent to which fine particles in urban 

and rural areas can be differentiated by 

size or chemistry; the extent to which 

the latest scientific information can be 

used to improve our understanding of 

the relationship of monitored pollution 

levels to human exposure; and on 

studies using concentrated ambient 

particles (CAPs) and their use in 

examining the toxicity of specific 

mixtures of pollutants or of particular 

source categories. 

The EPA received comparatively few 

public comments on issues related to 

the indicator for fine particles.23 Public 

comments from all major public and 

private sector groups received on the 

proposal were overwhelmingly in favor 

of EPA’s proposal to retain PM2.5 as the 

indicator for fine particles. Commenters 

who supported retaining PM2.5 as an 

indicator argued that current scientific 

evidence does not identify specific 

components or sources of concern and 

therefore, that a mass-based indicator 

remains the appropriate indicator for 

fine particles (Engine Manufacturers 

Association; American Lung 

Association et al.). Some commenters 

emphasized the need to conduct 

additional research to more fully 


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understand the effect of specific PM 

components and/or sources on public 

health. For example, the Electric Power 

Research Institute highlighted specific 

new research studies that had been 

completed since the close of the Criteria 

Document addressing issues related to 

fine particle components and source 

apportionment, and noted its ongoing 

research on component-related health 

effects that includes coordinated 

epidemiology, toxicology, and exposure 

assessment studies. The Administrator 

recognizes the work of the Electric 

Power Research Institute and agrees that 

additional research is important to 

improve future understanding of the 

role of specific fine particle components 

and/or sources of fine particles. The 

Administrator also recognizes the 

ongoing efforts of HEI to conduct 

additional multidisciplinary research 

targeted at expanding the available data 

on the health effects associated with 

specific PM components (HEI, 2005). 

Having considered the public 

comments on this issue, the 

Administrator concurs with the Staff 

Paper and CASAC recommendations 

and concludes that it is appropriate to 

retain PM2.5 as the indicator for fine 

particles. 

D. Averaging Time of Primary PM2.5

Standards 

In the last review, EPA established 

two PM2.5 standards, based on annual 

and 24-hour averaging times, 

respectively (62 FR 38668–70). This 

decision was based in part on evidence 

of health effects related to both shortterm (from less than 1 day to up to 

several days) and long-term (from a year 

to several years) measures of PM. The 

EPA noted that the large majority of 

community epidemiologic studies 

reported associations based on 24-hour 

averaging times or on multiple-day 

averages. Further, EPA noted that a 24- 

hour standard could also effectively 

protect against episodes lasting several 

days, as well as providing some degree 

of protection from potential effects 

associated with shorter duration 

exposures. The EPA also recognized that 

an annual standard would provide 

effective protection against both annual 

and multi-year, cumulative exposures 

that had been associated with an array 

of health effects, and that a much longer 

averaging time would complicate and 

unnecessarily delay control strategies 

and attainment decisions. The EPA 

considered the possibility of seasonal 

effects, although the very limited 

available evidence of such effects and 

the seasonal variability of sources of 

fine particle emissions across the 

country did not provide an adequate 

basis for establishing a seasonal 

averaging time. 

In considering whether the 

information available in this review 

supported consideration of different 

averaging times for PM2.5 standards, the 

Staff Paper concluded that the available 

information is generally consistent with 

and supportive of the conclusions 

reached in the last review to set PM2.5

standards with both annual and 24-hour 

averaging times. In considering the new 

information, the Staff Paper made the 

following observations (EPA, 2005, 

section 5.3.3): 

(1) There is a growing body of studies 

that provide additional evidence of 

effects associated with exposure periods 

shorter than 24-hours (e.g., one to 

several hours) (EPA, 2004a, section 

3.5.5.1). While the Staff Paper 

concluded that this information remains 

too limited to serve as a basis for 

establishing a shorter-than-24-hour fine 

particle primary standard at this time, it 

also noted that this information gives 

added weight to the importance of a 

standard with a 24-hour averaging time. 

(2) Some recent PM10 studies have 

used a distributed lag over several days 

to weeks preceding the health event, 

although this modeling approach has 

not been extended to studies of fine 

particles (EPA, 2004a, section 3.5.5). 

While such studies continue to suggest 

consideration of a multiple day 

averaging time, the Staff Paper noted 

that limiting 24-hour concentrations of 

fine particles will also protect against 

effects found to be associated with PM 

averaged over many days in health 

studies. Consistent with the conclusion 

reached in the last review, the Staff 

Paper concluded that a multiple-day 

averaging time would add complexity 

without providing more effective 

protection than a 24-hour average. 

(3) While some newer studies have 

investigated seasonal effects (EPA, 

2004a, section 3.5.5.3), the Staff Paper 

concluded that currently available 

evidence of such effects is still too 

limited to serve as a basis for 

considering seasonal standards. 

Based on the above considerations, 

the Staff Paper and CASAC (Henderson, 

2005a, p. 6) recommended retaining the 

current annual and 24-hour averaging 

times for PM2.5 primary standards. The 

Administrator concurred with the staff 

and CASAC recommendations and 

proposed that averaging times for PM2.5

standards should continue to include 

annual and 24-hour averages to protect 

against health effects associated with 

short-term (hours to days) and long-term 

(seasons to years) exposure periods. 

The EPA received very limited public 

comment on the issue of averaging time 

for the PM2.5 primary standards. A 

group of public health and 

environmental organizations agreed that 

‘‘the EPA has selected the appropriate 

averaging times for the fine particle 

standards’’ (American Lung Association 

et al.). 

Having considered the public 

comments on this issue, the 

Administrator concurs with the 

recommendations presented in the Staff 

Paper and recommendations made by 

CASAC (Henderson, 2005a) and 

concludes, as proposed, that it is 

appropriate to retain the current annual 

and 24-hour averaging times for the 

primary PM2.5 standards to protect 

against health effects associated with 

short-term and long-term exposure 

periods. 

E. Form of Primary PM2.5 Standards 

1. 24-Hour PM2.5 Standard 

In 1997 EPA established the form of 

the 24-hour PM2.5 standard as the 98th 

percentile of the annual 24-hour 

concentrations at each populationoriented monitor within an area, 

averaged over three years (62 FR 38671–

74). EPA found that, as compared to an 

exceedance-based form used in earlier 

PM standards, a concentration-based 

form is more reflective of the health risk 

posed by elevated PM2.5 concentrations 

because it 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 standard. Further, a 

concentration-based form better 

compensates for missing data and lessthan-every-day monitoring; and, when 

averaged over 3 years, it has greater 

stability and, thus, facilitates the 

development of more stable 

implementation programs. After 

considering a range of concentration 

percentiles from the 95th to the 99th, 

EPA selected the 98th percentile as an 

appropriate balance between adequately 

limiting the occurrence of peak 

concentrations and providing increased 

stability and robustness. Further, by 

basing the form of the standard on 

concentrations measured at populationoriented monitoring sites (as specified 

in 40 CFR part 58), EPA intended to 

provide protection for people residing 

in or near localized areas of elevated 

concentrations. 

In this review, the Staff Paper 

concluded that it is appropriate to retain 

a concentration-based form that is 

defined in terms of a specific percentile 

of the distribution of 24-hour PM2.5

concentrations at each populationoriented monitor within an area, 

averaged over 3 years. This staff 


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24See ATA III, 283 F. 3d at 374–375 which 

concludes it is legitimate for EPA to consider 

promotion of overall effectiveness of NAAQS 

implementation programs, including their overall 

stability, in setting a standard that is requisite to 

protect the public health. 

25See final rulemaking notice regarding revisions 

to ambient air monitoring requirements, elsewhere 

in today’s Federal Register. 

recommendation was based on the same 

reasons that were the basis for EPA’s 

selection of this type of form in the last 

review. As to the specific percentile 

value to be considered, the Staff Paper 

took into consideration (1) the relative 

risk reduction afforded by alternative 

forms at the same standard level, (2) the 

relative year-to-year stability of the air 

quality statistic to be used as the basis 

for the form of a standard, and (3) the 

implications from a public health 

communication perspective of the 

extent to which either form allows 

different numbers of days in a year to 

be above the level of the standard in 

areas that attain the standard. Based on 

these considerations, the Staff Paper 

recommended either retaining the 98th 

percentile form or revising it to be based 

on the 99th percentile form, and noted 

that primary consideration should be 

given to the combination of form and 

level, as compared to looking at the 

form in isolation (EPA, 2005, p. 5–44). 

In considering the information 

provided in the Staff Paper, most 

CASAC Panel members favored 

continued use of the 98th percentile for 

a concentration-based form because it is 

more robust than the 99th percentile, 

such that it would provide more 

stability to prevent areas from moving in 

and out of attainment from year to year 

(Henderson 2005a). In recommending 

retention of the 98th percentile form, 

the CASAC Panel recognized that it is 

the link between the form and level of 

a standard that determines the degree of 

public health protection the standard 

affords. 

In considering the available 

information and the Staff Paper and 

CASAC recommendations, the 

Administrator proposed to retain the 

form for the 24-hour standard. In so 

doing, the Administrator focused on the 

relative stability of the 98th and 99th 

percentile forms as a basis for selecting 

the 98th percentile form, while 

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. 

None of the public commenters raised 

objections to continuing the use of a 

concentration-based form for the 24- 

hour standard. Many of the individuals 

and groups who supported a more 

stringent 24-hour PM2.5 standard noted 

above in Section II.B, however, 

recommended a more restrictive 

concentration-based percentile form, 

specifically a 99th percentile form. The 

limited number of these commenters 

who provided a specific rationale for 

this recommendation generally 

expressed their concern that the 98th 

percentile form could allow too many 

days where concentrations exceeded the 

level of the standard, and thus fail to 

adequately protect public health. The 

EPA received comparatively few public 

comments from State and local air 

pollution control authorities and tribal 

organizations on the form of the 24-hour 

PM2.5 standard. Of the limited number 

of state air pollution control authorities 

that commented on the form of the 24- 

hour PM2.5 standard, all supported 

retaining the 98th percentile form. Of 

the limited number of local air pollution 

control authorities and tribal 

organizations that commented on the 

form of the 24-hour PM2.5 standard, 

some supported retaining the 98th 

percentile form while others supported 

the 99th percentile form. Beyond their 

support for retaining the current 24- 

hour PM2.5 standard, which has a 98th 

percentile form, commenters 

representing industry associations and 

businesses provided no specific 

comments regarding the form of the 24- 

hour PM2.5 standard. 

The EPA notes that the viewpoints 

represented in this review are similar to 

comments submitted in the last review 

and through various NAAQS reviews. 

The EPA recognizes that the selection of 

the appropriate form includes 

maintaining adequate protection against 

peak 24-hour values while also 

providing a stable target for risk 

management programs, which serves to 

provide for the most effective public 

health protection in the long run.24

Nothing in the commenters’ views has 

provided a reason to change the 

Administrator’s previous conclusion 

regarding the appropriate balance 

represented in the proposed form of the 

24-hour PM2.5 standard. Therefore, the 

Administrator concurs with CASAC 

recommendations and concludes that it 

is appropriate to retain the 98th 

percentile form for the 24-hour PM2.5

standard. 

In reaching this conclusion, EPA also 

recognizes that several states that 

otherwise supported EPA’s proposal to 

retain the 98th percentile form of the 24- 

hour PM2.5 standard raised concerns 

regarding a technical problem 

associated with a potential bias in the 

method used to calculate the 98th 

percentile concentration for this form. 

NESCAUM, in particular, noted that 

‘‘the existing and proposed 

methodology yields a lower (i.e., less 

stringent) value on average for a 1 in 3 

day frequency sample data-set 

compared to a daily sample data-set by 

approximately 1 μg/m 3’’ (NESCAUM, p. 

3), and recommended revisions to the 

methodology such that ‘‘the calculation 

becomes insensitive to data capture rate 

or sampling frequency’’ (NESCAUM, 

Attachment A, p.7). Another state 

commenter suggested the issue could be 

addressed by ‘‘the addition of language 

that requires areas that are near the 

daily NAAQS to continue to use every 

day FRM/FEM sampling’’ (Delaware 

Department of Natural Resources, p. 4). 

The EPA agrees with these commenters 

that the potential bias in calculating the 

design value of the 24-hour PM2.5

standard is a concern. To reduce this 

bias, EPA had proposed to increase the 

sampling frequency for monitoring sites 

that were within 10 percent of the 

standard to 1 in 3 day sampling (Part 58 

section 12(d)(1)). The EPA is persuaded 

by these comments that it is appropriate 

to adjust the proposed sampling 

frequency requirements in order to 

further reduce this bias. Accordingly, 

EPA is modifying the final monitoring 

requirements such that areas that are 

within 5 percent of the standard will be 

required to increase the frequency of 

sampling to every day (Part 58 section 

12(d)(1).25

2. Annual PM2.5 Standard 

In 1997 EPA established the form of 

the annual PM2.5 standard as an annual 

arithmetic mean, averaged over 3 years, 

from single or multiple communityoriented monitors. This form of the 

annual standard was intended to 

represent a relatively stable measure of 

air quality and to characterize area-wide 

PM2.5 concentrations in conjunction 

with a 24-hour standard designed to 

provide adequate protection against 

localized peak or seasonal PM2.5 levels. 

The current annual PM2.5 standard level 

is to be compared to measurements 

made at the community-oriented 

monitoring site recording the highest 

level, or, if specific constraints are met, 

measurements from multiple 

community-oriented monitoring sites 

may be averaged (Part 50 Appendix N 

section 1.0(c) and 2.1(a) and (b) and Part 

58 Appendix D section 2.8.1.6.1; 62 FR 

38672). Community-oriented monitoring 

sites were specified to be consistent 

with the intent that a spatially averaged 

annual standard protect persons living 

in smaller communities, as well as those 

in larger population centers. The 

constraints on allowing the use of 

spatially averaged measurements were 


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26The current constraints include the criteria that 

the correlation coefficient between monitor pairs to 

be averaged be at least 0.6, and that differences in 

mean air quality values between monitors to be 

averaged not exceed 20 percent and that areas in 

which monitoring results may be averaged should 

principally be affected by the same major emission 

source of PM2.5 (Part 58 App. D section 2.8.1.6.1). 

27As discussed in the Staff Paper (EPA, 2005; 

section 4.2.2), the monitored air quality values were 

used to determine the design value for the annual 

standard in each area, as applied to a ‘‘composite’’

monitor to reflect area-wide exposures. Changing 

the basis of the annual standard design value from 

the concentration at the highest monitor to the 

average concentration across all monitors changes 

the amount of reduction in PM2.5 levels that is 

needed to just meet the current or alternative 

annual standards. With averaging, less overall 

reduction in ambient PM2.5 is needed to just meet 

the standards. 

28For example, based on analyses conducted in 

three example urban areas, estimated mortality 

incidence associated with long-term exposure based 

on the use of spatial averaging is about 10 to more 

than 40 percent higher than estimated incidence 

based on the use of the highest monitor (EPA, 2005, 

p.5–41). 

29As summarized in section II.A.4 of the 

proposal, the Criteria Document notes that some 

epidemiologic study results, most notably the 

associations between total mortality and long-term 

PM2.5 exposure in the ACS cohort, have shown 

larger effect estimates in the cohort subgroup with 

lower education levels (EPA, 2004a, p. 8–103). The 

Criteria Document also notes that lower education 

level can be a marker for lower socioeconomic 

status that may be related to increased vulnerability 

to the effects of fine particle exposures, for example, 

as a result of greater exposure from proximity to 

sources such as roadways and industry, as well as 

other factors such as poorer health status and access 

to health care (EPA, 2004a, section 9.2.4.5). 

30 In CASAC’s review of the Second Draft Staff 

Paper, most of the members of the CASAC Review 

Panel found the fine particle sections to be 

‘‘generally well-written and scientifically wellreasoned’’ but, beyond their recommendation that 

the primary PM2.5 standards should be 

strengthened, CASAC provided no specific 

intended to limit averaging across 

poorly correlated or widely disparate air 

quality values.26 This approach was 

judged to be consistent with the shortterm epidemiologic studies on which 

the annual 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, 

not localized ‘‘hot spots’’ (62 FR 38672). 

These criteria and constraints were 

intended to ensure that spatial averaging 

would not result in inequities in the 

level of protection afforded by the PM2.5

standards (Id.). 

In this review, there now exists a 

much larger set of PM2.5 air quality data 

than was available in the last review. 

Consideration in the Staff Paper of the 

spatial variability across urban areas 

that is revealed by this new data base 

has raised questions as to whether an 

annual standard that allows for spatial 

averaging, within currently specified or 

alternative constraints, would provide 

appropriate public health protection. 

Analyses in the Staff Paper to assess 

these questions, as discussed below, 

took into account both aggregate 

population risk across an entire urban 

area and the potential for 

disproportionate impacts on potentially 

vulnerable subpopulations within an 

area. 

The effect of allowing the use of 

spatial averaging on aggregate 

population risk was considered in 

sensitivity analyses included in the 

health risk assessment (EPA, 2005, 

section 4.4.3.2). In particular, this 

included analyses of several urban areas 

that compared estimated mortality risks 

based on calculating compliance with 

alternative standards (1) using air 

quality values from the highest 

community-oriented monitor in an area 

and (2) using air quality values averaged 

across all such monitors within the 

constraints on spatial averaging allowed 

by the current standard.27 As expected, 

estimated risks associated with longterm exposures that remain upon just 

meeting the current annual standard are 

greater when spatial averaging is used 

than when the highest monitor is used 

(i.e., the estimated reductions in risk 

associated with just attaining the 

current or alternative annual standards 

are less when spatial averaging is used), 

as the use of the highest monitor leads 

to greater modeled reductions in 

ambient PM2.5 concentrations.28

In considering the potential for 

disproportionate impacts on potentially 

vulnerable subpopulations, EPA 

assessed whether any such groups are 

more likely than the general population 

to live in census tracts 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 tract level, including 

education level, income level, and 

percent minority population. Data from 

the census tract in each area in which 

the highest air quality value was 

monitored were compared to the areawide average value (consistent with the 

constraints on spatial averaging 

provided by the current standard) in 

each area (Schmidt et al., 2005). 

Recognizing the limitations of such 

cross-sectional analyses, the Staff Paper 

observed that the results suggest 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 

(EPA, 2005, p. 5–41).29 Noting the 

intended purposes of the form of the 

annual standard, as discussed above, the 

Staff Paper concluded that the existing 

constraints on spatial averaging may not 

be adequate to avoid substantially 

greater exposures in some areas, 

potentially resulting in disproportionate 

impacts on these potentially vulnerable 

subpopulations. 

In considering whether more stringent 

constraints on the use of spatial 

averaging may be appropriate, the Staff 

Paper presented results of an analysis of 

recent air quality data which assessed 

correlations and differences between 

monitor pairs in metropolitan areas 

across the country (Schmidt et al., 

2005). For all pairs of PM2.5 monitors, 

the median correlation coefficient based 

on annual air quality data is 

approximately 0.9, which is 

substantially higher than the current 

criterion (in Appendix D of Part 58, 

section 2.8.1.6.1) of a minimum 

correlation of at least 0.6, which was 

met by nearly all monitor pairs. The 

current criterion that differences in 

mean air quality values between 

individual monitors and the 

corresponding multi-site spatial average 

not exceed 20 percent on an annual 

basis also was met for most monitor 

pairs, while the actual annual median 

and mean differences for all monitor 

pairs were 5 percent and 8 percent, 

respectively. This analysis also showed 

that in some areas with highly seasonal 

air quality patterns (e.g., due to seasonal 

wood smoke emissions), substantially 

lower seasonal correlations and larger 

seasonal differences can occur relative 

to those observed on an annual basis. 

This analysis provided some 

perspective on the constraints on spatial 

averaging that were adopted in the last 

review before data were widely 

available on spatial distributions of 

PM2.5 air quality levels. 

In considering the results of the 

analyses discussed above, the Staff 

Paper concluded that it is appropriate to 

consider either eliminating the 

provision that allows for spatial 

averaging from the form of an annual 

PM2.5 standard or narrowing the 

constraints on spatial averaging to be 

based on more restrictive criteria. More 

specifically, based on the analyses 

discussed above, the Staff Paper 

recommended consideration of revised 

criteria such that the correlation 

coefficient between monitor pairs to be 

averaged be at least 0.9, determined on 

a seasonal basis, and annual mean 

differences between individual monitors 

and corresponding spatial averages not 

exceed 10 percent (EPA, 2005, p. 5–

42).30


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comments regarding the form of the annual 

standard (Henderson, 2005a, pp. 1–2). 

31 In so doing, EPA noted that an annual standard 

would focus control programs on annual average 

PM2.5 concentrations, which would generally 

control the overall distribution of 24-hour exposure 

levels, as well as long-term exposure levels, and 

would also result 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, EPA recognized that either standard could 

provide some degree of protection from both shortand 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). 

32See also ATA III, 283 F.3d at 373 (endorsing 

this reasoning). 

In considering the Staff Paper 

recommendations based on the results 

of the analyses discussed above, and 

focusing on a desire to be consistent 

with the epidemiologic studies on 

which the PM2.5 health effects are based 

and concern over the evidence of 

potential disproportionate impact on 

potentially vulnerable subpopulations, 

the Administrator proposed to revise the 

form of the annual PM2.5 standard 

consistent with the Staff Paper 

recommendation to change two of the 

criteria for use of spatial averaging such 

that the correlation coefficient between 

monitor pairs must be at least 0.9, 

determined on a seasonal basis, with 

differences between monitor values not 

to exceed 10 percent (71 FR 2647). The 

Administrator also solicited comment 

on the other Staff Paper-recommended 

alternative of revising the form of the 

annual PM2.5 standard to one based on 

the highest community-oriented 

monitor in an area, with no allowance 

for spatial averaging (Id. at 2647–48). 

Relatively few public comments were 

received on the form of the annual PM2.5

standard. Of the commenters noted 

above in Section II.B who supported a 

more stringent annual PM2.5 standard, 

those who commented on the form of 

the annual PM2.5 standard argued that 

the EPA analyses described above 

demonstrated that the current form of 

the standard results in uneven public 

health protection leading to 

disproportionate impacts on potentially 

vulnerable subpopulations, and thus a 

change in the form of the standard is 

needed. However, these commenters 

argued that the proposed modifications 

to the spatial averaging criteria were not 

stringent enough and, in order to reduce 

the possibility of pollution hotspots and 

disproportionate impacts, especially in 

areas meeting the annual PM2.5

standard, spatial averaging should be 

eliminated (American Lung Association 

et al., 2006, pp. 44–47; Schwartz, 2005, 

p. 2). Of the commenters noted above in 

Section II.B who supported retaining the 

current annual PM2.5 standard, those 

who commented specifically on the 

form of the standard supported retaining 

the current spatial averaging criteria. 

These views are most extensively 

presented in comments from UARG who 

argued that changes to the spatial 

averaging criteria, effectively increasing 

the stringency of the standard, are not 

needed as the current standards provide 

the requisite degree of public health 

protection (UARG, 2006. pp. 33–36). In 

addition, one state air pollution control 

agency supported a more stringent level 

for the annual PM2.5 standard in the 

range recommended by CASAC but also 

supported retaining the option for 

spatial averaging for the form of the 

standard arguing that ‘‘rarely is one 

monitor representative of an entire 

nonattainment area’’ especially in the 

western U.S. (Utah Department of 

Environmental Quality, 2006, p. 2). 

The Administrator emphasizes that 

the intent of the current spatial 

averaging criteria, as defined in 1997 

based on a limited set of PM2.5 air 

quality data, was to ensure that spatial 

averaging would not result in inequities 

in the level of protection provided by 

the PM2.5 standards against health 

effects associated with short- and longterm exposures to PM2.5. Based on the 

analyses described above (Schmidt et 

al., 2005), which are based on the much 

larger set of air quality data that has 

become available since the last review, 

EPA now believes that tighter 

constraints on spatial averaging are 

necessary to address concerns over 

potential disproportionate impacts on 

the populations that EPA has identified 

as being potentially vulnerable to PM2.5- 

related health effects. The EPA believes 

that current information and analyses 

indicate that application of the current 

form has the clear potential to result in 

disproportionate impacts on potentially 

vulnerable subpopulations in some 

areas. The EPA recognizes that the 

proposed constraints have the potential 

to increase the stringency of the annual 

PM2.5 standard in some areas in which 

a State might choose to use spatial 

averaging. The EPA believes that in 

such cases this increased stringency is 

warranted so as to address possible 

disproportionate impacts on potentially 

vulnerable populations and more 

generally to avoid inequities across all 

population groups. The EPA disagrees 

with those commenters who support 

eliminating spatial averaging altogether. 

The EPA believes that the proposed 

narrowing of the spatial averaging 

criteria will adequately address the 

concerns about disproportionate impact 

raised by some commenters, as analyzed 

in the Staff Paper, by substantially 

reducing the amount of spatial variation 

in long-term ambient levels that will be 

allowed to be averaged together in 

determining compliance with the 

standard. Therefore, the Administrator 

concludes that the current form of the 

standard should be retained with the 

proposed modifications. The form of the 

annual PM2.5 standard is retained as an 

annual arithmetic mean, averaged over 

3 years; however, the following two 

aspects of the spatial averaging criteria 

are narrowed: (1) The annual mean 

concentration at each site shall be 

within 10 percent of the spatially 

averaged annual mean, and (2) the daily 

values for each monitoring site pair 

shall yield a correlation coefficient of at 

least 0.9 for each calendar quarter. 

F. Level of Primary PM2.5 Standards 

In the last review, having concluded 

that it was appropriate to establish both 

24-hour and annual PM2.5 standards, 

EPA selected a level for each standard 

that was appropriate for the function to 

be served by each (62 FR 38674, 38676–

77). As noted above, EPA concluded at 

that time that the suite of PM2.5

standards could most effectively and 

efficiently protect public health by 

treating the annual standard as the 

generally controlling standard for 

lowering both short- and long-term 

PM2.5 concentrations.31 In conjunction 

with such an annual standard, the 24- 

hour standard was intended to provide 

protection against days with high peak 

PM2.5 concentrations, localized 

‘‘hotspots,’’ and risks arising from 

seasonal emissions that would not be 

well controlled by an annual standard.32

In selecting the level for the annual 

standard in the last review, EPA used an 

evidence-based approach that 

considered the evidence from both 

short- and long-term exposure studies. 

The risk assessment conducted in the 

last review, while providing qualitative 

insights about the distribution of risks, 

was considered by EPA to be too limited 

to serve as a quantitative basis for 

decisions on the standard levels. In 

accordance with Staff Paper and CASAC 

views on the relative strengths of the 

short- and long-term exposure studies, 

EPA placed greater emphasis on the 

short-term exposure studies. In so 

doing, EPA first determined a level for 

the annual standard based on the shortterm exposure studies, and then 

considered whether the long-term 

exposure studies suggested the need for 

a lower level. While recognizing that 

health effects could occur over the full 

range of concentrations observed in the 

studies, EPA concluded that the 


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33As discussed in the Staff Paper (EPA, 2005, p. 

5–30) and supporting staff memo (Ross and 

Langstaff, 2005), staff focused on U.S. and Canadian 

short-term exposure PM2.5 studies that had been 

reanalyzed as appropriate to address statistical 

modeling issues and considered the extent to which 

the reported associations are robust to co-pollutant 

confounding and alternative modeling approaches 

and are based on relatively reliable air quality data. 

Additional air quality data used in this analysis 

were documented in another staff memo (Ross and 

Langstaff, 2006) that was placed in the docket 

during the public comment period. 

34Of the four cities in this study that were within 

this range of air quality, statistically significant 

results were reported for Boston, St. Louis, and 

Knoxville, but not for Steubenville. 

strongest evidence for short-term PM2.5

effects occurs for air quality 

distributions with long-term 

concentrations near the long-term (e.g., 

annual) average in those studies 

reporting statistically significant health 

effects. Thus, in the last review, EPA 

selected a level for the annual standard 

that was somewhat below the lowest 

long-term average PM2.5 concentration 

in a short-term exposure study that 

reported statistically significant health 

effects. Further consideration of the 

average PM2.5 concentrations across the 

cities in the key long-term exposure 

studies available at that time did not 

provide a basis for establishing a lower 

annual standard level. 

In this review, the approach used in 

the Staff Paper as a basis for staff 

recommendations on standard levels 

built upon and broadened the general 

approach used by EPA in the last 

review. This broader approach reflected 

the more extensive and stronger body of 

evidence now available on health effects 

related to both short- and long-term 

exposure to PM2.5, together with the 

availability of much more extensive 

PM2.5 air quality data. This newly 

available information was used to 

conduct a more comprehensive risk 

assessment for PM2.5. As a consequence, 

the broader approach used in the Staff 

Paper discussed ways to take into 

account both evidence-based and 

quantitative risk-based considerations 

and placed relatively greater emphasis 

on evidence from long-term exposure 

studies than was done in the last 

review. 

Given the extensive body of new 

evidence based specifically on PM2.5

that is now available, and the resulting 

broader approach presented in the Staff 

Paper, the Administrator considered it 

appropriate to use a somewhat different 

evidence-based approach from that used 

in the last review to propose appropriate 

standard levels. In the Administrator’s 

view, the very large numbers of PM2.5

health effect studies that now make up 

the available body of evidence provide 

the most reliable basis for determining 

the level of the standards. More 

specifically, EPA’s proposal relied on an 

evidence-based approach that 

considered the much expanded body of 

evidence from short-term exposure 

PM2.5 studies as the principal basis for 

selecting the level of the 24-hour 

standard, with such standard aimed at 

protecting against health effects 

associated with short-term exposures to 

PM2.5. Likewise, the stronger and more 

robust body of evidence from the longterm exposure PM2.5 studies was 

considered as the principal basis for 

selecting the level of the annual 

standard, with such standard aimed at 

protecting against health effects 

associated with long-term exposures to 

PM2.5.

With respect to the quantitative risk 

assessment, the Administrator 

recognized at proposal that it rests on a 

more extensive body of data and is more 

comprehensive in scope than the 

assessment conducted in the last 

review, but was mindful that significant 

uncertainties continue to underlie the 

resulting risk estimates. Such 

uncertainties generally relate to a lack of 

clear understanding of a number of 

important factors, including, for 

example, the shape of concentrationresponse functions, particularly when, 

as here, effect thresholds can neither be 

discerned nor determined not to exist; 

issues related to selection of appropriate 

statistical models for the analysis of the 

epidemiologic data; the role of 

potentially confounding and modifying 

factors in the concentration-response 

relationships; issues related to 

simulating how PM2.5 air quality 

distributions will likely change in any 

given area upon attaining a particular 

standard, since strategies to reduce 

emissions are not yet defined; and 

whether there would be differential 

reductions in the many components 

within PM2.5 and, if so, whether this 

would result in differential reductions 

in risk. In the case of fine particles, the 

Administrator recognized that for 

purposes of developing quantitative risk 

estimates such uncertainties are likely 

to amplified by the complexity in the 

composition of the mix of fine particles 

generally present in the ambient air. 

Further, in the Administrator’s view, 

this risk assessment, which is based on 

studies that do not resolve the issue of 

a threshold, has important limitations as 

a basis for standard setting, since if no 

threshold is assumed the assessment 

necessarily predicts that ever lower 

standards result in ever lower risks. 

This has the effect of masking the 

increasing uncertainty in the risk 

estimates that exists as lower levels are 

considered, even when a range of 

assumed thresholds is included. As a 

result, at the time of proposal the 

Administrator viewed the risk 

assessment as providing supporting 

evidence for the conclusion that there is 

a need to revise the current suite of 

PM2.5 standards, but he judged that it 

did not provide an appropriate basis to 

determine what specific quantitative 

revisions are appropriate. 

1. 24-Hour PM2.5 Standard 

Based on the approach discussed 

above, the Administrator relied upon 

evidence from the short-term exposure 

PM2.5 studies as the principal basis for 

selecting the proposed level of the 24- 

hour standard. In considering these 

studies as a basis for the level of a 24- 

hour standard, and having provisionally 

selected a 98th percentile form for the 

standard, the Administrator agreed with 

the focus in the Staff Paper of looking 

at the 98th percentile values in these 

studies. In so doing, the Administrator 

recognized that these studies provide no 

evidence of clear effect thresholds or 

lowest-observed-effects levels. Thus, in 

focusing on 98th percentile values in 

these studies, the Administrator was 

seeking to establish a standard level that 

will require improvements in air quality 

generally in areas in which the 

distribution of daily short-term 

exposure to PM2.5 can reasonably be 

expected to be associated with serious 

health effects. Although future air 

quality improvement strategies in any 

particular area are not yet defined, most 

such strategies are likely to move a 

broad distribution of PM2.5 air quality 

values in an area lower, resulting in 

reductions in risk associated with 

exposures to PM2.5 levels across a wide 

range of concentrations. 

Based on the information in the Staff 

Paper and in a supporting staff 

memorandum,33 the Administrator 

observed an overall pattern of 

statistically significant associations 

reported in studies of short-term 

exposure to PM2.5 across a wide range of 

24-hour average 98th percentile values. 

More specifically, the Administrator 

observed a strong predominance of 

studies with 98th percentile values 

down to about 39 μg/m3 (in Burnett and 

Goldberg, 2003) reporting statistically 

significant associations with mortality, 

hospital admissions, and respiratory 

symptoms. For example, within this 

range of air quality, statistically 

significant associations were reported 

for mortality in the combined Six Cities 

study (and three of four individual cities 

within that study 34) (Klemm and 

Mason, 2003), the Canadian 8-City 

Study (Burnett and Goldberg, 2003), and 

in studies in Santa Clara County, CA 


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35The proposal incorrectly listed this as an 

association with ischemic heart disease. 

36The proposal incorrectly included Delfino et 

al., 1997 here as well as correctly including it in 

the next lower air quality range. 

37Of the studies within this group that evaluated 

multi-pollutant associations, as discussed above in 

section II.A.3, the results reported in Fairley (2003), 

Sheppard (2003), and Ito (2003) were generally 

robust to inclusion of gaseous co-pollutants. 

38The proposal incorrectly identified this as a 

statistically significant association. 

39For example, Delfino et al. (1997) report 

statistically significant associations between PM2.5

and respiratory emergency department visits for 

elderly people (>64 years old), but not children (<2 

years old), in one part of the study period (summer 

1993) but not the other (summer 1992). Peters et al. 

(2000) report new findings of associations between 

fine particles and cardiac arrhythmia, but the 

Criteria Document observes that the strongest 

associations were reported for a small subset of the 

study population that had experienced 10 or more 

defibrillator discharges (EPA, 2004a, p. 8–164). 

40The proposal incorrectly identified this as a 

statistically significant association. 

41As noted above, the proposed form of the 24- 

hour standard was the same as the current standard. 

(Fairley, 2003) and Philadelphia 

(Lipfert, 2000); for hospital admissions 

and emergency department visits in 

Seattle (Sheppard et al., 2003), Toronto 

(Burnett et al., 1997; Thurston et al., 

1994), Detroit (Ito, 2003, for heart 

failure 35 and pneumonia, but not for 

other causes), and Montreal (Delfino et 

al., 1998,36 for some but not all age 

groups and years); and for respiratory 

symptoms in panel studies in a 

combined Six Cities study (Schwartz et 

al., 1994, as reanalyzed in Schwartz and 

Neas, 2000) and in two Pennsylvania 

cities (Uniontown in Neas et al., 1995; 

State College in Neas et al., 1996).37

Studies in this air quality range that 

reported positive but not statistically 

significant associations include 

mortality studies in Detroit (Ito, 2003), 

Pittsburgh (Chock et al., 2000), 

Steubenville (Klemm and Mason, 2003), 

and Montreal (Goldberg and Burnett, 

2003), and a study of lung function in 

Philadelphia 38 (Neas et al., 1999). 

Within the range of 24-hour average 

98th percentile PM2.5 concentrations of 

about 35 to 30 μg/m3, the Administrator 

no longer observed this strong 

predominance of statistically significant 

results. Rather, within this range, one 

study reports statistically significant 

results (Mar et al., 2003), other studies 

report mixed results in which some 

associations reported in the study are 

statistically significant and others are 

not (Delfino et al., 1997; Peters et al., 

2000),39 and other studies report 

associations that are not statistically 

significant (Ostro, 2003; 40 two 

individual cities within Klemm and 

Mason, 2003). Further, the 

Administrator concluded that the very 

limited number of studies in which the 

98th percentile values are below this 

range (Stieb et al., 2000; Peters et al., 

2001) do not provide a basis for 

reaching conclusions about associations 

at such levels. Thus, in the 

Administrator’s view, this body of 

evidence provided confidence that 

statistically significant associations are 

occurring down close to this range, and 

it provided a clear basis for 

provisionally concluding that this range 

represents a range of reasonable values 

for a 24-hour standard level. The 

Administrator further noted that 

focusing on the range of 35 to 30 μg/m3

is consistent with the interpretation of 

the evidence held by most CASAC Panel 

members as reflected in their 

recommendation to select a 24-hour 

PM2.5 standard level within this range 

(Henderson, 2005a, p. 7). The 

Administrator recognized, however, the 

separate point that most CASAC Panel 

members favored the range of 35 to 30 

μg/m3 for the 24-hour PM2.5 standard in 

concert with an annual standard set in 

the range of 14 to 13 μg/m3 (Id.), as 

discussed in section II.F.2 below. 

At proposal, in considering what level 

would be appropriate for a 24-hour 

standard, the Administrator was 

mindful that this choice requires 

judgment based on an interpretation of 

the evidence that neither overstates nor 

understates the strength and limitations 

of the evidence, or the appropriate 

inferences to be drawn from the 

evidence. In the absence of evidence of 

any clear effects thresholds, EPA may 

select a specific standard level from 

within a range of reasonable values. In 

making this judgment, the 

Administrator noted that the general 

uncertainties related to the shape of the 

concentration-response functions and to 

the selection of appropriate statistical 

models affect the likelihood that 

observed associations are causal down 

to the lowest concentrations in the 

studies. Further, and more specifically, 

the variation in results found in the 

short-term exposure studies in which 

the 98th percentile values were below 

35 μg/m3 indicated an increase in 

uncertainty as to whether likely causal 

associations extend down below this 

level (71 FR 2649). 

In considering the extent to which the 

quantitative risk assessment should 

inform EPA’s selection of a 24-hour 

PM2.5 standard, the Administrator 

recognized that risk estimates based on 

simulating the attainment of standards 

set at lower levels within this range will 

inevitably suggest some additional 

reductions in risk at each lower 

standard level considered. However, 

these quantitative risk estimates largely 

depend upon assumptions made about 

the lowest level at which reported 

associations will likely persist and 

remain causal in nature. Thus, the 

Administrator was hesitant to use such 

risk estimates as a basis for proposing a 

specific standard level, particularly one 

below 35 μg/m3, and instead preferred 

to base the decision on level directly on 

the evidence in the studies themselves 

(71 FR 2649). 

Taking the above considerations into 

account, the Administrator proposed to 

set the level of the primary 24-hour 

PM2.5 standard at 35 μg/m3.41 In the 

Administrator’s judgment at that time, 

based on the currently available 

evidence, a standard set at this level 

would protect public health with an 

adequate margin of safety from serious 

health effects, including premature 

mortality and hospital admissions for 

cardiorespiratory causes that are likely 

causally associated with short-term 

exposure to PM2.5. This judgment 

appropriately considered the 

requirement for a standard that is 

neither more nor less stringent than 

necessary for this purpose and 

recognized that the CAA does not 

require that primary standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

At the time of proposal, the 

Administrator recognized that sharply 

divergent views on the appropriate level 

of this standard had been presented to 

EPA as part of the NAAQS review 

process, and solicited comment on a 

wide range of standard levels and 

alternative approaches to characterizing 

and addressing scientific uncertainties. 

One such alternative view focused very 

strongly on the uncertainties inherent in 

the epidemiologic and toxicologic 

studies and the quantitative risk 

assessment as the basis for concluding 

that no change to the current 24-hour 

PM2.5 standard of 65 μg/m3 was 

warranted. In sharp contrast, others 

viewed the epidemiologic evidence and 

other health studies as strong and 

robust, and generally placed much 

weight on the results of the quantitative 

risk assessment as a basis for concluding 

that a much stronger policy response is 

warranted, generally consistent with a 

standard level at or below 25 μg/m3. As 

discussed below, the same sharply 

divergent views were generally repeated 

in comments on the proposal by the two 

distinct groups of commenters 

identified in section II.B.2 above. 

In considering comments received on 

the proposal, the Administrator first 

notes that CASAC provided additional 

recommendations concerning the 


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42The EPA’s consideration of this examination is 

discussed more fully in the Response to Comments 

document. 

proposed PM standards in a letter to the 

Administrator (Henderson, 2006, p. 2), 

noting that members of the CASAC PM 

Panel were generally pleased that the 

proposed 24-hour PM2.5 primary 

standard was within the range that had 

previously been recommended by most 

members. Further, the Panel recognized 

that the proposed choice of the high end 

of the recommended range was a policy 

judgment. A number of commenters, 

including many States and Tribes, who 

supported the proposed level generally 

placed great weight on the 

recommendation of CASAC. 

Many more commenters expressed 

disagreement with the proposed level. 

As noted above, these commenters 

generally fell into two distinct groups 

that expressed sharply divergent views 

on their interpretations of the science 

(in some cases taking into consideration 

‘‘new’’ science not included in the 

Criteria Document), on the appropriate 

policy response based on the science, 

and on how the quantitative risk 

assessment should factor into a decision 

on the standard level. 

In interpreting the available scientific 

information, including consideration of 

‘‘new’’ science, and advocating a policy 

response based on the science, one 

group of commenters focused strongly 

on the uncertainties they saw in the 

scientific evidence as a basis for 

concluding that no change to the current 

level of the 24-hour PM2.5 standard was 

warranted. This group included 

virtually all commenters representing 

industry associations and businesses. In 

commenting on the proposed level, 

these commenters most generally relied 

on the same arguments presented above 

in section II.B.2 as to why they believed 

it was inappropriate for EPA to make 

any revisions to the suite of primary 

PM2.5 standards. That is, they asserted 

that the health effects of concern 

associated with short-term exposure to 

PM2.5 have not changed significantly 

since 1997; that the uncertainties in the 

underlying time-series epidemiologic 

studies are as great or greater than in 

1997; that the estimated risk upon 

attainment of the current PM2.5

standards is lower now than it was 

when the PM2.5 standards were set in 

1997; and that ‘‘new’’ science not 

included in the Criteria Document 

continues to increase uncertainty about 

possible health risks associated with 

exposure to PM2.5. These general 

comments are addressed above in 

section II.B.2. 

In more specific comments, UARG 

and other commenters in this group 

called into question EPA’s rationale for 

the proposed level of 35 μg/m3. In so 

doing, these commenters primarily 

relied on an examination of this 

rationale included in an attachment to 

UARG’s comments as the basis for 

concluding that the available studies do 

not support EPA’s view of the overall 

pattern of statistically significant 

associations in studies of short-term 

exposure to PM2.5 across a wide range of 

98th percentile PM2.5 values. This 

examination of such studies concluded 

that there is no consistent pattern of 

associations at levels up to (and above) 

the 65 μg/m3 98th percentile level of the 

current standard. This examination was 

based on an individual consultant’s 

ranking of a set of short-term exposure 

studies by what is characterized as the 

‘‘overall significance’’ of each study’s 

results. A number of studies were 

included in this examination that EPA 

did not include in looking at the pattern 

of associations. 

In considering the approach used in 

this examination, EPA concludes that 

the categorical rankings were 

inappropriately defined in a very 

restrictive way that overly emphasized 

certain studies based on selection 

criteria that favored multi-pollutant 

models and alternative model 

specifications, which had the effect of 

dismissing statistically significant 

results in some studies. This conclusion 

reflects EPA’s consideration of these 

issues as presented above in section 

II.B.2. As noted there, EPA believes in 

the importance of a comprehensive 

evaluation that considers and weighs a 

variety of evidence, including biological 

plausibility of associations between the 

various pollutants and health outcomes, 

and focuses on the stability of the size 

of the effect estimates in time-series 

studies using both single- and multipollutant models, rather than just 

looking at statistical significance in a 

large number of alternative models and 

using it simplistically to delineate 

between real and suspect associations. 

In addition, the examination included 

several studies that, for a variety of 

reasons, EPA does not believe are 

appropriate for such an analysis. The 

inclusion of such studies, many of 

which had low statistical power, served 

to dilute the pattern of associations seen 

in studies considered by EPA as 

providing a more appropriate basis for 

this type of examination. 

Further, even if this examination were 

to be accepted at face value, it still 

would support a distinction between the 

patterns of associations above and 

below the proposed level, in that over 

half of the cited studies with 98th 

percentile values above 35 μg/m3 were 

characterized as being of overall or 

mixed significance, and more than half 

of the cited studies with 98th percentile 

values below 35 μg/m3 were 

characterized as having no overall 

significant association. After fully 

considering this examination of patterns 

of study results, the Administrator 

believes that the observations of 

patterns of study results presented 

earlier in this section remain valid.42

The other group of commenters, 

including many medical groups, 

numerous physicians and academic 

researchers, many public health 

organizations, some States, and a large 

number of individual commenters, 

viewed the epidemiologic evidence and 

other health studies as strong and robust 

and expressed the belief that a much 

stronger policy response is warranted, 

generally consistent with a standard 

level at or below 25 μg/m3. Some of 

these commenters generally expressed 

the view that the level of the standard 

should be set below the lowest level 

observed in any of the studies that 

report any statistically significant 

association. Some also expressed the 

view that important uncertainties 

inherently present in the evidence 

warrant a highly precautionary policy 

response, particularly in view of the 

serious nature of the health effects at 

issue, and should be addressed by 

selecting a standard level that 

incorporates a large margin of safety. 

More specifically, American Lung 

Association et al. and other commenters 

noted three studies included in the 

Criteria Document with 98th percentile 

values below 35 μg/m3, including a 

mortality study in Phoenix (Mar et al., 

2000; reanalyzed in Mar et al., 2003) 

with a 98th percentile value of 32 μg/ 

m3, a study of emergency department 

visits in Montreal (Delfino et al., 1997) 

with a 98th percentile value of 31 μg/ 

m3, and a study of increase in 

myocardial infarction in Boston (Peters 

et al., 2001) with a 98th percentile value 

of 28 μg/m3. Further, these commenters 

expressed the view that EPA’s proposed 

approach to selecting a level of the 24- 

hour PM2.5 standard is fundamentally 

flawed because it ‘‘relies unreasonably 

on point estimates of statistical 

significance at various concentrations, 

rather than on trends, and because it 

completely fails to consider issues of 

statistical power’’ (American Lung 

Association et al., p. 57). In addition, 

these commenters found EPA’s 

justification for the proposed level to be 

‘‘simply irrational’’ in that it 

‘‘essentially fabricates uncertainty’’ as a 

basis for avoiding setting a standard that 


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the evidence ‘‘clearly indicates is 

necessary’’ (Id.). 

In considering these comments, the 

Administrator first notes that he 

generally agrees with CASAC’s view 

that selecting a level within the range of 

30 to 35 μg/m3 is a public health policy 

judgment and that the science does not 

dictate the selection of any specific level 

within this range. The Administrator 

also believes that this policy judgment 

should take into consideration the 

important uncertainties that remain in 

issues that are central to interpreting 

these types of time-series epidemiologic 

studies. While the Administrator 

believes that progress has been made 

since the last review in addressing key 

uncertainties, as discussed above in 

section II.B.2, EPA and the scientific 

community, including CASAC and the 

National Research Council (NRC), 

recognize that important uncertainties 

remain that warrant further research 

(e.g., see NRC, 2004). Thus, the 

Administrator does not agree that the 

Agency is ‘‘fabricating’’ uncertainties 

that do not exist. More specifically, in 

considering the studies cited in these 

comments as a basis for a standard level 

below 35 μg/m3, the Administrator 

continues to believe that it is necessary 

to consider not only the results of these 

studies and the inherent uncertainties in 

such studies, but also the pattern of 

results from other studies with similar 

air quality values. In so doing, EPA 

notes that the statistically significant 

results in Peters et al. (2001) were 

uniquely associated with 1 to 2 hour lag 

times, but not with 24-hour average 

PM2.5 concentrations, such that it would 

provide a very tenuous basis for the 

level of a 24-hour average national 

standard. While the studies in Phoenix 

and Montreal do provide some evidence 

of statistically significant associations 

within the range of 30 to 35 μg/m3, 

several other studies within this range 

of air quality that generally have 

somewhat greater statistical power and 

narrower confidence ranges do not 

provide such evidence. In making the 

public health policy judgment inherent 

in selecting a standard level, the 

Administrator believes that it is 

necessary to weigh the evidence and 

related uncertainties against the 

requirement that the standard is to be 

neither more nor less stringent than 

necessary to protect public health with 

an adequate margin of safety. See NRDC 

v. EPA, 902 F. 2d 962, 971 (D.C. Cir. 

1990) (in considering level of a NAAQS, 

EPA is required to take into account all 

of the relevant studies in the record and 

rationally determine what weight to give 

each study); API v. Costle, 665 F. 2d 

1176, 1187 (DC Cir. 1981) (same). In so 

doing, the Administrator does not agree 

that this evidence presented by 

American Lung Association et al. 

warrants a level below 35 μg/m3. 

These commenters also identified 

several ‘‘new’’ studies in support of 

their arguments for a lower level. As 

noted above, as in past NAAQS reviews, 

EPA is basing the final decisions in this 

review on the studies and related 

information included in the PM air 

quality criteria that have undergone 

CASAC and public review, and will 

consider the newly published studies 

for purposes of decision making in the 

next PM NAAQS review. Nonetheless, 

in provisionally evaluating commenters’

arguments (see Response to Comments 

document), EPA notes that its 

provisional assessment of ‘‘new’’

science found that such studies did not 

materially change the conclusions in the 

Criteria Document. 

With regard to the other studies, EPA 

notes that neither the Vancouver nor the 

Atlanta studies found statistically 

significant associations with PM2.5, and 

that the Atlanta and California studies 

were conducted in areas with 98th 

percentile PM2.5 values well above the 

proposed level. Thus, EPA concludes 

that, taken at face value, these studies 

would provide no basis for the 

commenters’ claim that they would 

require a lower standard level than one 

based on the science included in the 

Criteria Document. 

With regard to considering how the 

quantitative risk assessment should 

factor into a decision on the standard 

level, EPA notes that both groups of 

commenters generally consider the risk 

assessment in their comments on the 

standard level, but they reach 

diametrically opposed conclusions as to 

what standard level is supported by the 

assessment. The general views of both 

groups on the implications of the risk 

assessment are presented above in 

section II.B.2, with one group arguing 

that it supports a decision not to revise 

either of the current PM2.5 standards, 

and the other group arguing that it 

supports a decision to revise both PM2.5

standards. More specifically, some of 

the medical/environmental health 

commenters consider the magnitude of 

risk estimated to remain upon meeting 

the proposed 24-hour standard as a 

strong reason to select a lower level. 

These commenters generally assert that 

the risks are likely even higher than 

EPA’s primary estimates, in part 

because EPA incorporated a surrogate 

threshold of 10 μg/m3 even though there 

is no clear evidence of a threshold in the 

relevant time-series studies. On the 

other hand, the industry/business 

commenters generally assert that the 

risks are likely lower than EPA’s 

primary estimates, in part because EPA 

did not base its primary estimates on an 

assessment that included all statistical 

model results presented in the studies. 

Having considered comments based on 

the quantitative risk assessment from 

both groups of commenters, the 

Administrator finds no basis to change 

the position on the risk assessment that 

was taken at the time of proposal. That 

is, as discussed above, while the 

Administrator recognizes that the risk 

assessment rests on a more extensive 

body of data and is more comprehensive 

in scope than the assessment conducted 

in the last review, he is mindful that 

significant uncertainties continue to 

underlie the resulting quantitative risk 

estimates. Further, in the 

Administrator’s view, as noted above in 

this section, this risk assessment, which 

is based on studies that do not resolve 

the issue of a threshold, has important 

limitations as a basis for standard 

setting in this review, since if no 

threshold is assumed the assessment 

necessarily predicts that ever lower 

standards result in ever lower risks. 

This has the effect of masking the 

increasing uncertainty that exists as 

lower levels are considered, even when 

a range of assumed thresholds are 

considered. As a result, the 

Administrator judges that the 

quantitative risk assessment does not 

provide an appropriate basis for 

selecting the level of the 24-hour PM2.5

standard. 

After carefully taking the above 

comments and considerations into 

account, the Administrator has decided 

to set the level of the primary 24-hour 

PM2.5 standard at 35 μg/m3. In the 

Administrator’s judgment, based on the 

currently available evidence, a standard 

set at this level will protect public 

health with an adequate margin of safety 

from serious health effects including 

premature mortality and hospital 

admissions for cardiorespiratory causes 

that are likely causally associated with 

short-term exposure to PM2.5. A 

standard set at a higher level would not 

likely result in improvements in air 

quality in areas across the country in 

which short-term exposure to PM2.5 can 

reasonably be expected to be associated 

with serious health effects. A standard 

set at a lower level would only result in 

significant further public health 

protection if, in fact, there is a 

continuum of health risks down to the 

lower end of the ranges of air quality 

observed in the key epidemiologic 

studies and if the reported associations 

are, in fact, causally related to PM2.5 at 


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43 In the extended ACS study, significant lung 

cancer associations were found for those with high 

school education or less, but not for those with 

better than a high school education. When data are 

combined for all education levels, a significant 

association is found. 

those lower levels. Based on the pattern 

of results observed in the available 

evidence, the Administrator is not 

prepared to make those assumptions. 

Taking into account the uncertainties 

that remain in interpreting the available 

epidemiologic studies, the likelihood of 

obtaining benefits to public health 

decreases at lower levels while the 

likelihood of requiring reductions in 

ambient concentrations that go beyond 

those that are needed to reduce risks to 

public health increases. On balance, the 

Administrator does not believe that a 

lower standard is necessary to provide 

the requisite degree of public health 

protection. This judgment by the 

Administrator appropriately considers 

the requirement for a standard that is 

neither more nor less stringent than 

necessary for this purpose and 

recognizes that the CAA does not 

require that primary standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

2. Annual PM2.5 Standard 

Based on the approach discussed 

above at the beginning of section II.F, at 

the time of proposal the Administrator 

relied upon evidence from the long-term 

exposure PM2.5 studies as the principal 

basis for selecting the proposed level of 

the annual standard. In considering 

these studies as a basis for the level of 

an annual standard, the Administrator 

agreed with the evidence-based focus in 

the Staff Paper of looking at the longterm mean PM2.5 concentrations across 

the cities included in such long-term 

studies. In so doing, the Administrator 

recognized that these studies, like the 

short-term exposure studies, provide no 

evidence of clear effect thresholds or 

lowest-observed-effects levels. Thus, in 

focusing on the cross-city long-term 

mean concentrations in these studies, 

the Administrator was seeking to 

establish a standard level that will 

require improvements in air quality in 

areas in which long-term exposure to 

PM2.5 can reasonably be expected to be 

associated with serious health effects. 

Based on the characterization and 

assessment of the long-term PM2.5

exposure studies presented in the 

Criteria Document and Staff Paper, in 

the proposal the Administrator 

recognized the importance of the 

validation efforts and reanalyses that 

have been done since the last review of 

the original Six Cities and ACS 

mortality studies. These new 

assessments provide evidence of 

generally robust associations and 

provide a basis for greater confidence in 

the reported associations than in the last 

review, for example, in the extent to 

which they have made progress in 

understanding the importance of issues 

related to co-pollutant confounding and 

the specification of statistical models. 

Consistent with the information 

available in the last review, these two 

key long-term exposure mortality 

studies reported long-term mean PM2.5

concentrations across all the cities 

included in the studies of 18 and 21 μg/ 

m3, respectively. The Administrator also 

particularly recognized the importance 

of the extended ACS mortality study, 

published since the last review, which 

provides new evidence of mortality 

related to lung cancer and further 

substantiates the statistically significant 

associations with cardiorespiratoryrelated mortality observed in the 

original studies.43 The Administrator 

noted that the statistically significant 

associations reported in the extended 

ACS study, in a large number of cities 

across the U.S., provide evidence of 

effects at a lower long-term mean PM2.5

concentration (17.7 μg/m3) than had 

been observed in the original study, 

although the relative risk estimates are 

somewhat smaller in magnitude than 

those reported in the original study. The 

assessment in the Criteria Document of 

these mortality studies, taking into 

account study design, the strength of the 

study (in terms of statistical significance 

and precision of result), and the 

robustness of results, concluded that it 

would be appropriate to give the 

greatest weight to the reanalyses of the 

Six Cities and ACS studies, and in 

particular to the results of the extended 

ACS study (EPA, 2004a, p. 9–33) in 

weighing the evidence of mortality 

effects associated with long-term 

exposure to PM2.5. Consistent with that 

assessment, the Administrator placed 

greatest weight on these studies as a 

basis for selecting the proposed level of 

the annual PM2.5 standard. 

In addition to these mortality studies, 

the Administrator also recognized the 

availability of relevant morbidity 

studies providing evidence of 

respiratory morbidity, including 

decreased lung function growth, in 

children with long-term exposure to 

PM2.5. Studies conducted in the U.S. 

and Canada include the 24-Cities study 

considered in the last review and more 

recent studies of cohorts of children in 

southern California, in which the longterm mean PM2.5 concentrations in all 

the cities included in the studies are 

approximately 14.5 and 15 μg/m3, 

respectively. As discussed in section 

II.A. of the proposal (71 FR at 2632), in 

the 24 Cities study, statistically 

significant associations were reported 

between long-term fine particle 

exposures and lung function measures 

at a single point in time, whereas 

positive but generally not statistically 

significant associations were reported 

with prevalence of several respiratory 

conditions. As interpreted in the last 

review, the results from the 24-Cities 

study are uncertain as to the extent to 

which the association extends below a 

long-term mean PM2.5 concentration of 

approximately 15 μg/m3. The more 

recent Southern California children’s 

cohort study provides evidence of 

important respiratory morbidity effects 

in children, including evidence for a 

new measure of morbidity, decreased 

growth in lung function. Reports from 

this study suggest that long-term PM2.5

exposure is associated with decreases in 

lung function growth, as measured over 

a four-year follow-up period, although 

statistically significant associations are 

not consistently reported. The 

Administrator recognized that these are 

important new findings, indicating that 

long-term PM2.5 exposure may be 

associated with respiratory morbidity in 

children. However, the Administrator 

also observed that this is the only study 

reporting decreased lung function 

growth, conducted in just one area of 

the country, such that further study of 

this health endpoint in other areas of 

the country would be needed to increase 

confidence in the reported associations. 

Thus, the Administrator provisionally 

concluded that this study provides an 

uncertain basis for establishing the level 

of a national standard (Id. at 2651). 

The Administrator generally agreed 

that, as discussed in the Staff Paper 

(EPA, 2005, p. 5–22), it was appropriate 

to consider a level for an annual PM2.5

standard that is below the averages of 

the long-term PM2.5 concentrations 

across the cities in the key long-term 

exposure mortality studies, 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. 

The Administrator was mindful that 

considering what standard is 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. The 

Administrator provisionally concluded 

that these key mortality studies, together 


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with the morbidity studies, provide a 

basis for considering a standard level no 

higher than 15 μg/m3. This level is 

somewhat below the long-term mean 

concentrations in the key mortality 

studies and consistent with the 

interpretation of the evidence from the 

morbidity studies discussed above. 

Further, in the Administrator’s 

provisional view, these studies did not 

provide an appropriate basis for 

selecting a level lower than the current 

standard of 15 μg/m3. 

In considering the extent to which the 

quantitative risk assessment can help to 

inform these judgments with regard to 

the annual PM2.5 standard, the 

Administrator again recognized that risk 

estimates based on simulating the 

attainment of standards set at lower 

levels, as expected, continue to suggest 

some additional reductions in risk at the 

lower standard levels considered in the 

assessment, and that these estimates 

largely depend upon assumptions made 

about the lowest level at which reported 

associations will likely persist and 

remain causal in nature. Thus, the 

Administrator was again hesitant to use 

such risk estimates as a basis for 

proposing a lower annual standard level 

than 15 μg/m3, the level that is based 

directly on the evidence in the studies 

themselves, as discussed above. 

Taking the above considerations into 

account, the Administrator proposed to 

retain the level of the primary annual 

PM2.5 standard at 15 μg/m3. In the 

Administrator’s judgment at that time, 

based on the currently available 

evidence, a standard set at this level 

would be requisite to protect public 

health with an adequate margin of safety 

from serious health effects, including 

premature mortality and respiratory 

morbidity that are likely causally 

associated with long-term exposure to 

PM2.5. This judgment by the 

Administrator appropriately considered 

the requirement for a standard that is 

neither more nor less stringent than 

necessary for this purpose and 

recognized that the CAA does not 

require that primary standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

At the time of proposal, the 

Administrator recognized that the 

CASAC Panel did not endorse retaining 

the annual standard at the current level 

of 15 μg/m3 (Henderson, 2005a, p. 7). In 

weighing the recommendation of the 

CASAC Panel, the Administrator 

carefully considered CASAC’s stated 

rationale. In discussing its 

recommendation (Henderson, 2005a), 

the CASAC Panel first noted that 

changes to either the annual or 24-hour 

PM2.5 standard, or both, could be 

recommended. The Panel then gave 

three reasons for placing more emphasis 

on lowering the 24-hour standard than 

the annual standard: (1) The vast 

majority of studies indicating effects of 

short-term PM2.5 exposure were carried 

out in settings in which PM2.5

concentrations were largely below the 

current 24-hour standard level of 65 μg/ 

m3; (2) the amount of evidence on shortterm exposure effects, at least as 

reflected by the number of reported 

studies, is greater than for long-term 

exposure effects; and (3) toxicologic 

findings are largely related to the effects 

of short-term, rather than long-term, 

exposures. In not endorsing the option 

presented in the Staff Paper of retaining 

the level of the current annual standard 

in conjunction with lowering the 24- 

hour standard, the CASAC Panel 

observed that some cities have relatively 

high annual PM2.5 concentrations 

without much day-to-day variation and 

that such cities would only rarely 

exceed a 24-hour standard, even if it 

were set at a level below the current 

standard. In such a city, attaining a 24- 

hour standard would likely have 

minimal if any effect on the long-term 

mean PM2.5 concentration and 

consequently would be less likely to 

reduce health effects associated with 

long-term exposures. These observations 

indicate the desirability of lowering the 

level of the annual PM2.5 standard as 

well as that of the 24-hour standard, so 

as to ensure that revisions to the 

standards achieve appropriate 

reductions in long-term exposures. 

Based on these considerations and 

taking into account the results of the 

risk assessment, most CASAC Panel 

members favored setting an annual 

standard in the range of 14 to 13 μg/m3, 

along with lowering the 24-hour 

standard (Henderson, 2005a, p. 7). 

In considering these views, the 

Administrator noted that the 

appropriateness of setting an annual 

standard that would lower annual PM2.5

concentrations in cities across the 

country depends upon a policy 

judgment as to what annual level is 

required to protect public health with 

an adequate margin of safety from longterm exposures to PM2.5 in light of the 

available evidence. In considering the 

evidence of effects associated with longterm PM2.5 exposure as a basis for 

selecting an adequately health 

protective annual standard, as discussed 

above, the Administrator provisionally 

concluded that the evidence did not 

provide a basis for requiring annual 

levels below 15 μg/m3. Thus, the 

Administrator agreed conceptually with 

the CASAC Panel that any particular 24- 

hour standard may not result in 

reductions in the level of long-term 

exposures to PM2.5 in all areas with 

relatively higher than typical annual 

PM2.5 concentrations and lower than 

typical ratios of peak-to-mean values (71 

FR 2652). Further, the Administrator 

agreed that this general advice 

supported relying on the annual 

standard, and not the 24-hour standard, 

to achieve the appropriate level of 

protection from long-term exposures to 

PM2.5. However, the Administrator did 

not believe that this advice necessarily 

translated into a reason for setting the 

annual PM2.5 standard at a level below 

the current level of 15 μg/m3. As 

discussed above, the Administrator 

believed that the principal basis for 

selecting the appropriate level of an 

annual standard should be the evidence 

provided by the long-term studies, in 

conjunction with judgments concerning 

whether and over what range of 

concentrations the reported associations 

are likely causal, without reliance on 

the risk assessment, and that this 

evidence reasonably supported retaining 

the current level of the annual standard 

(Id.). 

Reflecting the great importance that 

EPA places on the advice of CASAC, the 

Administrator solicited broad public 

comment on the range of 15 down to 13 

μg/m3 the low end of the range 

recommended by CASAC for the level of 

the annual PM2.5 standard, and on the 

reasoning that formed the basis for that 

recommendation. The Administrator 

recognized that a decision to select a 

standard in this range below 15 μg/m3

would place greater weight on the 

strength of the associations reported in 

the key epidemiologic mortality and 

morbidity long-term exposure studies 

down to the lower part of the range of 

PM2.5 concentrations observed across all 

the cities included in these studies. 

Such a standard could also reflect 

greater reliance on the results of the 

quantitative risk assessment that 

suggested increased reductions in risk 

associated with meeting an annual 

standard at such lower levels (Id.). 

At the time of proposal, the 

Administrator also recognized that 

sharply divergent views on the 

appropriate level of this standard had 

been presented to EPA as part of the 

NAAQS review process, and solicited 

comments on a wider range of levels, 

down to 12 μg/m3 on alternative views 

of the appropriate interpretation of the 

epidemiologic evidence and related 

uncertainties, and on relevant research 

that would improve our understanding 

of key issues and analytic approaches to 


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44Two PM Panel members did not agree with the 

views of the majority, expressing the view that there 

was an adequate scientific basis to choose an 

annual PM2.5 standard level within the range of 12 

to 15 μg/m3 and that the choice of a specific level 

within that range was a policy decision (Henderson, 

2006, p. 6). 

45This is consistent with the approach taken in 

the Staff Paper, sections 5.3.4.1 and 5.3.5.1, for 

evaluating the evidence-based considerations 

related to setting the standards. The CASAC’s letter 

of June 6, 2005 states that the Second Draft of the 

Staff Paper was ‘‘Scientifically well-reasoned,’’ with 

the exception of a section not relevant to the fine 

PM (Henderson, 2005a, pp. 1–2). The CASAC’s 

general view thus includes this evidence-based 

approach presented in the Staff Paper. 

better inform policy judgments in the 

future. As was the case with the 24-hour 

PM2.5 standard, the same sharply 

divergent views were again expressed 

by the two distinct groups of 

commenters identified above in section 

II.B.2, as discussed below. 

In considering comments received on 

the proposal, the Administrator first 

notes that CASAC requested that EPA 

reconsider its proposed decision on the 

level of the annual PM2.5 standard and 

set the level within the range that 

CASAC had previously recommended, 

13 to 14 μg/m3 (Henderson, 2006, p. 

1).44 In so doing, CASAC reiterated and 

elaborated on the scientific basis for its 

earlier recommendation (Henderson, 

2006, pp. 3–4), which included 

consideration of the Agency’s risk 

assessment (as ‘‘the primary means of 

determining the effects on risk of 

changes in the 24-hour and annual 

PM2.5 standards in concert’’) as well as 

the observations that ‘‘a lower daily 

PM2.5 concentration limit alone cannot 

be relied on to provide protection 

against the adverse effects of higher 

annual average concentrations,’’ that 

‘‘there is evidence that effects of longterm PM2.5 concentrations occur at or 

below the level of the current standard,’’

and that ‘‘short-term effects of PM2.5

persist in cities with annual PM2.5

concentrations below the current 

standard’’ down to approximately 13 μg/ 

m3 (e.g., Burnett and Goldberg, 2003; 

Mar et al., 2003; and Lipsett et al., 

1997). The CASAC concluded: 

In summary, the epidemiologic evidence, 

supported by emerging mechanistic 

understanding, indicates adverse effects of 

PM2.5 at current annual average levels below 

15 μg/m3. The PM Panel realized the 

uncertainties involved in setting an 

appropriate, health-protective level for the 

annual standard, but noted that the 

uncertainties would increase rapidly below 

the level of 13 μg/m3. That is the basis for 

the PM Panel recommendation of a level at 

13–14 μg/m3 (Henderson, 2006, p. 4). 

In response to CASAC’s request for 

reconsideration, the Administrator has 

carefully considered its stated views 

and the scientific basis for the range it 

recommended. As an initial matter, the 

Administrator notes that CASAC’s 

recommendation to lower the level of 

the annual standard was based in large 

measure on the results of the Agency’s 

risk assessment, which examined 

changes in both the 24-hour and annual 

standard levels in concert. In 

considering this information 

qualitatively, as discussed above in 

section II.B, the Administrator believes 

that the estimates of risks likely to 

remain upon attainment of the current 

suite of PM2.5 standards are indicative of 

risks that can reasonably be judged to be 

important from a public health 

perspective, and thus support revision 

of the current suite of standards. In 

addressing what revisions to the current 

suite of PM2.5 standards are appropriate, 

the Administrator has determined that 

the evidence of health effects associated 

with short-term exposure to PM2.5 is 

such that it is appropriate to lower the 

level of the 24-hour PM2.5 standard (as 

discussed in section II.F.1 above). 

However, as discussed more fully above, 

the Administrator also believes that this 

risk assessment has important 

limitations as a basis for setting a 

standard level in this review, in part 

because the available studies do not 

resolve questions related to potential 

effect thresholds and because of other 

important uncertainties noted above in 

section II.A.3. As a result, the 

Administrator judges that the 

quantitative risk assessment does not 

provide an appropriate basis for 

selecting the level of either the 24-hour 

or the annual PM2.5 standard. Thus, the 

Administrator more heavily weighs the 

implications of the uncertainties 

associated with the Agency’s 

quantitative risk assessment than 

CASAC apparently does, and disagrees 

with CASAC that the risk assessment 

results appropriately serve as a primary 

basis for a decision on the level of the 

annual PM2.5 standard. 

The CASAC also considered the 

evidence from specific short-term 

exposure studies as part of the basis for 

its recommendation for a lower annual 

standard level, pointing to studies 

indicating that effects from short-term 

exposure of PM2.5 persist in cities with 

annual PM2.5 concentrations below the 

current standard. While the 

Administrator does not disagree with 

CASAC’s factual statements regarding 

the findings of the studies of short-term 

exposure effects, he believes that, based 

on the evidence available in this review, 

it is more appropriate to consider the 

short-term exposure studies as a basis 

for the level of the 24-hour standard and 

to consider the long-term exposure 

studies as a basis for the level of the 

annual standard. The Administrator 

recognizes that the Agency used 

available short-term exposure studies as 

the primary basis for setting the level of 

a ‘‘generally controlling’’ annual 

standard in the last review, with the 

purpose that the annual standard would 

provide protection against both shortterm exposures and long-term 

exposures, but notes that such a public 

health policy choice was made 

primarily because the short-term 

exposure studies were judged to be the 

strongest evidence available at that time 

and the evidence from long-term 

exposure studies was judged to be too 

limited to serve as other than a 

secondary consideration in setting the 

level of the annual standard. See 62 FR 

38675 n. 41 and 38676. In this review, 

however, the bodies of evidence for both 

short- and long-term exposures have 

been substantially extended and 

strengthened, such that each PM2.5

standard can appropriately be evaluated 

based on the most directly relevant body 

of scientific studies, and can be focused 

on providing protection from the health 

risks evaluated in that body of scientific 

studies. The Administrator continues to 

believe, consistent with the evidencebased approach presented in the Staff 

Paper, that using evidence of effects 

associated with periods of exposure that 

are most closely matched to the 

averaging time of each standard is the 

most appropriate public health policy 

approach to evaluating the scientific 

evidence in selecting the level of each 

standard, with each standard designed 

to provide protection from the health 

risks associated with exposures 

reflecting that averaging time. Thus, the 

Administrator believes that the 24-hour 

standard should be set so as to provide 

an appropriate degree of protection from 

health effects associated with short-term 

exposures to PM2.5, and the annual 

standard should be set so as to provide 

an appropriate degree of protection from 

health effects associated with long-term 

exposures to PM2.5. In determining the 

level of each standard, the 

Administrator believes it is appropriate 

to rely on the short-term studies for 

purposes of determining the level of the 

24-hour standard, and the long-term 

studies for purposes of determining the 

level of the annual standard.45

Therefore, the Administrator does not 

believe that evidence from short-term 

exposure studies is an appropriate basis 

for selecting any different level of the 

annual standard in this review than that 

selected based on the long-term 

exposure evidence. The EPA has instead 


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46The EPA does not believe that CASAC based 

this statement on the evidence it cites concerning 

effects associated with the long-term means of the 

short-term studies. These studies address effects 

from short-term exposures, and do not address 

effects from long-term exposures. 

47The CASAC did express the view that although 

the ‘‘new’’ scientific literature that was not 

included in the Criteria Document appears to 

support its findings, that literature was not needed 

to support its recommendation of a lower annual 

standard level (Henderson, 2006, p. 6). 

evaluated these short-term exposure 

studies in the context of determining the 

appropriate level for the 24-hour 

standard. 

Finally, CASAC also expressed the 

view that there is evidence that effects 

of long-term PM2.5 concentrations occur 

at or below the level of the current 

standard. While the Administrator 

agrees that any such evidence would be 

directly relevant to his decision on the 

level of the annual PM2.5 standard, 

CASAC did not provide any specific 

information as to what studies it felt 

provided such evidence nor the 

considerations that played a role in its 

interpretation of the studies, including 

its assessment of the uncertainties 

inherent in any such studies.46 As 

discussed below, the Administrator has 

considered the available studies of longterm exposure to PM2.5, together with 

the uncertainties inherent in that body 

of evidence, to reach his final decision 

on the level of the annual standard. 

However, since CASAC did not provide 

any more specific statements as to its 

assessment of such mortality or 

morbidity studies, the Administrator 

cannot determine in what ways his 

judgments about that evidence may 

differ from CASAC’s views.47 Lacking 

such specific statements to support 

CASAC’s view that there is evidence 

that effects of long-term PM2.5

concentrations occur at or below the 

level of the current standard, the 

Administrator cannot discern a clear 

line of scientific reasoning that would 

preclude the current level of 15 μg/m3

from being a reasonable policy choice 

based on the most relevant available 

evidence on the health effects of longterm exposures to PM2.5. 

As noted above, EPA received other 

comments on the proposal from two 

distinct groups of commenters. One 

group that included virtually all 

commenters representing industry 

associations and businesses agreed with 

the Agency’s proposed decision not to 

revise the level of the annual PM2.5

standard. The other group of 

commenters included many medical 

groups, numerous physicians and 

academic researchers, many public 

health organizations, many States, and a 

large number of individual commenters. 

They strongly disagreed with the 

Agency’s proposed decision and argued 

that EPA should lower the level of the 

annual PM2.5 standard. While some of 

these commenters felt that the level 

should be set within the range 

recommended by CASAC, most such 

commenters advocated a level of 12 μg/ 

m3. These commenters largely based 

their views on the same general 

considerations put forward by CASAC 

as a basis for its recommendation to 

lower the level of the annual PM2.5

standard. To the extent that these 

commenters, like CASAC, relied upon 

the Agency’s risk assessment or the 

evidence from short-term exposure 

studies as a basis for their views, their 

comments are addressed above. 

Comments that address how specific 

long-term PM2.5 exposure studies should 

be considered as a basis for the level of 

the annual PM2.5 standard are addressed 

below. 

A few commenters offered detailed 

comments on the key long-term 

exposure PM2.5 mortality studies 

discussed in the proposal, including the 

original analyses and reanalyses of the 

ACS and Six Cities cohorts and the 

extended ACS cohort study. In general, 

some medical/public health/researcher/ 

State commenters expressed the view 

that EPA has downplayed the results of 

these studies to the extent that they 

provide evidence of effects below the 

level of the current standard. For 

example, American Lung Association et 

al. and Schwartz (2006) asserted that the 

ACS cohort study and the HEI 

reanalysis provide direct evidence of 

premature mortality associated with 

annual exposures below 15 μg/m3 based 

on plots of the concentration-response 

function between long-term exposure to 

PM2.5 and risk of dying across 50 U.S. 

metropolitan areas that show no 

substantial deviation from linear, nonthreshold relationships down through 

levels well below 15 μg/m3. These 

commenters did not, however, discuss 

the uncertainties inherent in this type of 

epidemiologic study or the implications 

of these uncertainties on their 

interpretation of the results. 

In contrast, some industry/business 

commenters (e.g., Pillsbury et al.; 

Annapolis Center; UARG) emphasized 

that uncertainties remain in interpreting 

these studies with regard to issues such 

as potential confounding by copollutants, especially SO2, modeling to 

address spatial correlations in the data, 

and effect modification by education 

level or socioeconomic status. In 

addition, some industry/business 

commenters raised additional questions 

about the appropriate interpretation of 

these key studies in light of other 

studies, which EPA did not rely on, that 

provided either mixed or no evidence of 

PM2.5-mortality associations, and in 

light of their view that the studies that 

EPA relied on report implausibly large 

effect estimates. 

In considering these commenters’

sharply divergent assessments of the key 

mortality studies, the Administrator 

continues to believe that these studies 

provide strong evidence of an 

association between long-term exposure 

to PM2.5 and mortality. However, the 

Administrator believes that the 

remaining uncertainties weigh against 

reaching the conclusion that the level of 

the annual PM2.5 standard should be 

lowered on the basis of these studies. In 

reaching this conclusion, the 

Administrator notes that even though 

the long-term average PM2.5

concentration across the cities in the 

extended ACS study (17.7 μg/m3) is 

lower than in the original study (21 μg/ 

m3), the level of the current standard is 

still appreciably below the long-term 

average of the extended ACS study and 

that of the Six Cities study (18 μg/m3). 

In commenting on alternative 

approaches to interpreting the study 

results as a basis for setting a standard 

level, American Lung Association et al. 

expressed the view that the level of the 

standard should more appropriately be 

based on the concentration that is one 

standard deviation below the cross-city 

long-term average in each relevant longterm exposure study. In considering 

such an approach, the Administrator 

notes that while that approach would by 

definition lead to a more precautionary 

standard, there is no basis for 

concluding that it is a more 

scientifically defensible approach or 

that it is more appropriate in this case 

where a number of key uncertainties in 

the evidence remain to be addressed in 

future research, and where the basic 

decision is a judgment by the 

Administrator as to what level is neither 

more nor less stringent than is necessary 

to protect public health with an 

adequate margin of safety. The 

Administrator continues to believe that 

it is reasonable to base the decision on 

the standard level on long-term average 

PM2.5 concentrations in the key longterm exposure studies, because 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 (71 FR 

2651). 

Both groups of commenters also 

identified several ‘‘new’’ mortality 

studies not included in the Criteria 

Document in support of their various 

views. As noted above in Section I.C, as 

in past NAAQS reviews, EPA is basing 


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48The Gaudermann et al. (2004) study cited by 

these commenters is a ‘‘new’’ study, and EPA’s 

provisional consideration of this study is discussed 

in the Response to Comments document. 

49The Administrator notes that CASAC’s letter of 

March 21, 2006 did not note any objection to his 

views on these morbidity studies as discussed in 

the proposal, or provide any reason to reconsider 

such views (Henderson, 2006). 

50 Kim et al. (2004) is a ‘‘new’’ study and EPA’s 

provisional consideration of this study is discussed 

in the Response to Comments document. 

51For example, the California statute does not 

refer to setting a standard that is ‘‘requisite’’ to 

protect, as that term is used in the CAA, and 

California, unlike EPA, may take economic impacts 

into consideration in setting air quality standards. 

In addition, as with the WHO guidelines, the 

standards appear to be more in the nature of goals 

as compared to binding requirements that must be 

met. 

52The EPA is not required to base the level of the 

standard on either the highest or lowest level from 

any one study. Rather, the Administrator must 

‘‘make an informed judgment based on available 

evidence.’’ American Petroleum Inst v. Costle, 665 

F. 2d at 1187; NRDC v. EPA, 902 F. 2d at 971. Such 

an informed judgment can result in higher levels 

than shown in some of the studies in the record. 

See, e.g. NRDC v. EPA, 902 F. 2d at 971 (upholding 

1987 PM10 annual standard selected from ‘‘near the 

middle of the ‘range of interest’ ’’); API v. Costle, 

665 F. 2d at 1187 (upholding 1979 hourly standard 

for ozone selected at level higher than a number of 

studies in the record). 

the final decisions in this review on the 

studies and related information 

included in the PM air quality criteria 

that have undergone CASAC and public 

review, and will consider the newly 

published studies for purposes of 

decision making in the next PM NAAQS 

review. Nonetheless, in provisionally 

evaluating commenters’ arguments (see 

Response to Comments document), EPA 

notes that its provisional assessment of 

‘‘new’’ science found that such studies 

did not materially change the 

conclusions in the Criteria Document. 

Some commenters who supported a 

lower annual standard level also 

asserted that EPA failed to adequately 

consider long-term exposure PM2.5

morbidity studies, especially studies of 

effects in children. For example, the 

Children’s Health Protection Advisory 

Committee and other commenters noted 

that studies by Razienne et al. (1996) 

and Gauderman et al. (2002, 2004) 

showed effects on children’s lung 

function at long-term cross-city average 

PM2.5 concentrations of 14.5 μg/m3 and 

15 μg/m3, respectively. The proposal 

notice included a careful discussion of 

the 24-Cities study (Razienne et al., 

1996) and the earlier Southern 

California children’s health study 

(Gauderman et al., 2000, 2002), studies 

which were included in the Criteria 

Document,48 and explained the basis for 

the Administrator’s provisional 

conclusion that these studies provide an 

uncertain basis for establishing the level 

of a national standard (71 FR 2651). 

These commenters offered no 

information that would change the 

Administrator’s judgment with regard to 

these studies.49 In addition, the 

Children’s Health Advisory Committee 

also cited several studies of ‘‘trafficrelated’’ pollution (van Vliet et al., 1997; 

Brunekreef et al., 1997; Kim et al., 

2004 50) as showing associations 

between fine particles and adverse 

respiratory outcomes, including asthma 

in children who live near major 

roadways, with mean annual average 

fine particle concentrations near and 

below 15 μg/m3. 

In considering these comments, EPA 

first notes that studies of traffic-related 

pollution generally do not disentangle 

potential effects of fine particles from 

those of other traffic-related pollutants, 

and thus provide an uncertain basis for 

establishing the level of a PM2.5

standard. Further, two of the studies 

cited by this commenter are ‘‘new’’

studies not included in the Criteria 

Document. As discussed above in 

section I.C, EPA is basing the final 

decisions in this review on the studies 

and related information included in the 

PM air quality criteria that have 

undergone CASAC and public review, 

and will consider the newly published 

studies for purposes of decision making 

in the next PM NAAQS review. 

The CARB and some other 

commenters who supported a lower 

annual standard level discussed the 

rationale used by the CARB in deciding 

to set the State’s annual PM2.5 standard 

at a level of 12 μg/m3. Some of these 

commenters also pointed to the World 

Health Organization’s annual PM2.5

guideline value of 10 μg/m3 in support 

of their view that the scientific evidence 

supports an annual PM2.5 standard in 

the U.S. at a level no higher than 12 μg/ 

m3. In considering these comments, the 

Administrator notes that his decision is 

constrained by the provision of the CAA 

that requires that the NAAQS be 

requisite to protect public health with 

an adequate margin of safety. This 

requires that his judgment is to be based 

on an interpretation of the evidence that 

neither overstates nor understates the 

strength and limitations of the evidence, 

or the appropriate inferences to be 

drawn from the evidence. This is not the 

same legal framework that governs the 

standards set by the State of California 

or the guidelines established by a 

working group of scientists within the 

World Health Organization.51 Thus, the 

Administrator does not agree that the 

California standard or the WHO 

guideline provide an appropriate basis 

for setting the level of the annual PM2.5

NAAQS in the U.S. 

The Administrator further stresses, as 

explained at proposal, that he is placing 

the greatest weight in determining the 

level of the annual standard on the longterm means of the levels associated with 

mortality effects in the two key longterm studies in the record, the ACS and 

Six Cities studies (71 FR at 2651). The 

ACS and Six Cities studies are the two 

key long-term studies in this review, 

taking into account both ‘‘study design, 

strength of the study (in terms of 

statistical significance and precision of 

result), and the consistency and 

robustness of results’’ (71 FR 2651), and 

also the comprehensive reanalyses of 

these studies, which involved 

replication, validation, and sensitivity 

analyses. These reanalyses replicated 

the original results and confirmed the 

associations noted in the original 

studies (EPA 2005, p. 3–17). The 

Administrator has taken into account all 

the relevant studies but in evaluating 

the strengths and weaknesses of the 

various studies has determined that the 

greatest weight should be placed on 

these key studies, as compared to other 

studies, in determining the level of the 

annual standard. As discussed above, 

the level of the current annual standard 

is appropriate as it is appreciably below 

the long-term average of these key 

studies. This standard is also basically 

at the same level as the long-term 

average in the two morbidity studies, 

the 24 Cities study and the Southern 

California children’s cohort study. 

These morbidity studies provide an 

uncertain basis for setting the level of 

the national standard, and, therefore, in 

the judgment of the Administrator do 

not warrant setting a lower level for the 

annual standard than the level 

warranted based on the key mortality 

studies.52

After carefully taking the above 

comments and considerations into 

account, the Administrator has decided 

to retain the level of the primary annual 

PM2.5 standard at 15 μg/m3. In the 

Administrator’s judgment, based on the 

currently available evidence, a standard 

set at this level would be requisite to 

protect public health with an adequate 

margin of safety from serious health 

effects including premature mortality 

and respiratory morbidity that are likely 

causally associated with long-term 

exposure to PM2.5. A standard set at a 

lower level would only result in 

significant further public health 

protection if, in fact, there is a 

continuum of health risks in areas with 

long-term average PM2.5 concentrations 

that are well below the cross-city longterm average concentrations observed in 


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the key epidemiologic studies and if the 

reported associations are, in fact, 

causally related to PM2.5 at those lower 

levels. Based on the available evidence, 

the Administrator is not prepared to 

make these assumptions. As was the 

case in considering the 24-hour PM2.5

standard, taking into account the 

uncertainties that remain in interpreting 

the available long-term exposure 

epidemiologic studies, the likelihood of 

obtaining benefits to public health 

decreases with a standard set below the 

current level, while the likelihood of 

requiring reductions in ambient 

concentrations that go beyond those that 

are needed to reduce risks to public 

health increases. On balance, the 

Administrator does not believe that a 

lower standard is needed to protect 

public health with an adequate margin 

of safety. This judgment by the 

Administrator appropriately considers 

the requirement for a standard that is 

neither more nor less stringent than 

necessary for this purpose and 

recognizes that the CAA does not 

require that primary standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

G. Final Decisions on Primary PM2.5

Standards 

For the reasons discussed above, and 

taking into account the information and 

assessments presented in the Criteria 

Document and Staff Paper, the advice 

and recommendations of CASAC, 

including its request to reconsider parts 

of the proposal, and public comments 

received on the proposal, the 

Administrator is revising the current 

primary PM2.5 standards. The suite of 

standards as revised will provide 

increased protection from the health 

risks associated with exposure to PM2.5, 

and in the judgment of the 

Administrator will be requisite to 

protect public health with an adequate 

margin of safety. 

Specifically, the Administrator is 

making the following revisions: 

(1) The level of the primary 24-hour 

PM2.5 standard is revised to 35 μg/m3. 

(2) The form of the primary annual 

PM2.5 standard is revised with regard to 

the criteria for spatial averaging, such 

that averaging across monitoring sites is 

allowed if the annual mean 

concentration at each monitoring site is 

within 10 percent of the spatially 

averaged annual mean, and the daily 

values for each monitoring site pair 

yield a correlation coefficient of at least 

0.9 for each calendar quarter. Data 

handling conventions for the revised 

standards are specified in revisions to 

Appendix N, as discussed below in 

section V, and minor revisions to the 

reference method for monitoring PM as 

PM2.5 are specified in Appendix L, as 

discussed below in section VI. 

In a related rule on ambient air 

monitoring regulations (40 CFR Parts 53 

and 58) published elsewhere in today’s 

Federal Register, EPA is revising the 

requirements for reference and 

equivalent method determinations for 

fine particle monitors, monitoring 

network descriptions and periodic 

assessments, quality assurance, and data 

certification. 

Issues related to the implementation 

of revised PM2.5 standards are discussed 

below in section VII. The EPA plans to 

propose related revisions to the Air 

Quality Index for PM2.5 at a later date. 

III. Rationale for Final Decisions on 

Primary PM10 Standards 

A. Introduction 

1. Overview 

This section presents the 

Administrator’s final decisions on the 

review of the primary NAAQS for PM10. 

The rationale for the final decisions on 

the primary PM10 NAAQS includes 

consideration of: (1) Evidence of health 

effects related to short- and long-term 

exposures to thoracic coarse particles; 

(2) insights gained from a quantitative 

risk assessment prepared by EPA; and 

(3) specific conclusions regarding the 

need for revisions to the current 

standards and the elements of standards 

for thoracic coarse particles (i.e., 

indicator, averaging time, form, and 

level) that, taken together, would be 

requisite to protect public health with 

an adequate margin of safety. 

In developing this rationale, EPA has 

taken into account the information 

available from a growing, but still 

limited, body of evidence on health 

effects associated with thoracic coarse 

particles from studies that use PM10–2.5

as a measure of thoracic coarse particles. 

The EPA has drawn upon an integrative 

synthesis of the body of evidence on 

associations between exposure to 

ambient thoracic coarse particles and a 

range of health endpoints (EPA, 2004a, 

Chapter 9), focusing on those health 

endpoints for which the Criteria 

Document concludes that the 

associations are suggestive of possible 

causal relationships. In its policy 

assessment of the evidence judged to be 

most relevant to making decisions on 

elements of the standards, EPA has 

placed greater weight on U.S. and 

Canadian epidemiologic studies using 

thoracic coarse particle measurements, 

since studies conducted in other 

countries may well reflect different 

demographic and air pollution 

characteristics. 

While there is little question that 

particles in the thoracic coarse particle 

size range can present a risk of adverse 

effects to the most sensitive regions of 

the respiratory tract at sufficient 

exposure levels, the characterization of 

health effects attributable to various 

levels of exposure to ambient thoracic 

coarse particles is subject to 

uncertainties that are markedly greater 

than is the case for fine particles. As 

summarized below, however, there is a 

growing body of evidence available 

since the last review of the PM NAAQS, 

with important new information coming 

from epidemiologic, toxicologic, and 

dosimetric studies. Moreover, the newly 

available research studies have 

undergone intensive scrutiny through 

multiple layers of peer review and 

extended opportunities for public 

review and comment. While important 

uncertainties remain, the review of the 

health effects information has been 

extensive and deliberate. In the 

judgment of the Administrator, this 

intensive evaluation of the scientific 

evidence provides an adequate basis for 

making final regulatory decisions at this 

time. 

In addition, this review has already 

provided important input to EPA’s 

research and monitoring plans for 

improving our future understanding of 

the relationships between exposures to 

ambient thoracic coarse particles and 

health effects. As discussed in the 

proposal, the epidemiological evidence 

available in this review is almost 

entirely based on measurements of 

undifferentiated PM10–2.5 mass, without 

regard to the composition of thoracic 

coarse particles. Yet both fundamental 

toxicological considerations and the 

limited data available on this issue 

strongly suggest that the health effects 

could vary significantly depending 

upon the composition of the ambient 

coarse particle mix. The goal of the 

Agency’s research and monitoring 

programs going forward is to provide 

scientific advances that will enable 

future PM NAAQS reviews to make 

more informed decisions that will 

provide more effective and efficient 

protection against the effects of those 

coarse particles and related source 

emissions that prove to be of concern to 

public health. 

The health effects information and 

human risk assessment were 

summarized in sections III.A and III.B of 

the proposal and are only briefly 

outlined in subsections III.A.2 and 3 

below. Subsequent sections provide a 

more complete discussion of the 

Administrator’s rationale, in light of key 


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53The ‘‘thoracic’’ regions of the respiratory tract 

are located in the chest (thorax) and are comprised 

of the tracheo-bronchial region with connecting 

airways and the alveolar, or gas-exchange region of 

the lung. For ease of communication, ‘‘thoracic’’

particles penetrating to these regions are often 

called ‘‘inhalable’’ particles. 

54Quantitative risk estimates associated with 

recent air quality levels for these three cities are 

presented in Figures 4–11 and 4–12 of the Staff 

Paper. 

issues raised in public comments, for 

his decision to retain the current 24- 

hour primary PM10 standard and to 

revoke the current annual PM10

standard. Specifically, these sections 

present a more complete discussion of 

the Administrator’s rationale regarding 

the need to maintain protection against 

the health effects of coarse particles 

(section III.B) as well as the rationale for 

the decisions regarding specific 

elements of the primary PM10 standards 

including indicator (section III.C); and 

averaging time, level and form (section 

III.D). 

2. Overview of Health Effects Evidence 

The first PM NAAQS (36 FR 8186) 

used an indicator based solely on a 

preexisting monitor for total suspended 

particles (TSP) that was not designed to 

focus on particles of greatest risk to 

health. In preparing for the initial 

review of those standards, EPA placed 

a major emphasis on developing a new 

indicator that considered the significant 

amount of evidence on particle size, 

composition, and relative risk of effects 

from penetration and deposition to the 

major regions of the respiratory tract 

(Miller et al., 1979). The development 

and assessment of these lines of 

evidence in the PM Criteria Document 

and PM Staff Paper published between 

1979 and 1986 culminated in revised 

standards for PM that used PM10 as the 

indicator (52 FR 24634). The major 

conclusion from that review, which 

remained unchanged in the 1997 

review, was that ambient particles 

smaller than or equal to 10 μm in 

aerodynamic diameter are capable of 

penetrating to the deeper ‘‘thoracic’’ 53

regions of the respiratory tract and 

present the greatest concern to health 

(61 FR 65648). While considerable 

advances have been made, the available 

evidence in this review continues to 

support the basic conclusions reached 

in the 1987 and 1997 reviews regarding 

penetration and deposition of fine and 

thoracic coarse particles. As discussed 

in the Criteria Document, both fine and 

thoracic coarse particles penetrate to 

and deposit in the alveolar and 

tracheobronchial regions. For a range of 

typical ambient size distributions, the 

total deposition of thoracic coarse 

particles to the alveolar region can be 

comparable to or even larger than that 

for fine particles. For areas with 

appreciable coarse particle 

concentrations, thoracic coarse particles 

would tend to dominate particle 

deposition to the tracheobronchial 

region for mouth breathers (EPA, 2004a, 

p. 6–16). Deposition of particles to the 

tracheobronchial region is of particular 

concern with respect to aggravation of 

asthma. 

In the last review, little new 

toxicologic evidence was available on 

potential effects of thoracic coarse 

particles and there were few 

epidemiologic studies that had included 

direct measurements of thoracic coarse 

particles. Evidence of associations 

between health outcomes and PM10 that 

were conducted in areas where PM10

was predominantly composed of 

thoracic coarse particles was an 

important part of EPA’s basis for 

reaching conclusions about the requisite 

level of protection from coarse particles 

provided by the final standards. The 

new studies available in this review 

include epidemiologic studies that have 

reported associations with health effects 

using direct measurements of PM10–2.5, 

as well as new dosimetric and 

toxicologic studies. 

Section III.A of the proposal further 

outlines key information contained in 

the Criteria Document (Chapters 6–9) 

and the Staff Paper (Chapter 3) on 

known or potential effects associated 

with exposure to thoracic coarse 

particles and their major constituents. 

The information highlighted there 

includes: 

(1) New information available on 

potential mechanisms for health effects 

associated with exposure to thoracic 

coarse particles or their constituents. 

(2) The nature of the effects that have 

been associated with short-term 

exposures to ambient thoracic coarse 

particles, particularly in urban and 

industrial settings, including 

aggravation of respiratory and 

cardiovascular disease (as indicated by 

increased hospital admissions), 

increased respiratory symptoms in 

children, and premature mortality. 

(3) An integrative assessment of the 

evidence on health effects related to 

thoracic coarse particles, with an 

emphasis on the key issues raised in 

assessing the available communitybased epidemiologic studies, including 

alternative interpretations of the 

evidence, both for individual studies 

and the evidence as a whole. 

(4) Subpopulations that appear to be 

sensitive to effects from exposure to 

thoracic coarse particles, specifically 

including individuals with preexisting 

lung diseases such as asthma, and 

children and older adults. 

(5) Conclusions, based on the 

magnitude of these subpopulations and 

risks identified in health studies 

conducted in urban and industrial areas, 

that exposure to ambient thoracic coarse 

particles can have an important public 

health impact. 

The summary of the health effects 

evidence related to ambient coarse 

particles in the proposal will not be 

repeated here. The EPA emphasizes that 

the final decisions on these standards 

take into account the more 

comprehensive and detailed discussions 

of the scientific information on these 

issues contained in the Criteria 

Document and Staff Paper, which were 

reviewed by the CASAC and the public. 

For reasons summarized in section I.C 

above, EPA is not relying on studies 

published after completion of the 

Criteria Document as a basis for 

reaching final decisions on these 

standards. 

3. Overview of Quantitative Risk 

Assessment 

The general overview and discussion 

of key components of the risk 

assessment used to develop risk 

estimates for PM2.5 presented in section 

II.A above is also applicable to the 

assessment done for PM10–2.5 in this 

review. However, the scope of the risk 

assessment for PM10–2.5 is much more 

limited than that for PM2.5, reflecting the 

much more limited body of 

epidemiologic evidence and air quality 

information available for PM10–2.5. As 

discussed in chapter 4 of the Staff 

Paper, the PM10–2.5 risk assessment 

includes risk estimates for just three 

urban areas for two categories of health 

endpoints related to short-term 

exposure to PM10–2.5: hospital 

admissions for cardiovascular and 

respiratory causes, and respiratory 

symptoms. 

Estimates of hospital admissions 

attributable to short-term exposure to 

PM10–2.5 have been developed for Detroit 

(cardiovascular and respiratory 

admissions) and Seattle (respiratory 

admissions), and estimates of 

respiratory symptoms have been 

developed for St. Louis.54 While one of 

the goals of the PM10–2.5 risk assessment 

was to provide estimates of the risk 

reductions associated with just meeting 

alternative PM10–2.5 standards, the 

nature and magnitude of the 

uncertainties and concerns associated 

with this portion of the risk assessment 

weigh against use of these risk estimates 

as a basis for recommending specific 

standard levels (EPA, 2005, p. 5–69). 


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55The Administrator also proposed qualifications 

to the indicator, and corresponding revisions to the 

level and form of the 24-hour standard to provide 

protection that is generally equivalent to that 

afforded by the PM10 standard, and to revoke the 

annual PM10 standard. 

56EPA further concluded at that time that the 

risks of adverse health effects associated with 

deposition of particles in the thoracic region are 

‘‘markedly greater than for deposition in the 

extrathoracic (head) region,’’ and that risks from 

extrathoracic deposition are ‘‘sufficiently low that 

particles which deposit only in that region can 

safely be excluded from the standard indicator’’ (62 

FR 38666). 

These uncertainties and concerns are 

summarized in section III.B of the 

proposal and discussed more fully in 

the Staff Paper (Chapter 4) and the 

technical support document (Abt 

Associates, 2005). 

B. Need for Revision of the Current 

Primary PM10 Standards 

As presented in the proposal, taking 

into account both the nature of recent 

scientific evidence and legal 

considerations, this review of the 

primary PM10 standards has focused on 

whether to revise the indicator for 

thoracic coarse particles, and on the 

appropriate level, form and averaging 

time for any revised indicator. The basis 

for reaching a final decision on the 

indicator, as well as other facets of the 

standards, is presented below in 

sections III.C and III.D. This section 

provides an overview of the 

considerations that led to the 

Administrator’s provisional conclusion, 

at the time of proposal, that it would be 

appropriate to revise the PM10 standards 

by adopting a new indicator (PM10–2.5).55

The section then presents a summary of 

public comments concerning whether 

the available evidence supports 

retention, revision, or revocation of 

standards to protect against exposure to 

thoracic coarse particles. For the reasons 

discussed below, the Administrator has 

concluded, consistent with CASAC and 

Staff Paper recommendations and 

conclusions drawn at the time of 

proposal, that continued protection 

against health effects associated with 

short-term exposure to thoracic coarse 

particles is requisite. However, EPA 

notes that, having considered the issues 

raised in extensive public comment on 

the proposal, the Administrator’s final 

decision differs from that in the 

proposal regarding whether it is 

appropriate to revise the indicator in 

order to retain protection from coarse 

particles. This section, and the 

subsequent section on indicator, outline 

the rationale presented at the time of the 

proposal, and then describe how the 

Administrator has reached a different 

conclusion in his final decision. 

1. Overview of the Proposal 

The initial issue addressed in the 

current review of the primary PM10

standards was whether, in view of the 

advances in scientific knowledge 

reflected in the Criteria Document and 

Staff Paper, the current standards 

should be revised. The Staff Paper 

addressed this question by first 

considering the conclusions reached in 

the last review, the subsequent litigation 

of that decision, and the nature of the 

new information available in this 

review. 

In 1997, in conjunction with 

establishing new PM2.5 standards, EPA 

concluded that continued protection 

against potential effects associated with 

thoracic coarse particles in the size 

range of 2.5 to 10 μm was warranted 

based on particle dosimetry, toxicologic 

information, and limited epidemiologic 

evidence from studies that measured 

PM10 in areas where coarse particles 

were likely to dominate the distribution 

(62 FR 38677). This information 

indicated that thoracic coarse particles 

can deposit in those regions of the lung 

of most concern (i.e., the 

tracheobronchial and alveolar regions, 

which together make up the thoracic 

region),56 and that they can be expected 

to aggravate effects in individuals with 

asthma and contribute to increased 

upper respiratory illness (62 FR 38666–

8). 

Further, EPA decided that the new 

function of PM10 standard(s) would be 

to provide such protection against 

effects associated with particles in the 

narrower size range between 2.5 to 10 

μm. Although some consideration had 

been given to a more narrowly defined 

indicator that did not include fine 

particles (e.g., PM10–2.5), EPA decided 

that it was more appropriate to continue 

to use PM10 as the indicator for 

standards to control thoracic coarse 

particles. 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

in areas where the coarse fraction was 

the dominant fraction of PM10, namely 

two studies conducted in areas that 

substantially exceeded the 24-hour PM10

standard (62 FR 38679). The decision 

also reflected the fact that there were 

only very limited ambient air quality 

data then available specifically on 

thoracic coarse particles (i.e. PM10–2.5), 

in contrast to the extensive monitoring 

network already in place for PM10. In 

essence, EPA concluded at that time 

that it was appropriate to continue to 

control thoracic coarse particles, but 

that the only information available upon 

which to base such standards was 

indexed in terms of PM10. 

In subsequent litigation regarding the 

1997 PM NAAQS revisions, however, 

the U.S. Court of Appeals (D.C. Circuit) 

held in part that EPA had not provided 

a reasonable explanation justifying use 

of PM10 as an indicator for thoracic 

coarse particles. ATA I, 175 F.3d at 

1054–55. Although the court found 

‘‘ample support’’ (id. at 1054) for EPA’s 

decision to regulate thoracic coarse 

particles, it vacated the 1997 revised 

PM10 standards. The result of 

subsequent EPA actions, discussed 

above in section I.C, is that the 1987 

PM10 standards remain in place (65 FR 

80776, 80777, Dec. 22, 2000) and the 

present review is consequently of those 

1987 standards. 

In this review, the Staff Paper focused 

on the recent information available in 

the Criteria Document from a growing, 

but still limited, body of evidence on 

health effects associated with thoracic 

coarse particles from studies that use 

PM10–2.5 as the measure of thoracic 

coarse particles. In addition, there is 

now much more information available 

to characterize air quality in terms of 

PM10–2.5 than was available in the last 

review. In considering this information, 

the Staff Paper found that the major 

considerations that formed the basis for 

EPA’s 1997 decision to retain PM10 as 

the indicator for thoracic coarse 

particles, rather than a more narrowly 

defined indicator that does not include 

fine particles, no longer apply. More 

specifically, staff concluded that the 

continued use of PM10 as an indicator 

for standards intended to protect against 

health effects associated with thoracic 

coarse particles was no longer necessary 

since the information available in the 

Criteria Document could support the 

use of a more directly relevant indicator, 

PM10–2.5. Further, staff concluded that 

continuing to rely principally on health 

effects evidence indexed by PM10 to 

determine the appropriate averaging 

time, form, and level of a standard was 

no longer necessary or appropriate since 

a number of more directly relevant 

studies, indexed by PM10–2.5, were 

available. Thus, the Staff Paper 

concluded that it was appropriate to 

revise the current PM10 standards in 

part by revising the indicator for 

thoracic coarse particles, and by basing 

any such revised standard principally 

on the currently available evidence and 

air quality information indexed by 

PM10–2.5, but also considering evidence 

from studies using PM10 in locations 

where PM10–2.5 was the predominant 

fraction (EPA, 2005, section 5.4.1). As 

noted in the introduction to this section, 


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57The Criteria Document notes that toxicologic 

studies, in general, use exposure concentrations 

that are generally much higher than ambient 

concentrations (EPA, 2004a, p. 9–51). 

58Eventually, as a result of the data that will be 

gathered under EPA’s new research and monitoring 

plan , the Agency may be able to further refine its 

regulation of coarse particles to better target those 

coarse particles of greatest concern to health. 

59The Coachella Valley study, like the Seattle 

study noted above, is subject to additional 

measurement uncertainties because it used 

regression techniques to impute PM10–2.5

concentrations; this approach fills in missing 

PM10–2.5 data based on relationships developed 

using data from days when data are available for 

both PM10 and PM2.5. 

60Based on recent air quality data, as well as the 

summary information provided for PM 

concentrations used in the studies, the existing 

PM10 standards are not met in any of these study 

cities except Tucson, AZ. Based on 2002–2004 air 

quality data, the 98th percentile PM2.5

concentrations in three of these areas range from 15 

to 25 μg/m3, while in Utah Valley the 

concentrations range from 37 to 54 μg/m3. 

having considered public comments on 

this issue, EPA has reached different 

conclusions regarding the 

appropriateness of revising the current 

indicator in this final decision; this is 

described in more detail below in 

section III.C. 

Recognizing that dosimetric evidence 

formed the basis for the initial 

establishment of the PM10 indicator in 

1987 and supported the decision in 

1997 to retain the PM10 indicator, the 

Staff Paper also considered whether 

currently available dosimetric evidence 

continues to support the basic 

conclusions reached in those reviews of 

the standards. In particular, 

consideration was given to available 

information about patterns of 

penetration and deposition of thoracic 

coarse particles in the sensitive thoracic 

region of the lung and to whether an 

aerodynamic size of 10 μm remains a 

reasonable separation point for particles 

that penetrate and potentially deposit in 

the thoracic regions. The Staff Paper 

concluded that while considerable 

advances have been made in 

understanding particle dosimetry, the 

available evidence continues to support 

those basic conclusions from past 

reviews. More specifically, both fine 

particles, indexed by PM2.5, and thoracic 

coarse particles, indexed by PM10–2.5, 

penetrate to and deposit in the thoracic 

regions. Further, for a range of typical 

ambient size distributions, the total 

deposition of thoracic coarse particles to 

the alveolar region can be comparable to 

or even larger than that of fine particles 

(EPA, 2004a, p. 6–16). 

Beyond the dosimetric evidence, as 

noted in past reviews (EPA, 1982, 

1996b), toxicologic studies show that 

the deposition of a variety of particle 

types in the tracheobronchial region, 

including resuspended urban dust and 

coarse-fraction organic materials, has 

the potential to affect lung function and 

aggravate respiratory symptoms, 

especially in asthmatics. Of particular 

note are limited toxicologic studies that 

found urban road dust can produce 

cellular and immunological effects (e.g., 

Kleinman et al., 1995; Steerenberg et al., 

2003).57 In addition, some very limited 

in vitro toxicologic studies show some 

evidence that coarse particles may elicit 

pro-inflammatory effects (EPA, 2004a, 

section 7.4.4). Further, the Staff Paper 

assessment of the physicochemical 

properties and occurrence of ambient 

coarse particles suggests that both the 

chemical makeup and the spatial 

distribution of coarse particles are likely 

to be more heterogeneous than for fine 

particles (EPA, 2005, chapter 2). In 

particular, as discussed below in section 

III.C, coarse particles in urban areas can 

contain all of the components found in 

more rural areas, but can also be 

contaminated by a number of additional 

materials, from motor-vehicle-related 

emissions to metals and transition 

elements associated with industrial 

operations. The Staff Paper concluded 

that the weight of the dosimetric, 

limited toxicologic, and atmospheric 

science evidence, taken together, lends 

support to the plausibility of the 

PM10–2.5-related effects reported in the 

urban epidemiologic studies discussed 

below, and provides support for 

retaining some standard for thoracic 

coarse particles so as to continue 

programs to protect public health from 

such effects (EPA, 2005, p. 5–49).58

The available epidemiologic evidence, 

discussed in section III.A of the 

proposal, includes studies of 

associations between short-term 

exposure to thoracic coarse particles, 

indexed by PM10–2.5, and health 

endpoints. More specifically, several 

U.S. and Canadian studies now provide 

evidence of associations between shortterm exposure to PM10–2.5 and various 

morbidity endpoints. Three such studies 

conducted in Toronto (Burnett et al., 

1997), Seattle (Sheppard, 2003), and 

Detroit (Ito, 2003) report statistically 

significant associations between shortterm PM10–2.5 exposure and respiratoryand cardiac-related hospital admissions, 

and a fourth study (Schwartz and Neas, 

2000), conducted in six U.S. cities 

(Boston, St. Louis, Knoxville, Topeka, 

Portage, and Steubenville), reports 

statistically significant associations 

across these six areas with respiratory 

symptoms in children. These studies 

were mostly done in areas in which 

PM2.5, rather than PM10–2.5, is the larger 

fraction of ambient PM10, and they are 

not representative of areas with 

relatively high levels of thoracic coarse 

particles (EPA, 2005, p. 5–49). 

In evaluating the epidemiologic 

evidence from health studies on 

associations between short-term 

exposure to PM10–2.5 and mortality, the 

Criteria Document concluded that such 

evidence was ‘‘limited and clearly not 

as strong’’ as that for associations with 

PM2.5 or PM10 but nonetheless was 

suggestive of associations with mortality 

(EPA, 2004a, p. 9–28, 9–32). Statistically 

significant mortality associations were 

reported in short-term exposure studies 

conducted in areas with relatively high 

PM10–2.5 concentrations, including 

Phoenix (Mar et al., 2003), Coachella 

Valley, CA (Ostro et al., 2003),59 and in 

the initial analysis of data from 

Steubenville (as part of the Six Cities 

study, Schwartz et al., 1996; reanalysis, 

Schwartz, 2003). In a separate reanalysis 

of the Six Cities study, the PM10–2.5

mortality association was not 

statistically significant for Steubenville 

(Klemm and Mason, 2003). In areas with 

lower PM10–2.5 concentrations, including 

the remaining five cities in the Six 

Cities study, no statistically significant 

associations were reported with 

mortality, though most were positive. 

The Staff Paper also considered 

relevant epidemiologic studies indexed 

by PM10 that were conducted in areas 

where the coarse fraction of PM10 is 

typically much greater than the fine 

fraction. Such studies include findings 

of associations between short-term 

exposure to PM10 and hospitalization for 

cardiovascular diseases in Tucson, AZ 

(Schwartz, 1997), hospitalization for 

COPD in Reno/Sparks, NV (Chen et al., 

2000), and medical visits for asthma or 

respiratory diseases in Anchorage, AK 

(Gordian et al., 1996; Choudhury et al., 

1997). In addition, a number of 

epidemiologic studies have reported 

significant associations with mortality, 

respiratory hospital admissions and 

respiratory symptoms in the Utah Valley 

area (e.g., Pope, 1989 and 1991; Pope et 

al., 1992). This group of studies 

provides additional supportive evidence 

for associations between short-term 

exposure to thoracic coarse particles 

and health effects, particularly 

morbidity effects, generally in areas not 

meeting the PM10 standards (EPA, 2005, 

p. 5–50).60

In contrast to the findings from the 

short-term exposure studies discussed 

above, available epidemiologic studies 

do not provide evidence that long-term 

community-level exposure to thoracic 

coarse particles is associated with 

mortality or morbidity (EPA, 2005, p. 3–

25). More specifically, no association is 


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found between long-term exposure to 

thoracic coarse particles and mortality 

in the reanalyses and extended analysis 

of the ACS cohort (EPA, 2005, p. 8–306–

07). Further, little evidence is available 

on potential respiratory and 

cardiovascular morbidity effects of longterm exposure to thoracic coarse 

particles (EPA, 2005, p. 3–23–24). 

The Staff Paper concluded that the 

available body of health evidence, 

including dosimetric, toxicologic and 

epidemiologic study findings, supports 

retaining a NAAQS that would continue 

to provide protection against the effects 

associated with short-term exposure to 

thoracic coarse particles. However, the 

substantial uncertainties associated with 

this limited body of epidemiologic 

evidence on health effects related to 

exposure to PM10–2.5 suggest a high 

degree of caution in interpreting this 

evidence, especially at the lower levels 

of ambient particle concentrations in the 

morbidity studies discussed above 

(EPA, 2005, p. 5–50). 

Beyond this evidence-based 

evaluation, the Staff Paper also 

considered the extent to which PM10–2.5- 

related health risks estimated to occur at 

current levels of ambient air quality may 

be judged to be important from a public 

health perspective, taking into account 

key uncertainties associated with the 

estimated risks. Consistent with the 

approach used to address this issue for 

PM2.5-related health risks, discussed 

above in section II.A.3, the Staff Paper 

considered the results of a series of 

base-case analyses that reflect in part 

the uncertainty associated with the form 

of the concentration-response functions 

drawn from the studies used in the 

assessment. In this assessment 

summarized above in section III.A.3, 

which is much more limited than the 

risk assessment conducted for PM2.5, 

health risks were estimated for three 

urban areas (Detroit, Seattle, and St. 

Louis) by using the reported linear or 

log-linear concentration-response 

functions as well as modified functions 

that incorporate alternative assumed 

cutpoints as surrogates for potential 

population thresholds. In considering 

the risk estimates from this limited 

assessment, and recognizing the very 

substantial uncertainties inherent in 

basing an assessment on such limited 

information, the Staff Paper concluded 

that the results for the two areas in the 

assessment that did not meet the current 

PM10 standards are indicative of risks 

that can reasonably be judged to be 

important from a public health 

perspective, in contrast to the 

appreciably lower risks estimated for 

the area that did meet the current 

standards (EPA, 2005, p. 5–52). 

The Staff Paper recognized the 

substantial uncertainties associated with 

the limited available epidemiologic 

evidence and the inherent difficulties in 

interpreting the evidence for purposes 

of setting appropriate standards for 

thoracic coarse particles. Nonetheless, 

in considering the available evidence, 

the public health implications of 

estimated risks associated with current 

levels of air quality, and the related 

limitations and uncertainties, the Staff 

Paper concluded that this information 

supports (1) revising the current PM10

standards in part by revising the 

indicator for thoracic coarse particles, 

and (2) consideration of a standard that 

will continue to provide public health 

protection from short-term exposure to 

thoracic coarse particles of concern that 

have been associated with morbidity 

effects and possibly mortality at current 

levels in some urban areas (EPA, 2005, 

p. 5–52). 

In CASAC’s review of these Staff 

Paper recommendations, there was 

unanimous agreement among CASAC 

Panel members that ‘‘there was a need 

for a specific primary standard to 

address particles in the size range of 2.5 

to 10 microns’’ (Henderson, 2005b, p. 4). 

In making this recommendation, 

CASAC indicated its agreement with the 

summary of the scientific data regarding 

the potential adverse health effects from 

exposures to thoracic coarse particles in 

section 5.4 of the Staff Paper upon 

which the EPA staff recommendations 

were based. 

Unlike the case in the current PM2.5

review, neither EPA staff nor CASAC 

concluded that it was necessary to 

revise the PM10 standards to provide 

additional health protection against 

coarse particles beyond that afforded by 

the current standards. Rather, as noted 

above, staff and CASAC found that the 

most recent scientific information 

suggested it was possible to move to a 

more direct measurement of thoracic 

coarse particles via a PM10–2.5 indicator, 

and this was the major basis for 

recommending revisions to the current 

24-hour PM10 standard. In considering 

what level of protection was 

appropriate, staff and CASAC 

recommended consideration of a range 

of levels for alternative 24-hour coarse 

particle standards, from levels which 

would be more stringent than the 

current 24-hour PM10 standard to a level 

that would provide protection that was 

roughly equivalent to that provided by 

the current 24-hour PM10 standard. 

In considering whether the primary 

PM10 standards should be revised at the 

time of proposal, the Administrator 

considered the rationale and 

recommendations provided by the Staff 

Paper and CASAC, and the public 

comments received through the time of 

proposal. The Administrator 

provisionally concluded that the health 

evidence, including dosimetric, 

toxicologic and epidemiologic study 

findings, supported retaining a standard 

to provide continued protection against 

effects associated with short-term 

exposure to thoracic coarse particles. 

Further, the Administrator expressed 

the belief that the new evidence on 

health effects from studies that use 

PM10–2.5 as a measure of thoracic coarse 

particles, together with the much more 

extensive data now available to 

characterize air quality in terms of 

PM10–2.5, provided an appropriate basis 

for revising the current PM10 standards 

in part by revising the indicator to focus 

more narrowly on particles between 2.5 

and 10 μm. The Administrator also 

noted that the need for a standard for 

thoracic coarse particles had already 

been upheld based upon evidence of 

health effects considerably more limited 

than now available. ATA I, 175 F.3d at 

1054. Based on these considerations, the 

Administrator provisionally concluded 

that the current suite of PM10 standards 

should be revised, and that the revised 

standard(s) should be set at a level that 

would ensure an equivalent level of 

protection to the current suite of 

standards (71 FR 2665). 

2. Comments on the Need for Revision 

The vast majority of public comments 

on coarse particles raised issues related 

to the proposed revisions to the 

indicator for thoracic coarse standards, 

particularly the proposal to adopt a new 

PM10–2.5 indicator that was qualified to 

focus on particles associated with 

particular types of emissions sources 

and to impose stringent monitor sitesuitability criteria for NAAQScomparable monitors. These comments 

are addressed below in section III.C. 

Comments more specific to the 24-hour 

and annual standards (i.e., on averaging 

time, form, and level) are addressed 

below in section III.D. This section 

addresses those comments that, directly 

or indirectly, addressed the need to 

continue the kind of protection against 

coarse particles that is provided by the 

current PM10 standards. 

A substantial majority of commenters 

supported the Administrator’s 

provisional conclusion that it is 

necessary to maintain a standard to 

continue protection against the health 

effects associated with short-term 

exposure to thoracic coarse particles. 

Those advocating a coarse particle 

standard included public health 

organizations such as the American 

Lung Association, the American Heart 


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Association, and the American Cancer 

Society; environmental groups such as 

Environmental Defense, Earthjustice 

and Natural Resources Defense Council; 

the Children’s Health Protection 

Advisory Committee, which provides 

the EPA Administrator with advice on 

children’s health issues; all state and 

local air pollution control agencies 

commenting on the proposed coarse 

particle standard; and Tribal groups 

such as the National Tribal Caucus, the 

National Tribal Environmental Council, 

and numerous individual Tribes. 

These commenters agreed with EPA 

that the currently available scientific 

evidence clearly supports the need to 

provide continued protection from 

health effects associated with coarse 

particle exposure. Citing the Criteria 

Document and the Staff Paper, those 

commenters providing a more detailed 

rationale stressed the availability of 

epidemiologic, toxicologic and 

dosimetric studies showing associations 

between thoracic coarse particles and 

multiple morbidity and mortality 

endpoints. Many of these commenters 

also cited CASAC’s recommendation in 

favor of continued protection. Moreover, 

some of these commenters pointed to 

particular studies, such as Ito (2003), 

Mar et al. (2003) and Ostro et al. (2003), 

which they concluded show that coarse 

particles are associated with hospital 

admissions or mortality and that coarse 

particles may even have stronger effects 

than fine particles in some instances. 

Several also cited two recent 

independent reviews (Brunekreef and 

Forsberg, 2005; WHO, 2005) which 

considered many of the same scientific 

studies on the health effects of coarse 

particles that were included in the 

Criteria Document as support for 

separate standards for coarse particles, 

in addition to standards for fine 

particles. 

In general, this body of commenters 

opposed revisions that they believed 

would reduce the level of protection 

provided by the current PM10 standards. 

For example, the comments of the 

American Lung Association and five 

environmental groups stated (American 

Lung Association et al., p. 81): 

We strongly support the need for a coarse PM 

standard * * *. However, the coarse particle 

standard proposed by EPA is an egregious 

step backwards in protection of human 

health and welfare compared to the status 

quo * * *. If EPA feels it lacks adequate data 

to undertake the change in the coarse PM 

indicator to a PM10–2.5 standard, without 

reducing current protections * * * then the 

Agency must retain the existing PM10

NAAQS. 

Citing the more abundant evidence 

from studies focusing on short-term 

exposures, these commenters advocated 

maintaining a 24-hour standard for 

thoracic coarse particles, at a minimum. 

Several of them also recommended an 

annual standard for thoracic coarse 

particles to protect against possible 

long-term effects, despite a significantly 

more limited body of evidence (for 

specific comments on averaging time, 

see section III.D.1 below). 

Many of these commenters, while 

recognizing that the epidemiologic 

evidence available to support specific 

coarse particle standards is weaker than 

that for fine particles, believed that the 

weight of evidence required revisions 

that provided a greater degree of 

protection, on a national basis, than that 

afforded by the current PM10 standards 

(for specific comments on level, see 

section III.D.2 below). Some 

commenters favoring a coarse particle 

standard supported their arguments by 

reference to emerging science from new 

toxicologic and epidemiologic studies 

that were not included in the Criteria 

Document. In general, however, these 

‘‘new’’ studies were used in support of 

commenters’ concerns about the 

proposal to qualify the indicator 

(discussed in section III.C.2 below), and 

not to support their comments on the 

need for coarse particle standards. 

The EPA generally agrees with these 

commenters regarding the need to 

provide continued protection from 

short-term exposure to coarse particles 

that may be harmful. The scientific 

evidence cited by these commenters was 

generally the same as that discussed in 

the Criteria Document and the Staff 

Paper and the commenters’

recommendations for retaining a coarse 

particle standard are broadly consistent 

with staff and CASAC recommendations 

on this issue. To the limited extent that 

some commenters cited ‘‘new’’ scientific 

studies in support of their arguments in 

favor of retaining a coarse particle 

standard, EPA notes that it is basing the 

final decisions in this review on the 

studies and related information 

included in the PM air quality criteria 

that have undergone CASAC and public 

review. Although EPA is not basing its 

final decisions in this review on such 

information, the Agency will consider 

the newly published studies for 

purposes of decision making in the next 

PM NAAQS review, as discussed above 

in section I.C. Nonetheless, in 

provisionally evaluating commenters’

arguments concerning the need for 

revision to or elimination of the current 

standards, the Agency notes that its 

preliminary analysis suggests such 

studies would not materially change the 

conclusions in the Criteria Document. 

In sharp contrast, a number of 

commenters, including virtually all of 

those representing industry associations 

and businesses, recommended revising 

the PM10 standards by revoking both the 

24-hour and annual standards. These 

groups argued that the current body of 

scientific evidence is insufficient to 

justify either retaining the current PM10

standards or setting a revised standard 

for thoracic coarse particles at this time. 

These commenters included the 

National Cattlemen’s Beef Association, 

the National Mining Association, the 

American Farm Bureau Federation, the 

Alliance of Automobile Manufacturers, 

the Engine Manufacturers Association, 

the National Association of Home 

Builders, and the Coarse Particle 

Coalition, which includes the National 

Stone, Sand and Gravel Association, the 

Industrial Minerals Association, the 

American Forest and Paper Association, 

the Portland Cement Association and 

the National Cotton Council. These 

commenters stressed the uncertainties, 

particularly those associated with 

interpreting the limited number of 

epidemiologic studies focusing on 

coarse particle health effects, and stated 

that EPA had failed to demonstrate that 

a coarse particle standard is necessary to 

protect public health. These 

commenters recommended deferring the 

decision on the appropriateness of 

setting a coarse particle standard 

pending additional monitoring and 

scientific research on health effects 

associated with exposure to coarse 

particles. 

These commenters criticized the key 

epidemiologic studies cited by EPA, 

referring especially to the alternative 

interpretations of the evidence 

presented in the proposal and citing a 

review and critique of key studies 

prepared by an academic consultant. 

They also argued that all coarse particle 

epidemiologic studies are flawed to the 

extent that they rely on air quality data 

from central monitors in exposure 

assessments. Based on these arguments, 

the commenters asserted that EPA’s risk 

assessment cannot be used to 

demonstrate that ambient coarse 

particles present a significant risk to 

public health, and therefore EPA cannot 

maintain the existing PM10 NAAQS or 

establish a revised NAAQS to address 

coarse particles. Each of these issues is 

further summarized and discussed 

below. 

In discussing their disagreement with 

EPA’s interpretation of four key 

epidemiologic studies (Ito, 2003; 

Burnett et al., 1997; Mar et al., 2003; 

Ostro et al., 2003), these commenters 

placed significant weight on the 

alternative interpretations of these 


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61The Response to Comments document contains 

more detailed responses to the specific issues these 

commenters raise regarding the interpretation of the 

epidemiologic evidence, which is important in 

terms of the use of these studies for supporting a 

coarse standard (this section of the preamble) as 

well as their use in deciding upon an appropriate 

level of protection (section III.D.2 of this preamble). 

62Unlike more commonly used time series 

studies, the design used in this study has the 

advantage of controlling for confounding by having 

each case serve as its own control. The Criteria 

Document notes limitations in available 

measurement information and adjustment for 

season that may have influenced the relative results 

for fine and coarse particles (EPA, 2004a, pp. 185–

186). 

studies that EPA provided in the 

proposal to encourage additional public 

comment (71 FR 2671–72). In particular, 

they criticized EPA’s reliance on the 

single pollutant models in these and 

other studies as biased because the 

models omit PM2.5 and gaseous copollutants. The commenters argued that 

when PM2.5 or gaseous co-pollutants 

were added to the underlying models, 

the effects associated with PM10–2.5 lost 

statistical significance. These 

commenters also stated that EPA failed 

to consider and give appropriate weight 

to a significant number of studies which 

relied on larger and more powerful data 

sets, were of longer duration, and 

assessed PM10–2.5 using multi-pollutant 

models, but did not find any statistically 

significant associations, including 

Schwartz et al. (1996), Thurston et al. 

(1994), Sheppard (2003), Fairley (2003), 

and Lipfert et al. (2000). They further 

summarized and attached a ‘‘detailed 

review of the cited studies’’ prepared by 

an academic consultant, which they 

stated reveals numerous deficiencies 

that undermine the use of these studies 

to support the proposed coarse particle 

standard or any alternative standard. 

Based on all of the above, one 

commenter claimed that a ‘‘fair and 

sound’’ assessment of evidence would 

not conclude coarse particles have 

effects at ambient concentrations 

(National Mining Association, p. 14). 

The rationale for these commenters’

conclusions, however, do not consider 

important aspects of the rationale for 

retaining coarse particle protection and 

are inconsistent with CASAC and other 

recent reviews of the scientific 

evidence. As summarized in section 

III.A of the proposal, the scientific 

evidence contained in the Criteria 

Document and Staff Paper, both of 

which have been reviewed and found 

acceptable for use in regulatory decision 

making by CASAC, supports the need 

for some standard to provide continued 

protection from coarse particles.61 The 

alternative interpretation of the 

evidence espoused by these commenters 

essentially argues that it is more 

reasonable to presume that the positive 

results from one-pollutant PM10–2.5

statistical models is the result of bias 

associated with omitting co-pollutants, 

especially PM2.5, for which the evidence 

is much stronger. EPA does not accept 

this argument for both technical and 

public health policy reasons. The 

Criteria Document and Staff Paper 

explain the rationale for reliance on 

single pollutant models in these studies, 

while recognizing the significant 

uncertainties in the limited number of 

studies available (EPA, 2004, section 

8.4.3; EPA, 2005, p. 3–46). These 

documents illustrate the results of a 

number of studies that examined copollutants (Figures 8–16 through 8–18 

of the Criteria Document), where it can 

be seen that, in most cases, the 

inclusion of gaseous co-pollutants does 

little to change the effects estimate for 

PM2.5, although in some cases it does. 

Recognizing the additional uncertainties 

in measuring coarse particles (as 

discussed below), these documents 

further note the importance of the 

relative consistency in the size of effects 

estimates for coarse particles as well as 

the pattern of generally positive 

associations, and the need for 

considering the results of recent 

statistically significant associations 

found in PM10 studies where it is 

reasonable to expect that the coarse 

fraction dominated the distribution. It 

would be unwise to presume, in the face 

of this evidence, that the single 

pollutant result for coarse particles is 

generally the result of omitted gases in 

the model. 

EPA also believes that it is 

inappropriate to presume that coarse 

particle or PM10 associations in single 

or multi-pollutant models can be wholly 

explained by fine particles. In studies 

where PM2.5 and PM10–2.5 have similar 

effect estimates, it is difficult to 

determine whether one or both 

contribute to the result (e.g. EPA 2004a, 

p 8–61). The comparison of PM2.5 and 

PM10–2.5 is further complicated by the 

differential measurement error between 

the two pollutants, which is generally 

greater for coarse particles (as discussed 

below). When both pollutants have 

similar effect estimates, it is difficult to 

determine whether one or both 

contribute to the result (e.g. EPA, 2004a, 

p. 8–61). Some studies conducted in 

urban areas, however, have found 

significant associations for coarse 

particles, but not fine particles. The 

Criteria Document summarizes a case 

cross-over study (Lin et al., 2002) 

conducted in Toronto, that found a 

significant association of PM10–2.5 with 

asthma hospital admissions in children 

ages 6–12 that was robust to the 

inclusion of gaseous co-pollutants, but 

did not report significant associations 

for PM2.5.62 Three different studies used 

essentially the same air quality data set 

to examine coarse and fine particles in 

Phoenix (Mar et al., 2000, 2003; Clyde, 

2000; Smith et al., 2000). All three 

studies found significant associations 

between mortality and PM10–2.5, but only 

one found a significant association for 

PM2.5 (EPA, 2004a, p. 8–57 to 66). Ito 

(2003) found a significant association in 

Detroit between hospital admissions for 

ischemic heart disease and exposure to 

coarse particles, but not fine particles. 

While all of these studies have 

limitations, it is difficult to ignore the 

fact that, despite the differential 

measurement error associated with 

coarse particles, a number of these 

studies find statistically significant 

associations for coarse particles, but not 

for fine particles. For these reasons, EPA 

believes that it would be inappropriate, 

based on the limited data currently 

available, to presume that all of the 

effects associated with coarse particles 

in single pollutant models are actually 

the result of confounding by fine 

particles. 

It is also important to note that in the 

NAAQS reviews that concluded in 1987 

and 1997, EPA found that the scientific 

evidence then available supported the 

need to continue regulation of thoracic 

coarse particles through appropriate 

NAAQS. This evidence included 

mechanistic considerations developed 

from particle dosimetry and toxicology, 

as well as an integrated assessment of 

particle composition and both 

community and occupational 

epidemiologic studies. By 1997, EPA 

judged the evidence to be strong enough 

to propose separate standards for fine 

and coarse particles. While the D.C. 

Circuit found problems with the 

indicator for thoracic coarse particles 

promulgated in 1997, the court upheld 

EPA’s determination that a standard was 

needed (ATA I, 175 F.3d at 1054). In 

EPA’s judgment, the more recent studies 

included in the 2004 Criteria Document, 

even with their recognized limitations, 

serve to add to, not reduce, the concern 

present in previous reviews over 

ambient exposures to coarse particles, 

particularly in urban areas. 

The business and industry 

commenters also suggested that the 

epidemiologic studies were flawed by 

the reliance on data from central 

monitors to estimate community-level 

exposures to coarse fraction particles. 

According to these commenters, this 

would result in an overestimation of 


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63This issue is discussed in more detail in the 

Response to Comments document. 

64 In Phoenix, for example, two key sites were 

highly correlated with similar means. In Detroit/ 

Windsor, correlations were moderate to good, but 

absolute values were significantly higher in Detroit 

(Ross and Langstaff, 2005). 

65See e.g., American Petroleum Inst. v. Costle, 

665 F. 2d at 1186–87: ‘‘In setting margins of safety 

the Administrator need not regulate only the known 

dangers to health, but may ‘‘err’’ on the side of 

overprotection by setting a fully adequate margin of 

safety. Of course the Administrator’s conclusions 

must be supported by the record, and he may not 

engage in sheer guesswork. Where the 

Administrator bases his conclusion as to an 

adequate margin of safety on a reasoned analysis 

and evidence of risk, the court will not reverse.’’

exposure due to the significant spatial 

variability associated with coarse 

particle distributions. Such 

overestimation, in the commenters’

view, would invalidate any statistical 

associations found between ambient 

data, as measured by the central 

monitors, and adverse health effects. 

The National Mining Association (p. 

16–17), for example, noted: 

The spatial variability of coarse PM renders 

even the few, limited, uncertain 

epidemiological studies that have been cited 

by EPA invalid, as well as imprecise * * *. 

Given that the purported associations 

between PM coarse and health effects is 

small to begin with, 71 FR at 2659, the logical 

conclusion should be that the lack of a 

demonstrable connection between the 

monitored ambient data and the level of 

exposure of the subject population is a fatal 

flaw that precludes reliance on the studies 

for any connection between PM coarse and 

health effects. 

These commenters also provided 

supporting information regarding 

correlations among monitors and an air 

quality modeling analysis purporting to 

show that significant quantities of 

coarse particles cannot travel more than 

1 kilometer from sources.63

The Criteria Document and Staff 

Paper contain detailed analyses of the 

spatial variability of coarse particle 

concentrations, as well as other issues 

that generally result in greater exposure 

measurement error for coarse particles 

as compared to fine particles (EPA, 

2004a, p. 3–52–53, Appendix 3A; EPA, 

2005, pp. 2–36–40, 2–70–73). While 

EPA agrees that coarse particle 

measurements from central monitors is 

subject to potentially large measurement 

error when used to reflect population 

exposures in epidemiologic studies, the 

Agency disagrees with the commenters’

assessment of the direction of the 

resulting bias and with their conclusion 

that any statistically significant 

associations between centrally 

monitored air quality concentrations 

and adverse health effects measured in 

these studies are invalid as a result. This 

issue received substantial attention in 

the Criteria Document (EPA, 2004a, 

section 8.4.5). The Criteria Document 

concluded that such measurement 

errors are more likely to underestimate 

the strength and the significance of any 

association between coarse particles and 

any adverse health effects observed in 

the study (EPA, 2004a, pp. 5–126, 8–

341). While the spatial variation of 

coarse particle data is larger than for 

fine particles, the Staff Paper notes that, 

on a day-to-day basis, coarse particle 

data from monitor sites within an urban 

area can be fairly well correlated, even 

when substantial differences exist in the 

absolute concentrations between the 

sites (EPA, 2005, p. 3–41). The signal 

that drives statistical associations 

between ambient concentrations and 

health effects in time-series studies is 

the day-to-day changes in concentration, 

not the absolute daily values. To the 

extent possible, EPA examined both the 

day-to-day correlations and annual 

averages in PM10–2.5 taken from multiple 

monitors in key study locations, such as 

Detroit, Phoenix and Coachella Valley 

(Ross and Langstaff, 2005).64

In reacting to this issue in opposing 

comments, the California Air Resources 

Board similarly stated: 

The current scientific consensus suggests that 

measurement of coarse particles will 

typically involve greater errors than that of 

fine particles. However we reject the * * * 

implication that therefore these studies are 

not reliable. In fact, the larger measurement 

error, which is likely to be random, would 

make it more difficult to find an association 

with mortality. It is well accepted in the 

epidemiological literature that such 

measurement error will tend to obscure a 

relationship between an exposure and a 

given health outcome, assuming that such a 

relationship exists. Therefore, the 

measurement error argument cannot be used 

to nullify an effect that has been observed. If 

anything, it is likely that the real effects are 

likely to be larger than those that were 

estimated. (CARB, p. 11) 

The EPA agrees with CARB’s analysis 

of the issue. Therefore, for the purposes 

of determining whether public health 

protection is warranted in light of the 

available evidence, EPA believes that it 

has interpreted the evidence from these 

epidemiologic studies correctly, and 

that despite the uncertainties, the 

evidence of statistically significant 

relationships between exposure to 

coarse particles and adverse health 

effects is sufficiently strong to support 

continued regulation of coarse particles. 

Some commenters opposed to 

maintaining a coarse particle standard 

criticized EPA’s risk assessment. These 

commenters stated that current shortterm epidemiologic data are insufficient 

to serve as the basis for a scientifically 

sound quantitative risk assessment, 

without which, they claim, EPA lacks 

sufficient evidence to establish a 

standard based on those data. According 

to these commenters, while EPA may 

exercise its judgment about future risks 

and set standards that are preventive in 

nature, as long as an adequate scientific 

rationale is presented, the Agency does 

not have the authority to engage in 

‘‘crystal ball speculation’’ in the absence 

of support in the record considered as 

a whole. (See e.g., Coarse Particle 

Coalition, p. 8–9, citing Lead Industries 

Assoc v. EPA, 647 F. 2d 1130, 1146–7 

(DC Cir. 1980), NRDC v. EPA, 902 F.2d 

962, 968, 971 (D.C. Cir. 1990) and Ethyl 

Corp. v. EPA, 541 F.2d 1, 13 (D.C. Cir. 

1976).) These commenters stated that 

the NAAQS must address only 

‘‘significant risk’’, not any risk, and that 

EPA has failed to demonstrate that 

coarse particles pose a significant 

enough risk to human health to warrant 

a coarse particle standard. 

The EPA disagrees on technical, 

policy, and legal grounds. For reasons 

specified in the proposal and 

summarized above, EPA believes that 

the available scientific evidence is more 

than adequate to support a decision to 

continue regulation of coarse particles 

under the NAAQS. Although the data 

are weaker than for fine particles and 

subject to greater measurement error, in 

several of the studies where 

comparisons are possible, the 

normalized relative risk estimates for 

coarse particles from the new urban/ 

industrial-area studies that were 

included in the Criteria Document often 

fall into a similar range as those for fine 

particles (EPA, 2004a, p. 8–64; EPA, 

2005, pp. 3–13 and 3–20). Furthermore, 

as summarized above, EPA did produce 

a risk assessment for thoracic coarse 

particles, which was reviewed by 

CASAC and included in the Staff Paper 

(EPA, 2005, Chapter 4). While the 

limited number of cities and the 

significant uncertainties noted in the 

risk assessment and the proposal limit 

their quantitative usefulness, EPA staff 

concluded that the risk assessment 

results for the two urban areas in the 

assessment that did not meet the current 

PM10 standards are indicative of risks 

that can reasonably be judged to be 

important from a public health 

perspective. 

Furthermore, there is no requirement 

that EPA develop a ‘‘scientifically sound 

quantitative risk assessment’’ before 

adopting or revising a NAAQS (ATA III, 

283 F.3d at 374), or that the Agency 

must demonstrate significant risk before 

promulgating a NAAQS.65 EPA’s 

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66 In general, EPA believes it is appropriate to 

draw a distinction between two general types of 

ambient mixes of coarse particles: ‘‘urban’’ and 

‘‘non-urban’’. The first term characterizes the mix 

in more heavily populated urban areas, where 

sources such as motor vehicles and industry 

contribute heavily to ambient coarse particle 

concentrations and composition. The term ‘‘nonurban,’’ on the other hand, encompasses mixes in 

a variety of other locations outside of urbanized 

areas, including mixes in rural areas which are 

likely to be dominated by natural crustal materials 

(and where urban types of sources are largely absent 

or, in the case of motor vehicles, are not present to 

the same degree). It should be noted that some types 

of sources are present in both urban and non-urban 

areas. Industrial sources, for example, are found in 

non-urban areas, though they are more commonly 

located in urban areas. Similarly, agricultural and 

mining sources are primarily non-urban sources, 

but may be found in or near urban areas as well. 

reviewed scientific studies in this 

review, as well as its reliance on 

CASAC’s unanimous recommendation 

that there is a need for a standard for 

thoracic coarse particles, cannot be 

considered ‘‘crystal ball speculation.’’

After careful consideration of all of 

these comments, EPA continues to 

believe that the health evidence, 

including dosimetric, toxicologic and 

epidemiologic study findings, supports 

retaining a standard to protect against 

effects associated with short-term 

exposure to thoracic coarse particles. As 

noted above and summarized in section 

III.A of the proposal, there is a 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. As summarized in 

the proposal (71 FR 2659), the available 

body of evidence also suggests there is 

a lack of such effects associated with 

long-term exposure to thoracic coarse 

particles. Considering the magnitude of 

the risks identified in health studies, 

and the size of potentially susceptible 

subpopulations such as people with 

preexisting respiratory diseases, 

including asthma, and children and 

older adults, EPA concludes that shortterm exposure to thoracic coarse 

particles can have an important public 

health impact. The health evidence 

regarding effects of thoracic coarse 

particles is limited in some respects and 

still subject to significant uncertainty. 

The Administrator has concluded that it 

is a priority to establish a robust 

research program that will enable future 

PM NAAQS reviews to make more 

informed decisions that will provide 

more targeted protection against the 

effects only of those coarse particles and 

related source emissions that prove to 

be of concern to public health. The 

Administrator also notes that the need 

for a standard for thoracic coarse 

particles has already been upheld based 

upon evidence of health effects 

considerably more limited than now 

available (ATA I, 175 F.3d at 1054). 

In the judgment of the Administrator, 

it is appropriate at this time to retain a 

standard to address the known and 

potential public health risks associated 

with exposure to coarse particles. The 

Administrator’s specific decisions 

regarding the indicator, averaging time, 

level and form of a standard for thoracic 

coarse particles are described below. 

C. Indicator for Thoracic Coarse 

Particles 

1. Introduction 

As outlined above, at the time of 

proposal the Administrator judged it 

appropriate, based on an evaluation of 

the available scientific evidence, to 

propose a new indicator of thoracic 

coarse particles defined to include those 

particles between 2.5 and 10 μm in 

diameter, or PM10–2.5, and qualified to 

focus on the mix of thoracic coarse 

particles generally present in urban 

environments. In making this 

determination, the Administrator relied 

heavily on key findings and 

observations from the Criteria Document 

and Staff Paper, and on 

recommendations from CASAC. The 

Staff Paper made the following general 

observations about the PM10–2.5

indicator: 

(1) The most obvious choice for a 

thoracic coarse particle standard is the 

size-differentiated, mass-based indicator 

used in the epidemiologic studies that 

provide the most direct evidence of 

such health effects, PM10–2.5. 

(2) The upper size cut of a PM10–2.5

indicator is consistent with dosimetric 

evidence that continues to reinforce the 

finding from past reviews that an 

aerodynamic size of 10 μm is a 

reasonable separation point for particles 

that penetrate to and potentially deposit 

in the thoracic regions of the respiratory 

tract. 

(3) The lower size cut of such an 

indicator is consistent with the choice 

of 2.5 μm as a reasonable separation 

point between fine and coarse fraction 

particles. 

(4) Further, the limited available 

information is not sufficient to define an 

indicator for thoracic coarse particles 

solely in terms of metrics other than 

size-differentiated mass, such as specific 

chemical components. 

(5) The available epidemiologic 

evidence for effects of PM10–2.5 exposure 

is quite limited and is inherently 

characterized by large uncertainties, 

reflective in part of the more 

heterogeneous nature of the spatial 

distribution and chemical composition 

of thoracic coarse particles and the more 

limited and generally uncertain 

measurement methods that have 

historically been used to characterize 

their ambient concentrations. 

In evaluating relevant information 

from atmospheric sciences, toxicology, 

and epidemiology related to thoracic 

coarse particles, the Staff Paper also 

noted that there appear to be clear 

distinctions between (1) the character of 

the ambient mix of particles generally 

found in urban areas as compared to 

that found in non-urban and, more 

specifically, rural areas, and (2) the 

nature of the evidence concerning 

health effects associated with thoracic 

coarse particles generally found in 

urban versus rural areas.66 Based on 

such information, and on specific initial 

advice from CASAC (Henderson, 

2005a), the Staff Paper considered a 

more narrowly defined indicator for 

thoracic coarse particles that would 

focus on the mix of such particles that 

is characteristic of the mix generally 

found in urban areas where thoracic 

coarse particles are strongly influenced 

by traffic-related or industrial sources. 

In so doing, the Staff Paper focused on 

comparing the potential health effects 

associated with thoracic coarse particles 

in urban and rural settings, as discussed 

below. 

The Staff Paper also noted that 

atmospheric science and monitoring 

information indicates that exposures to 

thoracic coarse particles tend to be 

higher in urban areas than in nearby 

rural locations. Further, the mix of 

thoracic coarse particles typically found 

in urban areas contains a number of 

contaminants that are not commonly 

present to the same degree in the mix of 

natural crustal particles that is typical of 

rural areas. The elevation of PM10–2.5

levels in urban locations as compared to 

those at nearby rural sites suggests that 

sources located within urban areas are 

generally the cause of elevated urban 

concentrations; conversely, PM10–2.5

concentrations in such urban areas are 

not largely composed of particles blown 

in from more distant regions (EPA, 

2005, sections 2.4.5 and 5.4.2.1). 

Important sources of thoracic coarse 

particles in urban areas include dense 

traffic that suspends significant 

quantities of dust from paved roads, as 

well as industrial and combustion 

sources and construction activities that 

contribute to ambient coarse particles 

both directly and through deposition to 

soils and roads (EPA, 2005, Table 2–2). 


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The Staff Paper concluded that the mix 

of thoracic coarse particles in urban 

areas would likely differ in composition 

from that in rural areas, being 

influenced to a relatively greater degree 

by components from urban mobile and 

stationary source emissions. 

While detailed composition data are 

more limited for PM10–2.5 than for PM2.5, 

available measurements from some 

areas as well as studies of road dust 

components do show a significant 

influence of urban sources on both the 

composition and mass of thoracic coarse 

particles generally found in urban areas. 

Although crustal elements and natural 

biological materials represent a 

significant fraction of thoracic coarse 

particles in urban areas, both their 

relative quantity and character may be 

altered by urban sources (EPA, 2005, p. 

5–54). Traffic-related activities can also 

grind and resuspend vegetative 

materials into forms not as common in 

more natural areas (Rogge et al., 1993). 

Studies of urban road dusts find that 

levels of a variety of components are 

increased from traffic as well as from 

other anthropogenic urban sources, 

including products of incomplete 

combustion (e.g. polycyclic aromatic 

hydrocarbons) from motor vehicle 

emissions and other sources, brake and 

tire wear, rust, salt and biological 

materials (EPA, 2004a, p. 3D–3). 

Limited ambient coarse fraction 

composition data from various 

comparisons show that metals and 

sometimes elemental carbon contribute 

a greater proportion of thoracic coarse 

particle mass in urban areas than in 

nearby rural areas. In addition, while 

large uncertainties exist in emissions 

inventory data, the Staff Paper observed 

that major sources of PM10–2.5 emissions 

in the urban counties in which 

epidemiologic studies have been 

conducted are paved roads and ‘‘other’’

sources (largely construction), and that 

such areas also have larger contributions 

from industrial emissions, whereas 

unpaved roads and agriculture are the 

main sources of PM10–2.5 emissions 

outside of urban areas. 

In the proposal, EPA also stated that 

toxicologic studies, although quite 

limited, support the view that thoracic 

coarse particles from sources common 

in urban areas are of greater concern 

than uncontaminated materials of 

geologic origin. One major source of 

thoracic coarse particles in urban areas 

is paved road dust; the Criteria 

Document discussed results from a 

recent toxicologic study in which road 

tunnel dust particles had greater allergyrelated activity than several other 

particle samples (Steerenberg et al., 

2003; EPA, 2004a, pp. 7–136–137). This 

study supports evidence available in the 

last review regarding potential effects of 

road dust particles (EPA, 1996b, p. V–

70). In contrast, a number of studies 

have reported that Mt. St. Helens 

volcanic ash, an example of 

uncontaminated natural crustal material 

of geologic origin, has very little toxicity 

in animal or in vitro toxicologic studies 

(EPA, 2004a, p. 7–216). 

A few toxicologic studies have used 

ambient thoracic coarse particles from 

urban/suburban locations (PM10–2.5), 

and the results suggest that effects can 

be linked with several components of 

PM10–2.5. These in vitro toxicologic 

studies linked thoracic coarse particles 

with effects including cytotoxicity, 

oxidant formation, and inflammatory 

effects (EPA, 2005, sections 3.2 and 

5.4.1). While these studies cannot be 

used for quantitative assessment of 

morbidity or mortality effects, they 

suggest that several components (e.g., 

metals, endotoxin, other materials) may 

have roles in various health responses 

but do not suggest a focus on any 

individual component. 

Although largely focused on 

undifferentiated PM10, the series of 

epidemiologic observations and 

toxicologic experiments related to the 

Utah Valley suggest that directly 

emitted (fine and coarse) and 

resuspended (coarse) urban industrial 

emissions are of concern. Of particular 

interest are area studies spanning a 13- 

month period when a major source of 

PM10 in the area, a steel mill, was not 

operating. Observational studies found 

that respiratory hospital admissions for 

children were lower when the plant was 

shut down (Pope, 1989). More recently, 

a set of toxicologic and controlled 

human exposure studies have used 

particles extracted from filters from 

ambient PM10 monitors from periods 

when the plant did and did not operate. 

In both human volunteers and animals, 

greater lung inflammatory responses 

were reported with particles collected 

when the source was operating, as 

compared to the period when the plant 

was closed (EPA, 2004a, p. 9–73). In 

addition, in some studies it was 

suggested that the metal content of the 

particles was most closely related to the 

effects reported (EPA, 2004a, p. 9–74). 

While peak days in the Utah Valley 

occur in conditions that enhance fine 

particle concentrations, over the long 

run, over half of the PM10 was in the 

coarse fraction. The aggregation of 

particles collected on the filters during 

the study period reflects this long-term 

composition and represent the kinds of 

industrial components that would be 

incorporated in road dusts in the area. 

The Staff Paper also noted that 

epidemiologic studies that have 

examined exposures to thoracic coarse 

particles generally found in urban 

environments, together with studies that 

have taken into account exposures to 

natural crustal materials typical of rural 

areas, generally support the view that 

the mix of thoracic coarse particles 

generally found in urban areas is of 

concern to public health, in contrast to 

natural crustal dusts of geologic origin. 

With respect to the urban results, 

several recent studies have shown 

associations between PM10–2.5 and 

health outcomes in a few sites across the 

U.S. and Canada. Associations have 

been reported with morbidity in a few 

urban areas, some of which had 

relatively low PM10–2.5 concentrations. 

For mortality, statistically significant 

associations have been reported only for 

two urban areas that have notably 

higher ambient PM10–2.5 concentrations. 

These associations are with short-term 

exposures to aggregated PM10–2.5 mass, 

and no epidemiologic evidence is 

available on associations with different 

components or sources of PM10–2.5. 

However, these studies have all been 

conducted in urban areas of the U.S., 

and thus reflect effects associated with 

the ambient mix of thoracic coarse 

particles generally present in urban 

environments, which includes PM from 

traffic and industrial sources. 

The Staff Paper also pointed to other 

evidence from epidemiologic studies 

suggesting that mortality and possibly 

other health effects are not associated 

with thoracic coarse particles from dust 

storms or other such wind-related 

events that result in suspension of 

natural crustal materials of geologic 

origin. The clearest example is a study 

in Spokane, WA, which specifically 

assessed whether mortality was 

increased on dust-storm days using 

case-control analysis methods. The 

average PM10 level was more than 200 

μg/m3 higher on dust storm days than 

on control days, and the authors report 

no evidence of increased mortality on 

these specific days (Schwartz et al., 

1999). One caveat of note is the 

possibility that people may reduce their 

exposure to ambient particles on the 

dustiest days (e.g., Gordian et al., 1996; 

Ostro et al., 2000). Nevertheless, these 

studies provide no suggestion of 

significant health effects from 

uncontaminated natural crustal 

materials that would typically form a 

major fraction of coarse particles in 

rural areas. 

Beyond the urban and rural 

distinctions discussed above, the Staff 

Paper also considered the extent to 

which there is evidence of effects from 


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67As used in the Staff Paper, the term ‘‘mining 

sources’’ is intended to include all activities that 

encompass extraction and/or mechanical handling 

of natural geologic crustal materials. In the context 

of this rulemaking, neither mining nor agricultural 

sources are included in the more general category 

of ‘‘industrial sources.’’

exposure to the ambient thoracic coarse 

particles in communities predominantly 

influenced by agricultural or mining 

sources.67 For example, in the last 

review, EPA considered health evidence 

related to long-term silica exposures 

from mining activities, but found that 

there was a lack of evidence that such 

emissions contribute to effects linked 

with ambient PM exposures (EPA, 

1996b, p. V–28). Similarly in this 

review, there is an absence of evidence 

related to such community exposures. 

While crustal and organic dusts 

generated from agricultural activity can 

include a variety of biological materials, 

and some occupational studies 

discussed in the Criteria Document 

report effects at occupational exposure 

levels (EPA, 2004a, Table7B–3, p. 7B–

11), such studies do not provide 

relevant evidence for effects at the much 

lower levels of community exposure. 

Further, it is unlikely that such 

predominantly non-urban sources 

contribute to the effects reported in the 

recent urban epidemiologic studies. 

The Criteria Document concluded its 

integrated assessment of the effects of 

natural crustal materials as follows: 

Certain classes of ambient particles appear to 

be distinctly less toxic than others and are 

unlikely to exert human health effects at 

typical ambient exposure concentrations (or 

perhaps only under special circumstances). 

For example, particles of crustal origin, 

which are predominately in the coarse 

fraction, are relatively non-toxic under most 

circumstances, compared to combustionrelated particles (such as from coal and oil 

combustion, wood burning, etc.) However, 

under some conditions, crustal particles may 

become sufficiently toxic to cause human 

health effects. (EPA, 2004a, p. 8–344) 

The Staff Paper assessment of the 

available evidence relevant to the 

appropriate scope of an indicator for 

coarse particles can be summarized as 

follows. Ambient concentrations of 

thoracic coarse particles generally 

reflect contributions from local sources, 

and the limited information available 

from speciation of thoracic coarse 

particles and emissions inventory data 

indicate that the sources of thoracic 

coarse particles in urban areas generally 

differ from those found in non-urban 

areas. As a result, the mix of thoracic 

coarse particles people are typically 

exposed to in urban areas can be 

expected to differ appreciably from the 

mix typically found in non-urban or 

rural areas. Ambient PM10–2.5 exposure 

is associated with health effects in 

studies conducted in urban areas, and 

the limited available health evidence 

more strongly implicates the ambient 

mix of thoracic coarse particles that is 

dominated by traffic-related and 

industrial sources than that dominated 

by uncontaminated soil or geologic 

sources. The limited evidence does not 

support either the existence or the lack 

of causative associations for community 

exposures to thoracic coarse particles 

from agricultural or mining industries. 

Given the apparent differences in 

composition and in the epidemiologic 

evidence, the Staff Paper concluded that 

it is not appropriate to generalize the 

available evidence of associations with 

health effects that have been related to 

thoracic coarse particles generally found 

in urban areas and apply it to the mix 

of particles typically found in nonurban or rural areas (EPA, 2005, p. 5–

57). The Staff Paper concluded that the 

available evidence collectively suggests 

that a more narrowly defined indicator 

for thoracic coarse particles should be 

considered that would protect public 

health against effects that have been 

linked with the mix of thoracic coarse 

particles generally present in urban 

areas. Such an indicator would be 

principally based on particle size, but 

also reflect a focus on the mix of 

thoracic coarse particles that is 

generally present in urban environments 

and the sources that principally 

generate that mix. The Staff Paper 

recommended consideration of thoracic 

coarse urban particulate matter 

(UPM10–2.5) as an indicator for a thoracic 

coarse particle standard, referring to the 

mix of airborne particles between 2.5 

and 10 μm in diameter that are generally 

present in urban environments, which, 

as discussed above, are principally 

comprised of resuspended road dust 

typical of high traffic-density areas and 

emissions from industrial sources and 

construction activities (EPA, 2005, p. 5–

54, 5–57–58). The Staff Paper concluded 

that such an indicator would more 

likely be an effective indicator for 

standards to protect against health 

effects that have been associated with 

thoracic coarse particles than a more 

broadly focused PM10–2.5 indicator. This 

indicator would also be consistent with 

a cautious interpretation of the 

epidemiologic evidence that does not 

potentially over-generalize the results of 

the limited available studies. 

In conjunction with this 

recommendation of an indicator defined 

in terms of the mix of thoracic coarse 

particles that are generally present in 

urban areas, the Staff Paper also 

discussed the importance of a 

monitoring network designed to be 

consistent with the intent of such an 

indicator and to facilitate 

implementation of such a standard. It 

should be noted that EPA has 

historically used other implementationrelated policies, specifically its 

guidelines regarding the handling of 

data affected by exceptional or natural 

events, to address elevations in thoracic 

coarse particle levels that may occur in 

urban areas as a result of dust storms or 

other such events for which the staffrecommended indicator was not 

intended to apply. The Staff Paper 

recommended that both new criteria for 

monitor network design and revised 

natural/exceptional events policies 

should work in concert with a revised 

thoracic coarse particle indicator to 

ensure the most effective application of 

a thoracic coarse particle standard. 

In its review of the Staff Paper 

recommendation for a thoracic coarse 

particle indicator (Henderson, 2005b, p. 

4), the CASAC generally agreed that 

‘‘thoracic coarse particles in urban areas 

can be expected to differ in composition 

from those in rural areas;’’ that ‘‘coarse 

particles in urban or industrial areas are 

likely to be enriched by anthropogenic 

pollutants that tend to be inherently 

more toxic than the windblown crustal 

material which typically dominates 

coarse particle mass in arid rural areas;’’

and that ‘‘evidence of associations with 

health effects related to urban coarsemode particles would not necessarily 

apply to non-urban or rural coarse 

particles.’’ Further, most CASAC Panel 

members concurred that ‘‘the current 

scarcity of information on the toxicity of 

rural dusts makes it necessary’’ for EPA 

to base its standard for thoracic coarse 

particles ‘‘on the known toxicity of 

urban-derived coarse particles.’’ While 

most Panel members concurred with the 

thoracic coarse particle indicator 

recommended in the Staff Paper, a few 

members recommended specifying an 

unqualified PM10–2.5 indicator in 

conjunction with monitoring network 

design criteria and natural/exceptional 

events policies that would emphasize 

urban influences. In either case, CASAC 

indicated that the intent of any such 

indicator should be to ‘‘provide 

protection against those components of 

PM10–2.5 that arise from anthropogenic 

activities occurring in or near urban and 

industrial areas.’’

Based on these considerations, the 

Administrator proposed to establish a 

new indicator for thoracic coarse 

particles in terms of PM10–2.5, qualified 

so as to include any ambient mix of 

PM10–2.5 that is dominated by 

resuspended dust from high-density 

traffic on paved roads and PM generated 


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68Commenters cite the original publication. In 

the subsequent reanalysis, the investigators report 

‘‘our original findings remained unchanged’’ (Mar 

et al. 2003). 

by industrial sources and construction 

sources, and to exclude any ambient 

mix of PM10–2.5 that is dominated by 

rural windblown dust and soils and PM 

generated by agricultural and mining 

sources (71 FR 2667–68). Furthermore, 

EPA proposed that ‘‘[a]gricultural 

sources, mining sources, and other 

similar sources of crustal material shall 

not be subject to control in meeting this 

standard’’ (71 FR 2699). As summarized 

above in section I.E, the proposed 

standard also included specific monitor 

site-suitability requirements which any 

monitor would have to meet in order to 

be used for comparison to the NAAQS, 

including a requirement that such 

monitors be sited in urbanized areas 

with a minimum population of 100,000. 

These requirements were designed to 

ensure that the monitors were capturing 

the ambient mix of PM10–2.5 dominated 

by the sources of concern. 

Subsequent to the proposal, CASAC 

provided additional comments to the 

Administrator on the proposed indicator 

for thoracic coarse particles. In a letter 

dated March 21, 2006, the Committee 

stated that ‘‘the PM Panel was pleased 

to see that the indicator for coarse 

thoracic particles of concern to public 

health took into account some of the 

various approaches that the PM Panel 

identified for consideration’’

(Henderson 2006, p. 4). The CASAC 

reiterated its earlier statement that ‘‘the 

current scarcity of information on the 

toxicity of rural dusts makes it 

necessary for the Agency to base its 

regulations on the known toxicity of 

urban-derived coarse particles.’’

However, the Committee went on to say 

that ‘‘the CASAC neither foresaw nor 

endorsed a standard that specifically 

exempts all agricultural and mining 

sources, and offers no protection against 

episodes of urban-industrial PM10–2.5 in 

areas of populations less than 100,000.’’

The Committee recommended the 

‘‘expansion of our knowledge of the 

toxicity of rural dusts rather than 

exempting specific industries (e.g. 

mining, agriculture)’’ from control 

under the standard (id at 5). 

2. Comments on Indicator for Thoracic 

Coarse Particles 

The EPA received a large number of 

comments on its proposed decision with 

regard to the indicator of thoracic coarse 

particles which overwhelmingly 

opposed the proposed indicator. Few 

commenters unconditionally supported 

EPA’s proposal to replace the PM10

indicator with a qualified PM10–2.5

indicator that would provide targeted 

protection by including certain ambient 

mixes of thoracic coarse particles and 

excluding others. Support for the 

proposed approach came almost entirely 

from those industrial sectors whose 

sources were excluded from the 

proposed qualified PM10–2.5 indicator 

(i.e., agriculture and mining interests). 

While these commenters argued that 

EPA should not maintain any standard 

for thoracic coarse particles, they 

conditionally supported the qualified 

indicator if any standard were to be set. 

In contrast, all other commenters, 

including environmental and public 

health groups, State and local agencies, 

and industries not excluded from the 

proposed indicator (e.g., transportation 

and construction), opposed the 

proposed qualified indicator. 

Representatives from a variety of groups 

who otherwise disagreed on various 

aspects of the proposed indicator 

commented on the need for additional 

research to address the uncertainties in 

the current body of evidence regarding 

coarse particles and health effects. In 

addition, a variety of commenters urged 

EPA to deploy additional PM10–2.5

monitors in both urban and rural areas, 

consistent with the advice of CASAC, to 

provide a more robust and complete 

body of evidence regarding coarse 

particle effects. 

Commenters conditionally supporting 

the proposal expressed the view that 

EPA should exclude non-urban windblown dust and soil from the PM10–2.5

indicator. According to these 

commenters, ‘‘such particles have been 

shown to be nontoxic, and the scientific 

studies show that they are not 

associated with adverse health effects’

(American Farm Bureau Federation, p. 

1). Furthermore, these commenters 

agreed with the proposed exclusion for 

agricultural and mining sources, stating 

that ‘‘the preponderance of scientific 

evidence continues to demonstrate that 

fugitive dust from agricultural and 

mining operations presents no 

substantial health or welfare concerns’

(National Mining Association, p. 1; see 

also National Cattlemen’s Beef 

Association, p. 1). These commenters 

quoted extensively from the Criteria 

Document and Staff Paper, and made 

points that were in many cases 

conceptually similar to the arguments in 

these documents and in the proposal. 

These commenters also tended to argue 

that there is substantial scientific 

evidence showing an absence of health 

effects from rural particles. 

These commenters cited differences 

in the composition of the mix of 

particles in urban areas versus the mix 

of particles in non-urban areas, which 

they stated is dominated by wind-blown 

soil fractions including silicates, 

primary organic materials including 

ground plant matter, residential wood 

smoke, and dust from unpaved roads. 

Though the coarse particle mix in urban 

areas also contains significant crustal 

materials, the commenters stated that it 

is contaminated by a wide variety of 

industrial and combustion-related 

byproducts, such as metals and organic 

materials (tire and brake wear, vehicle 

exhaust, industrial emissions, 

residential fuel combustion). These 

commenters noted that studies 

conducted in urban areas have linked 

health effects specifically to these 

urban-industrial contaminants. For 

example, the American Farm Bureau 

Federation cited the distinction between 

studies that found health effects related 

to traffic emissions in urban areas 

(Pearson et al., 2000; Kramer et al., 

2000; and Lin et al., 2002) and a study 

they suggested found a strong 

association between cardiovascular 

mortality and motor vehicle exhaust 

components, but a negative association 

between soil and total mortality (Mar et 

al., 2000).68 Some of these commenters 

argued that coarse mode particles, 

especially crustal coarse mode particles, 

are unlikely to serve as carriers of 

urban-area contaminants because they 

have less surface area, do not adsorb 

contaminants easily, and have short 

atmospheric residence times. These 

commenters conditionally agreed with 

EPA’s proposed goal of focusing 

regulatory efforts on the sources known 

to be associated with toxic coarse 

particles, especially traffic (Coarse 

Particle Coalition). Some of these 

commenters cited new studies 

completed after the close of the Criteria 

Document as providing additional 

evidence of associations between trafficrelated emissions and adverse health 

effects (e.g. Kim et al., 2004; Ryan et al., 

2005; Garshick et al., 2003; McDonald et 

al., 2004; and Ostro et al., 2006). 

These commenters also stated that 

while urban contaminants may increase 

the toxicity of coarse particles, studies 

have demonstrated a lack of adverse 

effects associated with exposure to 

coarse particles in non-urban areas (e.g., 

Buist et al. (1983) study of exposure to 

Mount St. Helens’ ash among diabetic 

children). Furthermore, these 

commenters argued that studies have 

found a lack of effects associated with 

exposure to crustal materials in general. 

They cited the lack of an association 

between mortality and dust storms 

found in Schwartz et al. (1999) and also 

noted that studies such as the 6-city 

study by Laden et al. (2000) have found 


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69The American Farm Bureau Federation’s 

summary of the results of Mar et al. (2000), offered 

in support of their arguments about the lack of 

effect of soil or crustal materials, misses some 

important elements of the study results. A major 

finding of the original study as well as the 

reanalysis (Mar et al., 2003) was an association 

between PM10–2.5 particles and mortality. The 

analysis in this work that examined sources and 

components examined contributions to the effects 

of PM2.5, not to PM10–2.5. In the opinion of the 

authors, the factor commenters call motor vehicle 

exhaust ‘‘probably represents the influence of motor 

vehicle exhaust and resuspended road dust’’ (Mar 

et al., 2000, p. 351). The negative association for 

‘‘soil’’ in the fine fraction cited by the commenter 

was apparently related to problems in the PM2.5

measurement. When the data were reassessed for 

the period with an improved sampler, the authors 

report that the association between soil and 

mortality was ‘‘positive and significant at 0 days 

lag’’ (ibid., p. 352). 

70The Laden et al. (2000) study cited by 

commenters was reanalyzed in Schwartz (2003), 

with qualitatively similar findings. As in Mar et al. 

(2000, 2003), this study examined the associations 

of crustal materials in the fine particle fraction, in 

which they make up such a small fraction of fine 

mass that one of the six cities had to be excluded 

from the analysis (Laden et al., 2000, p. 945). While 

this result does not provide any support for 

associations between coarse crustal materials and 

mortality, given the lower concentrations of coarse 

particles in five of the six cities and the lack of 

examination of coarse particle composition, the 

results are inconclusive with respect to the 

potential effects of higher concentrations of coarse 

particles. 

that crustal material, in both the fine 

and coarse fractions, is not associated 

with increased mortality. Thus, these 

commenters argued that there is 

sufficient evidence to show that crustal 

particulate matter is essentially benign 

and therefore should be excluded from 

the coarse particle indicator. 

The EPA agrees with these 

commenters that the strongest available 

evidence relates to the toxicity of the 

ambient mix of coarse particles found in 

urban environments. The limited 

evidence available from epidemiologic 

and toxicologic studies indicates 

exposure to ambient thoracic coarse 

particulate in urban areas is associated 

with health effects, and the health 

evidence more strongly implicates 

coarse particles from urban types of 

sources such as resuspended dust from 

high-density traffic on paved roads and 

PM generated by industrial sources and 

construction sources than coarse 

particles from uncontaminated soil or 

geologic sources. The EPA also agrees 

that there is far more evidence 

concerning health effects associated 

with thoracic coarse particles in urban 

areas than in non-urban areas. However, 

EPA disagrees with these commenters 

that there is sufficient evidence to 

demonstrate that there are no adverse 

health effects from community-level 

exposure to coarse particles in nonurban areas. Rather, the existing 

evidence is inconclusive with regard to 

whether or not community-level 

exposures to thoracic coarse particles 

are associated with adverse health 

effects in non-urban areas. However, 

EPA does agree with these commenters 

that additional research is needed to 

clarify this issue and to reduce some of 

the other uncertainties regarding the 

effects associated with coarse particles. 

As discussed above, the EPA is, in fact, 

expanding both its research and 

monitoring programs to collect 

additional evidence on the differences 

between coarse particles typically found 

in urban areas and those typically found 

in rural areas. Specifically, EPA notes 

that the Agency’s National Center for 

Environmental Research recently issued 

a Request for Proposals on ‘‘Sources, 

Composition, and Health Effects of 

Coarse Particulate Matter’’ which is 

designed to (1) improve understanding 

of the type and severity of health 

outcomes associated with exposure to 

PM10–2.5; (2) improve understanding of 

subpopulations that may be especially 

sensitive to PM10–2.5 exposures 

including minority populations, highly 

exposed groups, and other susceptible 

groups; (3) characterize and compare the 

influence of mass, composition, source 

characteristics and exposure estimates 

in different locations and differences in 

health outcomes, including comparisons 

in rural and urban areas; and (4) 

characterize the composition and 

variability of PM10–2.5 in towns, cities or 

metropolitan areas, including 

comparisons of rural and urban areas. In 

addition, as described in the final 

monitoring rule published elsewhere in 

today’s Federal Register, EPA and the 

states will require measurement of 

PM10–2.5 at 75 new multipollutant 

monitoring sites around the country. 

These sites will provide continuous 

measurements of mass as well as 

chemical speciation. EPA will locate 55 

of these sites in urban areas and 20 in 

rural areas in order to gather 

information on the composition and 

transport of coarse particles in urban 

and rural areas. In addition, these 

monitors will employ the latest in 

speciation technology to advance the 

science so that future regulation will 

provide more targeted protection against 

the effects only of those coarse particles 

and related source emissions that prove 

to be of concern to public health. 

In addition, EPA disagrees with these 

commenters that there is sufficient 

evidence to exclude crustal materials 

from the coarse particle indicator 

regardless of the degree of 

contamination. Although there is some 

evidence that coarse particles of natural 

geologic origin are relatively non-toxic 

in their uncontaminated form, the 

Criteria Document notes that such 

particles may become sufficiently 

‘‘contaminated by toxic trace elements 

or other components from previously 

deposited fine PM,’’ to cause health 

effects (EPA, 2004a, 8–344). Indeed, the 

urban coarse PM associated with 

adverse health effects in the studies 

discussed above was, by mass, 

predominantly crustal in origin.69 As 

noted in the proposal and in the 

response to these commenters on the 

need to maintain a coarse particle 

standard, EPA is aware of the studies 

that found no effects on mortality at 

lower coarse particle concentrations, but 

believes, consistent with the Staff Paper 

and Criteria Document conclusions, that 

the evidence is suggestive of a coarse 

particle effect in urban or industrial 

areas.70 The EPA continues to believe 

that urban sources may significantly 

alter both the relative quantity and 

character of crustal and natural 

biological materials in ambient mixes in 

urban areas. As noted above in section 

III.C.1, metals and other contaminants 

such as elemental carbon tend to appear 

in higher concentrations in the urban 

PM10–2.5 mix, and vegetative materials 

are ground and resuspended by trafficrelated activities into forms not common 

outside urban areas. 

In contrast to those few commenters 

who conditionally supported EPA’s 

proposed indicator, the vast majority of 

commenters opposed one or more 

aspects of EPA’s proposed indicator, 

including: (1) The basic decision to 

qualify the indicator to focus on 

particles associated with certain types of 

sources and to exclude other ambient 

mixes; and (2) the particular 

qualifications applied to the indicator, 

including the proposed siting 

requirements for coarse particle 

monitors suitable for comparison with 

the NAAQS and the proposed exclusion 

of agricultural, mining, and other 

similar sources from control under the 

standard. This large group of 

commenters advanced scientific as well 

as legal and policy arguments against 

drawing a distinction between particles 

typical of urban versus non-urban or 

rural areas. These commenters included 

public health groups such as the 

American Lung Association, the 

American Heart Association, the 

American Cancer Society, the American 

Diabetes Association, and the American 

Public Health Association, and 

environmental groups such as 

Earthjustice, Environmental Defense, 

and the Natural Resources Defense 

Council. It also included the State and 

Territorial Air Pollution Program 


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Administrators and the Association of 

Local Air Pollution Control Officials 

(STAPPA/ALAPCO) and numerous 

individual State and local air pollution 

control agencies, as well as dozens of 

Tribes and Tribal organizations such as 

the National Tribal Caucus, the National 

Tribal Air Association and its parent 

organization, the National Tribal 

Environmental Council. In addition, a 

number of industry groups expressed 

opposition to the proposal to qualify the 

coarse particle indicator; in general, 

these comments came from groups 

representing industry categories that 

were not excluded from the proposed 

indicator, such as the Engine 

Manufacturers Association, the Alliance 

of Automobile Manufacturers, and the 

National Association of Home Builders. 

Though these industry commenters 

primarily argued against setting any 

coarse particle standard at this time, 

they stated that if a standard were to be 

adopted, scientific evidence did not 

support the proposal to qualify the 

indicator based on the mix of sources 

present. 

Commenters opposed to a qualified 

coarse particle indicator advanced 

numerous scientific arguments to 

support their position. They criticized 

EPA’s interpretation of key 

epidemiologic studies, such as Gordian 

et al. (1996), Choudhury et al. (1997), 

Ostro et al. (2003), Smith et al. (2000) 

and Mar et al. (2003), arguing that these 

studies linked thoracic coarse particles 

to adverse health effects in 

environments where crustal 

components formed a significant part of 

the ambient mix of PM10–2.5. For 

example, commenters argued that the 

study conducted by Ostro et al. (2003) 

in Coachella Valley, which found 

statistically significant associations 

between exposure to coarse particles 

and mortality, provides direct evidence 

of harm from exposure to rural particles. 

These commenters also challenged the 

results of Schwartz et al. (1999), 

attributing the lack of statistically 

significant mortality results in that 

study to avoidance behavior (i.e., people 

may stay inside during dust storms) and 

noting that the study might have drawn 

different conclusions if morbidity 

endpoints had been considered. In 

support of this argument, they pointed 

to Hefflin et al. (1994), which looked at 

hospitalizations for bronchitis and 

sinusitis during dust storms and did 

find a small increase in these effects in 

the same area. 

In addition, a number of commenters, 

including States, researchers, 

environmental and public health 

groups, and industry commenters, cited 

studies of particle composition as 

showing that the coarse PM found in 

rural areas is commonly contaminated 

with the same toxic components as 

particles found in urban areas (e.g. 

Alaska Department of Environmental 

Conservation; American Lung 

Association; Engine Manufacturers 

Association; Veranth). Moreover, these 

commenters noted that rural dusts may 

contain additional toxic contaminants 

such as molds, fungi, endotoxins, 

pesticides, and carbonaceous 

compounds including polycyclic 

aromatic hydrocarbons (PAHs), all of 

which are associated with rural sources 

and have been shown to produce toxic 

effects (citing studies including: Monn 

and Becker 1999; Soukup and Becker 

2001; Horvath et al., 1996; Offenberg 

and Baker, 2000; Eleftheriadis and 

Colbeck, 2001). (See American Lung 

Association et al., pp. 92–100.) In 

addition, some commenters pointed to 

studies of the composition of coarse 

particles in particular locations, such as 

Owens and Mono Lakes in California, as 

evidence of the dangerous nature of 

rural particles. Commenters noted that 

coarse particles from these areas are 

contaminated by heavy metals, arsenic, 

and other toxic contaminants, but 

would be excluded from the proposed 

indicator. 

Commenters critical of the proposed 

decision to qualify the coarse particle 

indicator also stated that EPA had 

inappropriately relied on the relatively 

few studies involving exposure to 

crustal materials, especially the Mt. St. 

Helens’ studies. These commenters 

expressed the view that EPA should not 

equate exposure to volcanic ash to 

exposure to coarse particles emitted 

from agricultural and mining industries. 

Commenters noted that volcanic ash 

lacks many of the organic components 

typical of rural coarse PM, including 

pesticides and PAHs. Commenters 

pointed to specific components of 

coarse particles emitted by agricultural 

or mining activities, including 

endotoxins, pesticides, and metals, that 

they claim are associated with adverse 

health effects. These commenters argued 

that coarse particles in rural and other 

non-urban areas are not generally 

‘‘uncontaminated materials of geologic 

origin’’ or ‘‘uncontaminated natural 

crustal dusts.’’ They argued that some of 

the effects noted in epidemiologic 

studies of thoracic coarse particles, such 

as Mar et al. (2003), occurred in areas 

dominated by agricultural or mining 

dusts (Maricopa County Air Quality 

Department, p. 3–4). Some commenters 

also stated that EPA had not 

demonstrated or even claimed that 

coarse particles associated with 

agricultural and mining activities are 

harmless. Citing a long history of 

occupational studies documenting 

effects and EPA’s statement in the 

proposal that ‘‘in the 1987 review, EPA 

found that occupational and 

toxicological studies provided ample 

cause for concern related to higher 

levels of thoracic coarse particles’ (71 

FR 2654), these commenters urged EPA 

to give greater weight to the results of 

such studies. 

A number of commenters opposing a 

qualified PM10–2.5 indicator referenced 

‘‘new’’ epidemiologic and toxicologic 

studies which were not included in the 

Criteria Document in support of their 

arguments in favor of an unqualified 

PM10–2.5 indicator. Specifically, the 

commenters pointed to recent 

epidemiologic studies showing 

statistically significant adverse health 

effects from exposure to coarse particles 

of varying composition, such as one 

study that found an association between 

exposure to volcanic ash and wheeze 

and exercise-induced 

bronchoconstriction (Forbes et al., 

2003). In addition, commenters cited 

several ‘‘new’’ studies of health effects 

associated with exposure to coarse 

particles during Asian dust storms 

(Chen Y-S et al., 2004; Chen and Yang, 

2005; Yang C-Y et al., 2005; Chang et al., 

2006). Commenters also pointed to 

‘‘new’’ toxicologic studies such as 

Schins et al. (2004), Veranth (2004, 

2006), Becker (2005), Labban et al. 

(2004, 2006), and Steerenberg et al. 

(2006), arguing that toxicological studies 

do not show consistent differences 

between urban and rural dusts. 

In response to these commenters’ first 

point regarding the epidemiologic 

studies that were included in the 

Criteria Document, EPA does not agree 

with the commenters that these 

epidemiologic studies provide direct 

evidence of harm from non-urban or 

rural crustal material. While EPA 

acknowledges that crustal particles may 

have dominated the ambient mix in 

some of the locations in which these 

studies were done, it is also the case 

that these areas are all urban, so the 

crustal materials in the ambient mix 

typically would be contaminated by 

metals, road dust, and other combustion 

byproducts. At the same time, EPA 

notes that CASAC cited the studies by 

Ostro et al. (2000, 2003) as suggestive of 

health effects associated with exposure 

to rural crustal materials: ‘‘Little is 

known about the potential toxicity of 

rural dusts, although the 2000 and 2003 

Coachella Valley, CA studies from Ostro 

et al. showed significant adverse health 

effects, primarily involving exposures to 

coarse-mode particles arising from 


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crustal sources’ (Henderson, 2005a, p. 

4). Thus while EPA does not agree with 

these commenters that the 

epidemiologic studies demonstrate that 

non-urban or rural crustal particles are 

harmful, at the same time EPA believes 

the studies do raise credible concerns 

and suggest the need to be cautious in 

interpreting the epidemiologic and other 

evidence. 

The EPA agrees with these 

commenters that the observations of 

Hefflin et al. (1994) suggest it is possible 

that the lack of mortality effects on dust 

storm days observed in Schwartz et al. 

(1999) may be due to avoidance 

behavior. As noted in the proposal (71 

FR 2666), there is a possibility that 

people may reduce their exposure to 

ambient particles on the most dusty 

days. This argues for caution in 

interpreting the results of Schwartz et 

al. (1999) with regard to the potential 

health effects associated with exposure 

to natural crustal material. 

The EPA acknowledges the 

limitations on the scientific evidence 

identified by these commenters 

regarding the differences in composition 

and toxicologic effects of urban and 

rural thoracic coarse particles. As noted 

in the Criteria Document and Staff 

Paper, there is clear evidence of toxicity 

of certain components of thoracic coarse 

particles, such as metals and 

endotoxins, as well as evidence that 

natural crustal materials of geologic 

origin, such as Mt. St. Helens volcanic 

ash, may have very little toxicity. There 

is largely an absence of evidence 

regarding the presence or absence of 

toxicologic effects associated with other 

types of coarse particles in non-urban 

areas. However, EPA agrees that 

thoracic coarse particles in non-urban 

areas may become contaminated with a 

wide variety of toxic materials (EPA, 

2004a, p. 8–344). Clearly, however, 

crustal material associated with 

particular locations, such as the dry 

lakebeds of Owens and Mono Lakes, can 

be highly contaminated with metals, 

salts, and other toxic constituents. The 

EPA agrees with commenters that the 

potential toxicity of these components is 

well recognized; however, such 

locations tend to be isolated and not 

representative of other locations. 

In response to other comments raised 

by this group of commenters, EPA 

continues to find it inappropriate to 

assume that effects observed in 

occupational studies should be 

considered representative of effects that 

would occur at community exposure 

levels. However, EPA agrees with 

commenters that the presence of 

occupational exposure studies 

demonstrating adverse effects lends 

further support to a cautious approach 

in considering revisions to the standards 

affording protection from thoracic 

coarse particles. Finally, to the extent 

that commenters cited new scientific 

studies that were not considered in the 

Criteria Document in support of their 

arguments against a qualified coarse 

particle indicator, EPA notes that as 

discussed above in section I.C, EPA it is 

basing the final decisions in this review 

on the studies and related information 

included in the PM air quality criteria 

that have undergone CASAC and public 

review, and will consider the newly 

published studies for purposes of 

decision making in the next PM NAAQS 

review. 

Overall, the scientific evidence 

supports a conclusion that the risks of 

adverse health effects associated with 

thoracic coarse particles typically found 

in urban or industrial areas warrant 

targeted protection. Although the 

limited and inconclusive evidence does 

not support such a conclusion 

concerning thoracic coarse particles 

typically found in non-urban or rural 

areas, it supports a cautious approach 

concerning thoracic coarse particles. 

The EPA agrees with all the commenters 

who pointed to the need for additional 

research to strengthen the current body 

of evidence to reduce some of the 

uncertainties regarding the health 

effects associated with coarse particles. 

In addition to their criticisms of the 

scientific basis for EPA’s proposed 

indicator, commenters opposed to a 

qualified indicator also advanced legal 

and policy arguments against EPA’s 

proposed approach. In particular, 

commenters criticized the proposal’s 

provision that ‘‘agricultural sources, 

mining sources, and other similar 

sources of crustal materials shall not be 

subject to control in meeting this 

standard’’ (71 FR 2699); a large number 

of commenters expressed the view that 

the exclusion is flatly illegal, citing CAA 

section 101 (a) (3) and case law in 

support. These commenters also pointed 

to CASAC’s March 21, 2006 letter to the 

Administrator which stated that EPA 

had misconstrued the finding of the 

Committee and that the proposed rule—

particularly the source-category 

exclusions—was not consistent with the 

Committee’s recommendations. 

These commenters also stated that 

EPA had failed to demonstrate that its 

proposed qualified indicator would 

protect public health with an adequate 

margin of safety. Pointing again to the 

relative paucity of data regarding health 

effects associated with coarse particles 

of differing compositions, and the 

almost complete lack of evidence 

regarding health effects in rural areas, 

these commenters expressed the view 

that EPA must demonstrate 

affirmatively that the coarse particle 

standards will ensure an absence of 

adverse effects on sensitive individuals 

(American Lung Association, p. 82, 

citing Lead Industries Ass’n v. EPA, 647 

F.2d 1130, 1153 (D.C. Cir. 1980) and 

American Lung Ass’n v. EPA, 134 F.3d 

388, 389 (D.C. Cir. 1998)), and that in 

the absence of evidence, or in the face 

of significant uncertainty, the CAA 

requirement to provide an adequate 

margin of safety obligates EPA to 

regulate all coarse particles equally 

(Lead Industries Ass’n v. EPA, 647 F.2d 

1154–55). Some of these commenters 

pointed to the DC Circuit Court’s 

instruction in ATA III that ‘‘[t]he Act 

requires EPA to promulgate protective 

primary NAAQS even where * * * the 

pollutant’s risks cannot be quantified or 

‘precisely identified as to nature or 

degree’ ’’ (ATA III, 283 F.3d 355, 369 

(quoting PM NAAQS, 62 FR 28653)). 

Commenters also argued that, under 

the CAA, EPA is charged with setting 

ambient standards that are national in 

scope and application, and that the 

proposed qualified indicator fails this 

test. Citing Whitman, 531 U.S. at 473, 

some of these commenters stated that 

the proposed qualified indicator is a 

thinly veiled attempt to establish a 

coarse particle standard that only 

applies to urban areas, and that it denies 

citizens in non-urban areas adequate 

health protection. Several commenters, 

including numerous Tribes, argued that 

the qualified indicator, by virtue of 

depriving non-urban populations of 

protection from coarse particles, 

violated principles of environmental 

justice and the government’s Trust 

Responsibility to Tribes. 

Commenters pointed to other 

concerns as well, many of them focused 

on specific aspects of the proposed 

PM10–2.5 indicator. First, some 

commenters stated that the proposed 

qualified indicator inadequately 

describes the substance(s) being 

regulated. These commenters argued 

that EPA is attempting to establish a 

composition-based indicator without 

being able to define adequately which 

particular chemical or physical 

components are associated with adverse 

health effects. Furthermore, commenters 

pointed out that the indicator was 

defined in large part through an 

implementation strategy—i.e. via the 

placement of monitors—rather than in 

scientific terms. The Alliance of 

Automobile Manufacturers expressed 

concern that the result would be that 

two sources of coarse particulate matter 

with similar composition that 

presumably produce similar health 


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impacts would be ‘‘given different 

regulatory treatment based merely on 

the non-scientific qualifiers established 

in EPA’s indicator’’ (Alliance of 

Automobile Manufacturers, p. 9). 

In addition, some commenters 

pointed to a logical paradox inherent in 

the proposed PM10–2.5 indicator, which 

is defined to include any ambient mix 

‘‘dominated by’’ particles from 

particular types of sources. Commenters 

noted the potential for the same 

concentration of ‘‘harmful’’ coarse 

particles—i.e. particles from highdensity traffic, industrial sources and 

construction sources—to be regulated 

differently in different locations 

depending on what percentage of the 

ambient mix it constitutes relative to 

‘‘crustal’’ particles. These commenters 

stated that the coarse particle standard 

must provide a consistent level of 

protection from particles of concern, 

and that use of a 50 percent domination 

threshold would result in a variable 

level of protection from particles of 

concern. 

The EPA also received an extremely 

large number of comments from diverse 

stakeholder groups—some of whom 

conditionally supported a qualified 

indicator—regarding perceived 

problems with implementing the 

proposed PM10–2.5 indicator. Many 

commenters pointed out that EPA failed 

to specify which source types were 

included in the broad source category 

descriptions listed in the indicator. 

They requested further definition of 

what could be considered an 

‘‘agricultural source,’’ a ‘‘mining 

source,’’ or ‘‘other similar sources of 

crustal material’’ (i.e. those sources that 

would be excluded from control under 

the proposed standard), and which 

‘‘industrial’’ and ‘‘construction’’ sources 

were included in the indicator. 

Furthermore, some commenters 

inquired about the treatment of sources 

that were neither explicitly included in 

nor excluded from the proposed 

indicator, such as residential and 

commercial sources. In addition, 

commenters wondered how EPA or the 

States would make the determination 

that one set of sources was ‘‘dominant,’’

given the scarcity of knowledge about 

coarse particle emissions and air quality 

concentrations, and the lack of suitable 

source attribution techniques. 

Commenters also objected to the 

proposed five-part test for siting 

NAAQS-comparable monitors, noting 

that as written, the monitor siting 

criteria arbitrarily would prohibit 

monitoring and regulation of coarse 

particles outside urbanized areas of 

100,000 population, regardless of the 

presence of large or numerous sources 

of the types of coarse particles of 

concern or the nature of the ambient 

mix. Commenters pointed out that the 

monitor siting criteria, by virtue of their 

highly prescriptive role in defining 

where the pollutant can and cannot be 

measured, in essence define the 

indicator itself, and artificially narrow 

its scope such that in many instances, 

coarse particles of concern would not be 

covered by the indicator. These 

commenters argued that by failing to 

provide protection from coarse particles 

of concern in non-urban areas even 

though the composition of those 

particles may be identical to that of 

coarse particles found in large urban 

areas, the qualified indicator, as EPA 

proposed to implement it, would be 

under inclusive. Many Tribes and some 

other commenters raised concerns about 

the environmental justice implications 

of the proposal and stated that EPA had 

violated its Trust Responsibility toward 

Tribes, because Tribal lands would be 

virtually excluded from coverage under 

the proposed monitor siting criteria, 

regardless of the mix of particles 

present. Furthermore, numerous 

commenters stated that the siting 

criteria would be impossible to 

implement, so the criteria undermined 

the proposed standard on a practical 

level. Commenters particularly objected 

to the fifth part of the monitor-site 

suitability test, which as proposed 

would require an affirmative 

demonstration that the ambient mix at 

the site was dominated by sources of 

concern, even if all of the other four 

monitor site-suitability criteria were 

met. Commenters stated that this 

demonstration would be impossible to 

execute due to the lack of suitable data 

and techniques, undermining the siting 

of any NAAQS-comparable PM10–2.5

monitors. 

In response to these perceived 

problems with the proposed qualified 

indicator, commenters suggested a 

number of remedies. A few commenters, 

mostly industry representatives who 

preferred that no coarse particle 

standard be set at the current time, 

stated that if EPA does set a standard, 

it should be based on a qualified 

PM10–2.5 indicator, but EPA should fix 

specific problematic aspects of the 

proposal (e.g. clarify the definition of 

included vs. excluded industries). Most 

commenters, including States, Tribes, 

and environmental and public health 

groups, urged EPA to adopt an 

unqualified PM10–2.5 indicator to ensure 

adequate public health protection and to 

avoid some of their perceived legal and/ 

or policy issues associated with the 

qualified indicator. A few of these 

commenters recommended that EPA 

utilize the Exceptional Events Rule, 

proposed on March 10, 2006 (71 FR 

12592–12610), to exclude violations 

caused by rural windblown dust. 

According to these commenters, this 

would be consistent with historical 

practice, because in the past the Natural 

Events Policy has been applied in many 

instances to exclude data associated 

with dust storms and other events from 

consideration under the PM10 standard 

(see New Mexico Air Quality Bureau, p. 

10). 

Some commenters advocating an 

unqualified PM10–2.5 indicator stated 

that, given the limitations on the 

scientific evidence, and in light of some 

of the other problems identified with 

the proposed qualified indicator, EPA 

should consider retaining the current 

PM10 standards to continue protection 

from coarse particles. They expressed 

particular concern about the absence of 

control in the interim period between 

the issuance of the final PM NAAQS 

rule (which as proposed would include 

the revocation of existing PM10

standards in almost all locations) and 

the completion of designations under a 

new PM10–2.5 standard (which would 

require deployment of a new monitoring 

network followed by 3 years of data 

collection). A few of the commenters 

advocating the retention of the PM10

standards suggested that measurements 

of PM10 could be adjusted by subtracting 

out PM2.5 to avoid double regulating the 

fine fraction, to satisfy a concern voiced 

by the D.C. Circuit in ATA I (e.g., 

Alliance of Automobile Manufacturers; 

also some Tribes and States). Some 

Tribal, State and local commenters 

suggested that the 24-hour PM10

standard be retained permanently in all 

areas where the PM10–2.5 standard did 

not apply by virtue of the monitoring 

requirements, which limited NAAQScomparable monitors to sites that met 

the five-point site suitability test 

outlined in the monitoring rule. 

While EPA proposed a qualified 

indicator that attempted to include 

certain ambient mixes of thoracic coarse 

particles and exclude others, EPA’s 

evaluation of the large number of 

adverse comments received on the 

proposed qualified indicator has led it 

to the conclusion that significant 

caution is warranted in considering 

such revisions to the scope of the 

indicator affording public health 

protection from coarse particles. As 

discussed below, there are two main 

issues that arise from consideration of a 

qualified indicator for thoracic coarse 

particles: (1) The inability to effectively 

and precisely identify which coarse 

particles are included in the indicator 


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71These concerns apply both to defining the 

qualified indicator and implementing the standard. 

and which are not; 71 and (2) the 

importance of providing some level of 

protection from exposure to all thoracic 

coarse particles while targeting 

protection at those kinds of thoracic 

coarse particles for which there is more 

evidence regarding adverse health 

effects. 

As explained earlier in this section, 

EPA continues to believe that, from a 

scientific standpoint, it is appropriate to 

draw a distinction between the 

character of the ambient mix of thoracic 

coarse particles generally found in 

urban areas and that found in non-urban 

and, more specifically, rural areas, 

recognizing that the mix of coarse 

particles in urban areas is influenced to 

a relatively greater degree by 

components from urban mobile and 

stationary source emissions and that the 

evidence of health effects associated 

with exposure to these urban types of 

coarse particles should not be 

generalized to other types of coarse 

particles. In the presence of significant, 

though limited, evidence of effects in 

urban areas, it remains EPA’s view that 

a targeted indicator that focuses control 

on areas with ambient mixes of coarse 

particles known to be associated with 

adverse health effects will provide the 

most certain and substantial public 

health benefits. 

However, EPA also recognizes a 

number of flaws in the proposed 

qualified indicator, as noted by 

numerous commenters, most 

specifically the difficulties inherent in 

attempting to effectively and precisely 

identify the ambient mixes of concern. 

These include: (1) The artificial 

constraints on the reach of the indicator 

resulting from the application of 

quantitative monitor site-suitability 

criteria such as the requirement that 

NAAQS-comparable monitors can only 

be sited in urbanized areas with 

minimum 100,000 population even if 

there is an ambient mix of concern 

around such an area; and (2) the 

difficulties associated with attempting 

to determine with any precision which 

sources ‘‘dominate’’ the ambient mix of 

coarse particles in different locations. 

The quantitative constraints in the 

monitor site-suitability criteria result in 

an under-inclusive indicator that fails to 

include all ambient mixes of concern. 

Smaller urban and/or industrial areas, 

for example, would not meet the 

proposed monitor siting criteria, but 

might have an ambient mix of concern. 

Consequently, EPA agrees with 

commenters that unless the constraints 

were changed, the proposed indicator 

would be under-inclusive. The EPA has 

considered several options to modify 

the quantitative criteria, including those 

discussed in the proposal (see 

Weinstock, 2006). For example, EPA 

evaluated different possible minimum 

population thresholds (e.g., 25,000 or 

50,000 instead of 100,000) for areas 

eligible to site NAAQS-comparable 

monitors, and/or the possibility of 

adding additional criteria to include 

areas that do not meet a quantitative 

population threshold but are dominated 

by industrial or traffic-oriented sources. 

Each of these options, however, was 

found too inflexible to capture all 

relevant areas or too difficult to 

implement in practice. Thus, EPA 

believes that even a more complex set 

of quantitative criteria would fail to 

resolve the basic problem inherent in 

precisely identifying those ambient 

mixes to include and those to exclude. 

Based on the data available to us in this 

review, there still remains a clear risk of 

failing to capture all ambient mixes of 

concern, or of capturing ambient mixes 

that are intended to be excluded from 

the qualified indicator. 

Moreover, as a general matter, the use 

of a qualified indicator without such 

objective monitor site-suitability criteria 

would still present serious problems 

because it is currently impossible to 

determine with any precision which 

sources ‘‘dominate’’ the ambient mix in 

many different locations. Although it 

may be easy in certain instances to 

identify an ambient mix dominated by 

urban and/or industrial sources, in 

many cases it would be difficult to 

determine whether that precise ambient 

mix presents the types of health risks 

identified in the epidemiologic and 

other studies. The EPA is currently 

unable to identify any set of objective 

criteria or techniques such as chemical 

air quality speciation or modeling that 

could be practically employed to ensure 

adequate inclusion of all areas with 

particles of concern, and exclusion of 

areas without such particles. 

The EPA is also aware that the legal 

concerns raised by commenters with 

regard to the exemption of agricultural 

and mining sources from control under 

the standard, and the specific sections 

of the Clean Air Act that speak to this 

issue, would require careful 

consideration if the proposed qualified 

indicator were to be adopted. The 

logical paradox noted by commenters is 

also a flaw in the qualified indicator 

that would need to be resolved. It is 

another example of the lack of precision 

in the use of such a qualified indicator. 

After careful consideration of the 

concerns raised by commenters and the 

options available, EPA now agrees with 

commenters that the proposed qualified 

indicator is fundamentally flawed, 

because it cannot effectively and 

precisely identify the ambient mixes of 

concern and because modifications to 

the indicator that could rectify this and 

other problems highlighted by the 

commenters have not been identified. 

At the present time, therefore, EPA 

believes that there is an inherent risk 

that a qualified indicator would not 

include all of the ambient mixes of 

concern which the indicator is intended 

to capture. 

Furthermore, in light of the significant 

scientific uncertainty surrounding the 

health effects associated with different 

ambient mixes of coarse particles, EPA 

agrees with commenters that the 

proposed qualified indicator would be 

insufficiently protective and further 

concludes that, given the limitations on 

the evidence regarding the health risks 

associated with different ambient mixes, 

some protection from exposure to 

thoracic coarse particles is warranted in 

all areas. The EPA recognizes that 

additional data will be collected and 

analyzed that will be useful to inform 

the next review. 

The EPA has already set out the 

reasons for providing protection from 

exposure to ambient mixes dominated 

by the types of thoracic coarse particles 

found in urban or industrial areas. With 

respect to other ambient mixes, some 

commenters have argued that the 

scientific evidence, including 

epidemiologic, dosimetric, toxicologic, 

and occupational studies, demonstrates 

that non-urban mixes of thoracic coarse 

particles are harmful, and therefore that 

EPA should maintain an unqualified 

indicator. Other commenters argue that 

the evidence demonstrates that nonurban mixes of thoracic coarse particles 

are benign and therefore EPA should 

retain a qualified indicator. The EPA 

disagrees with both of these views 

regarding the strength of the evidence. 

The existing evidence is inconclusive 

with regard to whether or not 

community-level exposures to thoracic 

coarse particles are associated with 

adverse health effects in non-urban 

areas. In light of this uncertainty and the 

need for caution in considering the 

evidence, and recognizing the large 

population groups potentially exposed 

to non-urban thoracic coarse particles 

and the nature and degree of the health 

effects at issue, it is the judgment of the 

Administrator that the proper response 

to this body of evidence is to provide 

some protection from thoracic coarse 

particles in all areas. Congress 

‘‘specifically directed the Administrator 

to allow an adequate margin of safety to 

protect against effects which have not 


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yet been uncovered by research and 

effects whose medical significance is a 

matter of disagreement * * * Congress’

directive to the Administrator to allow 

an ‘‘adequate margin of safety’’ alone 

plainly refutes any suggestion that the 

Administrator is only authorized to set 

primary air quality standards which are 

designed to protect against health effects 

that are known to be clearly harmful.’’

Lead Industries v. EPA, 647 F.2d at 

1154–55; see also American Petroleum 

Inst. v. Costle, 665 F.2d at 1186 (‘‘in 

setting margins of safety the 

Administrator need not regulate only 

the known dangers to health’’). 

The Administrator has carefully 

reviewed the scientific evidence and 

recommendations contained in the Staff 

Paper, the advice and recommendations 

from CASAC, and the public comments 

received regarding the appropriate 

indicator for coarse particles. After 

doing so, the Administrator has decided 

that it would not be appropriate at this 

time to revise the indicator for coarse 

particles by adopting a qualified 

PM10–2.5 indicator, either as proposed or 

with modifications. At the same time, 

the Administrator believes it is 

appropriate to target protection from 

thoracic coarse particles principally 

towards those types of coarse particles 

that have been demonstrated to be 

associated with significant adverse 

health effects, specifically urban and 

industrial ambient mixes of coarse 

particles. 

In general, EPA believes these 

conclusions regarding the potential 

health effects associated with thoracic 

coarse particles, and the conclusion that 

an unqualified indicator that provides 

targeted protection is the most 

appropriate approach for regulating 

coarse particles, are consistent with 

views expressed by CASAC. In its June 

6, 2005 letter, CASAC expressed the 

view that it was ‘‘important to qualify 

the PM10–2.5 standard by somehow 

allowing exceptions for regions where 

the coarse fraction was composed 

largely of material that was not 

contaminated by industrial- or motor 

vehicle traffic-associated sources. 

Options discussed by members of the 

Panel for attempting to achieve this 

approach included limiting the standard 

to cover ‘‘all’’ urban areas, the judicious 

siting of monitors with a focus on urban 

areas, or regulatory exceptions for 

regions where road dust is not an issue 

or where rural components dominate 

the source. No single option was 

favored’’ (Henderson, 2005a, p. 8, 

emphasis added). CASAC thus 

recognized that there were numerous 

ways to approach the need for targeted 

protection. In its September 2005 letter 

responding to the recommendations 

regarding a qualified PM10–2.5 indicator 

in the final Staff Paper, the PM Panel 

noted that some members did not favor 

adoption of a qualified indicator. 

Moreover, CASAC clearly anticipated 

the difficulties associated with adopting 

a qualified PM10–2.5 indicator: 

CASAC generally agrees with EPA staff 

conclusions that thoracic coarse particles in 

urban areas can be expected to differ in 

composition from those in rural areas and 

that evidence of associations with health 

effects related to urban coarse-mode particles 

would not necessarily apply to non-urban or 

rural coarse particles (although it is likely 

that there will be some overlap of the same 

contaminants in both areas). Most Panel 

members concurred that the current scarcity 

of information on the toxicity of rural dusts 

makes it necessary for the Agency to base its 

regulations on the known toxicity of urbanderived coarse particles, and that an urban 

coarse particle indicator should be specified 

as UPM10–2.5. Other Panel members 

recommended specifying a national PM10–2.5

standard accompanied by monitoring and 

exceptional-events guidance that emphasized 

urban influences. Some members also 

expressed concerns whether EPA would be 

able to specify a clear definition of ‘‘urban’’

to effectively determine in advance the 

specific conditions in which the standard 

would (and would not) apply. It is 

recognized that, as more information on the 

toxicity of rural dusts is acquired, the name 

and/or geographical focus of a coarse-particle 

indicator may need to be reconsidered* * *. 

There is a paucity of data currently available 

on health outcomes related to thoracic coarse 

particles in rural areas and limited 

information on the composition and toxicity 

of rural area coarse particles. (Henderson 

2005b, p. 4) 

CASAC also commented negatively on 

the proposed qualified indicator, raising 

concerns about the quantitative criteria 

for monitor siting and the source 

exclusions, as well as flagging the need 

for more information about health 

effects in non-urban areas (Henderson, 

2006, p.4). 

The comments and concerns 

expressed by CASAC are consistent 

with the difficulties EPA has 

encountered in attempting to craft a 

qualified indicator, and the Committee 

correctly anticipated these difficulties. 

Furthermore, CASAC’s advice is 

generally consistent with the ultimate 

decision by the Administrator not to 

move to a qualified PM10–2.5 indicator at 

present. The practical difficulties and 

imprecision associated with a qualified 

indicator, as well as the substantial 

scientific uncertainty regarding the 

health effects associated with different 

components and mixes of coarse 

particles, the large population groups 

potentially exposed to non-urban 

thoracic coarse particles and the nature 

and degree of the health effects at issue, 

have convinced the Administrator that 

it is inappropriate to adopt a qualified 

PM10–2.5 indicator at this time. In the 

following section, EPA considers what 

indicator would most appropriately 

provide the type of targeted but 

comprehensive protection judged 

appropriate based on its review of the 

scientific evidence. 

3. Decision Not To Revise PM10

Indicator 

For reasons discussed in the previous 

section, in the view of the Administrator 

it is not appropriate to revise the PM10

indicator by replacing it with a qualified 

indicator for thoracic coarse particles at 

this time. Based on the scientific 

evidence already summarized, the 

Administrator believes it is necessary to 

maintain some protection from all 

ambient mixes of thoracic coarse 

particles, and also to have that level of 

protection reflect the varying degree of 

public health concern presented by the 

different ambient mixes of thoracic 

coarse particulate matter. This would 

mean allowing lower ambient 

concentrations of thoracic coarse 

particles in urban areas, where the 

evidence indicates the public health 

risks to be significant, and higher levels 

in non-urban areas where the public 

health concerns are less certain. The 

difficulty of the task is compounded 

because there presently is no means of 

achieving this objective by linking 

allowable concentrations to specific 

coarse particle chemical components. 

As CASAC noted, ‘‘[s]ufficient data are 

lacking at the present time to set 

standards [for thoracic coarse 

particulate matter] based specifically on 

composition’’ (Henderson 2005b, p. 5). 

Given these objectives and 

constraints, EPA carefully considered 

various possibilities regarding the 

indicator for coarse particles, including 

adopting an unqualified PM10–2.5

indicator, retaining the existing PM10

indicator, and/or retaining the PM10

indicator with adjustment to avoid 

double-counting the PM2.5 fraction. 

These options are discussed below. 

a. Unqualified PM10–2.5 Indicator. The 

EPA evaluated whether an unqualified 

PM10–2.5 indicator would satisfy the 

goals for public health protection 

described above. However, if such an 

indicator were utilized as part of a 

standard with a single unvarying level, 

it would not reflect the critical 

difference in evidence regarding the 

relative public health risks associated 

with urban and non-urban thoracic 

coarse particles. If the level were 

selected to provide appropriate 

protection against effects associated 


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Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 61195 

with exposure to the ambient mixes 

typical of urban or industrial areas, the 

standard would likely be more stringent 

than necessary to protect against effects 

associated with exposure to the ambient 

mixes in non-urban areas. In the 

judgment of the Administrator, the 

evidence warrants a lower ambient 

concentration of ambient coarse 

particles in urban areas than in nonurban areas, where the coarse particles 

are typically from different sources and 

there is less evidence of public health 

risk. Conversely, if a less stringent level 

were adopted on the grounds that there 

is less certainty that the ambient mix in 

non-urban areas poses a health risk, 

then the standard would not provide 

sufficient protection from the ambient 

mix found in urban or industrial areas. 

In both instances the standard would 

not be requisite overall, i.e., ‘‘not lower 

or higher than is necessary,’’ to protect 

the public health with an adequate 

margin of safety. Whitman, 531 U.S. at 

476. 

Arguably this dilemma could be 

resolved by adopting a standard based 

on a PM10–2.5 indicator with a varying 

level depending on whether the area is 

urban or non-urban. However, 

determining appropriate levels for 

different kinds of ambient mixes is not 

feasible at this time. The EPA notes that 

given the variety of sources contributing 

to PM10–2.5 concentrations in different 

locations, a wide variety of ‘‘ambient 

mixes’’ are likely to exist, greatly 

complicating the determination of the 

appropriate standard level for each 

location. There is a lack of evidence to 

support establishing specific 

quantitative distinctions in level based 

on variations in coarse particle 

composition and differential toxicity. In 

addition, there is insufficient evidence 

regarding coarse particle composition in 

different areas to allow for the proper 

assignment of different standard levels 

in different locations, and the technical 

capabilities necessary to make such 

determinations are currently lacking. 

Even if EPA tried to assign only two 

levels, urban and non-urban, the same 

problems identified earlier with respect 

to a qualified indicator would apply 

here, given the inability to effectively 

and precisely identify different ambient 

mixes. Therefore, EPA finds that the 

current state of the science does not 

provide an adequate basis upon which 

to establish a PM10–2.5 standard with an 

appropriately varying level. As EPA’s 

new research program produces 

speciated monitoring data, thereby 

improving scientific knowledge, 

revealing more specific and precise 

information about coarse particle 

composition and relative toxicity, and 

about the distribution of ambient coarse 

particle mixes of varying composition, it 

will be appropriate in a future review to 

revisit the option of a PM10–2.5 standard 

with a variable level or a qualified 

indicator. 

b. PM10 Indicator. An alternative 

approach would be to retain PM10 as an 

indicator. The EPA recognizes, as did 

many commenters, that the D.C. Circuit 

concluded that EPA’s 1997 choice of 

PM10 as the indicator for coarse particles 

was arbitrary and capricious. ATA I, 175 

F.3d at 1027, 1054–55. In that case, the 

court noted the tension between EPA’s 

conclusion that coarse and fine particles 

are different kinds of particles and pose 

independent and distinct threats to 

public health, and its choice to address 

the public health risks associated with 

coarse particles indirectly, using an 

indicator for coarse particles that 

nonetheless includes both fine and 

coarse particles. Although EPA adopted 

PM10 as a ‘‘surrogate for coarse fraction 

particles,’’ the court also noted EPA’s 

recognition ‘‘that PM10–2.5 would have 

served as a satisfactory coarse particle 

indicator.’’ With this backdrop, the 

court evaluated EPA’s three bases for 

selecting PM10 as the indicator: (a) That 

the two epidemiologic studies 

underlying the standards for coarse 

particles used PM10 rather than PM10–2.5

as the indicator; (b) that the PM10

standards would work in conjunction 

with the PM2.5 standards ‘‘by regulating 

the portion of particulate pollution not 

regulated by the PM2.5 standards’’; and 

(c) that a nationwide monitoring 

network for PM10 already existed. Id. at 

1054. 

The court rejected the first two 

arguments for two interrelated reasons. 

First, use of PM10 as the indicator 

regulates both fine and coarse particles, 

contrary to EPA’s argument that the 

PM10 indicator would work in 

conjunction with the PM2.5 standard to 

regulate only the coarse particle fraction 

of PM10. The court concluded: ‘‘we 

cannot discern exactly how a PM10

standard, instead of a PM10–2.5 standard, 

will work alongside a PM2.5 standard to 

regulate only the coarse fraction of 

PM10. EPA provides no explanation to 

aid us in understanding its decision.’’

Id. at 1054. Second, because the PM10

indicator regulates both fine and coarse 

particles, the amount of coarse particles 

allowed ‘‘will depend (quite arbitrarily) 

on the amount of PM2.5 pollution in the 

air.’’ Id. EPA failed to explain why this 

result was consistent with its argument 

that a PM10 indicator would increase the 

likelihood that the standard would 

achieve the desired level of protection 

from exposure to coarse particles. The 

resulting combination of PM2.5 and PM10

standards would lead to double 

regulation of fine particles and the 

potential under-regulation of coarse 

particles, since the amount of allowable 

coarse particles would always depend 

on the amount of fine particles in the 

air. Id. The court rejected the third of 

EPA’s arguments, the pragmatic, 

administrative convenience of using the 

existing monitoring network, on the 

grounds that only factors related to 

public health can be considered in 

establishing a NAAQS. Id. at 1054–55. 

In sum, the court rejected EPA’s 

adoption of a PM10 indicator as arbitrary 

because of the inadequacy of the reasons 

provided by the Agency as support for 

the decision. 

Based on the current review of the 

scientific evidence, EPA feels it is now 

appropriate to reconsider utilizing PM10

as an indicator for coarse particles. 

Unlike its view in 1997, EPA views 

PM10–2.5 as an unsatisfactory indicator in 

this review, for the reasons described in 

the previous subsection. In addition, 

EPA is not maintaining, as it did in 

1997, that a PM10 indicator will work in 

conjunction with the PM2.5 standard to 

regulate coarse particles exclusively, nor 

is the Agency justifying its choice of the 

PM10 indicator on grounds of 

administrative convenience. Instead, 

after careful consideration, it is the view 

of the Administrator that the PM10

indicator will in fact provide the type of 

targeted protection from thoracic coarse 

particles which is justified by the 

emerging body of scientific evidence, 

that it will do so more effectively and 

more appropriately than all other 

indicators evaluated by EPA during the 

course of this review, and that the 

inclusion of PM2.5 in the PM10 indicator 

does not over-regulate fine particles or 

under-regulate coarse particles. 

To the contrary, the inclusion of PM2.5

in the PM10 indicator plays two 

important roles in effectively providing 

the kind of targeted health protection 

called for under the current state of the 

science. Because the PM10 indicator 

includes both coarse PM (PM10–2.5) 

and fine PM (PM2.5), the concentration 

of PM10–2.5 allowed by a PM10

standard set at a single level declines as 

the concentration of PM2.5 increases. 

Thus, the level of coarse particles 

allowed varies depending on the level of 

fine particles present. At the same time, 

PM2.5 levels tend to be lower in rural 

areas and higher in urban areas. EPA, 

2005, p. 2–54, and Figures 2–23 and 2–

24 at pp. 2–52 and 2–53. Thus, to the 

extent that higher PM2.5 levels lead to a 

lower allowable level of coarse particles 

in some areas compared to others, this 

will occur in precisely those locations—


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61196 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 

72The EPA 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. While 

currently available information does not allow any 

more precise adjustment for relative toxicity, EPA 

believes the standard will generally ensure that the 

coarse particle levels allowed will be lower in 

urban areas and higher in non-urban areas. While 

the allowable levels will vary with location due to 

differing levels of fine particles, that variability will 

ultimately be limited by implementation of the 

PM2.5 standards. Areas that do not meet these 

standards are taking steps to reduce PM2.5, 

Currently, the annual fine particle standard places 

limits on both the long- and short-term levels of fine 

particles in a number of cities, particularly in the 

east and in some California cities. In the long run, 

this will serve to make the ‘‘headroom’’ allowed for 

thoracic coarse particles (i.e. the allowable PM10

level minus the corresponding PM2.5 concentration) 

more uniform among cities. The new 24-hour PM2.5

standard of 35 μg/m3 will promote this same result. 

It should cause areas that now meet the annual 

PM2.5 standard, but have high 24-hour PM2.5

concentrations, to adopt additional controls, further 

reducing the variability in the ‘‘headroom’’ for 

allowable thoracic coarse particle concentrations. In 

combination with the annual standard, the revised 

24-hour PM2.5 standard thus will provide for more 

consistent allowable levels of thoracic coarse 

particles in cities under the PM10 standard. 

i.e. urban or industrial areas—where the 

science has shown the strongest 

evidence of adverse health effects 

associated with exposure to coarse 

particles. The EPA’s recent Particle 

Pollution Report (EPA, 2004b, Figure 5, 

p. 8) provides evidence that annual 

average concentrations of PM2.5 in 

selected eastern and western urban 

areas consistently exceed the annual 

average levels of PM2.5 in nearby rural 

areas. This means that a PM10 standard 

set at a single, unvarying level will 

permit, on average, lower levels of 

coarse particles in urban areas, where 

PM2.5 concentrations tend to be higher. 

The varying levels of coarse particles 

allowed by a PM10 indicator will 

therefore target protection in urban and 

industrial areas where the evidence of 

adverse health effects associated with 

exposure to coarse particles is strongest. 

For the same reason, lower levels of 

PM2.5 lead to a higher allowable level of 

coarse particles in non-urban areas, 

again an appropriate result given the 

inconclusive evidence of health risks 

associated with coarse particles in these 

areas. 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.72

This result is consistent with our 

current understanding of the strength of 

the evidence regarding the toxicity of 

different ambient mixes of thoracic 

coarse particles in urban and non-urban 

or rural areas, and also is in accord with 

our current understanding of the 

observed toxicity in urban and 

industrial areas. As noted in both the 

proposal and the Criteria Document, the 

observed toxicity of coarse particles in 

urban and industrial areas comes from 

the kind of coarse particles found in 

these environments, for example direct 

emissions from industrial sources or 

materials released to road dust from 

motor vehicles such as brake and tire 

wear, as well as from the contamination 

of coarse particles that can occur. This 

contamination can come from both 

mobile and stationary sources. In 

particular, specific components, such as 

byproducts of incomplete combustion 

(e.g. polycyclic aromatic hydrocarbons) 

most commonly emitted from motor 

vehicles and other sources in the form 

of PM2.5, as well as metals and other 

contaminants emitted from other 

anthropogenic sources, appear in higher 

levels in urban areas (EPA, 2004a, p. 8–

344; 71 FR 2665). Many of these 

contaminants in PM10–2.5 come 

originally from fine particles, which 

may become attached in the atmosphere 

or be deposited and mixed into coarse 

materials on the ground. Thus the 

greater the concentration of PM2.5, with 

higher levels typically found in urban 

areas, the greater the level of 

contamination of coarse particles by fine 

particles. This contamination increases 

the potential health risk posed by those 

coarse particles. For that reason, it is 

logical to allow lower levels of coarse 

particles when fine particle 

concentrations are high. In other words, 

inclusion of PM2.5 in the PM10 indicator 

for purposes of coarse particle 

protection would appropriately reflect 

the contribution that contaminants 

emitted in fine particle form can make 

to the overall health risk posed by 

coarse particles. 

Moreover, due to the contamination of 

PM10–2.5 by PM2.5, use of a PM10

indicator will not result in 

inappropriate double regulation of the 

PM2.5 component. To the extent that use 

of a PM10 indicator would result in any 

reduction in PM2.5 concentrations in an 

area, this would reduce the potential 

health risk from coarse particles in the 

area as well. There is no certainty that 

the contribution of PM2.5 to the health 

risk associated with exposure to 

contaminated coarse particles would be 

appropriately addressed through the 

fine particle standards alone. Thus, to 

the extent that the inclusion of the PM2.5

fraction in the PM10 indicator amounts 

to double regulation of PM2.5, its 

inclusion is non-duplicative and 

reasonable: it ensures that this risk of 

contamination of coarse particles by 

PM2.5 is addressed in the suite of fine 

and coarse PM standards. 

Some commenters nonetheless 

maintained that the court’s opinion in 

ATA I bars use of PM10 as an indicator 

for coarse particles, stressing the court’s 

statement that ‘‘[i]t is the very presence 

of a separate PM2.5 standard that makes 

retention of the PM10 indicator arbitrary 

and capricious.’’ 175 F. 3d at 1054. The 

EPA disagrees that the ATA I decision 

precludes use of a PM10 indicator. The 

court did not hold that it was unlawful 

per se to use PM10 as an indicator for 

thoracic coarse particles. Instead, the 

court noted two particular problems—

the variable level of allowable 

concentrations of PM10–2.5 and double 

regulation of PM2.5—and found that EPA 

either failed to address these issues, or 

provided explanations that were 

inconsistent and unsupported. Id. In 

large part, the court’s decision was an 

important factor in EPA’s close 

evaluation and subsequent proposal of a 

qualified PM10–2.5 indicator as part of 

this NAAQS review. See EPA, 2005, p. 

1–5. However, EPA now believes that a 

qualified PM10–2.5 indicator is 

inappropriate, and that an unqualified 

PM10–2.5 indicator is more problematic 

and less effective than a PM10 indicator 

at providing the requisite level of 

protection from the varying risks 

associated with thoracic coarse 

particles. Indeed, for the reasons 

described above, PM10 is an effective 

indicator for targeting coarse particles 

because it provides the desired 

variability in allowable coarse particle 

concentrations. 

Far from being arbitrary and 

capricious, inclusion of PM2.5 serves 

two important functions: first, it is the 

mechanism that provides for the 

variation in allowable PM10–2.5

concentrations, targeting lower 

allowable levels where there is greater 

public health concern; and second, to 

the extent that there is ‘‘double 

regulation’’ of PM2.5 by virtue of its 

inclusion in the PM10 indicator (175 

F.3d at 1054), regulation of PM2.5 via 

this indicator serves valid, nonduplicative purposes in providing 

requisite protection from thoracic coarse 

particles. The EPA also notes that 

‘‘double regulation’’ of a pollutant, in 

the context of multiple NAAQS 

standards, is neither impermissible nor 

even unusual. For example, there are 

both annual and 24-hour standards for 

PM2.5, as well as both primary and 

secondary standards for PM2.5. The key 

is that the different standards 

reasonably serve different purposes ‘‘

they are directed at different effects, or 


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are not inconsistent when directed at 

the same effect—as is the case here. 

The EPA also recognizes that 

selection of PM10 as the indicator for 

thoracic coarse particles differs in some 

degree from the specific advice 

provided by CASAC to use a qualified 

PM10–2.5 indicator directed at urban or 

industrial thoracic coarse particles (71 

FR 2665). However, EPA believes that 

the PM10 indicator is consistent with the 

central thrust of CASAC’s advice—to 

utilize an indicator directed at urban 

types of coarse particulate matter, given 

the known toxicity of these particles—

because it would generally allow lower 

levels of PM10–2.5 in urban areas. The 

EPA has also explained why it has 

rejected a qualified PM10–2.5 indicator at 

this time, and notes that CASAC itself 

considered multiple ways to achieve 

some degree of targeted protection and 

voiced strong objections to the qualified 

PM10–2.5 indicator which the Agency 

proposed (Henderson, 2006, p. 4). The 

EPA has carefully considered CASAC’s 

views in making its decision, and 

believes the final decision is consistent 

with the critical part of CASAC’s advice, 

i.e., to focus the indicator (and standard) 

on the type of thoracic coarse particles 

known to be harmful, which are found 

in urban and/or industrial 

environments. 

c. Unqualified PM10 Indicator, with 

Adjustment to the PM2.5 Component. 

EPA also solicited comment on an 

approach that would use PM10 as an 

indicator but subtract out the amount of 

PM2.5 in excess of the 24-hour daily 

standard for PM2.5 to avoid the double 

regulation of PM2.5 in the situations 

where this would have the most 

regulatory consequence (71 FR 2673). 

Specifically, this option would retain 

the indicator, form and level of the 1987 

PM10 standard, but on days when the 

measured concentration of PM10

exceeds the level of the standard and 

the measured concentration of PM2.5

exceeds the level of the daily PM2.5

standard, the amount of PM2.5 in excess 

of the daily PM2.5 standard would be 

subtracted from the total PM10. A few 

commenters, including certain industry 

commenters and several local agencies 

and Tribes, expressed conditional 

support for pursuing this approach: 

though they preferred either no coarse 

particle standard (in the case of industry 

commenters) or an unqualified PM10–2.5

standard applied nationally (in the case 

of Tribes or local agencies), they 

suggested that an adjusted PM10

indicator would be an acceptable 

alternative. This alternative, like an 

unadjusted PM10 indicator, would allow 

variable ambient concentrations of 

coarse particles. The net result, 

however, would be that PM10–2.5 levels 

would be allowed to increase relative to 

the current PM10 standard when PM2.5

levels are highest. As explained above, 

this is the opposite result from that 

desired from a public health 

perspective. There should be less 

allowable coarse particulate matter as 

PM2.5 levels increase because these are 

the conditions under which PM10–2.5

tends to become more contaminated and 

therefore more harmful. Furthermore, it 

would essentially relax the level of 

protection afforded by the current 24- 

hour PM10 standard because it would 

allow higher total PM10 levels on days 

with high PM2.5 levels. As explained 

below in section III.D.2, EPA believes it 

is important to maintain the current 

level of protection from health effects 

associated with exposure to thoracic 

coarse particles. For both of these 

reasons, therefore, EPA rejected this 

approach. 

4. Conclusions Regarding Indicator for 

Thoracic Coarse Particles 

After extensive evaluation of the 

evidence, the alternatives available to 

the Agency, the advice and 

recommendations of CASAC, and all of 

the public comments, EPA concludes 

that retaining the PM10 indicator will be 

more effective in providing targeted 

public health protection than all other 

options available and, based on the 

current state of the science, is the most 

appropriate indicator to protect against 

the health effects associated with 

exposure to thoracic coarse particles. 

Thus, in the judgment of the 

Administrator, it is appropriate to retain 

PM10 as the indicator for coarse particles 

at this time. The conclusions that led to 

this decision can be summarized as 

follows: 

(1) All thoracic coarse particulate 

matter can deposit in the sensitive 

regions of the lung of most concern, the 

tracheobronchial and alveolar regions. 

(2) It remains appropriate to provide, 

to the extent possible, targeted 

protection from thoracic coarse particles 

that have been demonstrated to be 

associated with significant adverse 

health effects. Urban or industrial 

ambient mixes of coarse particulate 

matter dominated by high density 

vehicular, industrial, and construction 

emissions are of greatest concern, and 

should be the focus of protection. 

(3) The proposed qualified PM10–2.5

indicator was beset by numerous 

problems. Possible modifications to the 

qualifications considered by EPA failed 

to resolve these problems, which stem 

from the basic inability at this time to 

effectively and precisely identify which 

ambient mixes are included in the 

indicator and which are not. 

(4) The evidence of health effects 

associated with non-urban ambient 

mixes of coarse particles is limited and 

inconclusive: in general, the evidence 

does not demonstrate that communitylevel exposures in non-urban areas are 

associated with either the existence or 

absence of adverse health effects. 

(5) In light of the entire body of 

evidence concerning thoracic coarse 

particles, and given the potentially 

serious nature of the health risks posed 

by at least some thoracic coarse particles 

and the potential size of the population 

exposed, it is appropriate to provide 

some protection for all types of thoracic 

coarse particles, consistent with the 

requirement of the Act to allow an 

adequate margin of safety. 

With all of the foregoing 

considerations in mind, the 

Administrator judges it appropriate not 

to revise the current PM10 indicator at 

this time. In the view of the 

Administrator, the PM10 indicator 

provides the type of targeted variation 

in allowable coarse particle 

concentrations that is justified by the 

emerging body of scientific evidence, 

while providing some protection in all 

areas. A decision not to revise the PM10

indicator reflects an appropriately 

cautious approach in two respects. First, 

it ensures inclusion of all ambient mixes 

of coarse particles of known concern in 

the indicator; and second, it addresses 

the potential that additional scientific 

research may reveal that non-urban or 

rural ambient mixes of thoracic coarse 

particles present public health risks that 

the evidence does not clearly identify at 

this time. It is EPA’s goal that its new 

research and speciated monitoring 

program will produce data to determine 

what effect differences in particle 

composition may have on health 

outcomes. Such results have the 

potential to provide the kind of 

certainty and specificity required for 

making future decisions on indicators 

for thoracic coarse particles that might 

incorporate qualifications, such as the 

proposed qualified indicator related to 

coarse particles from agriculture and 

mining. 

D. Conclusions Regarding Averaging 

Time, Form, and Level of the Current 

PM10 Standards 

1. Averaging Time 

In the last review, EPA retained both 

24-hour and annual PM10 standards to 

provide protection against the known 

and potential effects of short- and longterm exposures to thoracic coarse 

particles (62 FR 38677–79). That 


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73The only one of these studies (Gauderman et 

al., 2000) to include measurements of coarse 

particles found an association between lung 

function growth for PM10, PM2.5, PM10–2.5, NO2, and 

acids. The authors were unable to cite any single 

pollutant as responsible for these results, but they 

chose not to include measures for coarse particles 

in their follow-up study (Gauderman et al., 2002). 

As noted in the 1996 PM Staff Paper, the other 

major study of lung function and long-term air 

pollution in children found no associations with 

coarse particles (EPA, 1996, p. 5–67a). 

74The WHO panel essentially developed their 

recommendations for PM10 standards by deriving a 

ratio of fine particles to PM10 and adjusting their 

recommended levels for PM2.5 to derive an 

equivalent PM10 metric, for areas that do not yet 

have access to PM2.5 monitors (WHO, 2005, p. 8). 

75See EPA 2004a, pp. 8–306 to 307 (‘‘no 

statistically significant associations have been 

reported between long-term exposure to coarse 

fraction particles and cause-specific mortality’’); pp. 

8–313 to 314 (‘‘[t]he recent studies suggest that 

long-term exposure to fine particles is associated 

with development of chronic respiratory disease 

and reduced lung function growth; little evidence 

is available on potential effects of exposure to 

coarse fraction particles’’). 

76The Staff Paper analysis of PM10 air quality 

data indicates that the current 24-hour PM10

standard is ‘‘controlling’’ in virtually every area in 

the US; that is, virtually all areas that violate the 

PM10 standards violate the 24-hour PM10 standard. 

Some of them may violate the annual PM10 standard 

as well, but (depending on the year) few, if any, 

areas violate the annual PM without violating the 

24-hour PM10 standard (EPA, 2005, p. 2–31 to 32). 

A supplemental analysis in the Response to 

Comments document shows that for 2003–2005, all 

of the areas that would violate the annual PM10

standard also violate the 24-hour standard. 

decision was based in part on 

qualitative considerations related to the 

expectation that deposition of thoracic 

coarse particles in the respiratory 

system could aggravate effects in 

individuals with asthma. In addition, 

quantitative support for retaining a 24- 

hour standard came from limited 

epidemiologic evidence suggesting that 

aggravation of asthma and respiratory 

infection and symptoms may be 

associated with daily or episodic 

increases in PM10, where dominated by 

thoracic coarse particles including 

fugitive dust. The decision to retain an 

annual standard as well was generally 

based on considerations of the 

plausibility of the potential build-up of 

insoluble thoracic coarse particles in the 

lung after long-term exposures to high 

levels of such particles. 

New information available in this 

review, discussed above, includes 

several epidemiologic studies that 

report statistically significant 

associations between short-term (24- 

hour) exposure to PM10–2.5 and various 

morbidity effects and mortality. With 

regard to long-term exposure studies, 

while one study conducted in southern 

California reported a link between 

reduced lung function growth and longterm exposure to PM10–2.5 and PM2.5, 

other such studies reported no 

associations (EPA, 2005, p. 3–19, 3–23–

24). Thus, the Criteria Document 

concluded that the available evidence 

does not suggest an association with 

long-term exposure to PM10–2.5 (EPA, 

2004a, p. 9–79). 

Based on these considerations, the 

Staff Paper concluded that the newly 

available evidence continues to support 

a 24-hour averaging time for a standard 

intended to control thoracic coarse 

particles, based primarily on evidence 

suggestive of associations between 

short-term (24-hour) exposure and 

morbidity effects and, to a lesser degree, 

mortality. Noting the absence of 

evidence judged to be suggestive of an 

association with long-term exposures, 

the Staff Paper concluded that there is 

no quantitative evidence that directly 

supports an annual standard, while 

recognizing that it could be appropriate 

to consider an annual standard to 

provide a margin of safety against 

possible effects related to long-term 

exposure to thoracic coarse particles 

that future research may reveal. The 

Staff Paper observed, however, that a 

24-hour standard that would reduce 24- 

hour exposures would also likely reduce 

long-term average exposures, thus 

providing some margin of safety against 

the possibility of health effects 

associated with long-term exposures 

(EPA, 2005, p. 5–61). Based on its 

review of the Staff Paper, CASAC 

recommended retention of a 24-hour 

averaging time and agreed that an 

annual averaging time is not currently 

warranted for the coarse particle 

standard (Henderson, 2005b, p.5). 

The EPA received relatively few 

comments regarding the appropriate 

averaging time of the coarse particle 

standard. Most of those who did 

comment generally supported the 

retention of a 24-hour, but not annual, 

averaging time, as proposed. A few of 

the commenters who concurred with 

EPA’s proposal to revoke the annual 

standard urged reconsideration of the 

appropriateness of an annual averaging 

time in the next PM NAAQS review. 

Several commenters, however, 

including a few States and several 

environmental and public health 

groups, urged EPA to retain an annual 

standard as well as a 24-hour standard. 

The American Lung Association, in 

particular, stated that EPA had 

inappropriately ignored evidence of 

long-term morbidity effects in several 

studies, including Gauderman et al. 

(2000, 2002) and Avol et al. (2001), and 

had also ignored substantial evidence 

from European studies as well as the 

recommendations for an annual PM10

standard made by a WHO working 

group. These commenters argued that an 

annual standard was requisite to protect 

public health with an adequate margin 

of safety. 

EPA disagrees that it ignored the 

evidence that is relevant to evaluating 

the health effects associated with longterm exposure to thoracic coarse 

particles. The EPA’s assessment, both in 

this review and the previous review, 

placed greatest weight on studies that 

measured PM10–2.5 or on studies 

conducted in areas where it is 

reasonable to expect the PM10

measurements to be dominated by 

coarse particles (EPA, 2005). By 

contrast, these commenters have placed 

inappropriate reliance on studies that 

measured PM10, and were conducted in 

Southern California cities (Gauderman 

et al., 2000, 2002) or in European cities 

where it is not reasonable to assume that 

PM10 associations are dominated by 

coarse particles.73 In such cases, it is 

difficult to draw meaningful 

conclusions about the relative role of 

coarse as opposed to fine particles. The 

WHO panel recommendations for PM10

limits cited by commenters also do not 

provide any independent scientific 

justification regarding the need for a 

separate long-term standard for coarse 

particles.74

The long-term exposure studies of 

mortality and morbidity that permit 

comparisons of fine and coarse particles 

continue to suggest that, at current 

ambient levels in the US, fine particles 

are associated with health effects and 

coarse particles are not.75 The EPA 

believes that the PM2.5 standards it is 

establishing in today’s notice address 

the major risk suggested in the PM10

studies cited by commenters. To the 

extent that additional concerns may 

exist with regard to long-term exposures 

to coarse particles that have not been 

fully identified by scientific research, 

the Staff Paper notes that the short-term 

standard for coarse particles, which is 

generally controlling, has and will 

continue, as a practical matter, to limit 

such long-term exposures.76

After reviewing the available 

evidence, the Administrator concurs 

with staff and CASAC recommendations 

and concludes that the evidence 

continues to support a 24-hour 

averaging time for a coarse particle 

standard, based primarily on evidence 

suggestive of associations between 

short-term (24-hour) exposure and 

morbidity effects and, to a lesser degree, 

mortality. As noted above, a 24-hour 

standard would in effect also provide 

protection against any as yet 

unidentified potential effects of longterm exposure at ambient levels. 

Further, the Administrator concludes 


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77This examination of the evidence is based on 

air quality information and analyses presented in 

two staff memos which were part of the materials 

reviewed by CASAC (Ross and Langstaff, 2005; 

Ross, 2005). 

78As shown in air quality data trends reports: for 

Seattle, 1997 Air Quality Annual Report for 

Washington State, p. 17, at http://www.ecy.wa.gov/ 

pubs/97208.pdf; for Detroit, Michigan’s 2003 

Annual Air Quality Report, p. 46, at http:// 

www.deq.state.mi.us/documents/deq-aqd-airreports-03AQReport.pdf. 

that an annual coarse particle standard 

is not warranted at this time. Thus, the 

Administrator is retaining the 24-hour 

PM10 standard and revoking the annual 

PM10 standard. 

2. Level and Form of the 24-Hour PM10

Standard 

This section summarizes the major 

considerations that led to the proposed 

decision regarding the appropriate level 

and form for the 24-hour standard for 

thoracic coarse particles, summarizes 

and addresses public comments on the 

appropriate level of protection to be 

provided by the standard, and presents 

the Administrator’s final conclusions 

regarding the level and form of the 24- 

hour standard. The proposed level and 

form for the 24-hour standard for 

thoracic coarse particles were based 

primarily on an assessment of studies 

that measured PM10–2.5, as well as 

studies that measured PM10 in areas that 

were dominated by PM10–2.5. Now that 

the Administrator has concluded that it 

is appropriate to retain PM10 as the 

indicator for thoracic coarse particles, 

rather than adopting a PM10–2.5 indicator 

as proposed, the Administrator relied on 

this same body of studies as the 

principal basis for determining an 

appropriate level and form for a 

standard based on the PM10 indicator. 

Therefore, in this section EPA reviews 

the basis for its conclusions in the 

proposal, and then discusses how this 

evidence informs the choice of level and 

form for the 24-hour PM10 standard. 

In considering the available evidence 

as a basis for setting a 24-hour standard 

for thoracic coarse particles, the Staff 

Paper focused on relevant U.S. and 

Canadian epidemiologic studies 

showing associations between shortterm PM10–2.5 concentrations and 

morbidity and mortality effects, as 

discussed above in section III.A. As an 

initial matter, the Staff Paper recognized 

that these individual short-term 

exposure studies provide no evidence of 

clear population thresholds, or lowestobserved-effects levels, in terms of 24- 

hour average concentrations. As a 

consequence, this body of evidence is 

difficult to translate directly into a 

specific 24-hour standard that would 

protect against the range of effects that 

have been associated with short-term 

exposures to coarse particles. 

In considering the evidence, the Staff 

Paper noted the significant uncertainties 

and the limited nature of the available 

evidence. In examining the available 

evidence to identify a basis for a range 

of standard levels that would be 

appropriate for consideration, the Staff 

Paper focused on the upper end of the 

distributions of daily PM10–2.5

concentrations in the relevant studies in 

terms of the 98th and 99th percentile 

values.77

In looking first at the morbidity 

studies that report statistically 

significant associations with respiratoryand cardiac-related hospital admissions 

in Toronto (Burnett et al., 1997), Seattle 

(Sheppard, 2003), and Detroit (Ito, 

2003), the 98th percentile PM10–2.5

values reported in these studies range 

from approximately 30 to 36 μg/m3. To 

provide some perspective on these 

PM10–2.5 levels, the Staff Paper noted 

that the level of the 24-hour PM10

standard was exceeded on only a few 

occasions during the time periods of the 

studies in Detroit and Seattle.78 In the 

mortality studies that report statistically 

significant and generally robust 

associations with short-term exposures 

to PM10–2.5 in Phoenix (Mar et al., 2003) 

and Coachella Valley, CA (Ostro et al., 

2003), the reported 98th percentile 

values were approximately 70 and 107 

μg/m3, respectively. These studies were 

conducted in areas with air quality 

levels that did not meet the current 

PM10 standards. In addition, as part of 

the Six Cities study, Schwartz et al. 

(1996 and reanalysis 2003a) reported a 

statistically significant association 

between PM10–2.5 and mortality in 

Steubenville, where the PM10–2.5

concentrations were fairly high, with a 

reported 98th percentile value of 53 μg/ 

m3, although in a second reanalysis, the 

association did not remain statistically 

significant (Klemm and Mason, 2003). 

On the other hand, the Staff Paper noted 

that no statistically significant mortality 

associations were reported in a number 

of other studies, including those in the 

five other cities that were part of the Six 

Cities study (Boston, St. Louis, 

Knoxville, Topeka, and Portage), and in 

Santa Clara County, CA, Detroit, 

Philadelphia, and Pittsburgh. With the 

exception of Pittsburgh, these cities had 

much lower 98th percentile PM10–2.5

values, ranging from 18 to 49 μg/m3. 

Thus, in mortality studies that reported 

statistically significant associations, the 

reported 98th percentile PM10–2.5 values 

were all above 50 μg/m3, and all in areas 

that exceeded the level of the daily PM10

standard, whereas in the mortality 

studies that reported no statistically 

significant associations, the reported 

98th percentile PM10–2.5 values were 

generally below 50 μg/m3. 

In examining the air quality data used 

in the key morbidity and mortality 

studies considered in the Staff Paper, 

EPA recognized that the uncertainty 

related to exposure measurement error 

associated with using ambient 

concentrations to represent area-wide 

population exposure levels can be 

potentially quite large. For example, in 

looking specifically at the Detroit study, 

the Staff Paper noted that the PM10–2.5

air quality values were based on air 

quality monitors located in Windsor, 

Canada. While the study authors 

concluded that these monitors were 

appropriate for use in exploring the 

association between air quality and 

hospital admissions in Detroit, a close 

examination of air quality levels at 

Detroit and Windsor sites in recent 

years led to the conclusion that the 

statistically significant, generally robust 

association with hospital admissions in 

Detroit likely reflects population 

exposures that may be appreciably 

higher in the central city area, but not 

necessarily across the broader study 

area, than would be estimated using 

data from the Windsor monitors (EPA, 

2005, p. 5–64). 

The Staff Paper also looked more 

specifically at the Coachella Valley 

mortality study (Ostro et al., 2003), in 

which data were used from a single 

monitoring site in one city, Indio, 

within the study area where daily 

measurements were available. A close 

examination of air quality levels across 

the Coachella Valley suggested that 

while the association of mortality with 

PM10–2.5 measurements made at the 

Indio site was statistically significant, a 

portion of the study population would 

have been expected to experience 

appreciably lower ambient exposure 

levels. In contrast to the Detroit study, 

air quality data used in the mortality 

study conducted in Coachella Valley 

appeared to represent concentrations on 

the high end of PM10–2.5 levels for 

Coachella Valley communities. On the 

other hand, a close examination of the 

air quality data used in the other studies 

discussed above generally showed less 

disparity between air quality levels at 

the monitoring sites used in the studies 

and the broader pattern of air quality 

levels across the study areas than that 

described above in the Detroit and 

Coachella Valley studies. 

The Staff Paper noted that this close 

examination of air quality information 

generally reinforced the view that 

exposure measurement error is 

potentially quite large in studies 

focusing on thoracic coarse particles. As 


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a consequence, the air quality levels 

reported in these studies as measured by 

ambient concentrations at monitoring 

sites within the study areas are not 

necessarily good surrogates for 

population exposures that are likely 

associated with the observed effects in 

the study areas or that would likely be 

associated with effects in other urban 

areas across the country. The Detroit 

example suggests that population 

exposures were probably appreciably 

underestimated in the Detroit morbidity 

study, such that the observed effects are 

likely associated with higher PM10–2.5

levels than reported. In contrast, the 

Coachella Valley mortality study 

provides an example in which PM10–2.5

levels to which the study populations 

were exposed were probably 

appreciably overestimated, such that the 

observed effects may well be associated 

with lower PM10–2.5 levels than reported. 

At relatively low levels of air quality, 

population exposures implied by these 

studies as being associated with the 

observed effects become more uncertain, 

suggesting a high degree of caution in 

interpreting the air quality levels from 

the group of morbidity studies as a basis 

for identifying a standard level that 

would protect against the observed 

effects. See generally EPA, 2005, pp. 5–

65–66. 

Taking into account this close 

examination of the air quality data 

associated with health effects in these 

studies, the Staff Paper concluded that 

this evidence suggests that EPA could 

consider a standard for urban thoracic 

coarse particles at a PM10–2.5 level at 

least down to 50 μg/m3, in conjunction 

with a 98th percentile form. This view 

takes into account the conclusion that 

this evidence is particularly uncertain 

as to population exposures, especially 

from the morbidity studies reporting 

effects at relatively low concentrations, 

as well as the general lack of evidence 

of associations from the group of 

mortality studies with reported 

concentrations below these levels. Id. at 

p. 5–66. 

The Staff Paper also outlined another 

view that reflected a more cautious or 

restrained approach to interpreting the 

limited body of PM10–2.5 epidemiologic 

evidence. This approach would judge 

that the uncertainties as to population 

exposures associated with the observed 

effects in this whole group of studies 

were too large to permit direct use of the 

reported effects levels as a basis for 

setting a specific standard level. Such a 

judgment would be consistent with 

concluding that these studies, together 

with other dosimetric and toxicologic 

evidence, provide support for retaining 

standards for thoracic coarse particles at 

some level to protect against the 

morbidity and mortality effects observed 

in the studies, regardless of whether an 

associated population exposure level 

can be clearly discerned from the 

studies. 

Based on this more cautious 

approach, the Staff Paper concluded 

that it would be reasonable to interpret 

the available epidemiologic evidence 

more qualitatively. Considering the 

available evidence in this way led to the 

following observations: 

(1) The statistically significant 

mortality associations with short-term 

exposure to PM10–2.5 reported in the 

Phoenix and Coachella Valley studies 

were observed in areas that did not meet 

the current PM10 standards. 

(2) The statistically significant 

morbidity associations with short-term 

exposure to PM10–2.5 reported in the 

Detroit and Seattle studies were 

observed in areas that exceeded the 

level of the current 24-hour PM10

standard on just a few occasions during 

the time periods of the studies. 

(3) All but one of the statistically 

significant morbidity and mortality 

associations with short-term exposure to 

PM10 that were reported in areas in 

which PM10 was dominated by the 

coarse particle fraction (including Reno/ 

Sparks, NV, Tucson, AZ, Anchorage, 

AK, and the Utah Valley area) were 

observed in areas that did not meet the 

current PM10 standards. Id. at p. 5–67. 

Based on these considerations, the 

Staff Paper found 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. Further, the Staff Paper found 

little basis for concluding that a greater 

degree of protection is warranted in 

light of the very high degree of 

uncertainty in the relevant population 

exposures implied by the morbidity 

studies. The Staff Paper concluded, 

therefore, that it is reasonable to 

interpret the available evidence as 

supporting consideration of a short-term 

standard for urban thoracic coarse 

particles, so as to provide generally 

‘‘equivalent’’ protection to that afforded 

by the current 24-hour PM10 standard, 

recognizing that no one PM10–2.5 level 

will be strictly equivalent to a specific 

PM10 level in all areas (EPA, 2005, p. 5–

67). Such a standard would likely 

provide protection against morbidity 

effects especially in those urban areas 

where, unlike several of the study areas, 

PM10 is generally dominated by coarsefraction rather than fine-fraction 

particles. Such a standard would also 

likely provide protection against the 

more serious, but less certain, coarseparticle-related mortality effects 

observed in some studies, generally at 

somewhat higher concentrations. 

The Staff Paper went on to consider 

what level for a 24-hour PM10–2.5

standard for urban coarse particles 

would provide an equivalent level of 

protection to that afforded by the 

current 24-hour PM10 standard. This 

consideration of a PM10–2.5 standard 

providing generally ‘‘equivalent’’

protection reflected a judgment that 

while the epidemiologic evidence 

supported establishing a short-term 

standard for urban thoracic coarse 

particles at such a generally 

‘‘equivalent’’ level, the evidence 

concerning air quality levels of thoracic 

coarse particles in the studies was not 

strong enough to provide a basis for 

changing the level of protection 

generally afforded by the current PM10

standards (EPA, 2005, pp. 5–68–69). 

The Staff Paper examined various 

approaches to providing this equivalent 

level of protection, including 

establishing a level of 70 μg/m3 (98th 

percentile form) for the qualified 

PM10–2.5 standard (Id. at 5–67–68), 

which is what EPA proposed (71 FR 

2671). 

CASAC generally supported the 

Agency’s proposed range of 50–70 μg/ 

m3 (98th percentile) for the 24-hour 

PM10–2.5 standard. As noted, the upper 

end of this range was based on EPA’s 

assessment of a level for an urban coarse 

particle standard that would provide a 

generally equivalent level of protection 

to that afforded by the current PM10

standards. The lower end of the range 

was developed in consideration of an 

approach that would place greater 

weight on the effects levels reported in 

several studies with lower ambient 

coarse particle concentrations. The 

CASAC Panel noted that ‘‘there was 

general agreement among Panel 

members that Agency staff had 

presented a reasonable justification for 

the ranges of levels proposed’’

(Henderson 2005b, p. 6). 

Relatively few public commenters 

addressed the issue of whether ‘‘general 

equivalence’’ was an appropriate goal 

for the level and form of the proposed 

coarse particle standard. Some 

commenters, particularly those industry 

commenters advocating that no coarse 


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79As discussed in section III.B.2, these 

commenters call EPA’s interpretation of the key 

studies discussed in this section into question. 

EPA’s response to the criticisms of use of these 

studies for standard setting is summarized in 

section III.B.2 and presented in more detail in the 

Response to Comments document. 

80Commenters also suggested that, in 

promulgating revised PM10 standards in 1997, EPA 

did not consider whether the level of the PM10

standards it promulgated was lower than necessary 

and did not base the levels on coarse particle health 

effects data. While EPA disagrees with both of these 

claims—for example, EPA relied on two PM10

studies done in areas dominated by coarse particles 

in selecting the level (62 FR 38679)—this argument 

is not relevant to this review. 

81As detailed in the Response to Comment 

document, EPA had various reasons for not placing 

primary reliance on the reported air quality results 

in these studies for selecting a standard level. The 

Atlanta study (Tolbert et al, 2000), found a 

significant effect for PM10, but not for coarse 

particles. Both the Six Cities children’s diary study 

(Schwartz and Neas, 2000) and the Toronto hospital 

admissions study (Burnett et al.,, 1997) were 

conducted for a periods of less than one year, 

making it difficult to determine what peak value 

across all seasons in a year might represent 

exposures of concern. 

particle standard be adopted,79 stated 

that seeking ‘‘equivalence’’ to the PM10

standard was fundamentally flawed 

because, in their view: (1) The level of 

the current PM10 standard was not based 

on coarse particle studies; (2) the 

proposed standard is not equivalent to 

the PM10 standard; and (3) the court had 

already declared any standard based 

directly or indirectly on PM10 to be 

invalid. The EPA agrees that the 1987 

PM10 standards were designed to protect 

against the health effects of both fine 

and coarse particles, and based in part 

on epidemiological studies that 

variously measured particles both 

smaller and larger than PM10. However, 

the arguments regarding the origin of 

the 1987 standards as well as 

commenters’ claims about the basis for 

the PM10 standards promulgated in 

1997 80 are not relevant to the current 

review. In determining whether to 

revise the standards in this review, EPA 

has examined the degree of protection 

provided by the current 24-hour PM10

standard in light of the quantitative 

evidence from the expanded 

epidemiological data base that includes 

studies using direct PM10–2.5

measurements as well as studies using 

PM10 measurements in areas where 

coarse particles dominate the 

distribution. 

Because as discussed in section III.C.3 

above, the Administrator has decided 

that it is appropriate to retain PM10 as 

the indicator for thoracic coarse 

particles, there can be no uncertainty as 

to whether the final standard is 

equivalent to the current standard, 

making the commenters’ second point 

above moot. With regard to their third 

point, for reasons outlined in section 

III.C.3, EPA believes that it has 

addressed the concerns raised by the 

court regarding PM10 as an indicator, 

and in any case, the D.C. Circuit did not 

address the issue of the level of 

protection from thoracic coarse particles 

afforded by the 1997 or 1987 24-hour 

PM10 standard. 

Other commenters, particularly 

environmental and public health 

groups, disagreed with EPA’s proposal 

to seek an ‘‘equivalent level of 

protection’’ because they believe the 

scientific evidence mandates a lower 

level to protect against adverse health 

effects. These commenters cited studies 

reviewed in the Staff Paper and noted 

above, which they claimed showed 

significant associations between health 

effects and PM10–2.5 concentrations at 

levels between 30–40 μg/m3, and recent 

decisions by the European Union and 

the State of California to adopt 24-hour 

PM10 standards of 50 μg/m3. 

These commenters argued that, even 

considering EPA’s analyses of the 

uncertainties in the relevant ambient 

concentration measurements, these 

studies, particularly those in Atlanta, 

Seattle, and Toronto and the six-cities 

study of respiratory symptoms in 

children (Schwartz and Neas, 2000), 

demonstrate the need for a more 

stringent level of protection than that 

provided by the current standards. 

These commenters also argued that 

EPA’s approach to determining an 

equivalent level resulted in less 

protection than the current standard, 

even in urban areas. In addition, these 

commenters pointed to the study review 

conducted by Brunekreef and Forsberg 

(2005) and numerous ‘‘new’’ studies 

published too recently for inclusion in 

the Criteria Document such as Mar et al. 

(2004), Chen Y et al. (2005), and Lin et 

al. (2005), as supportive of lower levels. 

As noted above, EPA has conducted a 

careful assessment of the studies cited 

by commenters 81 from the Staff Paper 

assessment but reaches substantially 

different conclusions about their 

implications for the level of a 24-hour 

standard for thoracic coarse particles. 

Based on that assessment, EPA staff 

recommended consideration of a range 

of levels for a 24-hour PM10–2.5 standard 

extending from a level equivalent to the 

current PM10 standard down to a level 

of 50 μg/m3, which is clearly above that 

suggested by these commenters. CASAC 

found general agreement that the ‘‘staff 

had presented a reasonable 

justification’’ for this range oflevels. 

While EPA strongly agrees that the 

available scientific evidence supports 

and requires maintaining the level of 

protection provided by the current 24- 

hour PM10 standard, the limited extent 

of epidemiological evidence as well as 

the unusually large uncertainties in 

measuring exposures to thoracic coarse 

particles, particularly at lower levels, 

argue for the more cautious 

interpretation advocated by EPA staff 

and CASAC. Because the Administrator 

has decided to continue the use of PM10

as the indicator for coarse particles, 

commenters’ remaining concerns about 

whether the proposed levels for PM10–2.5

are as protective as current standards 

are no longer relevant. 

For reasons summarized in section 

II.F above, EPA does not believe that 

standards adopted by the State of 

California or, by extension, the 

European Union, which operates under 

a different legal and policy structure, 

provide a relevant guide for establishing 

U.S. National Ambient Air Quality 

Standards. While EPA agrees that the 

assessment of Brunekreef and Forsberg 

(2005) supports separate regulation of 

fine and coarse particles, these authors 

make no recommendations with respect 

to appropriate levels of protection. To 

the extent that commenters cited ‘‘new’’

studies in support of their argument for 

a more stringent standard to protect 

against health effects associated with 

exposure to coarse particles, EPA notes 

that as in past NAAQS reviews, it is 

basing the final decisions in this review 

on the studies and related information 

included in the PM air quality criteria 

that have undergone CASAC and public 

review, and will consider the newly 

published studies for purposes of 

decision making in the next PM NAAQS 

review, as discussed above in section 

I.C. As evidenced by the uncertainties 

found in the detailed assessment of key 

coarse particle studies in the Staff 

Paper, the kind of assessment and 

analysis provided by the formal criteria 

and standards review process is 

particularly crucial for coarse particle 

studies that may be relevant to selecting 

the level of the standard. 

After considering the public 

comments on this issue, EPA continues 

to believe that the available evidence 

leads to the conclusion that the degree 

of protection afforded by the current 24- 

hour PM10 standard is requisite to 

protect public health with an adequate 

margin of safety. Having chosen to 

retain the current indicator for the 

standard (PM10), and to retain the same 

degree of protection, it is still necessary 

to determine the appropriate form and 

level for the standard. In the context of 

proposing a standard based on a 

qualified PM10–2.5 indicator, EPA 

proposed to change the form of the 24- 

hour standard from a one-expected 


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exceedance form to a 98th percentile 

form. The 98th percentile form was 

intended to be consistent with the goal 

of providing protection equivalent to 

that afforded by the current 24-hour 

PM10 standard (71 FR at 2671; EPA, 

2005, p. 5–68). The few commenters 

addressing the proposed form supported 

it, largely because the 98th percentile 

would provide a more stable statistical 

basis for making nonattainment 

determinations. However, some 

commenters objected to the 98th 

percentile form because they felt it was 

inappropriate to allow as many as 21 

days over the level of the standard over 

the course of a three-year period. These 

commenters argued for a more 

restrictive form (generally 99th 

percentile) to ensure the protection of 

public health with an adequate margin 

of safety. The EPA notes that the current 

one-expected-exceedance form of the 

24-hour PM10 standard allows only 

three days above the standard over a 

three-year period. 

While EPA generally favors the 

concentration-based form for short-term 

standards for reasons noted above, EPA 

also notes that adopting such a form in 

this review without changing the level 

would result in a standard that would 

not provide the same protection as the 

current standard, and the level of the 

standard would have to be adjusted 

downward to achieve the desired 

protection. Given the overall decision to 

provide the same protection as the 

current standards, the Administrator 

concludes it is best to retain both the 

form and the level of the current 

primary 24-hour PM10 standard. 

In conclusion, it is EPA’s view, as 

expressed in the Staff Paper and 

proposal and supported by CASAC and 

by the available health effects evidence, 

that the level of protection afforded by 

the current 24-hour PM10 standard of 

150 μg/m3, one-expected-exceedance 

form, continues to be appropriate for the 

types of thoracic coarse particles 

typically found in urban or industrial 

areas. As explained above, mortality 

effects observed in epidemiologic 

studies for coarse particles are generally 

associated with exposure levels that 

exceed the current standards, and 

morbidity effects are generally 

associated with exposure levels that 

exceeded the current standards on only 

a few occasions. This suggests the level 

of protection afforded by the current 

PM10 standards is not greater than 

warranted. Furthermore, the very high 

degree of uncertainty in the relevant 

population exposures implied by the 

morbidity studies suggests there is little 

basis for concluding at this time that a 

greater degree of protection is 

warranted. 

Moreover, as explained above in 

section III.C.3.b, the PM10 indicator 

provides appropriate variation in 

allowable coarse particle concentrations 

in different areas based on the relative 

proportions of PM2.5 and PM10–2.5 in the 

ambient mix. In urban areas where 

PM2.5 concentrations tend to be higher, 

the current 24-hour PM10 standard level 

of 150 μg/m3 will result in lower 

allowable levels of PM10–2.5. In nonurban areas, the higher allowable levels 

of coarse particles provided by the 

current 24-hour PM10 standard will also 

provide appropriate protection of public 

health, given the body of evidence 

discussed above. The EPA therefore 

believes 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. Revising 

either the level or the form of this 

standard would alter the current level of 

protection and therefore would not be 

appropriate based on the scientific 

evidence available at this time. 

Therefore, after considering the 

available scientific evidence, the 

rationale and recommendations 

contained in the Staff Paper, the advice 

and recommendations of CASAC, and 

the public comments received regarding 

the appropriate level and form for a 24- 

hour standard intended to afford 

requisite protection of public health 

from effects associated with exposure to 

coarse particles, the Administrator has 

determined to retain the current level of 

150 μg/m3 for the 24-hour PM10

standard, and the current one-expectedexceedance form. In the Administrator’s 

judgment, based on the currently 

available evidence, a standard set at this 

level remains requisite to protect public 

health with an adequate margin of safety 

from the morbidity and possibly 

mortality effects that have been 

associated with short-term exposures to 

thoracic coarse particles in urban or 

industrial areas, as well as to protect 

against the potential for risks from 

exposure to thoracic coarse particles in 

other areas. The EPA intends to address 

the considerable uncertainties in the 

currently available information on 

thoracic coarse particles as part of the 

Agency’s ongoing PM research program. 

E. Final Decisions on Primary PM10

Standards 

For the reasons discussed above in 

this section, and taking into account the 

information and assessments presented 

in the Criteria Document and Staff 

Paper, the advice and recommendations 

of CASAC, and public comments 

received on the proposal, the 

Administrator is retaining the current 

primary 24-hour PM10 standard at the 

level of 150 μg/m3, which is met when 

this level is not exceeded more than 

once per year on average over a threeyear period measured at each monitor 

within an area. The Administrator also 

is revoking and not replacing the annual 

PM10 standard. 

As discussed in more detail in section 

VI, EPA is promulgating a new reference 

method (FRM) for measurement of mass 

concentrations of PM10–2.5 in the 

atmosphere. Although NAAQS for 

PM10–2.5 have not been established by 

EPA, this new FRM will nevertheless be 

defined as the standard of reference for 

measurements of PM10–2.5

concentrations in ambient air. This 

should provide a basis for approving 

Federal Equivalent Methods (FEMs) and 

promote the gathering of scientific data 

to support future reviews of the PM 

NAAQS. One of the reasons for not 

finalizing a PM10–2.5 standard was the 

limited body of evidence on health 

effects associated with thoracic coarse 

particles from studies that use PM10–2.5

measurements of ambient thoracic 

coarse particle concentrations. If an 

FRM is available, researchers will likely 

include PM10–2.5 measurements of 

thoracic coarse particles in health 

studies either by directly using the FRM 

or by utilizing approved equivalent 

methods based on the FRM. 

In addition, EPA published elsewhere 

in today’s Federal Register a 

requirement for a new multi-pollutant 

monitoring network that takes an 

integrated approach to air quality 

measurements. One of the required 

measurements at these multi-pollutant 

monitoring stations is PM10–2.5. The 

availability of an FRM, and 

subsequently approved equivalent 

methods for PM10–2.5, will support State 

and local agencies’ efforts to deploy 

robust methods at these monitoring 

stations for the measurement of thoracic 

coarse particles that do not include fine 

particles. These multi-pollutant 

monitoring stations will provide a 

readily available dataset at 

approximately 75 urban and rural 

locations for atmospheric and health 

researchers to compare particle and 

gaseous air pollutants. 

Finally, the PM10–2.5 FRM, by 

definition, provides a reference 

measurement. Because it is a filter based 

system, this method can itself be used 

to provide speciated data and EPA will 

be issuing guidance to ensure the use of 

a consistent national approach for 

speciated coarse particle monitors as 

soon as possible. The reference 

measurement from this instrument is 


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82The Administrator recognized in establishing 

the levels of the secondary standards for PM2.5 that 

these standards would work ‘‘in conjunction with 

implementation of a regional haze program’’ under 

Section 169A to provide appropriate national 

protection against visibility impairment in both 

urban and non-urban areas (62 FR 38683). 

83As noted in section I.A above, in establishing 

secondary standards that are requisite to protect the 

public welfare from any known or anticipated 

adverse effects, EPA may not consider the costs of 

implementing the standards. 

also important in the development of 

alternative PM10–2.5 speciation samplers. 

We will be developing dichotomous 

samplers to meet the requirements of 

SAFETEA–LU. Appropriate guidance to 

ensure that the use of a consistent 

national approach for speciated coarse 

particle monitors will be issued with 

this method. As discussed in more 

detail in the final monitoring rule 

published elsewhere in today’s Federal 

Register, EPA is requiring the 

deployment of PM10–2.5 speciation 

samplers at all 75 multi-pollutant 

monitoring stations. Such speciation 

monitoring will help States in 

developing SIPs and will address a key 

research need for thoracic coarse 

particles by providing a better 

understanding of the chemistry of the 

collected samples. 

IV. Rationale for Final Decisions on 

Secondary PM Standards 

This section presents the 

Administrator’s final decisions 

regarding the review of the current 

secondary NAAQS for PM. The existing 

suite of secondary PM standards, which 

is identical to the suite of primary PM 

standards, includes annual and 24-hour 

PM2.5 standards and annual and 24-hour 

PM10 standards. The existing suite of 

secondary standards is intended to 

address visibility impairment associated 

with fine particles,82 and materials 

damage and soiling related to both fine 

and coarse particles. The following 

discussion of the rationale for the final 

decisions on revising the secondary PM 

standards focuses on those 

considerations most influential in the 

Administrator’s decisions, first 

addressing visibility impairment as it 

relates to the PM2.5 secondary standards 

and then addressing the other welfare 

effects as they relate to both the PM2.5

and PM10 secondary standards. The 

other welfare effects considered in this 

review include effects on vegetation and 

ecosystems, materials damage and 

soiling, and climate change.83

Sections IV.A and IV.B of the 

proposal (71 FR 2675–2685) provide a 

detailed summary of key information 

contained in the Criteria Document 

(EPA, 2004a, Chapters 4 and 9) and in 

the Staff Paper (EPA, 2005, Chapters 6 

and 7) on the known and potential 

welfare effects associated with PM, 

including PM-related visibility 

impairment and PM-related effects on 

vegetation and ecosystems, materials 

damage and soiling, and climate change, 

respectively. This information is only 

briefly outlined in subsections IV.A.1 

and IV.B.1 below. Subsequent sections 

provide a more complete discussion of 

the Administrator’s rationale, having 

considered the evidence in light of 

public comments and his final decisions 

on the primary standards for PM, for his 

decision to revise the current PM 

secondary standards by making them 

identical in all respects to the revised 

suite of primary PM standards. 

A. Visibility Impairment 

This section presents the rationale for 

the Administrator’s decision to revise 

the current secondary PM2.5 standards to 

address PM-related visibility 

impairment by setting secondary 

standards identical in all respects to the 

revised PM2.5 primary standards. As 

discussed below, the rationale includes 

consideration of: (1) The latest scientific 

information on visibility effects 

associated with PM; (2) insights gained 

from assessments of correlations 

between ambient PM2.5 and visibility 

impairment prepared by EPA staff; 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 

of PM2.5 on visual air quality. 

1. Visibility Impairment Related to 

Ambient PM 

Section IV.A.1 of the proposal (71 FR 

2675–2678) outlined key information 

contained in the Criteria Document and 

Staff Paper relevant to considering 

visibility impairment that is related to 

ambient PM. The information 

highlighted there summarizes: 

(1) The nature of visibility 

impairment, including trends in visual 

air quality and the characterization of 

current visibility conditions, with a 

particular focus on visibility 

impairment in urban areas. 

(2) Direct, quantitative relationships 

that exist between ambient PM 

constituents and light extinction, and 

thus visibility impairment, based in part 

on analyses of the extensive new data 

now available on PM2.5 concentrations, 

primarily in urban areas, that explored 

factors that have historically 

complicated efforts to address visibility 

impairment nationally, including 

regional differences related to levels of 

primarily fine particles and to relative 

humidity. 

(3) The impacts of urban visibility 

impairment on public welfare, based in 

part on valuation studies of benefits 

associated with improvements in 

visibility and in part on recognition of 

a number of programs, standards, and 

planning efforts to address visibility 

impairment, in the U.S. and abroad, that 

illustrate the value that the public 

places on improved visibility. 

(4) Approaches to evaluating public 

perceptions and attitudes about 

visibility impairment, including new 

methods and tools that have been 

developed to communicate and evaluate 

public perceptions of varying visual 

effects associated with alternative levels 

of visibility impairment relative to 

varying pollution levels and 

environmental conditions. 

The summary of the evidence on 

visibility impairment related to ambient 

fine particles in the proposal will not be 

repeated here. The EPA emphasizes that 

the final decisions on the secondary 

standards take into account the more 

comprehensive and detailed discussions 

of the scientific information on visibility 

impairment contained in the Criteria 

Document and Staff Paper. 

2. Need for Revision of the Current 

Secondary PM2.5 Standards To Protect 

Visibility 

In 1997, EPA decided to address the 

effects of PM on visibility by setting 

secondary standards identical to the 

suite of PM2.5 primary standards, in 

conjunction with the future 

establishment of a regional haze 

program under sections 169A and 169B 

of the Act (62 FR 38679–83). In reaching 

this decision, EPA first concluded that 

PM, especially fine particles, impairs 

visibility in various locations across the 

country, including multi-state regions, 

urban areas, and remote Class I Federal 

areas (e.g., national parks and 

wilderness areas). The EPA also 

concluded that addressing visibility 

impairment solely through setting more 

stringent national secondary standards 

would not be an appropriate means to 

protect the public welfare from adverse 

impacts of PM on visibility in all parts 

of the country. As a consequence, EPA 

determined that an approach that 

combined national secondary standards 

with a regional haze program was the 

most appropriate and effective way to 

address visibility impairment (EPA 

2005, p. 7–2). 

As anticipated in the last review, EPA 

promulgated a regional haze program in 

1999 (65 FR 35713). That program 

requires States to establish goals for 

improving visibility in Class I areas and 


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84A dissenting view was expressed in one Panel 

member’s individual review comments to the effect 

that any urban visibility standard should be 

voluntary and locally adopted (Henderson, 2005a). 

to adopt control strategies to achieve 

these goals. Since strategies to meet 

these goals are to reflect a coordinated 

approach among States, multi-state 

regional planning organizations have 

been formed and are now developing 

strategies, to be adopted over the next 

few years, that will make reasonable 

progress in meeting these goals. 

The initial issue to be addressed in 

the current review of the secondary PM 

standards is whether, in view of the 

information now available, the existing 

secondary standards should be revised 

to provide requisite protection from PMrelated adverse effects on visual air 

quality. As discussed in the Criteria 

Document and Staff Paper, while new 

research has led to improved 

understanding of the optical properties 

of particles and the effects of relative 

humidity on those properties, it has not 

changed the fundamental 

characterization from the last review of 

the role of PM, and especially fine 

particles, in visibility impairment. 

However, extensive new information 

from visibility and fine particle 

monitoring networks since the last 

review has allowed for updated 

characterizations of visibility trends and 

current levels in urban areas, as well as 

Class I areas. As discussed in section 

IV.A.1.b. of the proposal (71 FR 2676–

2677), these new data were a critical 

component of analyses that better 

characterized visibility impairment in 

urban areas and the relationship 

between visibility and PM2.5

concentrations, and led to the finding 

that PM2.5 concentrations can be used as 

a general surrogate for visibility 

impairment in urban areas. 

Taking into account the most recent 

monitoring information and analyses, 

and recognizing that efforts are now 

underway to address all human-caused 

visibility impairment in Class I areas 

through the regional haze program 

implemented under sections 169A and 

169B of the CAA, as discussed above, 

this review focused on visibility 

impairment primarily in urban areas. In 

so doing, given the stronger link 

between visibility impairment and 

short-term PM2.5 concentrations, EPA 

gave significant consideration to the 

question of whether visibility 

impairment in urban areas allowed by 

the current 24-hour secondary PM2.5

standard can be considered adverse to 

public welfare. 

As discussed in section IV.A.1.c. of 

the proposal (71 FR 2677–2678), studies 

in the U.S. and abroad have provided 

the basis for the establishment of 

standards and programs to address 

specific visibility concerns in a number 

of local areas. These studies (e.g., in 

Denver, Phoenix, British Columbia) 

have produced reasonably consistent 

results in terms of the visual ranges 

found to be generally acceptable by the 

participants in the various studies, 

which spanned from approximately 40 

to 60 km in visual range. Standards 

targeting protection within this range 

have also been set by the State of 

Vermont and by California for the Lake 

Tahoe area, in contrast to the statewide 

California standard that targets a visual 

range of approximately 16 km. 

In addition to the information 

available from such programs, 

photographic representations (simulated 

images and actual photographs) of 

visibility impairment are available, as 

discussed in section IV.A.1.d of the 

proposal (71 FR 2678), to help inform 

judgments about the acceptability of 

varying levels of visual air quality in 

urban areas across the U.S. In 

considering these images for Phoenix, 

Washington, DC, and Chicago (for 

which PM2.5 concentrations are 

reported), the Staff Paper observed that: 

(1) At concentrations at or near the 

level of the current 24-hour PM2.5

standard (65 μg/m3), which equates to 

visual ranges roughly around 10 km (6 

miles), scenic views (e.g., mountains, 

historic monuments), as depicted in 

these images around and within the 

urban areas, are significantly obscured 

from view. 

(2) Appreciable improvement in the 

visual clarity of the scenic views 

depicted in these images occurs at PM2.5

concentrations below 35 to 40 μg/m3, 

which equate to visual ranges generally 

above 20 km for the urban areas 

considered (EPA, 2005, p. 7–6). 

(3) Visual air quality appears to be 

good in these images at PM2.5

concentrations generally below 20 μg/ 

m3, corresponding to visual ranges of 

approximately 25 to 35 km (EPA, 2005, 

p. 7–8). 

While being mindful of the 

limitations inherent in using visual 

representations from a small number of 

areas as a basis for considering national 

visibility-based secondary standards, 

the Staff Paper nonetheless concluded 

that these observations, together with 

information from the analyses and other 

programs discussed above, support 

revising the current secondary PM2.5

standards to improve visual air quality, 

particularly in urban areas. As 

discussed below, the Staff Paper 

recommended the establishment of a 

new short-term secondary PM2.5

standard to provide increased and more 

targeted protection, primarily in urban 

areas, from visibility impairment related 

to fine particles (EPA, 2005, p. 7–12). 

Based on its review of the Staff Paper, 

the CASAC advised the Administrator 

that most CASAC PM Panel members 

strongly supported the Staff Paper 

recommendation to establish a new 

distinct secondary PM2.5 standard to 

protect urban visibility (Henderson, 

2005a).84 Most Panel members 

considered such a standard to be a 

reasonable complement to the Regional 

Haze Rules that protect Class I areas. 

In the proposal, the Administrator 

carefully considered the rationale and 

recommendations in the Staff Paper, the 

advice and recommendations from 

CASAC, and initial public comments on 

the issue of whether the secondary PM 

standards should be revised to provide 

increased PM-related visibility 

impairment primarily in urban areas. In 

so doing, the Administrator first 

recognized that PM-related visibility 

impairment is principally related to fine 

particle levels, such that it is 

appropriate to focus the review on 

whether the current secondary PM2.5

standards should be revised. The 

Administrator also recognized that 

perception of visibility impairment is 

most directly related to instantaneous 

levels of visual air quality, such that in 

considering whether the current suite of 

secondary standards would provide the 

appropriate degree of protection, he first 

considered whether the current 24-hour 

secondary PM2.5 standard provides an 

appropriate level of protection from 

visibility impairment, principally in 

urban areas. 

In the proposal, the Administrator 

called attention to the Staff Paper 

finding that, at concentrations at or near 

the level of the current 24-hour PM2.5

secondary standard (65 μg/m3) visual 

ranges are degraded to a distance of 

about 10 km (6 miles) and images of 

scenic views (e.g., mountains, historic 

monuments, urban skylines) around and 

within a number of urban areas are 

significantly obscured from view. 

Further, the Administrator took note of 

the various State and local standards 

and programs that have been established 

to protect visual air quality beyond the 

degree of protection that would be 

afforded by the current 24-hour 

secondary PM2.5 standard. Based on all 

of the above considerations, the 

Administrator provisionally concluded 

that it was appropriate to revise the 

current 24-hour secondary PM2.5

standard to provide an appropriate level 

of protection from visibility impairment 

principally in urban areas, in 

conjunction with the regional haze 


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program for protection of rural air 

quality in Class I areas. 

The majority of commenters who 

expressed an opinion on the secondary 

standards, including NESCAUM, 

STAPPA/ALAPCO, a number of 

individual States, Tribal associations, 

and local organizations, and combined 

comments from various environmental 

groups supported the position that the 

secondary PM2.5 standards should be 

revised to increase protection against 

visibility impairment. A number of 

these commenters cited the studies and 

evidence in the PM Staff Paper, as well 

as the recommendations of CASAC, in 

support of their views that a more 

protective standard is warranted. 

NESCAUM noted that, though monitors 

in the northeast region do not exceed 

the current secondary PM2.5 standards, 

their regional haze camera network 

(CAMNET) routinely documents 

extremely hazy days obscuring city 

skylines and views. NESCAUM stated 

that ‘‘this shows that virtually all of 

PM2.5 effects on visibility in the 

Northeast are occurring below the 

present secondary standard, justifying 

EPA’s proposal to revise the existing 

standard to a more stringent level 

adequately protective of public welfare’’

(NESCAUM, attachment C, p. C–1) In 

general, EPA agrees with these 

commenters that the more recent 

information on visibility values, 

photographic evidence, and air quality/ 

visibility relationships supports the 

need to revise the current secondary 

PM2.5 standards. 

Other commenters, including UARG, 

American Public Power Association, 

and American Electric Power, opposed 

a revision to strengthen the secondary 

PM2.5 standards at this time. UARG 

stated that: 

Because the record does not establish that the 

risks to public welfare from ambient PM2.5

are greater, different in character, or more 

certain than was understood when the 

present standards were established, the 

Agency lacks a basis for revising its 

conclusion that those standards provide the 

requisite protection of public welfare. 

(UARG, p. 36). 

UARG questioned the usefulness of 

the photographic images and urban 

studies of acceptable visibility 

highlighted in the proposal for 

determining appropriate levels of urban 

visibility. They further noted that, for 

most areas, the annual PM2.5 standard 

would prevent any exceedances of 65 

μg/m3. 

While, as summarized above, the key 

optical aspects of the relationship 

between fine particles and visibility 

have been established for a long time, 

EPA strongly disagrees that the more 

recent visibility-related evidence and 

analyses presented in the Criteria 

Document and Staff Paper provide no 

basis for considering more protective 

PM2.5 standards. As discussed in the 

Staff Paper, one of the key issues in the 

last review was whether the differences 

in humidity between East and West 

complicated the establishment of a 

nationally uniform PM2.5 secondary 

standard, even for urban areas (EPA, 

2005, p. 7–3). 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 this review 

found that, in urban areas, visibility 

levels show 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 (EPA, 2005, p. 7–5). 

Of equal importance, more recent 

studies of visibility values conducted 

for several urbanized areas have found 

results generally consistent with an 

earlier study done for the city of Denver. 

While such studies are still limited in 

number and subject to uncertainty, they 

suggest a remarkable consistency in 

public reaction to urban visibility 

impairment caused by fine particles 

(EPA 2005, p. 6–18 to 23). 

Furthermore, staff and CASAC agreed 

on the utility of photographic evidence 

in characterizing the nature of particleinduced haze. At the level of the current 

24-hour PM2.5 standard, the potential 

subtleties associated with alternative 

photographic views alluded to by UARG 

would be obscured by the density of the 

accompanying haze, which would 

restrict the distance of the farthest 

discernable dark objects to only 6 miles 

and greatly reduce the contrast for 

objects at significantly shorter distances. 

Although, as suggested by these 

commenters, the annual standard serves 

to limit excursions above the level of the 

current 24-hour standard, particularly in 

eastern urban areas, continuation of the 

current 24-hr PM2.5 standard would 

permit a large number of exceedances of 

this level especially in some western 

urban areas, even when the standard is 

just attained. In summary, contrary to 

the views of this set of commenters, 

EPA believes that the combination of 

new insights from air quality analyses, 

the standards and studies developed to 

address urban visibility in several areas, 

as well as an evaluation of the 

photographic evidence, supports the 

need to revise the current secondary 

PM2.5 standards. 

Having considered the evidence and 

analysis of visibility and fine particles 

in the Criteria Document and Staff 

Paper, the advice and recommendations 

of the CASAC, as well as the public 

comments on this issue, the 

Administrator concludes that it is 

appropriate to revise the current 

secondary PM2.5 standards to provide 

increased protection from visibility 

impairment in urban areas. Consistent 

with the considerations and rationale 

summarized above and in the proposal, 

the Administrator believes that 

emphasis should be placed on revisions 

to the current 24-hour PM2.5 standard 

that would provide an appropriate level 

of protection against visibility 

impairment principally in urban areas, 

in conjunction with the regional haze 

program for protection of visual air 

quality in Class I areas. 

3. Indicator of PM for Secondary 

Standard To Address Visibility 

Impairment 

As discussed in the Staff Paper, fine 

particles contribute to visibility 

impairment directly in proportion to 

their concentration in the ambient air. 

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. In analyzing how well 

PM2.5 concentrations correlate with 

visibility in urban locations across the 

U.S. (see EPA, 2005, section 6.2.3), the 

Staff Paper concluded that the observed 

correlations are strong enough to 

support the use of PM2.5 as the indicator 

for such standards. More specifically, 

clear correlations exist between 24-hour 

average PM2.5 concentrations and 

reconstructed light extinction, which is 

directly related to visual range. These 

correlations are similar in the eastern 

and western regions of the U.S. Further, 

these correlations are less influenced by 

relative humidity and more consistent 

across regions when PM2.5

concentrations are averaged over 

shorter, daylight time periods (e.g., 4 to 

8 hours). Thus, the Staff Paper 

concluded that it is 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 of 

daylight hours. Based on its review of 

the Staff Paper, most CASAC Panel 

members endorsed a PM2.5 indicator for 

a secondary standard to address 

visibility impairment (Henderson, 

2005a, p. 9). 

The Administrator provisionally 

concurred with the EPA staff and 

CASAC recommendations, and 

proposed that PM2.5 should be retained 

as the indicator for fine particles as part 


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85The Staff Paper notes that a standard set at any 

specific PM2.5 concentration will 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 (EPA, 2005, p. 7–8). 

of a secondary standard to address 

visibility protection. No commenters 

disputed the appropriateness of 

continuing to use PM2.5 as the indicator 

for fine particle secondary standards to 

address visibility impairment. 

Having considered the scientific 

information discussed in the proposal 

and summarized above, as well as the 

recommendations of the staff and 

CASAC and the public comments on 

this issue, the Administrator concludes 

that PM2.5 should be retained as the 

indicator for fine particles as part of a 

secondary standard to address visibility 

protection. 

4. Averaging Time of a Secondary PM2.5

Standard for Visibility Protection 

As discussed in the Staff Paper, 

averaging times from 24 to 4 hours were 

considered for a revised standard to 

address visibility impairment. Within 

this range, clear and similarly strong 

correlations were found between 

visibility and 24-hour average PM2.5

concentrations in eastern and western 

areas, while somewhat stronger 

correlations were found with PM2.5

concentrations averaged over a 4-hour 

time period. In general, correlations 

between PM2.5 concentrations and light 

extinction were found to be generally 

less influenced by relative humidity and 

more consistent across regions as 

shorter, sub-daily averaging times, 

within daylight hours from 

approximately 10 a.m. to 6 p.m., were 

considered. The Staff Paper concluded 

that an averaging time from 4 to 8 hours, 

generally within this daylight time 

period, should be considered for a 

standard to address visibility 

impairment. 

In reaching this conclusion, the Staff 

Paper recognized that the PM2.5 Federal 

Reference Method (FRM) monitoring 

network provides 24-hour average 

concentrations, and, in some cases, on 

a third- or sixth-day sample schedule, 

such that implementing a standard with 

a less-than-24-hour averaging time 

would necessitate the use of continuous 

monitors that can provide hourly time 

resolution. Given that the data used in 

the Staff Paper analysis discussed above 

were from commercially available PM2.5

continuous monitors, such monitors 

clearly could provide the hourly data 

that would be needed for comparison 

with a potential visibility standard with 

a less-than-24-hour averaging time. 

Most CASAC Panel members 

supported the Staff Paper 

recommendation of a sub-daily (4 to 8 

daylight hours) averaging time, finding 

it to be an innovative approach that 

strengthens the quality of the PM2.5

indicator for visibility effects by 

targeting the driest part of the day 

(Henderson, 2005a, p. 9). In its advice 

to the Administrator, CASAC noted an 

indirect but important benefit to 

advancing EPA’s monitoring program 

goals that would come from the direct 

use of hourly data from a network of 

continuous PM2.5 mass monitors. 

In considering the Staff Paper 

recommendation and CASAC’s advice, 

the Administrator provisionally 

concluded that averaging times from 24 

hours to 4 daylight hours would 

represent a reasonable range of choices 

for a standard to address urban visibility 

impairment. A 24-hour averaging time 

could be selected and applied based on 

the extensive data base currently 

available from the existing PM2.5 FRM 

monitoring network, whereas a subdaily averaging time would necessarily 

depend upon an expanded network of 

continuous PM2.5 mass monitors. While 

the Administrator agreed that broader 

deployment of continuous PM2.5 mass 

monitors is a desirable goal, working 

toward that goal does not depend upon 

nor provide an appropriate basis for 

setting a sub-daily standard. The 

Administrator believed that it was 

appropriate to evaluate averaging time 

in conjunction with reaching decisions 

on the form and level of a standard. 

Public comments on these issues, as 

well as the rationale for the final 

decisions on averaging time, form, and 

level of the secondary standards, are 

presented in the following section. 

5. Final Decisions on Secondary PM2.5

Standards for Visibility Protection 

In considering PM2.5 standards that 

would provide an appropriate level of 

protection against PM-related 

impairment of visibility primarily in 

urban areas, the Administrator took into 

account the results of the public 

perception and attitude surveys in the 

U.S. and Canada, State and local 

visibility standards within the U.S., and 

visual inspection of photographic 

representations of several urban areas 

across the U.S. summarized in section 

IV.A.1 of the proposal. In the 

Administrator’s judgment, these sources 

provide 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. In considering 

alternative forms for such standards, the 

Administrator took into account the 

same general factors that were 

considered in selecting an appropriate 

form for the 24-hour primary PM2.5

standard (as discussed above in section 

II.E.1), as well as additional information 

on the percent of areas not likely to 

meet various alternative PM2.5

standards, consistent with CASAC 

advice to consider such information 

(Henderson, 2005a, p. 10). 

In considering the remaining elements 

of a secondary PM2.5 standard (averaging 

time, form, and level) for purposes of 

the proposal, the Administrator looked 

to the rationale presented in the Staff 

Paper and to CASAC’s advice and 

recommendations for such a standard. 

Based on photographic representations 

of varying levels of visual air quality, 

public perception studies, and local and 

State visibility standards, as discussed 

above, the Staff Paper concluded that 30 

to 20 μg/m3 PM2.5 represents a 

reasonable range for a national visibility 

standard primarily for urban areas, 

based on a sub-daily averaging time. 

The upper end of this range is below the 

levels at which the illustrative scenic 

views are significantly obscured, and 

the lower end is around the level at 

which visual air quality generally 

appears to be good based on observation 

of the illustrative views. Analyses of 4- 

hour average PM2.5 concentrations 

indicate that this concentration range 

can be expected generally to correspond 

to median visual ranges in urban areas 

within regions across the U.S. of 

approximately 25 to 35 km (see EPA, 

2005, Figure 7–1).85 This range of visual 

range values is 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 Staff Paper 

concluded that a concentration-based 

percentile form is appropriate for the 

same reasons as those discussed in 

section II.F.1 above (on the form of the 

24-hour primary PM2.5 standard). The 

Staff Paper also concluded that the 

upper end of the range of concentration 

percentiles should be consistent with 

the percentile used for the primary 

standard, which was proposed to be the 

98th percentile, and that the lower end 

of the range should be the 92nd 

percentile, which represents the mean 

of the distribution of the 20 percent 

most impaired days, as targeted in the 

regional haze program (EPA, 2005, p. 7–

11 to 12). 

In its advice to the Administrator, the 

CASAC Panel recognized that it is 

difficult to select any specific level and 


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86Some CASAC Panel members also 

recommended that such a standard be implemented 

in conjunction with an ‘‘exceptional events’’ policy 

so as to avoid having non-compliance with the 

standard be driven by natural source influences 

such as dust storms and wild fires (Henderson, 

2005a). 

87The information in these Tables is based on 

analysis of 2001–2003 air quality data, including 

562 counties with FRM monitors that met specific 

data completeness criteria for developing predicted 

percentages of counties not likely to meet the suite 

of primary PM2.5 standards and 168 counties with 

continuous PM2.5 monitors that met less restrictive 

data completeness criteria for developing predicted 

percentages for a 4-hour secondary PM2.5 standard. 

88The American Lung Association et al. 

disagreed with the Administrator’s view that the 

secondary standards should be focused primarily 

on providing protection in urban areas, with 

protection of Class I areas provided by the Regional 

Haze Rule. These commenters suggested that EPA 

should not rely on the regional haze program and 

must set national standards to protect all areas. As 

discussed in the Response to Comments document, 

EPA believes that this issue was settled in ATA I. 

(See 175 F.3d at 1056–1057.) 

form based on currently available 

information (Henderson, 2005a, p. 9). 

Some Panel members felt that the range 

of levels recommended in the Staff 

Paper was on the high side, but 

recognized that developing a more 

specific (and more protective) level in 

future reviews would require updated 

and refined public visibility valuation 

studies, which CASAC strongly 

encouraged the Agency to support prior 

to the next review. With regard to the 

form of the standard, the 

recommendations in the final Staff 

Paper reflected CASAC’s advice to 

consider percentiles in the range of the 

92nd to the 98th percentile. Some Panel 

members recommended considering a 

percentile within this range in 

conjunction with a level toward the 

upper end of the range recommended in 

the Staff Paper.86

Based on the above considerations, for 

purposes of the proposal the 

Administrator believed that it was 

appropriate to first consider the level of 

protection that would be afforded by the 

proposed suite of primary PM2.5

standards (71 FR 2681). The limited and 

uncertain evidence currently available 

for use in evaluating the appropriate 

level of protection suggested that a 

cautious approach was warranted in 

establishing a distinct secondary PM2.5

standard to address visibility 

impairment. While significantly more 

information is available since the last 

review concerning the relationship 

between fine PM levels and visibility 

across the country, there is 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. Given this, the 

Administrator first evaluated the level 

of protection that the proposed primary 

PM2.5 standards would likely provide, 

and then determined whether the 

available evidence warranted adopting a 

standard with a different level, form, or 

averaging time. 

In comparing 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, the 

Administrator looked to an analysis of 

the predicted percent of areas not likely 

to meet various alternative secondary 

and primary PM2.5 standards (EPA, 

2005, Tables 7A–1 and 5B–1(a) 87). In so 

doing, the Administrator observed that 

the predicted percent of counties with 

monitors not likely to meet the 

proposed suite of primary PM2.5

standards (i.e., a 24-hour standard set at 

35 μg/m3, with a 98th percentile form, 

and an annual standard of 15 μg/m3) 

was actually somewhat greater (27 

percent) 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 Staff 

Paper (e.g., up to 30 μg/m3), and a form 

within the recommended range (e.g., 

around the 95th percentile) (24 percent). 

A similar comparison was seen in 

considering the predicted percentages of 

the population living in such areas. 

Considering the evidence in light of 

these comparisons, the Administrator 

provisionally concluded that revising 

the current secondary 24-hour standard 

for PM2.5 to be identical to the proposed 

revised primary PM2.5 standard and 

retaining the current annual secondary 

PM2.5 standard was a reasonable policy 

approach to addressing visibility 

protection primarily in urban areas. 

Consistent with CASAC’s 

recommendation, the Administrator also 

solicited comment on a sub-daily (4- to 

8-hour averaging time) secondary PM2.5

standard. 

In additional comments responding to 

EPA’s proposed revision of the 

secondary PM2.5 standards for visibility 

protection (71 FR 2675–2781), the 

CASAC requested that a sub-daily 

standard to protect visibility be 

favorably reconsidered (Henderson, 

2006, p. 2). As noted above, most of the 

CASAC Panel recommended a sub-daily 

standard for PM2.5 with a level in the 20 

to 30 μg/m3 range for a four- to eighthour (4–8 hr) mid-day time period with 

a 92nd to 98th percentile form. The 

CASAC members noted three cautions 

regarding the Agency’s proposed 

reliance on a secondary PM2.5 standard 

identical to the proposed 24-hour 

primary PM2.5 standard (Id. at pp. 5–6): 

(1) They noted that the PM2.5 mass 

measurement is a better indicator of 

visibility impairment during daylight 

hours, when humidities are 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 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) They noted that CASAC and its 

monitoring subcommittees have 

repeatedly commended EPA’s initiatives 

promoting the introduction of 

continuous and near-continuous PM 

monitoring, and that expanded 

deployment of continuous PM2.5

monitors is consistent with setting a 

sub-daily standard to protect visibility. 

(3) They cautioned that the analysis 

showing a similarity between 

percentages of counties not likely to 

meet what they considered to be a 

lenient 4- to 8-hour secondary standard 

and a secondary standard identical to 

the proposed 24-hour primary standard 

is a numerical coincidence that is not 

indicative of any fundamental 

relationship between visibility and 

health. 

The CASAC Panel further stated that 

‘‘visual air quality is substantially 

impaired at PM2.5 concentrations of 35 

μg/m3’’ and that ‘‘it 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.’’ (Id. at p. 6.) 

Many of the public commenters who 

supported a more stringent visibility 

standard also supported the more 

specific EPA staff and CASAC 

recommendations and urged EPA to 

adopt a sub-daily (4- to 8-hour averaging 

time) PM2.5 standard to address 

visibility impairment, within the range 

of 20 to 30 μg/m3 and with a form 

within the range of the 92nd to 98th 

percentile. In general, these commenters 

based their recommendations on the 

same studies, analyses, and 

considerations presented in the Staff 

Paper and in section IV.A of the 

proposal.88

EPA agrees with several of the key 

technical points made in CASAC’s 

original recommendations and their 

request for reconsideration. The 

Administrator recognizes that there is a 

significant body of data and information 

indicating that a sub-daily standard has 


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strong technical merit. The fine particle/ 

visibility relationship is most consistent 

across regions for shorter averaging 

times during the daylight hours, when 

humidity tends to be lowest. The EPA 

also agrees that visibility impairment 

has the greatest impact on public 

welfare during the daylight hours, but 

notes that daylight is not limited to a 

four to eight hour period. 

The Administrator believes, however, 

that it is appropriate to consider the 

protection the revised suite of primary 

PM2.5 standards would provide against 

adverse effects on public welfare. The 

analysis summarized above found that 

the relative protection provided by the 

proposed primary standards was 

equivalent or more protective than 

several of the 4-hour secondary standard 

alternatives in the range recommended 

by the Staff Paper and CASAC. Given 

the limitations in the underlying studies 

and the subjective nature of the 

judgment required, the Administrator 

continues to believe that caution is 

warranted in establishing a distinct 

secondary standard for visibility 

impairment. Contrary to commenters 

who recommended a distinct standard 

providing greater protection, in this 

case, the Administrator does not believe 

that these studies 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. While EPA 

agrees that the use of 24-hour and 

annual averages will result in more 

variability in visibility across urban 

areas, as the Staff Paper notes, any PM2.5

secondary standard would result in 

some variability in protection in 

different locations (EPA, 2005, p. 7–8). 

While, as noted above and in the 

proposal, the Administrator agrees with 

CASAC’s point that broader deployment 

of continuous PM2.5 mass monitors is a 

desirable goal, working toward that goal 

does not depend upon nor provide an 

appropriate basis for setting a sub-daily 

standard. Moreover, pursuant to CASAC 

recommendations, EPA is today issuing 

modifications to the PM2.5 reference and 

equivalent methods that will encourage 

the certification and deployment of 

more continuous monitors (in a separate 

document published in today’s Federal 

Register). With respect to the third 

CASAC comment summarized above, 

EPA agrees that the result of the analysis 

showing a similarity in the percentages 

of counties not likely to meet the 

revised 24-hour primary PM2.5 standard 

or a sub-daily standard set toward the 

upper end of the range of protectiveness 

recommended by CASAC is not 

indicative of any fundamental 

relationship between visibility and 

public health. However, EPA does not 

believe that this coincidental similarity 

weighs against considering making the 

secondary standard identical to the 

revised primary standard. 

Having considered the evidence, the 

advice of CASAC, and public 

comments, the Administrator believes 

that revising the current secondary 

PM2.5 standards to be identical to the 

revised suite of primary PM2.5 standards 

adopted in today’s notice is a reasonable 

policy approach to addressing visibility 

impairment primarily in urban areas. 

The current annual and revised 24-hour 

secondary PM2.5 standards will result in 

improvements in visual air quality in as 

many or more urban areas across the 

country as would the alternative 

approach of setting a sub-daily standard 

consistent with the upper portion of the 

ranges recommended by CASAC. This 

approach recognizes the substantial 

limitations in the available hourly air 

quality data and in available studies of 

public perception and attitudes with 

regard to the acceptability of various 

degrees of visibility impairment in 

urban areas across the country. Given 

these limitations, the Administrator 

believes that a distinct secondary 

standard with a different averaging time, 

level, or form is not warranted at this 

time, because the available evidence 

does not support a decision to achieve 

a level of protection different from that 

provided by the revised suite of primary 

standards, and because no further 

change in averaging time, level, or form 

appears needed to achieve a comparable 

level of protection. A decision in this 

review to make secondary standards 

equivalent in all respects to the primary 

standards, as revised, does not limit the 

ability of the Agency to establish a 

distinct secondary standard in the 

future if and when the underlying 

evidence indicates that it is appropriate. 

Further, the Administrator notes that 

continuing to advance the use of 

continuous PM2.5 monitors is not 

dependant on establishing a sub-daily 

secondary PM2.5 standard. 

The Administrator believes that any 

secondary NAAQS for visibility 

protection should be considered 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 Class I areas 

across the country. Programs 

implemented to meet the national 

primary standards can be expected to 

improve visual air quality not just in 

urban areas but in surrounding nonurban areas as well; similarly, programs 

now being developed to address the 

requirements of the regional haze rule 

established for protection of visual air 

quality in Class I areas can be expected 

to improve visual air quality in 

surrounding areas as well. The 

Administrator further believes that the 

development of local programs 

continues to be an effective and 

appropriate approach to provide 

additional protection for unique scenic 

resources in and around certain urban 

areas that are highly valued by people 

living in those areas. 

Based on all of the considerations 

discussed above, the Administrator 

concludes that it is appropriate to revise 

the current secondary PM2.5 standards to 

be identical in all respects to the revised 

suite of primary PM2.5 standards 

adopted in today’s notice to provide an 

appropriate level of visibility protection 

primarily in urban areas. 

B. Other PM-Related Welfare Effects 

In considering the currently available 

evidence on non-visibility PM-related 

welfare effects, the Staff Paper noted 

that there was much information linking 

ambient PM to potentially adverse 

effects on vegetation and ecosystems 

and on materials damage and soiling, 

and on characterizing the role of 

atmospheric particles in climatic and 

radiative processes. However, given the 

evaluation of this information in the 

Criteria Document and Staff Paper, 

which highlighted the substantial 

limitations in the evidence, especially 

the lack of evidence linking various 

effects to specific levels of ambient PM, 

the Administrator provisionally 

concluded in the proposal that the 

available evidence did not provide a 

sufficient basis for establishing distinct 

secondary standards for PM based on 

any of these effects alone. 

In the proposal, the Administrator 

also addressed the question whether 

reductions in PM likely to result from 

the current secondary PM standards, or 

from the range of revised primary PM 

standards, would provide appropriate 

protection against any of these PMrelated welfare effects. As discussed 

below, these considerations included 

the latest scientific information 

characterizing the nature of these nonvisibility PM-related effects and 

judgments as to whether revision of the 

current secondary standards is 

appropriate based on that information. 

1. Evidence of Non-Visibility Welfare 

Effects Related to PM 

Particulate matter contributes to 

adverse effects on a number of welfare 

effects categories other than visibility 

impairment, including vegetation and 


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ecosystems, soiling and materials 

damage, and climate. These welfare 

effects result predominantly from 

exposure to excess amounts of specific 

chemical species, regardless of their 

source or predominant form (particle, 

gas, or liquid). Reflecting this fact, the 

Criteria Document concluded that 

regardless of size fraction, particles 

containing nitrates and sulfates have the 

greatest potential for widespread 

environmental significance. The nature 

of these welfare effects is discussed in 

the Criteria Document (Chapters 4 and 

9) and Staff Paper (Chapter 6) and 

summarized in section IV.B.1 of the 

proposal. The information highlighted 

there includes: 

(1) PM-related effects on vegetation, 

specifically those associated with excess 

levels of particulate nitrate and sulfate 

in acidifying deposition to foliage, 

leading to accelerated weathering of leaf 

cuticular surfaces; increased 

permeability of leaf surfaces to toxic 

materials, water, and disease agents; 

increased leaching of nutrients from 

foliage; and altered reproductive 

processes—all which serve to weaken 

trees so that they are more susceptible 

to other stresses (e.g., extreme weather, 

pests, pathogens). 

(2) PM-related effects on ecosystems, 

specifically those resulting from the 

nutrient or acidifying characteristics of 

deposited PM on both terrestrial and 

aquatic ecosystems, which contribute to 

adverse impacts on essential ecological 

attributes such as species shifts, loss of 

diversity, impacts to threatened and 

endangered species and alteration of 

native fire cycles. 

(3) Characterization of ecosystem 

exposure to PM deposition, specifically 

the currently available deposition 

monitoring network and the lack of 

sufficient long-term monitoring of 

ecosystem response needed for PMrelated ecological risk assessment. 

(4) The critical loads concept and its 

applicability as an assessment tool in 

the context of the PM secondary 

NAAQS review. 

(5) PM-related effects on materials, 

specifically the physical damage caused 

mainly by deposited particulate nitrates 

and sulfates and the impaired aesthetic 

qualities due to soiling caused mainly 

by particles consisting primarily of 

carbonaceous compounds. 

(6) PM-related effects on climate, 

specifically through scattering and 

absorption of radiation by ambient 

particles, as well as effects on the 

radiative properties of clouds through 

changes in the number and size 

distribution of cloud droplets, and by 

altering the amount of ultraviolet solar 

radiation (especially UV–B) penetrating 

through the atmosphere to ground level. 

2. Need for Revision of the Current 

Secondary PM Standards To Address 

Other PM-Related Welfare Effects 

At the time of proposal, in 

considering the currently available 

evidence on each type of PM-related 

welfare effects discussed above, the 

Administrator noted that there was 

much information linking the sulfurand nitrogen-containing components of 

ambient PM to potentially adverse 

effects on ecosystems and vegetation, as 

well as links between PM and its 

constituents and materials damage and 

soiling, as well as climatic and radiative 

processes. However, after reviewing the 

extent of relevant studies and other 

information available since the 1997 

review of the PM standards, which 

highlighted the substantial limitations 

in the evidence, especially with regard 

to the lack of evidence linking various 

effects to specific levels of ambient PM, 

the Administrator concurred with 

conclusions reached in the Staff Paper 

and by CASAC (Henderson, 2005a) that 

the available data do not provide a 

sufficient basis for establishing distinct 

secondary PM standards based on any of 

these non-visibility PM-related welfare 

effects. 

While recognizing that PM-related 

impacts on vegetation and ecosystems 

and PM-related soiling and materials 

damage are associated with chemical 

components in both fine and coarsefraction PM, the Administrator 

provisionally concluded that sufficient 

information was not available at this 

time to consider either an ecologically 

based indicator or an indicator based 

distinctly on soiling and materials 

damage, in terms of specific chemical 

components of PM. Further, consistent 

with the rationale and recommendations 

in the Staff Paper, the Administrator 

agreed that it was appropriate to 

continue control of ambient fine and 

coarse-fraction particles, especially 

long-term deposition of particles such as 

particulate nitrates and sulfates that 

contribute to adverse impacts on 

vegetation and ecosystems and/or to 

materials damage and soiling. The 

Administrator also agreed with the Staff 

Paper that the available information did 

not provide a sufficient basis for the 

development of distinct secondary 

standards to protect against such effects 

beyond the protection likely to be 

afforded by the proposed suite of 

primary PM standards. In considering 

those proposed standards in 

combination, including the proposed 

more protective 24-hour standard for 

PM2.5 and the proposed 24-hour 

standard for PM10–2.5, which was 

intended to provide an equivalent 

degree of protection to the current PM10

standards in areas where the proposed 

PM10–2.5 indicator would apply (which 

tend to be more densely populated areas 

where materials damage would be of 

greater concern), the Administrator 

believed that this proposed suite of 

standards would afford at least the 

degree of protection as that afforded by 

the current secondary PM standards. 

Finally, the Administrator believed 

that such standards should be 

considered in conjunction with the 

protection afforded by other programs 

intended to address various aspects of 

air pollution effects on ecosystems and 

vegetation, such as the acid deposition 

program and other regional approaches 

to reducing pollutants linked to nitrate 

or acidic deposition. Based on these 

considerations, and taking into account 

the information and recommendations 

discussed above, the Administrator 

proposed to revise the current 

secondary PM2.5 and PM10 standards to 

address these other welfare effects by 

making them identical in all respects to 

the proposed suite of primary PM2.5 and 

PM10–2.5 standards. 

In response to the proposal, in 

addition to their recommendation for a 

PM2.5 secondary standard, CASAC 

recommended (Henderson, 2006, p. 4) 

‘‘that a secondary PM10–2.5 standard be 

set at the same level as the primary PM 

coarse standard to protect against the 

various irritant, soiling and nuisance 

welfare or environmental effects of 

coarse particles. Since these effects are 

not uniquely related to urban sources or 

receptors, the standard should not be 

limited to urban areas.’’ Only limited 

public comments were received on this 

aspect of the proposal. 

In general, public comments relating 

to secondary standards and other 

welfare effects focused on issues related 

to the current secondary PM10

standards. Most of these commenters, 

including the groups who objected to 

the use of a qualified indicator for the 

primary thoracic coarse particle 

standard, argued that current levels of 

PM dust contribute or potentially 

contribute to nuisance, soiling, and 

irritant impacts on personal comfort and 

well being, especially in non-urban 

areas. The same commenters agreed 

with CASAC that, in the absence of a 

demonstration to the contrary, EPA is 

not justified in eliminating or reducing 

the level of protection to rural areas that 

is provided by the current suite of 

secondary standards. Most of these 

commenters recommended that EPA 

either retain the current PM10 secondary 

standard or replace it with a PM10–2.5


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standard set identical to the proposed 

primary standard without the proposed 

qualifications that limited application of 

the standard to urban areas. 

A few commenters argued against 

retaining any secondary standard for 

coarse particles. Many of these same 

commenters argued that if EPA did set 

a secondary PM10–2.5 standard, it should 

be set equal to the primary PM10–2.5

standard because there was insufficient 

evidence to support adoption of a 

distinct secondary standard for PM10–2.5

at this time. Furthermore, these 

commenters noted that in the proposal, 

EPA had correctly excluded from both 

primary and secondary standards ‘‘any 

ambient mix of PM10–2.5 that is 

dominated by rural windblown dust and 

soils and PM generated by agricultural 

and mining sources’’ because these 

particles are nontoxic and generally 

settle quickly. 

In reaching a final decision on the 

need to revise the PM secondary 

standards regarding these non-visibility 

related welfare effects, the 

Administrator has taken into account 

several key factors, including: (1) The 

latest scientific information on nonvisibility welfare effects associated with 

PM, as previously described; (2) the 

post-proposal recommendations of 

CASAC, (3) comments received during 

the public comment period, and (4) the 

final decisions reached in today’s notice 

on the primary standards for fine and 

coarse particles, as well as the decision 

presented above on secondary PM2.5

standards to protect against visibility 

impairment. The Administrator notes 

that extending today’s decision not to 

revise the current 24-hour primary PM10

standard to the secondary standard 

would be consistent with the 

recommendations of CASAC and would 

address the issues raised by the first 

group of commenters summarized 

above. Consistent with the assessment 

of the evidence in the Staff Paper and 

the CASAC recommendations, the 

Administrator disagrees with those who 

assert that no secondary standard is 

needed to protect against the welfare 

effects associated with coarse particles. 

On the other hand, the Administrator 

does not believe that distinct secondary 

standards for fine or coarse particles are 

warranted for any of the effects 

considered in this section. The available 

evidence is not sufficient to support the 

selection of an ecologically based 

indicator or an indicator based 

distinctly on materials damage, soiling, 

irritant or nuisance effects, or other 

effects of PM. However, the 

Administrator recognizes that it is 

appropriate to continue control of 

ambient fine and coarse particles, 

especially long-term deposition of 

particles such as particulate nitrates and 

sulfates that contribute to the total input 

of nitrogen and sulfur to ecosystems that 

has been shown to adversely affect 

sensitive aquatic and terrestrial 

ecosystems, and/or particles that 

contribute to materials damage and 

soiling. The Administrator notes that 

setting the secondary PM standards 

identical to the revised suite of primary 

standards directionally improves the 

level of protection afforded vegetation, 

ecosystems, and materials. In addition, 

the Administrator continues to believe 

that the secondary NAAQS should be 

considered in conjunction with the 

protection afforded by other programs 

intended to address various aspects of 

air pollution effects on ecosystems and 

vegetation, such as the acid deposition 

program and other regional approaches 

to reducing pollutants linked to nitrate 

or acidic deposition. 

Based on the above considerations, 

the Administrator concludes that it is 

appropriate to address the other welfare 

effects summarized in this section by 

revising the current suite of PM2.5

secondary standards, making them 

identical in all respects to the suite of 

primary PM2.5 standards, while 

retaining the current 24-hour PM10

secondary standard and revoking the 

current annual PM10 secondary 

standard. For the reasons noted in 

section III.D.1 above, the 24-hour PM10

standard will provide adequate 

protection against the known and 

potential effects related to long-term 

PM10 concentrations. 

C. Final Decisions on Secondary PM 

Standards 

For the reasons discussed above, and 

taking into account the information and 

assessments presented in the Criteria 

Document and Staff Paper, the advice 

and recommendations of CASAC, and 

public comments received on the 

proposal, the Administrator is revising 

the current secondary PM standards by 

making them identical in all respects to 

the suite of primary PM standards, as 

revised by today’s action. In the 

Administrator’s judgment, these 

standards, in conjunction with the 

regional haze program, will provide 

appropriate protection to address PMrelated welfare effects, including 

visibility impairment, effects on 

vegetation and ecosystems, materials 

damage and soiling, and effects on 

climate change. 

V. Interpretation of the NAAQS for PM 

This section presents EPA’s final 

decisions regarding the revision, 

addition, and/or revocation of 

appendices to 40 CFR Part 50 on 

interpreting the primary and secondary 

NAAQS for PM. 

A. Amendments to Appendix N—

Interpretation of the National Ambient 

Air Quality Standards for PM2.5

The EPA proposed to revise the data 

handling procedures in appendix N to 

40 CFR Part 50 for the annual and 24- 

hour PM2.5 standards (71 FR 2685–

2686). The proposed amendments to 

appendix N detailed the computations 

necessary for determining when the 

proposed primary and secondary PM2.5

NAAQS were met. The proposed 

amendments also addressed data 

reporting, monitoring considerations, 

and rounding conventions. Key 

elements of the proposed revisions to 

appendix N were presented in section V 

of the preamble to the proposed rule 

and are summarized below, together 

with EPA’s final decisions on revisions 

to appendix N. 

1. General 

As proposed, EPA is adding several 

new definitions to section 1.0 and using 

these definitions throughout the 

appendix, most notably ones for ‘‘design 

values.’’ Also, the 24-hour sampling 

timeframe has been clarified as 

representing ‘‘local standard (word 

inserted) time.’’ This revision reflects 

EPA’s previous intent as well as 

majority practice, and also avoids 

ambiguity since local clock time varies 

according to daylight savings periods. 

No opposing comments were received 

on these changes. 

2. PM2.5 Monitoring and Data Reporting 

Considerations 

As proposed, two new sections are 

being added to appendix N to more 

specifically stipulate and highlight 

monitoring and data considerations (71 

FR 2685). New section 2.0 includes 

statistical requirements for spatial 

averaging (which is part of the form of 

the annual standard for PM2.5). As 

discussed in section II.F.2 above, EPA is 

tightening two of the constraints on the 

use of spatial averaging to provide an 

adequate margin of safety to susceptible 

subpopulations by reflecting enhanced 

knowledge of typical monitor 

relationships in metropolitan areas. 

New section 3.0 to appendix N 

codifies aspects of raw data reporting 

and raw data time interval aggregation 

including specifications of number of 

decimal places. Previously, these 

reporting instructions resided only in 

associated guidance documents. Section 

3.0 also notes the process for 

assimilating monitored concentration 

data from collocated instruments into a 


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single ‘‘site’’ record; data for the site 

record would originate mainly from the 

designated ‘‘primary’’ monitor at the site 

location, but would be augmented with 

collocated Federal reference method 

(FRM) or Federal equivalent method 

(FEM) monitor data whenever valid data 

are not generated by the primary 

monitor. This procedure will enhance 

the opportunity for sites to meet data 

completeness requirements. This 

language likewise codifies existing 

practice, since the technique was 

previously documented in guidance 

documentation and implemented as 

EPA standard operating procedure. 

Commenters agreed that this was a valid 

approach and should be implemented. 

3. PM2.5 Computations and Data 

Handling Conventions 

As proposed, EPA is maintaining a 

spatially-averaged annual mean, with 

revisions to the criteria for when spatial 

averaging can be used (see section 1 

above, as well as section II.E.2), as the 

form of the annual PM2.5 standard and 

is retaining a 98th percentile 

concentration as the form of the 24-hour 

PM2.5 standard. Although no actual 

computational change was proposed for 

a spatially-averaged annual mean, the 

proposed Appendix N differentiated, in 

language and formulae, between a 

spatial average of more than one site 

and a spatial average of only one site. 

We are adopting these changes 

throughout Appendix N as appropriate 

to alleviate confusion caused by the 

current ‘‘catch-all’’ generic reference 

(i.e., ‘‘spatial average’’ or ‘‘spatially 

averaged’’) found throughout the 

existing Appendix N. 

As proposed, appendix N identifies 

the NAAQS metrics and explains data 

capture requirements and comparisons 

to the standards for the annual PM2.5

standard and the 24-hour standard (in 

sections 4.1, and 4.2, respectively); data 

rounding conventions (in section 4.3); 

and formulas for calculating the annual 

and 24-hour metrics (in sections 4.4 and 

4.5, respectively). A significant 

comment related to the 98th percentile 

formula and an associated bias for 

periodic sampling is discussed above in 

section II.E.1. 

With regard to the annual PM2.5

standard, EPA proposed to retain 

current data capture requirements with 

two exceptions. The current appendix N 

had reduced data capture requirements 

for years that exceeded the level of the 

annual NAAQS; specifically, a 

minimum of 11 valid samples per 

quarter as opposed to a more stringent 

75 percent (of scheduled samples) was 

considered sufficient in those instances 

where the annual mean exceeded the 

NAAQS level. See existing Part 50 App. 

N 2.1 (b). The EPA proposed to also 

allow 11 or more samples per quarter as 

an acceptable minimum if the 

calculated annual standard design value 

exceeds the level of the standard. The 

intent of this change was to prevent a 

site with a violating design value that is 

made up of one (or more) annual means 

under the level of the NAAQS from not 

being used for regulatory purposes just 

because one (or more) of the quarters of 

the year(s) under the NAAQS level has 

less than 75% data capture. One 

commenter voiced a general concern 

over the lack of uniformity in 

completeness criteria but the other 

commenters supported the change. 

Taking these comments into 

consideration, EPA is revising appendix 

N as proposed with regard to this issue. 

A second proposed change in the data 

completeness requirements would 

incorporate data substitution logic for 

situations where the proposed 11 

samples per quarter minimum is not 

met. Consistent with existing guidance 

and practice (implementing current 

App. N 2.1 (c)), EPA proposed to 

incorporate the following requirement 

into appendix N: a quarter with less 

than 11 samples would be complete and 

valid if, by substituting an historically 

low 24-hr value for the missing samples 

(up to the 11 minimum), the results 

yield an annual mean, spatially 

averaged annual mean, and/or annual 

standard design value that exceeds the 

level of the standard. The EPA proposed 

to implement this procedure for making 

comparisons to the NAAQS and not to 

permanently alter the reported data. The 

EPA considered this a very conservative 

means of imputing data (and increasing 

the opportunities for using monitoring 

data that otherwise are valid), but 

solicited comment on the proposed 

approach. Several comments were 

received on this approach and the 

majority favored it. However, two 

commenters (NESCAUM and a 

constituent State) suggested a limit of 

one quarter (out of the 12 in a 3-year 

period) where the substitutions could be 

made. They suggested the limitation 

because they were concerned that the 

absence of a significant amount of data 

is an indication that site operator and/ 

or equipment problems exist. The EPA 

shares this concern but observes that the 

method protocol itself guards against 

excessive utilization. The more missing 

values that are potentially substituted 

with the method effectively reduce the 

chance of a valid result (i.e., a usable 

design value). Taking these comments 

into consideration, EPA is revising 

appendix N as proposed with regard to 

this issue. 

With regard to the 24-hour PM2.5

standard, EPA proposed to revise 

appendix N to include a special formula 

(Equation 6 in the proposed rule, 71 FR 

2702) for computing annual 98th 

percentile values when a site operates 

on an approved seasonal sampling 

schedule. This formula was previously 

stated only in guidance documentation 

(EPA, 1999) but was utilized, where 

appropriate, in official OAQPS design 

value calculations. No adverse 

comments were received on this 

addition. 

The proposed revisions to appendix N 

also incorporated language explicitly 

stating that 98th percentiles (for both 

regular and seasonal sampling 

schedules) were to be based on the 

applicable number of samples rather 

than the actual number of samples. The 

EPA proposed that both annual 98th 

percentile equations (proposed 

Equations 5 and 6) would reflect this 

approach. The EPA acknowledges that it 

made an error in the placement of the 

‘‘applicable number of samples’’

references into the denominator of the 

special seasonal 98th percentile formula 

(Equation 6) and has restored the 

equation to its original form. The EPA 

notes that the special season formula 

already takes into consideration 

oversampling in low periods. 

Furthermore, because the ‘‘applicable 

number of samples’’ was removed from 

the seasonal formula, there was no need 

to stipulate that ‘‘seasons’’ could not 

divide months; that proposed 

requirement was only necessary to 

accommodate the calculation of 

‘‘applicable number.’’

The EPA solicited comment on the 

‘‘applicable number of samples’’

concept and calculation and received 

several comments on the concept. One 

commenter endorsed it without 

discussion, one commenter did not 

object to it but noted that it was difficult 

to program, and another commenter 

thought that the concept unnecessarily 

complicates matters and favored the use 

of ‘‘scheduled number of samples’’

instead. Two commenters said that it 

would be an acceptable approach if it 

still permitted ‘‘extra’’ sampling at the 

end of a month to make up for missed 

samples. The EPA notes that it has 

never endorsed this ‘‘extra’’ sampling 

practice for the 24-hour PM2.5 standard, 

so that the commenter’s premise is 

incorrect. The EPA agrees with 

comments that expressed concerns 

about this calculation being too 

complicated and, therefore, has 

simplified the procedure in a manner 

that corresponds to the calculation of 


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89EPA will answer all comments raising 

substantive issues relating to the natural events 

policy when it finalizes the pending exceptional 

events proposal. 

data capture. The applicable number of 

samples for a given year is now defined 

as simply the sum of the number of 

completed scheduled (‘‘creditable’’) 

samples for the year. The new appendix 

N defines the new term, ‘‘creditable’’

and describes its use in calculating data 

capture rates and ‘‘applicable number.’’

For sites that sample correctly (i.e. don’t 

oversample at the end of the month), the 

simpler ‘‘applicable number’’ procedure 

will produce the same result as the 

proposed calculation. 

To simplify the regulatory language, 

as proposed, EPA is revising appendix 

N to eliminate the equation 

computational examples. The EPA will 

provide extensive computational 

examples in forthcoming guidance 

documents. 

4. Conforming Revisions 

As proposed, EPA is revising 

terminology and data handling 

procedures associated with exceptional 

events to conform to rules which EPA 

proposed to implement the recent 

amendment to CAA section 319 (42 

U.S.C. 7619) by section 6013 of the Safe, 

Accountable, Flexible Efficient 

Transportation Equity Act: A Legacy for 

Users (SAFETEA–LU) (Pub. L. 109–59). 

The EPA proposed rules to address 

exceptional events on March 10, 2006 

(71 FR 12592). The EPA is replacing the 

term currently used in appendix 

N.1(b)—uncontrollable or natural 

events—with ‘‘exceptional events,’’

corresponding with the term used in the 

recent amendment. (Because this 

revision makes only a semantic change 

to existing appendix N, EPA believes 

the change is consistent with section 

6013(b)(4) of SAFETEA–LU, which 

provided that EPA continue to apply 

existing appendix N of part 50 (among 

others) until the effective date of rules 

implementing the exceptional event 

provisions in amended section 319 of 

the CAA.)89

B. Proposed Appendix P—Interpretation 

of the National Ambient Air Quality 

Standards for PM10–2.5

The EPA proposed to add appendix P 

to 40 CFR Part 50 in order to add data 

handling procedures for the proposed 

24-hour PM10–2.5 standard. Since the 

current 24-hour PM10 standard is being 

retained and a PM10–2.5 standard is not 

being implemented, the proposed new 

appendix P (on interpreting the 

proposed 24-hour PM10–2.5 standard) is 

not being added. 

C. Amendments to Appendix K—

Interpretation of the National Ambient 

Air Quality Standards for PM10

Because the Administrator has 

decided to retain the current 24-hour 

PM10 standard but to revoke and not 

replace the annual PM10 standard, some 

changes are required to appendix K to 

40 CFR Part 50 on interpreting the 

primary and secondary NAAQS for 

PM10. The modifications principally 

entailed simply removing the obsolete 

annual standard related sections. 

However some typographical 

corrections were also made to some of 

the remaining sections related to the 24- 

hour standard; a spelling error was 

corrected and certain equal signs (=) 

were changed to plus signs (+) in the 

illustrative examples found in section 3 

of the appendix in order to correct 

obvious mistakes in arithmetic. For 

readers’ convenience, EPA is reprinting 

the entire Appendix K in the rule 

section of this notice, but is not 

reopening or reconsidering any parts of 

the Appendix except those discussed 

above. 

VI. Reference Methods for the 

Determination of Particulate Matter as 

PM10–2.5 and PM2.5

A. Appendix O to Part 50—Reference 

Method for Determination of Coarse 

Particulate Matter as PM10–2.5 in the 

Atmosphere 

The EPA proposed a new reference 

method (FRM) for measuring mass 

concentrations of coarse particles 

(PM10–2.5) in ambient air as a new 

Appendix O to 40 CFR part 50.71 FR 

2703. Although this method can fulfill 

a variety of PM monitoring objectives, 

its primary purpose is to serve as the 

standard of comparison for determining 

the adequacy of alternative ‘‘equivalent’’

methods for use in lieu of the FRM. Id. 

at 2687–88. In conjunction with 

additional analysis, this method may be 

used to develop speciated data. The 

EPA expects to designate such 

alternative methods as equivalent 

methods (FEMs) under revised 

provisions of 40 CFR part 53, published 

elsewhere in today’s Federal Register. 

The EPA is finalizing the FRM for 

PM10–2.5, even though a NAAQS for 

PM10–2.5 is not being adopted. An 

official FRM will be an important 

element in facilitating consistent 

research on PM10–2.5 air quality and 

health effects and in promoting the 

commercial development of FEMs. In a 

separate final rule amending 40 CFR 

part 58 elsewhere in today’s Federal 

Register, the EPA is promulgating a 

requirement that States deploy about 60 

FRM or FEM PM10–2.5 monitors as part 

of a new National Core (NCore) multipollutant monitoring stations. The EPA 

also plans to negotiate with some States 

for additional NCore stations which 

would include PM10–2.5 monitors. 

The PM10–2.5 reference method is a 

difference method based on separate, 

concurrent measurements of PM10 and 

PM2.5, with the PM10–2.5 measurement 

being the result of subtraction of the 

PM2.5 measurement from the 

corresponding PM10 measurement. The 

24-hour integrated measurements are 

based on conventional, low-volume 

filter samples of particulate matter 

analyzed gravimetrically after a period 

of moisture and temperature 

equilibration. Although the component 

PM10 and PM2.5 filter samples can be 

subsequently analyzed chemically, no 

actual, physically separated PM10–2.5

sample is produced by the method for 

chemical species analysis. The EPA 

anticipates that one or more alternative 

methods that do provide PM10–2.5

samples that are completely or nearly 

completely separated physically for 

species analysis (such as the 

dichotomous sampler method) will 

become available as an FEM. 

The substantial advantages of the 

method and the rationale for its 

selection as the FRM for PM10–2.5 are 

discussed in the proposal (71 FR 2687). 

In that discussion, EPA acknowledges 

that the method does not provide a 

direct measurement of PM10–2.5, has 

some significant shortcomings, and 

likely will not ideally meet all needs for 

monitoring PM10–2.5 in the ambient air. 

The EPA indicated that although the 

method is readily usable in routine 

monitoring networks, it is clearly less 

than optimally suited for such use. 

Instead, EPA expects that alternative 

FEMs that typically offer some 

substantial advantage or advantages 

over the FRM will become the principle 

methods deployed for routine 

monitoring. Further, EPA anticipates 

that self-contained, automated FEMs 

will become available to provide near 

real-time, hourly monitoring data 

availability and ease the monitoring 

burdens of monitoring agencies. 

Although the FRM will likely be used 

initially in monitoring applications 

because of its conventional nature and 

similarity to the widely used PM2.5

FRM, ultimately its principle purpose 

will be as the standard of reference for 

determining the adequacy of alternative, 

candidate FEMs and for assessing the 

quality of PM10–2.5 monitoring data 

obtained in monitoring networks, 

particularly networks using alternative 

FEMs. The FRM may thus be used on 

a voluntary basis by states wishing to 

deploy PM10–2.5 monitors prior to the 


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January 1, 2011 deadline for operation 

of PM10–2.5 monitors at NCore multipollutant sites (a requirement of the 

final rule amending 40 CFR part 58, 

elsewhere in today’s Federal Register), 

although many of the required monitors 

operating at NCore sites in 2011 and 

beyond may be FEMs. 

After considering alternative 

methodologies and weighing the various 

pros and cons of other methods, as also 

discussed in the proposal preamble, the 

EPA concluded that the proposed 

method is the best method currently 

available to serve these purposes, while 

also being readily usable for many 

initial monitoring applications. The 

Ambient Air Monitoring and Methods 

Subcommittee of the Clean Air 

Scientific Advisory Committee (CASAC) 

concurs with this assessment and 

approach, recommending that EPA 

adopt the difference method as the 

FRM, but that it ultimately be used 

primarily as a benchmark for evaluating 

the performance of continuous as well 

as other direct-measuring filter-based 

integrated methods (Henderson, 2005c). 

Of the relatively few comments 

received on the proposed FRM, most 

raised concern about some of the same 

shortcomings of the method that had 

already been considered by EPA in 

selecting the method (and by the 

CASAC in concurring with EPA’s 

approach). No comments presented any 

issues that resulted in any changes to 

the method. Thus, the FRM is being 

promulgated today (in Appendix O), 

with the only change being deletion of 

the reference to national ambient air 

quality standards in section 1.1 of the 

method, since the EPA is not using 

PM10–2.5 as the indicator in the NAAQS 

addressing thoracic coarse particles. 

One comment raised concern about 

the relationship of the new PM10–2.5

FRM to the requirements of Section 

6012 of the SAFETEA–LU, under which 

the EPA is to ‘‘develop a Federal 

reference method to measure directly 

particles that are larger than 2.5 

micrometers in diameter without 

reliance on subtracting from coarse 

particle measurements those particles 

that are equal to or smaller than 2.5 

micrometers in diameter.’’ As discussed 

in the proposal preamble at 71 FR 2690, 

EPA believes that this FRM does not 

conflict with either the specific 

language or intent of the SAFETEA–LU 

Act. The new FRM, together with the 

additions to part 53 (published 

elsewhere in this Federal Register) that 

will allow designation of FEMs for 

monitoring PM10–2.5, will provide a 

strong incentive to stimulate the further 

commercial development and 

refinement of new or existing methods 

for PM10–2.5, most of which will not rely 

on subtraction of fine mode particle 

measurements from coarse mode 

particle measurements. Further, EPA is 

actively investigating the possibility that 

a dichotomous-based method might 

ultimately provide a more direct means 

of measuring the coarse fraction of 

PM10. Within the time frame prescribed 

by the SAFETEA–LU, it appears very 

likely that at least one such method will 

be shown to achieve an adequate level 

of performance and may therefore be 

identified and utilized as a ‘‘reference 

method’’. The terms of the SAFETEA–

LU Act do not require that the Agency 

promulgate a non-difference method as 

either the sole FRM or as an alternative 

FRM as specifically defined in part 53. 

Until such a new, more direct method 

is demonstrated to be suitable and 

adequate and becomes commercially 

available, the difference-based FRM of 

Appendix O provides a reliable, proven 

measurement method which can be 

successfully implemented immediately. 

The CASAC agreed that none of the 

direct sampling methods is presently 

sufficiently reliable for use as an FRM, 

Henderson, 2005c, but that suitable 

direct measurement methods could be 

developed quickly enough to become 

approved as equivalent methods in a 

planned monitoring network. 

The salient technical aspects of the 

FRM are provided in the proposal 

preamble (71 FR 2690). The dual 

samplers specified in the FRM are 

essentially identical to the sampler 

specified in the PM2.5 FRM (40 CFR part 

50, appendix L) except for removal of 

the PM2.5 WINS impactor particle 

separator from the sampler used for 

PM10. Operational procedures and most 

other aspects are also similar or 

identical to those for the PM2.5 FRM. 

One notable condition is that the PM10

sampler of the PM10–2.5 FRM must meet 

the higher standards of performance and 

manufacture of appendix L rather than 

the somewhat lesser requirements for 

conventional PM10 samplers in 40 CFR 

part 50, appendix J. Thus, conventional 

PM10 FRM samplers will not be 

acceptable for use as part of a PM10–2.5

FRM sampler pair. But both the PM10

and PM2.5 component measurements 

obtained incidental to PM10–2.5

measurements would be valid as PM10

or PM2.5 measurements under the 

monitoring requirements of 40 CFR part 

58, provided they are sited at the 

appropriate spatial scale. However, 

since such PM10 samplers meet higher 

standards of performance than 

conventional PM10 samplers, the 

measurements need to be differentiated 

from conventional PM10 measurements 

(e.g. by a descriptor such as PM10c). 

Also, conventional PM10 measurements 

are reported based on standard 

temperature and pressure, whereas 

PM10c measurements are reported based 

on actual local conditions of 

temperature and pressure. 

The EPA designation of specific, 

commercial candidate PM10–2.5 FRM 

samplers will be based on an 

application and on consideration in 

accordance with new or revised 

provisions of 40 CFR part 53, published 

elsewhere in this Federal Register. 

Since PM2.5 FRM samplers have been in 

use for several years and are readily 

available, EPA designation of PM10–2.5

FRM sampler models based on one or 

more currently available PM2.5 sampler 

models is expected to occur soon after 

promulgation. The two samplers of the 

PM10–2.5 FRM sampler pair would be 

required to be of the same make and 

model and matched design and 

fabrication so that they are essentially 

identical (except that one would not 

have a PM2.5 particle separator). The 

samplers may be of either single-filter or 

multiple-filter (sequential-sample) 

design, as long as both are of the same 

type, design, and configuration. For a 

commercial sampler that has already 

been designated as a PM2.5 FRM, no 

further testing under part 53 would be 

required for designation as a PM10–2.5

FRM, although the sampler 

manufacturer would have to submit a 

formal, brief application under part 53. 

Users may assemble their own PM10–2.5

sampler pair using existing PM2.5

samplers of matched model or design by 

converting one of the samplers to a 

PM10c sampler, provided that the 

specific sampler pair has been 

previously designated by the EPA as a 

PM10–2.5 FRM under part 53. 

A PM2.5 sampler pair consisting of 

samplers that are slightly dissimilar or 

have some minor design or model 

variations (and one sampler is 

configured as a PM10c sampler) may be 

considered for designation by EPA as a 

Class I FEM under revised part 53. An 

application for an FEM determination 

would need to be submitted under part 

53, and some supplemental or special 

tests may be required. Also, a pairing of 

slightly dissimilar samplers that has not 

been designated by EPA as an FRM or 

Class I FEM may be considered for 

approved use in PM10–2.5 monitoring 

networks as a user-modification of an 

FRM under section 2.8 of appendix C to 

40 CFR part 58. 


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90These comments and EPA’s responses to the 

issues raised by commenters are discussed in 

greater detail in the Response to Comments 

document. 

B. Amendments to Appendix L—

Reference Method for the Determination 

of Fine Particulate Matter (as PM2.5) in 

the Atmosphere 

In connection with the proposal of a 

new FRM for PM10–2.5, the EPA also 

proposed (71 FR 2691) minor technical 

changes to the FRM for PM2.5 (40 CFR 

Part 50, appendix L). EPA is adopting 

these changes as proposed. These 

changes are to provide improvements in 

the efficiency of the method in 

monitoring network operations without 

altering the method’s performance. 

The most significant change is the 

addition of an alternative PM2.5 particle 

size separator, specifically, a very sharp 

cut cyclone (VSCCTM) manufactured by 

BGI Incorporated, Waltham, MA. FRM 

samplers now may be configured with 

either the original WINS impactor or the 

alternative cyclone separator, and 

existing FRM samplers may be 

retrofitted by users with the cyclone, if 

desired. Sampler users wishing to 

retrofit their samplers should contact 

the sampler manufacturer to obtain the 

correct BGI VSCCTM model along with 

the associated installation, operation, 

and maintenance instructions specific to 

the sampler model, and a new 

designated method label to be attached 

to the sampler. The seven sampler 

models configured with the BGI 

VSCCTM that have been designated as 

FEMs will be re-designated as reference 

methods, and owners of such sampler 

should contact the sampler 

manufacturer to receive a new reference 

method label for the sampler. 

Another change is substitution of an 

improved type of impactor oil for the 

original PM2.5 WINS particle size 

separator to correct an occasional coldweather performance issue with the 

originally specified oil. Finally, minor 

increases in the time limits for sample 

retrieval and sample weighing were 

proposed, as were minor reductions in 

the sampler data output reporting 

requirements. Justifications for these 

changes are discussed in the proposal 

preamble. Of the very few comments 

received in connection with these 

proposed changes, all were supportive. 

Accordingly, the changes are adopted as 

proposed. 

VII. Issues Related to Implementation 

of PM10 Standards 

Issues related to implementation of 

the NAAQS are not relevant to the 

Administrator’s decisions regarding 

whether it is appropriate to set or revise 

a standard. For this reason, EPA has not 

addressed implementation-related 

issues in preceding sections, nor has it 

addressed public comments regarding 

implementation. The EPA identified 

issues regarding transition to or 

implementation of the standards 

promulgated in this rule in an advance 

notice of proposed rulemaking (ANPR) 

on Transition to New or Revised 

Particulate Matter National Ambient Air 

Quality Standards (71 FR 6718–6729, 

February 9, 2006). In the ANPR, EPA 

solicited comment on a wide range of 

issues related to both the fine and coarse 

particle NAAQS, including the 

schedules for implementation of these 

standards and the requirements that 

would be applicable if any PM NAAQS 

were revoked. The public comment 

period for the ANPR ended on July 10, 

2006. The EPA is currently reviewing 

the public comments received. In the 

near future, EPA intends to address, as 

necessary, issues such as designations, 

conformity, and new source review, 

related to implementation of today’s 

final rule. In this section, EPA 

highlights a few issues that may arise as 

an immediate consequence of today’s 

final decision to retain the 24-hour PM10

standards but revoke the annual PM10

standards, and restates existing policies 

and practices to address several 

concerns raised by commenters. 

A. Summary of Comments Received on 

Transition 

Many commenters, particularly State 

and local air pollution control agencies 

and Tribes, but also environmental and 

public health groups, voiced strong 

concerns about EPA’s proposal to 

revoke current annual PM10 standards 

everywhere upon promulgation of this 

final rule, and to revoke, upon 

finalization of a primary 24-hour 

standard for PM10–2.5, the current 24- 

hour PM10 standard everywhere except 

in 15 large urbanized areas (with 

population greater than 100,000) that 

have at least one monitor violating the 

24-hour PM10 standard based on the 

most recent three years of air quality 

data. For these few areas, EPA proposed 

to retain the 24-hour PM10 standard 

until designations were completed 

under a final 24-hour PM10–2.5 standard. 

While a few local government 

commenters recommended that one or 

another of the 15 areas be dropped from 

this list—i.e., recommended that the 24- 

hour PM10 standard should be retained 

in fewer locations—most commenters 

expressing views on transition 

suggested that EPA was being too hasty 

in dismantling existing PM10

protections. Pointing to long delays in 

the implementation timeline for the 

1997 PM2.5 standards due to litigation, 

such that designations were not 

completed for eight years after 

promulgation of the final rule, these 

commenters suggested that the 24-hour 

PM10 standard should remain in place 

everywhere until designations were 

complete under the 24-hour PM10–2.5

standard, or even until PM10–2.5 SIPs had 

been submitted by States. Some Tribal, 

State and local commenters suggested 

that the PM10 standard should be 

retained permanently in all areas where 

the PM10–2.5 standard did not apply by 

virtue of the monitoring requirements, 

which limited NAAQS comparable 

monitors to sites that met the five-point 

site suitability test outlined in the 

monitoring rule. Other commenters 

maintained that EPA has no authority to 

revoke the PM10 standards or the 

specific pollution controls mandated in 

Title I Subpart 4 for PM10 nonattainment 

areas.90

The EPA notes that the 

Administrator’s decision to retain the 

current 24-hour PM10 standard 

alleviates these concerns. Because the 

24-hour PM10 standard is generally 

controlling, as described above in 

section III.D.2, retention of this standard 

ensures the continuation of existing 

public health protections. The EPA 

further believes that it has the legal 

authority to revoke the annual PM10

standard, and addresses this issue in 

detail in the Response to Comments 

document. 

B. Impact of Decision on PM10

Designations 

The EPA notes that because it is 

retaining the current 24-hour PM10

standards, new nonattainment 

designations for PM10 will not be 

required under the provisions of the 

Clean Air Act. As established in Section 

107(d)(1) of the Act, the only time EPA 

is obligated to designate areas as 

attainment or nonattainment is after it 

promulgates or revises a NAAQS. Under 

an existing standard, all redesignations 

are at the Administrator’s discretion: 

EPA has no legal obligation to 

redesignate an area even if a monitor 

should register a violation of that 

standard (see CAA Section 107(d)(3)). 

Thus, this final decision does not affect 

existing PM10 nonattainment 

designations. This is consistent with 

past practice. For example, when EPA 

decided not to revise the ozone 

standards in 1993 or the SO2 standards 

in 1996, it did not revisit prior 

designations or designate any new areas 

as nonattainment. The EPA does regard 

air quality violations seriously, and does 

expect States to take actions to reduce 


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91 In addition, EPA notes that the Agency’s 

National Center for Environmental Research 

recently issued a Request for Proposals on 

‘‘Sources, Composition, and Health Effects of 

Coarse Particulate Matter’’ which is designed to (1) 

improve understanding of the type and severity of 

health outcomes associated with exposure to 

PM10–2.5; (2) improve understanding of 

subpopulations that may be especially sensitive to 

PM10–2.5 exposures including minority populations, 

highly exposed groups, and other susceptible 

groups; (3) characterize and compare the influence 

of mass, composition, source characteristics and 

exposure estimates in different locations and 

differences in health outcomes, including 

comparisons in rural and urban areas; and (4) 

characterize the composition and variability of 

PM10–2.5 in towns, cities or metropolitan areas, 

including comparisons of rural and urban areas. 

air quality to healthy levels in any areas 

that are experiencing violations. 

However, EPA recognizes that there are 

other ways to address such violations 

besides redesignating an area as 

nonattainment. For example, EPA can 

work directly with a State and nearby 

industries to take appropriate actions to 

reduce emissions that are contributing 

to the violation. The EPA has worked in 

this way with States in the past. Of 

course, States may request redesignation 

of an area, either from nonattainment to 

attainment, or from attainment to 

nonattainment, based on the most recent 

air quality data available, if they choose 

to do so. In addition, both transportation 

and general conformity will continue to 

apply to all PM10 nonattainment and 

maintenance areas since no designations 

are changing. However, because EPA is 

revoking the annual PM10 standard in 

this final rule, after the effective date of 

this rule conformity determinations in 

PM10 areas will only be required for the 

24-hour PM10 standard; conformity to 

the annual PM10 standard will no longer 

be required. The EPA will address 

specific conformity issues related to the 

revocation of the annual PM10 standard 

either in future guidance or in another 

public document. The EPA also notes 

that PSD increments and baseline years 

will not be affected by this decision. 

The EPA is retaining the current 24- 

hour PM10 standards and revoking the 

annual PM10 standards. Today’s rule 

does not change any existing guidance 

related to the PM10 NAAQS as it applies 

to the 24-hour PM10 standards, and to 

the extent that modifications to the 

existing guidance are needed in 

response to today’s action, EPA will 

make such modifications in the near 

future. 

As described in the revisions to Part 

53/58 appearing elsewhere in today’s 

Federal Register, EPA believes a 

reduction in the size of the existing 

monitoring networks for certain 

pollutants, including PM10, for which 

the large majority of monitors record no 

NAAQS violations, is appropriate as a 

way to free up resources for higher 

priority monitoring objectives. The 

current minimum PM10 network 

requirements are based on the 

population of a metropolitan statistical 

area (MSA) and its historical PM10 air 

quality. This focus on larger urban areas 

is consistent with EPA’s belief that it is 

appropriate to target an indicator for 

thoracic coarse particles toward urban 

and industrial areas, where the ambient 

mix of thoracic coarse particles is 

dominated by emissions from particular 

types of sources. See sections III.C.2 and 

III.C.3 above. To the extent that States 

and Tribes are considering reducing the 

total number of PM10 monitors 

deployed, EPA believes, consistent with 

the basis for retaining the 24-hour PM10

standard, that priority should be given 

to maintaining monitors sited in urban 

and industrial areas. 

In addition, if States and Tribes are 

considering deploying new PM10

monitors, EPA recommends, again 

consistent with the basis for retaining 

the 24-hour PM10 standard, that those 

monitors be placed in areas where there 

are urban and/or industrial sources of 

thoracic coarse particles. Furthermore, 

consistent with the monitors used in 

studies that informed the 

Administrator’s decision on the level of 

the standard (see section III.D above), 

EPA recommends that any new PM10

monitors be placed in locations that are 

reflective of community exposures at 

middle and neighborhood scales of 

representation, and not in sourceoriented hotspots. 

As summarized briefly above in 

section III.E and described in detail in 

section V.E.1 of the monitoring rule 

published elsewhere in today’s Federal 

Register, EPA is also establishing 

requirements for a new multi-pollutant 

monitoring network that will include 

approximately 75 PM10–2.5 monitors that 

will speciate according to the 

composition as well as size of the 

particles. These speciated PM10–2.5

monitors are a critical part of EPA’s 

research program on coarse particles, 

and will be sited in both urban and rural 

locations. It is EPA’s expectation that 

these monitors will help alleviate the 

current deficit of information regarding 

the public health impacts of PM10–2.5

mixes in different locations.91

C. Impact of Decision on State 

Implementation Plans (SIPs) and 

Control Obligations 

The EPA’s decision today to retain the 

PM10 NAAQS does not establish new 

legal obligations beyond those that 

already exist. Specifically, this final rule 

does not obligate States to revise SIPs or 

to create new obligations to control 

particular sources. In response to 

comments regarding potential impacts 

of any coarse particle standard on 

agricultural and mining sources, EPA 

notes that the NAAQS do not create 

emissions control obligations for 

individual sources or groups of sources. 

In this particular case, even if an 

individual source were shown to 

contribute to an exceedance of the 24- 

hour PM10 standard, this would not 

necessarily result in regulation of that 

source. Decisions about which sources 

to control are generally made by the 

State in the context of developing or 

revising SIPs. Given that the available 

evidence regarding adverse health 

effects associated with exposure to 

thoracic coarse particles is strongest 

with respect to urban and industrial 

ambient mixes of those particles, EPA 

encourages States to focus control 

programs on urban and industrial 

sources to the extent that those sources 

are contributing to air quality violations. 

This would help to ensure that 

resources expended on implementing 

the 24-hour PM10 standard realize the 

maximum public health and welfare 

benefits. 

With regard to emissions of thoracic 

coarse particles from agricultural 

sources, EPA recognizes that the United 

States Department of Agriculture 

(USDA) has been working with the 

agricultural community to develop 

conservation systems and activities to 

control coarse particle emissions. Based 

on current ambient monitoring 

information, these USDA-approved 

conservation systems and activities have 

proven to be effective in controlling 

these emissions in areas where coarse 

particles emitted from agricultural 

activities have been identified as a 

contributor to violation of the NAAQS. 

The EPA concludes that where USDAapproved conservation systems and 

activities have been implemented, these 

systems and activities have satisfied the 

Agency’s reasonably available control 

measure and best available control 

measure requirements. The EPA 

believes that in the future, when 

properly implemented, USDA-approved 

conservation systems and activities 

should satisfy the requirements for 

reasonably available control measures or 

best available control measures. The 

EPA will work with States to identify 

appropriate measures to meet their 

RACM or BACM requirements, 

including site-specific conservation 

systems and activities. The EPA will 

continue to work with USDA to 

prioritize the development of new 

conservation systems and activities; 


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demonstrate and improve, where 

necessary, the control efficiencies of 

existing conservation systems and 

activities; and ensure that appropriate 

criteria are used for identifying the most 

effective application of conservation 

systems and activities. 

The EPA does not construe the Clean 

Air Act (CAA) to require that the 

Agency make an independent 

determination as to whether a PSD 

increment is violated in any specific 

State or Tribal reservation. The EPA has 

the discretion to inquire into these 

matters and call for revisions to a State’s 

SIP if an EPA investigation concluded 

with EPA finding that the PSD 

increment is being exceeded. The EPA’s 

regulations at 40 CFR 51.166(a)(3) 

directs a state to make revisions to its 

SIP if EPA or a State finds such an 

exceedance. However, this regulation 

does not require that EPA conduct its 

own investigation and make such a 

finding in all cases where a State has 

completed a periodic review and 

submitted its findings to EPA. Oversight 

of this nature is a matter within EPA’s 

discretion. Likewise, section 110(k)(5) of 

the Clean Air Act does not require that 

EPA periodically investigate and 

determine whether a SIP is sufficient to 

protect the PSD increments. The EPA 

has the discretion to decide when it is 

appropriate to exercise its oversight 

authority and inquire into these issues 

in a specific State or Tribal reservation. 

When EPA exercises this discretion and 

finds an exceedance of the increments 

or another SIP deficiency, EPA is then 

required to issue a SIP call under 

section 110(k)(5) of the CAA. However, 

the CAA affords EPA discretion on 

whether to make a determination that a 

state SIP is deficient. See, New York 

Public Interest Research Group v. 

Whitman, 321 F.3d 316, 331 (2d Cir. 

2003) (considering analogous provision 

of the CAA addressing EPA oversight of 

state Title V operating permit programs). 

D. Consideration of Fugitive Emissions 

for New Source Review (NSR) Purposes 

Under the current NSR regulations, 

for purposes of determining whether a 

stationary source qualifies as a major 

stationary source, that source must 

include fugitive emissions in calculating 

the total amount of a pollutant directly 

emitted, or the potential to emit that 

pollutant, only if the source is 

associated with a source category listed 

by the Administrator pursuant to notice 

and comment rulemaking in accordance 

with Section 302(j) of the Clean Air Act 

(CAA). Agricultural and mining sources 

are generally not among those listed by 

the Administrator. Therefore, fugitive 

emissions from sources in these 

categories are generally not included in 

making major source determinations. 

However, the current NSR regulations 

require that once any source qualifies as 

a major stationary source, that source 

must count all fugitive emissions 

toward determining whether an 

emissions increase results in a major 

modification of that source regardless of 

whether the source is associated with a 

source category listed by the 

Administrator. On July 11, 2003, we 

received a petition for reconsideration 

of the current NSR regulations relating 

to whether fugitive emissions must be 

counted for purposes of determining 

whether a major modification occurs. In 

January 2004, we agreed to reconsider 

this issue, and we expect to propose 

changes to the existing regulations in 

the near future. 

E. Handling of PM10 Exceedances Due to 

Exceptional Events 

The EPA recognizes that PM10

exceedances may be caused, in whole or 

in part, by exceptional events, including 

natural events such as windstorms. In 

some of these instances, the PM10

exceedance(s) may also be associated 

with anthropogenic emissions that 

contribute to total PM10 concentrations. 

Under EPA’s March 2006 Proposed Rule 

on the Treatment of Data Influenced by 

Exceptional Events (71 FR 12592–

12610), and consistent with historical 

practice, an exceedance may be treated 

as an exceptional event even though 

anthropogenic sources such as 

agriculture and mining emissions 

contribute to the exceedance. (EPA’s 

Exceptional Events Rule will be 

finalized in March 2007 and will 

discuss this issue in more detail.) 

VIII. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under section 3(f)(1) of Executive 

Order (EO) 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, EPA submitted this action 

to the Office of Management and Budget 

(OMB) for review under EO 12866 and 

any changes made in response to OMB 

recommendations have been 

documented in the docket for this action 

(Docket ID No. EPA–HQ–OAR–2001–

0017). 

In addition, EPA prepared a 

regulatory impact analysis (RIA) of the 

potential costs and benefits associated 

with this action, entitled ‘‘Regulatory 

Impact Analysis for Particulate Matter 

National Ambient Air Quality 

Standards’’ (September 2006). The RIA 

estimates the nationwide costs and 

monetized human health and welfare 

benefits of attaining two alternatives to 

the current suite of PM2.5 NAAQS (15 

μg/m3 annual, 65 μg/m3 daily). 

Specifically, the RIA compares the 

current standards to the proposed 

alternative of 15 μg/m3 annual, 35 μg/m3

daily and a tighter alternative of 14 μg/ 

m3 annual, 35 μg/m3 daily. The RIA 

contains illustrative analyses that 

consider a limited number of emissions 

control scenarios that States and 

Regional Planning Organizations might 

implement to achieve the 1997 PM2.5

NAAQS and these alternative PM2.5

NAAQS. It calculates the incremental 

costs that might be incurred between the 

base year of 2015, which is the year by 

which States must all be in attainment 

with the 1997 PM2.5 standards (15 μg/m3

annual, 65 μg/m3 daily), and 2020, 

which is the final date by which States 

would implement controls to attain the 

revised PM2.5 standards. 

As discussed above in section I.B, the 

Clean Air Act and judicial decisions 

make clear that the economic and 

technical feasibility of attaining ambient 

standards are not to be considered in 

setting or revising NAAQS, although 

such factors may be considered in the 

development of State plans to 

implement the standards. Accordingly, 

although an RIA has been prepared, the 

results of the RIA have not been 

considered in issuing this final rule. 

B. Paperwork Reduction Act 

This action does not impose an 

information collection burden under the 

provisions of the Paperwork Reduction 

Act, 44 U.S.C. 3501 et seq. There are no 

information collection requirements 

directly associated with revisions to a 

NAAQS under section 109 of the CAA. 

Burden means the total time, effort, or 

financial resources expended by persons 

to generate, maintain, retain, or disclose 

or provide information to or for a 

Federal agency. This includes the time 

needed to review instructions; develop, 

acquire, install, and utilize technology 

and systems for the purposes of 

collecting, validating, and verifying 

information, processing and 

maintaining information, and disclosing 

and providing information; adjust the 

existing ways to comply with any 

previously applicable instructions and 

requirements; train personnel to be able 

to respond to a collection of 

information; search data sources; 

complete and review the collection of 

information; and transmit or otherwise 

disclose the information. 


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An agency may not conduct or 

sponsor, and a person is not required to 

respond to a collection of information 

unless it displays a currently valid OMB 

control number. The OMB control 

numbers for EPA’s regulations in 40 

CFR are listed in 40 CFR part 9. 

C. Regulatory Flexibility Act 

The EPA has determined that it is not 

necessary to prepare a regulatory 

flexibility analysis in connection with 

this final rule. For purposes of assessing 

the impacts of today’s rule on small 

entities, small entity is defined as: (1) A 

small business that is a small industrial 

entity as defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of today’s final rule on small 

entities, EPA has concluded that this 

action will not have a significant 

economic impact on a substantial 

number of small entities. This 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 ATA I at 1044–45 

(NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities). 

D. Unfunded Mandates Reform Act 

Title II of the Unfunded Mandates 

Reform Act of 1995 (UMRA), Public 

Law 104–4, establishes requirements for 

Federal agencies to assess the effects of 

their regulatory actions on State, local, 

and Tribal governments and the private 

sector. Under section 202 of the UMRA, 

EPA generally must prepare a written 

statement, including a cost-benefit 

analysis, for proposed and final rules 

with ‘‘Federal mandates’’ that may 

result in expenditures to State, local, 

and Tribal governments, in the 

aggregate, or to the private sector, of 

$100 million or more in any 1 year. 

Before promulgating an EPA rule for 

which a written statement is needed, 

section 205 of the UMRA generally 

requires EPA to identify and consider a 

reasonable number of regulatory 

alternatives and adopt the least costly, 

most cost-effective or least burdensome 

alternative that achieves the objectives 

of the rule. The provisions of section 

205 do not apply when they are 

inconsistent with applicable law. 

Moreover, section 205 allows EPA to 

adopt an alternative other than the least 

costly, most cost-effective or least 

burdensome alternative if the 

Administrator publishes with the final 

rule an explanation why that alternative 

was not adopted. Before EPA establishes 

any regulatory requirements that may 

significantly or uniquely affect small 

governments, including Tribal 

governments, it must have developed 

under section 203 of the UMRA a small 

government agency plan. The plan must 

provide for notifying potentially 

affected small governments, enabling 

officials of affected small governments 

to have meaningful and timely input in 

the development of EPA regulatory 

proposals with significant Federal 

intergovernmental mandates, and 

informing, educating, and advising 

small governments on compliance with 

the regulatory requirements. 

Today’s final rule contains no Federal 

mandates (under the regulatory 

provisions of Title II of the UMRA) for 

State, local, or Tribal governments or 

the private sector. The rule imposes no 

new expenditure or enforceable duty on 

any State, local or Tribal governments or 

the private sector, and EPA has 

determined that this rule contains no 

regulatory requirements that might 

significantly or uniquely affect small 

governments. Furthermore, as indicated 

previously, in setting a NAAQS 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 ATA I at 1043 (noting that because 

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). Accordingly, EPA has 

determined that the provisions of 

sections 202, 203, and 205 of the UMRA 

do not apply to this final decision. 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, EPA has addressed 

unfunded mandates in the notice that 

announces the revisions to 40 CFR part 

58, and will, as appropriate, address 

unfunded mandates when it proposes 

any revisions to 40 CFR part 51. 

E. Executive Order 13132: Federalism 

Executive Order 13132, entitled 

‘‘Federalism’’ (64 FR 43255, August 10, 

1999), requires EPA to develop an 

accountable process to ensure 

‘‘meaningful and timely input by State 

and local officials in the development of 

regulatory policies that have federalism 

implications.’’ ‘‘Policies that have 

federalism implications’’ is defined in 

the Executive Order to include 

regulations that 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.’’

At the time of proposal, EPA 

concluded that the proposed rule would 

not have federalism implications. The 

EPA stated that the proposed rule would 

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. However, EPA 

recognized that States would have a 

substantial interest in this rule and any 

corresponding revisions to associated 

SIP requirements and air quality 

surveillance requirements, 40 CFR part 

51 and 40 CFR part 58, respectively. 

Therefore, in the spirit of Executive 

Order 13132, and consistent with EPA 

policy to promote communications 

between EPA and State and local 

governments, EPA specifically solicited 

comment on the rule from State and 

local officials at the time of proposal. 

One commenter who opposed EPA’s 

proposed decision on the standards for 

thoracic coarse particles stated that the 

decision violated E.O. 13132. The 

commenter argued that EPA’s proposal 

to replace the PM10 standards with a 

new 24-hour PM10–2.5 standard based on 

a qualified indicator would 

substantially impact CAA section 107 

which establishes that the States have 

primary responsibility for 

implementation of the NAAQS. 

Specifically, the commenter stated that 

the proposed rule language establishing 

that ‘‘agricultural sources, mining 

sources, and other similar sources of 

crustal material shall not be subject to 

control in meeting this standard’’ was a 

clear infringement upon States’

authority with regard to implementation 

of the NAAQS. The EPA notes that in 

light of the final decision to retain the 

PM10 indicator, and the 24-hour PM10

NAAQS, the concern voiced by this 

commenter is no longer relevant. The 

final rule does not exclude any sources 


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from control under the 24-hour PM10

standard. 

Therefore, EPA concludes that this 

final rule 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, 

EPA is mandated to establish NAAQS; 

however, CAA section 116 preserves the 

rights of States to establish more 

stringent requirements if deemed 

necessary by a State. Furthermore, this 

rule does not impact CAA section 107 

which establishes that the States have 

primary responsibility for 

implementation of the NAAQS. Finally, 

as noted above in section E 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 rule. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

Executive Order 13175, entitled 

‘‘Consultation and Coordination with 

Indian Tribal Governments’’ (65 FR 

67249, November 9, 2000), requires EPA 

to develop an accountable process to 

ensure ‘‘meaningful and timely input by 

tribal officials in the development of 

regulatory policies that have tribal 

implications.’’ This rule concerns the 

establishment of PM 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. 

Although EPA determined at the time 

of proposal that Executive Order 13175 

did not apply to this rule, EPA 

contacted tribal environmental 

professionals during the development of 

this rule. The EPA staff participated in 

the regularly scheduled Tribal Air call 

sponsored by the National Tribal Air 

Association during the summer and fall 

of 2005 as the proposal was under 

development, as well as the call in the 

spring of 2006 during the public 

comment period on the proposed rule. 

The EPA sent individual letters to all 

federally recognized Tribes within the 

lower 48 states and Alaska to give Tribal 

leaders the opportunity for consultation, 

and EPA staff also participated in Tribal 

public meetings, such as the National 

Tribal Forum meeting in April 2006, 

where Tribes discussed their concerns 

regarding the proposed rule. 

Furthermore, the Administrator 

discussed the proposed PM NAAQS 

with members of the National Tribal 

Caucus and with leaders of individual 

Tribes during the spring and summer of 

2006, in advance of his final decision. 

During the course of these meetings 

and in written comments submitted to 

the Agency, Tribal commenters 

expressed significant concerns about the 

implications of the proposed rule for 

Tribes. In particular, Tribes strongly 

opposed the proposed qualified PM10–2.5

indicator and the proposed monitor sitesuitability requirements, especially the 

requirement that monitors used for 

comparison with the NAAQS be located 

within urbanized areas with a minimum 

population of 100,000. Tribal 

commenters pointed out that this would 

virtually exclude Tribes from applying 

the PM10–2.5 standards because very few 

Tribal sites would meet this criterion. 

Tribes stated that EPA had violated its 

Trust Responsibility to Tribes in three 

ways. First, the commenters claimed 

that EPA had failed to engage in 

meaningful consultation with Tribal 

leaders regarding the proposed qualified 

PM10–2.5 indicator and other aspects of 

the proposed rule. Second, commenters 

claimed that the proposed 24-hour 

PM10–2.5 standard would have serious 

adverse impacts on the existing level of 

health protection for Tribes. Third, 

Tribal commenters objected to the 

proposed exclusion of ‘‘agricultural 

sources, mining sources, and other 

similar sources of crustal material’’ from 

the proposed PM10–2.5 indicator; like 

States, Tribes felt this provision was 

illegal and Tribal commenters argued 

this violated Tribal sovereignty. The 

EPA notes that its final decision to 

retain the current 24-hour PM10

standard, for the reasons noted above in 

Section III, without any qualifications or 

changes to the monitor siting 

requirements, effectively resolves the 

concerns raised by these commenters. 

EPA has determined that this final 

rule does not have Tribal implications, 

as specified in Executive Order 13175. 

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. Thus, 

Executive Order 13175 does not apply 

to this rule. 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

Executive Order 13045, ‘‘Protection of 

Children from Environmental Health 

Risks and Safety Risks’’ (62 FR 19885, 

April 23, 1997) applies to any rule that: 

(1) Is determined to be ‘‘economically 

significant’’ as defined under Executive 

Order 12866, and (2) concerns an 

environmental health or safety risk that 

EPA has reason to believe may have a 

disproportionate effect on children. If 

the regulatory action meets both criteria, 

the Agency must evaluate the 

environmental health or safety effects of 

the rule on children, and explain why 

the regulation is preferable to other 

potentially effective and reasonably 

feasible alternatives considered by the 

Agency. 

This rule is subject to Executive Order 

13045 because it is an economically 

significant regulatory action as defined 

by Executive Order 12866, and we 

believe that the environmental health 

risk addressed by this action may have 

a disproportionate effect on children. 

The NAAQS constitute uniform, 

national standards for PM pollution; 

these standards are designed to protect 

public health with an adequate margin 

of safety, as required by CAA section 

109. However, the protection offered by 

these standards may be especially 

important for children because children, 

along with other sensitive population 

subgroups such as the elderly and 

people with existing heart or lung 

disease, are potentially susceptible to 

health effects resulting from PM 

exposure. Because children are 

considered a potentially susceptible 

population, we have carefully evaluated 

the environmental health effects of 

exposure to PM pollution among 

children. These effects and the size of 

the population affected are summarized 

in section 9.2.4 of the Criteria Document 

and section 3.5 of the Staff Paper, and 

the results of our evaluation of the effect 

of PM pollution on children are 

discussed in sections II and III of this 

preamble. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This rule is not a ‘‘significant energy 

action’’ as defined in Executive Order 

13211, ‘‘Actions Concerning Regulations 

That Significantly Affect Energy Supply, 

Distribution, or Use’’ (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 rule is to establish 

NAAQS for PM. The rule does not 


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prescribe specific pollution control 

strategies by which these ambient 

standards will be met. Such strategies 

will be developed by States on a caseby-case basis, and EPA cannot predict 

whether the control options selected by 

States will include regulations on 

energy suppliers, distributors, or users. 

Thus, EPA concludes that this rule is 

not likely to have any adverse energy 

effects and does not constitute a 

significant energy action as defined in 

Executive Order 13211. 

I. National Technology Transfer 

Advancement Act 

Section 12(d) of the National 

Technology Transfer Advancement Act 

of 1995 (NTTAA), Public Law 104–113, 

Section 12(d) (15 U.S.C. 272 note) 

directs 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 EPA to 

provide Congress, through OMB, 

explanations when the Agency decides 

not to use available and applicable 

voluntary consensus standards. 

The final rule establishes 

requirements for environmental 

monitoring and measurement. 

Specifically, it establishes the FRM for 

PM10–2.5 measurement (and slightly 

amends the FRM for PM2.5). The FRM is 

the benchmark against which all 

ambient monitoring methods are 

measured. While the FRM is not a 

voluntary consensus standard, the 

equivalency criteria established in 40 

CFR part 53 do allow for the utilization 

of voluntary consensus standards if they 

meet the specified performance criteria. 

To the extent feasible, 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 costeffective 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 requirements utilize 

performance standards for some aspects 

of monitor design, multiple performance 

standards defined for many 

combinations of PM type, concentration, 

and environmental conditions would be 

required to be sure that monitors 

certified to purely performance-based 

standards actually performed similarly 

in the field, which would in turn 

require extensive testing of each 

candidate monitor design. Therefore, it 

is not practically possible to fully define 

the FRM in performance terms. 

Nevertheless, our approach in the past 

has resulted in multiple brands of 

monitors qualifying as FRM for PM, and 

we expect this to continue. Also, the 

FRM described in this final rule and the 

equivalency criteria contained in the 

revisions to 40 CFR part 53 do 

constitute performance based criteria for 

the instruments that will actually be 

deployed for monitoring PM10–2.5. 

Therefore, for most of the measurements 

that will be made and most of the 

measurement systems that make them, 

EPA is not precluding the use of any 

method, whether it constitutes a 

voluntary consensus standard or not, as 

long as it meets the specified 

performance criteria. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898, ‘‘Federal 

Actions to Address Environmental 

Justice in Minority Populations and 

Low-Income Populations,’’ requires 

Federal agencies to consider the impact 

of programs, policies, and activities on 

minority populations and low-income 

populations. According to EPA 

guidance, agencies are to assess whether 

minority or low-income populations 

face a risk or a rate of exposure to 

hazards that are significant and that 

‘‘appreciably exceeds or is likely to 

appreciably exceed the risk or rate to the 

general population or to the appropriate 

comparison group’’ (EPA, 1998). 

In accordance with Executive Order 

12898, the Agency has considered 

whether these decisions may have 

disproportionate negative impacts on 

minority or low-income populations. 

This rule establishes uniform, national 

ambient air quality standards for 

particulate matter, and is not expected 

to have disproportionate negative 

impacts on minority or low income 

populations. The EPA notes that some 

commenters expressed concerns that 

EPA had failed to adequately assess the 

environmental justice implications of its 

proposed decisions, and that the 

proposed revisions to both the fine 

particle and coarse particle standards 

would violate the principles of 

environmental justice. In particular, 

numerous commenters criticized the 

proposed qualified PM10–2.5 indicator, 

arguing that the exclusive urban focus of 

the indicator failed to protect large 

segments of the U.S. population 

(including Tribes and lower-income 

rural populations). The EPA believes 

that the final decision to retain the 

current nationally applicable 24-hour 

PM10 standard adequately addresses the 

concerns raised by these commenters, as 

discussed above in section III. 

Further, some commenters were 

concerned that the proposed PM2.5

standards would permit the 

continuation of disproportionate 

adverse health effects on minority and 

low-income populations because those 

populations are concentrated in urban 

areas where exposures are higher and 

are generally more susceptible (given 

lack of access to health care and 

prevalence of chronic conditions such 

as asthma). The EPA believes that the 

implications of the newly strengthened 

suite of PM2.5 standards will reduce 

health risks precisely in the areas 

subject to the highest fine particle 

concentrations. Furthermore, the PM2.5

NAAQS established in today’s final rule 

are nationally uniform standards which 

in the Administrator’s judgment protect 

public health with an adequate margin 

of safety. In making this determination, 

the Administrator expressly considered 

the available information regarding 

health effects among vulnerable and 

susceptible populations, such as those 

with preexisting conditions. Thus it 

remains EPA’s conclusion that this rule 

is not expected to have disproportionate 

negative impacts on minority or low 

income populations. 

K. Congressional Review Act 

The Congressional Review Act, 5 

U.S.C. 801 et seq., as added by the Small 

Business Regulatory Enforcement 

Fairness Act of 1996, generally provides 

that before a rule may take effect, the 

agency promulgating the rule must 

submit a rule report, which includes a 

copy of the rule, to each House of the 

Congress and to the Comptroller General 

of the United States. EPA submitted a 

report containing this rule and other 

required information to the U.S. Senate, 

the U.S. House of Representatives, and 

the Comptroller General of the United 

States prior to publication of the rule in 

the Federal Register. A major rule 

cannot take effect until 60 days after it 

is published in the Federal Register. 

This action is a ‘‘major rule’’ as defined 

by 5 U.S.C. 804(2). This rule will be 

effective December 18, 2006. 

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List of Subjects in 40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

Dated: September 21, 2006. 

Stephen L. Johnson, 

Administrator. 

■ For the reasons set out in the 

preamble, title 40, chapter I of the Code 

of Federal Regulations is amended as 

follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

Authority: 42 U.S.C. 7401 et seq. 

■ 2. Section 50.3 is revised to read as 

follows: 

§ 50.3 Reference conditions. 

All measurements of air quality that 

are expressed as mass per unit volume 

(e.g., micrograms per cubic meter) other 

than for the particulate matter (PM2.5) 

standards contained in §§ 50.7 and 

50.13 shall be corrected to a reference 

temperature of 25 (deg) C and a 

reference pressure of 760 millimeters of 

mercury (1,013.2 millibars). 

Measurements of PM2.5 for purposes of 

comparison to the standards contained 

in §§ 50.7 and 50.13 shall be reported 

based on actual ambient air volume 

measured at the actual ambient 

temperature and pressure at the 

monitoring site during the measurement 

period. 

§ 50.6 [Amended] 

■ 3. Section 50.6 is amended by 

removing and reserving paragraph (b). 

■ 4. A new § 50.13 is added to read as 

follows: 

§ 50.13 National primary and secondary 

ambient air quality standards for PM2.5. 

(a) The national primary and 

secondary ambient air quality standards 

for particulate matter are 15.0 

micrograms per cubic meter (μg/m3) 

annual arithmetic mean concentration, 

and 35 μg/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 annual primary and secondary 

PM2.5 standards are met when the 

annual arithmetic mean concentration, 

as determined in accordance with 

appendix N of this part, is less than or 

equal to 15.0 μg/m3. 

(c) The 24-hour primary and 

secondary PM2.5 standards are met when 

the 98th percentile 24-hour 

concentration, as determined in 

accordance with appendix N of this 

part, is less than or equal to 35 μg/m3. 

■ 5. Appendix K to Part 50 is revised to 

read as follows: 

Appendix K to Part 50—Interpretation of the 

National Ambient Air Quality Standards for 

Particulate Matter 

1.0 General 

(a) This appendix explains the 

computations necessary for analyzing 

particulate matter data to determine 

attainment of the 24-hour standards specified 

in 40 CFR 50.6. For the primary and 

secondary standards, particulate matter is 

measured in the ambient air as PM10

(particles with an aerodynamic diameter less 

than or equal to a nominal 10 micrometers) 

by a reference method based on appendix J 

of this part and designated in accordance 

with part 53 of this chapter, or by an 

equivalent method designated in accordance 

with part 53 of this chapter. The required 

frequency of measurements is specified in 

part 58 of this chapter. 

(b) The terms used in this appendix are 

defined as follows: 

Average refers to the arithmetic mean of 

the estimated number of exceedances per 

year, as per Section 3.1. 

Daily value for PM10 refers to the 24-hour 

average concentration of PM10 calculated or 

measured from midnight to midnight (local 

time). 

Exceedance means a daily value that is 

above the level of the 24-hour standard after 

rounding to the nearest 10 μg/m3 (i.e., values 

ending in 5 or greater are to be rounded up). 

Expected annual value is the number 

approached when the annual values from an 

increasing number of years are averaged, in 

the absence of long-term trends in emissions 

or meteorological conditions. 

Year refers to a calendar year. 

(c) Although the discussion in this 

appendix focuses on monitored data, the 

same principles apply to modeling data, 

subject to EPA modeling guidelines. 

2.0 Attainment Determinations 

2.1 24-Hour Primary and Secondary 

Standards 

(a) Under 40 CFR 50.6(a) the 24-hour 

primary and secondary standards are attained 

when the expected number of exceedances 

per year at each monitoring site is less than 

or equal to one. In the simplest case, the 

number of expected exceedances at a site is 

determined by recording the number of 

exceedances in each calendar year and then 

averaging them over the past 3 calendar 

years. Situations in which 3 years of data are 

not available and possible adjustments for 

unusual events or trends are discussed in 

sections 2.3 and 2.4 of this appendix. 

Further, when data for a year are incomplete, 

it is necessary to compute an estimated 

number of exceedances for that year by 

adjusting the observed number of 

exceedances. This procedure, performed by 

calendar quarter, is described in section 3.0 

of this appendix. The expected number of 

exceedances is then estimated by averaging 

the individual annual estimates for the past 

3 years. 

(b) The comparison with the allowable 

expected exceedance rate of one per year is 

made in terms of a number rounded to the 

nearest tenth (fractional values equal to or 

greater than 0.05 are to be rounded up; e.g., 

an exceedance rate of 1.05 would be rounded 

to 1.1, which is the lowest rate for 

nonattainment). 

2.2 Reserved 

2.3 Data Requirements 

(a) 40 CFR 58.12 specifies the required 

minimum frequency of sampling for PM10. 

For the purposes of making comparisons 

with the particulate matter standards, all data 

produced by State and Local Air Monitoring 

Stations (SLAMS) and other sites submitted 

to EPA in accordance with the part 58 

requirements must be used, and a minimum 

of 75 percent of the scheduled PM10 samples 

per quarter are required. 

(b) To demonstrate attainment of the 24- 

hour standards at a monitoring site, the 

monitor must provide sufficient data to 

perform the required calculations of sections 

3.0 and 4.0 of this appendix. The amount of 

data required varies with the sampling 

frequency, data capture rate and the number 

of years of record. In all cases, 3 years of 

representative monitoring data that meet the 

75 percent criterion of the previous 

paragraph should be utilized, if available, 


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and would suffice. More than 3 years may be 

considered, if all additional representative 

years of data meeting the 75 percent criterion 

are utilized. Data not meeting these criteria 

may also suffice to show attainment; 

however, such exceptions will have to be 

approved by the appropriate Regional 

Administrator in accordance with EPA 

guidance. 

(c) There are less stringent data 

requirements for showing that a monitor has 

failed an attainment test and thus has 

recorded a violation of the particulate matter 

standards. Although it is generally necessary 

to meet the minimum 75 percent data capture 

requirement per quarter to use the 

computational equations described in section 

3.0 of this appendix, this criterion does not 

apply when less data is sufficient to 

unambiguously establish nonattainment. The 

following examples illustrate how 

nonattainment can be demonstrated when a 

site fails to meet the completeness criteria. 

Nonattainment of the 24-hour primary 

standards can be established by the observed 

annual number of exceedances (e.g., four 

observed exceedances in a single year), or by 

the estimated number of exceedances derived 

from the observed number of exceedances 

and the required number of scheduled 

samples (e.g., two observed exceedances with 

every other day sampling). In both cases, 

expected annual values must exceed the 

levels allowed by the standards. 

2.4 Adjustment for Exceptional Events and 

Trends 

(a) An exceptional event is an 

uncontrollable event caused by natural 

sources of particulate matter or an event that 

is not expected to recur at a given location. 

Inclusion of such a value in the computation 

of exceedances or averages could result in 

inappropriate estimates of their respective 

expected annual values. To reduce the effect 

of unusual events, more than 3 years of 

representative data may be used. 

Alternatively, other techniques, such as the 

use of statistical models or the use of 

historical data could be considered so that 

the event may be discounted or weighted 

according to the likelihood that it will recur. 

The use of such techniques is subject to the 

approval of the appropriate Regional 

Administrator in accordance with EPA 

guidance. 

(b) In cases where long-term trends in 

emissions and air quality are evident, 

mathematical techniques should be applied 

to account for the trends to ensure that the 

expected annual values are not 

inappropriately biased by unrepresentative 

data. In the simplest case, if 3 years of data 

are available under stable emission 

conditions, this data should be used. In the 

event of a trend or shift in emission patterns, 

either the most recent representative year(s) 

could be used or statistical techniques or 

models could be used in conjunction with 

previous years of data to adjust for trends. 

The use of less than 3 years of data, and any 

adjustments are subject to the approval of the 

appropriate Regional Administrator in 

accordance with EPA guidance. 

3.0 Computational Equations for the 24- 

Hour Standards 

3.1 Estimating Exceedances for a Year 

(a) If PM10 sampling is scheduled less 

frequently than every day, or if some 

scheduled samples are missed, a PM10 value 

will not be available for each day of the year. 

To account for the possible effect of 

incomplete data, an adjustment must be 

made to the data collected at each monitoring 

location to estimate the number of 

exceedances in a calendar year. In this 

adjustment, the assumption is made that the 

fraction of missing values that would have 

exceeded the standard level is identical to 

the fraction of measured values above this 

level. This computation is to be made for all 

sites that are scheduled to monitor 

throughout the entire year and meet the 

minimum data requirements of section 2.3 of 

this appendix. Because of possible seasonal 

imbalance, this adjustment shall be applied 

on a quarterly basis. The estimate of the 

expected number of exceedances for the 

quarter is equal to the observed number of 

exceedances plus an increment associated 

with the missing data. The following 

equation must be used for these 

computations: 

 Equation 1

e v N

n q q

q

q

= ×

















Where: 

eq = the estimated number of exceedances for 

calendar quarter q; 

vq = the observed number of exceedances for 

calendar quarter q; 

Nq = the number of days in calendar quarter 

q; 

nq = the number of days in calendar quarter 

q with PM10 data; and 

q = the index for calendar quarter, q = 1, 2, 

3 or 4. 

(b) The estimated number of exceedances 

for a calendar quarter must be rounded to the 

nearest hundredth (fractional values equal to 

or greater than 0.005 must be rounded up). 

(c) The estimated number of exceedances 

for the year, e, is the sum of the estimates for 

each calendar quarter. 

 Equation 2

e eq

q

= =

∑1

4

(d) The estimated number of exceedances 

for a single year must be rounded to one 

decimal place (fractional values equal to or 

greater than 0.05 are to be rounded up). The 

expected number of exceedances is then 

estimated by averaging the individual annual 

estimates for the most recent 3 or more 

representative years of data. The expected 

number of exceedances must be rounded to 

one decimal place (fractional values equal to 

or greater than 0.05 are to be rounded up). 

(e) The adjustment for incomplete data will 

not be necessary for monitoring or modeling 

data which constitutes a complete record, 

i.e., 365 days per year. 

(f) To reduce the potential for 

overestimating the number of expected 

exceedances, the correction for missing data 

will not be required for a calendar quarter in 

which the first observed exceedance has 

occurred if: 

(1) There was only one exceedance in the 

calendar quarter; 

(2) Everyday sampling is subsequently 

initiated and maintained for 4 calendar 

quarters in accordance with 40 CFR 58.12; 

and 

(3) Data capture of 75 percent is achieved 

during the required period of everyday 

sampling. In addition, if the first exceedance 

is observed in a calendar quarter in which 

the monitor is already sampling every day, 

no adjustment for missing data will be made 

to the first exceedance if a 75 percent data 

capture rate was achieved in the quarter in 

which it was observed. 

Example 1 

a. During a particular calendar quarter, 39 

out of a possible 92 samples were recorded, 

with one observed exceedance of the 24-hour 

standard. Using Equation 1, the estimated 

number of exceedances for the quarter is: 

eq = 1 × 92/39 = 2.359 or 2.36. 

b. If the estimated exceedances for the 

other 3 calendar quarters in the year were 

2.30, 0.0 and 0.0, then, using Equation 2, the 

estimated number of exceedances for the year 

is 2.36 + 2.30 + 0.0 + 0.0 which equals 4.66 

or 4.7. If no exceedances were observed for 

the 2 previous years, then the expected 

number of exceedances is estimated by: (1⁄3) 

× (4.7 + 0 + 0) = 1.57 or 1.6. Since 1.6 exceeds 

the allowable number of expected 

exceedances, this monitoring site would fail 

the attainment test. 

Example 2 

In this example, everyday sampling was 

initiated following the first observed 

exceedance as required by 40 CFR 58.12. 

Accordingly, the first observed exceedance 

would not be adjusted for incomplete 

sampling. During the next three quarters, 1.2 

exceedances were estimated. In this case, the 

estimated exceedances for the year would be 

1.0 + 1.2 + 0.0 + 0.0 which equals 2.2. If, as 

before, no exceedances were observed for the 

two previous years, then the estimated 

exceedances for the 3-year period would then 

be (1⁄3) × (2.2 + 0.0 + 0.0) = 0.7, and the 

monitoring site would not fail the attainment 

test. 

3.2 Adjustments for Non-Scheduled 

Sampling Days 

(a) If a systematic sampling schedule is 

used and sampling is performed on days in 

addition to the days specified by the 

systematic sampling schedule, e.g., during 

episodes of high pollution, then an 

adjustment must be made in the equation for 

the estimation of exceedances. Such an 

adjustment is needed to eliminate the bias in 

the estimate of the quarterly and annual 

number of exceedances that would occur if 

the chance of an exceedance is different for 

scheduled than for non-scheduled days, as 

would be the case with episode sampling. 

(b) The required adjustment treats the 

systematic sampling schedule as a stratified 

sampling plan. If the period from one 


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61226 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 

scheduled sample until the day preceding the 

next scheduled sample is defined as a 

sampling stratum, then there is one stratum 

for each scheduled sampling day. An average 

number of observed exceedances is 

computed for each of these sampling strata. 

With nonscheduled sampling days, the 

estimated number of exceedances is defined 

as: 

 Equation 3

e

N

m

v

k q

q

q

j

j j l

mq

=















 ×















 =

∑

Where: 

eq = the estimated number of exceedances for 

the quarter; 

Nq = the number of days in the quarter; 

mq = the number of strata with samples 

during the quarter; 

vj = the number of observed exceedances in 

stratum j; and 

kj = the number of actual samples in stratum 

j. 

(c) Note that if only one sample value is 

recorded in each stratum, then Equation 3 

reduces to Equation 1. 

Example 3 

A monitoring site samples according to a 

systematic sampling schedule of one sample 

every 6 days, for a total of 15 scheduled 

samples in a quarter out of a total of 92 

possible samples. During one 6-day period, 

potential episode levels of PM10 were 

suspected, so 5 additional samples were 

taken. One of the regular scheduled samples 

was missed, so a total of 19 samples in 14 

sampling strata were measured. The one 6- 

day sampling stratum with 6 samples 

recorded 2 exceedances. The remainder of 

the quarter with one sample per stratum 

recorded zero exceedances. Using Equation 3, 

the estimated number of exceedances for the 

quarter is: 

Eq = (92/14) × (2/6 + 0 +. . .+ 0) = 2.19. 

■ 6. Appendix L to part 50 is amended 

by: 

■ a. Revising section 1.1; 

■ b. Revising the heading of section 

7.3.4 and adding introductory text; 

■ c. Revising paragraph (a) of section 

7.3.4.3: 

■ d. Adding section 7.3.4.4; 

■ e. Revising Table L–1 in section 

7.4.19; 

■ f. Revising section 8.3.6; 

■ g. Revising the first sentence in 

section 10.10 and revising section 10.13; 

and 

■ h. Revising reference 2 in section 13.0 

to read as follows: 

Appendix L to Part 50—Reference Method 

for the Determination of Fine Particulate 

Matter as PM2.5 in the Atmosphere 

1.0 Applicability. 

1.1 This method provides for the 

measurement of the mass concentration of 

fine particulate matter having an 

aerodynamic diameter less than or equal to 

a nominal 2.5 micrometers (PM2.5) in ambient 

air over a 24-hour period for purposes of 

determining whether the primary and 

secondary national ambient air quality 

standards for fine particulate matter specified 

in § 50.7 and § 50.13 of this part are met. The 

measurement process is considered to be 

nondestructive, and the PM2.5 sample 

obtained can be subjected to subsequent 

physical or chemical analyses. Quality 

assessment procedures are provided in part 

58, appendix A of this chapter, and quality 

assurance guidance are provided in 

references 1, 2, and 3 in section 13.0 of this 

appendix. 

* * * * * 

7.3.4 Particle size separator. The sampler 

shall be configured with either one of the two 

alternative particle size separators described 

in this section 7.3.4. One separator is an 

impactor-type separator (WINS impactor) 

described in sections 7.3.4.1, 7.3.4.2, and 

7.3.4.3 of this appendix. The alternative 

separator is a cyclone-type separator 

(VSCCTM) described in section 7.3.4.4 of this 

appendix. 

* * * * * 

7.3.4.3 * * * 

(a) Composition. Dioctyl sebacate (DOS), 

single-compound diffusion oil. 

* * * * * 

7.3.4.4 The cyclone-type separator is 

identified as a BGI VSCCTM Very Sharp Cut 

Cyclone particle size separator specified as 

part of EPA-designated equivalent method 

EQPM–0202–142 (67 FR 15567, April 2, 

2002) and as manufactured by BGI 

Incorporated, 58 Guinan Street, Waltham, 

Massachusetts 20451. 

* * * * * 

7.4.19 * * * 

TABLE L–1 TO APPENDIX L OF PART 50.—SUMMARY OF INFORMATION TO BE PROVIDED BY THE SAMPLER

Information to be provided 

Appendix L 

section 

reference 

Availability Format 

Anytime 1 End of 

period 2

Visual 

display 3

Data 

output 4

Digital 

reading 5 Units 

Flow rate, 30-second maximum interval ..................................................... 7.4.5.1 ........ ✓ .................... ✓ * XX.X .......... L/min 

Flow rate, average for the sample period .................................................... 7.4.5.2 ........ * ✓ * ✓ XX.X .......... L/min 

Flow rate, CV, for sample period ......... 7.4.5.2 ........ * ✓ * ✓ XX.X .......... % 

Flow rate, 5-min. average out of spec. 

(FLAG 6) ............................................ 7.4.5.2 ........ ✓ ✓ ✓ ✓■ On/Off ........................

Sample volume, total ........................... 7.4.5.2 ........ * ✓ ✓ ✓ XX.X .......... m3

Temperature, ambient, 30-second interval ................................................. 7.4.8 ........... ✓ .................... ✓ .................... XX.X .......... °C 

Temperature, ambient, min., max., average for the sample period ............. 7.4.8 ........... * ✓ ✓ {■ XX.X .......... °C 

Baro. pressure, ambient, 30-second 

interval .............................................. 7.4.9 ........... ✓ .................... ✓ .................... XXX ........... mm Hg 

Baro. pressure, ambient, min., max., 

average for the sample period ......... 7.4.9 ........... * ✓ ✓ ✓■ XXX ........... mm Hg 

Filter temperature, 30-second interval 7.4.11 ......... ✓ .................... ✓ .................... XX.X .......... °C 

Filter temp. differential, 30-second interval, out of spec. (FLAG 6) ............ 7.4.11 ......... * ✓ ✓ ✓■ On/Off ........................

Filter temp., maximum differential from 

ambient, date, time of occurrence ... 7.4.11 ......... * * * * X.X, YY/ 

MM/DD 

HH.mm.

°C, Yr/Mon/ 

Day Hrs. min 

Date and Time ..................................... 7.4.12 ......... ✓ .................... ✓ .................... YY/MM/DD 

HH.mm.

Yr/Mon/Day 

Hrs. min 

Sample start and stop time settings .... 7.4.12 ......... ✓ ✓ ✓ ✓ YY/MM/DD 

HH.mm.

Yr/Mon/Day 

Hrs. min 

Sample period start time ...................... 7.4.12 ......... .................... ✓ ✓ ✓ YY/MM/DD 

HH.mm.

Yr/Mon/Day 

Hrs. min 


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TABLE L–1 TO APPENDIX L OF PART 50.—SUMMARY OF INFORMATION TO BE PROVIDED BY THE SAMPLER—Continued 

Information to be provided 

Appendix L 

section 

reference 

Availability Format 

Anytime 1 End of 

period 2

Visual 

display 3

Data 

output 4

Digital 

reading 5 Units 

Elapsed sample time ........................... 7.4.13 ......... * ✓ ✓ ✓ HH.mm ...... Hrs. min 

Elapsed sample time, out of spec. 

(FLAG 6) ............................................ 7.4.13 ......... .................... ✓ ✓ ✓■ On/Off ........................

Power interruptions ≤1 min., start time 

of first 10 .......................................... 7.4.15.5 ...... * ✓ * ✓ 1HH.mm, 

2HH.mm, 

etc..

Hrs. min 

User-entered information, such as 

sampler and site identification .......... 7.4.16 ......... ✓ ✓ ✓ ✓■ As entered.

✓ Provision of this information is required. 

* Provision of this information is optional. If information related to the entire sample period is optionally provided prior to the end of the sample 

period, the value provided should be the value calculated for the portion of the sampler period completed up to the time the information is provided. 

■ Indicates that this information is also required to be provided to the Air Quality System (AQS) data bank; see § 58.16 of this chapter. For 

ambient temperature and barometric pressure, only the average for the sample period must be reported. 

1. Information is required to be available to the operator at any time the sampler is operating, whether sampling or not. 

2. Information relates to the entire sampler period and must be provided following the end of the sample period until reset manually by the operator or automatically by the sampler upon the start of a new sample period. 

3. Information shall be available to the operator visually. 

4. Information is to be available as digital data at the sampler’s data output port specified in section 7.4.16 of this appendix following the end of 

the sample period until reset manually by the operator or automatically by the sampler upon the start of a new sample period. 

5. Digital readings, both visual and data output, shall have not less than the number of significant digits and resolution specified. 

6. Flag warnings may be displayed to the operator by a single flag indicator or each flag may be displayed individually. Only a set (on) flag 

warning must be indicated; an off (unset) flag may be indicated by the absence of a flag warning. Sampler users should refer to section 10.12 of 

this appendix regarding the validity of samples for which the sampler provided an associated flag warning. 

* * * * * 

8.3.6 The post-sampling conditioning and 

weighing shall be completed within 240 

hours (10 days) after the end of the sample 

period, unless the filter sample is maintained 

at temperatures below the average ambient 

temperature during sampling (or 4 °C or 

below for average sampling temperatures less 

than 4 °C) during the time between retrieval 

from the sampler and the start of the 

conditioning, in which case the period shall 

not exceed 30 days. Reference 2 in section 

13.0 of this appendix has additional guidance 

on transport of cooled filters. 

* * * * * 

10.10 Within 177 hours (7 days, 9 hours) 

of the end of the sample collection period, 

the filter, while still contained in the filter 

cassette, shall be carefully removed from the 

sampler, following the procedure provided in 

the sampler operation or instruction manual 

and the quality assurance program, and 

placed in a protective container. * * * 

* * * * * 

10.13 After retrieval from the sampler, 

the exposed filter containing the PM2.5

sample should be transported to the filter 

conditioning environment as soon as 

possible, ideally to arrive at the conditioning 

environment within 24 hours for 

conditioning and subsequent weighing. 

During the period between filter retrieval 

from the sampler and the start of the 

conditioning, the filter shall be maintained as 

cool as practical and continuously protected 

from exposure to temperatures over 25 °C to 

protect the integrity of the sample and 

minimize loss of volatile components during 

transport and storage. See section 8.3.6 of 

this appendix regarding time limits for 

completing the post-sampling weighing. See 

reference 2 in section 13.0 of this appendix 

for additional guidance on transporting filter 

samplers to the conditioning and weighing 

laboratory. 

* * * * * 

13.0 References 

* * * * * 

2. Quality Assurance Guidance Document 

2.12. Monitoring PM2.5 in Ambient Air Using 

Designated Reference or Class I Equivalent 

Methods. U.S. EPA, National Exposure 

Research Laboratory. Research Triangle Park, 

NC, November 1988 or later edition. 

Currently available at: http://www.epa.gov/ 

ttn/amtic/pmqainf.html. 

* * * * * 

■ 7. 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. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the annual 

and 24-hour primary and secondary national 

ambient air quality standards (NAAQS) for 

PM2.5 specified in § 50.7 and § 50.13 of this 

part are met. PM2.5, defined as particles with 

an aerodynamic diameter less than or equal 

to a nominal 2.5 micrometers, is 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. Data handling and 

computation procedures to be used in 

making comparisons between reported PM2.5

concentrations and the levels of the PM2.5

NAAQS are specified in the following 

sections. 

(b) Data resulting from exceptional events, 

for example structural fires or high winds, 

may be given special consideration. In some 

cases, it may be appropriate to exclude these 

data in whole or part because they could 

result in inappropriate values to compare 

with the levels of the PM2.5 NAAQS. In other 

cases, it may be more appropriate to retain 

the data for comparison with the levels of the 

PM2.5 NAAQS and then for EPA to formulate 

the appropriate regulatory response. 

(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. 

Creditable samples are samples that are 

given credit for data completeness. They 

include valid samples collected on required 

sampling days and valid ‘‘make-up’’ samples 

taken for missed or invalidated samples on 

required sampling days. 

Daily values for PM2.5 refers to the 24-hour 

average concentrations of PM2.5 calculated 

(averaged from hourly measurements) or 

measured from midnight to midnight (local 

standard time) that are used in NAAQS 

computations. 

Designated monitors are those monitoring 

sites designated in a State or local agency PM 

Monitoring Network Description in 

accordance with part 58 of this chapter. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

shown in section 4 of this appendix: 

(1) The 3-year average of annual means for 

a single monitoring site or a group of 

monitoring sites (referred to as the ‘‘annual 

standard design value’’). If spatial averaging 


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has been approved by EPA for a group of 

sites which meet the criteria specified in 

section 2(b) of this appendix and section 

4.7.5 of appendix D of 40 CFR part 58, then 

3 years of spatially averaged annual means 

will be averaged to derive the annual 

standard design value for that group of sites 

(further referred to as the ‘‘spatially averaged 

annual standard design value’’). Otherwise, 

the annual standard design value will 

represent the 3-year average of annual means 

for a single site (further referred to as the 

‘‘single site annual standard design value’’). 

(2) The 3-year average of annual 98th 

percentile 24-hour average values recorded at 

each monitoring site (referred to as the ‘‘24- 

hour standard design value’’). 

Extra samples are non-creditable samples. 

They are daily values that do not occur on 

scheduled sampling days and that can not be 

used as make-ups for missed or invalidated 

scheduled samples. Extra samples are used in 

mean calculations and are subject to 

selection as a 98th percentile. 

Make-up samples are samples taken to 

supplant missed or invalidated required 

scheduled samples. Make-ups can be made 

by either the primary or the collocated 

instruments. Make-up samples are either 

taken before the next required sampling day 

or exactly one week after the missed (or 

voided) sampling day. Also, to be considered 

a valid make-up, the sampling must be 

administered according to EPA guidance. 

98th percentile is the daily value out of a 

year of PM2.5 monitoring data below which 

98 percent of all daily values fall. 

Year refers to a calendar year. 

2.0 Monitoring Considerations. 

(a) Section 58.30 of this chapter specifies 

which monitoring locations are eligible for 

making comparisons with the PM2.5

standards. 

(b) To qualify for spatial averaging, 

monitoring sites must meet the criterion 

specified in section 4.7.5 of appendix D of 40 

CFR part 58 as well as the following 

requirements: 

(1) The annual mean concentration at each 

site shall be within 10 percent of the spatially 

averaged annual mean. 

(2) The daily values for each site pair 

among the 3-year period shall yield a 

correlation coefficient of at least 0.9 for each 

calendar quarter. 

(3) All of the monitoring sites should 

principally be affected by the same major 

emission sources of PM2.5. For example, this 

could be demonstrated by site-specific 

chemical speciation profiles confirming all 

major component concentration averages to 

be within 10 percent for each calendar 

quarter. 

(4) The requirements in paragraphs (b)(1) 

through (3) of this section shall be met for 3 

consecutive years in order to produce a valid 

spatially averaged annual standard design 

value. Otherwise, the individual (single) site 

annual standard design values shall be 

compared directly to the level of the annual 

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 a reduced 

frequency during a season of expected low 

concentrations (i.e., ‘‘seasonal sampling’’), 

are subject to the approval of EPA. Annual 

98th percentile values are to be calculated 

according to equation 6 in section 4.5 of this 

appendix when a site operates on a ‘‘seasonal 

sampling’’ schedule. 

3.0 Requirements for Data Used for 

Comparisons With the PM2.5 NAAQS and 

Data Reporting Considerations. 

(a) Except as otherwise provided in this 

appendix, only valid FRM/FEM/ARM PM2.5

data required to be submitted to EPA’s Air 

Quality System (AQS) shall be used in the 

design value calculations. 

(b) PM2.5 measurement data (typically 

hourly for continuous instruments and daily 

for filter-based instruments) shall be reported 

to AQS in micrograms per cubic meter (μg/ 

m3) to one decimal place, with additional 

digits to the right being truncated. 

(c) Block 24-hour averages shall be 

computed from available hourly PM2.5

concentration data for each corresponding 

day of the year and the result shall be stored 

in the first, or start, hour (i.e., midnight, hour 

‘0’) of the 24-hour period. A 24-hour average 

shall be considered valid if at least 75 

percent (i.e., 18) of the hourly averages for 

the 24-hour period are available. In the event 

that less than all 24 hourly averages are 

available (i.e., less than 24, but at least 18), 

the 24-hour average shall be computed on the 

basis of the hours available using the number 

of available hours as the divisor (e.g., 19). 24- 

hour periods with seven or more missing 

hours shall be considered valid if, after 

substituting zero for all missing hourly 

concentrations, the 24-hour average 

concentration is greater than the level of the 

standard. The computed 24-hour average 

PM2.5 concentrations shall be reported to one 

decimal place (the additional digits to the 

right of the first decimal place are truncated, 

consistent with the data handling procedures 

for the reported data). 

(d) Except for calculation of spatially 

averaged annual means and spatially 

averaged annual standard design values, all 

other calculations shown in this appendix 

shall be implemented on a site-level basis. 

Site level data shall be processed as follows: 

(1) The default dataset for a site shall 

consist of the measured concentrations 

recorded from the designated primary FRM/ 

FEM/ARM monitor. The primary monitor 

shall be designated in the appropriate State 

or local agency PM Monitoring Network 

Description. All daily values produced by the 

primary sampler are considered part of the 

site record (i.e., that site’s daily value); this 

includes all creditable samples and all extra 

samples. 

(2) Data for the primary monitor shall be 

augmented as much as possible with data 

from collocated FRM/FEM/ARM monitors. If 

a valid 24-hour measurement is not produced 

from the primary monitor for a particular day 

(scheduled or otherwise), but a valid sample 

is generated by a collocated FRM/FEM/ARM 

instrument (and recorded in AQS), then that 

collocated value shall be considered part of 

the site data record (i.e., that site’s daily 

value). If more than one valid collocated 

FRM/FEM/ARM value is available, the 

average of those valid collocated values shall 

be used as the daily value. 

(e) All daily values in the composite site 

record are used in annual mean and 98th 

percentile calculations, however, not all 

daily values are give credit towards data 

completeness requirements. Only 

‘‘creditable’’ samples are given credit for data 

completeness. Creditable samples include 

valid samples on scheduled sampling days 

and valid make-up samples. All other types 

of daily values are referred to as ‘‘extra’’

samples. 

4.0 Comparisons With the PM2.5 NAAQS. 

4.1 Annual PM2.5 NAAQS. 

(a) The annual PM2.5 NAAQS is met when 

the annual standard design value is less than 

or equal to 15.0 micrograms per cubic meter 

(μg/m3). 

(b) For single site comparisons, 3 years of 

valid annual means are required to produce 

a valid annual standard design value. In the 

case of spatial averaging, 3 years of valid 

spatially averaged annual means are required 

to produce a valid annual standard design 

value. Designated sites with less than 3 years 

of data shall be included in annual spatial 

averages for those years that data 

completeness requirements are met. A year 

meets data completeness requirements when 

at least 75 percent of the scheduled sampling 

days for each quarter have valid data. 

[Quarterly data capture rates (expressed as a 

percentage) 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.] 

However, years with at least 11 samples in 

each quarter shall be considered valid, 

notwithstanding quarters with less than 

complete data, if the resulting annual mean, 

spatially averaged annual mean 

concentration, or resulting annual standard 

design value concentration (rounded 

according to the conventions of section 4.3 of 

this appendix) is greater than the level of the 

standard. Furthermore, where the explicit 11 

sample per quarter requirement is not met, 

the site annual mean shall still be considered 

valid if, by substituting a low value 

(described below) for the missing data in the 

deficient quarters (substituting enough to 

meet the 11 sample minimum), the 

computation still yields a recalculated 

annual mean, spatially averaged annual mean 

concentration, or annual standard design 

value concentration over the level of the 

standard. The low value used for this 

substitution test shall be the lowest reported 

daily value in the site data record for that 

calendar quarter over the most recent 3-year 

period. If an annual mean is deemed 

complete using this test, the original annual 

mean (without substituted low values) shall 

be considered the official mean value for this 

site, not the result of the recalculated test 

using the low values. 

(c) The use of less than complete data is 

subject to the approval of EPA, which may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, and 

nearby concentrations in determining 

whether to use such data. 


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(d) The equations for calculating the 

annual standard design values are given in 

section 4.4 of this appendix. 

4.2 24-Hour PM2.5 NAAQS. 

(a) The 24-hour PM2.5 NAAQS is met when 

the 24-hour standard design value at each 

monitoring site is less than or equal to 35 μg/ 

m3. This comparison shall be based on 3 

consecutive, complete years of air quality 

data. A year meets data completeness 

requirements when at least 75 percent of the 

scheduled sampling days for each quarter 

have valid data. However, years shall be 

considered valid, notwithstanding quarters 

with less than complete data (even quarters 

with less than 11 samples), if the resulting 

annual 98th percentile value or resulting 24- 

hour standard design value (rounded 

according to the conventions of section 4.3 of 

this appendix) is greater than the level of the 

standard. 

(b) The use of less than complete data is 

subject to the approval of EPA which may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, and 

nearby concentrations in determining 

whether to use such data for comparisons to 

the NAAQS. 

(c) The equations for calculating the 24- 

hour standard design values are given in 

section 4.5 of this appendix. 

4.3 Rounding Conventions. For the 

purposes of comparing calculated values 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 standard design values 

shall be rounded to the nearest 0.1 μg/m3

(decimals 0.05 and greater are rounded up to 

the next 0.1, and any decimal lower than 0.05 

is rounded down to the nearest 0.1). 

(b) 24-hour PM2.5 standard design values 

shall be rounded to the nearest 1 μg/m3

(decimals 0.5 and greater are rounded up to 

the nearest whole number, and any decimal 

lower than 0.5 is rounded down to the 

nearest whole number). 

4.4 Equations for the Annual PM2.5 NAAQS. 

(a) An annual mean value for PM2.5 is 

determined by first averaging the daily values 

of a calendar quarter using equation 1 of this 

appendix: 

Equation 1

X l

n qys X

q

iqys

i l

nq

,, ,,, = =

∑

Where: 

X ̄ q,y,s = the mean for quarter q of the year y 

for site s; 

nq = the number of daily values in the 

quarter; and 

xi q,y,s = the ith value in quarter q for year y 

for site s. 

(b) Equation 2 of this appendix is then 

used to calculate the site annual mean: 

Equation 2

X X ys qys

q

, ,, = =

∑

1

4 1

4

Where: 

X ̄ y,s = the annual mean concentration for year 

y (y = 1, 2, or 3) and for site s; and 

X ̄ q,y,s = the mean for quarter q of year y for 

site s. 

(c) If spatial averaging is utilized, the sitebased annual means will then be averaged 

together to derive the spatially averaged 

annual mean using equation 3 of this 

appendix. Otherwise (i.e., for single site 

comparisons), skip to equation 4.B of this 

appendix. 

Equation 3

x n x y

s

y s

s

ns

= =

∑1

1

,

Where: 

x ̄ y = the spatially averaged mean for year y, 

x ̄ y,s = the annual mean for year y and site s 

for sites designated to be averaged that 

meet completeness criteria , and 

ns = the number of sites designated to be 

averaged that meet completeness criteria. 

(d) The annual standard design value is 

calculated using equation 4A of this 

appendix when spatial averaging and 

equation 4B of this appendix when not 

spatial averaging: 

Equation 4A

When spatial averaging

x xy

y

= =

∑

1

3 1

3

Equation 4B

When not spatial averaging

x xy s

y

= =

∑

1

3 1

3

,

Where: 

x ̄ = the annual standard design value (the 

spatially averaged annual standard 

design value for equation 4A of this 

appendix and the single site annual 

standard design value for equation 4B of 

this appendix); and 

x ̄ y = the spatially averaged annual mean for 

year y (result of equation 3 of this 

appendix) when spatial averaging is 

used, or 

x ̄ y,s the annual mean for year y and site s 

(result of equation 2 of this appendix) 

when spatial averaging is not used. 

(e) The annual standard design value is 

rounded according to the conventions in 

section 4.3 of this appendix before a 

comparison with the standard is made. 

4.5 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. Equation 5 of this appendix shall 

be used to compute annual 98th percentile 

values, except that where a site operates on 

an approved seasonal sampling schedule, 

equation 6 of this appendix shall be used 

instead. 

(1) Regular formula for computing annual 

98th percentile values. Calculation of annual 

98th percentile values using the regular 

formula (equation 5) 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. Sort all the 

daily values from a particular site and year 

by ascending value. (For example: (x[1], x[2], 

x[3], * * *, x[n]). In this case, x[1] is the 

smallest number and x[n] is the largest 

value.) The 98th percentile is determined 

from this sorted series of daily values which 

is ordered from the lowest to the highest 

number. Compute (0.98) x (cn) as the number 

‘‘i.d,’’ where ‘cn’ is the annual creditable 

number of samples, ‘‘i’’ is the integer part of 

the result, and ‘‘d’’ is the decimal part of the 

result. The 98th percentile value for year y, 

P0.98, y, is calculated using equation 5 of this 

appendix: 

Equation 5

P X 0 98 . ,y i 1 = [ ] +

Where: 

P0.98, y = 98th percentile for year y; 

x[i+1] = the (i+1)th number in the ordered 

series of numbers; 

i = the integer part of the product of 0.98 and 

cn. 

(2) Formula for computing annual 98th 

percentile values when sampling frequencies 

are seasonal. Calculate the annual 98th 

percentiles by determining the smallest 

measured concentration, x, that makes W(x) 

greater than 0.98 using equation 6 of this 

appendix: 


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61230 Federal Register / Vol. 71, No. 200 / Tuesday, October 17, 2006 / Rules and Regulations 

 Equation 6

W x

d

d d

F x d

d d

F High

High Low

High

Low

High Low

( ) = Lo + ( ) +

+ w ( ) x

Where: 

dHigh = number of calendar days in the 

‘‘High’’ season; 

dLow = number of calendar days in the ‘‘Low’’

season; 

dHigh+ = days in a year; and 

dLow

F x number of daily values in season a that are

number of daily v a ( ) = ≤ ×aluesin season a

Such that ‘‘a’’ can be either ‘‘High’’ or ‘‘Low’’; 

‘‘x’’ is the measured concentration; and 

‘‘dHigh/(dHigh + dLow) and dLow/(dHigh + dLow)’’

are constant and are called seasonal 

‘‘weights.’’

(b) The 24-hour standard design value is 

then calculated by averaging the annual 98th 

percentiles using equation 7 of this appendix: 

Equation 7

P

P y

y

0 98

0 98

1

3

3 .

. ,

= =

∑

(c) The 24-hour standard design value (3- 

year average 98th percentile) is rounded 

according to the conventions in section 4.3 

of this appendix before a comparison with 

the standard is made. 

■ 8. Appendix O is added to part 50 to 

read as follows: 

Appendix O to Part 50—Reference Method 

for the Determination of Coarse Particulate 

Matter as PM10–2.5 in the Atmosphere 

1.0 Applicability and Definition 

1.1 This method provides for the 

measurement of the mass concentration of 

coarse particulate matter (PM10–2.5) in 

ambient air over a 24-hour period. In 

conjunction with additional analysis, this 

method may be used to develop speciated 

data. 

1.2 For the purpose of this method, 

PM10–2.5 is defined as particulate matter 

having an aerodynamic diameter in the 

nominal range of 2.5 to 10 micrometers, 

inclusive. 

1.3 For this reference method, PM10–2.5

concentrations shall be measured as the 

arithmetic difference between separate but 

concurrent, collocated measurements of PM10

and PM2.5, where the PM10 measurements are 

obtained with a specially approved sampler, 

identified as a ‘‘PM10c sampler,’’ that meets 

more demanding performance requirements 

than conventional PM10 samplers described 

in appendix J of this part. Measurements 

obtained with a PM10c sampler are identified 

as ‘‘PM10c measurements’’ to distinguish 

them from conventional PM10 measurements 

obtained with conventional PM10 samplers. 

Thus, PM10–2.5 = PM10c ¥ PM2.5. 

1.4 The PM10c and PM2.5 gravimetric 

measurement processes are considered to be 

nondestructive, and the PM10c and PM2.5

samples obtained in the PM10–2.5

measurement process can be subjected to 

subsequent physical or chemical analyses. 

1.5 Quality assessment procedures are 

provided in part 58, appendix A of this 

chapter. The quality assurance procedures 

and guidance provided in reference 1 in 

section 13 of this appendix, although written 

specifically for PM2.5, are generally 

applicable for PM10c, and, hence, PM10–2.5

measurements under this method, as well. 

1.6 A method based on specific model 

PM10c and PM2.5 samplers will be considered 

a reference method for purposes of part 58 of 

this chapter only if: 

(a) The PM10c and PM2.5 samplers and the 

associated operational procedures meet the 

requirements specified in this appendix and 

all applicable requirements in part 53 of this 

chapter, and 

(b) The method based on the specific 

samplers and associated operational 

procedures have been designated as a 

reference method in accordance with part 53 

of this chapter. 

1.7 PM10–2.5 methods based on samplers 

that meet nearly all specifications set forth in 

this method but have one or more significant 

but minor deviations or modifications from 

those specifications may be designated as 

‘‘Class I’’ equivalent methods for PM10–2.5 in 

accordance with part 53 of this chapter. 

1.8 PM2.5 measurements obtained 

incidental to the PM10–2.5 measurements by 

this method shall be considered to have been 

obtained with a reference method for PM2.5

in accordance with appendix L of this part. 

1.9 PM10c measurements obtained 

incidental to the PM10–2.5 measurements by 

this method shall be considered to have been 

obtained with a reference method for PM10 in 

accordance with appendix J of this part, 

provided that: 

(a) The PM10c measurements are adjusted 

to EPA reference conditions (25 °C and 760 

millimeters of mercury), and 

(b) Such PM10c measurements are 

appropriately identified to differentiate them 

from PM10 measurements obtained with other 

(conventional) methods for PM10 designated 

in accordance with part 53 of this chapter as 

reference or equivalent methods for PM10. 

2.0 Principle 

2.1 Separate, collocated, electrically 

powered air samplers for PM10c and PM2.5

concurrently draw ambient air at identical, 

constant volumetric flow rates into specially 

shaped inlets and through one or more 

inertial particle size separators where the 

suspended particulate matter in the PM10 or 

PM2.5 size range, as applicable, is separated 

for collection on a polytetrafluoroethylene 

(PTFE) filter over the specified sampling 

period. The air samplers and other aspects of 

this PM10–2.5 reference method are specified 

either explicitly in this appendix or by 

reference to other applicable regulations or 

quality assurance guidance. 

2.2 Each PM10c and PM2.5 sample 

collection filter is weighed (after moisture 

and temperature conditioning) before and 

after sample collection to determine the net 

weight (mass) gain due to collected PM10c or 

PM2.5. The total volume of air sampled by 

each sampler is determined by the sampler 

from the measured flow rate at local ambient 

temperature and pressure and the sampling 

time. The mass concentrations of both PM10c

and PM2.5 in the ambient air are computed 

as the total mass of collected particles in the 

PM10 or PM2.5 size range, as appropriate, 

divided by the total volume of air sampled 

by the respective samplers, and expressed in 

micrograms per cubic meter (μg/m3)at local 

temperature and pressure conditions. The 

mass concentration of PM10–2.5 is determined 

as the PM10c concentration value less the 

corresponding, concurrently measured PM2.5

concentration value. 

2.3 Most requirements for PM10–2.5

reference methods are similar or identical to 

the requirements for PM2.5 reference methods 

as set forth in appendix L to this part. To 

insure uniformity, applicable appendix L 

requirements are incorporated herein by 

reference in the sections where indicated 

rather than repeated in this appendix. 

3.0 PM10–2.5 Measurement Range 

3.1 Lower concentration limit. The lower 

detection limit of the mass concentration 

measurement range is estimated to be 

approximately 3 μg/m3, based on the 

observed precision of PM2.5 measurements in 

the national PM2.5 monitoring network, the 

probable similar level of precision for the 

matched PM10c measurements, and the 

additional variability arising from the 

differential nature of the measurement 

process. This value is provided merely as a 

guide to the significance of low PM10–2.5

concentration measurements. 

3.2 Upper concentration limit. The upper 

limit of the mass concentration range is 

determined principally by the PM10c filter 


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mass loading beyond which the sampler can 

no longer maintain the operating flow rate 

within specified limits due to increased 

pressure drop across the loaded filter. This 

upper limit cannot be specified precisely 

because it is a complex function of the 

ambient particle size distribution and type, 

humidity, the individual filter used, the 

capacity of the sampler flow rate control 

system, and perhaps other factors. All PM10c

samplers are estimated to be capable of 

measuring 24-hour mass concentrations of at 

least 200 μg/m3 while maintaining the 

operating flow rate within the specified 

limits. The upper limit for the PM10–2.5

measurement is likely to be somewhat lower 

because the PM10–2.5 concentration represents 

only a fraction of the PM10 concentration. 

3.3 Sample period. The required sample 

period for PM10–2.5 concentration 

measurements by this method shall be at 

least 1,380 minutes but not more than 1,500 

minutes (23 to 25 hours), and the start times 

of the PM2.5 and PM10c samples are within 10 

minutes and the stop times of the samples are 

also within 10 minutes (see section 10.4 of 

this appendix). 

4.0 Accuracy (bias) 

4.1 Because the size, density, and 

volatility of the particles making up ambient 

particulate matter vary over wide ranges and 

the mass concentration of particles varies 

with particle size, it is difficult to define the 

accuracy of PM10–2.5 measurements in an 

absolute sense. Furthermore, generation of 

credible PM10–2.5 concentration standards at 

field monitoring sites and presenting or 

introducing such standards reliably to 

samplers or monitors to assess accuracy is 

still generally impractical. The accuracy of 

PM10–2.5 measurements is therefore defined 

in a relative sense as bias, referenced to 

measurements provided by other reference 

method samplers or based on flow rate 

verification audits or checks, or on other 

performance evaluation procedures. 

4.2 Measurement system bias for 

monitoring data is assessed according to the 

procedures and schedule set forth in part 58, 

appendix A of this chapter. The goal for the 

measurement uncertainty (as bias) for 

monitoring data is defined in part 58, 

appendix A of this chapter as an upper 95 

percent confidence limit for the absolute bias 

of 15 percent. Reference 1 in section 13 of 

this appendix provides additional 

information and guidance on flow rate 

accuracy audits and assessment of bias. 

5.0 Precision 

5.1 Tests to establish initial measurement 

precision for each sampler of the reference 

method sampler pair are specified as a part 

of the requirements for designation as a 

reference method under part 53 of this 

chapter. 

5.2 Measurement system precision is 

assessed according to the procedures and 

schedule set forth in appendix A to part 58 

of this chapter. The goal for acceptable 

measurement uncertainty, as precision, of 

monitoring data is defined in part 58, 

appendix A of this chapter as an upper 95 

percent confidence limit for the coefficient of 

variation (CV) of 15 percent. Reference 1 in 

section 13 of this appendix provides 

additional information and guidance on this 

requirement. 

6.0 Filters for PM10c and PM2.5 Sample 

Collection. Sample collection filters for both 

PM10c and PM2.5 measurements shall be 

identical and as specified in section 6 of 

appendix L to this part. 

7.0 Sampler. The PM10–2.5 sampler shall 

consist of a PM10c sampler and a PM2.5

sampler, as follows: 

7.1 The PM2.5 sampler shall be as 

specified in section 7 of appendix L to this 

part. 

7.2 The PM10c sampler shall be of like 

manufacturer, design, configuration, and 

fabrication to that of the PM2.5 sampler and 

as specified in section 7 of appendix L to this 

part, except as follows: 

7.2.1 The particle size separator specified 

in section 7.3.4 of appendix L to this part 

shall be eliminated and replaced by a 

downtube extension fabricated as specified 

in Figure O–1 of this appendix. 

7.2.2 The sampler shall be identified as a 

PM10c sampler on its identification label 

required under § 53.9(d) of this chapter. 

7.2.3 The average temperature and 

average barometric pressure measured by the 

sampler during the sample period, as 

described in Table L–1 of appendix L to this 

part, need not be reported to EPA’s AQS data 

base, as required by section 7.4.19 and Table 

L–1 of appendix L to this part, provided such 

measurements for the sample period 

determined by the associated PM2.5 sampler 

are reported as required. 

7.3 In addition to the operation/ 

instruction manual required by section 7.4.18 

of appendix L to this part for each sampler, 

supplemental operational instructions shall 

be provided for the simultaneous operation 

of the samplers as a pair to collect concurrent 

PM10c and PM2.5 samples. The supplemental 

instructions shall cover any special 

procedures or guidance for installation and 

setup of the samplers for PM10–2.5

measurements, such as synchronization of 

the samplers’ clocks or timers, proper 

programming for collection of concurrent 

samples, and any other pertinent issues 

related to the simultaneous, coordinated 

operation of the two samplers. 

7.4 Capability for electrical 

interconnection of the samplers to simplify 

sample period programming and further 

ensure simultaneous operation is encouraged 

but not required. Any such capability for 

interconnection shall not supplant each 

sampler’s capability to operate 

independently, as required by section 7 of 

appendix L of this part. 

8.0 Filter Weighing 

8.1 Conditioning and weighing for both 

PM10c and PM2.5 sample filters shall be as 

specified in section 8 of appendix L to this 

part. See reference 1 of section 13 of this 

appendix for additional, more detailed 

guidance. 

8.2 Handling, conditioning, and weighing 

for both PM10c and PM2.5 sample filters shall 

be matched such that the corresponding 

PM10c and PM2.5 filters of each filter pair 

receive uniform treatment. The PM10c and 

PM2.5 sample filters should be weighed on 

the same balance, preferably in the same 

weighing session and by the same analyst. 

8.3 Due care shall be exercised to 

accurately maintain the paired relationship 

of each set of concurrently collected PM10c 

and PM2.5 sample filters and their net weight 

gain data and to avoid misidentification or 

reversal of the filter samples or weight data. 

See Reference 1 of section 13 of this 

appendix for additional guidance. 

9.0 Calibration. Calibration of the flow 

rate, temperature measurement, and pressure 

measurement systems for both the PM10c and 

PM2.5 samplers shall be as specified in 

section 9 of appendix L to this part. 

10.0 PM10–2.5 Measurement Procedure 

10.1 The PM10c and PM2.5 samplers shall 

be installed at the monitoring site such that 

their ambient air inlets differ in vertical 

height by not more than 0.2 meter, if 

possible, but in any case not more than 1 

meter, and the vertical axes of their inlets are 

separated by at least 1 meter but not more 

than 4 meters, horizontally. 

10.2 The measurement procedure for 

PM10c shall be as specified in section 10 of 

appendix L to this part, with ‘‘PM10c’’

substituted for ‘‘PM2.5’’ wherever it occurs in 

that section. 

10.3 The measurement procedure for 

PM2.5 shall be as specified in section 10 of 

appendix L to this part. 

10.4 For the PM10–2.5 measurement, the 

PM10c and PM2.5 samplers shall be 

programmed to operate on the same schedule 

and such that the sample period start times 

are within 5 minutes and the sample 

duration times are within 5 minutes. 

10.5 Retrieval, transport, and storage of 

each PM10c and PM2.5 sample pair following 

sample collection shall be matched to the 

extent practical such that both samples 

experience uniform conditions. 

11.0 Sampler Maintenance. Both PM10c

and PM2.5 samplers shall be maintained as 

described in section 11 of appendix L to this 

part. 

12.0 Calculations 

12.1 Both concurrent PM10c and PM2.5

measurements must be available, valid, and 

meet the conditions of section 10.4 of this 

appendix to determine the PM10–2.5 mass 

concentration. 

12.2 The PM10c mass concentration is 

calculated using equation 1 of this section: 

Equation 1

PM

W W

V c

f i

a

10 = ( ) −

Where: 

PM10c = mass concentration of PM10c, μg/m3; 

Wf, Wi = final and initial masses (weights), 

respectively, of the filter used to collect 

the PM10c particle sample, μg; 

Va = total air volume sampled by the PM10c

sampler in actual volume units measured 

at local conditions of temperature and 

pressure, as provided by the sampler, m3. 

Note: Total sample time must be between 

1,380 and 1,500 minutes (23 and 25 hrs) for 

a fully valid PM10c sample; however, see also 

section 3.3 of this appendix. 


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12.3 The PM2.5 mass concentration is 

calculated as specified in section 12 of 

appendix L to this part. 

12.4 The PM10¥2.5 mass concentration, in 

μg/m3, is calculated using Equation 2 of this 

section: 

 Equation 2

PM PM PM 10 2 5 10 2 5 − . . = −c

13.0 Reference 

1. Quality Assurance Guidance Document 

2.12. Monitoring PM2.5 in Ambient Air Using 

Designated Reference or Class I Equivalent 

Methods. Draft, November 1998 (or later 

version or supplement, if available). 

Available at: www.epa.gov/ttn/amtic/ 

pgqa.html. 

14.0 Figures 

Figure O–1 is included as part of this 

appendix O. 

BILLING CODE 6560–50–P 


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[FR Doc. 06–8477 Filed 10–16–06; 8:45 am] 

BILLING CODE 6560–50–C 


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