Document ID: EPA-HQ-OAR-2001-0017-0707
Agency: epa
Document Type: Proposed Rule
Title: National Ambient Air Quality Standards for Particulate Matter
Posted Date: 2006-01-17T14:19:46Z

[Federal Register: January 17, 2006 (Volume 71, Number 10)]
[Proposed Rules]               
[Page 2619-2708]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr17ja06-21]                         

[[Page 2619]]

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Part II

Environmental Protection Agency

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40 CFR Part 50

National Ambient Air Quality Standards for Particulate Matter; Proposed 
Rule

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[OAR-2001-0017; FRL-8015-8]
RIN 2060-AI44

 
National Ambient Air Quality Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: Based on its review of the air quality criteria and national 
ambient air quality standards (NAAQS) for particulate matter (PM), EPA 
proposes to make revisions to the primary and secondary NAAQS for PM to 
provide requisite protection of public health and welfare, 
respectively, and to make corresponding revisions in monitoring 
reference methods and data handling conventions for PM.
    With regard to primary standards for fine particles (particles 
generally less than or equal to 2.5 micrometers ([mu]m) in diameter, 
PM2.5), EPA proposes to revise the level of the 24-hour 
PM2.5 standard to 35 micrograms per cubic meter ([mu]g/
m3), providing increased protection against health effects 
associated with short-term exposure (including premature mortality and 
increased hospital admissions and emergency room visits) and to retain 
the level of the annual PM2.5 standard at 15 [mu]g/
m3, continuing protection against health effects associated 
with long-term exposure (including premature mortality and development 
of chronic respiratory disease). The EPA solicits comment on 
alternative levels of the 24-hour PM2.5 standard (down to 25 
[mu]g/m3 and up to 65 [mu]g/m3) and the annual 
PM2.5 standard (down to 12 [mu]g/m3), and on 
alternative approaches for selecting the standard levels.
    With regard to primary standards for particles generally less than 
or equal to 10 [mu]m in diameter (PM10), EPA proposes to 
revise the 24-hour PM10 standard in part by establishing a 
new indicator for thoracic coarse particles (particles generally 
between 2.5 and 10 [mu]m in diameter, 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 by industrial sources and construction sources, and 
excludes 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 EPA proposes to set the new PM10-2.5 
standard at a level of 70 [mu]g/m3, continuing to provide a 
generally equivalent level of protection against health effects 
associated with short-term exposure (including hospital admissions for 
cardiopulmonary diseases, increased respiratory symptoms and possibly 
premature mortality). Also, EPA proposes to revoke, upon finalization 
of a primary 24-hour standard for PM10-2.5, the current 24-
hour PM10 standard in all areas of the country except in 
areas where there is at least one monitor located in an urbanized area 
(as defined by the U.S. Bureau of the Census) with a minimum population 
of 100,000 that violates the current 24-hour PM10 standard 
based on the most recent three years of data. In addition, EPA proposes 
to revoke the current annual PM10 standard upon promulgation 
of this rule. The EPA solicits comment on alternative approaches for 
selecting the level of a 24-hour PM10-2.5 standard, on 
alternative approaches based on retaining the current 24-hour 
PM10 standard, and on revoking and not replacing the 24-hour 
PM10 standard.
    With regard to secondary PM standards, EPA proposes to revise the 
current standards by making them identical to the suite of proposed 
primary standards for fine and coarse particles, providing protection 
against PM-related public welfare effects including visibility 
impairment, effects on vegetation and ecosystems, and materials damage 
and soiling. Also, EPA solicits comment on adding a new sub-daily 
PM2.5 standard to address visibility impairment.

DATES: Written comments on this proposed decision must be received by 
April 17, 2006.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2001-0017 by one of the following methods:
     http://www.regulations.gov: Follow the on-line 

instructions for submitting comments.
     E-mail: a-and-r-Docket@epa.gov.
     Fax: 202-566-1749.
     Mail: Docket ID No. EPA-HQ-OAR-2001-0017, Environmental 
Protection Agency, Mailcode: 6102T, 1200 Pennsylvania Avenue, NW., 
Washington, DC 20460. Please include a total of two copies.
     Hand Delivery: Environmental Protection Agency, EPA West 
Building, Room B102, 1301 Constitution Avenue, NW., Washington, DC. 
Such deliveries are only accepted during the Docket's normal hours of 
operation, and special arrangements should be made for deliveries of 
boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2001-0017. The EPA's policy is that all comments received will be 
included in the public docket without change and may be made available 
online at http://www.regulations.gov, including any personal 

information provided, unless the comment includes information claimed 
to be Confidential Business Information (CBI) or other information 
whose disclosure is restricted by statute. Do not submit information 
that you consider to be CBI or otherwise protected through http://www.regulations.gov or e-mail. The http://www.regulations.gov Web site 

is an ``anonymous access'' system, which means EPA will not know your 
identity or contact information unless you provide it in the body of 
your comment. If you send an e-mail comment directly to EPA without 
going through http://www.regulations.gov your e-mail address will be 

automatically captured and included as part of the comment that is 
placed in the public docket and made available on the Internet. If you 
submit an electronic comment, EPA recommends that you include your name 
and other contact information in the body of your comment and with any 
disk or CD-ROM you submit. If EPA cannot read your comment due to 
technical difficulties and cannot contact you for clarification, EPA 
may not be able to consider your comment. Electronic files should avoid 
the use of special characters, any form of encryption, and be free of 
any defects or viruses. For additional information about EPA's public 
docket visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm
.

    Docket: All documents in the docket are listed in the http://www.regulations.gov
 index. Although listed in the index, some 

information is not publicly available, e.g., CBI or other information 
whose disclosure is restricted by statute. Certain other material, such 
as copyrighted material, will be publicly available only in hard copy. 
Publicly available docket materials are available either electronically 
in http://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. The Public Reading Room is open 
from 8:30 a.m. to 4:30 p.m. Monday through Friday, excluding legal 
holidays. The telephone number for the Public Reading Room is 202-566-
1744 and the telephone number for the Air and Radiation Docket and 
Information Center is 202-566-1742.
    Public Hearings: The EPA intends to hold public hearings around the 
end of

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February in Philadelphia, Chicago, and San Francisco, and will announce 
in a separate Federal Register notice the date, time, and address of 
the public hearings on this proposed decision.

FOR FURTHER INFORMATION CONTACT: Dr. Erika Sasser, mail code C539-01, 
Air Quality Strategies and Standards Division, Office of Air Quality 
Planning and Standards, U.S. Environmental Protection Agency, Research 
Triangle Park, North Carolina 27711, telephone: (919) 541-3889, e-mail: 
sasser.erika@epa.gov.

SUPPLEMENTARY INFORMATION: 

General Information

A. What Should I Consider As I Prepare My Comments for EPA?

    1. Submitting CBI. Do not submit this information to EPA through 
http://www.regulations.gov or e-mail. Clearly mark the part or all of 

the information that you claim to be CBI. For CBI information in a disk 
or CD-ROM that you mail to EPA, mark the outside of the disk or CD-ROM 
as CBI and then identify electronically within the disk or CD-ROM the 
specific information that is claimed as CBI. In addition to one 
complete version of the comment that includes information claimed as 
CBI, a copy of the comment that does not contain the information 
claimed as CBI must be submitted for inclusion in the public docket. 
Information so marked will not be disclosed except in accordance with 
procedures set forth in 40 CFR part 2.
    2. Tips for Preparing Your Comments. When submitting comments, 
remember to:
     Identify the rulemaking by docket number and other 
identifying information (subject heading, Federal Register date and 
page number).
     Follow directions--The agency may ask you to respond to 
specific questions or organize comments by referencing a Code of 
Federal Regulations (CFR) part or section number.
     Explain why you agree or disagree; suggest alternatives 
and substitute language for your requested changes.
     Describe any assumptions and provide any technical 
information and/or data that you used.
     If you estimate potential costs or burdens, explain how 
you arrived at your estimate in sufficient detail to allow for it to be 
reproduced.
     Provide specific examples to illustrate your concerns, and 
suggest alternatives.
     Explain your views as clearly as possible, avoiding the 
use of profanity or personal threats.
     Make sure to submit your comments by the comment period 
deadline identified.

Availability of Related Information

    A number of documents are available on EPA Web sites. The Air 
Quality Criteria for Particulate Matter (Criteria Document) (two 
volumes, EPA/600/P-99/002aF and EPA/600/P-99/002bF, October 2004) is 
available on EPA's National Center for Environmental Assessment Web 
site. To obtain this document, go to http://www.epa.gov/ncea, and click 

on ``Particulate Matter''. The Staff Paper, human health risk 
assessment, and several other related technical documents are available 
on EPA's Office of Air Quality Planning and Standards (OAQPS) 
Technology Transfer Network (TTN) Web site. The Staff Paper is 
available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_sp.html
, and the risk assessment and technical documents are available 

at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_td.html. These 

and other related documents are also available for inspection and 
copying in the EPA docket identified above.

Table of Contents

    The following topics are discussed in today's preamble:

I. Background
    A. Legislative Requirements
    B. Review of Air Quality Criteria and Standards for PM
    C. Related Control Programs to Implement PM Standards
    D. Overview of Current PM NAAQS Review
II. Rationale for Proposed Decisions on Primary PM2.5 
Standards
    A. Health Effects Related to Exposure to Fine Particles
    1. Mechanisms
    2. Nature of Effects
    3. Integration and Interpretation of the Health Evidence
    4. Sensitive Subgroups for PM2.5-Related Effects
    5. PM2.5-Related Impacts on Public Health
    B. Quantitative Risk Assessment
    1. Overview
    2. Scope and Key Components
    3. Risk Estimates and Key Observations
    C. Need for Revision of the Current Primary PM2.5 
Standards
    D. Indicator of Fine Particles
    E. Averaging Time of Primary PM2.5 Standards
    F. Form of Primary PM2.5 Standards
    1. 24-Hour PM2.5 Standard
    2. Annual PM2.5 Standard
    G. Level of Primary PM2.5 Standards
    1. 24-Hour PM2.5 Standard
    2. Annual PM2.5 Standard
    H. Proposed Decisions on Primary PM2.5 Standards
III. Rationale for Proposed Decisions on the Primary PM10 
Standards
    A. Health Effects Related to Exposure to Thoracic Coarse 
Particles
    1. Mechanisms
    2. Nature of Effects
    3. Integration and Interpretation of the Health Evidence
    4. Sensitive Subgroups for Effects of Thoracic Coarse Particle 
Exposure
    5. Impacts on Public Health from Thoracic Coarse Particle 
Exposure
    B. Quantitative Risk Assessment
    C. Need for Revision of the Current Primary PM10 
Standards
    D. Indicator of Thoracic Coarse Particles
    E. Averaging Time of Primary PM10-2.5 Standard
    F. Form of Primary PM10-2.5 Standard
    G. Level of Primary PM10-2.5 Standard
    H. Proposed Decisions on Primary PM10-2.5 Standard
IV. Rationale for Proposed Decisions on Secondary PM Standards
    A. Visibility Impairment
    1. Visibility Impairment Related to Ambient PM
    2. Need for Revision of the Current Secondary PM Standards for 
Visibility Protection
    3. Indicator of PM for Secondary Standard to Address Visibility 
Impairment
    4. Averaging Time of a Secondary PM2.5 Standard for 
Visibility Protection
    5. Elements of a Secondary PM2.5 Standard for 
Visibility Protection
    B. Other PM-related Welfare Effects
    1. Nature of Effects
    2. Need for Revision of Current Secondary PM Standards to 
Address Other PM-related Welfare Effects
    C. Proposed Decision on Secondary PM Standards
V. Interpretation of the NAAQS for PM
    A. Proposed 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. Secondary Standard
    5. Conforming Revisions
    B. Proposed Appendix P--Interpretation of the National Ambient 
Air Quality Standards for PM10-2.5
    1. General
    2. PM2.5 Data Reporting Considerations
    3. PM10-2.5 Computations and Data Handling 
Conventions
    4. Exceptional Events
VI. Reference Methods for the Determination of Particulate Matter as 
PM2.5 and PM10-2.5
    A. Proposed Appendix O: Reference Method for the Determination 
of Coarse Particulate Matter (as PM10-2.5) in the 
Atmosphere
    1. Purpose of the New Reference Method
    2. Rationale for Selection of the New Reference Method
    3. Consideration of Other Methods for the Federal Reference 
Method
    4. Consideration of Automated Method
    5. Relationship of Proposed FRM to Transportation Equity Act 
Requirements
    6. Use of the Proposed Federal Reference Method

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    7. Basic Requirements of the Proposed Federal Reference Method 
Sampler
    8. Other Important Aspects of the Proposed Federal Reference 
Method Sampler
    B. Proposed Amendments to Appendix L--Reference Method for the 
Determination of Fine Particulate Matter (as PM2.5) in 
the Atmosphere
    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 and 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
References

I. Background

A. 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\
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    \1\ The legislative history of section 109 indicates that a 
primary standard is to be set at ``the maximum permissible ambient 
air level * * * which will protect the health of any [sensitive] 
group of the population,'' and that for this purpose ``reference 
should be made to a representative sample of persons comprising the 
sensitive group rather than to a single person in such a group'' [S. 
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970)].
    \2\ Welfare effects as defined in section 302(h) [42 U.S.C. 
7602(h)] include, but are not limited to, ``effects on soils, water, 
crops, vegetation, man-made materials, animals, wildlife, weather, 
visibility and climate, damage to and deterioration of property, and 
hazards to transportation, as well as effects on economic values and 
on personal comfort and well-being.''
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    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 ``requisite'' 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).
    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 background concentration 
levels (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 
Industries Association v. EPA, supra, 647 F.2d at 1161-62.
    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 * * 
*.'' 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.

B. Review of Air Quality Criteria and Standards 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.
    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 
[mu]m in

[[Page 2623]]

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 mean aerodynamic diameter less than or equal to 2.5 [mu]m), and 
retained PM10 standards for the purpose of regulating the 
coarse fraction of PM10 (referred to as thoracic coarse 
particles or coarse-fraction particles; generally including particles 
with a nominal mean aerodynamic diameter greater than 2.5 [mu]m and 
less than or equal to 10 [mu]m, or PM10-2.5). The EPA 
established two new PM2.5 standards: an annual standard of 
15 [mu]g/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 [mu]g/
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) (rehearing granted in part and denied in part, 195 F.3d 4 
(D.C. Cir. 1999), affirmed in part and reversed in part, Whitman v. 
American Trucking Associations, 531 U.S. 457 (2001). The Panel also 
found ``ample support'' for EPA's decision to regulate coarse particle 
pollution, but vacated the 1997 PM10 standards, concluding 
in part that PM10 is a ``poorly matched indicator for coarse 
particulate pollution'' because it includes fine particles. Id. at 
1053-55. Pursuant to the court's decision, EPA removed the vacated 1997 
PM10 standards from the Code of Federal 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 pre-existing 
1987 PM10 standards to the 1997 PM10 standards 
(65 FR 80776, December 22, 2000). The pre-existing 1987 PM10 
standards remained in place. Id. at 80777.
    More generally, the three-judge panel held (with one dissenting 
opinion) 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 drawing lines'' to determine where the standards should be set. 
Consistent with EPA's long-standing interpretation, 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. 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).
    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. 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 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.
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    \3\ The HEI is an independent research institute, jointly 
sponsored by EPA and a group of U.S. manufacturers and marketers of 
motor vehicles and engines, that conducts health effects research on 
major air pollutants related to motor vehicle emissions.
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    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.

[[Page 2624]]

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. 
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, 
issued in June, 2005, 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).\4\
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    \4\ The EPA has posted on its Web site (http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_index.html
) a second edition of the Staff 

Paper which was prepared for the purpose of including as an 
attachment this September 2005 letter from CASAC.
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    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.

C. 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 (42 U.S.C. 7470-7479) for these pollutants. In addition, 
Federal programs provide for nationwide reductions in emissions of 
these and other air pollutants through 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 or 
off-highway, 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. 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. 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 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) also have helped reduce carbon and nitrates, both of 
which are components of PM2.5. 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 
reduce SO2 and NOX emissions from electric 
generating units and industrial boilers across the eastern half of the 
U.S., regulations to implement the current 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.
    Modeling done by EPA indicates that by 2010, 18 of the 39 areas 
currently not attaining the PM2.5 standards will come into 
attainment just based on regulatory programs already in place, 
including CAIR, the Clean Diesel Rules, and other Federal measures. 
Four more PM2.5 areas are projected to attain the standards 
by 2015 based on the implementation of these programs. All areas in the 
eastern U.S. will have lower PM2.5 concentrations in 2015 
relative to present-day conditions. In most cases,

[[Page 2625]]

the predicted improvement in PM2.5 ranges from 10 percent to 
20 percent.

D. Overview of Current PM NAAQS Review

    This action presents the Administrator's proposed 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 separately below in sections II 
and III, respectively, consistent with the decision made by EPA in the 
last review and with the conclusions in the Criteria Document and Staff 
Paper that fine and thoracic coarse particles should continue to be 
considered as separate subclasses of PM pollution. Thus, the principal 
focus of this current review of the air quality criteria and primary 
standards for PM is on evidence of health effects and risks related to 
exposures to fine particles and to thoracic coarse particles. Secondary 
standards for fine and coarse-fraction particles are addressed below in 
section IV.
    Past and current decisions to address fine particles and thoracic 
coarse particles separately are based in part on long-established 
information on differences in sources, properties, and atmospheric 
behavior between fine and coarse particles (EPA, 2005a, 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, resuspension of 
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 approach 
in this review to continue to address fine and thoracic coarse 
particles separately is reinforced by new information that advances 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, 2004, 
section 3.2.3). See also American Trucking Associations v. EPA, 175 F. 
3d at 1053-54, 1055-56 (EPA justified in establishing separate 
standards for fine and thoracic coarse particles).
    Today's proposed decisions separately addressing fine and coarse 
particles are based on a thorough review in the Criteria Document of 
the latest 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 proposed decisions 
also take into account: (1) Staff assessments in the Staff Paper of the 
most policy-relevant information in the Criteria Document and as well 
as a quantitative risk assessment; (2) CASAC advice and 
recommendations, as reflected in the CASAC's letters to the 
Administrator, 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 \5\ (henceforth, 
``CASAC Panel''), and (3) public comments received during the 
development of these documents, either in connection with CASAC 
meetings or separately.
---------------------------------------------------------------------------

    \5\ The CASAC PM Review Panel is comprised of the seven members 
of the chartered CASAC, supplemented by fifteen subject-matter 
experts appointed by the Administrator to provide the types of 
scientific expertise relevant to this review of the PM NAAQS.
---------------------------------------------------------------------------

    The EPA is aware that a number of new scientific studies on the 
health effects of PM have been published since the 2002 cutoff date for 
inclusion in the Criteria Document. As in the last PM NAAQS review, EPA 
intends to conduct a review and assessment of any significant new 
studies published since the close of the Criteria Document, including 
studies submitted during the public comment period in order to ensure 
that, before making a final decision, the Administrator is fully aware 
of the new science that has developed since 2002. In this assessment, 
EPA will examine these new studies in light of the literature evaluated 
in the Criteria Document. This assessment and a summary of the key 
conclusions will be placed in the rulemaking docket. A preliminary list 
of potentially significant new studies identified to date has been 
compiled and placed in the rulemaking docket for this proposal, and EPA 
solicits comment on other relevant studies that may be added to this 
list. This list includes a wide array of different types of studies 
that are potentially relevant to various issues discussed in the 
following sections, including issues related to the elements of the 
standards under review.
    Throughout this preamble a number of conclusions, findings, and 
determinations by the Administrator are noted. It should be understood 
that these are all provisional and proposed in nature. While they 
identify the reasoning that supports this proposal, they are not 
intended to be final or conclusive in nature. The EPA invites comments 
on all issues involved with this proposal, including all such proposed 
judgments, conclusions, findings, and determinations.

II. Rationale for Proposed Decisions on Primary PM2.5 Standards

    As discussed more fully below, the rationale for the proposed 
revisions of 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, would be 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 of associations between 
exposure to ambient fine particles and a broad range of health 
endpoints (EPA, 2004, Chapter 9), focusing on those health endpoints 
for which the Criteria Document concludes 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 making 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. As discussed below, however, an 
unprecedented amount of new research has been conducted since the last 
review, with important new information coming from epidemiologic, 
toxicologic, controlled human exposure,

[[Page 2626]]

and dosimetric studies. Moreover, the newly available research studies 
evaluated in the Criteria Document 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 has provided 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.

A. Heath Effects Related to Exposure to Fine Particles

    This section 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 fine particles and their 
major constituents. The information highlighted here summarizes: (1) 
New information available on potential mechanisms for health effects 
associated with exposure to fine particles and constituents; (2) the 
nature of the effects that have been associated with ambient fine 
particles or fine particle constituents; (3) an integrative assessment 
of the evidence on fine particle-related health effects; (4) 
subpopulations that appear to be sensitive to effects of exposure to 
fine particles; and (5) the public health impact of exposure to ambient 
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 
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 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 new 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 particles levels and personal exposures to fine particles of 
ambient origin.
    (5) New analyses and approaches to addressing issues related to 
potential confounding by gaseous co-pollutants, possible thresholds for 
effects, and measurement error and exposure misclassification.\6\
---------------------------------------------------------------------------

    \6\ ``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, 2004, 
section 8.1.3.2; EPA, 2005a, section 3.6.4). A ``threshold'' is a 
concentration below which it is expected that effects are not 
observed (EPA, 2004, section 8.4.7; EPA, 2005a, section 3.6.6). 
``Gaseous co-pollutants'' generally refer to other commonly-occuring 
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, 2004, section 8.4.5; 
EPA, 2005a, section 3.6.2)
---------------------------------------------------------------------------

    (6) Preliminary attempts to evaluate the effects of fine particles 
from different sources (e.g., motor vehicles, coal combustion, 
vegetative burning, crustal \7\ ), using factor analysis or source 
apportionment methods with fine particle speciation data.
---------------------------------------------------------------------------

    \7\ ``Crustal'' is used here to describe particles of geologic 
origin, which can be found in both fine- and coarse-fraction PM.
---------------------------------------------------------------------------

    (7) Several 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 with findings from studies on 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.
1. Mechanisms
    In the last review, EPA 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. Much new evidence is now available on potential 
mechanisms or pathways for PM-related effects, ranging from effects on 
the respiratory system to indicators of cardiovascular response; these 
new findings are discussed in depth in Chapter 7 of the Criteria 
Document. While questions remain, the new findings have advanced our 
understanding of the complex and different patterns of particle 
deposition and clearance in the respiratory tract and provide insights 
into potential mechanisms for PM-related effects and support the 
plausibility of the findings of epidemiologic studies.
    Although there are differences among the size fractions of 
particles, fine particles, including accumulation mode and ultrafine 
particles, and thoracic coarse particles can all penetrate into and be 
deposited in the tracheobronchial and alveolar regions of the 
respiratory tract (i.e., the ``thoracic'' regions).\8\ Penetration into 
the tracheobronchial and alveolar regions is greater for accumulation 
mode particles than for coarse or ultrafine particles, since coarse and 
ultrafine particles are more efficiently removed from the air in the 
extrathoracic region than are accumulation-mode fine particles; the 
evidence from dosimetric studies is

[[Page 2627]]

reviewed in detail in Chapter 6 of the Criteria Document.
---------------------------------------------------------------------------

    \8\ Particles are often classified in modes based on their 
distribution by characteristics such as mass, surface area, and 
particle number. ``Coarse mode'' particles are those with diameters 
mostly greater than the minimum in the particle mass distribution, 
which generally occurs between about 1 and 3 [mu]m. ``Accumulation 
mode'' particles are those with diameters from about 0.1 [mu]m to 
between about 1 and 3 [mu]m. Ultrafine particles are generally those 
with diameters below about 0.1 [mu]m (EPA, 2004, pages 2-14).
---------------------------------------------------------------------------

    Fine particles have varying physical or chemical characteristics 
that may influence health responses. Physical characteristics that may 
be of importance are solubility or physical state of the particles 
(e.g., solid, liquid). Fine particle components include metals, acids, 
organic compounds, biogenic constituents, sulfate and nitrate salts, 
elemental carbon, and reactive components such as peroxides; size and 
surface area of the particles can also influence health responses. By 
way of illustration, Mauderly et al. (1998) discussed particle 
components or characteristics hypothesized to contribute to health, 
producing an illustrative list of 11 components or characteristics of 
interest for which some evidence existed. The list included: (1) 
Particle mass concentration, (2) particle size/surface area, (3) 
ultrafine particles, (4) metals, (5) acids, (6) organic compounds, (7) 
biogenic particles, (8) sulfate and nitrate salts, (9) peroxides, (10) 
soot, and (11) co-factors, including effects modification or 
confounding by co-occurring gases and meteorology. The authors stressed 
that this list is neither definitive nor exhaustive, and note that ``it 
is generally accepted as most likely that multiple toxic species act by 
several mechanistic pathways to cause the range of health effects that 
have been observed'' (Mauderly et al., 1998). The range of health 
outcomes linked with fine particle exposures is also broad, including 
effects on the cardiovascular and respiratory systems, and potential 
links with developmental effects in children (e.g., low birth weight) 
and death from lung cancer. It appears unlikely that the complex mixes 
of particles that are present in ambient air would act alone through 
any single pathway of response. Accordingly, it is plausible that 
several physiological responses might occur in concert to produce 
reported health endpoints.
    As discussed in section 7.10 of the Criteria Document, the 
potential pathways for direct effects on the respiratory system include 
lung injury and inflammation, increased airway reactivity and asthma 
exacerbation, and increased susceptibility to respiratory infections. 
New toxicologic or controlled human exposure studies have reported some 
evidence of inflammatory responses in animals, as well as increased 
susceptibility to infections. Toxicologic studies also report evidence 
of lung injury, inflammation, or altered host defenses with exposure to 
ambient particles or particle constituents. Some toxicologic evidence, 
particularly from results of studies using diesel exhaust particle 
exposures, also indicates that PM can aggravate asthmatic symptoms or 
increase airway reactivity.
    Potential pathways for fine particle-related effects also include 
systemic effects that are secondary to effects in the respiratory 
system. These include impairment of lung function leading to cardiac 
effects, pulmonary inflammation and cytokine production leading to 
systemic hemodynamic effects, lung inflammation leading to increased 
blood coagulability, and lung inflammation leading to hematopoiesis 
effects. While more limited than for direct pulmonary effects, some new 
toxicologic studies suggest that injury or inflammation in the 
respiratory system can lead to changes in heart rhythm, reduced 
oxygenation of the blood, changes in blood cell counts, and changes in 
the blood that can increase the risk of blood clot formation, a risk 
factor for heart attacks and strokes. In addition, health studies have 
suggested potential pathways for effects on the heart that include 
effects related to uptake of particles or particle constituents in the 
blood, and effects on the autonomic control of the heart and 
circulatory system. In the last review, little or no evidence was 
available from toxicologic studies on potential cardiovascular effects. 
More recent studies have provided some initial evidence that particles 
can have direct cardiovascular effects. Particle deposition in the 
respiratory system also could lead to cardiovascular effects, such as 
fine particle-induced pulmonary reflexes resulting in changes in the 
autonomic nervous system that then could affect heart rhythm. Also, 
inhaled fine particles could affect the heart or other organs if 
particles or particle constituents are released into the circulatory 
system from the lungs; some new evidence indicates that the smaller 
ultrafine particles or their soluble constituents can move directly 
from the lungs into systemic circulation.
    The potential mechanisms and/or general pathways for effects 
discussed above are primarily effects related to short-term rather than 
long-term exposure to fine particles; for the most part, air pollution 
toxicologic studies are not designed to assess long-term exposure 
effects. While repeated occurrences of some short-term insults, such as 
inflammation, might contribute to long-term effects, it is likely that 
wholly different mechanisms are involved in the development of chronic 
health responses. Some mechanistic evidence is available, however, for 
potential carcinogenic or genotoxic effects of ambient fine particles 
and combustion products of coal, wood, diesel, and gasoline (discussed 
in section 7.8 of the Criteria Document).
    Overall, the findings 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. In 
evaluating the new body of evidence, the Criteria Document states: 
``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 biologic responses are produced by PM 
whether it is composed of a single component or a complex mixture'' 
(EPA, 2004, p. 7-206).
2. Nature of Effects
    In the last review, evidence from health studies indicated that 
exposure to PM (using various indicators) was associated with premature 
mortality and indices of morbidity including respiratory hospital 
admissions and emergency room visits, school absences, work loss days, 
restricted activity days, effects on lung function and symptoms, 
morphological changes, and altered host defense mechanisms.\9\ As 
reviewed in Chapter 8 of the Criteria Document, recent epidemiologic 
studies have continued to report associations between short-term 
exposure to fine particles or fine particle indicators, and effects 
such as premature mortality, hospital admissions or emergency 
department visits for respiratory disease, and effects on lung function 
and symptoms. In addition, recent epidemiologic studies have provided 
some new evidence linking short-term fine particle exposures to effects 
on the cardivascular system, including cardiovascular hospital 
admissions and more subtle indicators of cardiovascular health. Long-
term exposure to PM2.5 and sulfates has also been associated 
with mortality from cardiopulmonary diseases and lung cancer, and 
effects on the respiratory system such as decreased lung function or 
the development of chronic respiratory disease. The

[[Page 2628]]

evidence for such effects is summarized below.

    \9\ Historical reports of dramatic pollution episodes, 
considered in the 1987 review of the PM NAAQS, provided clear 
evidence of mortality associated with high levels of PM and other 
pollutants, such as the air pollution episode that occurred in 
London in 1952 (EPA, 1996a, pp. 12-28 to 12-31).
---------------------------------------------------------------------------

BILLING CODE 6560-50-P
[GRAPHIC] [TIFF OMITTED] TP17JA06.048

BILLING CODE 6560-50-C

[[Page 2629]]

a. Effects Associated With Short-Term Exposure to Fine Particles

    Numerous epidemiologic studies have demonstrated statistical 
associations between short-term exposure to fine particles and health 
outcomes ranging from total mortality to respiratory symptoms, as 
discussed below. Figure 1 summarizes results from both multi-city and 
single-city epidemiologic studies using short-term exposures to 
PM2.5, including all U.S. and Canadian studies that used 
direct measurements of PM2.5 and for which effect estimates 
and confidence intervals were reported.\10\ The central effect estimate 
is indicated by a diamond for each study result, with the vertical bar 
representing the 95 percent confidence interval around the estimate. In 
the discussions that follow, an individual study result is considered 
to be statistically significant if the 95 percent confidence interval 
does not include zero. Positive effect estimates indicate increases in 
the health outcome with PM2.5 exposure. In considering these 
results as a whole, it is important to consider not only whether 
statistical significance at the 95 percent confidence level is reported 
in individual studies, but also the general pattern of results, 
focusing in particular on studies with greater statistical power that 
report relatively more precise results.
---------------------------------------------------------------------------

    \10\ In the following discussion of specific studies, results 
from single-pollutant models are referred to, as shown in Figure 1, 
unless otherwise noted.
---------------------------------------------------------------------------

i. Mortality
    Since the last review, a large number of new time-series studies of 
the relationship between short-term exposure to PM, including 
PM2.5, and mortality have been published, including several 
multi-city studies that are responsive to the recommendations from the 
last review. As discussed in section 8.2 of the Criteria Document, 
these include studies that have been conducted in single cities or 
locations in the U.S. or Canada, as well as Mexico City and locations 
in Europe, South America, Asia, and Australia.
    Several recent multi-city studies have been published since the 
last review that are of particular relevance for this review. The 
results of multi-city studies on associations between PM10 
and mortality across 90 U.S. cities (Dominici, 2003) and across ten 
U.S. cities (Schwartz, 2003b), while not specifically on fine 
particles, have provided important new information to help address 
uncertainties regarding a number of issues, including model 
specification, potential confounding by co-pollutants and the form of 
concentration-response functions (EPA, 2004, section 8.2.2.3). Two 
multi-city studies have included measurements of PM2.5; one 
was conducted in six U.S. cities (Schwartz et al., 2003a; Klemm and 
Mason, 2003) and the other in eight Canadian cities (Burnett and 
Goldberg, 2003). In the last review, results from one multi-city study 
(the Six Cities study) were available, in which the authors reported 
significant associations for total mortality with PM2.5 and 
PM10, but not with PM10-2.5. Reanalyses of Six 
Cities data have reported results consistent with the findings of the 
original study, with statistically significant increases for total 
mortality with short-term exposure to PM2.5 (Schwartz, 
2003a; Klemm and Mason, 2003). In a study using data from the eight 
largest Canadian cities, positive associations were reported for 
PM2.5, PM10, and PM10-2.5 with 
mortality, and the association with PM2.5 was statistically 
significant (Burnett and Goldberg, 2003).
    Single-city studies of mortality associations with short-term 
exposures to fine particles have also been conducted in areas across 
U.S. and Canada as well as in Europe, Australia and Mexico (some using 
fine particle indicators such as British Smoke). In general, it can be 
seen in Figure 1 that the effect estimates for associations between 
mortality and short-term exposure to PM2.5 are positive and 
a number are statistically significant, particularly when focusing on 
the results of studies with greater precision. For total nonaccidental 
mortality, the effect estimates from the multi-city and single-city 
studies with greater precision generally fall in a range of 2 to 6 
percent increases per 25 [mu]g/m3 PM2.5.\11\ 
Somewhat larger effect estimates have been reported for associations 
with cardiovascular or respiratory mortality than with total 
nonaccidental mortality although the confidence intervals may also be 
larger, especially for respiratory mortality since respiratory deaths 
comprise only a small proportion of total deaths (EPA, 2005a, p. 3-15). 
Some studies evaluated seasonal variation in effects, and there is no 
consistent pattern in results. The Criteria Document concludes that the 
results of recent epidemiologic studies are generally consistent with 
findings available in the previous review (EPA, 2004, p. 8-305).
---------------------------------------------------------------------------

    \11\ In general, the results of studies conducted over shorter 
time periods and/or smaller areas have a broader range or effect 
estimates with larger standard errors, as shown in Figure 1.
---------------------------------------------------------------------------

    In addition, associations have been reported between mortality and 
short-term exposure to a number of fine particle components, including 
sulfates, nitrates, metals, organic compounds and elemental carbon 
(EPA, 2004, Section 8.2.2.5.2), as well as gaseous precursors such as 
SO2 and NO2 and other gaseous pollutants such as 
CO. Further, three recent studies have used PM2.5 speciation 
data to evaluate the effects of air pollutant combinations or mixtures 
using factor analysis or source apportionment methods to evaluate 
potential associations between mortality and PM2.5 from 
different source categories. These studies reported that short-term 
exposures to fine particles from combustion sources, including motor 
vehicle emissions, coal combustion, oil burning and vegetative burning, 
were associated with increased mortality (EPA, 2004, Section 
8.2.2.5.3). However, different patterns of associations between various 
components or source categories of fine particles and total or 
cardiovascular mortality are seen in different studies (EPA, 2004, p. 
8-70, Tables 8-3, 8-4).
ii. Respiratory Morbidity
    As discussed in Section 8.4.6.4 of the Criteria Document, recent 
epidemiologic studies have provided further evidence for fine particle 
effects on morbidity, including effects such as hospital admissions or 
emergency department for respiratory diseases, respiratory symptoms and 
lung function changes.

(a) Hospital Admissions or Emergency Department Visits for Respiratory 
Diseases

    In the last review, results were available from one study that 
reported associations between PM2.5 and hospitalization for 
respiratory diseases; these findings were also supported by a number of 
studies using other fine particle indicators. Numerous studies had also 
reported statistically significant associations between hospital 
admissions or emergency department visits for respiratory diseases 
short-term exposures with various indicators ambient PM, especially 
PM10, in areas where fine particles are the predominant 
fraction of PM10, such as locations in the Eastern U.S. and 
in Ontario, Canada (EPA, 1996a, p. 13-39).
    The body of evidence has been expanded with numerous new studies in 
the U.S. and other countries that have reported associations between 
PM2.5 and hospitalization or emergency department visits 
(discussed more fully in Section 8.3.2 of the Criteria Document). As 
shown in Figure 1, all U.S. and Canadian studies report

[[Page 2630]]

associations between PM2.5 and hospitalization for all 
respiratory causes that are positive and statistically significant. A 
number of studies have also reported findings for hospital admissions 
for individual disease categories (COPD, pneumonia, and asthma) that 
are positive, but not always statistically significant, perhaps due to 
smaller sample sizes for the specific respiratory diseases. The effect 
estimates for respiratory hospital admissions tend to fall in the range 
of 5 to 15 percent per 25 [mu]g/m3 PM2.5.\12\ In 
addition, several studies have reported positive, statistically 
significant associations between exposure to PM2.5 and 
emergency department visits for respiratory diseases. The effect 
estimates for these associations range up to about 25 percent per 25 
[mu]g/m3 PM2.5 (EPA, 2005a, pp. 3-20, 3-21).
---------------------------------------------------------------------------

    \12\ Some studies have evaluated seasonal variation in effects, 
and no consistent pattern is apparent in the results. For example, 
stronger associations were reported between PM2.5 and 
asthma hospitalization in the warmer season in Seattle (Sheppard et 
al., 2003) but in the cooler season in Los Angeles (Nauenberg and 
Basu, 1999).
---------------------------------------------------------------------------

(b) Respiratory Symptoms and Lung Function Changes

    Associations between short-term exposure to PM2.5 and 
symptoms in U.S. and Canadian studies are presented in Figure 1. As 
discussed in Section 8.3.3 of the Criteria Document, a number of new 
studies have reported significant associations between short-term 
exposure to PM and increased respiratory symptoms (e.g., cough, wheeze, 
shortness of breath) and decreased lung function in people with asthma. 
In studies of nonasthmatic subjects, there were generally positive 
associations between short-term PM2.5 exposures and 
respiratory symptoms that often were not statistically significant and 
the results for changes in lung function were somewhat inconsistent. 
The Criteria Document concludes that the findings of these studies 
suggest associations with fine PM in reduced lung function and 
increased respiratory symptoms. For example, significant associations 
were reported between ambient PM2.5 and lower respiratory 
symptoms in children in a number of U.S. cities (Schwartz and Neas, 
2000), and significant associations were found with reduced lung 
function in Philadelphia (Neas et al., 1999). These findings are 
supported by results from numerous studies conducted in Europe and 
Central and South America. The Criteria Document finds that the recent 
epidemiologic findings are consistent with those of the previous review 
in showing associations with both respiratory symptom incidence and 
decreased lung function (EPA, 2004, Section 8.4.6.4).
iii. Cardiovascular Morbidity
    In the last review, none of the available studies had evaluated 
associations between exposure to PM and cardiovascular morbidity, 
though some studies had reported associations with cardiopulmonary 
morbidity. In this area, the evidence on PM-related effects has been 
greatly expanded. Numerous recent studies, including multi-city 
analyses, have reported significant associations between short-term 
exposures to PM and health endpoints related to cardiovascular 
morbidity, including hospitalization or emergency department visits for 
cardiovascular diseases, incidence of myocardial infarction, cardiac 
arrhythmia, changes in heart rate or heart rate variability and changes 
in cardiac health indicators such as fibrinogen or C-reactive protein 
(EPA, 2004, section 9.2.3.2.1).

(a) Hospital Admissions and Emergency Department Visits for 
Cardiovascular Diseases

    Several recent studies, including multi-city analyses, have 
reported significant associations between short-term exposures to 
various PM indicators and hospital admissions or emergency department 
visits for cardiovascular diseases. Among the studies using 
PM2.5 measurements are a number of single-city analyses of 
hospitalization or emergency department visits for cardiovascular 
diseases. As shown in Figure 1, studies conducted in Los Angeles, 
Toronto and Detroit have reported associations with hospital admissions 
or emergency department visits for all cardiovascular diseases that are 
positive and statistically significant or nearly so (Burnett et al., 
1997; Ito, 2003; Moolgavkar, 2003). As was true for respiratory 
diseases, the results for specific diseases (ischemic heart disease, 
dysrhythmia, congestive heart disease or heart failure, and stroke) are 
positive but often not statistically significant. The effect estimates 
reported for associations with hospitalization for cardiovascular 
diseases range from about 1 to 10 percent per 25 [mu]g/m3 
PM2.5 (EPA, 2004, p. 8-310); effect estimates reported for 
associations with emergency department visits are generally somewhat 
larger.

(b) Cardiovascular Health Indicators

    In addition to the greatly expanded body of evidence on 
hospitalization or emergency department visits for cardiovascular 
diseases, new epidemiologic studies have also reported associations 
with more subtle physiological changes in the cardiovascular system 
with short-term exposures to PM, particularly PM10 and 
PM2.5 (EPA, 2004, p. 9-67). Associations between short-term 
exposures to ambient PM (often using PM10) have been 
reported with measures of changes in cardiac function such as 
arrhythmia, alterations in electrocardiogram (ECG) patterns, heart rate 
or heart rate variability changes, although the Criteria Document urges 
caution in drawing conclusions regarding the effects of PM on heart 
rhythm, recognizing the need for further research to more firmly 
establish and understand links between particles and these more subtle 
endpoints. Recent studies have also reported increases in blood 
components or biomarkers such as increased levels of C-reactive protein 
and fibrinogen. Several of these studies report significant 
associations between various cardiovascular endpoints and short-term 
PM2.5 exposures, including one in which statistically 
significant associations were reported between onset of myocardial 
infarction and short-term PM2.5 exposures averaged over 2 
and 24 hours (EPA, 2004, p. 8-165; Peters et al., 2001). In this study, 
the effect estimates for the two averaging periods are quite similar in 
magnitude suggesting that for certain health outcomes very short-term 
fine particle concentration fluctuations are important (EPA, 2004, p. 
9-42; Peters et al., 2001). These new epidemiologic findings provide 
important insight into potential biologic mechanisms that could 
underlie associations between short-term PM exposure and cardiovascular 
mortality and hospitalization that have been reported previously.

b. Effects Associated With Long-Term Exposure to Fine Particles

    In the last review, results were available from several cohort 
studies that suggested associations between long-term exposure to PM 
(using various indicators) and both mortality and respiratory 
morbidity. Two studies of adult populations (the Six Cities and ACS 
studies) reported associations between increases in mortality and long-
term exposure to PM2.5, and results of a 24-city study 
indicated that long-term exposure to fine particles was associated with 
increased respiratory illness in children.
    As discussed below, the new evidence available in the current 
review includes an extensive reanalysis of data from the Six Cities and 
ACS studies, new analyses using updated data from the ACS and 
California Seventh Day

[[Page 2631]]

Adventist (AHSMOG) studies, and a new analysis using data from a cohort 
of veterans. In addition, new studies have been published on the 
association between long-term exposure to fine particles and 
respiratory morbidity using data from a cohort of schoolchildren in 
Southern California. In general, the newly available evidence has 
supported earlier findings, and the results of reanalyses have 
increased confidence in the associations reported in previous 
prospective cohort studies.
i. Mortality
    In the 1996 Criteria Document, statistically significant 
associations between long-term exposure to both PM2.5 and 
sulfates and mortality were reported in studies from the Six Cities and 
ACS cohorts (Dockery et al., 1993; Pope et al., 1995). These studies 
reported effect estimates of 6.6 percent (95 percent CI: 3.5, 9.8) 
increases in total mortality per 10 [mu]g/m3 
PM2.5 in the ACS study and 13 percent (95 percent CI: 4.2, 
23) increases in total mortality per 10 [mu]g/m3 
PM2.5 in the Six Cities study, with somewhat larger effect 
estimates reported for cardiopulmonary mortality (EPA, 2004, p. 8-117). 
A number of reviewers raised questions about the adequacy of 
adjustments for potential confounders and other issues (61 FR 65642, 
December 13, 1996). Subsequently, as discussed in more detail in 
Section 8.2.3 of the Criteria Document, the Health Effects Institute 
conducted a major reanalysis of the data from the Six Cities and ACS 
studies by a group of independent investigators to address questions 
and uncertainties raised about these prospective cohort studies. The 
reanalysis included two major components, a replication and validation 
study and a sensitivity analysis. In the first part of the reanalysis, 
the investigators validated the data used by the original investigators 
in both studies, and they were able to replicate the original results. 
The results confirmed the original investigators' findings of 
associations with both total and cardiorespiratory mortality, and the 
authors reported that the results were not dependent on the computer 
programs used in the original analyses (EPA, 2004, p. 8-91; Krewski et 
al., 2000, p. 91).
    The second component of the reanalysis project evaluated an array 
of different models and variables to determine whether the original 
results would remain robust to different analytic assumptions. This 
included controlling for other individual level variables, such as 
cigarette smoking, alcohol consumption, obesity and occupational 
exposures to dusts or other pollutants, and evaluation of the 
sensitivity of results to the addition of a range of additional city-
level variables such as population change, income, education levels, 
and access to health care. The sensitivity analysis included assessment 
of effects in different subgroups of the population. The investigators 
also evaluated the sensitivity of the results to the inclusion of 
gaseous co-pollutants, and tested the effects of different statistical 
modeling approaches, including methods to adjust for spatial patterns, 
such as the correlation in pollutant levels between cities.
    The authors found that adjustment for individual-level variables 
did not alter the results for the association between long-term 
PM2.5 or sulfate exposure and mortality (Krewski et al., 
2000, p. 218). In addition, in most (but not all) cases the 
associations between mortality and long-term exposure to 
PM2.5 and sulfates were unchanged when additional city-level 
variables were added to the models (Krewski et al., 2000, p. 233). 
Analyses to assess the potential modification of effects in different 
subgroups of the population found, for the most part, little difference 
in effects for different subgroups. However, education level was found 
to modify the estimated effect of fine particles, in that associations 
were statistically significant for those subgroups with lower education 
levels, whereas the effect estimates from associations for the subgroup 
with better than high school education were appreciably smaller and 
were statistically insignificant. The authors suggest that educational 
attainment may be a marker for lower socioeconomic status and thus 
greater vulnerability to fine particle-related effects (EPA, 2004, p. 
8-94; Krewski et al., 2000, p. 232).\13\
---------------------------------------------------------------------------

    \13\ 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).
---------------------------------------------------------------------------

    In single-pollutant models, none of the gaseous co-pollutants was 
significantly associated with mortality except SO2. Further 
reanalysis included multi-pollutant models with the gaseous pollutants, 
and the associations between mortality and both fine particles and 
sulfates were unchanged in these models, except when SO2 was 
included, which decreased the size of the effect estimates for 
PM2.5 to one-sixth of its original value and for sulfates to 
less than one-third of its original value (EPA, 2004, p. 8-136; Krewski 
et al., 2000, pp. 183-184).\14\ However, the regional association of 
SO2 and PM2.5 was relatively high, such that the 
effects of the separate pollutants could not be distinguished. The 
authors conclude that these findings support the notion that increased 
mortality may be attributable to more than one component of ambient air 
pollution, and that throughout the reanalyses, fine particles, 
sulfates, and SO2 demonstrated positive associations with 
mortality (Krewski et al., 2000, p. 233-234). As discussed more 
generally in the Criteria Document, this result may be reflecting the 
relatively high correlation between PM2.5 levels and 
SO2 levels that would be expected in cities across the 
industrial Midwest and northeastern states, the role that 
SO2 has as a precursor to sulfate components in the mix of 
PM2.5, and/or the likelihood that SO2 is part of 
the causal pathway linking exposure to PM2.5 to adverse 
health outcomes (EPA, 2004, section 8.1.3.2).
---------------------------------------------------------------------------

    \14\ For a 24.5 [mu]g/m3 change in PM2.5, 
the relative risk for the association between mortality and 
PM2.5 alone was 1.20 (95 percent CI: 1.11-1.29), and 
after adjustment for SO2 it was 1.03 (95 percent CI: 
0.95-1.13). The relative risk for SO2 alone was 1.49 (95 
percent CI: 1.36-1.64) and after adjustment for PM2.5 was 
1.46 (95 percent CI: 1.32-1.63) (Krewski et al., 2000, p. 184). The 
relative risk for sulfates alone was 1.28 (95 percent CI: 1.18-1.40) 
and after adjustment for SO2 it was 1.14 (95 percent CI: 
1.04-1.25) (Krewski et al., 2000, p. 184). These relative risks for 
PM2.5 are equivalent to effect estimates of 7.5 percent 
and 1.2 percent increases in mortality per 10 [mu]g/m3, 
in single-pollutant and two-pollutant models, respectively.
---------------------------------------------------------------------------

    Finally, Krewski and colleagues used several methods to address 
spatial patterns in the data; for example, concentrations of air 
pollutants may be correlated between cities within a region. These 
analyses were primarily based on sulfate concentrations, since more 
cities had data for sulfates than for fine particles. Addressing 
spatial patterns in the data generally reduced the size of the 
association between sulfates and mortality, but the models all 
continued to show associations between mortality risk and long-term 
sulfate exposures, although not all were statistically significant 
(Krewski et al., 2000, p. 228). Overall, considering the results of the 
extensive set of replication and sensitivity analyses, the authors 
report that the reanalysis confirmed the association between mortality 
and fine particle and sulfate exposures (EPA, 2004, p. 8-95; Krewski et 
al., 2000).
    In addition, extended analyses were conducted for the ACS cohort 
study that included follow-up health data and air quality data from the 
new fine particle

[[Page 2632]]

monitoring network for 1999-2000. In this study of the expanded ACS 
cohort, significant associations were reported between long-term 
exposure to fine particles (using various averaging periods for air 
quality concentrations) and premature mortality from all causes, 
cardiopulmonary diseases, and lung cancer (Pope et al., 2002; EPA, 
2004, 8-102). This extended analysis included the use of more recent 
data on fine particle concentrations, as well as data on gaseous co-
pollutant concentrations, though no multi-pollutant model results are 
presented. Further evaluation of the influence of other covariates 
(e.g., dietary intake data, occupational exposure) used methods similar 
to those in the reanalysis described above, and new statistical 
approaches were used for modeling the PM-mortality relationship as well 
as adjusting for spatial correlation (EPA, 2004, section 8.2.3.2.2). 
The investigators reported that the associations found with fine 
particle and sulfate concentrations were not markedly affected by 
adjustment for numerous socioeconomic variables, demographic factors, 
environmental variables, indicators of access to health services or 
personal health variables (e.g., dietary factors, alcohol consumption, 
body mass index). Similar to the results of Krewski et al. (2000), 
education level was found to be a modifier in the relationship between 
fine particles and mortality, in that associations were statistically 
significant for those subgroups with lower education levels, whereas 
effect estimates from associations for those with better than a high 
school education were close to zero and were statistically 
insignificant.
    There are also new analyses using updated data from the AHSMOG 
cohort. These include estimated PM2.5 concentrations from 
visibility data, along with new health information from continued 
follow-up of the Seventh Day Adventist cohort. Positive associations 
were reported for mortality with PM2.5 in males, but the 
estimates were generally not statistically significant (Abbey et al., 
1999; McDonnell et al., 2000; EPA, 2004, pp. 8-110 and 8-117). In 
addition, one new set of analyses was done using subsets of PM exposure 
and mortality time periods and data from a Veterans Administration (VA) 
cohort of hypertensive men. The investigators report inconsistent and 
largely nonsignificant associations between PM exposure (including, 
depending on availability, TSP, PM10, PM2.5, 
PM15 and PM15-2.5) and mortality (EPA, 2004, pp. 
8-110 to 8-111; Lipfert et al., 2000b).
    The Criteria Document and Staff Paper place greatest weight on the 
findings of the Six Cities and ACS studies (including reanalyses and 
extended analyses) that include measured fine particle data (in 
contrast with AHSMOG effect estimates based on TSP or visibility 
measurements), have study populations more similar to the general 
population than the VA study cohort, and have been replicated and 
examined through exhaustive reanalysis (EPA, 2005a, at 5-22; see also 
EPA, 2004, at 8.2.3.2.5.). In these studies, effect estimates for 
deaths from all causes fall in a range of 6 to 13 percent increased 
risk per 10 [mu]g/m3 PM2.5, while effect 
estimates for deaths from cardiopulmonary causes fall in a range of 6 
to 19 percent per 10 [mu]g/m3 PM2.5. For lung 
cancer mortality, the effect estimate was a 13 percent increase per 10 
[mu]g/m3 PM2.5 in the results of the extended 
analysis from the ACS cohort (Pope et al., 2002; CD, Table 8-12).
    The prospective cohort studies have used air quality measurements 
averaged over long periods of time, such as several years, to 
characterize the long-term ambient levels in the community. The 
exposure comparisons are basically cross-sectional in nature, and do 
not provide evidence concerning any temporal relationship between 
exposure and effect (EPA, 2004, p. 9-42). As discussed in the Criteria 
Document, it is not easy to differentiate the role of historic 
exposures from more recent exposures, leading to potential exposure 
measurement error that is increased if average PM concentrations change 
over time differentially between areas (EPA, 2004, p. 5-118). Several 
new studies have used different air quality periods for estimating 
long-term exposure and tested associations with mortality for the 
different exposure periods. As discussed in section 3.6.5.4 of the 
Staff Paper, these analyses indicate that averaging PM concentrations 
over a longer time period results in stronger associations, and that 
the longer series of data is likely a better indicator of cumulative 
exposure. Thus, in evaluating these findings, EPA has focused on the 
results of analyses using fine particle or sulfate measurements for the 
longer exposure periods in the studies.
ii. Respiratory Morbidity
    In the last review, several studies had reported that long-term PM 
exposure was linked with increased respiratory disease and decreased 
lung function. One study, using data from 24 U.S. and Canadian cities 
(``24 Cities'' study), reported associations with these effects and 
long-term exposure to fine particles or acidic particles, but not with 
PM10 exposure (Dockery et al., 1996; Raizenne et al., 1996). 
More specifically, statistically significant associations were reported 
between long-term exposure to fine particles and decreases in several 
measures of lung function evaluated at a single point in time (Raizenne 
et al., 1996). In addition, positive but not statistically significant 
associations were reported between long-term exposure to fine particles 
and prevalence of a range of respiratory conditions (e.g., asthma, 
bronchitis, chronic cough) (Dockery et al., 1996).
    In the current review, new studies conducted in the U.S. have been 
based on data from cohorts of schoolchildren in 12 Southern California 
Communities and an adult cohort of Seventh Day Adventists (AHSMOG) 
(EPA, 2004, section 8.3.3.2). Information specifically on associations 
with long-term PM2.5 exposures are available from the 
Southern California children's cohort study. Early findings from cross-
sectional analyses done at the beginning of the study suggested 
associations between long-term PM2.5 exposures and 
respiratory morbidity, but the findings were generally not 
statistically significant.\15\ Later publications from this cohort have 
reported associations with lung function growth in children over four-
year follow-up periods. In a study of a cohort of children followed 
from 4th to 7th grade, some measures of decreases in lung function 
growth were statistically significantly associated with increasing 
exposure to PM2.5, whereas in a second cohort of 4th 
graders, the associations generally did not reach statistical 
significance (Gauderman et al., 2002). Decreases in measures of lung 
function growth were also reported for cohorts of older children, but 
the associations did not reach statistical significance (Gauderman et 
al., 2000). The Criteria Document finds that these studies ``provide 
the best evidence'' on effects of long-term fine particle exposure 
(EPA, 2004, p. 8-314). However, this is the only cohort study to have 
evaluated associations with decreases in lung function growth in 
children over time. Considered together, the Criteria Document finds 
that the evidence from these studies indicates that long-term 
PM2.5 exposures may

[[Page 2633]]

result in chronic respiratory effects (EPA, 2004, p. 8-314).
---------------------------------------------------------------------------

    \15\ In an initial report on the prevalence of respiratory 
illnesses reported at the beginning of the study, positive 
associations, though not statistically significant, were reported 
between long-term PM2.5 exposure and risk of bronchitis 
and cough only in the subset of children with asthma (McConnell et 
al., 1999), and no significant associations with long-term 
PM2.5 exposure were reported for the full cohort (Peters 
et al., 1999a). In addition, long-term PM2.5 exposure was 
associated with decreases in some lung function measurements made at 
that time, but the associations were only statistically significant 
for females (Peters et al., 1999b).
---------------------------------------------------------------------------

3. Integration and Interpretation of the Health Evidence
    In evaluating the evidence from epidemiologic studies, the Criteria 
Document and Staff Paper focused on well-recognized criteria, including 
the strength of associations; robustness of reported associations to 
the use of alternative model specifications, potential confounding by 
co-pollutants, and exposure misclassification related to measurement 
error; consistency of findings in multiple studies of adequate power, 
and in different persons, places, circumstances and times; the nature 
of concentration-response relationships; and information from so-called 
natural experiments or intervention studies. These evaluations 
addressed key methodological issues that are relevant to interpretation 
of evidence from epidemiologic studies. Further, findings from 
epidemiologic studies were integrated with experimental (e.g., 
dosimetric and toxicologic) studies, in considering the extent of 
coherence and biological plausibility of effects observed in 
epidemiologic studies. This integrative assessment provided the basis 
for the judgments made in the Criteria Document and Staff Paper about 
the extent to which causal inferences can be made about observed 
associations between health endpoints and PM2.5 (as well as 
other indicators or constituents of ambient PM), acting alone and/or in 
combination with other pollutants. Key elements of these evaluations 
are briefly summarized below.
    (1) For short-term exposures to fine particles, in considering the 
magnitude and statistical strength of the associations, there is a 
pattern of positive and often statistically significant associations 
for cardiovascular and respiratory health outcomes with short-term 
exposure to PM10 and PM2.5. Of particular note 
are several multi-city studies that have yielded relative risk 
estimates for associations between short-term exposure to various 
indices of PM and mortality or morbidity. Although small in size, the 
effect estimates from multi-city studies have great precision due to 
the statistical power of the studies. New analyses of pre-existing 
cohorts with studies of long-term exposure to fine particles are 
available that confirm and strengthen conclusions from the previous 
review, although the effect estimates are sensitive to education level, 
co-pollutant effects of SO2, and spatial correlation, as 
discussed above.
    (2) The Criteria Document and Staff Paper have evaluated the 
robustness of epidemiologic associations in part by considering the 
effect of differences in statistical model specification, potential 
confounding by co-pollutants and exposure error on PM-health 
associations (EPA, 2004, section 9.2.2.2; EPA, 2005a, sections 3.4.2 
and 3.6).
    As discussed in section 8.4.2 of the Criteria Document and section 
3.6.3 of the Staff Paper, the influence of alternative modeling 
strategies on epidemiologic study results was assessed, with a 
particular focus on the recent set of analyses to address statistical 
modeling questions in epidemiologic studies for short-term PM 
exposures. Numerous recent studies used a certain type of statistical 
method (i.e., generalized additive methods (GAM)) in widely used 
statistical software (Splus), and it was discovered that the default 
program settings could potentially result in biased effect estimates 
for associations between pollutants and health outcomes. Results from a 
number of epidemiologic studies were reanalyzed to address this 
problem. These reanalyses also more broadly included the use of 
alternative statistical models and alternative methods of control for 
time-varying effects, such as weather or season (HEI, 2003). In 
general, the results of the reanalyses to address the use of default 
program settings in the Splus software showed little change in effect 
estimates for some studies; in others the effect estimates were reduced 
in size, though it was observed that the reductions were often not 
substantial (EPA, 2004, p. 9-35). For example, in comparing results for 
numerous studies of mortality associations with PM10, the 
Criteria Document found that the extent of reduction in effect 
estimates resulting from reanalysis was smaller than the variation in 
effect estimate size across studies (EPA, 2004, p. 8-229 and Figure 8-
15). A review panel commentary on the set of reanalysis studies (using 
various PM indicators) notes that most studies were considered to show 
``little or no change'' in results with initial reanalyses to address 
questions about the use of modeling specifications in the statistical 
software package (HEI, 2003, pp. 258-259).
    In addition, the reanalyses also refocused attention in general on 
the control for relationships between health effects and weather 
variables in time-series epidemiologic studies; such issues had been 
also discussed at length in the 1996 Criteria Document (EPA, 2004, 
section 8.4.3.5). The reanalysis results showed greater sensitivity to 
the modeling approach used to account for temporal effects and weather 
variables than to correcting the initial problem with default settings 
in the use of GAM in Splus software (EPA, 2004, p. 8-236). For example, 
in the review panel commentary, sixteen of the reanalyzed studies were 
considered to have ``little or no change'' in results of initial 
reanalyses, while only two studies showed ``substantial'' changes 
(Goldberg and Burnett, 2003; some results in Ito, 2003; HEI, 2003, pp. 
258-259). In contrast, four of the eight studies that were reanalyzed 
with additional methods to adjust for time-related variables were 
considered to show ``substantial'' changes in effect estimate size 
(HEI, 2003, p. 262).
    The recent time-series epidemiologic studies evaluated in the 
Criteria Document have included some degree of control for variations 
in weather and seasonal variables. 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 trade-off. 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, 2004, p. 8-236; HEI, 
2003, p. 266). The selection of approaches to address such variables 
depends in part on prior knowledge and judgments made by the 
investigators, for example, about weather patterns in the study area 
and expected relationships between weather and other time-varying 
factors and health outcomes considered in the study. 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, 2004, pp. 8-340, 8-
341). Further, considering the full set of reanalyses, the Criteria 
Document concludes that associations between short-term exposure to PM 
and various health outcomes are generally robust to the use of 
alternative modeling strategies, again recognizing that further 
evaluation of alternative modeling strategies was warranted (EPA, 2004, 
p. 9-48).

[[Page 2634]]

    For long-term exposure to fine particles, the reanalysis and 
extended analyses of data from prospective cohort studies, discussed 
above in section II.A.2, 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 extended 
analysis, Krewski et al. (2000) identified model sensitivities related 
to education level and spatial correlation, as well as to co-pollutant 
effects of SO2, as discussed below.
    The Criteria Document also included extensive evaluation of the 
sensitivity of PM-health responses to confounding by gaseous co-
pollutants (EPA, 2004, section 8.4.3, Figures 8-16 to 8-19). Results of 
new multi-city short-term exposure studies, that combine data from 
locations with different mixes of pollutants, provide important new 
results. Using PM10, the NMMAPS results indicated that 
associations with mortality were not confounded by co-pollutant 
concentrations across 90 U.S. cities (Dominici, 2003),\16\ and a 
similar lack of confounding was observed in a mortality study across 10 
U.S. cities (Schwartz, 2003b) (EPA, 2004, Figure 8-16). That is, in 
these studies, the size of the effect estimates are little changed and 
the associations remain statistically significant in multi-pollutant 
models including one or more of the gaseous co-pollutants. Similar 
results are seen in some single-city studies using PM2.5 for 
some health outcomes in which the single-pollutant model association 
was statistically significant (EPA, 2004, Figures 8-16 to 8-18), 
including the association with mortality in Santa Clara County, CA 
(Fairley, 2003); associations with hospital admissions in Detroit (for 
heart failure and pneumonia in Ito, 2003) and Seattle (for asthma in 
Sheppard et al., 2003); and associations with cardiovascular-related 
biomarkers in Boston (Gold et al., 2000). The size of the effect 
estimates were little changed in other studies as well in which the 
single-pollutant model associations were not statistically significant 
(e.g., for some health outcomes in Ito, 2003; for mortality in Chock et 
al., 2000). In yet other studies, however, for some combinations of 
pollutants in some areas, substantial reductions in the size of the 
effect estimates for PM2.5 were observed; notably, 
Moolgavkar (2003) reports substantial reductions in effect estimates 
when CO is included in models for mortality and hospitalization in Los 
Angeles, and Thurston et al. (1994) and Delfino et al. (1998) report 
substantial reductions when O3 is included in models for 
hospital admissions in Toronto and emergency department visits in 
Montreal, respectively.\17\ It is recognized that collinearity between 
co-pollutants can make interpretation of such multi-pollutant model 
results difficult (EPA, 2004, p. 8-253). Further, associations between 
long-term exposure to PM2.5 and mortality were not generally 
sensitive to inclusion of co-pollutants, with the notable exception of 
the inclusion of SO2 in multipollutant models used in the 
reanalysis of the ACS study, as discussed above in section II.A.2 (EPA, 
2004, p. 8-136). Overall, the Criteria Document concluded that these 
studies indicate that effect estimates for associations between 
mortality and morbidity and various PM indices are generally robust to 
confounding by co-pollutants, while recognizing that disentangling the 
effects attributable to various pollutants within an air pollution 
mixture is challenging (EPA, 2004, p. 9-37).
---------------------------------------------------------------------------

    \16\ In the HEI Review Panel commentary on the results of the 
NMMAPS multi-city analyses, the Panel stated that the results did 
not show a confounding effect of other pollutants, observing that 
the PM10 effects on mortality were not changed by 
addition of either O3, SO2, NO2 or 
CO to the models (HEI, 2000, p. 77).
    \17\ The correlation coefficients between concentrations of 
PM2.5 and the noted co-pollutants in these studies were 
high; the coefficient with CO in Los Angeles was 0.58, and the 
coefficients with O3 were 0.58 and 0.72 in Montreal and 
Toronto, respectively.
---------------------------------------------------------------------------

    Finally, as discussed in section 3.6.2, a number of recent studies 
have evaluated the influence of exposure error on PM-health 
associations. This includes both consideration of error in measurements 
of PM and other co-pollutants, and the degree to which measurements 
from an individual monitor reflect exposures to the surrounding 
community. As further discussed in section 3.6.2, several studies have 
shown that fairly extreme conditions (e.g., very high correlation 
between pollutants and no measurement error in the ``false'' pollutant) 
are needed for complete ``transfer of causality'' of effects from one 
pollutant to another (EPA, 2004, p. 9-38). In comparing fine and 
thoracic coarse particles, the Criteria Document observes that exposure 
error is likely to be more important for associations with 
PM10-2.5 than with PM2.5, since there is 
generally greater error in PM10-2.5 measurements, 
PM10-2.5 concentrations are less evenly distributed across a 
community, and less likely to penetrate into buildings (EPA, 2004, p. 
9-38). Therefore, while the Criteria Document concludes that 
associations reported with PM10, PM2.5 and 
PM10-2.5 are generally robust, it recognizes that factors 
related to exposure error may result in reduced precision for 
epidemiologic associations with PM10-2.5 (EPA, 2004, p. 9-
46).
    (3) Consistency refers to the persistent finding of an association 
between exposure and outcome in multiple studies of adequate power in 
different persons, places, circumstances and times (CDC, 2004). The 
1996 Criteria Document reported associations between short-term PM 
exposure and mortality or morbidity from studies conducted in locations 
across the U.S. as well as in other countries, and concluded that the 
epidemiologic data base had ``general internal consistency'' (EPA, 
1996a, p. 13-30). New multi-city studies have allowed evaluation of 
consistency in effect estimates across geographic locations, using 
uniform statistical modeling approaches; the results suggest that 
effect estimates differ from one location to another, but the extent of 
variation is not clear. For example, the Canadian 8-city study reported 
no evidence of heterogeneity in city-specific results in the initial 
study findings; however, in the reanalysis to address model 
specification issues, the findings suggested more evidence of 
heterogeneity in associations between mortality and short-term 
PM2.5 exposure (Burnett and Goldberg, 2003; EPA, 2004, p. 9-
39). The Criteria Document discussed a number of factors that would be 
likely to cause variation in PM-health outcomes in different 
populations and geographic areas in section 9.2.2.3, including 
indicators of exposure to traffic-related pollution, population 
characteristics that affect susceptibility or exposure differences, 
distribution of PM sources, or geographic features that would affect 
the spatial distribution of PM (EPA, 2004, p. 9-41). In addition, the 
use of data collected on a 1-in-6 or 1-in-3 day schedule results in 
reduced statistical power, resulting in less precision for estimated 
effect estimates for the individual cities and increased potential 
variability in results (EPA, 2004, p. 9-40). Overall, the Criteria 
document concluded that ``[f]ocusing on the studies with the most 
precision, it can be concluded that there is much consistency in 
epidemiologic evidence regarding associations between short-term and 
long-term exposures to fine

[[Page 2635]]

particles and cardiopulmonary mortality and morbidity.'' (EPA, 2004, p. 
9-47).
    (4) The form of concentration-response relationships (e.g., linear, 
sigmoid) and the potential existence of thresholds was one of the 
important research questions remaining in the previous review. The 
Criteria Document recognized that it is reasonable to expect that there 
likely are biologic thresholds for different health effects in 
individuals or groups of individuals with similar innate 
characteristics and health status (EPA, 2004, Section 9.2.2.5). 
Individual thresholds would presumably vary substantially from person 
to person due to individual differences in genetic-level susceptibility 
and pre-existing disease conditions (and could even vary from one time 
to another for a given person). Thus, it would be difficult to detect a 
distinct threshold at the population level, below which no individual 
would experience a given effect, especially if some members of a 
population are unusually sensitive even down to very low 
concentrations. The person-to-person difference in the relationship 
between personal exposure to PM of ambient origin and the concentration 
observed at a monitor may also add to the variability in observed 
concentration-response relationships, further obscuring potential 
population thresholds within the range of observed concentrations (CD, 
p. 9-43, 9-44).
    Several new epidemiologic studies have used different modeling 
methods to address this question, and most have been unable to detect 
threshold levels in the relationship between short-term PM exposure 
(generally using PM10) and mortality; in fact, one single-
city analysis suggests that statistical methods would allow detection 
of a threshold in the epidemiologic data if a clear threshold existed. 
However, a few analyses in individual cities have provided suggestions 
of some potential threshold levels, generally at fairly low ambient 
concentrations. One single-city study used PM2.5 and 
PM10-2.5 measurements in Phoenix and reported that there was 
suggestive evidence of a threshold for the association between 
mortality and short-term exposure to PM2.5 in the range of 
20-25 [mu]g/m3 (Smith et al., 2000; EPA, 2004, p. 8-322).
    The shape of the concentration-response function for long-term 
exposure to PM2.5 with mortality was evaluated using data 
from the ACS cohort. In the ACS reanalysis, the authors report that the 
concentration-response functions for PM2.5 and all-cause and 
cardiopulmonary mortality demonstrate near-linear increasing trends 
through the range of particle levels observed in the fine particle 
cohort (Krewski, p. 160). However, the HEI Review Committee concluded 
that these results show no clear evidence either for or against overall 
linearity (Krewski, p. 265). In the extended ACS study, the authors 
reported that the associations for all-cause, cardiovascular and lung 
cancer mortality ``were not significantly different from linear 
associations'' (Pope, et al., 2002).
    Thus, evaluation of the health effects data summarized in the 
Criteria Document provides no evidence to support selecting any 
particular population threshold for PM2.5. The Staff Paper 
also recognized, however, that it is reasonable to expect that, for 
individuals, there may be thresholds for specific health responses and 
that it is possible that such thresholds exist toward the lower end of 
these ranges (or below these ranges) but cannot be detected due to 
variability in susceptibility across a population. Even in those few 
studies with suggestive evidence of such thresholds, the potential 
thresholds are at fairly low concentrations (EPA, 2004, sections 8.4.7 
and 9.2.2.5).
    (5) Few studies are available that assess the extent to which 
reductions in ambient PM actually lead to reductions in health effects 
attributable to PM. As discussed in sections 8.2.3.4 and 9.2.2.6 of the 
Criteria Document, several epidemiologic studies were done in the Utah 
Valley area over a time period when a major source of PM was closed, 
resulting in markedly decreased PM10 concentrations. An 
epidemiologic study reported that respiratory hospital admissions 
decreased during the plant closure time period (EPA, 2004, p. 8-131; 
Pope et al., 1989). Newly available controlled human exposure and 
animal toxicology studies, using particles extracted from stored 
PM10 sampling filters from the Utah Valley, have shown 
inflammatory responses that are greater with extracts of particles 
collected during the time period of source operation than when the 
source was closed, suggesting that the PM from the steel mill was more 
harmful than other ambient PM on an equal mass basis (EPA, 2004, p. 9-
73). Epidemiologic studies in Dublin, Ireland and Hong Kong also 
provides evidence for reduced relative risks for mortality when PM 
(measured as BS or PM10) and SO2 were reduced as 
the result of interventions aimed at reducing air pollution. The 
Criteria Document concluded that this small group of studies add 
further support to the results of the hundreds of other epidemiologic 
studies linking ambient PM exposure to an array of health effects, and 
provide strong evidence that reducing emissions of PM and gaseous 
pollutants has beneficial public health impacts (EPA, 2004, p. 9-45 to 
9-46).
    (6) Several issues related to fine particle exposure time periods 
were assessed in the Criteria Document, as summarized in section 3.6.5 
of the Staff Paper. As discussed above in this section, these include 
the exposure time periods used in long-term exposure studies as well as 
health outcome associations with very short time periods (e.g., 2-hour 
average). An additional issue is the time period (``lag'') between fine 
particle exposure and health outcome that is reported in short-term 
exposure study results. In these epidemiologic studies, associations 
are often tested for a range of lag periods, for example, with PM 
concentrations from the same day as the effect, and one or more days 
preceding the effect. In evaluating these results, it is important to 
consider the pattern of results that is seen across the series of lag 
periods. If there is an apparent pattern of results across the 
different lags, with positive associations reported for a series of 
consecutive lag periods, then selecting the single-day lag with the 
largest effect from a series of positive associations is likely to 
underestimate the overall effect size, since single-day lag effect 
estimates do not fully capture the risk that may be distributed over 
adjacent or other days (EPA, 2004, sections 8.4.4 and 9.2.2.4). For 
many epidemiologic studies, the authors have reported just such a 
pattern of associations across several consecutive lag periods (EPA, 
2004, p. 8-279). However, if there is no apparent pattern or reported 
effects vary across lag days, any result for a single day may well be 
biased (CD, p. 9-42).
    Some new studies have used a ``distributed lag'' model approach, 
that captures an effect of PM over a series of days following 
exposure.\18\ Where effects are found for a series of lag periods, a 
distributed lag model will more accurately characterize the effect 
estimate size. A number of recent studies that have investigated 
associations with distributed lags provide effect estimates for health 
responses that persist over a period of time (days to weeks) after the 
exposure period. Effect estimates from distributed lag models are thus 
often, but not always, larger in size that those for single-day lag 
periods (EPA, 2004, p. 8-281).
---------------------------------------------------------------------------

    \18\ The available studies have generally used PM10, 
but not PM2.5 or PM10-2.5.

---------------------------------------------------------------------------

[[Page 2636]]

    The Criteria Document concludes that it is likely that the most 
appropriate lag period for a study will vary depending on the health 
outcome and the specific pollutant under study. For example, for a 
health outcome such as a delayed asthma response, the lag period of a 
day or several days might be expected between exposure and outcome; 
however, some cardiovascular responses might be expected to occur 
within a very short time period (e.g., an hour) after exposure (EPA, 
2004, p. 8-279). As shown in Figures 8-24 to 8-28, the Criteria 
Document notes a pattern of stronger associations between 
PM10 and mortality or cardiovascular hospitalization with 
shorter lag periods (e.g., same-day or 1-day lagged PM10). 
For other effects, however, such as respiratory symptoms, asthma 
emergency department visits or hospitalization, stronger effects were 
reported with PM concentrations averaged over several days (EPA, 2004, 
pp. 8-273 to 8-279). Thus, the Criteria Document concludes that one 
would expect to see different best-fitting lags for different health 
effects, based on potentially different biological mechanisms as well 
as individual variability in responses (EPA, 2004, p. 8-342). For some 
health outcomes, it is reasonable to expect associations to be observed 
with PM exposures on the same day or with very short lag periods, but 
not longer lag periods. In other cases, multi-day average exposure 
periods or distributed lag models would more appropriately estimate 
potential PM-related health risks.
    (7) Looking more broadly to integrate epidemiologic evidence with 
that from exposure-related, dosimetric and toxicologic studies, EPA has 
considered the coherence of the evidence and the extent to which the 
new evidence provides insights into mechanisms by which PM, especially 
fine particles, may be affecting human health. Progress made in gaining 
insights into potential mechanisms lends support to the biologic 
plausibility of results observed in epidemiologic studies. For 
cardiovascular effects, the convergence of important new epidemiologic 
and toxicologic evidence (especially from studies using concentrated 
ambient particles) builds support for the plausibility of causal 
associations, especially between fine particles and physiological 
endpoints indicative of increased risk of cardiovascular disease and 
changes in cardiac rhythm. This finding is supported by new 
cardiovascular effects research focused on fine particles that has 
notably advanced our understanding of potential mechanisms by which 
PM2.5 exposure, especially in susceptible individuals, could 
result in changes in cardiac function or blood parameters that are risk 
factors for cardiovascular disease. For respiratory effects, 
toxicologic studies have provided evidence that supports plausible 
biologic pathways for fine particles, including inflammatory responses, 
increased airway responsiveness, or altered responses to infectious 
agents. Further, coherence across a broad range of cardiovascular and 
respiratory health outcomes is supported by evidence from epidemiologic 
and toxicologic studies done in the same location, for example, in the 
series of studies conducted in or evaluating ambient PM from Boston and 
the Utah Valley (EPA, 2004, 7-42 to 43, 7-46 to 47, and 9-45). 
Toxicologic studies have suggested that some combustion-related 
particles, including particles from wood burning and diesel engine 
exhaust, but not others such as coal fly ash, may have carcinogenic 
effects (EPA, 2004, Section 7.8.4). This evidence supports the 
plausibility of the observed relationship between fine particles and 
lung cancer mortality. Evidence for PM-related infant mortality and 
developmental effects poses an emerging concern, but the current 
information is still very limited in support of the plausibility of 
potential ambient PM relationships. More generally, toxicologic animal 
studies often test effects of exposures to individual chemical 
components, and thus the physical and chemical characteristics may 
differ from those of particles in ambient air to which humans are 
exposed. These and other differences in toxicologic and epidemiologic 
study designs complicate the assessment of coherence in results from 
across disciplines (EPA, 2004, section 9.2.3.1; Schlesinger and Cassee, 
2003).
    Overall, the Criteria Document finds that much more evidence is now 
available related to the coherence and plausibility of effects than in 
the last review. For short-term exposures, integration of evidence from 
epidemiologic and toxicologic studies indicates both coherence and 
plausibility of effects on the cardiovascular and respiratory systems, 
especially for fine particles (EPA, 2004, p. 9-79). There is evidence 
supporting coherence and plausibility for the observed associations 
between long-term exposures to fine particles and lung cancer mortality 
(EPA, 2004, p. 9-78).
    (8) In summary, as discussed in the Staff Paper (section 3.5) and 
the Criteria Document (section 9.2.2), the extensive body of 
epidemiologic evidence now available continues to support likely causal 
associations between PM2.5 and a broad range of mortality 
and morbidity health outcomes based on an assessment of the strength of 
the evidence, including the strength and robustness of reported 
associations and the consistency of the results. 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 evidence now available provides 
strong support that both short-term and long-term exposures to fine 
particles are plausibly associated with a broad range of effects on the 
respiratory and cardiovascular systems. The Criteria Document 
concludes: ``the epidemiological evidence continues to support likely 
causal associations between PM2.5 and PM10 and 
both mortality and morbidity from cardiovascular and respiratory 
diseases, based on an assessment of strength, robustness, and 
consistency in results.'' (EPA, 2004, p. 9-48). In its integrative 
assessment, the Criteria Document finds that health evidence from 
various disciplines provides a strong and coherent basis for concluding 
that both short-term and long-term exposure to fine particles is 
associated with health effects ranging from subtle changes in lung 
function to premature mortality.
4. Sensitive Subgroups for PM2.5-Related Effects
    As described in the PM Criteria Document, the term susceptibility 
refers to innate (e.g., genetic or developmental) or acquired (e.g., 
personal risk factors, age) factors that make individuals more likely 
to experience effects with exposure to pollutants. A number of 
population subgroups have been identified as potentially susceptible to 
health effects as a result of PM exposure, including people with 
existing heart and lung diseases, including diabetes, and older adults 
and children. In addition, new attention has been paid to the concept 
of some population groups having increased vulnerability to pollution-
related effects due to factors such as socioeconomic status or factors 
that result in particularly elevated exposure levels, such as residence 
near sources such as roadways (EPA, 2004, p. 9-81).
    A good deal of evidence indicates that people with existing heart 
or lung diseases are more susceptible to PM-related effects. In 
addition, new studies have suggested that people with diabetes, who are 
at risk for cardiovascular disease, may have

[[Page 2637]]

increased susceptibility to PM exposures. As discussed in Section 
9.2.4.1 of the Criteria Document, this body of evidence includes 
findings from epidemiologic studies that associations with mortality or 
morbidity are greater in those with preexisting conditions, as well as 
evidence from toxicologic studies using animal models of 
cardiopulmonary disease. In addition, dosimetric evidence indicates 
that deposition of particles is increased, and can be focused in ``hot 
spots'' in the respiratory tract, in people with chronic respiratory 
diseases.
    Two age groups, older adults and the very young, are also 
potentially at greater risk for PM-related effects. Epidemiologic 
studies have generally not shown striking differences between adult age 
groups. However, some epidemiologic studies have suggested that serious 
health effects, such as premature mortality, are greater among older 
populations (EPA, 2005a, p. 8-328). In addition, preexisting 
respiratory or cardiovascular conditions are more prevalent in older 
adults than younger age groups; thus there is some overlap between 
potentially susceptible groups of older adults and people with heart or 
lung diseases.
    Epidemiologic evidence has reported associations with emergency 
hospital admissions for respiratory illness and asthma-related symptoms 
in children. Several factors may make children susceptible to PM-
related effects, including the greater ventilation rate per kilogram 
body weight in children, greater prevalence of chronic asthma, and the 
fact that children are more likely to be active outdoors and thus have 
greater exposures. In addition, there is a more limited body of new 
evidence from epidemiologic studies for potential PM-related health 
effects in infants, using various PM indicators. Results from this body 
of evidence, though mixed, are suggestive of possible effects; more 
research is needed to further elucidate the potential risks of PM 
exposure for these health outcomes (EPA, 2004, p. 8-222).
    In summary, there are several population groups that may be 
especially susceptible or vulnerable to PM-related effects. These 
groups include those with preexisting heart and lung diseases, older 
adults and children. Emerging evidence indicates that people from lower 
socioeconomic strata or who have particularly elevated exposures may be 
more vulnerable to PM-related effects.
5. PM2.5-Related Impacts on Public Health
    As just discussed, there are several population groups that may be 
especially susceptible or vulnerable to effects from exposure to PM. 
These population subgroups, such as young children or older adults, and 
people with pre-existing heart or lung diseases, constitute a large 
portion of the U.S. population. For example, approximately 22 million 
people, or 11 percent of the U.S. population, have received a diagnosis 
of heart disease, about 20 percent of the population has hypertension 
and about 9 percent of adults and 11 percent of children in the U.S. 
have been diagnosed with asthma. In addition, about 26 percent of the 
U.S. population is under 18 years of age,\19\ and about 12 percent is 
65 years of age or older (EPA, 2004, Table 9-4). EPA recognizes that 
combining fairly small risk estimates and small changes in PM 
concentrations with large groups of the U.S. population would result in 
large public health impacts.
---------------------------------------------------------------------------

    \19\ Health studies that have suggested that children are 
susceptible to PM-related effects include varying age ranges, for 
example, for hospital admissions in children up to 18 years of age, 
or respiratory symptoms in panels of 4th and 5th grade children.
---------------------------------------------------------------------------

    One issue that is important for interpreting the public health 
implications of the associations reported between mortality and short-
term exposure to PM is whether mortality is occurring only in very 
frail individuals (sometimes referred to as ``harvesting''), resulting 
in loss of just a few days of life expectancy. A number of new analyses 
assess the likelihood of such ``harvesting'' occurring in the short-
term exposure studies. Overall, the Criteria Document concludes from 
the time-series studies that there appears to be no strong evidence to 
suggest that short-term exposure to PM is only shortening life by a few 
days (EPA, 2004, Section 8.4.10). In addition to the evidence from 
short-term exposure studies discussed above, one new report used the 
mortality risk estimates from the ACS prospective cohort study to 
estimate potential loss of life expectancy from PM-related mortality in 
a population. The authors estimated that the loss of population life 
expectancy associated with long-term exposure to PM2.5 was 
on the order of a year or so (EPA, 2004, p. 8-334). The Criteria 
Document recognizes that these calculations were based on studies in 
adult populations, and potential population life shortening would be 
increased if the new, albeit limited, evidence from infant mortality 
studies was considered (EPA, 2004, p. 8-335). The Criteria Document 
also observes that the risk estimates reported for long-term fine 
particle exposures and lung cancer mortality are in about the same 
range as the risk seen for a nonsmoker living with a smoker (EPA, 2004, 
p. 9-94).
    Large subgroups of the U.S. population are included in 
subpopulations considered to be potentially sensitive to effects 
related to fine particle exposures (EPA, 2004, section 9.2.5.1). While 
individual epidemiologic effect estimates may be small in size, the 
public health impact of the mortality and morbidity associations can be 
quite large. In addition, it appears that mortality risks are not 
limited to the very frail. Taken together, these results suggest that 
exposure to ambient PM, especially PM2.5, can have 
substantial public health impacts (EPA, 2004, p. 9-93).

B. Quantitative Risk Assessment

    This section discusses the approach used to develop quantitative 
risk estimates associated with exposures to PM2.5 building 
upon a more limited risk assessment that was conducted during the last 
review.\20\ At that time, EPA conducted a very limited risk assessment 
covering a portion of two cities (i.e., Philadelphia County and 
Southeast Los Angeles County) for which ambient PM2.5 data 
were available. For short-term exposure mortality and morbidity health 
effects, the prior assessment relied on either pooled analyses that 
combined the results from several studies of individual cities or 
individual single- and multi-city studies, none of which included the 
two urban counties for which risks were estimated, to estimate 
concentration-response relationships for these two cities. EPA 
recognized that the lack of city-specific relative risks introduced 
substantial uncertainties in the risk estimates due to inherent 
differences (e.g., different population characteristics, PM size 
distributions) that might influence the concentration-response 
relationships. For long-term exposure mortality, the prior assessment 
relied on the concentration-response relationship reported in the 
original ACS study (Pope et al., 1995). Additional important 
uncertainties noted at the time of that assessment with respect to all 
health effects included: (1) The absence of clear evidence regarding 
mechanisms of

[[Page 2638]]

action for the various effects of interest, (2) uncertainties about the 
shape of the concentration-response relationships; and (3) concern 
about whether the use of ambient PM2.5 fixed-site monitoring 
data adequately reflected the relevant population exposures to PM that 
are responsible for the reported health effects (61 FR 65650).
---------------------------------------------------------------------------

    \20\ The methodology, scope, and results from the risk 
assessment conducted in the last review are described in Chapter 6 
of the 1996 Staff Paper (EPA, 1996b) and in several technical 
reports (Abt Associates, 1996; Abt Associates, 1997a,b) and 
publications (Post et al., 2000; Deck et al., 2001).
---------------------------------------------------------------------------

    In light of the substantial uncertainties in the prior risk 
estimates, EPA placed greater weight on the overall conclusions derived 
from the health effect studies--that ambient PM was likely causing or 
contributing to significant adverse effects at levels below those 
permitted by the then-existing PM10 standards--than on the 
specific concentration-response functions and quantitative risk 
estimates derived from them. Nevertheless, EPA judged that the 
assessment provided reasonable estimates as to the possible extent of 
risk for those effects given the available information (62 FR at 
38656).
1. Overview
    The updated risk assessment conducted as part of this review 
includes estimates of (1) risks of mortality, morbidity, and symptoms 
associated with recent ambient PM2.5 levels; (2) risk 
reductions and remaining risks associated with just meeting the current 
suite of PM2.5 NAAQS; and (3) risk reductions and remaining 
risks associated with just meeting various alternative PM2.5 
standards in a number of example urban areas. This risk assessment is 
more fully described and presented in the Staff Paper (EPA, 2005a, 
Chapter 4) and in a technical support document, Particulate Matter 
Health Risk Assessment for Selected Urban Areas (Abt Associates, 
2005a). The scope and methodology for this risk assessment were 
developed over the last few years with considerable input from the 
CASAC PM Panel and the public.\21\ The information presented in these 
documents included specific criteria for the selection of health 
endpoints and studies to include in the assessment. It also addressed 
which alternative statistical models (e.g., for control of time-varying 
factors such as weather and for various lags) to include in the 
assessment, recognizing that some of the health studies presented 
results from a large number of alternative models. In an advisory 
letter sent by CASAC to the Administrator documenting its advice in May 
2002 (Hopke, 2002), CASAC concluded that the general methodology and 
framework to be used in the assessment were appropriate.
---------------------------------------------------------------------------

    \21\ In June 2001, OAQPS released a draft document, PM NAAQS 
Risk Analysis Scoping Plan (EPA, 2001), for CASAC consultation and 
public comment, which described staff's general plan for this 
assessment. In January 2002, OAQPS released a more detailed draft 
document, Proposed Methodology for Particulate Matter Risk Analyses 
for Selected Urban Areas (Abt Associates, 2002), for CASAC review 
and public comment, which described staff's plans to assess (a) 
PM2.5-related risks for several health endpoints, 
including mortality, hospital admissions, and respiratory symptoms 
and (b) PM10-2.5-related risks for hospital admissions 
and respiratory symptoms (as discussed below in Section III.B).
---------------------------------------------------------------------------

    The goals of the PM2.5 risk assessment were: (1) To 
provide estimates of the potential magnitude of mortality and morbidity 
effects associated with current PM2.5 levels, and with 
meeting the current suite of PM2.5 NAAQS and alternative 
PM2.5 standards, in specific urban areas; (2) to develop a 
better understanding of the influence of various inputs and assumptions 
on the risk estimates; and (3) to gain insights into the distribution 
of risks and patterns of risk reductions associated with meeting 
alternative suites of PM2.5 standards. EPA recognizes that 
there are many sources of uncertainty and variability inherent in the 
inputs to this assessment and that there is a high degree of 
uncertainty in the resulting PM2.5 risk estimates. While 
some of these uncertainties have been addressed quantitatively in the 
form of estimated confidence ranges around central risk estimates, 
other uncertainties and the variability in key inputs are not reflected 
in these confidence ranges, but rather have been addressed through 
separate sensitivity analyses or characterized qualitatively.
2. Scope and Key Components
    The risk assessment estimates risks of various health effects 
associated with exposure to ambient PM2.5 in nine urban 
areas selected to illustrate the public health impacts associated with 
a recent year of air quality and potential reductions in risk 
associated with just meeting the current suite of PM2.5 
standards and alternative suites of standards. The selection of urban 
areas was largely determined by identifying areas in the U.S. for which 
acceptable epidemiological studies were available that estimated 
concentration-response relationships for PM2.5, which were 
then used in assessing the risks. Thus, unlike the prior risk 
assessment, the current risk assessment for short-term exposure 
mortality and morbidity health effects used concentration-response 
relationships reported in studies that included the urban areas for 
which risks were estimated. Based on a review of the evidence evaluated 
in the Criteria Document and Staff Paper, as well as the criteria 
discussed in Chapter 4 of the Staff Paper, the following broad 
categories of health endpoints were included in the risk assessment for 
PM2.5 associated with short-term exposure: Total (non-
accidental), cardiovascular, and respiratory mortality; hospital 
admissions for cardiovascular and respiratory causes; and respiratory 
symptoms not requiring hospitalization. Also included in the 
PM2.5 risk assessment were total, cardiopulmonary, and lung 
cancer mortality associated with long-term exposure.
    The available long-term exposure mortality concentration-response 
functions are all based on cohort studies, in which a cohort of 
individuals is followed over time. Based on the evaluation contained in 
the Criteria Document and EPA's assessment of the complete data base 
addressing mortality associated with long-term exposure to 
PM2.5, studies based on the following two cohorts were 
identified as being particularly relevant for the PM2.5 risk 
assessment: (1) The Six Cities study cohort (referred to as Krewski et 
al. (2000)--Six Cities) and (2) the ACS cohort (referred to Krewski et 
al. (2000)--ACS), which includes a much larger number of individuals 
from many more cities. In addition, Pope et al. (2002) extended the 
follow-up period for the ACS cohort to sixteen years and published 
findings on the relation of long-term exposure to PM2.5 and 
all-cause mortality as well as cardiopulmonary and lung cancer 
mortality (referred to as Pope et al. (2002)--ACS extended).\22\
---------------------------------------------------------------------------

    \22\ The use of these particular cohort studies to estimate 
health risks associated with long-term exposure to PM2.5 
is consistent with the views expressed in the National Academy of 
Sciences (2002) report, ``Estimating the Public Health Benefits of 
Proposed Air Pollution Regulations,'' and the Science Advisory Board 
Clean Air Act Compliance Council review of the proposed methodology 
to estimate the health benefits associated with the Clean Air Act 
(SAB, 2004).
---------------------------------------------------------------------------

    The available short-term exposure morbidity and mortality 
concentration-response functions used in the risk assessment are all 
from time series studies. The risk assessment included only those 
health endpoints for which the the Criteria Document concluded that 
there is likely to be a causal relationship with short-term exposure to 
PM2.5 based on the overall weight of the evidence from the 
collective body of available studies. Also, given the large number of 
endpoints and studies addressing PM2.5-related effects, the 
assessment only included the more severe and better understood (in 
terms of health consequences) health effects. As noted above, in 
contrast to the prior risk assessment, the concentration-response 
functions used in this assessment for each urban area are

[[Page 2639]]

based on results of studies for that specific area or from a multi-city 
study that included that specific area.
    The concentration-response relationships used in the assessment 
were based on findings from human epidemiological studies that have 
relied on fixed-site, population-oriented, ambient monitors as a 
surrogate for actual ambient PM2.5 exposures. The risk 
assessment addresses risks attributable to anthropogenic sources and 
activities (i.e., risk associated with concentrations above policy-
relevant background \23\ or above various selected higher cutpoints 
intended as surrogates for alternative assumed population thresholds). 
This approach of estimating risks in excess of background was judged to 
be more relevant to policy decisions regarding ambient air quality 
standards than risk estimates that include effects potentially 
attributable to uncontrollable background PM concentrations. For the 
base case analyses, an estimate of the annual average background level 
was used, rather than a maximum 24-hour value, since estimated risks 
were aggregated for each day throughout the year.
---------------------------------------------------------------------------

    \23\ Background PM concentrations used in the PM risk assessment 
were defined in Chapter 2 of the Staff Paper as the PM 
concentrations that would be observed in the U.S. in the absence of 
anthropogenic emissions of PM and its precursors in the U.S., 
Canada, and Mexico. For the initial base case risk estimates, the 
midpoints of the appropriate ranges of annual average estimates for 
PM2.5 background presented in the Staff Paper were used 
(i.e., eastern values were used for eastern study locations and 
western values were used for western study locations). Estimated 
policy-relevant background concentrations are 3.5 [mu]g/m\3\ in 
eastern cities, and 2.5 [mu]g/m\3\ in western cities.
---------------------------------------------------------------------------

    In order to estimate the incidence of a particular health effect 
associated with recent conditions in a specific county or set of 
counties attributable to ambient PM2.5 exposures in excess 
of background or various alternative cutpoints, as well as the change 
in incidence corresponding to a given change in PM2.5 levels 
resulting from just meeting a specified set of alternative 
PM2.5 standards, three elements are required. These elements 
are: (1) Air quality information (including recent air quality data for 
PM2.5 from ambient monitors for the selected location, 
estimates of background PM2.5 concentrations appropriate for 
that location, and a method for adjusting the recent data to reflect 
patterns of air quality estimated to occur when the area just meets a 
given set of PM2.5 standards); (2) relative risk-based 
concentration-response functions that provide an estimate of the 
relationship between the health endpoints of interest and ambient PM 
concentrations; and (3) annual or seasonal baseline health effects 
incidence rates and population data, which are needed to provide an 
estimate of the annual or seasonal baseline incidence of health effects 
in an area before any changes in PM air quality.
    The risk assessment for PM2.5 included a series of base 
case analyses that characterized the uncertainty associated with the 
form of the concentration-response relationship drawn from the studies 
used in the assessment--this uncertainty had by far the greatest impact 
on estimated risks. Other uncertainties addressed in various 
sensitivity analyses (e.g., the use of single-versus multi-pollutant 
models, single-versus multi-city models, use of a distributed lag 
model, alternative assumptions about the relevant air quality for long-
term exposure mortality, and alternative constant or varying background 
levels) all have a more moderate and often variable impact on the risk 
estimates in some or all of the cities.
    In estimating health risks remaining upon just meeting the current 
and alternative PM2.5 standards, the assessment includes a 
series of base cases, while noting that the confidence ranges in the 
estimates do not reflect all the identified uncertainties. As discussed 
above in section II.A.3, additional uncertainty for short-term exposure 
mortality is related to the use of alternative statistical models and 
methods to control for time-varying effects, such as weather or season, 
and to address alternative lag structures. To provide a consistent 
basis for comparison across studies and locations, the risk assessment 
used concentration-response functions based on the most common type of 
analysis (``generalized additive methods'') and on lag structures 
judged to be most appropriate for each specific health endpoint, as 
discussed in the Staff Paper (EPA, 2005a, p. 4-24). The risk assessment 
included a sensitivity analysis for one location where a wide array of 
statistical models and lags was reported in the health study for that 
location (Los Angeles, as reported in Moolgavkar, 2003). EPA recognizes 
that there is additional uncertainty associated with choices about 
appropriate modeling strategy (EPA, 2004, 8.4.2) and that this 
uncertainty is not included in the confidence ranges presented for the 
risk estimates.
    As noted earlier, EPA recognizes that while there are likely 
biological thresholds in individuals for specific health endpoints, the 
available epidemiologic studies do not support or refute the existence 
of thresholds at the population level for either long-term or short-
term PM2.5 exposures within the range of air quality 
observed in the studies (EPA, 2004, 9.2.2.5). Thus, base case risks 
were estimated using not only the linear or log-linear concentration-
response functions reported in the studies, but also using a series of 
modified linear functions, as discussed below, 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.
    For short-term exposure mortality and morbidity outcomes associated 
with PM2.5, the initial base case includes linear or log-
linear concentration-response models reported in the epidemiology 
studies which are applied down to the estimated policy-relevant 
background concentration level. Generally, the lowest measured 
concentrations in the short-term exposure studies were relatively near 
or below the estimated policy-relevant background levels such that 
little or no extrapolation was required beyond the range of data in the 
studies. In the case of the long-term exposure mortality studies for 
PM2.5 that have been included in the risk assessment, the 
lowest measured levels were in the range 7.5 to 11 [mu]g/m\3\. For the 
initial base case scenario for this endpoint, the reported linear 
models were applied down to 7.5 [mu]g/m\3\, which is the lowest 
measured level reported in the long-term studies. Going down to an 
estimated policy-relevant background level for short-term exposure 
studies and to 7.5 [mu]g/m\3\ for long-term studies provides a 
consistent framework which facilitates comparison of risk estimates 
across urban locations within each group of studies and avoids 
significant extrapolation beyond the range of concentrations included 
in these studies.
    Additional base case scenarios for both short- and long-term 
exposure health endpoints involved the use of alternative 
concentration-response functions that incorporated a modified linear 
slope with an imposed cutpoint (i.e., an assumed threshold). For 
mortality associated with short-term exposure, the base case analyses 
included risk estimates associated with cutpoints of 10, 15, and 20 
[mu]g/m. For mortality associated with long-term PM2.5 
exposure, cutpoints of 10 and 12 [mu]g/m\3\ were included. For the base 
case scenarios involving alternative cutpoints, the approach used to 
develop alternative functions incorporates a modified linear slope with 
an imposed cutpoint (i.e., an assumed population threshold) that is 
intended to reflect a

[[Page 2640]]

hypothetical inflection point in a typical non-linear, ``hockey-stick'' 
shaped function, below which there is little or no population response. 
More specifically, the slope of the concentration-response relationship 
has been adjusted assuming that the upward-sloping portion of the 
``hockey stick'' would be the slope estimated in the original 
epidemiologic study adjusted by the inverse of the proportion of the 
range of PM levels observed in the study that was above the cutpoint. 
The Staff Paper concludes that this simple slope adjustment approach 
represents a reasonable approach to illustrating the potential impact 
of possible non-linear concentration-response relationships. In its 
review of the Staff Paper and risk assessment, the CASAC PM Panel 
commented that for the purpose of estimating public health impacts, it 
``favored the primary use of an assumed threshold of 10 [mu]g/m\3\'' 
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 concentration-response curves at low levels of exposure that 
may exist'' (Henderson, 2005a).
3. Risk Estimates and Key Observations
    In focusing on the five study areas that do not meet the current 
PM2.5 standards based on 2001-2003 air quality data 
(Detroit, Los Angeles, Philadelphia, Pittsburgh, and St. Louis), the 
total mortality risk estimates associated with simulating air quality 
reductions to just meet the current PM2.5 standards (based 
on associations with long-term PM2.5 exposure, and using the 
lowest cutpoint of 7.5 [mu]g/m\3\) range from several hundred to over 
1500 deaths per year, which translate into an incidence rate of 
approximately 16 to 35 deaths per year per hundred thousand 
population.\24\ These estimated risks associated with long-term 
exposure represent approximately 2.6 to 3.2 percent of total mortality 
in those areas. Estimated risks associated with long-term exposure 
based on an assumed cutpoint of 10 [mu]g/m\3\ are roughly half as large 
as the estimates based on a cutpoint of 7.5 [mu]g/m\3\. In the same 
five areas, the estimates of mortality risk associated with short-term 
PM2.5 exposure, based on a cutpoint equal to policy-relevant 
background or 10 [mu]g/m, range from less than 20 percent to over 50 
percent of the estimates associated with long-term exposure.\25\
---------------------------------------------------------------------------

    \24\ The full range of quantitative risk estimates associated 
with just meeting the current PM2.5 standards are 
presented in Tables 4-9, 4-10, 4-12, and 4-13 in Chapter 4 of the 
Staff Paper.
    \25\ In some areas, the 95 percent confidence ranges associated 
with the risk estimates for short-term exposure (but not long-term 
exposure) extend to below zero, reflecting appreciably more 
uncertainty in estimates based on positive but not statistically 
significant associations.
---------------------------------------------------------------------------

    Reductions in risk associated with simulating air quality to just 
meet a range of lower alternative annual and 24-hour PM2.5 
standards were also estimated in this assessment. The estimated risk 
reductions are depicted graphically in the Staff Paper (EPA, 2005a, 
Figures 5-1 and 5-2 and Figures 5A-1 and 5A-2), showing patterns of 
estimated risk reductions associated with alternative suites of 
standards for all the various assumed cutpoints. As would be expected, 
patterns of increasing estimated risk reductions are observed as either 
the annual or 24-hour standard, or both, are reduced over the range 
considered in this assessment, and the estimated percentage reductions 
in risk are strongly influenced by the assumed cutpoint level.
    The discussion below highlights additional observations and 
insights from this PM2.5 risk assessment, together with 
important caveats and limitations.
    (1) With respect to short-term exposure mortality and morbidity, 
this risk assessment provides the basis for greater confidence in the 
results as compared to the prior assessment, given that studies are now 
available using PM2.5 as the indicator in a much greater 
number of locations, and the assessment is able to use city-specific 
functions that are matched to the locations for which risks are 
estimated. This contrasts with the use of pooled concentration-response 
functions in the prior assessment which did not include studies for the 
specific cities included in that assessment. However, EPA recognizes 
that the confidence ranges, which only reflect uncertainty associated 
with the precision of the study (related to the population size and 
duration of the study), may be larger for the current risk estimates 
due to the use of concentration-response functions from smaller, city-
specific studies now versus the use of concentration-response functions 
from pooled sets of studies that have greater statistical precision. 
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, is similar in one 
of the locations (Philadelphia) and the current estimate is lower in 
the other location (Los Angeles).
    (2) With respect to long-term exposure mortality risk estimates, 
the prior risk assessment focused on the estimates based on the 
original ACS study (Pope et al., 1995). Since that time additional 
cohort analyses have been published and evaluated in the Criteria 
Document. EPA has greater confidence in the current risk estimates for 
long-term exposure mortality, given the extensive review of these 
studies and the extension of the ACS study to additional years of data, 
as well as improvements in the statistical approach. However, ACS-based 
risk estimates remain sensitive to plausible changes in statistical 
model specifications. The choice of studies and concentration-response 
functions to use for the base case risk estimates is discussed in the 
Staff Paper (EPA, 2005a, p. 4-25) and risk assessment report (Abt 
Associates, 2005, pp.49-50) and is consistent with the advice provided 
by both the National Academy of Sciences and the Science Advisory board 
Clean Air Act Compliance Council (see footnote 22). At the same time, 
EPA recognizes that alternative statistical models were examined in the 
reanalysis of the ACS and Six-Cities studies, and that the uncertainty 
associated with model selection (such as multipollutant models and 
different effect estimates associated with different educational 
levels) is not reflected in the confidence ranges presented in this 
assessment. Thus, for long-term exposure mortality risk estimates there 
are additional unquantified uncertainties associated with a lack of 
understanding as to which statistical model best represents the actual 
concentration-response function. The relative risk estimates used in 
the current risk assessment from the ACS extended study are only 
slightly smaller (1.06 with 95 percent confidence interval of 1.02-
1.11) compared to the original ACS study (1.07 with 95 percent 
confidence interval 1.04-1.10) used in the prior assessment. In terms 
of the magnitude of the risk estimates, the estimates in terms of 
percentage of total incidence are very similar for the two specific 
locations included in both the prior and current assessments.
    (3) A fairly wide range of risk estimates are observed for 
PM2.5-related morbidity and mortality risk associated with 
recent air quality across the urban areas analyzed. The impact of 
adding additional co-pollutants to the models was variable; sometimes 
there was relatively little difference, while in other cases there were 
larger differences. The wide variability in risk estimates associated 
with a recent year of air

[[Page 2641]]

quality is to be expected given the wide range of PM2.5 
levels across the urban areas analyzed and the variation observed in 
the concentration-response relationships obtained from the original 
epidemiologic studies. Among other factors, this variability may 
reflect differences in the mixture of components or sources of fine 
particles, populations, exposure considerations (e.g., degree of air 
conditioning use), differences in co-pollutants and/or other stressors, 
differences in study design, and differences related to exposure and 
monitor measurement error.
    (4) The single most important factor influencing the quantitative 
estimates of risk is which of the alternative concentration-response 
functions included in this assessment are considered to best represent 
the unknown ``true'' concentration-response relationships. In 
comparison, the following uncertainties have only a moderate impact on 
the risk estimates in some or all of the cities: choice of an 
alternative estimated constant background level, use of a distributed 
lag model, and alternative assumptions about the relevant air quality 
for estimating exposure levels for long-term exposure mortality. Use of 
a distribution of daily background concentrations had very little 
impact on the risk estimates.
    The overall pattern of risk associated with short-term 
PM2.5 exposures across the distribution of PM2.5 
air quality, as typically observed in urban areas, is similar to that 
observed in the last review. That is, on an annual basis, the very 
highest days (which pose the greatest risk in terms of deaths per day) 
contribute less to the total annual health risk associated with short-
term exposures than the middle of the distribution, due to the much 
greater number of days that occur in this part of the air quality 
distribution.
    (5) Risk estimates associated with just meeting the current suite 
of PM2.5 standards in five urban areas that do not meet the 
current PM2.5 standards showed a wide range of 
PM2.5-related risk estimates for short-term exposure 
mortality and morbidity. This is likely due, in large part, to 
differences in concentration-response relationships among single-
location short-term exposure studies, differences in baseline incidence 
rates, and varying population sizes. Results of a sensitivity analysis 
which applied one multi-city concentration-response function to all 
five urban areas analyzed narrowed considerably the range of risk 
estimates when a risk metric was used that normalized for different 
population sizes. However, it is still unknown whether the wider range 
of estimates observed using single-city concentration-response 
functions reflect methodological differences between studies and/or 
real city-to-city differences related to exposure, population, 
composition of the particles, or other factors.
    (6) For the risk estimates associated with just meeting the current 
suite of PM2.5 standards and alternative suites of 
standards, the single most important factor influencing the short- and 
long-term exposure mortality and morbidity estimates is again which of 
the alternative concentration-response functions included in this 
assessment are considered to best represent the unknown ``true'' 
concentration-response relationships. Several additional sources of 
uncertainty are introduced into this portion of the risk assessment, 
including: (1) Uncertainty in the degree to which the pattern of air 
quality concentration reductions estimated for the risk assessment 
cities represents the distribution of actual PM concentration changes 
that would be observed in a given area (``rollback uncertainty'') and 
(2) uncertainty concerning the degree to which current PM risk 
coefficients may reflect contributions from other pollutants, or 
uncertainty concerning whether all of the constituents of 
PM2.5 would be reduced in similar proportion to the 
reduction in PM2.5 as a whole, and, if not, what impact this 
would have on estimated reductions in risk. For areas where the current 
annual standard is the controlling standard, one alternative approach 
to rolling back the distribution of daily PM2.5 
concentrations, in which the upper end of the distributions of 
concentrations was reduced by a greater amount than the rest of the 
distribution, had little impact on the risk estimates. This approach or 
alternative approaches to rolling back the distribution of daily 
concentrations may have a greater impact on the risk estimates in areas 
where the daily standard is the controlling standard.
    (7) For the risk estimates associated with just meeting the current 
or alternative suites of PM2.5 standards, there is a 
significant decrease in the mortality risk estimates based on short-
term PM2.5 exposure remaining as one considers alternative 
higher cutpoints. There also is a significant increase observed in the 
percent reduction in estimated risk upon just meeting alternative 
standards with higher alternative cutpoints. These findings are even 
more pronounced for the mortality risk estimates associated with long-
term PM2.5 exposure as higher alternative cutpoint levels 
are considered.

C. Need for Revision of the Current Primary PM2.5 Standards

    The initial issue to be addressed in the current review of the 
primary PM2.5 standards is whether, in view of the advances 
in scientific knowledge reflected in the Criteria Document and Staff 
Paper, the existing standards should be revised. Based on the 
information and conclusions presented in the Criteria Document, 
summarized above in section II.A., the Staff Paper concludes that the 
newly available information generally reinforces the associations 
between PM2.5 and mortality and morbidity effects observed 
in the last review. While important uncertainties and research 
questions remain, much progress has been made in reducing some key 
uncertainties since the last review. The examination of specific 
components, properties, and sources of fine particles that are linked 
with health effects remains an important research need. 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 time-varying 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. In addition, health 
effects associations reported in epidemiologic studies have been found 
to be generally robust to confounding by co-pollutants, there is now 
greater confidence in the results of long-term exposure studies due to 
reanalyses and extensions of the critical studies, and there is an 
increased understanding of susceptible populations. Based on these 
considerations, the Staff Paper finds clear support in the available 
evidence for fine particle standards that are at least as protective as 
the current PM2.5 standards (EPA, 2005a, p. 5-6).
    Having reached this initial conclusion, the Staff Paper addresses 
the question of whether the available evidence supports consideration 
of standards that are more protective than the current PM2.5 
standards. In so doing, the Staff Paper considers whether there is now 
evidence (1) that statistically significant health effects associations

[[Page 2642]]

with short-term exposures to fine particles occur in areas that would 
likely meet the current PM2.5 standards or (2) that such 
associations with long-term exposures to fine particles extend down to 
lower air quality levels than had previously been observed.\26\ This 
takes into consideration the bases for the decisions made in 1997 in 
setting the current PM2.5 standards. In generally 
considering what areas would likely meet the current PM2.5 
standards, the focus is principally on comparing the long-term average 
PM2.5 level in an area with the level of the current annual 
PM2.5 standard, since in 1997 that standard was set to be 
the ``generally controlling'' standard to provide protection against 
health effects related to both short- and long-term exposures to fine 
particles. 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.
---------------------------------------------------------------------------

    \26\ In addressing this question, the Staff Paper first 
recognizes, as discussed above in section II.A.3, 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, 2004, section 
9.2.2.5).
---------------------------------------------------------------------------

    In first considering the available epidemiologic evidence related 
to short-term exposures, the Staff Paper focuses on specific 
epidemiologic studies that show statistically significant associations 
between PM2.5 and health effects for which the Criteria 
Document judges associations with PM2.5 to be likely causal 
(EPA, 2005a, section 5.3.1.1). Many more U.S. and Canadian studies are 
now available that provide evidence of associations between short-term 
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 [mu]g/m3). Moreover, a few newly 
available short-term 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 focuses on those that include adequate gravimetric 
PM2.5 mass measurements, and where the associations are 
generally robust to alternative model specification and to the 
inclusion of potentially confounding co-pollutants. Three such 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 and cardiovascular mortality in 
areas in which long-term average PM2.5 concentrations ranged 
between 13 and 14 [mu]g/m3 and 98th percentile 
concentrations ranged between 32 and 59 [mu]g/m3.\27\
---------------------------------------------------------------------------

    \27\ As 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 the other study included multi-
pollutant model results in reanalyses, reporting that associations 
with PM2.5 remained significant with gaseous pollutants 
(Fairley, 2003).
---------------------------------------------------------------------------

    In also considering the new epidemiologic evidence available from 
U.S. and Canadian studies of long-term exposure to fine particles, the 
Criteria Document notes that new studies have built upon studies 
available in the last review and concludes that these studies have 
confirmed and strengthened the evidence of associations for both 
mortality and respiratory morbidity (EPA, 2004, section 9.2.3). For 
mortality, the Criteria Document places 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, 2004, p. 9-34), notwithstanding the lack of consistent 
results in other long-term exposure studies. For morbidity, the 
Criteria Document finds 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, 2004, pp. 9-33 to 9-34). In addition to 
strengthening the evidence of association, the new extended ACS 
mortality study 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, as well as other indicators or 
components of fine particles (e.g., sulfates, combustion-related 
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 source-
oriented studies (e.g., suggesting associations with combustion- and 
vehicle-related sources of fine particles). The Criteria Document also 
notes that new epidemiologic studies of asthma-related increased 
physicians 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, 2004, p. 9-
94).
    In reviewing this information, the Staff Paper recognizes that 
important limitations and uncertainties associated with this expanded 
body of evidence for PM2.5 and other indicators or 
components of fine particles, noted above in section II.A.2, 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 
notes that while 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, particularly 
SO2, or to the gaseous co-pollutants themselves. The 
Criteria Document concludes, however, that overall the various 
approaches that have now been used to evaluate this issue substantiate 
that associations for various PM indicators with mortality and 
morbidity are generally robust to confounding by co-pollutants (EPA, 
2004, 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 
concludes 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 notes 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 at levels

[[Page 2643]]

below the current standards and extending 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 
considers 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. 
In so doing, the Staff Paper first notes that the risk assessment 
addresses a number of key uncertainties through various base case 
analyses, as well as through several sensitivity analyses, as discussed 
above in section II.B. In considering the health risks estimated to 
occur upon attainment of the current PM2.5 standards, the 
Staff Paper focuses 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,\28\ 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 concentration-response functions 
reported in the studies or with the modified functions favored by 
CASAC,\29\ the risk assessment indicates 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 standards.\30\ Beyond 
the estimated incidences of premature mortality, the Staff Paper also 
recognizes 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, 
2005a, Chapter 4) and on the discussion related to this pyramid of 
effects in the Criteria Document (EPA, 2004, section 9.2.5). Based on 
these considerations, the Staff Paper concludes 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.
---------------------------------------------------------------------------

    \28\ As discussed above in section II.B.2, the reported linear 
or log-linear concentration-response functions were applied down to 
7.5 [mu]g/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 
[mu]g/m\3\ for eastern urban areas and 2.5 [mu]g/m\3\ for western 
urban areas).
    \29\ The CASAC PM Panel generally favored the primary use of an 
assumed threshold of 10 [mu]g/m\3\ for the various concentration-
response functions used in the risk assessment (Henderson, 2005a).
    \30\ The Staff Paper recognizes how highly dependent any 
specific risk estimates are on the assumed shape of the underlying 
concentration-response 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, 2005a, p. 5-15).
---------------------------------------------------------------------------

    In considering available evidence, risk estimates, and related 
limitations and uncertainties, the Staff Paper concludes 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 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 
increase 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).\31\
---------------------------------------------------------------------------

    \31\ Of the individual Panel members who submitted written 
comments expressing views on appropriate levels of the 
PM2.5 standards, only one did not suppport changes to 
either the 24-hour or annual standard to provide additional public 
health protection (Henderson, 2005a). In written comments, the 
health scientists on the CASAC Panel did not agree on whether the 
annual standard should be lowered.
---------------------------------------------------------------------------

    In considering whether the suite of primary PM2.5 
standards should be revised to provide requisite public health 
protection, the Administrator has 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 places primary consideration on the 
evidence obtained from the studies, and provisionally finds the 
evidence of serious health effects reported in short-term 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 
recognizes 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 PM2.5 standards. 
In considering the risk assessment presented in the Staff Paper, the 
Administrator notes 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 views 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 concludes that 
the current primary PM2.5 standards, taken together, are not 
requisite to protect public health with an adequate margin of safety 
and that revision is needed to provide increased public health 
protection.

D. Indicator of 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 size-
differentiated sample of fine particles or on one or more components 
within the mix of fine particles. Secondly, 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

[[Page 2644]]

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, July 18, 1997).
    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 [mu]m (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 [mu]m 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 [mu]m 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, 2004, 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, 2003a; Mar et 
al., 2003; Tsai et al., 2000; EPA, 2004, section 8.2.2.5). Schwartz 
(2003a) 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). 
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).\32\ No 
significant increase in mortality was reported with a source factor 
representing crustal material in fine particles (CD, 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, 2004, 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, 2004, section 7.10.2). In 
addition, some recent studies have suggested that the ultrafine subset 
of fine particles (generally including particles with a nominal mean 
aerodynamic diameter less than 0.1 [mu]m) may also be associated with 
adverse effects (EPA, 2004, pp. 8-67 to 68).
---------------------------------------------------------------------------

    \32\ More 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.
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    The Criteria Document recognizes 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 exposure to particles may elicit effects that are 
linked with a mixture of components more than with any individual PM 
component (EPA, 2004, section 9.2.3.1.3).
    Thus, epidemiologic and toxicologic studies have provided evidence 
for effects associated with various fine particle components or size-
differentiated subsets of fine particles. The Criteria Document 
concludes: ``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, 2004, p. 9-
31). Conversely, the Criteria Document provides no basis to conclude 
that any individual fine particle component cannot be associated with 
adverse health effects (EPA, 2005a, 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 continues 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 concludes that it remains appropriate to

[[Page 2645]]

control fine particles as a group; i.e., that total mass of fine 
particles is the most appropriate indicator for fine particle standards 
(EPA, 2005a, p. 5-17).
    With regard to an appropriate size cut for a size-based indicator 
of total fine particle mass, the Criteria Document concludes 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 [mu]m remains appropriate (EPA, 2004, p. 9-22). This 
conclusion follows from a recognition that within the intermodal range 
of 1 to 3 [mu]m 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 [mu]m and 2.5 [mu]m. 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 [mu]m, 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 [mu]m. The same considerations that led to the selection 
of a 2.5 [mu]m 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 by the Staff Paper (EPA, 
2005a, p. 5-18) and CASAC (Henderson, 2005a) in this review. In 
addition, the Staff Paper recognizes 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, 2005a, p. 5-18).
    Consistent with the Staff Paper and CASAC recommendations, the 
Administrator proposes to retain PM2.5 as the indicator for 
fine particles. Further, the Administrator provisionally concludes 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.
    The Administrator notes that some commenters have 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.\33\ 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 provide some limited evidence of 
such associations that is helping to focus research activities, the 
Administrator solicits 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 is 
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.
---------------------------------------------------------------------------

    \33\ Such comments have focused in part on newer studies that 
have become available since the close of the Criteria Document, 
which EPA intends to include in its assessment of potentially 
significant new studies discussed above in section I.D.
---------------------------------------------------------------------------

    More specifically, comment is also solicited on a number of related 
issues. One such issue is the extent to which reducing particular types 
of PM (differentiated by either size or chemistry) might alter the size 
and toxicity of remaining particles, and on the extent to which fine 
particles in urban and rural areas can be differentiated by size or 
chemistry. Another issue deals with assessment of human exposure and 
its relationship with pollution measurements at monitors (EPA, 2004, 
chapter 5); comment is solicited on 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. Comment 
is also solicited on studies using concentrated ambient particles 
(CAPs) and their use in examining the toxicity of specific mixtures of 
pollutants or of particular source categories.

E. 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 at 
38668-70). This decision was based in part on evidence of health 
effects related to both short-term (from less than 1 day to up to 
several days) and long-term (from a year to several years) measures of 
PM. 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. 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. 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 
supports consideration of different averaging times for 
PM2.5 standards, the Staff Paper concludes 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 makes the following observations 
(EPA, 2005a, 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, 2004, section 3.5.5.1). While 
the Staff Paper concludes

[[Page 2646]]

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, 2004, section 3.5.5). While such studies continue to suggest 
consideration of a multiple day averaging time, the Staff Paper notes 
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 concludes 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, 2004, section 3.5.5.3), the Staff Paper concludes 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) recommend retaining the current annual and 24-hour 
averaging times for PM2.5 primary standards. The 
Administrator concurs with the staff and CASAC recommendations and 
proposes 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.

F. 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 population-oriented monitor within an area, averaged over three 
years (62 FR at 38671-74). EPA selected such a concentration-based form 
because of its advantages over the previously used expected-exceedance 
form.\34\ 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 less-than-every-day monitoring; and, 
when averaged over 3 years, it has greater stability and, thus, 
facilitates the development of more stable implementation programs.\35\ 
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 population-oriented 
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.
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    \34\ The form of the 1987 24-hour PM10 standard is 
based on the expected number of days per year (averaged over 3 
years) on which the level of the standard is exceeded; thus, 
attainment of the one-expected exceedance form is determined by 
comparing the fourth-highest concentration in 3 years with the level 
of the standard.
    \35\ See American Trucking Associations v. EPA, 283 F. 3d at 
374-75 (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).
---------------------------------------------------------------------------

    In this review, the Staff Paper concludes 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 population-oriented monitor within an area, 
averaged over 3 years. This staff recommendation is 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 recommends either retaining the 98th 
percentile form or revising it to be based on the 99th percentile form, 
and notes that primary consideration should be given to the combination 
of form and level, as compared to looking at the form in isolation 
(EPA, 2005a, p. 5-44).
    In considering the information provided in the Staff Paper, most 
CASAC Panel members favored continued use of the 98th percentile form 
because it is more robust than the 99th percentile form, such that it 
would provide more stability to prevent areas from bouncing 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 afforded by a standard.
    In considering the available information and the Staff Paper and 
CASAC recommendations, the Administrator proposes that the form of the 
24-hour standard should be based on the 98th percentile form. In so 
doing, the Administrator has 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.
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 community-oriented 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 App. N section 2.1(a) and (b) and Part 58 App. D at 2.8.1.6.1; 62 FR 
38,672, July 18, 1997). Community-oriented monitoring sites were 
specified to be consistent with the intent that a spatially averaged 
annual standard protect those in smaller communities, as well as those 
in larger population centers. The constraints on allowing the use of 
spatially averaged measurements were intended to limit averaging across 
poorly correlated or widely disparate air quality values.\36\ This 
approach was judged to be consistent with the epidemiologic studies on 
which the PM2.5 standard

[[Page 2647]]

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.).
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    \36\ The 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 (Part 58 App. D at 
2.8.1.6.1).
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    In this review, there now exist much more PM2.5 air 
quality data than were available in the last review. Consideration in 
the Staff Paper of the spatial variability across urban areas that is 
revealed by this new database 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, have taken 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, 2005a). In particular, analyses were done 
in 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 allowed by the current standard.\37\ As expected, estimated 
risks associated with long-term exposures remaining 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.\38\
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    \37\ As discussed in the Staff Paper, 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 
ambient PM2.5 levels that are 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.
    \38\ For 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 over 
40 percent higher than estimated incidence based on the use of the 
highest monitor (EPA, 2005a, p. 5-41).
---------------------------------------------------------------------------

    In considering the potential for disproportionate impacts on 
potentially vulnerable subpopulations, analyses were done to assess 
whether any such groups are more likely to live in census tracts in 
which the monitors recording the highest air quality values in an area 
are located. Data were obtained for 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 
area-wide 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 observes 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 percentage minority 
levels (EPA, 2005a, p. 5-41).\39\ Noting the intended purposes of the 
form of the annual standard, as discussed above, the Staff Paper 
concludes 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 potentially 
vulnerable subpopulations.
---------------------------------------------------------------------------

    \39\ As summarized in section II.A.4 above, the Criteria 
Document notes that some epidemiologic study results, most notably 
the associations between 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, 2004, 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 to sources 
such as roadways. Lower education level may be associated with other 
potential risk factors, such as poorer health status or access to 
health care, that may also result in increased susceptibility to the 
effects of air pollution exposure (EPA, 2004, section 9.2.4.5)
---------------------------------------------------------------------------

    In considering whether more stringent constraints on the use of 
spatial averaging may be appropriate, the Staff Paper presents results 
of an analysis of recent air quality data on the 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 for correlation of at least 0.6, which was met by 
nearly all monitor pairs. Similarly, the current criterion that 
differences in mean air quality values between monitors not exceed 20 
percent was met for most monitor pairs, while the annual median and 
mean differences for all monitor pairs are 5 percent and 8 percent, 
respectively. This analysis also shows 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 provides some perspective on the constraints on 
spatial averaging that were put in place in the last review, before 
data were widely available on spatial distributions of PM2.5 
air quality levels, based on the extensive air quality data and related 
analyses that have become available since the last review.
    In considering the results of the analyses discussed above, the 
Staff Paper concludes 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 revising the allowance 
for spatial averaging to be based on more restrictive criteria. More 
specifically, based on the analyses discussed above, the Staff Paper 
recommends 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, with differences between monitor values 
not to exceed 10 percent (EPA, 2005a, p. 5-42).
    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 proposes to revise the form of the annual 
PM2.5 standard consistent with the Staff Paper 
recommendation to change 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. The Administrator also solicits 
comment on the other Staff Paper-recommended alternative of revising 
the form of the annual PM2.5

[[Page 2648]]

standard to one based on the highest community-oriented monitor in an 
area, with no allowance for spatial averaging.

G. Level of Primary PM2.5 Standards

    In the last review, having concluded that both 24-hour and annual 
PM2.5 standards were appropriate, EPA selected a level for 
each standard that was appropriate for the function to be served by 
such standard (62 FR 38652). As discussed 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.\40\ 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.\41\
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    \40\ 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 short- and long-term 
exposures, with the other standard serving to address situations 
where the daily peaks and annual averages are not consistently 
correlated (62 FR 38669).
    \41\ See also American Trucking Associations v. EPA, 283 F.3d at 
373 (endorsing this reasoning).
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    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 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, greater emphasis was 
placed on the short-term exposure studies. In so doing, EPA first 
determined a level for the annual standard based on the short-term 
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 strongest evidence for 
short-term PM2.5 effects occurs at 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 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 builds upon and broadens the 
general approach used by EPA in the last review. This broader 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, together with the availability of much more extensive 
PM2.5 air quality data. This newly available information has 
been used to conduct a more comprehensive risk assessment for 
PM2.5. As a consequence, the broader approach used in the 
Staff Paper discusses ways to take into account both evidence-based and 
quantitative risk-based considerations and places 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 considers it 
appropriate to use a different approach from that used in the last 
review to select appropriate standard levels. More specifically, the 
Administrator's proposal relies on an evidence-based approach that 
considers 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 and the stronger and more robust body of 
evidence from the long-term exposure PM2.5 studies as the 
principal basis for selecting the level of the annual standard. In the 
Administrator's view, the very large number of health effect studies 
that are now available provide the most reliable basis for standard 
setting. With respect to the quantitative risk assessment, the 
Administrator recognizes 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 is 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 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. 
In the case of fine particles, the Administrator recognizes that such 
uncertainties are likely to be unusually large due to the complexity in 
the composition of the mix of fine particles generally present in the 
ambient air. Further, in the Administrator's view, a risk assessment 
based on studies that do not resolve the issue of a threshold is 
inherently limited as a basis for standard setting, since it will 
necessarily predict that ever lower standards result in ever lower 
risks, which has the effect of masking the increasing uncertainty 
inherent as lower levels are considered. As a result, while the 
Administrator views the risk assessment as providing supporting 
evidence for the conclusion that there is a need to revise the current 
suite of PM2.5 standards, he judges that it does not provide 
a reliable basis to determine what specific quantitative revisions are 
appropriate.
1. 24-Hour PM2.5 Standard
    Based on the approach discussed above, the Administrator has relied 
upon evidence from the short-term exposure PM2.5 studies as 
the principal basis for selecting the level of the 24-hour standard. In 
considering these studies as a basis for the level of a 24-hour 
standard, and having selected a 98th percentile form for the standard, 
the Administrator agrees with the focus in the Staff Paper of looking 
at the 98th percentile values in these studies. In so doing, the 
Administrator recognizes 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 
is seeking to establish a standard level that will require improvements 
in air quality generally in areas in which short-term exposure to 
PM2.5 can reasonably be expected to be associated with 
serious

[[Page 2649]]

health effects. While strategies that may be employed in the future to 
bring about such improvements in air quality in any particular area are 
not yet defined, most such strategies are likely to move the 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 a supporting staff 
memo,\42\ the Administrator observes an overall pattern of 
statistically significant associations reported in studies of short-
term exposure to PM2.5 across a wide range of 98th 
percentile values. More specifically, there is a strong predominance of 
studies with 98th percentile values down to about 39 [mu]g/m\3\ (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 City study (and three of the individual cities within that study) 
(Klemm and Mason, 2003), the Canadian 8-City Study (Burnett and 
Goldberg, 2003), and in studies in Santa Clara County, CA (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 
ischemic heart disease and pneumonia, but not for other causes), and 
Montreal (Delfino et al., 1998, 1997, for some but not all age groups 
and years); for respiratory symptoms in panel studies in a combined Six 
City study (Schwartz et al., 1994) and in two Pennsylvania cities 
(Uniontown in Neas et al., 1995; State College in Neas et al., 1996); 
and for lung function in Philadelphia (Neas et al., 1999).\43\ Studies 
in this air quality range that reported positive but not statistically 
significant associations with mortality include studies in Detroit 
(Ito, 2003), Pittsburgh (Chock et al., 2000), and Montreal (Goldberg 
and Burnett, 2003).
---------------------------------------------------------------------------

    \42\ As discussed in the Staff Paper (EPA, 2005a, 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 the extent to which the studies used relatively 
reliable air quality data.
    \43\ Of the studies within this group that evaluated 
multipollutant associations, as discussed above in section II.A.3, 
the results reported in Fairley (2003), Sheppard et al. (2003), and 
Ito (2003) were generally robust to inclusion of gaseous co-
pollutants, whereas the effect estimate in Thurston et al. (1994) 
was substantially reduced with the inclusion of O3.
---------------------------------------------------------------------------

    Within the range of 98th percentile PM2.5 concentrations 
of about 35 to 30 [mu]g/m\3\, this strong predominance of statistically 
significant results is no longer observed. Rather, within this range, 
some studies report statistically significant results (Mar et al., 
2003; Ostro 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),\44\ and 
another study reports associations in two of six cities that are not 
statistically significant (Klemm and Mason, 2003). Further, the very 
limited number of studies in which the 98th percentile values are below 
this range do not provide a basis for reaching conclusions about 
associations at such levels (Stieb et al., 2000; Peters et al., 2001). 
Thus, in the Administrator's view, this body of evidence provides 
confidence that statistically significant associations are occurring 
down close to this range, and it provides a clear basis for concluding 
that this range represents a range of reasonable values and thus for 
selecting a 24-hour standard level from within this range. The 
Administrator further notes that focusing on the range of 35 to 30 
[mu]g/m\3\ 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). The Administrator recognizes, however, the separate 
point that most CASAC Panel members favored the range of 35 to 30 
[mu]g/m\3\ for the 24-hour PM2.5 standard in concert with an 
annual standard set in the range of 14 to 13 [mu]g/m3 (Henderson, 
2005a), as discussed in section II.G.2 below.
---------------------------------------------------------------------------

    \44\ For 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, 2004, p. 8-164).
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    In considering what 24-hour standard is requisite to protect public 
health with an adequate margin of safety, the Administrator is 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 effect 
thresholds, the Administrator may select a specific standard level from 
within a range of reasonable values. In making this judgment, the 
Administrator notes that the general uncertainties related to the shape 
of the concentration-response functions and 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 [mu]g/m\3\ indicates an increase in uncertainty as to 
whether likely causal associations extend down below this level.
    In considering the extent to which the quantitative risk assessment 
inform his selection of a 24-hour PM2.5 standard, the 
Administrator recognizes 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 is hesitant to use such risk estimates as a 
basis for proposing a standard level below 35 [mu]g/m\3\, and instead 
prefers to rely on inferences that are based directly on the evidence 
in the studies themselves.
    Taking the above considerations into account, the Administrator 
proposes to set the level of the primary 24-hour PM2.5 
standard at 35 [mu]g/m3. In the Administrator's judgment, 
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 
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. Being mindful that the available evidence 
does not provide a basis for identifying a bright line within the range 
of 35 to 30 [mu]g/m3 that clearly provides the appropriate 
degree of public health protection, the Administrator also

[[Page 2650]]

solicits comment on selecting a lower level within this range.
    Having reached this decision to propose a level of 35 [mu]g/
m3 for the 24-hour PM2.5 standard based on the 
approach to interpreting the available evidence described above, the 
Administrator recognizes that other approaches to selecting a standard 
level have been presented to the Agency. These other approaches reflect 
alternative views, principally expressed in public comments to date, as 
to the appropriate interpretation of the scientific evidence and the 
appropriate policy response in light of that interpretation. One such 
view focuses 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 [mu]g/m3 is 
warranted. Such commenters prefer greater weight, for example, on 
issues related to the sensitivity in the magnitude and statistical 
significance of relative risks reported in studies using different 
statistical models, noting that further research is needed to inform 
modeling strategies that will appropriately adjust for temporal trends 
and weather variables in time-series studies. Additional uncertainties 
arise from the potential confounding by co-pollutants, and the 
potential differential toxicity of components within the mix of fine 
particles. These commenters suggest that the magnitude of risks 
associated with fine particle exposures have decreased since the last 
review. Some such commenters also focus on considerations such as the 
absence of clear evidence from toxicologic studies and from studies 
focused on elucidating specific physiologic mechanisms by which 
PM2.5 may be causing the observed effects. Such commenters 
recognize a need for a 24-hour PM2.5 standard, but consider 
the evidence to be too uncertain overall to warrant any tightening of 
the standard and instead believe the appropriate policy response in 
light of this uncertainty is to retain the current level of the 24-hour 
standard.
    Other commenters who also focus strongly on the uncertainties 
inherent in the epidemiologic and toxicologic studies and the 
quantitative risk assessment reach a somewhat different conclusion as 
to the appropriate policy response in light of these uncertainties. 
This group of commenters sees a basis for lowering the level of the 24-
hour PM2.5 standard, but does not believe that a level as 
low as 35 [mu]g/m3 is warranted. Such commenters note that 
while many of the studies within the range of air quality from 
approximately 39 [mu]g/m3 up to the level of the current 
standard of 65 [mu]g/m3 report statistically significant 
results, only a few such studies independently evaluated confounding by 
co-pollutants. This lack of a broader assessment of co-pollutants, 
together with other types of uncertainties as noted above, leads such 
commenters to conclude that a standard level selected from below this 
range is not warranted, and that the appropriate policy response is to 
select a standard level from within the range of about 40 to 65 [mu]g/
m3.
    In sharp contrast, others view the epidemiologic evidence and other 
health studies as strong and robust, and generally place 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 [mu]g/m3. 
While recognizing that important uncertainties are inherently present 
in both the evidence and estimated risks, these commenters generally 
support a view that such uncertainties 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.
    The Administrator recognizes that these sharply divergent views on 
the appropriate level of the standard are based on very different 
interpretations of the science itself including its relative strengths 
and limitations and on very different judgments as to how such 
scientific evidence should be used in making policy decisions on 
proposed standards. Consistent with the goal of soliciting comments on 
a wide array of views, the Administrator also solicits broad public 
comment on these and other alternative approaches and on the related 
standard levels, such as levels from 35 [mu]g/m3 up to 65 
[mu]g/m3 or from 30 [mu]g/m3 down to 25 [mu]g/
m3, that commenters may believe are appropriate, along with 
the rationale supporting such approaches and levels. In addition, the 
Administrator solicits comments on issues related to the interpretation 
of relevant epidemiologic and toxicologic studies, including approaches 
to addressing uncertainties related to the sensitivity of results to 
alternative statistical modeling approaches, co-pollutant confounding, 
and the lack of a discernable threshold of effects, as well as 
approaches to more fully characterize uncertainties in quantitative 
risk assessments based on epidemiologic studies.
2. Annual PM2.5 Standard
    Based on the approach discussed at the beginning of this section, 
the Administrator has relied upon evidence from the long-term exposure 
PM2.5 studies as the principal basis for selecting the level 
of the annual standard. In considering these studies as a basis for the 
level of an annual standard, the Administrator agrees with the focus in 
the Staff Paper of looking at the long-term mean PM2.5 
concentrations across the cities included in such studies. In so doing, 
the Administrator recognizes 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 is 
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 
exposure PM2.5 studies presented in the Criteria Document 
and Staff Paper, the Administrator recognizes the importance of the 
validation efforts and reanalysis 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 [mu]g/m3, respectively. The 
Administrator also particularly recognizes 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 
cardiorespiratory-related mortality observed in the original studies. 
The Administrator notes 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 [mu]g/m3) than had been 
observed in the original study,

[[Page 2651]]

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 consistency and robustness of 
results, concludes 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, 2004, p. 9-
33) in weighing the evidence of mortality effects associated with long-
term exposure to PM2.5. Consistent with that assessment, the 
Administrator places greatest weight on these studies as a basis for 
selecting the level of the annual PM2.5 standard.
    In addition to these mortality studies, the Administrator also 
recognizes 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-city study 
considered in the last review and new studies of cohorts of children in 
southern California, in which the long-term mean PM2.5 
concentrations in all the cities included in the studies are 
approximately 14.5 and 15 [mu]g/m3, respectively. As 
discussed in section II.A. above, in the 24-city 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 not statistically significant associations were reported 
with prevalence of several respiratory conditions. As interpreted in 
the last review, the results from the 24-city study are uncertain as to 
the extent to which the association extends below a long-term mean 
PM2.5 concentration of approximately 15 [mu]g/m3. 
The new 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 recognizes 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 
observes 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, at 
this time, the Administrator provisionally concludes that this study 
provides an uncertain basis for establishing the level of a national 
standard.
    As discussed in the Staff Paper (EPA, 2005a, p. 5-22), the 
Administrator generally agrees that it is 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 is mindful that considering what 
standard is requisite to protect public health with an adequate margin 
of safety requires 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 concludes that these key mortality studies, together with 
the morbidity studies, provide a basis for considering a standard level 
no higher than 15 [mu]g/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 view, these 
studies do not provide a clear basis for selecting a level lower than 
the current standard of 15 [mu]g/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 recognizes 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 level 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 is again hesitant to use such 
risk estimates as a basis for proposing a lower annual standard level 
than 15 [mu]g/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 
proposes to retain the level of the primary annual PM2.5 
standard at 15 [mu]g/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. 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.
    In so doing, the Administrator recognizes that the CASAC Panel did 
not endorse retaining the annual standard at the current level of 15 
[mu]g/m3 (Henderson, 2005a, p. 7). In weighing the 
recommendation of the CASAC Panel, the Administrator has carefully 
considered the stated reasons for it. 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. Three reasons were then given 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 [mu]g/m3; (2) the amount of evidence on short-term 
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 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

[[Page 2652]]

were taken as an indication of the desirability of lowering the level 
of the annual PM2.5 standard as well as that of the 24-hour 
standard. 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 [mu]g/
m3, along with lowering the 24-hour standard (Henderson, 
2005a).
    In considering these views, the Administrator notes 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 long-term 
exposures to PM2.5 in light of the available evidence. In 
considering the evidence of effects associated with long-term 
PM2.5 exposure as a basis for selecting an adequately health 
protective annual standard, as discussed above, the Administrator 
provisionally concludes that the evidence does not provide a basis for 
requiring annual levels below 15 [mu]g/m3. Thus, the 
Administrator agrees 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. Further, the Administrator 
agrees that this general advice supports 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 does not believe that this advice 
necessarily translates into a reason for setting the annual 
PM2.5 standard at a level below the current level of 15 
[mu]g/m3. As discussed above, the Administrator believes 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 reported associations are likely causal, and this 
evidence reasonably supports retaining the current level of the annual 
standard.
    The Administrator places great importance on the advice of CASAC, 
and therefore solicits broad public comment on the range of 15 down to 
13 [mu]g/m3, the low end of the range recommended by CASAC, 
for the level of the annual PM2.5 standard as well as on the 
reasoning that formed the basis for that recommendation. A decision to 
select a standard from within this range 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.
    The Administrator recognizes that an even stronger view of the 
appropriate policy response to the currently available evidence has 
been expressed by some public commenters. These commenters have focused 
principally on the strength of the long-term exposure studies, 
including the new children's cohort study conducted in southern 
California, as well as on those results from the quantitative risk 
assessment that are based on the assumption that there is no threshold 
of effects down to the lowest levels observed in those studies. Such 
considerations generally have led these commenters to express views 
that support a highly precautionary policy response and the selection 
of a standard level that incorporates a large margin of safety, 
consistent with an annual PM2.5 standard level of 12 [mu]g/
m3. The Administrator recognizes that this view is based on 
a different interpretation of the science itself including its relative 
strengths and limitations and on different judgments as to how such 
scientific evidence should be used in making policy decisions on 
proposed standards. Consistent with the goal of soliciting comments on 
a wide array of views, the Administrator also solicits broad public 
comment on this alternative approach and on the related standard level 
of 12 [mu]g/m3.
    The Administrator also recognizes a contrasting view as to the 
interpretation of and weight to be accorded to the results from the 
ACS-based studies (Pope et al., 1995; Krewski et al., 2000; Pope et 
al., 2002). In this view, the ACS-based studies are not sufficiently 
robust to support a policy response that would tighten the annual 
PM2.5 standard based on the evidence. This view emphasizes 
the sensitivity of the results of these studies to plausible changes in 
model specification with regard to accounting for the geographical 
proximity of cities and the correlation of air pollutant concentrations 
within a region, effect modification by education level, and inclusion 
of SO2 in the model. In this view, these sensitivities 
suggest potential confounding or effect modification that has not been 
taken into account. For example, concern has been raised about the 
sensitivity of results in the reanalysis of data from the ACS cohort 
study (Krewski et al., 2000) to inclusion of SO2 in the 
models. As discussed in section II.A.2.b above, the reanalysis found 
that PM2.5, sulfates, and SO2 were each 
associated with mortality in single-pollutant models. However, in two-
pollutant models with SO2 and PM2.5, the relative 
risk for PM2.5 was substantially smaller and no longer 
statistically significant, whereas the effect estimates for 
SO2 were not sensitive to inclusion of PM2.5 or 
sulfates in two-pollutant models. In this view, the ACS-based risk 
estimates are more robust for SO2 than for PM2.5 
or sulfates. In further extended analyses, Pope et al. (2002) reported 
that effect estimates were not highly sensitive to spatial smoothing 
approaches intended to address spatial autocorrelation, while findings 
of effect modification by education level were reaffirmed. Results of 
multi-pollutant models were not reported by Pope et al. (2002). Because 
the correlation coefficient between PM2.5 and SO2 
was 0.50 in the ACS data, in this view it is plausible to believe that 
the independent effects of the two pollutants could be disentangled 
with additional study.
    In this view, there is a separate but related concern that 
tightening the annual standard now, without a clear understanding of 
which specific PM-related pollutants are most toxic, will have very 
uncertain public health payoffs. In response to the advice of the 
National Research Council (NRC) and other scientists, the Agency is 
undertaking, as one of its higher priorities, a substantial research 
program to clarify which aspects of PM-related pollution are 
responsible for elevated risks of mortality and morbidity. For example, 
the Health Effects Institute has issued a request for applications to 
analyze the largest database on specific components of PM that has ever 
been assembled for public health and medical researchers. The time line 
for this multi-million dollar research program is well designed to 
inform the Agency's next periodic reevaluation of the primary ambient 
air quality standard for PM2.5. In light of the degree of 
sensitivity of the ACS-based relative risk estimates to model 
specifications and the significant research underway, in this view, it 
would be wiser to consider modification of the annual standard with a 
fuller body of information in hand rather than initiate a change in the 
annual standard at this time.
    The Administrator solicits comment on this view and on the issues 
raised in interpreting the results of the ACS-based

[[Page 2653]]

studies. For example, comment is solicited on the extent to which the 
associations reported in the ACS-based studies suggest that 
SO2 should be considered as a surrogate for fine particles 
and/or the broader mix of air pollutants or as an independent pollutant 
exhibiting separate effects. Comment is also solicited on relevant 
research that would improve our understanding of issues related to 
model specification and alternative analytic approaches that would 
better inform judgments based on such epidemiologic studies in the 
future.

H. Proposed 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, and public 
comments to date, the Administrator proposes to revise the current 
primary PM2.5 standards. Specifically, the Administrator 
proposes to revise (1) the level of the 24-hour PM2.5 
standard to 35 [mu]g/m3, and (2) the form of the annual 
PM2.5 standard by changing the constraints on the 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. Data handling conventions 
are specified in proposed revisions to Appendix N, as discussed in 
Section V below, and the reference method for monitoring PM as 
PM2.5 is specified in proposed minor revisions to Appendix 
L, as discussed in Section VI below.
    In recognition of alternative views of the science and the 
appropriate policy response based on the currently available 
information, the Administrator also solicits comments on (1) 
alternative levels of the 24-hour PM2.5 standard within the 
range of 35 to 30 [mu]g/m3, and alternative approaches for 
selecting the level of the 24-hour PM2.5 standard, and 
related levels (such as approaches that suggest retaining the current 
level of 65 [mu]g/m3, setting a level no higher than 25 
[mu]g/m3, or setting a level within the range of 65 down to 
35 [mu]g/m3); (2) alternative levels of the annual 
PM2.5 standard below 15 [mu]g/m3 down to12 [mu]g/
m3; (3) issues related to consideration of alternative 
indicators of fine particle components; and (4) an alternative form of 
the annual PM2.5 standard based on the highest community-
oriented monitor in an area. Based on the comments received and the 
accompanying rationales, the Administrator may adopt other standards 
within the range of the alternatives identified above in lieu of the 
standards he is proposing today.
    The Administrator solicits comment on all aspects of this proposed 
decision. Comment is specifically invited on the methodology for 
evaluating the uncertainty and significance of risks to public health. 
The Administrator believes that it is important to further develop ways 
of addressing uncertainty when estimating such risk, recognizing the 
wide variety of information available in the underlying health effects 
and other studies. The Agency seeks comment on methods and approaches 
for conducting a more formalized uncertainty analysis. In addition, the 
Agency seeks comment on how to evaluate the results from a formalized 
uncertainty analysis or from the Staff Paper's risk assessment, which 
addresses multiple health effects across multiple populations, in the 
context of judging the public health importance of such risks and 
determining the requisite level of public health protection for the PM 
standards.
    To address issues related to the transition from the current 
PM2.5 standards to revised PM2.5 standards, the 
Administrator intends to seek public comment on EPA's implementation 
plans for the revised PM2.5 standards, including its plans 
for assuring an effective transition, as part of an advance notice of 
proposed rulemaking (ANPR) on NAAQS implementation that will be 
published in an early in 2006. In this ANPR, EPA will be discussing 
issues related to the timing and regulatory implications of this 
transition. The EPA intends to present and take comment on the need and 
potential approaches for revocation of the current 24-hour 
PM2.5 standard, and on issues related to the establishment 
of no-backsliding requirements, such as those adopted by the Agency in 
1997 with respect to the ozone NAAQS. The EPA also expects to address a 
variety of implementation issues concerning revised PM2.5 
standards in the ANPR. The ANPR will explain the designation process 
and its timing, and the timing of SIP submittals for both attainment 
and nonattainment areas. The EPA also expects to address issues 
regarding the attainment dates for areas designated nonattainment. The 
EPA will also discuss new source permitting requirements for both 
attainment and nonattainment areas, i.e., the PSD and Part D NSR 
programs. If the Administrator promulgates a revised PM2.5 
standard, EPA will determine the final implementation approach for that 
standard.

III. Rationale for Proposed Decisions on Primary PM10 
Standards

    This action presents the Administrator's proposed decisions on 
revision to the primary NAAQS for PM10. The rationale for 
the proposed revisions of 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 PM10 standards (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, 2004, 
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 epidemiological 
studies using thoracic coarse particles 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, 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 discussed 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

[[Page 2654]]

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 has 
provided an adequate basis for proposing regulatory decisions 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 thoracic coarse particles and health effects.

A. Evidence of Health Effects Related to Thoracic Coarse Particle 
Exposure

    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 [mu]m in 
aerodynamic diameter are capable of penetrating to the deeper 
``thoracic'' \45\ 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, 2004, p. 6-16). 
Deposition of particles to the tracheobronchial region is of particular 
concern with respect to aggravation of asthma.
---------------------------------------------------------------------------

    \45\ The `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 communications, `thoracic' particles 
penetrating to these regions are often called `inhalable' particles.
---------------------------------------------------------------------------

    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 the basis for reaching conclusions about the requisite level of 
protection provided against coarse particles for the final standards. 
The new studies available in this review include a number of 
epidemiologic studies that have reported associations with health 
effects using direct measurements of PM10-2.5, as well as a 
number of new toxicologic studies.
    This section 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 here 
summarizes: (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 ambient thoracic coarse particles or their 
constituents; (3) an integrative assessment of the evidence on health 
effects related to thoracic coarse particles; (4) subpopulations that 
appear to be sensitive to effects of exposure to thoracic coarse 
particles; and (5) the public health impact of exposure to ambient 
thoracic coarse particles.
1. Mechanisms
    As summarized above, the first review of the PM NAAQS found a 
strong basis for concluding that thoracic coarse particles could be 
plausibly linked to health effects. This was based on an integrated 
assessment of the physical and chemical characteristics of ambient 
coarse particles, the evidence regarding health effects that could be 
associated with deposition of coarse particulate substances in the 
different regions of the respiratory tract, and the relative potential 
for penetration and deposition of ambient distributions of coarse 
particles in the human respiratory tract (52 FR 24634). In the 1987 
review, EPA found that occupational and toxicologic studies provided 
ample cause for concern related to higher levels of thoracic coarse 
particles. Such findings indicated that elevated levels of thoracic 
coarse particles were linked with effects such as aggravation of asthma 
and increases in upper respiratory illness, which was consistent with 
dosimetric evidence of enhanced deposition of thoracic coarse particles 
in the respiratory tract (61 FR 65649).
    Toxicologic and controlled human exposure studies available in 
previous reviews have generally used particle exposures at levels 
higher than ambient levels, relying on various particle components or 
surrogates. Such studies reported some effects on the respiratory 
tract, indicative of inflammatory or irritant effects for particles in 
both the fine and thoracic coarse particle size range (EPA, 1982, 
chapters 12 and 13; EPA, 1996, chapters 10 and 11). As discussed above 
in section II.A, the results of numerous new toxicologic and controlled 
human exposure studies have implicated a number of potential mechanisms 
or pathways for effects associated with PM. Many of these studies have 
used particle exposures that are generally more relevant to studying 
the effects of fine particles than those of thoracic coarse particles. 
However, several studies, discussed more fully below, have suggested 
mechanisms or pathways for thoracic coarse particles to cause 
inflammatory and other effects on the respiratory system. This evidence 
generally supports previous conclusions that thoracic coarse particles 
can affect the respiratory system.
    Some limited evidence is available from recent toxicologic studies 
on effects of exposure to thoracic coarse particles, specifically using 
PM10-2.5, for either acute or chronic exposures (EPA, 2004, 
p. 9-55). This toxicologic evidence includes results from studies where 
respiratory cell cultures were exposed to ambient particles, thus 
providing insight into potential mechanisms for respiratory effects of 
thoracic coarse particles. The types of effects reported include 
inflammatory and allergic effects. For example, two recent studies 
report inflammatory responses in cells exposed to extracts of water-
soluble and water-insoluble materials from thoracic coarse particles 
and fine particles collected in Chapel Hill, NC (Monn and Becker, 1999; 
Soukup and Becker, 2001). One study focused on water-soluble materials, 
and reported significant immune system effects with water-soluble 
extracts of ambient PM10-2.5, in contrast to the lack of 
effects observed with extracts from

[[Page 2655]]

ambient PM2.5 as well as indoor-collected 
PM10-2.5 and PM2.5. The authors report that 
different components of PM10-2.5 appeared to have different 
effects, with endotoxin implicated in inflammatory effects, while 
coarse particulate metals appeared to have a role in cytotoxicity 
effects (Monn and Becker, 1999). A followup study in the same 
laboratory (Soukup and Becker, 2001) reports that the insoluble 
materials from thoracic coarse particles resulted in several effects on 
immune system cells.\46\ In this extract of thoracic coarse particles, 
endotoxin appeared to be the most pro-inflammatory component, but 
components other than endotoxin or metals appeared to contribute to 
other effects. Using particles collected in two urban areas in the 
Netherlands, Becker et al. (2003) reported that thoracic coarse 
particles, but not fine or ultrafine particles, resulted in effects 
related to inflammation and decreased pulmonary defenses. This small 
group of studies thus suggests that exposure to thoracic coarse 
particles may cause pro-inflammatory effects, as well as cytotoxicity 
and oxidant generation (EPA, 2004, section 7.4.2). While still limited, 
these emerging new studies provide additional insight into potential 
mechanisms for respiratory effects of thoracic coarse particles. The 
results also indicate that different health responses may be linked 
with different components of thoracic coarse particles.
---------------------------------------------------------------------------

    \46\ Examples of such effects include cytokine production, 
decreased phagocytic ability and oxidant generation.
---------------------------------------------------------------------------

    In contrast, one recent study exposed human red blood cell cultures 
to ambient coarse particles collected in Italy and found only limited 
effects on blood cells (Diociaiuti et al., 2001). The addition of 
thoracic coarse particles that were collected in Italy to human 
respiratory tract cell cultures produced only limited evidence of 
carcinogenic effects; some response was seen with thoracic coarse 
particles but greater response was reported with fine particle 
exposures (Hornberg et al., 1998). These latter results are consistent 
with the evidence from epidemiologic studies, which provide no direct 
evidence for carcinogenicity of thoracic coarse particles.
    As noted in past reviews (EPA, 1981b, 1996b), 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 symptoms, particularly 
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). Road dust is a 
major source of thoracic coarse particles in urban areas and is 
therefore representative of the components expected to be found in 
resuspended thoracic coarse particles. In the 1996 Staff Paper, results 
from the study by Kleinman and colleagues (1995) were highlighted in 
which effects were observed in rats with inhalation exposure to road 
dust. These effects included changes in the structure of the rat 
airways as well as effects on immune cells. Higher concentrations of 
road dust were needed to cause effects, compared with exposures to fine 
particle components (e.g., sulfates, nitrates), in part because of the 
limited penetration of coarse-sized particles past the nose of the rats 
studied (EPA, 1996b, p. V-70).\47\ Another study used a standard 
toxicologic approach to studying allergic responses, and the authors 
concluded that exposure to road tunnel dust particles resulted in 
greater allergy-related effects than did exposure to several other 
particle samples, including residual oil fly ash and diesel exhaust 
particles (Steerenberg et al., 2003).\48\ In this study, the particles 
were collected in a road tunnel and placed directly in the animal 
respiratory tract, so differences in inhalability of larger particles 
in rodents was not an issue. In contrast, a number of studies have 
reported that Mt. St. Helens volcanic ash, which is generally in the 
size range of thoracic coarse particles, has very little toxicity in 
animal or in vitro toxicologic studies (EPA, 2004, p. 7-216).
---------------------------------------------------------------------------

    \47\ The particles used in this study were collected by vacuum 
sweeping of freeway surfaces in California, and were generally 5 
[mu]m in diameter or lower (Kleinman et al., 1995).
    \48\ This approach, using ovalbumin-sensitized mice, is commonly 
used for comparing allergic potency of air pollutants. The authors 
also tested responses in an additional toxicologic model, based on 
pollen-sensitized rats, and reported responses only with diesel 
exhaust particles (Steerenberg et al., 2003, p. 1436).
---------------------------------------------------------------------------

    The Criteria Document finds that the limited number of recent 
toxicologic studies using PM10-2.5 provide some evidence 
that coarse fraction particle exposures can result in effects primarily 
linked to the respiratory system, related to inflammation or 
aggravation of allergic effects. Toxicologic studies have suggested 
potential pathways for effects from a few sources or components of 
thoracic coarse particles, such as road dust particles, metals or 
organic constituents. The need to better understand the relationship 
between different components or sources of thoracic coarse particles 
remains a key area of uncertainty with regard to the effects of 
thoracic coarse particles.
2. Nature of Effects
    In the last review, EPA considered a substantial number of 
epidemiological studies using PM10, which contains both fine 
and coarse particles, as a measure of exposure to PM. In many such 
studies in which fine and coarse particles occur at similar levels, it 
is difficult or impossible to determine whether fine and coarse 
particles both played major roles in the associations. Accordingly, 
considerable emphasis was placed on the more limited body of evidence 
from PM10 studies in locations where coarse particles were a 
much greater fraction of PM10 than were fine particles. 
These findings indicated that short-term exposure to thoracic coarse 
particles in such areas was linked with respiratory morbidity effects, 
such as aggravation of asthma, increases in respiratory symptoms and 
respiratory infections (62 FR 38677). The single available short-term 
exposure study that compared associations between mortality and fine 
and coarse particles reported a significant association between short-
term exposure to PM10-2.5 and mortality in one of six cities 
(Steubenville, OH). In this location, an unusually high correlation 
between high levels of fine and thoracic coarse particles suggested a 
common industrial source, and a clear conclusion about the relative 
contribution was not possible. The study found no association with 
thoracic coarse particles in a combined multi-city analysis (Schwartz 
et al., 1996; CD, p. 8-40 to 8-41).\49\ No studies in the past review 
provided clear epidemiologic evidence of mortality or morbidity effects 
related to long-term exposure to PM10-2.5. EPA observed that 
toxicologic studies offered some qualitative evidence suggesting the 
potential for effects on the respiratory system with long-term exposure 
to coarse particles or coarse particle constituents (62 FR 38678).
---------------------------------------------------------------------------

    \49\ Note that in more recent reanalyses of this study to 
investigate statistical modeling issues, the association for 
Steubenville was not statistically significant in most models 
reported in the two reanalyses (Klemm and Mason, 2003; Schwartz, 
2003a).
---------------------------------------------------------------------------

    In this review, epidemiologic studies have continued to support a 
relationship between short-term exposure to thoracic coarse particles 
and respiratory morbidity, with effects ranging from increased 
respiratory symptoms to hospitalization for respiratory diseases. As 
discussed below, the new studies also suggest associations with effects 
on the cardiovascular system and possibly with

[[Page 2656]]

mortality. Figure 2 summarizes results from both multi-city and single-
city epidemiologic studies using short-term exposures to 
PM10-2.5, including all U.S. and Canadian studies that used 
direct measurements of PM10-2.5\50\ and for which effect 
estimates and confidence intervals were reported. Consistent with the 
presentation of fine particle study results in Figure 1, the central 
effect estimate is indicated by a diamond for each study result, with 
the vertical bar representing the 95 percent confidence interval around 
the estimate. The results of these epidemiologic studies are discussed 
below.
---------------------------------------------------------------------------

    \50\ All epidemiologic studies discussed below included 
measurements of thoracic coarse particles either through monitors 
that collected thoracic coarse particles separately (e.g., 
dichotomous monitors) or using data from side-by-side (co-located) 
monitors for fine particles and PM10. Investigators have 
sometimes also used prediction models to ``fill'' or estimate PM 
concentrations where measurements are not available (most often 
where data are collected less frequently than daily). In one 
particular study in Coachella Valley, measurements were made of fine 
and thoracic coarse particle concentrations for two and a half 
years. The investigators predicted PM10-2.5 
concentrations for a longer time series, based on a ten-year data 
set for PM10 for use in the health study (Ostro et al., 
2003).

BILLING CODE 6560-50-P
[GRAPHIC] [TIFF OMITTED] TP17JA06.049

BILLING CODE 6560-50-C

a. Effects Associated With Short-Term Exposure to Thoracic Coarse 
Particles

    The discussion below focuses first on evidence related to 
respiratory morbidity effects, since information available in the 
previous review provided plausible evidence that short-term exposure to 
thoracic coarse particles was associated with such effects. This is 
followed by a discussion of new findings on potential cardiovascular 
effects of thoracic coarse particles, as well as new evidence from 
studies of mortality.
i. Morbidity

(a) Effects on the Respiratory System

    Evidence available in the last review suggested that aggravation of 
asthma

[[Page 2657]]

and respiratory infections and symptoms were associated with 
PM10 in areas where thoracic coarse particles were a much 
greater fraction of PM10 than were fine particles, such as 
Anchorage, AK, and southeast Washington (62 FR 38679). Only one 
epidemiologic study had used PM10-2.5 data; it reported a 
positive, but not statistically significant, association between 
respiratory hospital admissions and PM10-2.5 in Toronto 
(Thurston et al., 1994).
    Several new studies of respiratory symptoms and lung function have 
included both PM10-2.5 and PM2.5 data, and these 
results suggest a role for thoracic coarse particles as well as for 
fine particles in associations with respiratory symptoms (EPA, 2004, p. 
8-311). In the Six Cities study, a statistically significant increase 
in cough for children was found with PM10-2.5 but not with 
PM2.5, while the reverse was true for lower respiratory 
symptoms. When both PM10-2.5 and PM2.5 were 
included in models, the effect estimates were reduced for each, but 
PM10-2.5 retained significance in the association with cough 
and PM2.5 retained significance in the association with 
lower respiratory symptoms (Schwartz and Neas, 2000).\51\ Changes in 
lung function were evaluated in three cities in Pennsylvania, and in 
all three, short-term exposure to thoracic coarse particles was not 
significantly associated with peak flow rate, although some 
statistically significant associations were found with exposure to fine 
particles (EPA, 2004, p. 8-312).
---------------------------------------------------------------------------

    \51\ The authors conclude that for acute asthma-related 
responses as well as daily mortality, fine particles are a stronger 
predictor of health response that are thoracic coarse particles 
(Schwartz and Neas, 2000, p. 8).
---------------------------------------------------------------------------

    Three new U.S. and Canadian epidemiologic studies have reported 
associations between short-term exposure to PM10-2.5 with 
hospital admissions for respiratory diseases, including asthma, 
pneumonia and COPD (Burnett et al., 1997; Ito, 2003; Sheppard et al., 
2003). As shown in Figure 2, the effect estimates for these 
associations are positive and some are statistically significant. In 
these associations with respiratory hospitalization, the risk estimates 
tend to fall in the range of 5 to 15 percent per 25 [mu]g/m3 
PM10-2.5 (EPA, 2004, p. 8-193).
    Because fine particles and ozone, as well as other gaseous air 
pollutants, are known to cause respiratory effects, a key consideration 
for assessing this body of studies is assessment of potential 
confounding by these co-pollutants, as discussed in detail in Section 
8.4.3 of the Criteria Document. The associations reported between 
respiratory hospital admissions and short-term exposure to 
PM10-2.5 were largely unchanged in most cases when gaseous 
co-pollutants were included in the models (EPA, 2004, Figure 8-18; 
Burnett et al., 1997; Ito, 2003).\52\ Few investigators have evaluated 
potential confounding of PM10-2.5 effects with adjustment 
for PM2.5 in multi-pollutant models. Only the study 
conducted in Detroit included such multi-pollutant models for 
respiratory hospitalization and was reanalyzed to address potential 
statistical modeling questions. In this study, the simultaneous 
consideration of PM10-2.5 and PM2.5 resulted in 
reduction in the size of the effect estimate, as well as loss of 
statistical significance, for both pollutants. The authors report that 
the correlation between the two pollutants was ``modest'' (correlation 
coefficient of 0.42) (Lippmann et al., 2000, p. 33). The results in 
this study vary by health outcome; for example, for pneumonia 
hospitalization, effect estimates for PM2.5 were little 
changed but those for PM10-2.5 decreased substantially in 
magnitude in two-pollutant models. In contrast, effect estimates for 
PM2.5 with COPD hospitalization decreased dramatically, 
whereas those for PM10-2.5 were only slightly decreased in 
size in two-pollutant models (Ito, 2003, pp. 152, 153).
---------------------------------------------------------------------------

    \52\ More specifically, the effect estimates for associations 
between PM10-2.5 and hospitalization for COPD and 
pneumonia in Detroit are largely unchanged with the addition of 
gaseous co-pollutants to the models, except in one case where the 
PM10-2.5 effect estimate for COPD hospitalization is 
substantially reduced in size with the inclusion of O3 in 
the model (Ito, 2003). Results for the study in Toronto also show 
relatively consistent effect estimate size for associations between 
PM10-2.5 and respiratory hospitalization, except for the 
models including NO2 and all four gaseous pollutants 
(Burnett et al., 1997).
---------------------------------------------------------------------------

    Additional insight into the respiratory effects of coarse particles 
is provided by studies using PM10 in locations where 
thoracic coarse particles were a much greater fraction of 
PM10 than were fine particles. This review includes new 
PM10 studies in such relatively high coarse-fraction areas, 
such as Reno, NV and Anchorage, AK.\53\ In these areas, statistically 
significant associations have been reported between PM10 and 
hospitalization for respiratory diseases (Chen et al., 2000) and 
outpatient medical visits for asthma (Choudhury et al., 1997). These 
findings support the evidence from the limited group of studies 
discussed above that have reported associations between measured 
PM10-2.5 and respiratory morbidity.
---------------------------------------------------------------------------

    \53\ For example, Anchorage, AK and Reno, NV do not currently 
attain the PM10 24-hour standard which is set at 150 
[mu]g/m3. Based on 2002-2004 data, the 98th percentile 
PM2.5 concentrations in these areas were 21 and 25 [mu]g/
m3, respectively. As noted in the fine particle 
discussion above, no short-term exposure studies to date have shown 
statistically significant associations between fine particles and 
effects with 98th percentile values this low. This suggests that 
coarse particles either caused or contributed to the observed 
PM10 associations.
---------------------------------------------------------------------------

    Considering evidence from across a range of respiratory morbidity 
health outcomes, the Criteria Document concludes that the epidemiologic 
evidence indicates that both fine and thoracic coarse particles impact 
respiratory health (EPA, 2004, p. 8-311).

(b) Effects on the Cardiovascular System

    Two new studies conducted in the U.S. and Canada have also reported 
associations between short-term exposure to PM10-2.5 and 
hospital admissions for various cardiovascular diseases. The results of 
these studies are included in Figure 2, where it can be seen that the 
associations are generally positive and the results of the larger 
studies with more statistical power are statistically significant 
(Burnett et al., 1997, cardiovascular disease hospitalization; Ito, 
2003, ischemic heart disease hospitalization). The excess risks for 
hospital admissions for cardiovascular diseases range from about 1 to 
10 percent per 25 [mu]g/m3 PM10-2.5, as seen in 
the Detroit study (EPA, 2004, p. 8-310). In addition, a statistically 
significant association was reported between PM10 and 
increased hospitalization for cardiovascular diseases in Tucson, AZ, an 
urban area where thoracic coarse particles are a much greater fraction 
of PM10 than are fine particles (Schwartz, 1997).\54\ The 
Criteria Document finds that associations between cardiovascular 
hospitalization and short-term PM10-2.5 exposure were 
relatively unchanged when gaseous co-pollutants were included in the 
models (EPA, 2004, Figure 8-17; Burnett et al., 1997; Ito, 2003).\55\ 
In assessing potential confounding between PM2.5 and 
PM10-2.5, one new study in Detroit reported that 
simultaneous consideration of PM10-2.5 and PM2.5 
resulted in a reduction in effect estimate

[[Page 2658]]

size and a lack of statistical significance for both PM indicators 
(Ito, 2003). In the reanalysis for this study, for example, a 
significant association was reported between PM10-2.5 and 
hospitalization for ischemic heart disease in a single-pollutant model, 
and in a two-pollutant model the effect estimates for PM2.5 
and PM10-2.5 were both reduced in magnitude and neither 
remained statistically significant (Ito, 2003, pp. 152, 153).
---------------------------------------------------------------------------

    \54\ Tucson currently attains the PM10 standard, and 
the 98th percentile 24-hour average concentrations reported for 
PM2.5 are 15 and 17[mu]g/m3 at two monitoring 
sites in the area.
    \55\ The effect estimates for associations between 
PM10-2.5 and hospitalization for ischemic heart disease 
and heart failure in Detroit are largely unchanged with the addition 
of gaseous co-pollutants to the models (Ito, 2003). Results 
presented for the study in Toronto also show relatively consistent 
effect estimate size for associations between PM10-2.5 
and cardiovascular hospitalization, except for the models including 
NO2 and all four gaseous pollutants (Burnett et al., 
1997).
---------------------------------------------------------------------------

    Epidemiologic studies have also reported associations between 
short-term exposures to ambient PM (generally using PM10 or 
PM2.5) and more subtle cardiovascular health outcomes (e.g., 
changes in heart rhythm or cardiovascular biomarkers) (EPA, 2004, p. 8-
169). Only one of this new set of epidemiologic studies included 
PM10-2.5, and no significant associations were reported 
between onset of myocardial infarction and short-term 
PM10-2.5 exposures (EPA, 2005a, p. 8-165; Peters et al., 
2001).
ii. Mortality
    In the few epidemiologic studies available for the last review, 
only the Six City study summarized above evaluated the relationship 
between short-term exposure to PM10-2.5 and mortality. That 
study provided a suggestion of a potential effect of thoracic coarse 
particles only in the city with the highest coarse and fine particle 
concentrations, but it was not possible to separate fine and thoracic 
coarse particle contributions.
    As shown in Figure 2 for U.S. and Canadian studies, effect 
estimates for associations between mortality and short-term exposure to 
PM10-2.5 are generally positive and similar in magnitude to 
those for PM2.5 and PM10 though most are not 
statistically significant. In general, the confidence intervals 
(indicating uncertainty) are greater for associations between mortality 
and PM10-2.5 than for associations with PM2.5, as 
is apparent when directly comparing results from numerous studies as 
shown in Figure 8-5 of the Criteria Document (EPA, 2004, p. 8-61). In 
the same comparison, it can be seen that the size of the effect 
estimates for the associations are in the same range. In general, 
effect estimates are somewhat larger for respiratory and cardiovascular 
mortality than for total mortality. Two of the five effect estimates 
for cardiovascular mortality with short-term PM10-2.5 
exposure are positive and statistically significant (Mar et al., 2003; 
Ostro et al., 2003) while none of the effect estimates for total 
mortality reach statistical significance. The new studies include a 
multi-city study that uses data from the eight largest Canadian cities 
and reported associations between total mortality and 
PM10-2.5 as well as PM2.5 and PM10. 
The effect estimates were of similar magnitude for each PM indicator 
(Burnett and Goldberg, 2003), but the association with 
PM10-2.5 did not reach statistical significance. The 
magnitude of the effect estimates for PM10-2.5 are similar 
to those for PM2.5, generally falling in the range of 3 to 8 
percent for cardiovascular mortality per 25 [mu]g/m3 
PM10-2.5.
    Potential confounding by co-pollutant gases has been assessed in 
some of these mortality studies. As shown in Figures 8-16 through 8-18 
of the Criteria Document, the associations reported with 
PM10-2.5 are generally unchanged in effect size when co-
pollutant gases are included in multi-pollutant models. The evidence 
available on potential confounding between PM2.5 and 
PM10-2.5 is limited, but the Criteria Document includes 
results from two studies that showed effects of the two PM indicators 
to be relatively independent in multi-pollutant models, however, these 
particular analyses were not included in reanalyses to address 
statistical modeling questions.\56\
---------------------------------------------------------------------------

    \56\ One study was the Canadian 8-city study, in which multi-
pollutant models included PM2.5 and PM10-2.5 
and gaseous co-pollutants, with moderate reductions in the effect 
estimate size for both PM indicators (Burnett et al., 2000). 
Moolgavkar (2000) presented results of two-pollutant models for 
PM2.5 and PM10-2.5 with COPD hospitalization 
in Los Angeles, and again, effect estimates for both pollutants were 
generally reduced somewhat in size. The author also reports that 
associations with PM10-2.5 were generally reduced in size 
and lost statistical significance in two-pollutant models including 
CO. These two studies were reanalyzed to address potential issues 
with statistical model specification, but these multi-pollutant 
model results were not included in the reanalysis reports.
---------------------------------------------------------------------------

iii. Effects of Thoracic Coarse Particle Components or Sources in 
Epidemiologic Studies
    In considering the epidemiologic evidence on morbidity or mortality 
associations with short-term exposure to thoracic coarse particles, EPA 
recognizes that the issue of the relative toxicity of different PM 
components, discussed above in section II.A.1 for fine particles, is an 
important uncertainty for thoracic coarse particles as well. Several 
toxicologic studies, discussed above in section III.A.1, have reported 
evidence of effects with different components or sources of thoracic 
coarse particles. However, the available epidemiologic studies that 
have used PM10-2.5 did not evaluate associations with 
specific components of thoracic coarse particles (EPA, 2004, section 
8.2.2.5.2). As discussed in section II.A, several studies have reported 
that PM2.5 from combustion-related sources is more strongly 
linked with mortality than PM2.5 of crustal origin. However, 
these findings are not directly relevant to findings related to 
thoracic coarse particles. Combustion sources are a major contributor 
to PM2.5 emissions, but not to emissions of 
PM10-2.5, while crustal material is an important component 
of PM10-2.5 but only a small portion of PM2.5 
(EPA, 2005a, Table 2-2).
    One study that does have relevance to considering the effects of 
PM10-2.5 from different sources assessed the contribution of 
dust storms to PM10-related mortality. The authors focused 
on days when dust storms or high wind events occurred in Spokane, 
during which thoracic coarse particles from surrounding rural soils are 
the dominant fraction of PM10. No evidence was reported of 
increased mortality on days with high PM10 levels related to 
these dust storms (average PM10 level was 221 [mu]g/
m3 higher on dust storm days than on other study days) 
(Schwartz, et al., 1999), suggesting that PM10-2.5 from 
wind-blown rural dust is also not likely associated with mortality.\57\ 
EPA has also observed that the available epidemiologic studies using 
PM10-2.5 have been conducted in urban areas, such as 
Phoenix, Detroit and Seattle. Coarse particles are generally not 
distributed over broad areas, but rather reflect contributions from 
more localized sources, thus it is more difficult than for fine 
particles to generalize the results of these studies to areas with 
other types of sources.
---------------------------------------------------------------------------

    \57\ In addition, studies conducted in several areas in the 
western U.S. have reported that associations between PM10 
and mortality or morbidity remained unchanged or became larger and 
more precise when days indicative of wind-blown dust or high 
PM10 concentration days were excluded from the analyses 
(Pope et al., 1999; Schwartz, 1997; Chen et al., 2000; Hefflin et 
al., 1994). This group of studies does not provide conclusive 
evidence of any effects or lack of effects associated with wind-
blown dust or high concentration days, nor were the studies designed 
specifically for that purpose. The results do, however, indicate 
that associations between PM10 and health outcomes in 
these western areas are not overly influenced or ``driven by'' such 
days.
---------------------------------------------------------------------------

    The Criteria Document finds that the new epidemiologic studies 
support the conclusions drawn in the previous review, and indicate that 
short-term exposure to thoracic coarse particles is likely associated 
with respiratory morbidity. The epidemiologic studies report 
statistically significant associations between short-term 
PM10-2.5 exposure and outcomes ranging from respiratory 
symptoms to hospitalization for respiratory diseases (EPA, 2004, p. 8-
312). A limited body of new

[[Page 2659]]

epidemiologic evidence suggests that short-term exposure to thoracic 
coarse particles is associated with effects on the cardiovascular 
system. Finally, the Criteria Document finds that evidence from health 
studies on associations between short-term exposure to 
PM10-2.5 and mortality is ``limited and clearly not as 
strong'' as evidence for associations with PM2.5 or 
PM10 but nonetheless is suggestive of associations with 
mortality (EPA, 2004, p. 9-28, 9-32). As discussed briefly above, some 
epidemiologic evidence suggests that there are components of thoracic 
coarse particles (e.g., crustal material in non-urban areas) that are 
less likely to have adverse effects, at least at lower concentrations, 
than other components. Based on the epidemiologic evidence, the 
Criteria Document concluded that the limited body of evidence provided 
suggestive evidence for associations between throacic coarse particles 
and various mortality and morbidity effects ``in some locations'' (EPA, 
2004, p. 8-338).

b. Effects Related to Long-Term Exposure to Thoracic Coarse Particles

    In the last review, the available prospective cohort study results 
had shown no evidence of associations between long-term exposure to 
thoracic coarse particles and either mortality (Dockery et al., 1993; 
Pope et al., 1995) or morbidity (Dockery et al., 1996; Raizenne et al., 
1996). As discussed above for PM2.5, new studies available 
in this review include the reanalyses and extended analyses for the Six 
Cities and ACS cohort studies of mortality, and new analyses from the 
southern California children's cohorts of morbidity effects.
    In both the reanalyses and extended analyses of the ACS cohort 
study, long-term exposure to PM10-2.5 was not significantly 
associated with mortality (CD, p. 8-105; Krewski et al., 2000; Pope et 
al., 2002). Based on evidence from reanalyses and extended analyses 
using ACS cohort data, the Criteria Document concludes that the long-
term exposure studies find no associations between long-term exposure 
to thoracic coarse particles and mortality (EPA, 2004, p. 8-307).
    In the previous review, results from the Harvard 24-city study had 
shown associations between respiratory illness prevalence and decreased 
lung function in children with fine particles or fine particle 
indicators, but not with the larger size fractions (Dockery et al., 
1996; Raizenne et al., 1996). Further EPA staff evaluation of the data 
from this study that suggested that lung function decrements were not 
associated with long-term exposure to thoracic coarse particles (EPA, 
1996b, p. V-67a) . In this group of cities, mean thoracic coarse 
particle concentrations ranged from approximately 4 to 15 [mu]g/
m3. Several new studies have used data from the Southern 
California children's cohorts, one of which included 
PM10-2.5 data; in these cities, mean thoracic coarse 
particle concentrations ranged from 6 to 39 [mu]g/m3. In 
this study, decreases in several measures of lung function growth were 
associated with long-term exposure to PM10-2.5 (as well as 
PM10 and PM2.5) though not all associations 
reached statistical significance (Gauderman et al., 2000). Further, in 
analyses for a second cohort of children, no statistically significant 
associations were reported between lung function growth and long-term 
PM10-2.5 exposure (Gauderman et al., 2002, p. 81). The 
correlation reported between PM10-2.5 and PM2.5 
in this area was unusually high (r=0.76); in two-pollutant models, the 
authors observe that the effects reported with both pollutants were 
reduced in magnitude, and did not remain statistically significant, 
with somewhat larger reductions for PM10-2.5 associations 
than for PM2.5 (Gauderman et al., 2000, p. 1387). Thus, 
results from one children's cohort study provide no evidence of 
associations between long-term to exposure to PM10-2.5 and 
respiratory morbidity, while findings from a more recent cohort study 
provide only very limited evidence for such effects. Overall, EPA finds 
that the available evidence provides little support to link long-term 
exposures to thoracic coarse particles with respiratory morbidity (EPA, 
2004, p. 9-34).
3. Integration and Interpretation of the Health Evidence
    As discussed in section II.A.3, the Criteria Document and Staff 
Paper focused on well-recognized criteria in evaluating the 
epidemiologic evidence, including the strength of associations; 
robustness of reported associations to the use of alternative model 
specifications, potential confounding by co-pollutants, and exposure 
misclassification related to measurement error; consistency of findings 
in multiple studies of adequate power, and in different persons, 
places, circumstances and times; and the nature of concentration-
response relationships. These evaluations addressed key methodological 
issues that are relevant to interpretation of evidence from 
epidemiologic studies. Further, findings from epidemiologic studies 
were integrated with available experimental evidence (e.g., dosimetric 
and toxicologic), in considering the extent of coherence and biological 
plausibility of effects observed in epidemiologic studies. This 
integrative assessment formed the basis for the Criteria Document and 
Staff Paper to draw judgments about the extent to which causal 
inferences can be made about observed associations between health 
endpoints and thoracic coarse particles combination with other 
pollutants. The key elements of these evaluations are summarized below. 
Many of these issues are discussed in section II.A.3 above for fine 
particles, and are thus only briefly summarized here with regard to 
implications for thoracic coarse particles.
    (1) Effect estimates from associations between short-term exposures 
to thoracic coarse particles and various health outcomes are generally 
small in size. The Criteria Document observes that the associations are 
similar in size to those reported for PM2.5, but with less 
precision as the measurement error for PM10-2.5 is greater 
than that for PM2.5. Thus, the Criteria Document concludes 
that the magnitude of PM10-2.5 associations is similar to 
those for fine particles, but the lesser precision of the associations 
reduces the strength of the evidence for thoracic coarse particles 
(EPA, 2004, p. 9-41).
    (2) EPA has evaluated the robustness of epidemiologic associations 
in part by considering the effect of differences in statistical model 
specification, exposure error on PM-health associations, and potential 
confounding by co-pollutants.
    Sensitivity to model specification was discussed above for fine 
particles, and, in general, similar conclusions apply to studies using 
PM10-2.5. Section 8.4.2 of the Criteria Document discusses a 
series of reanalyses that address issues related to a specific type of 
statistical model (``generalized additive methods'') used in some 
recent epidemiologic studies. The results of the reanalyses showed 
little change in effect estimates for some studies; in others the 
effect estimates were reduced in size though it was observed that the 
reductions were often not substantial (EPA, 2004, p. 9-35). Overall, 
the Criteria Document concludes that associations between short-term 
exposure to PM and various health outcomes are generally robust to the 
use of alternative modeling strategies, recognizing that further 
evaluation of alternative modeling strategies is warranted. It was also 
observed that the results of reanalyses indicated that effect estimates 
were more sensitive to the modeling approach used to account for 
temporal effects and weather variables than to the specific model 
specifications, and thus

[[Page 2660]]

recommended further exploration of alternative modeling approaches for 
time-series analyses (EPA, 2004, pp. 8-236 to 8-237).
    Recent epidemiologic studies have also evaluated the influence of 
exposure error on PM-health associations. This includes both 
consideration of error in measurements of PM, and the degree to which 
measurements from an individual monitor reflect exposures to the 
surrounding community. As discussed in section 8.4.5 of the Criteria 
Document, several studies have shown that fairly extreme conditions 
(e.g., very high correlation between pollutants and no measurement 
error in the ``false'' pollutant) are needed for complete ``transfer of 
causality'' of effects from one pollutant to another (EPA, 2004, p. 9-
38). Exposure error is likely to be more important for associations 
with PM10-2.5 than with PM2.5, since there is 
generally greater error in PM10-2.5 measurements, 
PM10-2.5 concentrations are less evenly distributed across a 
community, and thoracic coarse particles are less likely to penetrate 
into buildings (EPA, 2004, p. 9-38). Thus, factors related to exposure 
error likely result in reduced precision for epidemiologic associations 
with PM10-2.5.
    There are two key implications of this uncertainty for this review. 
First, for an individual epidemiologic association, the increased 
uncertainty in measurements would tend to increase the standard error 
about the effect estimate, possibly reducing statistical significance 
of the findings. This would mean that a set of positive but generally 
not statistically significant associations between PM10-2.5 
and a health outcome could be reflecting a true association that is 
measured with error (EPA, 2004, p. 5-126). Second, this uncertainty 
about measurements is an important consideration in evaluating the air 
quality concentrations with which a statistical association is 
reported. 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 the 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 concentrations measured at one particular site 
may over-or under-estimate air quality levels in other parts of the 
area. In evaluating the air quality data from the locations in which 
epidemiologic associations were reported, as discussed in the Staff 
Paper and below in section III.G, examples of both cases are seen. For 
example, in Coachella Valley, mortality was statistically significantly 
associated with PM10-2.5 measurements made at one site 
(Ostro et al., 2003), but these air quality measurements appear to 
represent concentrations on the high end of PM10-2.5 levels 
for Coachella Valley communities. In contrast, statistically 
significant associations were reported with PM10-2.5 
measurements in Detroit (Ito, 2003), and in this case the data appear 
to represent concentrations on the low end of PM10-2.5 
levels for the Detroit area (EPA, 2005a, p. 5-65, 5-66).
    Finally, some investigators have assessed the robustness of 
associations between health outcomes and short-term exposures to 
PM10-2.5 in multi-pollutant models to potential confounding 
by the gaseous co-pollutants or fine particles. A high degree of 
correlation between the concentrations of thoracic coarse particles and 
other pollutants (either gaseous co-pollutants or fine particles) can 
make interpretation of the study results difficult. Multi-pollutant 
models including PM10-2.5 and gaseous co-pollutants are 
included in Figures 8-16 through 8-18 of the Criteria Document, where 
it can be seen that associations with PM10-2.5 are largely 
unchanged when gaseous co-pollutants are added to the models (EPA, 
2004, section 8.4.3). Further, in the available epidemiologic studies, 
it can be seen that correlations between the gaseous co-pollutants (CO, 
NO2, O3, SO2) and PM10-2.5 
concentrations are often lower than correlations between the gases and 
fine particles.\58\ While recognizing that disentangling the effects 
attributable to various pollutants within an air pollution mixture is 
challenging, the Criteria Document concludes that effect estimates for 
associations between PM, including PM10-2.5, and health 
endpoints are generally robust to confounding by gaseous co-pollutants 
(EPA, 2004, p. 9-37).
---------------------------------------------------------------------------

    \58\ For example, from the studies included in Figures 8-16 
through 8-18, correlation coefficients reported in Detroit between 
PM10-2.5 and the four gaseous co-pollutants ranged from 
0.13 to 0.32, whereas the correlation coefficients between 
PM2.5 and the gaseous co-pollutants range from 0.38-0.49 
(Ito, 2003).
---------------------------------------------------------------------------

    Less information is available from studies that specifically 
assessed potential confounding between fine and thoracic coarse 
particles, as noted above. The reported correlation coefficients 
between PM10-2.5 and PM2.5 are in the low to 
moderate range for most such studies, i.e., generally in a range of 
below 0.3 to 0.5, with some notably higher correlation coefficients 
reported in Phoenix (0.59) and Steubenville (0.69). As observed 
previously, one study in Detroit evaluated the effects of both 
PM2.5 and PM10-2.5 simultaneously where the 
correlation between the two pollutants was ``modest'' (correlation 
coefficient of 0.42). The authors report a reduction in coefficients 
for both PM10-2.5 and PM2.5 in associations with 
mortality and hospital admissions for respiratory or cardiovascular 
diseases (Ito, 2003, pp. 152-153); the degree of reduction in size 
varied for different health outcomes. Similarly, Schwartz and Neas 
(2000) report some reduction in effect estimate size for both 
PM10-2.5 and PM2.5 associations across six cities 
in two-pollutant models, but the association reported between 
PM10-2.5 and cough remains statistically significant.\59\ 
Two studies reported associations between PM10-2.5 and 
mortality (Ostro et al., 2003, Coachella Valley; Mar et al., 2003, 
Phoenix); stronger associations were reported with PM10-2.5 
than PM2.5 by Ostro et al., although the authors note the 
reduced sample size for PM2.5 may have influenced the 
statistical power (Ostro et al., 2003). Both areas have relatively low 
fine particle concentrations, with 98th percentile PM2.5 
concentrations of about 32 [mu]g/m3 in Phoenix and 34 [mu]g/
m3 in Coachella Valley, while the correlation coefficient 
reported between PM2.5 and PM10-2.5 was low in 
Coachella Valley (0.28) and fairly high in Phoenix (0.59). This limited 
body of evidence suggests that PM10-2.5 and PM2.5 
have associations with health outcomes that are likely independent of 
one another, but further work is needed to help distinguish the 
contributions of thoracic coarse particles on health outcomes from 
those of fine particles.
---------------------------------------------------------------------------

    \59\ The correlation coefficients between PM10-2.5 
and PM2.5 range from 0.23 to 0.45 in five of the six 
cities (Boston, Knoxville, Portage, Topeka, and St. Louis), with a 
correlation coefficient of 0.69 in Steubenville.
---------------------------------------------------------------------------

    Overall, the Criteria Document concludes that associations reported 
between health outcomes and short-term exposure to PM10-2.5 
are generally robust to the use of alternative modeling strategies, to 
adjustment for the potential confounding effects of gaseous co-
pollutants, and in terms of exposure error (EPA, 2004, p. 9-46). 
However, the remaining uncertainties are larger in assessing the degree 
to which effects observed with thoracic coarse particle exposures are 
independent from effects of fine particles. In addition, in 
interpreting the results of epidemiologic studies, it is difficult to 
determine how well PM10-2.5 concentrations measured at 
ambient monitoring stations

[[Page 2661]]

characterize the magnitude of population exposures to thoracic coarse 
particles.
    (3) In assessing consistency in effect estimates, the epidemiologic 
study results suggest that effect estimates may differ from one 
location to another, but the extent of variation is not clear. For 
example, in one multi-city study, some limited evidence was reported in 
the reanalysis to address model specification issues that suggested 
some heterogeneity among the 8 largest Canadian cities for associations 
with PM10-2.5, although there had been no evidence of 
heterogeneity in initial study findings (Burnett and Goldberg, 2003; 
EPA, 2004, p. 9-39). As was observed for fine particles, there are a 
number of factors that would be likely to cause variation in PM-health 
outcomes in different populations and geographic areas. The Criteria 
Document discusses such factors, including the mix of PM sources and 
composition, the mix of other gaseous pollutants, geographic features 
that would affect the spatial distribution of ambient PM, and 
population characteristics that affect susceptibility or exposure 
levels (EPA, 2004, p. 9-41). In addition, the use of data collected on 
a 1-in-6 or 1-in-3 day schedule results in reduced statistical power, 
resulting in less precision for estimated effect estimates for the 
individual cities and increased potential variability in results (EPA, 
2004, p. 9-40). Overall, the Criteria Document concludes that there is 
some consistency in effect estimates for hospitalization for 
respiratory and cardiovascular causes with short-term exposure to 
thoracic coarse particles, though fewer studies are available on which 
to make such an assessment than are available for fine particles (EPA, 
2004, p. 9-47).
    (4) Of the group of new epidemiologic studies that have evaluated 
the shape of concentration-response functions, many (generally using 
PM10) have been unable to detect threshold levels in the 
relationship between short-term PM exposure and mortality. One single-
city study used PM10-2.5 and PM2.5 measurements 
in Phoenix and reported that there was no indication of a threshold in 
the association between PM10-2.5 and mortality (Smith et 
al., 2000; EPA, 2004, p. 8-322). However, a few analyses have provided 
suggestions of some potential threshold levels, generally at fairly low 
ambient concentrations. Thus, the Criteria Document concludes that the 
evidence did not support selecting any particular population threshold 
for PM10-2.5, recognizing that there may be thresholds for 
specific health responses in individuals, and that it is possible that 
such thresholds exist toward the lower end of the range of air quality 
measurements in the health studies, but cannot be detected due to 
variability in susceptibility across a population. Even in those few 
studies with suggestive evidence of such thresholds, the potential 
thresholds are at fairly low concentrations (EPA, 2004, sections 8.4.7 
and 9.2.2.5).
    (5) Several issues related to exposure time periods were assessed 
in the Criteria Document, as summarized in section 3.6.5 of the Staff 
Paper. One key issue is the lag period between thoracic coarse particle 
exposure and health outcome in short-term exposure studies. In many 
epidemiologic studies, the authors have reported a pattern of positive 
associations across several consecutive lag periods for thoracic coarse 
particles, such that an effect estimate for any individual lag day for 
thoracic coarse particles likely underestimates the magnitude of the 
PM-health response. A number of recent studies that have investigated 
associations with distributed lags provide effect estimates for health 
responses that persist over a period of time (days to weeks) after the 
exposure period and the effect estimates are often, but not always, 
larger in size that those for single-day lag periods; however, 
available studies have generally not included PM10-2.5 (EPA, 
2004, p. 8-281). As reported for fine particles, the Criteria Document 
concludes that it is likely that the most appropriate lag period for a 
study will vary, depending on the health outcome and the specific 
pollutant under study. (EPA, 2004, p. 8-279).
    (6) In integrating evidence from across scientific disciplines, the 
Criteria Document and Staff Paper observed that the body of 
epidemiologic evidence on thoracic coarse particles is smaller than 
that for fine particles and the evidence available from toxicologic 
studies is also more limited. The clearest case for a causal 
relationship for coarse particles is for effects on the respiratory 
system. The epidemiologic results showing respiratory effects is 
consistent with the assessment of regional particle penetration and 
deposition, as well the observations from more limited toxicologic 
studies. The fractional deposition of elevated coarse particle 
concentrations is significant in the tracheobronchial region, which is 
particularly sensitive in asthmatic individuals. From the limited 
number of toxicologic studies using PM10-2.5, as noted above 
in section III.A.1, there is some evidence that exposure to thoracic 
coarse particles results in respiratory-related effects such as 
inflammation or oxidative stress. In addition, allergic adjuvant 
effects were linked with road dust exposures. These findings are 
generally consistent with epidemiologic evidence linking 
PM10-2.5 with respiratory morbidity, such as increased 
respiratory symptoms and hospitalization for respiratory diseases such 
as asthma or COPD.
    The evidence is less coherent for effects on the cardiovascular 
system. Some epidemiologic studies have reported significant 
associations with hospital admissions for cardiovascular diseases, and 
associations reported with cardiovascular mortality are positive and 
some are statistically significant (see Figure 2). However, the very 
limited available evidence from toxicologic studies or epidemiologic 
studies on more subtle cardiovascular effects has not provided evidence 
that demonstrates plausible mechanisms or pathways for these effects.
    Based on an integrative assessment of the evidence, the Criteria 
Document concludes that this growing but still limited body of health 
evidence is suggestive of causality in associations between short-term 
(but not long-term) exposures to thoracic coarse particles and health 
effects, particularly for associations with respiratory morbidity.
    (7) In summary, based on the available evidence and the evaluation 
of that evidence in the Criteria Document and Staff Paper, the Criteria 
Document concludes that the body of evidence on effects related to 
exposure to thoracic coarse particles is less strong than that for fine 
particles, but provides suggestive evidence of causality for short-term 
exposure to PM10-2.5 and morbidity, including 
hospitalization for respiratory diseases, increased respiratory 
symptoms and decreased lung function, and possibly mortality (EPA, 
2004, pp. 9-79, 9-80). The Staff Paper recognizes, however, that the 
substantial uncertainties associated with this limited body of evidence 
suggest that it should be interpreted with a high degree of caution 
(EPA, 2005a, p. 5-70).
4. Sensitive Subgroups for Effects of Thoracic Coarse Particle Exposure
    As described in section II.A.4, there are several population groups 
that may be susceptible or vulnerable to PM-related effects. These 
groups include those with preexisting lung diseases, such as asthma, 
and children and older adults. Emerging evidence indicates that people 
from lower socioeconomic strata or who have particularly elevated 
exposures may be more vulnerable to PM-related effects. However, the 
available evidence does not generally

[[Page 2662]]

allow distinctions to be drawn between the PM indicators, in terms of 
which groups might have greater susceptibility or vulnerability to 
PM2.5 or PM10-2.5 (EPA, 2005a pp. 3-35 to 36).
5. Impacts on Public Health From Thoracic Coarse Particle Exposure
    While recognizing that the health evidence regarding effects of 
thoracic coarse particles is more limited, the Criteria Document has 
concluded that the evidence suggests causal associations between short-
term exposure to thoracic coarse particles and morbidity effects, such 
as respiratory symptoms or hospital admissions for respiratory 
diseases, and possibly mortality. As observed above, the potentially 
susceptible populations for such effects include people with 
preexisting respiratory diseases, including asthma, and children and 
older adults. In focusing on respiratory effects likely associated with 
PM10-2.5, it can be observed that population groups with 
respiratory diseases such as asthma or COPD include tens of millions of 
people (EPA, 2004; Tables 9-4 and 9-5). Considering the magnitude of 
these subpopulations and risks identified in health studies, the 
Criteria Document concludes that exposure to thoracic coarse particles 
can have an important public health impact.

B. 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.B 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.
    Consistent with the approach used in the PM2.5 risk 
assessment, discussed above in section II.B, PM10-2.5-
related health risks attributable to anthropogenic sources and 
activities (i.e., risk associated with concentrations above background 
or above various selected higher cutpoints intended as surrogates for 
alternative assumed population thresholds) were estimated by using the 
reported linear or log-linear concentration-response functions from 
epidemiologic studies and available air quality data from the locations 
in which the studies had been conducted. A series of base case analyses 
were conducted, using the same assumed cutpoints as were used in the 
assessment of short-term exposures to PM2.5.
    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.\60\ Base case estimates of respiratory-related hospital 
admissions under recent air quality levels in Detroit are on the order 
of several hundred admissions per year across the range of assumed 
cutpoints considered in this assessment. The Detroit estimates are 
roughly one to two orders of magnitude greater than the range of 
estimated asthma-related admissions in Seattle, which can be attributed 
in part to differences in baseline risks related to respiratory-related 
health endpoints as well as to differences in PM10-2.5 air 
quality levels in these two areas. More specifically, recent (e.g., 
2001-2003) PM10-2.5 concentrations are substantially higher 
in Detroit, where the current 24-hour PM10 standard is not 
met, than they are in Seattle (where the 24-hour PM10 design 
value is well below the level of the current PM10 standard). 
In considering risk estimates for respiratory symptoms in St. Louis, 
the number of days of cough in children living in St. Louis associated 
with recent PM10-2.5 levels range from approximately 27,000 
days per year \61\ at the lowest assumed cutpoint to almost 3,000 days 
per year at the highest assumed cutpoint. For the same time period, 
PM10-2.5 air quality levels in St. Louis are high, where, 
like Detroit, the current 24-hour PM10 standard is not met.
---------------------------------------------------------------------------

    \60\ Quantitative risk estimates associated with recent air 
quality levels for these three cities are presented in Figures 4-11 
and 4-12 in Chapter 4 of the Staff Paper.
    \61\ This represents roughly 1100 days of cough per 100,000 
people in the general population, of which approximately 12 percent 
are children.
---------------------------------------------------------------------------

    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, 
2005a, p. 5-69). These uncertainties and concerns include, but are not 
limited to the following:
    (1) As noted above in section II.A and discussed more fully below 
in section III.G, the PM10-2.5 levels measured at ambient 
monitoring sites in recent years may be quite different from the levels 
used to characterize exposure in the original epidemiologic studies 
based on monitoring sites in different location, thus possibly over- or 
underestimating population risk levels.
    (2) There is greater uncertainty about the reasonableness of the 
use of proportional rollback to simulate just meeting alternative 
PM10-2.5 standards in any urban area relative to that for 
PM2.5 due to the limited availability of historic 
PM10-2.5 air quality data.
    (3) The locations used in the PM10-2.5 risk assessment 
are not representative of urban areas in the U.S. that experience the 
most significant 24-hour peak PM10-2.5 concentrations, and 
thus, observations about relative risk reductions associated with 
alternative standards may not be relevant to the areas expected to have 
the greatest health risks associated with elevated ambient 
PM10-2.5 levels.
    (4) The health effects database that supplies the concentration-
response relationships used in the PM10-2.5 risk assessment 
is much smaller than that available for PM2.5, which limits 
EPA's ability to evaluate the robustness of the risk estimates for the 
same health endpoints across different locations.

C. Need for Revision of the Current Primary PM10 Standards

    The initial issue to be addressed in the current review of the 
primary PM10 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. The Staff Paper 
addresses 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 [mu]m was warranted based on particle dosimetry, toxicologic 
information, and limited epidemiologic evidence (62 FR 38,677). This 
information indicated that thoracic coarse particles can deposit in the 
sensitive regions of the lung of most concern (e.g., the 
tracheobronchial and alveolar regions, which together make

[[Page 2663]]

up the thoracic region),\62\ and that they can be expected to aggravate 
effects in individuals with asthma and contribute to increased upper 
respiratory illness (62 FR 38,666-8).
---------------------------------------------------------------------------

    \62\ EPA 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 38,666).
---------------------------------------------------------------------------

    Further, EPA decided that the new function of PM10 
standard(s) would be to provide such protection against effects 
associated with particles in this narrower size range between 2.5 to 10 
[mu]m. 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 38,679). The decision also reflected the fact that there were 
only very limited ambient air quality data then available specifically 
on thoracic coarse particles, 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 court held in part that PM10 is a ``poorly 
matched indicator'' for thoracic coarse particles in the context of a 
rule that also includes PM2.5 standards because 
PM10 includes PM2.5. American Trucking 
Associations v. EPA, 175 F.3d. at 1054. Although the court found 
``ample support'' (id.) for EPA's decision to regulate thoracic coarse 
particles, it vacated the 1997 revised PM10 standards for 
that reason. 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 focuses on the information now 
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 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.\63\ In considering this information, the Staff Paper finds 
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, the 
continued use of PM10 as an indicator for standards intended 
to protect against health effects associated with thoracic coarse 
particles is no longer appropriate since information is now available 
that supports the use of a more directly relevant indicator, 
PM10-2.5. Further, continuing to rely principally on health 
effects evidence indexed by PM10 to determine the 
appropriate averaging time, form, and level of a standard is no longer 
necessary or appropriate since a number of more directly relevant 
studies, indexed by PM10-2.5, are also now available. Thus, 
separate from any legal considerations, the Staff Paper concludes it is 
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 is the predominant fraction (EPA, 2005a, 
section 5.4.1).
---------------------------------------------------------------------------

    \63\ Coarse particle concentrations from EPA's monitoring 
network are currently determined using a difference method in 
locations with same-day data from co-located PM10 and 
PM2.5 FRM monitors.
---------------------------------------------------------------------------

    Recognizing that dosimetric evidence formed the principal 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 considers whether currently available dosimetric 
evidence continues to support the basic conclusions reached in those 
reviews of the standards. In particular, consideration is 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 [mu]m remains a reasonable 
separation point for particles that penetrate and potentially deposit 
in the thoracic regions. The Staff Paper concludes 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 for fine particles (EPA, 2004, 
p. 6-16).
    Beyond the dosimetric evidence, as noted in past reviews (EPA, 
1981b, 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, 
particularly 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).\64\ In addition, some very limited in vitro toxicologic 
studies show some evidence that coarse particles may elicit pro-
inflammatory effects (EPA, 2004, 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, 2005a, chapter 2). In 
particular, as discussed below in section III.D, coarse particles in 
urban areas can contain all of the components found in more rural 
areas, but 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 concludes 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 urban epidemiologic 
studies, and provides support for retaining some standard for thoracic 
coarse particles so as to continue programs to protect public health 
from such effects (EPA, 2005a, p. 5-49).
---------------------------------------------------------------------------

    \64\ The Criteria Document notes that toxicologic studies, in 
general, use exposure concentrations that are generally much higher 
than ambient concentrations (EPA, 2004, p. 9-51).
---------------------------------------------------------------------------

    The available epidemiologic evidence, discussed above in section 
III.A, includes studies of associations between short-term exposure to 
thoracic coarse particles, indexed by PM10-2.5, and

[[Page 2664]]

health endpoints, as well as evidence from PM10 studies 
conducted in areas in which the coarse fraction is dominant. More 
specifically, several U.S. and Canadian studies now provide evidence of 
associations between short-term exposure to PM10-2.5 and 
various morbidity endpoints. Three such studies conducted in Toronto 
(Burnett et al., 1997), Seattle (Sheppard et al., 2003), and Detroit 
(Ito, 2003) report statistically significant associations between 
short-term PM10-2.5 exposure and respiratory- and cardiac-
related hospital admissions, and a fourth study (Schwartz and Neas, 
2000) conducted in six U.S. cities including 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, 
2005a, 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, 2004, 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), and in the initial analysis of data from 
Steubenville (as part of the Six Cities study, Schwartz et al., 1996), 
although in a reanalysis of this study, the results were generally not 
statistically significant (Klemm and Mason, 2003). In areas with lower 
PM10-2.5 concentrations, no statistically significant 
associations were reported with mortality, though most were positive.
    The Staff Paper also considers 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 et al., 1989; 1991; 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, 2005a, p. 5-50).\65\
---------------------------------------------------------------------------

    \65\ Based 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 [mu]g/m3, while in Utah Valley 
the concentrations range from 37 to 54 [mu]g/m3.
---------------------------------------------------------------------------

    In contrast to the findings from the short-term exposure studies 
discussed above, available epidemiologic studies do not provide 
evidence that long-term exposure to thoracic coarse particles is 
associated with mortality or morbidity (EPA, 2005a, p. 3-25). More 
specifically, no association is found between long-term exposure to 
thoracic coarse particles and mortality in the reanalyses and extended 
analysis of the ACS cohort (EPA, 2005a, p. 8-307). Further, little 
evidence is available on potential respiratory and cardiovascular 
morbidity effects of long-term exposure to thoracic coarse particles 
(EPA, 2005a, p. 3-23-24).
    Taken together, the Staff Paper concludes that the health evidence, 
including dosimetric, toxicologic and epidemiologic study findings, 
supports retaining some standard to protect against 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, including the difficulty in separating the effects 
of fine and thoracic coarse particles, 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, 2004, p. 5-50).
    Beyond this evidence-based evaluation, the Staff Paper also 
considers 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.B, 
the Staff Paper considers 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, which is much more limited than the 
risk assessment conducted for PM2.5, health risks were 
estimated for three urban areas 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 (discussed above in section III.B). 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 concludes 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, 2005a, p. 5-52).
    The Staff Paper recognizes 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 concludes 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, 2005a, p. 5-52).
    In CASAC's review of these Staff Paper recommendations, there was 
general concurrence among CASAC Panel members that there is a need to 
revise the current PM10 standards and establish a primary 
standard specifically targeted to address particles in the size range 
of 2.5 to 10 [mu]m (Henderson, 2005b). 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

[[Page 2665]]

section 5.4 of the Staff Paper upon which the EPA staff recommendations 
were based.
    In considering whether the primary PM10 standards should 
be revised, the Administrator has carefully considered the rationale 
and recommendations contained in the Staff Paper, the advice and 
recommendations of CASAC, and public comments to date on this issue. 
The Administrator provisionally concludes 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. Further, the 
Administrator believes 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, provide 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 [mu]m. 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. American Trucking Associations v. EPA, 175 F. 3d at 
1054. Based on these considerations, the Administrator provisionally 
concludes that the current suite of PM10 standards should be 
revised, and that the revised standard(s) should provide more targeted 
protection from short-term exposure to those thoracic coarse particles 
that are of concern to public health.

D. Indicator of Thoracic Coarse Particles

    In considering an appropriate indicator for a standard intended to 
afford protection from health effects associated with exposure to 
thoracic coarse particles of concern, the Staff Paper starts by making 
the following observations:
    (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 [mu]m 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 [mu]m 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 notes that there appears 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 nonurban 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. Based on such information, 
and on specific initial advice from CASAC (Henderson, 2005a), the Staff 
Paper considers a more narrowly defined indicator for thoracic coarse 
particles that focuses on the mix of such particles that is 
characteristic of that generally found in urban areas where thoracic 
coarse particles are strongly influenced by traffic-related or 
industrial sources. In so doing, the Staff Paper focuses on comparing 
the potential health effects associated with thoracic coarse particles 
in urban and rural settings, as discussed below.
    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, 2005a, 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, 2005a, Table 2-2). It follows that the mix of thoracic 
coarse particles in urban areas would 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. 
For example, in industrial cities, primary particle emissions from 
industrial sources and resuspended road dust can increase the relative 
amount of iron in the mix of PM10-2.5, one of the metals 
that has been noted as being of some interest in the studies of 
mechanisms of toxicity for PM, as well as other industrial process-
related and potentially toxic materials such as nickel, cadmium, and 
chromium (EPA, 2005a, 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, 2004, p. 3D-3). Limited ambient 
coarse fraction composition data from various comparisons find 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 observes that major sources of 
PM10-2.5 emissions in the urban counties in which 
epidemiologic studies have been conducted are paved roads and ``other''

[[Page 2666]]

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.
    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 discusses results from a recent toxicologic 
study in which road tunnel dust particles had greater allergic adjuvant 
activity than several other particle samples (Steerenberg et al., 2003; 
EPA, 2004, 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 natural crustal 
material of geologic origin, has very little toxicity in animal or in 
vitro toxicologic studies (EPA, 2004, 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, 2005a, sections 3.2 and 
5.4.1). These studies 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 et al., 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, 2004, 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, 2004, 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 reflect this 
long-term composition and represent the kinds of industrial components 
that would be incorporated in road dusts in the area.
    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.
    In contrast, recent evidence from epidemiologic studies has 
suggested 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 provided by 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 [mu]g/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 most dusty 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 non-urban or rural areas.
    Beyond the urban and rural distinctions discussed above, the Staff 
Paper also considers the extent to which there is evidence of effects 
with exposure to the ambient thoracic coarse particles in communities 
predominantly influenced by agricultural or mining sources.\66\ 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, 2004, Table 7B-3, p. 7B-11), such 
studies do not provide relevant evidence for effects at much lower 
levels of community exposures. Further, it is unlikely that such 
sources contribute to the effects that have been observed in the recent 
urban epidemiologic studies.
---------------------------------------------------------------------------

    \66\ Mining sources are intended to include all activities that 
encompass extraction and/or mechanical handling of natural geologic 
crustal materials.
---------------------------------------------------------------------------

    The Criteria Document concludes 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 
combustion-related 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, 2004, 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

[[Page 2667]]

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 nonurban 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 from 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 
concludes 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, 2005a, p. 5-57).
    Collectively, this evidence 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 recommends consideration 
of thoracic coarse urban particulate matter \67\ as an indicator for a 
thoracic coarse particle standard, referring to the mix of airborne 
particles between 2.5 and 10 [mu]m 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, 2005a, p. 5-54, 5-57-58). The Staff Paper concludes 
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 an appropriately cautious interpretation of the epidemiologic 
evidence that does not potentially over-generalize the results of the 
limited available studies.
---------------------------------------------------------------------------

    \67\ The acronym ``UPM10-2.5'' is used in the Staff 
Paper to refer to this indicator.
---------------------------------------------------------------------------

    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 discusses the importance 
of a monitoring network designed so as to be consistent with the intent 
of such an indicator and that would facilitate implementation of such a 
standard. EPA has historically used implementation policies 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 this 
staff-recommended indicator is not intended to apply. 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), 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 coarse-mode 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 a 
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.''
    In considering an appropriate indicator for a standard intended to 
afford protection from health effects associated with exposure to 
thoracic coarse particles of concern, the Administrator has 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 believes, despite 
the substantial limitations and uncertainties in the relevant 
information available, that it is appropriate to propose a new 
indicator for such particles at this time. Further, the Administrator 
believes that any such indicator should be defined not only by particle 
size, to generally include those particles between 2.5 and 10 [mu]m in 
diameter, but also by qualifications that narrow the scope of the 
indicator. In considering an indicator that is intended to focus on the 
mix of thoracic coarse particles generally present in urban 
environments and commonly derived from sources typically found in urban 
environments, consistent with Staff Paper and CASAC recommendations, 
the Administrator notes that identifying it as an ``urban'' thoracic 
coarse particle indicator could be misconstrued as meaning that the 
standard is limited to certain geographic locations and, thus, not a 
national standard. To avoid this semantic problem, the Administrator 
has sought to define the indicator in a way that more clearly focuses 
on the nature of the mix of thoracic coarse particles intended to be 
included and the sources that principally generate that mix, rather 
than just where they are found, and that also explicitly focuses on 
what would be excluded from such an indicator. In so doing, the 
Administrator intends the proposed indicator to be equivalent to the 
one recommended in the Staff Paper and endorsed by CASAC, but to do so 
in a manner that will be more clearly understood and less likely to be 
misinterpreted.
    Taking into account the considerations discussed above, the 
Administrator proposes to establish a new indicator for thoracic coarse 
particles in terms of PM10-2.5, the definition of which 
includes qualifications that identify both the mix of such particles 
that are of concern to public health, and are thus included in the 
indicator, and those for which currently available information is not 
sufficient to infer a public health concern, and are thus excluded. 
More specifically, the proposed PM10-2.5 indicator is 
qualified so as to include any ambient mix of PM10-2.5 that 
is

[[Page 2668]]

dominated by resuspended dust from high-density traffic on paved roads 
and PM generated by industrial sources and construction sources, and 
excludes any ambient mix of PM10-2.5 that is dominated by 
rural windblown dust and soils and PM generated by agricultural and 
mining sources. In short, the indicator is not defined by nor limited 
to any specific geographic area, but includes the mix of 
PM10-2.5 in any location that is dominated by these sources.
    With the indicator as defined above, each area in the country would 
fall into one or the other of these two categories: (1) Either the 
majority of the ambient mix of PM10-2.5 in an area is 
resuspended dust from high-density traffic on paved roads and PM 
generated by industrial sources and construction sources, or (2) the 
majority of the ambient mix is rural windblown dust and soils and PM 
generated by agricultural and mining sources. The indicator would apply 
when PM10-2.5 generated by one or more of these named 
sources in the first category constitutes a majority of the ambient mix 
of PM10-2.5. The EPA recognizes that in many cases it will 
be clear which of these two categories applies, while in other cases it 
may be difficult to determine the appropriate category. As described in 
more detail in the preamble to EPA's proposed monitor network design 
rule, published elsewhere in today's Federal Register, the proposed 
minimum monitor siting criteria would provide guidance on 
distinguishing between areas where the mix of PM10-2.5 of 
concern would likely be dominated by the named sources in the first 
category and those areas where it would not. Consequently, all 
PM10-2.5 captured by a monitor that is properly sited in 
light of the indicator described above, as discussed in the proposed 
monitoring rule, would be considered in applying the standard, since 
the monitor would be capturing the mix of ambient PM10-2.5 
covered by the proposed indicator. As such, the proposed indicator does 
not present the type of over-inclusion or under-inclusion problems 
noted by the court with respect to a PM10 indicator (see 
American Trucking Associations v. EPA, 175 F.3d at 1054), since the 
application of the proposed indicator would result in compliance being 
based on measurement of the mix of ambient PM10-2.5 at which 
the standard is directed.
    The regulation for the proposed thoracic coarse particle indicator 
states that ``[a]gricultural sources, mining sources, and other similar 
sources of crustal material shall not be subject to control in meeting 
this standard.'' This proposed language reflects that the information 
supporting the proposed standard for thoracic coarse particles does not 
support extending controls to thoracic coarse particles from 
agricultural, mining sources, and other similar sources of crustal 
material. This statement in the regulations therefore is designed to 
make clear that there is no need nor basis to control these sources to 
obtain the public health benefits intended by the proposed indicator.
    Although the Administrator believes that an indicator qualified 
through reference to these categories and named sources appropriately 
identifies the ambient mixes that the epidemiologic studies indicate 
are of concern to public health, he solicits comment as to whether 
there may be other classes of sources which should also be included or 
excluded from the indicator. More generally, comment is also solicited 
on the approach of defining the indicator in terms of both particle 
size and categories of named sources.
    The Administrator recognizes that the proposed indicator, which 
includes considerations beyond particle size in its definition, 
represents a shift in the way in which PM indicators have been defined 
historically, and thus poses new challenges in ensuring a common 
understanding of how it can be appropriately and consistently 
implemented in areas across the country. In the Administrator's view, 
the application of this proposed indicator in conjunction with the 
proposed monitoring network design criteria, published elsewhere in 
today's Federal Register, and proposed rules for the treatment of air 
quality data influenced by exceptional events that will be published in 
the near future, will facilitate appropriate and consistent 
implementation.

E. Averaging Time of Primary PM10-2.5 Standard

    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 long-term exposures to thoracic coarse 
particles (62 FR at 38,677-79). That 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 on thoracic coarse 
particles, 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 recent 
study conducted in southern California reported a link between reduced 
lung function growth and long-term exposure to PM10-2.5 and 
PM2.5, other such studies reported no associations (EPA, 
2005a, p. 3-19, 3-23-24). Thus, the Criteria Document concludes that 
the available evidence does not suggest an association with long-term 
exposure to PM10-2.5 (EPA, 2004, p. 9-79).
    Based on these considerations, the Staff Paper concludes 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 concludes 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 observes, 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, 2005a, p. 5-61).
    Based on its review of the Staff Paper, CASAC recommends retention 
of a 24-hour averaging time and agrees that an annual averaging time 
for PM10-2.5 is not currently warranted (Henderson, 2005b). 
Based on these considerations, the Administrator concurs with staff and 
CASAC recommendations, and provisionally concludes that the newly 
available evidence continues to support a 24-hour averaging time for a 
PM10-2.5 standard, based primarily on evidence suggestive of 
associations between

[[Page 2669]]

short-term (24-hour) exposure and morbidity effects and, to a lesser 
degree, mortality. Further, the Administrator agrees that an annual 
PM10-2.5 standard is not warranted at this time. Thus, the 
Administrator proposes to revoke the annual PM10 standard 
and is not proposing an annual PM10-2.5 standard.

F. Form of Primary PM10-2.5 Standard

    For reasons similar to those discussed above in section II.F.2 on 
the form of the 24-hour PM2.5 standard, the Staff Paper also 
recommends consideration of either the 98th or 99th percentile form for 
a 24-hour PM10-2.5 standard. The relative year-to-year 
stability of the air quality statistic to be used as the basis for the 
form of a PM10-2.5 standard is of particular importance for 
a PM10-2.5 standard, since the nature and magnitude of the 
uncertainties in the risk assessment conducted for thoracic coarse 
particles weighed against considering risk estimates as a basis for 
comparing alternative combinations of specific forms and levels of 
standards.
    In considering the information provided in the Staff Paper, CASAC 
strongly recommends use of the 98th percentile form because it is more 
statistically robust than the 99th percentile form, together with the 
use of a three-year average of this statistic (Henderson 2005b). In 
making this recommendation, CASAC notes that the use of this statistic 
will tend to minimize ``measurement error and spatial variability, 
which are larger for coarse-mode particles than for fine PM'' as well 
as ``the influence in arid areas of occasional but extreme excursion 
contributions from rural, coarse-mode dust sources that are thought to 
be inherently less toxic than coarse-mode particles heavily enriched 
with urban source contaminants'' (Henderson, 2005b).
    In considering the available information, the Administrator concurs 
with the CASAC recommendation and proposes that the form of the 24-hour 
PM10-2.5 standard be based on the annual 98th percentile 
statistic, averaged over three years.

G. Level of Primary PM10-2.5 Standard

    In considering the available evidence on associations between 
short-term PM10-2.5 concentrations and morbidity and 
mortality effects as a basis for setting a 24-hour standard for 
thoracic coarse particles, the Staff Paper focuses on relevant U.S. and 
Canadian epidemiologic studies, as discussed above in section II.A. As 
an initial matter, the Staff Paper recognizes that these individual 
short-term exposure studies provide no evidence of clear population 
thresholds, or lowest-observed-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.
    In considering the evidence, the Staff Paper notes 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 focuses 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.\68\
---------------------------------------------------------------------------

    \68\ This 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).
---------------------------------------------------------------------------

    In looking first at the morbidity studies that report statistically 
significant associations with respiratory- and cardiac-related hospital 
admissions in Toronto (Burnett et al., 1997), Seattle (Sheppard et al., 
2003), and Detroit (Ito, 2003), the 98th percentile values reported in 
these studies range from approximately 30 to 36 [mu]g/m\3\. To provide 
some perspective on these PM10-2.5 levels, the Staff Paper 
notes that the level of the 24-hour PM10 standard was 
exceeded only on a few occasions during the time periods of the studies 
in Detroit and Seattle.\69\ In looking also at 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 [mu]g/m\3\, 
respectively. These studies were conducted in areas with air quality 
levels that did not meet the current PM10 standards. In 
addition, a statistically significant association was reported between 
PM10-2.5 and mortality in Steubenville as part of the 
original Six Cities study (Schwartz et al., 1996), although in more 
recent reanalyses, the association did not remain statistically 
significant in most models (Schwartz, 2003a; Klemm and Mason, 2003)--
the PM10-2.5 concentrations in this eastern city were fairly 
high, with a reported 98th percentile value of 53 [mu]g/m\3\. In 
contrast to the statistically significant mortality associations with 
PM10-2.5 reported in these studies, the Staff Paper notes 
that no such 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), 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 [mu]g/m\3\. Thus, in 
mortality studies that reported statistically significant associations, 
the reported 98th percentile PM10-2.5 values were all above 
50 [mu]g/m\3\, whereas in the mortality studies that reported no 
statistically significant associations, the reported 98th percentile 
PM10-2.5 values were generally below 50 [mu]g/m\3\.
---------------------------------------------------------------------------

    \69\ As 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-air-reports-03AQReport.pdf.
---------------------------------------------------------------------------

    In looking more closely at air quality data used in the morbidity 
and mortality studies discussed above, however, the Staff Paper 
recognizes 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 notes 
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, 
2005a, p. 5-64).
    The EPA staff 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 suggests 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

[[Page 2670]]

study conducted in Coachella Valley appear 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 shows 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.
    This close examination of air quality information generally 
reinforces the view that exposure measurement error is potentially 
quite large in these PM10-2.5 studies. As 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 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 population levels 
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 
likely become more uncertain, suggesting a high degree of caution in 
interpreting the group of morbidity studies as a basis for identifying 
a standard level that would protect against the observed effects.
    Taking into account this close examination of the studies, the 
Staff Paper concludes 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 [mu]g/m\3\, 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.
    Another view that reflects a more cautious or restrained approach 
to interpreting the limited body of PM10-2.5 epidemiologic 
evidence would be to judge that the uncertainties in this whole group 
of studies as to population exposures that are associated with the 
observed effects are too large to use the reported air quality levels 
directly 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 concludes 
that it would be reasonable to interpret the available epidemiologic 
evidence more qualitatively. Considering the available evidence in this 
way leads 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 
reported in areas in which the thoracic coarse particle fraction of 
PM10 was much greater than was the fine 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.
    Based on these considerations, the Staff Paper finds 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 standards, but have not been 
associated with air quality levels that would generally meet the 
current standards, and morbidity effects have been associated with air 
quality levels that exceeded the current standards only a few times. 
Further, the Staff Paper finds 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 concludes, therefore, that it is 
reasonable to interpret the available evidence as supporting 
consideration of a short-term standard for thoracic coarse particles, 
so as to provide generally ``equivalent'' protection to that afforded 
by the current PM10 standards, recognizing that no one 
PM10-2.5 level will be strictly equivalent to a specific 
PM10 level in all areas (EPA, 2005a, p. 5-67). Such a 
standard would likely provide protection against morbidity effects 
especially in urban areas where, unlike the study areas, 
PM10 is generally dominated by coarse-fraction rather than 
fine-fraction particles. Such a standard would also likely provide 
protection against the more serious, but more uncertain, 
PM10-2.5-related mortality effects generally observed at 
somewhat higher air quality levels.
    To identify a range of levels for consideration for a 24-hour 
PM10-2.5 standard, based on the indicator proposed above and 
set so as to afford generally ``equivalent'' protection as the current 
PM10 standards, the Staff Paper presents the results of 
analyses of relevant data on PM10-2.5 and PM10 
24-hour average concentrations.\70\ In one such analysis of 205 
monitoring sites (Schmidt et al., 2005),\71\ a PM10-2.5 
level of approximately 60 [mu]g/m\3\, in terms of a 98th percentile 
form, would be roughly equivalent on average across the U.S. to the 
current PM10 standard level of 150 [mu]g/m\3\, in terms of 
the current one-expected-exceedance form.\72\ While noting appreciable 
variability in the estimated point of equivalence across individual 
sites, these levels of approximate average equivalence are quite 
consistent across each of the five regions in which all of the areas 
that do not meet the current PM10 standards are located 
(including the southern California, southwest, northwest, upper mid-
west, and southeast regions). Notably different average equivalence

[[Page 2671]]

levels were observed in the other two regions, i.e., approximately 40 
[mu]g/m\3\ in the northeast and over 70 [mu]g/m\3\ in the industrial 
mid-west.
---------------------------------------------------------------------------

    \70\ Consistent with PM10-2.5 monitoring network 
design criteria discussed in section 5.4.2.2 of the Staff Paper, 
monitors included in this analysis are those in CBSAs with at least 
100,000 population and in census block groups with a population 
density of at least 500, and that also had 3 years of complete data 
in each quarter for both PM10 and PM10-2.5 
(EPA, 2005a, p. 5-67).
    \71\ These analyses were based on collocated PM10 and 
PM10-2.5 data, and used linear regression methods to 
predict PM10-2.5 concentrations (98th percentile form) 
equivalent to the 24-hour PM10 standard level of 150 
[mu]g/m\3\ (one expected exceedence form) at a national and at 
regional levels.
    \72\ Across the U.S., the 95 percent confidence intervals around 
these point estimates are approximately  3 [mu]g/m\3\, 
while region-specific intervals are approximately  10 
[mu]g/m\3\ in the five regions in which all of the areas that do not 
meet the current PM10 standards are located (EPA, 2005a, 
p. 5-68).
---------------------------------------------------------------------------

    Another such analysis was based on comparing the number of areas, 
and the population in those areas, that would likely not meet a 
specific PM10-2.5 standard, set at a given level and form, 
with the same measures in areas that do not meet the current 
PM10 standards. This analysis, based on 2001 to 2003 data, 
provides some rough indication of the breadth of protection potentially 
afforded by alternative standards. The results of this analysis 
indicate that a PM10-2.5 standard of about 70 or 65 [mu]g/
m\3\, 98th percentile form, would impact approximately the same number 
of counties or number of people, respectively, as would the current 
PM10 standards.\73\
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    \73\ As shown in Tables 5B-2(a) and (b) of the Staff Paper, 
there are 585 counties with PM10 monitoring sites used in 
determining compliance with the PM10 standards, whereas 
only 309 of those counties have monitor sites that would be included 
in the monitoring network design criteria discussed in section 
5.4.2.2 of the Staff Paper. Of these 309 counties, 259 have 
PM10 and PM10-2.5 air quality data that meet 
the data completeness criteria defined for this analysis, which are 
somewhat less restrictive than the criteria that were applied in the 
regression analysis described above.
---------------------------------------------------------------------------

    In considering the relevant dosimetric, toxicologic, and 
epidemiologic evidence, related limitations and uncertainties, and 
analyses of relevant air quality information, the Staff Paper concludes 
that it is appropriate to consider a 24-hour PM10-2.5 
standard in the range of 50 to 70 [mu]g/m\3\, with a 98th percentile 
form.\74\ The lower end of this range is based on a close examination 
of the air quality patterns related to the limited number of relevant 
epidemiologic studies. The upper part of this range is based on a more 
cautious approach to interpreting the available information and 
reflects a generally ``equivalent'' degree of protection to that 
afforded by the current PM10 standards. The upper end of 
this range is also below the 98th percentile PM10-2.5 
concentrations in the two mortality studies that reported statistically 
significant associations. Consideration of a generally ``equivalent'' 
PM10-2.5 standard would reflect a judgment that while the 
epidemiologic evidence supports 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 is not strong enough to provide a basis for 
changing the level of protection generally afforded by the current 
PM10 standards.
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    \74\ Beyond looking directly at the relevant epidemiologic 
evidence and related air quality information, the Staff Paper also 
considers the extent to which the PM10-2.5 risk 
assessment, discussed above in section III.B, can help inform 
consideration of alternative 24-hour PM10-2.5 standards. 
The Staff Paper concludes that 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, 2005a, p. 5-69).
---------------------------------------------------------------------------

    Based on its review of the Staff Paper, there was general agreement 
among the CASAC Panel members that the Staff Paper-recommended range of 
50 to 70 [mu]g/m\3\, with a 98th percentile form, for a 24-hour 
PM10-2.5 standard was reasonably justified. Most CASAC Panel 
members favored levels at the upper end of that range, while several 
members supported the lower end of the range (Henderson, 2005b). 
Because of the significant uncertainties resulting from the limited 
number of studies to date in which PM10-2.5 has been 
measured and the potentially large exposure measurement errors in such 
studies, the CASAC Panel did not generally support a level below the 
Staff Paper-recommended range.
    In considering an appropriate level for a 24-hour 
PM10-2.5 standard intended to afford requisite protection of 
public health from health effects associated with exposure to thoracic 
coarse particles of concern, the Administrator has carefully considered 
the rationale and recommendations contained in the Staff Paper, the 
advice and recommendations of CASAC, and public comments to date on 
this issue. Taking these considerations into account, the Administrator 
proposes to set the level of the primary 24-hour PM10-2.5 
standard at 70 [mu]g/m\3\. In the Administrator's provisional 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 the morbidity and possibly mortality effects that have been 
associated with short-term exposures to thoracic coarse particles of 
concern. This proposed standard is expected to have the most impact in 
areas that do not meet the current 24-hour PM10 standard.
    In reaching this judgment, the Administrator recognizes that the 
epidemiologic evidence on morbidity and possible mortality effects 
related to PM10-2.5 exposure is very limited at this time, 
and that there are potentially quite large uncertainties inherent in 
interpreting the available evidence for PM10-2.5 as compared 
with the evidence related to fine particles. For example, 
PM10-2.5 concentrations can vary substantially across a 
metropolitan area and thoracic coarse particles are less able to 
penetrate into buildings than fine particles; thus, the ambient 
concentrations reported in epidemiologic studies may not well represent 
area-wide population exposure levels. It may also be difficult to 
disentangle effects associated with PM10-2.5 and 
PM2.5 in epidemiologic studies. Further, the Administrator 
is mindful that considering what standard is requisite to protect 
public health with an adequate margin of safety requires judgments that 
neither overstate nor understate the strength and limitations of the 
evidence or the appropriate inferences to be drawn from the evidence. 
Thus, the Administrator provisionally concludes that the selection of a 
level that provides generally equivalent protection to that provided by 
the current PM10 standards is an appropriate policy response 
to the very limited body of evidence that is available at this time. 
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.
    The Administrator also recognizes that there is no one level for a 
PM10-2.5 standard that would be equivalent to the current 
PM10 standards in every area across the country, and that 
there are likely additional approaches to identifying a generally 
equivalent standard level beyond those approaches considered in the 
Staff Paper upon which the proposed level is based. Thus, the 
Administrator also solicits comment on alternative approaches to 
identifying a generally ``equivalent'' standard level. While proposing 
to set the PM10-2.5 standard at a level that is generally 
equivalent to the 1987 PM10 standard, the Administrator 
solicits comment on whether it would be more appropriate to set the 
PM10-2.5 standard at a level that is generally equivalent to 
the PM10 standard set in 1997.
    Having decided to propose the 24-hour PM10-2.5 standard 
described above, the Administrator recognizes that there are important 
views on the information relating to the effects of coarse fraction PM 
that warrant consideration. For example, an alternative interpretation 
of the available health evidence presented in the Criteria Document and 
the Staff Paper questions the conclusions about PM10-2.5 
associations drawn from one-pollutant models. This interpretation of 
the available epidemiological evidence suggests that the results from 
one-pollutant PM10-2.5 models are confounded by fine 
particles and gaseous co-pollutants.
    The key PM10-2.5 epidemiologic results discussed in the 
Criteria Document and

[[Page 2672]]

Staff Paper are drawn from one-pollutant models; i.e., 
PM10-2.5 is the only variable used in the statistical model 
reflecting exposure to air pollution. There are four studies cited in 
these documents as being suggestive of a statistically significant role 
for PM10-2.5 in the reported associations: Ito (2003), 
Burnett et al. (1997), Mar et al. (2003), and Ostro et al. (2003). 
However, there is strong evidence that adverse health effects similar 
to those observed in these studies, including both cardiovascular and/
or respiratory health effects are associated with exposure to 
PM2.5. The authors of several of these studies focus on fine 
particles (and in some cases one or more of the gaseous pollutants) as 
playing an important role in ``explaining'' the association between PM 
and various health endpoints. For example, in these key epidemiologic 
studies, the correlation coefficients between PM2.5 and 
PM10-2.5 concentrations range from moderate to high (i.e., 
0.4 to 0.7), which increases the likelihood that associations between 
health effects and PM10-2.5 identified in one-pollutant 
models may instead simply reflect the effects of exposure to 
PM2.5 rather than independent health effects. With the 
positive correlations between pollutants and similar health effects, it 
generally would be appropriate for any assessment of the effect of 
exposure to PM10-2.5 to control for exposure to the 
PM2.5.
    In this light, it is important to review how the authors of the 
four key PM10-2.5 epidemiology studies have accounted for 
co-pollutants in their analysis. Ito (2003) noted significant estimates 
of the health effects of associations in one-pollutant models, but in a 
two-pollutant model with PM2.5 the PM10-2.5 
associations lost statistical significance. Burnett et al. (1997) 
concluded that the effect of PM10-2.5 in a one-pollutant 
model could be explained by gaseous co-pollutants. Mar et al. (2003) 
found PM10-2.5 to be positively associated with adverse 
health effects in a one-pollutant model, but also found similar 
associations with a range of other air pollutants. In addition, Mar et 
al. (2003) noted that even though all PM mass metrics included in the 
study were associated with an excess risk of cardiovascular death, the 
strongest associations were with PM2.5, followed by 
PM10 and PM10-2.5. Ostro et al. (2003) used a 
one-pollutant model to estimate the association between 
PM10-2.5 on mortality using an effectively linear construct 
of PM10 (as observed in Indio, CA) to represent 
PM10-2.5 for the entire study area. By using such a 
construct of PM10, the estimated associations simply reflect 
a PM10 association (i.e., the construct does not provide 
additional information on the effect of PM10-2.5). Moreover, 
roughly 75 percent of the cardiovascular mortality in this study 
occurred in or near Palm Springs, CA and PM characteristics differ 
significantly between Palm Springs and Indio (e.g., average 
PM10 concentrations are roughly 30 percent lower in Palm 
Springs and PM2.5 represents a higher fraction of 
PM10, with a correlation coefficient between 
PM2.5 and PM10-2.5 of 0.46 in Palm Springs). 
Thus, the Ostro et al. (2003) study suggests a positive association 
between PM10 monitored in Indio and mortality in Palm 
Springs, but some view this study as offering little basis for 
attributing significant mortality association to PM10-2.5 as 
observed in either city.
    The Criteria Document and Staff Paper also present and discuss 
other epidemiology studies in support of the proposal for both the 
PM2.5 and PM10-2.5 standards (as shown in Figure 
2 and discussed in Section III.A above): Burnett (1997), Fairley 
(2003), Ito (2003), Lipfert et al (2000), Mar et al (2003), Moolgavkar 
(2000), Sheppard et al (2003), Thurston et al (1994), Burnett (2000, 
2003), Klemm and Mason (2003), and Schwartz and Neas (2000). However, 
these studies report positive, statistically significant associations 
with PM2.5 that are more consistent and robust than the 
associations thus far identified for PM10-2.5. Indeed, 
several of these and other studies that specifically considered 
PM10-2.5, but did not find statistically significant 
associations, including Schwartz et al (1996), Thurston et al. (1994), 
Sheppard et al. (2003), Fairley (2003), Schwartz et al (1996) and 
Lipfert et al. (2000). With respect to mortality effects in the Six-
City study, Schwartz et al. (1996) concluded that the PM associations 
(in the six metropolitan areas--including Steubenville) were 
specifically associated with PM2.5, with little additional 
contribution from the PM10-2.5. Sheppard et al. (2003) noted 
that bias in model selection and reporting can result in inflated 
excess risk estimates for PM. Fairley (1999) noted that 
PM10-2.5 effects become negative and insignificant when 
modeled jointly with PM2.5. Lipfert et al. (2000) showed 
insignificant effects for PM10-2.5 in one- and two-pollutant 
models with O3. The authors also caution against drawing 
causal interpretations from results when comparing health effects from 
one region in a metropolitan area to air quality observations in 
another region. In addition, several of these studies also report 
positive, statistically significant associations with one or more of 
the gaseous pollutants. Both Thurston et al. (1994) and Burnett et al. 
(1997) reported substantial confounding with gaseous co-pollutants in 
Toronto, and Thurston et al. (1994, p. 282) reported that ``it seems 
clear that these apparent associations were merely a statistical by-
product of interpollutant confounding resulting from the shared day-to-
day variations in dispersion conditions.'' In addition, Burnett et al. 
(2000) concluded that gaseous pollutants played an important role in 
explaining the effect of urban air pollution on health. Similarly, 
Moolgavkar (2000) concludes that gases were more strongly associated 
with respiratory effects than PM in Los Angeles.
    Taken as a whole, evidence from PM10-2.5 epidemiologic 
studies could be interpreted to suggest that one-pollutant 
PM10-2.5 models suffer from bias due to omitting co-
pollutants in the statistical model, especially given the much stronger 
evidence (discussed above) that these effects are associated with 
exposure to PM2.5. As noted by many of the aforementioned 
authors, while significant health associations may be noted for coarse 
fraction PM in one-pollutant models, the actual association may be 
insignificant from zero due to confounding co-pollutants. Of course, 
the Administrator must conclude in the final rule that the evidence 
about the health effects of PM10-2.5 is sufficiently robust 
to finalize a standard for PM10-2.5.
    The Administrator, recognizing notably large uncertainties in the 
underlying evidence and information that formed the basis for this 
proposal as well as the challenges associated with moving toward a new 
PM10-2.5 indicator and a related new monitoring network, 
solicits comment on this and other alternative interpretations of the 
available health evidence and alternative policy responses. Several 
such alternative interpretations and policy responses are discussed 
below.
    (1) In light of the large uncertainties in the evidence and the 
challenges of moving to a new indicator, and provisionally recognizing 
the need for a standard to provide a requisite level of protection from 
the risks associated with thoracic coarse particles, the Administrator 
also believes it appropriate to consider a policy option that would 
retain the current 24-hour PM10 standard (with a one-
expected-exceedance form), while addressing issues such as the 
appropriateness of the indicator and the level of the standard.
    As discussed in section I.D, in response to a challenge to the 1997 
standards for thoracic coarse PM, the

[[Page 2673]]

U.S. Court of Appeals for the District of Columbia vacated the Agency's 
1997 PM10 standards. In its decision the Court noted that 
use of PM10 as an indicator to protect against the public 
health risks associated with thoracic coarse particles resulted in 
double regulation of PM2.5, since this size fraction is both 
a component of PM10 and the subject of its own standard. The 
Court further reasoned that, since PM2.5 concentrations vary 
from area to area, use of PM10 as a thoracic coarse particle 
indicator results in an arbitrary level of protection in public health 
from the risks associated with thoracic coarse particles on a national 
basis, as the level of protection would vary based on the concentration 
of PM2.5 in an area. See American Trucking Associations v. 
EPA, 175 F.3d at 1054-55.
    Under this option to retain the 24-hour PM10 standard, 
EPA would modify the standard to exclude the double-counted 
PM2.5 contribution in circumstances where this could present 
a concern. First, there will be some areas that may be in nonattainment 
with the PM10 standard because, and only because, they are 
in nonattainment with the PM2.5 standard. To remedy the 
double counting in this situation, EPA is requesting comment on 
subtracting from a daily measured PM10 concentration the 
value by which the concentration of PM2.5 measured at a 
collocated monitor is in excess of 35 [mu]g/m3 (i.e., the 
proposed level for the 24-hour PM2.5 standard). This 
adjustment would need to be made only on days when a 24-hour average 
PM10 concentration is measured in excess of 150 [mu]g/
m3. In such a case, the amount by which the PM2.5 
concentration exceeds 35 [mu]g/m3 would be subtracted from 
the measured PM10 concentration. The EPA would then use this 
adjusted value in any comparison to the PM10 standard.
    The second situation where the overlap between the PM2.5 
and PM10 standards may cause some concern is in areas where 
a daily PM2.5 level is below 35 [mu]g/m3. In 
those areas, the level of the PM10 standard would allow a 
higher concentration of thoracic coarse particles before triggering an 
exceedance than it would in other areas. The EPA is requesting comment 
on not requiring any adjustment to the daily measured PM10 
concentration in this situation, on the basis that any additional risk 
to public health that may be associated with this higher allowable 
concentration of thoracic coarse particles would reasonably be expected 
to present less concern from a public health perspective than would the 
otherwise allowable equivalent increase in the concentration of 
PM2.5.
    The EPA also believes that it would be appropriate in this option 
to focus the PM10 standard in a manner similar to that 
proposed above for the PM10-2.5 standard. While the 
indicator would remain specified as PM10, the focus would be 
on including only the mix of ambient thoracic coarse particles that are 
of concern to public health (and to exclude the mix for which 
information is not sufficient to infer a public health concern) and 
would be achieved in practice through the data handling requirements 
associated with the standard, which are linked to the proposed 
monitoring network design criteria (in the part 58 rule proposed 
elsewhere in today's Federal Register).
    The EPA invites comment on whether this option would provide the 
requisite level of public health protection from risks associated with 
thoracic coarse particles. Given the difference in form between the 24-
hour PM10 standard (one-expected-exceedance form) and the 
proposed PM10-2.5 standard (98th percentile form), and the 
adjustments noted above, in practice there may not be an appreciable 
difference in the degree of public health protection afforded by this 
option relative to that afforded by the proposed PM10-2.5 
standard. The EPA invites comment on whether this approach addresses 
one of the concerns about use of a PM10 indicator for 
thoracic coarse particles noted by the Court in its ATA decision, 
namely that the level of public health protection from thoracic coarse 
particles in an area would vary depending on the relative proportions 
of fine and thoracic coarse particles, by recognizing that the 
PM10 indicator and standard would cover both fine and 
thoracic coarse particles.
    With respect to revocation of the 1987 24-hour PM10 
standard, under this option EPA would apply the same approach to 
revocation as that proposed below in section III.H. in conjunction with 
the proposed PM10-2.5 standard. Since the 24-hour 
PM10 standard would be focused in basically the same manner 
as the proposed PM10-2.5 standard, it would be appropriate 
to follow the same approach to revocation of the current 24-hour 
PM10 standard under this option as well.
    The EPA solicits comment on all aspects of this approach, including 
views on whether a 24-hour PM10 standard revised as noted 
above would be requisite to protect public health from the risks 
associated with thoracic coarse particles, with an adequate margin of 
safety, as well as views on any legal, scientific, or policy issues 
associated with this alternative, and including comments on the 
consistency of this option with CASAC's recommendations. The EPA also 
solicits comment on whether a 98th percentile form should be considered 
for a 24-hour PM10 standard and on the appropriate level of 
such a standard.
    (2) The Administrator recognizes that some commenters hold the view 
that the uncertainties that exist at the present time are so great that 
no standards for thoracic coarse particles are warranted. Some such 
commenters point to conclusions reached in the Staff Paper in part as a 
basis for their view, including, for example, the conclusion that the 
``substantial uncertainties associated with this limited body of 
epidemiological evidence on health effects related to 
PM10-2.5 * * * suggests a high degree of caution in 
interpreting this evidence * * *.'' (EPA 2005, pp. 5-50). This view 
generally places significant weight on the issue of confounding between 
PM2.5 and PM10-2.5 (discussed above in section 
III.A), with some commenters stating that the correlation coefficients 
between fine and thoracic coarse particle levels are modest to high for 
all studies for which such data are available, increasing the 
possibility that the positive association identified in the 
PM10-2.5 one-pollutant models may instead reflect the 
effects of fine particles. Noting that the Staff Paper puts little 
weight on the health risk assessment because of the significant 
uncertainties in the underlying health studies, some commenters suggest 
that the risk assessment therefore does not provide a basis for 
determining whether the health effects possibly associated with 
PM10-2.5 constitute a meaningful public health risk. Some 
commenters take the view that, based either on the studies or the risk 
assessment, the magnitude of the health effects possibly associated 
with PM10-2.5 do not constitute a meaningful risk to public 
health. These commenters also maintain that significant uncertainty 
remains as to an appropriate level of a standard, even assuming that a 
meaningful public health risk exists. In consideration of these views, 
the Administrator also solicits comment on revoking the current 24-hour 
PM10 standard at this time (as well as the current annual 
PM10 standard, as proposed above), not adopting a thoracic 
coarse particle standard at this time, and taking into account any new 
relevant research that becomes available as a basis for considering a 
more targeted standard for thoracic coarse particles in the next 
periodic review of the PM NAAQS.

[[Page 2674]]

    (3) In sharp contrast to the views noted above, another view that 
the Administrator takes note of would place greater weight on the 
available epidemiologic evidence as a basis for selecting a level down 
to 50 [mu]g/m3 or below and/or for selecting an unqualified 
PM10-2.5 indicator. While recognizing that important 
uncertainties are present in the available evidence, this view would 
support incorporating a larger margin of safety consistent with a more 
highly precautionary policy response. In soliciting comments on a wide 
array of views, the Administrator solicits comment on this view and on 
standard levels that are consistent with this view.

H. Proposed Decisions on Primary PM10-2.5 Standard

    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 to date, the Administrator proposes to revise the current 
primary PM10 standards. In particular, to provide more 
targeted protection from thoracic coarse particles that are of concern 
to public health, the Administrator proposes to establish a new 
indicator for thoracic coarse particles in terms of 
PM10-2.5, the definition of which includes qualifications 
that identify both the mix of such particles that are of concern to 
public health, and are thus included in the indicator, and those for 
which currently available information is not sufficient to infer a 
public health concern, and are thus excluded. More specifically, the 
proposed PM10-2.5 indicator is qualified so as to include 
any ambient mix of PM10-2.5 that is dominated by particles 
generated by high-density traffic on paved roads, industrial sources, 
and construction sources, and to exclude any ambient mix of particles 
dominated by rural windblown dust and soils and agricultural and mining 
sources. The Administrator proposes to replace the current primary 24-
hour PM10 standard with a 24-hour standard defined in terms 
of this new PM10-2.5 indicator and set at a level of 70 
[mu]g/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. The proposed new standard would be met at an ambient air 
quality monitoring site \75\ when the 3-year average of the annual 98th 
percentile 24-hour average PM10-2.5 concentration is less 
than or equal to 70 [mu]g/m3.\76\ The Administrator also 
proposes to revoke and not replace the annual PM10 standard.
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    \75\ Monitoring sites that are appropriate for determining 
compliance with this standard are those that are consistent with the 
proposed indicator. Guidance on this can be found in the proposed 
monitoring network design criteria published elsewhere in today's 
Federal Register.
    \76\ Data handling conventions are specified in a new proposed 
Appendix P, as discussed in Section V below, and the reference 
method for monitoring PM as PM10-2.5 is specified in a 
new proposed Appendix L, as discussed in Section VI below.
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    In recognition of alternative views of the currently available 
scientific information and the appropriate policy response to this 
information, the Administrator also solicits comments on (1) 
alternative approaches to selecting the level of a 24-hour 
PM10-2.5 standard or to selecting an unqualified 
PM10-2.5 indicator, and (2) alternative approaches to 
providing continued protection from thoracic coarse particles based on 
retaining the current 24-hour PM10 standard. Alternatively, 
the Administrator also solicits comment on revoking and not replacing 
the 24-hour PM10 standard. Based on the comments received 
and the accompanying rationale, the Administrator may adopt other 
standards within the range of the alternatives identified above in lieu 
of the standard he is proposing today.
    The Administrator is also proposing to revoke the current annual 
PM10 standard upon promulgation of this rule. Further, if 
EPA finalizes a 24-hour primary PM10-2.5 standard, the 
Administrator is proposing to revoke the current 24-hour 
PM10 standard everywhere except in areas where there is at 
least one monitor that is located in an urbanized area \77\ with a 
minimum population of 100,000 people and that violates the 24-hour 
PM10 standard based on the most recent three years of data.
---------------------------------------------------------------------------

    \77\ As defined by the U.S. Bureau of the Census, an urbanized 
area has ``a minimum residential population of at least 50,000 
people'' and generally includes ``core census block groups or blocks 
that have a population density of at least 1,000 people per square 
mile and surrounding census blocks that have an overall density of 
at least 500 people per square mile.'' The Census Bureau notes that 
``under certain conditions, less densely settled territory may be 
part of each UA.'' See http://www.census.gov/ geo/www/ua/ua--

2k.html.
---------------------------------------------------------------------------

    EPA specifically proposes that the 24-hour PM10 standard 
would be revoked in this rulemaking in all areas except the following:

1. Birmingham urban area (Jefferson County, AL)
2. Maricopa and Pinal Counties; Phoenix planning area (AZ)
3. Riverside, Los Angeles, Orange and San Bernardino Counties; South 
Coast Air Basin (CA)
4. Fresno, Kern, Kings, Tulare, San Joaquin, Stanislaus, Maderia 
Counties; San Joaquin Valley planning area (CA)
5. San Bernardino County (part); excluding Searles Valley Planning Area 
and South Coast Air Basin (CA)
6. Riverside County; Coachella Valley Planning Area (CA)
7. Simi Valley urban area (CA)
8. Lake County; Cities of East Chicago, Hammond, Whiting, and Gary (IN)
9. Wayne County (part) (MI)
10. St. Louis urban area (MO)
11. Albuquerque urban area (NM)
12. Clark County; Las Vegas planning area (NV)
13. Columbia urban area (SC)
14. El Paso urban area (including those portions in TX and those 
portions in NM)
15. Salt Lake County (UT)

    A separate memorandum explaining the factual basis for our proposed 
determinations regarding each PM10 area where we are 
proposing to retain the current 24-hour standard is part of the 
administrative record for this proposed rule (Rosendahl, 2005).
    In essence, we are proposing to retain the current 24-hour 
PM10 standard only in areas which could be in violation of 
the proposed PM10-2.5 standard. While it is possible that 
some existing PM10 monitors may not be sited in accordance 
with all of the criteria for PM10-2.5 monitor siting 
proposed elsewhere in today's Federal Register (see section IV.E.2.b.ii 
of the preamble to the proposed changes to Part 53/58), it is not 
possible for EPA to make a case-by-case assessment of monitor placement 
within each area at this time. Therefore, EPA believes that all areas 
with violating PM10 monitors located in urbanized areas with 
a minimum population of 100,000 people should be considered areas that 
may violate the PM10-2.5 standard.
    For those areas where we propose to retain the 24-hour 
PM10 standard which were previously designated nonattainment 
for PM10 or which are currently designated nonattainment for 
PM10, EPA proposes, in the alternative, either that the 
standard would continue to apply in the entire attainment/nonattainment 
area, or that the area to which the standard would continue to apply 
should be limited to the urbanized area containing the violating 
monitor(s). For areas with violating monitor(s) which were never 
designated nonattainment, EPA proposes that the boundaries of the area 
to which the standard would continue to apply should be limited to the 
urbanized area containing the violating monitor(s). For

[[Page 2675]]

all areas in which the 24-hour PM10 standard would be 
retained, EPA invites comments on the appropriate boundaries within 
which the standard should continue to apply.
    Consistent with our request for comment in the Part 53/58 proposal, 
section IV.E.2.b.ii, on whether we should establish criteria for 
locating discretionary monitors appropriate for comparison with the 
proposed 24-hour PM10-2.5 standard in locations other than 
urbanized areas with population of at least 100,000 people, we also 
request comment on whether the 24-hour PM10 standard should 
be retained in areas that are either urbanized areas with a population 
less than 100,000 people or non-urbanized areas (i.e. population less 
than 50,000) but where the majority of the ambient mix of 
PM10-2.5 is generated by high density traffic on paved 
roads, industrial sources, and construction activities, and which have 
at least one monitor that violates the 24-hour PM10 
standard. The EPA requests comment on the criteria that should be used 
to determine whether such an area with a violating monitor must retain 
the 24-hour PM10 standard. Such criteria could include 
whether the area has one (or more) industrial source(s) listed in 
either the National Emissions Inventory or the Toxics Release Inventory 
located within a certain radius of the violating monitor, and whether 
these sources are in industrial categories that do not include 
agricultural or mining sources. One approach to defining such 
categories would be to utilize the U.S. Census Bureau's North American 
Industry Classification System,\78\ which defines separate 
classifications for agricultural and mining activities such as Crop 
Production (111), Animal Production (112), and Mining (112). The EPA 
requests comments on how this or another classification system, 
combined with information on the location of sources relative to the 
violating PM10 monitor, could be used to identify additional 
areas to which the 24-hour PM10 standard should continue to 
apply due to the presence of industrial sources. The EPA also requests 
comments on which areas would meet these criteria or other criteria 
that may be appropriate to determine in which, if any, areas the 24-
hour PM10 standard should be retained, and the appropriate 
boundaries within which the standard should continue to apply for these 
areas. A more detailed example of criteria that could be used to 
identify areas to which the standard should continue to apply, along 
with a list of all areas with violating PM10 monitors that 
meet these criteria, are part of the administrative record for this 
proposed rule (Rosendahl, 2005). For all areas where the 24-hour 
PM10 standard would be retained under this proposal, we 
contemplate that the 24-hour PM10 standard would be revoked 
after designations are completed under a final 24-hour 
PM10-2.5 standard.
---------------------------------------------------------------------------

    \78\ http://www.census.gov/epcd/naics02/naicod02.htm#N21.

---------------------------------------------------------------------------

    The EPA also recognizes that it is possible that some areas for 
which we are proposing to retain the PM10 daily standard 
would, upon a case-specific investigation (see section IV.E.2.c of the 
Part 53/58 preamble), warrant revocation as not being an area where the 
ambient coarse PM mix is dominated by the type of coarse PM described 
by the proposed indicator. The EPA is not in a position to conduct such 
case-by-case evaluation for this proposal, but could address revocation 
in such situations in a future rulemaking. The EPA invites comment on 
this issue.
    To address issues related to the transition from the current 
PM10 standards to a new PM10-2.5 standard, the 
Administrator intends to seek public comment on EPA's plans for 
assuring an effective transition as part of an ANPR that EPA intends to 
issue by the end of January 2006. In the forthcoming ANPR dealing with 
transition issues, EPA intends to address, among other things, the 
timing for revocation of the PM10 standard in areas in which 
we are proposing to retain that standard, and the consequences of 
revoking the PM10 standards on the PM10 PSD 
program (including PM10 increments), on the PM10 
nonattainment New Source Review (NSR) program, and on our existing 
policy of using PM10 as a surrogate for the PM2.5 
NSR program.

IV. Rationale for Proposed Decisions on Secondary PM Standards

    The Criteria Document and Staff Paper examined the effects of PM on 
such aspects of public welfare as visibility, vegetation and 
ecosystems, materials damage and soiling, and climate change. 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. This existing suite of secondary standards is intended to 
address visibility impairment associated with fine particles and 
materials damage and soiling related to both fine and coarse particles. 
The following discussion of the rationale for the proposed decisions on 
secondary PM standards focuses on those considerations most influential 
in the Administrator's proposed decisions, first addressing visibility 
impairment followed by the other welfare effects considered in this 
review.\79\
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    \79\ As 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.
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A. Visibility Impairment

    This section presents the rationale for the Administrator's 
proposed revision of the current secondary PM2.5 standard to 
address PM-related visibility impairment. 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 on visual air quality.
1. Visibility Impairment Related to Ambient PM
    This section outlines key information contained in the Criteria 
Document and Staff Paper on: (1) The nature of visibility impairment, 
including trends in visual air quality and the characterization of 
current visibility conditions; (2) quantitative relationships between 
ambient PM and visibility; (3) the impacts of visibility impairment on 
public welfare; and (4) approaches to evaluating public perceptions and 
attitudes about visibility impairment.
a. Nature of Visibility Impairment
    Visibility can be defined as the degree to which the atmosphere is 
transparent to visible light. Visibility conditions are determined by 
the scattering and absorption of light by particles and gases, from 
both natural and anthropogenic sources. Visibility is often described 
in terms of visual range, light extinction, or deciviews.\80\ The 
classes of fine particles principally responsible for visibility 
impairment are sulfates, nitrates, organic matter, elemental carbon, 
and soil dust. Fine

[[Page 2676]]

particles are more efficient per unit mass at scattering light than 
coarse particles. The scattering efficiency of certain classes of fine 
particles, such as sulfates, nitrates, and some organics, increases as 
relative humidity rises because these particles can absorb water and 
grow to sizes comparable to the wavelength of visible light. In 
addition to limiting the distance that one can see, the scattering and 
absorption of light caused by air pollution can also degrade the color, 
clarity, and contrast of scenes.
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    \80\ Visual range can be defined as the maximum distance at 
which one can identify a black object against the horizon sky. It is 
typically described in kilometers or miles. Light extinction is the 
sum of light scattering and absorption by particles and gases in the 
atmosphere. It is typically expressed in terms of inverse megameters 
(Mm-1), with larger values representing poorer 
visibility. The deciview metric describes perceived visual changes 
in a linear fashion over its entire range, analogous to the decibel 
scale for sound.
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    Visibility impairment is manifested in two principal ways: As local 
visibility impairment and as regional haze. Local visibility impairment 
may take the form of a localized plume, a band or layer of 
discoloration appearing well above the terrain that results from 
complex local meteorological conditions. Alternatively, local 
visibility impairment may manifest as an urban haze, sometimes referred 
to as a ``brown cloud.'' A ``brown cloud'' is predominantly caused by 
emissions from multiple sources in the urban area and is not typically 
attributable to a single nearby source or to long-range transport from 
more distant sources. The second type of visibility impairment, 
regional haze, generally results from pollutant emissions from a 
multitude of sources located across a broad geographic region. Regional 
haze impairs visibility in every direction over a large area, in some 
cases over multi-state regions. It is regional haze that is principally 
responsible for impairment in national parks and wilderness areas 
across the country (NRC, 1993).
    While visibility impairment in urban areas at times may be 
dominated by local sources, it often may be significantly affected by 
long-range transport of haze due to the multi-day residence times of 
fine particles in the atmosphere. Fine particles transported from urban 
and industrialized areas, in turn, may, in some cases, be significant 
contributors to regional-scale impairment in Class I areas \81\ and 
other rural areas.
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    \81\ There are 156 mandatory Class I Federal areas protected by 
the visibility provisions in sections 169A and 169B of the Act. 
These areas are defined in section 163 of the Act as those national 
parks exceeding 6000 acres, wilderness areas and memorial parks 
exceeding 5000 acres, and all international parks which were in 
existence on August 7, 1977.
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    As discussed in the Staff Paper (EPA, 2004, section 6.2), in Class 
I areas, visibility levels on the 20 percent haziest days in the West 
are about equal to levels on the 20 percent best days in the East. 
Despite improvement through the 1990's, visibility in the rural East 
remains significantly impaired, with an average visual range of 
approximately 20 km on the 20 percent haziest days (compared to the 
naturally occurring visual range in the eastern U.S. of about 150 
 45 km). In the rural West, the average visual range showed 
little change over this period, with an average visual range of 
approximately 100 km on the 20 percent haziest days (compared to the 
naturally occurring visual range in the western U.S. of about 230 
 40 km).
    In urban areas, visibility levels show far less difference between 
eastern and western regions. For example, the average visual ranges on 
the 20 percent haziest days in eastern and western urban areas are 
approximately 20 km and 27 km, respectively (Schmidt et al., 2005). 
Even more similarity is seen in considering 4-hour (12 to 4 p.m.) 
average PM2.5 concentrations, for which the average visual 
ranges on the 20 percent haziest days in eastern and western urban 
areas are approximately 26 km and 31 km, respectively (Schmidt et al., 
2005).
    Data on visibility conditions indicate that urban areas generally 
have higher loadings of PM2.5 and, thus, higher visibility 
impairment than monitored Class I areas. Since efforts are now underway 
to address all human-caused visibility in Class I areas through the 
regional haze program (EPA, 1999; 65 FR 35713), implemented under 
sections 169A and 169B of the CAA, and since the Clean Air Interstate 
Rule (CAIR) (70 FR 25162) is expected to result in improvements to 
visual air quality, particularly in eastern Class I and non-urban 
areas, new assessments included in the Staff Paper were primarily 
focused on visibility impairment in urban areas.
b. Correlations Between Urban Visibility and PM2.5 Mass
    Direct relationships exist between measured ambient pollutant 
concentrations and their contributions to light extinction and thus to 
visibility impairment. The contribution of each PM constituent to total 
light extinction is derived by multiplying the constituent 
concentration by its extinction efficiency to calculate a 
``reconstructed'' light extinction.\82\ For certain fine particle 
constituents, extinction efficiencies increase significantly with 
increases in relative humidity. As a consequence, while higher 
PM2.5 mass concentrations generally indicate higher levels 
of visibility impairment, it is not as precise a metric as the light 
extinction coefficient. Nonetheless, by using historic averages, 
regional estimates, or actual day-specific measurements of the 
component-specific percentage of total mass, one can develop reasonable 
estimates of light extinction from PM mass concentrations. As discussed 
below, the Staff Paper concludes that fine particle mass concentrations 
can be used as a general surrogate for visibility impairment (EPA, 
2005a, p. 2-74).
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    \82\ Extinction efficiencies vary by type of constituent and 
have been obtained for typical atmospheric aerosols by a combination 
of empirical approaches and theoretical calculations. As discussed 
in the Staff Paper, EPA's guidance for tracking progress under the 
regional haze program specifies an algorithm for calculating total 
light extinction as a function of the major fine particle components 
(EPA, 2005a, section 2.8.1). ``Reconstructed'' light extinction 
simply refers to the calculation of PM-related light extinction by 
the use of that formula.
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    In an effort to better characterize urban visibility, the Staff 
Paper presents results of analyses of the extensive new data now 
available on PM2.5 primarily in urban areas. This rapidly 
expanding national database includes federal reference method (FRM) 
\83\ measurements of PM2.5 mass, continuous measurements of 
hourly PM2.5 mass, and PM2.5 chemical speciation 
measurements. These data allowed for analyses 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. These analyses 
show a consistently high correlation between visibility, in terms of 
reconstructed light extinction, and hourly PM2.5 
concentrations for urban areas in a number of regions across the U.S. 
and, more generally, in the eastern and western U.S. These correlations 
in urban areas are generally similar in the East and West, in sharp 
contrast to the East/West differences observed in rural areas.
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    \83\ The PM2.5 Federal Reference Method (FRM) 
monitoring network provides 24-hour average PM2.5 
concentrations.
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    While the average daily relative humidity levels are generally 
higher in the East than in the West, in both regions relative humidity 
levels are appreciably lower during daylight as compared to night time 
hours. The reconstructed light extinction coefficient, for a given mass 
and concentration, increases sharply as relative humidity rises. Thus, 
with lower relative humidity levels, visibility impacts related to 
East/West differences in average relative humidity are minimized during 
daylight hours, when relative humidity is generally lower.
    Both 24-hour and shorter-term daylight hour averaging periods were

[[Page 2677]]

considered in evaluations of correlations between PM2.5 
concentrations in urban areas and visibility in eastern and western 
areas, as well as nationwide. Clear and similarly strong correlations 
are found between visibility and 24-hour average PM2.5 in 
eastern, western, and all urban areas (EPA, 2005a, Figure 6-3). 
Somewhat stronger correlations are observed between visibility and 
PM2.5 concentrations averaged over a 4-hour time period 
(EPA, 2005a, Figure 6-5). The correlations between visibility and 
PM2.5 concentrations during daylight hours in urban areas 
are relatively more reflective of PM2.5 mass rather than 
relative humidity effects, in comparison to correlations based on a 24-
hour averaging time.
c. Impacts of Urban Visibility Impairment on Public Welfare
    EPA has long recognized that impairment of visibility is an 
important effect of PM on public welfare, and that it is experienced 
throughout the U.S. in urban areas as well as in remote Class I areas 
(62 FR 38680). Visibility is an important welfare effect because it has 
direct significance to people's enjoyment of daily activities in all 
parts of the country. Individuals value good visibility for the sense 
of well-being it provides them directly, both in places where they live 
and work, and in places where they enjoy recreational opportunities.
    Survey research on public awareness of visual air quality using 
direct questioning typically reveals that 80 percent or more of the 
respondents are aware of poor visual air quality (Cohen et al., 1986). 
The importance of visual air quality to public welfare across the 
country has been demonstrated by a number of studies designed to 
quantify the benefits (or willingness to pay) associated with potential 
improvements in visibility (Chestnut and Dennis, 1997; Chestnut and 
Rowe, 1991). Economists have performed many studies in an attempt to 
quantify the economic benefits associated with improvements in current 
visibility conditions both in national parks and in urban areas 
(Chestnut and Dennis, 1997). These economic benefits may include the 
value of improved aesthetics during daily activities (e.g., driving or 
walking, daily recreations), for special activities (e.g., visiting 
parks and scenic vistas, hiking, hunting), and for viewing scenic 
photography. They may also include the value of improved road and air 
safety, and/or preservation of the resource for its own sake. As 
discussed in the Staff Paper and below, the value placed on protecting 
visual air quality is further demonstrated by the existence of a number 
of programs, goals, standards, and planning efforts that have been 
established in the U.S. and abroad to address visibility concerns in 
urban and non-urban areas.
    Protection against visibility impairment in special areas is 
provided for in sections 169A, 169B, and 165 of the CAA, in addition to 
that provided by the secondary NAAQS. Section 169A, added by the 1977 
CAA Amendments, established a national visibility goal to ``remedy 
existing impairment and prevent future impairment'' in 156 national 
parks and wilderness areas (Class I areas). The Amendments also called 
for EPA to issue regulations requiring States to develop long-term 
strategies to make ``reasonable progress'' toward the national goal. 
EPA issued initial regulations in 1980 focusing on visibility problems 
that could be linked to a single source or small group of sources. The 
1990 CAA Amendments placed additional emphasis on regional haze issues 
through the addition of section 169B. In accordance with this section, 
EPA established the Grand Canyon Visibility Transport Commission 
(GCVTC) in 1991 to address adverse visibility impacts on 16 Class I 
national parks and wilderness areas on the Colorado Plateau. The GCVTC 
issued its recommendations to EPA in 1996, triggering a requirement in 
section 169B for EPA issuance of regional haze regulations.
    EPA accordingly promulgated a final regional haze rule in 1999 
(U.S. EPA, 1999; 65 FR 35713). Under the regional haze program, States 
are required to establish goals for improving visibility on the 20 
percent most impaired days in each Class I area, and for allowing no 
degradation on the 20 percent least impaired days. Each state must also 
adopt emission reduction strategies which, in combination with the 
strategies of contributing States, assure that Class I area visibility 
improvement goals are met. The first State implementation plans are to 
be adopted in the 2003-2008 time period, with the first implementation 
period extending until 2018. Five multi-state planning organizations 
are evaluating the sources of PM2.5 contributing to Class I 
area visibility impairment to lay the technical foundation for 
developing strategies, coordinated among many States, in order to make 
reasonable progress in Class I areas across the country.
    A number of other programs, goals, standards, and planning efforts 
have also been established in the U.S. and abroad to address visibility 
concerns in urban and non-urban areas. These regulatory and planning 
activities are of interest because they are illustrative of the 
significant value that the public places on improving visibility, and 
because they have developed and applied methods for evaluating public 
perceptions and judgments about the acceptability of varying degrees of 
visibility impairment, as discussed below in the next section.
    Several state and local governments have developed programs to 
improve visual air quality in specific urban areas, including Denver, 
CO; Phoenix, AZ; and, Lake Tahoe, CA. At least two States have 
established statewide standards to protect visibility. In addition, 
interest in visibility protection in other countries, including Canada, 
Australia, and New Zealand has resulted in various studies, surveys, 
and programs. Examples of these efforts are highlighted below.
    In 1990, the State of Colorado adopted a visibility standard for 
the city of Denver. The Denver standard is a short-term standard that 
establishes a limit of a four-hour average light extinction level of 76 
Mm-1 (equivalent to a visual range of approximately 50 km) 
during the hours between 8 a.m. and 4 p.m. (Ely et al., 1991). In 2003, 
the Arizona Department of Environmental Quality created the Phoenix 
Region Visibility Index, which focuses on an averaging time of 4 hours 
during actual daylight hours. This visibility index establishes visual 
air quality categories (i.e., excellent to very poor) and establishes 
the goals of moving days in the poor/very poor categories up to the 
fair category, and moving days in the fair category up to the good/
excellent categories (Arizona Department of Environmental Quality, 
2003). This approach results in a focus on improving visibility to a 
visual range of approximately 48-36 km. In 1989, the state of 
California revised the visibility standard for the Lake Tahoe Air Basin 
and established an 8-hour visibility standard equal to a visual range 
of 30 miles (approximately 48 km) (California Code of Regulations).
    California and Vermont each have standards to protect visibility, 
though they are based on different measures. Since 1959, the state of 
California has had an air quality standard for particle pollution where 
the ``adverse'' level was defined as the ``level at which there will be 
* * * reduction in visibility or similar effects.'' California's 
general statewide visibility standard is a visual range of 10 miles 
(approximately 16 km) (California Code of Regulations). In 1985, 
Vermont established a state visibility standard that is expressed as a 
summer seasonal sulfate concentration of 2 [mu]g/m\3\, that equates to 
a visual range

[[Page 2678]]

of approximately 50 km. This standard was established to represent 
``reasonable progress'' toward attaining the congressional visibility 
goal for the Class 1 Lye Brook National Wilderness Area, and applies to 
this Class 1 area and to all other areas of the state with elevations 
greater than 2500 ft.
    Outside of the U.S., efforts have also been made to protect 
visibility. The Australian state of Victoria has established a 
visibility objective (State Government of Victoria, 1999 and 2000), and 
a visibility guideline is under consideration in New Zealand (New 
Zealand National Institute of Water & Atmospheric Research, 2000a and 
2000b; New Zealand Ministry of Environment, 2000). A survey was 
undertaken for the Lower Fraser Valley in British Columbia, with 
responses from this pilot study being supportive of a standard in terms 
of a visual range of approximately 40 km for the suburban township of 
Chilliwack and 60 km for the suburban township of Abbotsford, although 
no visibility standard has been adopted for the Lower Fraser Valley at 
this time.
d. Approaches to Evaluating Public Perceptions and Attitudes
    New methods and tools 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. New survey methods have 
been applied and evaluated in various studies, such as those done in 
Denver, Phoenix, and the Lower Fraser Valley in British Columbia. These 
methods are intended to assess public perceptions as to the 
acceptability of varying levels of visual air quality, considered in 
these studies to be an appropriate basis for developing goals and 
standards for visibility protection. A pilot study was also conducted 
in Washington, DC by EPA staff.\84\ Even with variations in each 
study's approaches, the public perception survey methods used for the 
Denver, Phoenix, and British Columbia studies produced reasonably 
consistent results from location to location, with each study 
indicating that a majority of participants find visual ranges within 
about 40 to 60 km to be acceptable.
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    \84\ This small pilot study was briefly discussed in the 
preliminary draft staff paper (Abt Associates, 2001).
---------------------------------------------------------------------------

    These public perception studies use images of urban and distant 
scenic views under different visibility conditions together with survey 
techniques designed to elicit judgments from members of the public 
about the acceptability of differing levels of visual air quality. 
Images used are either photographs or computer simulations using the 
WinHaze program.\85\ Examples of images that illustrate visual air 
quality in Denver, Phoenix, Washington, DC, and Chicago under a range 
of visibility conditions associated with a range of PM2.5 
concentrations are available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_sp.html
 (labeled as Appendix 6A: Images of Visual Air 

Quality in Selected Urban Areas in the U.S.). These examples include 
simulated images for Denver, Phoenix, and Washington, DC, and 
photographs of Chicago.
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    \85\ The Criteria Document discusses methods available to 
represent different levels of visual air quality (EPA, 2004, p. 4-
174). In particular, Molenar et al. (1994) describe a sophisticated 
visual air quality simulation technique, incorporated into the 
WinHaze program developed by Air Resources Specialists, Inc., which 
combined various modeling systems under development for the past 20 
years to produce images that standardize non-pollution related 
effects on visibility so that perceptions of these images are not 
biased due to these other factors.
---------------------------------------------------------------------------

    Survey techniques were developed in conjunction with the Denver 
study and relied on citizen judgments of acceptable and unacceptable 
levels of visual air quality (Ely et al., 1991; EPA, 2005a, section 
6.2.6.2). The studies in Phoenix and British Columbia, and the pilot 
study in Washington, DC used survey approaches based on that used in 
Denver. This approach involves conducting a series of meetings with 
civic and community groups to elicit individual ratings of a number of 
images of well-known local vistas having varying levels of visual air 
quality. Participants are told that the results are intended to provide 
input on setting a visibility standard, and they are asked to base 
their judgments on three factors: (1) The standard is for an urban 
area, not a pristine national park area where the standards might be 
more strict; (2) standard violations should be at visual air quality 
levels considered to be unreasonable, objectionable, and unacceptable 
visually; and (3) judgments of standard violations should be based on 
visual air quality only, not on any health effects that some may 
perceive as being linked with poor visual air quality. The Denver 
visibility survey process produced the following findings: (1) 
Individuals' judgments of an images's visual air quality and whether 
the image should be considered to violate a visibility standard are 
highly correlated with the group average; (2) when participants judged 
duplicate slides, group averages of the first and second ratings were 
highly correlated; and (3) group averages of visual air quality ratings 
and ``standard violations'' were highly correlated. The strong 
relationship of standard violation judgments with the visual air 
quality ratings is cited as the best evidence available from this study 
for the validity of this approach as input to a standard setting 
process (Ely et al., 1991).
    The Denver visibility standard was established based on a 50 
percent acceptability criterion. That is, under this approach, the 
standard was identified as the light extinction level that divides the 
images into two groups: those found to be acceptable and those found to 
be unacceptable by a majority of study participants. In fact, when 
researchers evaluated all citizen judgments made on all the 
photographic images at this level and above as a single group, more 
than 85 percent of the participants found visibility impairment at and 
above the level of the selected standard to be unacceptable.
    Generally consistent results were found in the Phoenix study, which 
used simulated images from the WinHaze program. The study carefully 
selected participants to be demographically representative of the 
Phoenix population. The Phoenix survey demonstrates that the rating 
methodology developed for gathering citizen input for establishing the 
Denver visibility standard can be reliably transferred to another city 
while relying on updated imaging technology to simulate a range of 
visibility impairment levels. Similarly, the British Columbia study 
reinforces the conclusion that the methodology originally developed for 
the Denver standard setting process is a sound and effective one for 
obtaining public participation in a standard setting process (EPA, 
2005a, p. 6-22).
2. Need for Revision of the Current Secondary PM Standards for 
Visibility Protection
    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 PM-related 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 of the 
role of PM, especially fine particles, in visibility impairment from 
the last review. However, extensive new information

[[Page 2679]]

now available from visibility and fine particle monitoring networks has 
allowed for updated characterizations of visibility trends and current 
levels in urban areas, as well as Class I areas. As discussed above, 
these new data are a critical component of analyses that better 
characterize visibility impairment in urban areas and the relationships 
between visibility and PM2.5 concentrations, 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 focuses on visibility impairment 
primarily in urban areas. In so doing, consideration is first given 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 above, 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 ranged 
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 above, 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 observes that:
    (1) At concentrations at or near the level of the current 24-hour 
PM2.5 standard (65 [mu]g/m3), which equates to 
visual ranges roughly around 10 km, 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 [mu]g/m3, which equate to 
visual ranges generally above 20 km for the urban areas considered 
(EPA, 2005a, p. 7-6).
    (3) Visual air quality appears to be good in these images at 
PM2.5 concentrations generally below 20 [mu]g/m3, 
corresponding to visual ranges of approximately 25 to 35 km (EPA, 
2005a, p. 7-8).
    While being mindful of the limitations in using visual 
representations from a small number of areas as a basis for considering 
national visibility-based secondary standards, the Staff Paper 
nonetheless concludes 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 
in the following sections, the Staff Paper recommends 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, 2005a, 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, secondary 
PM2.5 standard to protect urban visibility (Henderson, 
2005a).\86\ Most Panel members considered such a standard to be a 
reasonable complement to the Regional Haze Rules that protect Class I 
areas.
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    \86\ A dissenting view was expressed in one Panel member's 
invididual review comments to the effect that any urban visibility 
standard should be voluntary and locally adopted (Henderson, 2005a).
---------------------------------------------------------------------------

    In considering whether the secondary PM standards should be revised 
to target PM-related visibility impairment primarily in urban areas, 
the Administrator has carefully considered the rationale and 
recommendation in the Staff Paper, the advice and recommendations from 
CASAC, and public comments to date on this issue. In so doing, the 
Administrator first recognizes that PM-related visibility impairment is 
principally related to fine particle levels, such that it is 
appropriate to focus in this review on the current secondary 
PM2.5 standards to provide such targeted protection. The 
Administrator also recognizes that visibility is most directly related 
to instantaneous levels of visual air quality, such that it is 
appropriate to focus on a standard with a short-term averaging time 
(e.g., 24-hours or less). Thus, the Administrator has considered 
whether the current 24-hour secondary PM2.5 standard should 
be revised to provide a requisite level of protection from visibility 
impairment, principally in urban areas, in conjunction with the 
regional haze program for protection of visual air quality in Class I 
areas. The Administrator observes that at concentrations at or near the 
level of the current 24-hour PM2.5 standard (65 [mu]g/
m3), corresponding to visual ranges of about 10 km, 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 notes the various State and local 
standards and programs that have been established 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 concludes 
that it is appropriate to revise the current 24-hour secondary 
PM2.5 standard to provide requisite protection from 
visibility impairment principally in urban 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, 2005a, section 6.2.3), the Staff Paper concludes 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

[[Page 2680]]

8 hours). Thus, the Staff Paper concludes 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 PM Panel members endorsed a PM2.5 
indicator for a secondary standard to address visibility impairment.
    The Administrator concurs with the EPA staff and CASAC 
recommendations, and concludes that PM2.5 should be retained 
as the indicator for fine particles as part of a secondary standard to 
address visibility protection. In the Administrator's view, 
PM2.5 is the appropriate indicator for any such standard, 
whether averaged over 24-hours or over a shorter, sub-daily time 
period.
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 
have been considered for a standard to address visibility impairment. 
Within this range, as noted above, clear and similarly strong 
correlations are found between visibility and 24-hour average 
PM2.5 concentrations in eastern and western areas, while 
somewhat stronger correlations are found with PM2.5 
concentrations averaged over a 4-hour time period. In general, 
correlations between PM2.5 concentrations and light 
extinction are 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., are considered. 
The Staff Paper concludes 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 recognizes 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 analysis discussed above are 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.\87\
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    \87\ Decisions as to which PM2.5 continuous monitors 
are providing data of sufficient quality to be used in a sub-daily 
visibility standard would follow protocols for approval of Federal 
equivalent methods (FEMs) that can provide data in at least hourly 
intervals, as proposed in the revisions to Part 53, published 
elsewhere in today's Federal Register.
---------------------------------------------------------------------------

    Most CASAC PM 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 by targeting the driest part of the day 
(Henderson, 2005a). 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 concludes 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 sub-daily averaging time would 
necessarily depend upon an expanded network of continuous 
PM2.5 mass monitors. While the Administrator agrees that 
broader deployment of continuous PM2.5 mass monitors is a 
desirable goal, working toward that goal does not depend upon nor 
provide a basis for setting a sub-daily standard. The Administrator 
believes that it is appropriate to evaluate averaging time in 
conjunction with reaching decisions on the form and level of a 
standard, as discussed below.
5. Elements of a Secondary PM2.5 Standard for Visibility 
Protection
    In considering PM2.5 standards that would provide 
requisite protection against PM-related impairment of visibility 
primarily in urban areas, the Administrator has taken into account the 
results of 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. 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 has also taken 
into account the same general factors that were considered in selecting 
an appropriate form for the 24-hour primary PM2.5 standard, 
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).
    In considering elements of a secondary PM2.5 standard, 
the Administrator has 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 concludes that 30 to 20 
[mu]g/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, 2005a, 
Figure 7-1).\88\ 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.
---------------------------------------------------------------------------

    \88\ The 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, 2005a, p. 
7-8).
---------------------------------------------------------------------------

    In considering a reasonable range of forms for a PM2.5 
standard within this range of levels, the Staff Paper concludes that a 
concentration-based percentile form is appropriate for the same reasons 
as discussed above in section II.F.1 (on the form of the 24-hour 
primary PM2.5 standard). The Staff Paper also concludes that 
the upper end of the range of concentration percentiles should be 
consistent with the percentile used for the primary standard, which is 
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

[[Page 2681]]

of the 20 percent worst day, as targeted in the regional haze program 
(EPA, 2005a, p. 7-11 to 12).
    In its letter to the Administrator (Henderson, 2005a), the CASAC PM 
Panel recognizes that it is difficult to select any specific level and 
form based on currently available information. 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 recommend considering a percentile 
within this range in conjunction with a level toward the upper end of 
the range recommended in the Staff Paper.\89\
---------------------------------------------------------------------------

    \89\ Some CASAC Panel members also recommend that such a 
standard be implemented in conjunction 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).
---------------------------------------------------------------------------

    Based on the above considerations, the Administrator believes that 
it is appropriate to first consider the level of protection that would 
be afforded by the suite of primary PM2.5 standards proposed 
today. The limited and uncertain evidence currently available for use 
in evaluating the appropriate level of protection suggests that a 
cautious approach is warranted in establishing a secondary standard. 
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 setting 
the secondary standard. Given this, it is appropriate to first evaluate 
the level of protection that the proposed primary standards would 
likely provide, and then determine whether the available evidence 
warrants 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 
has looked to information on the predicted percent of areas not likely 
to meet various alternative secondary and primary PM2.5 
standards (EPA, 2005a, Tables 7A-1 and 5B-1(a) \90\). In so doing, the 
Administrator observes 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 [mu]g/
m3, with a 98th percentile form, and an annual standard of 
15 [mu]g/m3) is somewhat higher (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 to 8 daylight 
hours, a level toward the upper end of the range recommended in the 
Staff Paper (e.g., up to 30 [mu]g/m3), and a form within the 
recommended range (e.g., around the 95th percentile) (24 percent). A 
similar comparison is seen in considering the predicted percentages of 
the population living in such areas.
---------------------------------------------------------------------------

    \90\ The 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.
---------------------------------------------------------------------------

    The Administrator provisionally concludes that revising the current 
secondary PM2.5 to be identical to the proposed suite of 
primary PM2.5 standards is a reasonable policy approach to 
addressing visibility protection primarily in urban areas. Such an 
approach would 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 that generally 
recommended by CASAC. Such an approach also takes into account 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 
concludes, subject to consideration of public comment, that a secondary 
standard with a different averaging time, level, or form is not 
warranted, because the available evidence does not support a decision 
to achieve a level of protection different from that provided by the 
current primary standards, and because no change in averaging time, 
level, or form appears needed to achieve a comparable level of 
protection.
    The Administrator believes that a secondary NAAQS 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 a national standard focused 
primarily on urban areas can be expected to improve visual air quality 
in surrounding non-urban areas as well, as would programs now being 
developed to address the requirements of the regional haze rule 
established for protection of visual air quality in Class I areas. 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 particularly highly valued by people living in 
those areas. Based on these considerations, and taking into account the 
observations, analyses, and recommendations discussed above, the 
Administrator proposes to revise the current secondary PM2.5 
standards by making them identical in all respects to the proposed 
suite of primary PM2.5 standards.
    As discussed above, most CASAC PM Panel members strongly supported 
a sub-daily (4- to 8-hour averaging time) PM2.5 standard. 
The Administrator places great importance on the advice of CASAC, and 
therefore solicits public comment on such a standard.

B. Other PM-Related Welfare Effects

    This section presents the rationale for the Administrator's 
proposed revision of the current secondary PM standards to address PM-
related effects other than visibility impairment, including vegetation 
and ecosystems, materials damage and soiling, and climate change. In 
considering the currently available evidence on each of these types of 
PM-related welfare effects, the Staff Paper notes that there is much 
information linking ambient PM to potentially adverse effects on 
materials and ecosystems and vegetation, 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 
concludes that the available evidence does not provide a sufficient 
basis for establishing distinct secondary standards for PM based on any 
of these effects alone.

[[Page 2682]]

    The Administrator has also addressed the question of whether 
reductions in PM likely to result from the current secondary PM 
standards, or from the range of proposed revisions to the primary PM 
standards, would provide requisite protection against any of these PM-
related welfare effects. As discussed below, these considerations 
include the latest scientific information characterizing the nature of 
these PM-related effects and judgments as to whether revision of the 
current secondary standards are appropriate based on that information.

1. Nature of Effects

    Particulate matter contributes to adverse effects on a number of 
welfare effects categories other than visibility impairment, including 
vegetation and 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 concludes that regardless of size fraction, particles 
containing nitrates and sulfates have the greatest potential for 
widespread environmental significance, while effects are also related 
to other chemical constituents found in ambient PM, such as trace 
metals and organics.\91\ The following characterizations of the nature 
of these welfare effects are based on the information contained in the 
Criteria Document and Staff Paper.
---------------------------------------------------------------------------

    \91\ The Staff Paper notes that some of these other components 
are regulated under separate statutory authorities, e.g., section 
112 of the CAA.
---------------------------------------------------------------------------

a. Effects on Vegetation and Ecosystems
    Potentially adverse PM-related effects on vegetation and ecosystems 
are principally associated with particulate nitrate and sulfate 
deposition. In characterizing such effects, it is important to 
recognize that nitrogen and sulfur are necessary and beneficial 
nutrients for most organisms that make up ecosystems, with optimal 
amounts of these nutrients varying across organisms, populations, 
communities, ecosystems and time scales. Therefore, it is impossible to 
generalize to all species in all circumstances as to the amount at 
which inputs of these nutrients or acidifying compounds become 
stressors. The Staff Paper recognizes that the public welfare benefits 
from the use of nitrogen (N) and sulfur (S) nutrients in fertilizers in 
managed agricultural and commercial forest settings. The focus of this 
review, therefore, is on identifying risks to sensitive species and 
ecosystems where unintentional additions of these atmospherically 
derived nutrient and acidifying compounds may be contributing to 
undesired change in the nation's ecosystems and resulting in adverse 
impacts on essential ecological attributes such as species shifts, loss 
of species richness and diversity, impacts on threatened and endangered 
species, and alteration of native fire cycles. In these cases, 
deposited particulate nitrate and sulfate are appropriately termed 
ecosystem ``stressors.''
i. Vegetation Effects
    At current ambient levels, risks to vegetation from short-term 
exposures to dry deposited particulate nitrate or sulfate are low. 
However, when found in acid or acidifying deposition, such particles do 
have the potential to cause direct foliar injury. Specifically, the 
responses of forest trees to acid precipitation (rain, snow) include 
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). Acid deposition with levels of acidity associated with the 
foliar effects described above are currently found in some locations in 
the eastern U.S. (EPA, 2003). Even higher concentrations of acidity can 
be present in occult deposition (e.g. fog, mist or clouds) which more 
frequently impacts higher elevations. Thus, the risks of foliar injury 
occurring from acid deposition in some areas of the eastern U.S. is 
high. However, based on currently available information, the 
contribution of particulate sulfates and nitrates to the total acidity 
found at these locations is not clear.
ii. Ecosystem Effects
    The N- and S-containing components of PM have been associated with 
a broad spectrum of terrestrial and aquatic ecosystem impacts that 
result from either the nutrient or acidifying characteristics of the 
deposited compounds.
    Reactive nitrogen (Nr) is the form of N that is available to 
support the growth of plants and microorganisms. Since the mid-1960's, 
Nr creation through natural terrestrial processes has been overtaken by 
Nr creation as a result of human processes, and is now accumulating in 
the environment on all spatial scales--local, regional and global. Some 
Nr emissions are transformed into ambient PM and deposited onto 
sensitive ecosystems. Some of the most significant detrimental effects 
associated with excess Nr deposition are those associated with a 
syndrome known as ``nitrogen saturation.'' These effects include: (1) 
Decreased productivity, increased mortality, and/or shifts in 
terrestrial plant community composition, often leading to decreased 
biodiversity in many natural habitats wherever atmospheric Nr 
deposition increases significantly and critical thresholds are 
exceeded; (2) leaching of excess nitrate and associated base cations 
from terrestrial soils into streams, lakes and rivers and mobilization 
of soil aluminum; and (3) alteration of ecosystem processes such as 
nutrient and energy cycles through changes in the functioning and 
species composition of beneficial soil organisms (Galloway and Cowling 
2002). Thus, through its effects on habitat suitability, genetic 
diversity, community dynamics and composition, nutrient status, energy 
and nutrient cycling, and frequency and intensity of natural 
disturbance regimes (fire), excess Nr deposition is having profound and 
adverse impact on the essential ecological attributes associated with 
terrestrial ecosystems. In the U.S., numerous forests now show severe 
symptoms of nitrogen saturation. For other forested locations, ongoing 
expansion in nearby urban areas will increase the potential for 
nitrogen saturation unless there are improved emission controls.
    Excess nutrient inputs into aquatic ecosystems (e.g., streams, 
rivers, lakes, estuaries or oceans) either from direct atmospheric 
deposition, surface runoff, or leaching from nitrogen saturated soils 
into ground or surface waters can contribute to conditions of severe 
water oxygen depletion (hypoxia); eutrophication and algae blooms; 
altered fish distributions, catches, and physiological states; loss of 
biodiversity; habitat degradation; and increases in the incidence of 
disease. Estuaries are among the most intensely fertilized systems on 
Earth.
    Reactive nitrogen moves from one environmental reservoir to another 
through a number of sequential environmental processes. Though strong 
correlation between the stressor and adverse environmental response 
exists in many locations, and N-addition studies have confirmed the 
relationship between stressor and response, the ability to determine 
the temporal and spatial distribution of environmental effects for a 
given input of Nr are extremely limited by the large uncertainties 
associated with the rates at which Nr cascades through and

[[Page 2683]]

accumulates in various environmental reservoirs.
    Acid and acidifying deposition is another significant source of 
stress to forest and aquatic ecosystems. It changes the chemical 
composition of soils by depleting the content of available plant 
nutrient cations such as calcium (Ca2+), increasing the 
mobility of aluminum (Al), and increasing the S and N content (Driscoll 
et al., 2001).
    Leaching of soil nutrients is often of major importance in cation 
cycles, and many forest ecosystems show a net loss of base cations. In 
sensitive forest soils, acid deposition leads to a shift in chemical 
speciation of Al from organic to inorganic forms that are toxic to 
terrestrial and aquatic biota, and increases inorganic Al mobilization 
and transport into surface waters. The toxic effect of Al on forest 
vegetation is attributed to its interference with plant uptake of 
essential nutrients, such as Ca and Mg. There are large variations in 
Al sensitivity among ecotypes, between and within species, due to 
differences in nutritional demands and physiological status, that are 
related to age and climate, and which change over time.
    Acid deposition has been firmly implicated as a causal factor in 
the decline of red spruce in high elevation sites in the Northeast. Red 
spruce is valued commercially, for recreation and aesthetics, and as 
habitat for unique and endangered species. Dieback of red spruce trees 
has also been observed in mixed hardwood-conifer stands at relatively 
low elevations in the western Adirondack Mountains, where inputs of 
acid deposition are high. Exposure to acidic mist or cloud water 
reduces foliar calcium levels in red spruce needles, leading to 
increased susceptibility to freezing (winter injury). There is also the 
strong possibility that acid deposition altering of foliar calcium 
levels leading to reduced cold tolerance is not unique to red spruce 
but has been demonstrated in many other northern temperate forest tree 
species including yellow birch, white spruce, red maple, eastern white 
pine, and sugar maple. Less sensitive forests throughout the U.S. are 
experiencing gradual losses of base cation nutrients, which in many 
cases will reduce the quality of forest nutrition in the future 
(National Science and Technology Council, 1998).
    Inputs of acid deposition to regions with base-poor soils have also 
resulted in the acidification of soil waters, shallow ground waters, 
streams, and lakes in a number of locations within the U.S. 
Acidification has marked effects on the trophic structure of surface 
waters. Decreases in pH and increases in Al concentrations contribute 
to declines in species richness and in the abundance of zooplankton, 
macroinvertebrates, and fish. Numerous studies have shown that 
decreases in pH result in decreases in fish species richness (the 
number of fish species in a water body) by eliminating acid-sensitive 
species including important recreational fishes plus ecologically 
important minnows that serve as forage for sport fishes.
    Though significant decreases in sulfur emissions have occurred in 
the U.S. and Europe in recent decades, these decreases have not been 
accompanied by equivalent declines in net acidity related to sulfate in 
precipitation, and may have, to varying degrees, been offset by steep 
declines in atmospheric base cation concentrations over the past 10 to 
20 years (Hedin et al., 1994; Driscoll et al. 2001). Projections made 
using an acidification model (PnET-BGC) \92\ indicate that full 
implementation of the 1990 CAA Amendments will not afford substantial 
chemical recovery at Hubbard Brook Experimental Forest and at many 
similar acid-sensitive locations (Driscoll et al., 2001). Model 
calculations indicate that the magnitude and rate of recovery from acid 
deposition in the northeastern U.S. are directly proportional to the 
magnitude of emissions reductions. Model evaluations of policy 
proposals calling for additional reductions in utility SO2 
and NOX emissions, year round emissions controls, and early 
implementation indicate greater success in facilitating the recovery of 
sensitive ecosystems (Driscoll et al., 2001).
---------------------------------------------------------------------------

    \92\ PnET-BGC is designed to simulate element cycling in forest 
and interconnected aquatic ecosystems. The model PnET is a simple, 
generalized, and well validated model that provides estimates of 
forest net primary productivity, nutrient uptake by vegetation, and 
water balances. Recently, PnEt was coupled with a soil model that 
simulates abiotic soil processes, resulting in a comprehensive 
forest-soil-water model, PnET-BGC (Driscoll et al., 2001).
---------------------------------------------------------------------------

    Driscoll et al. (2001) envision a recovery process that will 
involve two phases: chemical and biological. Initially, a decrease in 
acid deposition following emissions controls will facilitate a phase of 
chemical recovery in forest and aquatic ecosystems. Recovery time for 
this phase will vary widely across ecosystems and will be a function of 
a number of factors. In most cases, it seems likely that chemical 
recovery will require decades, even with additional controls on 
emissions. The second phase in ecosystem recovery is biological 
recovery, which can occur only if chemical recovery is sufficient to 
allow survival and reproduction of plants and animals. The time 
required for biological recovery is uncertain. For terrestrial 
ecosystems, it is likely to be at least decades after soil chemistry is 
restored because of the long life of tree species and the complex 
interactions of soil, roots, microbes, and soil biota. For aquatic 
systems, research suggests that stream macroinvertebrate populations 
may recover relatively rapidly (approximately 3 years), whereas lake 
populations of zooplankton are likely to recover more slowly 
(approximately 10 years) (Gunn and Mills, 1998). Some fish populations 
may recover in 5 to 10 years after the recovery of zooplankton 
populations, perhaps sooner with fish stocking (Driscoll et al., 2001).
iii. Ecosystem Exposure to PM Deposition
    In order to establish exposure-response profiles useful in 
ecological risk assessments, two types of monitoring networks need to 
be in place. First, a deposition network is needed that can track 
changes in deposition rates of PM stressors (nitrates/sulfates) 
occurring in sensitive or symptomatic areas/ecosystems. Secondly, a 
network or system of networks should be established that measures the 
response of key sensitive ecological indicators over time to changes in 
atmospheric deposition of PM stressors.
    Data from existing deposition networks in the U.S. demonstrate that 
N and S compounds are being deposited in amounts known to be sufficient 
to affect sensitive terrestrial and aquatic ecosystems over time. 
Though the percentages of N and S containing compounds in PM vary 
spatially and temporally, nitrates and sulfates make up a substantial 
portion of the chemical composition of PM. In the future, speciated 
data from these networks may allow better understanding of the specific 
components of total deposition that are most strongly influencing PM-
related ecological effects.
    At this time, however, there are only a few sites where long-term 
monitoring of sensitive indicators of ecosystem response to excess 
nitrogen and/or acidic and acidifying deposition is taking place within 
the U.S. Because the complexities of ecosystem response make 
predictions of the magnitude and timing of chemical and biotic recovery 
uncertain, it is important that this type of long-term monitoring 
network be continued, and that biological monitoring be enhanced to 
support future evaluations of the response of forested watersheds and 
surface waters to a host of research and regulatory issues related to 
nutrient and acid and acidifying deposition.

[[Page 2684]]

iv. Critical Loads
    The critical load (CL) has been defined as a ``quantitative 
estimate of an exposure to one or more pollutants below which 
significant harmful effects on specified sensitive elements of the 
environment do not occur according to present knowledge'' (Lokke et 
al., 1996). The concept is useful for estimating the amounts of 
pollutants that ecosystems can absorb on a sustained basis without 
experiencing measurable degradation. The estimation of ecosystem 
critical loads requires an understanding of how an ecosystem will 
respond to different loading rates in the long term and is a direct 
function of the level of sensitivity of the ecosystem to the pollutants 
in question and its ability to ameliorate pollutant stress.
    The CL approach is very data-intensive, and, at the present time, 
there is a paucity of ecosystem-level data for most sites. However, for 
a limited number of areas which already have a long-term record of 
ecosystem monitoring, (e.g., Rocky Mountain National Park in Colorado 
and the Lye Brook Wilderness in Vermont), Federal Land Managers may be 
able to develop site specific CLs. More specifically, with respect to 
PM deposition, there are insufficient data for the vast majority of 
U.S. ecosystems that differentiate the PM contribution to total N or S 
deposition to allow for practical application of this approach as a 
basis for developing national standards to protect sensitive U.S. 
ecosystems from adverse effects related to PM deposition. Though 
atmospheric sources of Nr and acidifying compounds, including ambient 
PM, are clearly contributing to the overall excess load or burden 
entering ecosystems annually, insufficient data are available at this 
time to quantify the contribution of ambient PM to total Nr or acid 
deposition as its role varies both temporally and spatially along with 
a number of other factors. Thus, at the present time, a CL could not be 
developed that would address the portion of the total N or S input that 
is contributed by ambient PM.
b. Effects on Materials Damage and Soiling
    As discussed in the Staff Paper, the effects of the deposition of 
atmospheric pollution, including ambient PM, on materials are related 
to both physical damage and impaired aesthetic qualities. The 
deposition of PM (especially sulfates and nitrates) can physically 
affect materials, adding to the effects of natural weathering 
processes, by potentially promoting or accelerating the corrosion of 
metals, by degrading paints, and by deteriorating building materials 
such as concrete and limestone. As noted in the last review, only 
chemically active fine-mode or hygroscopic coarse-mode particles 
contribute to these physical effects. In addition, the deposition of 
ambient PM can reduce the aesthetic appeal of buildings and culturally 
important articles through soiling. Particles consisting primarily of 
carbonaceous compounds cause soiling of commonly used building 
materials and culturally important items such as statues and works of 
art. Available data indicate that particle-related soiling can result 
in increased cleaning frequency and repainting, and may reduce the 
useful life of the soiled materials. However, to date, no quantitative 
relationships between particle characteristics (e.g., concentrations, 
particle size, and chemical composition) and the frequency of cleaning 
or repainting have been established. Thus, the Administrator concludes 
that PM effects on materials can play no quantitative role in 
considering whether any revisions of the secondary PM standards are 
appropriate at this time.
c. Effects on Climate
    As discussed in the Staff Paper, atmospheric particles can alter 
the earth's energy balance by both scattering and absorbing radiation 
transmitted through the earth's atmosphere. Most components of ambient 
PM (especially sulfates) scatter and reflect incoming solar radiation 
back into space, thus tending to have a cooling effect on climate. In 
contrast, some components of ambient PM (especially black carbon) 
absorb incoming solar radiation or outgoing terrestrial radiation, thus 
tending to have a warming effect on climate. Other impacts of 
atmospheric particles are associated with their role in affecting the 
radiative properties of clouds, through changes in the number and size 
distribution of cloud droplets (which can have an effect on the climate 
in either direction), and by altering the amount of ultraviolet solar 
radiation (especially UV-B) penetrating through the atmosphere to 
ground level, where it can exert a variety of effects on human health, 
plant and animal biota, and other environmental components.
    The available information, however, provides no basis for 
estimating how localized changes in the temporal, spatial, and 
composition patterns of ambient PM likely to occur as a result of 
expected future emissions of particles and their precursor gases across 
the U.S., would affect local, regional, or global changes in climate or 
UV-B radiation penetration. Even the direction of such effects on a 
local scale remains uncertain. Moreover, similar concentrations of 
different particle components can produce opposite net effects, 
depending on other atmospheric parameters such as humidity. The 
Administrator thus concludes that, given this uncertainty, the 
potential indirect effects of ambient PM on public health and welfare, 
secondary to potential PM-related changes in climate and UV-B 
radiation, can play no quantitative role in considering whether any 
revisions of the primary or secondary PM standards are appropriate at 
this time.
2. Need for Revision of Current Secondary PM Standards To Address Other 
PM-Related Welfare Effects
    In considering the currently available evidence on each type of PM-
related welfare effects discussed above, the Administrator notes that 
there is much information linking the S- and N-containing components of 
ambient PM to potentially adverse effects on ecosystems and vegetation, 
materials damage and soiling, and on climatic and radiative processes. 
However, after reviewing the extent of relevant studies and other 
information provided 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 concurs with conclusions 
reached in the Staff Paper and by CASAC (Henderson, 2005a) that the 
available data do not provide a sufficient basis for establishing 
separate and 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 coarse-fraction PM, the 
Administrator provisionally concludes that sufficient information is 
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 agrees that it is 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

[[Page 2685]]

materials damage and soiling. The Administrator also agrees with the 
Staff Paper that the available information does not provide a 
sufficient basis for the development of distinct national 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 is intended to 
provide an equivalent degree of protection to the current 
PM10 standards in areas where the proposed 
PM10-2.5 indicator applies (which tend to be more densely 
populated areas where materials damage would be of greater concern), 
the Administrator believes 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 believes, as noted above, 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 therefore proposes 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.

C. Proposed 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 to date, the Administrator proposes to revise the current 
secondary PM2.5 and PM10 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. In 
recognition of an alternative view expressed by most members of the 
CASAC PM Panel, the Administrator also solicits comments on a sub-daily 
(4- to 8-hour averaging time) PM2.5 standard to address 
visibility impairment, within the range of 20 to 30 [mu]g/m3 
and with a form within the range of the 92nd to 98th percentile. Based 
on the comments received and the accompanying rationale, the 
Administrator may adopt other standards within the range of 
alternatives identified above in lieu of the standards he is proposing 
today.

V. Interpretation of the NAAQS for PM

A. Proposed Amendments to Appendix N--Interpretation of the National 
Ambient Air Quality Standards for PM2.5

    The EPA is proposing to revise the data handling procedures for the 
annual and 24-hour primary PM2.5 standards in appendix N to 
40 CFR part 50. The proposed amendments to appendix N would detail the 
computations necessary for determining when the proposed primary and 
secondary PM2.5 national ambient air quality standards 
(NAAQS) are met. The proposed amendments also would address data 
reporting, monitoring considerations, and rounding conventions. Key 
elements of the proposed revisions to appendix N are summarized below 
in sections V.A.1 through V.A.5 of this preamble.
1. General
    Several new definitions would be added to section 1.0 and utilized 
throughout the appendix, most notably ones for ``design values''. Also, 
the 24-hour time would be clarified as representing ``local standard 
(word inserted) time''. This proposal reflects EPA's previous intent as 
well as majority practice, and also avoids ambiguity since local clock 
time varies according to daylight savings periods.
2. PM2.5 Monitoring and Data Reporting Considerations
    Two new sections would be added to appendix N to more specifically 
stipulate and highlight monitoring and data considerations. New section 
2.0 would include statistical requirements for spatial averaging (which 
is part of the form of the current and proposed annual standard for 
PM2.5). As explained in section II.F.2 above, we are 
proposing to tighten the constraints on use of spatial averaging to 
reflect enhanced knowledge of typical monitor correlation coefficients 
in metropolitan areas. As also set out in section II.F.2, the 
Administrator is further soliciting comment on the other staff-
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.
    New section 3.0 would codify 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 would also note the 
process for assimilating monitored concentration data from collocated 
instruments into a 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 
would enhance the opportunity for sites to meet data completeness 
requirements. This proposed language likewise would codify existing 
practice, since the technique was previously documented in guidance 
documentation and implemented as EPA standard operating procedure.
3. PM2.5 Computations and Data Handling Conventions
    The EPA is proposing a spatially-averaged annual mean as the form 
of the annual PM2.5 standard and a 98th percentile 
concentration as the form of the 24-hour PM2.5 standard. 
Although no actual computational change is proposed for a spatially-
averaged annual mean, the proposed Appendix N now differentiates, in 
language and formulae, between a spatial average of more than one site 
and a spatial average of only one site. The intent of this change is to 
alleviate confusion caused by the current ``catch-all'' generic 
reference. The proposed revisions to appendix N would identify the 
NAAQS metrics and explain 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).
    With regard to the annual PM2.5 standard, we are 
proposing to retain current data capture requirements for the annual 
standard with two exceptions. Current appendix N has reduced data 
capture requirements for years that exceed 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) is 
currently considered sufficient in those instances where the annual 
mean exceeded the NAAQS level. See existing Part 50 App.

[[Page 2686]]

N 2.1(b). The EPA is proposing 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 EPA solicits 
comments on this proposed change.
    A second proposed change in the data completeness requirements 
would incorporate data substitution logic for situations where the 
proposed 11 sample per quarter minimum is not met. Consistent with 
existing guidance and practice (implementing current App. N 2.1(c)), 
EPA proposes to incorporate the following requirement into appendix N: 
a quarter with less than 11 samples would be complete and valid if, by 
substituting a 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 levels of the standard. The EPA proposes to implement this 
procedure for making comparisons to the NAAQS and not to permanently 
alter the reported data. The EPA considers this a very conservative 
means of inputing data (and increasing the opportunities for using 
monitoring data that otherwise are valid), but solicits comment on the 
proposed approach.
    With regard to the 24-hour PM2.5 standard, the proposed 
revisions to appendix N would include a special formula (Equation 6 in 
the proposed rule) 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 (``Guideline on 
Data Handling Conventions for the PM NAAQS'', April 1999) but was 
utilized, where appropriate, in official OAQPS design value 
calculations. Seasonal sampling has traditionally been implemented in 
periods that do not divide months; this criterion is explicitly stated 
in the proposed amendments.
    The proposed revisions to appendix N would also incorporate 
language explicitly stating that 98th percentiles (for both regular and 
seasonal sampling schedules) is to be based on the applicable number of 
samples rather than the actual number of samples. Both annual 98th 
percentile equations (proposed Equations 5 and 6) would now reflect 
this approach. To accommodate seasonal sampling, the calculation of 
``annual applicable number of samples'' would be changed from the sum 
of the ``quarterly applicable number of samples'' to a sum of the 
``monthly applicable number of samples''. The EPA welcomes comment on 
the ``applicable number of samples'' concept and calculation.
    To simplify the regulatory language, another proposed change to 
appendix N would eliminate the equation computational examples. The EPA 
will provide extensive computational examples in forthcoming guidance 
documents.
4. Secondary Standard
    The EPA is proposing that the secondary standards for 
PM2.5 be the same as the primary standards. However, the 
Administrator is soliciting comment on the alternative of a distinct 4-
hour secondary standard for visibility protection with a form of an 
annual percentile, in the range 92nd to 98th, for a 12 p.m. to 4 p.m. 
local standard time daily average, averaged over 3 years. The same 
basic data handling approach as used for the 24-hour 98th percentile 
primary standard would also be utilized for a 4-hour percentile-based 
secondary standard (should EPA ultimately adopt such a standard). For 
example, 75 percent of the hours in the averaging time (i.e., 3 hours) 
would be required to produce a valid daily measurement. Also, 75 
percent capture of sample days in a quarter would always make a 
complete quarter and four complete quarters, a complete year. Reduced 
capture (i.e., as little as one sample per year) would also suffice for 
high concentration years or 3-year periods. However, the percentile 
computational variation permitted for seasonal sampling for the 24-hour 
98th percentile would not be needed for the 4-hour 95th percentile 
since the predominant (if not only) monitoring instrument used for this 
standard would be a continuous PM2.5 sampler and EPA expects 
these continuous instruments to operate throughout the entire year. For 
this same reason, distinction between applicable number of samples and 
actual number of samples would not be necessary.
5. Conforming Revisions
    Terminology and data handling procedures associated with 
exceptional events would be revised to conform to rules which EPA plans 
to propose in the near future 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) (PL 109-59). At this time, EPA is proposing to replace 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 proposal would make only a 
semantic change to existing Appendix N, EPA believes the proposal is 
consistent with section 6013 (b) (4) of SAFETEA-LU, which provides that 
EPA shall 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.)

B. Proposed Appendix P--Interpretation of the National Ambient Air 
Quality Standards for PM10-2.5

    The EPA is proposing 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. The proposed appendix P would detail the 
computations necessary for determining when the proposed 
PM10-2.5 NAAQS is met. The proposed appendix also would 
address data reporting, sampling frequency considerations, and rounding 
conventions. The protocols described in proposed appendix P would 
mirror the general and 24-hour specific protocols proposed for the 
PM2.5 NAAQS in appendix N of 40 CFR part 50. Key elements of 
the proposed appendix P are summarized below in sections V.B.1 through 
V.B.3 of this preamble.
1. General
    Terms utilized throughout the proposed appendix would be defined in 
section 1.0.
2. PM2.5 Data Reporting Considerations
    Section 2.0 of the proposed appendix P would specify the input data 
to be used in the NAAQS computations. The section would address raw 
data reporting and raw data time interval aggregation (i.e., report/
calculate to one decimal place, truncate additional digits). Section 
2.0 would also note the process for assimilating monitored 
concentration data into a ``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 or Federal equivalent method monitor data whenever valid data 
are not generated by the primary monitor. This procedure would enhance 
the opportunity for sites to meet data completeness requirements.
3. PM10-2.5 Computations and Data Handling Conventions
    The EPA is proposing a site-based 98th percentile concentration as 
the form of the 24-hour PM2.5. The proposed appendix P would 
explain data handling conventions and computations for the 24-hour 
primary (and secondary) PM10-2.5 standards in section 3.1; 
data

[[Page 2687]]

rounding conventions in section 3.2; and sampling frequency 
considerations in section 3.3. The formulas used for calculating the 
24-hour NAAQS metric would be specified in section 3.4.
    The proposed appendix would include a special formula (Equation 2) 
for use in computing annual 98th percentile values when a site operates 
on an approved seasonal sampling schedule. The proposed appendix P also 
would incorporate language explicitly stating that 98th percentiles 
(for both regular and seasonal sampling schedules) is to be based on 
the applicable number of samples rather than actual number of samples. 
Both annual 98th percentile equations (Equations 1 and 2 of proposed 
appendix P) would reflect this approach. This approach parallels that 
proposed in appendix N for PM2.5 described in V.A.3. above, 
and is based on the same considerations.
4. Exceptional Events
    The EPA plans to use the terminology and adopt the data handling 
procedures associated with exceptional events consistent with rules 
which would implement the recent amendment to CAA section 319 discussed 
in section V.A.5 above. The EPA expects to propose such rules in the 
near future. In the present proposal, the term ``exceptional events'' 
is used, consistent with the term used in the recent amendment as well 
as the term EPA proposes to use in the parallel provision in Appendix N 
(see section V.A.5).

VI. Reference Methods for the Determination of Particulate Matter As 
PM2.5 and PM10-2.5

A. Proposed Appendix O: Reference Method for the Determination of 
Coarse Particulate Matter (as PM10-2.5) in the Atmosphere

1. Purpose of the New Reference Method
    The EPA is proposing a new Federal reference method (FRM) for the 
measurement of coarse particles (as PM10-2.5) in ambient air 
for the purpose of determining attainment of the proposed new 
PM10-2.5 standards. The FRM would also serve as the standard 
of comparison for determining the adequacy of alternative 
``equivalent'' methods for use in lieu of the FRM. The method is 
described in a proposed new appendix O to 40 CFR part 50, where it 
would join other FRM (or measurement principles) specified for the 
other criteria pollutants.
2. Rationale for Selection of the New Reference Method
    The proposed FRM for measuring PM10-2.5 is based on the 
combination of two conventional low-volume methods, one for measuring 
PM10 and the other for measuring PM2.5, and 
determining the PM10-2.5 measurement by subtracting the 
PM2.5 measurement from the concurrent PM10 
measurement. The proposed PM2.5 measurement method is 
identical to the PM2.5 FRM currently specified in appendix L 
to 40 CFR part 50, and the proposed PM10 measurement method 
is similar, utilizing the same sampler but without the PM2.5 
particle size separator. (Both samplers use identical PM10 
size-selective inlets.) Thus, this PM10-2.5 FRM is based on 
the same aerodynamic particle size separation and filter-based, 
gravimetric technology that is also the basis for FRMs for 
PM2.5 and (in a somewhat less rigorously specified form) for 
PM10.
    In selecting the FRM methodology, EPA's primary considerations were 
the ability of the method to provide: (1) Credible and reliable 
measurements of PM10-2.5; (2) reliable assessment of the 
quality of monitoring data; and (3) a credible and practical reference 
standard of comparison for candidate alternative measurement methods to 
determine their qualification as equivalent methods. In concept, a 
direct method for measuring PM10-2.5 would seem to be 
desirable for the FRM, rather than the indirect method proposed. The 
EPA tested and evaluated various types of direct measurement technology 
(Vanderpool et al., 2005), including other conventional, filter-based 
gravimetric methods. The results of these tests and other evaluations 
indicate that none of the available methods or alternative technologies 
was more suitable as a reference method for PM10-2.5 than 
the method proposed.
    Perhaps the most fundamental requirement for the 
PM10-2.5 FRM is the capability of the method to measure the 
subject particulate matter with a high degree of fidelity and 
faithfulness to the definition of PM10-2.5. In proposed 
appendix O, PM10-2.5 is defined as the mass concentration of 
ambient particles in the coarse-mode fraction of PM10, 
specifically the (nominal) size range of 2.5 to 10 micrometers. The 
lower and upper limits of this size range are formally defined by the 
existing FRMs for PM2.5 (40 CFR part 50, appendix L) and for 
PM10 (40 CFR part 50, appendix J). In both cases, the 
particle sizes are defined in terms of aerodynamic size, not actual 
physical size. Further, the particle size limits are not simple step 
functions but instead are defined by the corresponding PM2.5 
and PM10 measurement methodologies, which have inherent size 
fractionation curves with characteristic shapes and cutoff sharpness. 
The proposed PM10-2.5 FRM would utilize these same 
measurement methodologies to determine the PM10-2.5 
concentration as the difference between separate PM10 and 
PM2.5 measurements, thereby preserving and replicating the 
same particular PM10 and PM2.5 aerodynamic 
particle size limit characteristics previously established by the 
PM10 and PM2.5 FRMs.
    Also, the proposed PM10-2.5 FRM utilizes the same 
conventional integrated-sample, filter-collection, and mass-based 
gravimetric measurement technology that has been chosen for all 
previous FRM for the various formal particulate matter indicators. This 
well-established and reliable technology provides a high degree of 
credibility in the PM10-2.5 measurements, derived from its 
gravimetric basis and its extensive track record from wide utilization 
over many years in many government monitoring networks. Further, it 
allows for maximum compatibility and comparability among new and 
existing PM10-2.5, PM10, and PM2.5 
data sets and thus to much of the health effects data used as a basis 
for the proposed NAAQS. No costly studies are needed to assess the 
impact, effect, or degree of comparability of a new or changed 
measurement technology relative to previously acquired measurement 
data. Extensive wind tunnel tests have shown that the inlet, used on 
both the PM2.5 and PM10 samplers, is capable of 
aspirating large particles efficiently, even at high wind speeds. The 
presence of PM2.5 aerosols on the PM10 sample 
collection filter increases the adhesion of larger particles to the 
filter to minimize losses of large particles from the PM10 
filters during handling and transport. Such losses can be a problem 
with filter samples collected with a virtual impactor-type sampler, 
where the PM2.5 aerosols are not present on the 
PM10-2.5 filter in sufficient quantities to eliminate loss 
of coarse mode particles.
    An inherent advantage of a difference method is that some 
(additive) biases may be eliminated or substantially reduced by the 
subtraction. In the proposed PM10-2.5 FRM, the two samplers 
and their operational procedures are very closely matched (except for 
the particle size separator) to take maximum advantage of this feature, 
which helps to compensate for the additional variability resulting from 
dual measurement systems. Although a difference method could produce 
negative measurements on occasion,

[[Page 2688]]

considerable field testing of the method indicates that negative 
readings are rare, due in substantial part to the excellent precision 
of the base methods (Vanderpool et al., 2005). Moreover, measured 
negative PM10-2.5 concentrations, if observed, would likely 
occur only at low concentrations near the detection limit of the method 
and would thus be unlikely to adversely affect the accuracy of 
PM10-2.5 attainment decisions based on the proposed 24-hour 
NAAQS.
    The proposed method also has a number of secondary advantages. The 
samplers and operational procedures of the proposed FRM are similar to 
those of the PM2.5 FRM and will be familiar to most State 
monitoring agencies. In fact, the nature of the method allows for the 
possibility of readily and economically obtaining PM10-2.5 
samplers (actually sampler pairs) by reconfiguring existing 
PM2.5 samplers. PM10-2.5 sampler pairs based on 
currently designated PM2.5 FRM samplers could be quickly 
designated by EPA as PM10-2.5 FRM, as no additional 
qualification testing would be required. Existing PM2.5 FRM 
samplers can be easily reconfigured as PM10-2.5 FRM sampler 
pairs by converting some of them to the special PM10 
(PM10c) samplers by simply replacing the WINS impactor with 
the specified straight downtube adaptor. Thus, the PM10-2.5 
method could be rapidly and economically implemented into new or 
existing monitoring networks to begin collection of PM10-2.5 
monitoring data expeditiously, with minimal requirements for operator 
retraining or pilot operational periods.
    The proposed FRM provides readily accessible aerosol samples for 
subsequent chemical analyses, and the sampler's design allows use of a 
wide variety of filter materials including Teflon, quartz, nylon, and 
polycarbonate. Compared to PM2.5, the chemical composition 
of coarse-mode aerosols has not yet been extensively evaluated. The 
ability of the proposed FRM to provide speciated analyses of coarse 
aerosol samples would be an important tool for the States during 
development of effective implementation plans.
    In developing this new FRM for PM10-2.5, EPA staff 
consulted with a number of individuals and groups in the monitoring 
community, including instrument manufacturers, academics, consultants, 
and experts in State and local agencies. The approach and key 
specifications of the method were submitted for peer review to the 
Clean Air Scientific Advisory Committee (CASAC) Ambient Air Monitoring 
and Methods Subcommittee, which held public meetings to discuss methods 
and related monitoring issues on July 22, 2004 and September 21 and 22, 
2005. Comments on the proposed method were provided orally and in 
writing by Subcommittee members and by interested public entities. In a 
letter dated November 30, 2005 (Henderson, 2005c) forwarded by the 
CASAC to the Administrator, the CASAC provided its peer review 
consensus report stating that ``in general, the CASAC agrees that there 
are several important scientific or operational strengths of the 
proposed difference method PM10-2.5 to be used as the FRM, 
while noting that there are several prominent weaknesses as well. 
Despite these weaknesses, no other better, currently available 
candidate FRM method has been identified.'' The CASAC report noted that 
``A majority of the Subcommittee members expressed the opinion that the 
demonstrated data quality of the PM10-2.5 difference method 
and its documented value in correlations with health effects data 
support its being proposed as the PM coarse FRM''. However, the CASAC 
also indicated that the proposed FRM should not be intended for 
extensive implementation in national monitoring networks. Instead, it 
should be used primarily as a benchmark for evaluating the performance 
of continuous as well as other direct-measuring, filter-based, 
integrated methods and determining their acceptability for use in 
routine monitoring of PM10-2.5. As explained more fully 
below, this is the approach we intend to adopt for the national 
monitoring network.
3. Consideration of Other Methods for the Federal Reference Method
    Other measurement technologies considered for the FRM include a 
variety of alternative integrated-sample, filter-based methods as well 
as various automated methods providing continuous or semi-continuous 
measurements of PM10-2.5. One methodology that warranted 
particular consideration is integrated, filter sampling using a virtual 
impactor particle size separator (also known as a dichotomous 
fractionator). This technology provides for measuring 
PM10-2.5 more directly than the proposed difference method 
and also provides associated PM2.5 measurements, as well as 
PM10 measurements by addition. Like the proposed difference 
method, dichotomous samplers have been used in health studies that 
supported the basis for both the PM2.5 and proposed 
PM10-2.5 NAAQS. A dichotomous sampler can utilize the same 
PM10 sampler inlet, the same types of filters and filter 
processing, and similar quality assurance procedures as the proposed 
method. It also has a very important advantage in providing 
PM10-2.5 filter samples for chemical analysis. Such 
``speciation'' analysis is a critical tool used by States for 
developing effective PM10-2.5 control strategies. Speciated 
PM2.5 and PM10-2.5 data have supported 
epidemiological studies used to develop associations between exposure 
to ambient particulate matter and increased mortality and morbidity 
(Dockery, et al., 1993, Schwartz, 1994). Collected speciated samples 
from dichotomous samplers can also be used to conduct toxicological 
studies of the adverse health effects of PM exposure as a function of 
particle size (Demokritou, et al., 2003).
    However, some aspects of virtual impactor technology raise concerns 
regarding the technology's current suitability for use as a 
PM10-2.5 reference method. Various versions of virtual 
impactors have been designed and used, but their particle size 
separation characteristics have not been fully evaluated and 
independently characterized as extensively as those of the proposed 
method, resulting in considerable uncertainty about their performance 
relative to the conventional low-volume PM2.5 and 
PM10 FRMs. There is also concern about the impact and 
potential need to compensate for some inherent fine particle 
contamination on the PM10-2.5 filter. For example, for a 
virtual impactor which employs a 10 to 1 total flow rate to coarse flow 
rate ratio, 10 percent of the fine particles deposit on the coarse 
filter. Following each sampling event, the presence of these fine 
particles must be accounted for during subsequent calculation of the 
PM10-2.5 mass concentration. Depending upon the analyte of 
interest, the collected mass of the analyte, and the method detection 
limit of the analytical technique for that analyte, proper compensation 
for fine particle contamination will also need to be made when 
conducting speciation analysis of the coarse channel filter. Allen et 
al. (1999) also reported the tendency for some fraction (up to 16 
percent) of coarse mode particles to penetrate to the fine channel 
filter and thus positively bias calculated PM2.5 mass 
concentrations as well as concentrations of specific analytes. Because 
the level of coarse particle contamination depends upon the size 
distribution of the sampled aerosol and the physical nature of the 
coarse particles, this contamination cannot be accurately predicted and 
thus cannot be

[[Page 2689]]

accounted for during subsequent calculations.
    Loss of particles within virtual impactors is also well documented 
(Forney et al., 1982, Chen et al., 1985, Loo and Cork, 1988, Li and 
Lundgren, 1997, Allen, et al., 1999, Kim and Lee, 2000) and can 
substantially bias measured mass and species concentrations. As 
reported by Loo and Cork (1988), losses up to 50 percent have been 
reported during laboratory calibration of various virtual impactor 
designs when using liquid calibration aerosols. Moreover, these losses 
cannot be predicted and are very sensitive to virtual impactor geometry 
and component misalignment. Unlike conventional impactors where 
internal particle loss can be readily minimized, the design of virtual 
impactors must be optimized to ensure that particle loss is 
sufficiently low to enable accurate mass and species measurements 
during field use.
    In the proposed difference method, the high concentration of fine 
particles on the PM10 filter provides additional adhesive 
force for retaining large particles to the filter's surface. In the 
dichotomous sampler, however, the low concentration of fine particles 
on the coarse channel filter results in a significantly reduced 
adhesive force. If inertial forces (applied to the filter during its 
post-sampling handling and transport) are greater than the adhesive 
force, then coarse particles will be dislodged from the coarse channel 
filter and not be subsequently quantified. Depending upon the virtual 
impactor design, the nature of the collected aerosol, and the magnitude 
of the applied inertial force, large particle losses up to 50 percent 
have been documented (Dzubay and Barbour, 1983, Spengler and Thurston, 
1983). As in the case of coarse particle intrusion into the fine 
channel, the magnitude of this measurement bias is variable and cannot 
be accurately predicted nor compensated for.
    The CASAC, in their peer review report (Hendersen, 2005c) supports 
``* * * the possibility of specifying more than one FRM for 
PM10-2.5 (as it did for PM10) , if one or more of 
the current or evolving dichotomous sampler designs shows reasonable 
agreement with the difference method (assuming filter-handling 
procedures can be developed to minimize losses of coarse-only particles 
prior to weighing).'' We agree that the filter-handling procedures need 
to be investigated in addition to other issues described above. 
Therefore, at this point we believe the proposed FRM, based on the 
difference method, offers less uncertainty in PM10-2.5 
measurements and is the more prudent choice for the reference method. 
However, CASAC and EPA are both interested in utilizing dichotomous 
samplers in support of other monitoring objectives, such as providing 
samples for chemical speciation analysis, once a number of issues are 
worked through. Therefore, the Agency wishes to solicit public comment 
regarding consideration of a PM10-2.5 reference method or 
equivalent method based on the use of the virtual impactors to 
aerodynamically separate fine mode aerosols from coarse mode aerosols.
    Concerns have been expressed to EPA regarding the fact that the 
size separation devices of both the PM2.5 and 
PM10 FRMs, which are the basis of the proposed difference-
based PM10-2.5 FRM, have inherent size fractionation curves 
with characteristic shapes and cutoff sharpness rather than creating a 
perfectly sharp cutpoint at a specific aerodynamic particle size. For 
example, a portion of all ambient particles larger than 10 micrometers 
are included in the PM10-2.5 sample, while some particles 
smaller than 10 micrometers are not. A larger effect on measured 
PM10-2.5 will occur in environments with high concentrations 
of particles above 10 micrometers than in environments with low 
concentrations.
    Some commenters who have been concerned about this aspect of the 
PM2.5 and PM10 FRMs have supported the adoption 
of a PM10-2.5 FRM that would directly measure the coarse 
fraction of particles. We invite comment on this topic, in the context 
of today's proposal for a PM10-2.5 NAAQS and a FRM that 
would employ both PM2.5 and PM10 size separators.
4. Consideration of Automated Methods for the Federal Reference Method
    Other measurement technologies considered for the FRM included 
various types of automated analyzer methods that provide continuous or 
semi-continuous measurements of PM10-2.5. Such methods are 
particularly desirable for use in PM10-2.5 monitoring 
networks because they potentially offer substantially lower operational 
and maintenance costs, hourly averages or other short-term measurements 
in addition to 24-hour averages, and nearly real-time electronic, 
remote reporting of measurement data. However, recent field testing of 
many of these instruments (Vanderpool et al., 2005) indicated that none 
can yet achieve performance commensurate to that of the proposed 
method. The technologies employed by these methods usually represent a 
substantial, if not radical, departure from the well-characterized, 
conventional filter-collection and gravimetric determination. This 
departure raises inevitable questions of representativeness of particle 
size discrimination, treatment of volatile components, variability with 
differing site and climatic conditions, and the degree of comparability 
to conventionally obtained measurements. Also, since EPA is proposing a 
daily standard for PM10-2.5, hourly measurements are not 
required to support such a standard, although they would be of value to 
more closely investigate impacts of sources and exceptional events.
    Most, if not all, of these automated measurement technologies are 
proprietary. While that alone is not sufficient reason to preclude 
their consideration as FRM or as a ``reference measurement principle,'' 
it would be in the best interest of all stakeholders if multiple 
manufacturers could compete for this market. Adoption of the proposed 
FRM along with reasonable qualification requirements for equivalent 
methods leaves a fair and level playing field for any manufacturer to 
either produce the specified FRM samplers or to pursue the development 
and EPA approval of innovative new methods and technologies to strive 
for competitive marketing advantages.
5. Use of the Proposed Federal Reference Method
    The EPA acknowledges that the proposed FRM is quite labor-intensive 
and has other disadvantages that make it less than ideal for routine 
use in large monitoring networks. At the same time, as just described, 
alternative, automated methods are under continuing research and 
development, and some may soon demonstrate adequate performance and 
comparability to the FRM for use in monitoring networks. Accordingly, 
and consistent with the recommendations of the CASAC (Hendersen, 
2005c), EPA is providing for the possible designation of alternative 
methods as equivalent methods for PM10-2.5, as set forth in 
proposed amendments to 40 CFR part 53 published elsewhere in this 
Federal Register. Under these proposed equivalent method provisions, 
EPA anticipates that alternative methods--particularly filter based, 
virtual-impactor samplers as well as self-contained, automated 
analyzers--can be designated as equivalent methods. The dichotomous 
samplers could potentially lead to better speciation data, while 
automated equivalent methods would ease the potential 
PM10-2.5 monitoring burdens of monitoring agencies and would 
potentially provide substantial

[[Page 2690]]

monitoring advantages such as reduced operational cost, availability of 
1-hour (or other less-than-24-hour) average concentration measurements, 
and near real-time telemetered monitoring data. As explained in the 
preamble to the proposed Part 58 rule, if such automated methods are 
designated as equivalent, they would likely be used predominantly for 
much of the required PM10-2.5 network monitoring. The new 
PM10-2.5 FRM would thus be used primarily as the reference 
standard for designating qualified equivalent methods and for quality 
assurance activities, but used only minimally for routine network 
monitoring.
    Encouraging the further development of automated analyzers by 
providing for their designation as equivalent methods for 
PM10-2.5 could eventually lead to commercial, direct-reading 
instruments that would meet multiple monitoring objectives better than 
the FRM proposed today. In that event, the Agency may consider adopting 
such an automated method for the FRM (or as a ``measurement principle 
and calibration procedure'') under the provisions of 40 CFR 53.16, 
``Supersession of reference methods.''
6. Relationship of Proposed FRM to SAFETEA-LU Requirements
    Section 6012 of the SAFETEA-LU in part requires the Administrator, 
within two years, 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.'' We believe that our proposed action today is consistent 
with the goals of the new legislation, in that it actively promotes use 
of non-difference methods through the Part 53 equivalency designation 
process, and states our ultimate expectation that the monitoring 
network for PM10-2.5 will utilize primarily non-difference 
method monitors. Furthermore, we are actively investigating the 
possibility that a dichotomous method could be an alternative FRM 
within the time frame prescribed by this Act. However, we are proposing 
a difference method as the FRM for PM10-2.5, for the reasons 
explained above as we believe this is the only approach technically 
justified at this time. Since the new statutory language does not 
require that EPA promulgate a non-difference method as either the sole 
or alternative FRM, we believe this proposed approach is consistent 
with the express language of the provision as well as with its 
objectives.
7. Basic Requirements of the Proposed Federal Reference Method Sampler
    The proposed PM10-2.5 FRM ``sampler'' is actually a 
collocated pair of samplers, one for PM10 and one for 
PM2.5, operated simultaneously. The PM2.5 sampler 
is exactly as specified in the PM2.5 FRM (appendix L to 40 
CFR part 50). The operational and procedural requirements would be the 
same as those for PM2.5 FRM measurements. PM2.5 
measurements obtained as part of PM10-2.5 FRM measurements 
would be indistinguishable from conventional PM2.5 FRM 
measurements and would be usable for any PM2.5 monitoring 
purpose, provided they are sited at the appropriate spatial scale 
(e.g., neighborhood scale).
    In contrast, the PM10 sampler of the PM10-2.5 
sampler pair would be required to be identical in design and 
construction to the PM2.5 sampler, except that the 
PM2.5 particle size separator (WINS impactor) would be 
removed from the sampler and replaced with a straight downtube, thereby 
converting it to a PM10 sampler. This PM10 
sampler would have to meet the higher standards of manufacture and 
performance of appendix L to 40 CFR part 50 rather than the standards 
for conventional PM10 FRM samplers (which meet the lesser 
requirements of appendix J to 40 CFR part 50). Thus, PM10 
measurements obtained as part of or incidental to the 
PM10-2.5 FRM measurements must be distinguished from 
conventional PM10 measurements and need to be identified by 
a unique descriptor such as ``PM10c.'' Since 
PM10c measurements would meet a higher standard than 
conventional PM10 measurements, such measurements would also 
be acceptable for any conventional PM10 monitoring purpose. 
However, one subtle issue regarding conventional PM10 
measurements and new PM10c measurements needs clarification. 
Conventional PM10 measurement flow systems operate on 
conditions of standard temperature and pressure (STP). Flow systems for 
PM2.5 and the new PM10-2.5 FRM as proposed today 
and peer reviewed by the CASAC, all operate under conditions of actual 
local conditions.
    PM10-2.5 sampler pairs would be required to be 
specifically designated as PM10-2.5 FRM samplers by EPA 
under amendments to 40 CFR 53 proposed elsewhere in this Federal 
Register. The two samplers of the PM10-2.5 FRM sampler pair 
would be required to be of like manufacturer and of matched design and 
fabrication so that they are essentially identical, except that one 
would have a PM2.5 particle size separator while the other 
would not. Either single-filter samplers or multiple-filter, sequential 
samplers could constitute a PM10-2.5 sampler pair, as long 
as both were of the same type and design. For a manufacturer's sampler 
model that has already been designated as a PM2.5 FRM, no 
further testing would be required for designation as a 
PM10-2.5 FRM, although the sampler manufacturer would have 
to submit a formal application under 40 CFR part 53. Users could 
assemble their own PM10-2.5 sampler pair using existing 
PM2.5 samplers of the same model or design by converting one 
of the samplers to a PM10c sampler, provided the specific 
sampler pair has been previously designated by the EPA as a 
PM10-2.5 FRM under 40 CFR part 53.
    Pairings of qualified PM2.5 samplers that are dissimilar 
or have some minor design or model variations (and one sampler is 
converted to a PM10c sampler) could be designated by the EPA 
as Class I equivalent methods under proposed amendments to 40 CFR part 
53. Again, an application for an equivalent method determination for 
the sampler combination would have to be submitted to the EPA under 40 
CFR part 53, and not all combinations would necessarily be designated 
without further testing. For example, supplemental test or operational 
performance information would likely be required for designation of a 
PM10-2.5 sampler pair consisting of a single-filter sampler 
and a multiple-filter, sequential sampler. A pairing of dissimilar 
PM2.5 samplers that has not been designated as a Class I 
equivalent method for PM10-2.5 under 40 CFR part 53 could be 
considered by the EPA for approved use in PM10-2.5 
monitoring networks as a user modification under section 2.8 of 
appendix C to 40 CFR part 58.
8. Other Important Aspects of the Proposed Federal Reference Method 
Sampler
    The proposed method would require that both samplers of the 
PM10-2.5 sampler pair be located in close proximity and 
operated simultaneously. Operational procedures for both samplers of 
the pair would be similar or identical to those specified for 
PM2.5 FRM, and both samplers should be operated, serviced, 
and maintained similarly. Quality assurance procedures would parallel 
those for the PM2.5 FRM, although data quality assessment 
procedures would apply to the calculated PM10-2.5 
measurement data rather than (or in addition to) the individual 
PM10 and PM2.5

[[Page 2691]]

measurements. The proposed sample period would be nominally 24 hours 
(1 hour).
    Expected performance of the PM10-2.5 FRM--as measured by 
precision, lower concentration limit, and completeness--is similar to 
that of the PM2.5 FRM, but may be somewhat inferior because 
of the dual measurement components. Precision, defined as a goal for 
acceptable measurement uncertainty, is given as 15 percent coefficient 
of variation, as assessed according to quality assurance procedures for 
PM10-2.5 monitoring described in proposed revisions to 
appendix A of 40 CFR part 58, published elsewhere in this Federal 
Register.
    The lower concentration limit proposed for the method is 3 [mu]g/
m\3\. This value can vary with the level of quality control and 
precision achieved in implementing the method. It should not be 
interpreted as a specification but rather as a simple guide to the 
general significance of low-level measured concentrations. However, 
this proposed value may be used as a lower range limit for excluding 
low-concentration data from composite performance calculations that use 
percentages (where very low values in a denominator need to be avoided) 
or in types of statistical calculations of monitoring data that cannot 
accept zero or negative values (such as geometric distributions, where 
\1/2\ of this lower concentration limit may be substituted for any 
measurements less than that value). Comments are solicited on the 
usefulness of this lower concentration limit, its value, or how its 
value should be established and interpreted.

B. Proposed 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 Federal reference method 
(FRM) for PM10-2.5, EPA is proposing minor changes to the 
FRM for PM2.5 in appendix L to 40 CFR part 50. These 
proposed changes are based on new test information and extensive 
operational experience with the PM2.5 FRM acquired 
subsequent to its promulgation in 1997. Through the increased 
flexibility afforded by the proposed changes, significant improvements 
in the efficiency of the PM2.5 method in monitoring network 
operations are expected without altering the performance of the method. 
In fact, the changes have already been implemented in the national 
PM2.5 monitoring network through designated equivalent 
methods or duly approved user modifications. Further, the changes would 
also apply to the proposed PM10-2.5 FRM, so the benefits 
would be realized for PM10-2.5 measurements as well, and 
uniformity between the PM2.5 FRM and the PM2.5 
portion of the PM10-2.5 FRM would be maintained.
    The most significant proposed change is the addition of an 
alternative PM2.5 particle size separator. Since the 
promulgation of the PM2.5 FRM in 1997, a new, very sharp cut 
cyclone separator (VSCC\TM\) manufactured by BGI Incorporated, Waltham, 
MA has been shown to have performance equivalent to that of the 
originally specified separator (WINS impactor) (Kenny, et al., 2001; 
Kenny et al., 2004; EPA, 2002b). Although the original WINS impactor 
continues to show fully adequate performance in PM2.5 
samplers, the new VSCC provides the same level of performance and has a 
considerably longer service interval. Generally, the VSCC separator is 
also physically interchangeable with the WINS where both are 
manufactured for the same sampler. The proposed change would allow 
either the WINS or the VSCC separator to be used in a PM2.5 
FRM sampler. Currently, EPA has designated seven PM2.5 
samplers configured with VSCC separators as Class II equivalent 
methods.\93\ Upon promulgation of this change to appendix L, those 
seven methods would be re-designated as PM2.5 FRM.
---------------------------------------------------------------------------

    \93\ List of designated reference and equivalent methods 
available at http://www.epa.gov/ttn/amtic/criteria.html.

---------------------------------------------------------------------------

    Another minor change proposed for the PM2.5 FRM (and, 
hence, also applicable to the proposed PM10-2.5 FRM) would 
require an improved impactor oil for the PM2.5 WINS impactor 
particle size separator. The new oil corrects an occasional problem of 
crystallization of the original oil during sampling in cold and damp 
weather and has been tested and approved as a national user 
modification (EPA, 2000b). Also, the time limit specified for sample 
filter retrieval time would be increased from 96 hours to 177 hours 
following the end of the sample period. This change would allow the 
filter to be retrieved by the morning of the eighth day after sampling 
to permit recovery of up to three samples from a sequential sampler 
operating on a 1-in-3 day sample schedule. Based on a study (Papp, et 
al., 2002) at six sampling sites, this change has already been approved 
as a national user modification (EPA, 2002a). An associated change to 
ease the filter retrieval burden on monitoring agencies would modify 
the current requirement that retrieved filters be weighed within 10 
days after sampling, unless they are maintained at a temperature of 
4[deg]C or less at all times during transport. The filter recovery 
extension study (Papp, et al., 2002) showed that these limits can be 
relaxed somewhat (EPA, 2000a) to allow up to 30 days for weighing the 
filter if it is maintained below the average ambient temperature during 
the sampling period prior to the post-collection sample equilibration.
    Finally, some of the sampler data output reporting requirements 
specified in Table L-1 of appendix L to 40 CFR part 50 (e.g. flow rate 
CV, sample volume, minimum and maximum temperature, minimum and maximum 
pressure) have been determined to be unnecessary to report to the Air 
Quality System, and the reporting requirement for these data would be 
deleted. These data will be retained and available at the monitoring 
agency, if needed.

VII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under Executive Order 12866 (58 FR 51735, October 4, 1993), the 
Agency must determine whether a regulatory action is ``significant'' 
and therefore subject to Office of Management and Budget (OMB) review 
and the requirements of the Executive Order. The Order defines 
``significant regulatory action'' as one that is likely to result in a 
rule that may:
    1. Have an annual effect on the economy of $100 million or more or 
adversely affect in a material way the economy, a sector of the 
economy, productivity, competition, jobs, the environment, public 
health or safety, or State, local, or Tribal governments or 
communities;
    2. Create a serious inconsistency or otherwise interfere with an 
action taken or planned by another agency;
    3. Materially alter the budgetary impact of entitlements, grants, 
user fees, or loan programs or the rights and obligations of recipients 
thereof; or
    4. Raise novel legal or policy issues arising out of legal 
mandates, the President's priorities, or the principles set forth in 
the Executive Order.
    In view of its important policy implications and potential effect 
on the economy of over $100 million, this action has been judged to be 
an economically ``significant regulatory action'' within the meaning of 
the Executive Order. As a result, today's action was submitted to OMB 
for review. Changes made in response to OMB suggestions or 
recommendations

[[Page 2692]]

will be documented in the public record.

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 the establishment of 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.
    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 Regulatory Flexibility Act (RFA) generally requires an agency 
to prepare a regulatory flexibility analysis of any rule subject to 
notice and comment rulemaking requirements under the Administrative 
Procedure Act or any other statute unless the agency certifies that the 
rule will not have a significant economic impact on a substantial 
number of small entities. Small entities include small businesses, 
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of 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 proposed rule on 
small entities, I certify that this action will not have a significant 
economic impact on a substantial number of small entities. This 
proposed rule will not impose any requirements on small entities. 
Rather, this rule establishes national standards for allowable 
concentrations of particulate matter in ambient air as required by 
section 109 of the CAA. See also American Trucking Associations v. EPA. 
175 F. 3d at 1044-45 (NAAQS do not have significant impacts upon small 
entities because NAAQS themselves impose no regulations upon small 
entities). We continue to be interested in the potential impacts of the 
proposed rule on small entities and welcome comments on issues related 
to such impacts.

D. Unfunded Mandates Reform Act

    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 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 American Trucking Associations v. EPA, 175 F. 
3d 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 proposed 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 proposed 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.''
    This proposed rule does not have federalism implications. It will 
not have

[[Page 2693]]

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 in 
section E (above) on UMRA, this rule does not impose significant costs 
on State, local, or Tribal governments or the private sector. Thus, 
Executive Order 13132 does not apply to this rule.
    However, as also noted in section E (above) on UMRA, EPA recognizes 
that States will 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 solicits comment on this 
proposed rule from State and local officials.

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.
    This proposed 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.
    Although Executive Order 13175 does 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 this proposal was under development. Also, 
EPA is sending notice and an opportunity for comment to Tribal Leaders 
within the lower 48 states. Specifically, EPA solicits additional 
comment on this proposed rule from Tribal officials.

G. Executive Order 13045: Protection of Children From Environmental 
Health Risks and 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 planned rule on children, and explain why the 
planned regulation is preferable to other potentially effective and 
reasonably feasible alternatives considered by the Agency.
    This proposed 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 proposed NAAQS will establish 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.A, B, and C and 
III.A, B, and C of this preamble.

H. Executive Order 13211: Actions That Significantly Affect Energy 
Supply, Distribution or Use

    This proposed 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 prescribe 
specific pollution control strategies by which these ambient standards 
will be met. Such strategies will be developed by States on a case-by-
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 No. 104-113, Sec.  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 proposed rule establishes requirements for environmental 
monitoring and measurement. Specifically, it would establish the FRM 
for PM10-2.5 measurement (and slightly amend 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 proposed revisions to the FEM in 40 CFR part 53 
do allow for the utilization of voluntary consensus standards if they 
meet the specified performance criteria.

[[Page 2694]]

    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 cost-effective manner. It is intended to be more 
flexible and cost effective for the regulated community; it is also 
intended to encourage innovation in analytical technology and improved 
data quality. Though the FRM defines the particular specifications for 
ambient monitors, there is some variability with regard to how monitors 
measure PM, depending on the type and size of PM and environmental 
conditions. Therefore, it is not practically possible to fully define 
the FRM in performance terms. 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 
proposal and the equivalency criteria contained in the proposed 
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.
    The EPA welcomes comments on this aspect of the proposed rulemaking 
and, specifically, invites the public to identify potentially 
applicable voluntary consensus standards and to explain why such 
standards should be used in this regulation.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations

    Executive Order 12898, ``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 risks or a rate of exposure to hazards that 
are significant and that ``appreciably exceed 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 proposals, if promulgated, may have disproportionate 
negative impacts on minority or low income populations. The Agency 
expects these proposals would lead to the establishment of uniform 
NAAQS for PM.

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Peters, J. M.; Avol, E.; Navidi, W.; London, S. J.; Gauderman, W. 
J.; Lurmann, F.; Linn, W. S.; Margolis, H.; Rappaport, E.; Gong, H., 
Jr.; Thomas, D. C. (1999a). A study of twelve southern California 
communities with differing levels and types of air pollution. I. 
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159: 760-767.
Peters, J. M.; Avol, E.; Navidi, W.; London, S. J.; Gauderman, W. 
J.; Lurmann, F.; Linn, W. S.; Margolis, H.; Rappaport, E.; Gong, H., 
Jr.; Thomas, D. C. (1999b). A study of twelve southern California 
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mortality and PM10 pollution in Utah valley. Arch. 
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air pollution and daily mortality on Utah's Wasatch Front. Environ. 
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 October 18, 2004.

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[[Page 2698]]

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.

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(2000). Air pollution, aeroallergens and cardiorespiratory emergency 
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analysis of the relationship between mortality and the chemical 
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Site Evaluations of Candidate Methodologies for Determining Coarse 
Particulate Matter (PM10-2.5) Concentrations: August 
2005. Updated Report Regarding Second-Generation and New 
PM10-2.5 Samplers. In press.

List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide, 
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

    Dated: December 20, 2005.
Stephen L. Johnson,
Administrator.
    For the reasons set forth in the preamble, part 50 of chapter 1 of 
title 40 of the Code of Federal Regulations is proposed to be amended 
as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 
STANDARDS

    1. The authority citation for part 50 continues to read as follows:

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

    2. Section 50.3 is revised to read as follows:

Sec.  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 and PM10-2.5) standards 
contained in Sec. Sec.  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 and PM10-2.5 for purposes of comparison to 
the standards contained in Sec. Sec.  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.
    3. Section 50.6 is amended by adding new paragraphs (d) and (e) to 
read as follows:

Sec.  50.6  National primary and secondary ambient air quality 
standards for PM10.

* * * * *
    (d) The national primary and secondary 24-hour ambient air quality 
standards for particulate matter set forth in paragraph (a) of this 
section will no longer apply except in the following areas as of 
[effective date of final rule]:
    (1) Birmingham urban area (Jefferson County, AL).
    (2) Maricopa and Pinal Counties; Phoenix planning area (AZ).
    (3) Riverside, Los Angeles, Orange and San Bernardino Counties; 
South Coast Air Basin (CA).
    (4) Fresno, Kern, Kings, Tulare, San Joaquin, Stanislaus, Maderia 
Counties; San Joaquin Valley planning area (CA).
    (5) San Bernardino County (part); excluding Searles Valley Planning 
Area and South Coast Air Basin (CA).
    (6) Riverside County; Coachella Valley Planning Area (CA).
    (7) Simi Valley urban area (CA).
    (8) Lake County; Cities of East Chicago, Hammond, Whiting, and Gary 
(IN).
    (9) Wayne County (part) (MI).
    (10) St. Louis urban area (MO).
    (11) Albuquerque urban area (NM).
    (12) Clark County; Las Vegas planning area (NV).
    (13) Columbia urban area (SC).
    (14) El Paso urban area (including those portions in TX and those 
portions in NM).
    (15) Salt Lake County (UT).
    (e) The national primary and secondary annual ambient air quality 
standards for particulate matter set forth in paragraph (b) of this 
section will no longer apply in an area as of [effective date of final 
rule.]
    4. A new Sec.  50.13 is added, to read as follows:

Sec.  50.13  National primary and secondary ambient air quality 
standards for PM2.5 and PM10-2.5.

    (a) The national primary and secondary ambient air quality 
standards for particulate matter are:
    (1) 15.0 micrograms per cubic meter ([mu]g/m3) annual 
arithmetic mean concentration, and 35 [mu]g/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:
    (i) A reference method based on appendix L of this part and 
designated in accordance with part 53 of this chapter; or
    (ii) An equivalent method designated in accordance with part 53 of 
this chapter.
    (2)(i) 70 [mu]g/m3 24-hour average concentration 
measured in the ambient air as PM10-2.5 (particles with an 
aerodynamic diameter less than or equal to a nominal 10 micrometers and 
greater than a nominal 2.5 micrometers) by either:
    (A) A reference method based on appendix O of this part and 
designated in accordance with part 53 of this chapter; or
    (B) An equivalent method designated in accordance with part 53 of 
this chapter.
    (ii) The standard for PM10-2.5 includes any ambient mix 
of PM10-2.5 that is dominated by resuspended dust from high-
density traffic on paved roads and

[[Page 2699]]

PM generated by industrial sources and construction sources, and 
excludes any ambient mix of PM10-2.5 that is dominated by 
rural windblown dust and soils and PM generated by agricultural and 
mining sources. Agricultural sources, mining sources, and other similar 
sources of crustal material shall not be subject to control in meeting 
this standard.
    (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 
[mu]g/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 
[mu]g/m3. The 24-hour primary and secondary 
PM10-2.5 standards are met when the 98th percentile 24-hour 
concentration, as determined in accordance with appendix P of this 
part, is less than or equal to 70 [mu]g/m3.
    5. 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; revising paragraph (a) of section 7.3.4.3, adding section 
7.3.4.4; and revising Table L-1 in section 7.4.19;
    c. Revising section 8.3.6;
    d. Revising the first sentence in section 10.10 and revising 
section 10.13; and
    e. Revising reference 2 in section 13.0. The revisions and addition 
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 Sec.  
50.7 and Sec.  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 Design specifications. * * *
* * * * *
    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 Impactor oil specifications:
    (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 Data reporting requirements. * * *

                                Table L-1 to Appendix L of Part 50.--Summary of Information to be Provided by the Sampler
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                            Availability                                          Format
                                    Appendix L    ------------------------------------------------------------------------------------------------------
   Information to be provided         section                       End of period  Visual display   Data output     Digital reading
                                     reference       Anytime \1\         \2\             \3\            \4\               \5\                Units
--------------------------------------------------------------------------------------------------------------------------------------------------------
Flow rate, 30 second maximum     7.4.5.1           [b  ..............  [b  (*)            XX.X                L/min
 interval.                                          <]                   <]
Flow rate, average for the       7.4.5.2           (*)             [b  (*)             [b]                   >
                                                                                                    < ]
Flow rate, CV, for sample        7.4.5.2           (*)             [b  (*)             [b]                   >
                                                                                                    < ]
Flow rate, 5-min. average out    7.4.5.2           [b  [b  [b  [b]   <]   <]   >
                                                                                                    < ][squf]
Sample volume, total...........  7.4.5.2           (*)             [b  [b  [b]   <]   >
                                                                                                    < ]
Temperature, ambient, 30-second  7.4.8             [b  ..............  [b  .............  XX.X                [deg]C
 interval.                                          <]                   <]
Temperature, ambient, min.,      7.4.8             (*)             [b  [b  [b]   <]   >
 period.                                                                                            < ][squf]
Baro. pressure, ambient, 30-     7.4.9             [b  ..............  [b  .............  XXX                 mm Hg
 second interval.                                   <]                   <]
Baro. pressure, ambient, min.,   7.4.9             (*)             [b  [b  [b]   <]   >
 period.                                                                                            < ][squf]
Filter temperature, 30-second    7.4.11            [b  ..............  [b  .............  XX.X                [deg]C
 interval.                                          <]                   <]
Filter temp. differential, 30-   7.4.11            (*)             [b  [b  [b]   <]   >
 (FLAG \6\).                                                                                        < ][squf]
Filter temp., maximum            7.4.11            (*)             (*)             (*)             (*)            X.X, YY/MM/DD       [deg]C Yr/Mon/Day
 differential from ambient,                                                                                        HH.mm               Hrs.min
 date, time of occurrence.
Date and Time..................  7.4.12            [b  ..............  [b  .............  YY/MM/DD HH.mm      Yr/Mon/Day Hrs.min
                                                    <]                   <]
Sample start and stop time       7.4.12            [b  [b  [b  [b]   <]   <]   >
                                                                                                    < ]
Sample period start time.......  7.4.12            ..............  [b  [b  [b]   <]   >
                                                                                                    < ]
Elapsed sample time............  7.4.13            (*)             [b  [b  [b]   <]   >
                                                                                                    < ]
Elapsed sample time, out of      7.4.13            ..............  [b  [b  [b]   <]   >
                                                                                                    < ][squf]
Power interruptions <=1 min.,    7.4.15.5          (*)             [b  (*)             [b]                   >   etc* * *
                                                                                                    < ]
User-entered information, such   7.4.16            [b  [b  [b  [b]   <]   <]   >
 identification.                                                                                    < ][squf]
--------------------------------------------------------------------------------------------------------------------------------------------------------
[b] Provision of this information is required.

[[Page 2700]]

* 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.
[squf] Indicates that this information is also required to be provided to the Air Quality System (AQS) data bank; see Sec.   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 Weighing procedure.
* * * * *
    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[deg]C or below 
for average sampling temperatures less than 4[deg]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.
* * * * *
    10.0 PM2.5 Measurement Procedure. * * *
* * * * *
    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[deg]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
.

* * * * *
    6. 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 Sec.  50.7 and Sec.  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. 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.
    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).
    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 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'').
    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 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. This can 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.

[[Page 2701]]

    (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 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 ([mu]g/
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 insignificant digits to the right 
of the third 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 monitor. 
The primary monitor shall be designated in the appropriate State or 
local agency PM Monitoring Network Description.
    (2) Data for the primary monitor shall be augmented as necessary 
with data from collocated FRM/FEM monitors. If a valid 24-hour 
measurement is not produced from the primary monitor for a 
particular required sampling day, but a valid sample is generated by 
a collocated FRM/FEM instrument (and recorded in AQS), then that 
collocated value shall be considered part of the site data record. 
If more than one valid collocated FRM/FEM value is available, the 
average of those valid collocated values shall be used as the site 
value for the day.
    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 ([mu]g/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. However, years with high 
concentrations and 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 
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.
    (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 [mu]g/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 with 
high concentrations 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.
    (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 [mu]g/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 [mu]g/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:
[GRAPHIC] [TIFF OMITTED] TP17JA06.052

Where:

xq, y, s = the mean for quarter q of year y for site s;
nq = the number of monitored 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:
[GRAPHIC] [TIFF OMITTED] TP17JA06.053

Where:
xy,s = the annual mean concentration for year y (y = 1, 
2, or 3) and for site s; and
xq,y,s = the mean for quarter q of year y for site s.

    (c) If spatial averaging is utilized, the site-based 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.

[[Page 2702]]

[GRAPHIC] [TIFF OMITTED] TP17JA06.054

Where:

xy = the spatially averaged mean for year y,
xy,s = the annual mean for year y and site s, and
ns = the number of sites designated to be averaged.

    (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:
[GRAPHIC] [TIFF OMITTED] TP17JA06.055

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
xy = the spatially averaged annual mean for year y 
(result of equation 3 of this appendix) when spatial averaging is 
used, or
xy,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. Seasonal sampling, when 
approved, will be implemented in periods of calendar quarters or 
months; seasonal sampling seasons shall not divide months. 
Calculations of all annual 98th percentile values are based on the 
applicable number of samples (as described below), rather than on 
the actual number of samples. For the 24-hour NAAQS, credit will not 
be granted for more samples than the maximum number of scheduled 
sampling days in the sampling period. For each month, the applicable 
number of samples is the lower of the actual number of samples and 
the scheduled number of samples. The applicable number of samples 
for a year is the sum of the twelve monthly ``applicable number of 
samples'; the applicable number of samples for a season is the sum 
of the corresponding monthly ``applicable number of samples''. 98th 
percentile values shall be calculated as in equations 5 or 6 of this 
appendix using the applicable number of samples for the year or 
season. [The applicable number of samples will determine how deep to 
go into the data distribution, but all samples (scheduled or not) 
will be considered when making the percentile assignment.]
    (1) Regular formula for computing annual 98th percentile values. 
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 (an) as the number ``i.d'', where `an' is the 
annual applicable 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:
[GRAPHIC] [TIFF OMITTED] TP17JA06.056

Where:

P0.98,y = 98th percentile for year y;
x[i+1] = the (i+1)th number in the ordered series of numbers; and
i = the integer part of the product of 0.98 and an.

    (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:
[GRAPHIC] [TIFF OMITTED] TP17JA06.057

Where:

dHigh = number of calendar days in the ``High'' season;
dLow = number of calendar days in the ``Low'' season;
dHigh + dLow = days in a year; and

[GRAPHIC] [TIFF OMITTED] TP17JA06.058

[[Page 2703]]

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:
[GRAPHIC] [TIFF OMITTED] TP17JA06.059

    (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.

    7. Appendix O to part 50 is added 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 for purposes of determining whether 
the primary and secondary NAAQS for coarse particulate matter specified 
in Sec.  50.13 of this chapter are met.
    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 has 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[deg]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 
([mu]/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 
[mu]g/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 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

[[Page 2704]]

concentrations of at least 200 [mu]g/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). However, a PM10-2.5 measured concentration where 
the actual sample period for PM10c sample is less than 1,380 
minutes, but the corresponding PM2.5 sample period is at 
least 1,380 minutes, may be used as if it were a valid concentration 
measurement for the specific purpose of determining an exceedance of 
the NAAQS. For this purpose, the measured PM10c 
concentration is determined as the PM10c mass collected 
divided by the actual sampled air volume, multiplied by the actual 
number of minutes in the PM10c sample period and divided by 
1,440; the PM10-2.5 concentration is then calculated as 
prescribed in section 12.4 of this appendix. This value represents the 
minimum nominal PM10-2.5 concentration that could have been 
measured for the full sample period. Accordingly, if the value thus 
calculated is high enough to be an exceedance, such an exceedance would 
be a valid exceedance for the sample period. When reported to AQS, this 
data value should receive a special data qualifier code to identify it 
as having an insufficient sample period.
    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 Sec.  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

[[Page 2705]]

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:
[GRAPHIC] [TIFF OMITTED] TP17JA06.060

Where:

PM10c = mass concentration of PM10c, [mu]g/
m3;
Wf, Wi = final and initial masses (weights), 
respectively, of the filter used to collect the PM10c 
particle sample, [mu]g;
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.

    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 [mu]g/
m3, is calculated using Equation 2 of this section:
[GRAPHIC] [TIFF OMITTED] TP17JA06.061

    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: http://www.epa.gov/ttn/amtic/pgqa.html
.

    14.0 Figures.
    Figures O-1 is included as part of this appendix O.
BILLING CODE 6560-50-P

[[Page 2706]]

[GRAPHIC] [TIFF OMITTED] TP17JA06.050

BILLING CODE 6560-50-C
    8. Appendix P is added to part 50 to read as follows:

Appendix P to Part 50--Interpretation of the National Ambient Air 
Quality Standards for PM10-2.5

    1.0 General.
    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the 24-hour primary and 
secondary national ambient air quality standards (NAAQS) for 
PM10-2.5 specified in Sec.  50.13 of this part are met. 
PM10-2.5, defined as particles with an aerodynamic 
diameter more than a nominal 2.5 micrometers and less than or equal 
to a nominal 10.0 micrometers, is measured in the ambient air by a 
Federal reference method (FRM) based on appendix O 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. Data handling and 
computation procedures to be used in making comparisons between 
reported PM10-2.5 concentrations and the levels of the 
PM10-2.5 NAAQS are specified in the following sections.
    (b) Data resulting from exceptional events, for example 
structural fires or high winds, may require 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 PM10-2.5 NAAQS. In other cases, it 
may be more appropriate to retain the data for comparison with the 
levels of the PM10-2.5 NAAQS and then allow EPA to 
formulate the appropriate regulatory response.
    (c) The terms used in this appendix are defined as follows:
    Daily values for PM10-2.5 refers to the 24-hour 
average concentrations of PM10-2.5 calculated (averaged) 
or measured from midnight to midnight (local standard time).
    Designated monitors are those monitoring sites designated in a 
State or local agency PM

[[Page 2707]]

Monitoring Network Description in accordance with part 58 of this 
chapter.
    Design values are the metrics that are compared to the NAAQS 
levels to determine compliance and are comprised of the 3-year 
average of annual 98th percentile 24-hour average values recorded at 
each monitoring location, are referred to as ``24-hour standard 
design values,'' and are calculated as shown in section 3 of this 
appendix.
    Geographic area design value (e.g., one for a county or defined 
metropolitan area) is the highest valid site-level design value in 
that area.
    98th percentile means the daily value out of a year of 
PM10-2.5 monitoring data below which 98 percent of all 
values in the group fall.
    Year refers to a calendar year.
    2.0 Requirements for data used for comparisons with the 
PM10-2.5 NAAQS and data reporting considerations.
    (a) Appendix D to part 58 of this chapter specifies which 
monitors are eligible for making comparisons with the 
PM10-2.5 standards.
    (b) Except as otherwise provided in this appendix, only valid 
FRM/FEM PM10-2.5 data required to be submitted to EPA's 
Air Quality System (AQS) shall be used in the design value 
calculations.
    (c) Raw concentration data (typically hourly for automated 
continuous instruments and daily for manual, filter-based 
instruments) shall be reported to AQS in micrograms per cubic meter 
([mu]g/m\3\) to one decimal place, with additional digits to the 
right being truncated.
    (d) Block 24-hour averages shall be computed from available 
hourly PM10-2.5 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 7 or more missing hours shall be considered valid if, after 
substituting zero for the missing hourly concentrations, the 24-hour 
average concentration is greater than the level of the standard. The 
computed 24-hour average PM10-2.5 concentrations shall be 
reported to one decimal place (the insignificant digits to the right 
of the third decimal place are truncated, consistent with the data 
handling procedures for the reported data).
    (e) All calculations 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 monitor. 
The primary monitor shall be designated in the appropriate State or 
local agency PM Monitoring Network Description.
    (2) Data for the primary monitor shall be augmented as necessary 
with data from collocated FRM/FEM monitors. If a valid 24-hour 
measurement is not produced from the primary monitor for a 
particular required sampling day, but a valid sample is generated by 
a collocated FRM/FEM instrument (and recorded in AQS), then that 
collocated value shall be considered part of the site data record. 
If more than one valid collocated FRM/FEM value is available, the 
average of those valid collocated values shall be used as the site 
value for the day.
    3.0 Comparisons with the PM10-2.5 NAAQS.
    3.1 24-Hour PM10-2.5 NAAQS.
    (a) The 24-hour PM10-2.5 NAAQS is met when the 24-
hour standard design value at each monitoring site is less than or 
equal to 70 [mu]g/m\3\. 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 or 3-
year periods with high concentrations 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 3.2 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.
    (c) The equations for calculating the 24-hour standard design 
values are given in section 3.4 of this appendix.
    3.2 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 3.4 of this appendix. 24-hour PM10-2.5 
standard design values shall be rounded to the nearest 1 [mu]g/m\3\ 
(decimals 0.5 and greater are rounded up to nearest whole number, 
and any decimal lower than 0.5 is rounded down to the nearest whole 
number).
    3.3 Sampling Frequency Considerations. Section 58.12 of this 
chapter specifies the required minimum frequency of sampling for 
PM10-2.5. 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 2 in section 3.4 of this appendix when a site 
operates on a ``seasonal sampling'' schedule.
    3.4 Equations for the 24-Hour PM10-2.5 NAAQS.
    (a) When the data for a particular site and year meet the data 
completeness requirements in section 3.1 of this appendix, 
calculation of the 98th percentile is accomplished by the steps 
provided in paragraphs (a) through (c) of this section. Equation 1 
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 2 of this appendix shall be used 
instead. Seasonal sampling, when approved, will be implemented in 
periods of calendar quarters or months; seasonal sampling seasons 
shall not divide months. Calculations of all annual 98th percentile 
values are based on the applicable number of samples (as described 
below), rather than on the actual number of samples. For the 24-hour 
NAAQS, credit will not be granted for more samples than the maximum 
number of scheduled sampling days in the sampling period. For each 
month, the applicable number of samples is the lower of the actual 
number of samples and the scheduled number of samples. The 
applicable number of samples for a year is the sum of the twelve 
monthly ``applicable number of samples;'' the applicable number of 
samples for a season is the sum of the corresponding monthly 
``applicable number of samples.'' 98th percentile values shall be 
calculated as in equations 5 or 6 of this appendix using the 
applicable number of samples for the year or season. The applicable 
number of samples will determine how deep to go into the data 
distribution, but all samples (scheduled or not) will be considered 
when making the percentile assignment.
    (1) Regular formula for computing annual 98th percentile values. 
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. Compute (0.98) x (an) as the number ``i.d,'' where 
``an'' is the applicable 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 1 of this appendix:
[GRAPHIC] [TIFF OMITTED] TP17JA06.062

Where:

P0.98,y = 98th percentile for year y;
x[i+1] = the (i+1)th number in the ascending ordered series of 
numbers for year y; and
i = the integer part of the product of 0.98 and an.

    (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 2 of this appendix:

[[Page 2708]]

[GRAPHIC] [TIFF OMITTED] TP17JA06.063

Where:

dHigh = number of calendar days in the ``High'' season;
dLow = number of calendar days in the ``Low'' season;
dHigh + dLow = days in a year); and
[GRAPHIC] [TIFF OMITTED] TP17JA06.064

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 3-year average 98th percentile (24-hour standard design 
value) is then calculated by averaging the annual 98th percentiles 
using equation 3 of this appendix:
[GRAPHIC] [TIFF OMITTED] TP17JA06.065

    (c) The 24-hour standard design value (3-year average 98th 
percentile) is rounded according to the conventions in section 3.2 
of this appendix before a comparison with the standard is made.

[FR Doc. 06-177 Filed 1-13-06; 8:45 am]

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