National Ambient Air Quality Standards for Particulate Matter, 61144-61233 [06-8477]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 71, Number 200 (Tuesday, October 17, 2006)]
[Rules and Regulations]
[Pages 61144-61233]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 06-8477]



[[Page 61143]]

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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; Final 
Rule

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

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2001-0017; FRL-8225-3]
RIN 2060-AI44


National Ambient Air Quality Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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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 
is making revisions to the primary and secondary NAAQS for PM to 
provide increased protection of public health and welfare, 
respectively. With regard to primary standards for fine particles 
(generally referring to particles less than or equal to 2.5 micrometers 
([micro]m) in diameter, PM2.5), EPA is revising the level of 
the 24-hour PM2.5 standard to 35 micrograms per cubic meter 
([micro]g/m\3\) and retaining the level of the annual PM2.5 
standard at 15[micro]g/m\3\. With regard to primary standards for 
particles generally less than or equal to 10[mu]m in diameter 
(PM10), EPA is retaining the 24-hour PM10 and 
revoking the annual PM10 standard. With regard to secondary 
PM standards, EPA is making them identical in all respects to the 
primary PM standards, as revised.

DATES: This final rule is effective on December 18, 2006.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2001-0017. All documents in the docket are 
listed on the www.regulations.gov Web site. Although listed in the 
index, some information is not publicly available, e.g. confidential 
business information or other information whose disclosure is 
restricted by statute. Certain other material, such as copyrighted 
material, is not placed on the Internet and will be publicly available 
only in hard copy form. Publicly available docket materials are 
available either electronically through www.regulations.gov or in hard 
copy at the Air and Radiation Docket and Information Center, EPA/DC, 
EPA West, Room B102, 1301 Constitution Ave., NW., Washington, DC. This 
Docket Facility is open from 8:30 a.m. to 4:30 p.m., Monday through 
Friday, excluding legal holidays. The Docket telephone number is 202-
566-1741. The telephone number for the Public Reading Room is 202-566-
1744.
    The EPA Docket Center suffered damage due to flooding during the 
last week of June 2006. The Docket Center is continuing to operate. 
However, during the cleanup, there will be temporary changes to Docket 
Center telephone numbers, addresses, and hours of operation for people 
who wish to visit the Public Reading Room to view documents. Consult 
EPA's Federal Register notice at 71 FR 38147 (July 5, 2006) or the EPA 
Web site at www.epa.gov/epahome/dockets.htm for current information on 
docket status, locations and telephone numbers.

FOR FURTHER INFORMATION CONTACT: Ms. Beth M. Hassett-Sipple, Mail Code 
C504-06, Health and Environmental Impacts Division, Office of Air 
Quality Planning and Standards, U.S. Environmental Protection Agency, 
Research Triangle Park, North Carolina 27711, telephone: (919) 541-
4605, e-mail: hassett-sipple.beth@epa.gov.

SUPPLEMENTARY INFORMATION:

Table of Contents

    The following topics are discussed in today's preamble:

I. Background
    A. Summary of Revisions to the PM NAAQS
    B. Legislative Requirements
    C. Overview of Air Quality Criteria and Standards Review for PM
    D. Related Control Programs to Implement PM Standards
    E. Summary of Proposed Revisions to the PM NAAQS
    F. Organization and Approach to Final PM NAAQS Decisions
II. Rationale for Final Decisions on Primary PM2.5 
Standards
    A. Introduction
    1. Overview
    2. Overview of Health Effects Evidence
    3. Overview of Quantitative Risk Assessment
    B. Need for Revision of the Current Primary PM2.5 
Standards
    1. Introduction
    2. Comments on the Need for Revision
    3. Conclusions Regarding the Need for Revision
    C. Indicator for Fine Particles
    D. Averaging Time of Primary PM2.5 Standards
    E. Form of Primary PM2.5 Standards
    1. 24-Hour PM2.5 Standard
    2. Annual PM2.5 Standard
    F. Level of Primary PM2.5 Standards
    1. 24-Hour PM2.5 Standard
    2. Annual PM2.5 Standard
    G. Final Decisions on Primary PM2.5 Standards
III. Rationale for Final Decisions on Primary PM10 
Standards
    A. Introduction
    1. Overview
    2. Overview of Health Effects Evidence
    3. Overview of Quantitative Risk Assessment
    B. Need for Revision of the Current Primary PM10 
Standards
    1. Overview of the Proposal
    2. Comments on the Need for Revision
    C. Indicator for Thoracic Coarse Particles
    1. Introduction
    2. Comments on Indicator for Thoracic Coarse Particles
    3. Decision Not to Revise PM10 Indicator
    a. Unqualified PM10-2.5 Indicator
    b. PM10 Indicator
    c. Unqualified PM10 Indicator, with Adjustment to the 
PM2.5 Component
    4. Conclusions Regarding Indicator for Thoracic Coarse Particles
    D. Conclusions Regarding Averaging Time, Form, and Level of the 
Current PM10 Standards
    1. Averaging Time
    2. Level and Form of the 24-Hour PM10 Standard
    E. Final Decisions on Primary PM10 Standards
IV. Rationale for Final Decisions on Secondary PM Standards
    A. Visibility Impairment
    1. Visibility Impairment Related to Ambient PM
    2. Need for Revision of the Current Secondary PM2.5 
Standards to Protect Visibility
    3. Indicator of PM for Secondary Standard to Address Visibility 
Impairment
    4. Averaging Time of a Secondary PM2.5 Standard for 
Visibility Protection
    5. Final Decisions on Secondary PM2.5 Standards for 
Visibility Protection
    B. Other PM-Related Welfare Effects
    1. Evidence of Non-Visibility Welfare Effects Related to PM
    2. Need for Revision of the Current Secondary PM Standards to 
Address Other PM-Related Welfare Effects
    C. Final Decisions on Secondary PM Standards
V. Interpretation of the NAAQS for PM
    A. Amendments to Appendix N--Interpretation of the National 
Ambient Air Quality Standards for PM2.5
    1. General
    2. PM2.5 Monitoring and Data Reporting Considerations
    3. PM2.5 Computations and Data Handling Conventions
    4. Conforming Revisions
    B. Proposed Appendix P--Interpretation of the National Ambient 
Air Quality Standards for PM10-2.5
    C. Amendments to Appendix K--Interpretation of the 
National Ambient Air Quality Standards for PM10
VI. Reference Methods for the Determination of Particulate 
Matter as PM10-2.5 and PM2.5
    A. Appendix O to Part 50--Reference Method for the 
Determination of Coarse Particulate Matter as PM10-2.5 in 
the Atmosphere
    B. Amendments to Appendix L--Reference Method for the 
Determination of Fine Particulate Matter (as PM2.5) in 
the Atmosphere
VII. Issues Related to Implementation of PM10 Standards
    A. Summary of Comments Received on Transition

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    B. Impact of Decision on PM10 Designations
    C. Impact of Decision on State Implementation Plans (SIPs) and 
Control Obligations
    D. Consideration of Fugitive Emissions for New Source Review 
(NSR) Purposes
    E. Handling of PM10 Exceedances Due to Exceptional 
Events
VIII. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination with 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children from 
Environmental Health & Safety Risks
    H. Executive Order 13211: Actions that Significantly Affect 
Energy Supply, Distribution or Use
    I. National Technology Transfer Advancement Act
    J. Executive Order 12898: Federal Actions to Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    K. Congressional Review Act
References

I. Background

A. Summary of Revisions to the PM NAAQS

    Based on its review of the air quality criteria and national 
ambient air quality standards (NAAQS) for particulate matter (PM), EPA 
is making revisions to the primary and secondary NAAQS for PM to 
provide increased protection of public health and welfare, 
respectively.
    With regard to primary standards for fine particles (generally 
referring to particles less than or equal to 2.5 micrometers ([micro]m) 
in diameter, PM2.5), EPA is revising the level of the 24-
hour PM2.5 standard to 35 micrograms per cubic meter 
[micro]g/m\3\), providing increased protection against health effects 
associated with short-term exposure (including premature mortality and 
increased hospital admissions and emergency room visits), and retaining 
the level of the annual PM2.5 standard at 15 [micro]g/m\3\, 
continuing protection against health effects associated with long-term 
exposure (including premature mortality and development of chronic 
respiratory disease). The EPA is revising the form of the annual 
PM2.5 standard with regard to the criteria for spatial 
averaging, such that averaging across monitoring sites is allowed if 
the annual mean concentration at each monitoring site is within 10 
percent of the spatially averaged annual mean, and the daily values for 
each monitoring site pair yield a correlation coefficient of at least 
0.9 for each calendar quarter.
    With regard to primary standards for particles generally less than 
or equal to 10[micro]m in diameter (PM10), EPA is retaining 
the 24-hour PM10 standard to protect against the health 
effects associated with short-term exposure to coarse particles 
(including hospital admissions for cardiopulmonary diseases, increased 
respiratory symptoms and possibly premature mortality). Given that the 
available evidence does not suggest an association between long-term 
exposure to coarse particles at current ambient levels and health 
effects, EPA is revoking the annual PM10 standard.
    With regard to secondary PM standards, EPA is revising the current 
24-hour PM2.5 secondary standard by making it identical to 
the revised 24-hour PM2.5 primary standard, retaining the 
annual PM2.5 and 24-hour PM10 secondary 
standards, and revoking the annual PM10 secondary standard. 
This suite of secondary PM standards is intended to provide protection 
against PM-related public welfare effects, including visibility 
impairment, effects on vegetation and ecosystems, and materials damage 
and soiling.

B. Legislative Requirements

    Two sections of the Clean Air Act (CAA) govern the establishment 
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the 
Administrator to identify and list ``air pollutants'' that ``in his 
judgment, may reasonably be anticipated to endanger public health and 
welfare'' and whose ``presence * * * in the ambient air results from 
numerous or diverse mobile or stationary sources'' and to issue air 
quality criteria for those that are listed. Air quality criteria are 
intended to ``accurately reflect the latest scientific knowledge useful 
in indicating the kind and extent of identifiable effects on public 
health or welfare which may be expected from the presence of [a] 
pollutant in ambient air * * * .''
    Section 109 (42 U.S.C. 7409) directs the Administrator to propose 
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants 
listed under section 108. Section 109(b)(1) defines a primary standard 
as one ``the attainment and maintenance of which in the judgment of the 
Administrator, based on such criteria and allowing an adequate margin 
of safety, are requisite to protect the public health.'' \1\ A 
secondary standard, as defined in section 109(b)(2), must ``specify a 
level of air quality the attainment and maintenance of which, in the 
judgment of the Administrator, based on such criteria, is requisite to 
protect the public welfare from any known or anticipated adverse 
effects associated with the presence of [the] pollutant in the ambient 
air.'' \2\
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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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    The requirement that primary standards include an adequate margin 
of safety was intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It was also intended to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. Lead Industries Association v. EPA, 647 F.2d 1130, 1154 
(D.C. Cir 1980), cert. denied, 449 U.S. 1042 (1980); American Petroleum 
Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert. 
denied, 455 U.S. 1034 (1982). Both kinds of uncertainties are 
components of the risk associated with pollution at levels below those 
at which human health effects can be said to occur with reasonable 
scientific certainty. Thus, in selecting primary standards that include 
an adequate margin of safety, the Administrator is seeking not only to 
prevent pollution levels that have been demonstrated to be harmful but 
also to prevent lower pollutant levels that may pose an unacceptable 
risk of harm, even if the risk is not precisely identified as to nature 
or degree. The CAA does not require the Administrator to establish a 
primary NAAQS at a zero-risk level or at a background concentration 
level (see Lead Industries Association v. EPA, supra, 647 F.2d at 1156 
n. 51), but rather at a level that reduces risk sufficiently so as to 
protect public health with an adequate margin of safety.
    In addressing the requirement for an adequate margin of safety, EPA 
considers such factors as the nature and severity of the health effects 
involved, the size of the sensitive population(s) at risk, and the kind 
and degree of the uncertainties that must be addressed. The selection 
of any particular approach to providing an adequate margin of safety is 
a policy choice left specifically to the Administrator's judgment. Lead

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Industries Association v. EPA, supra, 647 F.2d at 1161-62.
    In setting standards that are ``requisite'' to protect public 
health and welfare, as provided in section 109(b), EPA's task is to 
establish standards that are neither more nor less stringent than 
necessary for these purposes. In establishing primary and secondary 
standards, EPA may not consider the costs of implementing the 
standards. See generally Whitman v. American Trucking Associations, 531 
U.S. 457, 465-472, 475-76 (2001).
    Section 109(d)(1) of the CAA requires that ``not later than 
December 31, 1980, and at 5-year intervals thereafter, the 
Administrator shall complete a thorough review of the criteria 
published under section 108 and the national ambient air quality 
standards * * * and shall make such revisions in such criteria and 
standards and promulgate such new standards as may be appropriate in 
accordance with [the provisions in section 109(b) on primary and 
secondary standards].'' This includes the authority to modify or revoke 
a standard or standards, as appropriate under these provisions. Section 
109(d)(2) requires that an independent scientific review committee 
``shall complete a review of the criteria * * * and the national 
primary and secondary ambient air quality standards * * * and shall 
recommend to the Administrator any new * * * standards and revisions of 
existing criteria and standards as may be appropriate * * *.'' This 
independent review function is performed by the Clean Air Scientific 
Advisory Committee (CASAC) of EPA's Science Advisory Board.

C. Overview of Air Quality Criteria and Standards Review for PM

    Particulate matter is the generic term for a broad class of 
chemically and physically diverse substances that exist as discrete 
particles (liquid droplets or solids) over a wide range of sizes. 
Particles originate from a variety of anthropogenic stationary and 
mobile sources as well as from natural sources. Particles may be 
emitted directly or formed in the atmosphere by transformations of 
gaseous emissions such as sulfur oxides (SOX), nitrogen 
oxides (NOX), and volatile organic compounds (VOC). The 
chemical and physical properties of PM vary greatly with time, region, 
meteorology, and source category, thus complicating the assessment of 
health and welfare effects.
    More specifically, the PM that is the subject of the air quality 
criteria and standards reviews includes both fine particles and 
thoracic coarse particles, which are considered as separate subclasses 
of PM pollution based in part on long-established information on 
differences in sources, properties, and atmospheric behavior between 
fine and coarse particles (EPA, 2005, section 2.2). Fine particles are 
produced chiefly by combustion processes and by atmospheric reactions 
of various gaseous pollutants, whereas thoracic coarse particles are 
generally emitted directly as particles as a result of mechanical 
processes that crush or grind larger particles or the resuspension of 
dusts. Sources of fine particles include, for example, motor vehicles, 
power generation, combustion sources at industrial facilities, and 
residential fuel burning. Sources of thoracic coarse particles include, 
for example, traffic-related emissions such as tire and brake lining 
materials, direct emissions from industrial operations, construction 
and demolition activities, and agricultural and mining operations. Fine 
particles can remain suspended in the atmosphere for days to weeks and 
can be transported thousands of kilometers, whereas thoracic coarse 
particles generally deposit rapidly on the ground or other surfaces and 
are not readily transported across urban or broader areas.
    The last review of PM air quality criteria and standards was 
completed in July 1997 with notice of a final decision to revise the 
existing standards (62 FR 38652, July 18, 1997). In that decision, EPA 
revised the PM NAAQS in several respects. While EPA determined that the 
PM NAAQS should continue to focus on particles less than or equal to 10 
[mu]m in diameter (PM10), EPA also determined that the fine 
and coarse fractions of PM10 should be considered 
separately. The EPA added new standards, using PM2.5 as the 
indicator for fine particles (with PM2.5 referring to 
particles with a nominal aerodynamic diameter less than or equal to 2.5 
[mu]m), and using PM10 as the indicator for purposes of 
regulating the coarse fraction of PM10 (referred to as 
thoracic coarse particles or coarse-fraction particles; generally 
including particles with a nominal aerodynamic diameter greater than 
2.5 [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) (``ATA I'') rehearing granted in part and denied in part, 
195 F.3d 4 (D.C. Cir. 1999) (``ATA II''), affirmed in part and reversed 
in part, Whitman v. American Trucking Associations, 531 U.S. 457 
(2001). The Panel also found ``ample support'' for EPA's decision to 
regulate coarse particle pollution, but vacated the 1997 
PM10 standards, concluding that EPA's justification for the 
use of PM10 as an indicator for coarse particles was 
arbitrary. 175 F.3d at 1054-55. Pursuant to the court's decision, EPA 
removed the vacated 1997 PM10 standards from the regulations 
(CFR) (69 FR 45592, July 30, 2004) and deleted the regulatory provision 
(at 40 CFR 50.6(d)) that controlled the transition from the 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 panel held (over one judge's dissent) that 
EPA's approach to establishing the level of the standards in 1997, both 
for PM and for ozone NAAQS promulgated on the same day, effected ``an 
unconstitutional delegation of legislative authority.'' Id. at 1034-40. 
Although the panel stated that ``the factors EPA uses in determining 
the degree of public health concern associated with different levels of 
ozone and PM are reasonable,'' it remanded the rule to EPA, stating 
that when EPA considers these factors for potential non-threshold 
pollutants ``what EPA lacks is any determinate criterion for

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drawing lines'' to determine where the standards should be set. 
Consistent with EPA's long-standing interpretation and D.C. Circuit 
precedent, the panel also reaffirmed prior rulings holding that in 
setting NAAQS EPA is ``not permitted to consider the cost of 
implementing those standards.'' Id. at 1040-41.
    Both sides filed cross appeals on these issues to the United States 
Supreme Court, and the Court granted certiorari. In February 2001, the 
Supreme Court issued a unanimous decision upholding EPA's position on 
both the constitutional and cost issues. Whitman v. American Trucking 
Associations, 531 U.S. 457, 464, 475-76 (2001). On the constitutional 
issue, the Court held that the statutory requirement that NAAQS be 
``requisite'' to protect public health with an adequate margin of 
safety sufficiently guided EPA's discretion, affirming EPA's approach 
of setting standards that are neither more nor less stringent than 
necessary. The Supreme Court remanded the case to the Court of Appeals 
for resolution of any remaining issues that had not been addressed in 
that court's earlier rulings. Id. at 475-76. In March 2002, the Court 
of Appeals rejected all remaining challenges to the standards, holding 
under the traditional standard of judicial review that EPA's 
PM2.5 standards were reasonably supported by the 
administrative record and were not ``arbitrary and capricious.'' 
American Trucking Associations v. EPA, 283 F. 3d 355, 369-72 (D.C. Cir. 
2002) (``ATA III'').
    In October 1997, EPA published its plans for the current periodic 
review of the PM criteria and NAAQS (62 FR 55201, October 23, 1997), 
including the 1997 PM2.5 standards and the 1987 
PM10 standards. The approach in this review continues to 
address fine and thoracic coarse particles separately. This approach 
has been reinforced by new information that has advanced our 
understanding of differences in human exposure relationships and 
dosimetric patterns characteristic of these two subclasses of PM 
pollution, as well as the apparent independence of health effects that 
have been associated with them in epidemiologic studies (EPA, 2004a, 
section 3.2.3). See also ATA I, 175 F. 3d at 1053-54, 1055-56 (EPA 
justified in establishing separate standards for fine and thoracic 
coarse particles).
    As part of the process of preparing an updated Air Quality Criteria 
Document for Particulate Matter (henceforth, the ``Criteria 
Document''), EPA's National Center for Environmental Assessment (NCEA) 
hosted a peer review workshop in April 1999 on drafts of key Criteria 
Document chapters. The first external review draft Criteria Document 
was reviewed by CASAC and the public at a meeting held in December 
1999. Based on CASAC and public comment, NCEA revised the draft 
Criteria Document and released a second draft in March 2001 for review 
by CASAC and the public at a meeting held in July 2001. A preliminary 
draft of a staff paper, Review of the National Ambient Air Quality 
Standards for Particulate Matter: Assessment of Scientific and 
Technical Information (henceforth, the ``Staff Paper'') prepared by 
EPA's Office of Air Quality Planning and Standards (OAQPS) was released 
in June 2001 for public comment and for consultation with CASAC at the 
same public meeting. Taking into account CASAC and public comments, a 
third draft Criteria Document was released in May 2002 for review at a 
meeting held in July 2002.
    Shortly after the release of the third draft Criteria Document, the 
Health Effects Institute (HEI) \3\ announced that researchers at Johns 
Hopkins University had discovered problems with applications of 
statistical software used in a number of important epidemiological 
studies that had been discussed in that draft Criteria Document. In 
response to this significant issue, EPA took steps in consultation with 
CASAC and the broader scientific community to encourage researchers to 
reanalyze affected studies and to submit them expeditiously for peer 
review by a special expert panel convened at EPA's request by HEI. The 
results of this reanalysis and peer-review process were subsequently 
incorporated into a fourth draft Criteria Document, which was released 
in June 2003 and reviewed by CASAC and the public at a meeting held in 
August 2003.
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    \3\ The HEI is a non-profit, independent research institute 
jointly and equally funded by EPA and multiple industries that 
conducts research on the health effects of air pollution.
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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. During that 
meeting, EPA also consulted with CASAC on a new framework for the final 
chapter (integrative synthesis) of the Criteria Document and on ongoing 
revisions to other Criteria Document chapters to address previous CASAC 
comments. The EPA held additional consultations with CASAC at public 
meetings held in February, July, and September 2004, leading to 
publication of the final Criteria Document in October 2004 (EPA, 
2004a). The second draft Staff Paper, based on the final Criteria 
Document, was released at the end of January 2005, and was reviewed by 
CASAC and the public at a meeting held in April 2005. The CASAC's 
advice and recommendations to the Administrator, based on its review of 
the second draft Staff Paper, were further discussed during a public 
teleconference held in May 2005 and are provided in a June 6, 2005 
letter to the Administrator (Henderson, 2005a). The final Staff Paper 
takes into account the advice and recommendations of CASAC and public 
comments received on the earlier drafts of this document. The 
Administrator subsequently received additional advice and 
recommendations from the CASAC, specifically on potential standards for 
thoracic coarse particles, in a teleconference on August 11, 2005, and 
in a letter to the Administrator dated September 15, 2005 (Henderson, 
2005b). The final Staff Paper was reissued in December 2005 to add 
CASAC's final letter as an attachment (EPA, 2005).
    The schedule for completion of this review is governed by a consent 
decree resolving a lawsuit filed in March 2003 by a group of plaintiffs 
representing national environmental organizations. The lawsuit alleged 
that EPA had failed to perform its mandatory duty, under section 
109(d)(1), of completing the current review within the period provided 
by statute. American Lung Association v. Whitman (No. 1:03CV00778, 
D.D.C. 2003). An initial consent decree was entered by the court in 
July 2003 after an opportunity for public comment. The consent decree, 
as modified by the court, provides that EPA will sign for publication 
notices of proposed and final rulemaking concerning its review of the 
PM NAAQS no later than December 20, 2005 and September 27, 2006, 
respectively.
    On December 20, 2005, EPA issued its proposed decision to revise 
the NAAQS for PM (71 FR 2620, January 17, 2006) (henceforth 
``proposal''). In the proposal, EPA identified proposed revisions to 
the standards, based on the air quality criteria for PM, and to related 
data handling conventions and federal reference methods for monitoring 
PM. The proposal solicited public comments on alternative primary and 
secondary standards and related matters.
    The EPA held several public hearings across the country to provide 
direct opportunities for public comment on the proposed revisions to 
the PM NAAQS. On March 8, 2006, EPA held three concurrent 12-hour 
public hearings in Philadelphia, PA; Chicago, IL; and San Francisco, 
CA. At these public hearings, EPA heard testimony

[[Page 61148]]

from 280 individuals representing themselves or specific interested 
organizations.
    More than 120,000 comments were received from members of the public 
and various interested groups on the proposed revisions to the PM NAAQS 
by the close of the public comment period on April 17, 2006. CASAC 
provided additional advice to EPA in a letter to the Administrator 
requesting reconsideration of CASAC's recommendations for both the 
primary and secondary PM2.5 standards as well as standards 
for thoracic coarse particles (Henderson, 2006). Major issues raised in 
the public comments are discussed throughout the preamble of this final 
action. A comprehensive summary of all significant comments, along with 
EPA's responses (henceforth ``Response to Comments''), can be found in 
the docket for this rulemaking (Docket No. EPA-HQ-OAR-2001-0017).
    In the proposal, EPA recognized that there were a number of new 
scientific studies on the health effects of PM that had been published 
recently and therefore were not included in the Criteria Document.\4\ 
The EPA committed to conduct a review and assessment of any significant 
``new'' studies, including studies submitted during the public comment 
period. The purpose of this review was to ensure that the Administrator 
was fully aware of the ``new'' science before making a final decision 
on whether to revise the current PM NAAQS. The EPA screened and 
surveyed the recent literature, including studies submitted during the 
public comment period, and conducted a provisional assessment (EPA, 
2006a) that places the results of those studies of potentially greatest 
policy relevance in the context of the findings of the Criteria 
Document.
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    \4\ For ease of reference, these studies will be referred to as 
``new'' studies or ``new'' science, using quotation marks around the 
word new. Referring to studies that were published too recently to 
have been included in the 2004 Criteria Document as ``new'' studies 
is intended to clearly differentiate such studies from those that 
have been published since the last review and are included in the 
2004 Criteria Document (these studies are sometimes referred to as 
new (without quotation marks) or more recent studies, to indicate 
that they were not included in the 1996 Criteria Document and thus 
are newly available in this review).
---------------------------------------------------------------------------

    The provisional assessment found that the ``new'' studies expand 
the scientific information and provide important insights on the 
relationship between PM exposure and health effects of PM. The 
provisional assessment also found that ``new'' studies generally 
strengthen the evidence that acute and chronic exposure to fine 
particles and acute exposure to thoracic coarse particles are 
associated with health effects; some of the ``new'' epidemiologic 
studies report effects in areas with lower concentrations of 
PM2.5 or PM10-2.5 than those in earlier reports; 
``new'' toxicology and epidemiologic studies link various health 
effects with a range of fine particle sources and components; and 
``new'' toxicology studies report effects of thoracic coarse particles 
but do not provide evidence to support distinguishing effects from 
exposure to urban and rural particles. Further, the provisional 
assessment found that the results reported in the studies do not 
dramatically diverge from previous findings, and, taken in context with 
the findings of the Criteria Document, the new information and findings 
do not materially change any of the broad scientific conclusions 
regarding the health effects of PM exposure made in the Criteria 
Document.
    The EPA believes it was important to conduct a provisional 
assessment in this case, so that the Administrator would be aware of 
the science that developed too recently for inclusion in the Criteria 
Document. However it is also important to note that EPA's review of 
that science to date has been limited to screening, surveying, and 
preparing a provisional assessment of these studies. Having performed 
this limited provisional assessment, EPA must decide whether to 
consider the newer studies in this review and take such steps as may be 
necessary to include them in the basis for the final decision, or to 
reserve such action for the next review of the PM NAAQS.
    As in prior NAAQS reviews, EPA is basing its decision in this 
review on studies and related information included in the Criteria 
Document and Staff Paper, which have undergone CASAC and public review. 
The studies assessed in the Criteria Document, and the integration of 
the scientific evidence presented in that document, have undergone 
extensive critical review by EPA, CASAC, and the public during the 
development of the Criteria Document. The rigor of that review makes 
these studies, and their integrative assessment, the most reliable 
source of scientific information on which to base decisions on the 
NAAQS, decisions that all parties recognize as of great import. NAAQS 
decisions can have profound impacts on public health and welfare, and 
NAAQS decisions should be based on studies that have been rigorously 
assessed in an integrative manner not only by EPA but also by the 
statutorily mandated independent advisory committee, as well as the 
public review that accompanies this process. As described above, the 
provisional assessment did not and could not provide that kind of in-
depth critical review.
    This decision is consistent with EPA's practice in prior NAAQS 
reviews. Since the 1970 amendments, the EPA has taken the view that 
NAAQS decisions are to be based on scientific studies and related 
information that have been assessed as a part of the pertinent air 
quality criteria. See e.g., 36 FR 8186 (April 30, 1971) (EPA based 
original NAAQS for six pollutants on scientific studies discussed in 
air quality criteria documents and limited consideration of comments to 
those concerning validity of scientific basis); 38 FR 25678, 25679-
25680 (September 14, 1973) (EPA revised air quality criteria for sulfur 
oxides to provide basis for reevaluation of secondary NAAQS). This 
longstanding interpretation was strengthened by new legislative 
requirements enacted in 1977, which added section 109(d)(2) of the Act 
concerning CASAC review of air quality criteria. EPA has consistently 
followed this approach. 52 FR 24634, 24637 (July 1, 1987) (after review 
by CASAC, EPA issued a post-proposal addendum to the PM Criteria 
Document, to address certain new scientific studies not included in the 
1982 Criteria Document); 61 FR 25566, 25568 (May 22, 1996) (after 
review by CASAC, EPA issued a post-proposal supplement to the 1982 
Criteria Document to address certain new health studies not included in 
the 1982 Criteria Document or 1986 Addendum). The EPA recently 
reaffirmed this approach in its decision not to revise the ozone NAAQS 
in 1993, as well as in its final decision on the PM NAAQS in the 1997 
review. 58 FR 13008, 13013-13014 (March 9, 1993) (ozone review); 62 FR 
38652, 38662 (July 18, 1997) (The EPA conducted a provisional 
assessment but based the final PM decision on studies and related 
information included in the air quality criteria that had been reviewed 
by CASAC).
    As discussed in EPA's 1993 decision not to revise the NAAQS for 
ozone, new studies may sometimes be of such significance that it is 
appropriate to delay a decision on revision of NAAQS and to supplement 
the pertinent air quality criteria so the new studies can be taken into 
account (58 FR at 13013-13014, March 9, 1993). In the present case, the 
provisional assessment of recent studies concludes that, taken in 
context, the new information and findings do not materially change any 
of the broad scientific conclusions regarding the health effects of PM 
exposure made in the Criteria

[[Page 61149]]

Document. For this reason, reopening the air quality criteria review 
would not be warranted even if there were time to do so under the court 
order governing the schedule for this rulemaking. Accordingly, EPA is 
basing the final decisions in this review on the studies and related 
information included in the PM air quality criteria that have undergone 
CASAC and public review. The EPA will consider the newly published 
studies for purposes of decision making in the next periodic review of 
the PM NAAQS, which will provide the opportunity to fully assess them 
through a more rigorous review process involving EPA, CASAC, and the 
public.
    In order to facilitate a comprehensive and timely review of the 
newly available science, the Administrator has directed EPA staff to 
begin the next review of the PM NAAQS immediately.\5\
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    \5\ The EPA has recently conducted a review of the process by 
which the Agency performs periodic NAAQS reviews to identify ways in 
which the process could be strengthened and streamlined (EPA, 
2006b). The EPA intends to incorporate recommendations from the 
NAAQS process review into the next PM NAAQS review.
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D. Related Control Programs To Implement PM Standards

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once EPA has established 
them. Under section 110 of the CAA (42 U.S.C. 7410) and related 
provisions, States are to submit, for EPA approval, State 
implementation plans (SIPs) that provide for the attainment and 
maintenance of such standards through control programs directed to 
sources of the pollutants involved. The States, in conjunction with 
EPA, also administer the prevention of significant deterioration (PSD) 
program under sections 160-169 of the CAA (42 U.S.C. 7470-7479) for 
these pollutants. In addition, the Act provides for nationwide 
reductions in emissions of these and other air pollutants through 
related programs, such as the Federal Mobile Source Control Program 
under Title II of the CAA (42 U.S.C. 7521-7574), which involves 
controls for automobile, truck, bus, motorcycle, nonroad and off-
highway engines and aircraft emissions; the new source performance 
standards under section 111 (42 U.S.C. 7411); and the national emission 
standards for hazardous air pollutants under section 112 (42 U.S.C. 
7412).
    As described in a recent EPA report, The Particle Pollution Report: 
Current Understanding of Air Quality and Emissions through 2003 (EPA, 
2004b), State and Federal programs have made substantial progress in 
reducing ambient concentrations of PM10 and 
PM2.5. For example, PM10 concentrations have 
decreased 31 percent nationally since 1988. Regionally, PM10 
concentrations decreased most in areas with historically higher 
concentrations--the Northwest (39 percent decline), the Southwest (33 
percent decline), and southern California (35 percent decline). Direct 
emissions of PM10 have decreased approximately 25 percent 
nationally since 1988.
    Programs aimed at reducing direct emissions of particles have 
played an important role in reducing PM10 concentrations, 
particularly in western areas. Some examples of PM10 
controls include paving unpaved roads and using best management 
practices for agricultural sources of resuspended soil. Of the 87 areas 
that were designated nonattainment for PM10 in the early 
1990s, 64 now meet those standards. In cities that have not attained 
the PM10 standards, the number of days above the standards 
is down significantly.
    Nationally, PM2.5 concentrations have declined by 10 
percent from 1999 to 2003. Generally, PM2.5 concentrations 
have also declined the most in regions with the highest 
concentrations--the Southeast (20 percent decline), southern California 
(16 percent decline), and the Industrial Midwest (9 percent decline). 
With the exception of the Northeast, the remaining regions posted 
modest declines in PM2.5 concentrations from 1999 to 2003. 
Direct emissions of PM2.5 have decreased by 5 percent 
nationally over the past 5 years.
    National programs that affect regional emissions have also 
contributed to lower sulfate concentrations and, consequently, to lower 
PM2.5 concentrations, particularly in the Industrial Midwest 
and Southeast. National ozone-reduction programs designed to reduce 
emissions of volatile organic compounds (VOCs) and nitrogen oxides 
(NOX) have also helped reduce carbon and nitrates, both of 
which are components of PM2.5. Additionally, EPA's Acid Rain 
Program has substantially reduced sulfur dioxide (SO2) 
emissions from power plants since 1995 in the eastern United States, 
contributing to lower PM concentrations. Nationally, SO2 
emissions have declined 9 percent, NOX emissions have 
declined 9 percent, and VOC emissions have declined by 12 percent from 
1999 to 2003. In eastern States affected by the Acid Rain Program, 
sulfates decreased 7 percent over the same period.
    Over the next 10 to 20 years, national and regional regulations 
will make major reductions in ambient PM2.5 levels. The 
Clean Air Interstate Rule (CAIR) and the NOX SIP Call will 
further reduce SO2 and NOX emissions from 
electric generating units and industrial boilers across the eastern 
half of the U.S.; regulations to implement the 1997 ambient air quality 
standards for PM2.5 will require direct PM2.5 and 
PM2.5 precursor controls in nonattainment areas; and new 
national mobile source regulations affecting heavy-duty diesel engines, 
highway vehicles, and other mobile sources will reduce emissions of 
NOX, direct PM2.5, SO2, and VOCs. The 
EPA estimates that these regulations for stationary and mobile sources 
will cut SO2 emissions by 6 million tons annually in 2015 
from 2001 levels. Emissions of NOX will be cut by 9 million 
tons annually in 2015 from 2001 levels. Emissions of VOCs will drop by 
3 million tons, and direct PM2.5 emissions will be cut by 
200,000 tons in 2015, compared to 2001 levels.
    In 2005, 39 nonattainment areas were designated as not attaining 
the PM2.5 standards established in 1997. SIPs for these 
areas are due in April 2008. Nonattainment areas are required to attain 
the standards as ``expeditiously as practicable'' based on 
implementation of federal measures already in place and the adoption of 
other reasonable control strategies for sources located in the 
nonattainment area and state. The presumptive timeframe for attainment 
is within five years of designation, although EPA may approve extended 
attainment dates of an additional one to five years for areas with more 
serious problems.
    Modeling done by EPA indicates that by 2010, 18 of the 39 currently 
designated nonattainment areas are projected to come into attainment 
with those standards just based on regulatory programs already in 
place, including CAIR, the Clean Diesel Rules, and other Federal 
measures. Between 2010 and 2015, further reductions in PM 
concentrations in the eastern U.S. are projected due to existing 
federal programs alone, on the order of 0.5 to 1.5 [mu]g/m\3\. All 
areas in the eastern U.S. will have lower PM2.5 
concentrations in 2015 relative to present-day conditions. In most 
cases, the predicted improvement in PM2.5 ranges from 10 
percent to 20 percent.

E. Summary of Proposed Revisions to the PM NAAQS

    For reasons discussed in the proposal, the Administrator proposed 
to revise the current primary and secondary PM2.5 and 
PM10 standards. With regard to the primary PM2.5 
standards, the Administrator proposed to revise the level of the 24-
hour PM2.5 standard to 35

[[Page 61150]]

[mu]g/m\3\, and to revise the form of the annual PM2.5 
standard by changing the constraints on the optional use of spatial 
averaging to include the criterion that the minimum correlation 
coefficient between monitor pairs to be averaged be 0.9 or greater, 
determined on a seasonal basis, and the criterion that differences 
between monitor values not exceed 10 percent. Related revisions for 
PM2.5 data handling conventions and for the reference method 
for monitoring PM as PM2.5 were also proposed.
    With regard to the primary PM10 standards, the 
Administrator proposed to revise the current standards to provide more 
targeted protection from thoracic coarse particles that are of concern 
to public health. In part, the Administrator proposed to establish a 
new indicator for thoracic coarse particles in terms of 
PM10-2.5, the definition of which included qualifications 
that identified both the mix of such particles that were provisionally 
determined to be of concern to public health, and were thus included in 
the indicator, and those for which currently available information was 
provisionally determined to be insufficient as a basis from which to 
infer a public health concern, and were thus excluded. More 
specifically, the proposed PM10-2.5 indicator was qualified 
so as to include any ambient mix of PM10-2.5 that is 
dominated by resuspended dust from high-density traffic on paved roads 
and PM generated by industrial sources and construction sources, and to 
exclude any ambient mix of PM10-2.5 that is dominated by 
rural windblown dust and soils and PM generated by agricultural and 
mining sources. The Administrator also proposed that agricultural 
sources, mining sources, and other similar sources of crustal material 
shall not be subject to control in meeting the proposed standard. The 
Administrator proposed to replace the current primary 24-hour 
PM10 standard with a 24-hour standard defined in terms of 
this new PM10-2.5 indicator. The proposed new standard would 
be met at an ambient air quality monitoring site when the 3-year 
average of the annual 98th percentile 24-hour average 
PM10-2.5 concentration is less than or equal to 70 [mu]g/
m\3\, which would generally maintain the degree of public health 
protection afforded by the current PM10 standards from 
short-term exposure to thoracic coarse particles of concern. 
Requirements for monitoring sites that would be appropriate for 
determining compliance with this proposed PM10-2.5 standard 
were included as part of proposed revisions to EPA's ambient air 
monitoring regulations (see 71 FR 2710, 2736-2728 and 71 FR 2706-2707 
(proposing to incorporate these requirements as part of the standard)). 
These proposed requirements included a five-part test for determining 
whether a potential monitoring site is suitable for comparison to the 
standard, all five parts of which had to be met. In summary, the 
suitability test included the following general provisions: a 
monitoring site must be within an urbanized area that has a population 
of at least 100,000 persons; the site must be within a block group with 
a population density greater than 500 people per square mile; the site 
must be a ``population-oriented'' site; the site may not be adjacent to 
a large emissions source or otherwise within the micro-scale 
environment affected by a large source; and, if the first four 
provisions are met, a site-specific assessment must show that the 
ambient mix of PM10-2.5 sampled at the site would be 
dominated by resuspended dust from high-density traffic on paved roads 
and PM generated by industrial sources and construction sources, and 
would not be dominated by rural windblown dust and soils and PM 
generated by agricultural and mining sources. Related new 
PM10-2.5 data handling conventions and a new reference 
method for monitoring PM as PM10-2.5 were also proposed. The 
Administrator also proposed to revoke and not replace the annual 
PM10 standard.
    With regard to the secondary PM2.5 and PM10 
standards, the Administrator proposed to revise the current standards 
by making them identical in all respects to the proposed primary 
PM2.5 and PM10-2.5 standards to address PM-
related welfare effects including visibility impairment, effects on 
vegetation and ecosystems, materials damage and soiling, and effects on 
climate change.

F. Organization and Approach to Final PM NAAQS Decisions

    This action presents the Administrator's final decisions on the 
review of the current primary and secondary PM2.5 and 
PM10 standards. Primary standards for fine particles and for 
thoracic coarse particles are addressed below in sections II and III, 
respectively. Consistent with the decisions made by EPA in the last 
review and with the conclusions in the Criteria Document and Staff 
Paper, fine and thoracic coarse particles continue to be considered as 
separate subclasses of PM pollution. Secondary standards for fine and 
thoracic coarse particles are addressed below in section IV. Related 
data handling conventions and federal reference methods for monitoring 
PM are addressed below in sections V and VI, respectively.
    Today's final decisions separately addressing fine and thoracic 
coarse particles are based on a thorough review in the Criteria 
Document of scientific information on known and potential human health 
and welfare effects associated with exposure to these subclasses of PM 
at levels typically found in the ambient air. These final decisions 
also take into account: (1) Staff assessments in the Staff Paper of the 
most policy-relevant information in the Criteria Document as well as a 
quantitative risk assessment based on that information; (2) CASAC 
advice and recommendations, as reflected in its letters to the 
Administrator, its discussions of drafts of the Criteria Document and 
Staff Paper at public meetings, and separate written comments prepared 
by individual members of the CASAC PM Review Panel \6\ (henceforth, 
``CASAC Panel''); (3) public comments received during the development 
of these documents, either in connection with CASAC meetings or 
separately; and (4) extensive public comments received on the proposed 
rulemaking.
---------------------------------------------------------------------------

    \6\ The CASAC PM Review Panel is comprised of the seven members 
of the chartered CASAC, supplemented by fifteen subject-matter 
experts appointed by the Administrator to provide additional 
scientific expertise relevant to this review of the PM NAAQS.
---------------------------------------------------------------------------

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

A. Introduction

1. Overview
    This section presents the Administrator's final decisions regarding 
the need to revise the current primary PM2.5 NAAQS, and, 
more specifically, regarding revisions to the level of the 24-hour 
standard and to the form of the annual standard. As discussed more 
fully below, the rationale for the final decision on appropriate 
revisions to the primary PM2.5 NAAQS includes consideration 
of: (1) Evidence of health effects related to short- and long-term 
exposures to fine particles; (2) insights gained from a quantitative 
risk assessment; and (3) specific conclusions regarding the need for 
revisions to the current standards and the elements of PM2.5 
standards (i.e., indicator, averaging time, form, and level) that, 
taken together, are requisite to protect public health with an adequate 
margin of safety.
    In developing this rationale, EPA has drawn upon an integrative 
synthesis of the entire body of evidence on associations between 
exposure to

[[Page 61151]]

ambient fine particles and a broad range of health endpoints (EPA, 
2004a, Chapter 9), focusing on those health endpoints for which the 
Criteria Document concluded that the associations are likely to be 
causal. This body of evidence includes hundreds of studies conducted in 
many countries around the world, using various indicators of fine 
particles. In its assessment of the evidence judged to be most relevant 
to decisions on elements of the primary PM2.5 standards, EPA 
has placed greater weight on U.S. and Canadian studies using 
PM2.5 measurements, since studies conducted in other 
countries may well reflect different demographic and air pollution 
characteristics.
    As with virtually any policy-relevant scientific research, there is 
uncertainty in the characterization of health effects attributable to 
exposure to ambient fine particles, most generally with regard to 
whether observed associations are likely causal in nature and, if so, 
whether there are exposure levels below which such associations are no 
longer likely. As discussed below, an unprecedented amount of new 
research has been conducted since the last review, with important new 
information coming from epidemiologic, toxicologic, controlled human 
exposure, and dosimetric studies. Moreover, the newly available 
research studies evaluated in the Criteria Document have undergone 
intensive scrutiny through multiple layers of peer review, with 
extended opportunities for review and comment by CASAC and the public. 
While important uncertainties remain, the review of the health effects 
information has been extensive and deliberate. In the judgment of the 
Administrator, this intensive evaluation of the scientific evidence 
provides an adequate basis for regulatory decision making at this time. 
This review also provides important input to EPA's research plan for 
improving our future understanding of the relationships between 
exposures to ambient fine particles and health effects.
    The health effects information and quantitative risk assessment 
were summarized in sections II.A and II.B of the proposal (71 FR 2626-
2641) and are only briefly outlined below in sections II.A.2 and 
II.A.3. Subsequent sections of this preamble provide a more complete 
discussion of the Administrator's rationale, in light of key issues 
raised in public comments, for concluding that it is appropriate to 
revise the current primary PM2.5 standards (section II.B), 
as well as a more complete discussion of the Administrator's rationale 
for retaining or revising the specific elements of the primary 
PM2.5 standards, namely the indicator (section II.C); 
averaging time (section II.D); form (section II.E); and level (section 
II.F). A summary of the final decisions on revisions to the primary 
PM2.5 standards is presented in section II.G.
2. Overview of Heath Effects Evidence
    This section briefly outlines the information presented in Section 
II.A of the proposal on the health effects associated with exposure to 
fine particles. As was true in the last review, evidence from 
epidemiologic studies plays a key role in the Criteria Document's 
evaluation of the scientific evidence. Some highlights of the new 
epidemiologic evidence available since the last review include:
    (1) New multi-city studies that use uniform methodologies to 
investigate the effects of various indicators of PM on health with data 
from multiple locations with varying climate and air pollution mixes, 
contributing to increased understanding of the role of various 
potential confounders, including gaseous co-pollutants, on observed 
associations with fine particles. These studies provide more precise 
estimates of the magnitude of an effect of exposure to PM, including 
fine particles, than most smaller-scale individual city studies.
    (2) More studies of various health endpoints evaluating 
associations between effects and exposures to fine particles and 
thoracic coarse particles (discussed below in section III), as well as 
ultrafine particles or specific components (e.g., sulfates, nitrates, 
metals, organic compounds, and elemental carbon) of fine particles.
    (3) Numerous studies of cardiovascular endpoints, with particular 
emphasis on assessment of cardiovascular risk factors or physiological 
changes.
    (4) Studies relating population exposure to fine particles and 
other pollutants measured at centrally located monitors to estimates of 
exposure to ambient pollutants at the individual level. Such studies 
have led to a better understanding of the relationship between ambient 
fine particle levels and personal exposures to fine particles of 
ambient origin.
    (5) New statistical approaches to addressing issues related to 
potential confounding by gaseous co-pollutants, possible thresholds for 
effects, and measurement error and exposure misclassification.\7\
---------------------------------------------------------------------------

    \7\ ``Confounding'' occurs when a health effect that is caused 
by one risk factor is attributed to another variable that is 
correlated with the causal risk factor; epidemiologic analyses 
attempt to adjust or control for potential confounders (EPA, 2004a, 
section 8.1.3.2; EPA, 2005, section 3.6.4). A ``threshold'' is a 
concentration below which it is expected that effects are not 
observed (EPA, 2004a, section 8.4.7; EPA, 2005, section 3.6.6). 
``Gaseous co-pollutants'' generally refer to other commonly-
occurring air pollutants, specifically O3, CO, 
SO2 and NO2. ``Measurement error'' refers to 
uncertainty in the air quality measurements, while ``exposure 
misclassification'' includes uncertainty in the use of ambient 
pollutant measurements in characterizing population exposures to PM 
(EPA, 2004a, section 8.4.5; EPA, 2005, section 3.6.2)
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    (6) Efforts to evaluate the effects of fine particles from 
different sources (e.g., motor vehicles, coal combustion, vegetative 
burning, crustal \8\), using factor analysis or source apportionment 
methods with fine particle speciation data.
---------------------------------------------------------------------------

    \8\ ``Crustal'' is used here to describe particles of geologic 
origin, which can be found in both fine- and coarse-fraction PM.
---------------------------------------------------------------------------

    (7) New ``intervention studies'' providing evidence for 
improvements in respiratory or cardiovascular health with reductions in 
ambient concentrations of particles and gaseous co-pollutants.
    In addition, the body of evidence on PM-related effects has greatly 
expanded since the last review with findings from studies of potential 
mechanisms or pathways by which particles may result in the effects 
identified in the epidemiologic studies. These studies include 
important new dosimetry, toxicologic and controlled human exposure 
studies, as highlighted below.
    (8) Animal and controlled human exposure studies using concentrated 
ambient particles (CAPs), new indicators of response (e.g., C-reactive 
protein and cytokine levels, heart rate variability), and animal models 
simulating sensitive human subpopulations. The results of these studies 
are relevant to evaluation of plausibility of the epidemiologic 
evidence and provide insights into potential mechanisms for PM-related 
effects.
    (9) Dosimetry studies using new modeling methods that provide 
increased understanding of the dosimetry of different particle size 
classes and in members of potentially sensitive subpopulations, such as 
people with chronic respiratory disease.
    Section II.A of the proposal provides a detailed summary of key 
information contained in the Criteria Document (EPA, 2004a, Chapters 6-
9), and in the Staff Paper (EPA, 2005, Chapter 3), on the known and 
potential effects associated with exposure to fine particles including 
information on specific constituents and information on the effects of 
fine particles in combination with other pollutants that are routinely 
present in the ambient air

[[Page 61152]]

(71 FR 2626-2637). The information highlighted there summarizes:
    (1) Multiple biologic mechanisms that may be responsible for 
morbidity/mortality effects associated with exposure to ambient fine 
particles, including potential mechanisms or pathways related to direct 
effects on the respiratory system, systemic effects that are secondary 
to effects in the respiratory system including cardiovascular effects, 
or direct cardiovascular effects.
    (2) The nature of the effects that have been reported to be 
ass