Document ID: EPA-HQ-OAR-2015-0072-1166
Agency: epa
Document Type: Rule
Title: Review of the National Ambient Air Quality Standards for Particulate Matter
Posted Date: 2020-12-18T05:00Z

[Federal Register Volume 85, Number 244 (Friday, December 18, 2020)]
[Rules and Regulations]
[Pages 82684-82748]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-27125]

[[Page 82683]]

Vol. 85

Friday,

No. 244

December 18, 2020

Part III

 Environmental Protection Agency

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40 CFR Part 50

 Review of the National Ambient Air Quality Standards for Particulate 
Matter; Final Rule

  Federal Register / Vol. 85 , No. 244 / Friday, December 18, 2020 / 
Rules and Regulations  

[[Page 82684]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2015-0072; FRL-10018-11-OAR]
RIN 2060-AS50

Review of the National Ambient Air Quality Standards for 
Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final action.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review 
of the air quality criteria and the national ambient air quality 
standards (NAAQS) for particulate matter (PM), the Administrator has 
reached final decisions on the primary and secondary PM NAAQS. With 
regard to the primary standards meant to protect against fine particle 
exposures (i.e., annual and 24-hour PM2.5 standards), the 
primary standard meant to protect against coarse particle exposures 
(i.e., 24-hour PM10 standard), and the secondary 
PM2.5 and PM10 standards, the EPA is retaining 
the current standards, without revision.

DATES: This final action is effective December 18, 2020.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2015-0072. Incorporated into this docket is a 
separate docket established for the Integrated Science Assessment 
(Docket ID No. EPA-HQ-ORD-2014-0859). All documents in the docket are 
listed in https://www.regulations.gov/. Although listed in the index, 
some information is not publicly available, e.g., Confidential Business 
Information (CBI) 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. With the exception of such material, publicly available 
docket materials are available electronically through https://www.regulations.gov/. Out of an abundance of caution for members of the 
public and our staff, the EPA Docket Center and Reading Room are closed 
to the public, with limited exceptions, to reduce the risk of 
transmitting COVID-19. Our Docket Center staff will continue to provide 
remote customer service via email, phone, and webform. For further 
information on EPA Docket Center services and the current status, 
please visit us online at https://www.epa.gov/dockets.

FOR FURTHER INFORMATION CONTACT: Dr. Lars Perlmutt, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail Code C539-04, 
Research Triangle Park, NC 27711; telephone: (919) 541-3037; fax: (919) 
541-5315; email: perlmutt.lars@epa.gov.

SUPPLEMENTARY INFORMATION:

Basis for Immediate Effective Date

    In accordance with section 307(d)(1)(V), the Administrator has 
designated this action as being subject to the rulemaking procedures in 
section 307(d) of the Clean Air Act (CAA). Section 307(d)(1) of the CAA 
states that: ``The provisions of section 553 through 557 * * * of Title 
5 shall not, except as expressly provided in this subsection, apply to 
actions to which this subsection applies.'' Thus, section 553(d) of the 
Administrative Procedure Act (APA), which requires publication of a 
substantive rule to be made ``not less than 30 days before its 
effective date'' subject to limited exceptions, does not apply to this 
action. In the alternative, the EPA concludes that it is s consistent 
with APA section 553(d) to make this action effective December 18, 
2020.
    Section 553(d)(3) of the APA, 5 U.S.C. 553(d)(3), provides that 
final rules shall not become effective until 30 days after publication 
in the Federal Register ``except . . . as otherwise provided by the 
agency for good cause found and published with the rule.'' ``In 
determining whether good cause exists, an agency should `balance the 
necessity for immediate implementation against principles of 
fundamental fairness which require that all affected persons be 
afforded a reasonable amount of time to prepare for the effective date 
of its ruling.'' Omnipoint Corp. v. Fed. Commc'n Comm'n, 78 F.3d 620, 
630 (D.C. Cir. 1996) (quoting United States v. Gavrilovic, 551 F.2d 
1099, 1105 (8th Cir. 1977)). The purpose of this provision is to ``give 
affected parties a reasonable time to adjust their behavior before the 
final rule takes effect.'' Id.; see also Gavrilovic, 551 F.2d at 1104 
(quoting legislative history).
    The EPA is determining that in light of the nature of this action, 
good cause exists to make this final action effective immediately 
because the Agency seeks to provide regulatory certainty as soon as 
possible and the Administrator's decision to retain the current NAAQS 
does not change the status quo or impose new obligations on any person 
or entity. As a result, there is no need to provide parties additional 
time to adjust their behavior, and no person will be harmed by making 
the action immediately effective as opposed to delaying the effective 
date by 30 days. Accordingly, the EPA is making this action effective 
immediately upon publication.

General Information

Availability of Information Related to This Action

    A number of the documents that are relevant to this final decision 
are available through the EPA's website at https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards. These documents include 
the Integrated Review Plan for the National Ambient Air Quality 
Standards for Particulate Matter (U.S. EPA, 2016), available at https://www3.epa.gov/ttn/naaqs/standards/pm/data/201612-final-integrated-review-plan.pdf, the Integrated Science Assessment for Particulate 
Matter (U.S. EPA, 2019), available at https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=347534, the Policy Assessment for the Review of 
the National Ambient Air Quality Standards for Particulate Matter (U.S. 
EPA, 2020), available at https://www.epa.gov/naaqs/particulate-matter-pm-standards-policy-assessments-current-review-0, and the notice of 
proposed rulemaking, available at https://www.epa.gov/naaqs/particulate-matter-pm-standards-federal-register-notices-current-review. These and other related documents are also available for 
inspection and copying in the EPA docket identified above.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related PM Control Programs
    C. History of the PM Air Quality Criteria and Standards
    1. Reviews Completed in 1971 and 1987
    2. Review Completed in 1997
    3. Review Completed in 2006
    4. Review Completed in 2012
    D. Current Review of the Air Quality Criteria and Standards
    E. Air Quality Information
    1. Distribution of Particle Size in Ambient Air
    2. Sources and Emissions Contributing to PM in the Ambient Air
    3. Ambient Concentrations and Trends
    a. PM2.5 Mass
    b. PM2.5 Components
    c. PM10
    d. PM10-2.5
    e. UFP
    4. Background PM
II. Rationale for Decisions on the Primary PM2.5 
Standards

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    A. Introduction
    1. Background on the Current Standards
    2. Overview of Health Effects Evidence
    a. Nature of Effects
    i. Mortality
    ii. Cardiovascular Effects
    iii. Respiratory Effects
    iv. Cancer
    v. Nervous System Effects
    vi. Other Effects
    b. At-Risk Populations
    c. Evidence-Based Considerations
    i. PM2.5 Concentrations Evaluated in Experimental 
Studies
    ii. Ambient Concentrations in Locations of Epidemiological 
Studies
    3. Overview of Risk and Exposure Assessment Information
    B. Conclusions on the Primary PM2.5 Standards
    1. CASAC Advice in This Review
    2. Basis for Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decision on the Primary PM2.5 Standards
III. Rationale for Decisions on the Primary PM10 Standard
    A. Introduction
    1. Background on the Current Standard
    2. Overview of Health Effects Evidence
    a. Nature of Effects
    i. Mortality
    ii. Cardiovascular Effects
    iii. Respiratory Effects
    iv. Cancer
    v. Metabolic Effects
    vi. Nervous System Effects
    B. Conclusions on the Primary PM10 Standard
    1. CASAC Advice in This Review
    2. Basis for the Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decision on the Primary PM10 Standard
IV. Rationale for Decision on the Secondary PM Standards
    A. Introduction
    1. Background on the Current Standards
    2. Overview of Welfare Effects Evidence
    a. Nature of Effects
    i. Visibility
    ii. Climate
    iii. Materials
    3. Overview of Air Quality and Quantitative Information
    a. Visibility Effects
    b. Non-Visibility Effects
    B. Conclusions on the Secondary Standards
    1. CASAC Advice in This Review
    2. Basis for the Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decision on the Secondary PM Standards
V. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Executive Order 13771: Reducing Regulations and Controlling 
Regulatory Costs
    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates Reform Act (UMRA)
    F. Executive Order 13132: Federalism
    G. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    H. Executive Order 13045: Protection of Children From 
Environmental Health Risks and Safety Risks
    I. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use
    J. National Technology Transfer and Advancement Act (NTTAA)
    K. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    L. Determination Under Section 307(d)
    M. Congressional Review Act (CRA)
References

Executive Summary

    This notice presents the Administrator's final decisions to retain 
the current primary (health-based) and secondary (welfare-based) 
National Ambient Air Quality Standards (NAAQS) for particulate matter 
(PM), without revision.
    In ambient air, PM is a mixture of substances suspended as small 
liquid and/or solid particles. Particles in the atmosphere range in 
size from less than 0.01 to more than 10 micrometers ([mu]m) in 
diameter. Particulate matter and its precursors are emitted from both 
anthropogenic sources (e.g., electricity generating units, cars and 
trucks, agricultural operations) and natural sources (e.g., sea salt, 
wildland fires, biological aerosols). When describing PM, subscripts 
are used to denote particle size. For example, PM2.5 
includes particles with diameters generally less than or equal to 2.5 
[mu]m and PM10 includes particles with diameters generally 
less than or equal to 10 [mu]m.
    The EPA has established primary (health-based) and secondary 
(welfare-based) NAAQS for PM2.5 and PM10. This 
includes two primary PM2.5 standards, an annual average 
standard with a level of 12.0 [mu]g/m\3\ and a 24-hour standard with a 
98th percentile form and a level of 35 [mu]g/m\3\. It also includes a 
primary PM10 standard with a 24-hour averaging time, a 1-
expected exceedance form, and a level of 150 [mu]g/m\3\. Secondary PM 
standards are set equal to the primary standards, except that the level 
of the secondary annual PM2.5 standard is 15.0 [mu]g/m\3\. 
In reaching decisions on these PM standards in the current review, the 
Administrator has considered the available scientific evidence assessed 
in the Integrated Science Assessment (ISA), analyses in the Policy 
Assessment (PA), advice from the Clean Air Scientific Advisory 
Committee (CASAC), and public comments on the proposal.
    For the primary PM2.5 standards, the Administrator 
concludes that there are important uncertainties in the evidence for 
adverse health effects below the current standards and in the potential 
for additional public health improvements from reducing ambient 
PM2.5 concentrations below those standards. Based on the 
available evidence, the Administrator has concluded that the current 
primary PM2.5 standards are requisite to protect public 
health, with an adequate margin of safety, from effects of 
PM2.5 in ambient air and should be retained, without 
revision. Therefore, the EPA is retaining those standards (i.e., both 
the annual and 24-hour standards), without revision.
    For the primary PM10 standard, the Administrator 
observes that, while the available health effects evidence has 
expanded, recent studies are subject to the same types of uncertainties 
that were judged important in the last review. He concludes that, based 
on the newly available evidence with its inherent uncertainties, the 
current primary PM10 standard is requisite to protect public 
health, with an adequate margin of safety, from effects of 
PM10 in ambient air, and should be retained, without 
revision. Therefore, the EPA is retaining that standard, without 
revision.
    For the secondary standards, the Administrator observes that the 
expanded evidence for non-ecological welfare effects is consistent with 
the last review \1\ and that updated quantitative analyses show results 
similar to those in the last review. Based on his consideration of the 
available evidence and quantitative information, he concludes that the 
current secondary PM standards are requisite to protect public welfare, 
against visibility effects and that there is insufficient information 
to establish distinct

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secondary PM standards to address materials and climate effects. 
Therefore, the EPA is retaining those standards, without revision.
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    \1\ The welfare effects considered in this review include 
visibility impairment, climate effects, and materials effects. 
Ecological effects associated with PM, and the adequacy of 
protection provided by the secondary PM standards for those effects, 
are being addressed in the separate review of the secondary NAAQS 
for oxides of nitrogen, oxides of sulfur and PM (U.S. EPA, 2016, 
section 5.2; U.S. EPA, 2020, section 5.1.1) in recognition of the 
linkages between oxides of nitrogen, oxides of sulfur, and PM with 
respect to atmospheric deposition and ecological effects. Addressing 
the pollutants together enables the EPA to take a comprehensive 
approach to considering the nature and interactions of the 
pollutants, which is important for ensuring that all scientific 
information relevant to ecological effects is thoroughly evaluated. 
Information on the current review of these secondary NAAQS can be 
found at https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards.
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    These decisions are consistent with the CASAC's consensus advice on 
the primary 24-hour PM2.5 standard, the primary 
PM10 standard, and the secondary standards. The CASAC 
provided differing views on the primary annual PM2.5 
standard, with some committee members recommending that the EPA retain 
the current standard and other members recommending revision of that 
standard.

I. Background

A. Legislative Requirements

    Two sections of the CAA govern the establishment and revision of 
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to 
identify and list certain air pollutants and then to issue air quality 
criteria for those pollutants. The Administrator is to list those 
pollutants ``emissions of which, in his judgment, cause or contribute 
to air pollution which may reasonably be anticipated to endanger public 
health or welfare''; ``the presence of which in the ambient air results 
from numerous or diverse mobile or stationary sources''; and for which 
he ``plans to issue air quality criteria . . . .'' (42 U.S.C. 
7408(a)(1)). Air quality criteria are intended to ``accurately reflect 
the latest scientific knowledge useful in indicating the kind and 
extent of all identifiable effects on public health or welfare which 
may be expected from the presence of [a] pollutant in the ambient air . 
. . .'' (42 U.S.C. 7408(a)(2)).
    Section 109 [42 U.S.C. 7409] directs the Administrator to propose 
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for 
which air quality criteria are issued [42 U.S.C. 7409(a)]. Section 
109(b)(1) defines primary standards as ones ``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.'' \2\ Under section 109(b)(2), a 
secondary standard 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.'' \3\
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    \2\ 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).
    \3\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on 
welfare include, but are not limited to, ``effects on soils, water, 
crops, vegetation, manmade materials, animals, wildlife, weather, 
visibility, and climate, damage to and deterioration of property, 
and hazards to transportation, as well as effects on economic values 
and on personal comfort and well-being.''
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    In setting primary and secondary standards that are ``requisite'' 
to protect public health and welfare, respectively, as provided in 
section 109(b), the EPA's task is to establish standards that are 
neither more nor less stringent than necessary. In so doing, the EPA 
may not consider the costs of implementing the standards. See generally 
Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-
76 (2001). Likewise, ``[a]ttainability and technological feasibility 
are not relevant considerations in the promulgation of national ambient 
air quality standards.'' American Petroleum Institute v. Costle, 665 
F.2d 1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corporation v. 
EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019).
    The requirement that primary standards provide an adequate margin 
of safety was intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It was also intended to provide a reasonable 
degree of protection against hazards that research has not yet 
identified. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 
(D.C. Cir 1980); American Petroleum Institute v. Costle, 665 F.2d at 
1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 
(D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 
2013). 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 Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 
F.3d at 1351, but rather at a level that reduces risk sufficiently so 
as to protect public health with an adequate margin of safety.
    In addressing the requirement for an adequate margin of safety, the 
EPA considers such factors as the nature and severity of the health 
effects involved, the size of the sensitive population(s), and the kind 
and degree of uncertainties. The selection of any particular approach 
to providing an adequate margin of safety is a policy choice left to 
the Administrator's judgment. See Lead Industries Ass'n v. EPA, 647 
F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
    Section 109(d)(1) of the Act requires the review every five years 
of existing air quality criteria and, if appropriate, the revision of 
those criteria to reflect advances in scientific knowledge on the 
effects of the pollutant on public health and welfare. Under the same 
provision, the EPA is also to review every five years and, if 
appropriate, revise the NAAQS, based on the revised air quality 
criteria.
    Section 109(d)(2) addresses the appointment and advisory functions 
of an independent scientific review committee. Section 109(d)(2)(A) 
requires the Administrator to appoint this committee, which is to be 
composed of ``seven members including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air pollution control agencies.'' Section 
109(d)(2)(B) provides that the independent scientific review committee 
``shall complete a review of the criteria . . . and the national 
primary and secondary ambient air quality standards . . . and shall 
recommend to the Administrator any new . . . standards and revisions of 
existing criteria and standards as may be appropriate. . . .'' Since 
the early 1980s, this independent review function has been performed by 
the Clean Air Scientific Advisory Committee (CASAC) of the EPA's 
Science Advisory Board. A number of other advisory functions are also 
identified for the committee by section 109(d)(2)(C), which reads:

    Such committee shall also (i) advise the Administrator of areas 
in which additional knowledge is required to appraise the adequacy 
and basis of existing, new, or revised national ambient air quality 
standards, (ii) describe the research efforts necessary to provide 
the required information, (iii) advise the Administrator on the 
relative contribution to air pollution concentrations of natural as 
well as anthropogenic activity, and (iv) advise the Administrator of 
any adverse public health, welfare, social, economic, or energy 
effects which may result from various strategies for attainment and 
maintenance of such national ambient air quality standards.

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    As previously noted, the Supreme Court has held that section 109(b) 
``unambiguously bars cost considerations from the NAAQS-setting 
process.'' Whitman v. Am. Trucking Associations, 531 U.S. 457, 471 
(2001). Accordingly, while some of these issues regarding which 
Congress has directed the CASAC to advise the Administrator are ones 
that are relevant to the standard setting process, others are not. 
Issues that are not relevant to standard setting may be relevant to 
implementation of the NAAQS once they are established.\4\
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    \4\ Some aspects of the CASAC's advice may not be relevant to 
the EPA's process of setting primary and secondary standards that 
are requisite to protect public health and welfare. Indeed, were the 
EPA to consider costs of implementation when reviewing and revising 
the standards ``it would be grounds for vacating the NAAQS.'' 
Whitman, 531 U.S. at 471 n.4. At the same time, the CAA directs the 
CASAC to provide advice on ``any adverse public health, welfare, 
social, economic, or energy effects which may result from various 
strategies for attainment and maintenance'' of the NAAQS to the 
Administrator under section 109(d)(2)(C)(iv). In Whitman, the Court 
clarified that most of that advice would be relevant to 
implementation but not standard setting, as it ``enable[s] the 
Administrator to assist the States in carrying out their statutory 
role as primary implementers of the NAAQS.'' Id. at 470 (emphasis in 
original). However, the Court also noted that the CASAC's ``advice 
concerning certain aspects of `adverse public health . . . effects' 
from various attainment strategies is unquestionably pertinent'' to 
the NAAQS rulemaking record and relevant to the standard setting 
process. Id. at 470 n.2.
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B. Related PM Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under sections 110 and 171-190 of the CAA, and 
related provisions and regulations, states are to submit, for the EPA's 
approval, state implementation plans (SIPs) that provide for the 
attainment and maintenance of such standards through control programs 
directed to sources of the pollutants involved. The states, in 
conjunction with the EPA, also administer the Prevention of Significant 
Deterioration (PSD) program (CAA sections 160 to 169). In addition, 
Federal programs provide for nationwide reductions in emissions of PM 
and other air pollutants through the Federal motor vehicle and motor 
vehicle fuel control program under title II of the Act (CAA sections 
202 to 250), which involves controls for emissions from mobile sources 
and controls for the fuels used by these sources, and new source 
performance standards for stationary sources under section 111 of the 
CAA.

C. History of the PM Air Quality Criteria and Standards

1. Reviews Completed in 1971 and 1987
    The EPA first established NAAQS for PM in 1971 (36 FR 8186, April 
30, 1971), based on the original Air Quality Criteria Document (AQCD) 
(DHEW, 1969).\5\ The federal reference method (FRM) specified for 
determining attainment of the original standards was the high-volume 
sampler, which collects PM up to a nominal size of 25 to 45 [micro]m 
(referred to as total suspended particulates or TSP). The primary 
standards were set at 260 [micro]g/m\3\, 24-hour average, not to be 
exceeded more than once per year, and 75 [micro]g/m\3\, annual 
geometric mean. The secondary standards were set at 150 [micro]g/m\3\, 
24-hour average, not to be exceeded more than once per year, and 60 
[micro]g/m\3\, annual geometric mean.
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    \5\ Prior to the review initiated in 2007 (see section I.C.4), 
the AQCD provided the scientific foundation (i.e., the air quality 
criteria) for the NAAQS. Beginning in that review, the Integrated 
Science Assessment (ISA) has replaced the AQCD.
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    In October 1979 (44 FR 56730, October 2, 1979), the EPA announced 
the first periodic review of the air quality criteria and NAAQS for PM. 
Revised primary and secondary standards were promulgated in 1987 (52 FR 
24634, July 1, 1987). In the 1987 decision, the EPA changed the 
indicator for particles from TSP to PM10,\6\ in order to 
focus on the subset of inhalable particles small enough to penetrate to 
the thoracic region of the respiratory tract (including the 
tracheobronchial and alveolar regions), referred to as thoracic 
particles. The level of the 24-hour standards (primary and secondary) 
was set at 150 [micro]g/m\3\, and the form was one expected exceedance 
per year, on average over three years. The level of the annual 
standards (primary and secondary) was set at 50 [micro]g/m\3\, and the 
form was annual arithmetic mean, averaged over three years.
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    \6\ PM10 refers to particles with a nominal mean 
aerodynamic diameter less than or equal to 10 [micro]m. More 
specifically, 10 [micro]m is the aerodynamic diameter for which the 
efficiency of particle collection is 50 percent.
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2. Review Completed in 1997
    In April 1994, the EPA announced its plans for the second periodic 
review of the air quality criteria and NAAQS for PM, and in 1997 the 
EPA promulgated revisions to the NAAQS (62 FR 38652, July 18, 1997). In 
the 1997 decision, the EPA determined that the fine and coarse 
fractions of PM10 should be considered separately. This 
determination was based on evidence that serious health effects were 
associated with short- and long-term exposures to fine particles in 
areas that met the existing PM10 standards. 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 [micro]m). The new 
primary standards were as follows: (1) An annual standard with a level 
of 15.0 [micro]g/m\3\, based on the 3-year average of annual arithmetic 
mean PM2.5 concentrations from single or multiple community-
oriented monitors; \7\ and (2) a 24-hour standard with a level of 65 
[micro]g/m\3\, based on the 3-year average of the 98th percentile of 
24-hour PM2.5 concentrations at each monitor within an area. 
Also, the 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 the health 
effects of the coarse fraction of PM10 (referred to as 
thoracic coarse particles or PM10-2.5; generally including 
particles with a nominal mean aerodynamic diameter greater than 2.5 
[micro]m and less than or equal to 10 [micro]m), the EPA retained the 
primary annual PM10 standard and revised the form of the 
primary 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 setting them equal 
in all respects to the primary standards.
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    \7\ The 1997 annual PM2.5 standard was compared with 
measurements made at the community-oriented monitoring site 
recording the highest concentration or, if specific constraints were 
met, measurements from multiple community-oriented monitoring sites 
could be averaged (i.e., ``spatial averaging''). In the last review 
(completed in 2012), the EPA replaced the term ``community-
oriented'' monitor with the term ``area-wide'' monitor. Area-wide 
monitors are those sited at the neighborhood scale or larger, as 
well as those monitors sited at micro- or middle-scales that are 
representative of many such locations in the same core-based 
statistical area (CBSA) (78 FR 3236, January 15, 2013).
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    Following promulgation of the 1997 p.m. NAAQS, petitions for review 
were filed by several parties, addressing a broad range of issues. In 
May 1999, the U.S. Court of Appeals for the District of Columbia 
Circuit (D.C. Circuit) upheld the 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, Inc. v. EPA, 175 
F. 3d 1027, 1055-56 (D.C. Cir. 1999). The D.C. Circuit also found 
``ample support'' for the EPA's decision to regulate coarse particle 
pollution, but vacated the 1997 PM10 standards, concluding 
that the

[[Page 82688]]

EPA had not provided a reasonable explanation justifying use of 
PM10 as an indicator for coarse particles. American Trucking 
Associations v. EPA, 175 F. 3d at 1054-55. Pursuant to the D.C. 
Circuit's decision, the EPA removed the vacated 1997 PM10 
standards, and the pre-existing 1987 PM10 standards remained 
in place (65 FR 80776, December 22, 2000). The D.C. Circuit also upheld 
the EPA's determination not to establish more stringent secondary 
standards for fine particles to address effects on visibility. American 
Trucking Associations v. EPA, 175 F. 3d at 1027.
    The D.C. Circuit also addressed more general issues related to the 
NAAQS, including issues related to the consideration of costs in 
setting NAAQS and the EPA's approach to establishing the levels of 
NAAQS. Regarding the cost issue, the court reaffirmed prior rulings 
holding that in setting NAAQS the EPA is ``not permitted to consider 
the cost of implementing those standards.'' American Trucking 
Associations v. EPA, 175 F. 3d at 1040-41. Regarding the levels of 
NAAQS, the court held that the EPA's approach to establishing the level 
of the standards in 1997 (i.e., both for PM and for the ozone NAAQS 
promulgated on the same day) effected ``an unconstitutional delegation 
of legislative authority.'' American Trucking Associations v. EPA, 175 
F. 3d at 1034-40. Although the court stated that ``the factors EPA uses 
in determining the degree of public health concern associated with 
different levels of ozone and PM are reasonable,'' it remanded the rule 
to the EPA, stating that when the EPA considers these factors for 
potential non-threshold pollutants ``what EPA lacks is any determinate 
criterion for drawing lines'' to determine where the standards should 
be set.
    The D.C. Circuit's holdings on the cost and constitutional issues 
were appealed to the U.S. Supreme Court. In February 2001, the Supreme 
Court issued a unanimous decision upholding the EPA's position on both 
the cost and constitutional 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 the EPA's discretion, affirming the EPA's 
approach of setting standards that are neither more nor less stringent 
than necessary.
    The Supreme Court remanded the case to the D.C. Circuit for 
resolution of any remaining issues that had not been addressed in that 
court's earlier rulings. Id. at 475-76. In a March 2002 decision, the 
D.C. Circuit rejected all remaining challenges to the standards, 
holding that the 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).
3. Review Completed in 2006
    In October 1997, the EPA published its plans for the third periodic 
review of the air quality criteria and NAAQS for PM (62 FR 55201, 
October 23, 1997). After the CASAC and public review of several drafts, 
the EPA's National Center for Environmental Assessment (NCEA) finalized 
the AQCD in October 2004 (U.S. EPA, 2004). The EPA's Office of Air 
Quality Planning and Standards (OAQPS) finalized a Risk Assessment and 
Staff Paper in December 2005 (Abt Associates, 2005; U.S. EPA, 2005).\8\ 
On December 20, 2005, the EPA announced its proposed decision to revise 
the NAAQS for PM and solicited public comment on a broad range of 
options (71 FR 2620, January 17, 2006). On September 21, 2006, the EPA 
announced its final decisions to revise the primary and secondary NAAQS 
for PM to provide increased protection of public health and welfare, 
respectively (71 FR 61144, October 17, 2006). With regard to the 
primary and secondary standards for fine particles, the EPA revised the 
level of the 24-hour PM2.5 standards to 35 [micro]g/m\3\, 
retained the level of the annual PM2.5 standards at 15.0 
[micro]g/m\3\, and revised the form of the annual PM2.5 
standards by narrowing the constraints on the optional use of spatial 
averaging. With regard to the primary and secondary standards for 
PM10, the EPA retained the 24-hour standards, with levels at 
150 [micro]g/m\3\, and revoked the annual standards.\9\ The 
Administrator judged that the available evidence generally did not 
suggest a link between long-term exposure to existing ambient levels of 
coarse particles and health or welfare effects. In addition, a new 
reference method was added for the measurement of PM10-2.5 
in the ambient air in order to provide a basis for approving federal 
equivalent methods (FEMs) and to promote the gathering of scientific 
data to support future reviews of the PM NAAQS.
---------------------------------------------------------------------------

    \8\ Prior to the review initiated in 2007, the Staff Paper 
presented the EPA staff's considerations and conclusions regarding 
the adequacy of existing NAAQS and, when appropriate, the potential 
alternative standards that could be supported by the evidence and 
information. More recent reviews present this information in the 
Policy Assessment (PA).
    \9\ In the 2006 proposal, the EPA proposed to revise the 24-hour 
PM10 standard in part by establishing a new 
PM10-2.5 indicator for thoracic coarse particles (i.e., 
particles generally between 2.5 and 10 [micro]m in diameter). The 
EPA proposed to include any ambient mix of PM10-2.5 that 
was dominated by resuspended dust from high density traffic on paved 
roads and by PM from industrial sources and construction sources. 
The EPA proposed to exclude any ambient mix of PM10-2.5 
that was dominated by rural windblown dust and soils and by PM 
generated from agricultural and mining sources. In the final 
decision, the existing PM10 standard was retained, in 
part due to an ``inability . . . to effectively and precisely 
identify which ambient mixes are included in the 
[PM10-2.5] indicator and which are not'' (71 FR 61197, 
October 17, 2006).
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    Several parties filed petitions for review following promulgation 
of the revised PM NAAQS in 2006. These petitions addressed the 
following issues: (1) Selecting the level of the primary annual 
PM2.5 standard; (2) retaining PM10 as the 
indicator of a standard for thoracic coarse particles, retaining the 
level and form of the 24-hour PM10 standard, and revoking 
the PM10 annual standard; and (3) setting the secondary 
PM2.5 standards identical to the primary standards. On 
February 24, 2009, the D.C. Circuit issued its opinion in the case 
American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009). 
The court remanded the primary annual PM2.5 NAAQS to the EPA 
because the Agency had failed to adequately explain why the standards 
provided the requisite protection from both short- and long-term 
exposures to fine particles, including protection for at-risk 
populations. Id. at 520-27. With regard to the standards for 
PM10, the court upheld the EPA's decisions to retain the 24-
hour PM10 standard to provide protection from thoracic 
coarse particle exposures and to revoke the annual PM10 
standard. Id. at 533-38. With regard to the secondary PM2.5 
standards, the court remanded the standards to the EPA because the 
Agency failed to adequately explain why setting the secondary PM 
standards identical to the primary standards provided the required 
protection for public welfare, including protection from visibility 
impairment. Id. at 528-32. The EPA responded to the court's remands as 
part of the next review of the PM NAAQS, which was initiated in 2007.
4. Review Completed in 2012
    In June 2007, the EPA initiated the fourth periodic review of the 
air quality criteria and the PM NAAQS by issuing a call for information 
(72 FR 35462, June 28, 2007). Based on the NAAQS review process, as 
revised in 2008 and again in 2009,\10\ the EPA held science/policy

[[Page 82689]]

issue workshops on the primary and secondary PM NAAQS (72 FR 34003, 
June 20, 2007; 72 FR 34005, June 20, 2007), and prepared and released 
the planning and assessment documents that comprise the review process 
(i.e., IRP (U.S. EPA, 2008), ISA (U.S. EPA, 2009c), REA planning 
documents for health and welfare (U.S. EPA, 2009b, U.S. EPA, 2009a), a 
quantitative health risk assessment (U.S. EPA, 2010a) and an urban-
focused visibility assessment (U.S. EPA, 2010b), and PA (U.S. EPA, 
2011)). In June 2012, the EPA announced its proposed decision to revise 
the NAAQS for PM (77 FR 38890, June 29, 2012).
---------------------------------------------------------------------------

    \10\ The history of the NAAQS review process, including 
revisions to the process, is discussed at https://www.epa.gov/naaqs/historical-information-naaqs-review-process.
---------------------------------------------------------------------------

    In December 2012, the EPA announced its final decisions to revise 
the primary NAAQS for PM to provide increased protection of public 
health (78 FR 3086, January 15, 2013). With regard to primary standards 
for PM2.5, the EPA revised the level of the annual 
PM2.5 standard \11\ to 12.0 [micro]g/m\3\ and retained the 
24-hour PM2.5 standard, with its level of 35 [micro]g/m\3\. 
For the primary PM10 standard, the EPA retained the 24-hour 
standard to continue to provide protection against effects associated 
with short-term exposure to thoracic coarse particles (i.e., 
PM10-2.5). With regard to the secondary PM standards, the 
EPA generally retained the 24-hour and annual PM2.5 
standards \12\ and the 24-hour PM10 standard to address 
visibility and non-visibility welfare effects.
---------------------------------------------------------------------------

    \11\ The EPA also eliminated the option for spatial averaging.
    \12\ Consistent with the primary standard, the EPA eliminated 
the option for spatial averaging with the annual standard.
---------------------------------------------------------------------------

    As with previous reviews, petitioners challenged the EPA's final 
rule. Petitioners argued that the EPA acted unreasonably in revising 
the level and form of the annual standard and in amending the 
monitoring network provisions. On judicial review, the revised 
standards and monitoring requirements were upheld in all respects. NAM 
v. EPA, 750 F.3d 921 (D.C. Cir. 2014).

D. Current Review of the Air Quality Criteria and Standards

    In December 2014, the EPA announced the initiation of the current 
periodic review of the air quality criteria for PM and of the 
PM2.5 and PM10 NAAQS and issued a call for 
information (79 FR 71764, December 3, 2014). From February 9 to 
February 11, 2015, the EPA's NCEA and OAQPS held a public workshop to 
inform the planning for the current review of the PM NAAQS (announced 
in 79 FR 71764, December 3, 2014). Workshop participants, including a 
wide range of external experts as well as EPA staff representing a 
variety of areas of expertise (e.g., epidemiology, human and animal 
toxicology, risk/exposure analysis, atmospheric science, visibility 
impairment, climate effects), were asked to highlight significant new 
and emerging PM research, and to make recommendations to the Agency 
regarding the design and scope of this review. This workshop provided 
for a public discussion of the key science and policy-relevant issues 
around which the EPA has structured the current review of the PM NAAQS 
and of the most meaningful new scientific information that would be 
available in this review to inform understanding of these issues.
    The input received at the workshop guided EPA staff in developing a 
draft IRP, which was reviewed by the CASAC Particulate Matter Review 
Panel and discussed on public teleconferences held in May 2016 (81 FR 
13362, March 14, 2016) and August 2016 (81 FR 39043, June 15, 2016). 
Advice from the chartered CASAC, supplemented by the Particulate Matter 
Review Panel, and input from the public were considered in developing 
the final IRP (U.S. EPA, 2016). The final IRP discusses the approaches 
to be taken in developing key scientific, technical, and policy 
documents in this review and the key policy-relevant issues.
    In May 2018, the Administrator issued a memorandum describing a 
``back-to-basics'' process for reviewing the NAAQS (Pruitt, 2018). This 
memo announced the Agency's intention to conduct the current review of 
the PM NAAQS in such a manner as to ensure that any necessary revisions 
are finalized by December 2020. Following this memo, on October 10, 
2018 the Administrator additionally announced that the role of 
reviewing the key assessments developed as part of the ongoing review 
of the PM NAAQS (i.e., drafts of the ISA and PA) would be performed by 
the seven-member chartered CASAC (i.e., rather than the CASAC 
Particulate Matter Panel that reviewed the draft IRP).\13\
---------------------------------------------------------------------------

    \13\ The CASAC charter is available at: https://
yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/2019casaccharter/$File/
CASAC%202019%20Renewal%20Charter%203.21.19%20-%20final.pdf. The 
Administrator's announcement is available at: https://archive.epa.gov/epa/newsreleases/acting-administrator-wheeler-announces-science-advisors-key-clean-air-act-committee.html.
---------------------------------------------------------------------------

    The EPA released the draft ISA in October 2018 (83 FR 53471, 
October 23, 2018). The draft ISA was reviewed by the chartered CASAC at 
a public meeting held in Arlington, VA in December 2018 (83 FR 55529, 
November 6, 2018) and was discussed on a public teleconference in March 
2019 (84 FR 8523, March 8, 2019). The CASAC provided its advice on the 
draft ISA in a letter to the EPA Administrator dated April 11, 2019 
(Cox, 2019b). In that letter, the CASAC's recommendations address both 
the draft ISA's assessment of the science for PM-related effects and 
the process under which this review of the PM NAAQS is being conducted.
    Regarding the assessment of the evidence, the CASAC letter states 
that ``the Draft ISA does not provide a sufficiently comprehensive, 
systematic assessment of the available science relevant to 
understanding the health impacts of exposure to particulate matter 
(PM)'' (Cox, 2019b, p. 1 of letter). The CASAC recommended that this 
and other limitations (i.e., ``[i]nadequate evidence for altered causal 
determinations'' and the need for a ``[c]learer discussion of causality 
and causal biological mechanisms and pathways'') be remedied in a 
revised ISA (Cox, 2019b, p. 1 of letter).
    Given the Administrator's timeline for this review, as noted above 
(Pruitt, 2018), the EPA did not prepare a second draft ISA (Wheeler, 
2019). Rather, the EPA has taken steps to address the CASAC's comments 
in the final ISA (U.S. EPA, 2019). In particular, the final ISA 
includes additional text and a new appendix to clarify the 
comprehensive and systematic process employed by the EPA to develop the 
ISA. In addition, several causality determinations were re-examined 
and, consistent with the CASAC advice, the final ISA reflects a revised 
causality determination for long-term ultrafine particle (UFP) 
exposures and nervous system effects (i.e., from ``likely to be 
causal'' to ``suggestive of, but not sufficient to infer, a causal 
relationship'').\14\ The final ISA also contains additional text to 
clarify the evidence for biological pathways of particular PM-related 
effects and the role of that evidence in causality determinations.
---------------------------------------------------------------------------

    \14\ Based on the CASAC's comments, the EPA also re-examined the 
causality determinations for cancer and for nervous system effects 
following long-term PM2.5 exposures. The EPA's 
consideration of these comments in the final ISA is described in 
detail in the proposal in sections II.B.1.d (85 FR 24111, April 30, 
2020) and II.B.1.e (85 FR 24113, April 30, 2020).
---------------------------------------------------------------------------

    Among its comments on the process, the chartered CASAC recommended 
``that the EPA reappoint the previous CASAC PM panel (or appoint a 
panel with similar expertise)'' (Cox, 2019b). The Agency's response to 
this advice was provided in a letter from the Administrator to the 
CASAC chair dated

[[Page 82690]]

July 25, 2019.\15\ In that letter, the Administrator announced his 
intention to identify a pool of non-member subject matter expert 
consultants to support the CASAC's review activities for the PM and 
ozone NAAQS. A Federal Register notice requesting the nomination of 
scientists from a broad range of disciplines ``with demonstrated 
expertise and research in the field of air pollution related to PM and 
ozone'' was published in August 2019 (84 FR 38625, August 7, 2019). The 
Administrator selected consultants from among those nominated, and 
input from members of this pool of consultants informed the CASAC's 
review of the draft PA.
---------------------------------------------------------------------------

    \15\ Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/
0/6CBCBBC3025E13B4852583D90047B352/$File/EPA-CASAC-19-
002_Response.pdf.
---------------------------------------------------------------------------

    The EPA released the draft PA in September 2019 (84 FR 47944, 
September 11, 2019). The draft PA drew from the assessment of the 
evidence in the draft ISA. It was reviewed by the chartered CASAC and 
discussed in October 2019 at a public meeting held in Cary, NC. Public 
comments were received via a separate public teleconference (84 FR 
51555, September 30, 2019). A public meeting to discuss the chartered 
CASAC letter and response to charge questions on the draft PA was held 
in Cary, NC in December 2019 (84 FR 58713, November 1, 2019), and the 
CASAC provided its advice on the draft PA, including its advice on the 
current primary and secondary PM standards, in a letter to the EPA 
Administrator dated December 16, 2019 (Cox, 2019a).
    With regard to the primary standards, the CASAC recommended 
retaining the current 24-hour PM2.5 and PM10 
standards but did not reach consensus on the adequacy of the current 
annual PM2.5 standard. With regard to the secondary 
standards, the CASAC recommended retaining the current standards. The 
CASAC's advice on the primary and secondary PM standards, and the 
Administrator's consideration of that advice in reaching proposed 
decisions, is discussed in detail in sections II.C.2 and II.C.3 
(primary PM2.5 standards), III.C.2 and III.C.3 (primary 
PM10 standards), and IV.D.2 and IV.D.3 (secondary standards) 
of the proposal notice (85 FR 24094, April 30, 2020).
    The CASAC additionally made a number of recommendations regarding 
the information and analyses presented in the draft PA. Specifically, 
the CASAC recommended that a revised PA include: (1) Additional 
discussion of the current CASAC and NAAQS review process; (2) 
additional characterization of PM-related emissions, monitoring and air 
quality information, including uncertainties in that information; (3) 
additional discussion and examination of uncertainties in the 
PM2.5 health evidence and the risk assessment; (4) updates 
to reflect changes in the ISA's causality determinations; and (5) 
additional discussion of the evidence for PM-related welfare effects, 
including uncertainties (Cox, 2019a, pp. 2-3 in letter). In response to 
the CASAC's comments, the final PA \16\ incorporated a number of 
changes, as described in detail in section I.C.5 of the proposal (85 FR 
24100, April 2020).
---------------------------------------------------------------------------

    \16\ Given the Administrator's timeline for this review, as 
noted above (Pruitt, 2018), the EPA did not prepare a second draft 
PA. Rather, the CASAC's advice was considered in developing the 
final PA (U.S. EPA, 2020).
---------------------------------------------------------------------------

    Drawing from his consideration of the scientific evidence assessed 
in the ISA and the analyses in the PA, including uncertainties in the 
evidence and analyses, and from his consideration of advice from the 
CASAC, on April 14, 2020 the Administrator proposed to retain all of 
the primary and secondary PM standards, without revision. These 
proposed decisions were published in the Federal Register on April 30, 
2020 (85 FR 24094, April 30, 2020). The EPA held virtual public 
hearings on the proposal on May 20-22, 2020 and May 27, 2020 (85 FR 
26634, May 5, 2020). In total, the EPA received more than 66,000 
comments on the proposal from members of the public and various 
stakeholder groups by the close of the public comment period on June 
29, 2020. Major issues raised in the public comments are discussed 
throughout the preamble of this final action. A more detailed summary 
of all significant comments, along with the EPA's responses (henceforth 
``Response to Comments''), can be found in the docket for this 
rulemaking (Docket No. EPA-HQ-OAR-2015-0072).
    As in prior NAAQS reviews, the EPA is basing its decision in this 
review on studies and related information included in the air quality 
criteria, which have undergone CASAC and public review. The studies 
assessed in the ISA \17\ and PA, and the integration of the scientific 
evidence presented in them, have undergone extensive critical review by 
the EPA, the CASAC, and the public. 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. 
Decisions on the NAAQS can have profound impacts on public health and 
welfare, and NAAQS decisions should be based on studies that have been 
rigorously assessed in an integrative manner not only by the EPA but 
also by the statutorily mandated independent scientific advisory 
committee, as well as the public review that accompanies this process. 
Some commenters have referred to and discussed individual scientific 
studies on the health effects of PM that were not included in the ISA 
(``'new' studies'') and that have not gone through this comprehensive 
review process. In considering and responding to comments for which 
such ``new'' studies were cited in support, the EPA has provisionally 
considered the cited studies in the context of the findings of the ISA. 
The EPA's provisional consideration of these studies did not and could 
not provide the kind of in-depth critical review described above, but 
rather was focused on determining whether they warranted reopening the 
review of the air quality criteria to enable the EPA, the CASAC, and 
the public to consider them further.
---------------------------------------------------------------------------

    \17\ Studies identified for the ISA were based on the review's 
opening ``call for information'' (79 FR 71764, December 3, 2014), as 
well as literature searches conducted routinely to identify and 
evaluate ``studies and reports that have undergone scientific peer 
review and were published or accepted for publication between 
January 1, 2009 and March 31, 2017. A limited literature update 
identified some additional studies that were published before 
December 31, 2017'' (U.S. EPA, 2019, Appendix, p. A-3). References 
that are cited in the ISA, the references that were considered for 
inclusion but not cited, and electronic links to bibliographic 
information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    This approach, and the decision to rely on studies and related 
information included in the air quality criteria, which have undergone 
CASAC and public review, is consistent with the EPA's practice in prior 
NAAQS reviews and its interpretation of the requirements of the CAA. 
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, and the EPA has consistently followed this approach. 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. See 71 FR 61144, 61148 
(October 17, 2006, final decision on review of NAAQS for particulate 
matter) for a detailed discussion of this issue and the EPA's past 
practice.
    As discussed in the EPA's 1993 decision not to revise the 
O3 NAAQS, ``new'' studies may sometimes be of

[[Page 82691]]

such significance that it is appropriate to delay a decision in a NAAQS 
review and to supplement the pertinent air quality criteria so the 
studies can be taken into account (58 FR at 13013-13014, March 9, 
1993). In the present case, the EPA's provisional consideration of 
``new'' studies concludes that, taken in context, the ``new'' 
information and findings do not materially change any of the broad 
scientific conclusions regarding the health and welfare effects of PM 
in ambient air made in the air quality criteria. For this reason, 
reopening the air quality criteria review would not be warranted.
    Accordingly, the EPA is basing the final decisions in this review 
on the studies and related information included in the PM air quality 
criteria that have undergone rigorous review by the EPA, CASAC and the 
public. The EPA will consider these ``new'' studies for inclusion in 
the air quality criteria for the next PM NAAQS review, which the EPA 
expects to begin soon after the conclusion of this review and which 
will provide the opportunity to fully assess these studies through a 
more rigorous review process involving the EPA, CASAC, and the public.

E. Air Quality Information

    This section provides a summary of basic information related to PM 
ambient air quality. It summarizes information on the distribution of 
particle size in ambient air (I.E.1), sources and emissions 
contributing to PM in the ambient air (I.E.2), ambient PM 
concentrations and trends in the U.S. (I.E.3), and background PM 
(I.E.4). Additional detail on PM air quality can be found in Chapter 2 
of the Policy Assessment (U.S. EPA, 2020; PA) and section I.D of the 
proposal (85 FR 24100, April 30, 2020).
1. Distribution of Particle Size in Ambient Air
    In ambient air, PM is a mixture of substances suspended as small 
liquid and/or solid particles (U.S. EPA, 2019, section 2.2) and 
distinct health and welfare effects have been linked with exposures to 
particles of different sizes. Particles in the atmosphere range in size 
from less than 0.01 to more than 10 [mu]m in diameter (U.S. EPA, 2019, 
section 2.2).The EPA defines PM2.5, also referred to as fine 
particles, as particles with aerodynamic diameters generally less than 
or equal to 2.5 [mu]m. The size range for PM10-2.5, also 
called coarse or thoracic coarse particles, includes those particles 
with aerodynamic diameters generally greater than 2.5 [mu]m and less 
than or equal to 10 [mu]m. PM10, which is comprised of both 
fine and coarse fractions, includes those particles with aerodynamic 
diameters generally less than or equal to 10 [mu]m. In addition, UFP 
are often defined as particles with a diameter of less than 0.1 [mu]m 
based on physical size, thermal diffusivity or electrical mobility 
(U.S. EPA, 2019, section 2.2). Atmospheric lifetimes are generally 
longest for PM2.5, which often remains in the atmosphere for 
days to weeks (U.S. EPA, 2019, Table 2-1) before being removed by wet 
or dry deposition, while atmospheric lifetimes for UFP and 
PM10-2.5 are shorter and are generally removed from the 
atmosphere within hours, through wet or dry deposition (U.S. EPA, 2019, 
Table 2-1; 85 FR 24100, April 30, 2020).
2. Sources and Emissions Contributing to PM in the Ambient Air
    PM is composed of both primary (directly emitted particles) and 
secondary particles. Primary PM is derived from direct particle 
emissions from specific PM sources while secondary PM originates from 
gas-phase chemical compounds present in the atmosphere that have 
participated in new particle formation or condensed onto existing 
particles (U.S. EPA, 2019, section 2.3). As discussed further in the 
ISA (U.S. EPA, 2019, section 2.3.2.1), secondary PM is formed in the 
atmosphere by photochemical oxidation reactions of both inorganic and 
organic gas-phase precursors. Sources and emissions of PM are discussed 
in more detail the PA (U.S. EPA, 2020, section 2.1.1) and in the 
proposal (85 FR 24101, April 30, 2020).
3. Ambient Concentrations and Trends
    This section summarizes available information on recent ambient PM 
concentrations in the U.S. and on trends in PM air quality. Sections 
I.E.3.a and I.E.3.b summarize information on PM2.5 mass and 
components, respectively. Section I.E.3.c summarizes information on 
PM10. Sections I.E.3.d and I.E.3.e summarize the more 
limited information on PM10-2.5 and UFP, respectively. 
Additional detail on PM air quality and trends can be found in the PA 
(U.S. EPA, 2020, section 2.3) and in the proposal (85 FR 24100, April 
30, 2020).
a. PM2.5 Mass
    At monitoring sites in the U.S., annual PM2.5 
concentrations from 2015 to 2017 averaged 8.0 [mu]g/m\3\ (and ranged 
from 3.0 to 18.2 [mu]g/m\3\) and the 98th percentiles of 24-hour 
concentrations averaged 20.9 [mu]g/m\3\ (and ranged from 9.2 to 111 
[mu]g/m\3\) (U.S. EPA, 2020, section 2.3.2.1). The highest ambient 
PM2.5 concentrations occur in the west, particularly in 
California and the Pacific northwest (U.S. EPA, 2020, Figure 2-8). Much 
of the eastern U.S. has lower ambient concentrations, with annual 
average concentrations generally at or below 12.0 [mu]g/m\3\ and 98th 
percentiles of 24-hour concentrations generally at or below 30 [mu]g/
m\3\ (U.S. EPA, 2020, section 2.3.2).
    Recent ambient PM2.5 concentrations reflect the 
substantial reductions that have occurred across much of the U.S. (U.S. 
EPA, 2020, section 2.3.2.1). From 2000 to 2017, national annual average 
PM2.5 concentrations have declined from 13.5 [mu]g/m\3\ to 
8.0 [mu]g/m\3\, a 41% decrease (U.S. EPA, 2020, section 2.3.2.1).\18\ 
These declines have occurred at urban and rural monitoring sites, 
although urban PM2.5 concentrations remain consistently 
higher than those in rural areas (Chan et al., 2018) due to the impact 
of local sources in urban areas. Analyses at individual monitoring 
sites indicate that declines in ambient PM2.5 concentrations 
have been most consistent across the eastern U.S. and in parts of 
coastal California, where both annual average and 98th percentiles of 
24-hour concentrations have declined significantly (U.S. EPA, 2020, 
section 2.3.2.1). In contrast, trends in ambient PM2.5 
concentrations have been less consistent over much of the western U.S., 
with no significant changes since 2000 observed at some sites in the 
Pacific northwest, the northern Rockies and plains, and the southwest, 
particularly for 98th percentiles of 24-hour concentrations (U.S. EPA, 
2020, section 2.3.2.1).
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    \18\ See https://www.epa.gov/air-trends/particulate-matter-pm25-trends and https://www.epa.gov/air-trends/particulate-matter-pm25-trends#pmnat for more information.
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    The recent deployment of PM2.5 monitors near major roads 
in large urban areas provides information on PM2.5 
concentrations near an important emissions source. Of the 25 CBSAs with 
valid design values at near-road monitoring sites,\19\ 52% measured the 
highest annual design value at the near-road site while 24% measured 
the highest 24-hour design value at the near-road site (U.S. EPA, 2020, 
section 2.3.2.2). Of the CBSAs with highest annual design values at 
near-road sites, those design values were, on average, 0.7 [mu]g/m\3\ 
higher than at the highest measuring non-near-road sites (range is 0.1 
to 2.0 [mu]g/m\3\ higher at near-road sites). Although most near-road 
monitoring sites do not have sufficient data to evaluate long-term 
trends in

[[Page 82692]]

near-road PM2.5 concentrations, analyses of the data at one 
near-road-like site in Elizabeth, NJ,\20\ show that the annual average 
near-road increment has generally decreased between 1999 and 2017 from 
about 2.0 [mu]g/m\3\ to about 1.3 [mu]g/m\3\ (U.S. EPA, 2020, section 
2.3.2.2).
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    \19\ A design value is considered valid if it meets the data 
handling requirements given in 40 CFR Appendix N to part 50.
    \20\ The Elizabeth Lab site in Elizabeth, NJ is situated 
approximately 30 meters from travel lanes of the Interchange 13 toll 
plaza of the New Jersey Turnpike and within 200 meters of travel 
lanes for Interstate 278 and the New Jersey Turnpike.
---------------------------------------------------------------------------

b. PM2.5 Components
    Based on recent air quality data, the major chemical components of 
PM2.5 have distinct spatial distributions. Sulfate 
concentrations tend to be highest in the eastern U.S., while in the 
Ohio Valley, Salt Lake Valley, and California nitrate concentrations 
are highest, and relatively high concentrations of organic carbon are 
widespread across most of the continental U.S. (U.S. EPA, 2020, section 
2.3.2.3). Elemental carbon, crustal material, and sea salt are found to 
have the highest concentrations in the northeast U.S., southwest U.S., 
and coastal areas, respectively.
    An examination of PM2.5 composition trends can provide 
insight into the factors contributing to overall reductions in ambient 
PM2.5 concentrations. The biggest change in PM2.5 
composition that has occurred in recent years is the reduction in 
sulfate concentrations due to reductions in SO2 emissions. 
Between 2000 and 2015, the nationwide annual average sulfate 
concentration decreased by 17% at urban sites and 20% at rural sites. 
This change in sulfate concentrations is most evident in the eastern 
U.S. and has resulted in organic matter or nitrate now being the 
greatest contributor to PM2.5 mass in many locations (U.S. 
EPA, 2019, Figure 2-19). The overall reduction in sulfate 
concentrations has contributed substantially to the decrease in 
national average PM2.5 concentrations as well as the decline 
in the fraction of PM10 mass accounted for by 
PM2.5 (U.S. EPA, 2019, section 2.5.1.1.6; U.S. EPA, 2020, 
section 2.3.1).
c. PM10
    At monitoring sites in the U.S., the 2015-2017 average of 2nd 
highest 24-hour PM10 concentration was 56 [mu]g/m\3\ 
(ranging from 18 to 173 [mu]g/m\3\) (U.S. EPA, 2020, section 
2.3.2.4).\21\ The highest PM10 concentrations tend to occur 
in the western U.S. Seasonal analyses indicate that ambient 
PM10 concentrations are generally higher in the summer 
months than at other times of year, though the most extreme high 
concentration events are more likely in the spring (U.S. EPA, 2019, 
Table 2-5). This is due to fact that the major PM10 emission 
sources, dust and agriculture, are more active during the warmer and 
drier periods of the year.
---------------------------------------------------------------------------

    \21\ The form of the current 24-hour PM10 standard is 
one-expected-exceedance, averaged over three years.
---------------------------------------------------------------------------

    Recent ambient PM10 concentrations reflect reductions 
that have occurred across much of the U.S. (U.S. EPA, 2020, section 
2.3.2.4). From 2000 to 2017, annual second highest 24-hour 
PM10 concentrations have declined by about 30% (U.S. EPA, 
2020, section 2.3.2.4).\22\ These PM10 concentrations have 
generally declined in the eastern U.S., while concentrations in much of 
the midwest and western U.S. have remained unchanged or increased since 
2000 (U.S. EPA, 2020, section 2.3.2.4). Analyses at individual 
monitoring sites indicate that annual average PM10 
concentrations have also declined at most sites across the U.S., with 
much of the decrease in the eastern U.S. associated with reductions in 
PM2.5 concentrations.
---------------------------------------------------------------------------

    \22\ For more information, see https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat.
---------------------------------------------------------------------------

d. PM10-2.5
    Since the last review, the availability of PM10-2.5 
ambient concentration data has greatly increased because of additions 
to the PM10-2.5 monitoring capabilities to the national 
monitoring network. As illustrated in the PA (U.S. EPA, 2020, section 
2.3.2.5), annual average and 98th percentile PM10-2.5 
concentrations exhibit less distinct differences between the eastern 
and western U.S. than for either PM2.5 or PM10. 
Additionally, compared to PM2.5 and PM10, changes 
in PM10-2.5 concentrations have been small in magnitude and 
inconsistent in direction (U.S. EPA, 2020, section 2.3.2.5).
e. UFP
    Compared to PM2.5 mass, there is relatively little data 
on U.S. particle number concentrations, which are dominated by UFP. 
Based on measurements in two urban areas (New York City, Buffalo) and 
at a background site (Steuben County) in New York, urban particle 
number counts were several times higher than at the background site 
(U.S. EPA, 2020, section 2.3.2.6; U.S. EPA, 2019, Figure 2-18). The 
highest particle number counts in an urban area with multiple sites 
(Buffalo) were observed at a near-road location.
    Long-term trends in UFP are not routinely available at U.S. 
monitoring sites. At one site in Illinois with long-term data 
available, the annual average particle number concentration declined 
between 2000 and 2017, closely matching the reductions in annual 
PM2.5 mass over that same period (U.S. EPA, 2020, section 
2.3.2.6). In addition, a small number of published studies have 
examined UFP trends over time. While limited, these studies also 
suggest that UFP number concentrations have declined over time along 
with decreases in PM2.5 (U.S. EPA, 2020, section 2.3.2.6).
4. Background PM
    In this review, background PM is defined as all particles that are 
formed by sources or processes that cannot be influenced by actions 
within the jurisdiction of concern. U.S. background PM is defined as 
any PM formed from emissions other than U.S. anthropogenic (i.e., 
manmade) emissions. Potential sources of U.S. background PM include 
both natural sources (i.e., PM that would exist in the absence of any 
anthropogenic emissions of PM or PM precursors) and transboundary 
sources originating outside U.S. borders. Background PM is discussed in 
more detail in the PA (U.S. EPA, 2020, section 2.4) and in the proposal 
(85 FR 24102, April 30, 2020). At annual and national scales, estimated 
background PM concentrations in the U.S. are small compared to 
contributions from domestic anthropogenic emissions.\23\ For example, 
based on zero-out modeling in the last review of the PM NAAQS, annual 
background PM2.5 concentrations were estimated to range from 
0.5-3 [micro]g/m\3\ across the sites examined. In addition, speciated 
monitoring data from IMPROVE sites can provide some insights into how 
contributions from different sources, including sources of background 
PM, may have changed over time. Such data suggests the estimates of 
background concentrations using speciated monitoring data from IMPROVE 
monitors are around 1-3 [micro]g/m\3\, and have not changed 
significantly since the last review. Contributions to background PM in 
the U.S. result

[[Page 82693]]

mainly from sources within North America. Contributions from 
intercontinental events have also been documented (e.g., transport from 
dust storms occurring in deserts in North Africa and Asia), but these 
events are less frequent and represent a relatively small fraction of 
background PM in most places.
---------------------------------------------------------------------------

    \23\ Sources that contribute to natural background PM include 
dust from the wind erosion of natural surfaces, sea salt, wildland 
fires, primary biological aerosol particles such as bacteria and 
pollen, oxidation of biogenic hydrocarbons such as isoprene and 
terpenes to produce secondary organic aerosols (SOA), and geogenic 
sources such as sulfate formed from volcanic production of 
SO2 and oceanic production of dimethyl-sulfide (U.S. EPA, 
2020, section 2.4). While most of these sources release or 
contribute predominantly to fine aerosol, some sources including 
windblown dust, and sea salt also produce particles in the coarse 
size range (U.S. EPA, 2019, section 2.3.3).
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II. Rationale for Decisions on the Primary PM2.5 Standards

    This section presents the rationale for the Administrator's 
decision to retain the current primary PM2.5 standards. This 
decision is based on a thorough review in the ISA of the latest 
scientific information, published through December 2017,\24\ on human 
health effects associated with long-and short-term exposures to 
PM2.5 in the ambient air. This decision also takes into 
account analyses in the PA of policy-relevant information from the ISA, 
as well as information on air quality; the analyses of human health 
risks in the PA; CASAC advice; and consideration of public comments 
received on the proposal.
---------------------------------------------------------------------------

    \24\ In addition to the review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), ``the current ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009 and March 31, 2017. A limited 
literature update identified some additional studies that were 
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p. 
A-3). References that are cited in the ISA, the references that were 
considered for inclusion but not cited, and electronic links to 
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    Section II.A provides background on the general approach for this 
review and the basis for the existing standard, and also presents brief 
summaries of key aspects of the currently available health effects and 
risk information. Section II.B summarizes the proposed conclusions and 
CASAC advice, addresses public comments received on the proposal and 
presents the Administrator's conclusions on the adequacy of the current 
standard, drawing on consideration of the scientific evidence and 
quantitative risk information, advice from the CASAC, and comments from 
the public. Section II.C summarizes the Administrator's decision on the 
primary PM2.5 standards.

A. Introduction

    As in prior reviews, the general approach to reviewing the current 
primary PM2.5 standards is based, most fundamentally, on 
using the EPA's assessment of current scientific evidence and 
associated quantitative analyses to inform the Administrator's judgment 
regarding primary PM2.5 standards that protects public 
health with an adequate margin of safety. In drawing conclusions with 
regard to the primary PM2.5 standards, the final decision on 
the adequacy of the standard is largely a public health policy judgment 
to be made by the Administrator. The Administrator's final decision 
draws upon scientific information and analyses about health effects, 
population risks, as well as judgments about how to consider the range 
and magnitude of uncertainties that are inherent in the scientific 
evidence and risk analyses. The approach to informing these judgments, 
discussed more fully below, generally reflects a continuum, consisting 
of levels at which scientists generally agree that health effects are 
likely to occur, through lower levels at which the likelihood and 
magnitude of the response become increasingly uncertain. This approach 
is consistent with the requirements of the NAAQS provisions of the CAA 
and with how the EPA and the courts have historically interpreted the 
Act. These provisions require the Administrator to establish primary 
standards that, in his judgment, are requisite to protect public health 
with an adequate margin of safety. In so doing, the Administrator seeks 
to establish standards that are neither more nor less stringent than 
necessary for this purpose. The Act does not require that primary 
standards be set at a zero-risk level, but rather at a level that 
avoids unacceptable risks to public health including the health of 
sensitive groups.\25\ The four basic elements of the NAAQS (indicator, 
averaging time, form, and level) are considered collectively in 
evaluating the health protection afforded by a standard.
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    \25\ As noted in section I.A above, such protection is specified 
for the sensitive group of individuals and not to a single person in 
the sensitive group (see S. Rep. No. 91-1196, 91st Cong., 2d Sess. 
10 [1970]).
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    In evaluating the appropriateness of retaining or revising the 
current primary PM2.5 standards, the EPA has adopted an 
approach that builds upon the general approach used in the last review 
and reflects the body of evidence of information now available. As 
summarized in section II.A.1 below, the Administrator's decisions in 
the prior review were based on an integration of information on health 
effects associated with exposure to PM2.5 with information 
on the public health significance of key health effects, as well as on 
policy judgments as to when the standard is requisite to protect public 
health with an adequate margin of safety and on consideration of advice 
from the CASAC and public comments. These decisions were also informed 
by air quality and related analyses and quantitative risk information.
    Similarly, in this review, as described in the PA, the proposal, 
and elsewhere in this document, we draw on the current evidence and 
quantitative assessments of public health risk of PM2.5 in 
ambient air. The past and current approaches are both based, most 
fundamentally, on the EPA's assessments of the current scientific 
information and associated quantitative analyses. The EPA's assessments 
are primarily documented in the ISA and PA, which have received CASAC 
review and public comment (83 FR 53471, October 23, 2018; 83 FR 55529, 
November 6, 2018; 84 FR 8523, March 8, 2019; 84 FR 47944, September 11, 
2019; 84 FR 51555, September 30, 2019; 84 FR 58713, September 30, 
2019). To bridge the gap between the scientific assessments of the ISA 
and quantitative assessments of the PA and the judgments required of 
the Administrator in determining whether the current standard remains 
requisite to protect public health with an adequate margin of safety, 
the PA evaluates the policy implications of the current evidence in the 
ISA and of the quantitative analyses in the PA.
    In considering the scientific and technical information, we 
consider both the information available at the time of the last review 
and information newly available since the last review, including most 
particularly that which has been critically analyzed and characterized 
in the current ISA. We additionally consider the quantitative risk 
information described in the PA that estimated population-level health 
risks associated with ambient PM2.5 concentrations that have 
been adjusted to simulate air quality scenarios of policy interest 
(e.g., ``just meeting'' the current standards) in multiple study areas. 
The evidence-based discussions presented below (and summarized more 
fully in the proposal) draw upon evidence from studies evaluating 
health effects related to exposures to PM2.5, as discussed 
in the ISA. The risk-based discussions also presented below (and 
summarized more fully in the proposal) have been drawn from the 
quantitative analyses for PM2.5, as discussed in the PA. 
Sections II.A.2 and II.A.3 below provide an overview for the current 
health effects evidence related to short- and long-term exposures to 
PM2.5 and quantitative risk information with a focus on 
specific policy-relevant questions identified for these categories of 
information in the PA.

[[Page 82694]]

1. Background on the Current Standards
    The last review of the primary PM NAAQS was completed in 2012 (78 
FR 3086, January 15, 2013). As noted above (section I.C.4), in the last 
review the EPA lowered the level of the primary annual PM2.5 
standard from 15.0 to 12.0 [mu]g/m\3\,\26\ and retained the existing 
24-hour PM2.5 standard with its level of 35 [mu]g/m\3\. The 
2012 decision to strengthen the suite of primary PM2.5 
standards was based on the prior Administrator's consideration of the 
extensive body of scientific evidence assessed in the 2009 ISA (U.S. 
EPA, 2009c); the quantitative risk analyses presented in the 2010 
health risk assessment (U.S. EPA, 2010a); the advice and 
recommendations of the CASAC (Samet, 2009; Samet, 2010c; Samet, 2010b); 
and public comments on the proposed rule (78 FR 3086, January 15, 2013; 
U.S. EPA, 2012). In particular, she noted the ``strong and generally 
robust body of evidence of serious health effect associated with both 
long- and short-term exposures to PM2.5'' (78 FR 3120, 
January 15, 2013). This included epidemiological studies reporting 
health effect associations based on long-term average PM2.5 
concentrations ranging from about 15.0 [micro]g/m\3\ or above (i.e., at 
or above the level of the then-existing annual standard) to 
concentrations ``significantly below the level of the annual standard'' 
(78 FR 3120, January 15, 2013). Based on her ``confidence in the 
association between exposure to PM2.5 and serious public 
health effects, combined with evidence of such an association in areas 
that would meet the current standards'' (78 FR 3120, January 15, 2013), 
the prior Administrator concluded that revision of the suite of primary 
PM2.5 standards was necessary in order to provide increased 
public health protection.
---------------------------------------------------------------------------

    \26\ The Agency also eliminated spatial averaging provisions as 
part of the form of the annual standard.
---------------------------------------------------------------------------

    The prior Administrator next considered what specific revisions to 
the existing primary PM2.5 standards were appropriate, given 
the available evidence and quantitative risk information. She 
considered both the annual and 24-hour PM2.5 standards, 
focusing on the basic elements of those standards (i.e., indicator, 
averaging time, form, and level). With regard to the indicator, the EPA 
recognized that the health studies available during the last review 
continued to link adverse health outcomes (e.g., premature mortality, 
hospital admissions, emergency department visits) with long- and short-
term exposures to PM2.5 (78 FR 3121, January 15, 2013). In 
assessing the appropriateness of PM2.5 mass as the 
indicator, the EPA also considered the available scientific evidence 
and information available related to ultrafine particles 
27 28 and PM components,\29\ noting the significant 
uncertainties and limitations associated with the evidence, as well as 
the availability of monitoring data. Consistent with the considerations 
and conclusions in the 2011 PA, the CASAC advised that it was 
appropriate to consider retaining PM2.5 as the indicator for 
fine particles. In light of the evidence and the CASAC's advice, the 
prior Administrator concluded that it was ``appropriate to retain 
PM2.5 as the indicator for fine particles'' (78 FR 3123, 
January 15, 2013).
---------------------------------------------------------------------------

    \27\ In the last review, the ISA defined ultrafine particles 
(UFP) as generally including particles with a mobility diameter less 
than or equal to 0.1 [micro]m. Mobility diameter is defined as the 
diameter of a particle having the same diffusivity or electrical 
mobility in air as the particle of interest and is often used to 
characterize particles of 0.5 [micro]m or smaller (U.S. EPA, 2009c, 
pp. 3-2 to 3-3).
    \28\ The 2011 PA noted the limited body of evidence assessed in 
the 2009 ISA (summarized in U.S. EPA, 2009c, section 2.3.5 and Table 
2-6) and the limited monitoring information available to 
characterized ambient concentrations of UFP (U.S. EPA, 2011, section 
1.3.2).
    \29\ The 2009 ISA concluded that ``the evidence is not yet 
sufficient to allow differentiation of those constituents or sources 
that are more closely related to specific health outcomes'' (U.S. 
EPA, 2009c, pp. 2-26 and 6-212; 78 FR 3123, January 15, 2013). The 
2011 PA further noted that ``many different constituents of the fine 
particle mixture as well as groups of components associated with 
specific source categories of fine particles are linked to adverse 
health effects'' (U.S. EPA, 2011, p. 2-55; 78 FR 3123, January 15, 
2013).
---------------------------------------------------------------------------

    With regard to averaging time, in the last review, the EPA 
considered issues related to the appropriate averaging time for 
PM2.5 standards, with a focus on evaluating support for the 
existing annual and 24-hour averaging times and for potential 
alternative averaging times based on sub-daily or seasonal metrics. 
Based on the evidence assessed in the 2009 ISA, the 2011 PA noted that 
the overwhelming majority of studies utilized annual (or multi-year) or 
24-hour PM averaging periods (U.S. EPA, 2011, section 2.3.2). Given 
this evidence-base, and limitations in the data for alternatives, the 
2011 PA reached the overall conclusions that the available information 
provided strong support for considering retaining the existing annual 
and 24-hour averaging times (U.S. EPA, 2011, p. 2-58). The CASAC agreed 
that these conclusions were reasonable (Samet, 2010a, p. 2-58). The 
prior Administrator concurred with the CASAC's advice. Specifically, 
she judged that it was ``appropriate to retain the current annual and 
24-hour averaging times for the primary PM2.5 standards to 
protect against health effects associated with long- and short-term 
exposure periods'' (78 FR 3124, January 15, 2013).
    With regard to form, the EPA first noted that the form of the 
annual PM2.5 standard was established in 1997 as an annual 
arithmetic mean, averaged over 3 years, from single or multiple 
community-oriented monitors.\30\ That is, the level of the annual 
standard was to be compared to measurements made at each community-
oriented monitoring site, or if criteria were met, measurements from 
multiple community-oriented monitoring sites could be averaged together 
(i.e., spatial averaging) \31\ (62 FR 38671-38672, July 18, 1997). In 
the 1997 review, the EPA also established the form of the 24-hour 
PM2.5 standard as the 98th percentile of 24-hour 
concentrations at each monitor within an area (i.e., no spatial 
averaging), averaged over three years (62 FR 38671-38674, July 18, 
1997). In the 2006 review, the EPA retained these standard forms but 
tightened the criteria for using spatial averaging with the annual 
standard (71 FR 61167, October 17, 2006).\32\
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    \30\ In the last review, the EPA replaced the term ``community-
oriented'' monitor with the term ``area-wide'' monitor (U.S. EPA, 
2020, section 1.3). Area-wide monitors are those sited at the 
neighborhood scale or larger, as well as those monitors sited at 
micro- or middle scales that are representative of many such 
locations in the same core-based statistical area (CBSA; 78 FR 3236, 
January 15, 2013). CBSAs are required to have at least one area-wide 
monitor sited in the area of expected maximum PM2.5 
concentration.
    \31\ The original criteria for spatial averaging included: (1) 
The annual mean concentration at each site shall be within 20% of 
the spatially averaged annual mean, and (2) the daily values for 
each monitoring site pair shall yield a correlation coefficient of 
at least 0.6 for each calendar quarter (62 FR 38671-38672, July 18, 
1997).
    \32\ Specifically, the Administrator revised spatial averaging 
criteria such that ``(1) [t]he annual mean concentration at each 
site shall be within 10 percent of the spatially averaged annual 
mean, and (2) the daily values for each monitoring site pair shall 
yield a correlation coefficient of at least 0.9 for each calendar 
quarter'' (71 FR 61167, October 17, 2006).
---------------------------------------------------------------------------

    At the time of the last review, the EPA again considered the form 
of the standard with a focus on the issue of spatial averaging. An 
analysis of air quality and population demographic information 
indicated that the highest PM2.5 concentrations in a given 
area tended to be measured at monitors in locations where the 
surrounding populations were more likely to live below the poverty line 
and to include larger percentages of racial and ethnic minorities (U.S. 
EPA, 2011, p. 2-60). Based on this analysis, the 2011 PA concluded that 
spatial averaging could result in disproportionate impacts in at-risk 
populations and populations with

[[Page 82695]]

lower socioeconomic status (SES). Therefore, the PA concluded that it 
was appropriate to consider revising the form of the annual 
PM2.5 standard such that it did not allow for the use of 
spatial averaging across monitors (U.S. EPA, 2011, p. 2-60). The CASAC 
agreed with the PA conclusions that it was ``reasonable'' for the EPA 
to eliminate the spatial averaging provisions (Samet, 2010c, p. 2).
    With regard to the form of the annual PM2.5 standard, 
the prior Administrator concluded that public health would not be 
protected with an adequate margin of safety in all locations if 
disproportionately higher PM2.5 concentrations in low income 
and minority communities were averaged together with lower 
concentrations measured at other sites in a larger urban area. 
Therefore, she concluded that the form of the annual PM2.5 
standard should be revised to eliminate spatial averaging provisions 
(78 FR 3124, January 15, 2013).
    With regard to the form of the 24-hour PM2.5 standard, 
the EPA recognized that the existing 98th percentile form was 
originally selected to provide a balance between limiting the 
occurrence of peak 24-hour PM2.5 concentrations and 
identifying a stable target for risk management programs.\33\ Updated 
air quality analyses in the last review provided additional support for 
the increased stability of the 98th percentile PM2.5 
concentration, compared to the 99th percentile (U.S. EPA, 2011, Figure 
2-2, p. 2-62). Consistent with the PA conclusions based on this 
analysis, the prior Administrator concluded that it was appropriate to 
retain the 98th percentile form for the 24-hour PM2.5 
standard (78 FR 3127, January 15, 2013).
---------------------------------------------------------------------------

    \33\ See ATA III, 283 F.3d at 374-76 which concludes that it is 
legitimate for the EPA to consider overall stability of the standard 
and its resulting promotion of overall effectiveness of NAAQS 
control programs in setting a standard that is requisite to protect 
the public health.
---------------------------------------------------------------------------

    With regard to alternative levels of the annual and 24-hour 
PM2.5 standards, in the last review, the EPA considered the 
public health protection provided by the standards, taken together, 
against mortality and morbidity effects associated with long- or short-
term PM2.5 exposures. This approach recognized that it is 
appropriate to consider the protection provided by attaining the air 
quality needed to meet the suite of standards, and that there is no 
bright line clearly directing the choice of levels. Rather, the choice 
of what is appropriate is a public health policy judgment entrusted to 
the Administrator. See Mississippi, 744 F.3d at 1358, Lead Industries 
Ass'n, 647 F.2d at 1147.
    In selecting the levels of the annual and 24-hour PM2.5 
standards, the prior Administrator placed the greatest emphasis on 
health endpoints for which the evidence was strongest, based on the 
assessment of the evidence in the ISA and on the ISA's causality 
determinations (U.S. EPA, 2009c, section 2.3.1). She particularly noted 
that the evidence was sufficient to conclude a causal relationship 
exists between PM2.5 exposures and mortality and 
cardiovascular effects (i.e., for both long- and short-term exposures) 
and that the evidence was sufficient to conclude a causal relationship 
is ``likely'' to exist between PM2.5 exposures and 
respiratory effects (i.e., for both long- and short-term exposures). 
She also noted additional, but more limited, evidence for a broader 
range of health endpoints, including evidence ``suggestive of a causal 
relationship'' between long-term exposures and developmental and 
reproductive effects as well as carcinogenic effects (78 FR 3158, 
January 15, 2013).
    To inform her decisions on an appropriate level for the annual 
standard, the Administrator considered the degree to which 
epidemiological studies indicate confidence in the reported health 
effect associations over distributions of PM2.5 
concentrations in ambient air. She noted that a level of 12.0 [micro]g/
m\3\ was below the long-term mean PM2.5 concentrations 
reported in key epidemiological studies that provided evidence of an 
array of serious health effects (78 FR 3161, January 15, 2013). She 
further noted that 12.0 [micro]g/m\3\ generally corresponded to the 
lower portions (i.e., about the 25th percentile) of distributions of 
health events in the limited number of epidemiological studies for 
which population-level information was available. A level of 12.0 
[micro]g/m\3\ also reflected placing some weight on studies of 
reproductive and developmental effects, for which the evidence was more 
uncertain (78 FR 3161-3162, January 15, 2013).
    Given the uncertainties remaining in the scientific evidence, the 
Administrator judged that an annual standard level below 12.0 [micro]g/
m\3\ was not supported. She specifically noted uncertainties related to 
understanding the relative toxicity of the different components in the 
fine particle mixture, the role of PM2.5 in the complex 
ambient mixture, exposure measurement error in epidemiological studies, 
and the nature and magnitude of estimated risks at relatively low 
ambient PM2.5 concentrations. Furthermore, she noted that 
epidemiological studies had reported heterogeneity in effect estimates 
both within and between cities and in geographic regions of the U.S. 
She recognized that this heterogeneity may be attributed, in part, to 
difference in PM2.5 composition in different regions and 
cities. With regard to evidence for reproductive and developmental 
effects, the prior Administrator recognized that there were a number of 
limitations associated with this body of evidence, including the 
limited number of studies evaluating such effects; uncertainties 
related to identifying the relevant exposure time periods of concern, 
and limited toxicologic evidence providing information on the mode of 
action(s) or biological plausibility for an association between long-
term PM2.5 exposures and adverse birth outcomes. On balance, 
she found that the available evidence, interpreted in light of these 
remaining uncertainties, did not justify an annual standard level set 
below 12.0 [micro]g/m\3\ as being requisite to protect public health 
with an adequate margin of safety (i.e., a standard with a lower level 
would have been more stringent than necessary).
    In conjunction with a revised annual standard with a level of 12.0 
[micro]g/m\3\, the prior Administrator concluded that the evidence 
supported retaining the 35 [micro]g/m\3\ level of the 24-hour 
PM2.5 standard. She noted that the existing 24-hour 
standard, with its 35 [micro]g/m\3\ level and 98th percentile form, 
would provide supplemental protection, particularly for areas with high 
peak-to-mean ratios possibly associated with strong seasonal sources 
and for areas with PM2.5-related effects that may be 
associated with shorter than daily exposure periods (78 FR 3163, 
January 15, 2013). Thus, she concluded that the available evidence and 
information, considered together with its inherent uncertainties and 
limitations, supported an annual standard with a level of 12.0 
[micro]g/m\3\ combined with a 24-hour standard with a level of 35 
[micro]g/m\3\.
2. Overview of Health Effects Evidence
    In this section, we provide an overview of the policy-relevant 
aspects of the health effects evidence available for consideration in 
this review. Section II.B of the proposal provides a detailed summary 
of key information contained in the ISA (U.S. EPA, 2019) and in the PA 
(U.S. EPA, 2020) on the health effects associated with PM2.5 
exposures, and the related public health implications, focusing 
particularly on the information most relevant to consideration of 
effects associated with the presence of PM2.5 in ambient 
air. The subsections below briefly

[[Page 82696]]

summarize the information discussed in more detail in section II.B of 
the proposal (85 FR 24106 to 24114, April 30, 2020).
a. Nature of Effects
    Drawing from the assessment of the evidence in the ISA (U.S. EPA, 
2019), and the summaries of that assessment in the PA (U.S. EPA, 2020), 
the sections below summarize the evidence for relationships between 
long- or short-term PM2.5 exposures and mortality 
(II.A.2.a.i), cardiovascular effects (II.A.2.a.ii), respiratory effects 
(II.A.2.a.iii), cancer (II.A.2.a.iv), nervous system effects 
(II.A.2.a.v), and other effects (II.A.2.a.vi). For these outcomes, the 
ISA concludes that the evidence supports either a ``causal'' or a 
``likely to be causal'' relationship with PM2.5 
exposures.\34\
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    \34\ In this review of the PM NAAQS, the EPA considers the full 
body of health evidence, placing the greatest emphasis on the health 
effects for which the evidence has been judged in the ISA to 
demonstrate a ``causal'' or a ``likely to be causal'' relationship 
with PM exposures.
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i. Mortality
Long-Term PM2.5 Exposures
    In the last review, the 2009 ISA reported that the evidence was 
``sufficient to conclude that the relationship between long-term 
PM2.5 exposures and mortality is causal'' (U.S. EPA, 2009c, 
p. 7-96). The strongest evidence supporting this conclusion was 
provided by epidemiological studies, particularly those examining two 
seminal cohorts, the American Cancer Society (ACS) cohort and the 
Harvard Six Cities cohort. Analyses of the Harvard Six Cities cohort 
included demonstrations that reductions in ambient PM2.5 
concentrations are associated with reduced mortality risk (Laden et 
al., 2006) and with increases in life expectancy (Pope et al., 2009). 
Further support was provided by other cohort studies conducted in North 
America and Europe that reported positive associations between long-
term PM2.5 exposures and risk of mortality (U.S. EPA, 
2009c).
    Recent cohort studies, which have become available since the 2009 
ISA, continue to provide consistent evidence of positive associations 
between long-term PM2.5 exposures and mortality. These 
studies add support for associations with total and non-accidental 
mortality,\35\ as well as with specific causes of death, including 
cardiovascular disease and respiratory disease (U.S. EPA, 2019, section 
11.2.2). Many of these recent studies have extended the follow-up 
periods originally evaluated in the ACS and Harvard Six Cities cohort 
studies and continue to observe positive associations between long-term 
PM2.5 exposures and mortality (U.S. EPA, 2019, section 
11.2.2.1, Figures 11-18 and 11-19). Adding to recent evaluations of the 
ACS and Six Cities cohorts, studies conducted with other cohorts also 
show consistent, positive associations between long-term 
PM2.5 exposure and mortality across various demographic 
groups (e.g., age, sex, occupation), spatial and temporal extents, 
exposure assessment metrics, and statistical techniques (U.S. EPA, 
2019, sections 11.2.2.1 and 11.2.5). This includes some of the largest 
cohort studies conducted to date, with analyses of the U.S. Medicare 
cohort that include nearly 61 million enrollees (Di et al., 2017b) and 
studies that control for a range of individual and ecological 
covariates.
---------------------------------------------------------------------------

    \35\ The majority of these studies examined non-accidental 
mortality outcomes, though some Medicare studies lack cause-specific 
death information and, therefore, examine total mortality.
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    A recent series of accountability studies has additionally tested 
the hypothesis that past reductions in ambient PM2.5 
concentrations have been associated with increased life expectancy or a 
decreased mortality rate (U.S. EPA, 2019, section 11.2.2.5). Pope et 
al. (2009) conducted a cross-sectional analysis using air quality data 
from 51 metropolitan areas across the U.S., beginning in the 1970s 
through the early 2000s, and found that a 10 [micro]g/m\3\ decrease in 
long-term PM2.5 concentration was associated with a 0.61-
year increase in life expectancy. In a subsequent analysis, the authors 
extended the period of analysis to include 2000 to 2007 (Correia et 
al., 2013), a time period with lower ambient PM2.5 
concentrations. In this follow-up study, a decrease in long-term 
PM2.5 concentrations continued to be associated with an 
increase in life expectancy, though the magnitude of the increase was 
smaller than during the earlier time period (i.e., a 10 [micro]g/m\3\ 
decrease in long-term PM2.5 concentration was associated 
with a 0.35-year increase in life expectancy). Additional studies 
conducted in the U.S. or Europe similarly report that reductions in 
ambient PM2.5 are associated with improvements in longevity 
(U.S. EPA, 2019, section 11.2.2.5).
    The ISA concludes that positive associations between long-term 
PM2.5 exposures and mortality are robust across analyses 
examining a variety of study designs (e.g., U.S. EPA, 2019, section 
11.2.2.4), approaches to estimating PM2.5 exposures (U.S. 
EPA, 2019, section 11.2.5.1), approaches to controlling for confounders 
(U.S. EPA, 2019, sections 11.2.3 and 11.2.5), geographic regions and 
populations, and temporal periods (U.S. EPA, 2019, sections 11.2.2.5 
and 11.2.5.3). Recent evidence further demonstrates that associations 
with mortality remain robust in copollutant analyses (U.S. EPA, 2019, 
section 11.2.3), and that associations persist in analyses restricted 
to long-term exposures below 12 [mu]g/m\3\ (Di et al., 2017b) or 10 
[mu]g/m\3\ (Shi et al., 2016).
    Another important consideration in characterizing the potential for 
additional public health improvements associated with changes in 
PM2.5 exposure is whether concentration-response 
relationships are linear across the range of concentrations or if 
nonlinear relationships exist along any part of this range. Several 
recent studies examine this issue, and continue to provide evidence of 
linear, no-threshold relationships between long-term PM2.5 
exposures and all-cause and cause-specific mortality (U.S. EPA, 2019, 
section 11.2.4). However, interpreting the shapes of these 
relationships, particularly at PM2.5 concentrations near the 
lower end of the air quality distribution, can be complicated by 
relatively low data density in the lower concentration range, the 
possible influence of exposure measurement error, and variability among 
individuals with respect to air pollution health effects. These sources 
of variability and uncertainty tend to smooth and ``linearize'' 
population-level concentration-response functions, and thus could 
obscure the existence of a threshold or nonlinear relationship (85 FR 
24107, April 30, 2020).
    The biological plausibility of PM2.5-attributable 
mortality is supported by the coherence of effects across scientific 
disciplines (i.e., animal toxicologic, controlled human exposure 
studies, and epidemiologic). The ISA outlines the available evidence 
for plausible pathways by which inhalation exposure to PM2.5 
could progress from initial events (e.g., respiratory tract 
inflammation, autonomic nervous system modulation) to endpoints 
relevant to population outcomes, particularly those related to 
cardiovascular diseases such as ischemic heart disease, stroke and 
atherosclerosis (U.S. EPA, 2019, section 6.2.1), and to metabolic 
disease and diabetes (U.S. EPA, 2019, section 7.2.1). The ISA notes 
``more limited evidence from respiratory morbidity'' (U.S. EPA, 2019, 
p. 11-101) to support the biological plausibility of mortality due

[[Page 82697]]

to long-term PM2.5 exposures (U.S. EPA, 2019, section 
11.2.1).
    Taken together, recent studies reaffirm and further strengthen the 
body of evidence from the 2009 ISA for the relationship between long-
term PM2.5 exposure and mortality. Recent epidemiological 
studies consistently report positive associations with mortality across 
different geographic locations, populations, and analytic approaches. 
Recent experimental and epidemiological evidence for cardiovascular 
effects, and respiratory effects to a more limited degree, supports the 
plausibility of mortality due to long-term PM2.5 exposures. 
The 2019 ISA concludes that, ``collectively, this body of evidence is 
sufficient to conclude that a causal relationship exists between long-
term PM2.5 exposure and total mortality'' (U.S. EPA, 2019, 
section 11.2.7; p. 11-102).
Short-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship exists between 
short-term exposure to PM2.5 and mortality'' (U.S. EPA, 
2009c). This conclusion was based on the evaluation of both multi- and 
single-city epidemiological studies that consistently reported positive 
associations between short-term PM2.5 exposure and non-
accidental mortality. Examination of the potential confounding effects 
of gaseous copollutants was limited, though evidence from single-city 
studies indicated that gaseous copollutants have minimal effect on the 
PM2.5-mortality relationship (i.e., associations remain 
robust to inclusion of other pollutants in copollutant models). The 
evaluation of cause-specific mortality found that effect estimates were 
larger in magnitude, but also had larger confidence intervals, for 
respiratory mortality compared to cardiovascular mortality. Although 
the largest mortality risk estimates were for respiratory mortality, 
the interpretation of the results was complicated by the limited 
coherence from studies of respiratory morbidity. However, the evidence 
from studies of cardiovascular morbidity provided both coherence and 
biological plausibility for the relationship between short-term 
PM2.5 exposure and cardiovascular mortality.
    Recent multicity studies evaluated since the 2009 ISA continue to 
provide evidence of primarily positive associations between daily 
PM2.5 exposures and mortality, with percent increases in 
total mortality ranging from 0.19% (Lippmann et al., 2013) to 2.80% 
(Kloog et al., 2013) \36\ at lags of 0 to 1 days in single-pollutant 
models. Whereas most studies rely on assigning exposures using data 
from ambient monitors, associations are also reported in recent studies 
that employ hybrid modeling approaches using additional 
PM2.5 data (i.e., from satellites, land use information, and 
modeling, in addition to monitors), allowing for the inclusion of more 
rural locations in analyses (Kloog et al., 2013, Shi et al., 2016, Lee 
et al., 2015).
---------------------------------------------------------------------------

    \36\ As detailed in the ISA, risk estimates are for a 10 
[micro]g/m\3\ increase in 24-hour average PM2.5 
concentrations, unless otherwise noted (U.S. EPA, 2019, Preface).
---------------------------------------------------------------------------

    Some recent studies have expanded the examination of potential 
confounders (e.g., U.S. EPA, 2019, section 11.1.5.1) to include not 
only copollutants, but also systematic evaluations of the potential 
impact of inadequate control from long-term temporal trends and 
weather. Associations between short-term PM2.5 exposures and 
mortality remain positive and relatively unchanged in copollutant 
models with both gaseous pollutants and PM10-2.5 (U.S. EPA, 
2019, Section 11.1.4). Additionally, the low (r <0.4) to moderate 
correlations (r = 0.4-0.7) between PM2.5 and gaseous 
pollutants and PM10-2.5 increase the confidence in 
PM2.5 having an independent effect on mortality (U.S. EPA, 
2019, section 11.1.4).
    The generally positive associations reported with mortality are 
supported by a small group of studies employing causal inference or 
quasi-experimental statistical approaches (U.S. EPA, 2019, section 
11.1.2.1). For example, a recent study examined whether a specific 
regulatory action in Tokyo, Japan (i.e., a diesel emission control 
ordinance) resulted in a subsequent reduction in daily mortality 
(Yorifuji et al., 2016). The authors report a reduction in mortality in 
Tokyo due to the ordinance, compared to Osaka, which did not have a 
similar diesel emission control ordinance in place.
    Positive associations with total mortality are further supported by 
analyses reporting positive associations with cause-specific mortality, 
including cardiovascular and respiratory mortality (U.S. EPA, 2019, 
section 11.1.3). For cause-specific mortality, there has been only a 
limited assessment of potential copollutant confounding, though initial 
evidence indicates that associations remain positive and relatively 
unchanged in models with gaseous pollutants and PM10-2.5. 
The evidence for ischemic events and heart failure, as detailed in the 
assessment of cardiovascular morbidity (U.S. EPA, 2019, chapter 6), 
provides biological plausibility for PM2.5-related 
cardiovascular mortality, which comprises the largest percentage of 
total mortality (i.e., ~33%) (U.S. National Institutes of Health, 
2013). Although there is evidence for exacerbations of chronic 
obstructive pulmonary disease (COPD) and asthma, the collective body of 
evidence for respiratory effects, particularly from controlled human 
exposure studies, provides only limited support for the biological 
plausibility of PM2.5-related respiratory mortality (U.S. 
EPA, 2019, chapter 5).
    In the 2009 ISA, one of the main uncertainties identified was the 
regional and city-to-city heterogeneity in PM2.5-mortality 
associations. Recent studies examine both city-specific as well as 
regional characteristics to identify the underlying contextual factors 
that could contribute to this heterogeneity (U.S. EPA, 2019, section 
11.1.6.3). Collectively, these studies indicate that the heterogeneity 
in PM2.5-mortality risk estimates cannot be attributed to 
one factor, but instead to a combination of factors including, but not 
limited to, PM composition and sources as well as community 
characteristics that could influence exposures (U.S. EPA, 2019, section 
11.1.12).
    A few recent studies have conducted analyses comparing the 
traditional 24-hour average exposure metric with a sub-daily metric 
(i.e., 1-hour max). These initial studies provide evidence of a similar 
pattern of associations for both the 24-hour average and 1-hour max 
metric, with the association larger in magnitude for the 24-hour 
average metric (U.S. EPA, 2019, section 11.1.8.1).
    Recent multicity studies indicate that positive and statistically 
significant associations with mortality persist in analyses restricted 
to short-term PM2.5 exposures below 35 [mu]g/m\3\ (Lee et 
al., 2015),\37\ below 30 [mu]g/m\3\ (Shi et al., 2016), and below 25 
[mu]g/m\3\ (Di et al., 2017a). Additional studies examine the shape of 
the concentration-response relationship and whether a threshold exists 
specifically for PM2.5 (U.S. EPA, 2019, section 11.1.10). 
These studies have used various statistical approaches and consistently 
found linear relationships with no evidence of a threshold. Recent 
analyses provide initial evidence indicating that PM2.5-
mortality associations persist and may be stronger (i.e., a steeper 
slope) at lower

[[Page 82698]]

concentrations (e.g., Di et al., 2017a; U.S. EPA, 2019, Figure 11-12). 
However, given the limited data available at the lower end of the 
distribution of ambient PM2.5 concentrations, the shape of 
the concentration-response curve remains uncertain at these low 
concentrations and, to date, studies have not conducted extensive 
analyses exploring alternatives to linearity when examining the shape 
of the PM2.5-mortality concentration-response relationship.
---------------------------------------------------------------------------

    \37\ Lee et al. (2015) also report that positive and 
statistically significant associations between short-term 
PM2.5 exposures and mortality persist in analyses 
restricted to areas with long-term concentrations below 12 [mu]g/
m\3\.
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    Overall, recent epidemiological studies build upon and extend the 
conclusions of the 2009 ISA for the relationship between short-term 
PM2.5 exposures and total mortality. Supporting evidence for 
PM2.5-related cardiovascular morbidity, and more limited 
evidence from respiratory morbidity, provides biological plausibility 
for mortality due to short-term PM2.5 exposures. The 
primarily positive associations observed across studies conducted in 
diverse geographic locations is further supported by the results from 
copollutant analyses indicating robust associations, along with 
evidence from analyses of the concentration-response relationship. The 
2019 ISA states that, collectively, ``this body of evidence is 
sufficient to conclude that a causal relationship exists between short-
term PM2.5 exposure and total mortality'' (U.S. EPA, 2019, 
p. 11-58).
ii. Cardiovascular Effects
Long-Term PM2.5 Exposures
    The scientific evidence reviewed in the 2009 ISA was ``sufficient 
to infer a causal relationship between long-term PM2.5 
exposure and cardiovascular effects'' (U.S. EPA, 2009c). The strongest 
line of evidence comprised findings from several large epidemiological 
studies of U.S. cohorts that consistently showed positive associations 
between long-term PM2.5 exposure and cardiovascular 
mortality (Pope et al., 2004, Krewski et al., 2009, Miller et al., 
2007, Laden et al., 2006). Studies of long-term PM2.5 
exposure and cardiovascular morbidity were limited in number. 
Biological plausibility and coherence with the epidemiological findings 
were provided by studies using genetic mouse models of atherosclerosis 
demonstrating enhanced atherosclerotic plaque development and 
inflammation, as well as changes in measures of impaired heart 
function, following 4- to 6-month exposures to PM2.5 
concentrated ambient particles (CAPs), and by a limited number of 
studies reporting CAPs-induced effects on coagulation factors, vascular 
reactivity, and worsening of experimentally induced hypertension in 
mice (U.S. EPA, 2009c).
    Studies conducted since the last review continue to support the 
relationship between long-term exposure to PM2.5 and 
cardiovascular effects. As discussed above, results from recent U.S. 
and Canadian cohort studies consistently report positive associations 
between long-term PM2.5 exposure and cardiovascular 
mortality (U.S. EPA, 2019, Figure 6-19) in evaluations conducted at 
varying spatial scales and employing a variety of exposure assessment 
and statistical methods (U.S. EPA, 2019, section 6.2.10). Positive 
associations between long-term PM2.5 exposures and 
cardiovascular mortality are generally robust in copollutant models 
adjusted for ozone, NO2, PM10-2.5, or 
SO2. In addition, most of the results from analyses 
examining the shape of the concentration-response relationship for 
cardiovascular mortality support a linear relationship with long-term 
PM2.5 exposures and do not identify a threshold below which 
effects do not occur (U.S. EPA, 2019, section 6.2.16, Table 6-52).\38\
---------------------------------------------------------------------------

    \38\ As noted above for mortality, uncertainty in the shape of 
the concentration-response relationship increases near the upper and 
lower ends of the concentration distribution where the data are 
limited.
---------------------------------------------------------------------------

    The available evidence examining the relationship between long-term 
PM2.5 exposure and cardiovascular morbidity has greatly 
expanded since the 2009 ISA, with positive associations reported in 
several cohorts examining a range of cardiovascular outcomes (U.S. EPA, 
2019, section 6.2). Though results for cardiovascular morbidity are 
less consistent than those for cardiovascular mortality (U.S. EPA, 
2019, section 6.2), recent studies provide some evidence for 
associations between long-term PM2.5 exposures and the 
progression of cardiovascular disease, including cardiovascular 
morbidity (e.g., coronary heart disease, stroke) and atherosclerosis 
progression (e.g., coronary artery calcification) (U.S. EPA, 2019, 
sections 6.2.2. to 6.2.9). Associations reported in such studies are 
supported by toxicologic evidence for increased plaque progression in 
mice following long-term exposure to PM2.5 collected from 
multiple locations across the U.S. (U.S. EPA, 2019, section 6.2.4.2). A 
small number of epidemiological studies also report positive 
associations between long-term PM2.5 exposure and heart 
failure, changes in blood pressure, and hypertension (U.S. EPA, 2019, 
sections 6.2.5 and 6.2.7). Associations with heart failure are 
supported by animal toxicologic studies demonstrating decreased cardiac 
contractility and function, and increased coronary artery wall 
thickness following long-term PM2.5 exposure (U.S. EPA, 
2019, section 6.2.5.2). Similarly, a limited number of animal 
toxicologic studies demonstrating a relationship between long-term 
exposure to PM2.5 and consistent increases in blood pressure 
in rats and mice are coherent with epidemiological studies reporting 
positive associations between long-term exposure to PM2.5 
and hypertension. Further, a recent animal toxicologic study also 
demonstrates increased plaque progression in mice following long-term 
exposure to PM2.5 and provides coherent results with 
epidemiological evidence reporting positive associations between long-
term exposure to PM2.5 and indicators of atherosclerosis 
(U.S. EPA, 2019, section 6.2.4.2).
    Longitudinal epidemiological analyses also report positive 
associations with markers of systemic inflammation (U.S. EPA, 2019, 
section 6.2.11), coagulation (U.S. EPA, 2019, section 6.2.12), and 
endothelial dysfunction (U.S. EPA, 2019, section 6.2.13). These results 
are coherent with animal toxicologic studies generally reporting 
increased markers of systemic inflammation, oxidative stress, and 
endothelial dysfunction (U.S. EPA, 2019, section 6.2.12.2 and 6.2.14).
    In summary, the 2019 ISA concludes that there is consistent 
evidence from multiple epidemiological studies illustrating that long-
term exposure to PM2.5 is associated with mortality from 
cardiovascular causes. Associations with CHD, stroke and 
atherosclerosis progression were observed in several additional 
epidemiological studies providing coherence with the mortality 
findings. Results from copollutant models generally support an 
independent effect of PM2.5 exposure on mortality. 
Additional evidence of the independent effect of PM2.5 on 
the cardiovascular system is provided by experimental studies in 
animals, which support the biological plausibility of pathways by which 
long-term exposure to PM2.5 could potentially result in 
outcomes such as CHD, stroke, CHF and cardiovascular mortality. The 
combination of epidemiological and experimental evidence results in the 
ISA conclusion that ``a causal relationship exists between long-term 
exposure to PM2.5 and cardiovascular effects'' (U.S. EPA, 
2019, p. 6-222).
Short-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship exists between 
short-

[[Page 82699]]

term exposure to PM2.5 and cardiovascular effects'' (U.S. 
EPA, 2009c). The strongest evidence in the 2009 ISA was from 
epidemiological studies of emergency department visits and hospital 
admissions for ischemic heart disease (IHD) and heart failure (HF), 
with supporting evidence from epidemiological studies of cardiovascular 
mortality (U.S. EPA, 2009c). Animal toxicologic studies reported 
evidence of reduced myocardial blood flow during ischemia and studies 
indicating altered vascular reactivity (i.e., vascular function), which 
provided coherence and biological plausibility for the effects observed 
in epidemiological studies. In addition, both animal toxicologic and 
epidemiological panel studies reported effects of PM2.5 
exposure on ST segment depression, an electrocardiogram change that 
potentially indicates ischemia.\39\ Key uncertainties from the last 
review included inconsistent results across disciplines with respect to 
the relationship between short-term exposure to PM2.5 and 
changes in blood pressure, blood coagulation markers, and markers of 
systemic inflammation. In addition, while the 2009 ISA identified a 
growing body of evidence from controlled human exposure and animal 
toxicologic studies, uncertainties remained with respect to biological 
plausibility.
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    \39\ Some animal studies included in the 2009 ISA examined 
exposures to mixtures, such as motor vehicle exhaust or woodsmoke. 
In these studies, it was unclear if the resulting cardiovascular 
effects could be attributed specifically to the particulate 
components of the mixture.
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    A large body of recent evidence confirms and extends the evidence 
from the 2009 ISA supporting the relationship between short-term 
PM2.5 exposure and cardiovascular effects. This includes 
generally positive associations observed in multicity epidemiological 
studies of emergency department visits and hospital admissions for IHD, 
HF, and combined cardiovascular-related endpoints. In particular, 
nationwide studies of older adults (65 years and older) report positive 
associations between PM2.5 exposures and hospital admissions 
for HF (U.S. EPA, 2019, section 6.1.3.1). Single-city epidemiological 
studies contribute some support, though associations reported are less 
consistently positive than in multicity studies, and include a number 
of studies reporting null associations (U.S. EPA, 2019, sections 6.1.2 
and 6.1.3).
    In addition, a number of more recent controlled human exposure, 
animal toxicologic, and epidemiological panel studies provide evidence 
that PM2.5 exposure could plausibly result in IHD or HF 
through pathways that include endothelial dysfunction, arterial 
thrombosis, and arrhythmia (U.S. EPA, 2019, section 6.1.1). The most 
consistent evidence from recent controlled human exposure studies is 
for endothelial dysfunction, as measured by changes in brachial artery 
diameter or flow mediated dilation (U.S. EPA, 2019, section 6.1.13.2). 
These studies report variable results regarding the timing of the 
effect and the mechanism by which reduced blood flow occurs (i.e., 
availability of vs. sensitivity to nitric oxide). Some controlled human 
exposure studies using PM2.5 CAPs report evidence for small 
increases in blood pressure (U.S. EPA, 2019, section 6.1.6.3). In 
addition, although not entirely consistent, there is also some evidence 
across controlled human exposure studies for conduction abnormalities/
arrhythmia (U.S. EPA, 2019, section 6.1.4.3), changes in heart rate 
variability (HRV) (U.S. EPA, 2019, section 6.1.10.2), changes in 
hemostasis that could promote clot formation (U.S. EPA, 2019, section 
6.1.12.2), and increases in inflammatory cells and markers (U.S. EPA, 
2019, section 6.1.11.2). Thus, when taken as a whole, controlled human 
exposure studies are coherent with epidemiological studies in that they 
provide evidence that short-term exposures to PM2.5 may 
result in the types of cardiovascular endpoints that could lead to 
emergency department visits and hospital admissions for IHD or HF.
    Animal toxicologic studies published since the 2009 ISA also 
support a relationship between short-term PM2.5 exposure and 
cardiovascular effects. A recent study demonstrating decreased cardiac 
contractility and left ventricular pressure in mice is coherent with 
the results of epidemiological studies that report associations between 
short-term PM2.5 exposure and heart failure (U.S. EPA, 2019, 
section 6.1.3.3). In addition, similar to results of controlled human 
exposure studies, there is generally consistent evidence in animal 
toxicologic studies for indicators of endothelial dysfunction (U.S. 
EPA, 2019, section 6.1.13.3). Studies in animals also provide evidence 
for changes in a number of other cardiovascular endpoints following 
short-term PM2.5 exposure. Although not entirely consistent, 
these studies provide some evidence of conduction abnormalities and 
arrhythmia (U.S. EPA, 2019, section 6.1.4.4), changes in HRV (U.S. EPA, 
2019, section 6.1.10.3), changes in blood pressure (U.S. EPA, 2019, 
section 6.1.6.4), and evidence for systemic inflammation and oxidative 
stress (U.S. EPA, 2019, section 6.1.11.3).
    In summary, recent evidence supports the conclusions reported in 
the 2009 ISA indicating relationships between short-term 
PM2.5 exposures and hospital admissions and ED visits for 
IHD and HF, along with cardiovascular mortality. Epidemiological 
studies reporting robust associations in copollutant models are 
supported by direct evidence from controlled human exposure and animal 
toxicologic studies reporting independent effects of PM2.5 
exposures on endothelial dysfunction as well as endpoints indicating 
impaired cardiac function, increased risk of arrhythmia, changes in 
HRV, increases in BP, and increases in indicators of systemic 
inflammation, oxidative stress, and coagulation (U.S. EPA, 2019, 
section 6.1.16). Epidemiological panel studies, although not entirely 
consistent, provide some evidence that PM2.5 exposures are 
associated with cardiovascular effects, including increased risk of 
arrhythmia, decreases in HRV, increases in BP, and ST segment 
depression. Overall, the results from epidemiological panel, controlled 
human exposure, and animal toxicologic studies (in particular those 
related to endothelial dysfunction, impaired cardiac function, ST 
segment depression, thrombosis, conduction abnormalities, and changes 
in blood pressure) provide coherence and biological plausibility for 
the consistent results from epidemiological studies reporting positive 
associations between short-term PM2.5 exposures and IHD and 
HF, and ultimately cardiovascular mortality. The 2019 ISA concludes 
that, overall, ``there continues to be sufficient evidence to conclude 
that a causal relationship exists between short-term PM2.5 
exposure and cardiovascular effects'' (U.S. EPA, 2019, p. 6-138).
iii. Respiratory Effects
Long-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship is likely to 
exist between long-term PM2.5 exposure and respiratory 
effects'' (U.S. EPA, 2009c). This conclusion was based mainly on 
epidemiological evidence demonstrating associations between long-term 
PM2.5 exposure and changes in lung function or lung function 
growth in children. Biological plausibility was provided by a single 
animal toxicologic study examining pre- and post-natal exposure to 
PM2.5 CAPs, which found impaired lung development. 
Epidemiological evidence for associations between long-term 
PM2.5 exposure and other respiratory outcomes, such as the

[[Page 82700]]

development of asthma, allergic disease, and COPD; respiratory 
infection; and the severity of disease was limited, both in the number 
of studies available and the consistency of the results. Experimental 
evidence for other outcomes was also limited, with one animal 
toxicologic study reporting that long-term exposure to PM2.5 
CAPs results in morphological changes in the nasal airways of healthy 
animals. Other animal studies examined exposure to mixtures, such as 
motor vehicle exhaust and woodsmoke, and effects were not attributed 
specifically to the particulate components of the mixture.
    Recent cohort studies provide additional support for the 
relationship between long-term PM2.5 exposure and decrements 
in lung function growth (as a measure of lung development), indicating 
a robust and consistent association across study locations, exposure 
assessment methods, and time periods (U.S. EPA, 2019, section 5.2.13). 
This relationship is further supported by a recent retrospective study 
that reports an association between declining PM2.5 
concentrations and improvements in lung function growth in children 
(U.S. EPA, 2019, section 5.2.11). Epidemiological studies also examined 
asthma development in children (U.S. EPA, 2019, section 5.2.3), with 
recent prospective cohort studies reporting generally positive 
associations, though several are imprecise (i.e., they report wide 
confidence intervals). Supporting evidence is provided by studies 
reporting associations with asthma prevalence in children, with 
childhood wheeze, and with exhaled nitric oxide, a marker of pulmonary 
inflammation (U.S. EPA, 2019, section 5.2.13). A recent animal 
toxicologic study showing the development of an allergic phenotype and 
an increase in a marker of airway responsiveness supports the 
biological plausibility of the development of allergic asthma (U.S. 
EPA, 2019, section 5.2.13). Other epidemiological studies report a 
PM2.5-related acceleration of lung function decline in 
adults, while improvement in lung function was observed with declining 
PM2.5 concentrations (U.S. EPA, 2019, section 5.2.11). A 
recent longitudinal study found declining PM2.5 
concentrations are also associated with an improvement in chronic 
bronchitis symptoms in children, strengthening evidence reported in the 
2009 ISA for a relationship between increased chronic bronchitis 
symptoms and long-term PM2.5 exposure (U.S. EPA, 2019, 
section 5.2.11). A common uncertainty across the epidemiological 
evidence is the lack of examination of copollutants to assess the 
potential for confounding. While there is some evidence that 
associations remain robust in models with gaseous pollutants, a number 
of these studies examining copollutant confounding were conducted in 
Asia, and thus have limited generalizability due to high annual 
pollutant concentrations.
    When taken together, the 2019 ISA concludes that ``the collective 
evidence is sufficient to conclude a likely to be causal relationship 
between long-term PM2.5 exposure and respiratory effects'' 
(U.S. EPA, 2019, p. 5-220).
Short-Term PM2.5 Exposures
    The 2009 ISA (U.S. EPA, 2009c) concluded that a ``causal 
relationship is likely to exist'' between short-term PM2.5 
exposure and respiratory effects. This conclusion was based mainly on 
the epidemiological evidence demonstrating positive associations with 
various respiratory effects. Specifically, the 2009 ISA described 
epidemiological evidence as consistently showing PM2.5-
associated increases in hospital admissions and emergency department 
visits for COPD and respiratory infection among adults or people of all 
ages, as well as increases in respiratory mortality. These results were 
supported by studies reporting associations with increased respiratory 
symptoms and decreases in lung function in children with asthma, though 
the available epidemiological evidence was inconsistent for hospital 
admissions or emergency department visits for asthma. Studies examining 
copollutant models showed that PM2.5 associations with 
respiratory effects were robust to inclusion of CO or SO2 in 
the model, but often were attenuated (though still positive) with 
inclusion of O3 or NO2. In addition to the 
copollutant models, evidence supporting an independent effect of 
PM2.5 exposure on the respiratory system was provided by 
animal toxicologic studies of PM2.5 CAPs demonstrating 
changes in some pulmonary function parameters, as well as inflammation, 
oxidative stress, injury, enhanced allergic responses, and reduced host 
defenses. Many of these effects have been implicated in the 
pathophysiology for asthma exacerbation, COPD exacerbation, or 
respiratory infection. In the few controlled human exposure studies 
conducted in individuals with asthma or COPD, PM2.5 exposure 
mostly had no effect on respiratory symptoms, lung function, or 
pulmonary inflammation. Available studies in healthy people also did 
not clearly find respiratory effects following short-term 
PM2.5 exposures.
    Recent epidemiological studies provide evidence for a relationship 
between short-term PM2.5 exposure and several respiratory-
related endpoints, including asthma exacerbation (U.S. EPA, 2019, 
section 5.1.2.1), COPD exacerbation (U.S. EPA, 2019, section 5.1.4.1), 
and combined respiratory-related diseases (U.S. EPA, 2019, section 
5.1.6), particularly from studies examining emergency department visits 
and hospital admissions. The generally positive associations between 
short-term PM2.5 exposure and asthma and COPD emergency 
department visits and hospital admissions are supported by 
epidemiological studies demonstrating associations with other 
respiratory-related effects such as symptoms and medication use that 
are indicative of asthma and COPD exacerbations (U.S. EPA, 2019, 
sections 5.1.2.2 and 5.4.1.2). The collective body of epidemiological 
evidence for asthma exacerbation is more consistent in children than in 
adults. Additionally, epidemiological studies examining the 
relationship between short-term PM2.5 exposure and 
respiratory mortality provide evidence of consistent positive 
associations, demonstrating a continuum of effects (U.S. EPA, 2019, 
section 5.1.9).
    Building on the studies evaluated in the 2009 ISA, recent 
epidemiological studies expand the assessment of potential copollutant 
confounding. There is some evidence that PM2.5 associations 
with asthma exacerbation, combined respiratory-related diseases, and 
respiratory mortality remain relatively unchanged in copollutant models 
with gaseous pollutants (i.e., O3, NO2, 
SO2, with more limited evidence for CO) and other particle 
sizes (i.e., PM10-2.5) (U.S. EPA, 2019, section 5.1.10.1).
    Insight into whether there is an independent effect of 
PM2.5 on respiratory health is provided by findings from 
animal toxicologic studies. Specifically, short-term exposure to 
PM2.5 has been shown to enhance asthma-related responses in 
an animal model of allergic airways disease and lung injury and 
inflammation in an animal model of COPD (U.S. EPA, 2019, sections 
5.1.2.4.4 and 5.1.4.4.3). The experimental evidence provides biological 
plausibility for some respiratory-related endpoints, including limited 
evidence of altered host defense and greater susceptibility to 
bacterial infection as well as consistent evidence of respiratory 
irritant effects. Animal toxicologic evidence for other respiratory 
effects is inconsistent and controlled human exposure studies

[[Page 82701]]

provide limited evidence of respiratory effects (U.S. EPA, 2019, 
section 5.1.12).
    The 2019 ISA concludes that ``[t]he strongest evidence of an effect 
of short-term PM2.5 exposure on respiratory effects is 
provided by epidemiological studies of asthma and COPD exacerbation. 
While animal toxicologic studies provide biological plausibility for 
these findings, some uncertainty remains with respect to the 
independence of PM2.5 effects'' (U.S. EPA, 2019, p. 5-155). 
When taken together, the ISA concludes that this evidence ``is 
sufficient to conclude a likely to be causal relationship between 
short-term PM2.5 exposure and respiratory effects'' (U.S. 
EPA, 2019, p. 5-155).
iv. Cancer
    The 2009 ISA concluded that the overall body of evidence was 
``suggestive of a causal relationship between relevant PM2.5 
exposures and cancer'' (U.S. EPA, 2009c). This conclusion was based 
primarily on positive associations observed in a limited number of 
epidemiological studies of lung cancer mortality. The few 
epidemiological studies that had evaluated PM2.5 exposure 
and lung cancer incidence or cancers of other organs and systems 
generally did not show evidence of an association. Toxicologic studies 
did not focus on exposures to specific PM size fractions, but rather 
investigated the effects of exposures to total ambient PM, or other 
source-based PM such as wood smoke. Collectively, results of in vitro 
studies were consistent with the larger body of evidence demonstrating 
that ambient PM and PM from specific combustion sources are mutagenic 
and genotoxic. However, animal inhalation studies found little evidence 
of tumor formation in response to chronic exposures. A small number of 
studies provided preliminary evidence that PM exposure can lead to 
changes in methylation of DNA, which may contribute to biological 
events related to cancer.
    Since the 2009 ISA, additional cohort studies provide evidence that 
long-term PM2.5 exposure is positively associated with lung 
cancer mortality and with lung cancer incidence, and provide initial 
evidence for an association with reduced cancer survival (U.S. EPA, 
2019, section 10.2.5), with limited evidence of cancer in other organ 
systems. Reanalyses of the ACS cohort using different years of 
PM2.5 data and follow-up, along with various exposure 
assignment approaches, provide consistent evidence of positive 
associations between long-term PM2.5 exposure and lung 
cancer mortality (U.S. EPA, 2019, Figure 10-3). Additional support for 
positive associations with lung cancer mortality is provided by recent 
epidemiological studies using individual-level data to control for 
smoking status, in studies of people who have never smoked), and in 
analyses of cohorts that relied upon proxy measures to account for 
smoking status (U.S. EPA, 2019, section 10.2.5.1.1). Although studies 
that evaluate lung cancer incidence, including studies of people who 
have never smoked, are limited in number, recent studies generally 
report positive associations with long-term PM2.5 exposures 
(U.S. EPA, 2019, section 10.2.5.1.2). In addition, a subset of the 
studies focusing on lung cancer incidence also examined histological 
subtypes, providing some evidence of positive associations for 
adenocarcinomas, the predominate subtype of lung cancer observed in 
people who have never smoked (U.S. EPA, 2019, section 10.2.5.1.2). 
Associations between long-term PM2.5 exposure and lung 
cancer incidence were found to remain relatively unchanged, though in 
some cases confidence intervals widened, in analyses that attempted to 
reduce exposure measurement error by accounting for length of time at 
residential address or by examining different exposure assignment 
approaches (U.S. EPA, 2019, section 10.2.5.1.2).
    To date, relatively few studies have evaluated the potential for 
copollutant confounding of the relationship between long-term 
PM2.5 exposure and lung cancer mortality or incidence. The 
small number of such studies have generally focused on O3 
and report that PM2.5 associations remain relatively 
unchanged in copollutant models (U.S. EPA, 2019, section 10.2.5.1.3). 
However, available studies have not systematically evaluated the 
potential for copollutant confounding by other gaseous pollutants or by 
other particle size fractions (U.S. EPA, 2019, section 10.2.5.1.3). 
Compared to total (non-accidental) mortality (discussed above), fewer 
studies have examined the shape of the concentration-response curve for 
cause-specific mortality outcomes, including lung cancer. Several of 
these studies have reported no evidence of deviations from linearity in 
the shape of the concentration-response relationship (Lepeule et al., 
2012; Raaschou-Nielsen et al., 2013; Puett et al., 2014), though 
authors provided only limited discussions of results (U.S. EPA, 2019, 
section 10.2.5.1.4).
    In support of the biological plausibility of an independent effect 
of PM2.5 on cancer, the 2019 ISA notes evidence from recent 
experimental studies demonstrating that PM2.5 exposure can 
lead to a range of effects indicative of mutagenicity, genotoxicity, 
and carcinogenicity, as well as epigenetic effects (U.S. EPA, 2019, 
section 10.2.7). For example, both in vitro and in vivo toxicologic 
studies have shown that PM2.5 exposure can result in DNA 
damage (U.S. EPA, 2019, section 10.2.2). Although such effects do not 
necessarily equate to carcinogenicity, the evidence that PM exposure 
can damage DNA, and elicit mutations, provides support for the 
plausibility of epidemiological associations with lung cancer mortality 
and incidence. Additional supporting studies indicate the occurrence of 
micronuclei formation and chromosomal abnormalities (U.S. EPA, 2019, 
section 10.2.2.3), and differential expression of genes that may be 
relevant to cancer pathogenesis, following PM exposures. Experimental 
and epidemiological studies that examine epigenetic effects indicate 
changes in DNA methylation, providing some support for PM2.5 
exposure contributing to genomic instability (U.S. EPA, 2019, section 
10.2.3).
    Epidemiological evidence for associations between PM2.5 
exposure and lung cancer mortality and incidence, together with 
evidence supporting the biological plausibility of such associations, 
contributes to the 2019 ISA's conclusion that the evidence ``is 
sufficient to conclude there is a likely to be causal relationship 
between long-term PM2.5 exposure and cancer'' (U.S. EPA, 
2019, p. 10-77).
    In its letter to the Administrator on the draft ISA, the CASAC 
states that ``the Draft ISA does not present adequate evidence to 
conclude that there is likely to be a causal relationship between long-
term PM2.5 exposure and . . . cancer'' (Cox, 2019a, p. 1 of 
letter). The CASAC specifically states that this causality 
determination ``relies largely on epidemiology studies that . . . do 
not provide exposure time frames that are appropriate for cancer 
causation and that there are no animal studies showing direct effects 
of PM2.5 on cancer formation'' (Cox, 2019a, p. 20 of 
consensus responses).
    With respect to the latency period, it is well recognized that 
``air pollution exposures experienced over an extended historical time 
period are likely more relevant to the etiology of lung cancer than air 
pollution exposures experienced in the more recent past'' (Turner et 
al. 2011). However, many epidemiological studies conducted within the 
U.S. that examine long-term

[[Page 82702]]

PM2.5 exposure and lung cancer incidence and lung cancer 
mortality rely on more recent air quality data because routine 
PM2.5 monitoring did not start until 1999-2000. An exception 
to this is the ACS study that had PM2.5 concentration data 
from two time periods, 1979-1983 and from 1999-2000. Turner et al. 
(2011), conducted a comparison of PM2.5 concentrations 
between these two time periods and found that they were highly 
correlated (r >0.7), with the relative rank order of metropolitan 
statistical areas (MSAs) by PM2.5 concentrations being 
``generally retained over time.'' Therefore, areas where 
PM2.5 concentrations were high remained high over decades 
(or were low and remained low) relative to other locations. Long-term 
exposure epidemiological studies rely on spatial contrasts between 
locations; therefore, if a location with high PM2.5 
concentrations continues to have high concentrations over decades 
relative to other locations a relationship between the PM2.5 
exposure and cancer should persist. This was confirmed in a sensitivity 
analysis conducted by Turner et al. (2011), where the authors reported 
a similar hazard ratio (HR) for lung cancer mortality for participants 
assigned exposure to PM2.5 (1979-1983) and PM2.5 
(1999-2000) in two separate analyses.
    While experimental studies showing a direct effect of 
PM2.5 on cancer formation were limited to an animal model of 
urethane-induced tumor initiation, a large number of experimental 
studies report that PM2.5 exhibits several key 
characteristics of carcinogens, as indicated by genotoxic effects, 
oxidative stress, electrophilicity, and epigenetic alterations, all of 
which provide biological plausibility that PM2.5 exposure 
can contribute to cancer development. The experimental evidence, in 
combination with multiple recent and previously evaluated 
epidemiological studies examining the relationship between long-term 
PM2.5 exposure and both lung cancer incidence and lung 
cancer mortality that reported generally positive associations across 
different cohorts, exposure assignment methods, and in analyses of 
never smokers further addresses uncertainties identified in the 2009 
ISA. Therefore, upon re-evaluating the causality determination for 
cancer, when considering CASAC comments on the draft ISA and applying 
the causal framework as described (U.S. EPA, 2015; U.S. EPA, 2019, 
section A.3.2.1), the EPA continues to conclude in the 2019 ISA that 
the evidence for long-term PM2.5 exposure and cancer 
supports a ``likely to be causal relationship'' (U.S. EPA, 2019, p. 10-
77).
v. Nervous System Effects
    Reflecting the very limited evidence available in the last review, 
the 2009 ISA did not make a causality determination for long-term 
PM2.5 exposures and nervous system effects (U.S. EPA, 
2009c). Since the last review, this body of evidence has grown 
substantially (U.S. EPA, 2019, section 8.2). Recent studies in adult 
animals report that long-term PM2.5 exposures can lead to 
morphologic changes in the hippocampus and to impaired learning and 
memory. This evidence is consistent with epidemiological studies 
reporting that long-term PM2.5 exposure is associated with 
reduced cognitive function (U.S. EPA, 2019, section 8.2.5). Further, 
while the evidence is limited, early markers of Alzheimer's disease 
pathology have been reported in rodents following long-term exposure to 
PM2.5 CAPs. These findings support reported associations 
with neurodegenerative changes in the brain (i.e., decreased brain 
volume), all-cause dementia, and hospitalization for Alzheimer's 
disease in a small number of epidemiological studies (U.S. EPA, 2019, 
section 8.2.6). Additionally, loss of dopaminergic neurons in the 
substantia nigra, a hallmark of Parkinson's disease, has been reported 
in mice following long-term PM2.5 exposures (U.S. EPA, 2019, 
section 8.2.4), though epidemiological studies provide only limited 
support for associations with Parkinson's disease (U.S. EPA, 2019, 
section 8.2.6). Overall, the lack of consideration of copollutant 
confounding introduces some uncertainty in the interpretation of 
epidemiological studies of nervous system effects, but this uncertainty 
is partly addressed by the evidence for an independent effect of 
PM2.5 exposures provided by experimental animal studies.
    In addition to the findings described above, which are most 
relevant to older adults, several recent studies of neurodevelopmental 
effects in children have also been conducted. Epidemiological studies 
provided limited evidence of an association between PM2.5 
exposure during pregnancy and childhood on cognitive and motor 
development (U.S. EPA, 2019, section 8.2.5.2). While some studies 
report positive associations between long-term exposure to 
PM2.5 during the prenatal period and autism spectrum 
disorder (ASD) (U.S. EPA, 2019, section 8.2.7.2). Interpretation of 
these epidemiological studies is limited due to the small number of 
studies, their lack of control for potential confounding by 
copollutants, and uncertainty regarding the critical exposure windows. 
Biological plausibility is provided for the ASD findings by a study in 
mice that found inflammatory and morphologic changes in the corpus 
collosum and hippocampus, as well as ventriculomegaly (i.e., enlarged 
lateral ventricles) in young mice following prenatal exposure to 
PM2.5 CAPs.
    Taken together, the 2019 ISA concludes that the strongest evidence 
of an effect of long-term exposure to PM2.5 on the nervous 
system is provided by toxicologic studies that show inflammation, 
oxidative stress, morphologic changes, and neurodegeneration in 
multiple brain regions following long-term exposure of adult animals to 
PM2.5 CAPs. These findings are coherent with epidemiological 
studies reporting consistent associations with cognitive decrements and 
with all-cause dementia. The ISA determines that ``[o]verall, the 
collective evidence is sufficient to conclude a likely to be causal 
relationship between long-term PM2.5 exposure and nervous 
system effects'' (U.S. EPA, 2019, p. 8-61).
    In its letter to the Administrator on the draft ISA, the CASAC 
states that ``the Draft ISA does not present adequate evidence to 
conclude that there is likely to be a causal relationship between long-
term PM2.5 exposure and nervous system effects'' (Cox, 
2019a, p. 1 of letter). The CASAC specifically states that ``[f]or a 
likely causal conclusion, there would have to be evidence of health 
effects in studies where results are not explained by chance, 
confounding, and other biases, but uncertainties remain in the overall 
evidence'' (Cox, 2019a, p. 20 of consensus responses). These 
uncertainties in the eyes of CASAC reflect that animal toxicologic 
studies ``have largely been done by a single group,'' and for 
epidemiological studies that examined brain volume that ``brain volumes 
can vary . . . between normal people'' and the results from studies of 
cognitive function were ``largely non-statistically significant'' (Cox, 
2019a, p. 20 of consensus responses).
    With these concerns in mind, and as noted in the proposed rule (85 
FR 24114, April 30, 2020), the EPA re-evaluated the evidence and note 
that animal toxicologic studies were conducted in ``multiple research 
groups [and show a range of effects including] inflammation, oxidative 
stress, morphologic changes, and neurodegeneration in multiple brain 
regions following long-term exposure of adult animals to 
PM2.5 CAPs'' (U.S. EPA, 2019, p. 8-61). The results from the

[[Page 82703]]

animal toxicologic studies ``are coherent with a number of 
epidemiological studies reporting consistent associations with 
cognitive decrements and with all-cause dementia'' (U.S. EPA, 2019, p. 
8-61). Additionally, as discussed in the Preamble to the ISAs (U.S. 
EPA, 2015):

``. . . the U.S. EPA emphasizes the importance of examining the 
pattern of results across various studies and does not focus solely 
on statistical significance or the magnitude of the direction of the 
association as criteria of study reliability. Statistical 
significance is influenced by a variety of factors including, but 
not limited to, the size of the study, exposure and outcome 
measurement error, and statistical model specifications. Statistical 
significance . . . is just one of the means of evaluating confidence 
in the observed relationship and assessing the probability of chance 
as an explanation. Other indicators of reliability such as the 
consistency and coherence of a body of studies as well as other 
confirming data may be used to justify reliance on the results of a 
body of epidemiologic studies, even if results in individual studies 
lack statistical significance . . . [Therefore, the U.S. EPA] . . . 
does not limit its focus or consideration to statistically 
significant results in epidemiologic studies.''

Therefore, upon re-evaluating the causality determination, when 
considering the CASAC comments on the draft ISA and applying the causal 
framework as described (U.S. EPA, 2015; U.S. EPA, 2019, section 
A.3.2.1), the EPA continues to conclude in the 2019 ISA that the 
evidence for long-term PM2.5 exposure and nervous system 
effects supports a ``likely to be causal relationship'' (U.S. EPA, 
2019, p. 8-61).
vi. Other Effects
    For other categories of health effects and PM2.5 
exposures,\40\ the currently available evidence is ``suggestive of, but 
not sufficient to infer, a causal relationship,'' mainly due to 
inconsistent evidence across specific outcomes and uncertainties 
regarding exposure measurement error, the potential for confounding, 
and potential modes of action (U.S. EPA, 2019, sections 7.1.4, 7.2.10, 
8.1.6, and 9.1.5). These causality determinations are revised from 
``inadequate to infer a causal relationship'' or not evaluated in the 
2009 ISA this review; however, the ``suggestive of, but not sufficient 
to infer, a causal relationship'' causality determinations reflect 
continued uncertainties in the evidence.
---------------------------------------------------------------------------

    \40\ The other categories evaluated in the ISA include nervous 
system effects and short-term exposures; metabolic effects; 
reproduction and fertility; and pregnancy and birth outcomes (U.S. 
EPA, 2019, Table ES-1).
---------------------------------------------------------------------------

b. At-Risk Populations
    In this review, we use the term ``at-risk populations'' to describe 
populations with a quality or characteristic in common (e.g., a 
specific pre-existing illness or specific lifestage) that contributes 
to them having a greater likelihood of experiencing PM2.5-
related health effects. In the current review, consistent with the last 
review, the 2019 ISA cites extensive evidence indicating that ``both 
the general population as well as specific populations and lifestages 
are at risk for PM2.5-related health effects'' (U.S. EPA, 
2019, p. 12-1). For example, in support of its ``causal'' and ``likely 
to be causal'' determinations, the ISA cites substantial evidence for: 
PM-related mortality and cardiovascular effects in older adults (U.S. 
EPA, 2019, sections 11.1, 11.2, 6.1, and 6.2); PM-related 
cardiovascular effects in people with pre-existing cardiovascular 
disease (U.S. EPA, 2019, section 6.1); PM-related respiratory effects 
in people with pre-existing respiratory disease, particularly asthma 
exacerbations in children (U.S. EPA, 2019, section 5.1); and PM-related 
impairments in lung function growth and asthma development in children 
(U.S. EPA, 2019, sections 5.1 and 5.2; 12.5.1.1).
    The ISA additionally notes that stratified analyses (i.e., analyses 
that directly compare PM-related health effects across groups) provide 
support for racial and ethnic differences in PM2.5 exposures 
and in PM2.5-related health risk (U.S. EPA, 2019, section 
12.5.4). Drawing from such studies, the ISA concludes that ``[t]here is 
strong evidence demonstrating that black and Hispanic populations, in 
particular, have higher PM2.5 exposures than non-Hispanic 
white populations'' and that ``there is consistent evidence across 
multiple studies demonstrating an increase in risk for nonwhite 
populations'' (U.S. EPA, 2019, p. 12-38). Stratified analyses focusing 
on other groups also suggest that populations with pre-existing 
cardiovascular or respiratory disease, populations that are overweight 
or obese, populations that have particular genetic variants, 
populations that are of low socioeconomic status, and current/former 
smokers could be at increased risk for PM2.5-related adverse 
health effects (U.S. EPA, 2019, Chapter 12).
    Thus, the groups at greater risk of PM2.5-related health 
effects represent a substantial portion of the total U.S. population. 
In evaluating the primary PM2.5 standards, an important 
consideration is the potential for additional public health 
improvements in these populations.
c. Evidence-Based Considerations
    The sections below summarize the PA's evaluation of the 
PM2.5 exposure concentrations that have been examined in 
controlled human exposure studies, animal toxicology studies, and 
epidemiological studies.
i. PM2.5 Concentrations Evaluated in Experimental Studies
    Evidence for a particular PM2.5-related health outcome 
is strengthened when results from experimental studies demonstrate 
biologically plausible mechanisms through which adverse human health 
outcomes could occur (U.S. EPA, 2015, p. 20). Two types of experimental 
studies are of particular importance in understanding the effects of PM 
exposures: Controlled human exposure and animal toxicologic studies. In 
such studies, investigators expose human volunteers or laboratory 
animals, respectively, to known concentrations of air pollutants under 
carefully regulated environmental conditions and activity levels. Thus, 
controlled human exposure and animal toxicology studies can provide 
information on the health effects of experimentally administered 
pollutant exposures under well-controlled laboratory conditions (U.S. 
EPA, 2015, p. 11).
    Controlled human exposure studies have reported that 
PM2.5 exposures lasting from less than one hour up to five 
hours can impact cardiovascular function (U.S. EPA, 2019, section 6.1). 
The most consistent evidence from these studies is for impaired 
vascular function (U.S. EPA, 2019, section 6.1.13.2). Table 3-2 in the 
PA (U.S. EPA, 2020) summarizes information from the ISA on available 
controlled human exposure studies that evaluate effects on markers of 
cardiovascular function following exposures to PM2.5. Most 
of the controlled human exposure studies in Table 3-2 of the PA have 
evaluated average PM2.5 exposure concentrations at or above 
about 100 [micro]g/m\3\, with exposure durations typically up to about 
two hours. Statistically significant effects on one or more indicators 
of cardiovascular function are often, though not always, reported 
following 2-hour exposures to average PM2.5 concentrations 
at and above about 120 [micro]g/m\3\, with less consistent evidence for 
effects following exposures to lower concentrations. Impaired vascular 
function, the effect identified in the ISA as the most consistent 
across studies (U.S. EPA, 2019, section 6.1.13.2), is shown following 
2-hour

[[Page 82704]]

exposures to PM2.5 concentrations at and above 149 [micro]g/
m\3\. Mixed results are reported in the few studies that evaluate 
longer exposure durations (i.e., longer than 2 hours) and lower 
PM2.5 concentrations (U.S. EPA, 2020, section 3.2.3.1).
    To provide some insight into what these studies may indicate 
regarding the primary PM2.5 standards, analyses in the PA 
examine monitored 2-hour PM2.5 concentrations at sites 
meeting the current standards (U.S. EPA, 2020, section 3.2.3.1). At 
these sites, most 2-hour concentrations are below 11 [mu]g/m\3\, and 
they almost never exceed 32 [mu]g/m\3\. Even the highest 2-hour 
concentrations remain well-below the exposure concentrations 
consistently shown to cause effects in controlled human exposure 
studies (i.e., 99.9th percentile of 2-hour concentrations is 68 [mu]g/
m\3\ during the warm season). Thus, while controlled human exposure 
studies support the plausibility of the serious cardiovascular effects 
that have been linked with ambient PM2.5 exposures (U.S. 
EPA, 2019, Chapter 6), the PA notes that the PM2.5 exposures 
evaluated in most of these studies are well-above the ambient 
concentrations typically measured in locations meeting the current 
primary standards (U.S. EPA, 2020, section 3.2.3.2.1).
    With respect to animal toxicology studies, the ISA relies on animal 
toxicology studies to support the plausibility of a wide range of 
PM2.5-related health effects. While animal toxicology 
studies often examine more severe health outcomes and longer exposure 
durations than controlled human exposure studies, there is uncertainty 
in extrapolating the effects seen in animals, and the PM2.5 
exposures and doses that cause those effects, to human populations.
    As with controlled human exposure studies, most of the animal 
toxicology studies assessed in the ISA have examined effects following 
exposures to PM2.5 concentrations well-above the 
concentrations likely to be allowed by the current PM2.5 
standards. Such studies have generally examined short-term exposures to 
PM2.5 concentrations from 100 to >1,000 [mu]g/m\3\ and long-
term exposures to concentrations from 66 to >400 [mu]g/m\3\ (e.g., see 
U.S. EPA, 2019, Table 1-2). Two exceptions are a study reporting 
impaired lung development following long-term exposures (i.e., 24 hours 
per day for several months prenatally and postnatally) to an average 
PM2.5 concentration of 16.8 [mu]g/m\3\ (Mauad et al., 2008) 
and a study reporting increased carcinogenic potential following long-
term exposures (i.e., 2 months) to an average PM2.5 
concentration of 17.7 [mu]g/m\3\ (Cangerana Pereira et al., 2011). 
These two studies report serious effects following long-term exposures 
to PM2.5 concentrations close to the ambient concentrations 
reported in some PM2.5 epidemiological studies (U.S. EPA, 
2019, Table 1-2), though still above the ambient concentrations likely 
to occur in areas meeting the current primary standards. Thus, as is 
the case with controlled human exposure studies, animal toxicology 
studies support the plausibility of various adverse effects that have 
been linked to ambient PM2.5 exposures (U.S. EPA, 2019), but 
have not evaluated PM2.5 exposures likely to occur in areas 
meeting the current primary standards.
ii. Ambient Concentrations in Locations of Epidemiological Studies
    As summarized above in section II.A.2.a, epidemiological studies 
examining associations between daily or annual average PM2.5 
exposures and mortality or morbidity represent a large part of the 
evidence base supporting several of the ISA's ``causal'' and ``likely 
to be causal'' determinations for cardiovascular effects, respiratory 
effects, cancer, and mortality. The PA considers what information from 
these epidemiological studies may indicate regarding primary 
PM2.5 standards. The use of information from epidemiological 
studies to inform conclusions on the primary PM2.5 standards 
is complicated by the fact that such studies evaluate associations 
between distributions of ambient PM2.5 and health outcomes, 
but do not identify the specific exposures that cause reported effects. 
Rather, health effects can occur over the entire distributions of 
ambient PM2.5 concentrations evaluated, and epidemiological 
studies do not identify a population-level threshold below which it can 
be concluded with confidence that PM-associated health effects do not 
occur (U.S. EPA, 2020, section 3.2.3.2). Therefore, the PA evaluates 
the PM2.5 air quality distributions over which 
epidemiological studies support health effect associations. As 
discussed further in the PA (U.S. EPA, 2020, section 3.2.3.2.1), 
studies of daily PM2.5 exposures examine associations 
between day-to-day variation in PM2.5 concentrations and 
health outcomes, often over several years. While there can be 
considerable variability in daily exposures over a multi-year study 
period, most of the estimated exposures reflect days with ambient 
PM2.5 concentrations around the middle of the air quality 
distributions examined (i.e., ``typical'' days rather than days with 
extremely high or extremely low concentrations). Similarly, for studies 
of annual PM2.5 exposures, most of the estimated exposures 
reflect annual average PM2.5 concentrations around the 
middle of the air quality distributions examined. In both cases, 
epidemiological studies provide the strongest support for reported 
health effect associations for this middle portion of the 
PM2.5 air quality distribution, which corresponds to the 
bulk of the underlying data, rather than the extreme upper or lower 
ends of the distribution. Consistent with this, and as noted in the PA 
(U.S. EPA, 2020, section 3.2.1.1), several epidemiological studies 
report that associations persist in analyses that exclude the upper 
portions of the distributions of estimated PM2.5 exposures, 
indicating that ``peak'' PM2.5 exposures are not 
disproportionately responsible for reported health effect associations.
    Thus, in considering PM2.5 air quality data from 
epidemiological studies, the PA evaluates study-reported means (or 
medians) of daily and annual average PM2.5 concentrations as 
proxies for the middle portions of the air quality distributions that 
support reported associations. In Figure 3-7, the PA highlights the 
overall mean (or median) PM2.5 concentrations reported in 
key U.S. and Canadian epidemiological studies that use ground-based 
monitors alone to estimate long- or short-term PM2.5 
exposures. In Figure 3-8, the PA also considers the emerging body of 
studies that use hybrid modeling methods to estimate long- or short-
term PM2.5 exposures. Hybrid methods incorporate data from 
several sources, often including satellites and models, in addition to 
ground-based monitors.
    Epidemiological studies using hybrid methods are generally new in 
this review. These modeling methods have improved the ability to 
estimate PM2.5 exposure for populations throughout the 
conterminous U.S. compared with the earlier approaches based on 
monitoring data alone. Excellent performance in cross-validation tests 
suggests that hybrid methods are reliable for estimating 
PM2.5 exposure in many applications. As discussed in Chapter 
3 of the PA, good agreement in health study results between monitor- 
and model-based methods for urban areas (McGuinn et al., 2017) and 
general consistency in results for the conterminous U.S. (Jerrett et 
al., 2017; Di et al., 2016) also suggests that the fields are reliable 
for use in health studies. However, there are also important 
limitations associated with the modeled fields that should be kept in 
mind. First, performance evaluations

[[Page 82705]]

for the methods are weighted toward densely monitored urban areas at 
the scales of representation of the monitoring networks. Predictions at 
different scales or in sparsely monitored areas are relatively 
untested. Second, studies have reported heterogeneity in performance 
with relatively weak performance in parts of the western U.S., at low 
concentrations, at greater distance to monitors, and under conditions 
where the reliability and availability of key input datasets (e.g., 
satellite retrievals and air quality modeling) are limited. Lastly, 
differences in predictions among different hybrid methods have also 
been reported and tend to be most important under conditions with the 
performance issues just noted. Differences in predictions can be 
related to the different approaches used to create long-term 
PM2.5 fields (e.g., averaging daily PM2.5 fields 
vs. developing long-term average fields), which can be impacted by 
variability in monitoring schedules, and the spatial scale at which 
these fields are created. Future work to further characterize the 
performance of modeled fields will be useful in informing our 
understanding of the implications of using these fields to estimate 
PM2.5 exposures in health studies (U.S. EPA, 2020, section 
2.3.3.1.4).
    In assessing how the overall mean (or median) PM2.5 
concentrations reported in key epidemiological studies can inform 
conclusions on the primary PM2.5 standards, there are some 
important considerations. As noted in the PA, study-reported 
PM2.5 concentrations reflect the averages of daily or annual 
PM2.5 air quality concentrations or exposure estimates in 
the study population over the years examined by the study, and are not 
the same as the PM2.5 design values used by the EPA to 
determine whether areas meet or violate the PM NAAQS (U.S. EPA, 2020, 
section 3.2.3.2.1). Overall mean PM2.5 concentrations in key 
studies reflect averaging of short- or long-term PM2.5 
exposure estimates across locations (i.e., across multiple monitors or 
across modeled grid cells) and over time (i.e., over several years). In 
contrast, to determine whether areas meet or violate the NAAQS, the EPA 
measures air pollution concentrations at individual monitors (i.e., 
concentrations are not averaged across monitors) and calculates design 
values at monitors meeting appropriate data quality and completeness 
criteria. For the annual PM2.5 standard, design values are 
calculated as the annual arithmetic mean PM2.5 
concentration, averaged over 3 years (described in Appendix N of 40 CFR 
part 50). For an area to meet the NAAQS, all valid design values in 
that area, including the highest monitored values, must be at or below 
the level of the standard.
    In the context of epidemiological studies that use ground-based 
monitors, analyses of recent air quality in U.S. CBSAs indicate that 
maximum annual PM2.5 design values for a given three-year 
period are often 10% to 20% higher than average monitored 
concentrations (i.e., averaged across multiple monitors in the same 
CBSA) (U.S. EPA, 2020, Appendix B, section B.7). This comparison is 
more difficult for epidemiological studies that use hybrid methods. To 
try to address this issue, the PA also considered a second approach to 
evaluating information from epidemiological studies. In this approach, 
the PA calculated study area air quality metrics similar to 
PM2.5 design values (i.e., referred to in the PA as pseudo-
design values; U.S. EPA, 2020, section 3.2.3.2.2) and considered the 
degree to which such metrics indicate that study area air quality would 
likely have met or violated the current standards during study periods. 
This approach was generally not well received by commenters during the 
review of the PA.
3. Overview of Risk and Exposure Assessment Information
    Beyond the consideration of the scientific evidence, discussed 
above in section II.A.2, the EPA also considers the extent to which new 
or updated quantitative analyses of PM2.5 air quality, 
exposure, or health risks could inform conclusions on the adequacy of 
the public health protection provided by the current primary 
PM2.5 standards. Conducting such quantitative analyses, if 
appropriate, could inform judgments about the potential for additional 
public health improvements associated with PM2.5 exposure 
and related health effects and could help to place the evidence for 
specific effects into a broader public health context.
    To this end, the PA includes a risk assessment that estimates 
population-level health risks associated with PM2.5 air 
quality that has been adjusted to simulate air quality scenarios of 
policy interest (e.g., ``just meeting'' the current standards). The 
general approach to estimating PM2.5-associated health risks 
combines concentration-response functions from epidemiological studies 
with model-based PM2.5 air quality surfaces, baseline health 
incidence data, and population demographics for 47 urban study areas 
(U.S. EPA, 2020, section 3.3, Figure 3-10 and Appendix C).
    The risk assessment estimates that the current primary 
PM2.5 standards could allow a substantial number of 
PM2.5-associated deaths in the U.S. For example, when air 
quality in the 47 study areas is adjusted to simulate just meeting the 
current standards, the risk assessment estimates from about 16,000 to 
17,000 long-term PM2.5 exposure-related deaths from ischemic 
heart disease in a single year (i.e., confidence intervals range from 
about 12,000 to 21,000 deaths).\41\ Compared to the current annual 
standard, meeting a revised annual standard with a lower level is 
estimated to reduce PM2.5-associated health risks by about 7 
to 9% for a level of 11.0 [micro]g/m\3\, 14 to 18% for a level of 10.0 
[micro]g/m\3\, and 21 to 27% for a level of 9.0 [micro]g/m\3\.
---------------------------------------------------------------------------

    \41\ For the only other cause-specific mortality endpoint 
evaluated (i.e., lung cancer), substantially fewer deaths were 
estimated (U.S. EPA, 2020, section 3.3.2, e.g., Figure 3-5). Risk 
estimates were not generated for other ``likely to be causal'' 
outcome categories (i.e., respiratory effects, nervous system 
effects).
---------------------------------------------------------------------------

    Uncertainty in risk estimates (e.g., in the size of risk estimates) 
can result from a number of factors, including assumptions about the 
shape of the concentration-response relationship with mortality at low 
ambient PM concentrations, the potential for confounding and/or 
exposure measurement error in the underlying epidemiological studies, 
and the methods used to adjust PM2.5 air quality. The PA 
characterizes these and other sources of uncertainty in risk estimates 
using a combination of quantitative and qualitative approaches (U.S. 
EPA, 2020, Appendix C, section C.3). As detailed further below in 
II.B.1, some members of CASAC advised that the risk assessment 
estimates did not provide useful information about whether the current 
standard is protective, while other members thought they were useful to 
understand potential impacts of alternative standards.

B. Conclusions on the Primary PM2.5 Standards

    In drawing conclusions on the adequacy of the current primary 
PM2.5 standards, in view of the advances in scientific 
knowledge and additional information now available, the Administrator 
has considered the evidence base, information, and policy judgments 
that were the foundation of the last review and reflects upon the body 
of evidence and information newly available in this review. In so 
doing, he considered the large body of evidence presented and assessed 
in the ISA (U.S.

[[Page 82706]]

EPA, 2019), the policy-relevant and risk-based conclusions and 
rationales as presented in the PA (U.S. EPA, 2020), views expressed by 
CASAC, and public comments. The Administrator has taken into account 
both evidence- and risk-based considerations in developing final 
conclusions on the adequacy of the current primary PM2.5 
standards. Evidence-based considerations include the assessment of 
epidemiological, animal toxicologic, and controlled human exposure 
studies evaluating long- or short-term exposures to PM2.5 
and the integration of evidence across each of these disciplines. These 
considerations, as assessed in the ISA (U.S. EPA, 2019), focus on the 
policy-relevant considerations, as discussed in II.A.2 above and in the 
PA (U.S. EPA, 2020, section 3.2.1). Risk-based considerations draw from 
the results of the quantitative analyses and policy-relevant 
considerations as discussed in II.A.3 above and in the PA (U.S. EPA, 
2020, section 3.3.2).
    Section II.B.1 summarizes the advice and recommendations of the 
CASAC. Section II.B.2 below summarizes the basis for the 
Administrator's proposed decision, drawing from section II.C.3 of the 
proposal, and section II.B.3 addresses public comments on the proposed 
decision. The Administrator's conclusions in this review regarding the 
adequacy of the current primary standard and whether any revisions are 
appropriate are described in section II.B.4.
1. CASAC Advice in This Review
    With regard to the process for reviewing the PM NAAQS, the CASAC 
requested the opportunity to review a second draft ISA (Cox, 2019b, p. 
1 of letter) and recommended that ``the EPA reappoint the previous 
CASAC PM panel (or appoint a panel with similar expertise)'' (Cox, 
2019b, p. 2 of letter). As discussed above in section I.D, the Agency's 
responses to these recommendations were described in a letter from the 
Administrator to the CASAC chair (Wheeler, 2019).
    As part of its review of the draft PA, the CASAC provided advice on 
the adequacy of the public health protection afforded by the current 
primary PM2.5 standards.\42\ Its advice is documented in a 
letter sent to the EPA Administrator (Cox, 2019a). In this letter, the 
committee recommended retaining the current 24-hour PM2.5 
standard but did not reach consensus on whether the scientific and 
technical information support retaining or revising the current annual 
standard. In particular, though the CASAC agreed that there is a long-
standing body of health evidence supporting relationships between 
PM2.5 exposures and various health outcomes, including 
mortality and serious morbidity effects, individual CASAC members 
``differ[ed] in their assessments of the causal and policy significance 
of these associations'' (Cox, 2019a, p. 8 of consensus responses). 
Drawing from this evidence, ``some CASAC members'' expressed support 
for retaining the current annual standard while ``other members'' 
expressed support for revising that standard in order to increase 
public health protection (Cox, 2019a, p.1 of letter). These views are 
summarized below.
---------------------------------------------------------------------------

    \42\ The CASAC also provided advice on the draft ISA's 
assessment of the scientific evidence (Cox, 2019b). That advice, and 
the resulting changes made in the final ISA and final PA, are 
summarized in section II.B.3 of the proposal (85 FR 24114, April 30, 
2020).
---------------------------------------------------------------------------

    The CASAC members who supported retaining the current annual 
standard expressed the view that substantial uncertainty remains in the 
evidence for associations between PM2.5 exposures and 
mortality or serious morbidity effects. These committee members 
asserted that ``such associations can reasonably be explained in light 
of uncontrolled confounding and other potential sources of error and 
bias'' (Cox, 2019a, p. 8 of consensus responses). They noted that 
associations do not necessarily reflect causal effects, and they 
contended that recent epidemiological studies reporting positive 
associations at lower estimated exposure concentrations mainly confirm 
what was anticipated or already assumed in setting the 2012 NAAQS. In 
particular, they concluded that such studies have some of the same 
limitations as prior studies and do not provide new information calling 
into question the existing standard. They further asserted that 
``accountability studies provide potentially crucial information about 
whether and how much decreasing PM2.5 causes decreases in 
future health effects'' (Cox, 2019a, p. 10), and they cited recent 
reviews (i.e., Henneman et al., 2017; Burns et al., 2019) to support 
their position that in such studies, ``reductions of PM2.5 
concentrations have not clearly reduced mortality risks'' (Cox, 2019a, 
p. 8 of consensus responses). Thus, the committee members who supported 
retaining the current annual standard advise that, ``while the data on 
associations should certainly be carefully considered, this data should 
not be interpreted more strongly than warranted based on its 
methodological limitations'' (Cox, 2019a, p. 8 of consensus responses).
    These members of the CASAC further concluded that the 
PM2.5 risk assessment does not provide a valid basis for 
revising the current standards. This conclusion was based on concerns 
that (1) ``the risk assessment treats regression coefficients as causal 
coefficients with no justification or validation provided for this 
decision;'' (2) the estimated regression concentration-response 
functions ``have not been adequately adjusted to correct for 
confounding, errors in exposure estimates and other covariates, model 
uncertainty, and heterogeneity in individual biological (causal) 
[concentration-response] functions;'' (3) the estimated concentration-
response functions ``do not contain quantitative uncertainty bands that 
reflect model uncertainty or effects of exposure and covariate 
estimation errors;'' and (4) ``no regression diagnostics are provided 
justifying the use of proportional hazards . . . and other modeling 
assumptions'' (Cox, 2019a, p. 9 of consensus responses). These 
committee members also contended that details regarding the derivation 
of concentration-response functions, including specification of the 
beta values and functional forms, were not well-documented, hampering 
the ability of readers to evaluate these design details. Thus, these 
members ``think that the risk characterization does not provide useful 
information about whether the current standard is protective'' (Cox, 
2019a, p. 11 of consensus responses).
    Drawing from their evaluation of the evidence and the risk 
assessment, these committee members concluded that ``the Draft PM PA 
does not establish that new scientific evidence and data reasonably 
call into question the public health protection afforded by the current 
2012 PM2.5 annual standard'' (Cox, 2019a, p.1 of letter).
    In contrast, ``[o]ther members of CASAC conclude[d] that the weight 
of the evidence, particularly reflecting recent epidemiology studies 
showing positive associations between PM2.5 and health 
effects at estimated annual average PM2.5 concentrations 
below the current standard, does reasonably call into question the 
adequacy of the 2012 annual PM2.5 [standard] to protect 
public health with an adequate margin of safety'' (Cox, 2019a, p.1 of 
letter). The committee members who supported this conclusion noted that 
the body of health evidence for PM2.5 not only includes the 
repeated demonstration of associations in epidemiological studies, but 
also includes support for biological

[[Page 82707]]

plausibility established by controlled human exposure and animal 
toxicology studies. They pointed to recent studies demonstrating that 
the associations between PM2.5 and health effects occur in a 
diversity of locations, in different time periods, with different 
populations, and using different exposure estimation and statistical 
methods. They concluded that ``the entire body of evidence for PM 
health effects justifies the causality determinations made in the Draft 
PM ISA'' (Cox, 2019a, p. 8 of consensus responses).
    The members of the CASAC who supported revising the current annual 
standard particularly emphasized recent findings of associations with 
PM2.5 in areas with average long-term PM2.5 
concentrations below the level of the annual standard and studies that 
show positive associations even when estimated exposures above 12 
[mu]g/m\3\ are excluded from analyses. They found it ``highly 
unlikely'' that the extensive body of evidence indicating positive 
associations at low estimated exposures could be fully explained by 
confounding or by other non-causal explanations (Cox, 2019a, p. 8 of 
consensus responses). They additionally concluded that ``the risk 
characterization does provide a useful attempt to understand the 
potential impacts of alternate standards on public health risks'' (Cox, 
2019a, p. 11 of consensus responses). These committee members concluded 
that the evidence available in this review reasonably calls into 
question the protection provided by the current primary 
PM2.5 standards and supports revising the annual standard to 
increase that protection (Cox, 2019a).
2. Basis for Proposed Decision
    On April 14, 2020, the Administrator proposed to retain the current 
primary PM2.5 standards. This proposal was published in the 
Federal Register on April 30, 2020 (85 FR 24094, April 30, 2020). In 
reaching his proposed decision to retain the current PM2.5 
standards (i.e., annual and 24-hour PM2.5 standards), the 
Administrator considered the assessment of the available evidence and 
conclusions reached in the ISA (U.S. EPA, 2019); the analyses in the PA 
(U.S. EPA, 2020), including uncertainties in the evidence and analyses; 
and the advice and recommendations from the CASAC. These considerations 
are summarized briefly below and discussed in detail in the proposal 
notice (85 FR 24094, April 30, 2020).
    As described further in section II.A.2 of the proposal, the 
Administrator's consideration of the public health protection provided 
by the current primary PM2.5 standards were based on his 
consideration of the combination of the annual and 24-hour standards, 
including the indicators (PM2.5), averaging times, forms 
(arithmetic mean and 98th percentile, averaged over three years), and 
levels (12.0 [micro]g/m\3\, 35 [micro]g/m\3\) of those standards.
    The Administrator's proposed decision noted that one of the 
methodological limitations highlighted by the CASAC members who support 
retaining the annual standard (see section II.B.1 above) is that 
associations reported in epidemiological studies are not necessarily 
indicative of causal relationships and such associations ``can 
reasonably be explained in light of uncontrolled confounding and other 
potential sources of error and bias'' (Cox, 2019a, p.8). In the 
proposed decision, the Administrator recognized that epidemiological 
studies examine associations between distributions of PM2.5 
air quality and health outcomes, and they do not identify particular 
PM2.5 exposures that cause effects, as noted in the PA (U.S. 
EPA, 2020, section 3.1.2). The Administrator's proposed decision noted 
that experimental studies do provide evidence for health effects 
following particular PM2.5 exposures under carefully 
controlled laboratory conditions and further notes that the evidence 
for a given PM2.5-related health outcome is strengthened 
when results from experimental studies demonstrate biologically 
plausibility mechanisms through which such an outcome could occur. In 
the proposed decision, therefore, the Administrator expressed greatest 
confidence in the potential for PM2.5 exposures to cause 
adverse effects at concentrations supported by multiple types of 
studies, including experimental studies as well as epidemiological 
studies.
    In the proposed decision, in light of this approach to considering 
the evidence, the Administrator recognized that controlled human 
exposure and animal toxicology studies report a wide range of effects, 
many of which are plausibly linked to the serious cardiovascular and 
respiratory outcomes reported in epidemiological studies (including 
mortality), though he noted that the PM2.5 exposures 
examined in these studies are above the concentrations typically 
measured in areas meeting the current annual and 24-hour standards 
(U.S. EPA, 2020, section 3.2.3.1). The Administrator was cautious about 
placing too much weight on reported PM2.5 health effect 
associations for air quality meeting the current annual and 24-hour 
standards. He concluded in the proposed decision that such associations 
alone, without supporting experimental evidence at similar 
PM2.5 considerations, left important questions unanswered 
regarding the degree to which the typical PM2.5 exposures 
likely to occur in areas meeting the current standard could cause the 
mortality and morbidity outcomes reported in epidemiological studies. 
Given this concern, the Administrator noted in the proposal that he did 
not think that recent epidemiological studies reporting health effect 
associations at PM2.5 air quality concentrations likely to 
have met the current primary standards support revising those 
standards. Rather, he judged that the overall body of evidence, 
including controlled human exposure and animal toxicologic studies, in 
addition to epidemiological studies, indicated continuing uncertainty 
in the degree to which adverse effects could result from 
PM2.5 exposure in areas meeting the current annual and 24-
hour standards.
    The Administrator also considered the emerging body of evidence 
from accountability studies examining past reductions in ambient 
PM2.5, and the degree to which those reductions resulted in 
public health improvements, but also recognized that interpreting such 
studies in the context of the current primary PM2.5 
standards was complicated by the fact that some of the available 
accountability studies have not evaluated PM2.5 
specifically, did not show changes in PM2.5 air quality, or 
have not been able to disentangled health impacts of the interventions 
from background trends in health. The Administrator also recognized 
that the small number of available studies that do report public health 
improvements following past declines in ambient PM2.5 have 
not examined air quality meeting the current standard. Together with 
the Administrator's concerns regarding the lack of experimental studies 
examining PM2.5 exposures typical of areas meeting the 
current standards, the lack of demonstrated health improvements in 
areas with air quality meeting the current standards led him to 
conclude, at the time of proposal, that there was considerable 
uncertainty in the potential for increased public health protection 
from further reductions in ambient PM2.5 concentrations 
beyond those achieved under the current primary PM2.5 
standards.
    In addition to the evidence, the Administrator also considered the 
potential implications of the risk assessment for his proposed 
decision, noting that all risk assessments have limitations. He noted 
that such limitations in risk estimates can result

[[Page 82708]]

from uncertainty in the shapes of concentration-response functions, 
particularly at low concentrations; uncertainties in the methods used 
to adjust air quality; and uncertainty in estimating risks for 
populations, locations and air quality distributions different from 
those examined in the underlying epidemiological study. The 
Administrator noted agreement with some members of the CASAC who 
expressed concerns regarding limitations in the epidemiological 
evidence, which provides key inputs to the risk assessment. Thus, he 
judged it appropriate to place little weight on quantitative estimates 
of PM2.5-associated mortality risk in reaching proposed 
conclusions on the primary PM2.5 standards.
    In reaching his proposed decision to retain the current primary 
PM2.5 standards, the Administrator concluded that the 
scientific evidence assessed in the ISA (U.S. EPA, 2019), and the 
analyses based on that evidence in the PA (U.S. EPA, 2020), do not call 
into question the public health protection provided by the current 
annual and 24-hour PM2.5 standards. In particular, the 
Administrator judged that there is considerable uncertainty in the 
potential for additional public health improvements from reducing 
ambient PM2.5 below the concentrations achieved under the 
current primary standards and, therefore, that standards more stringent 
than the current standards (e.g., with lower levels) are not supported. 
That is, he judged that such standards would be more than requisite to 
protect the public health with an adequate margin of safety. This 
judgment reflected his consideration of the uncertainties in the 
potential implications of recent epidemiological studies due in part to 
the lack of supporting evidence from experimental studies and 
accountability studies conducted at PM2.5 concentrations 
meeting the current standards.
    In addition, based on the Administrator's review of the science, 
including experimental and accountability studies conducted at levels 
just above the current standard, he judged that the degree of public 
health protection provided by the current standard is not greater than 
warranted. This judgment, together with the fact that no CASAC member 
expressed support for a less stringent standard, led the Administrator 
to conclude that standards less stringent than the current standards 
(e.g., with higher levels) are also not supported.
    Thus, based on his consideration of the available scientific 
evidence and technical information and his consideration of advice from 
the CASAC, the Administrator proposed to conclude that the current 
suite of primary standards, including the current indicators 
(PM2.5), averaging times (annual and 24-hour), forms 
(arithmetic mean and 98th percentile, averaged over three years) and 
levels (12.0 [micro]g/m\3\, 35 [micro]g/m\3\), remain requisite to 
protect the public health. As discussed in detail in the proposal (85 
FR 24094, April 30, 2020), this proposed conclusion reflected his 
judgment that limitations in the science lead to considerable 
uncertainty regarding the potential public health implications of 
revising the existing suite of PM2.5 standards. Therefore, 
the Administrator proposed to retain the current standards, without 
revision.
3. Comments on the Proposed Decision
    Overall, the EPA received a large number of unique public comments 
on the proposed decision to retain the annual and 24-hour 
PM2.5 standards. These comments generally fall into one of 
two broad groups that expressed sharply divergent views. The first 
group is comprised of the many commenters, representing industries and 
industry groups, some state and local governments, and independent 
organizations, that support the Administrator's proposed decision to 
retain the primary PM2.5 standards. The second group of 
commenters are those who asserted that the current primary 
PM2.5 standards are not sufficient to protect public health 
with an adequate margin of safety. These commenters disagree with the 
EPA's proposed decision to retain the current PM2.5 
standards and generally recommend a revised annual standard of between 
8-10 [micro]g/m\3\ and a revised 24-hour standard between a range of 
25-30 [micro]g/m\3\. Among those calling for revisions to the current 
primary PM2.5 standards were commenters representing 
national public health, medical, and environmental nongovernmental 
organization, tribes and tribal groups, some state and local 
governments and independent organizations and individuals.
    We address the key public comments received on the proposal (85 FR 
24094, April 30, 2020) and present the EPA's responses to those 
comments below. A more detailed summary of all significant comments, 
along with the EPA's responses (henceforth ``Response to Comments''), 
can be found in the docket for this rulemaking (Docket No. EPA-HQ-OAR-
2015-0072). This document is available for review in the docket for 
this rulemaking and through the EPA's NAAQS website (https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards).
    With respect to the various elements of the standards, the EPA 
received very few comments related to indicator and none advocate for 
revising the current PM2.5 indicator for fine particles. 
Those who express explicit support for retaining the current 
PM2.5 indicator generally endorse the rationale put forward 
in the PA. The EPA agrees with these commenters, noting that the 
scientific evidence in this review, as in the last review, continues to 
provide strong support for health effects following short- and long-
term PM2.5 exposures and that the available information 
remains too limited to support a distinct standard for any specific 
PM2.5 component or group of components or to support a 
distinct standard for the ultrafine fraction.
    The EPA also received very few comments on averaging time and form. 
Those who did provide comments are mostly affiliated with public health 
organizations and environmental advocacy groups and generally discuss 
the need for future evaluation of the form and averaging time of the 
current 24-hour standard (98th percentile, averaged over three years). 
These commenters, acknowledging the current limitations and 
uncertainties in the available evidence, suggest that in future reviews 
the EPA should evaluate how well the current form of the 24-hour 
standard protects against potential sub-daily exposures based on new 
epidemiological and experimental evidence that considers sub-daily 
exposures, but these commenters support retaining the current 
indicators, averaging times, and forms.
    The EPA acknowledges the comments related to averaging time and 
form of the 24-hour standard and agrees that the current information 
does not support a revision to the averaging time or form. The EPA will 
continue to evaluate the form and averaging time of the current 24-hour 
standard in future reviews based on any new relevant information.
    With respect to the level of the 24-hour standard, commenters 
supporting revision generally support a revised level in the range of 
25-30 [micro]g/m\3\. They contend the available scientific evidence 
supports that lower levels within this range are required to protect 
public health, including the health of at-risk populations, with an 
adequate margin of safety, and that lower levels within this range will 
provide additional margin of safety. The commenters cite controlled 
human exposure studies that assess short-term exposures (i.e., 2 to 5 
hours) and epidemiological studies that report

[[Page 82709]]

associations between adverse health effects and concentrations below 
the current standard level as supporting the need for this revision. 
They further add that while revising the 24-hour level to 25 [micro]g/
m\3\ would offer more health protection than 30 [micro]g/m\3\, it would 
still not reduce the risk of adverse health outcomes to zero.
    With respect to the level of the annual PM2.5 standard, 
numerous comments were received that specifically focus on the 
Administrator's consideration of epidemiological evidence in this 
review. Commenters who support revision generally disagree with the 
Administrator's conclusions and judgments about the uncertainties in 
the epidemiological evidence and suggest that these studies support 
revision of the PM2.5 annual standard to a level of 8-10 
[micro]g/m\3\. These commenters state that uncertainties in the 
epidemiological studies, alone, do not negate positive associations 
seen in studies using diverse study designs and capturing large 
geographic and population domains. These commenters note that the 
possibility of confounders and the other referenced uncertainties have 
been investigated and found not to be material given the overall 
strength and consistency of results from varying approaches. The 
commenters who support revising the primary PM2.5 standards 
generally place substantial weight on epidemiologic evidence from 
multi-city U.S. and Canadian studies that captured a larger geographic 
domain and population size, and were included in the ISA and in the 
study-related analyses in the PA (U.S. EPA, 2020). Further, they also 
cite epidemiological studies in the ISA (U.S. EPA, 2019) that performed 
restricted/truncated analyses with populations living in areas of lower 
PM2.5 concentrations and contend that associations still 
exist in these studies at the concentrations below the levels of the 
current annual and daily standards. Moreover, they state that there was 
no evidence for an ambient concentration threshold for adverse health 
effects at the lowest observed levels of either annual or 24-hour 
PM2.5 concentrations.
    The EPA disagrees with these commenters. First, the EPA notes that, 
consistent with past practices, the foremost consideration is the 
adequacy of the public health protection as provided by the combination 
of the annual and 24-hour standards together. The annual standard 
limits ``typical'' daily PM2.5 concentrations that make up 
the bulk of the distribution, while the 24-hour standard adds 
supplemental protection against ``peak'' daily PM2.5 
concentrations. In the judgment of the Administrator, therefore, the 
current annual standard (arithmetic mean, averaged over three years) 
remains appropriate for targeting protection against the annual and 
daily PM2.5 exposures around the middle portion of the 
PM2.5 air quality distribution, while the current 24-hour 
standard (98th percentile, averaged over three years) continues to 
provide an appropriate balance between limiting the occurrence of peak 
24-hour PM2.5 concentrations and identifying a stable target 
for risk management programs (U.S. EPA, 2020, section 3.5.2.3). 
Further, the Administrator notes that changes in PM2.5 air 
quality to meet an annual standard would likely result not only in 
lower short- and long-term PM2.5 concentrations near the 
middle of the air quality distribution, but also in fewer and lower 
short-term peak PM2.5 concentrations. Similarly, the 
Administrator recognizes that changes in air quality to meet a 24-hour 
standard, would result not only in fewer and lower peak 24-hour 
PM2.5 concentrations, but also in lower annual average 
PM2.5 concentrations.
    Thus, in considering the adequacy of the 24-hour standard, an 
important consideration is whether additional protection is needed 
against short-term exposures to peak PM2.5 concentrations. 
In examining the scientific evidence, the EPA notes that controlled 
human exposure studies do provide evidence for health effects following 
single, short-term PM2.5 exposures to concentrations. These 
types of exposures correspond best to those to ambient exposures that 
might be experienced in the upper end of the PM2.5 air 
quality distribution in the U.S. (i.e., ``peak'' concentrations). 
However, and as noted above in section II.A.2.c.i, most of these 
studies examine exposure concentrations considerably higher than are 
typically measured in areas meeting the current standards (U.S. EPA, 
2020, section 3.2.3.1). In particular, controlled human exposure 
studies often report statistically significant effects on one or more 
indicators of cardiovascular function following 2-hour exposures to 
PM2.5 concentrations at and above 120 [mu]g/m\3\ (at and 
above 149 [mu]g/m\3\ for vascular impairment, the effect shown to be 
most consistent across studies). Commenters did specifically note one 
study (Hemmingsen et al., 2015b) and contend that this study shows 
significant effects on some outcomes at lower concentrations, following 
5-hour exposures to 24 [mu]g/m\3\. The PA notes that this study does 
not report effects consistent with other studies in the ISA that 
evaluate longer exposure durations (i.e., longer than 2 hours) and 
lower PM2.5 concentrations (e.g., Br[auml]uner et al., 2008 
and Hemmingsen et al., 2015a). Furthermore, analyses in the PA show 
that the exposure concentrations included in this study are not 
observed in areas meeting the current standards (U.S. EPA, 2020, Figure 
A-2), suggesting that the current standards provide protection against 
these exposure concentrations. To provide insight into what these 
studies may indicate regarding the primary PM2.5 standards, 
the PA (U.S. EPA, 2020, p.3-49) notes that 2-hour ambient 
concentrations of PM2.5 at monitoring sites meeting the 
current standards almost never exceed 32 [mu]g/m\3\. In fact, even the 
extreme upper end of the distribution of 2-hour PM2.5 
concentrations at sites meeting the current standards remains well-
below the PM2.5 exposure concentrations consistently shown 
in controlled human exposure studies to elicit effects (i.e., 99.9th 
percentile of 2-hour concentrations at these sites is 68 [mu]g/m\3\ 
during the warm season). Thus, available PM2.5 controlled 
human exposure studies do not indicate the need for additional 
protection against exposures to peak PM2.5 concentrations, 
beyond the protection provided by the combination of the current 24-
hour standard and the current annual standard (U.S. EPA, 2020, section 
3.2.3.1). With respect to the epidemiological evidence and as noted 
above in section II.A.2.c.ii, the information from such studies is most 
applicable to examining potential health impacts associated with 
typical (i.e., average or mean) exposures and thus are most applicable 
in informing decisions on the annual standard (with its arithmetic mean 
form). Furthermore, as noted above, the available epidemiological 
studies do not indicate that associations in these studies are strongly 
influenced by exposures to peak concentrations in the air quality 
distribution, and thus do not indicate the need for additional 
protection against short-term exposures to peak PM2.5 
concentrations. As discussed above, the annual standard provides 
protection against the typical 24-hour and annual PM2.5 
exposures. Thus, in the context of a 24-hour standard that is meant to 
provide supplemental protection (i.e., beyond that provided by the 
annual standard alone) against short-term exposures to peak 
PM2.5 concentrations, the available evidence supports the 
Administrator's proposed conclusion to retain the current 24-hour 
standard with its level of 35 [micro]g/m\3\.
    With respect to commenters that support revision of the annual 
standard,

[[Page 82710]]

the EPA recognizes that there are a large number of studies, many of 
which include a variety of study populations and geographic locations, 
that show positive associations between mortality and morbidity and 
short-term and long-term PM2.5 exposure. Furthermore, the 
EPA recognizes that while uncertainties exist, when the epidemiological 
evidence is viewed together in the context of the full body of 
evidence, the scientific information supports that exposure to 
PM2.5 may cause adverse health effects (U.S.EPA, 2019, 
section 1.7.3, Table 1-4). Therefore, the EPA does not dispute 
commenters that note epidemiological studies support the conclusion 
that exposure to PM2.5 is associated with morbidity and 
mortality.
    However, while the epidemiological evidence when considered 
together with the full body of evidence supports health effects 
associated with PM2.5 exposure, the EPA recognizes that 
important uncertainties and limitations in the health effects evidence 
remain. Epidemiological studies evaluating health effects associated 
with long- and short-term PM2.5 exposures have reported 
heterogeneity in associations between cities and geographic regions 
within the U.S. Heterogeneity in the associations observed across 
PM2.5 epidemiological studies may be due in part to exposure 
error related to measurement-related issues, the use of central fixed-
site monitors to represent population exposure to PM2.5, 
models used in lieu of or to supplement ambient measurements, 
limitations in hybrid models and our limited understanding of factors 
that may influence exposures (e.g., topography, the built environment, 
weather, source characteristics, ventilation usage, personal activity 
patterns, photochemistry) (U.S. EPA, 2020, p.3-25), all of which can 
introduce bias and/or increased uncertainty is associated health 
effects estimates. Heterogeneity is expected when the methods or 
underlying distribution of covariates vary across studies (U.S. EPA, 
2019, p. 6-221). In addition, where PM2.5 and other 
pollutants (e.g., ozone, nitrogen dioxide, and carbon monoxide) are 
correlated, it can be difficult to distinguish whether attenuation of 
effects in some studies results from copollutant confounding or 
collinearity with other pollutants in the ambient mixture (U.S. EPA, 
2019, section 1.5.1). The EPA also recognizes that methodological study 
designs to address confounding, such as causal inference methods, are 
an emerging field of study (U.S. EPA, 2019, section 11.2.2.4 or U.S. 
EPA, 2020, p. 3-24). The Administrator weighs these uncertainties in 
the reported associations of PM2.5 concentrations in the 
studies and considers them in the context of the entire body of 
evidence before the Agency when reviewing the standards.
    Additionally, while epidemiological studies indicate associations 
between exposure to PM2.5 and health effects, they do not 
identify particular PM2.5 exposures that cause effects 
(section II.A.2.c.ii above and U.S. EPA, 2020, section 3.1.2). Further, 
using information from epidemiological studies to inform decisions on 
PM2.5 standards is complicated by the recognition that no 
population threshold, below which it can be concluded with confidence 
that PM2.5-related effects do not occur, can be discerned 
from the available evidence. As a result, any general approach to 
reaching decisions on what standards are appropriate necessarily 
requires judgments about how to translate the information available 
from the epidemiological studies into a basis for appropriate 
standards. This includes consideration of how to weigh the 
uncertainties in the reported associations in the epidemiological 
studies and the uncertainties in quantitative estimates of risk, in the 
context of the entire body of evidence before the Agency. Such 
approaches are consistent with setting standards that are neither more 
nor less stringent than necessary, recognizing that a zero-risk 
standard is not required by the CAA.
    Commenters who support revising the PM2.5 standards 
further contend that the Administrator has arbitrarily rejected an 
established practice of relying on epidemiological studies and of 
setting the standard below the long-term mean PM2.5 
concentrations reported in each of the studies that provide evidence of 
an array of serious health effects. The commenters state that in 
declaring that the latest epidemiological studies cannot justify a 
decision to strengthen the PM NAAQS, the Administrator has rejected--
without acknowledgment or explanation--the EPA's long history of 
relying on such research as the basis for its primary standards.
    As recognized in this and previous PM NAAQS reviews, including 
those completed in 2006 and 2012, evidence of an association in any 
epidemiological study is ``strongest at and around the long-term 
average where the data in the study are most concentrated.'' In the PA 
(U.S. EPA, 2020, section 3.2.3.2.1), the EPA assessed air quality 
distributions reported in key epidemiological studies included in the 
ISA, with a focus on characterizing the long-term average or mean 
PM2.5 concentrations. In doing this, key studies \43\ were 
identified that examined short- and long-term exposure and showed 
positive associations with either mortality or morbidity health 
outcomes. The studies either estimated PM2.5 exposure using 
ground-based monitored data or using hybrid modeling data, which 
incorporate data from several sources, often including satellites and 
models, as well as ground-based monitors (U.S. EPA, 2020, section 
2.3.3). The PA notes some important considerations in using study 
reported concentrations to inform conclusions on the primary 
PM2.5 standards. In particular, it notes that the overall 
mean PM2.5 concentrations reported by key epidemiological 
studies are not the same as the ambient concentrations used by the EPA 
to determine whether areas meet or violate the PM NAAQS. Mean 
PM2.5 concentrations in key studies reflect averaging of 
short- or long-term PM2.5 exposure estimates across 
locations (i.e., across multiple monitors or across modeled grid cells) 
and over time (i.e., over several years). In contrast, to determine 
whether areas meet or violate the PM NAAQS, the EPA measures air 
pollution concentrations at individual monitors (i.e., concentrations 
are not averaged across monitors) and calculates design values \44\ at 
monitors meeting appropriate data quality and completeness 
criteria.\45\ For an area to meet the NAAQS, all valid design values in 
that area, including the highest annual and highest 24-hour monitoring 
values, must be at or below the standards. As a result, study reported 
mean concentration values are generally lower than the design value of 
the highest monitor in an area, which determines compliance.
---------------------------------------------------------------------------

    \43\ Studies included were multi-city studies in Canada and the 
U.S. that examined health endpoints with `causal' or `likely to be 
causal' determinations in the ISA.
    \44\ A design value is a statistic that summarizes the air 
quality data for a given area in terms of the indicator, averaging 
time, and form of the standard. Design values can be compared to the 
level of the standard and are typically used to designate areas as 
meeting or not meeting the standard and assess progress towards 
meeting the NAAQS.
    \45\ For the annual PM2.5 standard, design values are 
calculated as the annual arithmetic mean PM2.5 
concentration, averaged over 3 years (described in appendix N of 40 
CFR part 50). For the 24-hour standard, design values are calculated 
as the 98th percentile of the annual distribution of the 24-hour 
PM2.5 concentrations, averaged over three years.
---------------------------------------------------------------------------

    The PA first presents results from key epidemiological studies that 
used ground-based monitoring data to estimate population exposure (U.S. 
EPA, 2020, section 3.2.3.2.1). Study reported mean (or medians) \46\ 
were

[[Page 82711]]

examined from the air quality distributions reported in key 
epidemiological studies included in the ISA exposures (U.S. EPA, 2020, 
Figure 3-7). The PA noted that these values are most useful in the 
context of considering the level of the primary PM2.5 annual 
standard. This is because the mean concentration values from these 
studies, which include studies examining both short- and long-term 
exposures, represent ``typical'' or mean exposures, which are most 
relevant to the form and averaging time of the annual standard, and not 
as relevant to the daily standard, whose form and averaging time 
focuses on protecting against peak concentrations. Further, the PA 
noted that in using these data it should be recognized that these mean 
concentrations are generally below the design values in the 
corresponding areas. In fact, analyses included in the PA of recent air 
quality in U.S. CBSAs indicate that maximum annual PM2.5 
design values for a given three-year period are often 10% to 20% higher 
than average monitored concentrations (i.e., averaged across multiple 
monitors in the same CBSA) (U.S. EPA, 2020, Appendix B, section B.7). 
As noted in the PA, the difference between the maximum annual design 
value and the average concentrations in an area will depend on a number 
of factors including the numbers of monitors, monitor citing 
characteristics, and the distribution of ambient PM2.5 
concentrations. The PA also recognized that the recent requirement for 
PM2.5 monitoring at near-road locations in large urban areas 
may further increase the ratios of maximum annual design values to 
average concentrations in some areas (U.S. EPA, 2020, section 
3.2.3.2.1).
---------------------------------------------------------------------------

    \46\ Some epidemiological studies report median versus mean air 
quality concentrations offering that median is a better metric since 
it is less skewed by outlying concentrations. In most studies, the 
mean and median concentrations are very similar and are generally 
used here interchangeably.
---------------------------------------------------------------------------

    As detailed more in section II.A.2.c.ii, the PA next presents data 
from the epidemiological studies that used hybrid modeling approaches 
to estimate exposures (U.S. EPA, 2020, Figure 3-8). While studies using 
hybrid modeling approaches provide valid methods to estimate exposures 
in epidemiological studies and can expand the characterization of 
PM2.5 exposures in areas with sparse monitoring networks, 
these exposure estimation methods provide additional challenges to 
comparing study reported mean concentrations to the annual standard 
level. In these studies, PM2.5 concentrations are typically 
estimated based on a hybrid approach of ``fusing'' data from air 
quality models, satellites and ground-based monitors. As such, the 
reported mean concentrations in an area (e.g., county or zip-code) from 
these studies are calculated using the estimated concentrations from 
thousands of grid cells across the area. Generally, this means a larger 
number of lower concentration grid cells being included in the 
calculation of the mean, resulting in a mean concentration even further 
below the design value of the highest monitor in the area (which is 
used for determining whether the area is meeting the current standard) 
and even further below the mean concentration reported in 
epidemiological studies utilizing ground-based monitors to estimate 
exposure.
    It is also important to note that the performance of these hybrid 
modeling approaches in estimating PM2.5 concentrations, 
which are being used as surrogates for population exposure in the 
epidemiological study, depends on the availability of monitoring data, 
air quality model and the ability of the satellite to estimate ground 
level concentration and, thus, varies by location. Factors that 
contribute to poorer model performance often coincide with relatively 
low ambient PM2.5 concentrations (U.S. EPA 2020, 2.3.3) 
Thus, uncertainty in hybrid model predictions becomes an increasingly 
important issue as lower predicted concentrations are considered. This 
additional source of uncertainty is an important consideration, 
particularly when all grid cell estimates are being used to calculate 
the study mean concentration, and further adds to why using study 
reported mean concentrations from epidemiological studies that use 
hybrid approaches to inform conclusions on the primary PM2.5 
standards is a challenge.
    Given all of this, the EPA concludes that the overall mean 
PM2.5 concentrations in hybrid modeling studies are more 
difficult to directly compare to design values than ground-based 
monitoring concentrations in the context of setting a standard level. 
In fact, recognizing this challenge, the PA tried to assess information 
from hybrid modelling studies by calculating ``pseudo-design values'' 
in locations of the key epidemiological studies (U.S. EPA, 2020, 
section 3.2.3.2.2), as noted above in section II.A.2.c.ii and detailed 
further in section II.C.1.a.ii of the proposal (85 FR 24117, April 30, 
2020). However, this analysis and the associated approach were highly 
criticized by most commenters, with none suggesting the methodology be 
carried forward in the review. While the EPA believes that the PA's 
``pseudo-design value'' approach was a step in the right direction, the 
specific methodology itself needs further development.
    Given these considerations, and in light of the comments received, 
the EPA believes it is reasonable to focus on study reported mean (or 
median) concentrations \47\ from key U.S.\48\ epidemiological studies 
that used ground-based monitors when considering information most 
comparable to the current annual standard, while also weighing the 
uncertainties associated with these studies and considering support 
provided by other lines of evidence. Based on the information shown in 
Figure 3-7 of the PA (U.S. EPA, 2020), the mean concentrations in 19 of 
the 21 these studies were equal to or greater than the level of the 
current annual standard of 12 [micro]g/m\3\. There were two studies, 
both included in last review, for which the mean concentration (11.8 
[micro]g/m\3\; Peng et al., 2009) or median concentration (10.7 
[micro]g/m\3\ (Central Region); Zeger et al., 2008 \49\) was somewhat 
below 12 [micro]g/m\3\. While these studies were included in the last 
review, the air quality distributions were not used by the prior 
Administrator in making a judgment on the level of the standard. The 
reported study mean concentration for one other study was 12 [micro]g/
m\3\ (Kioumourtzoglou et al., 2016). The mean \50\ of the study 
reported means (or medians) of these 21 studies is 13.5 [micro]g/m\3\, 
a concentration level above the current level of the primary annual 
standard of 12 [micro]g/m\3\. Additionally, based on analyses in the 
PA, it would be expected that most of the design values (the metric 
most relevant for comparison to the standard level) in the areas 
included in these studies would be greater than 12 [micro]g/m\3\ \51\ 
(section II.A.2.c.ii above and U.S.

[[Page 82712]]

EPA 2020, Appendix B, section B.7). This is also supported by the 
pseudo-design value analysis in Figure 3-9 of the PA (U.S. EPA, 2020).
---------------------------------------------------------------------------

    \47\ Some epidemiological studies report median versus mean air 
quality concentrations offering that median is a better metric since 
it is less skewed by outlying concentrations. In most studies, the 
mean and median concentrations are very similar and are generally 
used here interchangeably.
    \48\ Given how air quality monitors in other countries differ 
from the U.S. EPA FRM monitors discussed here, a focus on U.S. 
studies ensures that the results most closely compare to the data 
being used for calculating the design values and for compliance of 
the standard.
    \49\ We note that in this study the population was divided into 
regions of the country, with statistically significant associations 
in the Central and Eastern Regions and with median long-term 
PM2.5 concentrations of: Central: 10.7 [micro]g/m\3\; 
Western: 13.1 [micro]g/m\3\ and Eastern: 14.0 [micro]g/m\3\.
    \50\ The median of the study reported mean (or median) 
PM2.5 concentrations is 13.3 [micro]g/m\3\.
    \51\ Recent air quality in U.S. CBSAs in the PA indicate that 
maximum annual PM2.5 design values for a given three-year 
period are often 10% to 20% higher than average monitored 
concentrations (i.e., averaged across multiple monitors in the same 
CBSA) (U.S. EPA, 2020, Appendix B, section B.7).
---------------------------------------------------------------------------

    Therefore, although recognizing that the proposal identified 
certain concerns about the proper weight to be placed on 
epidemiological studies, the EPA finds that its assessment of the mean 
concentrations of the key short-term and long-term epidemiological 
studies in the U.S. that use ground-based monitoring (i.e., those 
studies that can provide information most directly comparable to the 
current annual standard) is fundamentally consistent with the 
assessment in the last review, which established the current primary 
PM2.5 standards.
    Some commenters supporting revision of the primary PM2.5 
standards contend that the quantitative risk assessment finds the 
number of avoided deaths resulting from retention of the standards will 
likely number in the many thousands, and a substantial reduction in 
these events could be achieved by a more stringent PM2.5 
standard. While commenters who support revising the PM2.5 
standards support the recommendation of the PA to use the evidence-
based approach, as opposed to the risk-based approach, as a basis for 
ascertaining whether and how to revise the primary standards, the 
commenters state that the risk assessment does provide qualitative 
support to revise the standards.
    With regard to the quantitative risk assessment described by some 
commenters as showing health impacts that would be avoided by a more 
stringent standard, the EPA notes that these analyses utilize 
epidemiological study effect estimates as concentration-response 
functions to predict the occurrence of primarily premature mortality 
under different air quality conditions (characterized by the metric 
used in the epidemiological study). While the epidemiological studies 
that are inputs to the quantitative risk assessment are part of the 
evidence base that supports the conclusion of a ``causal'' or ``likely 
to be causal'' determination in the ISA (U.S. EPA, 2019), there are 
uncertainties inherent in the derivation of estimates of health effects 
(e.g., total mortality or ischemic heart disease mortality) ascribed to 
PM2.5 exposures using effect estimates from these studies. 
For example, the PA recognized several important uncertainties 
associated with aspects of the quantitative risk assessment approach 
and that the EPA concluded to have a medium or greater magnitude on 
risk estimates (U.S. EPA, 2020, section C.3.1 and table C-32). These 
uncertainties limit the applicability of the risk results for selecting 
a specific standard. Uncertainties in the shapes of concentration-
response functions, particularly at low concentrations; uncertainties 
in the methods used to adjust air quality; and uncertainty in 
estimating risks for populations, locations and air quality 
distributions different from those examined in the underlying 
epidemiological study all limit utility (U.S. EPA, 2020, section 
3.3.2.4). Further, the approach to weighing evidence-based and risk-
based considerations is not a new approach and as in previous reviews, 
the selection of a specific approach to reaching final decisions on the 
primary PM2.5 standards will reflect the judgments of the 
Administrator as to what weight to place on the various types of 
information available in the current review. The EPA notes that in the 
previous review, evidence-based considerations were given greater 
weight in the selection of standard levels than risk-based approaches 
(e.g., 78 FR 3086, 3098-99, January 15, 2013) due to a recognition of 
similar limitations.
    Some commenters who support the Administrator's rationale to retain 
the PM2.5 standards contend that, due to uncertainties in 
extrapolating health effects observed in animal toxicology studies to 
humans, animal toxicology studies are of limited regarding the adequacy 
of the current standard. On the other hand, commenters who support 
revisions to the current suite of PM2.5 standards generally 
contend that for experimental studies the Administrator: (1) 
Inappropriately tied the concept of biological plausibility to a 
specific concentration; (2) incorrectly interpreted animal/controlled 
human exposure studies; (3) ignored the limitations of experimental 
studies in relation to informing NAAQS levels and (4) gave inadequate 
weight to all of the evidence because the Administrator saw no absolute 
corroboration from clinical and accountability studies. The commenters 
emphasize their view that experimental studies provide important 
information regarding biological plausibility of numerous health 
effects (e.g., cardiovascular, respiratory, nervous system, and cancer 
effects) associated with PM2.5 exposure. Therefore, the 
commenters contend that experimental studies provide biological 
plausibility for human health effects linked to PM exposure in 
epidemiological studies and when viewed together, support revision of 
the current PM2.5 standards.
    The EPA notes that controlled human exposures studies provide 
crucial evidence in assessing whether protection is provided for short-
term exposure concentrations consistently shown to elicit effects. In 
examining the controlled human exposure studies, the PA notes these 
studies provide evidence for health effects following single, short-
term PM2.5 exposures to concentrations, and thus, can be 
useful to assess whether these effects are likely to occur in the upper 
end of the PM2.5 air quality distribution in the U.S. (i.e., 
``peak'' concentrations) (U.S. EPA, 2020, section 3.2.3.1). As noted by 
the commenters, most of these studies examine exposure concentrations 
considerably higher than are typically measured in areas meeting the 
current standards (U.S. EPA, 2020, section 3.2.3.1). As detailed in 
section II.A.2.c.i above, even the extreme upper end of the 
distribution of 2-hour PM2.5 concentrations at sites meeting 
the current standards remains well-below the PM2.5 exposure 
concentrations consistently shown to elicit effects. Further, human 
exposure studies have not reported health effects at PM2.5 
air quality concentrations likely to be seen in areas meeting the 
current primary PM2.5 standards. As such, these studies do 
not call into question the protection provided by the current primary 
PM2.5 standards.
    Additionally, with respect to the experimental evidence, the EPA 
agrees that animal toxicologic studies can be useful in understanding 
and supporting the biological plausibility of various effects linked to 
PM2.5 exposures. However, it is important to remember that 
for this body of evidence there is uncertainty in extrapolating from 
effects in animals to those in human populations. As such, animal 
toxicology studies are of limited utility in directly informing 
conclusions on the appropriate level of the standard. Thus, the 
available evidence from animal toxicologic studies do not call into 
question the protection provided by the current primary 
PM2.5 standards.
    Further, the ISA assesses both human exposures studies and animal 
toxicologic studies to evaluate the biological plausibility of various 
effects linked to PM2.5 exposures, and thus, we agree with 
the commenters on the importance of experimental evidence on this 
account. Within the ISA's weight of evidence evaluation, which is based 
on the integration of findings from various lines of evidence, 
considerations in making causality determinations

[[Page 82713]]

include: ``determining whether laboratory studies of humans and 
animals, in combination with epidemiological studies, inform the 
biological mechanisms by which PM can impart health effects and provide 
evidence demonstrating that PM exposure can independently cause a 
health effect'' (U.S. EPA, 2019, p. ES-8). However, the ISA also notes 
that the strength of the PM2.5 exposure-health effects 
relationship varies depending on the exposure duration (i.e., short- or 
long-term) and broad health effects category (e.g., cardiovascular 
effects, respiratory effects) examined, and that across the broad 
health effects categories examined, the evidence supporting biological 
plausibility varies. Additionally, while assessing plausible biological 
pathways is an important step in evaluating causality determinations, 
the degree of biological plausibility for different mechanisms and end 
points can also vary depending on the evidence available. As a result, 
without a more clear linkage between concentrations below the current 
standard levels and adverse health effects, the Administrator noted in 
the proposal that he was ``cautious about placing too much weight on 
reported PM2.5 health effect associations'' observed in 
epidemiological studies (85 FR 24119, April 30, 2020). As discussed in 
the proposal, the Administrator's proposed decision was based on his 
evaluation of ``the overall body of evidence, including controlled 
human exposure and animal toxicologic studies, in addition to 
epidemiological studies'' (85 FR 24120, April 30, 2020). Thus, the 
experimental evidence does not suggest that the epidemiological 
evidence must be viewed differently than the Administrator has viewed 
such evidence in his proposed decision to retain the current primary 
standards.
    Some commenters who support retaining the current primary 
PM2.5 standards assert that the currently available 
accountability studies do not demonstrate that further reduction of the 
PM NAAQS would achieve a measurable improvement in public health. In 
contrast, commenters opposing the Administrator's proposed decision to 
retain the PM2.5 standards criticize the Administrator's 
heavy reliance on accountability studies to guide his decision, while 
emphasizing that accountability studies are just one line of evidence 
to inform causality. The commenters acknowledge the importance of well-
designed and conducted accountability studies but warn that 
accountability studies measuring past interventions that are highly 
localized may have actual effects too small to be reliably measured. 
Considering the limitations of the accountability studies, including 
findings leading to false negative results, such studies are not 
considered essential for the proof of evidence required by statute, 
according to these commenters.
    The EPA agrees with the commenters that well-designed and conducted 
accountability studies can be informative and should be considered as 
one line of evidence, recognizing that that these studies offer insight 
into examples of how public health has responded to implementation of 
PM2.5 reduction strategies. As discussed in the PA (U.S. 
EPA, 2020, section 3.2.3.2.1) and in section III.C.3 of the proposal 
(85 FR 24120, April 30, 2020), the EPA notes the availability of 
several such accountability studies and other retrospective health 
studies examining periods of declining PM2.5 concentrations. 
As indicated in Table 3-3 of the PA (U.S. EPA, 2020), these studies 
conducted in the U.S. indicate that declines in ambient 
PM2.5 concentrations over a period of years have been 
associated with decreases in mortality rates and increases in life 
expectancy, improvements in respiratory development, and decreased 
incidence of respiratory disease in children. When considering the 
overall means in these studies (i.e., the part of the air quality 
distribution over which the studies provide the strongest support for 
reported health effect associations), we find that ``starting'' annual 
average PM2.5 concentrations (i.e., mean concentration prior 
to reductions being evaluated) range from 13.2-31.5 [micro]g/m\3\ and 
``ending'' concentrations ranging from 11.6-17.8 [micro]g/m\3\. As 
such, the EPA notes that these retrospective studies tend to focus on 
time periods during which ambient PM2.5 concentrations were 
substantially higher than those measured more recently, as well as 
``starting'' annual average PM2.5 concentrations above those 
allowed by the current primary PM2.5 standards. As a result, 
the EPA believes that while these studies do provide evidence of public 
health improvements as ambient PM2.5 has declined over time, 
no current studies have examined public health improvements following 
reductions in ambient PM2.5 concentrations in areas where 
the ``starting'' concentration met the current primary standards. Thus, 
while acknowledging that this is an emerging field of study for 
PM2.5-related health effects, the available evidence 
supports the Administrator's recognition that currently, there is a 
lack of accountability studies that clearly demonstrate that revising 
the current primary PM2.5 standards would result in public 
health improvements.
    Commenters opposed to the Administrator's proposed decision to 
retain the PM2.5 standards contend that the EPA's proposed 
decision is a violation of the CAA because it fails to consider 
sensitive populations and contains no margin of safety for them, as 
required under the CAA. In particular, these commenters pointed to 
evidence drawn from epidemiological studies that included specific at-
risk groups in their study design and results.
    The EPA disagrees with these comments. As discussed above, the 
Administrator's proposed decision to retain the current primary 
PM2.5 standards followed the same general approach used in 
previous reviews for reaching conclusions on what standards are 
appropriate. As such, the Administrator recognized that judgments of 
how to translate information available from epidemiological studies 
into a basis for appropriate standards must be considered in 
conjunction with the uncertainties in the epidemiological studies and 
in the context of the entire body of evidence before the Agency. This 
approach recognizes that the Administrator's judgment is particularly 
important for a pollutant where a population threshold cannot be 
clearly discerned with confidence from the evidence and where clinical 
evidence does not demonstrate health effects at typical ambient 
concentrations that meet the current standards. This approach is also 
consistent with the CAA requirement to set standards that are neither 
more nor less stringent than necessary, recognizing that a zero-risk 
standard is not required by the CAA.
    With respect to protection of at-risk populations, the EPA has 
carefully evaluated and considered evidence of effects in at-risk 
populations. Unlike some of the other NAAQS reviews where the 
epidemiological evidence may be less complete, this PM NAAQS review has 
the benefit of having an ISA that considered many epidemiological 
studies that assessed impacts for populations considered at-risk (e.g., 
populations of older adults, children, or those with preexisting 
conditions, like cardiovascular disease). In addition, some of the key 
epidemiology studies that the EPA assessed (included in Figure 3-7 of 
the PA) also specifically focused on and evaluated at-risk populations, 
including epidemiology studies that assessed morbidity and mortality 
associations for age-specific

[[Page 82714]]

populations (e.g., Medicare populations), as well as epidemiology 
studies that evaluated associations between PM2.5 exposure 
and specific health endpoints, like hospital admissions for 
cardiovascular effects in populations age 65 and older. The Agency 
takes note that it considered these studies to inform its review of the 
primary PM2.5 standards, which include at-risk populations, 
as well as other studies in the full body of scientific evidence in 
evaluating effects associated with long or short-term PM2.5 
exposures (i.e., premature mortality, cardiovascular effects, cancer, 
and respiratory effects).
    More specifically, in weighing the scientific evidence to inform 
his decision on requisite PM2.5 standards with an adequate 
margin of safety, including protection for at-risk populations, the 
Administrator's proposed conclusions recognized that epidemiological 
studies, many of which by design include at-risk populations, examine 
associations between distributions of PM2.5 air quality and 
health outcomes. Further, in noting that epidemiological studies do not 
identify particular PM2.5 exposures that cause effects, the 
PA focused on the reported mean concentrations from key epidemiological 
studies with the aim of providing a potential translation of 
information from epidemiological studies into the basis for 
consideration on standard levels (U.S. EPA, 2020, section 3.1.2). As 
discussed in more detail above, for the mean concentrations of the key 
epidemiological studies in the U.S. that use ground-based monitoring 
(i.e., those studies that can provide information most directly 
comparable to the current annual standard), the majority of studies 
have long-term mean (or median) concentrations above the current NAAQS 
(12.0 [micro]g/m\3\), with the mean of the study reported means or 
medians equal to 13.5 [micro]g/m\3\, a concentration level above the 
current level of the primary annual standard of 12 [micro]g/m\3\. The 
EPA notes that study reported mean (or median) concentration values are 
generally 10-20% lower than the design value of the highest monitor in 
an area, which determines compliance, and suggesting that that the 
current level of the standard provides even more protection than is 
suggested by the reported means.\52\ In the proposal, the Administrator 
recognized that important uncertainties and limitations do remain in 
the epidemiological evidence and the Administrator weighed these 
uncertainties, while also considering support provided by other lines 
of evidence, in judging whether the current standards are requisite 
with an adequate margin of safety. The Administrator further considered 
the emerging body of evidence from accountability studies examining 
past reductions in ambient PM2.5 and the degree to which 
those reductions have resulted in public health improvements. As 
discussed above, such studies have focused on time periods during which 
ambient PM2.5 concentrations were substantially higher than 
those measured more recently and therefore do not demonstrate public 
health improvements attributable to reduction in ambient 
PM2.5 at concentrations below the current standard.
---------------------------------------------------------------------------

    \52\ Analyses of recent air quality in U.S. CBSAs indicate that 
maximum annual PM2.5 design values for a given three-year 
period are often 10% to 20% higher than average monitored 
concentrations (i.e., averaged across multiple monitors in the same 
CBSA) (U.S. EPA, 2020, Appendix B, section B.7).
---------------------------------------------------------------------------

    Thus, the Administrator judged that the overall body of evidence 
indicates continued uncertainty in the degree to which adverse effects 
could result from PM2.5 exposures in areas meeting the 
current annual and 24-hour standards. Additionally, the current annual 
standard is below the lowest ``starting'' concentration in the 
available accountability studies (i.e., 13.2 [micro]g/m\3\) and below 
the reported mean concentration in the majority of the key U.S. 
epidemiological studies using ground-based monitoring data \53\ (i.e., 
mean of the reported means was 13.5 [micro]g/m\3\). In addition, 
concentrations in areas meeting the current 24-hour and annual 
standards remain well-below the PM2.5 exposure 
concentrations consistently shown to elicit effects in controlled human 
exposure studies. In specifically assessing his proposed decision, the 
Administrator noted that more stringent standards would be more than 
requisite to protect public health with an adequate margin of safety.
---------------------------------------------------------------------------

    \53\ As discussed above, the means from these studies are most 
relatable to the level of the annual standard. However, because the 
reported means in these studies are based on averaging the monitored 
concentration across an area, they are lower than the design value 
for that same area, since attainment of the standard is based on the 
measurements at the highest monitor (and not the average across 
multiple monitors.)
---------------------------------------------------------------------------

4. Administrator's Conclusions
    This section summarizes the Administrator's conclusions and final 
decisions related to the current primary PM2.5 standards and 
presents his decision to retain those standards, without revision. As 
described above (section I.D) and in section II.A.2 of the proposal (85 
FR 24105, April 30, 2020), the Administrator's approach to considering 
the adequacy of the current standards focuses on evaluating the public 
health protection afforded by the annual and 24-hour standards, taken 
together, against mortality and morbidity associated with long- or 
short-term PM2.5 exposures. This approach recognizes that 
changes in PM2.5 air quality designed to meet either the 
annual or the 24-hour standard would likely result in changes to both 
long-term average and short-term peak PM2.5 concentrations 
and that the protection provided by the suite of standards results from 
the combination of all of the elements of those standards (i.e., 
indicator, averaging time, form, level). Thus, the Administrator's 
consideration of the public health protection provided by the current 
primary PM2.5 standards is based on his consideration of the 
combination of the annual and 24-hour standards, including the 
indicators (PM2.5), averaging times, forms (arithmetic mean 
and 98th percentile, averaged over three years), and levels (12.0 
[mu]g/m\3\, 35 [mu]g/m\3\) of those standards.
    In establishing primary standards under the Act that are 
``requisite'' to protect public health with an adequate margin of 
safety, the Administrator is seeking to establish standards that are 
neither more nor less stringent than necessary for this purpose. He 
recognizes that the requirement to provide an adequate margin of safety 
was intended to address uncertainties associated with inconclusive 
scientific and technical information and to provide a reasonable degree 
of protection against hazards that research has not yet identified. 
However, the Act does not require that primary standards be set at a 
zero-risk level; rather, the NAAQS must be sufficiently protective, but 
not more stringent than necessary.
    Given these requirements, the Administrator's final decision in 
this review is a public health policy judgment drawing upon scientific 
and technical information examining the health effects of 
PM2.5 exposures, including how to consider the range and 
magnitude of uncertainties inherent in that information. This public 
health policy judgment is based on an interpretation of the scientific 
and technical information that neither overstates nor understates its 
strengths and limitations, nor the appropriate inferences to be drawn, 
and is informed by the Administrator's consideration of advice from the 
CASAC and public comments received on the proposal notice.
    As an initial matter, the Administrator recognizes that, with 
regard to effects classified as having evidence of a causal

[[Page 82715]]

or likely causal relationship with long or short-term PM2.5 
exposures (i.e., premature mortality, cardiovascular effects, cancer, 
and respiratory effects), the EPA considered the full range of studies 
evaluating these effects, including studies of at-risk populations, to 
inform its review of the primary PM2.5 standards. Thus, the 
Administrator notes that his judgment in this final decision reflects 
placing the greatest weight on evidence of effects for which the ISA 
determined there is a causal or likely causal relationship with long- 
and short-term PM2.5 exposures.
    With respect to the indicator, the Administrator recognizes that 
the scientific evidence in this review, as in the last review, 
continues to provide strong support for health effects following short- 
and long-term PM2.5 exposures. He notes the PA conclusion 
that the available information continues to support the 
PM2.5 mass-based indicator and remains too limited to 
support a distinct standard for any specific PM2.5 component 
or group of components, and too limited to support a distinct standard 
for the ultrafine fraction. Further, the Administrator notes that the 
EPA received very few comments on the indicator, with no commenters 
advocating for revising the current PM2.5 indicator for fine 
particles. Thus, as proposed, the Administrator concludes that it is 
appropriate to retain PM2.5 as the indicator for the primary 
standards for fine particulates.
    With respect to averaging time and form, the Administrator notes 
that the scientific evidence continues to provide strong support for 
health effects associations with both long-term (e.g., annual or multi-
year) and short-term (e.g., mostly 24-hour) exposures to 
PM2.5 and, consistent with the conclusions in the PA, judges 
that the current evidence does not support considering alternatives 
(U.S. EPA, 2020, section 3.5.2). The Administrator also notes that very 
few comments were received related to averaging time and form and none 
directly advocated for changing the form or averaging time. In the 
current review, epidemiological and controlled human exposure studies 
have examined a variety of PM2.5 exposure durations. 
Epidemiological studies continue to provide strong support for health 
effects associated with short-term PM2.5 exposures based on 
24-hour PM2.5 averaging periods, and the EPA notes that 
associations with sub-daily estimates are less consistent and, in some 
cases, smaller in magnitude (U.S. EPA, 2019, section 1.5.2.1; U.S. EPA, 
2020, section 3.5.2.2). In addition, controlled human exposure and 
panel-based studies of sub-daily exposures typically examine 
subclinical effects, as the commenters acknowledge, rather than the 
more serious population-level effects that have been reported to be 
associated with 24-hour exposures (e.g., mortality, hospitalizations). 
Taken together, the ISA concludes that epidemiological studies do not 
indicate that sub-daily averaging periods are more closely associated 
with health effects than the 24-hour average exposure metric (U.S. EPA, 
2019, section 1.5.2.1). Additionally, while recent controlled human 
exposure studies provide consistent evidence for cardiovascular effects 
following PM2.5 exposures for less than 24 hours (i.e., <30 
minutes to 5 hours), exposure concentrations in these studies are well-
above the ambient concentrations typically measured in locations 
meeting the current standards (U.S. EPA, 2020, section 3.2.3.1). Thus, 
these studies also do not suggest the need for additional protection 
against sub-daily PM2.5 exposures, beyond that provided by 
the current primary standards. Therefore, the Administrator's judgment 
is that the current 24-hour averaging time remains appropriate.
    In relation to the form of the 24-hour standard (98th percentile, 
averaged over three years), the Administrator notes that 
epidemiological studies continue to provide strong support for health 
effect associations with short-term (e.g., mostly 24-hour) 
PM2.5 exposures (U.S. EPA, 2020, section 3.5.2.3) and that 
controlled human exposure studies provide evidence for health effects 
following single short-term ``peak'' PM2.5 exposures. Thus, 
the evidence supports retaining a standard focused on providing 
supplemental protection against short-term peak exposures and supports 
a 98th percentile form for a 24-hour standard. The Administrator 
further notes that this form also provides an appropriate balance 
between limiting the occurrence of peak 24-hour PM2.5 
concentrations and identifying a stable target for risk management 
programs (U.S. EPA, 2020, section 3.5.2.3). As such, the Administrator 
concludes, as proposed, to retain the form and averaging time of the 
current 24-hour standard (98th percentile, averaged over three years) 
and annual standard (annual average, averaged over three years).
    The Administrator also proposed to retain the current levels of the 
24-hour standard (98th percentile, averaged over three years) at 35 
[micro]g/m\3\ and annual standard (annual average, averaged over 3 
years) at 12 [micro]g/m\3\. The majority of the comments received 
focused on this proposed decision to retain the current levels of both 
standards. In reaching his final decision regarding the level of the 
standards, the Administrator considered the large body of evidence 
presented and assessed in the ISA (U.S. EPA, 2019), the policy-relevant 
and risk-based conclusions and rationales as presented in the PA (U.S. 
EPA, 2020), views expressed by the CASAC, and public comments. In 
particular, in considering the ISA and PA, he considers key 
epidemiological studies that evaluate associations between 
PM2.5 air quality distributions and mortality and morbidity, 
including key ``accountability studies''; the availability of 
experimental studies to support biological plausibility; controlled 
human exposure studies examining effects following short-term 
PM2.5 exposures; air quality analyses; and the important 
uncertainties and limitations associated with this information.
    As an initial matter, the Administrator recognizes that the current 
annual standard is most effective in controlling PM2.5 
concentrations near the middle of the air quality distribution (i.e., 
around the mean of the distribution), but can also provide some control 
over short-term peak PM2.5 concentrations. On the other 
hand, the 24-hour standard, with its 98th percentile form, is most 
effective at limiting peak 24-hour PM2.5 concentrations, but 
in doing so will also have an effect on annual average PM2.5 
concentrations. Thus, while either standard could be viewed as 
providing some measure of protection against both average exposures and 
peak exposures, the 24-hour and annual standards are not expected to be 
equally effective at limiting both types of exposures. Thus, consistent 
with previous reviews, the Administrator's consideration of the public 
health protection provided by the current primary PM2.5 
standards is based on his consideration of the combination of the 
annual and 24-hour standards. Specifically, he recognizes that the 
annual standard is more likely to appropriately limit the ``typical'' 
daily and annual exposures that are most strongly associated with the 
health effects observed in epidemiological studies. The Administrator 
concludes that an annual standard (arithmetic mean, averaged over three 
years) remains appropriate for targeting protection against the annual 
and daily PM2.5 exposures around the middle portion of the 
PM2.5 air quality distribution. Further, recognizing that 
the 24-hour standard (with its 98th percentile form) is more directly 
tied to short-term peak PM2.5 concentrations, and thus more 
likely to appropriately limit exposures to such concentrations, the 
Administrator concludes that the

[[Page 82716]]

current 24-hour standard (98th percentile, averaged over three years) 
remains appropriate to provide a balance between limiting the 
occurrence of peak 24-hour PM2.5 concentrations and 
identifying a stable target for risk management programs. However, the 
Administrator recognizes that changes in PM2.5 air quality 
to meet an annual standard would likely result not only in lower short- 
and long-term PM2.5 concentrations near the middle of the 
air quality distribution, but also in fewer and lower short-term peak 
PM2.5 concentrations. The Administrator further recognizes 
that changes in air quality to meet a 24-hour standard, with a 98th 
percentile form, would result not only in fewer and lower peak 24-hour 
PM2.5 concentrations, but also in lower annual average 
PM2.5 concentrations.
    Thus, in considering the adequacy of the 24-hour standard, the 
Administrator notes the importance of considering whether additional 
protection is needed against short-term exposures to peak 
PM2.5 concentrations. In examining the scientific evidence, 
he notes that controlled human exposure studies provide evidence for 
health effects following single, short-term PM2.5 exposures 
to concentrations. These types of exposures correspond best to those to 
ambient exposures that might be experienced in the upper end of the 
PM2.5 air quality distribution in the U.S. (i.e., ``peak'' 
concentrations). However, most of these studies examine exposure 
concentrations considerably higher than are typically measured in areas 
meeting the current standards (U.S. EPA, 2020, section 3.2.3.1). In 
particular, controlled human exposure studies often report 
statistically significant effects on one or more indicators of 
cardiovascular function following 2-hour exposures to PM2.5 
concentrations at and above 120 [mu]g/m\3\ (at and above 149 [mu]g/m\3\ 
for vascular impairment, the effect shown to be most consistent across 
studies). To provide insight into what these studies may indicate 
regarding the primary PM2.5 standards, the PA (U.S. EPA, 
2020, p.3-49) notes that 2-hour ambient concentrations of 
PM2.5 at monitoring sites meeting the current standards 
almost never exceed 32 [mu]g/m\3\. In fact, even the extreme upper end 
of the distribution of 2-hour PM2.5 concentrations at sites 
meeting the current standards remains well-below the PM2.5 
exposure concentrations consistently shown in controlled human exposure 
studies to elicit effects (i.e., 99.9th percentile of 2-hour 
concentrations at these sites is 68 [mu]g/m\3\ during the warm season). 
Additionally, the Administrator notes the limited utility of the animal 
toxicologic studies in directly informing conclusions on the 
appropriate level of the standard given the uncertainty in 
extrapolating from effects in animals to those in human populations. 
Thus, the available experimental evidence does not indicate the need 
for additional protection against exposures to peak PM2.5 
concentrations, beyond the protection provided by the combination of 
the current 24-hour standard and the current annual standard (U.S. EPA, 
2020, section 3.2.3.1).
    With respect to the epidemiological evidence, the Administrator 
notes that the available epidemiological studies do not indicate that 
associations in those studies are strongly influenced by exposures to 
peak concentrations in the air quality distribution and thus do not 
indicate the need for additional protection against short-term 
exposures to peak PM2.5 concentrations (U.S. EPA 2020, 
section 3.5.1). Lastly, the Administrator notes CASAC consensus support 
for retaining the current 24-hour standard. Thus, the Administrator 
concludes that the 24-hour standard with its level of 35 [micro]g/m\3\ 
is adequate to provide supplemental protection (i.e., beyond that 
provided by the annual standard alone) against short-term exposures to 
peak PM2.5 concentrations.
    In reviewing the level of the annual standard, the Administrator 
recognizes that the annual standard, with its form based on the 
arithmetic mean concentration, is most appropriately meant to limit the 
``typical'' daily and annual exposures that are most strongly 
associated with the health effects observed in epidemiological studies. 
However, the Administrator also recognizes that while epidemiological 
studies examine associations between distributions of PM2.5 
air quality and health outcomes, they do not identify particular 
PM2.5 exposures that cause effects and thus, they cannot 
alone identify a specific level at which the standard should be set, as 
such a determination necessarily requires the Administrator's judgment. 
Thus, any approach that uses epidemiological information in reaching 
decisions on what standards are appropriate necessarily requires 
judgments about how to translate the information available from the 
epidemiological studies into a basis for appropriate standards. This 
includes consideration of how to weigh the uncertainties in the 
reported associations between daily or annual average PM2.5 
exposures and mortality or morbidity in the epidemiological studies. 
Such an approach is consistent with setting standards that are neither 
more nor less stringent than necessary, recognizing that a zero-risk 
standard is not required by the CAA.
    The Administrator recognizes that important uncertainties and 
limitations that were present in epidemiological studies in previous 
reviews, remain in the current review. As discussed above, these 
uncertainties include exposure measurement error; potential confounding 
by copollutants; increasing uncertainty of associations at lower 
PM2.5 concentrations; and heterogeneity of effects across 
different cities or regions. The Administrator also recognizes the 
advice given by the CASAC on this matter. As discussed above (section 
II.B.1), the CASAC members who support retaining the annual standard 
expressed their concerns with available PM2.5 
epidemiological studies. They assert that recent epidemiological 
studies do not provide a sufficient basis for revising the current 
standards. They also identify several key concerns regarding the 
associations reported in PM2.5 epidemiological studies and 
conclude that ``while the data on associations should certainly be 
carefully considered, this data should not be interpreted more strongly 
than warranted based on its methodological limitations'' (Cox, 2019a, 
p. 8 consensus responses).
    Taking into consideration the views expressed by these CASAC 
members, the Administrator recognizes that epidemiological studies 
examine associations between distributions of PM2.5 air 
quality and health outcomes, and they do not identify particular 
PM2.5 exposures that cause effects (U.S. EPA, 2020, section 
3.1.2). While the Administrator remains concerned about placing too 
much weight on epidemiological studies to inform conclusions on the 
adequacy of the current primary standards, he notes that several 
commenters advocated for using the epidemiological studies in a manner 
they characterized as similar to the last review, to determine the 
level of the annual standard. The previous PM NAAQS review completed in 
2012 noted that the evidence of an association in any epidemiological 
study is ``strongest at and around the long-term average where the data 
in the study are most concentrated'' (78 FR 3140, January 15, 2013). 
Accordingly, the Administrator notes the characterization of study 
reported short-term and long-term mean PM2.5 concentrations 
(section II.A.2.c.ii). As discussed in more detail above in section 
II.B.3 in responding to comments, when assessing the mean 
concentrations of the key short-term and

[[Page 82717]]

long-term epidemiological studies in the U.S. that use ground-based 
monitoring (i.e., those studies that can provide information most 
directly comparable to the current annual standard), the majority of 
studies (i.e., 19 out of 21) have mean concentrations at or above the 
level of the current annual standard (12.0 [micro]g/m\3\), with the 
mean of the study reported means or medians equal to 13.5 [micro]g/
m\3\, a concentration level above the current level of the primary 
annual standard of 12 [micro]g/m\3\.\54\ The Administrator further 
notes his caution in directly comparing the reported study mean values 
to the standard level given that, as discussed in more detail above, 
study-reported mean concentrations, by design, are generally lower than 
the design value of the highest monitor in an area, which determines 
compliance. In fact, analyses of recent air quality in U.S. CBSAs 
indicate that maximum annual PM2.5 design values for a given 
three-year period are often 10% to 20% higher than average monitored 
concentrations (i.e., averaged across multiple monitors in the same 
CBSA) (U.S. EPA, 2020, Appendix B, section B.7). He further notes his 
concern in placing too much weight on any one epidemiological study but 
instead feels that it is more appropriate to focus on the body of 
studies together and therefore takes note of the calculation of the 
mean of study-reported means (or medians). Thus, in summary, while the 
Administrator is cautious about placing too much weight on the 
epidemiological evidence on its own, he notes: (1) The reported mean 
concentration in the majority of the key U.S. epidemiological studies 
using ground-based monitoring data are above the level of the current 
annual standard; (2) the mean of the reported study means (or medians) 
(i.e., 13.5 [micro]g/m\3\) is above the level of the current standard; 
\55\ (3) air quality analyses show the study means to be lower than 
their corresponding design values by 10-20%; and (4) that these 
analyses must be considered in light of uncertainties inherent in the 
epidemiological evidence. When taken together, the Administrator judges 
that, even if he were to place greater weight on the epidemiological 
evidence, this information would not call into question the adequacy of 
the current standards.
---------------------------------------------------------------------------

    \54\ There were two studies, both included in the last review, 
for which the mean concentration (11.8 [micro]g/m\3\; Peng et al., 
2009) or median concentration (10.7 [micro]g/m\3\ (Central Region); 
Zeger et al., 2008) was somewhat below 12 [micro]g/m\3\.
    \55\ The median of the study reported mean (or median) 
PM2.5 concentrations is 13.3 [micro]g/m\3\, which is also 
above the level of the current standard.
---------------------------------------------------------------------------

    In addition to the evidence, the Administrator also considers the 
potential implications of the risk assessment. He notes that all risk 
assessments have limitations and that he remains concerned about the 
uncertainties in the underlying epidemiological data used in the risk 
assessment. The Administrator also notes that in previous reviews, 
these uncertainties and limitations have often resulted in less weight 
being placed on quantitative estimates of risk than on the underlying 
scientific evidence itself (e.g., 78 FR 3086, 3098-99, January 15, 
2013). These uncertainties and limitations have included uncertainty in 
the shapes of concentration-response functions, particularly at low 
concentrations; uncertainties in the methods used to adjust air 
quality; and uncertainty in estimating risks for populations, locations 
and air quality distributions different from those examined in the 
underlying epidemiological study (U.S. EPA, 2020, section 3.3.2.4). 
Additionally, the Administrator notes similar concern expressed by 
members of the CASAC who support retaining the current standards; they 
highlighted similar uncertainties and limitations in the risk 
assessment (Cox, 2019a). In light of all of this, the Administrator 
judges it appropriate to place little weight on quantitative estimates 
of PM2.5-associated mortality risk in reaching conclusions 
about the level of the primary PM2.5 standards.
    The Administrator additionally considers the emerging body of 
evidence from accountability studies examining past reductions in 
ambient PM2.5, and the degree to which those reductions have 
resulted in public health improvements. The Administrator agrees with 
public commenters who note that well-designed and conducted 
accountability studies can be informative. However, the Administrator 
also recognizes that interpreting such studies in the context of the 
current primary PM2.5 standards is complicated by the fact 
that some of the available studies have not evaluated PM2.5 
specifically (e.g., as opposed to PM10 or total suspended 
particulates), did not show changes in PM2.5 air quality, or 
have not been able to disentangle health impacts of the interventions 
from background trends in health (U.S. EPA, 2020, section 3.5.1). He 
further recognizes that the small number of available studies that do 
report public health improvements following past declines in ambient 
PM2.5 have not examined air quality meeting the current 
standards (U.S. EPA, 2020, Table 3-3). This includes recent U.S. 
studies that report increased life expectancy, decreased mortality, and 
decreased respiratory effects following past declines in ambient 
PM2.5 concentrations. Such studies have examined 
``starting'' annual average PM2.5 concentrations (i.e., 
prior to the reductions being evaluated) ranging from about 13.2 to >20 
mg/m\3\ (i.e., U.S. EPA, 2020, Table 3-3). Given the lack of available 
accountability studies reporting public health improvements 
attributable to reductions in ambient PM2.5 in locations 
meeting the current standards, together with his broader concerns 
regarding the lack of experimental studies examining PM2.5 
exposures typical of areas meeting the current standards (discussed 
above), the Administrator judges that there is considerable uncertainty 
in the potential for increased public health protection from further 
reductions in ambient PM2.5 concentrations beyond those 
achieved under the current primary PM2.5 standards.
    When the above considerations are taken together, the Administrator 
concludes that the scientific evidence that has become available since 
the last review of the PM NAAQS, together with the analyses in the PA 
based on that evidence and consideration of CASAC advice and public 
comments, does not call into question the adequacy of the public health 
protection provided by the current annual and 24-hour PM2.5 
standards. In particular, the Administrator judges that there is 
considerable uncertainty in the potential for additional public health 
improvements from reducing ambient PM2.5 concentrations 
below the concentrations achieved under the current primary standards 
and, therefore, that standards more stringent than the current 
standards (e.g., with lower levels) are not supported. That is, he 
judges that such standards would be more than requisite to protect the 
public health with an adequate margin of safety. This judgment reflects 
the Administrator's consideration of the uncertainties in the potential 
implications of the lower end of the air quality distributions from the 
epidemiological studies due in part to the lack of supporting evidence 
from experimental studies and retrospective accountability studies 
conducted at PM2.5 concentrations meeting the current 
standards.
    In reaching this conclusion, the Administrator notes that the 
current standards provide an adequate margin of safety. With respect to 
the annual standard, the level of 12 [micro]g/m\3\ is below the lowest 
``starting'' concentration (i.e.,

[[Page 82718]]

13.2 [micro]g/m\3\) in the available accountability studies that show 
public health improvements attributable to reductions in ambient 
PM2.5. In addition, while the Administrator places less 
weight on the epidemiological evidence for the purposes of selecting a 
standard, he notes that the current level of the annual standard is 
below the reported mean (and median) concentrations in the majority of 
the key U.S. epidemiological studies using ground-based monitoring data 
\56\ (noting that these means tend to be 10-20% lower than their 
corresponding area design values which is the more relevant metric when 
considering the level of the standard) and below the mean of the 
reported means (or medians) of these studies (i.e., 13.5 [micro]g/
m\3\). In addition, the Administrator recognizes that concentrations in 
areas meeting the current 24-hour and annual standards remain well-
below the PM2.5 exposure concentrations consistently shown 
to elicit effects in human exposure studies.
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    \56\ As discussed above, the means from these studies are most 
relatable to the level of the annual standard. However, because the 
reported means in these studies are based on averaging the monitored 
concentration across an area, they tend to be lower than the design 
value for that same area, since attainment of the standard is based 
on the measurements at the highest monitor (and not the average 
across multiple monitors.)
---------------------------------------------------------------------------

    In addition, based on the Administrator's review of the science, 
including controlled human exposure studies examining effects following 
short-term PM2.5 exposures, the epidemiological studies 
described above, and accountability studies conducted at levels just 
above the current standard, he judges that the degree of public health 
protection provided by the current standard is not greater than 
warranted. This judgment, together with the fact that no CASAC member 
expressed support for a less stringent standard, leads the 
Administrator to conclude that standards less stringent than the 
current standards (e.g., with higher levels) are also not supported.
    When the above information is taken together, the Administrator 
concludes that the available scientific evidence and technical 
information continue to support the current annual and 24-hour 
PM2.5 standards. This conclusion reflects the fact that 
important limitations in the evidence remain. The Administrator 
concludes that these limitations lead to considerable uncertainty 
regarding the potential public health implications of revising the 
existing suite of PM2.5 standards. Given this uncertainty, 
and the advice from some CASAC members, he concludes that the current 
suite of primary standards, including the current indicators 
(PM2.5), averaging times (annual and 24-hour), forms 
(arithmetic mean and 98th percentile, averaged over three years) and 
levels (12.0 [mu]g/m\3\, 35 [mu]g/m\3\), when taken together, remain 
requisite to protect the public health. Therefore, the Administrator 
reaches the final conclusion that the current suite of primary 
PM2.5 standards is requisite to protect public health from 
fine particles with an adequate margin of safety, including the health 
of at-risk populations, and is retaining the standards, without 
revision.

C. Decision on the Primary PM2.5 Standards

    For the reasons discussed above and taking into account information 
and assessments presented in the ISA and PA, the advice from the CASAC, 
and consideration of public comments, the Administrator concludes that 
the current annual and 24-hour primary PM2.5 standards are 
requisite to protect public health from fine particles with an adequate 
margin of safety, including the health of at-risk populations, and is 
retaining the current standards without revision.

III. Rationale for Decisions on the Primary PM10 Standard

    This section presents the rationale for the Administrator's 
decision to retain the existing primary PM10 standard. This 
decision is based on a thorough review of the latest scientific 
information, published through December 2017,\57\ and assessed in the 
ISA, on human health effects associated with PM10-2.5 in 
ambient air. This decision also accounts for considerations in the PA 
of the policy-relevant information, CASAC advice, and consideration of 
public comments received on the proposal.
---------------------------------------------------------------------------

    \57\ In addition to the review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), ``the current ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009 and March 31, 2017. A limited 
literature update identified some additional studies that were 
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p. 
A-3). References that are cited in the ISA, the references that were 
considered for inclusion but not cited, and electronic links to 
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    Section III.A provides background on the general approach for this 
review and the basis for the existing standard, and also presents a 
brief summary of key aspects of the currently available health effects 
information. Section III.B summarizes the CASAC advice and the 
Administrator's proposed decision to retain the existing primary 
PM10 standard, addresses public comments received on the 
proposal, and presents the Administrator's conclusions on the adequacy 
of the current standard, drawing on consideration of information in the 
ISA and the PA information, advice from the CASAC, and comments from 
the public. Section III.C summarizes the Administrator's decision on 
the primary PM10 standard.

A. Introduction

    As in prior reviews, the general approach to reviewing the current 
primary PM10 standard is based, most fundamentally, on using 
the EPA's assessment of the current scientific evidence to inform the 
Administrator's judgment regarding a primary PM10 standard 
that protects public health with an adequate margin of safety. In 
drawing conclusions with regard to the primary PM10 
standard, the final decision on the adequacy of the current standard is 
largely a public health policy judgment to be made by the 
Administrator. The Administrator's final decision draws upon the 
scientific information about health effects, as well as judgments about 
how to consider the range and magnitude of uncertainties that are 
inherent in the scientific evidence. The approach to informing these 
judgments, discussed more fully below, is based on the recognition that 
the available health effects evidence generally reflects a continuum, 
consisting of levels at which scientists generally agree that health 
effects are likely to occur, through lower levels at which the 
likelihood and magnitude of the response become increasingly uncertain. 
This approach is consistent with the requirements of the NAAQS 
provisions in the CAA and with how the EPA and the courts have 
interpreted the Act. These provisions require the Administrator to 
establish primary standards that, in his judgment, are requisite to 
protect public health with an adequate margin of safety. In so doing, 
the Administrator seeks to establish standards that are neither more 
nor less stringent for this purpose. The Act does not require that 
primary standards be set at a zero-risk level, but rather at a level 
that avoids unacceptable risks to public health including the health of 
sensitive groups. The four basic elements of the NAAQS (indicator, 
averaging time, form, and level) are considered collectively in 
evaluating the health protection afforded by a standard.
    In evaluating the appropriateness of retaining or revising the 
current primary

[[Page 82719]]

PM10 standard, the EPA has adopted an approach which is 
similar to that used in the last review and which reflects the body of 
evidence and information now available. As summarized in section 
III.A.1 below, the Administrator's decisions in the prior review were 
based on an integration of information on health effects associated 
with exposure to PM10-2.5, on the public health significance 
of key health effects, on policy judgments as to whether the standard 
is requisite to protect public health with an adequate margin of 
safety, and on consideration of the CASAC advice and public comments.
    Similarly, in this review, as described in the PA, the proposal, 
and elsewhere in this document, we draw on the current evidence 
pertaining to the public health risk of PM10-2.5 in ambient 
air. The past and current approaches are both based, most 
fundamentally, on the EPA's assessment of the current scientific and 
technical information. The EPA's assessments are primarily documented 
in the ISA and the PA, which have received CASAC review and public 
comment (83 FR 53471, October 23, 2018; 84 FR 47944, September 11, 
2019). To bridge the gap between the scientific assessment of the ISA 
and the judgments required of the Administrator in determining whether 
the current standard is requisite to protect public health with an 
adequate margin of safety, the PA evaluates the policy implications of 
the current evidence in the ISA.
    In considering the scientific and technical information, we 
consider both the information available at the time of the last review 
and information newly available since the last review, including most 
particularly that which has been critically analyzed and characterized 
in the current ISA. The evidence-based discussions presented below in 
section III.A.2 (and summarized more fully in the proposal) draw upon 
evidence from studies evaluating health effects related to exposures to 
PM10-2.5, as discussed in the ISA.
1. Background on the Current Standard
    The last review of the PM NAAQS was completed in 2012 (78 FR 3086, 
January 15, 2013). In that review, the EPA retained the existing 
primary 24-hour PM10 standard, with its level of 150 
[micro]g/m\3\ and its one-expected-exceedance form on average over 
three years, to continue to provide public health protection against 
exposures to PM10-2.5. In support of this decision, the 
prior Administrator emphasized her consideration of three issues: (1) 
The extent to which it was appropriate to retain a standard that 
provides some measure of protection against all PM10-2.5 
(regardless of composition or source of origin), (2) the extent to 
which a standard with a PM10 indicator can provide 
protection against exposures to PM10-2.5, and (3) the degree 
of public protection provided by the existing PM10 standard.
    First, the prior Administrator judged that the evidence provided 
``ample support for a standard that protects against exposures to all 
thoracic coarse particles, regardless of their location or source of 
origin'' (78 FR 3176, January 15, 2013). In support of this, she noted 
that the epidemiological studies had reported positive associations 
between PM10-2.5 and mortality or morbidity in a large 
number of cities across North America, Europe, and Asia, encompassing a 
variety of environments where PM10-2.5 sources and 
composition were expected to vary widely. Though most of the available 
studies examined associations in urban areas, the Administrator noted 
that some studies had also found associations between mortality and 
morbidity and relatively high ambient concentrations of particles of 
non-urban crustal origin. In the last review, in considering this body 
of evidence, and consistent with the CASAC's advice, the Administrator 
concluded that it was appropriate to maintain a standard that provides 
some measure of protection against exposures to all thoracic coarse 
particles, regardless of their composition, location, or source of 
origin (78 FR 3176, January 15, 2013).
    With regard to the appropriateness of retaining a PM10 
indicator for a standard meant to protect against exposures to 
PM10-2.5 in ambient air, the prior Administrator noted that 
PM10 mass included both coarse PM (PM10-2.5) and 
fine PM (PM2.5). As a result, the concentration of thoracic 
coarse particles (PM10-2.5) allowed by a PM10 
standard set at a single level declines as the concentration of 
PM2.5 increases. Because PM2.5 concentrations 
tend to be higher in urban areas than in rural areas, she observed that 
a PM10 standard would generally allow lower 
PM10-2.5 concentrations in urban areas than in rural areas. 
She judged it appropriate to maintain such a standard given that much 
of the evidence for PM10-2.5 toxicity, particularly at 
relatively low particle concentrations, came from study locations where 
thoracic coarse particles were of urban origin, and given that 
contaminants in urban areas would increase PM10-2.5 particle 
toxicity. Therefore, in the last review, the Administrator concluded 
that it remained appropriate to maintain a standard that requires lower 
concentrations of PM10-2.5 in ambient air in urban areas, 
where the strongest evidence was for associations between mortality and 
morbidity, and allows higher concentrations of PM10-2.5 in 
non-urban areas, where the evidence of public health concerns was less 
certain. The Administrator concluded that the varying concentrations of 
coarse particles that would be permitted in urban versus non-urban 
areas under the 24-hour PM10 standard, based the varying 
levels of PM2.5 present, appropriately reflected the 
differences in the strength of evidence regarding the health effects of 
coarse particles.
    With regard to evaluating the degree of public health protection 
provided by the current primary PM10 standard, with its 
level of 150 [micro]g/m\3\ and its one-expected-exceedance form on 
average over three years, the Administrator recognized that the 
available scientific evidence and air quality information was much more 
limited for PM10-2.5 than for PM2.5. In 
particular, the strongest evidence for PM10-2.5-related 
health effects was for cardiovascular effects, respiratory effects, and 
premature mortality following short-term exposures. For each of these 
categories of effects, the 2009 ISA concluded that the evidence was 
``suggestive of a causal relationship'' (U.S. EPA, 2009c, section 
2.3.3). The Administrator noted the significant uncertainties and 
limitations associated with the PM10-2.5 scientific evidence 
leading to these causal determinations and questioned whether 
additional public health improvements would be achieved by revising the 
existing primary PM10 standard. She specifically took note 
of several uncertainties and limitations, including the following:
     There were a limited number of epidemiological studies 
that employed copollutant models to address the potential for 
confounding, particularly by PM2.5, that would further the 
understanding of the extent to which PM10-2.5 itself, rather 
than copollutants, contributed to the reported health effects.
     The plausibility of the associations between 
PM10-2.5 and mortality and morbidity reported in 
epidemiological studies was uncertain given the limited number of 
experimental studies providing support for these associations.
     Limitations in PM10-2.5 monitoring data (i.e., 
limited data available from FRM/FEM sampling methods) and the different 
approaches used to estimate PM10-2.5 concentrations across 
epidemiological studies resulted in uncertainties in the ambient 
PM10-2.5 concentrations at which the reported

[[Page 82720]]

effects occur, increasing uncertainty in estimates of the extent to 
which changes in ambient PM10-2.5 concentrations would 
likely impact public health.
     While PM10-2.5 effect estimates reported for 
mortality and morbidity were generally positive, most were not 
statistically significant, even in single pollutant models. This 
included effect estimates reported in some study locations where the 
ambient PM10 concentrations were above those allowed by the 
current 24-hour PM10 standard.
     The composition of PM10-2.5, and the effects 
associated with specific components, were also key uncertainties in the 
evidence. With a lack of information on the chemical speciation of 
PM10-2.5, the apparent variability in associations across 
study locations was difficult to characterize.
    In considering these uncertainties and limitations, the prior 
Administrator particularly took note of degree of uncertainty 
associated with the extent to which health effects reported in the 
epidemiological studies are due to PM10-2.5 itself, as 
opposed to one or more copollutants, especially PM2.5. This 
uncertainty reflects the relatively small number of studies available 
for PM10-2.5 in ambient air that had evaluated copollutant 
models, and the very limited evidence from controlled human exposure 
studies supporting the plausibility of adverse health effects 
attributable to PM10-2.5 at ambient concentrations.
    When considering the available evidence overall, the prior 
Administrator concluded that the degree of public health protection 
provided by the current PM10 standard against exposures to 
PM10-2.5 should be maintained (i.e., neither increased nor 
decreased). Her judgment that a more stringent standard to provide 
additional protection was not necessary was supported by her 
consideration of the uncertainties in the overall body of evidence. Her 
judgment that a less stringent standard was not needed and that the 
degree of public health protection provided by the current standard was 
not greater than warranted was supported by the positive and 
statistically significant associations with mortality observed in some 
single-city study locations that were likely to have violated the 
current PM10 standard. Therefore, the prior Administrator 
concluded that the existing 24-hour standard, with its one-expected 
exceedance form on average over three years and a level of 150 
[micro]g/m\3\, was requisite to protect public health with an adequate 
margin of safety against effects that have been associated with 
PM10-2.5. In light of this conclusion, the EPA retained the 
existing primary PM10 standard.
2. Overview of Health Effects Evidence
    In this section, we provide an overview of the policy-relevant 
aspects of the PM10-2.5-related health effects evidence 
available for consideration in this review. Section III.B of the 
proposal provides a detailed summary of key information contained in 
the ISA and the PA on the health effects associated with 
PM10-2.5 exposures, and the related public health 
implications. As described in the proposal, the ISA does not identify 
any PM10-2.5-related health outcomes for which the evidence 
supports either a ``causal'' or ``likely to be causal relationship'' 
(85 FR 24122, April 30, 2020). Therefore, for PM10-2.5, we 
consider the evidence determined to be ``suggestive of, but not 
sufficient to infer, a causal relationship,'' recognizing the greater 
uncertainty in such evidence.\58\
---------------------------------------------------------------------------

    \58\ As noted in the Preamble to the ISA, ``suggestive'' 
evidence is ``limited, and chance, confounding, and other biases 
cannot be ruled out'' (U.S. EPA, 2015, Table II).
---------------------------------------------------------------------------

    While studies conducted since the time of the last review have 
strengthened support for relationships between PM10-2.5 
exposures and some key health outcomes, several key uncertainties from 
the last review have, to date, ``still not been addressed'' (U.S. EPA, 
2019, section 1.4.2, p. 1-41). For example, in the last review, 
epidemiological studies relied on a number of methods to estimate 
PM10-2.5 exposures, but the methods had not been 
systematically compared to evaluate spatial and temporal correlations 
in exposure estimates. Methods employed by these studies included: (1) 
Calculating the difference between PM10 and PM2.5 
at co-located monitors, (2) calculating the difference between county-
wide averages of monitored PM10 and PM2.5 based 
on monitors that are not necessarily co-located, and (3) direct 
measurement of PM10-2.5 using a dichotomous sampler (U.S. 
EPA, 2019, section 1.4.2). More recent epidemiological studies, 
available since the last review, continue to use these approaches to 
estimate PM10-2.5 concentrations. Some recent studies 
estimate long-term PM10-2.5 exposures as the difference 
between PM10 and PM2.5 concentrations based on 
information from spatiotemporal or land use regression (LUR) models, in 
addition to monitors. As in the last review, the methods used to 
estimate PM10-2.5 concentrations have not been 
systematically evaluated (U.S. EPA, 2019, section 3.3.1.1), 
contributing to the uncertainty regarding spatial and temporal 
correlations in PM10-2.5 concentrations across methods and 
in PM10-2.5 exposure estimates used in epidemiological 
studies (U.S. EPA, 2019, sections 2.5.1.2.3 and 2.5.2.2.3). Given the 
greater spatial and temporal variability of PM10-2.5 and 
fewer PM10-2.5 monitoring sites compared to 
PM2.5, this uncertainty is particularly important for the 
coarse size fraction.
    In addition to the uncertainty associated with PM10-2.5 
exposure estimates in the epidemiological studies, information in the 
current review remains limited with regard to the potential for 
confounding by copollutants and provides limited support for the 
biological plausibility of serious effects following 
PM10-2.5 exposures; both of these limitations continue to 
contribute broadly to uncertainty in the PM10-2.5 health 
evidence. Uncertainty related to potential confounding is related to 
the relatively few epidemiological studies that have evaluated 
PM10-2.5 health effect associations in copollutant models 
with both gaseous pollutants and other PM size fractions. Uncertainty 
related to the biological plausibility of serious effects caused by 
PM10-2.5 exposures results from the limited number of 
controlled human exposure and animal toxicology \59\ studies that have 
evaluated the health effects of experimental PM10-2.5 
inhalation exposures. The evidence supporting the ISA's ``suggestive'' 
causality determinations for PM10-2.5 and health effects, 
including the uncertainties in the evidence, are summarized in the 
sections below.
---------------------------------------------------------------------------

    \59\ Compared to humans, smaller fractions of inhaled 
PM10-2.5 penetrate into the thoracic regions of rats and 
mice (U.S. EPA, 2019, section 4.1.6), contributing to the relatively 
limited evaluation PM10-2.5 exposures in animal studies.
---------------------------------------------------------------------------

a. Nature of Effects
i. Mortality
    With regard to long-term PM10-2.5 exposure and 
mortality, very few studies were available at the time of the last 
review. As such, the 2009 ISA concluded that the evidence was 
``inadequate to determine if a causal relationship exists'' (U.S. EPA, 
2009c). Since the time of the last review, there is limited new 
evidence and many of the limitations noted in the 2012 review persist. 
In the current review, some recent cohort studies conducted in the U.S. 
and Europe reported positive associations between long-term 
PM10-2.5 exposure and total (nonaccidental) mortality, 
though results are

[[Page 82721]]

inconsistent across studies (U.S. EPA, 2019, Table 11-11). The 
examination of copollutant models in these studies remains limited, and 
when copollutants are included, PM10-2.5 effect estimates 
are often attenuated after adjusting for PM2.5 (U.S. EPA, 
2019, Table 11-11). These studies employed a number of approaches for 
estimating PM10-2.5 exposures, including direct measurements 
from dichotomous samplers, calculating the difference between 
PM10 and PM2.5 measured at co-located monitors, 
and calculating the difference of area-wide PM10 and 
PM2.5 concentrations. As discussed above as a limitation in 
the last review, temporal and spatial correlations between these 
approaches still have not been evaluated, contributing to uncertainty 
regarding the potential for exposure measurement error (U.S. EPA, 2019, 
section 3.3.1.1, Table 11-11). The 2019 ISA concludes that this 
uncertainty ``reduces the confidence in the associations observed 
across studies'' (U.S. EPA, 2020, p. 11-125) and that the evidence for 
long-term PM10-2.5 exposures and cardiovascular effects, 
respiratory morbidity, and metabolic disease provide limited biological 
plausibility for PM10-2.5-related mortality (U.S. EPA, 2019, 
sections 11.4.1 and 11.4). Taken together, the 2019 ISA concludes that 
``this body of evidence is suggestive, but not sufficient to infer, 
that a causal relationship exists between long-term PM10-2.5 
exposure and total mortality'' (U.S. EPA, 2019, p. 11-125).
    With regard to short-term PM10-2.5 exposures and 
mortality, the 2009 ISA concluded that the evidence is ``suggestive of 
a causal relationship between short-term exposure to 
PM10-2.5 and mortality'' (U.S. EPA, 2009c). Since the last 
review, multicity epidemiological studies conducted primarily in Europe 
and Asia continue to provide consistent evidence of positive 
associations between short-term PM10-2.5 exposure and total 
(nonaccidental) mortality (U.S. EPA, 2019, Table 11-9). These studies 
contribute to increasing confidence in the relationship between the 
short-term PM10-2.5 exposures and mortality, however, the 
use of varying approaches to estimate PM10-2.5 exposures 
continue to contribute uncertainty to the associations observed. 
Additionally, the 2019 ISA notes than an analysis by Adar et al. (2014) 
indicates ``possible evidence of publications bias, which was not 
observed for PM2.5'' (U.S. EPA, 2019, section 11.3.2, p. 11-
106). Studies newly available in this review expand the assessment of 
potential copollutant confounding of the short-term 
PM10-2.5-mortality relationship and provide evidence that 
PM10-2.5 associations generally remain positive in 
copollutant models, although associations are attenuated in some 
instances (U.S. EPA, 2019, section 11.3.4.1, Figure 11-28, Table 11-
10). The 2019 ISA concludes that, overall, the assessment of potential 
copollutant confounding is limited by a lack of information on the 
correlation between PM10-2.5 and gaseous pollutants and the 
small number of locations where copollutant analyses have been 
conducted. Associations with cause-specific mortality provide some 
support for associations with total (nonaccidental) mortality, though 
associations with cause-specific mortality, particularly respiratory 
mortality, are more uncertain (i.e., wider confidence intervals) and 
less consistent (U.S. EPA, 2019, section 11.3.7). As discussed further 
below, the ISA concludes that evidence for PM10-2.5-related 
cardiovascular and respiratory effects provides only limited support 
for the biological plausibility of a relationship between short-term 
PM10-2.5 exposure and cause-specific mortality (U.S. EPA, 
2019, section 11.3.7). Based on the overall evidence, the 2019 ISA 
concludes that ``this body of evidence is suggestive, but not 
sufficient to infer, that a causal relationship exists between short-
term PM10-2.5 exposure and total mortality'' (U.S. EPA, 
2019, p. 11-120).
ii. Cardiovascular Effects
    With regard to long-term exposures, the evidence available in the 
last review describing the relationship between long-term exposure to 
PM10-2.5 and cardiovascular effects was characterized in the 
2009 ISA as ``inadequate to infer the presence or absence of a causal 
relationship.'' The limited number of epidemiological studies available 
at that time reported contradictory results and experimental evidence 
demonstrating an effect of PM10-2.5 on the cardiovascular 
system was lacking (U.S. EPA, 2019, section 6.4).
    The evidence of long-term PM10-2.5 exposures and 
cardiovascular mortality remains limited, with no consistent pattern of 
associations across studies, and as discussed above, uncertainty from 
the use of various approaches for estimating PM10-2.5 
concentrations (U.S. EPA, 2019, Table 6-70). The evidence for 
associations between PM10-2.5 and cardiovascular morbidity 
has grown and, while results across studies are not entirely 
consistent, some epidemiological studies report positive associations 
with IHD and myocardial infarction (MI) (U.S. EPA, 2019, Figure 6-34); 
stroke (U.S. EPA, 2019, Figure 6-35); atherosclerosis (U.S. EPA, 2019, 
section 6.4.5); venous thromboembolism (VTE) (U.S. EPA, 2019, section 
6.4.7); and blood pressure and hypertension (U.S. EPA, 2019, section 
6.4.6). With respect to copollutant confounding, the effect estimates 
for PM10-2.5-cardiovascular mortality are often attenuated, 
but remain positive, in copollutant models adjusted for 
PM2.5. For cardiovascular morbidity outcomes, associations 
are inconsistent in copollutant models that adjust for 
PM2.5, NO2, and chronic noise pollution (U.S. 
EPA, 2019, p. 6-276). The 2019 ISA concluded that ``evidence from 
experimental animal studies is of insufficient quantity to establish 
biological plausibility'' (U.S. EPA, 2019, p. 6-277). Despite this 
substantial data gap in the toxicologic evidence for long-term 
PM10-2.5 exposures and based largely on the observation of 
positive associations in some high-quality epidemiological studies, the 
ISA concludes that ``evidence is suggestive of, but not sufficient to 
infer, a causal relationship between long-term PM10-2.5 
exposure and cardiovascular effects'' (U.S. EPA, 2019, p. 6-277).
    With regard to short-term PM10-2.5 exposures and 
cardiovascular effects, the 2009 ISA found the available evidence was 
``suggestive of a causal relationship,'' based primarily on several 
epidemiological studies reporting associations between short-term 
PM10-2.5 exposure and cardiovascular effects, including IHD 
hospitalizations, supraventricular ectopy, and changes in heart rate 
variability (HRV). In addition, studies found increases in 
cardiovascular disease emergency department visits and hospital 
admissions linked to dust storm events resulting in high concentrations 
of crustal material. However, the 2009 ISA noted the potential for 
exposure measurement error and copollutant confounding in these 
studies. Moreover, there was only limited evidence of cardiovascular 
effects from a small number of controlled human exposure and animal 
toxicologic studies that examined PM10-2.5 exposures (U.S. 
EPA, 2009c, section 6.2.12.2). Therefore, the potential for exposure 
measurement error and copollutant confounding, along with the limited 
evidence of biological plausibility for cardiovascular effects 
following inhalation exposure, contributed uncertainty to the 
scientific evidence available at the time of the last review (U.S. EPA, 
2009c, section 6.3.13).
    The evidence related to short-term PM10-2.5 exposure and 
cardiovascular

[[Page 82722]]

effects has somewhat expanded since the last review, but a number of 
important uncertainties persist. The 2019 ISA notes that there are a 
small number of epidemiological studies reporting positive associations 
between short-term PM10-2.5 exposures and cardiovascular 
morbidity. There continues to be limited evidence, however, to suggest 
that these associations are biologically plausible, or independent of 
copollutant confounding. Additionally, the ISA concludes that it 
remains unclear how the approaches used to estimate PM10-2.5 
concentrations in epidemiological studies may impact exposure 
measurement error. The 2019 ISA concludes that overall ``the evidence 
is suggestive of, but not sufficient to infer, a causal relationship 
between short-term PM10-2.5 exposures and cardiovascular 
effects'' (U.S. EPA, 2019, p. 6-254).
iii. Respiratory Effects
    With regard to short-term PM10-2.5 exposures and 
respiratory effects, the 2009 ISA concluded that, based on a small 
number of epidemiological studies observing some respiratory effects 
and limited evidence to support biological plausibility, the 
relationship is ``suggestive of a causal relationship.'' 
Epidemiological findings were consistent for respiratory infection and 
combined respiratory-related diseases, but not for COPD. Studies were 
characterized by overall uncertainty in the exposure assignment 
approach and limited information regarding potential copollutant 
confounding. Controlled human exposure studies of short-term 
PM10-2.5 exposures found no lung function decrements and 
inconsistent evidence of pulmonary inflammation. Animal toxicologic 
studies were limited to those that used non-inhalation (e.g., intra-
tracheal instillation) routes of PM10-2.5 exposure.
    Recently available epidemiological studies link short-term 
PM10-2.5 exposure with asthma exacerbation and respiratory 
mortality. Some associations remained positive in copollutant models 
including PM2.5 or gaseous pollutants, although associations 
were attenuated in some studies of mortality. Limited evidence is 
available that observes positive associations with other respiratory 
outcomes, including COPD exacerbation, respiratory infection, and 
combined respiratory-related diseases (U.S. EPA, 2019, Table 5-36). The 
lack of systematic evaluation of the various methods used to estimate 
PM10-2.5 concentrations and the resulting spatial and 
temporal variability in PM10-2.5 concentrations compared to 
PM2.5 continues to be an uncertainty in this evidence (U.S. 
EPA, 2019, sections 2.5.1.2.3 and 3.3.1.1). Based on the overall 
evidence, the 2019 ISA concludes that the ``evidence is suggestive of, 
but not sufficient to infer, a causal relationship between short-term 
PM10-2.5 exposure and respiratory effects'' (U.S. EPA, 2019, 
p. 5-270).
iv. Cancer
    In the last review, little information was available from studies 
of cancer following inhalation exposures to PM10-2.5. Thus, 
the 2009 ISA concluded that the evidence was ``inadequate to assess the 
relationship between long-term PM10-2.5 exposures and 
cancer'' (U.S. EPA, 2009c). Since the last review, the available 
studies of long-term PM10-2.5 exposure and cancer remain 
limited, with a few recent epidemiological studies that report 
positive, but imprecise, associations with lung cancer incidence. 
Uncertainty remains in these studies due to exposure measurement error 
from the use of PM10-2.5 predictions that have not been 
validated by monitored PM10-2.5 concentrations (U.S. EPA, 
2019, sections 3.3.2.3 and 10.3.4). Very few experimental studies of 
PM10-2.5 exposures have been conducted, although the 
available studies indicate that PM10-2.5 exhibits 
genotoxicity and oxidative stress, two key characteristics of 
carcinogens. While limited, these studies provide some evidence of 
biological plausibility for the findings in a small number of 
epidemiological studies (U.S. EPA, 2019, section 10.3.4). Taken 
together, the small number of available epidemiological and 
experimental studies, along with uncertainty related to exposure 
measurement error, contribute to the 2019 ISA conclusion that ``the 
evidence is suggestive of, but not sufficient to infer, a causal 
relationship between long-term PM10-2.5 exposure and 
cancer'' (U.S. EPA, 2019, p. 10-87).
v. Metabolic Effects
    The 2009 ISA did not make a causality determination for 
PM10-2.5-related metabolic effects. Since the last review, 
one epidemiological study shows an association between long-term 
PM10-2.5 exposure and incident diabetes, while additional 
cross-sectional studies report associations with effects on glucose or 
insulin homeostasis (U.S. EPA, 2019, section 7.4). Uncertainties with 
this evidence include the potential for copollutant confounding and 
exposure measurement error (U.S. EPA, 2019, Tables 7-14 7-15). There is 
limited evidence to support biological plausibility of metabolic 
effects, although a cross-sectional study that investigated biomarkers 
of insulin resistance and systemic and peripheral inflammation may 
support a pathway leading to type 2 diabetes (U.S. EPA, 2019, sections 
7.4.1 and 7.4.3). Based on the somewhat expanded evidence available in 
this review, the 2019 ISA concludes that ``the evidence is suggestive 
of, but not sufficient to infer, a causal relationship between [long]-
term PM10-2.5 exposures and metabolic effects'' (U.S. EPA, 
2019, p. 7-56).
vi. Nervous System Effects
    The 2009 ISA did not make a causal determination for 
PM10-2.5 exposures and nervous system effects. Newly 
available evidence since that time includes epidemiological studies 
that report associations between long-term PM10-2.5 
exposures and impaired cognition and anxiety in adults in longitudinal 
analyses (U.S. EPA, 2019, Table 8-25, section 8.4.5). Associations of 
long-term PM10-2.5 exposure with neurodevelopmental effects 
are not consistently reported in children (U.S. EPA, 2019, section 
8.4.4 and 8.4.5). Uncertainties in these studies include the potential 
for copollutant confounding, given that no studies examined copollutant 
models (U.S. EPA, 2019, section 8.4.5), and exposure measurement error 
based on the various methods used across studies to estimate 
PM10-2.5 concentrations (U.S. EPA, 2019, Table 8-25). 
Additionally, there is very limited animal toxicologic evidence to 
provide support for biological plausibility of nervous system effects 
(U.S. EPA, 2019, sections 8.4.1 and 8.4.5). Considering the available 
studies and associated limitations, the 2019 ISA concludes that ``the 
evidence is suggestive of, but not sufficient to infer, a causal 
relationship between long-term PM10-2.5 exposure and nervous 
system effects'' (U.S. EPA, 2019, p. 8-75).

B. Conclusions on the Primary PM10 Standard

    In drawing conclusions on the adequacy of the current primary 
PM10 standard, in view of the advances in scientific 
knowledge and additional information now available, the Administrator 
has considered the evidence base, information, and policy judgments 
that were the foundation of the last review and reflects upon the body 
of evidence and information newly available in this review. In so 
doing, the Administrator has taken into account the evidence-based 
considerations, as well as advice from the CASAC and public comments. 
Evidence-based considerations draw upon the EPA's assessment and 
integrated synthesis of the scientific evidence from animal

[[Page 82723]]

toxicologic, controlled human exposure studies, and epidemiological 
studies evaluating health effects related to exposures to 
PM10-2.5 as presented in the ISA and discussed in section 
III.A.2. In addition to the evidence, the Administrator has weighed a 
range of policy-relevant considerations as discussed in the PA and 
summarized in sections III.B and III.C of the proposal and summarized 
in section III.B.2 below. These considerations, along with the advice 
from the CASAC (section III.B.1) and public comments (section III.B.3), 
are discussed below. A more detailed summary of all significant 
comments, along with the EPA's responses (henceforth ``Response to 
Comments''), can be found in the docket for this rulemaking (Docket No. 
EPA-HQ-OAR-2015-0072). This document is available for review in the 
docket for this rulemaking and through the EPA's NAAQS website (https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards). The 
Administrator's conclusions in this review regarding the adequacy of 
the current primary PM10 standard and whether any revisions 
are appropriate are described in section III.B.4.
1. CASAC Advice in This Review
    As a part of the review of the draft PA, the CASAC has provided 
advice on the adequacy of the public health protection afforded by the 
current primary PM10 standard. As for PM2.5 
(section II.B.1 above), the CASAC's advice is documented in a letter 
sent to the EPA Administrator (Cox, 2019a).
    In its comments on the draft PA, the CASAC concurs with the draft 
PA's overall preliminary conclusions that it is appropriate to consider 
retaining the current primary PM10 standard without 
revision. The CASAC agrees with the draft PA ``that key uncertainties 
identified in the last review remain'' (Cox, 2019a, p. 13 of consensus 
responses) and that ``none of the identified health outcomes linked to 
PM10-2.5'' were judged to be causal or likely causal. (Cox, 
2019a, p. 12 of consensus responses). To reduce these uncertainties in 
future reviews, the CASAC recommends improvements to 
PM10-2.5 exposure assessment, including a more extensive 
network for direct monitoring of the PM10-2.5 fraction (Cox, 
2019a, p. 13 of consensus responses). The CASAC also recommends 
additional controlled human exposure and animal toxicology studies of 
the PM10-2.5 fraction to improve the understanding of 
biological causal mechanisms and pathway (Cox, 2019a, p. 13 of 
consensus responses). Overall, the CASAC agrees with the EPA that ``. . 
. the available evidence does not call into question the adequacy of 
the public health protection afforded by the current primary 
PM10 standard and that evidence supports consideration of 
retaining the current standard in this review'' (Cox, 2019a, p. 3 of 
letter).
2. Basis for the Proposed Decision
    At the time of the proposal, the Administrator carefully considered 
the assessment of the current evidence and conclusions reached in the 
ISA, considerations and staff conclusions and associated rationales 
presented in the PA, and the advice and recommendations of the CASAC 
(85 FR 24125, April 30, 2020). In reaching his proposed decision on the 
primary PM10 standard, the Administrator first noted the 
decision to retain the primary PM10 standard in the last 
review recognized that epidemiological studies had reported positive 
associations between PM10-2.5 and mortality and morbidity in 
cities across North America, Europe, and Asia. The studies encompassed 
a variety of environments where PM10-2.5 sources and 
composition were expected to vary widely. Although many of the studies 
examined associations between PM10-2.5 and health effects in 
urban areas, some of the studies also linked mortality and morbidity 
with relatively high ambient concentrations of particles of non-urban 
crustal origin. Drawing on this information, the EPA judged that it was 
appropriate to maintain a standard that provides some measure of 
protection against exposures to PM10-2.5, regardless of 
location, source of origin, or particle composition (78 FR 3176, 
January 15, 2013).
    The Administrator noted that the evidence for several 
PM10-2.5-related health effects, particularly for long-term 
exposures, has expanded since the time of the last review. Recently 
available epidemiological studies conducted in North America, Europe, 
and Asia continue to report positive associations with mortality and 
morbidity in cities where PM10-2.5 sources and composition 
are expected to vary widely, but uncertainties remain with respect to 
the methods used to assign exposure in the studies. While the 
Administrator recognized that important uncertainties persist in the 
scientific evidence, as described below and in section III.A.2 above, 
he also recognized that PM10-2.5 exposures may be associated 
with a broader range of health effects that have been linked with 
PM10-2.5 exposures. These studies provide an important part 
of the body of evidence supporting the ISA's revised causality 
determinations, including new determinations, for long-term 
PM10-2.5 exposures and mortality, cardiovascular effects, 
metabolic effects, nervous system effects, and cancer (U.S. EPA, 2019; 
U.S. EPA, 2020, section 4.2). Drawing on this information, the 
Administrator proposed to conclude that the scientific studies 
available since the last review continue to support a primary 
PM10 standard that provides some measure of public health 
protection against PM10-2.5 exposures, regardless of 
location, source of origin, or particle composition.
    With regard to the uncertainties in the scientific evidence, the 
Administrator noted that the decision in the last review highlighted 
limitations in the estimates of ambient PM10-2.5 
concentrations used in epidemiological studies, the limited evaluation 
of copollutant models to address potential confounding, and the limited 
number of experimental studies to support biologically plausible 
pathways for PM10-2.5-related health effects. These and 
other limitations raised questions as to whether additional public 
health improvements would be achieved by revising the existing 
PM10 standard.
    Despite some additional new evidence available in this review, the 
Administrator recognized that, similar to the last review, 
uncertainties remain in the scientific evidence for 
PM10-2.5-related health effects. As summarized above 
(section III.A.2), these include uncertainties in the 
PM10-2.5 exposure estimates used in epidemiological studies, 
in the independence of PM10-2.5 health effect associations, 
and in support for the biologic plausibility of PM10-2.5-
related effects from controlled human exposure and animal toxicologic 
studies (U.S. EPA, 2020, section 4.2). These uncertainties contributed 
to the conclusions in the 2019 ISA that the evidence for key 
PM10-2.5 health effects is ``suggestive of, but not 
sufficient to infer'' causal relationships (U.S. EPA, 2019). In light 
of his emphasis on evidence supporting ``causal'' or ``likely to be 
causal'' relationships in the current review, the Administrator judged 
that the evidence of health effects associated with PM10-2.5 
in ambient air provides an uncertain scientific foundation for making 
decisions for standard setting. As such, he further judged that, 
consistent with the last review, limitations in the evidence raise 
questions as to whether additional public health protections would be 
achieved by revising the existing PM10 standard.
    In reaching his proposed conclusions on the primary PM10 
standard, the Administrator additionally considered the advice and 
recommendations from the CASAC. As described above (section III.B.1), 
the CASAC recognized the

[[Page 82724]]

uncertainties in the evidence for PM10-2.5-related health 
effects, stating that ``key uncertainties identified in the last review 
remain'' (Cox, 2019a, p. 13 of consensus responses). Given these 
uncertainties, the CASAC agreed with the PA conclusion that the 
evidence available in this review ``does not call into question the 
adequacy of the public health protection afforded by the current 
primary PM10 standard'' (Cox, 2019a, p. 3 of letter). The 
CASAC further recommended that this evidence ``supports consideration 
of retaining the current standard in this review'' (Cox, 2019a, p. 3 of 
letter).
    In considering the information above, the Administrator proposed to 
conclude that the available scientific evidence continues to support a 
PM10 standard to provide some measure of protection against 
PM10-2.5 exposures. This conclusion reflected the expanded 
evidence available in this review for health effects from 
PM10-2.5 exposures. However, important uncertainties and 
limitations in the evidence remain. Consistent with the decision in the 
last review, the Administrator proposed to conclude that these 
limitations contribute to considerable uncertainty regarding the 
potential public health implications of revising the existing 
PM10 standard. Given this uncertainty, and consistent with 
the advice from the CASAC, the Administrator proposed to conclude that 
the available evidence does not call into question the adequacy of the 
public health protection afforded by the current primary 
PM10 standard. Therefore, he proposed to retain the primary 
PM10 standard, without revision.
3. Comments on the Proposed Decision
    Of the public comments received on the proposal, very few 
commenters provided comments on the primary PM10 standard. 
Of those commenters who did provide comments on the primary 
PM10 standard, the majority supported the Administrator's 
proposed decision to retain the current primary PM10 
standard, without revision. This group includes primarily industries 
and industry groups. All of these commenters generally note their 
agreements with the rationale provided in the proposal and the CASAC 
concurrence with the PA conclusion that the current evidence does not 
support revision to the standard. Most also cite the EPA and CASAC 
statements that the newly available information in this review does not 
call into question the adequacy of the current standard. The EPA agrees 
with these comments and with the CASAC advice regarding the adequacy of 
the current primary standard and the lack of support for revision of 
the standard.
    Some commenters disagreed with the Administrator's proposed 
conclusion to retain the current primary PM10 standard, 
primarily focusing their comments on the need for revisions to the form 
of the standard or the level of the standard. With regard to comments 
on the form of the standard, some commenters assert that the EPA should 
revise the standard by adopting a separate form (or a ``compliance 
threshold'' in their words)--the 99th percentile, averaged over three 
years--for the primary PM10 standard for continuous 
monitors, which provide data every day, while maintaining the current 
form of the standard (one exceedance, averaged over three years) for 1-
in-6 samplers, given the widespread use of continuous monitoring and to 
ease the burden of demonstrating exceptional events. These commenters, 
in support of their comment, contend that the 99th percentile would 
effectively change the form from the 2nd high to the 4th high and would 
allow no more than three exceedances per year, averaged over three 
years. These commenters additionally highlight the EPA's decision in 
the 1997 review to adopt a 99th percentile form, averaged over three 
years, citing to advantages of a percentile-based form in the 
Administrator's rationale in that review. The comments further assert 
that a 99th percentile form for the primary PM10 standard is 
still more conservative than the form for other short-term NAAQS (e.g., 
PM2.5 and NO2).
    First, the EPA has long recognized that the form is an integral 
part of the NAAQS and must be selected together with the other elements 
of the NAAQS to ensure the appropriate stringency and requisite degree 
of public health protection. Thus, if the EPA were to change the form 
according to the monitoring method it would be establishing two 
different NAAQS, varying based on the monitoring method. The EPA has 
not done this to date, did not propose such an approach, and declines 
to adopt it for the final rule, as we believe such a decision in this 
final rule is beyond the scope of the proposal, and that each PM 
standard should have a single form, indicator, level and averaging 
time, chosen by the Administrator as necessary and appropriate. While 
certain continuous monitors may be established and approved as a 
Federal Equivalent Method (FEM) for PM10, as an alternative 
to a Federal Reference Method (FRM), the use of an FEM is intended as 
an alternative means of determining compliance with the NAAQS, not as 
authorizing a different NAAQS.
    Even if the commenters had asked that the change in form be made 
without regard to monitoring method, the EPA does not believe such a 
change would be warranted. The change in form for continuous monitors 
suggested by the commenters, without also lowering the level of such a 
standard, would allow more exceedances and thereby markedly reduce the 
public health protection provided against exposures to 
PM10-2.5 in ambient air. These commenters have not provided 
new evidence or analyses to support their conclusion that an 
appropriate degree of public health protection could be achieved by 
allowing the use of an alternative form (i.e., 99th percentile), while 
retaining the other elements of the standard.
    With regard to the commenters' assertion that an alternate form of 
the standard would ease the burden of demonstrating exceptional events, 
the EPA first recognizes, consistent with the CAA, that it may be 
appropriate to exclude monitoring data influenced by ``exceptional'' 
events when making certain regulatory determinations. However, the EPA 
notes that the cost of implementation of the standards may not be 
considered by the EPA in reviewing the standards \60\ and further the 
EPA believes it is unnecessary to alter the standard for the purpose of 
reducing the burden of demonstrating exceptional events. The EPA 
continues to update and develop documentation and tools to facilitate 
the implementation of the 2016 Exceptional Events Rule, including new 
documents intended to assist air agencies with the development of 
demonstrations for specific types of exceptional events. Moreover, with 
regard to the commenters' specific concerns for wildfires or high 
winds, the EPA released updated guidance documents on the preparation 
of exceptional event demonstrations related to wildfires in September 
2016, high wind dust events in April 2019, and prescribed fires in 
August 2019. These guidance documents outline the regulatory 
requirements and provide examples for air agencies preparing 
demonstrations for wildfires, high wind dust, and prescribed fire 
events.
---------------------------------------------------------------------------

    \60\ See generally Whitman v. American Trucking Associations, 
531 U.S. 457, 465-472, 475-76 (2001).
---------------------------------------------------------------------------

    For all of the reasons discussed above, the EPA does not agree with 
the commenters that the form of the primary PM10 standard 
should be revised to a 99th percentile for continuous monitors.

[[Page 82725]]

    Some commenters who disagreed with the proposal to retain the 
current standard advocate for revision to the primary PM10 
standard to protect public health with an adequate margin of safety. In 
their recommendations for revising the standard, some commenters 
contend that the current standard, with its indicator of 
PM10 to target exposures to PM10-2.5, has become 
less protective as ambient concentrations of PM2.5 have been 
reduced with revisions to that standard. These commenters assert that 
the current primary PM10 standard allows increased exposure 
to PM10-2.5 in ambient air because retaining the primary 
PM10 would allow proportionately more PM10-2.5 
mass as the PM2.5 standard has been revised downward. 
Moreover, in support of their recommendations, the commenters note that 
the available evidence of PM10-2.5-related health effects 
has been expanded and strengthened since the time of the last review. 
Taken together, the commenters contend that the primary PM10 
standard should be revised and failure to do so would be arbitrary and 
capricious.
    We disagree with the commenters that the primary PM10 
standard should be revised because reductions in ambient concentrations 
of PM2.5 result in a less protective PM10 
standard. As an initial matter, we note that overall, ambient 
concentrations of both PM10 and PM2.5 have 
declined significantly over time. Ambient concentrations of 
PM10 have declined by 46% across the U.S. from 2000 to 
2019,\61\ while PM2.5 concentrations in ambient air have 
declined by 43% during this same time period.\62\ While trends data is 
not currently available for PM10-2.5 concentrations in 
ambient air, the expanded availability of monitoring data from the 
NCore network in this review can provide insight into the relative 
contributions of fine and coarse PM to total PM10 
concentrations.
---------------------------------------------------------------------------

    \61\ PM10 concentrations presented as the annual 
second maximum 24-hour concentration (in [micro]g/m\3\) at 262 sites 
in the U.S. For more information, see: https://www.epa.gov/air-trends/particulate-matter-pm10-trends.
    \62\ PM2.5 concentrations presented as the 
seasonally-weighted annual average concentration (in [micro]g/m\3\) 
at 406 sites in the U.S. For more information, see: https://www.epa.gov/air-trends/particulate-matter-pm25-trends.
---------------------------------------------------------------------------

    The 2019 ISA provides a comparison of the relative contribution of 
PM2.5 and PM10-2.5 to PM10 
concentrations by region and season using the more comprehensive 
monitoring data from the NCore network available in this review (U.S. 
EPA, 2019, section 2.5.1.1.4). The data indicate that, for urban areas, 
there are roughly equivalent amounts of PM2.5 and 
PM10-2.5 contributing to PM10 in ambient air, 
while rural locations have a slightly higher contribution of 
PM10-2.5 contributing to PM10 concentrations than 
PM2.5 (U.S. EPA, 2019, section 2.5.1.1.4, Table 2-7). There 
is generally a greater contribution from the PM2.5 fraction 
in the East and a greater contribution from the PM10-2.5 
fraction in the West and Midwest. However, as described in the 2019 
ISA, PM10 has become considerably coarser across the U.S. 
compared to similar observations in the 2009 ISA (U.S. EPA, 2019, 
section 2.5.1.1.4; U.S. EPA, 2009c).
    The EPA recognizes that when the primary annual PM2.5 
standard was revised from 15 [micro]g/m\3\ to 12 [micro]g/m\3\ while 
leaving the 24-hour PM2.5 standards unchanged at 35 
[micro]g/m\3\ and the 24-hour PM10 standard unchanged at 150 
[micro]g/m\3\, the PM10-2.5 fraction of PM10 
could increase in some areas as the PM2.5 fraction 
decreases. Moreover, the EPA recognizes that in most areas of the 
country PM2.5 and PM10 concentrations have 
declined and are well below their respective 24-hour standards, which 
may also allow the relative ratio of PM2.5 to 
PM10-2.5 to vary. In considering the available health 
effects evidence in this review, there continue to be significant 
uncertainties and limitations that make it difficult to fully assess 
the public health implications of revising the primary PM10 
standard even considering the possibility for additional variability in 
the relative ratio of PM2.5 to PM10-2.5 in 
current PM10 air quality across the U.S. As described in 
detail above in section III.A.2 and in the proposal (85 FR 24125, April 
30, 2020), these uncertainties contribute to the determinations in the 
2019 ISA that the evidence for key PM10-2.5 health effects 
is ``suggestive of, but not sufficient to infer, a causal 
relationship'' (U.S. EPA, 2019). Beyond these uncertainties, the EPA 
also notes that, while the NCore monitoring network has been expanded 
since the time of the last review, epidemiological studies available in 
this review do not use PM10-2.5 NCore data in evaluating 
associations between PM10-2.5 in ambient air and long- or 
short-term exposures. In the absence of such evidence, the public 
health implications of changes in ambient PM10 
concentrations as PM2.5 concentrations decrease remain 
unclear. Therefore, the EPA continues to recognize this as an area for 
future research, to address the existing uncertainties (U.S. EPA, 2020, 
section 4.5), and inform future reviews of the PM NAAQS.
    Taken together, at the time of proposal, the Administrator 
concluded that these and other limitations in the PM10-2.5 
evidence raised questions as to whether additional public health 
improvements would be achieved by revising the existing PM10 
standard. Therefore, the EPA does not agree with the commenters that 
the currently available air quality information or scientific evidence 
support revisions to the primary PM10 standard in this 
review.
4. Administrator's Conclusions
    Having carefully considered advice from the CASAC and the public 
comments, as discussed above, the Administrator believes that the 
fundamental scientific conclusions on health effects of 
PM10-2.5 in ambient air that were reached in the ISA and 
summarized in the PA remain valid. Additionally, the Administrator 
believes the judgments he proposed (85 FR 24125, April 30, 2020) with 
regard to the evidence remain appropriate. Further, in considering the 
adequacy of the current primary PM10 standard in this 
review, the Administrator has carefully considered the policy-relevant 
evidence and conclusions contained in the ISA; the rationale and 
conclusions presented in the PA; the advice and recommendations from 
the CASAC; and public comments, as addressed in section III.B.3 above. 
In the discussion below, the Administrator gives weight to the PA 
conclusions, with which the CASAC has concurred, as summarized in 
section III.D of the proposal, and takes note of the key aspects of the 
rationale for those conclusions that contribute to his decision in this 
review. After giving careful consideration to all of this information, 
the Administrator believes that the conclusions and policy judgments 
supporting his proposed decision remain valid, and that the current 
primary PM10 standard provides requisite protection of 
public health with an adequate margin of safety and should be retained.
    In considering the PA evaluations and conclusions, the 
Administrator specifically notes that, while the health effects 
evidence is somewhat expanded since the last review, the overall 
conclusions are generally consistent with what was considered in the 
last review (U.S. EPA, 2020, section 4.4). In so doing, he additionally 
notes that the CASAC supports retaining the current standard, agreeing 
with the EPA that ``the available evidence does not call into question 
the adequacy of the public health protection afforded by the current 
primary PM10 standard'' (Cox, 2019a, p. 3 of letter). As 
noted below, the newly available evidence for several 
PM10-2.5-related health effects has

[[Page 82726]]

expanded since the last review, in particular for long-term exposures. 
The Administrator recognizes, however, that there are a number of 
uncertainties and limitations associated with the available 
information, as described in the proposal (85 FR 24125, April 30, 2020) 
and below.
    With regard to the current evidence on PM10-2.5-related 
health effects, the Administrator takes note of recent epidemiological 
studies that continue to report positive associations with mortality 
and morbidity in cities across North America, Europe, and Asia, where 
PM10-2.5 sources and composition are expected to vary 
widely. While significant uncertainties remain, as described below, the 
Administrator recognizes that this expanded body of evidence has 
broadened the range of effects that have been linked with 
PM10-2.5 exposures. These studies provide an important part 
of the scientific foundation supporting the ISA's revised causality 
determinations (and new determinations) for long-term 
PM10-2.5 exposures and mortality, cardiovascular effects, 
metabolic effects, nervous system effects, and cancer (U.S. EPA, 2019; 
U.S. EPA, 2020, section 4.2). Drawing from his consideration of this 
evidence, the Administrator concludes that the scientific information 
available since the time of the last review supports a decision to 
maintain a primary PM10 standard to provide public health 
protection against PM10-2.5 exposures, regardless of 
location, source of origin, or particle composition.
    With regard to uncertainties in the evidence, the Administrator 
first notes that a number of limitations were identified in the last 
review related to: (1) Estimates of ambient PM10-2.5 
concentrations used in epidemiological studies; (2) limited evaluation 
of copollutant models to address the potential for confounding; and (3) 
limited experimental studies supporting biological plausibility for 
PM10-2.5-related effects. In the current review, despite the 
expanded body of evidence for PM10-2.5 exposures and health 
effects, the Administrator recognizes that similar uncertainties 
remain. As summarized in section III.B.1 above and in responding to 
public comments, uncertainties in the current review continue to 
include those associated with the exposure estimates used in 
epidemiological studies, the independence of the PM10-2.5 
health effect associations, and the biologically plausible pathways for 
PM10-2.5 health effects (U.S. EPA, 2020, section 4.2). These 
uncertainties contribute to the 2019 ISA determinations that the 
evidence is ``suggestive of, but not sufficient to infer'' causal 
relationships (U.S. EPA, 2019). In light of his emphasis on evidence 
supporting ``causal'' or ``likely to be causal'' relationships 
(sections II.A.2 and III.A.2 above), recognizing that the NAAQS should 
allow for a margin of safety but finding that there is too much 
uncertainty that a more stringent standard would improve public health, 
the Administrator judges that the available evidence provides support 
for his conclusion that the current standard provides the requisite 
level of protection from the effects of PM10-2.5.
    In making this judgment, the Administrator considers whether this 
level of protection is more than what is requisite and whether a less 
stringent standard would be appropriate to consider. He notes that 
there continues to be uncertainty associated with the evidence, for 
example exposure measurement error, as reflected by the ``suggestive 
of, but not sufficient to infer'' causal determinations. The 
Administrator recognizes that the CAA requirement that primary 
standards provide an adequate margin of safety, as summarized in 
section I.A above, is intended to address uncertainties associated with 
inconclusive scientific evidence and technical information, as well as 
to provide a reasonable degree of protection against hazards that 
research has not yet identified. Based on all of the considerations 
noted here, and considering the current body of evidence, including 
uncertainties and limitations, the Administrator concludes that a less 
stringent standard would not provide the requisite protection of public 
health, including an adequate margin of safety.
    The Administrator also considers whether the level of protection 
associated with the current standard is less than what is requisite and 
whether a more stringent standard would be appropriate to consider. In 
so doing, the Administrator considers, as discussed above, the level of 
protection offered from exposures for which public health implications 
are less clear. In so doing, he again notes the significant 
uncertainties and limitations that persist in the scientific evidence 
in this review. In particular, he notes limitations in the approaches 
used to estimate ambient PM10-2.5 concentrations in 
epidemiological studies, limited examination of the potential for 
confounding by co-occurring pollutants, and limited support for the 
biological plausibility of the serious effects reported in many 
epidemiological studies that are reflected by the ``suggestive of, but 
not sufficient to infer'' causal determinations. Thus, in light of the 
currently available information, including the uncertainties and 
limitations of the evidence base available to inform his judgments 
regarding protection against PM10-2.5-related effects, the 
Administrator does not find it appropriate to increase the stringency 
of the standard in order to provide the requisite public health 
protection. Rather, he judges it appropriate to maintain the level of 
protection provided by the current PM10 standard for 
PM10-2.5 exposures and he does not judge the available 
information and the associated uncertainties to indicate the need for a 
greater level of public health protection.
    In reaching his conclusions on the primary PM10 
standard, the Administrator also considers advice from the CASAC, 
including that regarding uncertainties that remain in this review 
(summarized in section III.B.1 above). In their comments, the CASAC 
noted that uncertainties persist in the evidence for 
PM10-2.5-related health effects, stating that ``key 
uncertainties identified in the last review remain'' (Cox, 2019a, p. 13 
of consensus responses). In considering these comments, the 
Administrator takes note of the CASAC consideration of the 
uncertainties related to the evidence and its conclusion that 
``evidence does not call into question the adequacy of the public 
health protection afforded by the current primary PM10 
standard'' (Cox, 2019a, p. 3 of letter). The Administrator further 
notes the CASAC overall conclusion in this review that the current 
evidence ``supports consideration of retaining the current standard in 
this review'' (Cox, 2019a, p. 3 of letter).
    Thus, in light of the currently available information, including 
uncertainties and limitations in the evidence base available to inform 
his judgments regarding public health protection, as well as CASAC 
advice, the Administrator does not find it appropriate to revise the 
standard. Rather, he judges it appropriate to retain the primary 
PM10 standard to provide the requisite degree of public 
health protection against PM10-2.5 exposures, regardless of 
location, source of origin, or particle composition.
    With regard to the uncertainties identified above, the 
Administrator notes that his final decision in this review is a public 
health policy judgment that draws upon scientific information, as well 
as judgments about how to consider the range and magnitude of 
uncertainties that are inherent in the information. Accordingly, he 
recognizes that his decision requires judgments based on

[[Page 82727]]

the interpretation of the evidence that neither overstates nor 
understates the strength or limitations of the evidence nor the 
appropriate inferences to be drawn. He recognizes, as described in 
section I.A above, that the Act does not require that primary standards 
be set at a zero-risk level; rather, the NAAQS must be sufficient but 
not more stringent than necessary to protect public health, including 
the health of sensitive groups with an adequate margin of safety.
    Recognizing and building upon all of the above considerations and 
judgments, the Administrator has reached his conclusion in the current 
review. As an initial matter, he recognizes the control exerted by the 
current primary PM10 standard against exposures to 
PM10-2.5 in ambient air. With regard to key aspects of the 
specific elements of a standard, the Administrator recognizes continued 
support in the current evidence base for PM10 as the 
indicator for the standard. In so doing, he notes that such an 
indicator provides protection from exposure to all coarse PM, 
regardless of location, source of origin, or particle composition. 
Similarly, with regard to averaging time, form, and level of the 
standard, the Administrator takes note of uncertainties in the 
available evidence and information and continues to find that the 
current standard, as defined by its current elements, is requisite. He 
has additionally considered the public comments regarding revisions to 
these elements of the standard and continues to judge that the existing 
level and the existing form, in all its aspects, together with the 
other elements of the existing standard provide an appropriate level of 
public health protection.
    For all of the reasons discussed above, and recognizing the CASAC 
conclusion that the current evidence provides support for retaining the 
current standard, the Administrator concludes that the current primary 
PM10 standard (in all of its elements) is requisite to 
protect public health with an adequate margin of safety from effects of 
PM10-2.5 in ambient air, and should be retained without 
revision.

C. Decision on the Primary PM10 Standard

    For the reasons discussed above and taking into account information 
and assessments presented in the ISA and PA, the advice from the CASAC, 
and consideration of public comments, the Administrator concludes that 
the current primary PM10 standard is requisite to protect 
public health with an adequate margin of safety, including the health 
of at-risk populations, and is retaining the current standard without 
revision.

IV. Rationale for the Decision on the Secondary PM Standards

    This section presents the rationale for the Administrator's 
decision to retain the current secondary PM standards, without 
revision. This decision is based on a thorough review of the latest 
scientific information generally published through December 2017,\63\ 
as presented in the ISA, on non-ecological public welfare effects 
associated with PM and pertaining to the presence of PM in ambient air, 
specifically visibility, climate, and materials effects. This decision 
also accounts for analyses in the PA of policy-relevant information 
from the ISA and quantitative analyses of air quality related to 
visibility impairment; CASAC advice; and consideration of public 
comments received on the proposal.
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    \63\ In addition to the review's opening ``call for 
information'' (79 FR 71764, December 3, 2014), ``the current ISA 
identified and evaluated studies and reports that have undergone 
scientific peer review and were published or accepted for 
publication between January 1, 2009 and March 31, 2017. A limited 
literature update identified some additional studies that were 
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p. 
A-3). References that are cited in the ISA, the references that were 
considered for inclusion but not cited, and electronic links to 
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    The EPA is separately reviewing the ecological effects associated 
with PM in conjunction with reviews of other pollutants that, along 
with PM, contribute jointly to atmospheric deposition. As explained in 
both the PM IRP (U.S. EPA, 2016, p. 1-17) and the IRP for review of the 
secondary NAAQS for oxides of nitrogen, oxides of sulfur and PM (U.S. 
EPA, 2017, p. 1-1), and discussed in the proposal for this review (85 
FR 24127, April 30, 2020), in recognition of the linkages between 
oxides of nitrogen, oxides of sulfur, and PM with respect to 
atmospheric deposition, and with respect to the ecological effects, the 
reviews of the ecological effects evidence and the secondary standards 
for these pollutants are being conducted together. Addressing the 
pollutants together enables the EPA to take a comprehensive approach to 
considering the nature and interactions of the pollutants, which is 
important for ensuring that all scientific information relevant to 
ecological effects is thoroughly evaluated. This combined review of the 
ecological criteria for oxides of nitrogen, oxides of sulfur, and 
particulate matter is ongoing.\64\
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    \64\ The final ISA was released in October 2020: https://www.epa.gov/isa/integrated-science-assessment-isa-oxides-nitrogen-oxides-sulfur-and-particulate-matter.
---------------------------------------------------------------------------

    Section IV.A provides background on the general approach for this 
review and the basis for the existing secondary PM standards, and also 
presents brief summaries of key aspects of the currently available 
welfare effects evidence and quantitative information. Section IV.B 
summarizes the proposed conclusions and CASAC advice, addresses public 
comments received on the proposal, and presents the Administrator's 
conclusions on the adequacy of the current standards, drawing on 
consideration of this information, advice from the CASAC, and comments 
from the public. Section IV.C summarizes the Administrator's decision 
on the secondary PM standards.

A. Introduction

    As in prior reviews, the general approach to reviewing the current 
secondary standards is based, most fundamentally, on using the EPA's 
assessment of the current scientific evidence and associated 
quantitative analyses to inform the Administrator's judgment regarding 
secondary standards for PM that are requisite to protect the public 
welfare from known or anticipated adverse effects associated with the 
presence of PM in the ambient air. The EPA's assessments are primarily 
documented in the ISA and PA, both of which have received CASAC review 
and public comment (83 FR 53471, October 23, 2018; 84 FR 47944, 
September 11, 2019). To bridge the gap between the scientific 
assessments of the ISA and judgments required of the Administrator in 
determining whether the current standards provide the requisite welfare 
protection, the PA evaluates the policy implications of the assessment 
of the current evidence in the ISA and of the quantitative air quality 
information documented in the PA. In evaluating the public welfare 
protection afforded by the current standards, the four basic elements 
of the NAAQS (indicator, averaging time, level, and form) are 
considered collectively.
    The secondary standard is to ``specify a level of air quality the 
attainment and maintenance of which in the judgment of the 
Administrator . . . is requisite to protect the public welfare from any 
known or anticipated adverse effects associated with the presence of 
such air pollutant in the ambient air'' (CAA, section 109(b)(2)). The 
secondary

[[Page 82728]]

standard is not meant to protect against all known or anticipated PM-
related effects, but rather those that are judged to be adverse to the 
public welfare, and a bright-line determination of adversity is not 
required in judging what is requisite (78 FR 3212, January 15, 2013; 80 
FR 65376, October 26, 2015). Thus, the level of protection from known 
or anticipated adverse effects to public welfare that is requisite for 
the secondary standard is a public welfare policy judgment to be made 
by the Administrator. In exercising that judgment, the Administrator 
seeks to establish standards that are neither more nor less stringent 
than necessary for this purpose. The Act does not require that the 
standards be set at a zero-risk level, but rather at a level that 
reduces risk to protect the public welfare from known or anticipated 
adverse effects. In reaching conclusions on the standards, the 
Administrator's final decision draws upon the scientific information 
and analyses about welfare effects, environmental exposure and risks, 
and associated public welfare significance, as well as judgment about 
how to consider the range and magnitude of uncertainties that are 
inherent in the scientific evidence and quantitative analyses. The 
approach to informing these judgments is based on the recognition that 
the available evidence generally reflects a continuum, consisting of 
levels at which scientists generally agree that effects are likely to 
occur, through lower levels at which the likelihood and magnitude of 
the responses become increasingly uncertain. This approach is 
consistent with the requirements of the CAA and with how the EPA and 
the courts have historically interpreted the Act.
    In considering the scientific and technical information, we 
consider both the information available at the time of the last review 
and information newly available since the last review, including most 
particularly that which has been critically analyzed and characterized 
in the current ISA. We additionally consider the quantitative 
information described in the PA that estimated visibility impairment 
associated with current air quality conditions in areas with monitoring 
data that met completeness criteria (U.S. EPA, 2020, chapter 5). The 
evidence-based discussions presented below (and summarized more fully 
in the proposal) draw upon evidence from studies evaluating visibility, 
climate, and materials effects related to PM in ambient air, as 
discussed in the ISA. The quantitative-based discussions also presented 
below (and summarized more fully in the proposal) have been drawn from 
the quantitative analyses for PM-related visibility impairment, as 
discussed in the PA.
1. Background on the Current Standards
    In the last review, completed in 2012,\65\ the EPA retained the 
secondary 24-hour PM2.5 standard, with its level of 35 
[micro]g/m\3\, and the 24-hour PM10 standard, with its level 
of 150 [micro]g/m\3\ (78 FR 3228, January 15, 2013). The EPA also 
retained the level, set at 15 [micro]g/m\3\, and averaging time of the 
secondary annual PM2.5 standard, while revising the form. 
With regard to the form of the annual PM2.5 standard, the 
EPA removed the option for spatial averaging (78 FR 3228, January 15, 
2013). Key aspects of the Administrator's decisions on the secondary PM 
standards in the last review for non-visibility effects and visibility 
effects are described below. In the previous PM NAAQS review, the prior 
Administrator concluded that there was insufficient information 
available to base a national ambient air quality standard on climate 
impacts associated with ambient air concentrations of PM or its 
constituents (78 FR 3225-3226, January 15, 2013; U.S. EPA, 2011, 
section 5.2.3). In reaching this decision, the prior Administrator 
considered the scientific evidence, noting the 2009 ISA conclusion 
``that a causal relationship exists between PM and effects on climate'' 
and that aerosols \66\ alter climate processes directly through 
radiative forcing and by indirect effects on cloud brightness, changes 
in precipitation, and possible changes in cloud lifetimes (U.S. EPA, 
2009c, section 9.3.10). She also noted that the major aerosol 
components with the potential to affect climate processes (i.e., black 
carbon (BC), organic carbon (OC), sulfates, nitrates and mineral dusts) 
vary in their reflectivity, forcing efficiencies, and direction of 
climate forcing (U.S. EPA, 2009c, section 9.3.10). The prior 
Administrator recognized the strong evidence indicating that aerosols 
affect climate and further considered what the available information 
indicated regarding the adequacy of protection provided by the 
secondary PM standards. In particular, she noted that a number of 
uncertainties in the scientific information (i.e., the spatial and 
temporal heterogeneity of PM components that contribute to climate 
forcing, uncertainties in the measurement of aerosol components, 
inadequate consideration of aerosol impacts in climate modeling, 
insufficient data on local and regional microclimate variations and 
heterogeneity of cloud formations) affected our ability to conduct a 
quantitative analysis to determine a distinct secondary standard based 
on climate.
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    \65\ The 2012 decision on the adequacy of the secondary PM 
standards was based on consideration of the protection provided by 
those standards for visibility and for the non-visibility effects of 
materials damage, climate effects and ecological effects. As noted 
earlier, the current review of the public welfare protection 
provided by the secondary PM standards against ecological effects is 
occurring in the separate, on-going review of the secondary NAAQS 
for oxides of nitrogen and oxides of sulfur (U.S. EPA, 2016, Chapter 
1, section 5.2; U.S. EPA, 2020, Chapter 1, section 5.1.1). Thus, the 
consideration of ecological effects in the 2012 review is not 
discussed here.
    \66\ In the climate sciences research community, PM is 
encompassed by what is typically referred to as aerosol. An aerosol 
is defined as a solid or liquid suspended in a gas, but PM refers to 
the solid or liquid phase of an aerosol. In this review of the 
secondary PM NAAQS the discussion on climate effects of PM uses the 
term PM throughout for consistency with the ISA (U.S. EPA, 2019) as 
well as to emphasize that the climate processes altered by aerosols 
are generally altered by the PM portion of the aerosol. Exceptions 
to this practice include the discussion of climate effects in the 
last review, when aerosol was used when discussing suspended aerosol 
particles, and for certain acronyms that are widely used by the 
climate community that include the term aerosol (e.g., aerosol 
optical depth, or AOD).
---------------------------------------------------------------------------

    In the last review, the prior Administrator concluded that that it 
is generally appropriate to retain the existing secondary standards and 
that it is not appropriate to establish any distinct secondary PM 
standards to address PM-related materials effects (78 FR 3225-3226, 
January 15, 2013; U.S. EPA, 2011, p. 5-29). In reaching this 
conclusion, she considered materials effects associated with the 
deposition of PM (i.e., dry and wet deposition), including both 
physical damage (materials effects) and aesthetic qualities (soiling 
effects). She noted the 2009 ISA conclusion that evidence was 
``sufficient to conclude that a causal relationship exists between PM 
and effects on materials'' (U.S. EPA, 2009c, sections 2.5.4 and 9.5.4), 
but also recognized that the 2011 PA noted that quantitative 
relationships were lacking between particle size, concentrations, and 
frequency of repainting and repair of surfaces and that considerable 
uncertainty exists in the contributions of co-occurring pollutants to 
materials damage and soiling processes (U.S. EPA, 2011, p. 5-29).
    In considering non-visibility welfare effects in the last review, 
as discussed above, the prior Administrator concluded that, while it is 
important to maintain an appropriate degree of control of fine and 
coarse particles to address non-visibility welfare effects, ``[i]n the 
absence of information that would support any different standards . . . 
it is appropriate to retain the

[[Page 82729]]

existing suite of secondary standards'' (78 FR 3225-3226, January 15, 
2013). Her decision was consistent with the CASAC advice related to 
non-visibility effects. Specifically, the CASAC agreed with the 2011 PA 
conclusions that, while these effects are important, ``there is not 
currently a strong technical basis to support revisions of the current 
standards to protect against these other welfare effects'' (Samet, 
2010a, p. 5). Thus, in considering non-visibility welfare effects, the 
prior Administrator concluded that it was appropriate to retain all 
aspects of the existing 24-hour PM2.5 and PM10 
secondary standards. With regard to the secondary annual 
PM2.5 standard, she concluded that it was appropriate to 
retain a level of 15.0 [micro]g/m\3\ while revising only the form of 
the standard to remove the option for spatial averaging (78 FR 3225-
3226, January 15, 2013).
    Having reached the conclusion it is generally appropriate to retain 
the existing secondary standards and that it is not appropriate to 
establish any distinct secondary PM standards to address non-visibility 
PM-related welfare effects, the prior Administrator next considered the 
level of protection that would be requisite to protect public welfare 
against PM-related visibility impairment and whether to adopt a 
distinct secondary standard to achieve this level of protection. In 
reaching her final decision that the existing 24-hour PM2.5 
standard provides sufficient protection against PM-related visibility 
impairment (78 FR 3228, January 15, 2013), she considered the evidence 
assessed in the 2009 ISA (U.S. EPA, 2009c) and the analyses included in 
the Urban-Focused Visibility Assessment (2010 UFVA; U.S. EPA, 2010b) 
and the 2011 PA (U.S. EPA, 2011). She also considered the degree of 
protection for visibility that would be provided by the existing 
secondary standard, focusing specifically on the secondary 24-hour 
PM2.5 standard with its level of 35 [micro]g/m\3\. These 
considerations, and the prior Administrator's conclusions regarding 
visibility are summarized below and discussed in more detail in the 
proposal (85 FR 24128-24129, April 30, 2020).
    In the last review, the ISA concluded that, ``collectively, the 
evidence is sufficient to conclude that a causal relationship exists 
between PM and visibility impairment'' (U.S. EPA, 2009c, p. 2-28). In 
consideration of the potential public welfare implication of various 
degrees of PM-related visibility impairment, the prior Administrator 
considered the available visibility preference studies that were part 
of the overall body of evidence in the 2009 ISA and reviewed as a part 
of the 2010 UFVA. These preference studies provided information about 
the potential public welfare implications of visibility impairment from 
surveys in which participants were asked questions about their 
preferences or the values they placed on various visibility conditions, 
as displayed to them in scenic photographs or in images with a range of 
known light extinction levels.\67\
---------------------------------------------------------------------------

    \67\ Preference studies were available in four urban areas in 
the last review. Three western preference studies were available, 
including one in Denver, Colorado (Ely et al., 1991), one in the 
lower Fraser River valley near Vancouver, British Columbia, Canada 
(Pryor, 1996), and one in Phoenix, Arizona (BBC Research & 
Consulting, 2003). A pilot focus group study was also conducted for 
Washington, DC (Abt Associates, 2001), and a replicate study with 26 
participants was also conducted for Washington, DC (Smith and 
Howell, 2009). More details about these studies are available in 
Appendix D of the PA.
---------------------------------------------------------------------------

    In noting the relationship between PM concentrations and PM-related 
light extinction, the prior Administrator focused on identifying an 
adequate level of protection against visibility-related welfare 
effects. She first concluded that a standard in terms of a 
PM2.5 visibility index would provide a measure of protection 
against PM-related light extinction that directly takes into account 
the factors (i.e., PM species composition and relative humidity) that 
influence the relationship between PM2.5 in ambient air and 
PM-related visibility impairment. A PM2.5 visibility index 
standard would afford a relatively high degree of uniformity of visual 
air quality protection in areas across the country by directly 
incorporating the effects of differences of PM2.5 
composition and relative humidity. In defining a target level of 
protection in terms of a PM2.5 visibility index, as 
discussed below, she considered specific elements of the index, 
including the basis for its derivation, as well as an appropriate 
averaging time, level, and form.
    The prior Administrator concluded that it was appropriate to use an 
adjusted version of the original IMPROVE algorithm,\68\ in conjunction 
with monthly average relative humidity data based on long-term 
climatological means, as the basis for deriving a visibility index. In 
so concluding, she noted the CASAC conclusion on the reasonableness of 
reliance on a PM2.5 light extinction indicator calculated 
from PM2.5 chemical composition and relative humidity, and 
she recognized that the mass monitoring methods available at that time 
were unable to measure the full water content of ambient 
PM2.5 and did not provide information on the composition of 
PM2.5, both of which contribute to visibility impacts (77 FR 
38980, June 29, 2012). As noted at the time of the proposal, the prior 
Administrator recognized that suitable equipment and performance-based 
verification procedures did not then exist for direct measurement of 
light extinction and could not be developed within the time frame of 
the review (77 FR 38980-38981, June 29, 2012).
---------------------------------------------------------------------------

    \68\ The revised IMPROVE algorithm (Pitchford et al., 2007) uses 
major PM chemical composition measurements and relative humidity 
estimates to calculate light extinction. For more information about 
the derivation of and input data required for the original and 
revised IMPROVE algorithms, see 78 FR 3168-3177, January 15, 2013.
---------------------------------------------------------------------------

    The prior Administrator concluded that a 24-hour averaging time 
would be appropriate for a visibility index (78 FR 3226, January 15, 
2013). Although she recognized that hourly or sub-daily (4- to 6-hour) 
averaging times, within daylight hours and excluding hours with 
relatively high humidity, are more directly related to the short-term 
nature of the perception of PM-related visibility impairment and 
relevant exposure periods for segments of the viewing public than a 24-
hour averaging time, she also noted that there were data quality 
uncertainties associated with the instruments used to provide the 
hourly PM2.5 mass measurements required for an averaging 
time shorter than 24 hours. She also considered the results of analyses 
that compared 24-hour and 4-hour averaging times for calculating the 
index. These analyses showed good correlation between 24-hour and 4-
hour average PM2.5 light extinction, as evidenced by 
reasonably high city-specific and pooled R-squared values, generally in 
the range of over 0.6 to over 0.8. Based on these analyses and the 2011 
PA conclusions regarding them, the prior Administrator concluded that a 
24-hour averaging time would be a reasonable and appropriate surrogate 
for a sub-daily averaging time.
    The statistical form of the index, 3-year average of annual 90th 
percentile values, was based on the prior Administrator's consideration 
of the analyses conducted in the 2011 UFVA of three different 
statistics and consistency of this statistical form with the Regional 
Haze Program, which targets the 20 percent most impaired days for 
improvements in visual air quality in Federal Class I areas. Moreover, 
the prior Administrator noted that a 3-year average form provided 
stability from the occasional effect of inter-annual meteorological 
variability that can result in unusually high pollution levels for a 
particular year (78 FR 3198, January 15, 2013; U.S. EPA,

[[Page 82730]]

2011, p. 4-58).\69\ The Administrator also noted that the available 
studies on people's preferences did not address frequency of occurrence 
of different levels of visibility and did not identify a basis for a 
different target for urban areas than that for Class I areas (U.S. EPA, 
2011, p. 4-59). These considerations led the prior Administrator to 
conclude that 90th percentile form was the most appropriate annual 
statistic to be averaged across three years (78 FR 3226, January 15, 
2013).
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    \69\ The EPA recognized that a percentile form averaged over 
multiple years offers greater stability to the air quality 
management process by reducing the possibility that statistically 
unusual indicator values will lead to transient violations of the 
standard, thus reducing the potential for disruption of programs 
implementing the standard and reducing the potential for disruption 
of the protections provided by those programs.
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    In selecting a level for the index, the prior Administrator 
considered the ``candidate protection levels'' (CPLs) \70\ identified 
in the 2011 PA based on the visibility preference studies, ranging from 
20 to 30 deciviews (dv),\71\ while noting the uncertainties and 
limitations in these public preference studies.\72\ She concluded that 
that the current substantial degrees of variability and uncertainty 
inherent in the public preference studies should be reflected in a 
higher target protection level than would be appropriate if the 
underlying information were more consistent and certain. Therefore, she 
concluded that it was appropriate to set a target level of protection 
in terms of a 24-hour PM2.5 visibility index at 30 dv (78 FR 
3226-3227, January 15, 2013).
---------------------------------------------------------------------------

    \70\ For comparison, 20 dv, 25 dv, and 30 dv are equivalent to 
64, 112, and 191 megameters (Mm-1), respectively.
    \71\ Deciview (dv) refers to a scale for characterizing 
visibility that is defined directly in terms of light extinction. 
The deciview scale is frequently used in the scientific and 
regulatory literature on visibility.
    \72\ Uncertainties and limitations in the public preference 
studies included the small number of stated preference studies 
available; the relatively small number of study participants and the 
extent to which the study participants may not be representative of 
the broader study area population in some of the studies; and the 
variations in the specific materials and methods used in each study.
---------------------------------------------------------------------------

    Based on her considerations and conclusions summarized above, the 
prior Administrator concluded that the protection provided by a 
secondary standard based on a 3-year visibility metric, defined in 
terms of a PM2.5 visibility index with a 24-hour averaging 
time, a 90th percentile form averaged over 3 years, and a level of 30 
dv, would be requisite to protect public welfare with regard to visual 
air quality (78 FR 3227, January 15, 2013). Having reached this 
conclusion, she next determined whether an additional distinct 
secondary standard in terms of a visibility index was needed given the 
degree of protection from visibility impairment afforded by the 
existing secondary standards. Specifically, she noted that the air 
quality analyses showed that all areas meeting the existing 24-hour 
PM2.5 standard, with its level of 35 [micro]g/m\3\, had 
visual air quality at least as good as 30 dv, based on the visibility 
index defined above (Kelly et al., 2012b, Kelly et al., 2012a). Thus, 
the secondary 24-hour PM2.5 standard would likely be 
controlling relative to a 24-hour visibility index set at a level of 30 
dv. Additionally, areas would be unlikely to exceed the target level of 
protection for visibility of 30 dv without also exceeding the existing 
secondary 24-hour standard. Thus, the prior Administrator judged that 
the 24-hour PM2.5 standard ``provides sufficient protection 
in all areas against the effects of visibility impairment--i.e., that 
the existing 24-hour PM2.5 standard would provide at least 
the target level of protection for visual air quality of 30 dv which 
[she] judges appropriate'' (78 FR 3227, January 15, 2013). She further 
judged that ``[s]ince sufficient protection from visibility impairment 
would be provided for all areas of the country without adoption of a 
distinct secondary standard, and adoption of a distinct secondary 
standard will not change the degree of over-protection for some areas 
of the country . . . adoption of such a distinct secondary standard is 
not needed to provide requisite protection for both visibility and 
nonvisibility related welfare effects'' (78 FR 3228, January 15, 2013).
2. Overview of Welfare Effects Evidence
    In this section, we provide an overview of the policy-relevant 
aspects of the welfare effects evidence available for consideration in 
this review. Sections IV.B and IV.C of the proposal provide a detailed 
summary of key information contained in the ISA and in the PA on the 
visibility and non-visibility welfare effects associated with PM in 
ambient air, and the related public welfare implications (85 FR 24129, 
April 30, 2020). The subsections below briefly summarize the nature of 
PM-related visibility and non-visibility effects.
a. Nature of Effects
    The evidence base available in the current review includes decades 
of research on visibility impairment, climate effects, and materials 
effects associated with PM (U.S. EPA, 2004, 2009c, 2019). Visibility 
impairment can have implications for people's enjoyment of daily 
activities and for their overall sense of well-being (U.S. EPA, 2009c, 
section 9.2). The strongest evidence for PM-related visibility 
impairment comes from the fundamental relationship between light 
extinction and PM mass (U.S. EPA, 2009c), as well as studies of the 
public perception of visibility impairment (U.S. EPA, 2010b), which 
confirm a well-established ``causal relationship exists between PM and 
visibility impairment'' (U.S. EPA, 2009c, p. 2-28). Beyond its effects 
on visibility, the 2009 ISA also identified a causal relationship 
``between PM and climate effects, including both direct effects of 
radiative forcing and indirect effects that involve cloud and feedbacks 
that influence precipitation formation and cloud lifetimes'' (U.S. EPA, 
2009, p. 2-29). The evidence also supports a causal relationship 
between PM and effects on materials, including soiling effects and 
materials damage (U.S. EPA, 2009, p. 2-31).
    The evidence newly available in this review is consistent with the 
evidence available at the time of the last review and supports the 
conclusions of causal relationships between PM and visibility, climate, 
and materials effects (U.S. EPA, 2019, chapter 13). Evidence newly 
available in this review augments the previously available evidence of 
the relationship between PM and visibility impairment (U.S. EPA, 2019, 
section 13.2), climate effects (U.S. EPA, 2019, section 13.3), and 
materials effects (U.S. EPA, 2019, section 13.4).
i. Visibility
    Visibility refers to the visual quality of a human's view with 
respect to color rendition and contrast definition. It is the ability 
to perceive landscape form, colors, and textures. Visibility involves 
optical and psychophysical properties involving human perception, 
judgment, and interpretation. Light between the observer and the object 
can be scattered into or out of the sight path and absorbed by PM or 
gases in the sight path. Consistent with conclusions of causality in 
the last review, the 2019 ISA concludes that ``the evidence is 
sufficient to conclude that a causal relationship exists between PM and 
visibility impairment'' (U.S. EPA, 2019, section 13.2.6). These 
conclusions are based on the strong and consistent evidence that 
ambient PM can impair visibility in both urban and remote areas (U.S. 
EPA, 2019, section 13.1; U.S. EPA, 2009c, section 9.2.5).
    The fundamental relationship between light extinction and PM mass, 
and the EPA's understanding of this relationship, has changed little 
since the

[[Page 82731]]

2009 ISA (U.S. EPA, 2009c). The combined effect of light scattering and 
absorption by particles and gases is characterized as light extinction, 
i.e., the fraction of light that is scattered or absorbed per unit of 
distance in the atmosphere. Light extinction is measured in units of 1/
distance, which is often expressed in the technical literature as 
visibility per megameter (abbreviated Mm-1). Higher values 
of light extinction (usually given in units of Mm-1 or dv) 
correspond to lower visibility. When PM is present in the air, its 
contribution to light extinction is typically much greater than that of 
gases (U.S. EPA, 2019, section 13.2.1). The impact of PM on light 
scattering depends on particle size and composition, as well as 
relative humidity. All particles scatter light, as described by the Mie 
theory, which relates light scattering to particle size, shape, and 
index of refraction (U.S. EPA, 2019, section 13.2.3; Van de Hulst, 
1981; Mie, 1908). Fine particles scatter more light than coarse 
particles on a per unit mass basis and include sulfates, nitrates, 
organics, light-absorbing carbon, and soil (Malm et al., 1994). 
Hygroscopic particles like ammonium sulfate, ammonium nitrate, and sea 
salt increase in size as relative humidity increases, leading to 
increased light scattering (U.S. EPA, 2019, section 13.2.3).
    As at the time of the last review, direct measurements of PM light 
extinction, scattering, and absorption continue to be considered more 
accurate for quantifying visibility than PM mass-based estimates 
because measurements do not depend on assumptions about particle 
characteristics (e.g., size, shape, density, component mixture, etc.) 
(U.S. EPA, 2019, section 13.2.2.2). Measurements of light extinction 
can be made with high time resolution, allowing for characterization of 
sub-daily temporal patterns of visibility impairment. A number of 
measurement methods have been used for visibility impairment (e.g., 
transmissometers, integrating nephelometers, teleradiometers, 
telephotometers, and photography and photographic modeling), although 
each of these methods has its own strengths and limitations (U.S. EPA, 
2019, Table 13-1). As recognized in the last review, there are no 
common performance-based criteria to evaluate these methods and none 
have been deployed broadly across the U.S. for routine measurement of 
visibility impairment.
    In the absence of a robust monitoring network for the routine 
measurement of light extinction across the U.S., estimation of light 
extinction based on existing PM monitoring can be used. The theoretical 
relationship between light extinction and PM characteristics, as 
derived from Mie theory (U.S. EPA, 2019, Equation 13.5), and can be 
used to estimate light extinction by combining mass scattering 
efficiencies of particles with particle concentrations (U.S. EPA, 2019, 
section 13.2.3; U.S. EPA, 2009c, sections 9.2.2.2 and 9.2.3.1). This 
estimation of light extinction is consistent with the method used in 
the last review. The algorithm used to estimate light extinction, known 
as the IMPROVE algorithm,\73\ provides for the estimation of light 
extinction (bext), in units of Mm-1, using 
routinely monitored components of fine (PM2.5) and coarse 
(PM10-2.5) PM. Relative humidity data are also needed to 
estimate the contribution by liquid water that is in solution with the 
hygroscopic components of PM. To estimate each component's contribution 
to light extinction, their concentrations are multiplied by extinction 
coefficients and are additionally multiplied by a water growth factor 
that accounts for their expansion with moisture. Both the extinction 
efficiency coefficients and water growth factors of the IMPROVE 
algorithm have been developed by a combination of empirical assessment 
and theoretical calculation using particle size distributions 
associated with each of the major aerosol components (U.S. EPA, 2019, 
section 13.2.3.1, section 13.2.3.3).
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    \73\ The algorithm is referred to as the IMPROVE algorithm as it 
was developed specifically to use monitoring data generated at 
IMPROVE network sites and with equipment specifically designed to 
support the IMPROVE program and was evaluated using IMPROVE optical 
measurements at the subset of monitoring sites that make those 
measurements (Malm et al., 1994).
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    At the time of the last review, two versions of the IMPROVE 
algorithm were available in the literature--the original IMPROVE 
algorithm (Malm and Hand, 2007; Ryan et al., 2005; Lowenthal and Kumar, 
2004) and the revised IMPROVE algorithm (Pitchford et al., 2007). As 
described in detail in the proposal (85 FR 24130, April 30, 2020) and 
the ISA (U.S. EPA, 2019, section 13.2.3), the algorithm has been 
further evaluated and refined since the time of the last review 
(Lowenthal and Kumar, 2016), particularly for PM characteristics and 
relative humidity in remote areas. All three versions of the IMPROVE 
algorithm were considered in evaluating visibility impairment in this 
review.
    Consistent with the evidence available at the time of the last 
review, our understanding of public perception of visibility impairment 
comes from visibility preference studies conducted in four areas in 
North America.\74\ The detailed methodology for these studies are 
described in the proposal (85 FR 24131, April 30, 2020), the 2019 ISA 
(U.S. EPA, 2019), and the 2009 ISA (U.S. EPA, 2009c). In summary, the 
study participants were queried regarding multiple images that were 
either photographs of the same location and scenery that had been taken 
on different days on which measured extinction data were available or 
digitized photographs onto which a uniform ``haze'' had been 
superimposed. Results of the studies indicated a wide range of 
judgments on what study participants considered to be acceptable 
visibility across the different study areas, depending on the setting 
depicted in each photograph. Based on the results of the four cities, a 
range encompassing the PM2.5 visibility index values from 
images that were judged to be acceptable by at least 50 percent of 
study participants across all four of the urban preference studies was 
identified (U.S. EPA, 2010b, p. 4-24; U.S. EPA, 2020, Figure 5-2). Much 
lower visibility (considerably more haze resulting in higher values of 
light extinction) was considered acceptable in Washington, DC, than was 
in Denver, and 30 dv reflected the level of impairment that was 
determined to be ``acceptable'' by at least 50 percent of study 
participants (78 FR 3226-3227, January 15, 2013). As noted in the 
proposal (85 FR 24131, April 30, 2020), the evidence base for public 
preferences of visibility impairment has not been augmented since the 
last review. There are no new visibility preference studies that have 
been conducted in the U.S. since the time of the last review and there 
is very little new information available regarding acceptable levels of 
visibility impairment in the U.S.
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    \74\ Preference studies were available in four urban areas in 
the last review: Denver, Colorado (Ely et al., 1991), Vancouver, 
British Columbia, Canada (Pryor, 1996), Phoenix, Arizona (BBC 
Research & Consulting, 2003), and Washington, DC (Abt Associates, 
2011; Smith and Howell, 2009).
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ii. Climate
    The current evidence continues to support the conclusion of a 
causal relationship between PM and climate effects (U.S. EPA, 2019, 
section 13.3.9). Since the last review, climate impacts and been 
extensively studied and recent research reinforces and strengthens the 
evidence evaluated in the 2009 ISA. New evidence provides greater 
specificity about the details of radiative

[[Page 82732]]

forcing effects \75\ and increases the understanding of additional 
climate impacts driven by PM radiative effects. The Intergovernmental 
Panel on Climate Change (IPCC) assesses the role of anthropogenic 
activity in past and future climate change, and since the last review, 
has issued the Fifth IPCC Assessment Report (AR5; IPCC, 2013) which 
summarizes any key scientific advances in understanding the climate 
effects of PM since the previous report. As in the last review, the ISA 
draws substantially on the IPCC report to summarize climate effects. As 
discussed in more detail in the proposal (85 FR 24131, April 30, 2020), 
the general conclusions are similar between the IPCC AR4 and AR5 
reports with regard to effects of PM on global climate. Consistent with 
the evidence available in the last review, the key components, 
including sulfate, nitrate, organic carbon (OC), black carbon (BC), and 
dust, that contribute to climate processes vary in their reflectivity, 
forcing efficiencies, and direction of forcing. Since the last review, 
the evidence base has expanded with respect to the mechanisms of 
climate responses and feedbacks to PM radiative forcing; however, the 
new literature published since the last review does not reduce the 
considerable uncertainties that continue to exist related these 
mechanisms.
---------------------------------------------------------------------------

    \75\ Radiative forcing (RF) for a given atmospheric constituent 
is defined as the perturbation in net radiative flux, at the 
tropopause (or the top of the atmosphere) caused by that 
constituent, in watts per square meter (Wm-2), after 
allowing for temperatures in the stratosphere to adjust to the 
perturbation but holding all other climate responses constant, 
including surface and tropospheric temperatures (Fiore et al., 2015; 
Myhre et al., 2013). A positive forcing indicates net energy trapped 
in the Earth system and suggests warming of the Earth's surface, 
whereas a negative forcing indicates net loss of energy and suggests 
cooling (U.S. EPA, 2019, section 13.3.2.2).
---------------------------------------------------------------------------

    As described in the proposal (85 FR 24133, April 30, 2020), PM has 
a very heterogeneous distribution globally and patterns of forcing tend 
to correlate with PM loading, with the greatest forcings centralized 
over continental regions. The climate response to this PM forcing, 
however, is more complicated since the perturbation to one climate 
variable (e.g., temperature, cloud cover, precipitation) can lead to a 
cascade of effects on other variables. While the initial PM radiative 
forcing may be concentrated regionally, the eventual climate response 
can be much broader spatially or be concentrated in remote regions, and 
may be quite complex, affecting multiple climate variable with possible 
differences in the direction of the forcing in different regions or for 
different variables (U.S. EPA, 2019, section 13.3.6). The complex 
climate system interactions lead to variation among climate models, 
which have suggested a range of factors which can influence large-scale 
meteorological processes and may affect temperature, including local 
feedback effects involving soil moisture and cloud cover, changes in 
the hygroscopicity of the PM, and interactions with clouds (U.S. EPA, 
2019, section 13.3.7). Further research is needed to better 
characterize the effects of PM on regional climate in the U.S. before 
PM climate effects can be quantified.
iii. Materials
    Consistent with the last review, the current evidence continues to 
support the conclusion that there is a causal relationship between PM 
deposition and materials effects. Effects of deposited PM, particularly 
sulfates and nitrates, to materials include both physical damage and 
impaired aesthetic qualities, generally involving soiling and/or 
corrosion (U.S. EPA, 2019, section 13.4.2; 85 FR 24133, April 30, 
2020). Because of their electrolytic, hygroscopic, and acidic 
properties and their ability to sorb corrosive gases, particles 
contribute to materials damage by adding to the effects of natural 
weathering processes, by potentially promoting or accelerating the 
corrosion of metals, degradation of painted surfaces, deterioration of 
building materials, and weakening of material components.\76\ There is 
a limited amount of new data for consideration in this review from 
studies primarily conducted outside of the U.S. on buildings and other 
items of cultural heritage. However, these studies involved 
concentrations PM in ambient air greater than those typically observed 
in the U.S. (U.S. EPA, 2019, section 13.4).
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    \76\ As discussed in the ISA (U.S. EPA, 2019, section 13.4.1), 
corrosion typically involves reactions of acidic PM (i.e., acidic 
sulfate or nitrate) with material surfaces, but gases like 
SO2 and nitric acid (HNO3) also contribute. 
Because ``the impacts of gaseous and particulate N and S wet 
deposition cannot be clearly distinguished'' (U.S. EPA, 2019, p. 13-
1), the assessment of the evidence in the ISA considers the combined 
impacts.
---------------------------------------------------------------------------

    Building on the evidence available in the 2009 ISA, and as 
described in detail in the proposal (85 FR 24134, April 30, 2020) and 
in the 2019 ISA (U.S. EPA, 2019, section 13.4), research has progressed 
on: (1) The theoretical understanding of soiling of items of cultural 
heritage; (2) the quantification of degradation rates and further 
characterization of factors that influence damage of stone materials; 
(3) materials damage from PM components besides sulfate and black 
carbon and atmospheric gases besides SO2; (4) methods for 
evaluating soiling of materials by PM mixtures; (5) PM-attributable 
damage to other materials, including glass and photovoltaic panels; (6) 
development of dose-response relationships for soiling of building 
materials; and (7) damage functions to quantify material decay as a 
function of pollutant type and load. While the evidence of PM-related 
materials effects has expanded somewhat since the last review, there 
remains insufficient evidence to relate soiling or damage to specific 
PM levels in ambient air or to establish a quantitative relationship 
between PM and materials degradation. The current evidence is generally 
similar to the evidence available in the last review, including 
associated limitations and uncertainties and a lack of evidence to 
inform quantitative relationships between PM and materials effects, 
therefore leading to similar conclusions about the PM-related effects 
on materials.
3. Overview of Air Quality and Quantitative Information
a. Visibility Effects
    In the current review, quantitative analyses were conducted to 
further our understanding of the relationship between recent air 
quality and calculated light extinction. As at the time of the last 
review, these analyses explored this relationship as an estimate of 
visibility impairment in terms of the 24-hour PM2.5 standard 
and the visibility index. Generally, the results of the updated 
analyses are similar to those based on the data available at the time 
of the last review (U.S. EPA, 2020, section 5.2.1.1). Compared to the 
last review, updated analyses incorporate several refinements, 
including: (1) The evaluation of three versions of the IMPROVE equation 
\77\ to calculate light extinction (U.S. EPA, 2020, Appendix D, 
Equations D-1 through D-3) in order to better understand the influence 
of variability in equation inputs; \78\ (2) the

[[Page 82733]]

use of 24-hour relative humidity data, rather than monthly average 
relative humidity as was used in the last review (U.S. EPA, 2020, 
section 5.2.1.2, Appendix D); and (3) the inclusion of the coarse 
fraction in the estimation of light extinction in the subset of areas 
with PM10-2.5 monitoring data available for the time period 
of interest (U.S. EPA, 2020, section 5.2.1.2, Appendix D). The analyses 
in the current review are updated from the last review and include 67 
monitoring sites that measure PM2.5, including 20 sites that 
measure both PM10 and PM2.5, that are 
geographically distributed across the U.S. in both urban and rural 
areas (U.S. EPA, 2020, Appendix D, Figure D-1).
---------------------------------------------------------------------------

    \77\ Given the lack of new information to inform a different 
visibility metric, the metric used in the updated analyses is that 
defined by the EPA in the last review as the target level of 
protection for visibility (discussed above in section IV.A.1): A 
PM2.5 visibility index with a 24-hour averaging time, a 
90th percentile form averaged over 3 years, and a level of 30 dv 
(U.S. EPA, 2020, section 5.2.1.2).
    \78\ While the PM2.5 monitoring network has an 
increasing number of continuous FEM monitors reporting hourly 
PM2.5 mass concentrations, there continue to be data 
quality uncertainties associated with providing hourly 
PM2.5 mass and component measurements that could be input 
into IMPROVE equation calculations for sub-daily visibility 
impairment estimates. As detailed in the PA, there are uncertainties 
associated with the precision and bias of 24-hour PM2.5 
measurements (U.S. EPA, 2020, p. 2-18), as well as to the fractional 
uncertainty associated with 24-hour PM component measurements (U.S. 
EPA, 2020, p. 2-21). Given the uncertainties present when evaluating 
data quality on a 24-hour basis, the uncertainty associated with 
sub-daily measurements may be even greater. Therefore, the inputs to 
these light extinction calculations are based on 24-hour average 
measurements of PM2.5 mass and components, rather than 
sub-daily information.
---------------------------------------------------------------------------

    In areas that meet the current 24-hour PM2.5 standard 
for the 2015-2017 time period, all sites have light extinction 
estimates at or below 27 dv using the original and revised IMPROVE 
equations (and most areas are below 25 dv; U.S. EPA, 2020, section 
5.2.1.2). In the one location that exceeds the current 24-hour 
PM2.5 standard, light extinction estimates are at or below 
27 dv (U.S. EPA, 2020, Figure 5-3). These findings are consistent with 
the findings of the analysis in the last review with older air quality 
data (Kelly et al., 2012b; 78 FR 3201, January 15, 2013).
    Using the recently modified IMPROVE equation from Lowenthal and 
Kumar (2016), new in this review, the resulting 3-year visibility index 
is slightly higher at all of the sites compared to the original and 
revised IMPROVE equation estimates (U.S. EPA, 2020, Figure 5-4). These 
higher estimates are to be expected, given the higher OC multiplier 
included in the IMPROVE equation from Lowenthal and Kumar (2016), which 
reflects the use of data from remote areas with higher concentrations 
of organic PM when validating the equation. As such, it is important to 
note that the Lowenthal and Kumar (2016) version of the equation may 
overestimate light extinction in non-remote areas, including the urban 
areas in the updated analyses in this review.
    Nevertheless, when light extinction is calculated using the 
Lowenthal and Kumar (2016) equation for those sites that meet the 
current 24-hour PM2.5 standard, the 3-year visibility metric 
is generally at or below 30 dv. The one exception to this is a site in 
Fairbanks, Alaska that just meets the current 24-hour PM2.5 
standard in 2015-2017 and has a 3-year visibility index value just 
above 30 dv, rounding to 31 dv (compared to 27 dv when light extinction 
is calculated with the original IMPROVE equations) (U.S. EPA, 2020, 
Appendix D, Table D-3). The unique conditions at this urban site (e.g., 
higher OC concentrations, much lower temperatures, and the complete 
lack of sunlight for long periods) that affect quantitative 
relationships between OC, OM and visibility (e.g., Hand et al., 2012; 
Hand et al., 2013) may differ considerably from those under which the 
Lowenthal and Kumar (2016) equation has been evaluated, making the most 
appropriate approach for characterizing light extinction in this area 
unclear.
    At the time of the last review, the EPA noted that PM2.5 
is the size fraction of PM responsible for most of the visibility 
impairment in urban areas (77 FR 38980, June 29, 2012). Data available 
at the time of the last review suggested that PM10-2.5 was a 
minor contributor to visibility impairment (U.S. EPA, 2010b), although 
this fraction may be responsible for a larger contribution in some 
areas in the desert southwestern region of the U.S. However, at the 
time of the last review, there was very little data available from 
PM10-2.5 monitors to quantify the contribution of coarse PM 
to calculated light extinction.
    Since the last review, the expansion of PM10-2.5 
monitoring efforts has increased the availability of data for use in 
estimating light extinction. As such, both PM2.5 and 
PM10-2.5 concentrations can be included as inputs in the 
equations in the updated analyses in this review. For 2015-2017, 20 of 
the 67 PM2.5 sites analyzed have collocated 
PM10-2.5 monitoring data available. These 20 sites meet both 
the 24-hour PM2.5 and 24-hour PM10 standards. All 
of these sites have 3-year visibility metrics at or below 30 dv 
regardless of whether light extinction is calculated with or without 
the coarse fraction, and for all three versions of the IMPROVE 
equation. Generally, the coarse fraction contribution to light 
extinction is minimal, contributing less than 1 dv to the 3-year 
visibility metric. The 20 locations with collocated PM2.5 
and PM10-2.5 monitoring data available in this review would 
be expected to have relatively low concentrations of coarse PM. In 
areas with higher concentrations of coarse PM, such as the southwestern 
U.S., the coarse fraction may be a more important contributor to light 
extinction and visibility impairment than in the locations included in 
the updated analyses in this review.
    Overall, the results of the updated analyses in this review are 
consistent with those in the last review. The 3-year visibility metric 
is generally at or below 27 dv in areas that meet the current secondary 
standards, with only small differences observed for the three versions 
of the IMPROVE equation. Though such differences are modest, the 
IMPROVE equation from Lowenthal and Kumar (2016) results in higher 
light extinction values, which were expected given the higher OC 
multiplier in the equation and its validation using data from remote 
areas far away from emission sources. There are only small differences 
in estimates of light extinction when the coarse fraction is included 
in the equation, although a somewhat larger coarse fraction 
contribution to light extinction would be expected in areas with higher 
concentrations of coarse PM. Overall, the updated analyses indicate 
that the current secondary PM standards provide a degree of protection 
against visibility impairment similar to the target level of protection 
identified in the last review, in terms of a 3-year visibility index.
b. Non-Visibility Effects
    Consistent with the evidence available at the time of the last 
review, and as described in detail in the PA (U.S. EPA, 2020, section 
5.2.2.2), the data remain insufficient to conduct quantitative analyses 
for PM effects on climate and materials. For PM-related climate 
effects, as explained in more detail in the proposal (85 FR 24131-
24133, 24136, April 30, 2020), our understanding of PM-related climate 
effects is still limited by significant key uncertainties. The newly 
available evidence does not appreciably improve our understanding of 
the spatial and temporal heterogeneity of PM components that contribute 
to climate forcing (U.S. EPA, 2020, sections 5.2.2.1.1 and 5.4). 
Significant uncertainties also persist related to quantifying the 
contributions of PM and PM components to the direct and indirect 
effects on climate forcing, such as changes to the pattern of rainfall, 
changes to wind patterns, and effects on vertical mixing in the 
atmosphere (U.S. EPA, 2020, sections 5.2.2.1.1 and 5.4). Additionally, 
while improvements have been made to climate models since the time of 
the last review, the models continue to exhibit variability in 
estimates of the PM-related climate effects on regional scales (e.g., 
~100 km) compared to simulations at the global scale (U.S. EPA, 2020, 
sections 5.2.2.1.1 and 5.4). While our understanding of

[[Page 82734]]

climate forcing on a global scale is somewhat expanded since the last 
review, significant limitations remain to quantifying potential adverse 
PM-related climate effects in the U.S. and how they would vary in 
response to incremental changes in PM concentrations across the U.S. As 
such, while new research is available on climate forcing on a global 
scale, the remaining limitations and uncertainties are significant, and 
the new global scale research does not translate directly for use at 
regional spatial scales. Therefore, the evidence does not provide a 
clear understanding at the necessary spatial scales for quantifying the 
relationship between PM mass in ambient air and the associated climate-
related effects in the U.S. that would be most relevant to informing 
consideration of a national PM standard on climate in this review (U.S. 
EPA, 2020, section 5.2.2.2.1; U.S. EPA, 2019, section 13.3).
    For PM-related materials effects, as explained in more detail in 
the proposal (85 FR 24133-24134, 24137, April 30, 2020), the available 
evidence has been somewhat expanded to include additional information 
about the soiling process and the types of materials impacted by PM. 
This evidence provides some limited information to inform dose-response 
relationships and damage functions associated with PM, although most of 
these studies were conducted outside of the U.S. where PM 
concentrations in ambient air are typically above those observed in the 
U.S. (U.S. EPA, 2020, section 5.2.2.1.2; U.S. EPA, 2019, section 13.4). 
The evidence available in this review also includes studies examining 
effects of PM on the energy efficiency of solar panels and passive 
cooling building materials, although the evidence remains insufficient 
to establish quantitative relationships between PM in ambient air and 
these or other materials effects (U.S. EPA, 2020, section 5.2.2.1.2). 
While the available evidence is somewhat expanded since the time of the 
last review, quantitative relationships have not been established for 
PM-related soiling and corrosion and frequency of cleaning or repair 
that would help inform our understanding of the public welfare 
implications of materials effects (U.S. EPA, 2020, section 5.2.2.2.2; 
U.S. EPA, 2019, section 13.4). Therefore, there is insufficient 
information to inform quantitative analyses assessing materials effects 
to inform a consideration of a national PM standard on materials in 
this review (U.S. EPA, 2020, section 5.2.2.2.2; U.S. EPA, 2019, section 
13.4).

B. Conclusions on the Secondary Standards

    In drawing conclusions on the adequacy of the current secondary PM 
standards, in view of the advances in scientific knowledge and 
additional information now available, the Administrator has considered 
the evidence base, information, and policy judgments that were the 
foundation of the last review and reflects upon the body of information 
and evidence available in this review. In so doing, the Administrator 
has taken into account both evidence-based and quantitative 
information-based considerations, as well as advice from the CASAC and 
public comments. Evidence-based considerations draw upon the EPA's 
assessment and integrated synthesis of the scientific evidence from 
studies evaluating welfare effects related to visibility, climate, and 
materials associated with PM in ambient air as discussed in the PA 
(summarized in sections IV..B, V.C, and IV.D.1 of the proposal, and 
section IV.A.2 above). The quantitative information-based 
considerations draw from the results of the quantitative analyses of 
visibility impairment presented in the PA (as summarized in section 
IV.D.1 of the proposal and section IV.A.3 above) and consideration of 
these results in the PA.
    Consideration of the evidence and quantitative information in the 
PA and by the Administrator is framed by consideration of a series of 
policy-relevant questions. Section IV.B.2 below summarizes the 
rationale for the Administrator's proposed decision, drawing from 
section IV.D.3 of the proposal. The advice and recommendations of the 
CASAC and public comments on the proposed decision are addressed below 
in sections IV.D.2 and IV.D.3, respectively. The Administrator's 
conclusions in this review regarding the adequacy of the secondary PM 
standards and whether any revisions are appropriate are described in 
section IV.D.4.
1. CASAC Advice in This Review
    In comments on the draft PA, the CASAC concurred with the staff's 
overall preliminary conclusions that it is appropriate to consider 
retaining the current secondary standards without revision (Cox, 
2019a). The CASAC ``finds much of the information . . . on visibility 
and materials effects of PM2.5 to be useful, while 
recognizing that uncertainties and controversies remain about the best 
ways to evaluate these effects'' (Cox, 2019a, p. 13 of consensus 
responses). Regarding climate, while the CASAC agreed that research on 
PM-related effects has expanded since the last review, it also 
concluded that ``there are still significant uncertainties associated 
with the accurate measurement of PM to the direct and indirect effects 
of PM on climate'' (Cox, 2019a, pp. 13-14 of consensus responses). The 
committee recommended that the EPA summarize the ``current scientific 
knowledge and quantitative modeling results for effects of reducing 
PM2.5'' on several climate-related outcomes (Cox, 2019a, p. 
14 of consensus responses), while also recognizing that ``it is 
appropriate to acknowledge uncertainties in climate change impacts and 
resulting welfare impacts in the United States of reductions in 
PM2.5 levels'' (Cox, 2019a, p. 14 of consensus responses). 
When considering the overall body of scientific information for PM-
related effects on visibility, climate, and materials, the CASAC agreed 
that ``the available evidence does not call into question the 
protection afforded by the current secondary PM standards and concurs 
that they should be retained'' (Cox, 2019a, p. 3 of letter).
2. Basis for the Proposed Decision
    At the time of the proposal, the Administrator carefully considered 
the assessment of the current evidence and conclusions reached in the 
ISA; the currently available quantitative information, including 
associated limitations and uncertainties, described in detail and 
characterized in the PA; considerations and staff conclusions and 
associated rationales presented in the PA; and the advice and 
recommendations from the CASAC (85 FR 24137, April 30, 2020).
    In reaching his proposed decision on the secondary PM standards, 
the Administrator first recognized the longstanding body of evidence 
for PM-related visibility impairment. The Administrator recognized that 
visibility impairment can have implications for people's enjoyment of 
daily activities and for their overall sense of well-being. In so 
doing, and consistent with the approach used in the last review 
(section IV.A.1 above), the Administrator first defined a target level 
of protection in terms of a PM visibility index that accounts for the 
factors that influence the relationship between PM in ambient air and 
visibility (i.e., size fraction, species composition, and relative 
humidity). He then considered air quality analyses examining the 
relationship between this PM visibility index and the current 24-hour 
PM2.5 and 24-hour PM10 standards in areas that

[[Page 82735]]

met data completeness criteria for inclusion in the analyses.\79\
---------------------------------------------------------------------------

    \79\ As described in detail in section IV.A.3.a above, the EPA's 
updated quantitative analyses in this review included 67 areas that 
met data completeness criteria for inclusion in the analyses (see 
U.S. EPA, 2020, Appendix D for details of the criteria). Of those 
monitoring locations that met the data completeness criteria, all 
but one location met the current secondary PM2.5 standard 
(U.S. EPA, 2020, Table D-7).
---------------------------------------------------------------------------

    To identify a target level of protection, the Administrator first 
defined the specific characteristics of the visibility index, noting 
that in the last review, the EPA used an index based on estimates of 
light extinction by PM2.5 components calculated using the 
IMPROVE algorithm. As described in section IV.A.2 above, the IMPROVE 
algorithm estimates light extinction using routinely monitored 
components of PM2.5 and PM10-2.5,\80\ along with 
estimates of relative humidity. The Administrator recognized that, 
despite revisions to the IMPROVE algorithm since the last review (U.S. 
EPA, 2020, section 5.2.1.1), our fundamental understanding of the 
relationship between PM in ambient air and light extinction has changed 
little and that the various IMPROVE algorithms can appropriately 
reflect this relationship across the U.S. In the absence of a robust 
monitoring network to measure light extinction (85 FR 24130, 24135, 
April 30, 2020), the Administrator judged that estimated light 
extinction, as calculated using the IMPROVE algorithms, continued to 
provide a reasonable basis for defining a target level of protection 
against PM-related visibility impairment in the current review.
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    \80\ In the last review, the focus was on PM2.5 
components given their prominent role in PM-related visibility 
impairment in urban areas and the limited data available for 
PM10-2.5 (77 FR 38980, June 29, 2010; U.S. EPA, 2020, 
section 5.2.1.2).
---------------------------------------------------------------------------

    In further defining the characteristics of a visibility index based 
on estimates of light extinction, the Administrator considered the 
appropriate averaging time, form, and level of the index. The 
Administrator judged that the decisions made in the last review with 
regard to averaging time and form remain reasonable. In the last 
review, a 24-hour averaging time was judged to be an appropriate 
surrogate for the sub-daily periods relevant for visual perception,\81\ 
recognizing the relatively strong correlations between 24-hour and sub-
daily (i.e., 4-hour) average PM2.5 light extinction and that 
this longer averaging time may be less influenced by atypical 
conditions and/or atypical instrument performance (78 FR 3226, January 
15, 2013). In the decision to set the form as the 3-year average of 
annual 90th percentile values in the last review, it was noted that: 
(1) A 3-year average provided stability from the occasional effect of 
interannual meteorological variability (78 FR 3198, January 15, 2013); 
(2) the 90th percentile corresponds to the 20 percent worst days for 
visibility, which are targeted in Class I areas by the Regional Haze 
program; and (3) available studies on people's visibility preferences 
did not identify a basis for a different target than that identified 
for Class I areas (U.S. EPA, 2011, p. 4-59). Recognizing that the 
information available in the current review is similar to that 
available in the last review, at the time of proposal the Administrator 
judged that these decisions remain reasonable, and it remains 
appropriate to define a visibility index based on estimated light 
extinction in terms of a 24-hour averaging time and a form based on the 
3-year average of annual 90th percentile values.
---------------------------------------------------------------------------

    \81\ While the PM2.5 monitoring network has an 
increasing number of continuous FEM monitors reporting hourly 
PM2.5 mass concentrations, there continue to be data 
quality uncertainties associated with providing hourly 
PM2.5 mass and component measurements that could be input 
into IMPROVE equation calculations for sub-daily visibility 
impairment estimates. As detailed in the PA, there are uncertainties 
associated with the precision and bias of 24-hour PM2.5 
measurements (U.S. EPA, 2020, p. 2-18), as well as to the fractional 
uncertainty associated with 24-hour PM component measurements (U.S. 
EPA, 2020, p. 2-21). Given the uncertainties present when evaluating 
data quality on a 24-hour basis, the uncertainty associated with 
sub-daily measurements may be even greater. Therefore, the inputs to 
these light extinction calculations are based on 24-hour average 
measurements of PM2.5 mass and components, rather than 
sub-daily information.
---------------------------------------------------------------------------

    At the time of the last review, the level of the visibility index 
was set at 30 dv, based on the upper end of the range of levels of 
visibility impairment judged to be acceptable by at least 50% of study 
participants in the available visibility preference studies (U.S. EPA, 
2020, section 5.2.1.1). (78 FR 3226-27, January 15, 2013; 85 FR 24131 
April 30, 2020).\82\ In the last review, the Administrator concluded 
that the substantial degree of variability and uncertainty in the 
public preference studies should be reflected in a target protection 
level at the upper end of the 20 dv to 30 dv range of CPLs. Therefore, 
she concluded that it was appropriate to set a target level of 
protection in terms of a 24-hour PM2.5 visibility index at 
30 dv (78 FR 3226-27, January 15, 2013).
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    \82\ Based on the preference studies, the 2011 PA identified a 
range of levels from 20 to 30 deciviews (dv) as being a reasonable 
range of ``candidate protection levels'' or ``CPLs'' for a 
visibility index (U.S. EPA, 2011, p. 4-61; U.S. EPA, 2020, section 
5.2.1.1).
---------------------------------------------------------------------------

    In considering the preference studies in this review, the 
Administrator first noted that, as a part of the last review, a range 
of levels was identified for the PM2.5 visibility index 
based on an aggregated evaluation of the results of these studies that 
reflected variability in levels of visibility that were considered 
acceptable in the four study areas (U.S. EPA, 2010b). Because no 
visibility preference studies have been conducted in the U.S. since the 
last review, and given the general lack of new preference studies over 
the last several reviews, the Administrator proposed to conclude that 
the range considered in the last review remained appropriate to 
consider in the current review.
    The Administrator highlighted the following uncertainties and 
limitations in the underlying public preference studies (U.S. EPA, 
2020, section 5.2.1.1), consistent with those identified in the last 
review:
     The available studies may not capture the full range of 
visibility preferences in the U.S. population, particularly given the 
potential for preferences to vary based on the visibility conditions 
commonly encountered and the types of scenes being viewed.
     The available preference studies were conducted 15 to 30 
years ago and may not reflect visibility preferences in the U.S. 
population today.
     The available preference studies have used a variety of 
methods, potentially influencing responses as to what level of 
visibility impairment is deemed acceptable.
     Factors that are not captured by the methods used in 
available preference studies may influence people's judgments on 
acceptable visibility, including the duration of visibility impairment, 
the time of day during which light extinction is greatest, and the 
frequency of episodes of visibility impairment.
    After considering these preference studies, along with their 
inherent uncertainties and limitations, the Administrator judged in the 
proposal that a level of 30 dv continued to be an appropriate target 
level of protection for the visibility index in the current review.\83\
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    \83\ As noted above, in the last review, the Administrator 
explained that the current substantial degrees of variability and 
uncertainty inherent in the public preference studies should be 
reflected in a higher target protection level than would be 
appropriate if the underlying information were more consistent and 
certain (78 FR 3216, January 15, 2013).
---------------------------------------------------------------------------

    Having defined a target level of protection in terms of a 
visibility index based on the elements described above, (i.e., with a 
24-hour averaging time; a 3-year average of the annual 90th

[[Page 82736]]

percentile form; and a level of 30 dv), the Administrator next 
considered the degree of protection from visibility impairment afforded 
by the existing secondary standards. In so doing, he considered the 
updated analyses of PM-related visibility impairment (U.S. EPA, 2020, 
section 5.2.1.2), specifically noting the improvements over the 
analyses in the last review, in particular the use of multiple versions 
of the IMPROVE algorithm, including the version incorporating revisions 
since the last review (85 FR 24135-24136, April 30, 2020). The analyses 
in this review expand upon our understanding of how variation in 
equation inputs impacts calculated light extinction (U.S. EPA, 2020, 
Appendix D) and also better characterizes the influence of the coarse 
fraction on light extinction for the subset of sites with available 
PM10-2.5 monitoring data (U.S. EPA, 2020, section 5.2.1.2).
    The Administrator noted that the results of the updated analyses 
are consistent with the results from the last review, regardless of the 
IMPROVE equation used. The results of the analyses demonstrated that, 
in areas meeting the 24-hour PM2.5 standard, the 3-year 
visibility metric is at or below about 30 dv,\84\ and is below 25 dv in 
most of the areas. In those locations with PM10-2.5 
monitoring data available, which met both the current 24-hour 
PM2.5 and 24-hour PM10 standards, 3-year 
visibility metrics were at or below 30 dv regardless of if the coarse 
fraction was included in the calculation (U.S. EPA, 2020, section 
5.2.1.2). In considering these updated analyses, the Administrator 
proposed to conclude that the scientific and quantitative information 
available in this review support the adequacy of the current secondary 
PM2.5 and PM10 standards to protect against PM-
related visibility impairment.
---------------------------------------------------------------------------

    \84\ As discussed above and in the PA (U.S. EPA, 2020, section 
5.2.1.2), one site in Fairbanks, Alaska just meets the current 24-
hour PM2.5 standard and has a 3-year visibility index 
value of 27 dv based on the original IMPROVE equation and 31 dv 
based on the Lowenthal and Kumar (2016) equation. At this site, use 
of the Lowenthal and Kumar (2016) equation may not be appropriate 
given that PM composition and meteorological conditions may differ 
considerably from those under which revisions to the equation have 
been validated.
---------------------------------------------------------------------------

    With respect to non-visibility welfare effects, the Administrator 
considered the evidence related to climate and materials effects and 
proposed to conclude that it is generally appropriate to retain the 
existing secondary standards and that it is not appropriate to 
establish any distinct secondary PM standards to address non-visibility 
PM-related welfare effects. With regard to climate, the Administrator 
recognized that a number of improvements and refinements have been made 
to climate models since the last review, while also noting that 
significant limitations continue to exist in quantifying the 
contributions of the direct and indirect effects of PM and PM 
components on climate forcing (85 FR 24139, April 30, 2020; U.S. EPA, 
2020, sections 5.2.2.1.1 and 5.4). The Administrator also recognized 
that climate models continue to exhibit considerable variability in 
estimates of PM-related climate impacts at regional scales (e.g., ~100 
km) compared to simulations at global scales (85 FR 24139, April 30, 
2020; U.S. EPA, 2020, section 5.2.2.1.1 and 5.4). In considering this 
uncertainty, the Administrator proposed to conclude that the scientific 
information available in the current review remains insufficient to 
quantify the impacts of ambient PM on climate in the U.S. with 
confidence (85 FR 34139, April 30, 2020; U.S. EPA, 2020, sections 
5.2.2.1.1 and 5.4) and that there is insufficient information available 
in this review to base a national ambient air quality standard on 
climate impacts.
    With respect to materials effects, the Administrator recognized 
that deposition of fine or coarse particles can result in physical 
damage and/or impaired aesthetic qualities. Particles can contribute to 
materials damage by adding to the effects of weathering processes and 
by promoting the corrosion of metals, the degradation of painted 
surfaces, the deterioration of building materials, and the weakening of 
material components. The Administrator, while recognizing that some new 
evidence of PM-related materials effects is available in this review, 
noted that this evidence is primarily from studies conducted outside of 
the U.S. with PM concentrations that are higher than those typically 
observed in ambient air in the U.S. (U.S. EPA, 2019, section 13.4). 
Consistent with the information available at the time of the last 
review, the Administrator recognized a limited amount of information 
available on the quantitative relationships between PM and materials 
effects in the U.S., and uncertainties in the degree to which those 
effects could be adverse to public welfare. Therefore, at the time of 
proposal, the Administrator judged that the scientific information 
available in this review remains insufficient to quantify the public 
welfare impacts of PM in ambient air on materials with confidence and 
that there is insufficient information available in this review to 
support a distinct national ambient standard based on materials 
effects.
    Thus, based on consideration of the scientific and quantitative 
information available in this review, with its uncertainties and 
limitations, and information that might inform his public welfare 
judgments, as well as consideration of advice from the CASAC, including 
their concurrence with the PA conclusions that the current evidence 
does not support revision of the secondary PM standards (discussed in 
section IV.B.1 above). The Administrator proposed to conclude that it 
is appropriate to retain the current secondary PM standards without 
revision based on his judgment that the current secondary PM standards 
are requisite to protect against PM-related effects on visibility and 
that there is insufficient information available in this review to base 
a national ambient air quality standard for PM on climate and materials 
impacts.
3. Comments on the Proposed Decision
    Of the public comments received on the proposal, very few were 
specific to the secondary PM standards. Of those commenters who did 
provide comments on the secondary PM standards, the majority support 
the Administrator's proposed decision to retain the current standards. 
Some commenters disagree with the Administrator's proposed conclusion 
to retain the current secondary standards, primarily focusing their 
comments on the need for a revised standard to protect against 
visibility impairment. In addition to the comments addressed in this 
notice, the EPA has prepared a Response to Comments document that 
addresses other specific comments related to setting the secondary PM 
standards. This document is available for review in the docket for this 
rulemaking and through the EPA's NAAQS website (https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards).
    Of the comments addressing the proposed decision, many of the 
commenters support the Administrator's proposed decision to retain the 
current secondary PM standards, without revision. This group includes 
industries and industry groups and state and local governments and 
organizations. All of these commenters generally note their agreement 
with the rationale provided in the proposal and with the views 
expressed by the CASAC that the current evidence does not support 
revision to the standards. Most also cite the EPA and CASAC statements 
that the scientific evidence and quantitative information in this 
review has not substantially altered our previous

[[Page 82737]]

understanding of the effects of PM on visibility, climate, and 
materials beyond what was previously examined and does not call into 
question the adequacy of the current standards. They all find the 
proposed decision to retain the current standards to be well supported 
and a reasonable exercise of the Administrator's public welfare policy 
judgment under the CAA. The EPA agrees with these comments and with the 
CASAC advice regarding the adequacy of the current secondary PM 
standards and the lack of support for revision of these standards.
    Of the commenters who disagree with the proposal to retain the 
current standards, nearly all of these commenters recommend more 
stringent standards, primarily to protect against visibility 
impairment. These comments were submitted primarily by national public 
health, medical, and environmental nongovernmental organizations, and 
some individuals. The commenters who recommend strengthening the 
standards state their support for revisions to provide greater public 
welfare protection, generally claiming that the current standards are 
inadequate and do not provide the requisite protection against known or 
anticipated welfare effects. Additionally, some of the commenters who 
disagree with the proposal did not specifically recommend revising the 
current standards, but instead recommend additional research to address 
key uncertainties and limitations in the available scientific and 
quantitative information that would inform decisions regarding a 
national standard to protect against PM-related non-visibility and 
visibility effects.
    The EPA received relatively few comments on the proposed decision 
that it is not appropriate to establish any distinct secondary PM 
standards to address PM-related climate effects. The majority of the 
comments that were received agree with the EPA that the currently 
available information is not sufficient for supporting quantitative 
analyses for the climate effects of PM in ambient air. These commenters 
support the Administrator's proposed decision not to set a distinct 
standard for climate. Several commenters note, however, that the EPA 
should frequently reconsider the available evidence and quantitative 
information and should revise the standard as necessary to provide 
requisite protection against PM-related climate effects. The EPA agrees 
with the commenters that quantitative analyses of the relationship 
between PM and climate effects are not supported by the available 
information in this review, and new information about PM-related 
welfare effects, including climate, will be assessed consistent with 
CAA requirements in the next review of the PM NAAQS.
    There were also very few commenters who commented on the proposed 
decision that it is not appropriate to establish any distinct secondary 
PM standards to address PM-related material effects. As with comments 
on climate effects, commenters generally agree with the EPA that the 
evidence is not sufficient to support quantitative analyses for PM-
related materials effects. However, some commenters contend that the 
EPA failed to consider the following information: (1) Studies conducted 
outside of the U.S. on the cost of soiling of materials that are also 
found in the U.S.; (2) recent work related to soiling of photovoltaic 
modules and other surfaces, and; (3) quantitative relationships between 
PM in ambient air and materials effects used in several studies. These 
commenters further assert that the EPA failed to specify a level of air 
quality that protects against adverse effects of PM on materials and 
failed to propose a standard that provides requisite protection against 
materials effects attributable to PM.
    We disagree with the commenters that the EPA failed to consider the 
relevant scientific information about materials effects available in 
this review. As an initial matter, the ISA considered and included 
studies related to materials effects of PM, including studies conducted 
in and outside of the U.S., on newly studied materials including 
photovoltaic modules that were published prior to the cutoff date for 
the literature search.\85\ These include the Besson et al. (2017) study 
referenced by the commenters (U.S. EPA, 2019, section 13.4.2). The 
Gr[oslash]ntoft et al. (2019) study referenced by the same commenters 
was published after the cutoff date for the literature search. However, 
the EPA has provisionally considered new studies, including the new 
studies highlighted by the commenters, in the context of the findings 
of the ISA (see Appendix in Response to Comments document).\86\ Based 
on this provisional consideration, the EPA concludes that the new 
studies are not sufficient to alter the conclusions reached in the ISA 
regarding PM and materials effects.
---------------------------------------------------------------------------

    \85\ As noted earlier in section IV, ``the current ISA 
identified and evaluated studies and reports that that have 
undergone scientific peer review and were published or accepted for 
publication between January 1, 2009 and March 31, 2017. A limited 
literature update identified some additional studies that were 
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p. 
A-3).
    \86\ As discussed in section I.D, the EPA has provisionally 
considered studies that were highlighted by commenters and that were 
published after the ISA. These studies are generally consistent with 
the evidence assessed in the ISA, and they do not materially alter 
our understanding of the scientific evidence or the Agency's 
conclusions based on that evidence or warrant reopening of the air 
quality criteria.
---------------------------------------------------------------------------

    Moreover, we disagree with the commenters that the EPA failed to 
consider quantitative information from studies available in this 
review. As detailed in section 5.2.2.1.2 of the PA, a number of new 
studies are available that apply new methods to characterize PM-related 
effects on previously studied materials; however, the evidence remains 
insufficient to relate soiling or damage to specific levels of PM in 
ambient air or to establish quantitative relationships between PM and 
materials degradation. The uncertainties in the evidence identified in 
the last review persist in the evidence in the current review, with 
significant uncertainties and limitations to establishing quantitative 
relationships between particle size, concentration, chemical 
components, and frequency of painting or repair of materials. While 
some new evidence is available in this review, overall, the data are 
insufficient to conduct quantitative analyses for PM-related materials 
effects. Quantitative relationships have not been established between 
characteristics of PM and frequency of repainting or cleaning of 
materials, including photovoltaic panels and other energy-efficient 
materials, that would help inform our understanding of the public 
welfare implications of soiling (U.S. EPA, 2020, section 5.2.2.2.2; 
U.S. EPA, 2019, section 13.4). Similarly, the information does not 
support quantitative analyses between microbial deterioration of 
surfaces and the contribution of carbonaceous PM to the formation of 
black crusts that contribute to soiling (U.S. EPA, 2020, section 
5.2.2.2.2; U.S. EPA, 2019, section 13.4). We also note that 
quantitative relationships are difficult to assess, in particular those 
characterized using damage functions as these approaches depend on 
human perception of the level of soiling deemed to be acceptable and 
evidence in this area remains limited in the current review (U.S. EPA, 
2020, section 5.2.2.1.2). Additionally, we note the CASAC's concurrence 
with conclusions in the PA that uncertainties remain about the best way 
to evaluate materials effects of PM in ambient air (Cox, 2019a, p. 13 
of consensus responses). Further, no new studies are available in this 
review to link human perception of reduced aesthetic appeal of 
buildings

[[Page 82738]]

and other objects to materials effects and PM in ambient air. Finally, 
uncertainties remain about deposition rates of PM in ambient air to 
surfaces and the interaction of PM with copollutants on these surfaces 
(U.S. EPA, 2020, p. 5-34).
    As summarized above and in the proposal, the evidence in this 
review for PM effects on materials is not substantively changed from 
that in the last review. There continues to be a lack of evidence 
related to materials effects that establishes quantitative 
relationships and supports quantitative analyses of PM-related 
materials soiling or damage. While the information available in this 
review continues to support a causal relationship between PM in ambient 
air and materials effects (U.S. EPA, 2019, section 13.4), the EPA is 
unable to relate soiling or damage to specific levels of PM in ambient 
air and is unable to evaluate or consider a level of air quality to 
protect against such materials effects. Although the EPA did not 
propose a distinct level of air quality or a national standard based on 
air quality impacts (85 FR 24139, April 30, 2020), we did identify data 
gaps that prevented us from doing so. The EPA identified a number of 
key uncertainties and areas of future research (U.S. EPA, 2020, p. 5-
42) that may inform consideration of the materials effects of PM in 
ambient air in future reviews of the PM NAAQS.
    Commenters who disagreed with the Administrator's proposed decision 
to retain the current secondary PM standards provided a number of 
comments on the scientific evidence and quantitative analyses for 
visibility impairment. These commenters criticize various aspects of 
the EPA's proposal to retain the standards, including specific aspects 
of the visibility index, the target level of protection identified by 
the Administrator, and the appropriateness of a single national 
standard for purposes of protecting against PM-related visibility 
impairment. In general, these comments indicated support for a more 
stringent standard for visibility impairment, although the commenters 
did not necessarily specify the alternative standard that would, in 
their judgment, address the concerns raised. Rather, most of these 
commenters focused on particular aspects of the visibility metric 
underlying the current secondary standard, including the form, 
averaging time, and target level of protection necessary to protect 
against visibility impairment.
    Several commenters argue that the evidence does not support a 
single level of ``acceptable'' visibility. Commenters expressed the 
view that the public preference studies present important evidence 
related to the importance of visibility, but that they do not provide 
enough information to set a national standard for visibility impairment 
because the results show that visibility preferences vary regionally 
and/or locally for a variety of reasons. Commenters additionally state 
that the EPA failed to explain and analyze the uncertainties associated 
with the public preference studies, including: (1) The different 
methods used in the studies and their influence on the responses; (2) 
the impact of different scenes being viewed on the full range of public 
preferences; and (3) factors that were not considered in the study 
methods that could impact judgments in the studies. These commenters 
suggest that the secondary standards should account for regional 
variability, although they did not provide specific recommendations 
regarding how to accomplish this.
    The EPA agrees with commenters that the available scientific 
evidence indicates that public preferences for ``acceptable'' 
visibility and air quality depends in large part on the characteristics 
of the scene being viewed. The EPA understands that there is a wide 
range of urban and rural scenes within the U.S. and included in the 
public preference studies, including natural vistas such as the Rocky 
Mountains in Colorado and man-made urban structures such as the 
Washington Monument. However, the EPA disagrees with commenters that 
the available evidence cannot support a national standard to protect 
against PM-related visibility impairment. As at the time of the last 
review, the EPA believes that the scenes presented in the public 
preference studies include important types of valued scenic views, and 
therefore, when considered together, can inform consideration of an 
acceptable level of visual air quality at the national scale, taking 
into account variation across the U.S. as evidenced in the studies.
    With regard to the comments that these studies do not provide 
enough information to account for regional variability that is 
important to consider when setting a national standard for visibility 
protection, the EPA recognizes that there may be regional variability 
in the available evidence but believes that these studies provide 
significant information that is useful for the Administrator to 
consider in his judgments on the public welfare implications of PM-
related visibility effects. While the EPA acknowledges that there may 
be regional differences in the stated preferences for visibility, the 
EPA finds there is not enough information available at this time to 
take such regional differences into account. The commenter did not 
provide specific recommendations for the EPA's consideration of such 
information even if such information were available, and the EPA finds 
the question of how, or if, to account for regional preferences in 
setting a national standard is a substantial question that should be 
addressed when it is presented by the available information.
    With regard to the commenters' assertion that the current secondary 
standards are inadequate to protect the public welfare from PM-related 
visibility impairment, the EPA disagrees that the currently available 
information is sufficient to suggest that a more stringent standard is 
warranted. The EPA identified and addressed in great detail the 
limitations and uncertainties associated with the public preference 
studies as a part of the last review (78 FR 3210, January 15, 2013). 
Given that the evidence related to public preferences is the same in 
this review as it was at the time of the last review, the EPA 
reiterated the limitations and uncertainties inherent in this evidence 
as a part of the PA (U.S. EPA, 2020, section 5.5). The PA highlights 
key uncertainties associated with public perception of visibility 
impairment and identifies areas for future research to inform future PM 
NAAQS reviews, including those raised by the commenters (U.S. EPA, 
2020, p. 5-41). For example, the PA notes the critical need for 
information to further our understanding of human perception of 
visibility impairment in public preference studies in order to address 
uncertainties and limitations in the evidence, including an expansion 
of the number and geographic coverage of preference studies in urban, 
rural, and Class I areas to account for the potential for people to 
have different preferences based on the conditions that they commonly 
encounter and potential differences in preferences based on the scene 
types (U.S. EPA, 2020, p. 5-41).
    These same commenters further argue that the EPA omitted recent 
studies that could further inform our understanding of the public 
welfare implications of visibility impairment. Commenters specifically 
point to a recent meta-analysis of available preference studies (Malm 
et al., 2019) and also cites to several related studies (Malm et al., 
2011; Malm, 2013, 2016; Molenar and Malm, 2012). Commenters 
additionally contend that studies of the economic effects of impaired 
visibility were omitted from the ISA and PA and were

[[Page 82739]]

not considered in the EPA's approach for evaluating visibility.
    The EPA disagrees with the commenters that studies related to 
visibility were inappropriately omitted from the ISA in this review. As 
an initial matter, the ISA considered and included studies related to 
PM-related visibility impairment and public preferences that were 
published prior to the cutoff date for the literature search.\87\ As 
described in the Preamble to the ISA, ``studies and reports that have 
undergone scientific peer review and have been published (or accepted 
for publication) are considered for inclusion in the ISA'' (U.S. EPA, 
2015, p. 6). The meta-analysis by Malm et al. (2019) was published 
after the cutoff date for the literature search for the ISA, and 
therefore, was not included in the ISA. Malm et al. (2019) was 
provisionally considered, along with other studies published after the 
cut-off date, and the EPA concluded that these studies did not 
materially change the broad scientific conclusions of the ISA regarding 
welfare effects, including visibility impairment. Moreover, the other 
citations provided by the commenters (Malm et al., 2011; Malm, 2013, 
2016; Molenar and Malm, 2012) are not peer-reviewed publications and as 
such do not meet the criteria for inclusion in the ISA. With regard to 
studies of economic effects, these studies were not considered to be 
within the scope of the ISA, and therefore were not included in this 
review (U.S. EPA, 2019, p. P-16). The studies submitted by the 
commenters, together with other new evidence, will be assessed 
consistent with CAA requirements in the next review of the PM NAAQS.
---------------------------------------------------------------------------

    \87\ As noted earlier in section IV, ``the current ISA 
identified and evaluated studies and reports that that have 
undergone scientific peer review and were published or accepted for 
publication between January 1, 2009 and March 31, 2017. A limited 
literature update identified some additional studies that were 
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p. 
A-3).
---------------------------------------------------------------------------

    Some commenters contend that the EPA's visibility analyses only 
focused on locations that met the current standards. These commenters 
argue that the EPA concluded at the beginning of the analysis that the 
current standards do not need to be revised and that the EPA's approach 
ignores information available since the last review, leading to the 
Administrator to propose no revisions to the standards based on this 
flawed approach.
    We disagree with commenters that the updated analyses of visibility 
impairment in this review only considered air quality in areas that 
meet the current standards. As described in detail in the PA, locations 
included in the analyses were those that met specific data completeness 
criteria for the monitoring data required as inputs to the IMPROVE 
equations for estimating light extinction (U.S. EPA, 2020, Appendix D). 
The data set used for the updated analyses is comprised of sites with 
data for the 2015-2017 period that supported a valid 24-hour 
PM2.5 design value and met strict criteria for PM species. 
For PM2.5 concentrations, data were screened so that all 
days either had a valid filter-based 24-hour concentration measurement 
or at least 18 valid hourly concentration measurements (U.S. EPA, 2020, 
section D.2.1.2).\88\ For coarse PM concentrations, data were included 
for sites with >=11 valid days for each quarter of 2015-2017. For 
PM2.5 component concentrations, data were included for days 
with valid data for all chemical components listed in Table D-1 in the 
PA and for sites with >=11 valid days for each quarter of 2015-
2017.\89\ Of all of the PM monitoring locations in the U.S., 67 
monitoring sites met the data completeness criteria and light 
extinction was calculated without the coarse fraction in the IMPROVE 
equations. Of these 67 monitoring sites, 20 locations met the data 
completeness criteria for coarse PM, and as such, light extinction was 
also estimated with the coarse fraction as an input to the IMPROVE 
equation at these sites (U.S. EPA, 2020, section 5.2.1.2, Appendix D). 
For the sites that met the data completeness criteria for inclusion in 
the analyses, all of the sites met the annual PM2.5 and 24-
hour PM10 standards, and all but one site (located in 
southern California) met the 24-hour PM2.5 standard. 
Therefore, we disagree with the commenters that the analysis was 
designed to consider only locations that met the current standards and 
did not consider locations that did not meet the current secondary PM 
standards. Moreover, the EPA notes that data from areas exceeding the 
current standard are generally of limited use in deciding whether to 
retain the standard, or lower it, because it is not representative or 
informative of circumstances and effects that would be expected to be 
seen upon attainment of the standard.
---------------------------------------------------------------------------

    \88\ A valid filter-based 24-hour concentration measurement is 
one collected via FRM, and that has undergone laboratory 
equilibration (at least 24 hours at standardized conditions of 20-23 
[deg]C and 30-40% relative humidity) prior to analysis (see Appendix 
L of 40 CFR part 50 for the 2012 NAAQS for PM).
    \89\ For coarse PM and PM2.5 components, data 
completeness criteria were selected for the quantitative analyses 
consistent with those in Appendix N of 40 CFR part 50 for the 2012 
NAAQS for PM.
---------------------------------------------------------------------------

    Furthermore, it is unclear what additional information the 
commenters contend that the EPA omitted from its consideration in this 
review. All scientific information available in this review has been 
considered and integrated as a part of the ISA. The Administrator, in 
considering the adequacy of the current secondary PM standards, 
considered the available scientific evidence and quantitative 
information in this review, along with CASAC advice and public 
comments, and concluded that the current secondary PM standards provide 
requisite protection against visibility impairment.
    Some commenters additionally contend that the EPA's evaluation of 
public welfare effects of PM in the proposal solely focuses on fine PM 
and ignores coarse PM. These commenters assert that trends data show 
that coarse PM is increasing, which they believe to be a concern to 
public welfare.
    We disagree with the commenters that the EPA's proposal failed to 
consider the public welfare implications of coarse PM. First, we note 
that there is limited new scientific evidence available in this review 
on climate- and materials-related effects of coarse PM beyond that of 
the last review (85 FR 24131, April 30, 2020). With regard to the 
contribution of coarse PM to visibility impairment, we first note that 
at the time of the last review, the EPA noted that PM2.5 is 
the size fraction of PM responsible for most of the visibility 
impairment in urban areas (U.S. EPA, 2020, p. 5-22). Data available for 
PM10-2.5 was very limited in the last review and was not 
used in quantitative analyses of estimated PM2.5 light 
extinction (U.S. EPA, 2020, Appendix D, section D-1). Since the time of 
the last review, an expansion of PM10-2.5 monitoring efforts 
has increased the availability of data for use in estimating light 
extinction with both fine and coarse fractions of PM. As described in 
the PA, the analyses of visibility impairment were updated in this 
review to include consideration of the coarse fraction of PM in 
estimating light extinction in the subset of areas with 
PM10-2.5 monitoring data available for the time period of 
interest (U.S. EPA, 2020, section 5.2.1.2, Appendix D). The updated 
analyses in this review included 20 sites that measured both 
PM10 and PM2.5 (U.S. EPA, 2020, section 5.2.1.2, 
Appendix D), all of which meet the current 24-hour PM2.5 and 
PM10 standards. All of these sites have 3-year visibility at 
or below 30 dv regardless of whether light extinction is calculated

[[Page 82740]]

with or without the coarse fraction, and for all three versions of the 
IMPROVE equation used in this review. Generally, the contribution of 
the coarse fraction of PM to light extinction in these locations was 
minimal, contributing less than 1 dv to the 3-year visibility metric 
(U.S. EPA, 2020, section 5.2.1.2, Appendix D). While there were not 
monitoring data available to evaluate the impact of coarse PM on 
estimates of light extinction in locations expected to have higher 
concentrations of coarse PM, the coarse fraction may be a more 
important contributor to light extinction and visibility impairment 
than in those areas included in the PA analyses in this review. As 
additional information and monitoring data become available to further 
evaluate the impact of coarse PM on estimates of light extinction in 
more locations, including geographical locations expected to have high 
concentrations of coarse PM, such information will be considered in a 
future PM NAAQS review.
    Several commenters in support of revising the secondary PM 
standards to protect against visibility impairment, generally recommend 
revisions to elements of the secondary standard and visibility index 
(indicator, averaging time, form, and level) consistent with those 
supported by the CASAC and public comments in previous PM reviews. We 
address comments on the elements of a visibility index and a revised 
standard for visibility effects below.
    With regard to an indicator for the secondary standards to protect 
against visibility impairment, a number of commenters suggest that the 
EPA failed to explain why the current indicator is adequate and pointed 
to recommendations from the CASAC in the PM reviews completed in 2012 
and 2006 with regard to alternate indicators. As noted by the 
commenters, in the 2012 review, the CASAC recommended three alternate 
indicators for a secondary standard to protect against visibility 
impairment: (1) Using direct, continuous measurement of PM light 
extinction to support hourly or multi-hour daylight-only averaging 
time(s); (2) using PM speciation data to calculate seasonal (or 
monthly) regional species and relative humidity values to combine with 
the denser continuous PM2.5 monitoring network to calculate 
hourly PM light extinction; or, (3) using hourly PM2.5 as a 
basis for a sub-daily (hourly or multi-hour) daylight-only indicator, 
which would intentionally remove the variable influence of water from 
the regulatory metric. In the 2006 review, as noted by the commenters, 
the CASAC recommended a PM2.5 mass indicator, coupled with 
revisions to the averaging time, form, and level of the standard, to 
protect against visibility impairment.
    The EPA generally agrees with commenters that an indicator based on 
directly measured light extinction would provide the most direct link 
between PM in ambient air and PM-related visibility impairment. 
However, as noted in the proposal (85 FR 24138, April 30, 2020, 
sections IV.B.1 and IV.D.1), the Administrator concluded that in the 
absence of a monitoring network to directly measure light extinction, 
he judged that estimated light extinction, as calculated using the 
IMPROVE algorithms, continues to provide a reasonable basis for 
defining a target level of protection against PM-related visibility 
impairment in the current review. There has been little progress in 
development of such a monitoring network since the time of the last 
review when CASAC concluded that, in the absence of such a monitoring 
network, relying on a calculated PM2.5 light extinction 
indicator based on PM2.5 components and relative humidity 
represented a reasonable approach and that the inputs for calculating 
light extinction were readily available through existing monitoring 
networks and approved monitoring protocols (78 FR 3205, January 15, 
2013). Further, in this review, the CASAC generally agreed with the EPA 
that the available evidence does not call into question the protection 
afforded by the current secondary PM standards and concurs that they 
should be retained.
    With regard to the elements of the visibility index, in considering 
the adequacy of the current secondary PM standards to protect against 
visibility impairment, as described in the proposal (85 FR 24135, April 
30, 2020), the Administrator first defined an appropriate target level 
of protection in terms of a PM visibility index. In defining this 
target level of protection, the Administrator first considered the 
indicator of such an index. He noted that, given the lack of 
availability of methods and an established network for directly 
measuring light extinction, a visibility index based on estimates of 
light extinction by PM2.5 components derived from an 
adjusted version of the original IMPROVE algorithm would be most 
appropriate, consistent with the last review. As described in the 
proposal and above (section IV.A.2.a.i), the IMPROVE algorithm 
estimates light extinction using routinely monitored components of 
PM2.5 and PM10-2.5, along with estimates of 
relative humidity. The Administrator, while recognizing that some 
revisions to the IMPROVE algorithm have been made since the time of the 
last review, noted that the fundamental relationship between ambient PM 
and light extinction has changed very little and the different versions 
of the IMPROVE algorithms can appropriately reflect this relationship 
across the U.S. (85 FR 24138, April 30, 2020). As such, he judged that 
defining a target level of protection in terms of estimated light 
extinction continues to be a reasonable approach in the current review.
    With regard to averaging time, commenters were critical of the 24-
hour averaging time to protect against visibility impairment and argue 
for a sub-daily averaging time. While some comments clearly focused on 
the averaging time of the current secondary PM2.5 standard, 
other comments were unclear as to whether they recommended a sub-daily 
averaging time for the secondary PM2.5 standard or for the 
visibility index used in defining a target level of the protection. 
Nonetheless, all of these commenters contend that people do not 
perceive visibility impairment over a 24-hour period, but rather their 
perception of impairment ranges from minutes to multiday, and that 
daylight hours are much more important in terms of visibility 
impairment, particularly in urban areas. As with comments on the 
indicator of the standard, some commenters also point to previous CASAC 
advice on the need for a sub-daily standard.
    In defining the characteristics of a visibility index, the EPA 
continues to believe that a 24-hour averaging time is reasonable. This 
is in part based on analyses conducted in the last review that showed 
relatively strong correlations between 24-hour and sub-daily (i.e., 4-
hour average) PM2.5 light extinction from the analyses 
conducted in the last review (85 FR 24138, April 30, 2020; 78 FR 3226, 
January 15, 2013), indicating that a 24-hour averaging time is an 
appropriate surrogate for the sub-daily time periods relevant for 
visual perception. The EPA believes that these analyses continue to 
provide support for consideration of a 24-hour averaging time for the 
visibility index in this review. The EPA also recognizes that the 
longer averaging time may be less influenced by atypical conditions 
and/or atypical instrument performance (85 FR 24138, April 30, 2020; 78 
FR 3226, January 15, 2013). When taken together, the available 
scientific information and updated analyses of calculated light 
extinction available in this review continue to support that a 24-hour 
averaging time is appropriate when defining a target level of 
protection

[[Page 82741]]

against visibility impairment in terms of a visibility index.
    Moreover, the EPA disagrees with commenters that a secondary 
PM2.5 standard with a 24-hour averaging time does not 
provide requisite protection against the public welfare impacts of 
visibility impairment. At the time of the last review, the EPA 
recognized that hourly or sub-daily (i.e., 4- to 6-hour) averaging 
times, within daylight hours and excluding hours with high relative 
humidity, are more directly related to the short-term nature of 
visibility impairment and the relevant viewing periods for segments of 
the viewing public than a 24-hour averaging time. At that time, the EPA 
agreed that a sub-daily averaging time would generally be preferable. 
However, the Agency noted significant data quality uncertainties 
associated with the instruments that would provide hourly 
PM2.5 mass concentrations necessary to inform a sub-daily 
averaging time. These uncertainties, as described in the last review, 
included short-term variability in hourly data from available 
continuous monitoring methods, which would prohibit establishing a sub-
daily averaging time (78 FR 3209, January 15, 2013). For all of these 
reasons, the EPA continues to believe that a sub-daily averaging time 
is not supported by the information available in this review.
    With regard to the form of the visibility index, many of the 
commenters contend that the form used in evaluating visibility 
impairment is not appropriate. First, commenters contend that a 90th 
percentile form is too low and excludes too many days that could have 
visibility impairment. These same commenters also suggest that a 3-year 
average form is not justified and does not protect visibility and 
public welfare. These commenters also argue that the EPA failed to 
consider the 98th percentile form for the visibility index as a part of 
the proposal. Second, some commenters recommend a form for the 
visibility index within the range of 95th to 98th percentile, coupled 
with a multi-hour sub-daily averaging time, consistent with the CASAC 
advice in the 2006 review.
    The EPA disagrees with these commenters on both points. With regard 
to the form of the visibility index, the EPA continues to conclude that 
a 3-year average of annual 90th percentile values is appropriate. In so 
doing, the EPA notes that a 3-year average form provides stability from 
the occasional effect of inter-annual meteorological variability that 
can result in unusually high pollution levels for a particular year, 
consistent with the decision in the last review (78 FR 3198, January 
15, 2013; U.S. EPA, 2011, p. 4-58). With regard to the annual 
statistical form to be averaged over 3-years, the EPA considers the 
evaluation in the 2010 UFVA of three different statistics: 90th, 95th, 
and 98th percentiles (U.S. EPA, 2010b, chapter 4). In considering the 
alternative statistical forms, the 2011 PA noted that the Regional Haze 
Program targets the 20 percent most impaired days for improvements in 
visual air quality in Federal Class I areas and that the median of the 
distribution of these 20 percent worst days would be the 90th 
percentile. The 2011 PA further noted that strategies that are 
implemented so that 90 percent of days would have visual air quality 
that is at or below the level of the standard would reasonably be 
expected to lead to improvements in visual air quality for the 20 
percent most impaired days. Finally, the 2011 PA recognized that the 
public preference studies available at the time of the last review did 
not address frequency of occurrence of different levels of visibility 
and did not identify a basis for a different target for urban areas 
than for Federal Class I areas (U.S. EPA, 2011, p. 4-59). The analyses 
and considerations for the form of a visibility index from the 2011 PA 
continue to provide support for a 90th percentile form, averaged across 
three years, in defining the characteristics of a visibility index in 
this review.
    Some commenters contend that the EPA's proposal to retain the level 
of 30 dv for a visibility index is arbitrary, capricious, and not 
technically sound. These commenters assert that the EPA failed to 
consider recent research studies that provide a meta-analysis of 
visibility preference studies that suggest that a level of 30 dv is 
unacceptable to study participants included in the meta-analysis.
    As an initial matter, as described above, the studies cited by the 
commenters in support of their rationale were either published after 
the cutoff date for the literature search for the ISA (Malm et al., 
2019) or were not peer-reviewed studies that met the inclusion criteria 
for the ISA (Malm et al., 2011; Malm, 2013, 2016; Molenar and Malm, 
2012). The EPA provisionally considered the Malm et al. (2019) study 
and concludes that this study does not sufficiently alter the 
conclusions reached in the ISA regarding PM and visibility effects.
    With regard to a level of 30 dv for the visibility index, the EPA 
believes that it is appropriate to establish a target level of 
protection based on the upper end of the range of levels of visibility 
impairment judged to be acceptable by at least 50% of study 
participants in the available visibility preference studies (U.S. EPA, 
2020, section 5.2.1.1). The 2011 PA identified a range of levels from 
20 to 30 dv based on the responses in the public preference studies 
available at that time. Given the lack of new preferences studies 
available in this review, the EPA again relies on the same studies and 
the range of levels identified in those studies in the current review. 
As described in detail in the PA (U.S. EPA, 2020, sections 5.2.1.1 and 
5.5), there are a number of uncertainties and limitations associated 
with the public preference studies, including those described in 
section IV.B.2 above. Recognizing these uncertainties and limitations, 
the EPA concludes that substantial degrees of variability and 
uncertainty in the public preference studies should be reflected in a 
target level of protection at the upper end of the range than if the 
information was more consistent and certain. Therefore, the EPA 
believes that 30 dv is an appropriate level for a visibility index in 
this review.
    A number of commenters advocate for a more stringent standard, 
recommending that the level of the secondary PM2.5 standards 
be lowered. Some commenters reference the recommendations of previous 
CASAC panels for revisions to the secondary 24-hour PM2.5 
standard. Additionally, some commenters contend that the secondary 
PM2.5 standards should be set equal to the primary 
PM2.5 standards, with some of the commenters aligning their 
support for their position with their recommendations for revisions to 
the primary PM2.5 standards in this review.
    We disagree with the commenters that the secondary PM2.5 
standard should be revised to provide additional public welfare 
protection beyond that achieved under the current standard. Based on 
the available scientific and quantitative information, and for the 
reasons discussed above, the EPA concludes that it is appropriate to 
define a target level of protection in terms of a visibility index 
based on estimated light extinction with a 24-hour averaging time, a 3-
year 90th percentile form, and a level of 30 dv. In having concluded 
that this visibility index is appropriate, the EPA then considers the 
degree of protection from visibility impairment afforded by the 
existing standard. In so doing, we consider results of updated analyses 
of calculated light extinction that demonstrate that, in areas meeting 
the current PM mass-based standards, the target level of protection in 
terms of a visibility index is also achieved (85 FR 24135, April 30, 
2020; U.S. EPA, 2020, section 5.2.1.2). The results of these analyses 
(as described in detail in

[[Page 82742]]

section IV.A.3.a above and in section 5.2.1.2 of the PA) demonstrate 
that the 3-year visibility metric is at or below about 30 dv in all 
areas meeting the current PM2.5 standard, and below 25 dv in 
most areas. For those areas with available PM10-2.5 
monitoring data, which met both the current 24-hour PM2.5 
and PM10 standards, 3-year visibility metrics were at or 
below 30 dv regardless of if the coarse fraction was included in the 
calculation (U.S. EPA, 2020, section 5.2.1.2). Given the results of 
these analyses, the Administrator concluded at the time of proposal 
that the updated scientific evidence and quantitative information 
support the adequacy of the current secondary PM2.5 and 
PM10 standards to protect against PM-related visibility 
impairment (85 FR 24138-24139, April 30, 2020).
    With regard to comments recommending to set the secondary 
PM2.5 standards equal to the current primary 
PM2.5 standards, these commenters do not provide a basis for 
their recommendation, nor do they provide a rationale for revising the 
secondary PM2.5 standards to their recommended revised 
levels of the primary PM2.5 standards. However, we note that 
the primary annual PM2.5 standard, with its lower level, 
would be the controlling standard. The EPA disagrees that such 
revisions would be appropriate, for all of the reasons discussed above.
4. Administrator's Conclusions
    In considering the adequacy of the current secondary PM standards 
in this review, the Administrator has carefully considered the: (1) 
Policy-relevant evidence and conclusions contained in the ISA; (2) the 
quantitative information presented and assessed in the PA; (3) the 
evaluation of this evidence, the quantitative information, and the 
rationale and conclusions presented in the PA; (4) the advice and 
recommendations from the CASAC; and (5) public comments, as addressed 
in section IV.B.3 above. In the discussion below, the Administrator 
gives weight to the PA conclusions, with which the CASAC concurred, as 
summarized in section IV.D of the proposal, and takes note of key 
aspects of the rationale for those conclusions that contribute to his 
decision in this review. After giving careful consideration to all of 
this information, the Administrator believes that the conclusions and 
policy judgments supporting his proposed decision remain valid and the 
secondary PM standards should be retained.
    In considering the PA evaluations and conclusions, the 
Administrator specifically takes note of the overall conclusions that 
the welfare effects evidence and quantitative information are generally 
consistent with what was considered in the last review (U.S. EPA, 2020, 
section 5.4). In so doing, he additionally notes that the CASAC 
supports retaining the current standard agreeing with the EPA ``that 
the available evidence does not call into question the protection 
afforded by the current secondary PM standards'' (Cox, 2019a, p. 3 of 
letter). As noted below, the newly available welfare effects evidence, 
critically assessed in the ISA as part of the full body of current 
evidence, reaffirms conclusions on the visibility, climate, and 
materials effects recognized in the last review, including key 
conclusions on which the current standard is based. Further, as 
discussed in more detail above, the updated quantitative analyses of 
visibility impairment for areas meeting the current standards support 
the adequacy of the current secondary PM2.5 and 
PM10 standards to protect against PM-related visibility 
impairment. The Administrator also recognizes limitations and 
uncertainties continue to be associated with the available information.
    With regard to the current evidence on visibility effects, as 
summarized in the PA and discussed in detail in the ISA, the 
Administrator takes note of the long-standing body of evidence for PM-
related visibility impairment. This evidence, which is based on the 
fundamental relationship between light extinction and PM mass, 
demonstrates that ambient PM can impair visibility in both urban and 
remote areas, and has changed very little since the last review (U.S. 
EPA, 2019, section 13.1; U.S. EPA, 2009a, section 9.2.5). The evidence 
related to public perception of visibility impairment comes from 
studies from four areas in North America. These studies provide 
information to inform our understanding of levels of visibility 
impairment that the public judged to be ``acceptable'' (U.S. EPA, 
2010b; 85 FR 24131, April 30, 2020). In considering these public 
preference studies, the Administrator notes that, as described in the 
ISA, no new visibility studies have been conducted in the U.S. and 
there is little newly available information with regard to acceptable 
levels of visibility impairment in the U.S. The Administrator 
recognizes that visibility impairment can have implications for 
people's enjoyment of daily activities and their overall well-being, 
and therefore, considers the degree to which the current secondary 
standards protect against PM-related visibility impairment.
    Based on the considerations discussed above in sections IV.B.2 and 
IV.B.3, the Administrator first concludes, consistent with the last 
review, that a target level of protection for a secondary PM standard 
is most appropriately defined in terms of a visibility index that 
directly takes into account the factors (i.e., species composition and 
relative humidity) that influence the relationship between 
PM2.5 in ambient air and PM-related visibility impairment. 
In defining a target level of protection, the Administrator has 
considered the specific aspects of such an index, including the 
appropriate indicator, averaging time, form, and level.
    First, with regard to indicator, the Administrator notes that in 
the last review, the EPA used an index based on estimates of light 
extinction by PM2.5 components calculated using an adjusted 
version of the IMPROVE algorithm. As described above (section IV.A.3), 
this algorithm allows the estimation of light extinction using 
routinely monitored components of PM2.5 and 
PM10-2.5, along with estimates of relative humidity. The 
Administrator recognizes that, while there have been some revisions to 
the IMPROVE algorithm since the time of the last review, our 
fundamental understanding of the relationship between PM in ambient air 
and light extinction has changed little and the various IMPROVE 
algorithms can appropriately reflect this relationship across the U.S. 
In the absence of a monitoring network for direct measurement of light 
extinction (section IV.A.3), he concludes that calculated light 
extinction indicator that utilizes the IMPROVE algorithms continues to 
provide a reasonable basis for defining a target level of protection 
against PM-related visibility impairment in the current review.
    In further defining the characteristics of a visibility index, the 
Administrator next considers the appropriate averaging time, form, and 
level of the index. Given the available scientific information in this 
review, and in considering the CASAC's advice and public comments, the 
Administrator concludes that, consistent with the decision in the last 
review, a visibility index with a 24-hour averaging time and a form 
based on the 3-year average of annual 90th percentile values remains 
reasonable in this review. With regard to the averaging time and form 
of such an index, the Administrator takes note of analyses conducted in 
the last review that demonstrated relatively strong correlations 
between 24-hour and sub-daily (i.e., 4-hour average) PM2.5 
light extinction (78 FR 3226, January 15,

[[Page 82743]]

2013), indicating that a 24-hour averaging time is an appropriate 
surrogate for the sub-daily time periods of the perception of PM-
related visibility impairment and the relevant exposure periods for 
segments of the viewing public. This decision also recognized that a 
24-hour averaging time may be less influenced by atypical conditions 
and/or atypical instrument performance (78 FR 3226, January 15, 2013). 
The Administrator recognizes that there is no new information in the 
current review to support updated analyses of this nature, and 
therefore, he believes these analyses continue to provide support for 
consideration of a 24-hour averaging time for a visibility index in 
this review. With regard to the statistical form of the index, the 
Administrator notes that, consistent with the last review: (1) A multi-
year percentile form offers greater stability from the occasional 
effect of inter-annual meteorological variability (78 FR 3198, January 
15, 2013; U.S. EPA, 2011, p. 4-58); (2) a 90th percentile represents 
the median of the distribution of the 20 percent worst visibility days, 
which are targeted in Federal Class I areas by the Regional Haze 
Program; and (3) public preference studies did not provide information 
to identify a different target than that identified for Federal Class I 
areas (U.S. EPA, 2011, p. 4-59). Therefore, the Administrator judges 
that a visibility index based on estimates of light extinction, with a 
24-hour averaging time and a 90th percentile form, averaged over three 
years, remains appropriate.
    With regard to the level of a visibility index, the Administrator 
judges that it is appropriate to establish a target level of protection 
of 30 dv, reflecting the upper end of the range of visibility 
impairment judged to be acceptable by at least 50% of study 
participants in the available public preference studies (78 FR 3226, 
January 15, 2013). The 2011 PA identified a range of levels from 20 to 
30 dv based on the responses in the public preference studies available 
at that time. At the time of the last review, the Administrator noted a 
number of uncertainties and limitations in public preference studies, 
including the small number of stated preference studies available, the 
relatively small number of study participants and the extent to which 
the study participants may not be representative of the broader study 
area population in some of the studies, and the variations in the 
specific materials and methods used in each study. In considering the 
available preference studies, with their inherent uncertainties and 
limitations, the prior Administrator concluded that the substantial 
degree of variability and uncertainty in the public preference studies 
should be reflected in a target level of protection based on the upper 
end of the range of CPLs.
    Given that there are no new preference studies available in this 
review, the Administrator notes that his judgments are based on the 
same studies, with the same range of levels, available in the last 
review. The Administrator recognizes a number of limitations and 
uncertainties associated with these studies, as identified in the PA 
(U.S. EPA, 2020, section 5.5), including the following: (1) Available 
studies may not represent the full range of preferences for visibility 
in the U.S. population, particularly given the potential variability in 
preferences based on the conditions commonly encountered and the scenes 
being viewed; (2) available preference studies were conducted 15 to 30 
years ago and may not accurately represent the current day preferences 
of people in the U.S.; (3) the variety of methods used in the 
preference studies may potentially influence the responses as to what 
level of impairment is deemed acceptable; and (4) factors that are not 
captured in the methods of the preference studies, such as the time of 
day when light extinction is the greatest or the frequency of 
impairment episodes, may influence people's judgment on acceptable 
visibility (U.S. EPA, 2020, section 5.2.1.1). Therefore, in considering 
the scientific information, with its uncertainties and limitations, as 
well as public comments on the level of the target level of protection 
against visibility impairment, the Administrator concludes that it is 
appropriate to again use a level of 30 dv for the visibility index.
    Having concluded that the protection provided by a standard defined 
in terms of a PM2.5 visibility index, with a 24-hour 
averaging time, and a 90th percentile form, averaged over 3 years, set 
at a level of 30 dv, is requisite to protect public welfare with regard 
to visual air quality, the Administrator next considers the degree of 
protection from visibility impairment afforded by the existing 
secondary PM standards. This determination requires considering such 
protection not in isolation but in the context of the full suite of 
secondary standards.
    In this context, the Administrator has considered the degree of 
protection from visibility afforded by the existing secondary 
PM2.5 standards. The Administrator has considered both 
whether the existing 24-hour PM2.5 standard of 35 [micro]g/
m\3\ is sufficient (i.e., not under-protective) and whether it is not 
more stringent than necessary (i.e., not over-protective).
    As discussed in section IV.A.3 above, the Administrator considers 
the updated analyses of visibility impairment presented in the PA (U.S. 
EPA, 2020, section 5.2.1.2), which reflect a number of improvements 
since the last review. Specifically, the updated analyses examine 
multiple versions of the IMPROVE equation, including the version 
incorporating revisions since the time of the last review (section 
IV.A.3.a above). These updated analyses provide a further understanding 
of how variation in the inputs to the algorithms impact the estimates 
of light extinction (U.S. EPA, 2020, Appendix D). Additionally, for a 
subset of monitoring sites with available PM10-2.5 data, the 
updated analyses better characterize the influence of coarse PM on 
light extinction than in the last review (U.S. EPA, 2020, section 
5.2.1.2).
    As discussed above in section IV.A.3.a, the results of the updated 
analyses are consistent with those from the last review. Regardless of 
which version of the IMPROVE equation is used, the analyses demonstrate 
that, based on 2015-2017 data, the 3-year visibility metric is at or 
below about 30 dv in all areas meeting the current 24-hour 
PM2.5 standard, and below 25 dv in most of those areas. In 
locations with available PM10-2.5 monitoring, which met both 
the current 24-hour secondary PM2.5 and PM10 
standards, 3-year visibility index metrics were at or below 30 dv 
regardless of whether the coarse fraction was included as an input to 
the algorithm for estimating light extinction (U.S. EPA, 2020, section 
5.2.1.2). While the inclusion of the coarse fraction had a relatively 
modest impact on the estimates of light extinction, as noted in 
responding to comments in section IV.B.3 above, the Administrator 
recognizes the continued importance of the PM10 standard 
given the potential for larger impacts on light extinction in areas 
with higher coarse particle concentrations, which were not included in 
the PA's analyses due to a lack of available data (U.S. EPA, 2019, 
section 13.2.4.1; U.S. EPA, 2020, section 5.2.1.2). He notes that the 
air quality analyses showed that all areas meeting the existing 24-hour 
PM2.5 standard, with its level of 35 [micro]g/m\3\, had 
visual air quality at least as good as 30 dv, based on the visibility 
index. Thus, the secondary 24-hour PM2.5 standard would 
likely be controlling relative to a 24-hour visibility index set at a 
level of 30 dv. Additionally, areas would be unlikely to exceed the 
target level of protection for visibility of 30 dv without

[[Page 82744]]

also exceeding the existing secondary 24-hour standard. Thus, the 
Administrator judges that the 24-hour PM2.5 standard 
provides sufficient protection in all areas against the effects of 
visibility impairment--i.e., that the existing 24-hour PM2.5 
standard would provide at least the target level of protection for 
visual air quality of 30 dv which he judges appropriate.
    With respect to the non-visibility welfare effects of PM in ambient 
air, the Administrator concludes that it is generally appropriate to 
retain the existing standards and that there is insufficient 
information to establish any distinct secondary PM standards to address 
climate and materials effects of PM. With regard to climate, he 
recognizes that there have been a number of improvements and 
refinements to climate models since the last review. However, as 
discussed in sections IV.A.3.b and IV.B.3 above, while the evidence 
continues to support a causal relationship between PM and climate 
effects (U.S. EPA, 2019, section 13.3.9), the Administrator notes that 
significant limitations continue to exist related to quantifying the 
contributions of direct and indirect effects of PM and PM components on 
climate forcing (U.S. EPA, 2020, sections 5.2.2.1.1 and 5.4). He also 
recognizes that that models continue to exhibit considerable 
variability in estimates of PM-related climate impacts at regional 
scales (e.g., ~100 km) as compared to simulations at global scales. 
Therefore, the resulting uncertainty leads the Administrator to 
conclude that the available scientific information in this review 
remains insufficient to quantify climate impacts associated with 
particular concentrations of PM in ambient air (U.S. EPA, 2020, section 
5.2.2.2.1) or to evaluate or consider a level of PM air quality in the 
U.S. to protect against climate effects and that there is insufficient 
information available at this time to base a national ambient standard 
on climate impacts.
    With regard to materials effects, the Administrator notes that the 
evidence available in this review continues to support a causal 
relationship between materials effects and PM deposition (U.S. EPA, 
2019, section 13.4). He recognizes that the deposition of fine and 
coarse particles to materials can lead to physical damage and/or 
impaired aesthetic qualities. Particles can contribute to materials 
damage by adding to the natural weathering processes and by promoting 
the corrosion of metals, the degradation of painted surfaces, the 
deterioration of building materials, and the weakening of material 
components. While some new information is available in this review, as 
discussed in sections IV.A.3.b and IV.B.3 above, this information is 
primarily conducted outside the U.S. in areas where PM concentrations 
in ambient air are typically higher than those observed in the U.S. 
(U.S. EPA, 2020, section 13.4). Additionally, the newly available 
information in this review does not support quantitative analyses of 
PM-related materials effects in this review (U.S. EPA, 2020, section 
5.2.2.2.2). Given the limited amount of information available and its 
inherent uncertainties and limitations, the Administrator concludes 
that he is unable to relate soiling or damage to specific levels of PM 
in ambient air or to evaluate or consider a level of air quality to 
protect against such materials effects, and that there is insufficient 
information available in this review to support a distinct national 
ambient standard based on materials effects.
    With regard to the secondary PM standards, the Administrator 
concludes that it is appropriate to retain the existing secondary PM 
standards, without revision. This conclusion is based on the 
considerations discussed above in sections IV.A.3.b and IV.B.2, 
including the latest scientific information and the advice of the 
CASAC, and the public comments received on the proposal, as discussed 
above in section IV.B.3. For visibility effects, this decision also 
reflects his consideration of the evidence for PM-related light 
extinction, together with his consideration of the updated analyses of 
the protection provided against visibility impairment by the current 
secondary PM2.5 and PM10 standards. For climate 
and materials effects, this conclusion reflects his judgment that, 
although it remains important to maintain secondary PM2.5 
and PM10 standards to provide some degree of control over 
long- and short-term concentrations of both fine and coarse particles, 
there is insufficient information to establish distinct secondary PM 
standards to address non-visibility PM-related welfare effects. The 
Administrator concurs with the advice of the CASAC, which agrees ``that 
the available evidence does not call into question the protection 
afforded by the current secondary PM standards'' and recommends that 
the secondary standards ``should be retained'' (Cox, 2019a, p. 3 of 
letter). This is also consistent with the conclusions at the time of 
the proposal (IV.B.2) and with the majority of public comments received 
on the proposed decision (section IV.B.3).
    In addition, the Administrator judges that, based on his review of 
the science and his judgment that air quality should be maintained to 
provide the target level of protection for visual air quality of 30 dv 
(as discussed in more detail above), the degree of public welfare 
protection provided by the current secondary standards is not greater 
than warranted. This judgment, together with the fact that no CASAC 
member expressed support for a less stringent standard, leads the 
Administrator to conclude that standards less stringent than the 
current secondary standards (e.g., with higher levels) are also not 
supported.
    Thus, based on his consideration of the evidence and analyses for 
welfare effects, his consideration of the CASAC's advice and public 
comments on the secondary standards, and in the absence of information 
that would support establishment of any different standards, the 
Administrator concludes that it is appropriate to retain the current 
24-hour and annual PM2.5 standards and the 24-hour 
PM10 standard, without revision.

D. Decision on the Secondary PM Standards

    For the reasons discussed above and taking into account information 
and assessments presented in the ISA and PA, advice from the CASAC, and 
consideration of public comments, the Administrator concludes that the 
current secondary PM standards are requisite to protect public welfare 
from known or anticipated adverse effects and is retaining the 
standards, without revision.

V. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive Orders 
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 13563: Improving Regulation and Regulatory Review

    The Office of Management and Budget (OMB) determined that this 
action is a significant regulatory action and it was submitted to OMB 
for review. Changes made during Executive Order 12866 review have been 
documented in the docket. Because this action does not change the 
existing PM NAAQS, it does not impose costs or benefits relative to the 
baseline of continuing with the current NAAQS in effect. Thus, the EPA 
has not prepared a Regulatory Impact Analysis for this action.

[[Page 82745]]

B. Executive Order 13771: Reducing Regulations and Controlling 
Regulatory Costs

    This action is not an Executive Order 13771 regulatory action. 
There are no costs or cost savings compared to the current baseline for 
this action because EPA is retaining the current standards.

C. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under 
the PRA. There are no information collection requirements directly 
associated with a decision to retain a NAAQS without any revision under 
section 109 of the CAA and this action retains the current PM NAAQS 
without any revisions.

D. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. Rather, this 
action retains, without revision, existing national standards for 
allowable concentrations of PM in ambient air as required by section 
109 of the CAA. See also American Trucking Associations v. EPA, 175 
F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have significant 
impacts upon small entities because NAAQS themselves impose no 
regulations upon small entities), reviewed in part on other grounds, 
Whitman v. American Trucking Associations, 531 U.S. 457 (2001).

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely 
affect small governments. This action imposes no enforceable duty on 
any state, local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have 
substantial direct effects on the states, on the relationship between 
the national government and the states, or on the distribution of power 
and responsibilities among the various levels of government.

G. Executive Order 13175: Consultation and Coordination With Indian 
Tribal Governments

    This action 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. This action does not change existing 
regulations; it retains the existing PM NAAQS, without revision. 
Executive Order 13175 does not apply to this action.

H. Executive Order 13045: Protection of Children From Environmental 
Health Risks and Safety Risks

    This action is not subject to Executive Order 13045 because it is 
not economically significant as defined in Executive Order 12866. The 
health effects evidence for this action, which includes evidence for 
effects in children, is summarized in section II.B above and is 
described in the ISA and PA, copies of which are in the public docket 
for this action.

I. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use

    This action is not a ``significant energy action'' as defined by 
Executive Order 13211 (66 FR 28355, May 22, 2001) because it is not 
likely to have a significant adverse effect on the supply, 
distribution, or use of energy and has not otherwise been designated as 
a significant energy action by the Administrator of the Office of 
Information and Regulatory Affairs (OIRA).

J. National Technology Transfer and Advancement Act (NTTAA)

    This action does not involve technical standards.

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

    The EPA believes that this action does not have disproportionately 
high and adverse human health or environmental effects on minority, 
low-income populations and/or indigenous peoples, as specified in 
Executive Order 12898 (59 FR 7629, February 16, 1994). The action 
described in this document is to retain without revision the existing 
PM NAAQS based on the Administrator's conclusions that the existing 
primary standards protect public health, including the health of 
sensitive groups, with an adequate margin of safety, and the existing 
secondary standards protect public welfare from known or anticipated 
adverse effects. As discussed in section II, the EPA expressly 
considered the available information regarding health effects among at-
risk populations in reaching the decision that the existing standard is 
requisite.

L. Determination Under Section 307(d)

    Section 307(d)(1)(V) of the CAA provides that the provisions of 
section 307(d) apply to ``such other actions as the Administrator may 
determine.'' Pursuant to section 307(d)(1)(V), the Administrator 
determines that this action is subject to the provisions of section 
307(d).

M. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule 
report to each House of the Congress and to the Comptroller General of 
the United States. The Administrator of OIRA has not determined that 
this action is a ``major rule'' as defined by 5 U.S.C. 804(2).

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List of Subjects in 40 CFR Part 50

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

    Dated: December 4, 2020.
Andrew Wheeler,
Administrator.
[FR Doc. 2020-27125 Filed 12-17-20; 8:45 am]
BILLING CODE 6560-50-P