Petition for Writ of Certiorari — Kentucky, et al., Petitioners v. Environmental Protection Agency, et al.

Supreme Court briefAug 28, 2026

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No. 26-___

In the Supreme Court of the United States

COMMONWEALTH OF KENTUCKY,

STATE OF WEST VIRGINIA, ET AL.

Petitioners,

U.S. ENVIRONMENTAL PROTECTION AGENCY, ET AL.

Respondents.

On Petition for Writ of Certiorari

to the United States Court of Appeals

for the District of Columbia Circuit

APPENDIX – Volume I of II

RUSSELL COLEMAN

Attorney General

Office of the Kentucky

Attorney General

1024 Capital Center

Drive, Suite 200

Frankfort, KY 40601

(502) 696-5300

Matt.Kuhn@ky.gov

MATTHEW F. KUHN

Solicitor General

Counsel of Record

JOHN H. HEYBURN

Principal Deputy

Solicitor General

JACOB M. ABRAHAMSON

Deputy Solicitor General

(Additional Counsel Below)

Office of the

West Virginia

Attorney General State

Capitol Complex

Building 1, Room E-26

Charleston, West

Virginia 25305

(304) 558-2021

mwilliams@wvago.gov

JOHN B. MCCUSKEY

Attorney General

MICHAEL R. WILLIAMS

Solicitor General

(Additional Counsel Listed After Signature Block)

APPENDIX TABLE OF CONTENTS

Page

Appendix A: Opinion, United States Court of

Appeals for the District of Columbia Circuit,

June 26, 2026 .......................................................

Appendix B: Reconsideration of the National

Ambient Air Quality Standards for Particulate

Matter, 89 Fed. Reg. 16202 (March 6, 2024) ......

39a

Appendix C: Primary Statutes ............................ 899a

APPENDIX A

UNITED STATES COURT OF APPEALS

FOR THE DISTRICT OF COLUMBIA CIRCUIT

————

Argued December 16, 2024

Decided June 26, 2026

No. 24-1050

————

COMMONWEALTH OF KENTUCKY, et al.,

Petitioners

ENVIRONMENTAL PROTECTION AGENCY AND LEE M.

ZELDIN, IN HIS OFFICIAL CAPACITY AS ADMINISTRATOR

OF THE U.S. ENVIRONMENTAL PROTECTION AGENCY,

Respondents

ALLIANCE OF NURSES FOR

HEALTHY ENVIRONMENTS, et al.,

Intervenors

————

Consolidated with 24-1051, 24-1052,

24-1073, 24-1091

————

On Petitions for Review of a Final Action

of the Environmental Protection Agency

————

Elbert Lin argued the cause for Industry Petitioners.

With him on the briefs were Lucinda Minton

Langworthy, Erica N. Peterson, Andrew R. Varcoe,

Stephanie A. Maloney, Nate Curtisi, Michael A.

Tilghman II, Brunn (Beau) W. Roysden III, Kristina

(Tina) R. Van Bockern, Emily C. Schilling, and Sydney

J. Sell.

Jacob M. Abrahamson, Assistant Solicitor General,

Office of the Attorney General for the Commonwealth

of Kentucky, argued the cause for State Petitioners.

With him on the briefs were Russell Coleman,

Attorney General, Matthew F. Kuhn, Solicitor General,

Lindsey R. Keiser, Assistant Attorney General, Patrick

Morrisey, Attorney General, Office of the Attorney

General for the State of West Virginia, Michael R.

Williams, Solicitor General, Steve Marshall, Attorney

General, Office of the Attorney General for the State of

Alabama, Edmund G. LaCour Jr., Solicitor General,

Tim Griffin, Attorney General, Office of the Attorney

General for the State of Arkansas, Nicholas J. Bronni,

Solicitor General, Dyland L. Jacobs, Deputy Solicitor

General, Treg R. Taylor, Attorney General, Office of the

Attorney General for the State of Alaska, Jennifer J.

Seely, Assistant Attorney General, Ashley Moody,

Attorney General, Office of the Attorney General for

the State of Florida, Henry C. Whitaker, Solicitor

General, James H. Percival, Chief of Staff, Christopher

M. Carr, Attorney General, Office of the Attorney

General for the State of Georgia, Stephen J. Petrany,

Solicitor General, Theodore E. Rokita, Attorney

General, Office of the Attorney General for the State of

Indiana, James A. Barta, Solicitor General, Raul R.

Labrador, Attorney General, Office of the Attorney

General for the State of Idaho, Alan M. Hurst, Solicitor

General, Brenna Bird, Attorney General, Office of the

Attorney General for the State of Iowa, Eric H. Wessan,

Solicitor General, Kris Kobach, Attorney General,

Office of the Attorney General for the State of Kansas,

Anthony J. Powell, Solicitor General, Lynn Fitch,

Attorney General, Office of the Attorney General for

the State of Mississippi, Justin L. Matheny, Deputy

Solicitor General, Liz Murrill, Attorney General, Office

of the Attorney General for the State of Louisiana, J.

Benjamin Aguinaga, Solicitor General, Andrew Bailey,

Attorney General, Office of the Attorney General for

the State of Missouri, Joshua M. Divine, Solicitor

General, Austin Knudsen, Attorney General, Office of

the Attorney General for the State of Montana,

Christian B. Corrigan, Solicitor General, Drew

Wrigley, Attorney General, Office of the Attorney

General for the State of North Dakota, Philip Axt,

Solicitor General, Michael T. Hilgers, Attorney

General, Office of the Attorney General for the State of

Nebraska, Grant D. Strobl, Assistant Solicitor

General, Dave Yost, Attorney General, Office of the

Attorney General for the State of Ohio, T. Elliot Gaiser,

Solicitor General, Mathura Sridharan, Deputy

Solicitor General, Gentner F. Drummond, Attorney

General, Office of the Attorney General for the State of

Oklahoma, Garry M. Gaskins, II, Solicitor General,

Jennifer L. Lewis, Deputy Attorney General, Marty J.

Jackley, Attorney General, Office of the Attorney

General for the State of South Dakota, Steven Blair,

Deputy Attorney General, Alan Wilson, Attorney

General, Office of the Attorney General for the State of

South Carolina, J. Emory Smith, Jr., Deputy Solicitor

General, Thomas T. Hydrick, Assistant Deputy

Solicitor General, Joseph D. Spate, Assistant Deputy

Solicitor General, Jonathan Skrmetti, Attorney

General and Reporter, Office of the Attorney General

for the State of Tennessee, Whitney Hermandorfer,

Director of Strategic Litigation, Harrison Gray

Kilgore, Strategic Litigation Counsel and Assistant

Solicitor General, Sean D. Reyes, Attorney General,

Office of the Attorney General for the State of Utah,

Stanford E. Purser, Solicitor General, Bridget Hill,

Attorney General, Office of the Attorney General for

the State of Wyoming, D. David DeWald, Deputy

Attorney General, Ken Paxton, Attorney General,

Office of the Attorney General for the State of Texas,

Kellie E. Billings-Ray, Chief, Environmental

Protection Division, and Clayton Smith, Assistant

Attorney General. Kateland R. Jackson, Attorney,

Office of the Attorney General for the State of Texas,

entered an appearance. Matthew D. Hardin and

Christopher C. Horner were on the brief for amicus

curiae Government Accountability & Oversight in

support of petitioners.

Kathy G. Beckett, David M. Flannery, Keeleigh S.

Huffman, and Edward L. Kropp, were on the brief for

amicus curiae Midwest Ozone Group in support of

Industry and Arizona Coalition petitioners.

Sarah A. Buckley and Alexandra L. St. Romain,

Attorneys, U.S. Department of Justice, argued the

causes for respondents. With them on the brief were

Todd Kim, Assistant Attorney General, and David P.

W. Orlin, Attorney, U.S. Environmental Protection

Agency.

Jonathan A. Wiener, Deputy Attorney General,

Office of the Attorney General for the State of

California, argued the cause for respondent-intervenors

State and Local Government. With him on the brief

were Rob Bonta, Attorney General, Stacy Lau and

Corey M. Moffat, Deputy Attorneys General, Kristin K.

Mayes, Attorney General, Office of the Attorney

General for the State of Arizona, Kristin M. Wrobel,

Assistant Attorney General, Kwame Raoul, Attorney

General, Office of the Attorney General for the State of

Illinois, Jason E. James, Assistant Attorney General,

Matthew J. Dunn, Chief, Environmental Enforcement/

Asbestos Litigation Division, William Tong, Attorney

General, Office of the Attorney General for the State of

Connecticut, Matthew I. Levine, Deputy Associate

Attorney General, Jill Lacedonia, Assistant Attorney

General, Anthony G. Brown, Attorney General, Office

of the Attorney General for the State of Maryland,

Michael F. Strande, Assistant Attorney General,

Andrea Joy Campbell, Attorney General, Office of

the Attorney General for the Commonwealth of

Massachusetts, Turner Smith, Assistant Attorney

General & Deputy Chief, Keith Ellison, Attorney

General, Office of the Attorney General for the State of

Minnesota, Peter N. Surdo, Special Assistant Attorney

General, Elizabeth Morrisseau, Assistant Attorney

General, Office of the Attorney General for the State of

Michigan, Matthew J. Platkin, Attorney General,

Office of the Attorney General for the State of New

Jersey, Lisa Morelli, Deputy Attorney General, Letitia

James, Attorney General, Office of the Attorney

General for the State of New York, Barbara D.

Underwood, Solicitor General, Judith N. Vale, Deputy

Solicitor General, Elizabeth A. Brody, Assistant

Solicitor General, Michael J. Myers, Senior Counsel,

Nicholas C. Buttino, Assistant Attorney General, Dave

Sunday, Attorney General, Office of the Attorney

General for the Commonwealth of Pennsylvania, Ann

R. Johnston, Assistant Chief Deputy Attorney General,

Dan Rayfield, Attorney General, Office of the Attorney

General for the State of Oregon, Paul Garrahan,

Attorney-in-Charge, Steve Novick, Special Assistant

Attorney General, Peter F. Neronha, Attorney General,

Office of the Attorney General for the State of Rhode

Island, Alison Hoffman Carney, Assistant Attorney

General, Charity R. Clark, Attorney General, Office of

the Attorney General for the State of Vermont, Melanie

Kehne, Assistant Attorney General, Joshua L. Kaul,

Attorney General, Office of the Attorney General for

the State of Wisconsin, Bradley J. Motl, Assistant

Attorney General, Nick Brown, Attorney General,

Office of the Attorney General for the State of

Washington, Christopher H. Reitz, Assistant Attorney

General, Brian L. Schwalb, Attorney General, Office of

the Attorney General for the District of Columbia,

Caroline S. Van Zile, Solicitor General, Christopher

Gene King, and Sarah Jane Utley. Jennifer Slocum,

Attorney, Office of the Attorney General for the State

of Washington, entered an appearance.

Seth L. Johnson, Marvin C. Brown IV, Shaun A.

Goho, Hayden W. Hashimoto, John Walke, and Emily

Davis were on the brief for respondent-intervenors

Health, Environmental, and Community Group.

Keri N. Powell was on the brief for amicus curiae WE

ACT for Environmental Justice, et al. in support of

respondents.

Jaclyn Lopez and Rachel Curran were on the brief

for amicus curiae Environmental Protection Network

in support of respondents.

Elizabeth J. Hubertz was on the brief for amicus

curiae National Parks Conservation Association in

support of respondents.

Jessica H. Arnell and Jason A. Schwartz were on the

brief for amicus curiae the Institute for Policy

Integrity at New York University School of Law in

support of respondents.

Before: MILLETT and CHILDS, Circuit Judges, and

GINSBURG, Senior Circuit Judge.

Opinion for the Court filed by Senior Circuit Judge

GINSBURG.

I. Background ...............................................

A. Statutory Framework ..........................

B. The 2020 and 2024 Final Rules ..........

C. Procedural History ..............................

II. Analysis .....................................................

A. Statutory Authority .............................

1. Reconsideration or revision? ..........

2. Authority to revise NAAQS ...........

B. Arbitrary and Capricious .....................

1. Impermissible considerations ........

2. Considerations other than public

health ..............................................

3. Other objections .............................

III. Conclusion .................................................

GINSBURG, Senior Circuit Judge: Two groups of

petitioners challenge a 2024 Environmental Protection

Agency rule revising the National Ambient Air Quality

Standards (NAAQS) for particulate matter. Their

claims specifically concern the decision of the

Administrator to revise the primary annual standard

for fine particulate matter (PM2.5) from 12 μg/m3 to 9

μg/m3. In so doing, he expressly reconsidered a decision

of the previous Administrator who in 2020 had decided

to leave the 12 μg/m3 standard in place based upon the

scientific evidence then available.1

In 2025, after this case had been fully briefed and argued,

President Trump appointed a new Administrator of the EPA. For

purposes of this opinion, “the Administrator” refers to the

Administrator appointed by President Biden, who approved the

A group of industry petitioners argue that the EPA

lacked statutory authority to promulgate the new rule;

several states contend the EPA’s decision-making

was tainted by impermissible “environmental justice”

considerations; and all claim the agency action was

arbitrary and capricious in violation of the Clean Air

Act, 42 U.S.C. § 7607(d)(9)(A).

After initially defending the new rule, the EPA now

moves to vacate the rule on the grounds that it

exceeded its statutory authority and acted unreasonably by failing to consider costs. Because these

arguments lack merit, we deny the petitions for review

and the motion for vacatur.

I. Background

The Clean Air Act (CAA) requires that the

Administrator (1) publish a list of air pollutants,

(2) issue air quality criteria for those pollutants, and

(3) promulgate a primary and a secondary air quality

standard for each pollutant for which criteria have

been issued. See generally 42 U.S.C. §§ 7408-09.

Although this case most directly concerns a revision to

the primary standard for PM2.5, it necessarily

implicates steps antecedent to setting the NAAQS for

fine particulate matter.

A. Statutory Framework

The NAAQS-setting process begins with the statutory

requirement in § 7408 that the Administrator publish

and “from time to time thereafter revise” a list of air

pollutants “[f]or the purpose of establishing [NAAQS].”

§ 7408(a)(1). Once the Administrator lists an air

2024 Final Rule. References to “the previous Administrator” are

to the Administrator during President Trump’s first term, who

approved the 2020 Final Rule.

pollutant, he must publish air quality criteria for that

pollutant, which form the scientific basis upon which

the corresponding standard relies. § 7408(a)(2).

Accordingly, the criteria must “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 such pollutant in the ambient air, in

varying quantities.” Id.2

Further, § 7408(c) provides that “[t]he Administrator

shall from time to time review, and, as appropriate,

modify, and reissue any criteria . . . issued pursuant to

this section.” The EPA complies with the requirement

by preparing an “Integrated Science Assessment,”

which is a “comprehensive evaluation and synthesis of

the policy-relevant science” that “serves as the

scientific foundation for the review of ” the NAAQS.

EPA, INTEGRATED SCIENCE ASSESSMENT FOR PARTICULATE MATTER P-9 (2019).

When the Administrator issues criteria for a pollutant, he must “simultaneously” propose a corresponding

standard. § 7409(a)(2). After a period for public

As far as “practicable,” the criteria shall include the following

information:

(A)

those variable factors (including atmospheric

conditions) which of themselves or in combination

with other factors may alter the effects on public

health or welfare of such air pollutant;

(B)

the types of air pollutants which, when present in

the atmosphere, may interact with such pollutant

to produce an adverse effect on public health or

welfare; and

(C)

any known or anticipated adverse effects on

welfare.

§ 7408(a)(2).

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comment, the proposed standard may be promulgated

and become law. § 7409(a)(1)(B). Substantively, the

Administrator must base the standard upon the

scientific criteria and exercise his “judgment” to set

the standard at a level “requisite to protect the

public health” with “an adequate margin of safety.”

§ 7409(b)(1).3 A NAAQS “may be revised in the same

manner as promulgated.” Id.

In short, pursuant to § 7408 the Administrator

maintains a list of air pollutants and publishes

scientific criteria for each one. Then, under § 7409(a)

and (b), the Administrator promulgates a NAAQS for

each listed pollutant and may later revise that

standard in the manner in which it was promulgated.

That brings us to § 7409(d)(1), the interpretation of

which the parties vigorously dispute. In full, the

provision states:

Not later than December 31, 1980, and at fiveyear intervals thereafter, the Administrator

shall complete a thorough review of the

criteria published under section 7408 of this

title and the national ambient air quality

standards promulgated under this section

and shall make such revisions in such criteria

and standards and promulgate such new

National primary ambient air quality standards,

prescribed under subsection (a) shall be ambient air

quality standards 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. Such

primary standards may be revised in the same manner

as promulgated.

§ 7409(b)(1).

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standards as may be appropriate in accordance with section 7408 [and section 7409(b)].

The Administrator may review and revise

criteria or promulgate new standards earlier

or more frequently than required under this

paragraph.

As this court has recognized, the first sentence

requires that the Administrator “complete a ‘thorough’

review of the NAAQS every five years” and “revise the

criteria and standards or promulgate new standards

as appropriate.” Murray Energy Corp. v. EPA, 936 F.3d

597, 605 (2019).

By statutory design, an independent council of

experts — the Clean Air Scientific Advisory Committee

(CASAC) — “assist[s] in this process.” Id. At five-year

intervals, the CASAC is required to “complete a

review” of existing NAAQS and criteria, and “recommend

to the Administrator any new [NAAQS] and revisions

of existing criteria and standards as may be

appropriate.” § 7409(d)(2)(B). The CASAC’s recommendations are timed to arrive approximately one year

prior to the date by which the Administrator must

complete his mandatory quinquennial review. Compare

§ 7409(d)(2)(B) (five-year intervals for the CASAC

beginning on January 1, 1980), with § 7409(d)(1) (fiveyear intervals for the Administrator beginning on

December 31, 1980). “[T]he ultimate decision to revise

the NAAQS — and the determination of the new

level — rests with the Administrator.” Murray, 936

F.3d at 605. To the extent the Administrator’s proposed

or promulgated rule “differs in any important respect

from any of [the CASAC’s] recommendations,” however,

it must be accompanied by a statement that includes

“an explanation of the reasons for such differences.” §

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7607(d)(3); see § 7607(d)(6)(A); Mississippi v. EPA, 744

F.3d 1334, 1355 (D.C. Cir. 2013).

B. The 2020 and 2024 Final Rules

In December 2020, the previous Administrator

promulgated a final rule that retained the preexisting

primary annual standard for PM2.5 of 12.0 μg/m3.

Review of the National Ambient Air Quality Standards

for Particulate Matter (2020 Final Rule), 85 Fed. Reg.

82684, 82685/2-3. The Administrator’s judgment at

that time was based upon the EPA’s science and policy

assessments as well as comments from the CASAC

and the public. Id. On the question whether to retain

the standard of 12.0 μg/m3 for PM2.5, however, the

CASAC “did not reach consensus.” Id. at 82706/1-2.

Those who supported keeping the standard at 12.0

μg/m3 “expressed the view that substantial uncertainty

remains in the evidence for associations between PM2.5

exposures and mortality or serious morbidity effects.”

Id. at 82706/2. Those who supported lowering the

standard “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 μg/m3 are excluded from

analyses.” Id. at 82707/1.

The previous Administrator’s approach in 2020

ultimately tracked the views of those CASAC members

who supported keeping the standard at 12.0 μg/m3. He

emphasized “important uncertainties and limitations”

in the epidemiological studies, expressed concern

“about placing too much weight” on those studies, and

noted his “caution in directly comparing the reported

study mean values to the standard level.” Id. at

82716/3-82717/1. He therefore found it “more appropriate to focus on the body of studies together” and

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took note of “the mean of study-reported means” —

13.5 μg/m3 — which was “above the level of the current

standard.” Id. at 82717/1. Several groups filed

petitions challenging the 2020 Final Rule, which

petitions were consolidated in this court as California

v. EPA, No. 21-1014.

Meanwhile, President Biden assumed office in

January 2021, and in February the EPA filed an

unopposed motion to hold the consolidated petitions in

abeyance for 90 days. In its motion, the EPA explained

that the President had issued an executive order

directing agency review of certain actions, including

the 2020 Final Rule. See Exec. Order No. 13990, 86

Fed. Reg. 7037 (2021). We granted the EPA’s motion

and its eight subsequent motions to extend the period

of abeyance, as a result of which the case has remained

in abeyance for more than five years, spanning the

Biden and Trump Administrations. During that time,

the Administrator initiated the review that culminated in the 2024 Final Rule before us in this case. See

Reconsideration of the National Ambient Air Quality

Standards for Particulate Matter (2024 Final Rule),

89 Fed. Reg. 16202, 16210/2 (explaining the EPA

announced its decision to reconsider the 2020 Final

Rule in June 2021 “because the available scientific

evidence and technical information indicate that the

current standards may not be adequate to protect

public health”).

As part of its review, the EPA reopened and revised

the underlying air quality criteria to account for recent

scientific literature. Id. at 16211/1-2. The result was

the EPA’s Supplement to the 2019 Integrated Science

Assessment for Particulate Matter and a new policy

assessment. See id. at 16212/1-3. The CASAC also

conducted a new review and this time unanimously

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concluded that the 12.0 μg/m3 primary standard

should be lowered. See id. at 16204/2, 16256/3. A

majority of the CASAC recommended a standard of

8 to 10 μg/m3, while a minority deemed a standard of

10 to 11 μg/m3 adequate to protect public health. See

id. at 16204/2.

The EPA promulgated the 2024 Final Rule, titled

“Reconsideration of the National Ambient Air Quality

Standards for Particulate Matter,” in March of that

year. Id. at 16202/1. The Administrator considered the

various positions of the CASAC members and

concluded that setting the standard at 9 μg/m3, the

middle of the range recommended by the CASAC

majority, was appropriate. Id. at 16204/3.

C. Procedural History

Industry groups and several states thereafter filed

petitions challenging the 2024 Final Rule, which this

court consolidated. Other states intervened in support

of the rule, as did a group of health, environmental,

and community entities.

We heard oral argument on the petitions for review

on December 16, 2024. Two months later, the new

Administrator appointed by President Trump filed a

motion to hold the case in abeyance to give the agency

time to review the 2024 Final Rule. We granted that

motion and three more extending the period of

abeyance. In two of those motions, the EPA stated its

intention to begin a new rulemaking to replace the

2024 Final Rule.

In November 2025, however, the EPA filed a motion

for vacatur of the 2024 Final Rule and reversed course

in two ways. First, the EPA now agreed with the

Industry Petitioners that the EPA had exceeded its

statutory authority in issuing the 2024 Final Rule and

15a

had acted unreasonably by ignoring costs. Second, the

EPA seemingly disavowed any intention to begin a

new rulemaking to replace the 2024 Final Rule. The

EPA explained that it had simply been considering

next steps when it said that it would begin a new

rulemaking and now called any future rulemaking

“irrelevant.”

The intervenors opposed the EPA’s motion for

vacatur. The Health Group Intervenors filed a crossmotion for this court to lift the abeyance and decide

the merits of the petitions for review. The EPA agrees

that we should lift the abeyance and asks us to reach

the merits, hold the 2024 Final Rule unlawful, and

vacate it. We now grant the cross-motion to lift the

abeyance and proceed to the merits of the petitions for

review and the motion for vacatur.

II. Analysis

We have jurisdiction under 42 U.S.C. § 7607(b)(1).

We review the 2024 Final Rule under the same

standard as applies to claims brought under the

Administrative Procedure Act. Heating, Air Conditioning

& Refrigeration Distribs. Int’l v. EPA, 71 F.4th 59, 63

(D.C. Cir. 2023). As relevant here, we will set aside the

rule if it is “arbitrary, capricious, an abuse of

discretion,” or “in excess of statutory jurisdiction,

authority, or limitations, or short of statutory right.”

§ 7607(d)(9)(A), (C).

The standard by which we determine whether the

Administrator acted within his statutory authority

differs from the standard applicable when reviewing

the Administrator’s judgment on the merits. With

respect to the former, this court exercises its

“independent judgment in deciding whether an agency

has acted within its statutory authority.” Loper Bright

16a

Enters. v. Raimondo, 603 U.S. 369, 412 (2024). When as

here “a particular statute delegates authority to an

agency consistent with constitutional limits, courts

must respect the delegation, while ensuring that the

agency acts within it.” Id. at 413; see § 7409(b)(1)

(delegating NAAQS-setting authority to the “judgment of the Administrator” within statutorily defined

limits); Whitman v. Am. Trucking Ass’ns, 531 U.S. 457,

474 (2001) (upholding the constitutionality of this

delegation).

With respect to this court’s review of the

Administrator’s decision on its merits, we “defer to the

EPA’s scientific judgment while examining the record

to ensure the Agency has considered the relevant

factors and reasonably explained how it reached its

conclusions.” Am. Farm Bureau Fed’n v. EPA, 559 F.3d

512, 519-20 (D.C. Cir. 2009). “We will give an extreme

degree of deference to the agency when it is evaluating

scientific data within its technical expertise.” Ctr. for

Biological Diversity v. EPA, 749 F.3d 1079, 1087-88

(D.C. Cir. 2014) (cleaned up).

A. Statutory Authority

The Industry Petitioners, the State Petitioners, and

the EPA contend the Administrator lacked statutory

authority to promulgate the 2024 Final Rule. They

argue first that the Administrator acted unlawfully

when he reconsidered the 2020 Final Rule and issued

the Final Rule only four years after the previous

rulemaking; any implicit authority the EPA may have

had to reconsider its rules was displaced, they say, by

§ 7409(d)(1), which requires the Administrator to

review and revise (if appropriate) criteria and NAAQS

every five years and allows him to review and revise

them more frequently than required. They argue

second that insofar as the 2024 Final Rule can be

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characterized as a revision rather than a reconsideration, § 7409(d)(1) requires the Administrator to

perform a “thorough review” of criteria and NAAQS as

part of any revision — including any off-cycle revision

such as this one. Because the EPA did not perform

such a review, they ask us to set aside the 2024 Final

Rule.

According to the State and Health Group Intervenors, § 7409(d)(1) does not displace but supplements

the Administrator’s authority to revise a NAAQS

under § 7409(b)(1); § 7409(d)(1) establishes a mandatory duty — that the Administrator perform a

“thorough review” of NAAQS every five years — that

is “distinct from EPA’s authority to otherwise reconsider or revise its NAAQS.”

1. Reconsideration or revision?

We need not delineate the precise boundaries of the

Administrator’s reconsideration authority because the

2024 Final Rule is best understood as a revision to the

criteria and NAAQS for particulate matter. Although

titled a “reconsideration” of the 2020 Final Rule, it is

in substance a revision of an individual NAAQS.

Indeed, the first paragraph unambiguously states

that it provides the “rationale for the Administrator’s

final decisions to revise the primary annual PM2.5

standard.” 89 Fed. Reg. at 16203/1.

The Industry Petitioners argue the 2024 Final Rule

must be a reconsideration because the Administrator

“prepared only an ‘[Integrated Science Assessment]

Supplement’” and “deferred consideration of a number

of available scientific studies” until the next quinquennial review. They say those facts are controlling

because a true revision “must build a record from

scratch” while a “reconsideration would take as its

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starting point the previous decision not to revise and

the associated record.” Nothing in the statutory

scheme, however, reflects that distinction. We see no

reason the Administrator cannot revise a NAAQS and,

in the process, reconsider a prior rule that left the

existing NAAQS unchanged.4

The Industry Petitioners call upon SEC v. Chenery

Corp., 318 U.S. 80 (1943), to argue that the EPA cannot

“recast” its reconsideration as a revision. “Under the

Chenery doctrine, a reviewing court must confine itself

to the grounds upon which the record discloses that

the agency’s action was based.” Byers v. Comm’r, 740

F.3d 668, 680 (D.C. Cir. 2014). The Chenery doctrine,

however, has no application here because the EPA

consistently characterized its rule as a revision. It did

so in its proposed and final rules, see 88 Fed. Reg. 5558,

5560/2 (2023); 89 Fed. Reg. at 16203/1, and it

repeatedly referred to its decision to “revise” the

“existing” NAAQS in its responses to comments, see

EPA, Responses to Significant Comments on the 2023

Proposed Rule for the Reconsideration of the National

Ambient Air Quality Standards for Particulate Matter

(Responses to Comments) at 27, 34, 46, 56, 118 (2024).

This characterization is unsurprising because the

principal change worked by the 2024 Final Rule was

its revision of the primary annual standard for PM2.5.

To the extent the Industry Petitioners argue that the

Administrator lacks any reconsideration authority, they are

mistaken. The CAA explicitly requires the Administrator to

“convene a proceeding for reconsideration” under certain circumstances. § 7607(d)(7)(B). We need not, however, fully define the

interplay between the Administrator’s reconsideration authority

and his other powers. It is enough that we conclude the

Administrator may revise criteria and NAAQS and, in the course

of so doing, reconsider a previous rule.

19a

2. Authority to revise NAAQS

Our conclusion that the 2024 Final Rule is best

understood as a revision does not answer whether the

Administrator acted within his statutory authority

to revise a NAAQS. The question is particularly

important here because the parties dispute the source

of the Administrator’s revision authority. The Industry

and State Petitioners — joined by the EPA in its

motion for vacatur — argue that § 7409(d)(1) provides

the sole source of the Administrator’s revision

authority and authorizes him to revise NAAQS offcycle only if he first performs a “thorough review” of

the criteria and NAAQS, which the EPA concedes it

did not do before promulgating the 2024 Final Rule.

The intervenors respond that § 7409(b) authorizes the

Administrator to revise a standard and does not

require him to first perform a “thorough review.”

“Statutory interpretation, as we always say, begins

with the text.” Ross v. Blake, 578 U.S. 632, 638 (2016).

As relevant here, two provisions of § 7409 refer to the

Administrator’s authority to revise a NAAQS. Section

7409(b) provides the substantive principles the

Administrator is to apply when setting a primary or

secondary standard and states that each “may be

revised in the same manner as promulgated.” This

provision was added to the CAA in 1970, Pub. L. No.

91-604, § 4(a), 84 Stat. 1676, 1679-80, and predates the

addition of § 7409(d)(1) by seven years, Clean Air Act

Amendments of 1977, Pub. L. No. 95-95, § 106(a),

91 Stat. 685, 691. Consequently, the Administrator

indisputably had authority to revise primary and

secondary NAAQS at any time. So far, so clear.

The first sentence of § 7409(d)(1) then requires the

Administrator to “complete a thorough review” of the

existing criteria and standards every five years and to

20a

revise them if appropriate. The reason for that requirement is obvious: By requiring a “thorough review” the

Congress ensured the Administrator would comprehensively evaluate each and every existing criterion

and standard on a regular basis. In other words, it

serves as an action-forcing device to ensure NAAQS do

not fall too far behind the evolving scientific evidence.

Together § 7409(b) and the first sentence of

§ 7409(d)(1) provide that the Administrator may revise

NAAQS “in the same manner as promulgated” and

must revise NAAQS as appropriate every five years.5

The second sentence of § 7409(d)(1) then adds that

“[t]he Administrator may review and revise criteria or

promulgate new standards earlier or more frequently

than required under this paragraph,” i.e., under the

first sentence. The plain objective of this sentence is to

clarify that the quinquennial review required by the

first sentence of § 7409(d)(1) does not limit the

When questioned about § 7409(b)(1) at oral argument, the

Industry Petitioners again invoked Chenery to argue that the

EPA could not rely upon § 7409(b)(1) as the source of its revision

authority. As an initial matter, we have long held the Chenery

doctrine inapplicable “when the question presented [was] one of

statutory construction.” Bldg. & Constr. Trades Dep’t, AFL-CIO v.

U.S. Dep’t of Lab. Wage Appeals Bd., 829 F.2d 1186, 1189 (1987);

see also Canonsburg Gen. Hosp. v. Burwell, 807 F.3d 295, 304 (D.C.

Cir. 2015) (“We have explained that Chenery only limits judicial

review of factual determinations or policy judgments that the

agency alone is authorized to make” (cleaned up)). In any event,

the Chenery doctrine does not apply here because the EPA raised

§ 7409(b) as a basis for the 2024 Final Rule in response to

comments: “The EPA understands [§ 7409(b)] to authorize the

Administrator to revise the NAAQS.” Responses to Comments at

120. The EPA also cited its “implicit and explicit authority to

revisit earlier decisions on the NAAQS,” id. at 118, a matter we

discuss below at 21, and its “authority under [§ 7409]” generally,

id. at 118, 122, as authorizing its revision.

21a

Administrator’s authority to revise a standard sooner;

nothing in § 7409(d)(1) displaces the Administrator’s

revision authority in § 7409(b).

The petitioners and the EPA raise several objections

to this conclusion. The EPA agrees that it had revision

authority before the Congress added § 7409(d)(1), but

it disputes the source of that authority. The EPA

claims it had “inherent authority” to revise a standard

before 1977, and § 7409(b) provided only the procedure

for doing so. As we have explained, however, “the term

‘inherent’ is misleading because it is axiomatic that

administrative agencies may act only pursuant to

authority delegated to them by Congress. Thus, the

more accurate label for the power [the] EPA describes

is ‘statutorily implicit.’” Nat. Res. Def. Council v. Regan,

67 F.4th 397, 401 (2023) (cleaned up). More to the

point, § 7409(b) expressly recognizes that a NAAQS

“may be revised.” The EPA offers no evidence the

Congress intended to limit the Administrator’s preexisting revision authority in 1977 when it added the

periodic-review requirement. We therefore do not

agree with the EPA and the petitioners that § 7409(d)(1)

provides the “exclusive source of [the Administrator’s]

revision authority.”

Even if § 7409(b) provides revision authority, says

the EPA, we must read that provision in harmony with

§ 7409(d)(1). The petitioners and the EPA reason that

the “thorough review” requirement applies not only to

the quinquennial review mandated by the first sentence

of § 7409(d)(1), but also to the off-cycle revisions

referenced in the second sentence. We do not, however,

understand § 7409(d)(1) to require a “thorough review”

as a precondition to an off-cycle revision.

We begin again with the statutory text. The requirement that the Administrator “complete a thorough

22a

review” of the existing criteria and standards every

five years resides in the first sentence of § 7409(d)(1).

The phrase “thorough review,” however, does not

appear in the second sentence of § 7409(d)(1): “The

Administrator may review and revise criteria or

promulgate new standards earlier or more frequently

than required under this paragraph.” The Industry

Petitioners and the EPA argue this sentence nonetheless implicitly cross-references the first sentence

through the phrases “review” and “under this paragraph.” Per this reading of § 7409(d)(1), the sole

difference between the two sentences is one of timing;

the process for revising a standard — whether at the

required intervals or off-cycle — is the same.

We are not persuaded. Starting with “review,” the

absence of the word “thorough” in the second sentence

of § 7409(d)(1) is significant: “When Congress includes

particular language in one section of a statute but

omits it in another — let alone in the very next

provision — this Court presumes that Congress

intended a difference in meaning.” Loughrin v. United

States, 573 U.S. 351, 358 (2014) (cleaned up). Omitting

the word “thorough” in the second sentence of

§ 7409(d)(1) suggests the Congress did not intend to

require the Administrator to perform such a review

when making an off-cycle revision.

The phrase “under this paragraph” does refer to the

first sentence of § 7409(d)(1), but we do not read that

reference as importing into the second sentence all the

requirements in the first sentence. As the State

Intervenors explain, that reference simply clarifies

that the Administrator may act sooner than required

by the first sentence of § 7409(d)(1), that is, “earlier or

more frequently” than every five years. Indeed, the

Industry Petitioners themselves characterize that

23a

phrase as “a cross-reference to the five-year intervals

mandated in the first sentence of [§ 7409(d)(1)].” To

read it as also importing the “thorough review”

requirement into the second sentence, as the Industry

Petitioners and the EPA ask us to do, would stretch its

meaning to the point of frustrating it. That seems

certain when one considers that this interpretation

would undermine rather than advance the plain

objective of the first sentence: Requiring a full review

whenever the Administrator seeks to revise a single

criterion or standard off-cycle would prevent the

Administrator from responding promptly to changes

in the relevant science.

When pressed on this point at oral argument, the

Industry Petitioners suggested a “thorough review”

outside the five-year cycle would require only that the

Administrator thoroughly review the scientific criteria

underlying the standard he seeks to revise. Oral Arg.

Tr. 39:24-40:15. Even this more limited understanding

of “thorough review” in the second sentence would

undercut the Administrator’s ability to keep a NAAQS

up-to-date. Requiring a comprehensive review of all

the scientific inputs to the criteria for a single

standard would unnecessarily delay promulgation of a

new standard.

This case illustrates the problem. By 2021 the EPA

became aware of certain scientific studies likely to

require a change in the standard for fine particulate

matter and began updating its science and policy

assessments. That process resulted in the EPA’s

Supplement to the 2019 Integrated Science Assessment for Particulate Matter and a new policy assessment, both published in May 2022. See 89 Fed. Reg.

16212/3-16213/1. Requiring the EPA to expend limited

resources to review other scientific studies unlikely to

24a

affect the final standard could delay but not improve

the result.6 That the Congress gave the agency

discretion to address the criteria most relevant to

protecting public health is the only sensible reading of

the statute.

The EPA says its reading of § 7409(d)(1) better

consists with other provisions of the Clean Air Act,

particularly the requirement that the Administrator

establish air quality criteria based upon the “latest

scientific knowledge.” § 7408(a)(2). To the contrary,

as the Health Group Intervenors argue, importing

the “thorough review” requirement into the second

sentence of § 7409(d)(1) would create tension with

several other parts of the statutory scheme. For

example, the provision requiring the Administrator to

issue criteria for newly designated air pollutants also

requires that the Administrator “from time to time

review, and, as appropriate, modify, and reissue any

criteria . . . issued pursuant to this section.” § 7408(c).

It says nothing of performing a thorough review first

and instead affords the Administrator maximum

flexibility to update criteria so as to reflect the

prevailing science. Under the Industry Petitioners’

and the EPA’s interpretation of the second sentence of

§ 7409(d)(1), however, the Administrator could “revise

criteria . . . more frequently than required” by the

At oral argument, the Industry Petitioners said that in order

to perform a “thorough review” of a NAAQS off-cycle the EPA

would not need to revisit studies that it had considered during its

prior review. Oral Arg. Tr. 35:21-36:7. As for new studies, the

notice-and-comment process provides an opportunity for interested parties to argue that the EPA did not consider relevant

studies or that it did not provide a reasoned explanation for the

studies it chose to rely upon. Indeed, the Industry Petitioners did

the former in this rulemaking proceeding, but they did not raise

that argument on appeal. See Oral Arg. Tr. 86:23-88:7.

25a

mandatory review only if he performs the “thorough

review” required by the first sentence. That effectively

reads the thoroughness requirement into § 7408(c),

which is inconsistent with the broad and flexible terms

used in that provision. See Loper Bright Enters., 603

U.S. at 395 (recognizing “appropriate” as a term that

“leaves agencies with flexibility”); cf. Waterkeeper All.

v. EPA, 140 F.4th 1193, 1215 (9th Cir. 2025) (explaining

that the use of the phrase “from time to time” in the

Clean Water Act “suggests that EPA has discretion

about when to revise such standards”).

The Industry Petitioners and the EPA resist this

conclusion by arguing that an off-cycle revision

without a thorough review would undermine the

periodic-review requirement. We do not see how.

Acting sooner than the five-year review cycle required

by § 7409(d)(1) does not excuse the Administrator from

doing a thorough review every fifth year. Nor are the

Industry Petitioners correct that an off-cycle revision

under our interpretation of § 7409 would be “without

any limitations or requirements whatsoever.” Any

revision must satisfy the substantive standard in

§ 7409(b) and comply with the strictures of the CAA,

including its requirement of reasoned decisionmaking.

See Heating, Air Conditioning & Refrigeration

Distribs., 71 F.4th at 63. Those requirements are not

toothless and have been applied by this court when

reviewing NAAQS on many occasions.

In sum, the Administrator must “complete a

thorough review” and, if appropriate, revise criteria

and NAAQS every five years pursuant to the first

sentence of § 7409(d)(1), but he may revise them more

frequently without completing a “thorough review.”

The Administrator therefore acted within his

statutory authority by promulgating the 2024 Final

26a

Rule revising the air quality standard for fine

particulate matter.

B. Arbitrary and Capricious

An agency rule will be held arbitrary and capricious

if, among other reasons, in formulating it the agency

(1) relied upon an impermissible consideration,

(2) entirely failed to consider an important aspect of a

problem, (3) explained the rule in a way that runs

counter to the evidence before it, or (4) adopted a rule

that “is so implausible that it could not be ascribed to

a difference in view or the product of agency expertise.”

Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins.,

463 U.S. 29, 43 (1983). The Industry and State

Petitioners claim the EPA violated the CAA in each of

these respects, and the EPA itself, in moving for

vacatur, now joins the petitioners in asserting that the

Administrator unreasonably ignored the costs

associated with the mid-cycle review giving rise to the

2024 Final Rule. Each of these arguments is either

foreclosed by precedent or unsupported by the record.

1. Impermissible considerations

The State Petitioners argue that the Administrator

relied upon impermissible considerations — namely,

promoting environmental justice and countering

climate change — to justify the 2024 Final Rule. They

begin by correctly observing that the Administrator’s

remit under the CAA is to set primary NAAQS

“requisite to protect the public health” with “an

adequate margin of safety.” § 7409(b)(1). They then

point to President Biden’s Executive Order No. 13990,

which served as the initial catalyst for the EPA’s

review, as evidence of the Administrator’s impermissible environmental goals. See 89 Fed. Reg. at 16210/2

27a

(explaining the order “directed review of certain

agency actions” including the 2020 Final Rule).

The EPA, for its part, did not dispute in its response

brief the State Petitioners’ premise that environmental justice and climate change are impermissible

reasons for setting primary NAAQS under § 7409(b)(1).

Instead, the agency defended the record it made and

characterized the State Petitioners’ argument as little

more than an “unsubstantiated accusation that climate

or environmental justice considerations improperly

influenced the Agency’s course of action.” We agree

with the latter point.

The EPA here is on solid ground; the administrative

record simply does not support the State Petitioners’

claim. The 2024 Final Rule accurately sets out and

adheres to the appropriate legal framework for setting

primary NAAQS consistent with § 7409(b)(1). See 89

Fed. Reg. at 16206/1-16207/1. The 2024 Final Rule

explained that the Administrator announced his decision to reconsider the 2020 Final Rule in June 2021

“because the available scientific evidence and technical

information indicate[d] that the current standards may

not be adequate to protect public health.” Id. at

16210/2. Indeed, in the lead-up to its review the EPA

“preliminarily concluded” that certain new studies

“were likely to be relevant” and “would potentially

warrant a reopening of the air quality criteria.” Id.

at 16211/1. None of these statements suggests the

Administrator considered environmental justice or

climate change. On the contrary, the EPA made clear

in its response to public comments that it had “no

intention or goal to reduce greenhouse gases through

this rulemaking.” Response to Comments at 135.

Against that, the best the State Petitioners can

muster are repeated references to Executive Order No.

28a

13990, which they unilaterally christen “the Climate

Order.” The references to climate change in that order,

however, are not specific to NAAQS-setting and cannot

overcome the evidence — or lack of it — in the

administrative record.

In any event, the EPA addressed this issue head-on

when responding to comments regarding the proposed

rule. The agency made clear that the decision to revisit

the NAAQS for particulate matter “rested with the

EPA” and was grounded upon “additional available

information, as well as advice from the CASAC and

public comment.” Id. at 134. Notably, although the

Administrator ultimately adopted the recommendation of the CASAC, the State Petitioners do not accuse

the CASAC of improper motives. At bottom, the

State Petitioners’ mixture of conjecture and strident

accusations falls far short of demonstrating that the

Administrator relied upon something other than

public health when setting the NAAQS for particulate

matter.

2. Considerations other than public health

The Industry and State Petitioners, now joined by

the EPA, also argue that the 2024 Final Rule is invalid

because the Administrator failed to consider matters

other than public health, principally costs and attainability. Both the Supreme Court and this court have

repeatedly rejected similar efforts to inject considerations unrelated to public health into the NAAQSsetting process. See Am. Trucking, 531 U.S. at 464

(citing D.C. Circuit cases holding that “economic

considerations may play no part in the promulgation

of ambient air quality standards” (cleaned up));

Murray, 936 F.3d at 621-22; Am. Petroleum Inst. v.

Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981). These

cases are controlling.

29a

The petitioners invite us to read these precedents

narrowly in light of the Supreme Court’s opinion in

Michigan v. EPA, 576 U.S. 743 (2015). In that case the

Court considered whether the phrase “appropriate and

necessary” in a different provision of the CAA required

the EPA to consider costs. Id. at 752. Justice Scalia,

writing for the Court, concluded that it did. Id. He

explained that it was not “rational, never mind

‘appropriate,’ to impose billions of dollars in economic

costs in return for a few dollars in health or environmental benefits.” Id. Yet Justice Scalia explicitly

distinguished that case from American Trucking, an

opinion he had authored a decade earlier. He explained

that the provision at issue in the earlier case —

§ 7409(b)(1) — “expressly” directed the EPA to set

NAAQS at levels requisite to protect public health and

therefore should not be read “as implicitly allowing the

[EPA] to consider cost anyway.” Id. at 755-56.

Consistent with that understanding of the statutory

scheme, in Murray this court rejected an invitation

based upon Michigan to incorporate cost considerations into § 7409(d)(1). 936 F.3d at 622 (explaining the

word “appropriate” in § 7409(d)(1) did not require

consideration of economic costs and that the outcome

in Michigan turned upon its “statutory context”).

In other words, Michigan reinforces rather than

undermines American Trucking.

Murray similarly forecloses the petitioners’ argument that the Administrator was required to consider

attainability. In that case the petitioners argued the

CAA “requires EPA to set NAAQS that are attainable.”

Id. To the contrary, we explained that the Congress

“recognized the possibility that some states could not

achieve attainment,” but “rather than watering down

the nationally applicable standards, [it] allowed EPA

to relax enforcement on a case-by-case basis.” Id. at

30a

623. The EPA, that is, may consider attainability

“during enforcement, not when setting standards.” Id.;

see also Costle, 665 F.2d at 1190 (“[T]he question of

attainability is not relevant to the setting of ambient

air quality standards under the Clean Air Act”).

The Industry and State Petitioners attempt to elude

these cases by distinguishing between setting a

NAAQS (to which § 7409(b) applies) and the anterior

decision to revise a NAAQS. In their view, the EPA

must consider costs and attainability when it initially

decides “whether a revision ‘may be appropriate.’” They

reason that the first sentence of § 7409(d)(1) provides

for revisions “as may be appropriate in accordance

with [§ 7408] and [§ 7409(b)],” and Michigan says the

word “appropriate . . . requires at least some attention

to cost,” 576 U.S. at 752. The Industry Petitioners

attempt to bolster this conclusion by pointing to

§ 7409(d)(2)(C), which requires the CASAC to “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.” It is only when the EPA formulates a

standard that the Industry Petitioners concede the

EPA may disregard costs and attainability. The EPA,

for its part, asserts that nothing in § 7409(d)(1)

prevents it from considering costs, but it tellingly fails

to address American Trucking or Murray.

The Industry Petitioners’ and the EPA’s interpretation does not square with that controlling precedent:

Their approach necessarily, but impermissibly, injects

economic considerations and attainability into the

NAAQS-setting process. See Am. Trucking, 531 U.S.

at 464-65; Murray, 936 F.3d at 622 23. Whether the

Administrator considers costs and attainability when

31a

deciding whether to revise a NAAQS or when setting

the NAAQS would have the same impermissible effect.

Moreover, nothing in § 7409(d)(1) suggests the

Congress intended to distinguish between the Administrator considering whether to revise a standard and

his setting a standard. That section requires that the

Administrator revise a NAAQS “as may be appropriate

in accordance with [§ 7408] and [§ 7409(b)].” In this

way, the Congress tied the word “appropriate” in

§ 7409(d)(1) to § 7409(b), which in turn prescribes

NAAQS that “are requisite to protect the public

health.” See Am. Trucking Ass’ns v. EPA, 175 F.3d 1027,

1040 (D.C. Cir. 1999) (explaining that “the clause

immediately following ‘appropriate’ . . . affirmatively

precludes consideration of costs in revising NAAQS”).7

Indeed, this court in Murray explained that we had

“already rejected the idea that ‘appropriate’ in

[§ 7409(d)(1)] requires consideration of economic costs.”

936 F.3d at 622 (citing Am. Trucking Ass’ns, 175 F.3d

at 1040 41, aff’d in relevant part, 531 U.S. at 464-65).

Nor does § 7409(d)(2)(c) support the petitioners’

two-step approach. In Murray we explained why

§ 7409(d)(2)(C) did not imply that economic costs were

permissible considerations in the NAAQS-setting

process: “[T]his provision was intended to ‘enable the

[EPA] to assist the States in carrying out their

The petitioners briefly argue that this conclusion results in

an interpretation of the statutory scheme that violates the nondelegation doctrine. That argument is foreclosed by American

Trucking, in which the Supreme Court held that the “scope of

discretion [§ 7409(b)(1)] allows is in fact well within the outer

limits of our nondelegation precedents.” 531 U.S. at 474. The

petitioners have preserved the issue whether that case was

correctly decided expressly for further review before the Supreme

Court.

32a

statutory role as primary implementers of the NAAQS,’

but had ‘no bearing upon whether cost considerations

are to be taken into account in formulating the

[NAAQS].’” Id. (quoting Am. Trucking, 531 U.S. at 47071). Although Murray spoke of “formulating the

NAAQS,” and not the anterior decision whether to

revise a standard, we do not see how its reasoning

would permit the Administrator to consider costs at

either stage.8

The State Petitioners take a slightly different tack

and, without quite saying so, challenge the holding of

American Trucking directly. They point to the Supreme

Court’s statement that “ozone and particulate matter

are ‘nonthreshold’ pollutants that inflict a continuum

of adverse health effects at any airborne concentration

greater than zero, and hence require the EPA to make

judgments of degree.” 531 U.S. at 475. Consequently,

when setting NAAQS for these pollutants the

Administrator must exercise “discretion” guided by the

requirements of § 7409(b)(1). Id. at 475-76. Then,

quoting Justice Breyer’s concurrence, the State

Petitioners attempt to read consideration of costs into

the exercise of the Administrator’s discretion. See id.

The Industry Petitioners also argue, based upon their twostep interpretation of § 7409(d)(1), that the Administrator was

required to consider current air quality when deciding whether

to revise the NAAQS. They claim the Administrator “overstated

the benefits of a NAAQS revision by assuming a fictional world in

which no area of the country had better air quality than the

NAAQS standard of 12 μg/m3,” when in fact “the current air

quality in most areas significantly out-performs that standard.”

As we have explained, however, it was “not unreasonable for the

EPA to measure expected benefits from the new NAAQS in part

upon the assumption that, if the new NAAQS were not adopted,

then each area would in the future just meet the existing

standard.” Am. Petroleum Inst. v. EPA, 684 F.3d 1342, 1352 (2012).

33a

at 494 (Breyer, J., concurring in part and concurring in

the judgment) (The CAA “does not require the EPA to

eliminate every health risk, however slight, at any

economic cost, however great, to the point of hurtling

industry over the brink of ruin, or even forcing

deindustrialization” (cleaned up)).

The opinion of the Court to the contrary, however,

was clear: § 7409(b)(1) “unambiguously bars cost

considerations from the NAAQS-setting process.” 531

U.S. at 471. As we explained in Murray in response to

the petitioners’ reliance on the same passage from

Justice Breyer’s concurrence, “the concurrence does

not govern our decision, and in any event, Justice

Breyer agreed with the majority that economic costs

could not be considered in setting NAAQS.” 936 F.3d

at 622 (citing Am. Trucking, 531 U.S. at 490).

In short, “when Congress directs an agency to

consider only certain factors in reaching an administrative decision, the agency is not free to trespass

beyond the bounds of its statutory authority by taking

other factors into account.” Lead Indus. Ass’n v. EPA,

647 F.2d 1130, 1150 (D.C. Cir. 1980). Here the EPA

properly followed the Congress’s direction and declined

to consider non-public health factors throughout the

NAAQSsetting process.

3. Other objections

The petitioners also contend the Administrator’s

decision was inadequately explained and insufficiently

supported by the evidence. Their arguments are of

three types. First, the Administrator did not sufficiently justify performing an off-cycle review. Second,

the Administrator failed adequately to explain his

reasons for departing from the conclusions of the 2020

34a

Final Rule. Third, the scientific evidence does not

justify a standard of 9 μg/m3.

Off-Cycle Revision. The petitioners’ first argument

requires little additional discussion. Their charge is

that the Administrator’s decision to perform an offcycle revision was unprecedented and without regard

for the reliance interests of those affected by the

revised standard. As already explained, however, the

Congress authorized the Administrator to revise

criteria and NAAQS outside the five-year cycle in the

interests of public health.

Changed Course. The petitioners’ second argument

merits more attention but is equally unavailing. From

our review of the 2024 Final Rule, we hold that the

Administrator satisfactorily explained his basis for

revising the standard his predecessor had decided not

to revise in the 2020 Final Rule. Far from ignoring the

earlier rule, the Administrator acknowledged that his

analysis differed from that of “the prior Administrator,”

89 Fed. Reg. at 16276/2, and he explained the bases for

his decision at length, id. at 16273/3-16277/1. His

reasons included newly available scientific evidence,

the unanimous recommendation of CASAC, and a

reweighing of the previously available evidence. Id.

With respect to new evidence, the Administrator

explained that “a number of studies” were published

after the previous literature cutoff date, and he

therefore had “additional information for consideration in reaching his final conclusions.” Id. at 16275/2.

Several of these studies improved upon those that

were available to the previous Administrator, see id. at

16276/1 (explaining the studies “employed statistical

approaches that attempted to more extensively

account for confounders and are more robust to model

misspecification”); and at 16276/3 (identifying “several

35a

[new] accountability studies”), including the “study

that report[ed] the long-term mean PM2.5 concentration of 9.3 μg/m3,” id. at 16275/3. The last-referenced

study was particularly significant because the

Administrator set the standard “somewhat below the

lowest long-term study-reported mean PM2.5 concentration reported in key U.S. epidemiologic studies,

which is 9.3 μg/m3.” Id.

The Administrator also explained that the CASAC

had unanimously agreed that the 2020 standard was

inadequate based upon the newly available scientific

evidence. Id. at 16204/3, 16256/3, 16275/1. Although

most of the CASAC’s membership had changed since

its earlier review, two members remained the same. Of

those, one had previously concluded that the annual

PM2.5 standard should remain at 12 μg/m3 only later to

conclude, as the Administrator did, that this standard

was inadequate to protect public health based upon

the latest evidence. See id. at 16257 n.93.

With respect to his evaluation of the scientific

evidence, the Administrator explained that he disagreed

with the approach of the previous Administrator in

certain respects. Recall that the previous Administrator

took a mean-of-means approach to the scientific

studies — i.e., he “placed weight on the average of the

study-reported means (or medians) across the U.S.

monitor-based studies of 13.5 μg/m3.” Id. at 16275/3.

The new Administrator, as already explained, set the

revised standard “below the lowest long-term studyreported mean PM2.5 concentration.” Id. Additionally,

the new Administrator gave weight to “U.S. hybrid

model-based epidemiologic studies,” which he considered

“an advancement in the available science.” Id.

Considering the deference we owe the Administrator’s

scientific judgments, we readily conclude that he

36a

adequately explained his reasons for not retaining the

standard set in the 2020 Final Rule. See Mississippi,

744 F.3d at 1344 (explaining that the Administrator is

not bound by previous NAAQS).

Appropriate Standard. The petitioners also argue

that the evidence does not support a standard of 9

μg/m3. Because § 7409(b)(1) expressly delegates the

setting of NAAQS to the “judgment” of the Administrator,

however, we must respect the Congress’s delegation

and take care not to substitute our judgment for that

of the Administrator, see Loper Bright Enters., 603 U.S.

at 412-13, which “necessarily requires the exercise of

policy judgment,” Mississippi, 744 F.3d at 1358. Put

another way, we must “defer to the EPA’s scientific

judgment while examining the record to ensure the

agency has considered the relevant factors and

reasonably explained how it reached its conclusions.”

Am. Farm Bureau Fed’n, 559 F.3d at 520.

Here the petitioners do not claim the Administrator

overlooked an important scientific study or ignored a

key public-health factor. Nor do they provide meaningful evidence that the Administrator misapplied the

relevant scientific evidence; rather, the Administrator

followed a methodology that this court has upheld on

several occasions. See Nat’l Ass’n of Mfrs. v. EPA, 750

F.3d 921, 924 (D.C. Cir. 2014) (collecting cases in which

this court has upheld a NAAQS set “somewhat below

the lowest long-term mean concentration shown by

certain key epidemiologic studies to cause adverse

health effects”). Under these circumstances, we cannot

agree that the Administrator’s decision was unreasoned

or contrary to the evidence.

That is not to say the Administrator’s application of

§ 7409(b)(1) was the only permissible judgment on

this record. See Mississippi, 744 F.3d at 1348 (“That

37a

the evidence in the record may also support other

conclusions . . . does not prevent us from concluding

that [these] decisions were rational” (cleaned up)).

Setting NAAQS for particulate matter entails the

exercise of judgment in the face of scientific

uncertainty. See 89 Fed. Reg. at 16276/2 (recognizing

“uncertainties and limitations associated with the

epidemiologic studies”); Am. Trucking, 531 U.S. at 475

(explaining that “judgments of degree” are necessary

for “nonthreshold” pollutants). Unsurprisingly, then,

all members of the CASAC recommended revising the

NAAQS downward to levels ranging from 8-11 μg/m3.

See 89 Fed. Reg. at 16280/1 (explaining that “the

majority and minority of the CASAC . . . weighed the

studies in different ways”). The Administrator

ultimately set the standard near the middle of the

range recommended by the CASAC majority.

The Industry Petitioners fault the Administrator for

not sufficiently explaining why he did not select 10

μg/m3, which also would have fallen within the range

staked out by the majority and minority members of

the CASAC. The Industry Petitioners contend that the

Administrator’s “own evidence and reasoning pointed

more toward 10.0 μg/m3 than 9.0 μg/m3.” So they say

because the 2024 Final Rule recognized that “an

annual standard level that is no more than 15-18%

higher than the study-reported means[, or 10.7 to

11.0 μg/m3,] would generally maintain air quality

exposures to be below those . . . for which we have the

strongest support for adverse health effects occurring.”

89 Fed. Reg. at 16241/3. Yet the Administrator

explained that he was setting the standard “below the

lowest study-reported mean” to ensure that those

living where PM2.5 concentrations are highest “will be

exposed to PM2.5 concentrations below the PM2.5

concentrations reported in the epidemiologic studies

38a

where there is the highest confidence of an association” with adverse health effects. Id. at 16263/2. The

Administrator’s regard for populations in areas with

the highest PM2.5 concentration was permissible. See

Nat’l Ass’n of Mfrs., 750 F.3d at 926 (finding the

Administrator’s consideration of populations near

heavily trafficked roads reasonable “[i]n the context of

[§ 7409]”).

In short, the Administrator “offered reasoned

explanations for how [he] approached and weighed the

evidence, and why the scientific evidence supported

[his] revision of the NAAQS.” Id. at 924. That is

sufficient for us to uphold his decision on its merits.

III. Conclusion

For the reasons stated, the petitions for review and

the EPA’s motion for vacatur are

Denied.

39a

APPENDIX B

16202 Federal Register/Vol. 89, No. 45/Wednesday,

March 6, 2024/Rules and Regulations

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, and 58

[EPA–HQ–OAR–2015–0072; FRL–8635–02– OAR]

RIN 2060–AV52

Reconsideration of the National Ambient Air Quality

Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

————

SUMMARY: Based on the Environmental Protection

Agency’s (EPA’s) reconsideration of the air quality

criteria and the national ambient air quality standards (NAAQS) for particulate matter (PM), the EPA

is revising the primary annual PM2.5 standard by

lowering the level from 12.0 μg/m3 to 9.0 μg/m3. The

Agency is retaining the current primary 24-hour

PM2.5 standard and the primary 24-hour PM10 standard. The Agency also is not changing the secondary

24-hour PM2.5 standard, secondary annual PM2.5

standard, and secondary 24-hour PM10 standard at

this time. The EPA is also finalizing revisions to

other key aspects related to the PM NAAQS,

including revisions to the Air Quality Index (AQI)

and monitoring requirements for the PM NAAQS.

DATES: This final rule is effective May 6, 2024.

ADDRESSES: The EPA has established a docket for

this action under Docket ID No. EPA–HQ–OAR–

2015–0072. All documents in the docket are listed on

40a

the https://www.regulations.gov website. Although

listed in the index, some information is not publicly

available, e.g., CBI or other information whose

disclosure is restricted by statute. Certain other

material, such as copyrighted material, is not placed

on the internet and will be publicly available only in

hard copy form. Publicly available docket materials

are available electronically through https://

www.regulations.gov.

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:

Table of Contents

The following topics are discussed in this preamble:

Executive Summary

I. Background

A. Legislative Requirements

B. Related PM Control Programs

C. Review of the Air Quality Criteria and

Standards for Particulate Matter

1. Reviews Completed in 1971 and 1987

2. Review Completed in 1997

3. Review Completed in 2006

4. Review Completed in 2012

5. Review Initiated in 2014

41a

a. 2020 Proposed and Final Decisions

b. Reconsideration of the 2020 PM NAAQS

Final Action

D. Air Quality Information

1. Distribution of Particle Size in Ambient Air

2. Sources and Emissions Contributing to PM

in the Ambient Air

3. Monitoring of Ambient PM

4. Ambient Concentrations and Trends

a. PM2.5 Mass

b. PM2.5 Components

c. PM10

d. PM10–2.5

e. UFP

5. Characterizing

Ambient

Concentrations for Exposure

PM2.5

a. Predicted Ambient PM2.5 and Exposure

Based on Monitored Data

b. Comparison

PM2.5

Fields

Estimating Exposure and Relative to

Design Values

6. Background PM

II. Rationale for Decisions on the Primary PM2.5

Standards

A. Introduction

1. Background on the Current Standards

2. Overview of the Health Effects Evidence

42a

a. Nature of Effects

i. Mortality

ii. Cardiovascular Effects

iii. Respiratory Effects

iv. Cancer

v. Nervous System Effects

vi. Other Effects

b. Public Health Implications and At-Risk

Populations

c. PM2.5 Concentrations in Key Studies

Reporting Health Effects

i. PM2.5

Exposure

Concentrations

Evaluated in Experimental Studies

ii. Ambient PM2.5 Concentrations in

Locations of Epidemiologic Studies

d. Uncertainties in the Health Effects

Evidence

3. Summary of Exposure and Risk Estimates

a. Key Design Aspects

b. Key Limitations and Uncertainties

c. Summary of Risk Estimates

B. Conclusions on the Primary PM2.5 Standards

1. CASAC Advice

2. Basis for the Proposed Decision

3. Comments on the Proposed Decision

4. Administrator’s Conclusions

C. Decisions on the Primary PM2.5 Standards

43a

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

2. Basis for the Proposed Decision

3. Comments on the Proposed Decision

4. Administrator’s Conclusions

C. Decisions on the Primary PM10 Standard

IV. Communication of Public Health

A. Air Quality Index Overview

B. Air Quality Index Category Breakpoints for

PM2.5

1. Summary of Proposed Revisions

a. Air Quality Index Values of 50, 100, and

150

b. Air Quality Index Values of 200 and

Above

44a

2. Summary of Significant Comments on

Proposed Revisions

a. Air Quality Index Values of 50, 100, and

150

b. Air Quality Index Values of 200 and

Above

c. Other Comments

3. Summary of Final Revisions

C. Air Quality Index Category Breakpoints for

PM10

D. Air Quality Index Reporting

1. Summary of Proposed Revisions

2. Summary of Significant Comments on

Proposed Revisions

3. Summary of Final Revisions

V. Rationale for Decisions on the Secondary PM

Standards

A. Introduction

1. Background on the Current Standards

a. Non-Visibility Effects

b. Visibility Effects

2. Overview of Welfare Effects Evidence

a. Nature of Effects

i. Visibility

ii. Climate

iii. Materials

45a

3. Summary of Air Quality and Quantitative

Information

a. Visibility Effects

i. Target Level of Protection in Terms

of a PM2.5 Visibility Index

ii. Relationship Between the PM2.5

Visibility Index and the Current

Secondary 24-Hour PM2.5 Standard

b. Non-Visibility Effects

B. Conclusions on the Secondary PM Standards

1. CASAC Advice

2. Basis for the Proposed Decision

3. Comments on the Proposed Decision

4. Administrator’s Conclusions

C. Decisions on the Secondary PM Standards

VI. Interpretation of the NAAQS for PM

A. Amendments to Appendix K: Interpretation of

the NAAQS for Particulate Matter

B. Amendments to Appendix N: Interpretation of

the NAAQS for PM2.5

VII. Amendments to Ambient Monitoring

Quality Assurance Requirements

and

A. Amendment to 40 CFR Part 50 (Appendix L):

Reference Method for the Determination of

Fine Particulate Matter as PM2.5 in the

Atmosphere—Addition of the Tisch Cyclone as

an Approved Second Stage Separator

B. Issues Related to 40 CFR Part 53 (Reference

and Equivalent Methods)

46a

C. Changes to 40 CFR Part 58 (Ambient Air

Quality Surveillance)

D. Incorporating

Technologies

Data

From

Next-Generation

VIII. Clean Air Act Implementation Requirements for

the Revised Primary Annual PM2.5 NAAQS

A. Designation of Areas

B. Section 110(a)(1) and (2) Infrastructure SIP

Requirements

C. Implementing the Revised Primary Annual

PM2.5 NAAQS in Nonattainment Areas

D. Implementing the Primary and Secondary

PM10 NAAQS

E. Prevention of Significant Deterioration and

Nonattainment New Source Review Programs

for the Revised Primary Annual PM2.5 NAAQS

F. Transportation Conformity Program

G. General Conformity Program

IX. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning

and Review and Executive Order 14094:

Modernizing Regulatory Review

B. Paperwork Reduction Act (PRA)

C. Regulatory Flexibility Act (RFA)

D. Unfunded Mandates Reform Act (UMRA)

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and

Coordination With Indian Tribal Governments

47a

G. Executive Order 13045: Protection of Children

From Environmental Health and Safety Risks

H. Executive Order 13211: Actions Concerning

Regulations That Significantly Affect Energy

Supply, Distribution or Use

I. National

Technology

Advancement Act (NTTAA)

Transfer

and

J. Executive Order 12898: Federal Actions To

Address Environmental Justice in Minority

Populations and Low-Income Populations and

Executive Order 14096: Revitalizing Our

Nation’s Commitment to Environmental

Justice for All

K. Congressional Review Act (CRA) References

Executive Summary

This document presents the Administrator’s final

decisions for the reconsideration of the 2020 final

decision on the primary (health-based) and secondary

(welfare-based) National Ambient Air Quality Standards

(NAAQS) for Particulate Matter (PM). More specifically, this document summarizes the background and

rationale for the Administrator’s final decisions to

revise the primary annual PM2.5 standard by lowering

the level from 12.0 μg/m3 to 9.0 μg/m3; to retain the

current primary 24-hour PM2.5 standard (at a level of

35 μg/m3); to retain the primary 24-hour PM10

standard; and, not to change the secondary PM

standards at this time. In reaching his final decisions, the Administrator considered the currently

available scientific evidence in the 2019 Integrated

Science Assessment (2019 ISA) and the Supplement

to the 2019 ISA (ISA Supplement), quantitative and

policy analyses presented in the 2022 Policy

48a

Assessment (2022 PA), advice from the Clean Air

Scientific Advisory Committee (CASAC), and public

comments on the proposal. The EPA has established

primary and secondary standards for PM2.5, which

includes particles with diameters generally less than

or equal to 2.5 mm, and PM10, which includes

particles with diameters generally less than or equal

to 10 mm. The standards include two primary PM2.5

standards: an annual average standard, averaged

over three years, with a level of 12.0 μg/m3, and a 24hour standard with a 98th percentile form, averaged

over three years, and a level of 35 μg/m3. It also

includes a primary PM10 standard with a 24-hour

averaging time, and a level of 150 μg/m3, not to be

exceeded more than once per year on average over

three years. Secondary PM standards are set equal to

the primary standards, except that the level of the

secondary annual PM2.5 standard is 15.0 μg/m3.

The most recent of the PM NAAQS was completed

in December 2020. In that review, the EPA retained

the primary and secondary NAAQS, without revision

(85 FR 82684, December 18, 2020). Following publication of the 2020 final action, several parties filed

petitions for review and petitions for reconsideration

of the EPA’s final decision.

In June 2021, the Agency announced its decision to

reconsider the 2020 PM NAAQS final action.1 The

EPA decided to reconsider the December 2020

decision because the available scientific evidence and

technical information indicated that the current

standards may not be adequate to protect public

The press release for this announcement is available at:

https://www.epa.gov/newsreleases/epa-reexamine-health-stand

ards-harmful-soot-previous-administration-left-unchanged.

49a

health and welfare, as required by the Clean Air Act.

The EPA noted that the 2020 PA concluded that

the scientific evidence and information called into

question the adequacy of the primary PM2.5 standards

and supported consideration of revising the level of

the primary annual PM2.5 standard to below the

current level of 12.0 μg/m3 while retaining the

primary 24-hour PM2.5 standard (U.S. EPA, 2020b).

The EPA also noted that the 2020 PA concluded that

the available scientific evidence and information did

not call into question the adequacy of the primary

PM10 or secondary PM standards and supported

consideration of retaining the primary PM10 standard

and secondary PM standards without revision (U.S.

EPA, 2020b).

The final decisions presented in this document on

the primary PM2.5 standards have been informed by

key aspects of the available health effects evidence

and conclusions contained in the 2019 ISA and ISA

Supplement, quantitative exposure/risk analyses and

policy evaluations presented in the 2022 PA, advice

from the CASAC2 and public comment received as

part of this reconsideration. 3 The health effects

In 2021, the Administrator announced his decision to

reestablish the membership of the CASAC. The Administrator

selected seven members to serve on the chartered CASAC, and

appointed a PM CASAC panel to support the chartered

CASAC’s review of the draft ISA Supplement and the draft PA

as a part of this reconsideration (see section I.C.6.b below for

more information).

More information regarding the CASAC review of the draft

ISA Supplement and the draft PA, including opportunities for

public comment, can be found in the following Federal Register

notices: 86 FR 54186, September 30, 2021; 86 FR 52673,

September 22, 2021; 86 FR 56263, October 8, 2021; 87 FR 958,

January 7, 2022.

50a

evidence newly available in this reconsideration, in

conjunction with the full body of evidence critically

evaluated in the 2019 ISA, supports a causal relationship between long- and short-term exposures and

mortality and cardiovascular effects, and the evidence

supports a likely to be a causal relationship between

long-term exposures and respiratory effects, nervous

system effects, and cancer. The longstanding evidence

base, including animal toxicological studies, controlled

human exposure studies, and epidemiologic studies,

reaffirms, and in some cases strengthens, the conclusions from past reviews regarding the health effects

of PM2.5 exposures. Epidemiologic studies available in

this reconsideration demonstrate generally positive,

and often statistically significant, PM2.5 health effect

associations. Such studies report associations between

estimated PM2.5 exposures and non-accidental,

cardiovascular, or respiratory mortality; cardiovascular or respiratory hospitalizations or emergency room

visits; and other mortality/morbidity outcomes (e.g.,

lung cancer mortality or incidence, asthma development). The scientific evidence available in this

reconsideration, as evaluated in the 2019 ISA and

ISA Supplement, includes a number of epidemiologic

studies that use various methods to characterize

exposure to PM2.5 (e.g., ground-based monitors and

hybrid modeling approaches) and to evaluate associations between health effects and lower ambient PM2.5

concentrations. There are a number of recent

epidemiologic studies that use varying study designs

that reduce uncertainties related to confounding and

exposure measurement error. The results of these

analyses provide further support for the robustness of

associations between PM2.5 exposures and mortality

and morbidity. Moreover, the Administrator notes

that recent epidemiologic studies strengthen support

51a

for health effect associations at lower PM2.5 concentrations, with these new studies finding positive and

significant associations when assessing exposure in

locations and time periods with lower annual mean

and 25th percentile concentrations than those evaluated in epidemiologic studies available at the time of

previous reviews. Additionally, the experimental

evidence (i.e., animal toxicological and controlled

human exposure studies) strengthens the coherence

of effects across scientific disciplines and provides

additional support for potential biological pathways

through which PM2.5 exposures could lead to the overt

population-level outcomes reported in epidemiologic

studies for the health effect categories for which a

causal relationship (i.e., short- and long-term PM2.5

exposure and mortality and cardiovascular effects) or

likely to be causal relationship (i.e., short- and longterm PM2.5 exposure and respiratory effects; and

long-term PM2.5 exposure and nervous system effects

and cancer) was concluded.

The available evidence in the 2019 ISA continues

to provide support for factors that may contribute to

increased risk of PM2.5-related health effects including lifestage (children and older adults), pre-existing

diseases (cardiovascular disease and respiratory

disease), race/ethnicity, and socioeconomic status.

For example, the 2019 ISA and ISA Supplement

conclude that there is strong evidence that Black and

Hispanic populations, on average, experience higher

PM2.5 exposures and PM2.5-related health risks than

non-Hispanic White populations. In addition, studies

evaluated in the 2019 ISA and ISA Supplement also

provide evidence indicating that communities with

lower socioeconomic status (SES), as assessed in

epidemiologic studies using indicators of SES

including income and educational attainment are, on

52a

average, exposed to higher concentrations of PM2.5

compared to higher SES communities.

The quantitative risk assessment, as well as policy

considerations in the 2022 PA, also inform the final

decisions on the primary PM2.5 standards. The risk

assessment in this reconsideration focuses on allcause or nonaccidental mortality associated with

long- and short-term PM2.5 exposures. The primary

analyses focus on exposure and risk associated with

air quality that might occur in an area under air

quality conditions that just meet the current and

potential alternative standards. The risk assessment

estimates that the current primary PM2.5 standards

could allow a substantial number of PM2.5-associated

premature deaths in the United States, and that

public health improvements would be associated with

just meeting all of the alternative (more stringent)

annual and 24-hour standard levels modeled.

Additionally, the results of the risk assessment

suggest that for most of the U.S., the annual

standard is the controlling standard and that

revision to that standard has the most potential to

reduce PM2.5 exposure-related risk. The analyses are

summarized in this document and in the proposal

and are described in detail in the 2022 PA.

In its advice to the Administrator, in its review of

the 2021 draft PA, the CASAC concurred that the

currently available health effects evidence calls into

question the adequacy of the primary annual PM2.5

standard. With regard to the primary annual PM2.5

standard, the majority of the CASAC concluded that

the level of the standard should be revised within the

range of 8.0 to 10.0 μg/m3, while the minority of the

CASAC concluded that the primary annual PM2.5

standard should be revised to a level of 10.0 to 11.0

53a

μg/m . With regard to the primary 24-hour PM2.5

standard, the CASAC did not reach consensus on the

adequacy of the current standard. The majority of the

CASAC concluded that the primary 24- hour PM2.5

was not adequate and that the level of the standard

should be revised to within the range of 25 to 30

μg/m3, while the minority of the CASAC concluded

that the standard was adequate and should be

retained, without revision. Additionally, in their

review of the 2019 draft PA, the CASAC did not

reach consensus on the adequacy of the primary

annual PM2.5 standard, with the minority recommending revision and the majority recommending the

standard be retained. In their review of the 2019

draft PA, the CASAC reached consensus regarding

the adequacy of the primary 24-hour PM2.5 standard,

concluding that the standard should be retained.

In considering how to revise the suite of primary

PM2.5 standards to provide the requisite degree of

protection, the Administrator recognizes that the

current annual standard and 24-hour standard,

together, are intended to provide public health

protection against the full distribution of short- and

long- term PM2.5 exposures. Further, he 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.

As in 2012, the Administrator concludes that the

most effective way to reduce total population risk

associated with both long- and short-term PM2.5

exposures is to set a generally controlling annual

standard, and to provide supplemental protection

against the occurrence of peak 24-hour PM2.5

concentrations by means of a 24-hour standard set at

54a

the appropriate level. Based on the current evidence

and quantitative information, as well as consideration of CASAC advice and public comments, the

Administrator concludes that the current primary

annual PM2.5 standard is not adequate to protect

public health with an adequate margin of safety. The

Administrator notes that the CASAC was unanimous

in its advice on the 2021 draft PA regarding the need

to revise the annual standard. In considering the

appropriate level for a revised annual standard, the

Administrator concludes that a standard set at a

level of 9.0 μg/m3 reflects his judgment about placing

the most weight on the strongest available evidence

while appropriately weighing the uncertainties.

With regard to the primary 24-hour PM2.5 standard, the Administrator finds the available scientific

evidence and quantitative information to be insufficient to call into question the adequacy of the public

health protection afforded by the current 24-hour

standard. He further notes that a more stringent

annual standard set at a level of 9.0 μg/m3 is expected

to reduce both average (annual) concentrations and

peak (daily) concentrations. The Administrator also

notes that, in their review of the 2021 draft PA, the

CASAC did not reach consensus on whether revisions

to the primary 24-hour PM2.5 standard are warranted

at this time. He also notes that, in their review of

the 2019 draft PA, the CASAC did reach consensus

that the primary 24-hour PM2.5 standard should be

retained. The Administrator concludes that the 24hour standard should be retained to continue to

provide requisite protection against short-term peak

PM2.5 concentrations, particularly when considered in

conjunction with the protection provided by the suite

of standards and the decision to revise the annual

standard to a level of 9.0 μg/m3.

55a

The primary PM10 standard is intended to provide

public health protection against health effects related

to exposures to PM10–2.5, which are particles with a

diameter between 10 mm and 2.5 mm. The final

decision to retain the current 24-hour PM10 standard

has been informed by key aspects of the available

health effects evidence and conclusions contained in

the 2019 ISA, the policy evaluations presented in

the 2022 PA, advice from the CASAC and public

comments. Specifically, the health effects evidence for

PM10–2.5 exposures is somewhat strengthened since

past reviews, although the strongest evidence still

only provides support for a suggestive of, but not

sufficient to infer, causal relationship with long- and

short-term exposures and mortality and cardiovascular effects, short-term exposures and respiratory

effects, and long-term exposures and cancer, nervous

system effects, and metabolic effects. In reaching his

final decision on the primary PM10 standard, the

Administrator recognizes that, while the available

health effects evidence has expanded, recent studies

are subject to the same types of uncertainties that

were judged to be important in previous reviews. He

also recognizes that, in their review of the 2019 draft

PA and the 2021 draft PA, the CASAC generally

agreed that it was reasonable to retain the primary

24-hour PM10 standard given the available scientific

evidence, including retaining PM10 as the indicator.

He concludes that the newly available evidence does

not call into question the adequacy of the current

primary PM10 standard, and retains that standard,

without revision.

With respect to the secondary PM standards, this

reconsideration focuses on visibility, climate, and

56a

materials effects. The Administrator’s final decision

to not change the current secondary standards at this

time has been informed by key aspects of the currently available welfare effects evidence as well as

the conclusions contained in the 2019 ISA and ISA

Supplement; quantitative analyses of visibility

impairment; policy evaluations presented in the 2022

PA; advice from the CASAC; and public comments.

Specifically, the welfare effects evidence available in

this reconsideration is consistent with the evidence

available in previous reviews and supports a causal

relationship between PM and visibility, climate, and

materials effects. With regard to visibility effects, the

Administrator notes that he judges that the evidence

supports a target level of protection of 27 dv. He

further notes that the results of quantitative analyses of

visibility impairment suggest that in areas that meet

the current secondary 24-hour PM2.5 standard that

estimated light extinction in terms of a 3-year

visibility metric would be at or well below the target

level of protection. With regard to climate and

materials effects, while the evidence has expanded

since previous reviews, significant limitations and

Consistent with the 2016 Integrated Review Plan (U.S.

EPA, 2016), other welfare effects of PM, such as ecological

effects, are being considered in the separate, on-going review of

the secondary NAAQS for oxides of nitrogen, oxides of sulfur

and PM. Accordingly, the public welfare protection provided by

the secondary PM standards against ecological effects such as

those related to deposition of nitrogen- and sulfur-containing

compounds in vulnerable ecosystems is being considered in that

separate review. Thus, the Administrator’s conclusion in this

reconsideration of the 2020 final decision is focused only and

specifically on the adequacy of public welfare protection

provided by the secondary PM standards from effects related to

visibility, climate, and materials and hereafter “welfare effects”

refers to those welfare effects.

57a

uncertainties remain in the evidence. While the

evidence has expanded since previous reviews, the

available scientific evidence remains insufficient to

allow the Administrator to make a reasoned judgment about what specific standard(s) would be

requisite to protect against known or anticipated

adverse effects to public welfare from PM’s effects on

materials damage or climate.- In their review of the

2019 draft PA and the 2021 draft PA, the CASAC did

not recommend revising the secondary PM standards.

In considering the available evidence and quantitative information, with its inherent uncertainties and

limitations, the Administrator judges that it is

appropriate not to change the secondary PM

standards at this time.

The final revisions to the primary annual PM2.5

NAAQS trigger a process under which States (and

Tribes, if they choose) make recommendations to the

Administrator regarding designations, identifying

areas of the country that either meet or do not meet

the new or revised PM NAAQS. Those areas that do

not meet the revised PM NAAQS will need to develop

plans that demonstrate how they will meet the

standards. As part of these plans, states have the

opportunity to advance environmental justice, in this

case for overburdened communities in areas with

high PM concentrations above the NAAQS, by using

the tools described in the current PM NAAQS

implementation guidance (80 FR 58010, 58136,

August 25, 2016). The EPA is not making changes to

any of the current PM NAAQS implementation

programs in this final rulemaking.

On other topics, the EPA is finalizing two sets of

changes to the PM2.5 sub-index of the Air Quality

Index (AQI). First, the EPA is continuing to use the

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approach used in the revisions to the AQI in 2012 (77

FR 38890, June 29, 2012) of setting the lower

breakpoints (50, 100 and 150) based on the levels of

the primary annual and 24-hour PM2.5 standards. In

so doing, the EPA is revising the AQI value of 50 to

9.0 μg/m3 and is retaining the AQI values of 100 and

150 at 35.4 μg/m3 and 55.4 μg/m3, respectively.

Second, the EPA is revising the upper AQI

breakpoints (200 and above), and replacing the

linear- relationship approach used in 1999 (64 FR

42530, August 4, 1999) to set these breakpoints, with

an approach that more fully considers the PM2.5

health effects evidence from controlled human

exposure and epidemiologic studies that has become

available in the last 20 years. The EPA is also

revising the AQI values of 200, 300 and 500 to 125.4

μg/m3, 225.4 μg/m3, and 325.4 μg/m3, respectively. In

addition, this final rule revises the daily reporting

requirement from 5 days per week to 7 days per

week, while also reformatting appendix G and

providing clarifications.

With regard to monitoring-related activities, the

EPA finalizes revisions to data calculations and

ambient air monitoring requirements for PM to

improve the usefulness and appropriateness of data

used in regulatory decision making and to better

characterize air quality in communities that are at

increased risk of PM2.5 exposure and health risk.

These changes are found in 40 CFR part 50

(appendices K, L, and N), part 53, and part 58 with

associated appendices (A, B, C, D, and E). These

changes include addressing updates in data calculations, approval of reference and equivalent methods,

updates in quality assurance statistical calculations

to account for lower concentration measurements,

updates to support improvements in PM methods, a

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revision to the PM2.5 network design to account for atrisk populations, and updates to the Probe and

Monitoring Path Siting Criteria for NAAQS pollutants.

In setting the NAAQS, the EPA may not consider

the costs of implementing the standards. This was

confirmed by the Supreme Court in Whitman v.

American Trucking Associations, 531 U.S. 457, 465–

472, 475–76 (2001), as discussed in section II.A of

this document. As has traditionally been done in

NAAQS rulemaking, the EPA prepared a Regulatory

Impact Analysis (RIA) to provide the public with

information on the potential costs and benefits of

attaining several alternative PM2.5 standard levels. In

NAAQS rulemaking, the RIA is done for informational purposes only, and the final decisions on the

NAAQS in this rulemaking are not based on consideration of the information or analyses in the RIA. The

RIA fulfills the requirements of Executive Orders

14094, 13563, and 12866. The RIA estimates the

costs and monetized human health benefits of attaining the revised and two alternative annual PM2.5

standard levels and one alternative 24-hour PM2.5

standard level. Specifically, the RIA examines the

revised annual standard level of 9.0 μg/m3 in

combination with the current 24-hour standard of 35

μg/m3 (i.e., 9.0/35 μg/m3), as well as the following less

and more stringent alternative standard levels: (1)

An alternative annual standard level of 10.0 μg/m3 in

combination with the current 24-hour standard (i.e.,

10.0/35 μg/m3), (2) an alternative annual standard

level of 8.0 μg/m3 in combination with the current 24hour standard (i.e., 8.0/35 μg/m3), and (3) an

alternative 24-hour standard level of 30 μg/m3 in

combination with an alternative annual standard

level of 10 μg/m3 (i.e., 10.0/30 μg/m3). The RIA

presents estimates of the costs and benefits of

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applying illustrative national control strategies in

2032 after implementing existing and expected

regulations and assessing emissions reductions to

meet the current annual and 24-hour particulate

matter NAAQS (12.0/35 μg/m3).

I. Background

A. Legislative Requirements

Two sections of the Clean Air Act (CAA) govern the

establishment and revision of the NAAQS. Section

108 (42 U.S.C. 7408) directs the Administrator to

identify and list 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

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requisite to protect the public health.”5 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.”6

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

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).

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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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,

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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 specifically 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)(1)

also provides that the Administrator may review and

revise criteria or promulgate new standards earlier

or more frequently.

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

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Scientific Advisory Committee (CASAC) of the EPA’s

Science Advisory Board.

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.

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

section 110, Part C, and Part D, Subparts 1 and 4 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 the NAAQS for PM

through control programs directed to sources of the

pollutants involved. The States, in conjunction with

the EPA, also administer the prevention of significant deterioration of air quality program that covers

these pollutants (see 42 U.S.C. 7470–7479). In

addition, Federal programs provide for or result in

nationwide reductions in emissions of PM and its

precursors under Title II of the Act, 42 U.S.C. 7521–

7574, which involves controls for motor vehicles and

nonroad engines and equipment; the new source

performance standards under section 111 of the Act,

42 U.S.C. 7411; and the national emissions standards

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for hazardous pollutants under section 112 of the Act,

42 U.S.C. 7412.

C. Review of the Air Quality Criteria

Standards for Particulate Matter

and

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). 7 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 mm

(referred to as total suspended particulates or TSP).

The primary standards were set at 260 μg/m3, 24hour average, not to be exceeded more than once per

year, and 75 μg/m3, annual geometric mean. The

secondary standards were set at 150 μg/m3, 24- hour

average, not to be exceeded more than once per year,

and 60 μg/m3, annual geometric mean.

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, 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

Prior to the review initiated in 2007 (see below), 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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thoracic particles. The level of the 24- hour

standards (primary and secondary) was set at 150

μg/m3, 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 μg/m3, and the form was the annual arithmetic

mean, averaged over three years.

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 mm). The new primary standards were

as follows: (1) An annual standard with a level of 15.0

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

arithmetic mean PM2.5 concentrations from single or

multiple community-oriented monitors;9 and (2) a 248

PM10 refers to particles with a nominal mean aerodynamic

diameter less than or equal to 10 mm. More specifically, 10 mm

is the aerodynamic diameter for which the efficiency of particle

collection is 50 percent.

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

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hour standard with a level of 65 μg/m3, 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 mm and less than or equal to 10

mm), 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.

Following promulgation of the 1997 PM 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 and to regulate

coarse particle pollution, but vacated the 1997 PM10

standards, concluding that the EPA had not provided

a reasonable explanation justifying use of PM10 as an

indicator for coarse particles. American Trucking

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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Associations, Inc. v. EPA, 175 F. 3d 1027 (D.C. Cir.

1999). 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 holding on the cost and constitutional issues were appealed to the United States

Supreme Court. In February 2001, the Supreme

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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, 2004a). 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). 10 On

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

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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 24hour PM2.5 standards to 35 μg/m3, retained the level

of the annual PM2.5 standards at 15.0 μg/m3, 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 μg/m3, and

revoked the annual standards. The then- 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.

Several parties filed petitions for review following

promulgation of the revised PM NAAQS in 2006. 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

appropriate, the potential alternative standards that could be

supported by the evidence and information. More recent reviews

present this information in the Policy Assessment.

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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 (discussed below).

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 2009,11 the EPA held

science/policy 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., Integrated

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.

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Review Plan, (IRP; U.S. EPA, 2008), Integrated

Science Assessment (ISA; U.S. EPA, 2009a), Risk and

Exposure Assessment (REA) planning documents for

health and welfare (U.S. EPA, 2009b, U.S. EPA,

2009c), a quantitative health risk assessment (U.S.

EPA, 2010a) and an urban-focused visibility assessment (U.S. EPA, 2010b), and a Policy Assessment

(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).

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 standard12 to 12.0 μg/m3 and retained

the 24-hour PM2.5 standard, with its level of 35 μg/m3.

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 standards13

and the 24-hour PM10 standard to address visibility

and non-visibility welfare effects.

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

The EPA also eliminated the option for spatial averaging.

Consistent with the primary standard, the EPA eliminated

the option for spatial averaging with the annual standard.

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in all respects. NAM v. EPA, 750 F.3d 921 (D.C. Cir.

2014).

5. Review Initiated in 2014

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). On February 9 to 11, 2015,

the EPA’s NCEA and OAQPS held a public workshop

to inform the planning for the 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 the 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 the review. This workshop

provided for a public discussion of the key science

and policy-relevant issues around which the EPA

structured the review of the PM NAAQS and of the

most meaningful new scientific information that

would be available in the review to inform

understanding of these issues.

The input received at the workshop guided the

EPA staff in developing a draft IRP, which was

reviewed by the CASAC Particulate Matter 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

CASAC, supplemented by the Particulate Matter

Panel, and input from the public were considered in

developing the final IRP (U.S. EPA, 2016). The final

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IRP discusses the approaches to be taken in developing key scientific, technical, and policy documents in

the review and the key policy-relevant issues that

frame the EPA’s consideration of whether the

primary and/or secondary NAAQS for PM should be

retained or revised.

In May 2018, the then-Administrator issued a

memorandum announcing the Agency’s intention to

conduct the review of the PM NAAQS in such a

manner as to ensure that any necessary revisions

were finalized by December 2020 (Pruitt, 2018).

Following this memo, on October 10, 2018, the thenAdministrator 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 sevenmember chartered CASAC (i.e., rather than the

CASAC Particulate Matter Panel that reviewed the

draft IRP).14

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 then-Administrator

dated April 11, 2019 (Cox, 2019a). The EPA

addressed these comments in the final ISA, which

was released in December 2019 (U.S. EPA, 2019a).

The EPA released the draft PA in September 2019

(84 FR 47944, September 11, 2019). The draft PA was

Announcement available at: https://www.regulations.gov/

document/EPA-HQ-OAR- 2015-0072-0223.

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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 October 2019 (84 FR 51555,

September 30, 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 then-Administrator dated December 16,

2019 (Cox, 2019b). 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. Some CASAC members expressed

support for retaining the current primary annual

PM2.5 standard while other members expressed

support for revising that standard in order to increase

public health protection (Cox, 2019b, p. 1 of letter).

These views are described in greater detail in the

letter to the then-Administrator (Cox, 2019b) and in

the notice of final rulemaking (85 FR 82706–82707,

December 18, 2020), as well as below. With regard to

the secondary standards, the CASAC recommended

retaining the current standards. In response to the

CASAC’s comments, the 2020 final PA incorporated a

number of changes (Cox, 2019b, U.S. EPA, 2020b), as

described in detail in section I.C.5 of the 2020

proposal document (85 FR 24100, April 30, 2020).

a. 2020 Proposed and Final Actions

On April 14, 2020, the EPA 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,

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April 30, 2020). The EPA’s final decision on the PM

NAAQS was published in the Federal Register on

December 18, 2020 (85 FR 82684, December 18,

2020). In the 2020 rulemaking, the EPA retained the

primary and secondary PM2.5 and PM10 standards,

without revision. The then-Administrator’s rationale

for his decisions is described in more detail in section

II, III, and V below, and is briefly summarized here.

In reaching his final decision to retain the primary

annual and 24-hour PM2.5 standards, the thenAdministrator considered the available scientific

evidence, quantitative information, CASAC advice,

and public comments in his supporting rationale in

the 2020 final action (85 FR 82714, December 18,

2020). In so doing, he concluded that the available

controlled human exposure studies did not provide

support for additional public health protection against

exposures to peak PM2.5 concentrations, beyond the

protection provided by the combination of the current

primary annual and 24-hour PM2.5 standards. He also

noted that the available epidemiologic studies did not

indicate that associations in those studies are strongly

influenced by exposures to peak concentrations in the

air quality distribution and thus did not indicate the

need for additional protection against short-term

exposures to peak PM2.5 concentrations. Accordingly,

and taking into account consensus CASAC advice to

retain the current primary 24-hour PM2.5 standard,

the then-Administrator concluded the primary 24hour PM2.5 standard should be retained.

With respect to the annual PM2.5 standard, the

then-Administrator recognized that important uncertainties and limitations that were present in

epidemiologic studies in previous reviews remained

in the evidence assessed in the 2019 ISA. In

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considering the epidemiologic evidence, the thenAdministrator noted that: (1) The reported mean

concentration in the majority of the key U.S.

epidemiologic 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 μg/m3) is above the level of the

current primary annual PM2.5 standard of 12 μg/m3;

(3) air quality analyses show the study means to be

lower than their corresponding design by 10–20%;

and (4) that these analyses must be considered in

light of uncertainties inherent in the epidemiologic

evidence. The then-Administrator further considered

other available information, including the risk

assessment, accountability studies, and controlled

human exposure studies, and found that, in

considering all of the evidence together along with

advice from the CASAC, the suite of primary PM2.5

standards were requisite to protect public health

with an adequate margin of safety, and should be

retained, without revision.

With regard to the primary PM10 standard, the

then-Administrator noted that the expanded body of

evidence has broadened the range of effects that have

been linked with PM10–2.5 exposures. In light of that

information, as well as continued uncertainties in the

evidence and advice from the CASAC to retain the

standard, the then-Administrator judged 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 (85 FR 82725,

December 18, 2020).

With regard to the secondary PM standards, the

then-Administrator concluded that there was insuffi-

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cient information available to establish any distinct

secondary PM standards to address climate and

materials effects of PM. For visibility effects, he

found that in the absence of a monitoring network for

direct measurement of light extinction, a calculated

light extinction indicator that utilizes the IMPROVE

algorithms continued to provide a reasonable basis

for defining a target level of protection against PMrelated visibility impairment. He further found that a

visibility index with a 24-hour averaging time was

reasonable based on its stability and suitability for

representing subdaily periods, and a form based on

the 3-year average of annual 90th percentile values

was reasonable based on its stability and that it

represents the median of the 20 percent worst

visibility days which are targeted under the Regional

Haze program. With regard to the level of a visibility

index, the then-Administrator judged it 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, taking into consideration the variability,

limitations and uncertainties of the public preference

studies. The then-Administrator judged that the

secondary 24-hour PM2.5 standard with its level of 35

μg/m3 would provide at least the target level of

protection for visual air quality of 30 dv which he

judged appropriate. Accordingly, taking into consideration the advice of the CASAC to retain the current

secondary PM standards, the then-Administrator

found the current secondary standards provide the

requisite degree of protection and that they should be

retained (85 FR 82742, December 18, 2020).

Following publication of the 2020 final action,

several parties filed petitions for review and petitions

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for reconsideration of the EPA’s final decision. The

petitions for review were filed in the D.C. Circuit and

the Court consolidated the cases.15 Following EPA’s

decision to reconsider the 2020 final decision, the

Court ordered the consolidated cases to be held in

abeyance.

b. Reconsideration of the 2020 PM NAAQS

Final Action

Executive Order 13990 directed review of certain

agency actions (86 FR 7037, January 25, 2021).16 An

accompanying fact sheet provided a non-exclusive list

of agency actions that agency heads should review in

accordance with that order, including the 2020

Particulate Matter NAAQS Decision.17

On June 10, 2021, the Agency announced its

decision to reconsider the 2020 PM NAAQS final

action because the available scientific evidence and

technical information indicate that the current

standards may not be adequate to protect public

health and welfare, as required by the Clean Air

Act.18 The Administrator reached this decision in part

based on the fact that the EPA noted that the 2020

PA concluded that the scientific evidence and

See California v. EPA, (D.C. Cir., No. 21–2014 consolidated

with Nos. 21–1027, 21–1054).

See https://www.whitehouse.gov/briefing-room/presidenti

al-actions/2021/01/20/executive-order-protecting-public-healthand-environment-and- restoring-science-to-tackle-climate-crisis/.

See https://www.whitehouse.gov/briefing-room/statemen

ts-releases/2021/01/20/fact-sheet-list-of-agency-actions-for-revi

ew/.

The press release for this announcement is available at:

https://www.epa.gov/newsreleases/epa-reexamine-health-stand

ards-harmful-soot-previous-administration-left-unchanged.

80a

information called into question the adequacy of the

primary annual PM2.5 standard and supported

revising the level to below the current level of 12.0

μg/m3 while retaining the primary 24-hour PM2.5

standard (U.S. EPA, 2020b). The EPA also noted that

the 2020 PA concluded that the available scientific

evidence and information supported retaining the

primary PM10 standard and secondary PM standards

without revision (U.S. EPA, 2020b).

The EPA staff conclusions detailed in the 2020 PA

in combination with the CASAC advice that informed

the Administrator’s decisions regarding the 2020

final action, studies highlighted by public comments

on the 2020 proposal, and the numerous studies

published since the literature cutoff date of the 2019

ISA all informed the scope of the reconsideration.

In its review of the 2019 draft PA, some members

of the CASAC had recommended that greater attention should be given to accountability studies and

epidemiologic studies that employ alternative methods

for confounder control (also referred to as causal

inference or causal modeling studies) in order to

“more fully account for effects of confounding, measurement and estimation errors, model uncertainty,

and heterogeneity” in epidemiologic studies (Cox,

2019b, p. 8 of consensus responses). In addition,

public commenters submitted a number of recent

studies published after the literature cutoff date for

the 2019 ISA that would have been considered within

the scope of the 2019 ISA. While the EPA

provisionally considered these studies in responding

to public comments,19 it was determined that, at the

The list of provisionally considered studies is included in

Appendix A to the 2020 Response to Comments document (U.S.

EPA, 2020a).

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time of the 2020 final action, these studies were

generally consistent with the evidence assessed in

the 2019 ISA (85 FR 82690, December 18, 2020; U.S.

EPA, 2020a). As such, and consistent with previous

NAAQS reviews, the EPA concluded that the new

studies did 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, and therefore, reopening of the air

quality criteria was not warranted (85 FR 82691,

December 18, 2020). However, at that time, the EPA

recognized that its “provisional consideration of these

studies did not and could not provide the kind of indepth critical review” (85 FR 82690, December 18,

2020) that studies undergo in the development of an

ISA.

In preparing to reconsider the 2020 final decision

for the PM NAAQS, the Agency revisited the need to

reopen the air quality criteria, given the amount of

time that had passed since the literature cutoff date

of the 2019 ISA (i.e., approximately January 2018)

and the volume of literature that had become available, including those studies provisionally considered

in responding to comments in 2020. In so doing, the

EPA preliminarily concluded that at least some of

these studies were likely to be relevant to its

reconsideration of the air quality criteria and the PM

NAAQS and that, in considering public comments on

any proposed decisions for the reconsideration, these

studies were likely to be raised by public commenters

and would potentially warrant a reopening of the air

quality criteria. For example, on February 16, 2021,

the EPA received two petitions to reconsider the PM

NAAQS. One petition objected to the EPA’s provisional consideration of studies submitted in public

comments on the 2020 proposal and suggested that

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the provisional consideration was inadequate because

the studies could be important in determining whether

the existing standards are adequately protective. See,

Petition for Reconsideration of National Ambient Air

Quality Standards for Particulate Matter, submitted

by American Lung Association, et al, dated Feb. 16,

2020. The other petition identified a number of new

studies, including one epidemiologic study that was

published after the provisional consideration was

completed that could further inform the concern

expressed by the CASAC that associations reported

in epidemiologic studies do not adequately account

for “uncontrolled confounding and other potential

sources of error and bias.” See Petition for

Reconsideration of “Review of the National Ambient

Air Quality Standards for Particulate Matter,”

submitted by the State of California, dated Feb. 16,

2020. This was also an uncertainty noted by the thenAdministrator in the 2020 decision, who also

recognized “that methodological study designs to

address confounding, such as causal inference

methods, are an emerging field of study.” Thus, the

Agency concluded it was appropriate to reconsider

not only the standards but also the air quality

criteria, in light of public comments during the 2020

PM NAAQS proposal and recent studies published

since the cutoff date of the 2019 ISA, as reflected in

petitions. In deciding to reopen the air quality

criteria, the Agency concluded it was reasonable to

focus on studies that were most likely to inform

decisions on the appropriate standard, but not to

reassess areas which, based on the assessment of

available science published since the cutoff date of

the 2019 ISA and through 2021, were judged unlikely

to have new information that would be useful for the

Administrator’s decision making. The Agency accord-

83a

ingly announced that, in support of the reconsideration,

it would develop a supplement to the 2019 ISA and a

revised PA.

The EPA also explained that the draft ISA

Supplement and draft PA would be reviewed at a

public meeting by the CASAC, and the public would

have opportunities to comment on these documents

during the CASAC review process, as well as to

provide input during the rulemaking through the

public comment process and public hearings on the

proposed rulemaking.

On March 31, 2021, the Administrator announced

his decision to reestablish the membership of the

CASAC to “ensure the agency received the best

possible scientific insight to support our work to

protect human health and the environment.” 20

Consistent with this memorandum, a call for

nominations of candidates to the EPA’s chartered

CASAC was published in the Federal Register (86 FR

17146, April 1, 2021). On June 17, 2021, the

Administrator announced his selection of the seven

members to serve on the chartered CASAC. 21 22

Additionally, a call for nominations of candidates to a

PM- specific panel was published in the Federal

The press release for this announcement is available at:

https://www.epa.gov/newsreleases/administrator-regan-dire

cts-epa-reset-critical- science-focused-federal-advisory

The press release for this announcement is available at:

https://www.epa.gov/newsreleases/epa-announces-selections-ch

arter-members-clean-air- scientific-advisory-committee.

The list of members of the chartered CASAC and their

biosketches are available at: https://casac.epa.gov/ords/sab/r/

sab_apex/casac/mems?p14_committeeon=2021%20CASAC%20P

M%20Panel &session=17433386035954.

84a

of the PM CASAC panel were announced on August

30, 2021.23

The draft ISA Supplement was released in

September 2021 (U.S. EPA, 2021a; 86 FR 54186,

September 30, 2021), and included a discussion of the

rationale and scope of the Supplement. As explained

therein, the ISA Supplement focuses on a thorough

evaluation of some studies that became available

after the literature cutoff date of the 2019 ISA that

could either further inform the adequacy of the

current PM NAAQS or address key scientific topics

that have evolved since the literature cutoff date for

the 2019 ISA. In selecting the health effects to

evaluate within the ISA Supplement, the EPA

focused on health effects for which the evidence

supported a “causal relationship” because those were

the health effects that were most useful in informing

conclusions in the 2020 PA (U.S. EPA, 2022a, section

1.2.1).24 Consistent with the rationale for the focus on

The list of members of the PM CASAC panel and their

biosketches are available at: https://casac.epa.gov/ords/sab/

f?p=105:14:9979229564047:::14:P14_COMMITTEEON:2021%20

CASAC %20PM%20Panel.

As described in section 1.2.1 of the ISA Supplement: “In

considering the public health protection provided by the current

primary PM2.5 standards, and the protection that could be

provided by alternatives, [the U.S. EPA, within the 2020 PM

PA] emphasized health outcomes for which the ISA determined

that the evidence supports either a ‘causal’ or a ‘likely to be

causal’ relationship with PM2.5 exposures” (U.S. EPA, 2020b).

Although the 2020 PA initially focused on this broader set of

evidence, the basis of the discussion on potential alternative

standards primarily focused on health effect categories where

the 2019 PM ISA concluded a ‘causal relationship’ (i.e., shortand long-term PM2.5 exposure and cardiovascular effects and

mortality) as reflected in Figures 3–7 and 3–8 of the 2020 PA

(U.S. EPA, 2020b).”

85a

certain health effects, in selecting the non- ecological

welfare effects to evaluate within the ISA Supplement,

the EPA focused on the non-ecological welfare effects

for which the evidence supported a “causal relationship”

and for which quantitative analyses could be supported

by the evidence because those were the welfare

effects that were most useful in informing conclusions in the 2020 PA.25 Specifically, for non- ecological

welfare effects, the focus within the ISA Supplement

is on visibility effects. The ISA Supplement also

considers recent health effects evidence that addresses

key scientific topics where the literature has evolved

since the 2020 review was completed, specifically

since the literature cutoff date for the 2019 ISA.26

As described in section 1.2.1 of the ISA Supplement: “The

2019 PM ISA concluded a ‘causal relationship’ for each of the

welfare effects categories evaluated (i.e., visibility, climate

effects and materials effects). While the 2020 PA considered the

broader set of evidence for these effects, for climate effects and

material effects, it concluded that there remained ‘substantial

uncertainties with regard to the quantitative relationships with

PM concentrations and concentration patterns that limit[ed]

[the] ability to quantitatively assess the public welfare

protection provided by the standards from these effects’ (U.S.

EPA, 2020b).”

These key scientific topics include experimental studies

conducted at near-ambient concentrations, epidemiologic studies

that employed alternative methods for confounder control or

conducted accountability analyses, studies that assess the

relationship between PM2.5 exposure and severe acute

respiratory syndrome coronavirus 2 (SARS–CoV–2) infection

and coronavirus disease 2019 (COVID–19) death; and in

accordance with recent EPA goals on addressing environmental

justice, studies that examine disparities in PM2.5 exposure and

the risk of health effects by race/ethnicity or socioeconomic

status (SES) (U.S. EPA, 2022a, section 1.2.1).

86a

Building on the rationale presented in section

1.2.1, the ISA Supplement considers peer-reviewed

studies published from approximately January 2018

through March 2021 that meet the following criteria:

• Health Effects

 U.S. and Canadian epidemiologic studies for

health effect categories where the 2019 ISA

concluded a “causal relationship” (i.e., short- and

long-term PM2.5 exposure and cardiovascular effects

and mortality).

 U.S. and Canadian epidemiologic studies that

employed alternative methods for confounder control

or conducted accountability analyses (i.e., examined

the effect of a policy on reducing PM2.5

concentrations).

• Welfare Effects

 U.S. and Canadian studies that provide new

information on public preferences for visibility

impairment and/or developed methodologies or

conducted quantitative analyses of light extinction.

• Key Scientific Topics

 Experimental studies (i.e., controlled human

exposure and animal toxicological) conducted at nearambient PM2.5 concentrations experienced in the U.S.

 U.S.- and Canadian-based epidemiologic studies

that examined the relationship between PM2.5

exposures and severe acute respiratory syndrome

coronavirus

(SARS–CoV–2)

infection

and

coronavirus disease 2019 (COVID–19) death.

 At-Risk Populations.

87a

 U.S.- and Canadian-based epidemiologic or

exposure studies examining potential disparities in

either PM2.5 exposures or the risk of health effects by

race/ethnicity or socioeconomic status (SES).

Given the narrow scope of the ISA Supplement, it

is important to recognize that the evaluation does not

encompass the full multidisciplinary evaluation

presented within the 2019 ISA that would result in

weight-of-evidence conclusions on causality (i.e.,

causality determinations). The ISA Supplement critically evaluates and provides key study-specific

information for those recent studies deemed to be of

greatest significance for informing preliminary

conclusions on the PM NAAQS in the context of the

body of evidence and scientific conclusions presented

in the 2019 ISA.

In developing a revised PA to support the

reconsideration, the EPA considered the available

scientific evidence, including the evidence presented

in the 2019 ISA and ISA Supplement. The 2022 PA

considered the quantitative and technical information presented in the 2020 PA, in addition to new

and updated analyses conducted since the 2020 final

decision. For those health and welfare effects for

which the ISA Supplement evaluated recently available studies (i.e., PM2.5-related health effects and

visibility effects), new updated quantitative analyses

were conducted as a part of the development of the

2022 PA. The newly available scientific and technical

information presented in the 2022 PA were considered in reaching conclusions regarding the adequacy

of the current standards and any potential

alternative standards. For those health and welfare

effects for which newly available scientific and

technical information were not evaluated (i.e., PM10–

88a

2.5-related health effects and non-visibility welfare

effects), the conclusions presented in the 2022 PA

rely heavily on the information that supported the

conclusions in the 2020 PA.

The CASAC PM panel met at a virtual public

meeting in November 2021 to review the draft ISA

Supplement (86 FR 52673, September 22, 2021). A

virtual public meeting was then held in February

2022, and during this meeting the chartered CASAC

considered the CASAC PM panel’s draft letter to the

Administrator on the draft ISA Supplement (87 FR

958, January 7, 2022).

The chartered CASAC provided its advice on the

draft ISA Supplement in a letter to the EPA

Administrator dated March 18, 2022 (Sheppard,

2022b). In its review of the draft ISA Supplement,

the CASAC noted that they found “the Draft ISA

Supplement to be a well- written, comprehensive

evaluation of the new scientific information published

since the 2019 PM ISA” (Sheppard, 2022b, p. 2 of

letter). Furthermore, the CASAC stated that “the

final Integrated Science Assessment (ISA) Supplement

. . . deserve[s] the Administrator’s full consideration

and [is] adequate for rulemaking” (Sheppard, 2022b,

p. 2 of letter). The CASAC generally endorsed EPA’s

decisions regarding the limited scope of the draft ISA

Supplement, stating that “this limitation [on scope] is

appropriate for the targeted purpose of the Draft ISA

Supplement” although the CASAC noted it would not

be appropriate for ISAs generally, and recommended

that the EPA provide additional acknowledgment

and explanation for the limited scope (Sheppard,

2022b, p. 2 of letter; see also pp. 2–3 of consensus

responses). The EPA specifically noted in the final

ISA Supplement, which was released in May 2022

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(U.S. EPA, 2022a; hereafter referred to as the ISA

Supplement throughout this document) that the

“targeted approach to developing the Supplement to

the 2019 PM ISA for the purpose of reconsidering the

2020 PM NAAQS decision does not reflect a change to

EPA’s approach for developing ISAs for NAAQS

reviews.” Thus, the evidence presented within the

2019 ISA, along with the targeted identification and

evaluation of new scientific information in the ISA

Supplement, provides the scientific basis for the

reconsideration of the 2020 PM NAAQS final

decision.

The draft PA was released in October 2021 (86 FR

56263, October 8, 2021). The CASAC PM panel met

at a virtual public meeting in December 2021 to

review the draft PA (86 FR 52673, September 22,

2021). A virtual public meeting was then held in

February 2022 and March 2022, and during this

meeting the chartered CASAC considered the CASAC

PM panel’s draft letter to the Administrator on the

draft PA (87 FR 958, January 7, 2022). The chartered

CASAC provided its advice on the draft PA in a letter

to the EPA Administrator dated March 18, 2022

(Sheppard, 2022a). The EPA took steps to address

these comments in revising and finalizing the PA.

The 2022 PA considers the scientific evidence

presented in the 2019 ISA and ISA Supplement and

considers the quantitative and technical information

presented in the 2020 PA, along with updated and

newly available analyses since the completion of the

2020 review. For those health and welfare effects for

which the ISA Supplement evaluated recently available evidence and for which updated quantitative

analyses were supported (i.e., PM2.5-related health

effects and visibility effects), the 2022 PA includes

consideration of this newly available scientific and

90a

technical information in reaching preliminary

conclusions. For those health and welfare effects for

which newly available scientific and technical

information were not evaluated (i.e., PM10–2.5-related

health effects and non-visibility effects), the

conclusions presented in the 2022 PA rely heavily on

the information that supported the conclusions in the

2020 PA. The final PA was released in May 2022

(U.S. EPA, 2022b; hereafter referred to as the 2022

PA throughout this document).

Drawing from his consideration of the scientific

evidence assessed in the 2019 ISA and ISA

Supplement and the analyses in the 2022 PA,

including the uncertainties in the evidence and

analyses, and from his consideration of advice from

the CASAC, on January 5, 2023, the Administrator

proposed to revise the level of the primary annual

PM2.5 standard and to retain the primary 24-hour

PM2.5 standard, the primary 24-hour PM10 standard,

and the secondary PM standards. These proposed

decisions were published in the Federal Register on

January 27, 2023 (88 FR 5558, January 27, 2023).

The EPA held a multi-day virtual public hearing on

February 21–23, 2023 (88 FR 6215, January 31,

2023). In total, the EPA received nearly 700,000

comments on the proposal from members of the

public by the close of the public comment period on

March 28, 2023. 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” document), can

be found in the docket for this rulemaking (Docket

No. EPA–HQ–OAR–2015–0072).

91a

As in prior reviews, the EPA is basing its decision

in this reconsideration on studies and related information in the air quality criteria, which have

undergone CASAC and public review. These studies

assessed in the 2019 ISA 27 and ISA Supplement 28

and the 2022 PA, and the integration of the scientific

evidence presented in them, have undergone

extensive critical review by the EPA, the CASAC, and

the public. Decisions on the NAAQS 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. It is for this reason that

the EPA preliminarily concluded that the scientific

evidence available since the completion of the 2019

ISA, including those raised in public comments on

the proposal in 2020, warranted a partial reopening

of the air quality criteria and prepared an ISA

Supplement to enable the EPA, the CASAC, and the

public to consider them further. Some commenters

have referred to and discussed additional individual

scientific studies on the health effects of PM that

In addition to the 2020 review’s opening “call for

information” (79 FR 71764, December 3, 2014), the 2019 ISA

identified and evaluated studies and reports that have

undergone scientific peer review and were published or accepted

for publication between January 1, 2009, through approximately

January 2018 (U.S. EPA, 2019a, p. ES–2). References that are

cited in the 2019 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.

As described above, the ISA Supplement represents an

evaluation of recent studies that are of greatest policy relevance

and utility to the reconsideration of the 2020 final decision on

the PM NAAQS (U.S. EPA, 2022a).

92a

were not included in the 2019 ISA or ISA Supplement

(“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 2019 ISA and ISA Supplement. 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 further reopening

the review of the air quality criteria to enable the

EPA, the CASAC, and the public to consider them

further.

This approach, and the decision to rely on the

studies and related information 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 6114, 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

review the O3 NAAQS, “new” studies may sometimes

be of such significance that it is appropriate to delay

93a

a decision in a NAAQS review and to supplement the

pertinent air quality criteria so the studies can be

taken into account (58 FR 13013–13014, March 9,

1993). In the present case, the EPA decided to

partially reopen the air quality criteria and prepared

an ISA Supplement as a part of the reconsideration

to facilitate evaluation of these studies by the EPA,

the CASAC, and the public. The narrow scope of the

ISA Supplement is supported by EPA’s provisional

consideration of “new” studies submitted in response

to public comments on the 2020 proposal which concluded 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. Therefore, a full reopening of the air

quality criteria was not warranted to assess the

health and welfare effects of PM for purposes of the

review.

Accordingly, the EPA is basing the final decisions

in this reconsideration on the studies and related

information included in the PM air quality criteria

(including the 2019 PM ISA and ISA Supplement)

that have undergone rigorous review by the EPA, the

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

reconsideration and which will provide the

opportunity to fully assess these studies through a

more rigorous review process involving the EPA, the

CASAC, and the public.

D. Air Quality Information

This section provides a summary of basic information

related to PM ambient air quality. It summarizes

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information on the distribution of particle size in

ambient air (section I.D.1), sources and emissions

contributing to PM in the ambient air (section I.D.2),

monitoring ambient PM in the U.S. (section I.D.3),

ambient PM concentrations and trends in the U.S.

(I.D.4), characterizing ambient PM2.5 concentrations

for exposure (section I.D.5), and background PM

(section I.D.6). Additional detail on PM air quality

can be found in Chapter 2 of the 2022 PA (U.S. EPA,

2022b).

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, 2019a, 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 [m

in diameter (U.S. EPA, 2019a, 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 μm. 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 μm and less than or equal

to 10 μm. PM10, which is comprised of both fine and

coarse fractions, includes those particles with

aerodynamic diameters generally less than or equal

to 10 μm. In addition, ultrafine particles (UFP) are

often defined as particles with a diameter of less than

0.1 μm based on physical size, thermal diffusivity or

electrical mobility (U.S. EPA, 2019a, section 2.2).

Atmospheric lifetimes are generally longest for PM2.5,

which often remains in the atmosphere for days to

weeks (U.S. EPA, 2019a, Table 2–1) before being

removed by wet or dry deposition, while atmospheric

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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,

2019a, Table 2–1; U.S. EPA, 2022b, section 2.1).

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 gasphase precursor chemical compounds present in the

atmosphere that have participated in new particle

formation or condensed onto existing particles (U.S.

EPA, 2019a, section 2.3). As discussed further in the

2019 ISA (U.S. EPA, 2019a, section 2.3.2.1), secondary PM is formed in the atmosphere by photochemical

oxidation reactions of both inorganic and organic gasphase precursors. Precursor gases include sulfur

dioxide (SO2), nitrogen oxides (NOX), and volatile

organic compounds (VOC) (U.S. EPA, 2019a, section

2.3.2.1). Ammonia also plays an important role in the

formation of nitrate PM by neutralizing sulfuric acid

and nitric acid. Sources and emissions of PM are

discussed in more detail the 2022 PA (U.S. EPA,

2022b, section 2.1.1). Briefly, anthropogenic sources

of PM include both stationary (e.g., fuel combustion

for electricity production and other purposes, industrial processes, agricultural activities) and mobile

(e.g., diesel- and gasoline-powered highway vehicles

and other engine-driven sources) sources. Natural

sources of PM include dust from the wind erosion of

natural surfaces, sea salt, wildfires, primary biological

aerosol particles (PBAP) such as bacteria and pollen,

oxidation of biogenic hydrocarbons, such as isoprene

and terpenes to produce secondary organic aerosol

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(SOA), and geogenic sources, such as sulfate formed

from volcanic production of SO2. Wildland fire, which

encompass both wildfire and prescribed fire, accounts

for 44% of emissions of primary PM2.5 emissions (U.S.

EPA, 2021b). Emissions from wildfire comprises 29%

of primary PM2.5 emissions.

In recent years, the frequency and magnitude of

wildfires have increased (U.S. EPA, 2019a). The

magnitude of the public health impact of wildfires is

substantial both because of the increase in PM2.5

concentrations as well as the duration of the wildfire

smoke season, which is considered to range from May

to November. Wildfire can make a large contribution

to air pollution (including PM2.5), and wildfire events

can

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