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

No. 26-___

In the Supreme Court of the United States

COMMONWEALTH OF KENTUCKY,

STATE OF WEST VIRGINIA, ET AL.

Petitioners,

v.

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 II 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 .......................................................

1a

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

417a

A. Introduction

The general approach for this reconsideration of

the 2020 final decision on the primary PM10 standard

relies on the scientific information available for this

review, as well as the Administrator’s judgments

regarding the available public health effects evidence,

and the appropriate degree of public health protection for the existing standards. With the 2020

decision, the then-Administrator retained the existing

primary 24-hour PM10 standard, with its level of 150

μg/m3 and its one-expected-exceedance form on

average over three years, to continue to provide

public health protection against short-term exposures

to PM10–2.5 (85 FR 82725, December 18, 2020).

1. Background on the Current Standard

Consistent with the 2009 ISA, the 2019 ISA

concluded that the available epidemiologic, controlled

human exposure, and animal toxicological studies,

including uncertainties, provided support for the

causality determinations of “suggestive of, but not

sufficient to infer, a causal relationship” between

short-term exposures to PM10–2.5 and cardiovascular

effects, respiratory effects, and mortality (U.S. EPA,

2019a, section 1.4.2). The 2019 ISA also reached the

conclusion that the evidence supports a “suggestive

of, but not sufficient to infer, a causal relationship”

between short-term PM10–2.5 exposures and metabolic

effects, an endpoint that was not evaluated in the

2009 ISA (U.S. EPA, 2019a, section 1.4.2).

Compared to the 2009 ISA, the 2019 ISA includes

expanded evidence for the relationships between

long-term exposures and cardiovascular effects,

metabolic effects, nervous system effects, cancer, and

mortality. The 2019 ISA concluded that the small

418a

number of epidemiologic and experimental studies,

including uncertainties, contribute to the determination that, “the evidence is suggestive of, but not

sufficient to infer, a causal relationship between longterm PM10–2.5 exposure and cardiovascular effects,

metabolic effects, nervous system effects, cancer, and

mortality and cancer (U.S. EPA, 2019a, p. 10–87).

For long-term exposures and cardiovascular effects,

cardiovascular effects, and cancer, this is an upgrade

from the “inadequate to infer the presence or absence

of a causal relationship” conclusions in the 2009 ISA

(U.S. EPA, 2019a, section 1.4.2). This determination

is also the first for long-term exposures and metabolic

effects, as the 2009 ISA did not include metabolic

effects as an endpoint (U.S. EPA, 2019a section

1.4.2).

In considering the available body of evidence, it

was noted in the 2020 review there were considerable

uncertainties and limitations associated with the

experimental evidence for PM2.5 exposures and health

effects, and as such more weight was placed on the

available epidemiologic evidence. Therefore, the

primary focus in the 2020 review was on multi-city

and single-city epidemiologic studies that evaluated

associations between short-term PM10–2.5 and mortality,

cardiovascular effects (hospital admissions and

emergency department visits, as well as blood

pressure and hypertension), and respiratory effects.

Despite differences in the approaches 127 used to

127

As discussed further below, methods employed by the

epidemiologic studies to estimate ambient PM10–2.5 concentrations include: (1) Calculating the difference between PM10 and

PM2.5 at co-located monitors, (2) calculating the difference

between county-wide averages of monitored PM10 and PM2.5

based on monitors that are not necessarily co-located, and (3)

419a

estimate ambient PM10–2.5 concentrations, the majority

of the studies reported positive, though often not

statistically significant, associations with short-term

PM10–2.5 exposures. Most PM10–2.5 effect estimates

remained positive in copollutant models that included

either gaseous pollutants or other particulate matter

size fractions (e.g., PM2.5). In U.S. study locations

likely to have met the PM10 standard during the

study period, a few studies reported positive associations between PM10–2.5 and mortality that were

statistically significant and remained so in copollutant

models (U.S. EPA, 2019a). In addition to the

epidemiologic studies, there were a small number of

controlled human exposure studies evaluated in the

2019 ISA that reported alterations in heart rate

variability or increased pulmonary inflammation

following short-term exposure to PM10–2.5, providing

some support for the associations in the epidemiologic studies. Animal toxicological studies examined

the effect of short-term PM10–2.5 exposures using noninhalation (e.g., intratracheal instillation) route.128

Therefore, these studies provided limited evidence

for the biological plausibility of PM10–2.5-induced

effects (U.S. EPA, 2019a). Although the scientific

evidence available in the 2019 ISA expanded the

understanding of health effects associated with PM10–

2.5 exposures, a number of important uncertainties

remained. These uncertainties, and their implicadirect measurement of PM10–2.5 using a dichotomous sampler

(U.S. EPA, 2019a, section 1.4.2).

128

Non-inhalation exposure experiments (i.e., intratracheal

[IT] instillation) are informative for size fractions (e.g., PM10–2.5)

that cannot penetrate the airway of a study animal and may

provide information relevant to biological plausibility and

dosimetry (U.S. EPA, 2019a, section A–12).

420a

tions for interpreting the scientific evidence, include

the following:

• The potential for confounding by copollutants,

notably PM2.5, was addressed with copollutant models

in a relatively small number of PM10–2.5 epidemiologic

studies (U.S. EPA, 2019a). This was particularly

important given the relatively small body of experimental evidence (i.e., controlled human exposure and

animal toxicological studies) available to support the

independent effect of PM10–2.5 on human health. This

increases the uncertainty regarding the extent to

which PM10–2.5 itself, rather than one or more

copollutants, is responsible for the mortality and

morbidity effects reported in epidemiologic studies.

• There was greater spatial variability in PM10–2.5

concentrations than PM2.5 concentrations, resulting

in the potential for increased exposure error for PM10–

2.5 (U.S. EPA, 2019a). Available measurements did

not provide sufficient information to adequately

characterize the spatial distribution of PM10–2.5

concentrations (U.S. EPA, 2019a). The limitations in

estimates of ambient PM10–2.5 concentrations “would

tend to increase uncertainty and make it more

difficult to detect effects of PM10–2.5 in epidemiologic

studies” (U.S. EPA, 2019a).

• Estimation of PM10–2.5 concentrations over which

reported health outcomes occur remain highly uncertain. When compared with PM2.5, there is uncertainty

spanning all epidemiologic studies examining

associations with PM10–2.5 including deficiencies in the

existing monitoring networks, the lack of a

systematic evaluation of the various methods used to

estimate PM10–2.5 concentrations and the resulting

uncertainty in the spatial as well as the temporal

variability in PM10–2.5 concentration (U.S. EPA, 2019a).).

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Given these limitations in routine monitoring,

epidemiologic studies employed a number of different

approaches for estimating PM10–2.5 concentrations,

including (1) calculating the difference between PM10

and PM2.5 at co-located monitors, (2) calculating the

difference between county-wide averages of monitored

PM10 and PM2.5 based on monitors that are not

necessarily co-located, and (3) direct measurement of

PM10–2.5 using a dichotomous sampler (U.S. EPA,

2019a, section 1.4.2). Given the relatively small

number of PM10–2.5 monitoring sites, the relatively

large spatial variability in ambient PM10–2.5 concentrations, the use of different approaches to estimating

ambient PM10–2.5 concentrations across epidemiologic

studies, and the limitations inherent in such

estimates, the distributions of PM10–2.5 concentrations

over which reported health outcomes occur remain

highly uncertain (U.S. EPA, 2019a).

There was relatively little information available to

characterize potential exposure differences that may

inform the apparent variability in associations

between short-term PM10–2.5 exposures and health

effects across study locations (U.S. EPA, 2019a).

Specifically, the potential spatial and temporal

variability in PM10–2.5 exposures complicates the

interpretation of results between study locations as

well as the relative lack of information on the

chemical and biological composition of PM10–2.5 (U.S.

EPA, 2009a U.S. EPA, 2019a).

In reaching his decision in 2020 to retain the

existing 24-hour primary PM10 standard, the thenAdministrator specifically noted that, while the

health effects evidence was somewhat expanded since

the prior reviews, the overall conclusions in the 2019

ISA, including uncertainties and limitations, were

422a

generally consistent with what was considered in the

2012 review (85 FR 82725, December 18, 2020). In

addition, the then-Administrator recognized that

there were still a number of uncertainties and

limitations associated with the available evidence.

With regard to the evidence on PM10–2.5-related

health effects, the then-Administrator noted that

epidemiologic studies continued to report positive

associations with mortality and morbidity in cities

across North America, Europe, and Asia, where PM10–

2.5 sources and composition were expected to vary

widely. While significant uncertainties remained in

the 2020 review, the then-Administrator recognized

that this expanded body of evidence had broadened

the range of effects that have been linked with PM10–

2.5 exposures. The studies evaluated in the 2019 ISA

expanded the scientific foundation presented in the

2009 ISA and led to revised causality determinations

(and new determinations) for long-term PM10–2.5

exposures and mortality, cardiovascular effects,

metabolic effects, nervous system effects, and cancer

(85 FR 82726, December 18, 2020). Drawing from

his consideration of this evidence, the thenAdministrator concluded that the scientific information available since the time of the last review

supported a decision to maintain a primary PM10

standard to provide public health protection against

PM10–2.5 exposures, regardless of location, source of

origin, or particle composition (85 FR 82726,

December 18, 2020). With regard to uncertainties in

the available evidence, the then-Administrator first

noted that a number of limitations were identified in

the 2012 review related to: (1) Estimates of ambient

PM10–2.5 concentrations used in epidemiologic studies;

(2) limited evaluation of copollutant models to

address the potential for confounding; and (3) limited

423a

experimental studies supporting biological plausibility for PM10–2.5-related effects. Despite the expanded

body of evidence for PM10–2.5 exposures and health

effects, the then-Administrator recognized that

uncertainties in the 2020 review continued to include

those associated with the exposure estimates used in

epidemiologic studies, the independence of the PM10–

2.5 health effect associations, and the biologically

plausible pathways for PM10–2.5 health effects (85 FR

82726, December 18, 2020). These uncertainties

contributed to the 2019 ISA determinations that the

evidence is at most “suggestive of, but not sufficient

to infer” causal relationships (85 FR 82726,

December 18, 2020). In considering the available

evidence in his basis for the decision, the thenAdministrator emphasized evidence supporting

“causal” and “likely to be causal” relationships, and

therefore, judged that the PM10–2.5-related health

effects evidence provided an uncertain scientific

foundation for making standard-setting decisions. He

further judged limitations in the evidence raised

questions as to whether additional public health

improvements would be achieved by revising the

existing PM10 standard (85 FR 24126, April 30, 2020).

In the 2020 decision, for all of the reasons discussed

above and recognizing the CASAC conclusion that

the evidence provided support for retaining the

current standard, the then- Administrator concluded

that it was appropriate to retain the existing primary

PM10 standard, without revision. His decision was

consistent with the CASAC advice related to the

primary PM10 standard. Specifically, the CASAC

agreed with the 2020 PA conclusions that, while

these effects are important, the “evidence does not

call into question the adequacy of the public health

protection afforded by the current primary PM10

424a

standard” and “supports consideration of retaining

the current standard in this review” (Cox, 2019b, p. 3

of consensus letter). Thus, the then-Administrator

concluded that the primary PM10 standard (in all of

its elements (i.e., indicator, averaging time, form, and

level)) was requisite to protect public health with an

adequate margin of safety against effects that have

been associated with PM10–2.5. In light of this

conclusion, the EPA retained the existing PM10

standard.

2. Overview of the Health Effects Evidence

The information summarized here is based on the

scientific assessment of the health effects evidence

available in this reconsideration; this evaluation is

documented in the 2019 ISA and its policy

implications are discussed further in the 2022 PA. As

noted above, the ISA Supplement does not include an

evaluation of studies for PM10–2.5, and the 2019 ISA

continues to serve as the scientific foundation for this

reconsideration.

a. Nature of Effects

For the health effect categories and exposure

duration combinations evaluated, the 2019 ISA

concludes that the evidence supports causality

determinations for PM10–2.5 that are at most

“suggestive of, but not sufficient to infer, a causal

relationship”. While the evidence supporting the

causal nature of relationships between exposure to

PM10–2.5 has been strengthened for some health effect

categories since the completion of the 2009 ISA, the

2019 ISA concludes that overall “the uncertainties in

the evidence identified in the 2009 ISA have, to date,

still not been addressed” (U.S. EPA, 2019a, section

1.4.2, p. 1–41; U.S. EPA, 2022b, section 4.3.1).

425a

Specifically, epidemiologic studies available in the

2012 review relied on various methods to estimate

PM10–2.5 concentrations, and these methods had not

been systematically compared to evaluate spatial and

temporal correlations in PM10–2.5 concentrations.

Methods included: (1) Calculating the difference

between PM10 and PM2.5 concentrations at co-located

monitors, (2) calculating the difference between

county-wide averages of monitored PM10- and PM2.5based on monitors that are not necessarily co-located,

and (3) direct measurement of PM10–2.5 using a

dichotomous sampler (U.S. EPA, 2019a, section

1.4.2). As described in the 2019 ISA, there continues

to be variability across epidemiologic studies in the

approaches used to estimate PM10–2.5 concentrations.

Additionally, some studies estimate long-term PM10–

2.5 exposures as the difference between PM10 and

PM2.5 concentrations based on information from

spatiotemporal or land use regression (LUR) models,

in addition to monitors. The various methods used to

estimate PM10–2.5 concentrations have not been

systematically evaluated (U.S. EPA, 2019a, section

3.3.1.1), contributing to uncertainty regarding the

spatial and temporal correlations in PM10–2.5 concentrations across methods and in the PM10–2.5 exposure

estimates used in epidemiologic studies (U.S. EPA,

2019a, section 2.5.1.2.3). Given the greater spatial

and temporal variability of PM10–2.5 and the lower

number of PM10–2.5 monitoring sites, compared to

PM2.5, this uncertainty is particularly important for

the coarse size fraction. Beyond the uncertainty

associated with PM10–2.5 exposure estimates in

epidemiologic studies, the limited information on the

potential for confounding by copollutants and the

limited support available for the biological plausibility of health effects following PM10–2.5 exposures also

426a

continue to contribute to uncertainty in the PM10–2.5

health evidence. Uncertainty related to potential

confounding stems from the relatively small number

of epidemiologic studies that have evaluated PM10–2.5

health effect associations in copollutants models with

both gaseous pollutants and other PM size fractions.

On the other hand, uncertainty related to the

biological plausibility of effects attributed to PM10–2.5

exposures results from the small number of

controlled human exposure and animal toxicological

studies that have evaluated the health effects of

experimental PM10–2.5 inhalation exposures. The

evidence supporting the 2019 ISA’s “suggestive of,

but not sufficient to infer, a causal relationship”

causality determinations for PM10–2.5, including

uncertainties in this evidence, is summarized below

in sections III.B.1.a through III.B.1.f.

i. Mortality

Due to the dearth of studies examining the

association between long-term PM10–2.5 exposure and

mortality, the 2009 ISA concluded that the evidence

was “inadequate to determine if a causal relationship

exists” (U.S. EPA, 2009a). As reported in the 2019

ISA, some cohort studies conducted in the U.S. and

Europe report positive associations between longterm PM10–2.5 exposure and total (nonaccidental)

mortality, though results are inconsistent across

studies (U.S. EPA, 2019a, Table 11–11). The

examination of copollutant models in these studies

remains limited and, when included, PM10–2.5 effect

estimates are often attenuated after adjusting for

PM2.5 (U.S. EPA, 2019a, Table 11–11). Across studies,

PM10–2.5 exposure concentrations are estimated using

a variety of approaches, including direct measurements from dichotomous samplers, calculating the

427a

difference between PM10 and PM2.5 concentrations

measured at collocated monitors, and calculating

difference of area-wide concentrations of PM10 and

PM2.5. As discussed above, temporal and spatial

correlations between these approaches have not been

evaluated, contributing to uncertainty regarding the

potential for exposure measurement error (U.S. EPA,

2019a, section 3.3.1.1 and Table 11–11). The 2019

ISA concludes that this uncertainty “reduces the

confidence in the associations observed across

studies” (U.S. EPA, 2019a, p. 11–125). The 2019 ISA

additionally concludes that the evidence for longterm PM10–2.5 exposures and cardiovascular effects,

respiratory morbidity, and metabolic disease provide

limited biological plausibility for PM10–2.5-related

mortality (U.S. EPA, 2019a, sections 11.4.1 and 11.4).

Taken together, the 2019 ISA concludes that, “this

body of evidence is suggestive, but not sufficient to

infer, that a causal relationship exists between longterm PM10–2.5 exposure and total mortality” (U.S.

EPA, 2019a, p. 11–125).

With regard to short-term PM10–2.5 exposures and

mortality, the 2009 ISA concluded that the evidence

is “suggestive of a causal relationship between shortterm exposure to PM10–2.5 and mortality” (U.S. EPA,

2009a). The 2019 ISA included multicity epidemiologic studies conducted primarily in Europe and Asia

that continue to provide consistent evidence of

positive associations between short-term PM10–2.5

exposure and total (nonaccidental) mortality (U.S.

EPA, 2019a, Table 11–9). Although these studies

contribute to increasing confidence in the PM10–2.5mortality relationship, the use of various approaches

to estimate PM10–2.5 exposures continues to contribute

uncertainty to the associations observed. Recent

studies expand the assessment of potential

428a

copollutant confounding of the PM10–2.5-mortality

relationship and provide evidence that PM10–2.5

associations generally remain positive in copollutant

models, though associations are attenuated in some

instances (U.S. EPA, 2019a, section 11.3.4.1, Figure

11–28, Table 11–10). The 2019 ISA concludes that,

overall, the assessment of potential copollutant

confounding is limited due to the lack of information

on the correlation between PM10–2.5 and gaseous

pollutants and the small number of locations in

which copollutant analyses have been conducted.

Associations with cause-specific mortality (i.e., cardiovascular and respiratory mortality) provide some

support for associations with total (nonaccidental)

mortality, though associations with respiratory

mortality are more uncertain (i.e., wider confidence

intervals) and less consistent (U.S. EPA, 2019a,

section 11.3.7). The 2019 ISA concludes that the

evidence for PM10–2.5-related cardiovascular effects

provides only limited support for the biological

plausibility of a relationship between short-term

PM10–2.5 exposure and cardiovascular mortality (U.S.

EPA, 2019a, section 11.3.7). Based on the overall

evidence, the 2019 ISA concludes that, “this body of

evidence is suggestive, but not sufficient to infer, that

a causal relationship exists between short-term PM10–

2.5 exposure and total mortality” (U.S. EPA, 2019a, p.

11–120).

ii. Cardiovascular Effects

In the 2009 ISA, the evidence describing the

relationship between long-term exposure to PM10–2.5

and cardiovascular effects was characterized as

“inadequate to infer the presence or absence of a

causal relationship.” The limited number of

epidemiologic studies reported contradictory results

429a

and experimental evidence demonstrating an effect of

PM10–2.5 on the cardiovascular system was lacking

(U.S. EPA, 2019a, section 6.4).

The evidence relating long-term PM10–2.5 exposures

to cardiovascular mortality remains limited, with no

consistent pattern of associations across studies and,

as discussed above, uncertainty stemming from the

use of various approaches to estimate PM10–2.5

concentrations (U.S. EPA, 2019a, Table 6–70). The

evidence for associations with cardiovascular morbidity

has grown and, while results across studies are not

entirely consistent, some epidemiologic studies report

positive associations with ischemic heart disease

(IHD) and MI (U.S. EPA, 2019a, Figure 6–34); stroke

(U.S. EPA, 2019a, Figure 6–35); atherosclerosis (U.S.

EPA, 2019a, section 6.4.5); venous thromboembolism

(VTE) (U.S. EPA, 2019a, section 6.4.7); and blood

pressure and hypertension (U.S. EPA, 2019a, Section

6.4.6). PM10–2.5 cardiovascular mortality effect

estimates are often attenuated, but remain positive,

in copollutants models that adjust for PM2.5. For

morbidity outcomes, associations are inconsistent in

copollutant models that adjust for PM2.5, NO2, and

chronic noise pollution (U.S. EPA, 2019a, p. 6–276).

The lack of toxicological evidence for long-term PM10–

2.5 exposures represents a data gap (U.S. EPA, 2019a,

section 6.4.10), resulting in the 2019 ISA conclusion

that “evidence from experimental animal studies is of

insufficient quantity to establish biological plausibility” (U.S. EPA, 2019a, p. 6–277). Based largely on the

observation of positive associations in some

epidemiologic studies, the 2019 ISA concludes that

“evidence is suggestive of, but not sufficient to infer,

a causal relationship between long-term PM10–2.5

exposure and cardiovascular effects” (U.S. EPA,

2019a, p. 6–277).

430a

With regard to short-term PM10–2.5 exposures and

cardiovascular effects, the 2009 ISA found that the

available evidence for short-term PM10–2.5 exposure

and cardiovascular effects was “suggestive of a causal

relationship.” This conclusion was based on several

epidemiologic studies reporting associations between

short-term PM10–2.5 exposure and cardiovascular

effects, including IHD hospitalizations, supraventricular

ectopy, and changes in heart rate variability (HRV).

In addition, dust storm events resulting in high

concentrations of crustal material were linked to

increases in total cardiovascular disease emergency

department visits and hospital admissions. However,

the 2009 ISA noted the potential for exposure

measurement error primarily due to the different

methods used across studies to estimate PM10–2.5

concentrations and copollutant confounding in these

epidemiologic studies. In addition, there was only

limited evidence of cardiovascular effects from a

small number of experimental studies (e.g. animal

toxicological studies and controlled human exposure

studies) that examined short-term PM10–2.5 exposures

(U.S. EPA, 2009a, section 6.2.12.2). In the 2019 ISA,

key uncertainties included the potential for exposure

measurement error, copollutant confounding, and

limited evidence of biological plausibility for cardiovascular effects following inhalation exposure (U.S.

EPA, 2019a, section 6.3.13).

The evidence for short-term PM10–2.5 exposure and

cardiovascular outcomes has expanded since the 2009

ISA, though important uncertainties remain. The

2019 ISA notes that there are a small number of

epidemiologic studies reporting positive associations

between short-term exposure to PM10–2.5 and

cardiovascular-related morbidity outcomes. However,

the 2019 ISA notes that there is limited evidence to

431a

support that these associations are biologically

plausible, or independent of copollutant confounding.

The 2019 ISA also concludes that it remains unclear

how the approaches used to estimate PM10–2.5

concentrations in epidemiologic studies compare

amongst one another and subsequently how exposure

measurement error varies between each method.

Specifically, it is unclear how well-correlated PM10–2.5

concentrations are both temporally and spatially

across these methods and therefore whether exposure

measurement error varies across these methods.

Taken together, the 2019 ISA concludes that “the

evidence is suggestive of, but not sufficient to infer, a

causal relationship between short-term PM10–2.5

exposures and cardiovascular effects” (U.S. EPA,

2019a, p. 6–254).

iii. Respiratory Effects

With regard to short-term PM10–2.5 exposures and

respiratory effects, the 2009 ISA (U.S. EPA, 2009a)

concluded that the relationship between short-term

exposure to PM10–2.5 and respiratory effects is

“suggestive of a causal relationship” based on a small

number of epidemiologic studies observing associations with some respiratory effects and limited

evidence from experimental studies to support

biological plausibility. Epidemiologic findings were

consistent for respiratory infection and combined

respiratory-related diseases, but not for COPD.

Studies were characterized by overall uncertainty in

the exposure assignment approach and limited

information regarding potential copollutant confounding.

Controlled human exposure studies of short-term

PM10–2.5 exposures found no lung function decrements

and inconsistent evidence for pulmonary inflammation. Animal toxicological studies were limited to

432a

those using non-inhalation (e.g., intra-tracheal

instillation) routes of PM10–2.5 exposure.

Recent epidemiologic findings consistently link

PM10–2.5 exposure to asthma exacerbation and

respiratory mortality, with some evidence that

associations remain positive (though attenuated in

some studies of mortality) in copollutant models that

include PM2.5 or gaseous pollutants. Epidemiologic

studies provide limited evidence for positive associations with other respiratory outcomes, including

COPD exacerbation, respiratory infection, and

combined respiratory-related diseases (U.S. EPA,

2019a, Table 5–36). As noted above for other

endpoints, an uncertainty in these epidemiologic

studies is the lack of a systematic evaluation of the

various methods used to estimate PM10–2.5 concentrations and the resulting uncertainty in the spatial and

temporal variability in PM10–2.5 concentrations compared

to PM2.5 (U.S. EPA, 2019a, sections 2.5.1.2.3 and

3.3.1.1). Specifically, the existing monitoring

networks do not provide a good characterization of

how well correlated concentrations are both spatially

and temporally across the PM10–2.5 estimation

methods and overall spatial and temporal patterns in

PM10–2.5 concentrations. Taken together, the 2019 ISA

concludes that “the collective evidence is suggestive

of, but not sufficient to infer, a causal relationship

between short-term PM10–2.5 exposure and respiratory

effects” (U.S. EPA, 2019a, p. 5–270).

iv. Cancer

In the 2012 review, little information was available

from studies of cancer following inhalation exposures

to PM10–2.5. Thus, the 2009 ISA determined the

evidence was “inadequate to evaluate the relationship

between long-term PM10–2.5 exposures and cancer”

433a

(U.S. EPA, 2009a). The scientific information

evaluated in the 2019 ISA of long-term PM10–2.5

exposure and cancer remains limited, with a few

recent epidemiologic studies reporting positive, but

imprecise, associations with lung cancer incidence

(U.S. EPA, 2019a). Moreover, uncertainty remains in

these studies with respect to exposure measurement

error due to the use of PM10–2.5 predictions that have

not been validated by monitored PM10–2.5 concentrations (U.S. EPA, 2019a, sections 3.3.2.3 and

10.3.4). Relatively few experimental studies of PM10–

2.5 have been conducted, though available studies

indicate that PM10–2.5 exhibits two key characteristics

of carcinogens: genotoxicity and oxidative stress.

While limited, such experimental studies provide

some evidence of biological plausibility for the

findings in a small number of epidemiologic studies

(U.S. EPA, 2019a, section 10.3.4).

Taken together, the small number of epidemiologic

and experimental studies, along with uncertainty

with respect to exposure measurement error,

contribute to the determination in the 2019 ISA that,

“the evidence is suggestive of, but not sufficient to

infer, a causal relationship between long-term PM10–

2.5 exposure and cancer” (U.S. EPA, 2019a, p. 10–87).

v. Metabolic Effects

The 2009 ISA did not make a causality

determination for PM10–2.5-related metabolic effects.

One epidemiologic study in the 2019 ISA reports an

association between long-term PM10–2.5 exposure and

incident diabetes, while additional cross-sectional

studies report associations with effects on glucose or

insulin homeostasis (U.S. EPA, 2019a, section 7.4).

As discussed above for other outcomes, uncertainties

with the epidemiologic evidence include the potential

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for copollutant confounding and exposure measurement error due to the different methods used across

studies to estimate PM10–2.5 concentrations (U.S. EPA,

2019a, Tables 7–14 and 7–15). The evidence base to

support the biological plausibility of metabolic effects

following PM10–2.5 exposures is limited, but a crosssectional study that investigated biomarkers of

insulin resistance and systemic and peripheral

inflammation may support a pathway leading to type

2 diabetes (U.S. EPA, 2019a, sections 7.4.1 and 7.4.3).

Based on the expanded, though still limited evidence

base, the 2019 ISA concludes that, “[o]verall, the

evidence is suggestive of, but not sufficient to infer, a

causal relationship between [long]-term PM10–2.5 exposure

and metabolic effects” (U.S. EPA, 2019a, p. 7–56).

vi. Nervous System Effects

The 2009 ISA did not make a causality determination

for PM10–2.5-related nervous system effects. In the

2019 ISA, available epidemiologic studies report

associations between PM10–2.5 and impaired cognition

and anxiety in adults in longitudinal analyses (U.S.

EPA, 2019a, Table 8–25, section 8.4.5). Associations

of long-term exposure with neurodevelopmental

effects are not consistently reported in children (U.S.

EPA, 2019a, sections 8.4.4 and 8.4.5). Uncertainties

in these studies include the potential for copollutant

confounding, as no studies examined copollutants

models (U.S. EPA, 2019a, section 8.4.5), and for

exposure measurement error, given the use of various

methods to estimate PM10–2.5 concentrations (U.S.

EPA, 2019a, Table 8–25). In addition, there is limited

animal toxicological evidence supporting the biological

plausibility of nervous system effects (U.S. EPA,

2019a, sections 8.4.1 and 8.4.5). Overall, the 2019

ISA concludes that, “the evidence is suggestive of, but

435a

not sufficient to infer, a causal relationship” between

long-term PM10–2.5 exposure and nervous system

effects (U.S. EPA, 2019a, p. 8–75).

B. Conclusions on the Primary PM10 Standard

In drawing conclusions on the adequacy of the

current primary PM10 standard, in view of the

advances in scientific knowledge and additional

information now available, the Administrator has

considered the evidence base, information, and policy

judgments that were the foundation of the 2020

review and reflects upon the body of information and

evidence available in this reconsideration. In so

doing, the Administrator has taken into account both

evidence-based and quantitative information-based

considerations, as well as advice from the CASAC

and public comments. Evidence-based considerations

draw upon the EPA’s integrated synthesis of the

scientific evidence from animal toxicologic, controlled

human exposure, and epidemiologic studies

evaluating health effects related to exposures to

PM10–2.5 as presented in the 2019 ISA and discussed

in section III.A.2. In addition to the evidence, the

Administrator has weighed a range of policy-relevant

considerations as discussed in the 2022 PA and

summarized in sections III.B and III.C of the

proposal and summarized in section III.B.2 below.

These considerations, along with the advice from the

CASAC (section III.B.1) and public comments

(section III.B.3), are discussed below. A more detailed

summary of all significant comments, along with the

EPA’s responses in the Response to Comments

document, can be found in the docket for this

rulemaking (Docket No. EPA–HQ–OAR–2015–

00072). This document is available for review in the

docket for this rulemaking and through EPA’s

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NAAQS website (link). The Administrator’s conclusions

in this reconsideration regarding the adequacy of the

current primary PM10 standard and whether any

revisions are appropriate are described in section

III.B.4.

1. CASAC Advice

As described in section I.X, the EPA decided to

prepare a revised PA for the reconsideration of the

2020 final decision. The CASAC’s advice on the 2019

draft PA and the 2021 draft PA was documented in

letters to the prior and current Administrators (Cox,

2019b; Sheppard, 2022a) and is summarized below.

In reviewing both the 2019 draft PA and the 2021

draft PA, the CASAC agreed with the EPA’s

preliminary conclusion that the available scientific

evidence, including its uncertainties and limitations,

does not call into question the adequacy of the

current primary PM10 standard and that the

standard should be retained, without revision.

In its review of the 2019 draft PA, the CASAC

concurred with the overall preliminary conclusion

that it is appropriate to consider retaining the

current primary PM10 standard, without revision. In

their agreement with the conclusions in the 2019

draft PA, the CASAC stated that “that key

uncertainties identified in the last review remain”

(Cox, 2019b) and that “none of the identified health

outcomes linked to PM10–2.5” were judged to be causal

or likely to be causal (Cox, 2019b, p. 12 of consensus

responses).

Moreover, to reduce these uncertainties in future

reviews, the CASAC recommended improvements to

PM10–2.5 exposure assessment, including a more

extensive network for direct monitoring of the PM10–2.5

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fraction (Cox, 2019b, p. 13 of consensus responses).

The CASAC also recommended additional controlled

human exposure and animal toxicological studies of

the PM10–2.5 fraction to improve the understanding of

biological mechanisms and pathways (Cox, 2019b, p.

13 of consensus responses). Overall, the CASAC

agreed with the EPA’s preliminary conclusion in the

2019 draft PA that “. . . the available evidence does

not call into question the adequacy of the public

health protection afforded by the current primary

PM10 standard and that evidence supports

consideration of retaining the current standard in

this review” (Cox, 2019b, p. 3 of letter).

In its review of the 2021 draft PA, the CASAC

provided advice on the adequacy of the current

primary PM10 standard in the context of its review of

the revised PA for this reconsideration (Sheppard,

2022a)129.)130 In this context, the CASAC supported

the preliminary conclusion in the 2021 draft PA that

the evidence reviewed in the 2019 ISA does not call

into question the public health protection provided by

the current primary PM10 standard against PM10–2.5

exposures and concurs with the 2021 draft PA’s

overall preliminary conclusion that it is appropriate

to consider retaining the current primary PM10

standard (Sheppard, 2022a, p. 4 of consensus letter).

129

As described in section I.C.5.b above, the scope of the ISA

Supplement did not include consideration of studies of health

effects associated with exposure to PM10–2.5. Therefore, the

information and conclusions presented in the 2022 PA are very

similar to those in the 2020 PA.

130

As described in section I.C.5.b above, the scope of the ISA

Supplement did not include consideration of studies of health

effects associated with exposure to PM10–2.5. Therefore, the

information and conclusions presented in the 2022 PA are very

similar to those in the 2020 PA.

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Additionally, the CASAC concurred that “. . . at this

time, PM10 is an appropriate choice as the indicator

for PM10–2.5” and “that it is important to retain the

level of protection afforded by the current PM10

standard” (Sheppard, 2022a, p. 4 of consensus letter).

The CASAC also recognized uncertainties associated

with the scientific evidence, including “compared to

PM2.5 studies, the more limited number of epidemiology studies with positive statistically significant

findings, and the difficulty in extracting the sole

contribution of coarse PM to observed adverse health

effects” (Sheppard, 2022a, p. 19 of consensus responses).

The CASAC recommended several areas for

additional research to reduce uncertainties in the

PM10–2.5 exposure estimates used in the epidemiologic

studies, to evaluate the independence of PM10–2.5

health effect associations, to evaluate the biological

plausibility of PM10–2.5-related effects, and to increase

the number of studies examining PM10–2.5-related

health effects in at-risk populations (Sheppard, 2022a,

p. 20 of consensus responses). Furthermore, the

CASAC “recognizes a need for, and supports investment in research and deployment of measurement

systems to better characterize PM10–2.5” and to

“provide information that can improve public health”

(Sheppard, 2022a, p. 20 of consensus responses).

2. Basis for the Proposed Decision

At the time of the proposal, the Administrator

carefully considered the assessment of the current

evidence and conclusions reached in the 2019 ISA,

considerations and staff conclusions and associated

rationales presented in the 2020 PA and 2022 PA,

and advice and recommendations of the CASAC (88

FR 5634, January 27, 2023). Consistent with

previous reviews, the Administrator first considered

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the available scientific evidence for PM10–2.5-related

exposures and health effects, as evaluated in the

2019 ISA. As an initial matter, the Administrator

recognized that the scientific evidence for PM10–2.5related effects available in this reconsideration is the

same body of evidence that was available at the time

of the 2020 review, as evaluated in the 2019 ISA and

summarized in section III.A.2 above. The 2019 ISA

concludes that the evidence supports “suggestive of,

but not sufficient to infer” causal relationships

between short- and long-term exposures to PM10–2.5

and cardiovascular effects, cancer, and mortality and

long-term PM10–2.5 exposures and metabolic effects

and nervous system effects (U.S. EPA, 2019a). The

Administrator noted that the evidence for several

PM10–2.5-related health effects has expanded since the

completion of the 2009 ISA, but important uncertainties remain. The uncertainties in the epidemiologic

studies contribute to the determinations in the 2019

ISA that the evidence for short and long-term PM10–2.5

exposures and mortality, cardiovascular effects,

metabolic effects, nervous system effects, and cancer

is “suggestive of, but not sufficient to infer” causal

relationships (U.S. EPA, 2019a; U.S. EPA, 2022b,

section 4.3.1). Drawing from the evidence evaluated

in the 2019 ISA and consideration of the scientific

evidence in the 2022 PA, the Administrator noted

that, consistent with previous reviews, the 2019 ISA

and the 2022 PA highlight a number of uncertainties

associated with the evidence, including: (1) PM10–2.5

exposure estimates used in epidemiologic studies, (2)

independence of PM10–2.5 health effect associations,

and (3) biological plausibility of the PM10–2.5-related

effects. These uncertainties contribute to the determinations in the 2019 ISA that the evidence for

short-term PM10–2.5 exposures and key health effects

440a

is “suggestive of, but not sufficient to infer” causal

relationships. In considering the available scientific

evidence, consistent with approaches employed in

past NAAQS reviews, the Administrator placed the

most weight on evidence supporting “causal” and

“likely to be causal” relationships. In so doing, he

noted that the available evidence for short- and longterm PM10–2.5 exposures and health effects does

not support causality determinations of a “causal

relationship” or “likely to be causal relationship.”

Furthermore, the Administrator recognized that,

because of the uncertainties and limitations in the

evidence base, the 2022 PA does not include a

quantitative assessment of PM10–2.5 exposures and

risk that might further inform decisions regarding

the adequacy of the current 24-hour primary PM10

standard. Therefore, in light of the 2019 ISA

conclusions that the evidence supports “suggestive of,

but not sufficient to infer” causal relationships. The

Administrator judged that there are substantial

uncertainties that raise questions regarding the

degree to which additional public health improvements would be achieved by revising the existing

PM10 standard. In considering the available evidence

for long-term PM10–2.5 exposures, the Administrator

noted that there is limited evidence that would

support consideration of an annual standard to

provide protection against such effects, in conjunction

with the current primary 24-hour PM10 standard. He

preliminarily concluded that the current primary 24hour PM2.5 standard that reduces 24-hour exposures

also likely reduces long-term average exposures, and

therefore provides some margin of safety against the

health effects associated with long-term PM10–2.5

exposures.

441a

In reaching his proposed decision on the adequacy

of the current primary 24-hour PM10 standard, the

Administrator also considered advice from the

CASAC. As noted above in section III.B.1, the

CASAC recognized uncertainties associated with the

scientific evidence and agreed with the 2019 draft PA

and 2021 draft PA conclusions that the scientific

evidence does not call into question the adequacy of

the primary PM10 standard and supports consideration of retaining the current standard.

When considering the above information together,

the Administrator proposed to conclude that the

available scientific evidence continues to support a

PM10 standard to provide some measure of protection

against PM10–2.5 exposures. Additionally, he recognized

that there are important uncertainties and limitations associated with the available evidence for PM10–

2.5-related health effects, for both short and long-term

exposure, as evaluated in the 2019 ISA. Consistent

with the decisions in the previous reviews, the

Administrator proposed to conclude that these limitations lead to considerable uncertainty regarding the

potential public health implications of revising the

level of the current primary 24-hour PM10 standard.

Thus, based on his consideration of the evidence and

associated uncertainties and limitations for PM10–2.5related health effects and his consideration of CASAC

advice on the primary PM10 standard, the Administrator

proposed to retain the current primary PM10

standard, without revision.

3. Comments on the Proposed Decision

Of the public comments received on the proposal,

very few commenters provided comments on the

primary PM10 standard. Of those commenters who

did provide comments on the primary PM10 standard,

442a

the majority agree with the EPA’s proposed decision

to retain the primary PM10 standard. In so doing,

these commenters agree with the EPA’s rationale

regarding the available scientific information, including

uncertainties and limitations, for informing decisions

on the standard. These commenters state that no new

scientific evidence or quantitative information has

emerged since the 2020 decision to retain the current

standard. Furthermore, these commenters note that

the EPA did not evaluate any new scientific evidence

related to PM10–2.5 exposures and health effects as a

part of the 2022 ISA Supplement developed for this

reconsideration, nor did the revised 2022 PA consider

any new or different information from the 2020 PA,

and therefore, the EPA reached the same conclusion

as is the 2020 PA that the current standard is

adequate and should be retained. This group includes

industries and industry groups, as well as some State

and local governments. All of these commenters

generally note their agreements with the rationale

provided in the proposal and the CASAC concurrence

with the 2021 draft PA conclusion that the available

information does not call into question the adequacy

of the current standard, and therefore, does not

support revision and that the current standard

should be retained.

Some commenters, including those from environmental and public health organizations and groups,

some states, and individuals, disagreed with the

Administrator’s proposed decision to retain the

current primary PM10 standard. These commenters

recommend that the EPA revise the primary PM10

standard to a lower level to provide increased public

health protection, citing to the available scientific

evidence, as well as the proposed revision to the

primary PM2.5 standard.

443a

Commenters who disagreed with the proposal to

retain the current standard state that revision to the

primary PM10 standard is necessary to protect public

health with an adequate margin of safety. In their

recommendations for revising the standard, some

commenters contend that the current standard, with

its indicator of PM10 to target exposures to PM10–2.5,

has become less protective as ambient concentrations

of PM2.5 have been reduced with revisions to that

standard. These commenters assert that the current

primary PM10 standard allows increased exposure to

PM10–2.5 in ambient air because retaining the primary

PM10 would allow proportionately more PM10–2.5 mass

as the PM2.5 standard has been revised downward.

Moreover, in support of their recommendations, the

commenters note that the available evidence of

PM10–2.5-related health effects has been expanded and

strengthened since the time of the last review. Taken

together, the commenters contend that the primary

PM10 standard should be revised and failure to do so

would be arbitrary and capricious. Some of these

commenters assert that the level of the primary PM10

standard should be revised to 140 or 145 μg/m3,

concurrent with a strengthened primary 24-hour

PM2.5 standard, while other commenters recommend

revising the level of the standard to within the range

of 65–75 μg/m3, to provide increased public health

protection.

We disagree with the commenters that the primary

PM10 standard should be revised because of reductions

in ambient concentrations of PM2.5. As an initial

matter, we note that overall, ambient concentrations

of both PM10 and PM2.5 have declined significantly

over time. Ambient concentrations of PM10 have

444a

declined by 46% across the U.S. from 2000 to 2019,131

while PM2.5 concentrations in ambient air have

declined by 43% during this same time period.132 As

noted in the 2022 PA (p. 2–41), the majority of PM10–

2.5 sites have generally remained steady and do

notexhibit a trend of increasing or decreasing

concentrations during this time period, reflecting the

relatively consistent level of dust emission across the

U.S. from 2000 to 2019 (U.S. EPA, 2022b).

The 2019 ISA provides a comparison of the relative

contribution of PM2.5 and PM10–2.5 to PM10 concentrations by region and season using the more

comprehensive monitoring data from the NCore

network available in this reconsideration (U.S. EPA,

2019, section 2.5.1.1.4). The data indicate that, for

urban areas, there are roughly equivalent amounts of

PM2.5 and PM10–2.5 contributing to PM10 in ambient

air, while rural locations have a slightly higher

contribution of PM10–2.5 contributing to PM10

concentrations than PM2.5 (U.S. EPA, 2019, section

2.5.1.1.4, Table 2–7). There is generally a greater

contribution from the PM2.5 fraction in the East and a

greater contribution from the PM10–2.5 fraction in the

West and Midwest.

The EPA recognizes that when the primary annual

PM2.5 standard was revised from 15.0 μg/m3 to 12.0

μg/m3 while leaving the 24-hour PM2.5 standards

131

PM10 concentrations presented as the annual second

maximum 24-hour concentration (in μg/m3) at 262 sites in the

U.S. For more information, see: https://www.epa.gov/airtrends/particulate-matter-pm10-trends

132

PM2.5 concentrations presented as the seasonally-weighted

annual average concentration (in μg/m3) at 406 sites in the U.S.

For

more

information,

see:

https://www.epa.gov/airtrends/particulate-matter-pm25-trends

445a

unchanged at 35 μg/m3 and the 24-hour PM10

standard unchanged at 150 μg/m3, the PM10–2.5

fraction of PM10 could increase in some areas as the

PM2.5 fraction decreases (78 FR 3085, March 03,

2013). As described in the 2019 ISA, PM10 has become

considerably coarser across the U.S. compared to

similar observations in the 2009 ISA such that, in

urban areas, the mass of the coarse fraction of PM is

similar to or greater than the mass of the fine

fraction of PM (U.S. EPA, 2019, section 2.5.1.1.4; U.S.

EPA, 2009c). However, in considering recent air

quality data, the EPA notes that in most areas of the

country PM2.5 and PM10 concentrations have declined

and are well below their respective 24-hour

standards. While the contribution of fine and coarse

PM to PM10 mass concentrations may vary spatially

and temporally, based on the trends in recent air

quality data, the Administrator concludes that the

current primary 24-hour PM10 standard is

maintaining air quality at level that provides

requisite protection against PM10–2.5. That is, recent

air quality data does not suggest that PM10–2.5

concentrations have been increasing as PM2.5

concentrations have been decreasing. In considering

the available PM10–2.5 health effects evidence in this

reconsideration, there continue to be significant

uncertainties and limitations, specifically with

respect to the exposure assessment methods used to

estimate PM10–2.5 concentrations, that make it

difficult to fully assess the public health implications

of revising the primary PM10 standard even considering the possibility for additional variability in the

relative ratio of PM2.5 to PM10–2.5 in current PM10 air

quality across the U.S. As described in detail above in

section III.A.2 and in the proposal (85 FR 5558,

January 27, 2023), the uncertainties and limitations

446a

in the health effects evidence for PM10–2.5 contributed

to the determinations in the 2019 ISA that the

evidence for key PM10–2.5 health effects is “suggestive

of, but not sufficient to infer, a causal relationship” or

“inadequate to infer the presence, or absence of a

causal relationship” (U.S. EPA, 2019a). While the

evidence base for PM10–2.5-related health effects has

somewhat expanded since the 2009 ISA, the

Administrator concludes that the evidence remains

too limited to inform judgments regarding whether a

more protective primary PM10 standard is warranted

at this time.

Beyond the uncertainties and limitations associated

with the available scientific evidence, the EPA also

notes that, while the NCore monitoring network has

been expanded since the time of the last review,

epidemiologic studies available in this review do not

use PM10–2.5 NCore data in evaluating associations

between PM10–2.5 in ambient air and long- or shortterm exposures. In the absence of such evidence, the

public health implications of changes in ambient

PM10–2.5 concentrations as PM2.5 concentrations

decrease remain unclear. Therefore, the EPA continues

to recognize this as an area for future research, to

address the existing uncertainties (U.S. EPA, 2022b,

section 4.6), and inform future reviews of the PM

NAAQS. Taken together, as at the time of proposal,

the Administrator concludes that these and other

limitations in the PM10–2.5 evidence raised questions

as to whether additional public health improvements

would be achieved by revising the existing PM10

standard, particularly when considering such judgments along with his decision to retain the current

primary 24-hour PM2.5 standard. Therefore, the EPA

does not agree with the commenters that the currently

available air quality information or scientific

447a

evidence support revisions to the primary PM10

standard in this reconsideration.

Consistent with their comments on the 2020

proposal, some commenters disagreed with the

Administrator’s proposed conclusion to retain the

current primary PM10 standard, primarily focusing

their comments on the need for revisions to the form

of the standard or the level of the standard. With

regard to comments on the form of the standard,

some commenters assert that the EPA should revise

the standard by adopting a separate form (or a

“compliance threshold” in their words)—the 99th

percentile, averaged over three years—for the primary

PM10 standard for continuous monitors, which provide

data every day, while maintaining the current form

of the standard (one exceedance, averaged over three

years) for 1-in-6 samplers, given the increased use of

continuous monitoring and to ease the burden of

demonstrating exceptional events. These commenters, in

support of their comment, contend that the 99th

percentile would effectively change the form from the

2nd highest to the 4th highest and would allow no

more than three exceedances per year, averaged over

three years. These commenters additionally highlight

the EPA’s decision in the 1997 review to adopt a

99th percentile form, averaged over three years,

citing to advantages of a percentile-based form in the

Administrator’s rationale in that review. The

comments further assert that a 99th percentile form

for the primary PM10 standard is still more

conservative than the form for other short-term

NAAQS (e.g., PM2.5 and NO2).

First, the EPA has long recognized that the form is

an integral part of the NAAQS and must be selected

together with the other elements (i.e., indicator,

448a

averaging time, level) of the NAAQS to ensure the

appropriate stringency and requisite degree of public

health protection. Thus, if the EPA were to change

the form according to the monitoring method it would

be establishing two different NAAQS, varying based

on the monitoring method. The EPA has not done

this to date, did not propose such an approach, and

declines to adopt it for the final rule, as we believe

such a decision in this final rule is beyond the scope

of the proposal, and that each PM standard should

have a single form, indicator, level and averaging

time, chosen by the Administrator as necessary and

appropriate. While certain continuous monitors may

be established and approved as a Federal Equivalent

Method (FEM) for PM10, as an alternative to a

Federal Reference Method (FRM), the use of an FEM

is intended as an alternative means of determining

compliance with the NAAQS, not as authorizing a

different NAAQS.

Even if the commenters had asked that the change

in form be made without regard to monitoring

method, the EPA does not believe such a change

would be warranted. The change in form for continuous monitors suggested by the commenters, without

also lowering the level of such a standard, would

allow more exceedances and thereby reduce the

public health protection provided against exposures

to PM10–2.5 in ambient air, resulting in a less stringent

primary PM10 standard than the current standard.

These commenters have not provided new evidence or

analyses to support their conclusion that an

appropriate degree of public health protection could

be achieved by allowing the use of an alternative

form (i.e., 99th percentile), while retaining the other

elements of the standard.

449a

With regard to the commenters’ assertion that an

alternate form of the standard would ease the burden

of demonstrating exceptional events, the EPA

recognizes, consistent with the CAA, that it may be

appropriate to exclude monitoring data influenced by

“exceptional” events when making certain regulatory

determinations. However, the EPA notes that the

cost of implementation of the standards may not be

considered by the EPA in reviewing the standards.

The EPA continues to update and develop documentation and tools to facilitate the implementation of the

2016 Exceptional Events Rule, including new PM2.5

implementation focused products under development

that are intended to assist air agencies with the

development of demonstrations for specific types of

exceptional events. With regard to the commenters’

specific concerns for wildfires or high winds, the EPA

released updated guidance documents on the

preparation of exceptional event demonstrations

related to wildfires in September 2016, high wind

dust events in April 2019, and prescribed fires in

August 2019. These guidance documents outline the

regulatory requirements and provide examples for air

agencies preparing demonstrations for wildfires, high

wind dust, and prescribed fire events. For all of the

reasons discussed above, the EPA does not agree with

the commenters that the form of the primary PM10

standard should be revised to a 99th percentile for

continuous monitors.

4. Administrator’s Conclusions

This section summarizes the Administrator’s considerations and conclusions related to the current

primary PM10 standard. In establishing primary

standards under the Act that are “requisite” to

protect the public health with an adequate margin of

450a

safety, the Administrator is seeking to establish

standards that are neither more nor less stringent

than necessary for this purpose. In so doing, the

Administrator notes that his final decision in this

reconsideration is a public health policy judgment

that draws upon scientific information, as well as

judgments about how to consider the range and

magnitude of uncertainties that are inherent in the

information. Accordingly, he recognizes that his

decision requires judgments based on the interpretation of the evidence that neither overstates nor

understates the strength or limitations of the

evidence nor the appropriate inferences to be drawn.

He recognizes, as described in section I.A above, that

the Act does not require that primary standards be

set at a zero-risk level; rather, the NAAQS must be

sufficient but not more stringent than necessary to

protect public health, including the health of

sensitive groups with an adequate margin of safety.

Given these requirements, and consistent with the

primary PM2.5 standards discussed above (section

II.C.3), the Administrator’s final decision in this

reconsideration of the current primary PM10 standard

will be a public health policy judgment that draws

upon the scientific information examining the health

effects of PM10–2.5 exposures, including how to consider

the range and magnitude of uncertainties inherent in

that information. The Administrator’s final decision

is based on an interpretation of the scientific evidence

that neither overstates nor understates its strengths

and limitations, nor the appropriate inferences to be

drawn.

Having carefully considered advice from the

CASAC and public comments, as discussed above, the

Administrator notes that the fundamental scientific

451a

conclusions on health effects of PM10–2.5 in ambient

air that were reached in the 2019 ISA and

summarized in the 2020 PA and 2022 PA remain

valid. Additionally, the Administrator believes the

judgments he proposed (85 FR 5558, January 27,

2023) with regard to the evidence remain appropriate. Further, in considering the adequacy of the

current primary PM10 standard in this reconsideration, the Administrator has carefully considered the

policy-relevant evidence and conclusions contained

in the 2019 ISA; the rationale and conclusions

presented in the 2020 PA and 2022 PA; the advice

and recommendations from the CASAC in their

reviews of the 2019 draft PA and 2021 draft PA; and

public comments, as addressed in section III.B.3

above and in the RTC document. In the discussion

below, the Administrator gives weight to the

conclusions in the 2020 PA and 2022 PA, with which

the CASAC has concurred, as summarized in section

III.C of the proposal and takes note of the key aspects

of the rationale for those conclusions that contribute

to his decision in this review. In considering this

information, the Administrator concludes that the

preliminary conclusions and policy judgments

supporting his proposed decision remain valid, and

that the current primary PM10 standard provides

requisite protection of public health with an adequate

margin of safety and should be retained. In considering the 2020 PA and 2022 PA evaluations and

conclusions, the Administrator notes that, while the

health effects evidence is somewhat expanded since

the 2009 ISA as described in section III.A.2 above,

the overall conclusions are generally consistent with

those reached in the 2009 ISA (U.S. EPA, 2020b,

section 4.4). In so doing, he additionally notes that

the CASAC supported the preliminary conclusion in

452a

the 2019 draft PA and 2021 draft PA that the

evidence reviewed in the 2019 ISA does not call into

question the public health protection provided by the

current primary PM10 standard against PM10–2.5

exposures and concurs that it is appropriate to consider

retaining the current primary PM10 standard (Cox,

2019b, p. 13 of consensus responses; Sheppard,

2022a, p. 4 of consensus letter).

As noted below, the scientific evidence for PM10–2.5related health effects has expanded somewhat since

the 2012 review, in particular for long-term exposures.

The Administrator recognizes, however, that there

are a number of uncertainties and limitations associated

with the available information, as described in the

proposal (85 FR 5558, January 27, 2023) and below.

With regard to the current evidence on PM10–2.5related health effects, the Administrator takes note of

recent epidemiologic studies that continue to report

positive associations with mortality and morbidity in

cities across North America, Europe, and Asia, where

PM10–2.5 sources and composition are expected to vary

widely. While significant uncertainties remain, as

described below, the Administrator recognizes that

this expanded body of evidence has broadened the

range of effects that have been linked with PM10–2.5

exposures. These studies provide an important part

of the scientific foundation supporting the 2019 ISA’s

revised causality determinations (and new determinations) for long-term PM10–2.5 exposures and mortality,

cardiovascular effects, metabolic effects, nervous

system effects, and cancer (U.S. EPA, 2019a; U.S.

EPA, 2022b, section 4.2). Drawing from his consideration of this evidence, the Administrator concludes

that the available scientific information supports a

decision to maintain a primary PM10 standard to

provide public health protection against PM10–2.5

453a

exposures, regardless of location, source of origin, or

particle composition. With regard to uncertainties in

the evidence, the Administrator first notes that a

number of limitations were identified in the 2012

review related to: (1) Estimates of ambient PM10–2.5

concentrations used in epidemiologic studies; (2) limited

evaluation of copollutant models to address the potential

for confounding; and (3) limited experimental studies

supporting biological plausibility for PM10–2.5-related

effects. Despite the expanded body of evidence for

PM10–2.5 exposures and health effects assessed in the

2019 ISA, the Administrator recognizes that uncertainties remain, similar to those in the 2012 review.

As summarized in section III.A.2 above and in

responding to public comments, uncertainties in the

available scientific evidence continue to include those

associated with the exposure estimates used in

epidemiologic studies, the independence of the PM10–

2.5 health effect associations, and the biologically

plausible pathways for PM10–2.5 health effects (U.S.

EPA, 2022b, section 4.3). These uncertainties contribute to the 2019 ISA determinations that the evidence

is “suggestive of, but not sufficient to infer” causal

relationships (U.S. EPA, 2019a). The Administrator

recognizes that the NAAQS must allow for a margin

of safety but also places emphasis on evidence

supporting “causal” or “likely to be causal” relationships (as described in sections II.A.2 and III.A.2

above). Finding that there is too much uncertainty

that a more stringent standard would improve public

health, the Administrator judges that the available

evidence provides support for his conclusion that the

current standard provides the requisite level of

protection from the effects of PM10–2.5. In making this

judgment, the Administrator considers whether this

level of protection is more than what is requisite and

454a

whether a less stringent standard would be appropriate to consider. He notes that there continues to be

uncertainty associated with the evidence, as reflected

by the “suggestive of, but not sufficient to infer”

causal determinations. The Administrator recognizes

that the CAA requirement that primary standards

provide an adequate margin of safety, as summarized

in section I.A above, is intended to address uncertainties associated with inconclusive scientific evidence

and technical information, as well as to provide a

reasonable degree of protection against hazards that

research has not yet identified. In light of these

considerations and the current body of evidence,

including uncertainties and limitations, the Administrator concludes that a less stringent standard

would not provide the requisite protection of public

health, including an adequate margin of safety. The

Administrator also considers whether the level of

protection associated with the current standard is

less than what is requisite and whether a more

stringent standard would be appropriate to consider.

In so doing, the Administrator considers, as discussed

above, the level of protection offered from exposures

for which public health implications are less clear. In

so doing, he again notes the significant uncertainties

and limitations that persist in the scientific evidence.

In particular, he notes limitations in the approaches

used to estimate ambient PM10–2.5 concentrations in

epidemiologic studies, limited examination of the

potential for confounding by co-occurring pollutants,

and limited support for the biological plausibility of

the serious effects reported in many epidemiologic

studies that are reflected by the “suggestive of, but

not sufficient to infer” causal determinations. Thus,

in light of the currently available information,

including the uncertainties and limitations of the

455a

evidence base available to inform his judgments

regarding protection against PM10–2.5-related effects,

the Administrator does not find it appropriate to

increase the stringency of the standard in order to

provide the requisite public health protection.

Rather, he judges it appropriate to maintain the level

of protection provided by the current primary PM10

standard for PM10–2.5 exposures and he does not judge

that the available information and the associated

uncertainties indicate the need for a greater level of

public health protection.

In reaching his conclusions on the primary PM10

standard, the Administrator also considers advice

from the CASAC. In their comments, the CASAC

noted that uncertainties that were identified in the

2012 review persist in the evidence for PM10–2.5related health effects (Cox, 2019b, p. 13 of consensus

responses; Sheppard, 2022a, p. 4 of consensus letter)

In considering these comments, the Administrator

takes note of the CASAC consideration of the evidence,

and associated uncertainties, and its conclusion that

the evidence reviewed in the 2019 ISA does not call

into question the adequacy of the public health

protection afforded by the current primary PM10

standard (Cox, 2019b, p. 3 of letter; Sheppard, 2022a,

p. 4 of consensus letter). The Administrator further

notes the unanimous conclusions of the CASAC that

evidence supports consideration of retaining the

current primary PM10 standard (Cox, 2019b, p. 3 of

consensus letter; Sheppard, 2022a, p. 4 of consensus

letter). In addition to the CASAC’s advice, the

Administrator also considers public comments, the

majority of which supported retaining the primary

PM10 standard, citing to and agreeing with the

Administrator’s rationale for his proposed decision.

The Administrator also recognizes that a few public

456a

commenters supported revising the primary PM10

standard in order to provide increased protection

against PM10–2.5-related health effects.

The Administrator also notes that the scientific

record for his decision on the primary PM10 standard is

the same as the record before the then-Administrator

in 2020, as the scope of the ISA Supplement focused

on health effect categories where the 2019 ISA

concluded a causal relationship (i.e., short- and longterm PM2.5 exposure and cardiovascular effects and

mortality). Therefore, because no health outcome

categories for short- or long-term PM10–2.5 exposure in

the 2019 ISA were greater than “suggestive of, but

not sufficient to infer, a causal relationship”, the

ISA Supplement did not evaluate studies published

after the literature cutoff date of the 2019 ISA

related to PM10–2.5 exposures and health effects. The

Administrator further notes his decision is consistent

with the decision of the prior Administrator in 2020

to retain the primary PM10 standard.

With regard to the indicator for the primary PM10

standard, the Administrator recognizes that the 2022

PA notes that the evidence continues to support

retaining the PM10 indicator to provide public health

protection against PM10–2.5-related effects. He notes

that, consistent with the approaches in previous

reviews, a standard with a PM10 mass-based indicator,

in conjunction with a PM2.5 mass-based standard, will

result in controlling allowable concentrations of PM10–

2.5. The Administrator also takes note of the 2019 ISA

comparison that showed that the relative contribution of PM2.5 and PM10–2.5 to PM10 concentrations can

vary across the U.S. by region and season, with

urban locations having a somewhat higher contribution of PM2.5 contributing to PM10 concentrations than

457a

PM10–2.5 (U.S. EPA, 2019a, section 2.5.1.1.4, Table

2–7). In these urban locations, where PM2.5 concentrations are somewhat higher than in rural locations,

the toxicity of the PM10 may be higher due to

contaminating PM2.5. Further, although uncertainties

with the evidence persist, the strongest health effects

evidence associated with PM10–2.5 comes from

epidemiologic studies conducted in urban areas. He

also notes that the CASAC agreed with the EPA’s

conclusions that a PM10 indicator remained appropriate (Cox, 2019b, p. 13 of consensus responses;

Sheppard, 2022a, p. 4 of letter). In light of this

information, the Administrator concludes that the

PM10 indicator remains appropriate and provides

protection from exposure to all coarse PM, regardless

of location, source of origin, or particle composition.

Similarly, with regard to averaging time, form, and

level of the standard, the Administrator takes note of

uncertainties in the available evidence and information

and continues to find that the current standard, as

defined by in all of its elements, is requisite. As an

initial matter, the Administrator notes that the

current primary PM10 standard, with its level of 150

μg/m3, 24-hour averaging time, not to be exceeded

more than once per year on average over three

years, is intended to protect against short-term peak

PM10–2.5 exposures. In so doing, while the Administrator notes that changes in PM2.5 concentrations in

ambient air can influence the contribution of the

fine and coarse fractions to PM10 mass, such that

reductions in PM2.5 concentrations can lead to more

allowable PM10–2.5 under the current primary PM10

standard, he recognizes that there is no new information available in this reconsideration to suggest

that the public health protection provided by the

current standard is not requisite or that a more

458a

stringent standard is warranted at this time. The

Administrator concludes that, particularly in light of

his decision to retain the primary 24-hour PM2.5

standard with its level of 35 μg/m3 as described in

section II.B.4 above, the primary PM10 standard

would be expected to maintain PM10–2.5 concentrations

in ambient air below those that have been considered

to be associated with serious health effects in past

NAAQS reviews. The Administrator also notes that

while the scientific evidence available in the 2019

ISA has expanded since the completion of the 2009

ISA, he concludes that this information does not

provide support for the causal or likely to be causal

relationships upon which he places the greatest

weight in considering the adequacy of the current

standards. He further concludes that the uncertainties and limitations of the scientific evidence, along

with the absence of information to inform a quantitative exposure or risk assessment, make it difficult to

reach decisions regarding whether a more protective

standard is warranted at this time. He has

additionally considered the public comments regarding

revisions to these elements of the standard and

continues to judge that the existing level and the

existing form, in all its aspects, together with the

other elements of the existing standard provide an

appropriate level of public health protection. For all

of the reasons discussed above and recognizing the

CASAC’s conclusion that the current evidence

provides support for retaining the current standard,

the Administrator concludes that the current primary

PM10 standard (in all of its elements) is requisite to

protect public health with an adequate margin of

safety from effects of PM10–2.5 in ambient air and

should be retained without revision.

459a

C. Decision on the Primary PM10 Standard

For the reasons discussed above and considering

information and assessments presented in the 2019

ISA and the 2022 PA, the advice from the CASAC,

and public comments, the Administrator concludes

that the current primary PM10 standard is requisite

to protect public health with an adequate margin of

safety, including the health of at-risk populations,

and is retaining the current standard without

revision.

IV. Communication of Public Health

A. Air Quality Index Overview

Information about the public health implications of

ambient concentrations of criteria pollutants is

communicated to the public using the Air Quality

Index (AQI) reported on the EPA’s AirNow website.133

The current AQI has been in use since its inception in

1999. 134 It provides useful, timely, and easily

understandable information about the daily degree of

pollution. The goal of the AQI is to establish a

nationally uniform system of indexing pollution

concentrations for ozone, carbon monoxide, nitrogen

dioxide, PM, and sulfur dioxide. The AQI is recognized internationally as a proven tool to effectively

communicate air quality information to the public as

demonstrated by the fact that many countries have

created similar indices based on the AQI.

133

134

See http://www.airnow.gov/.

In 1976, the EPA established a nationally uniform air

quality index, then called the Pollutant Standard Index (PSI),

for use by State and local agencies on a voluntary basis (41 FR

37660, September 7, 1976; 52 FR 24634, July 1,1987). In August

1999, the EPA adopted revisions to this air quality index (64 FR

42530, August 4, 1999) and renamed the index the AQI.

460a

The AQI converts an individual pollutant concentration in a community’s air to a number on a scale

from 0 to 500. Reported AQI values for specific

pollutants enable the public to know whether air

pollution levels in a particular location are characterized as good (0–50), moderate (51–100), unhealthy for

sensitive groups (101–150), unhealthy (151–200),

very unhealthy (201–300), or hazardous (301+).

Across criteria pollutants, the AQI value of 100

typically corresponds to the level of the short-term

(e.g., 24-hour, 8-hour, or 1-hour standard) NAAQS for

each pollutant. Below an index value of 100, an

intermediate value of 50 is defined either as the level

of the annual standard if an annual standard has

been established (e.g., PM2.5, nitrogen dioxide), a

concentration equal to one-half the value of the 24hour standard used to define an index value of 100

(e.g., carbon monoxide), or a concentration based

directly on health effects evidence (e.g., ozone). An

AQI value greater than 100 means that a pollutant is

in one of the unhealthy categories (i.e., unhealthy for

sensitive groups, unhealthy, very unhealthy, or

hazardous). An AQI value at or below 100 means that

a pollutant concentration is in one of the satisfactory

categories (i.e., moderate or good). The scientific

evidence on pollutant-related health effects for each

NAAQS review support decisions related to pollutant

concentrations at which to set the various AQI

breakpoints, which delineate the AQI categories for

each individual pollutant (i.e., the pollutant concentrations corresponding to index values of 150, 200,

300, and 500). The AQI is reported three ways by the

EPA and State, local and Tribal agencies, all of which

are useful and complementary. The daily AQI is

reported for the previous day and used to observe

trends in community air quality, the AQI forecast

461a

helps people plan their outdoor activities for the next

day, and the near-real-time AQI, or NowCast AQI,

tells people whether it is a good time for outdoor

activity.

Historically, State and local agencies have primarily used the AQI to provide general information to the

public about air quality and its relationship to public

health. For more than two decades, many State and

local agencies, as well as the EPA and other Federal

agencies, have been developing new and innovative

programs and initiatives to provide more information

related to air quality and health messaging to the

public in a more timely way. These initiatives,

including air quality forecasting, near real-time data

reporting through the AirNow website, use of data

from air quality sensors on the EPA and U.S. Forest

Service’s (USFS) Fire and Smoke Map, and air

quality action day programs, provide useful, up-todate, and timely information to the public about air

pollution and its health effects. Such information can

help the public learn when their well-being may be

compromised, so they can take actions to avoid or to

reduce exposures to ambient air pollution at

concentrations of concern. This information can also

encourage the public to take actions that will reduce

air pollution on days when concentrations are

projected to be of concern to local communities (e.g.,

air quality action day programs can encourage

individuals to drive less or carpool).

B. Air Quality Index Category Breakpoints for

PM2.5

Recognizing the scientific information available

and current AQI reporting practices, the EPA proposed

several revisions to the AQI PM2.5 breakpoints. EPA

solicited and received comments on these proposed

462a

revisions. Upon reviewing the information in the

proposal and considering the comments received EPA

is making final revisions to the AQI category

breakpoints for PM2.5. This section summarizes the

proposed revisions, which can be read in full in the

proposal (88 FR 5638, January 27, 2023), significant

comments, and final revisions.

1. Summary of Proposed Revisions

One purpose of the AQI is to communicate to the

public when air quality is poor and thus when they

should consider taking actions to reduce their

exposures. The higher the AQI value, the higher the

level of air pollution and the greater the health

concern. In recognition of the scientific information

available that is informing the reconsideration of the

2020 final decision on the primary PM2.5 standards,

including a number of new controlled human

exposure and epidemiologic studies published since

the completion of the 2009 ISA, as well as additional

epidemiologic studies from other peer reviewed

documents that evaluate the health effects of wildfire

smoke exposure and that can inform the selection of

AQI breakpoints at higher PM2.5 concentrations, 135

135

In evaluating the scientific evidence available to inform

decisions regarding the AQI breakpoints, the EPA considered

studies that were included as a part of the 2019 ISA and ISA

Supplement, but also considered other studies that were not

included as a part of the review of the air quality criteria. The

ISAs have specific criteria for study inclusion and consideration

in reaching conclusions regarding causal relationships, and

some studies that may not have met those criteria (e.g.,

epidemiologic studies that evaluate the health effects of wildfire

smoke exposure that would have higher PM2.5 concentrations,

which are outside of the scope of the ISA) were identified as

studies that could be used to inform decisions on the AQI,

particularly for the upper breakpoints.

463a

the EPA proposed to make two sets of changes to the

PM2.5 sub-index of the AQI. First, the EPA proposed

to continue to use the 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) to be

based on the levels of the primary PM2.5 annual and

24-hour standards and proposed to revise the lower

breakpoints to be consistent with changes to the

primary PM2.5 standards that are part of this

reconsideration. Second, the EPA proposed to revise

the upper AQI breakpoints (200 and above) and to

replace the linear-relationship approach used in 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 have become available in the last 20 years (64

FR 42530, August 4, 1999).

a. Air Quality Index Values of 50, 100

and 150

With respect to the lower AQI breakpoints in the

proposal (88 FR 5638, January 27, 2023), the EPA

proposed to conclude that it is appropriate to

continue setting these breakpoints to be consistent

with the primary annual and 24-hour PM2.5 standard

levels. The lowest AQI value of 50 provides the

breakpoint between the “good” and “moderate” categories. At and below this concentration, air quality is

considered “good” for everyone. Above this concentration, in the “moderate” category, the AQI contains

advisories for unusually sensitive individuals. The

EPA has historically set this breakpoint at the level

of the primary annual PM2.5 standard. In doing so,

the EPA has recognized that: (1) The annual

standard is set to provide protection to the public,

including at-risk populations, from PM2.5 concentra-

464a

tions, which, when experienced on average for a year,

have the potential to result in adverse health effects;

and (2) the AQI exposure period represents a shorter

exposure period (e.g., 24-hour (or less)) while focusing

on the most sensitive individuals. The EPA saw no

basis for deviating from this approach in this

reconsideration. Thus, the EPA proposed to set the

AQI value of 50 at a daily (i.e., 24-hour) average

concentration equal to the level of the primary

annual PM2.5 standard that is promulgated.

The historical approach to setting an AQI value of

100, which is the breakpoint between the “moderate”

and “unhealthy for sensitive groups” categories, and

above which advisories are generated for sensitive

groups, is to set it at the same level as the primary

24-hour PM2.5 standard. In so doing, the EPA has

recognized that the primary 24-hour PM2.5 standard

is set to provide protection to the public, including atrisk populations, from short-term exposures to PM2.5

concentrations that have the potential to result

in adverse health effects. Given this, it is appropriate

to generate advisories for sensitive groups at

concentrations above this level. In the past, State,

local, and Tribal air quality agencies have expressed

strong support for this approach (78 FR 3086,

January 15, 2013). The EPA saw no basis to deviate

from this approach in this reconsideration. In the

proposal (88 FR 5638, January 27, 2023), the EPA

proposed to retain the current primary 24-hour PM2.5

standard with its level of 35 μg/m3 but took comment

on revising the level of that standard to 25 μg/m3

(section II.D.3.b). Thus, the EPA proposed to retain

the AQI value of 100 set at the level of the current

primary 24-hour PM2.5 standard concentration of 35

μg/m3 (i.e., 24-hour average).

465a

With respect to an AQI value of 150, which is the

breakpoint between the “unhealthy for sensitive

groups” and “unhealthy categories,” this breakpoint

concentration in this reconsideration is based upon

the considering the same health effects information,

as assessed in the 2019 ISA and ISA Supplement and

described in section II above, that informs the

proposed decisions on the level of the 24-hour

standard and the AQI value of 100. Previously, the

Agency has used a proportional adjustment in which

the AQI value of 150 was set proportionally to the

AQI value of 100. This proportional adjustment

inherently recognizes that the available epidemiologic studies provide no evidence of discernible

thresholds, below which effects do not occur in either

sensitive groups or in the general population, that

could inform conclusions regarding concentrations at

which to set this breakpoint. Given that the epidemiologic evidence continues to be the most relevant

health effects evidence for informing this range of

AQI values, the EPA saw no basis to deviate from

this approach in this reconsideration. Therefore, the

EPA proposed to set an AQI value of 150 proportionally, depending on the breakpoint concentration

of the AQI value of 100 (i.e., 55.4 for a 24-hour

standard of 35 μg/m3).

b. Air Quality Index Values of 200 and

Above

In the proposal (88 FR 5639, January 27, 2023), the

EPA summarized the history of setting the AQI

values of 300 and above in the 1999 rule (64 FR

42530, August 4, 1999) and established breakpoints

for PM2.5 in that range. In general, the AQI values

between 100 and 500 were based on PM2.5 concentrations that generally reflected a linear relationship

466a

between increasing index values and increasing PM2.5

concentrations. 136 It was found that this linear

relationship was generally consistent with the health

effects evidence, which suggested that as PM2.5

concentrations increase, increasingly larger numbers

of people are likely to experience serious health

effects in this range of PM2.5 concentrations (64 FR

42536, August 4, 1999). For the AQI breakpoint of

500, the concentration was based on the method used

to establish a previously existing PM10 breakpoint

that was informed by studies conducted in London

using the British Smoke method, which uses a

different particle size cutpoint as noted in the

proposal (88 FR 5639, January 27, 2023). Due to

limited ambient PM2.5 monitoring data available at

that time, the decision on the 500 value concentration

for PM2.5 was based on the stated assumption that

PM concentrations measured by the British Smoke

method were approximately equivalent to PM2.5

concentrations (64 FR 42530, August 4, 1999). Given

that the British Smoke method has a larger particle

size cutpoint than the current PM2.5 monitoring

method, which has a cutpoint of 2.5 microns, a

concentration of 500 μg/m3 based on the British

Smoke method would be equivalent to a lower PM2.5

concentration. With respect to the upper breakpoints

of the AQI, the EPA has historically been concerned

about establishing these upper breakpoints using

evidence based on larger size fractions of PM, given

that PM2.5 is the indicator for the AQI. While

monitoring data for higher PM2.5 concentrations in

136

The AQI breakpoint at 150 was originally set in 1999 to be

linearly related to the concentrations at the 100 and 500

breakpoints but then revised in 2012 to be proportional to the

AQI breakpoint concentration at 100 (78 FR 3181, January 15,

2013).

467a

ambient air has been available for many years, the

health effects evidence has only recently become

available for consideration in informing decisions on

the upper breakpoints of the AQI.

As part of this reconsideration, the EPA recognized

that the health effects evidence associated with PM2.5

exposure has greatly expanded in recent years.

Multiple controlled human exposure studies have

become available that provide information about

health effects across a range of concentrations. While

many of the new studies evaluated in the 2019 ISA

focused on examining health effects associated with

exposure to lower PM2.5 concentrations, there are also

several new controlled human exposure studies that

provide information about the health effects observed

in study participants at concentrations well above the

standard levels. Additionally, there are also epidemiologic studies now available and evaluated in

other Agency peer-reviewed documents that can

inform health effects associated with higher PM2.5

concentrations (U.S. EPA, 2021b).137 Thus, the EPA

concluded that it is appropriate to reevaluate the

upper AQI breakpoints, taking into account the

expanded body of scientific evidence, particularly

given several new epidemiologic studies conducted

during high pollution events like wildfires and

multiple controlled human exposure studies. While it

remains unclear the exact PM2.5 concentrations at

which specific health effects occur, the more recent

137

In this reconsideration, the controlled human exposure

studies were evaluated in the 2019 ISA, whereas the

epidemiologic studies of wildfire smoke exposures were included

in the EPA Comparative Assessment of the Impacts of

Prescribed Fire Versus Wildfire (CAIF): A Case Study in the

Western U.S. (U.S. EPA 2021b).

468a

studies do provide more refined information about

the concentration range in which these effects might

occur in some populations. These studies provide

support for coherence of effects across scientific

disciplines and potentially biologically plausible

pathways for the overt population-level health effects

observed in epidemiologic studies. Therefore, taking

into account the short exposure time period in these

studies (e.g., 1–6 hours) and that the studies generally

do not include at-risk (or sensitive) populations, but

rather young, healthy adults, these studies, in conjunction with information from epidemiologic studies,

the EPA preliminarily concluded it would be appropriate to be more cautionary and offer advisories to

the public for reducing exposures at lower concentrations than recommended with the current AQI

breakpoints. The AQI value of 200 is the breakpoint

between the “unhealthy” and “very unhealthy”

categories. At AQI values above 200, the AQI would

be providing a health warning that the risk of anyone

experiencing a health effect following short-term

exposures to these PM2.5 concentrations has increased.

To inform proposed decisions on this breakpoint, the

EPA takes note of studies indicating the potential for

respiratory or cardiovascular effects that are on their

own representative of or are on the biologically

plausible pathway to more serious health outcomes

(e.g., emergency department visits, hospital admissions). The controlled human exposure studies

evaluated in the 2009 and 2019 ISAs provide

evidence of inflammation as well as cardiovascular

effects in healthy subjects at and above 120 μg/m3.

For example, Ramanathan et al. (2016) observed a

transient reduction in antioxidant/anti-inflammatory

function after exposing healthy young subjects to a

mean concentration of 150 μg/m3 of PM2.5 for 2 hours.

469a

Urch et al. (2010) also reported increased markers of

inflammation when exposing both asthmatic and

non-asthmatic subjects to a mean concentration of

140 μg/m3 of PM2.5 for 3 hours. In studies specifically

examining cardiovascular effects, Ghio et al. (2000)

and Ghio et al. (2003) exposed healthy subjects to a

mean concentration of 120 μg/m3 for 2 hours and

reported significantly increased levels of fibrinogen, a

marker of coagulation that increases during inflammation. Sivagangabalan et al. (2011) exposed healthy

subjects to a mean concentration of 150 μg/m3 of

PM2.5 for 2 hours and noted an increased QT interval

(3.4 ± 1.4) indicating some evidence for conduction

abnormalities, an indicator of possible arrhythmias.

Lastly, Brook et al. (2009) reported a transient

increase of 2.9 mm Hg in diastolic blood pressure in

healthy subjects during the 2-hour exposure to a

mean concentration of 148 μg/m3 of PM2.5.

In addition to epidemiologic studies evaluated in

the 2019 ISA that analyzed exposures at ambient

PM2.5 concentrations, there are a number of recent

epidemiologic studies focusing on wildfire smoke that

have become available that were evaluated in the

EPA’s recently released peer-reviewed assessment on

wildland fire (U.S. EPA, 2021b). One of these studies,

Hutchinson et al. (2018), conducted a bidirectional

case-crossover analysis to examine associations between

wildfire specific PM2.5 exposure and respiratoryrelated healthcare encounters (i.e., ED visits,

inpatient hospital admissions, and outpatient visits)

prior and during the 2007 San Diego wildfires. This

study found positive and significant associations to

PM2.5 exposures and respiratory-related healthcare

encounters. Further, during the initial 5-day period

of the wildfire event, the study observed that there

was evidence of increases in a number of respiratory-

470a

related outcomes particularly ED visits for asthma,

upper respiratory infection, respiratory symptoms,

acute bronchitis, and all respiratory-related visits

(Hutchinson et al., 2018). When examining the air

quality

during

the

wildfire

event,

PM2.5

concentrations were highest during the initial five

days of the wildfire, with 24-hour average PM2.5

concentrations of 89.1 μg/m3 across all zip codes and

with the highest 24-hour average of 160 μg/m3 on the

first day (Hutchinson et al., 2018).

When considering this collective body of evidence

from controlled human exposure and epidemiologic

studies, the Agency proposed to set an AQI value of

200 at a daily (i.e., 24-hour average) concentration of

PM2.5 of 125 μg/m3. As discussed above and in the

proposal (88 FR 5640, January 27, 2023), this

concentration is at the lower end of the concentrations consistently shown to be associated with

respiratory and cardiovascular effects in controlled

human exposure studies following short-term exposures

(e.g., 2–3 hours) and in young, healthy adults (Ghio et

al., 2000; Ghio et al., 2003; Urch et al., 2010;

Ramanathan et al., 2016; Sivagangabalan et al.,

2011; and Brook et al., 2009) and also within the

range of 5-day average and maximum concentrations

observed to be associated with respiratory-related

outcomes following exposure to wildfire smoke

(Hutchinson et al., 2018).

The AQI value of 300 denotes the breakpoint

between the “very unhealthy” and “hazardous”

categories, and thus marks the beginning of the

“hazardous” AQI category. At AQI values above 300,

the AQI provides a health warning that everyone is

likely to experience effects following short-term

exposures to these PM2.5 concentrations. To inform

471a

decisions on this AQI breakpoint, the EPA takes note

of controlled human exposure studies that consistently show subclinical effects which are often

associated with more severe cardiovascular outcomes.

As discussed above, Brook et al. (2009) reported a

transient increase of 2.9 mm Hg in diastolic blood

pressure in healthy subjects during the 2-hour

exposure to a mean concentration of 148 μg/m3 of

PM2.5. Bellavia et al. (2013) exposed healthy subjects

to an average PM2.5 concentration of 242 μg/m3 for

2 hours and reported increased systolic blood

pressure (2.53 mm Hg). Tong et al. (2015) exposed

healthy subjects to an average PM2.5 concentration of

253 μg/m3 for 2 hours and observed a significant

increase in diastolic blood pressure (2.1 mm Hg) and

a nonsignificant increase in systolic blood pressure

(2.5 mm Hg). Lucking et al. (2011) reported impaired

vascular function and increased potential for

coagulation when exposing healthy subjects to diesel

exhaust (DE) with an average PM2.5 concentration of

320 μg/m3 for a duration of 1 hour.138 These studies

all provided evidence of impaired vascular function,

including vasodilatation impairment and increased

thrombus formation, with Tong et al. (2015), Bellavia

et al. (2013), Brook et al. (2009) all reporting increases

in blood pressure. Additionally, Behbod et al. (2013)

reported increased inflammatory markers following a

2-hour exposure to an average PM2.5 concentration of

250 μg/m3 in healthy subjects.

138

Although participants in Lucking et al. (2011) were

exposed to diesel exhaust (DE), the authors also conducted

analyses using a particle trap, and as noted in the 2019 ISA,

this type of study design allows for the assessment of the role of

PM2.5 on the health effects observed by removing PM from the

DE mixture.

472a

In addition to the controlled human exposure

studies discussed above, the epidemiologic study

conducted by DeFlorio-Barker et al. (2019) examined

the relationship between wildfire smoke and

cardiopulmonary hospitalizations among adults 65

years of age and older from 2008–2010 in 692 U.S.

counties. The authors reported a 2.22% increase in

all-cause respiratory hospitalizations on wildfire

smoke days for a 10 μg/m3 increase in 24-hour

average PM2.5 concentrations (DeFlorio-Barker et al.,

2019). The maximum 24-hour average concentration

in this study on wildfire smoke days was 212.5 μg/m3

(DeFlorio-Barker et al., 2019). In considering this

study, the EPA notes the increased probability that

even healthy adults experience effects at this

maximum exposure concentration, particularly given

that this maximum concentration is near the exposure

concentrations in controlled human exposure studies

that consistently reported evidence of impaired

vascular function and several that reported increases

in blood pressure in healthy adults following 2-hour

exposures.

Based on the information discussed above and in

the proposal (88 FR 5640, January 27, 2023), the

EPA proposed to revise the 300 level of the AQI,

which marks the beginning of the “hazardous” AQI

category, to a concentration that is consistent with

the PM2.5 concentrations associated with health

effects as reported in the controlled human exposure

(Brook et al., 2009; Bellavia et al., 2013; Tong et al.,

2015; Behbod et al., 2013) and epidemiologic studies

(DeFlorio-Barker et al. (2019). Specifically, the

Agency proposed to set an AQI value of 300 at a daily

(i.e., 24-hour average) PM2.5 concentration of 225

μg/m3. This concentration falls between the 2-hour

average concentrations reported in controlled human

473a

exposure studies found to be consistently associated,

in healthy adults, with impaired vascular function

and/or increases in blood pressure, which could both

be a precursor to more severe cardiovascular effects

following short-term (1- to 2-hour) exposures, and the

maximum 24-hour average PM2.5 concentrations on

wildfire smoke days reported in the epidemiologic

study conducted by DeFlorio-Barker et al. (2019).

c. Air Quality Index Value of 500

Lastly, the EPA also proposed revisions to the 500

value of the AQI. The 500 value of the AQI is within

the “hazardous” category but is specified and used to

calculate the slope of the AQI values in the

“hazardous category” above and below AQI values of

500. In the past, this breakpoint had a very

prominent role in determining the current upper AQI

values given that it was used as part of the linear

relationship with the concentration at the AQI value

of 100 to determine the AQI values of 200 and 300 in

1999 (64 FR 42530, August 4, 1999).

As discussed above and in the proposal (88 FR

5641, January 27, 2023), the current breakpoint

concentration for the 500 value of the AQI was set in

1999 at a 24-hour average PM2.5 concentration of 500

μg/m3 and was based on studies conducted in London

using the British Smoke method, which used a

different particle size cutpoint and likely overestimated the PM2.5 concentration. In looking to improve

upon that approach, the EPA considered several

recent controlled human exposure studies that

observe health effects that are on the biologically

plausible pathway to more severe cardiovascular

outcomes and note that these seem to follow

exposures to high PM2.5 concentrations that are well

above those typically observed in ambient air. More

474a

specifically, in controlled human exposure studies,

Vieira et al. (2016a) and Vieira et al. (2016b) exposed

healthy subjects and subjects with heart failure to

diesel exhaust (DE) with a mean PM2.5 concentration

of 325 μg/m3 for 21 minutes and reported decreased

stroke volume, and increased arterial stiffness (an

indicator of endothelial dysfunction) in both healthy

and heart failure subjects. 139 Also as summarized

above and discussed in the proposal (88 FR 5641,

January 27, 2023), Lucking et al. (2011) exposed

healthy subjects to DE with a mean PM2.5

concentration of 320 μg/m3 for 1 hour.140 Epidemiologic

studies have linked the types of cardiovascular effects

observed in these controlled human exposure studies

with the exacerbation of ischemic heart disease (IHD)

and heart failure as well as myocardial infarction

(MI) and stroke.

In addition to the controlled human exposure

studies discussed in the proposal (88 FR 5641,

January 27, 2023) and summarized above, recent

epidemiologic studies examining the relationship

between concentrations of PM2.5 during wildfires and

respiratory health also informed the proposed

decisions on the concentration for the AQI value of

500. As discussed in the proposal (88 FR 5641,

January 27, 2023) and summarized earlier in this

section, Hutchinson et al. (2018) reported increases

139

These effects were attenuated when the DE was filtered, to

reduce PM2.5 concentrations, indicating the effects were likely

associated with PM2.5 exposure.

140

When applying a particle trap, PM2.5 concentrations were

reduced, and effects associated with cardiovascular function

including impaired vascular function, as measured by vasodilatation and thrombus formation were attenuated indicating

associations with PM2.5.

475a

in a number of respiratory-related ED visits for

asthma, upper respiratory infection, respiratory

symptoms, acute bronchitis, and all combined

respiratory-related visits based on data from MediCal claims for emergency department presentations,

inpatient hospitalizations, and outpatient visits

during the initial 5-day period of the 2007 San Diego

fire. During the initial 5-day window, PM2.5

concentrations were found to be at their highest with

the 95th percentile of 24-hour average concentrations

of 333 μg/m3.

Although studies of short-term (i.e., daily)

exposures to wildfire smoke are more informative in

considering alternative level for the AQI value of 500

since they mirror the 24-hour exposure timeframe,

additional information from epidemiologic studies of

longer-term exposures (i.e., over many weeks) during

wildfire events can provide supporting information.

As discussed in the proposal (88 FR 5641, January

27, 2023) and summarized here, Orr et al. (2020)

conducted a longitudinal study that reported exposure

to wildfire smoke from a multi-month fire resulted in

reduced lung function in subsequent years and

concluded that exposure to high PM2.5 concentrations

during a multi-week fire event may lead to health

consequences, such as declines in lung function.

During the 2017 wildfire event (August 1 to

September 19, 2017), Orr et al. (2020) reported that

many days during the multi-month fire had PM2.5

concentrations above 300 μg/m3, resulting in a daily

average PM2.5 concentration of 220.9 μg/m3 with a

maximum PM2.5 concentration of 638 μg/m3.

The controlled human exposure studies provide

biological plausibility for results of epidemiologic

studies that document increases in respiratory-

476a

related health care events during the wildfires. The

collective evidence from controlled human exposure

and epidemiologic studies, which includes decreases

in stroke volume, increased arterial stiffness, impaired

vascular function and respiratory-related healthcare

encounters provide health-based evidence that

informed the proposed decisions on the level of the

AQI value of 500. Given the concentrations observed

in these studies, the Agency proposed to revise the

AQI value of 500 to a level set at a daily (i.e., 24-hour

average) PM2.5 concentration of 325 μg/m3. This

concentration is at or below the lowest concentrations

observed in the controlled human exposure studies

associated with more severe effects discussed above

and also at the low end of the daily concentrations

observed in the epidemiologic studies conducted by

Hutchinson et al. (2018) and Orr et al. (2020).

Table 1 below summarizes

breakpoints for the PM2.5 sub-index.

the

proposed

477a

478a

479a

2. Summary of Significant Comments on

Proposed Revisions

The EPA received many comments on the proposed

changes to the PM2.5 AQI breakpoints. Many commenters generally supported all the proposed

revisions to the AQI breakpoints based on the

revisions to the primary annual and daily PM2.5

standards and recent scientific evidence discussed in

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

However, we received specific comments on proposed

revisions to the breakpoints in the lower end of the

AQI, related to their linkage to the annual and daily

PM2.5 standards, and proposed revisions to the

breakpoints at the upper end of the AQI, based on

EPA’s interpretation of available health effects

evidence.

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

150

Some commenters agreed with using the historical

approach of setting the 50, 100 and 150 breakpoints

of the AQI to be consistent with the primary PM2.5

standards. Some cited the reason that this approach

creates consistent communication with respect to air

quality and the standards, and this is how the other

AQI sub-indices are set. A few commenters disagreed

with the historical approach and suggested instead

that the 50 breakpoint of the AQI should not be

revised at all, or that the 50 and 100 breakpoints of

the AQI should be supported directly by health data

similar to the basis for the proposed 200, 300 and 500

breakpoints.

The few commenters that disagreed with the

historical approach of the 50 breakpoint of the AQI

noted that setting a short-term breakpoint to annual

480a

standard was not logical since it is a long-term

standard and not meant to be interpreted for shortterm messaging with the AQI, in particular when

reported hourly via the NowCast. These commenters

also noted that additional studies are needed to

identify the health impacts of short-term exposures

at low concentrations. They also noted that lowering

the 50 breakpoint of the AQI in conjunction with the

annual standard may cause confusion with the public

because some State programs and policy decisions

are connected to the AQI while others are based on

PM concentrations, which could lead to inconsistent

messaging reducing the public’s trust. These

comments were supported by noting that revised

breakpoints could lead to more moderate days than

in the past, but the monitor values would be the same

as before when the commenters considered it

“healthy,” possibly eroding trust in air agencies’

messaging. Commenters also noted if the breakpoints

are revised, the public will not visually be able to

detect the difference between what was considered a

good AQI day versus a now moderate AQI day.

The EPA disagrees with these commenters. With

respect to setting a short-term breakpoint to the level

of a much longer-term (annual) standard, setting the

lower AQI breakpoints at the level of the annual and

daily PM2.5 standards for communication purposes

was discussed in the proposed reconsideration (88 FR

5558, January 27, 2023) and previously supported by

State organizations in the 2012 PM Final Rule (77

FR 38890, June 29, 2012). Both the AQI and the

Pollutant Standards Index, which came before it,

have historically been normalized across pollutants

by defining an index value of 50 and 100 as the

numerical level of the annual (when defined) and

short-term (i.e., averaging time of 24-hours or less)

481a

primary NAAQS for each pollutant. This approach

clearly communicates the air quality to the public.

The EPA considers this approach to be appropriate

given the available evidence and structure of the

standard. As discussed in section II.B above and in

the notice of final rulemaking for the 2012 review (77

FR 38890, June 29, 2012), the primary annual and

24-hour PM2.5 standards work together in concert to

provide public health protection. The annual PM2.5

standard is generally viewed as the principal means

of providing public health protection against “typical”

daily and annual PM2.5 exposures, while the 24-hour

PM2.5 standard is generally viewed as a means of

providing protection against short-term exposures to

“peak” PM2.5 concentrations, such as can occur in

areas with strong contributions from local or seasonal

sources, even when annual average PM2.5 concentrations remain relatively low. Because the annual

standard provides public health protection for typical

daily PM2.5 exposures, the EPA thinks it is

appropriate to use that level for the 50 breakpoint of

the AQI and describe daily air quality at and below

the level of the annual standard “Good.” Since an

annual standard allows for days with air quality

above that level, it is appropriate to call days just

above it “Moderate.” If the 50 breakpoint of the AQI

was set at a level above the annual standard, it

would be possible for the majority of days to be called

“good” in a year when an area exceeds the annual

standard. This could cause confusion with the public

about air quality if the general perception is that

local air quality is “good,” but the area fails to meet

the annual standard. In addition, the EPA continues

to find it appropriate to use the NowCast with

the PM2.5 AQI index to provide more real-time

information to the public. As discussed in the AQI

482a

Technical Assistance Document, while the NowCast

algorithm is approximating a 24-hour average

exposure, it can reflect concentrations observed over

shorter averaging times when air quality is changing

rapidly (U.S. EPA, 2018a). The EPA continues to

consider the use of the primary annual standard level

suitable in the NowCast given the health evidence

supporting the standard and given that the reported

concentrations are an approximation of “typical”

daily exposure. Additionally, the EPA reflects the

nature of the NowCast in the associated health

messaging.

With regard to the commenter stating the public

may not be able to visually detect a difference in the

air quality, the EPA notes that the AQI is intended to

be a communication tool for public awareness precisely

because it is generally difficult for the public to

visually judge air quality risks when air pollution is

“moderate.” Moreover, since the establishment of the

AQI, the EPA and State and local air agencies and

organizations have developed experience in educating

the public about changes in the standards and,

concurrently, related changes to AQI breakpoints and

advisories. When the standards change, the EPA and

State and local agencies have sought to help the

public understand that air quality is not getting

worse, it’s that the health evidence underlying the

standards and the AQI has changed. The EPA’s Air

Quality System (AQS), the primary repository for air

quality monitoring data, is also adjusted to reflect the

revised breakpoints. Specifically, all historical AQI

values in AQS are recomputed with the revised

breakpoints, so that all data queries and reports

downstream of AQS will show appropriate trends in

AQI values over time. If any State, local or Tribal air

agency is concerned that people are or will be

483a

confused on a moderate AQI day, then they could use

the communication information that has been

developed with this rulemaking.

Some commenters stated that the AQI should not

necessarily be linked to the primary PM2.5 standards.

One example is the comment that if the annual

standard is not lowered to 8 μg/m3, the EPA should

lower the 50 breakpoint of the AQI to that level to

better inform the public of the need for behavioral

modifications to reduce the harm to health from PM2.5

exposure. Similar to the reasons discussed above, the

EPA concludes that setting the 50 breakpoint of the

AQI at the level of the annual PM2.5 standard is

appropriate from a health perspective and for

communication purposes. The Administrator has

judged the primary annual standard (in conjunction

with the other primary standards) as revised in this

final action to be requisite to protect public health

with an adequate margin of safety, based on the

health evidence discussed in section II.A.2. Setting

the 50 breakpoint lower than the annual standard

also has the potential to cause confusion with the

public since it does not reflect the standards and the

Administrator’s judgments about the standards as

well.

With regard to the 100 breakpoint of the AQI,

several commenters expressed the view that the level

of the 24-hour PM2.5 standard and an AQI value of

100 should be set at 25 μg/m3 based on the body of

evidence and lower end of the range recommended by

CASAC. These commenters noted that if the current

24- hour standard and AQI value of 100 is retained at

35 μg/m3 then the public will not be able to make

informed decisions about actions to take to protect

their health. Many of these commenters further

484a

recommended that the AQI value of 100 should be

lowered to 25 μg/m3 even if the standard is retained.

Commenters expressed the view that this would more

adequately allow the public to take health- protective

actions.

The EPA disagrees with these commenters and

notes that many State, Tribal and local air agencies

have expressed strong support for aligning the 100

breakpoint of the AQI with the short-term 24-hour

primary PM2.5 standards as discussed in the proposal

(88 FR 5558, January 27, 2023). The EPA agrees

with the view, expressed by State, local and Tribal

entities, that aligning the lower breakpoints with the

standards enables clear communication of the standards. This alignment approach is also utilized in the

other AQI sub-indices lower breakpoints and taking a

different approach with the PM2.5 AQI could cause

confusion. Additionally, the Administrator has judged

that it is appropriate to retain the 24-hour standard

at a level of 35 μg/m3 (in conjunction with the other

primary standards) to protect public health with an

adequate margin of safety, based on the health

evidence discussed in section II.A.2. Thus, EPA

disagrees that it is necessary or appropriate to set

the 100 breakpoint at a lower concentration to

provide further information to the public. The 50

breakpoint, which is set at a level below 25 μg/m3,

will continue to provide information to members of

the public particularly concerned about exposures to

PM2.5. As with the 50 breakpoint, aligning the

breakpoint with the standard both reflects the

Administrator’s judgment about the health risks and

eliminates the potential to cause confusion in the

public about those risks.

485a

b. Air Quality Index Values of 200 and

Above

Some commenters supported the proposed revisions

to the 200, 300 and 500 breakpoints that recognize

the expanded body of scientific evidence, particularly

several new epidemiologic studies conducted during

high pollution events such as wildfires and multiple

controlled human exposure studies. A few commenters

agreed with incorporating the expanded body of

scientific evidence into the 200, 300 and 500 breakpoints, but suggested a modified linear approach

between 200 (115 μg/m3) and 500 (312 μg/m3, setting

the 300 breakpoint to 187 μg/m3) based on recent

epidemiologic wildfire smoke studies.

Other commenters disagreed with the proposed

revisions and suggested the EPA should continue

using the previous breakpoints that follow the 1999

linear approach (64 FR 42530, August 4, 1999),

because not changing the breakpoints would simplify

communications. A few commenters stated the

proposed revisions to the AQI upper breakpoints are

not justified because the scientific evidence supporting

the revisions is inadequate. To support this view, the

commenters suggest that only three epidemiologic

studies were used in determining the upper

breakpoints and none of them were representative of

potential effects in the general public; of the 13

studies cited only three were near the proposed

revised breakpoints; four of the studies involved

exposure to PM from diesel and traffic pollution,

which is different than PM from wildfire smoke; and

the data supporting the revisions only indicated

“mild” health effects that were mostly in sensitive

populations.

486a

The EPA agrees with the majority of commenters

that supported utilizing the expanded body of

scientific evidence to revise the 200, 300 and 500

breakpoints of the AQI. The EPA appreciates the

suggestion of using a revised linear approach from

200 to 500. But rather than using the available

evidence to only set the breakpoint of 500, the EPA

finds it appropriate to set the breakpoints for 200,

300 and 500 using an evidence-based approach, by

relying on information presented in both controlled

human exposure studies and epidemiologic studies

that examine relationships between high PM2.5

exposure episodes (i.e., periods of wildfire smoke) and

various health outcomes. Setting these breakpoints

based directly on health effects evidence, which can

be communicated, is more useful and appropriate

than using a linear approach, because it can better

describe the potential health effects and symptoms

which also helps the public better understand why

more health protective actions are needed. By its

nature, a linear approach does not evaluate and

identify associated health effects and risk factors.

The EPA disagrees with the commenters that

expressed the view that these upper breakpoints

should not be revised based largely on the numerous

peer-reviewed studies published since the 200, 300

and 500 breakpoints were originally established in

1999 (64 FR 42530, August 4, 1999). As discussed in

the proposal (88 FR 5641, January 27, 2023), the

rationale behind the proposed revisions is rooted in

the fact the upper AQI breakpoints are based on

outdated scientific evidence. Specifically, the traditional

linear approach was predicated on the 500 value of

the AQI, which was estimated using health studies

that used the British Smoke Method. The British

Smoke Method is based on a particle size fraction (4.5

487a

microns) that is larger than PM2.5. Given that the

British Smoke method has a larger particle size

cutpoint than the current PM2.5 monitoring method,

which has a cutpoint of 2.5 microns, a concentration

of 500 μg/m3 based on the British Smoke method

would be equivalent to a lower PM2.5 concentration

(88 FR 5641, January 27, 2023). The combination of a

larger particle size fraction informing previous

decisions around upper AQI breakpoints and more

recent scientific evidence than the London Fog

Episode, on the potential health consequences of

what we currently consider to be high PM2.5 exposures,

provides the underlying basis for revising the upper

breakpoints to better inform the public about air

quality to allow the public to take health protective

actions as appropriate. Moreover, as discussed above,

until recently there was limited information upon

which to base the breakpoints between 150 and 500,

so the linear approach was a reasonable substitute.

While not changing the breakpoints may be easier

because there is no change to communicate, using a

health-based approach is more appropriate, because

it helps the public better understand that more

health protective actions are needed.

The Agency disagrees that the scientific evidence

discussed in the proposal is inadequate to revise the

200, 300 and 500 breakpoints of the AQI (88 FR 5640,

January 27, 2023). The EPA disagrees that these

studies should not be considered because they “indicated

mild health effects in sensitive populations.” The

EPA notes that many of the subclinical effects

discussed in the proposal (88 FR 5640, January 27,

2023) that informed the breakpoints are on the

biologically plausible pathway (see 2019 ISA, section

6.1.1 and Figure 6–1) to more severe cardiovascular

outcomes, such as ED visits, hospital admissions, and

488a

death as depicted in the large number of

epidemiologic studies evaluated in the 2019 ISA and

ISA Supplement. From a public health perspective,

the purpose of the AQI is to inform the public when

air quality could adversely affect their health. The

scientific evidence informed revisions to the

breakpoints at the upper end of the AQI allow it to

better reflect the risk of experiencing health effects at

higher PM2.5 concentrations. In addition, the EPA

disagrees with the commenter that the effects

reported at these higher concentrations were observed

only in sensitive populations as these effects were

also reported in healthy populations (Ghio et al.,

2000; Ghio et al., 2003; Urch et al., 2010;

Ramanathan et al., 2016; Sivagangabalan et al.,

2011; Brook et al., 2009; Bellavia et al. (2013); Tong

et al. (2015); Behbod et al. (2013); Vieira et al.

(2016a) Vieira et al. (2016b); and Lucking et al.

(2011)).

c. Other Comments

The EPA received a few additional comments on

elements of the PM2.5 AQI, including the averaging

time. Some commenters expressed the view that the

24-hour averaging time was not useful when informing

the public how to protect their health, particularly

during rapidly changing conditions such as wildfire

smoke events. Instead, they suggested a subdaily

averaging time of 1–3 hours would be more effective

because it more closely aligns with how people

breathe.

A few of these commenters suggested that instead

of changing the AQI averaging time, which aligns

with the short-term standard, the EPA could create a

public health warning system for unhealthy PM2.5

levels. The commenters noted that aligning the AQI

489a

averaging time with the short-term standard could be

useful for consistent communication with the standards

and attainment but suggested that a subdaily

warning system could better allow the public to take

health protective actions.

The EPA disagrees that a shorter averaging period

for the PM2.5 AQI sub-index would be better. The

health effects evidence supporting a subdaily metric

is limited and inconsistent. As part of its review of

the health effects evidence, the 2019 ISA evaluated

whether a subdaily metric would be more closely

related to health effects. Most epidemiologic studies

that examined the relationship between short-term

PM2.5 exposures and health effects evaluated an

exposure metric averaged over 24-hours. Some recent

studies, focusing on respiratory and cardiovascular

effects and mortality, have examined whether there

is evidence that subdaily exposure metrics are more

closely related to health effects than a traditional 24hour average metric. After evaluating this limited

newer evidence, the 2019 ISA concluded that

“collectively, the available evidence does not indicate

that subdaily averaging periods for PM2.5 are more

closely associated with health effects than the 24hour avg exposure metric,” (2019 ISA, chapter 1,

section 1.5.2.1, pp. 146–147; U.S. EPA, 2022a).

In addition, there are communication benefits to

aligning the averaging time of the AQI with the daily

standard, as some of these commenters note, such as

providing consistent messages about when it may be

beneficial for people to take actions to reduce PM2.5

exposures. Furthermore, with regard to an additional

warning system, the EPA is concerned that having

two air quality communication systems operating at

490a

the same time would likely be confusing to the public

and reduce the effectiveness of the systems.

At the same time, the EPA recognizes that when

air quality is rapidly changing, such as during

wildfire smoke events, reporting information based

on a 24-hour metric may not be as useful for the

public as reporting more frequently would be. The

EPA has balanced concerns about being able to

provide timely communication of air quality hazards

when conditions are changing quickly with the goal of

limiting the number of air quality communications

systems and its judgment that the evidence supports

a 24-hour-based metric linked to the daily standard

by establishing the NowCast, which takes into

consideration subdaily PM2.5 concentrations and

provides a near real-time AQI value based on the

AQI colors and scale. Specifically, the NowCast

shows air quality conditions for the most current

hour of PM2.5 data available by using a calculation

that involves multiple hours of past data. As noted in

the AQI Technical Assistance Document, the NowCast

currently uses longer averages during periods of

stable air quality and shorter averages (down to a

3-hour average) when air quality is changing rapidly,

such as during a wildfire (U.S. EPA, 2018a). As

discussed further in section IV.D.2 of this notice, the

EPA uses the NowCast to approximate the complete

daily AQI (24-hour average) during any given hour.

This means the subdaily NowCast is approximating a

24-hour average exposure, which aligns with the

health evidence and the existing AQI communications

network, while also being capable of communicating

rapidly changing conditions to the public.

491a

3. Summary of Final Revisions

Upon reviewing and considering the comments on

the proposed revisions (summarized above in Section

IV.C) along with the scientific evidence outlined in

the proposal (88 FR 5639, January 27, 2023) and

summarized above in section IV.A, the EPA is

finalizing the proposed changes to the AQI.

Thus, as discussed in section IV of the preamble

(88 FR 5639, January 27, 2023) to the proposed rule,

the EPA is taking final action to revise the AQI value

of 50 to 9.0 μg/m3, 24-hour average, consistent with

the final decision on the primary annual PM2.5

standard level as summarized in section II.C of the

preamble to the final rule; retain the AQI value of

100 at 35 μg/m3, 24-hour average, consistent with the

final decision on the primary 24-hour PM2.5 standard

level as summarized in section II.C of the preamble

to the final rule; and retain the AQI value of 150 at

55 μg/m3, 24-hour average. The EPA is also taking

action to revise the AQI value of 200 to 125 μg/m3,

24-hour average; 300 to 225 μg/m3, 24-hour average;

and 500 to 325 μg/m3, 24-hour average, consistent

with the rationale discussed above and the health

evidence discussed in section IV of the preamble

(88 FR 5639, January 27, 2023) to the proposed rule.

The EPA has prepared communications materials to

assist States with adjusting to the revised AQI and

looks forward to working with, and learning from the

experiences of, State, local, and Tribal governments

in implementing these changes.

C. Air Quality Index Category Breakpoints for

PM10

The EPA proposed to retain the PM10 sub-index of

the AQI consistent with the proposed decision to

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retain the primary PM10 standard, and consistent

with the health effects information that supports this

proposed decision, as discussed in section III.D of the

proposal (88 FR 5632, January 27, 2023). EPA did

not receive comments on this and is taking final

action to retain the PM10 sub-index of the AQI for the

reasons stated in the preamble to the proposed rule

(88 FR 5642, January 27, 2023).

D. Air Quality Index Reporting

With respect to the reporting requirements for the

AQI and as noted in the proposal (88 FR 5642,

January 27, 2023) there have been many

technological advances in air quality monitoring and

data reporting since the appendix G to 40 CFR part

58 was last revised in 1999. Federal, State, local, and

Tribal agencies have used these changes to make

health information and air quality data more readily

available and easier to access. Given this, it is useful

to update the reporting requirements and recommendations to match current practices and ensure the

public has the most useful and timely information to

take health-protective behaviors.

1. Summary of Proposed Revisions

Currently, appendix G defines daily reporting as

five days per week. When this reporting requirement

was originated in 1999 the technology available at

that time was not sufficient to calculate and report

the AQI more than five days per week without

requiring additional staffing on the weekends. Since

that time, advances in technology have allowed for

reporting seven days per week automatically without

expending additional resources on weekends. As a

result, most State, local, and Tribal air agencies now

report the AQI seven days per a week. Given these

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technological advances and noting that reporting

agencies currently report the AQI seven days per

week, the EPA proposed that State, local, and Tribal

agencies that report the AQI be required to report it

seven days a week, ensuring that the members of the

public continue to have access to daily air quality and

health information that they can use to take steps to

protect their health.

Improvements in monitoring networks and

modeling capabilities have also enabled the ability to

report the AQI in near real-time. This allows State,

local, and Tribal air agencies to provide timely air

quality information to the public for making healthprotective decisions and to help satisfy AQI reporting

requirements. The availability of near real-time AQI

data also allows for more timely responses by the

public when air quality conditions are changing

rapidly, such as during wildfire smoke events.

Subdaily reporting of the AQI can be critical when

there are rapidly change conditions and/or high

pollution events so that the public is able to make

informed decisions to protect their health. Many

State, local, and Tribal air agencies currently report

the AQI hourly to ensure that the public has access to

accurate and timely information. In recognition of

these advances, and to continue to provide for nearreal time AQI reporting that the public has come to

rely on, the EPA proposed to recommend that State,

local, and Tribal agencies report the AQI in near-real

time.

In lieu of or along with reporting the near-real-time

AQI directly to the public, most State/local and Tribal

agencies submit hourly air quality data to the EPA.

The EPA and some State, local and Tribal air quality

agencies use this near-real-time data to create

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products for use by the public, weather service

providers and the media as discussed in the proposal

(88 FR 5643, January 27, 2023). To continue to

ensure the availability of the products that the public

and many stakeholders rely upon, the EPA proposed

to recommend that State, local, and Tribal air quality

agencies submit hourly data to the EPA’s air quality

database. Submitting hourly data to the EPA for use

on the AirNow website and in other products also

enables State, local, and Tribal air quality agencies to

meet the recommendation to report the AQI in nearreal-time.

In addition to the proposed updates to the

reporting requirements and recommendations for

near-real-time reporting and data submission

recommendations, the Agency also proposed reformatting the question-and-answer format used in

appendix G to align with the current standard

formatting used in the Code of Federal Regulations.

In proposing to update the format, the EPA did not

reopen the language that has merely been moved or

rearranged as there are no substantive changes.

Another change the EPA proposed to make to

appendix G is with regard to Table 2—Breakpoints

for the AQI for purposes of clarity. As discussed in

the proposal (88 FR 5642, January 27, 2023) and

summarized here, the EPA proposed to collapse the

two rows presented for the Hazardous Category into

one. The two rows in the current table specify

pollutant concentrations for two AQI ranges within

the Hazardous category (301–400 and 401–500), with

an intermediate break at 400. The 400 breakpoint for

all criteria pollutants in the current Table 2 is set at

the proportional pollutant concentration approximately

halfway between the Index values of 300 and 500. In

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proposing updated AQI breakpoints for PM2.5, the

EPA considered adjusting the 400 breakpoint

similarly. However, the EPA concluded that

collapsing the two rows into a single range (301–500)

would provide a more transparent and easy-to-follow

presentation of the pollutant concentrations corresponding to the AQI range for the Hazardous

category. Moreover, collapsing the Hazardous

category into a single row in Table 2 has no

substantive effect on the Emergency Episode

program in 40 CFR part 51, appendix L. Thus, the

EPA proposed to remove the breakpoint of 400 from

the table in appendix G but this change would not

substantively affect the derivation of the AQI for any

pollutant.

In addition, the EPA proposed to move some

information currently in appendix G into the

Technical Assistance Document for the Reporting of

Daily Air Quality, or TAD (U.S. EPA, 2018a), so that

it can be updated in a more timely manner to reflect

current scientific and health effects evidence and

current communication methods, thereby assisting

State, local, and Tribal agencies in providing accurate

and timely information to the public. Information

that was proposed to be moved from appendix G to

the TAD included the definitions of the sensitive (atrisk) populations for each pollutant.

This definition is typically evaluated and updated,

as warranted, in most NAAQS reviews, even if the

standard is not revised. Generally, if the standard is

not revised in a review of the NAAQS, then appendix

G is also not revised. Moving the definitions of

sensitive groups to the TAD allows them to be

updated even when a NAAQS is not revised to be

consistent with the definitions of the sensitive (at-

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risk) populations identified in the ISA for that

NAAQS review. Also, the proposal (88 FR 5642,

January 27, 2023) recognized that the ways that air

quality and health information is supplied to the

news media and public changes regularly and thus

proposed that information about suggested approaches

for public communication be taken out of appendix G

and discussed in the TAD.

2. Summary of Significant Comments on

the Proposed Revisions

The EPA received many comments on the proposed

changes to AQI reporting, many of which supported

the proposed revisions. EPA discusses several of the

topics that received the most attention from

commenters below. Discussion of other comments

received on the proposed changes to the AQI can be

found in section IV of the Responses to Significant

Comments on the 2023 Proposed Reconsideration of

the National Ambient Air Quality Standards for

Particulate Matter.

Most commenters expressed support for revising

the definition of “daily reporting” from five days a

week to seven days a week. A commenter did not

support this change and recommended the EPA

maintain the definition of daily as five days per week,

noting that State and local air agencies do not

routinely work seven days per week and would not be

available to perform quality control of this data and

report it reliably on weekends.

The EPA appreciates the support for this proposed

revision and disagrees that the proposed change

would require personnel to perform quality control of

AQI data on weekends. 40 CFR part 58 Appendix D

defines continuous monitoring requirements for agencies

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participating in the State/Local Air Monitoring

Stations (SLAMS) network, and Appendix G states

that agencies “ . . . must use concentration data from

State/Local Air Monitoring Stations (SLAMS)

required by 40 CFR 58.10” when reporting the AQI.

Therefore, as noted in Appendix D and G, Agencies

are required to report the AQI using monitors within

SLAMS, which are not subject to daily quality

control/ validation.

A few commenters noted that the proposal

preamble language mentioned AQI is reported three

ways (88 FR 5637, 5638, January 27, 2023): “The AQI

is reported three ways all of which are useful and

complementary. The daily AQI is reported for the

previous day and used to observe trends in

community air quality, the AQI forecast helps people

plan their outdoor activities for the next day, and the

near-real-time AQI, or NowCast AQI, tells people

whether it is a good time for outdoor activity.” These

commenters suggested that the NowCast is being

codified in 40 CFR part 58 Appendix G as a method of

calculating the AQI, which they oppose, saying that

codifying its use is inappropriate given the shortest

averaging period of the PM2.5 NAAQS remains at 24hours. Some stated that NowCast values have no

direct correlation to the AQI calculation methodology

codified in 40 CFR part 58 Appendix G. These

commenters say that codifying the NowCast would

impose a significant burden on States’ forecasting

staff.

However, some other commenters noted they

appreciate the public-friendly format and near realtime data the NowCast provides and use it in their

clinical encounters with patients. One air agency

recognized the importance of the NowCast near real-

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time AQI during high pollution events and suggested

the EPA should provide more “concrete” health

messaging for these short-term spikes.

The EPA disagrees that the preamble language

proposed to codify the NowCast or to impose a burden

on reporting agencies. The preamble to the proposed

rule references the AQI being reported in three ways

and it does so because the EPA and many State, local

and Tribal air quality agencies already report it these

three ways. However, text included in the preamble

is generally explanatory and does not alter regulatory

provisions. Comments that State that EPA is

codifying the NowCast into Appendix G are incorrect.

Further, in proposed revisions to 40 CFR part 58

Appendix G, the EPA recommended, but did not

propose to require, the use of air quality forecasts

and a subdaily AQI. Consistent with the proposal, the

EPA is therefore not finalizing any additional

requirement or burden on States’ forecasting staff

relative to forecasts or a subdaily AQI.

The EPA disagrees with the comment that the

NowCast values have no direct correlation to the AQI

calculation methodology codified in 40 CFR part 58

Appendix G. As noted in the AQI Technical

Assistance Document (Technical Assistance Document

for the Reporting of Daily Air Quality—the Air

Quality Index (AQI)), the NowCast algorithm is

based on the AQI methodology but provides more

real-time information to the public (U.S. EPA,

2018a). While the NowCast algorithm is approximating a 24-hour average exposure, it can reflect

concentrations observed over shorter averaging times

when air quality is changing rapidly (U.S. EPA,

2018a). The EPA reflects the nature of the NowCast

in the health messaging provided there.

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As noted in the above discussion of the AQI, air

quality can change quickly during the day. A central

purpose of the AQI is to help the public know when it

is prudent to take action to reduce their exposure to

pollution. Accordingly, the EPA developed the

NowCast to estimate the 24-hour AQI for the current

hour to give people information and tools to reduce

their exposures to protect their health, particularly

when air quality may be changing. The NowCast

gives people the knowledge and ability to take timely

action. They can use this information to reduce their

exposure—reducing exposures if PM2.5 is high only

during a few hours a day will help reduce a person’s

24-hour exposure—or be active when air quality is

better.

The first NowCast method was developed in 2003

and was designed so “current conditions” represent

the 24-hour PM2.5 standard as closely as possible.

This method proved to be slow to respond during

rapid air quality changes. In 2013, the EPA

developed an updated NowCast method for PM2.5141

that responds more quickly to rapidly changing air

quality conditions, such as those we see during

wildfires, to make air quality alerts more timely. We

analyzed millions of data points in developing this

NowCast method and presented this information to

State, local and Tribal air agencies. The updated

NowCast, which is still in use, was launched August

1, 2013, on AirNow.gov. It was designed to represent

a shorter average (target 3-hour) when air quality is

141

U.S. EPA. (2013). Transitioning to a New NowCast

Method. Presentation available in the Rulemaking Docket for

the Review of the National Ambient Air Quality Standards for

Particulate Matter (EPA–HQ–OAR–2015–0072), at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 0072.

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changing rapidly, in part because 3-hour averages

from some continuous monitors are more stable than

1-hour averages. The NowCast reflects a longer-term

(12-hour) average when air quality is stable.

After evaluating the 2013 NowCast method, the

EPA concluded that it matched the desired characteristics. The NowCast method responds to rapid

changes in air quality yet still reflects a longer-term

average when air quality is stable; will work in any

location with adequate air quality data and for any

air quality situation; gives people the best possible

estimate of a 24-hour exposure; allows the EPA to

caution people in time for them to take protective

action and reduce their 24-hour exposure; and ensures

that AQI maps on AirNow more closely match what

people see.

The AQI is designed to allow people to reduce their

exposure when pollution levels are higher and be

active outdoors when pollution levels are lower. Since

air quality almost always changes during the day,

that level of granularity is not possible with a 24hour forecast. If the public has only the 24-hour

forecast, they may miss the times to be active

outdoors when air quality is better and may be active

outdoors when air quality is worse.

Also as noted above, many entities appreciate the

near real-time reporting of the AQI that the NowCast

provides and suggested more specific messaging is

needed. The EPA appreciates this insight and will

continue to consider ways to communicate air quality

information most effectively to the public. For

example, in light of recent wildfire events, the EPA

worked with the USFS to pilot the AirNow Fire and

Smoke Map.

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3. Summary of Final Revisions

Upon reviewing and considering the comments on

the proposed revisions (summarized above in Section

IV.C) along with the rationale outlined in the

proposal (88 FR 5638, January 27, 2023) and

summarized above in section IV.C, the EPA is

finalizing the proposed changes to the AQI reporting

requirements. Thus, as discussed in section IV of the

preamble to the proposed rule, the EPA is taking

final action to require the AQI be reported seven days

a week; recommend that State, local, and Tribal

agencies report the AQI in near-real time;

recommend that State, local, and Tribal air quality

agencies submit hourly data to the EPA’s air quality

database; reformat appendix G to align with the

current standard formatting used in the Code of

Federal Regulations; collapse the two rows in Table 2

presented for the Hazardous Category into one by

removing the 400 breakpoint; and move some

information currently in appendix G into the

Technical Assistance Document for the Reporting of

Daily Air Quality, or TAD (U.S. EPA, 2018a) such as

including the definitions of the sensitive (at-risk)

populations for each pollutant and suggested

approaches for public communication as stated in the

revised Appendix G.

Table 2 below summarizes the breakpoints for the

PM2.5 sub-index.

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503a

V. Rationale for Decisions on the Secondary PM

Standards

This section presents the rationale for the

Administrator’s decision that no change to the

current secondary PM standards is required at this

time to provide requisite protection against the public

welfare effects of PM within the scope of this

reconsideration (i.e., visibility, climate, and materials

effects). 142 This decision is based on a thorough

review of the scientific evidence generally published

through December 2017,143 as presented in the 2019

ISA (U.S. EPA, 2019a), on the non- ecological public

welfare effects of PM pertaining to the presence of

PM in ambient air, specifically visibility, climate, and

142

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

EPA, 2016), other welfare effects of PM, including 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 decision in this

reconsideration will be 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 non-ecological

welfare effects (i.e., visibility, climate, and materials effects).

143

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.

504a

materials effects. Additionally, this decision is based

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, generally through March 2021,

as presented in the ISA Supplement 144 (U.S. EPA,

2022a). The selection of welfare effects evaluated

within the ISA Supplement was based on the

causality determinations reported in the 2019 ISA

and the subsequent use of scientific evidence in the

2020 PA.145 Specifically, for welfare effects, the focus

144

As described in more detail in the ISA Supplement, “the

scope of this Supplement provides specific criteria for the types

of studies considered for inclusion within the Supplement.

Specifically, studies must be peer reviewed and published

between approximately January 2018 and March 2021” (U.S.

EPA, 2022a, section 1.2.2).

145

As described in section 1.2.1 of the ISA Supplement, “the

selection of welfare effects to evaluate within this Supplement is

based on the causality determinations reported in the 2019 PM

ISA and the subsequent use of scientific evidence in the 2020

PM PA. 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 PM 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). Given these

uncertainties and limitations, the basis of the discussion on

conclusions regarding the secondary standards in the 2020 PM

PA primarily focused on visibility effects. Therefore, this

Supplement focuses only on visibility effects in evaluating newly

available scientific information and is limited to studies

505a

within the ISA Supplement is on visibility effects.

The ISA Supplement does not include an evaluation

of studies on climate or materials effects. The

Administrator’s decision also takes into account the

2022 PA evaluation of the policy-relevant information

in the 2019 ISA and ISA Supplement and presentation of quantitative analysis of air quality related to

visibility impairment; CASAC advice and recommendations, as reflected in discussions of the drafts of the

ISA Supplement and 2022 PA at public meetings and

in the CASAC’s letters to the Administrator; and

public comments received on the proposal.

In presenting the rationale for the Administrator’s

final decision and its foundations, section V.A

provides background on the 2020 final decision to

retain the secondary PM standards (section V.A.1),

and also provides brief summaries of key aspects of

the currently available welfare effects evidence

(section V.A.2) and quantitative information (section

V.A.3) Section V.B summarizes the CASAC’s advice

(section V.B.1) and the proposed conclusions (section

V.B.2), addresses public comments received on the

proposal (section V.B.3), and presents the Administrator’s conclusions on the adequacy of the current

standards (section V.B.4), drawing on consideration

of the available scientific and quantitative information, advice from the CASAC, and comments from

the public. Section V.C summarizes the Administrator’s

decision on the secondary PM standards.

A. Introduction

The general approach for this reconsideration of

the 2020 final decision on the secondary PM

conducted in the U.S. and Canada” (U.S. EPA, 2022a, section

1.2.1).

506a

standards relies on the EPA’s assessments of the

current scientific evidence and associated quantitative

analyses to inform the Administrator’s judgments

regarding secondary standards that are requisite to

protect the public welfare from known or anticipated

adverse effects associated with the pollutant’s

presence in the ambient air. The EPA’s assessments

are primarily documented in the 2019 ISA, ISA

Supplement, and 2022 PA, which builds on the 2020

PA, all of which have received CASAC review and

public comment (83 FR 53471, October 23, 2018; 83

FR 55529, November 6, 2018; 85 FR 4655, January

27, 2020; 86 FR 52673, September 22, 2021; 86 FR

54186, September 30, 2021; 86 FR 56263, October 8,

2021; 87 FR 958, January 7, 2022; 87 FR 22207, April

14, 2022; 87 FR 31965, May 26, 2022). In bridging the

gap between the scientific assessments of the 2019

ISA and ISA Supplement and the judgments required

of the Administrator in determining whether the

current standards provide the requisite public

welfare protection, the 2022 PA evaluates policy

implications of the evaluation of the current evidence

in the 2019 ISA and ISA Supplement, and the

quantitative information documented in the 2022 PA.

In evaluating the public welfare protection afforded

by the current standards against PM-related effects

within the scope of this reconsideration, the four

basic elements of the NAAQS (indicator, averaging

time, level, and form) are considered collectively.

The final decision on the adequacy of the current

secondary standards is a public welfare policy

judgment to be made by the Administrator. In

reaching conclusions with regard to the standard, the

decision draws on the scientific information and

analyses about welfare effects, and associated public

welfare significance, as well as judgments about how

507a

to consider the range and magnitude of uncertainties

that are inherent in the scientific evidence and

analyses. This approach is based on the recognition

that the available evidence generally reflects a

continuum that includes ambient air exposures at

which scientists agree that effects are likely to occur

through lower levels at which the likelihood and

magnitude of responses become increasingly uncertain.

This approach is consistent with the requirements of

the provisions of the Clean Air Act related to the

review of NAAQS and with how the EPA and the

courts have historically interpreted the Act. These

provisions require the Administrator to establish

secondary standards that, in the judgment of the

Administrator, are requisite to protect public welfare

from known or anticipated adverse effects associated

with the presence of the pollutant in the ambient air.

In so doing, the Administrator seeks to establish

standards that are neither more nor less stringent

than necessary for this purpose. The Act does not

require that standards be set at a zero-risk level, but

rather at a level that reduces risk sufficiently so as to

protect the public welfare from known or anticipated

adverse effects.

1. Background on the Current Standards

The current secondary PM standards were retained

in 2020 based on the scientific and technical

information available at that time, as well as the

then-Administrator’s judgments regarding the

available welfare effects evidence, the appropriate

degree of public welfare protection for the existing

standards, and available air quality information on

visibility impairment that may be allowed by such a

standard (85 FR 82684, December 18, 2020). With

the 2020 decision, the then-Administrator retained

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the secondary 24-hour PM2.5 standard, with its level

of 35 μg/m3, the annual PM2.5 standard, with its level

of 15.0 μg/m3, and the 24-hour PM10 standard, with

its level of 150 μg/m3. The subsections below focus on

the key considerations and the then-Administrator’s

conclusions in the 2020 final decision for climate and

materials effects (section V.A.1.a) and visibility

effects (section V.A.2.b).

a. Non-Visibility Effects

In light of the robust evidence base, the 2019 ISA

concluded there to be causal relationships between

PM and climate effects and materials effects (U.S.

EPA, 2019a, sections 13.3.9 and 13.4.2). The 2020

final decision was based on a thorough review in the

2019 ISA of the scientific information on PM- induced

climate and materials effects. The decision also took

into account: (1) Assessments in the 2020 PA of the

most policy-relevant information in the 2019 ISA

regarding evidence of adverse effects of PM to climate

and materials, (2) uncertainties in the available

evidence to inform a quantitative assessment of PMrelated climate and materials effects, (3) CASAC

advice and recommendations, and (4) public comments

received during the development of these documents

and on the proposal document.

In considering non-visibility welfare effects in the

2020 decision, the then-Administrator concluded

that, while it is important to maintain an appropriate

degree of control of fine and coarse particles to

address non-visibility welfare effects, “it is generally

appropriate to retain the existing standards and that

there is insufficient information to establish any

distinct secondary PM standards to address climate

and materials effects of PM” (85 FR 82744, December

18, 2020).

509a

With regard to climate, the then- Administrator

recognized that there were a number of improvements and refinements to climate models since the

2012 review. However, while the evidence continued

to support a causal relationship between PM and

climate effects, the then-Administrator noted that

significant limitations continued to exist related to

quantifying the contributions of direct and indirect

effects of PM and PM components on climate forcing

(U.S. EPA, 2020b, sections 5.2.2.1.1 and 5.4). He also

recognized that the models continued to exhibit

considerable variability in estimates of PM-related

climate impacts at regional scales (e.g., ~100 km) as

compared to simulations at global scales. Therefore,

the resulting uncertainty led the then-Administrator

to conclude in the 2020 decision that the available

scientific information remained insufficient to quantify

climate impacts associated with particular concentrations of PM in ambient air (U.S. EPA, 2020b, section

5.2.2.2.1) or to evaluate or consider a level of PM air

quality in the U.S. to protect against climate effects

and that there was insufficient information available

to base a national ambient standard on climate

impacts (85 FR 82744, December 18, 2020).

With regard to materials effects, the thenAdministrator noted that the evidence available in

the 2019 ISA continued to support a causal relationship between materials effects and PM deposition

(U.S. EPA, 2019a, section 13.4). He recognized that

the deposition of fine and coarse particles to

materials can lead to physical damage and/or

impaired aesthetic qualities. Particles can contribute

to materials damage by adding to the natural

weathering processes and by promoting the corrosion

of metals, the degradation of building materials, and

the weakening of material components. While some

510a

new information was available in the 2019 ISA, the

information was from studies primarily conducted

outside of the U.S. in areas where PM concentrations

in ambient air are higher than those observed in the

U.S. (U.S. EPA, 2020b, section 13.4). Additionally,

the information assessed in the 2019 ISA did not

support quantitative analyses of PM-related materials

effects in the 2020 PA (U.S. EPA, 2020b, section

5.2.2.2.2). Given the limited amount of information

available and its inherent uncertainties and limitations, the Administrator concluded that he was

unable to relate soiling or damage to specific levels of

PM in ambient air or to evaluate or consider a level of

air quality to protect against such materials effects,

and that there was insufficient information available

to support a distinct national am

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