 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued December 18, 2018            Decided August 23, 2019

                        No. 15-1385

              MURRAY ENERGY CORPORATION,
                      PETITIONER

                             v.

          ENVIRONMENTAL PROTECTION AGENCY,
                    RESPONDENT

           AMERICAN LUNG ASSOCIATION, ET AL.,
                     INTERVENORS


   Consolidated with 15-1392, 15-1490, 15-1491, 15-1494


     On Petitions for Review of Final Agency Action of
     the United States Environmental Protection Agency


    Seth L. Johnson argued the cause for Public Health and
Environmental Petitioners. With him on the briefs were Joshua
Stebbins, Joshua Berman, David S. Baron, and Paul Cort.
                               2
     Dominic E. Draye, Deputy Solicitor General, Office of the
Attorney General for the State of Arizona, argued the cause for
State Petitioners. With him on the briefs were Mark Brnovich,
Attorney General, John R. Lopez, IV, Solicitor General, Keith
Miller, Associate Solicitor, Joshua L. Kaul, Attorney General,
Office of the Attorney General for the State of Wisconsin,
Misha Tseytlin, Solicitor General, Daniel P. Lennington,
Assistant Attorney General, Leslie C. Rutledge, Attorney
General, Office of the Attorney General for the State of
Arkansas, Lee Rudofsky, Solicitor General, Jamie L. Ewing,
Assistant Attorney General, Jeff Landry, Attorney General,
Office of the Attorney General for the State of Louisiana,
Steven B. ABeaux@ Jones, Assistant Attorney General, Wayne
Stenehjem, Attorney General, Office of the Attorney General
for the State of North Dakota, Margaret I. Olson, Assistant
Attorney General, Peter S. Glaser, Carroll W. AMack@
McGuffey, Andy Beshear, Attorney General, Office of the
Attorney General for the Commonwealth of Kentucky, Joseph
A. Newberg, II, Assistant Attorney General, Lara Katz,
Assistant Attorney General, Office of the Attorney General for
the State of New Mexico, Ken Paxton, Attorney General,
Office of the Attorney General for the State of Texas, Priscilla
M. Hubenak, Chief, Craig J. Pritzlaff, Assistant Attorney
General, E. Scott Pruitt, Attorney General at the time the brief
was filed, Office of the Attorney General for the State of
Oklahoma, P. Clayton Eubanks, Deputy Solicitor General, and
Sean Reyes, Attorney General, Office of the State of Utah.
Mithun Mansinghani, Attorney, Office of the Attorney General
for the State of Oklahoma, Lisa M. Mitchell, Assistant Attorney
General, Office of the Attorney General for the State of Texas,
Oramel H. Skinner, III, Solicitor, Office of the Attorney
General for the State of Arizona, Sarah Adkins and Samuel R.
Flynn, Assistant Attorneys General, and Gregory T. Dutton,
Counsel,       Office of the Attorney General for the
Commonwealth of Kentucky, Steven C. Kilpatrick, Assistant
                               3
Attorney General, Office of the Attorney General for the State
of Wisconsin, Elizabeth B. Murrill and Harry J. Vorhoff,
Assistant Attorneys General, Office of the Attorney General
for the State of Louisiana, entered appearances.

    James R. Bieke argued the cause for Industry Petitioners.
With him on the briefs were Roger R. Martella, Joel F. Visser,
Scott C. Oostdyk, E. Duncan Getchell, Jr., Michael H. Brady,
Thomas A. Lorenzen, Robert J. Meyers, Linda E. Kelly,
Quentin Riegel, Leland P. Frost, Michael B. Schon, Elizabeth
L. Horner, Lucinda Minton Langworthy, Aaron M. Flynn,
Steven P. Lehotsky, Sheldon B. Gilbert, Stacy Linden, and
Richard S. Moskowitz.       Peter C. Tolsdorf entered an
appearance.

     Hope M. Babcock and Sarah J. Fox were on the brief for
amici curiae American Thoracic Society, et al. in support of
petitioners Sierra Club, et al.

    Thomas J. Ward was on the brief for amicus curiae The
National Association of Home Builders in support of Industry
and State Petitioners.

     Justin Heminger and Simi Bhat, Trial Attorneys, U.S.
Department of Justice, argued the causes for respondent. With
them on the brief were John C. Cruden, Assistant Attorney
General at the time the brief was filed, and David Orlin, Steven
Silverman, and Brian Doster, Attorneys, U.S. Environmental
Protection Agency.       Jon M. Lipshultz, Attorney, U.S.
Department of Justice, entered an appearance.

     James R. Bieke argued the cause for Industry Respondent-
Intervenors. With him on the brief were Roger R. Martella,
Joel F. Visser, Lucinda Minton Langworthy, Aaron M. Flynn,
Thomas A. Lorenzen, Robert J. Meyers, Stacy Linden, Richard
                              4
S. Moskowitz, Steven P. Lehotsky, Sheldon B. Gilbert, Linda E.
Kelly, Quentin Riegel, Leland P. Frost, Michael B. Schon,
Elizabeth Horner, and Leslie A. Hulse. Peter C. Tolsdorf
entered an appearance.

    Seth L. Johnson argued the cause for Health and
Environmental Respondent-Intervenors. With him on the brief
was David S. Baron. Joshua A. Berman and Joshua R. Stebbins
entered appearances.

     Jonathan Weiner, Deputy Attorney General, Office of the
Attorney General for the State of California, argued the cause
for State Amici in support of respondent. With him on the brief
were Kamala D. Harris, Attorney General at the time the brief
was filed, Robert W. Byrne, Senior Assistant Attorney General,
and Gavin G. McCabe, Supervising Deputy Attorney General.
Melinda Pilling, Attorney, entered an appearance.

    Richard L. Revesz, Denise A. Grab, Jack Lienke, Michael
A. Livermore, and Jason A. Schwartz were on the brief for
amicus curiae The Institute for Policy Integrity at New York
University School of Law in support of respondent.

    Before: GRIFFITH, PILLARD and WILKINS, Circuit Judges.

    Opinion for the Court filed PER CURIAM.

     PER CURIAM: In this opinion, we consider various
challenges to the Environmental Protection Agency’s 2015
revisions to the primary and secondary national ambient air
quality standards for ozone. For the reasons given below, we
deny the petitions, except with respect to the secondary ozone
standard, which we remand for reconsideration, and
grandfathering provision, which we vacate.
                                5
                         I. Background

A. Statutory and Procedural Background

     Congress enacted the modern version of the Clean Air Act
(the “Act”), codified at 42 U.S.C. § 7401 et seq., in 1970 to
control and reduce contaminants responsible for air pollution
with the overarching goal to protect human health and the
environment. Pursuant to Title I, EPA must establish, publish,
and periodically review primary and secondary national
ambient air quality standards (“NAAQS”) for air pollutants that
“may reasonably be anticipated to endanger public health or
welfare.” 42 U.S.C. § 7408(a)(1)(A). The primary NAAQS
are to be set at levels “the attainment and maintenance of which
in the judgment of the Administrator, . . . allowing an adequate
margin of safety, are requisite to protect the public health.” Id.
§ 7409(b)(1). The secondary NAAQS “shall specify a level of
air quality the attainment and maintenance of which in the
judgment of the Administrator, . . . is requisite to protect the
public welfare from any known or anticipated adverse effects.”
Id. § 7409(b)(2). Thus, primary NAAQS protect the public
health, while the secondary NAAQS protect the public welfare.
“Public health” includes adverse health effects for both the
population at large and sensitive populations such as children,
older adults, and people with asthma or other lung diseases.
The term “public welfare” encompasses a wide variety of
effects on soil, plants, wildlife and biota, property damage,
aesthetic concerns, and other non-health-related impacts such
as hazards to economic values and personal comfort. Id.
§ 7602(h).

     Pursuant to section 109(d) of the Clean Air Act, EPA must
complete a “thorough” review of the NAAQS every five years.
Id. § 7409(d)(1). During this review, EPA must revise the
criteria and standards or promulgate new standards as
                               6
appropriate. Id. To assist in this process, the Clean Air
Scientific Advisory Committee (“CASAC”) undertakes an
examination of the current air quality criteria, primary
NAAQS,       and     secondary    NAAQS,      and     submits
recommendations to EPA for “any new [NAAQS] and
revisions of existing criteria and standards as may be
appropriate.” Id. § 7409(d)(2)(A)-(B). Congress required EPA
to take CASAC’s recommendations into account when
promulgating revised NAAQS and to fully explain its reasons
when it departs from CASAC’s advice. However, the ultimate
decision to revise the NAAQS—and the determination of the
new level—rests with the Administrator. Id. § 7407(d)(3).

     These petitions concern EPA’s promulgation of revised
NAAQS related to ozone. Ozone is a colorless gas that occurs
both in the Earth’s upper atmosphere and at ground level.
Although ozone is an “essential presence in the atmosphere’s
stratospheric layer,” ground-level ozone is an air pollutant that
is harmful to breathe and damages crops, trees, and other
vegetation. S. Coast Air Quality Mgmt. Dist. v. EPA, 472 F.3d
882, 887 (D.C. Cir. 2006); see Allied Local & Reg’l Mfrs.
Caucus v. EPA, 215 F.3d 61, 66 (D.C. Cir. 2000). Ground-
level ozone is not a direct product of human activity, but
instead forms when atmospheric pollutants (including nitrogen
oxides and volatile organic compounds) react in the presence
of sunlight. See Mississippi v. EPA, 744 F.3d 1334, 1340 (D.C.
Cir. 2013). These precursor atmospheric pollutants are created
primarily from emissions produced by cars, power plants, and
chemical solvents. NRDC v. EPA, 777 F.3d 456, 459 (D.C.
Cir. 2014).

    In 1979, EPA issued primary and secondary NAAQS for
ozone with a limit of 0.12 parts per millions (ppm), and a one-
hour averaging time. See id. This “one-hour standard”
measured average ozone levels over one-hour periods, and
                               7
EPA would deem an area in compliance with this standard if it
did not exceed the level for more than one day per calendar
year. Id. EPA next revised the ozone NAAQS in 1997, having
determined that no revisions to the standards were necessary in
1993. National Ambient Air Quality Standards for Ozone, 58
Fed. Reg. 13,008 (Mar. 9, 1993). The agency replaced the one-
hour, 0.12 ppm standard with a 0.08 ppm standard measured
over eight hours. National Ambient Air Quality Standards for
Ozone, 62 Fed. Reg. 38,856 (July 18, 1997). EPA also altered
the form of compliance to the annual fourth-highest daily
maximum 8-hour concentration, averaged over three years. Id.

     In 2008, EPA lowered the primary and secondary
standards to 0.075 ppm but kept the same eight-hour averaging
time and form as in 1997. NRDC, 777 F.3d at 462-63; National
Ambient Air Quality Standards for Ozone, 73 Fed. Reg. 16,436
(Mar. 27, 2008). In developing the 2008 standards, EPA relied
on scientific evidence showing that ozone causes health effects
at and above 0.08 ppm and examined two new clinical studies
that found negative health effects from ozone at lower levels.
See Mississippi, 744 F.3d at 1340. While CASAC reviewed
this same evidence and recommended a primary level between
0.06 ppm and 0.07 ppm, EPA departed from this advice and
explained that the scientific data regarding negative health
effects at 0.06 ppm was too limited and inconclusive to support
a standard below 0.075 ppm. See id. at 1340-41. We upheld
EPA’s primary standard on this basis but found that EPA had
not adequately explained its revision of the secondary standard.
Id. at 1359-62. We noted that EPA had not properly
determined what level of air quality was requisite to protect the
public welfare. Id. Accordingly, we remanded for further
explanation and reconsideration of the secondary level.

   Following the promulgation of the 2008 ozone NAAQS,
EPA undertook another comprehensive review of the ozone
                               8
standards and scientific data. EPA consolidated its review on
remand of the 2008 secondary standard with its ongoing review
for the 2015 ozone NAAQS. After conducting public hearings
and receiving approximately 430,000 written comments on its
proposed revision of the primary and secondary ozone
NAAQS, EPA published its final 2015 ozone NAAQS on
October 26, 2015. National Ambient Air Quality Standards for
Ozone (“2015 Rule”), 80 Fed. Reg. 65,292 (Oct. 26, 2015). For
both the primary and secondary standards, EPA lowered the
level from 0.075 ppm to 0.07 ppm, while retaining the 2008
indicator (ozone), averaging time (8 hours), and form (three-
year average of the fourth-highest daily maximum 8-hour
concentration). See id. at 65,294, 65,301, 65,347, 65,349-50,
65,352.

     Based on scientific data and CASAC’s recommendation,
the Administrator concluded that the 2008 primary ozone
standard (0.075 ppm) was not at a level requisite to protect
public health while allowing an adequate margin of safety. Id.
at 65,326, 65,344, 65,346.         The Administrator placed
significant weight on new clinical studies linking short-term
ozone exposure with respiratory effects, including lung
inflammation. See id. at 65,302-03, 65,341, 65,352, 65,359.
EPA conducted its own exposure study and found that a revised
primary NAAQS standard of 0.07 ppm would “eliminate the
occurrence of two or more exposures of concern to [ozone]
concentrations at and above [0.08 ppm]” and would “virtually
eliminate the occurrence of two or more exposures of concern
to [ozone] concentrations at and above [0.07 ppm], even in the
worst-case urban study area and year evaluated.” Id. at 65,353.
The Administrator additionally found that a level of 0.07 ppm
would “protect the large majority of children in the urban study
areas (i.e., about 96% to more than 99% of children in
individual urban study areas) from experiencing two or more
exposures of concern at or above the [0.06 ppm] benchmark.”
                               9
Id. at 65,353, 65,360-64. EPA also evaluated hundreds of
epidemiologic studies that provided statistically relevant
information about a broader population of individuals who are
exposed to uncontrolled air pollution. See id. at 65,304,
65,364.

     Further, the Administrator considered CASAC’s advice on
the new primary ozone standard. See id. at 65,346, 65,361. In
advance of the 2015 Rule, CASAC stated that “there is clear
scientific support for the need to revise the standard” in place
since 2008 and recommended setting the standard within a
range of 0.06 ppm to 0.07 ppm, while leaving the form,
averaging time, and indicator unchanged. Id. at 65,322,
65,361.

     EPA also assessed the secondary standard and concluded
that the 2008 secondary standard (0.075 ppm) was not requisite
to protect public welfare. Id. at 65,382, 65,389-90. Rather,
EPA set the secondary standard at 0.07 ppm and kept the
indicator, averaging time, and form the same. Id. at 65,369,
65,403, 65,409-10. EPA noted that more than four hundred
new studies examining the interplay between ozone and public
welfare had been developed since the promulgation of the 2008
ozone NAAQS. Id. at 65,369. These studies strengthened and
expanded the scientific understanding of ozone’s effects on
plants. The Administrator analyzed the causal relationship
between ozone exposure and vegetation effects, examining tree
growth impacts, crop yield loss, and visible leaf injury. Id. at
65,370; see id. at 65,294, 65,369-70. The Administrator gave
“primary consideration” to tree growth impacts, judging that it
was more difficult to assess the welfare significance of crop
yield loss (given that crops are heavily managed) and visible
leaf injury (citing the “lack of established criteria or
objectives”). Id. at 65,407.
                               10
     In setting the secondary standard, EPA departed from
CASAC’s recommendations as to the form and standard of the
secondary ozone NAAQS. CASAC recommended using a
single-year averaging approach, but EPA opted for a three-year
average because “the public welfare significance of effects
associated with multiple years of critical exposures are
potentially greater than those associated with a single year of
such exposure.” Id. at 65,404. The Administrator also rejected
CASAC’s recommendation to use the W126 exposure index, a
cumulative, seasonal ozone exposure metric, as the form of the
secondary standard, rather than the same form as the primary
standard (three-year average of the fourth-highest daily level),
finding that the latter form was adequate to restrict cumulative
ozone exposures that are detrimental to vegetation. Id. at
65,408.

      In addition to revising the ozone standards, the 2015
NAAQS also updated the regulations for the Prevention of
Significant Deterioration (“PSD”) permitting program. See id.
at 65,431-34. Under the PSD program, no construction on a
major emitting facility may be commenced in an area that has
attained the air quality standards for any criteria pollutant
unless “the owner or operator of such facility demonstrates . . .
that emissions from construction or operation of such facility
will not cause, or contribute to, air pollution in excess of any
. . . [NAAQS].” 42 U.S.C. § 7475(a)(3)(B). The permitting
authority (either a state or EPA) must grant or deny a completed
preconstruction permit application under the PSD program
within one year of filing. Id. § 7475(c).

    EPA claimed that sections 7475(a)(3)(B) and 7475(c) have
the “potential for conflict,” which it was authorized under
Chevron to resolve. EPA Br. 132; see also 80 Fed. Reg. at
65,433-34. EPA worried that, for a limited subset of pending
permit applications, complying with section 7475(a)(3)(B)’s
                               11
demonstration requirement for the 2015 NAAQS “could hinder
compliance with the requirement under section [7475](c) to
issue a permit within one year of the completeness
determination.” 80 Fed. Reg. at 65,434. In other words, EPA
was unsure how to handle completed permit applications where
sources would have complied with the NAAQS in existence at
the time the application was filed (the 2008 ozone standards),
but where the NAAQS were revised before the permit was
approved. See id. at 65,433. EPA resolved this apparent
conflict by implementing a grandfathering provision for
pending permit applications that satisfy one of two permitting
criteria. See id. at 65,431-34. These two applicable categories
include: (1) permit applications that are deemed complete on
or before the signature date of the final rule revising the ozone
NAAQS; and (2) permit applications “for which the reviewing
authority has first published a public notice of the draft permit
or preliminary determination before the effective date of the
revised NAAQS.” Id. at 65,431, 65,433. If a permit
application satisfies either requirement, the owner or operator
must show compliance with the 2008 rather than the 2015
NAAQS before initiating construction.

     Petitioners from the oil and gas industry (“Industry
Petitioners”) and from several states (“State Petitioners”) claim
that the primary and secondary NAAQS are too protective.
Conversely, petitioners from public health and environmental
groups (“Environmental Petitioners”) claim that these NAAQS
are not protective enough. Environmental Petitioners also
contend that EPA’s decision to allow limited grandfathering of
certain permit applications was inconsistent with the Clean Air
Act.

     This opinion proceeds in five parts. Part II considers the
challenges to the primary standard, and Part III the challenges
to the secondary standard. Part IV reviews the cross-cutting
                               12
challenges to EPA’s promulgation of the NAAQS, and Part V
addresses the challenge to the grandfathering provision.

B. Jurisdiction and Standard of Review

     This court has jurisdiction to review EPA’s 2015 Rule
pursuant to 42 U.S.C. § 7607(b)(1). Under section 7607,
agency action may be reversed if it is arbitrary, capricious, an
abuse of discretion, not in accordance with law, or in excess of
statutory authority. Id. § 7607(d)(9)(A), (C); see Am.
Petroleum Inst. v. EPA, 684 F.3d 1342, 1347 (D.C. Cir. 2012).
To withstand review, an agency must have examined all
relevant facts and data, and articulated a rational explanation
for its decision, including a reasonable connection between the
facts and ultimate outcome. See Allied Local, 215 F.3d at 68.
We cannot look at EPA’s decision as would a scientist, but
instead must exercise our “narrowly defined duty of holding
agencies to certain minimal standards of rationality.”
Mississippi, 744 F.3d at 1342 (quoting Nat’l Envtl. Dev.
Ass’n’s Clean Air Project v. EPA, 686 F.3d 803, 810 (D.C. Cir.
2012)); see id. at 1348 (“We repeat: it is not our job to referee
battles among experts; ours is only to evaluate the rationality
of EPA’s decision . . . .”).

     On questions of statutory interpretation, the court must
review EPA’s actions in accordance with the standard set forth
in Chevron, U.S.A., Inc. v. Natural Resources Defense Council,
Inc., 467 U.S. 837, 842-44 (1984). See Michigan v. EPA, 135
S. Ct. 2699, 2706-07 (2015); NRDC, 777 F.3d at 463. Chevron
deference involves a two-step analysis. First, if a statute is
clear, the court must give effect to Congress’s unambiguous
language and intent. Chevron, 467 U.S. at 842-43. Where a
statute that an agency administers is ambiguous, the court must
turn to the second Chevron principle and give deference to the
                               13
agency’s reasonable interpretation of the statute. Id. at 843; see
also S. Coast Air Quality Mgmt. Dist., 472 F.3d at 891.

              II. Primary Standard Challenges

     Industry and State Petitioners contend that EPA’s
promulgation of the 2015 Rule’s primary standard was
arbitrary and capricious because, they say, EPA failed to
provide a reasoned explanation for departing from the 2008
NAAQS. Environmental Petitioners argue that the primary
ozone standard is too lenient because it occasionally permits
ozone levels to exceed 0.07 ppm and will allegedly tolerate
adverse health effects. For the reasons below, we hold that
these arguments lack merit.

A. Industry and State Petitioners’ Challenge

     The Clean Air Act requires EPA to set primary NAAQS
that are “requisite” to protect public health with an adequate
margin of safety. 42 U.S.C. § 7409(b)(1). The term “requisite”
means “sufficient, but not more than necessary.” See Whitman
v. Am. Trucking Ass’ns, 531 U.S. 457, 473 (2001). While the
determination of what is “requisite” may require a contextual
assessment of acceptable risk, our precedent is clear that prior
NAAQS are not sacrosanct and are not granted presumptive
validity. See Mississippi, 744 F.3d at 1343. Prior NAAQS
need not remain the governing standard until every aspect of
those prior NAAQS is undermined. Id. To hold otherwise
“would bind EPA to potential deficiencies in past reviews
because discrepancies between past and current judgments as
easily reflect problems in the past as in the present.” Id. Thus,
when EPA reviews and revises the NAAQS, it does so against
current policy considerations and existing scientific
knowledge. Id. Accordingly, “[t]he statutory framework
requires us to ask only whether EPA’s proposed NAAQS is
                               14
‘requisite’; we need not ask why the prior NAAQS once was
‘requisite’ but is no longer up to the task.” Id.

     Given our decision in Mississippi, we must reject Industry
and State Petitioners’ argument that EPA departed from the
2008 NAAQS without adequate explanation. EPA has
adequately explained why on the record before it the revised
standard is requisite to protect the public health. As the record
makes clear, the Administrator considered the entire body of
scientific evidence available, including clinical studies,
epidemiologic evidence, human exposure and health risk
assessments, CASAC’s recommendations, and over 430,000
public comments. See 80 Fed. Reg. at 65,293, 65,299, 65,301,
65,323, 65,326. And consistent with CASAC’s advice, the
Administrator placed the most weight on clinical studies. See
id. at 65,302-03, 65,341, 65,352, 65,359. State Petitioners take
particular issue with EPA’s reliance on the 2009 Schelegle
study, which, they argue, does not support EPA’s finding of
adverse effects. But the 2009 Schelegle study is merely one of
many clinical studies that EPA relied on. State Petitioners
ignore the fact that additional clinical studies and
epidemiologic evidence show substantial health effects at
ozone levels as low as 0.06 ppm. See id. at 65,331, 65,334-35
& n.96, 65,344, 65,364.

     In addition to evaluating clinical studies, the Administrator
also examined epidemiologic evidence and EPA’s exposure
and risk assessments. See 80 Fed. Reg. at 65,304, 65,314-17,
65,364. While the Administrator placed less weight on the
epidemiologic results than on clinical studies, see id. at 65,320,
65,324, 65,335, she nonetheless found that recent
epidemiologic studies suggested adverse health effects would
occur below the 2008 NAAQS standard. While Industry
Petitioners challenge the use of epidemiologic evidence given
the uncertainties presented in these studies, even CASAC
                              15
concluded that the epidemiologic evidence would have alone
been strong enough to justify revision of the 2008 NAAQS.

B. Environmental Petitioners’ Challenges

     In contrast to Industry and State Petitioners,
Environmental Petitioners argue that the primary ozone
standard is too lenient. They offer two reasons. First,
Environmental Petitioners contend that the form of the revised
standard is not health-protective because it permits ozone
levels to exceed 0.07 ppm on some days. Second, they argue
that ozone exposures of 0.07 ppm will cause adverse health
effects, particularly in sensitive populations. For the reasons
below, we hold that these arguments lack merit.

           EPA reasonably explained its decision to retain
           the form of the primary standard.

     Environmental Petitioners first take issue with the form of
the revised standard—that the average, over three years, of the
annual fourth-highest maximum daily 8-hour average ozone
level must not exceed 0.07 ppm. By using this form, EPA
“allows ozone levels to exceed—multiple times in any year—
levels that EPA itself agrees cause adverse health effects.”
Envtl. Pet’rs Br. 20. Environmental Petitioners contend that
because areas with ozone levels above the 0.07 ppm threshold
can still comply with EPA’s standards, the standard is unlawful
and arbitrary. We disagree.

     The Administrator’s decision to retain the same form used
in the 1997 and 2008 primary standards was in part based on
how many people are estimated to experience unhealthy ozone
exposures. To this end, the Administrator utilized EPA’s
exposure assessment to gauge how often various population
subgroups will be exposed to potentially health-impairing
ozone concentrations while experiencing elevated breathing
                              16
rates. See 80 Fed. Reg. at 65,356. Rather than considering the
ozone level of an area in isolation, the Administrator also
“consider[ed] activity patterns in the exposed population”
because adverse health responses to ozone exposure are
critically dependent on breathing rates. Id. at 65,363; see also
id. (“Not considering activity patterns, and corresponding
ventilation rates, can result in a standard that provides more
protection than is requisite.”). This use of the exposure
assessment was rational.

     Further, the Administrator adequately interpreted the
exposure assessment. The Administrator correctly observed
that the exposure assessment only chronicled how many people
with an elevated breathing rate will be exposed to a specific
ozone level, not how many will have an adverse effect. Given
this limitation, the Administrator focused on estimates of two
or more exposures of concern to assess the potential for adverse
effects at and above an ozone concentration of 0.07 ppm. See
id. at 65,310, 65,325, 65,330, 65,343, 65,345-46, 65,354,
65,358, 65,361, 65,363. Based on the data, the Administrator
concluded that a level of 0.07 ppm ozone would eliminate the
occasions on which school-age children experience two or
more exposures of concern at ozone concentrations at and
above 0.08 ppm and, even in the worst-case years and
locations, would “virtually eliminate” the occasions on which
such children experience two or more exposures of concern at
0.07 ppm. Id. at 65,353. In particular, the Administrator noted
that a primary level of 0.07 ppm would protect approximately
96% to 99% of children in the urban study areas from
experiencing two or more exposures of concern above the 0.06
ppm benchmark. Id. at 65,353; see also id. at 65,360-64. That
Environmental Petitioners cite data suggesting that 18,000
children would experience multiple exposures of concern at or
above 0.07 ppm during the worst year and location is not
determinative because the primary standard for a non-threshold
                               17
pollutant like ozone is not required to produce zero risk, and
“[t]he task of determining what standard is ‘requisite’ to protect
the qualitative value of public health or what margin of safety
is ‘adequate’ to protect sensitive subpopulations necessarily
requires the exercise of policy judgment.” Mississippi, 744
F.3d at 1351, 1358. EPA has reasonably exercised its
judgment.

     Given that the Administrator appropriately examined and
considered EPA’s exposure assessment, her decision to retain
the form of the primary standard was also rational. The
Administrator determined that, when combined with an ozone
level of 0.07 ppm, the form of the standard (three-year average
of the fourth-highest daily level) was requisite. In particular,
the Administrator found that most areas that meet the revised
standard will have an 8-hour ozone concentration below 0.07
ppm. See 80 Fed. Reg. at 65,363. Environmental Petitioners,
however, criticize this form for allegedly failing to account for
individual ozone days above 0.07 ppm. But Environmental
Petitioners elide a crucial detail of EPA’s methodology: the
exposure assessment study that “informed the Administrator’s
consideration of the degree of public health protection provided
by various standard levels” considered the air quality at
“various standards with the current 8-hour averaging time and
fourth-high, 3-year average form.” Id. at 65,351-52 (emphasis
added). As such, the Administrator reasonably accounted for
days when ozone levels may exceed 0.07 ppm. Taken together,
we conclude that the Administrator’s decision to retain the
form of the standard was appropriate and neither arbitrary nor
capricious.
                              18
           EPA reasonably set the primary standard at 0.07
           ppm.

    Environmental Petitioners next argue that the
Administrator’s decision to lower the primary standard from
from 0.075 to 0.07 ppm is an unexplained departure from
CASAC’s recommendation and from EPA’s prior position
regarding the adversity of certain lung function decrements.
We reject both contentions.

     Environmental Petitioners argue that EPA failed to fulfill
its duty under the Act to provide “an explanation of the
reasons” for departing from CASAC’s scientific
recommendations. 42 U.S.C. § 7607(d)(3); see also id.
§ 7607(d)(6). They contend CASAC told EPA that “[a]t [0.07
ppm] there is substantial scientific certainty of a variety of
adverse effects, including decrease in lung function, increase
in respiratory symptoms, and increase in airway
inflammation,” Envtl. Pet’rs Br. 31 (quoting J.A. 531), and
EPA failed to rationally dispute or refute this finding.

      EPA adequately considered CASAC’s advice when setting
the primary NAAQS to 0.07 ppm. While CASAC advised EPA
to “set the level of the standard lower than [0.07 ppm],” J.A.
531, that recommendation constituted policy—not scientific—
advice. This distinction is crucial because we defer to EPA’s
judgment on issues of policy but require EPA to “fully explain
its reasons for any departure from” “CASAC’s expert scientific
recommendations.” Mississippi, 744 F.3d at 1358, 1354-55
(emphasis added). CASAC’s letter to EPA makes clear that
“based on the scientific evidence” it “recommend[ed] a range
of levels for a revised primary ozone standard from [0.07 ppm]
to [0.06 ppm].” J.A. 531. CASAC then “acknowledge[d] that
the choice of a level within the range recommended based on
scientific evidence is a policy judgment.” Id.
                                19
     In an effort to influence EPA’s policy judgment, CASAC
noted that given the “substantial scientific certainty of a variety
of adverse effects” at 0.07 ppm, that level may not be adequate
to protect public health. Id. So, CASAC’s “policy advice
[was] to set the level of the standard lower than [0.07 ppm]
within a range down to [0.06 ppm], taking into account [EPA’s]
judgment regarding the desired margin of safety to protect
public health.” Id. EPA did not take CASAC’s policy advice
because the Administrator found that 0.07 ppm would still
“provide substantial protection against the broader range of
[ozone] exposure concentrations that have been shown in
controlled human exposure studies to result in respiratory
effects, including exposure concentrations below [0.07 ppm].”
80 Fed. Reg. at 65,363. EPA nonetheless chose a level for the
primary standard that was within CASAC’s scientifically
recommended range. In so doing, EPA did not abrogate its
duty under the Clean Air Act to consider CASAC’s scientific
recommendations but instead made a valid policy decision.

     Environmental Petitioners also argue that EPA should
have set the primary standard lower than 0.07 ppm, given the
controlled human exposure studies that had been published
since the agency’s 2008 NAAQS review. Specifically,
Environmental Petitioners highlight two new studies
evaluating exposures to 0.06-0.063 ppm ozone that concluded
that exposures at 0.06 ppm caused lung function decrements of
10% or more. Taken with EPA’s statement in the 2008 ozone
NAAQS rule that a lung function decrement of 10% or more
“should be considered adverse for sensitive populations,” 73
Fed. Reg. at 16,454-55, Environmental Petitioners contend that
EPA acted arbitrarily by giving insufficient weight to those
new studies. We are unpersuaded by this argument, which fails
to appreciate the dynamic nature of adversity determinations at
each NAAQS review.
                               20
     In determining the appropriate level for the 2015 primary
standard, the Administrator was not bound by adversity
judgments in the 2008 NAAQS review. Indeed, “as the
contours and texture of scientific knowledge change, the
epistemological posture of EPA’s NAAQS review necessarily
changes as well.” Mississippi, 744 F.3d at 1344. Thus, we
consider only whether EPA’s 2015 Rule offers a rational
explanation of why EPA chose a new adversity definition and
whether the Administrator reasonably evaluated the evidence
of adversity. We hold that EPA’s actions were reasonable and
reasonably explained. Rather than applying a rigid test for
determining what level of decrement is adverse, the
Administrator took a more comprehensive approach provided
by American Thoracic Society (ATS) guidelines. ATS
guidelines provide that “reversible loss of lung function in
combination with the presence of [respiratory] symptoms
should be considered adverse.” 80 Fed. Reg. at 65,357
(internal quotation marks omitted). EPA reasonably explained
that it chose to adopt the ATS definition of adversity because
“the available evidence does not provide information on the
extent to which a short-term, transient decrease in lung function
in a population,” without more, could “change the risk profile
of the population,” and that CASAC was “conditional” about
whether “the lung function decrements observed in some
people at [0.06 ppm] . . . are adverse.” Id. at 65,358. The
clinical studies that Environmental Petitioners contend EPA
dismissed concluded that lung function decrements (such as
10%) occurred in some individuals at lower ozone
concentrations, including 0.06 and 0.063 ppm, but not in
“combination [with] statistically significant increases in
respiratory symptoms.” See id. at 65,357. Under ATS
guidance, the Administrator declined to designate such loss of
lung function as adverse. This decision was rational, and
Environmental Petitioners cannot show that the evidence
required EPA to decide differently.
                                 21
            III. Secondary Standard Challenges

     Next, we turn to the challenges to EPA’s secondary
standard. The Environmental Petitioners contend that: (1) in
considering tree growth loss, EPA acted arbitrarily in setting
the target level of air quality and therefore fell short of the
statutory requirement to set a standard “requisite” to protect
against such harm; (2) EPA arbitrarily used a three-year
average rather than a single-year, cumulative measurement of
ozone exposure as a benchmark to gauge the protectiveness of
its standard, and arbitrarily declined to adopt the single-year
cumulative exposure index as the form and averaging time for
the secondary standard; and (3) EPA unreasonably failed to
identify a level of air quality requisite to protect the public
welfare against adverse effects from visible leaf injury.
Industry Petitioners argue only that EPA failed to justify its
decision to lower the secondary standard from its 2008 level.
We hold that EPA has not explained its decision to set a target
level of protection against tree growth loss based on a three-
year average of cumulative, single-year ozone exposures, nor
has it justified its decision not to specify any level of air quality
requisite to protect against visible leaf injury. We reserve
judgment on whether EPA reasonably declined to adopt the
cumulative exposure index as the form and averaging time, and
deny the remainder of the challenges.

     We begin by reviewing EPA’s secondary standard-setting
process. EPA concentrated its review on the association
between ozone exposure and “vegetation effects,” focusing on
tree growth loss (also referred to as “relative biomass loss”),
crop yield loss, and visible leaf (or “foliar”) injury. 80 Fed.
Reg. at 65,370-71. CASAC agreed that those three effects
were “appropriate surrogates [for] a wide range of damage that
is adverse to public welfare” because ozone damage to trees,
leaves, and crops can directly affect numerous resources and
                               22
ecosystem services that are important to the public and
indirectly affect a wide array of ecosystem components and
functions. J.A. 532-33.

     In performing its analysis, EPA used an “exposure metric”
called the “W126 index” to gauge how differing levels of ozone
exposure correspond to effects on tree growth, crop yields, and
visible leaf injury. 80 Fed. Reg. at 65,373. The W126 index,
which EPA deemed to be the “most biologically relevant
metric[] for consideration of [ozone] exposures eliciting
vegetation-related effects,” measures the cumulative amount of
ozone to which a plant is exposed over a single three-month
growing season. Id. at 65,373-74. W126 levels are expressed
as parts-per-million hours (ppm-hrs). Id.

     CASAC recommended that EPA use the single-year W126
exposure index as the form and averaging time for the
secondary standard, J.A. 518, meaning that compliance with
the standard would be measured based on a single growing
season’s worth of cumulative ozone exposure. CASAC
advised EPA to set the level for the secondary standard within
a range of 7 ppm-hrs and 15 ppm-hrs. Based on the data
CASAC used, that corresponded to median annual tree growth
loss between 2% and 5.2%. J.A. 518, 534-36, 631. CASAC
cautioned that “at 17 ppm-hrs, the median tree species has 6%
relative biomass loss,” which would be “unacceptably high.”
J.A. 518. Regarding the other two surrogates, CASAC
counseled that, to protect against loss of crop yield, “a level of
15 ppm-hrs for the highest 3-month sum in a single year is
requisite,” and to reduce foliar injury, a “level below 10 ppm-
hrs is required.” Id. All of those levels were based on single-
year measuring periods, so CASAC advised that if EPA were
to base its secondary standard on a three-year average of the
relevant measurements, it should lower the level of the standard
                              23
“to protect against single unusually damaging years that will be
obscured in the average.” J.A. 536.

     EPA agreed with CASAC’s recommendation that 6% tree
growth loss would be “unacceptably high.” 80 Fed. Reg. at
65,406. However, as EPA explained, id. at 65,384, 65,392
n.197, CASAC’s advice on that was based in part on a study of
cottonwood trees, which are uniquely ozone-sensitive, and
CASAC itself advised EPA that the “cottonwood data . . .
receive[d] too much emphasis” in EPA’s analysis, J.A. 533.
After excluding the cottonwood data, EPA concluded that
ozone exposure of 19 ppm-hrs was associated with 6% tree
growth loss, and that 17 ppm-hrs of ozone exposure brought it
down to 5.3% loss. 80 Fed. Reg. at 65,396, 65,407. EPA
accordingly chose to focus on a standard “somewhat below”
the 19 ppm-hrs level associated with 6% median tree growth
loss. Id. at 65,406-07.

     EPA then departed from CASAC’s advice in several ways.
First, it chose not to use the W126 cumulative seasonal
exposure index as the form and averaging time of the standard,
opting instead to retain the averaging time (8 hours) and form
it had used in the 2008 rule (the three-year average of the
fourth-highest daily level). EPA purported to use as a
“benchmark,” or target level of protectiveness, the high end of
the range of W126 levels CASAC had recommended. Recall
that CASAC had recommended that the maximum seasonal
ozone exposure not exceed (the cottonwood-adjusted) 17 ppm-
hrs in any single growing season. In deciding what exposure
level correlated to CASAC’s 17 ppm-hrs, however, EPA used
a three-year average of anticipated seasonal exposure levels.
Thus, EPA chose to set the standard at a level that it projected
(based on a statistical analysis of past ozone exposure data)
would “in nearly all instances” going forward restrict the
average of three growing seasons’ ozone exposures to the
                               24
equivalent of 17 ppm-hrs. Id. at 65,407. But by using that
method to set the level, EPA arrived at a standard that
statistically tolerates cumulative ozone exposures in a single
growing season that are higher than CASAC’s maximum
recommended level. EPA also did not specify any level of
ozone to protect against visible leaf injury or crop loss. The
agency reasoned that data for those public-welfare harms were
too uncertain to permit it to discern a level that would provide
the requisite protections, and that the standard it set to protect
against adverse effects from tree growth loss would at least
provide “some increased protection” against visible leaf injury
and crop damage. Id. at 65,407-08.

    In sum, EPA ultimately chose to set the level of the
secondary standard at 0.07 ppm while retaining the form and
averaging time it had previously used. Id. at 65,410.

A. Environmental Petitioners’ Challenges

     The essence of the Environmental Petitioners’ petition is
that EPA did not adequately explain its deviations from
CASAC’s advice. “In order to enable judicial review and to
satisfy its statutory obligation to explain its reasons for
departing from CASAC, EPA must be precise in describing the
basis for its disagreement with CASAC.” Mississippi, 744 F.3d
at 1355. Where EPA diverges from CASAC’s scientific
advice, there must be “substantial evidence in the record when
considered as a whole which supports the Administrator’s
determinations.” Id. (quoting Lead Indus. Ass’n v. EPA, 647
F.2d 1130, 1146 (D.C. Cir. 1980)).
                              25
           EPA reasonably used 17 ppm-hrs as the
           benchmark level to protect against adverse effects
           associated with tree growth loss.

    The Environmental Petitioners argue that, by establishing
17 ppm-hrs as the target level of protection against adverse
welfare effects associated with tree growth loss, EPA
impermissibly departed upward from CASAC’s advice in two
ways—first, by excluding the data from the cottonwood tree
study from its tree growth loss analysis and, second, by
focusing on limiting tree growth loss to under 6%, rather than
the 2% target they say CASAC’s analysis required. Setting
aside for the moment EPA’s decision to average the benchmark
over three years, we conclude that EPA adequately explained
its decision to exclude the cottonwood data and acted
consistently with CASAC’s advice in choosing to limit tree
growth loss to under 6%.

     Cottonwood Data. EPA reviewed seedling studies of
twelve different tree species to determine the median percentile
of tree growth loss at varying ozone levels. See 80 Fed. Reg.
at 65,372. One of those studies—of cottonwood seedlings—
significantly lowered the ozone level at which the twelve tree
species experience 6% median growth loss. See id. at 65,384,
65,391-92. CASAC had itself relied on that study, see J.A.
537, but advised EPA that the cottonwood data had received
“too much emphasis” because the study “did not control for
ozone and climatic conditions,” and the results “show[ed]
extreme sensitivity to ozone compared to other studies,” J.A.
533. EPA accordingly excluded the cottonwood data from its
tree growth analysis, which increased the ozone level
associated with 6% median growth tree loss from 17 ppm-hrs
up to 19 ppm-hrs. 80 Fed. Reg. at 65,384, 65,391-92.
                               26
      The Environmental Petitioners object that EPA arbitrarily
disregarded CASAC’s advice because, they say, CASAC never
expressly advised EPA to disregard the cottonwood data
altogether, and CASAC relied on it in setting its recommended
exposure range. But we can only discern an unreasonable
deviation where CASAC itself has been “precise about the
basis for its recommendations.” Mississippi, 744 F.3d at 1358.
It was far from precise on the disputed point. It warned EPA
to treat the cottonwood data with caution, but recommended a
range of ozone levels in reliance on the median tree growth loss
estimates (outlined in Table 6-1 of EPA’s staff’s second draft
Policy Assessment), which weigh the cottonwood data equally
with other seedling studies. See J.A. 537 (citing EPA, Policy
Assessment for the Review of the Ozone NAAQS, Second
External Review Draft 6-19, tbl. 6-1 (2014), J.A. 631). The
Environmental Petitioners suggest that CASAC was only
warning EPA to take care in describing the cottonwood data in
the narrative portion of its Policy Assessment, not suggesting
anything about how the data was actually used, but that makes
little sense in the context of an exchange about quantitative
growth loss estimates. In light of CASAC’s mixed messages,
we cannot say that EPA’s decision to exclude the cottonwood
data was arbitrary.

     The 6% Target. Emphasizing CASAC’s admonition that
6% tree growth loss would be “unacceptably high,” EPA
decided “to focus on a standard that would generally limit
cumulative exposures to those for which the median [tree
growth loss] estimate would be somewhat lower than 6%.” 80
Fed. Reg. at 65,406-07. The Environmental Petitioners
contend that CASAC “plainly specified” that 2% median
growth loss was the requisite level to avoid adverse welfare
effects related to tree growth loss, Envtl. Pet’rs Br. 42-44, but
they misconstrue CASAC’s advice.
                               27
     CASAC did not identify 2% growth loss as the only
sufficiently protective level, but as “an appropriate
scientifically based value to consider as a benchmark of
adverse impact,” elaborating that it would be “appropriate to
identify a range of levels of alternative W126-based standards
that includes levels that aim for not greater than 2% [growth
loss] for the median tree species.” J.A. 537 (emphases added).
CASAC in fact ultimately recommended that EPA set the
standard between 7 ppm-hrs and 15 ppm-hrs, J.A. 518, which,
based on the seedling data upon which CASAC relied
(including the cottonwood data), corresponds to median annual
tree growth loss between 2% and 5.2%, see J.A. 631. In other
words, CASAC recommended that EPA consider limiting tree
growth loss to 2% as the lower end of a range of permissible
target levels. EPA followed CASAC’s advice by considering
a 2% growth loss target as part of a range of growth loss targets
and determined that the studies underlying CASAC’s low-end
recommendation were insufficiently reliable to base the
secondary standard on that low level of tree growth loss. See
80 Fed. Reg. at 65,394.

     The Environmental Petitioners passingly disparage EPA’s
assessment of the studies associated with CASAC’s 2% growth
loss target, but that argument is equally unavailing. EPA
reasonably explained why it thought those studies were
unreliable, and its “evaluation of ‘scientific data within its
technical expertise’” is entitled significant deference. See
Miss. Comm’n on Envtl. Quality v. EPA, 790 F.3d 138, 150,
155-56 (D.C. Cir. 2015) (quoting City of Waukesha v. EPA,
320 F.3d 228, 247 (D.C. Cir. 2003)). For instance, in support
of its advice that EPA consider limiting median annual tree
growth loss to 2%, CASAC highlighted a study purporting to
show that stands of aspen that experienced ozone-induced
biomass loss of 2% annually had cumulative biomass loss of
21% over seven years, suggesting that even relatively minor
                              28
annual growth loss can lead to significant loss over longer
periods of time. J.A. 537. But CASAC had misread the study.
As EPA explained, the aspen in that study in fact experienced
more than 20% biomass loss annually, suggesting that their
cumulative biomass loss was not much worse than their loss in
any single year. 80 Fed. Reg. at 65,394.

     EPA also clarified that the only other report CASAC relied
on for its 2% target recommendation provided no explicit
rationale for selecting that level and did not identify any new
data in support of its recommendations. See id. at 65,394-95 &
n.200. While it is true that EPA has cited that same report
favorably in the past, see, e.g., National Ambient Air Quality
Standards for Ozone, 75 Fed. Reg. 2,938, 3,025 (Jan. 19, 2010),
that alone does not make EPA’s current assessment arbitrary.
EPA must simply provide a “reasoned explanation” for its
departure from its past position—which it has done. See
Encino Motorcars, LLC v. Navarro, 136 S. Ct. 2117, 2125-26
(2016); see also Mississippi, 744 F.3d at 1351.

           EPA did not adequately explain its decision to use
           a three-year average of the W126 index as the
           benchmark.

     The Environmental Petitioners raise two challenges
pertaining to the W126 index, both as a benchmark and as a
form and averaging time. First, they argue that EPA
impermissibly departed from CASAC’s advice by setting the
secondary standard level using a three-year average W126
benchmark without lowering the level to protect against single-
year exposures associated with median annual tree growth loss
of 6%, which CASAC had advised was “unacceptably high.”
J.A. 518. They also contend that EPA arbitrarily disregarded
CASAC’s advice to adopt the W126 index as the form and
                              29
averaging time for the secondary standard. We remand to EPA
on the first issue and do not reach the second.

     The Three-Year Average Benchmark. CASAC advised
EPA that basing the secondary standard on a “single-year
period” would provide “more protection for annual crops and
for the anticipated cumulative effects on perennial species”
than a three-year average. J.A. 518. It explained that EPA’s
proposal to use a three-year averaging period was “not
supported by the available data,” J.A. 536, and that if EPA
chose to “base the secondary standard on a three-year
averaging period,” then “the level of the standard should be
revised downward such that the level for the highest three-
month summation in any given year of the three-year period
would not exceed [its] scientifically recommended” range of
single-year, W126 exposure levels, J.A. 518. This was
necessary, CASAC explained, to “protect against single
unusually damaging years that will be obscured in the
average.” J.A. 536.

     EPA argues it gave effect to CASAC’s recommendation
by using a three-year average benchmark of 17 ppm-hrs,
which, after adjusting for the cottonwood data, was “somewhat
below” the 19 ppm-hrs associated with 6% annual growth loss
that CASAC had advised was “unacceptably high.” 80 Fed.
Reg. at 65,406-07 (quoting J.A. 518). But CASAC had advised
a maximum level associated with 5.2% annual biomass loss,
see J.A. 631, and it expressly cautioned that 6% median growth
loss in a single year was unacceptable, see J.A. 518. EPA’s use
of a benchmark that averages out to less than 6% biomass loss
over three years does not accord with CASAC’s advice.
Indeed, as commenters informed EPA during the rulemaking,
see J.A. 1836-40, EPA’s own air quality data suggests that
many large national parks and wilderness areas that have met
EPA’s chosen three-year average 17 ppm-hrs benchmark—
                              30
areas that Congress considers significant to the public welfare,
see 42 U.S.C. §§ 7470(2), 7472(a)—have meanwhile recorded
single-year W126 values at and above 19 ppm-hrs, which is
associated with “unacceptably high” annual biomass loss of
6% and higher. See J.A. 1061-64; 80 Fed. Reg. at 65,391. EPA
critiques that data as marred by outdated handling procedures,
but the agency acknowledged that other data derived through
updated procedures produced results “similar to” those
showing harmful exposure spikes. J.A. 1213. Critically, EPA
points us to no data or analysis (based on new or old
procedures) suggesting that the chosen benchmark prevents
single seasonal exposures of 19 ppm-hrs or higher. In short,
EPA has not demonstrated how its chosen benchmark protects
against “unusually damaging years that will be obscured in the
average.” J.A. 536.

     EPA alternatively defends its decision to use the three-
year-average benchmark as providing a focus on public welfare
effects of “potentially greater” significance than effects
“associated with a single year” of exposure. 80 Fed. Reg. at
65,404. This position, however, is inconsistent with EPA’s
other actions. In establishing the secondary standard, for
example, EPA heavily relied on data showing annual 6%
median tree growth loss at 19 ppm-hrs, see id. at 65,406, and
acknowledged the potential for a single season of high ozone
exposure to “alter biomass allocation and plant reproduction in
seasons subsequent to [that season’s] exposure,” thereby
leading to “a negative impact on species regeneration in
subsequent years,” id. at 65,371-72; see also J.A. 740-41. EPA
additionally recognized that “ozone effects in plants are
cumulative,” id. at 65,373 (quoting EPA, Integrated Science
Assessment 2-44 (2013)), meaning that the adverse vegetative
effects from single, high-ozone years are not offset by
subsequent low-ozone years. EPA has identified no contrary
                              31
evidence in the record demonstrating why these single-year
effects matter less than a three-year average.

     We therefore remand this issue for EPA to either lower the
standard to protect against unusually damaging cumulative
seasonal exposures that will be obscured in its three-year
average, or explain its conclusion that the unadjusted average
is an appropriate benchmark notwithstanding CASAC’s
contrary advice. Alternatively, EPA could adopt the single-
year W126 exposure index as the form and averaging time,
which would presumably moot any problems with the way it
translated that index to use as a benchmark.

     The Form and Averaging Time.              CASAC also
recommended that EPA use the single-year W126 index as the
form and averaging time for the secondary standard. J.A. 518.
EPA instead chose to retain the existing form and averaging
time—the three-year average of the fourth-highest daily
maximum eight-hour concentration. Adopting the W126 index
as the form and averaging time was unnecessary, EPA
explained, because the ozone exposure levels associated with
the existing form and averaging time and a three-year average
of the W126 index are “highly correlated,” especially at lower
levels, and “future control programs designed to help meet a
primary [ozone] standard based on the” current form and
averaging time should “provide similar improvements in terms
of the 3-year average of the annual W126 metric.” J.A. 1253;
see also 80 Fed Reg. at 65,400-01, 408-09.

     The Environmental Petitioners argue that EPA did not
justify its decision not to adopt the W126 index as the form and
averaging time. We lack any basis to assess the reasonableness
of EPA’s actions, however, because a critical piece of the
puzzle is missing. To review: EPA chose not to use the W126
index as the form and averaging time because it found that
                               32
ozone exposure levels associated with the existing form and
averaging time are “highly correlated” to a three-year average
of the W126 index. But, as discussed, EPA never explained
why it is reasonable to focus on a three-year average of the
W126 index in the first place. Therefore, we cannot assess the
relevance of the claim that the two are “highly correlated.”
EPA’s reconsideration on remand of the three-year averaging
issue should supply us with the information necessary to
resolve this question, or, if EPA chooses to follow CASAC’s
advice to lower the standard to control for unusually high single
years, potentially moot the Environmental Petitioners’ concern
that the current form tolerates even three-year average W126
levels higher than 17 ppm-hrs during periods when a 0.07 ppm,
8-hour level is met. Accordingly, we decline to reach this
question.

           EPA arbitrarily failed to identify a level of air
           quality requisite to protect against adverse effects
           from visible leaf injury.

     The Environmental Petitioners also fault EPA for deciding
not to specify a level of air quality to protect against adverse
welfare effects from ozone-induced visible leaf injury.
Because EPA failed to offer a reasoned explanation, we remand
for it to reconsider.

     EPA has found that the “strongest evidence for effects
from [ozone] exposure on vegetation is from controlled
exposure studies, which ‘have clearly shown that exposure to
[ozone] is causally linked to visible foliar injury.’” 80 Fed.
Reg. at 65,370 (quoting EPA, Integrated Science Assessment
1-15). Relying on that evidence, CASAC advised EPA of its
“scientific judgment” that “a level of 10 ppm-hrs is required to
reduce foliar injury.” J.A. 538. EPA nonetheless concluded
that there were too many “uncertainties and complexities” in
                                33
the evidence to specify a level of air quality to protect against
leaf injury. See 80 Fed. Reg. at 65,407-08. But the mere
invocation of “substantial uncertainty” is not a justification for
the agency’s failure to fulfill its statutory mandate. Motor
Vehicle Mfrs. Ass’n of U.S., Inc. v. State Farm Mut. Auto. Ins.
Co., 463 U.S. 29, 52 (1983) (internal quotation marks omitted).
EPA “must explain the evidence which is available, and must
offer a ‘rational connection between the facts found and the
choice made.’” Id. (quoting Burlington Truck Lines, Inc. v.
United States, 371 U.S. 156, 168 (1962)). Where CASAC has
“reached a scientific conclusion that adverse [welfare] effects
[are] likely to occur,” EPA must “explain why the evidence on
which CASAC relied cannot support the degree of confidence
CASAC placed in it.” Mississippi, 744 F.3d at 1357; see also
Am. Farm Bureau Fed’n v. EPA, 559 F.3d 512, 529-30 (D.C.
Cir. 2009). We defer to EPA’s judgment that the available
evidence is too uncertain only when the agency reasonably
explains its decision, see State Farm, 463 U.S. at 51-53, and
EPA has failed to carry that burden here.

      None of the identified uncertainties justifies EPA’s
decision not to set a standard to protect against visible leaf
injury. For instance, EPA argues that it lacked criteria for
assessing the welfare effects of visible leaf injury, but that does
not square with its conclusion in the Integrated Science
Assessment (compiling all available scientific criteria) that
visible leaf injury “has been well characterized and
documented over several decades.” J.A. 985-86. EPA had at
its fingertips a wealth of new data regarding visible leaf injury,
including new controlled exposure studies, multiyear field
surveys, and biomonitoring data of ozone-induced visible leaf
injury on public lands. J.A. 749. Using that data, EPA
analyzed how visible leaf injury affects “ecosystem services”
like aesthetic value and recreation on “Class I” lands—lands
that “have particular public welfare significance.” J.A. 749-50,
                               34
1036-44; see also 42 U.S.C. §§ 7470(2), 7472(a). CASAC
pointed out that those same ecosystem services are relevant in
identifying “damage that is adverse to public welfare.” J.A.
532-33.

     CASAC concluded, based on the same data available to
EPA, that “a level of 10 ppm-hrs is required to reduce foliar
injury.” J.A. 538. EPA counters that “decreases in leaf injury
[also occur] with decreasing ozone exposures across a range of
values well above 10 ppm-hrs,” and that CASAC
recommended a range of ozone levels that included values
above 10 ppm-hrs. See EPA Br. 89; 80 Fed. Reg. at 65,395-
96. Be that as it may, EPA has not explained why, unlike
CASAC, it was unable to choose a level at all. Indeed,
“[b]ecause the EPA failed to identify any target level, we need
not decide whether it was reasonable for the agency to reject
the target recommended by . . . CASAC.” Am. Farm Bureau,
559 F.3d at 530.

     EPA objects that it only found that visible leaf injury has
the “potential to be adverse to the public welfare,” and that the
Clean Air Act does not require EPA “to identify a precise,
quantified level of public welfare protection for every
potentially adverse public welfare impact” it considers when
revising the secondary standard. EPA Br. 87-88 (first quoting
80 Fed. Reg. at 65,388). But EPA has failed to justify its
inaction where CASAC unequivocally found that “damage to
resource use from foliar injury” was an “adverse welfare
effect.” J.A. 518. In light of CASAC’s “scientific conclusion
that adverse [welfare] effects were likely to occur” from visible
leaf injury, EPA’s inaction in the face of such effects is
“unacceptable.” Mississippi, 744 F.3d at 1357. Nor was it
sufficient for EPA simply to conclude that the standard set to
protect against tree growth loss sufficed because it provided
“additional” protection against leaf injury. 80 Fed. Reg. at
                               35
65,407-08. Nothing in the record suggests that tree growth loss
is a surrogate for leaf injury. EPA’s assurance that the standard
set to protect against tree growth loss will provide
“incidental[]” protection against visible leaf injury “cannot
save its decision.” Am. Farm Bureau, 559 F.3d at 529-30.

     Other purported uncertainties do not support EPA’s
inaction. For instance, EPA asserts that the Administrator
“lacked evidence that would allow her to measure the
relationship between leaf injury and other vegetation effects
that she might find adverse,” EPA Br. 87, but EPA and CASAC
both determined that visible leaf injury itself can “impact the
public welfare” by harming “aesthetic value and outdoor
recreation” in public lands, 80 Fed. Reg. at 65,379; J.A. 533.
Visible leaf injury impairs a variety of outdoor activities,
including “scenic viewing, wildlife watching, hiking, and
camping, that are of significance to the public welfare” and
generate “millions of dollars in economic value” annually. 80
Fed. Reg. at 65,381; J.A. 749. Much of the documented visible
leaf injury is in Class I areas, see 80 Fed. Reg. at 65,378, the
preservation and enjoyment of which is a “clear public
interest,” id. at 65,377 (quoting 73 Fed. Reg. at 16,496). Those
interests are manifest whether or not the relationship between
leaf injury and other vegetative effects has been well
characterized.

      This case is a far cry from Center for Biological Diversity
v. EPA, where we upheld EPA’s determination that the
available data was too uncertain to support setting a standard
for acid rain precursors. 749 F.3d 1079 (D.C. Cir. 2014). In
the administrative record in that case, EPA “explained in great
detail” why the scientific uncertainties were so “unusually
profound” that EPA “could not form” a reasoned judgment as
to a requisite level of protection. Id. at 1088, 1090-91. CASAC
concurred with that assessment. Id. at 1086 n.11. Here, in
                               36
contrast, CASAC expressed its scientific judgment that a target
level of 10 ppm-hrs was “required to reduce” leaf injury. J.A.
538. EPA’s failure to explain why it could not accept data that
CASAC deemed informative falls short of reasoned
decisionmaking.

B. Industry Petitioners’ Challenge

     The Industry Petitioners glancingly claim that EPA failed
to explain why, when faced with the same scientific evidence
that was available in 2008, it revised the secondary standard
downward. As we have explained in reference to the primary
standard, it is a “conceptual error” to assume that “EPA is
somehow bound by” a prior standard in a subsequent review.
Mississippi, 844 F.3d at 1344. What is more, EPA did in fact
rely on new scientific evidence, including more than 400 new
studies and various new analyses of existing data, which
“strengthen[ed] [its] confidence” that the 2008 secondary
standard was no longer as demanding as was “requisite” to
protect the public welfare. See 80 Fed. Reg. at 65,369-70,
65,384. “[A]dditional certainty . . . that the line marked by the
term ‘requisite’ has shifted” is a reasonable—if not
paradigmatic—basis for EPA to revise its prior standards.
Mississippi, 844 F.3d at 1344.

          IV. Cross-Cutting NAAQS Challenges

     We now turn to the arguments of State and Industry
Petitioners that EPA failed to take into account all the factors
required by law when setting the primary and secondary
standards. First, they fault EPA for not considering the “overall
adverse economic, social, and energy impacts” of the revised
NAAQS. Second, they argue that the Clean Air Act requires
EPA to consider the impact of background ozone on the ability
of states to attain the revised NAAQS. And, lastly, they argue
that EPA’s interpretation of the Act, which excluded
                                37
consideration of background ozone, leaves no “intelligible
principle” by which to set NAAQS, creating a constitutional
nondelegation issue. None of these arguments has merit.

A. Adverse Impacts

     According to section 109(b) of the Clean Air Act, EPA
must set the NAAQS at a level “requisite to protect the public
health” and “the public welfare.” 42 U.S.C. § 7409(b). The
NAAQS should be revised periodically “as may be
appropriate.” Id. § 7409(d)(1). Industry Petitioners invoke the
term “appropriate” and claim that “an evaluation of
‘appropriateness’ must take into account the adverse
socioeconomic and energy impacts” of the revised NAAQS.
Indus. Pet’rs Br. 33. To bolster this argument, they point to
Michigan v. EPA, which held that the phrase “appropriate and
necessary” in section 112(n) of the Act—governing power
plants’ hazardous air pollution—“requires at least some
attention to cost” because “‘appropriate’ is ‘the classic broad
and all-encompassing term that naturally and traditionally
includes consideration of all the relevant factors.’” 135 S. Ct.
at 2707 (quoting White Stallion Energy Ctr., LLC v. EPA, 748
F.3d 1222, 1266 (D.C. Cir. 2014) (Kavanaugh, J., concurring
in part and dissenting in part)).

     In Whitman, the Supreme Court considered a similar
argument regarding NAAQS-setting, with challengers
asserting that the words “requisite” and “adequate” in section
109(b)(1) meant that EPA must consider the economic costs of
implementing revised NAAQS. 531 U.S. at 468. The Whitman
Court rejected this reading, finding that the plain text of the Act
“unambiguously bars cost considerations from the NAAQS-
setting process.” Id. at 471. The text of the Act refers only to
the “public health,” and though it uses the terms “requisite” and
“adequate,” the Whitman Court found it “implausible that
                              38
Congress would give to the EPA through these modest words
the power to determine whether implementation costs should
moderate national air quality standards,” especially given that
other Clean Air Act provisions “explicitly permit[] or require[]
economic costs to be taken into account in implementing
[other] air quality standards.” Id. at 467-68.

     According to Whitman, the Act commands EPA to follow
a specific process when setting primary NAAQS: “[I]dentify
the maximum airborne concentration of a pollutant that the
public health can tolerate, decrease the concentration to
provide an ‘adequate’ margin of safety, and set the standard at
that level.” Id. at 465. Industry Petitioners argue that this
account of the NAAQS-setting process is incomplete, and that
consideration of other “adverse economic, social, and energy
impacts” is required. But at bottom this is the same argument
rejected in Whitman, with the “costs” of the revised and more
stringent NAAQS merely reframed as “impacts.” In listing
those impacts, Industry Petitioners emphasize that stricter
standards could reduce gross domestic product, drive up energy
costs, “stymie economic growth by forcing the early retirement
of facilities unable to implement controls, contributing to job
losses[,] discourag[e] existing businesses from expanding in
nonattainment regions[,] and driv[e] away potential new
investments,” Indus. Pet’rs Br. 35, but these “impacts” are no
different than the “economic costs” that the petitioners in
Whitman worried “might produce health losses sufficient to
offset the health gains achieved in cleaning the air—for
example, by closing down whole industries and thereby
impoverishing the workers and consumers dependent upon
those industries,” 531 U.S. at 466. Whitman forbids EPA from
taking these considerations into account, however
denominated.
                              39
     As for Petitioners’ reliance on Michigan and the Act’s use
of the word “appropriate,” that argument fails twice over. We
have already rejected the idea that “appropriate” in section
109(d) requires consideration of economic costs. Am. Trucking
Ass’ns, Inc. v. EPA, 175 F.3d 1027, 1040-41 (D.C. Cir. 1999).
Further, Michigan involved a different provision of the Clean
Air Act, and the Court was careful to emphasize that its reading
of “appropriate” was dependent on the statutory context,
explaining that “[t]here are undoubtedly settings in which the
phrase ‘appropriate and necessary’ does not encompass cost.”
135 S. Ct. at 2707. Indeed, Michigan explicitly distinguished
section 112(n) from section 109(b)(1), explaining yet again that
the criteria for setting NAAQS “do[] not encompass cost.” Id.
at 2709.

     Industry Petitioners also point to section 109(d)(2)(C) of
the Act, which requires CASAC to advise EPA “of any adverse
public health, welfare, social, economic, or energy effects
which may result from various strategies for attainment and
maintenance” of revised NAAQS. 42 U.S.C. § 7409(d)(2)(C).
According to Petitioners, the fact that CASAC is required to
supply information to EPA about the “social, economic, or
energy effects” of the revised NAAQS implies that EPA is
obliged to consider that information in setting the NAAQS.
That argument, however, was also raised and rejected in
Whitman, where the Supreme Court concluded that this
provision was intended to “enable the [EPA] to assist the States
in carrying out their statutory role as primary implementers of
the NAAQS,” but had “no bearing upon whether cost
considerations are to be taken into account in formulating the
[NAAQS].” 531 U.S. at 470-71 (second emphasis added).

    Petitioners’ remaining arguments rely heavily on Justice
Breyer’s concurrence in Whitman, in which he urged that the
Act “does not require the EPA to eliminate every health risk,
                               40
however slight, at any economic cost, however great, to the
point of hurtling industry over the brink of ruin, or even forcing
deindustrialization.” Id. at 494 (Breyer, J., concurring in part
and concurring in the judgment) (internal quotation marks
omitted). But the concurrence does not govern our decision,
and in any event, Justice Breyer agreed with the majority that
economic costs could not be considered in setting NAAQS. Id.
at 490.

B. Background Ozone

     Under the Clean Air Act, “background ozone” is ozone
“that would exist in the absence of any man-made emissions
inside the U.S.” 80 Fed. Reg. at 65,436. This includes ozone
generated by both natural sources anywhere (e.g., a wildfire)
and foreign man-made sources (e.g., a factory in Mexico). See
id. According to the Industry and State Petitioners, EPA failed
to take into account background ozone when setting the new
NAAQS. As a result, certain areas of the country with high
levels of background ozone fail right off the bat to meet the
new standards, which is unlawful, Petitioners argue, because
the Act requires EPA to set NAAQS that are attainable.
Therefore, they contend EPA’s failure to consider background
ozone when revising the NAAQS was arbitrary and capricious.

     The text of the Act forecloses this argument. Section
109(b) directs EPA to set NAAQS “requisite to protect the
public health” and “the public welfare.” 42 U.S.C. § 7409(b).
Accepting Petitioners’ argument would mean that, if the level
of background ozone in any part of the country exceeds the
level of ozone that is “requisite to protect the public health,”
EPA must set the NAAQS at the higher, unhealthy level. The
statutory text leaves no room for this hidden caveat: “[W]hen
Congress directs an agency to consider only certain factors in
reaching an administrative decision, the agency is not free to
                               41
trespass beyond the bounds of its statutory authority by taking
other factors into account.” Lead Indus., 647 F.2d at 1150.

     Petitioners argue that states cannot be required to do the
impossible, and the presence of background ozone will make it
impossible to achieve attainment. But Congress recognized the
possibility that some states could not achieve attainment
because of the presence of background ozone and, rather than
watering down the nationally applicable standards, allowed
EPA to relax enforcement on a case-by-case basis. When
ozone exceedances are caused by events beyond a state’s
control, three enforcement exceptions in the Act allow the state
some leeway as a practical matter: A showing of an
“Exceptional Event” may allow a state to avoid a
nonattainment designation. See 42 U.S.C. § 7619(b) (defining
“exceptional event” as one caused by “human activity that is
unlikely to recur at a particular location” or “a natural event,”
and that “affects air quality” but is “not reasonably controllable
or preventable”). And a state that meets the requirements of
either the “International Transport” or “Rural Transport”
provision may avoid having its state implementation plan (SIP)
rejected due to the exceedance. See id. §§ 7509a(a)-(b)
(directing EPA to approve a SIP if attainment is not met due to
“emissions emanating from outside of the United States”),
7511a(h) (excluding nonattainment regions that themselves
“do not make a significant contribution” to ozone levels, so
long as they are not within or next to a “Metropolitan Statistical
Area”). These provisions make little sense under Petitioners’
reading of the Act. And even if, as the states claim, it is more
difficult to meet the terms of these exceptions than EPA asserts,
State Pet’rs Br. 34-44, the fact remains that Congress decided
that EPA should account for background ozone during
enforcement, not when setting standards.
                                42
     We rejected a version of Petitioners’ argument in
American Petroleum Institute v. Costle, 665 F.2d 1176 (D.C.
Cir. 1981). There, the City of Houston urged us to find that
EPA’s primary ozone standard was arbitrary because “natural
factors ma[de] attainment impossible” for that City. Id. at
1185. We emphasized that “Congress [was] aware that some
regions [were] having difficulty in meeting the national
standard,” and had responded not by requiring EPA to ease the
NAAQS but by developing a distinct program for
nonattainment areas that gives states resources to bring those
areas into compliance while also protecting the public health
and welfare. Id. at 1185-86. Industry Petitioners argue that
Costle, a case about one city, is not binding when “numerous
areas of the country cannot attain the NAAQS due to
background levels” of ozone. Indus. Pet’rs Br. 26-27
(emphasis added). But neither our precedent nor the Act allows
for such a distinction. Indeed, in Costle, we rejected the
argument that EPA had ignored comments related to the issue
of whether “attainment of the proposed standards would be
precluded in most areas of the nation by natural background
levels of ozone.” 665 F.2d at 1190 (emphasis added) (internal
quotation marks omitted). Simply put, “the question of
attainability is not relevant to the setting of ambient air quality
standards under the Clean Air Act.” Id.

     State Petitioners argue that Costle is distinguishable
because the petitioner there was not a state, and several parts of
the Act place the burden to attain NAAQS on states rather than
cities. Indus. Pet’rs Br. 25-26 (citing 42 U.S.C. §§ 7407(a),
7410(a)(2)(C)); State Pet’rs Br. 32-33 (citing 42 U.S.C.
§ 7407(a)). True, if Houston cannot attain NAAQS due to
background ozone, the Act places the burden on Texas, not the
City, to identify a plan to bring the area into attainment. But
Costle was not limited to its particular facts—it relied instead
on the premise that “[a]ttainability and technological feasibility
                               43
are not relevant considerations in the promulgation of
[NAAQS],” 665 F.2d at 1185, a premise that does not change
whether it is a state or city arguing that the NAAQS are
unattainable. Congress intended NAAQS to be national
ambient air quality standards, and EPA is not required to “tailor
national regulations to fit each region or locale.” Id. Because
the Clean Air Act prohibits EPA from adjusting for background
ozone in setting the NAAQS, EPA did not act unlawfully or
arbitrarily and capriciously in setting the NAAQS without
regard for background ozone.

C. Nondelegation

     Finally, State Petitioners argue that EPA’s interpretation
of the Clean Air Act runs afoul of the Constitution’s limit on
the authority of Congress to delegate lawmaking power to an
agency. “Congress generally cannot delegate its legislative
power to another Branch” unless it provides an “intelligible
principle” for the agency to follow in exercising that delegated
power. Mistretta v. United States, 488 U.S. 361, 372 (1989)
(second quoting J.W. Hampton, Jr., & Co. v. United States, 276
U.S. 394, 409 (1928)). We refer to this as the nondelegation
doctrine. Id.

     State Petitioners do not argue that the Act lacks an
intelligible principle. Nor could they, as the Supreme Court
held in Whitman that Congress provided one when it directed
EPA to set NAAQS “requisite to protect public health”—
meaning “sufficient, but not more than necessary.” 531 U.S. at
473-74 (internal quotation marks omitted). Instead, Petitioners
claim that EPA’s interpretation of the Act has created a
nondelegation problem by “ignor[ing] the ‘intelligible’
principle[] that Congress provided.” State Pet’rs Br. 45. But
in a nondelegation challenge, “the constitutional question is
whether the statute has delegated legislative power to the
                                44
agency.” Whitman, 531 U.S. at 472 (emphasis added). EPA
cannot alter the text of the Clean Air Act, so EPA’s
interpretation of the Act cannot alter whether the Act included
an intelligible principle. Cf. id. at 473 (“Whether the statute
delegates legislative power is a question for the courts, and an
agency’s [interpretation] has no bearing upon the answer.”).
There is no nondelegation issue here.

                V. Grandfathering Challenge

     Finally, we address EPA’s decision to allow those who
completed applications for preconstruction permits before the
2015 Rule was adopted to demonstrate compliance with the
previous NAAQS rather than the new, more stringent
standards. See 80 Fed. Reg. at 65,431-35; 40 C.F.R.
§§ 51.166(i)(11), 52.21(i)(12). The Environmental Petitioners
argue that this “grandfathering” provision violates the Clean
Air Act, which requires permit applicants to demonstrate
compliance with “any” NAAQS regardless of when their
application was completed. See Envtl. Pet’rs Br. 57-62. EPA
argues that the Act is ambiguous with respect to the treatment
of permit applications pending at the time that new NAAQS
are adopted, and that the grandfathering provision is lawful as
a reasonable interpretation of the Act. We find no such
ambiguity.

    Under section 165(a) of the Act, before beginning
construction of any “major emitting facility,” the owner or
operator must demonstrate “that emissions from construction
or operation of such facility will not cause, or contribute to, air
pollution in excess of any . . . national ambient air quality
standard.” 42 U.S.C. § 7475(a). This “demonstration
requirement” is part of EPA’s PSD program. Generally, EPA
requires permit applicants to demonstrate compliance with the
NAAQS in effect at the time a permit is issued. 80 Fed. Reg.
                               45
at 65,433; EPA Br. 130. The revised ozone NAAQS were
effective on December 28, 2015, so under the usual rule no
permit could be granted for the construction of a major emitting
facility after that date unless the operator demonstrated that the
facility would not cause or contribute to ozone levels in excess
of the revised NAAQS. But when EPA issued the 2015 Rule,
it waived that requirement for projects that either (1) had
complete permit applications as of October 1, 2015, or (2) had
a draft permit or preliminary determination publicly noticed
before December 28, 2015. 1 See 80 Fed. Reg. at 65,460
(codified at 40 C.F.R. §§ 51.166(i)(11), 52.21(i)(12)).

     In the 2015 Rule, EPA asserted the need for this
grandfathering provision because section 165(c) of the Act
instructs EPA and state permitting authorities that “[a]ny
completed permit application . . . shall be granted or denied not
later than one year after the date of filing such completed
application.” 42 U.S.C. § 7475(c). As EPA sees it, requiring
applicants with already-completed permit applications to
demonstrate compliance with the newly revised NAAQS
“could hinder compliance with the requirement under
section 165(c) to issue a permit within one year.” 80 Fed. Reg.
at 65,433-34. According to EPA, the Act does not “clearly
address” how to handle such a situation, therefore it was
“permissible under the discretion provided by the [Act] for the
EPA to craft a reasonable implementation regulation that
balances competing objectives of the statutory PSD program.”
Id. at 65,433.

   We evaluate EPA’s interpretation of the Act under
Chevron. “If the Act unambiguously authorizes or forecloses
EPA’s . . . rule, step one of the Chevron analysis requires that

    1
      The second type of waiver was necessary because not all
agencies that issue preconstruction permits issue formal
completeness determinations. See 80 Fed. Reg. at 65,432-33.
                              46
we follow Congress’s express policy choice. If the Act is
unclear on the matter, step two of Chevron requires that we
defer to EPA’s reasonable interpretation.” Sierra Club v. EPA,
536 F.3d 673, 677 (D.C. Cir. 2008) (citing Chevron, 467 U.S.
at 842-43). We hold, under step one of Chevron, that the Act
unambiguously      precludes    EPA’s      interpretation   of
section 165(a) and vacate the grandfathering provision of the
Final Rule.

     The revised NAAQS were effective on December 28,
2015. As a result of the grandfathering provision, a major
emitting facility that demonstrates compliance with the
previous NAAQS of 0.075 ppm, but not the revised NAAQS
of 0.07 ppm, can still be built even after the new NAAQS take
effect, provided the project had completed its application
within certain time limits. That is exactly what the plain text
of the Act forbids: the “construction” of a “major emitting
facility” with emissions that will “cause, or contribute to, air
pollution in excess of any . . . [NAAQS].” 42 U.S.C. § 7475(a)
(emphasis added). “Read naturally, the word ‘any’ has an
expansive meaning, that is ‘one or some indiscriminately of
whatever kind.’” United States v. Gonzalez, 520 U.S. 1, 5
(1997) (quoting WEBSTER’S THIRD NEW INTERNATIONAL
DICTIONARY 97 (1976)). EPA gives no reason why, in the
context of section 165(a), the word “any” should not carry its
natural meaning and hence refer to the newly revised NAAQS
once they have become effective.

    In New York v. EPA, we observed that the meaning of
“any” can differ if, for instance, the “Supreme Court has
required heightened standards of clarity to avoid upsetting
fundamental policies,” or if there would be “‘strange and
indeterminate results’ that would emerge from adopting the
natural meaning of ‘any’” in a statute. 443 F.3d 880, 885-86
(D.C. Cir. 2006) (quoting Nixon v. Mo. Mun. League, 541 U.S.
                               47
125, 132-33 (2004)). But none of those factors is present here.
Indeed, NAAQS are set based on the “maximum airborne
concentration of a pollutant that the public health can tolerate,”
Whitman, 531 U.S. at 465, so the “strange” result would be if
the Act, focused as it is on the protection of public health, left
the door open for construction projects that will cause or
contribute to ozone levels higher than the “public health can
tolerate.” Thus, we read section 165(a) to forbid construction
of any facility that cannot demonstrate compliance with “any”
effective NAAQS.

    There is no ambiguity created by reading section 165(a)
alongside the timeliness requirement of section 165(c). EPA
thought the grandfathering provision necessary to allow
permitting authorities to “issue a permit within one year.” 80
Fed. Reg. at 65,433-34. But the Act does not require that a
permit be “issued” within one year. It requires only that the
permitting authority “grant[] or den[y]” completed permit
applications within one year. 42 U.S.C. § 7475(c) (emphasis
added). “[N]othing in the [Act] provides for issuance of a []
permit as a matter of right.” Am. Corn Growers Ass’n v. EPA,
291 F.3d 1, 12 (D.C. Cir. 2002) (internal quotation marks
omitted). If a permit applicant has not shown that it can meet
the new NAAQS, EPA or a state permitting authority can
comply with the timeliness requirement of section 165(c) by
denying the application.

    This reading is confirmed by the structure of the Act.
When amending the Act to add section 165(a)’s demonstration
requirement, Congress expressly exempted projects that had
begun construction prior to passage of that amendment. See 42
U.S.C. § 7475(a); Pub. L. No. 95-95, § 127(a), 91 Stat. 685,
735-39 (Aug. 7, 1977). The inclusion of this grandfather clause
implies that Congress did not intend EPA to have some
inherent grandfathering authority, and that, in the future,
                              48
NAAQS would be enforced as enacted. See Andrus v. Glover
Constr. Co., 446 U.S. 608, 616-17 (1980) (“Where Congress
explicitly enumerates exceptions to a general prohibition,
additional exceptions are not to be implied, in the absence of
evidence of a contrary legislative intent.”). Congress has
spoken on this question, and EPA cannot displace the statutory
determination simply because the agency’s “preferred
approach [might] be better policy.” See Engine Mfrs. Ass’n v.
EPA, 88 F.3d 1075, 1089 (D.C. Cir. 1996). Nor does it matter
that EPA has evidently relied on this grandfathering authority
in the past. See, e.g., National Ambient Air Quality Standards
for Particulate Matter, 78 Fed. Reg. 3,086, 3,258-59 (Jan. 15,
2013) (adopting grandfathering provision for revised
NAAQS); Regulations for Implementing Revised Particulate
Matter Standards, 52 Fed. Reg. 24,672, 24,683 (July 1, 1987)
(same). “[P]revious statutory violations cannot excuse the one
now before the court.” New Jersey v. EPA, 517 F.3d 574, 583
(D.C. Cir. 2008).

     The Ninth Circuit addressed a related issue in Sierra Club
v. EPA, 762 F.3d 971 (9th Cir. 2014). EPA had failed to grant
or deny a completed permit application within one year as
required by section 165(c), and in the meantime the agency had
lowered the relevant NAAQS. See id. at 974-75. After the
applicant filed suit to compel issuance of the permit under the
old standards, EPA initially took the position—contrary to its
stance here, but consistent with our holding—that the Act
prohibited it “from granting the Permit unless [the applicant]
complies with the superseding standards.” Id. at 975. Then,
after a “policy review,” EPA changed its mind and asserted
“inherent grandfathering authority,” id., relying on the alleged
tension that it advances here between sections 165(a) and (c),
see id. at 978. The Ninth Circuit discerned no such tension and
held that, under Chevron step one, EPA must “apply the
                               49
regulations in effect at the time of the permitting decision.” Id.
at 979.

     After concluding that EPA had violated the plain terms of
the Act, the Sierra Club court added in dicta that the agency
could create a grandfathering exception through “formal notice
and comment rulemaking.” Id. at 982. The Ninth Circuit cited
42 U.S.C. § 7601(a), which includes a general grant of
authority to the EPA Administrator to “prescribe such
regulations as are necessary” to carry out the Act. EPA argues
that we should follow the Ninth Circuit’s dicta because EPA
used formal rulemaking to grandfather these applications. But
we decline to do so. Dicta is never binding on any court, Glus
v. Brooklyn E. Dist. Terminal, 359 U.S. 231, 235 (1959), nor is
it persuasive here, because it is fundamentally incorrect. A
general grant of authority cannot displace the clear, specific
text of the Act. Air All. Hous. v. EPA, 906 F.3d 1049, 1061
(D.C. Cir. 2018) (“[A]n agency may not circumvent specific
statutory limits on its actions by relying on separate, general
rulemaking authority.”). As the Sierra Club court recognized,
the Act requires compliance with the NAAQS that are in effect
“at the time of the permitting decision.” 762 F.3d at 979. EPA
has no authority to change that provision of the Act, whether
by ad hoc waiver or rulemaking. See also Citizens to Save
Spencer Cty. v. EPA, 600 F.2d 844, 873 (D.C. Cir. 1979)
(explaining that a general power cannot “provide the [EPA]
Administrator with Carte blanche authority to promulgate any
rules, on any matter relating to the [Act], in any matter that the
Administrator wishes”).

     The grandfathering provision of the 2015 Rule, as codified
at 40 C.F.R. §§ 51.166(i)(11) and 52.21(i)(12), contradicts
Congress’s “express policy choice” not to allow construction
which will “cause or contribute to” nonattainment of “any”
effective NAAQS, regardless of when they are adopted or
                               50
when a permit was completed. Accordingly, we grant the
petition for review with respect to the grandfathering provision,
and vacate that portion of the Rule.

                           *   *    *

     In setting the secondary standard, EPA failed to justify its
decision to use a three-year average benchmark without
lowering the level to account for single-year spikes in ozone
exposures, and it arbitrarily declined to set a level to protect
against adverse welfare effects associated with visible leaf
injury. EPA also impermissibly allowed sources that had
completed applications for preconstruction permits before the
2015 Rule was adopted to demonstrate compliance with the
previous NAAQS rather than the new, more stringent primary
and secondary standards. Accordingly, we grant those portions
of the Environmental Petitioners’ petition, vacate the
grandfathering provision, and remand to EPA for
reconsideration of the secondary standard.

                                                    So ordered.
