United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued November 16, 2012 Decided July 23, 2013

Reissued December 11, 2013

No. 08-1200

STATE OF MISSISSIPPI,

PETITIONER

v.

ENVIRONMENTAL PROTECTION AGENCY,

RESPONDENT

COUNTY OF NASSAU, ET AL.,

INTERVENORS

Consolidated with 08-1202, 08-1203, 08-1204, 08-1206

On Petition for Review of a Final Rule Issued 

by the United States Environmental Protection Agency

F. William Brownell argued the cause for State of 

Mississippi and Industry Petitioners. With him on the briefs 

were Allison D. Wood, Lucinda Minton Langworthy, Harold 

E. Pizzetta, III, Special Assistant Attorney General, Office of 

the Attorney General for the State of Mississippi, Robert R. 

Gasaway, Jeffrey Bossert Clark, and William H. Burgess. 
2

David S. Baron argued the cause for Environmental 

Petitioners. With him on the briefs was Seth L. Johnson. 

Michael J. Myers, Assistant Attorney General, Office of 

the Attorney General for the State of New York, argued the 

cause for State Petitioners. With him on the briefs were Eric 

T. Schneiderman, Attorney General, Barbara D. Underwood, 

Solicitor General, Morgan A. Costello, Assistant Attorney 

General, Kamala D. Harris, Attorney General, Office of the 

Attorney General for the State of California, Nicholas Stern, 

Deputy Attorney General, George Jepsen, Attorney General, 

Office of the Attorney General for the State of Connecticut, 

Kimberly P. Massicotte and Scott Koshwitz, Assistant 

Attorneys General, Joseph R. Biden, III, Attorney General, 

Office of the Attorney General for the State of Delaware, 

Valerie M. Satterfield, Deputy Attorney General, Lisa 

Madigan, Attorney General, Office of the Attorney General 

for the State of Illinois, Gerald T. Karr, Senior Assistant 

Attorney General, William J. Schneider, Attorney General, 

Office of the Attorney General for the State of Maine, Gerald 

D. Reid, Assistant Attorney General, Douglas F. Gansler, 

Attorney General, Office of the Attorney General for the State 

of Maryland, Roberta R. James, Assistant Attorney General, 

Martha Coakley, Attorney General, Office of the Attorney 

General for the Commonwealth of Massachusetts, Carol 

Iancu, Assistant Attorney General, Michael A. Delaney, 

Attorney General, Office of the Attorney General for the State 

of New Hampshire, K. Allen Brooks, Assistant Attorney 

General, Gary K. King, Attorney General, Office of the 

Attorney General for the State of New Mexico, Stephen R. 

Farris, Assistant Attorney General, Ellen F. Rosenblum, 

Attorney General, Office of the Attorney General for the State 

of Oregon, Paul S. Logan, Assistant Attorney-in-Charge, 

Peter Kilmartin, Attorney General, Office of the Attorney 

General for the State of Rhode Island, Gregory S. Schultz, 

Special Assistant Attorney General, Irvin B. Nathan, Attorney 
3

General, Office of the Attorney General for the District of 

Columbia, Amy E. McDonnell, Deputy General Counsel, and 

Christopher King. William L. Pardee, Assistant Attorney 

General, Office of the Attorney General for the 

Commonwealth of Massachusetts at the time the brief was 

filed, Tricia K. Jedele, Special Assistant Attorney General, 

Office of the Attorney General for the State of Rhode at the

time the brief was filed, Maureen Smith, Senior Assistant 

Attorney General, Office of the Attorney General for the State 

of Rhode Island, Donna M. Murasky, Deputy Solicitor, 

Office of the Attorney General for the District of Columbia, 

and Katherine Kennedy entered appearances. 

Richard A. Wegman and Harold G. Bailey, Jr. were on 

the brief for amicus curiae Province of Ontario in support of 

petitioners State of New York, et al. and petitioners American 

Lung Association, et al.

Madeline Fleisher, Attorney, and David J. Kaplan, 

Senior Attorney, U.S. Department of Justice, argued the 

causes for respondent. With them on the brief was John 

Hannon, Attorney, U.S. Environmental Protection Agency.

David S. Baron and Seth L. Johnson were on the brief for 

Environmental Intervenors. 

F. William Brownell, Allison D. Wood, Lucinda Minton 

Langworthy, Robert R. Gasaway, Jeffrey Bossert Clark, and 

William H. Burgess were on the brief for industry intervenorrespondents Ozone NAAQS Litigation Group, et al. in 

support of respondent. Duane J. Desiderio entered an 

appearance.

Before: TATEL, BROWN, and GRIFFITH, Circuit Judges.
4

Opinion for the Court filed PER CURIAM.

PER CURIAM: In this opinion, we consider several 

challenges to the Environmental Protection Agency’s most 

recent 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. 

I.

The Clean Air Act directs EPA to establish and 

periodically review and revise primary and secondary 

National Ambient Air Quality Standards (“NAAQS”) for 

certain pollutants the “emissions of which . . . cause or 

contribute to air pollution which may reasonably be 

anticipated to endanger public health or welfare.” 42 U.S.C. 

§ 7408(a)(1)(A). Under section 109(b)(1), 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). Under section 109(b)(2), 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). The Act provides that the public welfare

protected by secondary NAAQS includes “effects on soils, 

water, crops, vegetation, manmade materials, animals, 

wildlife, weather, visibility, and climate, damage to and 

deterioration of property, and hazards to transportation, as 

well as effects on economic values and on personal comfort 

and well-being.” Id. § 7602(h). 
5

“Once EPA establishes NAAQS for a particular 

pollutant, the standards become the centerpiece of a complex 

statutory regime aimed at reducing the pollutant’s 

atmospheric concentration.” American Trucking Ass’ns v. 

EPA (“ATA III”), 283 F.3d 355, 358–59 (D.C. Cir. 2002).

EPA must “complete a thorough review” of each NAAQS at 

five-year intervals and “make such revisions . . . as may be 

appropriate.” 42 U.S.C. § 7409(d)(1). Pursuant to section

109(d)(2), the Clean Air Scientific Advisory Committee 

(“CASAC”) must periodically review NAAQS and 

“recommend to [EPA] any new [NAAQS] and revisions of 

existing criteria and standards as may be appropriate.” Id. 

§ 7409(d)(2)(A)–(B). In proposing to issue new NAAQS or 

revise existing ones, EPA must “set forth or summarize and 

provide a reference to any pertinent findings, 

recommendations, and comments by [CASAC]” and explain 

the reasons for any “important” divergences from CASAC’s 

recommendations. Id. § 7607(d)(3), (6).

These consolidated cases concern the NAAQS for ozone

(O3). Ozone is a colorless, odorless gas that is not a direct 

product of human activity but instead forms when other 

atmospheric pollutants react in the presence of sunlight. ATA 

III, 283 F.3d at 359. EPA has identified several health effects 

linked to ozone, including decreased lung function and 

respiratory symptoms. Proposed National Ambient Air 

Quality Standards for Ozone (“2007 Proposed Rule”), 72 Fed. 

Reg. 37,818, 37,827 (July 11, 2007). EPA has also found that 

ozone is associated with more serious health effects such as 

increased asthma medication use, emergency department 

visits, and hospital admissions. See id. at 37,827–29, 37,832. 

Furthermore, EPA has determined that ozone has a broad 

array of effects on trees, vegetation, and crops and can 

indirectly affect other ecosystem components such as soil, 

water, and wildlife. Id. at 37,883.
6

EPA last revised the ozone NAAQS in 1997, instituting

an “8-hour” primary standard—based on the annual fourthhighest daily maximum 8-hour average ozone 

concentration—of 0.08 parts per million. National Ambient 

Air Quality Standards for Ozone (“1997 Final Rule”), 62 Fed. 

Reg. 38,856, 38,873 (July 18, 1997). EPA also set the

secondary NAAQS to be identical to this primary standard in 

both form (measured over an 8-hour period) and level (0.08 

ppm). Id. at 38,877–78. In American Trucking Associations v. 

EPA, 175 F.3d 1027 (D.C. Cir. 1999), several parties 

challenged these revisions, as well as the NAAQS for 

particulate matter that EPA had issued at the same time. After 

the Supreme Court reversed this court’s conclusion that the 

Clean Air Act unconstitutionally delegated Congress’s 

legislative authority, see Whitman v. American Trucking 

Ass’ns, 531 U.S. 457, 473–76 (2001), we rejected all of 

petitioners’ challenges to both the primary and secondary 

ozone NAAQS. ATA III, 283 F.3d at 378–80.

EPA initiated the current review of the ozone NAAQS in 

2000. Proceeding under a schedule adopted by consent 

decree, and after receiving significant public comment on 

proposed changes, EPA issued revised primary and secondary 

standards on March 27, 2008. National Ambient Air Quality 

Standards for Ozone (“2008 Final Rule”), 73 Fed. Reg. 

16,436 (Mar. 27, 2008). In reaching its final decision, EPA

examined “the entire body of evidence relevant to examining 

associations between exposure to ambient O3 and a broad 

range of health endpoints.” Id. at 16,439. Of particular 

relevance here, EPA emphasized new clinical studies, 

including human exposure studies, showing respiratory 

effects at ozone levels below 0.08 ppm. Id. at 16,449–50, 

16,470–71. EPA also cited new epidemiological evidence 

suggesting associations between “serious morbidity 
7

outcomes” and ozone exposure at levels below 0.08 ppm, as 

well as risk assessments estimating the effects of various 

levels of ozone on the population. Id. at 16,446, 16,450–51, 

16,471–72. On the basis of this evidence, EPA concluded that 

the existing 0.08 ppm primary standard was not requisite to 

protect the public health with an adequate margin of safety. 

Id. at 16,470–71.

Assessing the proper level for a revised standard, EPA 

found that a level just below 0.08 ppm would be inappropriate 

because “such a level would not be appreciably below the 

level in controlled human exposure studies at which adverse 

effects have been demonstrated.” Id. at 16,482. Although 

acknowledging that CASAC had recommended a level as low 

as 0.060 to 0.070 ppm, see id., EPA determined that “[a] 

standard set at a level lower than 0.075 [ppm] would only 

result in significant further public health protection if, in fact, 

there is a continuum of health risks in areas with . . . O3

concentrations that are well below the concentrations 

observed in the key controlled human exposure studies and if 

the reported associations observed in epidemiological studies 

are, in fact, causally related to O3 at those lower levels,” id. at 

16,483. Stating that it was “not prepared to make these 

assumptions,” EPA found that, with a standard set below 

0.075 ppm, “the likelihood of obtaining benefits to public 

health . . . decreases, while the likelihood of requiring 

reductions in ambient concentrations that go beyond those 

that are needed to protect public health increases.” Id.

“[J]udg[ing] that the appropriate balance to be drawn, based 

on the entire body of evidence and information available in 

this review, is a standard set at 0.075 [ppm],” EPA concluded

that “[a] standard set at this level provides a significant 

increase in protection compared to the current standard, and is 

appreciably below 0.080 ppm, the level in controlled human 
8

exposure studies at which adverse effects have been 

demonstrated.” Id.

EPA also determined that the secondary standard should 

be revised to be identical to the new primary standard. Id. at 

16,500. Noting new evidence that had become available since 

the last review, EPA found that the ozone level of the existing 

secondary standard would cause significant effects on 

vegetation and sensitive ecosystems. Id. at 16,496–97. EPA 

acknowledged CASAC’s recommendation that a revised 

secondary standard should measure ozone exposure

cumulatively over a seasonal period, rather than the 8-hour 

period of the primary standard. Id. at 16,498–500. EPA 

agreed with CASAC that “a cumulative, seasonal standard is 

the most biologically relevant way to relate exposure to plant 

growth response.” Id. at 16,500. Nonetheless, conducting a 

comparison between the revised primary standard and a range 

of proposed levels for a cumulative, seasonal standard, EPA 

found “significant overlap between the revised 8-hour 

primary standard and selected levels of the [seasonal] 

standard form being considered.” Id. at 16,499. Although 

recognizing that “there would be the potential for not 

providing the appropriate degree of protection for vegetation 

in areas with air quality distributions that result in a high 

cumulative, seasonal exposure but do not result in high 8-hour 

average exposures,” the agency determined that “establishing 

a new secondary standard with a cumulative, seasonal form at 

this time would result in uncertain benefits beyond those 

afforded by the revised primary standard and therefore may 

be more than necessary to provide the requisite degree of 

protection.” Id. at 16,500. EPA therefore concluded that the 

revised primary standard “would be sufficient to protect 

public welfare from known or anticipated adverse effects,

and . . . that an alternative cumulative, seasonal standard is

[not] needed to provide this degree of protection.” Id.
9

Challenging the revised primary and secondary NAAQS, 

various parties, including several states, the District of 

Columbia, New York City, and several industry, 

environmental, and public health groups, filed these petitions 

for review. We then granted EPA’s unopposed motion to hold 

these cases in abeyance to allow the agency to review the 

2008 revisions and determine whether they should be 

reconsidered. In September 2011, EPA indicated that it was 

withdrawing its reconsideration proceedings and would 

instead be completing the reconsideration in conjunction with 

the next periodic review. Several parties filed petitions for

review, challenging EPA’s withdrawal of the reconsideration 

rulemaking. Finding that we lacked jurisdiction over EPA’s 

non-final action, we dismissed the petitions and set a briefing 

schedule for the present case. 

We now confront the parties’ competing petitions for 

review. One set of petitioners—comprising several states, the 

District of Columbia, New York City, and a number of 

environmental and public health groups—thinks the primary 

and secondary NAAQS are not protective enough, while the 

other set—comprising the state of Mississippi and several 

industry groups—thinks they are too protective.

This opinion considers each of these claims in turn. We

reject Mississippi and the industry groups’ challenge to the 

primary and secondary standards in Part II. We explain our 

denial of the governmental and environmental petitions with 

respect to the primary standard in Part III and our grant of 

these petitions with respect to the secondary standard in Part

IV.

In considering challenges to NAAQS, “we apply the 

same highly deferential standard of review that we use under 
10

the Administrative Procedure Act.” ATA III, 283 F.3d at 362. 

Accordingly, “we will set aside the Agency’s determination 

only if it is ‘arbitrary, capricious, an abuse of discretion, or 

otherwise not in accordance with law.’ ” National 

Environmental Development Ass’n’s Clean Air Project v. 

EPA, 686 F.3d 803, 809–10 (D.C. Cir. 2012) (quoting 42 

U.S.C. § 7607(d)(9)(A)). And “we do not look at the decision 

as would a scientist, but as a reviewing court exercising our 

narrowly defined duty of holding agencies to certain minimal 

standards of rationality.” Id. at 810 (internal quotation marks 

omitted). That said, “we perform a searching and careful 

inquiry into the underlying facts.” ATA III, 283 F.3d at 362 

(internal quotation marks omitted).

II.

Mississippi and the industry petitioners (collectively 

“Mississippi”) challenge EPA’s threshold decision to revise

the primary NAAQS level. According to Mississippi, several 

aspects of EPA’s decision were arbitrary, including its 

allegedly unsupported finding that the revised NAAQS would 

provide increased protection; its failure to compare the 2008 

risk assessment with the 1997 risk assessment; and the 

allegedly inadequate and distorted science on which the 

agency relied. We reject these arguments. Mississippi also 

claims the secondary NAAQS is improper because it tracks 

the primary NAAQS, which Mississippi believes is unlawful, 

but this argument falls as collateral damage from our rejection 

of Mississippi’s challenge to the primary NAAQS.

A.

The Clean Air Act requires EPA to set primary NAAQS 

that are “requisite” to protect the public health with an 

adequate margin of safety. 42 U.S.C. § 7409(b)(1). 

“Requisite” means the NAAQS must be “sufficient, but not 
11

more than necessary.” Whitman, 531 U.S. at 473 (internal 

quotation marks omitted). Mississippi now tells us the agency 

cannot determine why further risk reduction is “requisite” 

without “putting risk in the context of earlier NAAQS 

decisions (and other risk-based decisions).” Mississippi’s Br. 

24. EPA’s failure to do so, Mississippi explains, means the 

NAAQS revision is nothing more than the legally inadequate 

determination that a lower level is more protective. Not so. 

The force of Mississippi’s position, generated by the 

Whitman Court’s malleable definition of “requisite,” assumes 

only one standard at any given time can be “requisite” 

because, by definition, that standard is neither higher nor 

lower than necessary. Any other standard would therefore 

miss the mark. But of course, this idea presupposes scientific 

certainty in an area actually governed by policy-driven 

approaches to uncertain science. See Lead Industries Ass’n v. 

EPA, 647 F.2d 1130, 1146–47 (D.C. Cir. 1980); see also

Motor Vehicle Manufacturers Ass’n of the U.S. v. State Farm 

Mutual Automobile Insurance Co., 463 U.S. 29, 57 (1983). 

Mississippi’s position—though perhaps an arguable thesis—

collapses under the weight of reality.

Determining what is “requisite” to protect the “public 

health” with an “adequate” margin of safety may indeed 

require a contextual assessment of acceptable risk. See 

Whitman, 531 U.S. at 494–95 (Breyer, J., concurring in part 

and concurring in the judgment). Such is the nature of policy. 

But that does not mean the initial assessment is sacrosanct and 

remains the governing standard until every aspect of it is 

undermined. Every time EPA reviews a NAAQS, it 

(presumably) does so against contemporary policy judgments 

and the existing corpus of scientific knowledge. See 42 U.S.C. 

§§ 7408–09. It would therefore make no sense to give prior 

NAAQS the sort of presumptive validity Mississippi insists 
12

upon. The statutory framework requires us to ask only 

whether EPA’s proposed NAAQS is “requisite”; we need not 

ask why the prior NAAQS once was “requisite” but is no 

longer up to the task. Following Mississippi’s approach 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. We decline 

Mississippi’s invitation to enter that funhouse and will defer 

as long as EPA reasonably explains its actions. American 

Farm Bureau Federation v. EPA, 559 F.3d 512, 521 (D.C. 

Cir. 2009) (per curiam).

Mississippi argues at length that EPA should have 

compared the evidence available in 2008 to the evidence

available in 1997—in particular, the clinical, epidemiological, 

and toxicological studies, risk assessments, and EPA’s 

protocol for sensitive populations. We need not respond point 

by point; suffice to say that EPA reasonably explained how 

the scientific evidence had in fact changed since the 1997 

review. To name just one example, whereas in reviewing 

EPA’s 1997 NAAQS-setting we emphasized “the absence of 

any human clinical studies at ozone concentrations below 

0.08,” ATA III, 283 F.3d at 379, EPA here explained that “two 

new controlled human-exposure studies . . . are now available 

that examine respiratory effects associated with prolonged O3

exposures at levels at and below 0.080 ppm.” 2008 Final 

Rule, 73 Fed. Reg. at 16,454.1 But to frame it more broadly, 

 1 Because Mississippi independently challenges EPA’s failure 

to compare its 2008 and 1997 risk assessments, however, we also 

acknowledge the reasonableness of EPA’s explanation for not 

doing so—namely, that a comparison would be “factually 

inappropriate,” would not account for the fact that “with similar 

risks, increased certainty in the risks” would engender greater 

concern, and would obscure the qualitativeness of EPA’s approach. 
13

we note, first, that the NAAQS review process includes 

EPA’s public health policy judgments as well as its analysis 

of scientifically certain fact, and, second, that as the contours 

and texture of scientific knowledge change, the 

epistemological posture of EPA’s NAAQS review necessarily 

changes as well; additional certainty about what was merely a 

thesis might very well support a determination that the line 

marked by the term “requisite” has shifted. In short, 

Mississippi seeks to eliminate any adumbration of the 

inevitable scientific uncertainties that shadow and shape 

EPA’s statutory mandate to take a preventative approach. See, 

e.g., ATA III, 283 F.3d at 378.

B.

Mississippi looses the rest of its arrows at the evidence 

on which EPA relied, although in doing so, Mississippi again 

 

2008 Final Rule, 73 Fed. Reg. at 16,466. First, the 2008 risk 

assessment analyzed a number of health effects not included in the 

1997 risk assessment, so the ultimate value of comparing the two 

assessments would be limited. Second, logic rejects comparisons of 

apples and oranges, which is how we would describe two data 

analyses subject to different geographic and demographic 

parameters. See, e.g., 2007 Proposed Rule, 72 Fed. Reg. at 37,851–

52 (explaining that 2008 exposure analysis, an input to part of the 

risk assessment, relied on a model different from the one used in 

1997). And finally, even if EPA should have compared the two risk 

assessments where they overlapped, EPA’s failure to do so does not 

necessarily render EPA’s ultimate NAAQS decision improper. See 

ATA III, 283 F.3d at 369–70; see also Lead Industries Ass’n, 647 

F.2d at 1162. The risk assessment turned on more than just those 

risks amenable to comparison with the 1997 risk assessment, and in 

setting the NAAQS, EPA relied on much more than just the risk 

assessment. See, e.g., 2008 Final Rule, 73 Fed. Reg. at 16,467, 

16,476, 16,479.
14

showcases its apparent preference for exuberance over 

precision. Ultimately, Mississippi’s arguments that the 2008 

science added nothing new to the 1997 NAAQS conversation 

and that EPA misrepresented the science on which it relied 

are largely dependent on the conceptual error that EPA is 

somehow bound by the 1997 NAAQS and on the legal error 

that it is our job to “weigh the evidence anew.” American 

Farm Bureau, 559 F.3d at 533 (internal quotation marks 

omitted). Nonetheless, we address each argument in turn, 

construing Mississippi’s arguments to articulate a weightier 

challenge: that the available evidence did not support EPA’s 

threshold decision to revise the NAAQS. We again disagree.

1.

The 1997 standard was “set at a level of 0.08 ppm, which 

is equivalent to 0.084 ppm using the standard rounding 

conventions.” 2008 Final Rule, 73 Fed. Reg. at 16,437. By 

framing the issue in terms of EPA’s decision to lower the 

NAAQS level below 0.080 ppm, Mississippi fails to capture 

the full significance of the 2008 NAAQS revision.2 Indeed, 

Mississippi’s imprecision undermines its case: by conceding 

that health effects are linked to ozone levels of 0.080 ppm, 

Mississippi rebuts its claim that science in 2008 did not 

support a NAAQS set at an effective level lower than 0.084 

ppm. 

In any event, after reviewing the record, we think it quite 

clear EPA’s rejection of the 1997 NAAQS was proper. EPA 

 2 For that matter, EPA commits the same error, referring to 

“0.080” when it should refer to “0.08,” compare 2008 Final Rule, 

73 Fed. Reg. at 16,444, with 1997 Final Rule, 62 Fed. Reg. at 

38,859, but Mississippi does not note it, so we have no reason to 

think this reflects anything other than sloppy editing.
15

relied on a broad array of scientific studies, quantified 

models, and input from CASAC, EPA staff, and commenters, 

and it considered not only what was known, but also what was 

not known. See, e.g., 2008 Final Rule, 73 Fed. Reg. at 

16,438–40. It then evaluated the evidence as a whole through 

an “integrative synthesis,” what it called a “weight of 

evidence approach.” Id. at 16,439, 16,479. And appropriately 

so: one type of study might be useful for interpreting 

ambivalent results from another type, see Ethyl Corp. v. EPA, 

541 F.2d 1, 26 (D.C. Cir. 1976) (en banc), and though a new 

study does little besides confirm or quantify a previous 

finding, such incremental (and arguably duplicative) studies 

are valuable precisely because they confirm or quantify 

previous findings or otherwise decrease uncertainty. See, e.g.,

2008 Final Rule, 73 Fed. Reg. at 16,450 (noting that post1997 evidence “increased the Administrator’s confidence” 

that particular health endpoints are causally related to ozone 

exposure). EPA made this point when it explained that 

controlled human exposure studies provide “the most directly 

applicable” evidence (and engender “the highest level of 

confidence”) about the causal relationship between ozone 

exposure and health effects; that epidemiological studies 

provide evidence both about health effects from exposure to 

ambient air and about the effect of ozone exposure on “more 

serious” health effects like hospital admissions and mortality; 

and that animal toxicology studies generally support the 

biological plausibility of effects noted in clinical and 

epidemiological studies. 2007 Proposed Rule, 72 Fed. Reg. at 

37,823, 37,825; see 2008 Final Rule, 73 Fed. Reg. at 16,440 

(incorporating discussion of scientific evidence in proposed 

rule). Given that the record includes, among other things, 

numerous epidemiological studies linking health effects to 

exposure to ozone levels below 0.08 ppm and clinical human

exposure studies finding a causal relationship between health 

effects and exposure to ozone levels at and below 0.08 ppm, 
16

we will not second-guess EPA’s interpretations of, or the 

conclusions it drew from, this evidence.

Reasonable people might disagree with EPA’s 

interpretations of the scientific evidence, but any such 

disagreements must come from those who are qualified to 

evaluate the science, not us. We are satisfied that EPA’s 

interpretations are permissible, and that is enough. Indeed, 

CASAC unanimously concluded that “[t]here is no scientific 

justification for retaining the current primary 8-hr NAAQS of 

0.08 parts per million,” that the primary NAAQS “needs to be 

substantially reduced to protect human health,” and that the 

primary NAAQS should be set at a level somewhere between 

0.060 and 0.070 ppm. Letter from Dr. Rogene Henderson, 

CASAC Chair, to Stephen L. Johnson, EPA Administrator 

(“Oct. 2006 CASAC Letter”), at 1–2 (Oct. 24, 2006), EPACASAC-07-001. If, as we have explained, EPA may give 

“significant weight” to propositions about the appropriate 

NAAQS level implicitly accepted by otherwise-disagreeing 

CASAC members, see ATA III, 283 F.3d at 378–79, surely it 

may rely on an explicit recommendation by the unanimous 

CASAC panel.

And given the reasonableness of EPA’s interpretation of 

the science, its determination that the 1997 NAAQS was 

insufficiently protective of public health follows as a matter 

of course. EPA concluded sensitive populations like 

asthmatics are affected by ozone in a more severe way and at 

lower levels than are healthy individuals and that ozonerelated health effects might be adverse for sensitive 

individuals though comparable effects would not be 

considered adverse for healthy individuals—conclusions 

Mississippi does not challenge. See 2008 Final Rule, 73 Fed. 

Reg. at 16,454–55, 16,466. EPA could properly decide that a 

NAAQS set at the level of 0.08 ppm does not protect the 
17

public health with an adequate margin of safety when healthy

individuals experience adverse health effects from exposure 

to ozone at and below that level. See, e.g., American Farm 

Bureau, 559 F.3d at 525–26. “That petitioners . . . find a basis 

to disagree” with EPA is “hardly surprising.” Ethyl Corp., 541 

F.2d at 26. But that does not make EPA’s decision to revise 

the NAAQS arbitrary.

2.

Mississippi also contends that section 108(a)(2) of the 

Clean Air Act, 42 U.S.C. § 7408(a)(2), requires EPA “to 

consider and rely upon all scientific information that is 

capable of being put to use and serviceable for [identifying 

the effect that a given pollutant has on public health and 

welfare] and that is free from error (accurate).” Mississippi’s 

Br. 47. This requirement, Mississippi explains, incorporates 

information standards under the Information Quality Act

(“IQA”), Pub. L. No. 106-554, sec. 1(a)(3), § 515, 114 Stat. 

2763, 2763A-153 to 154 (2000) (H.R. 5658) (codified at 44 

U.S.C. § 3516 note). According to Mississippi (as we 

understand its argument), EPA violated both the Clean Air

Act and the IQA by inaccurately characterizing some studies 

and by relying on other, flawed studies. These arguments are 

difficult to parse because they fill two roles in Mississippi’s 

shadow play: reinforcing the implied point that the available 

evidence did not support EPA’s decision to revise the 

NAAQS level and unmasking a violation of the Clean Air

Act’s procedural requirements. Having already discussed the 

reasonableness of EPA’s threshold decision to revise the 

NAAQS, we now consider only Mississippi’s suggestion that 

EPA violated the Clean Air Act’s and the IQA’s procedural 

standards and that these violations independently render 

EPA’s NAAQS determination unlawful. 
18

The Clean Air Act implicitly divides the NAAQS review 

process into three stages. First, members of the scientific 

community publish studies, which may be more or less 

flawed. Second, EPA must issue and periodically revise air 

quality criteria that “accurately reflect the latest scientific 

knowledge useful in indicating the kind and extent of all 

identifiable effects on public health or welfare which may be 

expected from the presence of such pollutant in the ambient 

air, in varying quantities.” 42 U.S.C. §§ 7408(a)(2), 7409(d). 

Finally, EPA must “base[]” its NAAQS determinations on the 

criteria. 42 U.S.C. § 7409(b)(1). From start to finish, this 

system is vulnerable to error. In particular, even if the 

foundational scientific studies are not flawed in any material 

way, transmission errors may nevertheless occur when EPA 

drafts the criteria or when it subsequently decides what 

NAAQS to set. Mississippi’s point appears to be that the

Clean Air Act and the IQA impose safeguards to ensure 

accuracy throughout this entire process. While that may be a 

fair characterization, it overstates the practical effect of the 

statutory schemes.

First, though the Clean Air Act requires the air quality 

criteria to “accurately reflect” the scientific evidence, that 

requirement says nothing about the accuracy of the science 

itself or the precision of the relationship between the criteria 

and EPA’s NAAQS decision. The criteria, which are neither 

“standards” nor “guidelines,” simply “provide the scientific 

basis for promulgation of air quality standards.” Lead 

Industries Ass’n, 647 F.2d at 1136–37. We do not reweigh the 

evidence or second-guess technical judgments but are limited 

to determining whether EPA made a rational judgment. See 

American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 

(D.C. Cir. 1981). Nor do we look through the microscope to 

scrutinize EPA’s use of the criteria: there are limits to EPA’s 

discretion in using the criteria, see Michigan v. EPA, 213 F.3d 
19

663, 696 (D.C. Cir. 2000) (per curiam), but EPA’s translation 

of the criteria into a NAAQS decision is not frictionless, and 

ignoring this fact would squeeze considerations of policy and 

the role of CASAC out of the equation. See 42 U.S.C. 

§ 7607(d)(3); see also Catawba County, North Carolina v. 

EPA, 571 F.3d 20, 35 (D.C. Cir. 2009) (per curiam). 

Second, Mississippi fails to show the IQA is an 

independent measure of EPA’s NAAQS decision. The IQA 

requires the Director of the Office of Management and 

Budget to provide “policy and procedural guidance” to 

“ensur[e] and maximiz[e] the quality, objectivity, utility, and 

integrity of information . . . disseminated by Federal 

agencies.” 44 U.S.C. § 3516 note. OMB, in turn, issued 

flexible, “generic” guidelines that it recognized “cannot be 

implemented in the same way by each agency.” Guidelines 

for Ensuring and Maximizing the Quality, Objectivity, Utility, 

and Integrity of Information Disseminated by Federal 

Agencies; Republication, 67 Fed. Reg. 8,452, 8,452–53 (Feb. 

22, 2002). EPA’s implementing guidelines, meanwhile, 

purport to provide only “non-binding policy and procedural 

guidance.” EPA, GUIDELINES FOR ENSURING AND 

MAXIMIZING THE QUALITY, OBJECTIVITY, UTILITY, AND 

INTEGRITY OF INFORMATION DISSEMINATED BY THE 

ENVIRONMENTAL PROTECTION AGENCY 4 (2002). Mississippi 

points to nothing indicating that any part of this scheme 

committed EPA to having done things differently. See 

American Petroleum Institute v. EPA, 684 F.3d 1342, 1348 

(D.C. Cir. 2012); see also Salt Institute v. Leavitt, 440 F.3d 

156, 159 (4th Cir. 2006). 

Measuring Mississippi’s challenge to EPA’s use of the 

scientific evidence against the agency’s legal obligations, we 

see nothing to suggest EPA acted improperly, particularly 

given our approval of its ultimate decision to revise the 
20

NAAQS. To start, Mississippi’s challenge to EPA’s use of the 

Adams studies—a set of controlled human exposure studies 

that played a relatively significant role in the NAAQS review 

process—is nothing more than a claim that EPA did wrong by 

disagreeing with Adams’s interpretation of his data. 

(Although Adams concluded his 2006 study did not show 

statistically significant lung function decrements at the 0.06 

ppm ozone exposure level, EPA explained that it believed a 

different statistical model would more directly address the 

precise question with which it was concerned—namely, the 

effects of prolonged ozone exposure versus exposure to 

filtered air—and that application of this model yielded 

statistically significant results at the 0.06 ppm level.) Yet 

nothing in the Clean Air Act or the IQA prohibits EPA from 

independently analyzing the science—for example, by asking 

“different questions” from those asked by the study’s author, 

2008 Final Rule, 73 Fed. Reg. at 16,455—and the only 

objections Mississippi offers to EPA’s independent analysis 

are either conclusory or require us to weigh in on what is 

apparently legitimate scientific debate. See id. (noting 

approval by members of CASAC of the statistical approach 

used in the reanalysis).3

Mississippi’s challenge to EPA’s use of the 

epidemiological evidence fares no better. Though it claims 

EPA improperly relied on studies using ambient ozone data as 

a proxy for personal exposure, Mississippi neither challenges 

 3 To the extent Mississippi’s complaint centers around EPA’s 

failure to peer review its reanalysis, we note that EPA’s IQA 

guidelines expressly disclaim a categorical peer-review policy, so 

even assuming Mississippi is right that the reanalysis was not peer

reviewed, Mississippi’s failure to explain why the alleged lack of 

peer review was improper is fatal. See American Petroleum 

Institute, 684 F.3d at 1348–49. 
21

EPA’s explanation that very few epidemiological studies 

directly measuring personal exposure exist in the literature 

nor acknowledges EPA’s recognition that ambient 

measurements do not necessarily represent personal exposure 

levels and must therefore be used with caution. We have no 

problem with EPA’s reliance on actual, rather than 

nonexistent, evidence, and in any event, Mississippi does not 

challenge EPA’s interpretation of the measurement 

disparity—that if the disparity biases the epidemiological 

evidence, it does so by underestimating ozone’s health 

effects. See 2007 Proposed Rule, 72 Fed. Reg. at 37,839. 

Finally, Mississippi’s belief that EPA ignored contradictory 

evidence is an example of its own confirmation bias. 

Mississippi insists EPA failed to account for studies 

suggesting that findings of ozone-related health effects may 

be confounded by the presence of other pollutants, but this 

challenge boils down to a claim about two epidemiological 

studies. EPA mentions only one of these studies—and only 

once—in the final rule, and in doing so, it also cites two other 

studies (which Mississippi does not challenge) for the same 

proposition. See 2008 Final Rule, 73 Fed. Reg. at 16,446.

Even granting the substance of Mississippi’s assertions 

(which we do not), it is hard to imagine how eliminating both 

studies from EPA’s NAAQS calculation would have altered 

EPA’s ultimate decision.4

We repeat: it is not our job to referee battles among 

experts; ours is only to evaluate the rationality of EPA’s 

decision, and as we have explained, the agency did its part. 

And because we reject Mississippi’s challenge to the primary 

 4 EPA also cited the two studies a total of five times in the 

proposed rule, but we think this immaterial in light of Mississippi’s 

failure to explain their importance to EPA’s final decision.
22

NAAQS, we must also reject its challenge to the secondary 

NAAQS. Mississippi’s petition for review is therefore denied.

III.

As discussed above, EPA’s review of the available 

scientific evidence led it to adopt a primary ozone NAAQS of 

0.075 ppm. While Mississippi criticized EPA’s decision to 

reduce this standard from its prior level of 0.08 ppm, multiple 

state and local governments, environmental advocacy nonprofits, and public health non-profits contend that EPA did 

not go far enough. Thus, EPA finds itself in a situation 

reminiscent of Goldilocks and the Three Bears. On one side, 

Mississippi argued that EPA is too stringent with its ozone 

NAAQS; on the other side, the governmental and 

environmental petitioners argue that the NAAQS is too lax. 

But unlike Goldilocks, this court cannot demand that EPA get 

things “just right.” Rather, for EPA’s decision to survive these 

challenges, it need do no more than meet the statutory 

standards found in the Clean Air Act. “That the evidence in 

the record may also support other conclusions, even those that 

are inconsistent with [EPA’s], does not prevent us from 

concluding that [its] decisions were rational . . . .” Lead 

Industries Ass’n, 647 F.2d at 1160 (footnote and citations 

omitted).

The Act requires us to overturn any EPA action that is 

arbitrary, capricious, an abuse of discretion, or contrary to 

law. 42 U.S.C. § 7607(d)(9)(A). The governmental and 

environmental petitioners argue that EPA’s judgment—that a 

primary ozone NAAQS of 0.075 ppm is “requisite to protect 

the public health,” 42 U.S.C. § 7409(b)(1)—was arbitrary and 

capricious because EPA failed to rationally consider scientific 

evidence demonstrating adverse health effects at ozone levels 

below 0.075 ppm. They also argue that EPA acted contrary to 

law because it failed to calculate an adequate margin of 
23

safety, as required by section 109(b)(1) of the Act, 42 U.S.C. 

§ 7409(b)(1). Finally, they argue that EPA violated its 

statutory duty to explain and defend its decision to depart 

from CASAC’s recommendations. See id. § 7607(d)(3), (6).

We address each of these arguments in turn. 

A.

It is true that “[a]n agency’s failure adequately to 

consider a relevant and significant aspect of a problem may 

render its rulemaking arbitrary and capricious.” American

Farm Bureau, 559 F.3d at 520. But the corollary to EPA’s 

obligation to “weigh the entire record,” Achernar 

Broadcasting Co. v. FCC, 62 F.3d 1441, 1446 (D.C. Cir. 

1995), is that no single piece of evidence is dispositive. See 

American Farm Bureau, 559 F.3d at 525; see also ATA III, 

283 F.3d at 379. Moreover, “we do not determine the 

convincing force of evidence, nor the conclusion it should 

support, but only whether the conclusion reached by EPA is 

supported by substantial evidence when considered on the 

record as a whole.” Coalition for Responsible Regulation, Inc. 

v. EPA, 684 F.3d 102, 122 (D.C. Cir. 2012) (per curiam).

Provided EPA meets its obligation “to explain and 

expose every step of its reasoning,” American Lung Ass’n v. 

EPA, 134 F.3d 388, 392 (D.C. Cir. 1998), the governmental 

and environmental petitioners have a heavy burden to show 

that the totality of the evidence required EPA to decide 

differently than it did. Lead Industries Ass’n, 647 F.2d at 

1160. This approach to giant administrative records is 

consistent with the deference principles discussed above. See 

Part II, supra at 14. Our role is circumscribed. We are merely

to “determin[e] if [EPA] made a rational judgment,” not to 

“weigh the evidence anew and make technical judgments.” 

Costle, 665 F.2d at 1185. 
24

The governmental and environmental petitioners argue 

that EPA failed to grapple with three major types of evidence 

that they claim favor a lower primary ozone NAAQS: 

controlled human exposure studies, epidemiological studies, 

and human exposure and health risk assessments. The record 

reveals, and petitioners do not dispute, that EPA considered 

the entire body of scientific evidence available to it, 

discussing each type of evidence at each stage of its analysis. 

See, e.g., 2007 Proposed Rule, 72 Fed. Reg. at 37,864–68; 

2008 Final Rule, 73 Fed. Reg. at 16,452–70; see also id. at 

16,439 (describing the range of evidence considered in the 

years-long review process). The petitioners argue instead that

EPA’s conclusion that a level of 0.075 ppm is “requisite” to 

protect public health cannot be rationally drawn from this 

evidence. We disagree. EPA’s treatment of the evidence 

satisfies our deferential standard of review.

Petitioners argue that the controlled human exposure 

studies—in particular, the Adams studies—support a more 

protective primary NAAQS because they demonstrate adverse 

effects at the 0.060 ppm level. The Adams studies, published 

in 2002 and 2006, analyzed the results of laboratory 

experiments that directly measured the effects of ozone on 

humans’ respiratory health by exposing thirty subjects to 

ozone in a controlled environment. Adams tested his subjects 

at various ozone concentrations, including 0.08 ppm and 0.06 

ppm, but at no levels in between. He found that a small 

number of subjects exposed to ozone at 0.06 ppm experienced 

lung function decrements of at least ten percent—a level EPA 

considers to be harmful (or “adverse”) to asthmatics. See 2008 

Final Rule, 73 Fed. Reg. at 16,454–55. Petitioners argue that 

the 0.06 ppm Adams results were “unrebutted ‘substantial 

evidence’ ” favoring a lower standard, and that EPA’s 

decision to set the standard as high as 0.075 ppm “ ‘is not 

supported by substantial evidence.’ ” Environmental 
25

Petitioners’ Br. 19 (quoting City of Naples Airport Authority 

v. FAA, 409 F.3d 431, 436 (D.C. Cir. 2005)). The crux of the 

dispute is whether EPA rationally treated this evidence of 

adverse effects as not dispositive. 

EPA relied on the Adams studies and other clinical 

studies to justify its decision to lower the primary ozone 

NAAQS from the 0.08 ppm level, concluding that they 

“provide[d] the most certain evidence of adverse health 

effects” at 0.080 ppm available. 2008 Final Rule, 73 Fed. 

Reg. at 16,478. EPA also conducted a reanalysis of the 

Adams (2006) study that found “small group mean 

decrements in lung function responses to be statistically 

significant at the 0.060 ppm exposure level.” Id. at 16,454. 

But EPA further concluded that the data at the 0.060 ppm 

level was too limited to support a reduction in the NAAQS to 

that level. 

Each Adams study involved only thirty subjects, of which 

six at most experienced lung function decrements of ten 

percent or more at exposure levels below 0.080 ppm.5 For this 

 5 The record includes conflicting accounts of the number of 

participants experiencing lung function decrements of ten percent 

or larger at 0.06 ppm. All accounts indicate that it was a small 

number, and never more than six. The precise number appears to 

depend on one’s method of measuring lung function decrements. 

Compare OFFICE OF AIR QUALITY PLANNING AND STANDARDS,

EPA, REVIEW OF THE NATIONAL AMBIENT AIR QUALITY 

STANDARDS FOR OZONE: POLICY ASSESSMENT OF SCIENTIFIC AND 

TECHNICAL INFORMATION (“STAFF PAPER”), § 3.3.1.1.1 (2007), 

EPA-452/R-07-007, with Letter from Dr. Rogene Henderson, 

CASAC Chair, to Stephen L. Johnson, EPA Administrator (“Mar. 

2007 CASAC Letter”), at C-31–32 (Mar. 26, 2007), EPA-CASAC07-002.
26

reason, the CASAC scientists had mixed views about the 

Adams studies. For instance, one scientific advisor stated that 

the number of data points in the Adams studies was “pitiful,” 

and that the limited nature of the data was “astounding.” 

Letter from Dr. Rogene Henderson, CASAC Chair, to 

Stephen L. Johnson, EPA Administrator (“Mar. 2007 CASAC 

Letter”), at C-31–32 (Mar. 26, 2007), EPA-CASAC-07-002.

Another cautioned that the responses Adams recorded at the 

0.06 ppm level might merely reflect “normal variations” in 

human lung function rather than “real ozone responses.” Oct. 

2006 CASAC Letter, at D-14. In other words, other factors 

apart from a change in ozone levels—for example, participant 

fatigue or diminished effort—might explain the “decrements” 

that Adams observed. Adams himself was critical of those 

who drew strong conclusions from his results at the 0.06 ppm 

level because he determined that the average responses were 

not statistically significant. See William C. Adams, Comment 

on EPA Memorandum: The Effects of Ozone on Lung 

Function at 0.06 ppm in Healthy Adults (Oct. 9, 2007), EPAHQ-OAR-2005-0172-4783. Ultimately, although EPA 

disagreed with Adams regarding the statistical significance of 

some results, it found that the study’s small sample size could 

not “appropriately be generalized to the U.S. population.” 

2008 Final Rule, 73 Fed. Reg. at 16,454, 16,478. 

Thus, while the 0.08 ppm results were robust, EPA 

rationally treated the 0.06 ppm results as inconclusive. 

Perhaps more studies like the Adams studies will yet reveal 

that the 0.060 ppm level produces significant adverse 

decrements that simply cannot be attributed to normal 

variation in lung function. But at the time of EPA’s 

rulemaking, it was rational to treat the 0.06 ppm results with 

skepticism. The Adams results at 0.06 ppm indicate some

degree of risk that some number of individuals might continue 

to experience health effects at and below 0.075 ppm, but we 
27

have previously acknowledged the impossibility of 

eliminating all risk of health effects from “non-threshold” 

pollutants like ozone. See ATA III, 283 F.3d at 360 (“The lack 

of a threshold concentration below which these pollutants are 

known to be harmless makes the task of setting primary 

NAAQS difficult, as EPA must select standard levels that 

reduce risks sufficiently to protect public health even while 

recognizing that a zero-risk standard is not possible.” (internal 

quotation marks and original alterations omitted)). 

Petitioners counter that EPA has relied on even 

statistically nonsignificant results in the past when setting the 

primary ozone NAAQS, so the limitations of the Adams 

studies provide no basis for dismissing the evidence of 

adverse effects at that level. Environmental Petitioners’ Br. 

20–21. Be that as it may, the question for this court is not 

what EPA has done in the past, or even what levels it 

rationally could have settled on, but only whether it has 

provided a rational explanation of how it treated the evidence 

before it. See ATA III, 283 F.3d at 374 (“[W]e review [EPA’s] 

scientific judgments . . . not as the chemist, biologist, or 

statistician that we are qualified neither by training nor 

experience to be, but as a reviewing court exercising our 

narrowly defined duty of holding agencies to certain minimal 

standards of rationality.” (quoting Troy Corp. v. Browner, 120 

F.3d 277, 283 (D.C. Cir. 1997)). Statistical quality affords a 

perfectly rational basis for assigning different weights to 

different pieces of scientific data when evaluating the totality 

of the evidence. While EPA is certainly permitted to look to 

statistically uncertain results, it is by no means required to 

rely on them. Its failure to do so in this case did not render its 

decision irrational.

The governmental and environmental petitioners next 

argue that EPA gave short shrift to the epidemiological 
28

studies. By using statistical techniques to analyze vast bodies 

of health and environmental data across large populations, 

epidemiological studies allow scientists to draw inferences 

about the harms of ozone without carefully calibrated 

laboratory experiments. EPA relied on over 250 such studies 

during its 2008 rulemaking. 2008 Final Rule, 73 Fed. Reg. at 

16,455, 16,479. Petitioners point out that some studies found 

significant correlations between ozone concentration and 

adverse health outcomes at levels well below 0.075 ppm. See 

Comments of the American Lung Association, Environmental 

Defense, Sierra Club on the U.S. Environmental Protection 

Agency’s Proposed Revisions to the NAAQS for Ozone (Oct. 

9, 2007), EPA-HQ-OAR-2005-0172-4261; Comments of 

American Thoracic Society, et al. (Oct. 9, 2007), EPA-HQOAR-2005-0172-4305. The studies were relatively consistent, 

and the results—as EPA admits—may help establish a causal 

relationship between the presence of ozone and the 

occurrence of adverse health effects. 2008 Final Rule, 73 Fed. 

Reg. at 16,450.

As with the Adams studies, EPA relied on the 

epidemiological studies to conclude that the existing standard 

of 0.08 ppm was too high. EPA noted that many 

epidemiological studies reported “statistically significant 

associations that generally extend down to ambient O3

concentrations that are below the level of the current 

standard” and considered these studies as part of the body of 

“new evidence demonstrating that exposures to O3 at levels 

below the level of the current standard are associated with a 

broad array of adverse health effects.” Id. at 16,471. EPA also 

explained, however, that “the epidemiological studies are not 

themselves direct evidence of a causal link between exposure 

to O3 and the occurrence of [health] effects,” id. at 16,479, 

and that evidence of this causal relationship “becomes 

increasingly uncertain at lower levels of exposure.” Id. at 
29

16,478. EPA explained this uncertainty by reference to 

intrinsic indicators of reliability and extrinsic sources of 

corroboration, both of which provide substantial evidence for 

EPA’s decision. For example, at much lower levels of ozone 

exposure, EPA questioned whether it could attribute the 

epidemiological effects to ozone alone “rather than to the 

broader mix of air pollutants present in the ambient air.” Id. at 

16,456; see also EPA, Responses to Significant Comments on 

the 2007 Proposed Rule, at 29 (Mar. 2008), EPA-HQ-OAR2005-0172-7185. Additionally, EPA relied on controlled 

studies like the Adams studies to lend “biological plausibility” 

to the inferences of causation drawn from epidemiological 

studies. According to EPA, while “[t]he biological plausibility 

of the epidemiological associations is generally supported by 

controlled human exposure and toxicological evidence of 

respiratory morbidity effects for levels at and below 0.080 

ppm,” that “biological plausibility becomes increasingly 

uncertain at much lower levels.” 2008 Final Rule, 73 Fed. 

Reg. at 16,456. EPA’s discussion of the limitations of the 

epidemiological studies at lower levels of ozone exposure 

satisfies the “minimal standards of rationality” to which we 

hold the agency. See National Environmental Development 

Ass’n’s Clean Air Project, 686 F.3d at 810 (internal quotation 

marks omitted).

Petitioners also challenge EPA’s interpretation of its own 

risk and exposure assessments. EPA did not rely heavily on 

them, though petitioners think it should have. These 

assessments model real-world interactions between a host of 

variables in order to predict health outcomes based on 

available data. 2008 Final Rule, 73 Fed. Reg. at 16,441. As 

such, they adhere to the inviolable law of data analysis,

“garbage in; garbage out.” That is, as CASAC cautioned EPA, 

the risk and exposure assessments are only as reputable as the 

inputs upon which they rely to produce their predictions. Oct. 
30

2006 CASAC Letter, at 12; see also Letter from Dr. Rogene

Henderson, CASAC Chair, to Stephen L. Johnson, EPA 

Administrator, at D-39 (Feb. 10, 2006), EPA-CASAC-06-003

(discussing similar weaknesses in risk assessment for the 

secondary NAAQS). In this case, the inputs were the very 

data whose reliability EPA questioned at lower levels.

Recognizing their limitations, we have previously approved 

EPA’s cautious treatment of risk and exposure assessments 

when EPA “consider[s] all aspects of the problem” and 

“catalogue[s] its concerns.” See American Farm Bureau, 559

F.3d at 527. We do the same now.

Having reasonably explained the limitations it believed 

existed in each of these bodies of scientific evidence, EPA 

concluded that the standard “must be set at a level appreciably 

below 0.080 ppm, the level at which there is considerable 

evidence of effects in healthy people.” 2008 Final Rule, 73 

Fed. Reg. at 16,480; see also id. at 16,483 (“0.080 ppm [is] 

the level in controlled human exposure studies at which 

adverse effects have been demonstrated.”). EPA concluded 

that a standard set at 0.075 ppm “would be requisite to protect 

public health with an adequate margin of safety, including the 

health of sensitive subpopulations.” Id. at 16,483. EPA 

explained that a standard lower than 0.075 ppm was not 

required because it “would only result in significant further 

public health protection if, in fact, there is a continuum of 

health risks in areas with 8-hour average O3 concentrations 

that are well below the concentrations observed in the key 

controlled human exposure studies and if the reported 

associations observed in epidemiological studies are, in fact, 

causally related to O3 at those lower levels.” Id. Based on the 

uncertainties EPA had identified “in interpreting the evidence 

from available controlled human exposure and 

epidemiological studies at very low levels,” EPA was “not 

prepared to make these assumptions.” Id. Finding that “the 
31

likelihood of obtaining benefits to public health with a 

standard set below 0.075 ppm O3 decreases, while the 

likelihood of requiring reductions in ambient concentrations 

that go beyond those that are needed to protect public health 

increases,” EPA judged that “the appropriate balance to be 

drawn” was a standard set at 0.075 ppm. Id. We see nothing 

arbitrary and capricious about EPA’s balancing of these 

considerations.

B.

The governmental and environmental petitioners next 

argue that, even if the scientific evidence of adverse effects at 

ozone levels below 0.075 ppm remained uncertain, the 

overwhelming evidence of adverse effects at 0.080 ppm 

required a primary NAAQS lower than 0.075 ppm to ensure 

an adequate margin of safety. EPA is required to “allow[] an 

adequate margin of safety” in setting a primary NAAQS that 

is “requisite to protect the public health.” 42 U.S.C. 

§ 7409(b)(1). By requiring an “adequate margin of safety,” 

Congress was directing EPA to build a buffer to protect 

against uncertain and unknown dangers to human health. 

Lead Industries Ass’n, 647 F.2d at 1154; see also ATA III, 283 

F.3d at 368. Our case law has left EPA with a wide berth 

when it comes to deciding how best to account for an 

adequate margin of safety. In Lead Industries Association, we 

held that the choice of how to set a margin of safety is “a 

policy choice of the type that Congress specifically left to the 

Administrator’s judgment.” 647 F.2d at 1162. And in 

American Trucking Associations, we clarified that EPA need 

not “identify[] a ‘safe level’ and then apply[] an additional 

margin of safety”; instead, it may “take into account margin 

of safety considerations throughout the process as long as 

such considerations are fully explained and supported by the 

record.” ATA III, 283 F.3d at 368 (internal quotation marks 

omitted).
32

In light of this deferential standard, we have only rarely

found that the agency failed to build in a margin of safety. 

See, e.g., American Farm Bureau, 559 F.3d at 525–26

(granting the petition for review in part because EPA failed to 

account for a margin of safety). When we have, it has not 

been on the basis of our own untutored judgment about how 

large a margin is necessary, but rather because EPA acted 

arbitrarily and capriciously by, for example, failing to give 

appropriate consideration to key aspects of the “margin of 

safety” inquiry, such as how pollution thresholds might 

impact sensitive sub-populations, such as asthmatics, 

children, or the elderly. See id. In this case, no such problem 

presents itself; EPA regularly and consistently considered the 

effects of its rules on these sensitive groups. See, e.g., 2008 

Final Rule, 73 Fed. Reg. at 16,476. EPA acknowledged that 

some of these subpopulations are more likely to experience 

adverse effects at all levels of exposure, requiring it to select a 

primary NAAQS level below the level at which adverse 

effects occur “with reasonable scientific certainty.” See id. at 

16,437 (explaining the purpose of the margin of safety); id. at 

16,449 (describing CASAC’s conclusion that existing studies 

do not adequately cover sensitive subpopulations); id. at 

16,452 (adopting that conclusion in part). As a result, EPA set 

the standard “appreciably below” 0.080 ppm, the lowest level 

at which EPA expressed confidence that ozone causes adverse 

health effects in healthy individuals. Id. at 16,480. Petitioners 

have given us no reason to doubt EPA’s characterization of 

the 0.075 ppm level as “appreciably below” 0.080 ppm. EPA 

complied with Congress’s command in section 109(b)(1) to 

build in a margin of safety, and its judgment that this margin 

is adequate was not arbitrary or capricious.
33

C.

The governmental and environmental petitioners next 

argue that EPA failed to uphold its duty under the Act to 

provide “an explanation of the reasons” for departing from 

CASAC’s recommendations. 42 U.S.C. § 7607(d)(3); see also 

id. § 7607(d)(6)(A). Congress created CASAC in the 1977 

Clean Air Act Amendments and tasked it with providing 

scientific advice to aid EPA in setting NAAQS. See id. 

§ 7409(d)(2). Expressing its “desire for continued 

independent scientific review of the Environmental Protection 

Agency’s exercise of judgment,” H. Rep. No. 95-294, at 182 

(1977), Congress directed CASAC to complete a review of 

the air quality criteria and primary and secondary NAAQS 

every five years and to “recommend to the Administrator any 

new national ambient air quality standards and revisions of 

existing criteria and standards as may be appropriate,” 42 

U.S.C. § 7409(d)(2)(B). 

When Congress created CASAC, the promulgation of 

NAAQS was in its infancy. In describing the role it 

envisioned for CASAC, Congress emphasized the valuable 

role that advisory committees and expert groups had played in 

reviewing the first criteria documents and air quality 

standards issued in the late 1960s and early 1970s, explaining 

that “[f]or nearly 10 years the scientific basis for setting 

ambient air quality standards has been reviewed, evaluated, 

subjected to outside criticism, and reevaluated.” H. Rep. No. 

95-294, at 179–81. CASAC was intended to replicate this role 

by “provid[ing] an independent source of review and advice 

to the Administrator and to the Congress.” Id. at 182. Thus, 

Congress explained that it established CASAC “[b]ecause of 

the admitted need for greater research, the importance of the 

national ambient air quality standards, the continuing 

controversy over the standards, and the committee’s desire for 
34

continued independent scientific review of the Environmental 

Protection Agency’s exercise of judgment.” Id. 

Congress expected that CASAC’s central role would be 

one of scientific analysis, explaining that CASAC’s “main 

function” was “to assess the health and environmental effects 

of ambient air pollution.” Id. at 183. CASAC would “provide 

an outside mechanism for evaluating whether any pollutant 

may reasonably be anticipated to endanger public health or 

environment, for evaluating the scientific and medical data 

which might bear on this question, and for reviewing gaps in 

the available data and recommending additional needs for 

research.” Id. at 182. Given these functions, Congress 

expected that CASAC members would “be selected on the 

basis of their special expertise” in fields such as 

“environmental toxicology, epidemiology and/or clinical 

medicine.” Id. at 183. 

Congress also required EPA to take CASAC’s expert 

scientific recommendations into account in promulgating 

NAAQS. Although EPA is not bound by CASAC’s 

recommendations, it must fully explain its reasons for any 

departure from them. Specifically, section 307(d)(3) of the 

Act mandates that when EPA proposes to issue new NAAQS 

or revise existing NAAQS, the proposed rule must include a 

“statement of its basis and purpose” that “set[s] forth or 

summarize[s] and provide[s] a reference to any pertinent 

findings, recommendations, and comments by [CASAC].” 42 

U.S.C. § 7607(d)(3). If EPA’s “proposal differs in any 

important respect from any of [CASAC’s] recommendations,” 

the proposed rule must provide “an explanation of the reasons 

for such differences.” Id. Section 307(d)(6) of the Act 

requires that the final promulgated rule must also “be 

accompanied by . . . a statement of basis and purpose like that 

referred to in paragraph (3) with respect to a proposed rule.” 
35

Id. § 7607(d)(6)(A). Thus if, as here, EPA departs from 

CASAC’s recommendations in the final rule, EPA must also 

explain there its reasons for doing so. See American Farm 

Bureau, 559 F.3d at 521 (concluding that EPA failed in the 

final rule “adequately to explain its reason for not accepting 

the CASAC’s recommendations”). 

Congress intended that CASAC’s expert scientific 

analysis aid not only EPA in promulgating NAAQS but also 

the courts in reviewing EPA’s decisions. As Congress 

explained, CASAC’s “views are to be included in the record 

of any such rulemaking proceeding and, therefore, to be 

considered by the courts in reviewing the Administrator’s 

action or inaction.” H. Rep. No. 95-294, at 182–83. 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. If EPA’s quarrel is with CASAC’s scientific 

analysis, then in order to preserve the integrity of CASAC’s 

scientific role, EPA must give a sound scientific reason for its 

disagreement. In reviewing such scientific explanations, we 

undertake a “searching and careful” inquiry into the facts “to 

ascertain whether there is substantial evidence in the record 

when considered as a whole which supports the 

Administrator’s determinations.” Lead Industries Ass’n, 647 

F.2d at 1145–46 (internal quotation marks omitted). 

Alternatively, EPA could accept CASAC’s scientific analysis 

yet explain the policy considerations that led it to select a 

different level than that recommended by CASAC. See id. at 

1147. Of course, EPA’s policy judgments “are not susceptible 

to the same type of verification or refutation by reference to 

the record as are some factual questions,” and thus “our 

paramount objective” in reviewing them “is to see whether

the agency, given an essentially legislative task to perform, 

has carried it out in a manner calculated to negate the dangers 

of arbitrariness and irrationality.” National Lime Ass’n v. 
36

EPA, 627 F.2d 416, 431 n.48 (D.C. Cir. 1980) (internal 

quotation marks omitted).

In this case, the CASAC Ozone Review Panel was 

composed of twenty-three scientists who are professors, 

analysts, and other practitioners in fields such as medicine, 

anatomy, environmental science, and chemical engineering. 

Drawing on this substantial expertise, the twenty-three

members of the panel, in an October 2006 letter to EPA 

following CASAC’s peer review of the second draft of the 

agency’s Ozone Staff Paper, unanimously recommended that 

“the current primary ozone NAAQS be revised and that the 

level that should be considered for the revised standard be 

from 0.060 to 0.070 ppm.” Oct. 2006 CASAC Letter, at 5. In 

explaining the basis for this recommendation, CASAC noted 

that “[a] large body of data clearly demonstrates adverse 

human health effects at the current level of the 8-hr primary 

ozone standard.” Id. According to CASAC, “[r]etaining this 

standard would continue to put large numbers of individuals 

at risk for respiratory effects and/or significant impact on 

quality of life including asthma exacerbations, emergency 

room visits, hospital admissions and mortality.” Id.

CASAC also noted a large body of studies providing 

“evidence for adverse health effects at concentrations lower 

than the current standard.” Id. at 3. Among this evidence was 

a “broad range of epidemiologic and controlled exposure 

studies” observing multiple “adverse health effects due to 

low-concentration exposure to ambient ozone.” Id. at 4. In 

addition, CASAC explained that the Adams (2006) study had 

observed “[s]tatistically-significant decrements in lung 

function . . . at the 0.08 ppm exposure level,” as well as 

“adverse lung function effects . . . in some individuals at 0.06 

ppm.” Id. at 3. CASAC also noted that “these findings were 

observed in healthy volunteers” and that asthmatics and 
37

children had been found in other studies “to be more sensitive 

and to experience larger decrements in lung function in 

response to ozone exposures than would healthy volunteers.”

Id. at 4. Finally, pointing to the exposure and risk 

assessments, CASAC explained that “a significant decrease in 

adverse effects due to ozone exposures can be achieved by 

lowering the exposure concentrations below the current 

standard,” noting that “[b]eneficial effects in terms of 

reduction of adverse health effects were calculated to occur at 

the lowest concentration considered (i.e., 0.064 ppm).” Id. On 

the basis of all this evidence, CASAC concluded that “the 

current primary 8-hr standard of 0.08 ppm needs to be

substantially reduced to be protective of human health, 

particularly in sensitive subpopulations” and that the standard 

should be set within the range of 0.060 to 0.070 ppm. Id. at 4–

5. CASAC reiterated this recommendation in a March 2007 

letter to EPA, underscoring that “overwhelming scientific 

evidence” supported its recommendation “that the level of the 

current primary ozone standard should be lowered from 0.08 

ppm to no greater than 0.070 ppm.” Mar. 2007 CASAC 

Letter, at 2.

When EPA issued its notice of proposed rulemaking, it 

proposed to revise the primary ozone standard to within a 

range from 0.070 ppm, the high end of CASAC’s 

recommended range, to 0.075 ppm. 2007 Proposed Rule, 72 

Fed. Reg. at 37,878. EPA explained why it believed a 

standard set below 0.070 ppm would be inappropriate. Id. at 

37,880. In the final rule, EPA departed from CASAC’s 

recommended range and set the standard at 0.075 ppm. EPA

acknowledged that this standard was “above the range 

recommended by the CASAC.” 2008 Final Rule, 73 Fed. Reg. 

at 16,482. In explaining its departure, EPA catalogued its 

disputes with CASAC over the interpretation of specific 

bodies of scientific evidence and also noted that “the basis for 
38

[CASAC’s] recommendation appears to be a mixture of 

scientific and policy considerations.” Id. “[T]here is,” EPA 

stated, “no bright line clearly directing the choice of level, and 

the choice of what is appropriate is clearly a public health 

policy judgment entrusted to the Administrator.” Id. at 

16,482–83. In explaining this policy judgment, EPA reasoned 

that “[a] standard set at a level lower than 0.075 would only 

result in significant further public health protection if, in fact, 

there is a continuum of health risks in areas with 8-hour 

average O3 concentrations that are well below the 

concentrations observed in the key controlled human 

exposure studies and if the reported associations observed in 

epidemiological studies are, in fact, causally related to O3 at 

those lower levels.” Id. at 16,483. “Based on the available 

evidence,” EPA declared that it was “not prepared to make 

these assumptions.” Id. “Taking into account the uncertainties 

that remain in interpreting the evidence from available 

controlled human exposure and epidemiological studies at 

very low levels,” EPA concluded that “the likelihood of 

obtaining benefits to public health with a standard set below 

0.075 ppm O3 decreases, while the likelihood of requiring 

reductions in ambient concentrations that go beyond those 

that are needed to protect public health increases.” Id. EPA 

thus “judge[d] that the appropriate balance to be drawn, based 

on the entire body of evidence and information available in 

this review, is a standard set at 0.075.” Id.

This explanation rests largely on EPA’s policy judgment 

about the appropriate NAAQS level. We have explained that, 

where EPA operates within the realm of uncertain science, its 

decisions about the appropriate NAAQS level must 

“necessarily . . . rest largely on policy judgments.” Lead 

Industries Ass’n, 647 F.2d 1147 (internal quotation marks 

omitted). But this presupposes that the scientific evidence is 

actually uncertain—a question that itself requires a scientific 
39

determination. EPA did not make such a specific scientific 

determination about the 0.070 ppm level that served as the 

ceiling of CASAC’s recommendation; instead, EPA referred 

generally to declining certainty below 0.075 ppm. Had

CASAC reached a scientific conclusion that adverse health 

effects were likely to occur at the 0.070 ppm level, EPA’s 

failure to justify its uncertainty regarding the existence of 

adverse health effects at this level would be unacceptable.

6

Indeed, it is a familiar principle that agencies may not 

“merely recite the terms ‘substantial uncertainty’ as a 

justification for [their] actions”; instead, they “must explain 

the evidence which is available, and must offer a rational 

connection between the facts found and the choice made.” 

State Farm, 463 U.S. at 52 (internal quotation marks omitted).

In other words, EPA must explain why the evidence on which 

CASAC relied cannot support the degree of confidence 

CASAC placed in it. This is especially true given the added 

layer of stringency imposed by EPA’s obligations under 

section 307(d)(6).

But we are unable to determine whether CASAC reached 

any such scientific conclusion. Although CASAC stated that 

“overwhelming scientific evidence” supported its 

recommendation that the standard be set no higher than 0.070 

ppm, Mar. 2007 CASAC Letter, at 2, it never explained 

whether this proposal was based on its scientific judgment 

that adverse health effects would occur at that level or instead 

based on its more qualitative judgment that the range it 

proposed would be appropriately protective of human health 

 6 This conclusion concerns only disagreements regarding the 

certainty of the science; of course, EPA could also have accepted 

CASAC’s scientific conclusion and explained its view that any 

health effects at that level were not severe enough to be considered 

“adverse.”
40

with an adequate margin of safety. Indeed, although CASAC 

concluded that “there is no longer significant scientific 

uncertainty regarding [its] conclusion that the current 8-hr 

primary NAAQS must be lowered,” given the “large body of 

data clearly demonstrat[ing] adverse human health effects at 

the current level,” CASAC recognized that “[s]cientific 

uncertainty does exist with regard to the lower level of ozone 

exposure that would be fully-protective of human health.”

Oct. 2006 CASAC Letter, at 5.

To be sure, EPA’s statutory obligation to respond to 

CASAC does not evaporate whenever CASAC exercises 

judgment amidst scientific uncertainty. Quite to the contrary, 

had CASAC acknowledged uncertainty in the scientific 

evidence but explained that, based on its expert scientific 

judgment, it nonetheless believed adverse health effects were 

likely to occur at the 0.070 ppm level, then section 307(d)(6) 

would have required EPA to explain why it disagreed with 

this scientific conclusion. Put differently, to the extent that 

CASAC has exercised scientific judgment, EPA must respond 

in kind. But because CASAC never made clear the precise 

basis for its recommendation, all we know for certain is this: 

both CASAC and EPA believed the existence of adverse 

health effects to be certain at the 0.08 ppm level and reached 

differing conclusions about what level below 0.08 ppm was 

requisite to protect the public health with an adequate margin 

of safety.

The 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. Here, 

EPA’s policy judgment was informed by its view of the 

limitations of the scientific evidence—namely, that at lower 

levels of ozone exposure, the clinical and epidemiological 
41

studies provide less conclusive evidence of the existence of 

adverse health effects. See 2008 Final Rule, 73 Fed. Reg. at 

16,483 (noting “the uncertainties that remain in interpreting 

the evidence from available controlled human exposure and 

epidemiological studies at very low levels”). Striking a 

balance between “the increasing uncertainty associated with 

[its] understanding of the likelihood of such effects at lower 

O3 exposure levels” and “concern about the potential for 

health effects and their severity,” id. at 16,477, EPA set the 

standard at 0.075 ppm, a level the agency believed to be 

“appreciably below” the 0.08 ppm level at which both EPA 

and CASAC expressed certainty about the existence of 

adverse health effects, id. at 16,483. Absent a definitive 

scientific conclusion from CASAC that adverse health effects 

would occur at the 0.070 ppm level, we must assume that it 

too took these same considerations into account and simply 

exercised its judgment to recommend a standard set at a lower

level. Although both CASAC and EPA must exercise public 

health policy judgment when confronted with scientific 

evidence that does not direct it to a specific outcome, it is to 

EPA’s judgment that we must defer.

In our view, this conclusion is perfectly consistent with 

the role Congress intended CASAC to play in the NAAQSsetting process. In order to ensure that EPA’s NAAQS 

decisions rest on sound scientific judgment, Congress 

required EPA not only to describe CASAC’s 

recommendations in any rulemaking but also, if it departs 

from such recommendations, to explain its reasons for doing 

so. See 42 U.S.C. § 7607(d)(3), (6). But in order for EPA to 

explain adequately its reasons for disagreeing with CASAC, 

CASAC itself must be precise about the basis for its 

recommendations. Because in this case CASAC failed to 

specify whether the 0.070 ppm level it recommended as a 

maximum rested on a scientific conclusion about the 
42

existence of adverse health effects at that level, EPA’s 

invocation of scientific uncertainty and more general public 

health policy considerations satisfies its obligations under the 

statute.

IV.

We turn finally to EPA’s decision to set the secondary 

ozone NAAQS “identical in every way to the revised primary 

standard.” 2008 Final Rule, 73 Fed. Reg. at 16,500. The 

governmental and environmental petitioners argue, among 

other things, that EPA’s failure to “specify a level of air 

quality . . . [that] is requisite to protect the public welfare,” 42 

U.S.C. § 7409(b)(2), violates the statute as interpreted in our 

decision in American Farm Bureau. Because we agree that 

EPA’s justification for the secondary standard is inadequate 

under American Farm Bureau, we need not reach petitioners’ 

other arguments. 

As described above, the Clean Air Act requires secondary 

NAAQS to “specify a level of air quality the attainment and 

maintenance of which . . . is requisite to protect the public 

welfare from any known or anticipated adverse effects 

associated with the presence of such air pollutant in the 

ambient air.” Id. Regarding ozone, EPA set the secondary 

NAAQS in 1997 to protect against harmful effects on 

vegetation and indirect effects on other ecosystem 

components. See 2008 Final Rule, 73 Fed. Reg. at 16,485. In 

the current review of the secondary standard, before EPA 

came to its final decision, agency staff examined new 

scientific evidence and risk assessments that evaluated 

ozone’s welfare effects. Id. On the basis of this new evidence, 

EPA staff concluded that the existing 0.08 ppm 8-hour 

standard was inadequate because ozone at that level directly 

causes adverse effects to vegetation and has indirect adverse 

effects on soil, water, and wildlife. 2007 Proposed Rule, 72 
43

Fed. Reg. at 37,883, 37,897–99. EPA staff also considered 

whether the evidence still justified a standard that measured

ozone over an 8-hour interval or whether instead the 

secondary standard should measure ozone cumulatively over 

a seasonal period. Id. at 37,882–83. In the end, EPA staff 

found that new evidence about the cumulative effect of ozone 

on vegetation supported a seasonal standard and 

recommended that the agency consider a range of seasonal 

levels between 7 and 21 ppm-hours. Id. at 37,900, 37,903.

CASAC unanimously agreed with EPA staff that adverse 

effects on vegetation occur under the existing standard and 

that “it is not appropriate to try to protect vegetation . . . by 

continuing to promulgate identical primary and secondary 

standards for O3.” 2008 Final Rule, 73 Fed. Reg. at 16,492.

All but one member of the CASAC panel “encourage[d] 

[EPA] to establish an alternative cumulative secondary 

standard for O3 and related photochemical oxidants that is 

distinctly different in averaging time, form and level from the 

currently existing or potentially revised 8-hour primary 

standard.” 2007 Proposed Rule, 72 Fed. Reg. at 37,899 & 

n.62 (internal quotation marks omitted). CASAC also agreed 

with EPA staff that the lowest seasonal level that the agency 

should consider was 7 ppm-hours, but recommended that 

EPA consider a level no higher than 15 ppm-hours. Id. at 

37,903.

In the final rule, EPA agreed that new evidence indicates 

that ozone causes adverse effects on vegetation and related 

ecosystems at the current level of the secondary standard and 

concluded that it was appropriate to revise the secondary 

standard to provide increased protection. 2008 Final Rule, 73 

Fed. Reg. at 16,496, 16,499–500. Regarding the 

recommendations to adopt a cumulative seasonal standard, 

EPA cited a staff analysis that found “significant overlap” 
44

between counties expected to meet the revised 8-hour primary 

standard and counties that would meet a cumulative seasonal 

standard. Id. at 16,499. EPA “focused [its] consideration on a 

level for an alternative [seasonal] standard at the upper end of 

the proposed range (i.e., 21 ppm-hours)” and found 

“essentially no counties with air quality that would be 

expected both to exceed such an alternative [seasonal] 

standard and to meet the revised 8-hour primary standard.” Id.

at 16,499–500. From this comparison, EPA concluded that 

merely revising the secondary standard to match the revised 

primary standard would “provide a significant degree of 

additional protection for vegetation” and that “a [seasonal] 

standard would be unlikely to provide additional protection in 

any areas beyond that likely to be provided by the revised 

primary standard.” Id. at 16,499–500. Citing the “significant 

uncertainties in determining or quantifying the degree of risk 

attributable to varying levels of O3 exposure, the degree of 

protection that any specific cumulative, seasonal standard 

would produce, and the associated potential for error in 

determining the standard that will provide a requisite degree 

of protection,” EPA rejected a cumulative seasonal standard 

in favor of a secondary standard that was identical to the 

revised primary standard. Id. at 16,500.

In American Farm Bureau, we rejected EPA’s 

explanation for setting the fine particulate matter secondary 

NAAQS—which protects public welfare from adverse 

visibility effects—identical to the primary fine particulate 

matter standard. 559 F.3d at 530–31. While the primary 

standard measured fine particulate matter levels annually, 

EPA staff and CASAC had recommended that the secondary 

standard measure fine particulate matter over 4- or 8-hour 

periods, suggesting a range of appropriate levels for this 

alternatively measured standard. Id. at 528. EPA rejected 

these recommendations on the ground that the evidence 
45

supporting the recommended alternative standard was 

“limited and uncertain,” instead adopting a secondary 

standard that was identical to the primary standard. Id. at 529

(internal quotation marks omitted). In so doing, EPA relied on 

a comparison purporting to show that the revised primary 

NAAQS would actually provide slightly more visibility 

protection than one proposed level of the alternative standard. 

Id.

Relying on the statute’s plain language—EPA “shall 

specify a level of air quality the attainment and maintenance 

of which . . . is requisite to protect the public welfare from 

any known or anticipated adverse effects,” 42 U.S.C. 

§ 7409(b)(2)—we rejected EPA’s explanation, finding that 

EPA must “determine what level of visibility protection is 

requisite to protect the public welfare,” American Farm 

Bureau, 559 F.3d at 530. We also found that EPA’s reliance 

on the comparison between the primary standard and the 

recommended secondary standards “fail[ed] on its own 

terms.” Id. “[T]wo-thirds of the potential standards within the 

CASAC’s recommended range,” we explained, “would be 

substantially more protective than the primary standards,” and 

“EPA failed to explain why it looked only at one of the few 

potential standards that would be less protective.” Id.

Furthermore, we faulted EPA’s failure to respond to technical 

problems with the comparison identified by CASAC and EPA 

staff. Id. at 530–31. 

Although American Farm Bureau was decided after EPA 

issued the rule challenged here, the decision is binding on us 

now—a proposition that EPA nowhere disputes. Bradley v. 

School Board of City of Richmond, 416 U.S. 696, 714 (1974) 

(“[A]n appellate court must apply the law in effect at the time 

it renders its decision.” (quoting Thorpe v. Housing Authority 

of City of Durham, 393 U.S. 268, 281 (1969))). Indeed, the 
46

statutory requirement that the secondary NAAQS “specify a 

level of air quality the attainment and maintenance of 

which . . . is requisite to protect the public welfare,” 42 U.S.C. 

§ 7409(b)(2), existed when EPA issued the rules at issue in 

American Farm Bureau and here.

EPA’s explanation for setting the secondary standard 

identical to the primary standard fails under American Farm 

Bureau. As we explained there, it is insufficient for EPA 

merely to compare the level of protection afforded by the 

primary standard to possible secondary standards and find the 

two roughly equivalent. EPA must expressly “determine what 

level of . . . protection is requisite to protect the public 

welfare,” American Farm Bureau, 559 F.3d at 530, and 

explain why this is so. Here EPA found “significant overlap” 

between the revised primary standard and “selected levels” of

a seasonal standard, 2008 Final Rule, 73 Fed. Reg. at 16,499, 

and it did say that the revised primary standard “would be 

sufficient to protect public welfare from known or anticipated 

adverse effects,” id. at 16,500. But it justified this conclusion 

only by comparing the revised primary standard to a seasonal 

level of 21 ppm-hours that EPA never “specif[ed]” was

“requisite to protect the public welfare,” 42 U.S.C. 

§ 7409(b)(2)—exactly what American Farm Bureau held is 

inconsistent with the statute.

EPA argues that it “identified a target level of protection 

in terms of a cumulative, seasonal standard.” Respondent’s

Br. 122. In support, the agency points to the sentence in the 

final rule stating that EPA “focused [its] consideration on a 

level . . . at the upper end of the proposed range (i.e., 21 ppmhours).” 2008 Final Rule, 73 Fed. Reg. at 16,499–500. But 

neither this statement nor anything else EPA said indicated 

that the 21 ppm-hours level was “requisite to protect the 

public welfare.” Perhaps more importantly, EPA never 

explained why a 21 ppm-hours level would, in fact, be 
47

requisite to protect vegetation. That a seasonal standard of 21 

ppm-hours was one of the levels proposed by EPA staff 

hardly shows that the level was “requisite to protect the public 

welfare.”

Also as in American Farm Bureau, EPA’s comparison 

between the primary and secondary standards “fails on its 

own terms.” 559 F.3d at 530. Although the comparison 

between the revised 8-hour standard and a seasonal standard 

showed that the level of protection afforded by the revised 

primary standard would be arguably equivalent to the level of 

protection afforded by a 21 ppm-hours seasonal standard, the 

comparison also showed that the primary standard would 

offer less protection than other seasonal levels within the 

range recommended by CASAC and EPA staff. See 2007 

Proposed Rule, 72 Fed. Reg. at 37,892–93. In American Farm 

Bureau, “EPA failed to explain why it looked only at one of 

the few potential standards that would be less protective . . . 

than the primary standard,” 559 F.3d at 530; in this case, EPA 

failed to explain why it looked only at one potential seasonal 

standard that the primary standard would arguably protect as 

well as.

At oral argument, counsel for EPA repeatedly insisted 

that petitioners “tacitly conceded” that the agency identified a 

target level of protection by “criticiz[ing] the reason EPA 

focused on 21 [ppm-hours].” Oral Arg. Rec. 1:50:43–51:06; 

see also id. 1:50:02–16. But counsel confuses assuming a 

premise for the sake of argument with conceding the point. 

Petitioners argue both that EPA failed to identify a target level 

of protection and that, even if EPA had in fact determined that 

the “requisite” level was 21 ppm-hours, that finding was 

irrational. See Environmental Petitioners’ Br. 35–37 (“EPA 

Acted Illegally and Arbitrarily in Failing to Identify the Level 

of Air Quality Requisite to Protect Against Adverse 

Vegetation Impacts.”); id. at 37–39 (“EPA’s Decision on the 
48

Secondary Standard Was Irrational.”); id. at 39–40 (“EPA’s 

Attempts to Justify Its Secondary Standard Were 

Groundless.”). Contending that settling on 21 ppm-hours 

would be senseless hardly precludes petitioners from arguing 

that EPA never expressly made the required determination.

Because EPA failed to determine what level of protection 

was “requisite to protect the public welfare,” EPA’s 

explanation for the secondary standard violates the Act. We 

therefore remand this portion of the final rule for further 

explanation or reconsideration by EPA. In the meantime, we 

leave the standard in place rather than vacating the rule. 

“First, the EPA’s failure adequately to explain itself is in 

principle a curable defect. Second, vacating a standard 

because it may be insufficiently protective would sacrifice 

such protection as it now provides, making the best an enemy 

of the good.” American Farm Bureau, 559 F.3d at 528. Given 

these principles, neither EPA nor petitioners advocate vacatur.

V.

For the foregoing reasons, we remand the secondary 

NAAQS to EPA for reconsideration in view of this opinion. 

In all other respects, the petitions for review are denied.

So ordered.