United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued May 3, 2012 Decided July 20, 2012

No. 10-1252

NATIONAL ENVIRONMENTAL DEVELOPMENT ASSOCIATION'S

CLEAN AIR PROJECT,

PETITIONER

v.

ENVIRONMENTAL PROTECTION AGENCY,

RESPONDENT

LOUISIANA DEPARTMENT OF ENVIRONMENTAL QUALITY, ET

AL.,

INTERVENORS

Consolidated with 10-1254, 10-1255, 10-1256, 10-1258,

10-1259, 11-1073, 11-1080, 11-1081, 11-1090, 11-1092

On Petitions for Review of a Final Action of the 

United States Environmental Protection Agency

Paul M. Seby, Special Assistant Attorney General, Office of

the Attorney General for the State of North Dakota, et al.,

argued the cause for petitioner State of North Dakota. Mark W.

DeLaquil argued the cause for petitioner ASARCO LLC. With

them on the briefs were Wayne Stenehjem, Attorney General,

Office of the Attorney General for the State of North Dakota,
2

Margaret I. Olson, Assistant Attorney General, Andrea Bear

Field, Lucinda Minton Langworthy, Robert N. Steinwurtzel,

Leslie Sue Ritts, Benjamin Joseph Alke, Robert K. Baldwin,

Marty J. Jackley, Attorney General, Office of the Attorney

General for the State of South Dakota, Roxanne Giedd, Deputy

Attorney General, Catherine Cortez Masto, Attorney General,

Office of the Attorney General for the State Nevada, Jasmine K.

Mehta, Deputy Attorney General, James D. “Buddy” Caldwell,

Attorney General, Office of the Attorney General for the State

of Louisiana, Megan K. Terrell, Assistant Attorney General,

Herman Robinson, Donald J. Trahan, Deidra L. Johnson,

Gregory W. Abbott, Attorney General, Office of the Attorney

General for the State of Texas, and Nancy E. Olinger, Assistant

Attorney General. Nhu Q. Nguyen, Senior Deputy Attorney

General, Office of the Attorney General for the State of Nevada,

entered an appearance.

Madison B.C. Miller was on the brief for amicus curiae

Oklahoma Department of Environmental Quality in support of

state petitioners, non-state petitioners and supporting

intervenors.

Norman L. Rave Jr. , Attorney, U.S. Department of Justice,

argued the cause for respondent. With him on the brief were

Steven Silverman, Attorney, United States Environmental

Protection Agency, and Michael Thrift, Attorney. 

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

respondent-intervenors American Lung Association, et al. 

Before: SENTELLE, Chief Judge, KAVANAUGH, Circuit

Judge, and GINSBURG, Senior Circuit Judge.

Opinion for the Court filed by Chief Judge SENTELLE.
3

SENTELLE, Chief Judge: Several states and state regulatory

agencies, together with corporations and industrial associations,

petition for review of the Environmental Protection Agency’s

rule entitled “Primary National Ambient Air Quality Standard

for Sulfur Dioxide,” and of the subsequent denial of petitions for

reconsideration of the standard. Petitioners contend, first, that

EPA failed to follow notice-and-comment rulemaking

procedures, and second, that the agency arbitrarily set the

maximum sulfur dioxide (SO2) concentration at a level lower

than statutorily authorized. For the reasons discussed more fully

below, we conclude that the challenge to the rulemaking

procedure is not within our jurisdiction and must be dismissed. 

We further conclude that EPA did not act arbitrarily in setting

the level of SO2 emissions and therefore deny that portion of the

petitions for review. 

I. Background

A. The Clean Air Act

The Clean Air Act (CAA) in §§ 108 and 109 requires EPA

to establish, review, and revise air quality criteria and standards,

allowing an “adequate margin of safety.” 42 U.S.C. §§ 7408,

7409. The 1970 amendments to the Act required the

Administrator to publish a list of air pollutants it intended to

regulate under the Act, including all those pollutants the

Administrator found reasonably could be anticipated to

endanger public health. 42 U.S.C. § 7408(a)(1). For each listed

pollutant, the Administrator had to issue air quality criteria that

“accurately reflect[ed] 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,”

including the effects of a pollutant when it combines with other

factors such as atmospheric conditions or other pollutants. 42
4

U.S.C. § 7408(a)(2). 

The CAA required the Administrator to promulgate a

primary and secondary National Ambient Air Quality Standard

(NAAQS) for each listed pollutant by 1971 and to review and

revise those standards as appropriate every five years. 42 U.S.C.

§ 7409(a), (d)(1). The Act requires that the primary standards

“be ambient air quality standards the attainment and

maintenance of which in the judgment of the Administrator,

based on such criteria and allowing an adequate margin of

safety, are requisite to protect the public health.” 42 U.S.C.

§ 7409(b)(1). 

The Act vests each State with “the primary responsibility

for assuring air quality within the entire geographic area

comprising such State . . . .” 42 U.S.C. § 7407(a). After EPA

promulgates a new final standard, the Act gives States a chance

to recommend whether areas within their boundaries should be

designated as “nonattainment,” “attainment,” or

“unclassifiable,” and the Agency makes the final designation. 

42 U.S.C. § 7407(d). States then must submit State

Implementation Plans (SIPs), which, after receiving EPA

approval, impose federally enforceable controls on air pollution

sources so States can attain and maintain the NAAQS. 42

U.S.C. §§ 7410, 7502, 7514-7514a. 

B. Regulatory Background: Sulfur Dioxide NAAQS

Sulfur dioxide, a “highly reactive colorless gas,” derives

mostly from fossil fuel combustion. It smells like rotting eggs

and, at elevated concentrations in the air, can cause acid rain. Its

presence in the ambient air can cause adverse health effects,

particularly in asthmatics. See Am. Lung Ass’n v. EPA, 134

F.3d 388, 389 (D.C. Cir. 1998). 
5

On April 30, 1971, EPA promulgated the first primary

NAAQS for SO2 concentrations in the ambient air. 36 Fed. Reg.

8186 (Apr. 30, 1971). The standard set a 24-hour concentration

limit of 140 parts per billion (ppb) SO2, and an annual average

limit of 30 ppb. Id. at 8187. Over the next three decades, EPA

reviewed the standard, but did not revise it. See 74 Fed. Reg.

64810, 64813 (Dec. 8, 2009) (providing history of the SO2

NAAQS). 

In 1988, EPA declined to revise the NAAQS, but requested

comment on a proposal to add a new 1-hour primary standard of

400 ppb to protect against five- to ten-minute bursts of SO2

concentrations. 53 Fed. Reg. 14926 (Apr. 26, 1988). In

response to those comments and other developments, in 1994,

EPA offered several more options for comment, including the

addition of a five-minute standard of 600 ppb. 59 Fed. Reg.

58958 (Nov. 15, 1994). After concluding its review of these

proposals and comments in 1996, EPA announced it would not

revise the NAAQS. In its review, it found that under the current

standards at that time, thousands of asthmatics could be exposed

to enough short-term bursts of SO2 that their lung function could

be impaired. 61 Fed. Reg. 25566, 25572 (May 22, 1996). EPA

concluded, however, that such effects “do not pose a broad

public health problem when viewed from a national perspective”

and did not warrant revisions to the SO2 NAAQS. Id. at 25572,

25575. 

The American Lung Association and the Environmental

Defense Fund challenged before this Court the Administrator’s

decision not to implement a five-minute standard. We found

that EPA had failed to explain adequately how it reached its

decision not to revise the NAAQS, given that the Administrator

had found that short-term exposures to bursts of SO2 could

significantly affect the lung function of thousands of asthmatics. 

Am. Lung Ass’n, 134 F.3d at 392-93 (D.C. Cir. 1998). 
6

Accordingly, we remanded the decision to EPA.

In response, EPA initiated the review of the SO2 NAAQS

that eventually led to this proceeding. See 71 Fed. Reg. 28023

(May 15, 2006). Based on that review, EPA proposed a rule to

revise the primary SO2 standard. EPA proposed, inter alia, to

revoke the current 24-hour and annual standards and to establish

a standard to target short-term bursts of SO2

exposure—specifically, a 99th percentile 1-hour daily maximum

standard level set somewhere between 100 ppb and 50 ppb. 74

Fed. Reg. 64810, 64845-86 (Dec. 8, 2009). EPA also proposed

to amend ambient air monitoring, reporting, and network design

requirements. The proposal focused on increasing and updating

the monitoring network to support the proposed 1-hour standard. 

74 Fed. Reg. at 64846-47. 

C. The Final Rule

After receiving comments on its rule proposal, EPA issued

a final rule addressing the primary SO2 standard. 75 Fed. Reg.

35520 (June 22, 2010). Petitioners challenge two parts of the

final rulemaking, which we describe here—the level at which

EPA set the standard and a portion of its statements regarding

the implementation plan for the standard.

1. Level

EPA mandated that States must meet a new 1-hour SO2

standard using a 99th percentile form, set at 75 ppb maximum

SO2 concentration. 75 Fed. Reg. at 35548. The goal of the new

standard is to prevent asthmatics from being exposed to shortterm, five- to ten-minute bursts of SO2, which EPA found could

cause lung function decrements in asthmatics. Id. 
7

EPA explained that it conducted substantial amounts of new

research to determine the appropriate level for the 1-hour SO2

NAAQS. 75 Fed. Reg. at 35524. In 2008, EPA staff prepared

an Integrated Science Assessment (ISA), which summarized the

latest scientific knowledge regarding effects of exposure to SO2. 

In 2009, EPA staff prepared a Risk and Exposure Assessment

(REA) to quantify the public health effects of exposure to SO2

in the ambient air. See 75 Fed. Reg. at 35523-24 (discussing

development of ISA and REA). The ISA and REA focused on

two types of studies—controlled human exposure clinical

studies and epidemiologic studies. See 75 Fed. Reg. at 35525. 

The controlled human exposure studies examined the effects of

varying levels of SO2 on unmedicated asthmatics performing

exercises. The studies did not test subjects with severe asthma

because of ethical concerns. 75 Fed. Reg. at 35533. The

epidemiologic studies considered whether a statistical

association exists between levels of SO2 in the ambient air and

the occurrence of events such as hospital admissions and

emergency room visits for respiratory ailments. The ISA and

REA also reviewed animal studies. 75 Fed. Reg. at 35525. 

The epidemiologic studies showed that in geographic areas

meeting the previous 24-hour and annual concentration limits,

there were positive associations between ambient air

concentrations of SO2 and respiratory symptoms in children,

emergency department visits, and hospitalizations for respiratory

conditions. Id. 

Clinical studies showed that mild and moderate asthmatics

exposed to SO2 concentrations as low as 200 to 300 ppb for five

to ten minutes experienced moderate or greater decrements in

lung function. See 75 Fed. Reg. at 35525. As SO2 exposure

increased, both the severity of the decrements and the number of

asthmatics affected increased. Id. At 400 ppb and greater, the

effects often were statistically significant at the group mean
8

level and were accompanied by respiratory symptoms. Id. In

the REA, EPA determined that a 1-hour NAAQS set at 50 to 100

ppb could limit exposures of exercising asthmatic children to

five-minute peak SO2 levels greater than or equal to 400 ppb. A

1-hour 150 ppb standard could limit their exposure to fiveminute 400 ppb concentrations, but would provide “appreciably

less” protection to five-minute exposures of 200 ppb

concentrations. 75 Fed. Reg. at 35528-29. 

The EPA Administrator determined that the studies showed

that the NAAQS should protect asthmatics from 200 ppb shortterm bursts of SO2. She concluded that a 1-hour standard level

set at 75 ppb would accomplish this goal and provide an

adequate margin of safety. 75 Fed. Reg. at 35548. 

2. Implementation

 In the preamble to the final rule, EPA also explained that,

based on comments it received, “we are revising our general

anticipated approach toward implementation of the new 1-hour

NAAQS.” 75 Fed. Reg. at 35550. Instead of assessing

attainment of the standard primarily by monitoring the ambient

air, as it had stated it would in the proposed rule, EPA suggested

it would use a “hybrid analytic approach” that would combine

monitoring with computer modeling to determine compliance. 

75 Fed. Reg. at 35551.

Several states and environmental regulatory bodies of states

charged with implementing the SO2 standards, along with

several companies and coalitions that represent industries that

emit SO2 as a byproduct of their industrial activities, petition for

review of the EPA’s rulemaking. Petitioners allege two errors

in the EPA’s proceedings. All Petitioners contend that EPA

violated the notice-and-comment rulemaking provisions of the

Administrative Procedure Act (APA), 5 U.S.C. § 553, by
9

mandating a hybrid modeling-monitoring implementation

approach rather than a monitoring-only approach. Only the nonstate petitioners bring the second challenge, contending that the

Agency’s decision to set the new 1-hour SO2 standard at 75 ppb

was arbitrary and capricious. 

II. Analysis

A. Computer Modeling Provisions

Petitioners argue that EPA failed to follow the notice-andcomment rulemaking provisions of the APA because it did not

include in its proposed rule any suggestion that EPA was

considering changing its method of determining attainment of

the SO2 standard from an air-monitoring approach to a hybrid

approach using computer modeling in combination with air

monitoring. The APA requires agencies to publish proposed

rules in the Federal Register and afford interested parties

opportunity to comment on the proposals. 5 U.S.C. § 553; see

PPG Indus., Inc. v. Costle, 659 F.2d 1239, 1241 (D.C. Cir.

1981) (requiring EPA to comply with APA notice-and-comment

rulemaking procedures when changing a NAAQS

implementation program). Petitioners point out that statements

in the preamble to the final rule suggesting a computer-modeling

and air-monitoring approach were not part of the original

proposed rule submitted for notice and comment, and that

therefore they had no opportunity to comment thereon.

We do not have jurisdiction, however, to consider whether

EPA has violated APA rulemaking procedures because the

challenged statements do not constitute final agency action. The

CAA provides that this Court has jurisdiction to review “any

national primary or secondary ambient air quality standard . . .

or final action taken . . . by the Administrator.” 42 U.S.C.

§ 7607(b) (emphasis added); see Indep. Equip. Dealers Ass’n v.
10

EPA, 372 F.3d 420, 426 (D.C. Cir. 2004). An agency action is

“final” if it meets two conditions: the action must “mark the

consummation of the agency’s decisionmaking process,” and the

action must be “one by which rights or obligations have been

determined, or from which legal consequences will flow.” 

Bennett v. Spear, 520 U.S. 154, 177-78 (1997) (internal

quotation marks and citation omitted). The challenged

statements in the preamble here do not meet these conditions.

 Petitioners compare the preamble’s statements here to the

statements in the preamble to the final rule challenged in

Natural Resources Defense Council v. EPA, 571 F.3d 1245

(D.C. Cir. 2009). There, we recognized that the challenged

statements were “not conjectural” and that their “terms [were]

clear,” so therefore it was “fair to infer that the EPA intended

the statements to create binding legal consequences.” Id. at

1252 n.2. Here, the preamble’s statements about the hybrid

approach are much less clear, demonstrating that the statements

do not mark the consummation of agency action, and they do not

create obligations from which legal consequences will flow. 

The preamble explains that given the comments EPA received

regarding the proposed monitoring approach, “we are revising

our general anticipated approach toward implementation of the

new 1-hour NAAQS.” 75 Fed. Reg. at 35550. It cautions that

the discussions here “explain our expected and intended future

action in implementing the new 1-hour NAAQS—in other

words, they constitute guidance, rather than final agency

action—and it is possible that our approaches may continue to

evolve as we, States, and other stakeholders proceed with actual

implementation.” Id. In discussing the specifics of its intended

hybrid approach, EPA states that, “[w]e believe that some type

of hybrid approach is more consistent with our historical

approach and longstanding guidance toward SO2 than what we

originally proposed.” Id. at 35551. EPA explains that it

“anticipates making the determination of when monitoring alone
11

is ‘appropriate’ for a specific area on a case-by-case basis,

informed by the area’s factual record, as part of the designations

process,” citing as an example of a situation in which

monitoring might be the preferred approach an area in which a

shipping port is the only significant stationary source of SO2. Id.

at 35552 n.22. EPA notes that it “intends to solicit public

comment prior to finalizing this guidance.” Id. at 35552. This

language all suggests an indefinite, anticipated plan.

We do not suggest that if the language had imposed definite

requirements upon states or regulated industries we would be

bound by the agency’s characterization. Certainly if that were

the case, we could consider rejecting the characterization and

consider Petitioners’ challenges to what might then be final

agency action for purposes of judicial review. See Barrick

Goldstrike Mines Inc. v. Browner, 215 F.3d 45, 48 (D.C. Cir.

2000) (agency labels regarding finality or lack thereof are not

determinative). But the preamble imposes no such

requirements. EPA explained that it expected to make initial

attainment designations in 2012 based on existing monitoring

capabilities, as well as “any refined modeling the State chooses

to conduct specifically for initial area designations.” 75 Fed.

Reg. at 35552. That language does not impose new legal

obligations to use modeling. 

To be sure, because EPA now intends to use this hybrid

approach, it has scaled back its proposed plans to develop a

more extensive monitoring network. 75 Fed. Reg. at 35551

(“This projected change in approach would necessarily result in

a lesser emphasis on the less appropriate, more expensive, and

slower to establish monitoring tool than did the proposed rule. 

Therefore, the minimum requirements for the SO2 monitoring

network in this final rule are of a smaller scale than proposed

. . . .”). Petitioners do not argue, however, that they have

suffered an injury by not being required to build a more
12

extensive monitoring network. Petitioners will be free to

challenge any final action EPA takes that imposes an obligation

Petitioners must meet. The challenged provisions here do not

meet that standard.

B. NAAQS Level

Some Petitioners also challenge the level at which EPA set

the maximum concentration for the new 1-hour SO2 standard,

arguing that 75 ppb is lower than “requisite” to protect public

health. We have jurisdiction to consider this challenge under

CAA Section 307(b)(1), which provides that this Court has

exclusive jurisdiction over petitions for review of national

ambient air quality standards promulgated by the EPA

Administrator. 42 U.S.C. § 7607(b)(1). Under the CAA, 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.” 42 U.S.C. § 7607(d)(9)(A); see also

APA Section 706 (5 U.S.C. § 706); Ethyl Corp. v. EPA, 51 F.3d

1053, 1064 (D.C. Cir. 1995) (arbitrary and capricious standard

under the CAA is interpreted in “essentially the same” way as

the same standard under the APA). We owe deference to the

Administrator’s determination regarding the reliability of

scientific evidence. Am. Trucking Ass’ns v. EPA, 283 F.3d 355,

374 (D.C. Cir. 2002). Although we must perform a “searching

and careful” inquiry into the facts, 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.” Ethyl Corp. v. EPA, 541 F.2d 1, 36-37

(D.C. Cir. 1976) (en banc). 

Under the Clean Air Act, the EPA Administrator must set

NAAQS at a level “requisite to protect the public health,”

“allowing an adequate margin of safety.” 42 U.S.C.

§ 7409(b)(1). This Court has recognized that Congress defined
13

public health broadly, requiring NAAQS to “protect not only

average healthy individuals, but also ‘sensitive citizens,’” such

as children or people afflicted with asthma, emphysema, or other

conditions causing sensitivity to air pollution. Am. Lung Ass’n,

134 F.3d at 389 (citing SEN. REP. No. 91-1196, at 10 (1970)). 

The Supreme Court has held that NAAQS are set at the

“requisite” level if they are set at a level “not lower or higher

than is necessary” to protect public health. Whitman v. Am.

Trucking Ass’ns, 531 U.S. 457, 475-76 (2001); see also 75 Fed.

Reg. at 35521 (EPA must “establish standards that are neither

more nor less stringent than necessary.”). 

1. Interpretation of Human Clinical Studies

Petitioners first argue that the Administrator’s decision to

adopt a 75 ppb standard was arbitrary and capricious because

EPA misinterpreted the controlled human exposure clinical

studies. Specifically, they criticize how EPA applied guidelines

published by the American Thoracic Society (ATS) regarding

what constitutes an adverse effect of air pollution. 

The ATS guidelines recommend that reversible loss of lung

function in individuals in combination with respiratory

symptoms should be considered adverse. See 75 Fed. Reg. at

35531. The guidelines also recommend that an increased risk to

a population caused by a pollutant, even if the risk to a single

individual is not increased, should be considered an adverse

effect because individuals within that group would have

diminished reserve function and would be at an increased risk if

affected by another agent. 75 Fed. Reg. at 35526. 

Petitioners argue that EPA concedes that the clinical studies

only have shown that five-minute exposures to SO2 produce

adverse effects at 400 ppb and above. Pet. Br. at 45-46 (citing 75

Fed. Reg. at 35526-27). Thus, Petitioners reason, EPA has not
14

shown that five-minute exposures to SO2 levels below 400 ppb

cause adverse effects in individuals. Regarding the populationlevel standard, petitioners claim first that EPA failed to find a

causal relationship between five-minute exposures to SO2 at

levels below 400 ppb and the decrements in lung function and,

second, that EPA extrapolated individual data from the clinical

studies to represent the effect of SO2 on a population level. Pet.

Br. at 46-48. 

EPA, however, was not bound to set the SO2 standard

according to the ATS guidelines. The guidelines merely

provided one reference point to help EPA and the public

understand what should be considered an adverse effect of SO2

on human health. 

On the other hand, the EPA Administrator is bound by

statute to promulgate NAAQS that are “requisite to protect the

public health” “allowing an adequate margin of safety.” 42

U.S.C. § 7409(b)(1). It could not then exceed EPA’s authority

to choose a level below that which produced adverse effects in

the clinical studies in order to set a standard that allows an

adequate margin of safety. Further, the clinical studies did not

test severe asthmatics or very young children. EPA concluded

that it was reasonable to assume that those vulnerable

populations would suffer more serious health effects than mild

and moderate asthmatics. 75 Fed. Reg. at 35526 & n.5. We

cannot say it was unreasonable for EPA to consider these

vulnerable populations in setting the standard. 

Further, in issuing the final rule, EPA considered more than

the ATS adversity standards. EPA explains it considered the

advice and recommendations it received from Clean Air

Scientific Advisory Committee, an independent scientific review

committee, and the conclusions drawn from previous NAAQS

reviews. Id. at 35526. EPA also considered epidemiologic
15

studies, which we discuss in greater detail below, to inform its

view of the population-level risk.

 

2. Choice of Epidemiologic Studies

Petitioners contend that the decision to adopt a 75 ppb

standard was arbitrary and capricious because EPA “cherrypick[ed]” studies that supported its preferred result, while

ignoring studies that would support a higher standard. Pet. Br.

at 42. To make this argument, Petitioners point to language in

the final rule which they claim affirms that the epidemiologic

studies upon which EPA relied to set the 75 ppb standard

“suffered from a ‘major methodological issue’ arising from ‘the

extent to which other air pollutants,’ particularly fine particulate

matter, ‘may confound or modify SO2-related effect estimates.’” 

Pet. Br. at 44 (quoting 75 Fed. Reg. at 35531); see also Pet. Br.

at 50-51, 53. Petitioners suggest that EPA “assumed away” the

problem by relying on controlled human exposure studies to lend

“biological plausibility” to the Agency’s preferred results. Pet.

Br. at 44.

 The quotations selected by Petitioners, however, only

support Petitioners’ arguments when taken out of their original

context. In the final rule, EPA explains that it has conducted

substantial amounts of new research to determine the appropriate

level for the 1-hour SO2 NAAQS. 75 Fed. Reg. at 35524. As

described above, EPA reviewed controlled human exposure

studies in which exercising asthmatics were exposed to five- to

ten-minute bursts of SO2, epidemiologic studies relying mostly

on 1-hour and 24-hour standards, and animal toxicologic studies

examining animal exposures to SO2 for short periods of time. 75

Fed. Reg. at 35525. Out of context, the language Petitioners

quote, as presented above, would suggest that when reviewing

the epidemiologic studies, EPA failed to consider how other

pollutants might affect the results of those studies. In fact, the
16

full quotation states as follows:

Although EPA has recognized that multiple factors can

contribute to the etiology of respiratory disease and that

more than one air pollutant could independently impact

respiratory health, we continue to judge, as discussed in

the ISA, that the available evidence supports the

conclusion that there is an independent effect of SO2 on

respiratory morbidity. In reaching this judgment, we

recognize that a major methodological issue affecting

SO2 epidemiologic studies concerns the evaluation of

the extent to which other air pollutants, particular [sic]

PM2.5, may confound or modify SO2-related effect

estimates. The use of multi-pollutant regression models

is a common approach for evaluating potential

confounding by co-pollutants in epidemiologic studies. 

It is therefore important to note that when the ISA

evaluated U.S. and international epidemiologic studies

employing multi-pollutant models, SO2 effect estimates

generally remained positive and relatively unchanged

when co-pollutants, including PM, were included.

75 Fed. Reg. at 35531. Thus, in context, the language Petitioners

cite to support their argument that EPA failed to consider the

effects of other pollutants in the epidemiologic studies actually

demonstrates that EPA did take other pollutants into

consideration, and even when it did so, the results “remained

positive and relatively unchanged.” EPA concedes that there are

“uncertainties” associated with separating the various pollutants’

effects, but that “the limited available evidence indicates that the

effect of SO2 on respiratory health outcomes appears to be

generally robust and independent of the effects of gaseous copollutants, including NO2 and O3, as well as particulate copollutants, particularly PM2.5.” 75 Fed. Reg. at 35531. 
17

Further, although Petitioners accuse EPA of “cherrypicking” its friends, EPA offers a reasonable explanation for why

it relied most heavily on three particular epidemiologic studies. 

EPA explains that it relied on those three studies, out of the more

than fifty peer reviewed studies available, precisely because

these three were conducted in the United States and included

multi-pollutant models to help address the “major

methodological issue” that Petitioners contend EPA ignored. See

75 Fed. Reg. at 35547 (recognizing that “there is special

sensitivity in this review in disentangling PM-related effects . . . 

from SO2-related effects in interpreting the epidemiologic

studies”). EPA concluded that the epidemiologic evidence

supported a 75 ppb standard, 75 Fed. Reg. at 35548, and the

controlled human exposure studies supported the plausibility of

the associations reported in the epidemiologic studies, 75 Fed.

Reg. at 35544. 

Based on its review of all of those studies, EPA found a

“causal relationship between respiratory morbidity and shortterm (5-minutes to 24-hours) exposure to SO2.” 75 Fed. Reg. at

35525. A “causal relationship” finding is the strongest finding

the ISA can make. Id. EPA concluded that the collected

evidence showed that five- to ten-minute exposures to SO2

concentrations at least as low as 200 ppb can result in adverse

health effects in five to thirty percent of the exercising

asthmatics tested in the controlled human exposure studies, 75

Fed. Reg. at 35526, and that a 75 ppb 1-hour limit would

“substantially limit asthmatics’ exposure” to such concentrations,

allowing a reasonable margin for safety, 75 Fed. Reg. at 35548. 

Based on the record discussed above, we cannot conclude

that the choice EPA made to give especial weight to the three

studies conducted in the United States that accounted for the

effects of SO2 concentrations using multi-pollutant regression

models was arbitrary or capricious.
18

3. Statistical Significance

Petitioners also argue that EPA acted arbitrarily in setting

the 1-hour standard at 75 ppb because EPA did not rely on

studies that showed a statistically significant association between

exercising asthmatics’ lung function decrements and short-term

exposures to air concentrations of 200 ppb SO2. Pet. Br. at 43,

45-47, 48. We disagree. EPA found that five- to ten-minute

exposures to SO2 caused statistically significant lung function

decrements when asthmatics were exposed to 400 ppb SO2 or

greater. 75 Fed. Reg. at 35525. Although EPA recognized that

the results did not remain statistically significant when the

concentrations sank to 200 to 300 ppb, EPA reasoned that the

clinical study results “could reasonably indicate an SO2-induced

shift in these lung function measurements for [exercising

asthmatics].” 75 Fed. Reg. at 35526. Further, the studies did not

include severe asthmatics. EPA concluded that it was

“reasonable to presume” that people with more severe asthma

would suffer more serious health consequences from short-term

exposures to 200 ppb SO2. 75 Fed. Reg. at 35526.

We have held before that EPA has discretion to set a

NAAQS at a concentration level below a level that has been

demonstrated to have a statistically significant association with

negative health effects. See Am. Trucking Ass’ns, 283 F.3d at

371. Just so now. We cannot say that the studies necessitated a

75 ppb standard, but we also cannot say that such a standard is

unreasonable or unsupported by the record before us. See id. at

372.

4. Current Air Quality Standards

Finally, Petitioners argue that the new SO2 standard is

arbitrary and capricious because EPA ignored its own finding

that the new standard would create few new health benefits
19

compared to current air quality standards and other CAA

provisions that would prevent air quality from deteriorating to

the level of the existing NAAQS. Pet. Br. at 55 (citing 75 Fed.

Reg. at 35533-34). Petitioners explain that the CAA only gives

EPA authority to revise NAAQS “as appropriate” and reason that

it is inappropriate for EPA to revise the standards when current

air quality does not warrant a revision to protect public health.

Pet. Br. at 57-58 (citing 42 U.S.C. § 7409(d)(1)). 

Nothing in the CAA requires EPA to give the current air

quality such a controlling role in setting NAAQS. And as

Petitioners themselves note, the CAA gives EPA significant

discretion to decide whether to revise NAAQS. Further, in the

final rule, EPA cites evidence that current levels of SO2 in the

ambient air, even when the air quality meets the current SO2

NAAQS, still cause respiratory effects in some areas. 75 Fed.

Reg. at 35530-31. In short, EPA had discretion to revise the

NAAQS and Petitioners’ argument is unavailing. 

III. Conclusion

For the foregoing reasons, we dismiss the petitions in part

and deny in part.