Amer Trk Assn Inc v. EPA

United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued December 18, 2001 Decided March 26, 2001

No. 97-1440

American Trucking Associations, Inc., et al.,
Petitioners

v.

Environmental Protection Agency,
Respondent

Commonwealth of Massachusetts, et al.,
Intervenors

Consolidated with
97-1546, 97-1548, 97-1551, 97-1552, 97-1553, 97-1555,
97-1559, 97-1561, 97-1562, 97-1565, 97-1567, 97-1571,
97-1573, 97-1574, 97-1576, 97-1578, 97-1579, 97-1582,
97-1585, 97-1586, 97-1587, 97-1588, 97-1592, 97-1594,
97-1596, 97-1597, 97-1598

 ---------

No. 97-1441

American Trucking Associations, Inc., et al.,
Petitioners

v.

Environmental Protection Agency,
Respondent

Commonwealth of Massachusetts, et al.,
Intervenors

Consolidated with
97-1502, 97-1505, 97-1508, 97-1509, 97-1510, 97-1512,
97-1513, 97-1514, 97-1518, 97-1519, 97-1526, 97-1531,
97-1539, 97-1566, 97-1568, 97-1570, 97-1572, 97-1575,
97-1584, 97-1589, 97-1591, 97-1595, 97-1619

On Remand from the United States Supreme Court

F. William Brownell and Norman W. Fichthorn argued
the causes for State and Business Petitioners, Non-Environ-
mental Petitioners, and Petitioners on Ozone Issues in
97-1440 and 97-1441. With them on the briefs were Henry
V. Nickel, Thomas Richichi, Betty D. Montgomery, Attorney
General, Judith L. French and Bryan F. Zima, Assistant
Attorneys General, State of Ohio, Jennifer M. Granholm,
Attorney General, Thomas Casey, Solicitor General, Alan F.
Hoffman and Pamela J. Stevenson, Assistant Attorneys Gen-
eral, State of Michigan, Mark J. Rudolph, Senior Counsel,

State of West Virginia Department of Environmental Protec-
tion, Robert R. Gasaway, Daryl Joseffer, David E. Menotti,
Jeffrey A. Knight, G. William Frick, M. Elizabeth Cox, Robin
S. Conrad, Jan Amundson, Beth L. Law, Robert S. Digges,
Harold P. Quinn Jr., David M. Flannery, Gale Lea, Russell
S. Frye, Richard Wasserstrom, Julie C. Becker, Jeffery L.
Leiter, Chet M. Thompson, Douglas I. Greenhaus, Maurice
H. McBride, Gary H. Baise, David F. Zoll, Ronald A.
Shipley, Peter S. Glaser, Grant Crandall, Timothy L. Hark-
er, Eugene M. Trisko, Thomas J. Graves, Kurt E. Blase,
Erika Z. Jones, Timothy S. Bishop, Adam C. Sloane, Duane
J. Desiderio, and David M. Friedland.

Robert E. Yuhnke argued the cause for Environmental
Group and Citizen Petitioners in 97-1440. With him on the
briefs was Joy E. Herr-Cardillo.

James M. Rinaca, Robert R. Gasaway and Daryl Joseffer
were on the brief of intervenors Atlantic City Electric Com-
pany and American Road and Transportation Builders Associ-
ation in 97-1440 and 97-1441.

Norman L. Rave Jr. and David J. Kaplan, Attorneys, U.S.
Department of Justice, argued the causes for respondent in
97-1440 and 97-1441. With them on the briefs were John C.
Cruden, Assistant Attorney General, Thomas A. Lorenzen,
Attorney, John Hannon, Gerald Gleason, Carol S. Holmes
and Steven Silberman, Attorneys, U.S. Environmental Pro-
tection Agency.

Thomas F. Reilly, Attorney General, Edward G. Bohlen,
Assistant Attorney General, Commonwealth of Massachu-
setts, John J. Farmer Jr., Attorney General, Howard Gedul-
dig, Deputy Attorney General, State of New Jersey, Eliot
Spitzer, Attorney General, J. Jared Snyder, Assistant Attor-
ney General, State of New York, Philip T. McLaughlin,
Attorney General, Maureen D. Smith, Senior Assistant Attor-
ney General, State of New Hampshire, William Sorrell,
Attorney General, Erick Titrud, Assistant Attorney General,
State of Vermont, Richard Blumenthal, Attorney General,
Kimberly Massicotte, Assistant Attorney General, State of

Connecticut, and Howard I. Fox were on the brief for inter-
venors Massachusetts, New Jersey and American Lung Asso-
ciation, and amici curiae New York, et al. in 97-1440 and
97-1441.

Before: Ginsburg, Chief Judge, Tatel, Circuit Judge, and
Williams, Senior Circuit Judge.

Opinion for the Court filed by Circuit Judge Tatel.

Tatel, Circuit Judge: In these consolidated cases, we
consider challenges to the Environmental Protection Agency's
National Ambient Air Quality Standards for particulate mat-
ter and ozone. Petitioners originally raised a broad range of
issues, including the constitutionality of the Clean Air Act, the
contours of EPA's authority to promulgate air quality stan-
dards, and the lawfulness of the challenged standards. We
addressed many of these issues in an earlier ruling that the
Supreme Court subsequently reversed in part and affirmed in
part. On remand, only Petitioners' specific challenges to the
air quality standards remain unresolved. Rejecting the argu-
ment that the language and reasoning of our earlier decision
determine the outcome of these remaining claims, and finding
the challenged air quality standards neither arbitrary nor
capricious, we deny the petitions for review except to the
extent the Supreme Court's and our earlier decisions require
further action by EPA.

I.

The Clean Air Act, 42 U.S.C. ss 7401-7671q, directs the
Environmental Protection Agency to establish and periodical-
ly review primary and secondary National Ambient Air Quali-
ty Standards ("NAAQS"), id. s 7409, for any pollutant the
"emissions of which ... cause or contribute to air pollution
which may reasonably be anticipated to endanger public
health or welfare," id. s 7408(a)(1)(A). Section 109(b)(1) of
the Act directs EPA to set the primary NAAQS 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. s 7409(b)(1).
Secondary NAAQS must be set at "level[s] ... the attain-
ment and maintenance of which in the judgment of the
Administrator ... [are] requisite to protect the public welfare
from any known or anticipated adverse effects...." Id.
s 7409(b)(2). Under the Act, secondary NAAQS protect such
aspects of the public "welfare" as "soils, water, crops, vegeta-
tion, manmade materials, [domesticated] animals, wildlife,
weather, visibility, ... climate," and property values. Id.
s 7602(h).

The Act calls for the appointment of "an independent
scientific review committee," the Clean Air Scientific Advisory
Committee ("CASAC"), and tasks this committee with period-
ically reviewing the NAAQS and advising EPA of any need
for new standards or for revisions to existing standards. 42
U.S.C. s 7409(d)(2)(A), (B); see also Nat'l Ambient Air Quali-
ty Standards for Particulate Matter, 62 Fed. Reg. 38,652,
38,653 (Jul. 18, 1997) (codified at 40 C.F.R. s 50.7 (1999))
("Particulate Matter NAAQS"). The seven-member commit-
tee comprises "at least one member of the National Academy
of Sciences, one physician, and one person representing State
air pollution control agencies." 42 U.S.C. s 7409(d)(2)(A).
The Act directs CASAC to "advise the [EPA] Administrator
of areas in which additional knowledge is required to appraise
the adequacy and basis of existing, new, or revised
[NAAQS]," and to "describe the research efforts necessary to
provide the required information[.]" Id. s 7409(d)(2)(C).
When EPA proposes to issue new or revise existing NAAQS,
it must "set forth or summarize and provide a reference to
any pertinent findings, recommendations, and comments by
[CASAC]." Id. s 7607(d)(3). If the proposed rule "differs in
any important respect from any of [CASAC's] recommenda-
tions," the Agency must provide "an explanation of the rea-
sons for such differences." Id.

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 concen-
tration. EPA and the States must first designate areas of

the country that fail to meet the standards--that is, areas in
which atmospheric concentrations of the pollutant exceed
allowable levels. 42 U.S.C. s 7407(d)(1)-(2). Each State
must then adopt a plan that "provides for implementation,
maintenance, and enforcement of [the] primary" NAAQS, id.
s 7410(a)(1), through, for example, regulation of wood fires or
automobile or power plant emissions. States must submit
their plans to EPA for approval, and may have to make
revisions if the Agency finds the plans inadequate. States
that fail to develop adequate plans are subject to sanctions,
id. s 7509, or to imposition of a federal implementation plan,
id. s 7410(c)(1).

These consolidated cases concern NAAQS for particulate
matter and ozone, two ubiquitous atmospheric pollutants.
The term "particulate matter," or "PM," refers to all "solid
particles and liquid droplets found in air." Office of Air &
Radiation, U.S. Envtl. Prot. Agency, EPA-454/R-00-005, Air
Quality Index: A Guide to Air Quality and Your Health 11
(2000) ("EPA, Air Quality Index"). Although these particles
and droplets come in varying sizes, only particulate matter
less than 2.5 micrometers in diameter--so-called "fine PM" or
"PM2.5"--is relevant here. As originally filed, these cases
also concerned "coarse" particulate matter, or particles and
droplets between 2.5 and 10 micrometers in diameter, but we
resolved all issues relating to this "coarse particulate matter"
in our earlier ruling. See Am. Trucking Ass'ns v. EPA, 175
F.3d 1027, 1053-55 (D.C. Cir. 1999) ("ATA I"), reh'g granted
in part and denied in part, 195 F.3d 4 (D.C. Cir. 1999) ("ATA
II"), aff'd in part and rev'd in part, Whitman v. Am. Truck-
ing Ass'ns, 531 U.S. 457 (2001) ("Whitman").

PM2.5 is associated with a range of adverse health effects
such as coughing; shortness of breath; aggravation of existing
respiratory conditions like asthma and chronic bronchitis;
increased susceptibility to respiratory infections; and height-
ened risk of premature death. EPA, Air Quality Index at 11.
High PM2.5 concentrations also impair visibility, reducing
people's "well-being ..., both where they live and work, and
in places [like national parks and wilderness areas] where
they enjoy recreational opportunities." Particulate Matter

NAAQS, 62 Fed. Reg. at 38,680. Sources of fine PM include
vehicle engines, power plants, and wood fires. EPA, Air
Quality Index at 11.

Unlike PM, ozone is a colorless, odorless gas. EPA, Air
Quality Index at 7. Not a direct product of human activity,
ozone forms when other atmospheric pollutants--ozone "pre-
cursors"--react in the presence of sunlight. Office of Air
Quality Planning & Standards, U.S. Envtl. Prot. Agency,
EPA/451-K-97-002, Ozone: Good Up High, Bad Nearby 2-3,
(1997) ("EPA, Ozone Facts"). Significant health effects asso-
ciated with ozone pollution include coughing; throat irritation;
aggravation of existing conditions like asthma, bronchitis,
heart disease, and emphysema; and lung tissue damage. Id.
Ozone pollution can also interfere with plants' ability to
produce and store food, rendering them more susceptible to
disease, insect pests, and other stressors. Id. EPA esti-
mates that ozone "is responsible for 500 million dollars in
reduced crop production in the United States each year." Id.
Ozone levels tend to be highest in urban areas, in part
because cars, power plants, and chemical solvents are the
principal sources of ozone precursors. Id.

In EPA's judgment, ozone is, and PM may be, a non-
threshold pollutant--that is, a pollutant that causes adverse
health effects at any non-zero atmospheric concentration.
Nat'l Ambient Air Quality Standards for Ozone, 62 Fed. Reg.
38,856, 38,863 (July 18, 1997) (codified at 40 C.F.R. ss 50.9,
50.10 (1999)) ("Ozone NAAQS") ("Nor does it seem possible,
in the Administrator's judgment, to identify [an ozone concen-
tration] level at which it can be concluded with confidence
that no 'adverse' effects are likely to occur."); Nat'l Ambient
Air Quality Standards for Particulate Matter: Proposed Rule,
61 Fed. Reg. 65,638, 65,651 (Dec. 13, 1996) ("Particulate
Matter NPRM") ("[T]he single most important factor influ-
encing the uncertainty associated with the risk estimates [for
PM] is whether or not a threshold concentration exists below
which PM-associated health risks are not likely to occur.");
see also ATA I, 175 F.3d at 1034 (making the same point).
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
level[s] that ... reduce risks sufficiently to protect public
health" even while recognizing that "a zero-risk standard is
[not] possible." Ozone NAAQS, 62 Fed. Reg. at 38,863.

On July 18, 1997, EPA revised the primary and secondary
NAAQS for particulate matter and ozone. See Particulate
Matter NAAQS, 62 Fed. Reg. 38,652; Ozone NAAQS, 62 Fed.
Reg. 38,856. For particulate matter, the Agency abandoned
its approach of regulating both coarse and fine particles and
droplets under the same standards. Observing that the
"epidemiological evidence suggest[s] stronger associations of
mortality and some morbidity effects with fine particles than
with ... coarse particles," Nat'l Ambient Air Quality Stan-
dards for Ozone and Particulate Matter, Advance Notice of
Proposed Rulemaking, 61 Fed. Reg. 29,719, 29,723 (June 12,
1996) ("Advance NPRM"), the Agency adopted new, PM2.5-
specific standards: an annual primary standard of 15 micro-
grams per cubic meter ("ug/m3"); a daily primary standard of
65 ug/m3; and secondary standards equal to the primary
standards. Particulate Matter NAAQS, 62 Fed. Reg. at
38,652.

EPA also made significant changes to the ozone NAAQS.
Citing new information that suggests a positive correlation
between prolonged (six-to eight-hour) exposures to relatively
low levels of ozone and "a wide range of health effects,"
Ozone NAAQS, 62 Fed. Reg. at 38,861, the Agency adopted
new primary and secondary standards under which eight-
hour-average ozone concentrations may not exceed 0.08 parts
per million ("ppm"), in place of the old, one-hour-average
standards of 0.12 ppm, id. at 38,856.

Soon after EPA issued the revised particulate matter and
ozone NAAQS, various parties, including American Trucking
Associations, other businesses and business associations, envi-
ronmental groups, citizens, and several States, petitioned for
review of the revised standards. Other parties intervened
(some supporting Petitioners and others supporting EPA),
four additional States filed amicus briefs supporting EPA,
and Senator Orrin Hatch and Representative Thomas Bliley

filed amicus briefs supporting Petitioners. Petitioners and
supporting Intervenors and Amici challenged the NAAQS
from all sides, arguing (among other things) that the Clean
Air Act delegates excessive legislative authority to EPA in
violation of Article I of the Constitution and that the Agency
failed to consider certain relevant factors, including imple-
mentation costs, prior to setting the NAAQS. Invoking
Clean Air Act section 307(d)(9), which directs federal appeals
courts to vacate Agency action "found to be ... arbitrary,
capricious, an abuse of discretion, or otherwise not in accor-
dance with law," 42 U.S.C. s 7607(d)(9)(A), Petitioners also
challenged specific aspects of the new particulate matter and
ozone NAAQS ("the 307(d)(9) claims").

We addressed most of these arguments in ATA I and II.
With respect to the nondelegation argument, we agreed with
Senator Hatch and Representative Bliley that neither the
Act's requirement that EPA "set each [primary] standard at
the level 'requisite to protect the public health' with an
'adequate margin of safety,' " ATA I, 175 F.3d at 1034
(quoting 42 U.S.C. s 7409(b)(1)), nor the Agency's interpreta-
tion of that requirement in the challenged rules, see id.,
identified an "intelligible principle" limiting EPA's rulemak-
ing authority, id. at 1034-40 (internal quotation marks omit-
ted). But see id. at 1057-62 (Tatel, J., dissenting); ATA II,
195 F.3d at 14-16 (Silberman, J., dissenting from denial of
rehearing en banc); id. at 16-17 (Tatel, J., same). Although
we concluded that the Clean Air Act, as interpreted by EPA,
effected an unconstitutional delegation of Congress's legisla-
tive authority, we struck down neither the Act nor the
challenged NAAQS, choosing instead to remand the stan-
dards to the Agency to give it an opportunity to identify a
determinate standard that would limit its discretion. ATA I,
175 F.3d at 1038; see also ATA II, 195 F.3d at 6-8.

Having remanded the challenged NAAQS for other rea-
sons, we never addressed Petitioners' 307(d)(9) claims. We
did, however, address Petitioners' more general claims, hold-
ing that EPA: may not consider either the implementation
costs or the indirect environmental consequences of NAAQS
prior to setting the standards, ATA I, 175 F.3d at 1040-41;

has no obligation to comply with certain requirements of the
National Environmental Policy Act, the Unfunded Mandates
Reform Act, or the Regulatory Flexibility Act before issuing
NAAQS, id. at 1041-45 (citing NEPA, 42 U.S.C.
s 4332(2)(C)-(D); UMRA, 2 U.S.C. ss 1532, 1535; and RFA,
5 U.S.C. ss 603(a), 605(b)); should have considered any bene-
ficial effects of ozone before revising the ozone NAAQS, id. at
1051-53; has only limited statutory authority to enforce the
new ozone NAAQS, id. at 1046-51; see also ATA II, 195 F.3d
at 8-10; need not identify the specific biological mechanism
by which fine particulate matter affects human health, ATA I,
175 F.3d at 1055-56; and has no obligation to set secondary
NAAQS at levels "sufficient to eliminate all adverse visibility
effects" of a pollutant, id. at 1056-57 (emphasis added).

The Supreme Court granted certiorari with respect to four
questions: whether the Clean Air Act violates the nondelega-
tion doctrine; whether EPA may consider implementation
costs in setting NAAQS; whether this court had jurisdiction
to review the Agency's interpretation of the Clean Air Act
provisions governing implementation of the ozone NAAQS;
and if this court had jurisdiction, whether the Agency's
interpretation of those provisions was permissible. Whitman,
531 U.S. at 462. Disagreeing with our nondelegation ruling,
the Supreme Court held that the Clean Air Act requirement
that EPA "set air quality standards at the level that is
'requisite'--that is, not lower or higher than is necessary--to
protect the public health with an adequate margin of safety,
fits comfortably within the scope of [Agency] discretion per-
mitted by [Court] precedent." Id. at 475-76. Writing for the
Court, Justice Scalia next confirmed our long-held view that
" 'economic considerations [may] play no part in the promul-
gation of ambient air quality standards,' " id. at 464 (quoting
Lead Indus. Ass'n v. EPA, 647 F.2d 1130, 1148 (D.C. Cir.
1980)); see also ATA I, 175 F.3d at 1040-41 (making the
same point), and agreed with ATA I that this court had
jurisdiction to review EPA's interpretation of the Act's provi-
sions governing implementation of the ozone NAAQS, Whit-
man, 531 U.S. at 477-80. Turning to the merits of the latter
issue, the Court found the Agency's ozone NAAQS "imple-

mentation policy to be unlawful" and indicated that after this
court's final disposition of the parties' 307(d)(9) claims "it is
left to the EPA to develop a reasonable interpretation of the
nonattainment implementation provisions insofar as they ap-
ply to revised ozone NAAQS." Id. at 486.

The Supreme Court remanded the cases for us to consider
Petitioners' as-yet-unaddressed 307(d)(9) challenges to the
particulate matter and ozone NAAQS. On remand, three
sets of Petitioners filed additional briefs, two addressing the
particulate matter NAAQS and one addressing the ozone
NAAQS. With respect to the particulate matter NAAQS, one
group of Petitioners, led by American Trucking Associations
("State and Business Petitioners"), urges us to vacate the
NAAQS under section 307(d)(9) because EPA "[f]ailed" either
"to [a]rticulate and [a]pply" the Act's "requisite to protect"
standard as elucidated in Whitman, State & Bus. Pet'rs' Br.
at 35, or to determine PM2.5 levels "at which [the] public
health risk is acceptable," id. at 40. The second group of
PM2.5 Petitioners, led by Citizens for Balanced Transporta-
tion ("Environmental Petitioners"), believes that the daily
primary standard is too lenient to protect against known
adverse health effects of fine particulate matter and that the
annual and daily secondary standards are insufficient to
reduce visibility impairment. The third set of Petitioners,
again led by American Trucking Associations ("Ozone Peti-
tioners"), challenges the ozone NAAQS on grounds similar to
those raised by State and Business Petitioners in their attack
on the particulate matter NAAQS.

In considering these section 307(d)(9) claims, we apply the
same highly deferential standard of review that we use under
the Administrative Procedure Act, 5 U.S.C. s 706(2)(A). See,
e.g., Allied Local & Reg'l Mfrs. Caucus v. EPA, 215 F.3d 61,
68 (D.C. Cir. 2000) ("To determine whether [the Agency's]
rules are 'arbitrary and capricious,' we apply the same stan-
dard of review under the Clean Air Act as we do under the
Administrative Procedure Act."). Thus, we presume the va-
lidity of agency action as long as "a rational basis for it is
presented." Lead Indus. Ass'n, 647 F.2d at 1145. That said,
however, we perform a "searching and careful" inquiry into

the underlying facts. Id. (internal quotation marks and cita-
tions omitted). In a similar case involving a challenge to
ambient air quality standards, Lead Industries, we elaborated
on this standard of review as follows:

It is not our function to resolve disagreement among the
experts or to judge the merits of competing expert views.
Our task is the limited one of ascertaining that the
choices made by the [EPA] Administrator were reason-
able and supported by the record. That the evidence in
the record may also support other conclusions, even
those that are inconsistent with the Administrator's, does
not prevent us from concluding that [her] decisions were
rational and supported by the record.

Id. at 1160 (internal citations omitted).

II.

Before evaluating the merits of Petitioners' claims, we must
address State, Business, and Ozone Petitioners' argument
that the language and reasoning of ATA I and II "govern[ ]"
"the outcome of th[ese] case[s]." State & Bus. Pet'rs' Br. at
32. In essence, these Petitioners argue that ATA I and II
signal this court's belief that EPA failed to apply the Clean
Air Act's "requisite to protect the public health" standard in
setting the primary NAAQS. 42 U.S.C. s 7409(b)(1). Be-
cause EPA chose not to appeal this purported "holding" to
the Supreme Court, Petitioners reason, the holding has be-
come the law of these cases and constrains our review of all
remaining claims.

Petitioners base this law of the case argument primarily on
our statement in ATA II that although EPA's "petition for
rehearing ... argue[d] that s 109 of the Clean Air Act
contains [a] principle limiting the agency's discretion"--name-
ly, that the NAAQS must be set at levels "necessary for
public health protection: neither more nor less stringent than
necessary, but 'requisite' "--EPA never "identif[ied]" this
principle "as a limit upon its discretion" anywhere in the
challenged rules. 195 F.3d at 6-7 (internal quotation marks
and citations omitted). In addition, Petitioners point to a

later passage in ATA II in which we declined to "express [an]
opinion upon the sufficiency of [the newly identified limiting]
principle," choosing to wait until "after the [Agency] itself has
applied [the principle] in setting a NAAQS" to determine
"whether the principle, in practice, fulfills the purposes of the
nondelegation doctrine." Id. at 7 (emphasis added). Finally,
Petitioners cite the following sentences from ATA I:

[The Agency's] explanations for its decisions amount to
assertions that a less stringent standard would allow the
relevant pollutant to inflict a greater quantum of harm on
public health, and that a more stringent standard would
result in less harm. Such arguments only support the
intuitive proposition that more pollution will not benefit
public health, not that keeping pollution at or below any
particular level is "requisite" or not requisite to "protect
the public health" with an "adequate margin of safety,"
the formula set out by [Clean Air Act section 109(b)(1)'s
definition of a primary NAAQS].

175 F.3d at 1035. According to Petitioners, these passages
express our view that EPA failed to follow the Clean Air Act
in setting the PM and ozone NAAQS, and that the NAAQS
themselves are therefore "arbitrary, capricious, an abuse of
discretion, or otherwise not in accordance with law." 42
U.S.C. s 7607(d)(9)(A).

Petitioners misread ATA I and II. Both decisions address
the broad claim that the Clean Air Act effects an unconstitu-
tional delegation of legislative authority. To evaluate that
claim, we looked first to the Act for an "intelligible principle"
limiting the Agency's "application of the[ ] factors" it "uses in
determining the degree of public health concern associated
with different levels of ozone and PM[.]" ATA I, 175 F.3d at
1034 (internal quotation marks omitted). Finding none, we
next looked to see whether the Agency developed "binding
standards for itself." ATA I, 175 F.3d at 1038 (emphasis
added). Again finding none, we remanded the NAAQS to
EPA to give it "an opportunity to extract [from the Act] a
determinate standard" that would "channel" its rulemaking
authority. Id. at 1034, 1038; see also ATA II, 195 F.3d at 6-

8 (affirming that holding). At no point in either ATA I or II,
however, did we address the narrower question now before
us: Did EPA abuse its discretion or otherwise violate the
Clean Air Act in setting the challenged NAAQS? Indeed, in
ATA I we emphasized this very distinction:

[The Agency] cites prior decisions of this Court holding
that when there is uncertainty about the health effects of
concentrations of a particular pollutant within a particu-
lar range, EPA may use its discretion to make the
"policy judgment" to set the standards at one point
within the relevant range rather than another.... We
agree. But none of those panels addressed the claim of
undue delegation that we face here, and accordingly had
no occasion to ask EPA for ... a "principle[ ]" ... in
making its "policy judgment."

175 F.3d at 1037. As this language makes clear, we recog-
nized that the search for a binding principle guiding Agency
policy judgments differs in kind and degree from the familiar
administrative law inquiry into whether an agency abused its
discretion. When we stated that EPA failed to "appl[y]" its
newly identified limiting principle "in setting [the] NAAQS,"
therefore, we spoke in constitutional terms: In our pre-
Whitman view, EPA never agreed that it "could not (or in a
later rulemaking would not)" promulgate a NAAQS either
"more [ ]or less stringent than necessary." ATA II, 195 F.3d
at 6-7 (emphasis removed). We in no way implied, however,
that the particulate matter and ozone NAAQS themselves are
either more or less stringent than necessary.

In short, ATA I and II address only whether the Act (or
EPA's reading of the Act) adequately limits the Agency's
discretion. Here, we ask whether EPA reasonably exercised
that discretion--an entirely different question that we now
answer differently.

III.

We start with Petitioners' challenge to the PM2.5 NAAQS.
Fulfilling our obligation to "undertake a 'substantial inquiry'
into the facts" underlying challenged agency actions, Lead

Indus. Ass'n, 647 F.2d at 1146 (quoting Citizens to Preserve
Overton Park v. Volpe, 401 U.S. 402, 415 (1971), overruled on
other grounds by Califano v. Sanders, 430 U.S. 99, 105
(1977)), we first summarize the Agency's rulemaking process
with respect to both the primary and secondary NAAQS and
then consider State, Business, and Environmental Petitioners'
specific arguments.

The Rulemaking

In late 1996, EPA issued a public notice of proposed
rulemaking that announced possible changes to the particu-
late matter NAAQS. Particulate Matter NPRM, 61 Fed.
Reg. 65,638. Most relevant to these cases, EPA said it
planned to supplement the existing primary NAAQS, which
regulated both coarse particulate matter and PM2.5 under a
single set of standards, by adding two new primary standards
specific to fine PM: an annual PM2.5 standard of 15 ug/m3
and a daily PM2.5 standard of 50 ug/m3. Id. EPA sought
comment on that proposal and on a range of alternative
values for the new standards, from 12 to 20 ug/m3 for the
annual standard, and 20 to 65 ug/m3 for the daily standard.
Id. at 65,658-61. According to EPA, an annual standard of
12 ug/m3 and a daily standard of 20-50 ug/m3 would be
"maximally precautionary," id., at 65,659; by comparison, an
annual standard of 20 ug/m3 and a daily standard of 65 ug/m3
would reflect "the judgment that the current scientific evi-
dence has not demonstrated adverse public health effects
from fine particle concentrations ... below those correspond-
ing to the [old, composite, PM] standard," id. Between those
extremes, the proposed annual and daily levels of 15 and 50
ug/m3, respectively, would reflect an effort "to limit annual
PM2.5 concentrations to somewhat below those where the
body of epidemiological evidence is most consistent and co-
herent." Id. at 65,660.

To facilitate public input on the proposed changes, EPA
established a toll-free telephone hotline and a system for
submission of comments via the Internet. Particulate Matter
NAAQS, 62 Fed. Reg. at 38,654. In addition, Agency staff

conducted public hearings in communities across the country
and held two national satellite telecasts. Id. By the end of
the comment period, EPA had received more than 50,000
comments. Id.

In setting the final NAAQS, EPA considered these public
comments, in addition to CASAC's recommendations and
Agency staff's "thorough review ... of the latest scientific
information on known and potential human health effects
associated with exposure to PM." Id. at 38,655-56. EPA
also considered an independent (though partially Agency-
funded) research institute's study of the correlation between
particulate matter levels and mortality in six urban areas, the
same institute's extended analyses of the mortality data for
Philadelphia alone (the "Philadelphia studies"), see id. at
38,660, and Agency staff's quantitative assessment of other
PM-related health risks in Philadelphia and Los Angeles
("risk assessment"), see id. at 38,656 (citing Office of Air
Quality Planning & Standards, U.S. Envtl. Prot. Agency, A
Particulate Matter Risk Assessment for Philadelaphia and
Los Angeles (1996) ("Risk Assessment")). EPA emphasized,
however, that it "place[d] greater weight on the overall
conclusions derived from the[se] studies--that PM air pollu-
tion is likely causing or contributing to significant adverse
effects at levels below those permitted by the [old] stan-
dards--than on the [more uncertain] concentration-response
functions and quantitative risk estimates derived from them."
Id.

Ultimately, EPA adopted the proposed annual PM2.5 stan-
dard of 15 g/m3, explaining that it would assess compliance
based on the three-year average of annual arithmetic mean
PM2.5 concentrations at monitoring stations in a given area.
See Particulate Matter NAAQS, 62 Fed. Reg. at 38,652. For
the daily standard, EPA chose a less restrictive level than it
had originally proposed, setting the standard at 65 ug/m3,
with a "form"--or method of assessing compliance--based on
the three-year average of the 98th percentile of daily PM2.5
concentrations at each monitoring site. Id. EPA provided a
lengthy explanation of its selection of these new standards,
four aspects of which are relevant here: its discussions of (1)
the need to revise the old NAAQS for particulate matter; (2)

the reasons for adopting both annual and daily standards; (3)
the grounds for choosing 15 and 65 ug/m3, respectively, as the
levels for the new standards; and (4) the rationale behind the
new daily standard's rather peculiar form.

Explaining the first point--the need for the new PM2.5
NAAQS--EPA began by documenting evidence of the old PM
standards' inadequacy. Examination of an "extensive ...
epidemiological data base," EPA observed, showed that chil-
dren, as well as the elderly and other "sensitive populations,"
were experiencing adverse, PM-related health effects--some-
times including mortality--even "in areas [and] at times when
the levels of the [old] ... standards [were] met." Id. at
38,657. In addition, a majority of CASAC members recom-
mended strengthening "the health protection[s] provided by
the [old] PM standard[s]." Id. at 38,666. Finally, a majority
of commenters agreed that "based on the available scientific
information, the [old PM] standards [were] not of themselves
sufficient to protect public health." Id. at 38,657.

Responding to more critical commenters, EPA acknowl-
edged that in several of the cited epidemiological studies, the
effects of particulate matter were difficult to isolate from
those of other air pollutants. The Agency soundly rejected,
however, "the suggestion that such multi-pollutant studies are
in any way 'negative' with respect to [the] conclusion[ ] that
PM, alone or in combination with other pollutants, is associat-
ed with adverse effects at levels below those allowed by the
current standards." Particulate Matter NAAQS, 62 Fed.
Reg. at 38,661. "This conclusion is based not only on the
consistency of PM effects across areas with widely varying
concentrations of potentially confounding copollutants," EPA
explained, "but also on the extended analyses of the Philadel-
phia studies," in which "PM can reasonably be distinguished
from potential effects of all pollutants except [sulfur dioxide]."
Id. Moreover, various characteristics of fine particulate mat-
ter, including its greater ability to "penetrate and remain
indoors where ... sensitive population[s] reside[ ]," id., and
to infiltrate "to the airways and gas exchange regions of the
lung," id. at 38,662, convinced the Agency that even in

Philadelphia, PM rather than sulfur dioxide "play[ed] an
important direct role in the observed mortality effects," id.

Turning to the specific characteristics of the new PM2.5
NAAQS, EPA gave only a brief explanation of its decision to
adopt a primary standard with a twenty-four-hour averaging
period: The standard is "consistent with [most] community
epidemiological studies," which suggest that same-day and
previous-day PM concentrations correlate positively with ad-
verse health effects. Particulate Matter NAAQS, 62 Fed.
Reg. at 38,668. EPA dismissed as insufficiently quantitative
the few studies suggesting an association between health
effects and shorter-term (minutes to hours) PM exposures,
noting that in any case, regulating daily-average PM concen-
trations would also reduce shorter-term-average concentra-
tions in most areas. Addressing the studies reporting
"stronger associations [between PM and] multiple-
day[-]average concentrations," EPA decided that a twenty-
four-hour-average standard would effectively prevent both
single-day and multi-day PM "episodes." Id. (emphasis add-
ed).

EPA then discussed the annual NAAQS' one-year averag-
ing time, explaining that it adopted the standard to reduce
the likelihood of long-term and cumulative PM exposures,
which "appear" to pose "larger" risks than shorter-term
exposures. Id. Although EPA recognized that either a multi-
year or a single-season standard would also be effective
against long-term exposures, it concluded that the annual
standard would provide adequate protection against multi-
year PM events and that "the current evidence does not
provide a satisfactory quantitative basis for setting a national
fine particle standard in terms of a seasonal averaging time."
Id. at 38,669.

Explaining its decision to establish both annual and daily
standards, EPA indicated that although "either standard
could be viewed as providing both short-and long-term pro-
tection" from PM2.5, the use of two standards with very
different averaging times would "serv[e] to address situations
where the daily peaks and annual averages are not consistent-

ly correlated." Particulate Matter NAAQS, 62 Fed. Reg. at
38,669. In such situations, the annual standard would "low-
er[ ] both short- and long-term PM2.5 concentrations," while
the daily standard would "protect[ ] against ... localized 'hot
spots,' and ... seasonal emissions," neither of which would be
"well controlled by a national annual standard" alone. Id.

EPA next made a number of key points about the levels it
selected for the new NAAQS. Implementing its view that the
annual standard should do most of the work in mitigating the
risks of PM2.5, EPA selected the level of the standard "so as
to protect against the range of effects associated with both
short-and long-term exposures to PM." Id. at 38,675. In
those studies reporting statistically significant correlations
between health effects and PM exposures of any duration,
the mean annual PM concentrations ranged from 16 to 21
ug/m3, while in those studies reporting nearly significant
correlations, the mean annual concentrations ranged from 11
to 30 ug/m3. Id. at 38,676. "[P]lacing greatest weight on
those studies that were clearly statistically significant," there-
fore, the Agency adopted an annual standard level of 15
ug/m3--just "below the range of annual data most strongly
associated with both short- and long-term exposure effects,"
id., and in the lower half of the 12 to 20 ug/m3 range
mentioned in the Particulate Matter NPRM, 61 Fed. Reg. at
65,658-61. Although EPA acknowledged it could not rule out
"the possibility of [health] effects at lower annual concentra-
tions," it nevertheless decided not only that the evidence for
such effects is "highly uncertain," but that "the likelihood of
significant health risk" decreases as annual-average PM con-
centrations approach background levels. Particulate Matter
NAAQS, 62 Fed. Reg. at 38,676 (emphasis added).

Having settled on a moderate level for the annual NAAQS,
EPA selected the level of the daily standard "to provide
supplemental protection against peak concentrations that
might occur over limited areas and/or for limited time peri-
ods." Id. This selection process posed difficult questions
because in all of the available studies in which short-term
exposures correlated positively with adverse health effects,
the long-term annual-average PM2.5 concentration exceeded

the new 15 ug/m3 NAAQS. EPA thus had no way to evaluate
the "incremental risk associated with single peak exposures to
PM2.5 in areas where the [new] annual standard is met." Id.
at 38,677. Because it did not yet know how effective the new
annual standard would be in reducing the risk of peak short-
term exposures, therefore, EPA decided it could not justify a
restrictive daily standard, instead selecting 65 ug/m3--a level
"at the upper end of the range recommended by [Agency]
staff and most CASAC ... members." Id.

In justifying the chosen levels for the new NAAQS, EPA
made one final point relevant to these cases. Responding to
commenters who advocated lower levels for both the daily
and annual standards, EPA emphasized that considerable
uncertainty remains about whether PM2.5 is a threshold
pollutant--that is, whether there is a concentration below
which PM2.5 is harmless. See id. at 38,675. As a result,
EPA could not be sure that lowering the NAAQS would
produce corresponding reductions in health risks. "[T]he
inherent scientific uncertainties are too great to support"
lower levels for the NAAQS, the Agency explained. Id.

EPA also gave a lengthy explanation for the form of the
primary standards. Only one aspect of that form is relevant
here: the Agency's decision to base compliance with the daily
standard on the average of the 98th percentile of daily PM2.5
concentrations at each monitoring station. See Particulate
Matter NAAQS, 62 Fed. Reg. at 38,652. This form permits
monitoring stations to exceed the 65 ug/m3 level of the
standard two percent of the time, or about seven days each
year assuming the stations monitor PM levels every day.
EPA justified these authorized exceedances on the ground
that they will increase the "stability" of the standard by
permitting States to design long-term PM2.5 control pro-
grams without worrying about the effects of "single high
exposure event[s] that may be due to unusual meteorological
conditions alone." Id. at 38,673. In addition, EPA pointed
out that "the [Clean Air] Act provides for emergency State or
Federal action to address" any such high exposure events.
Id. That said, EPA rejected alternative daily-standard forms
(such as a 90th percentile form) that would have allowed more

exceedances per year, on the ground that a daily standard
that permitted such "a large number of days with peak PM2.5
concentrations above the standard level" would not "serve as
an effective supplement to the annual standard." Id.

Turning finally to the secondary PM2.5 NAAQS, EPA
focused primarily on the impact of particulate matter on
visibility, "an important welfare effect [that] has direct signifi-
cance to people's enjoyment of daily activities in all parts of
the country." Id. at 38,680. EPA predicted that attainment
of the new PM2.5 primary standards would improve visibility
in the eastern United States. Conceding that the new stan-
dards would have little or no effect in the west "except in and
near certain urban areas," id. at 38,681, EPA observed that
no single suite of secondary standards would solve visibility
problems everywhere in the country because, due to regional
differences in relative humidity, natural background PM lev-
els, and other factors, "a national secondary standard intend-
ed to maintain or improve visibility conditions [in] ... the
West would have to be set at or even below natural back-
ground levels in the East," while a national secondary stan-
dard intended to "achieve an appropriate degree of visibility
improvement in the East would permit further degradation in
the West," id. at 38,680. EPA therefore decided simply to
establish secondary PM2.5 NAAQS identical to the new pri-
mary NAAQS, and to establish a regional haze program
under Clean Air Act section 169A, 42 U.S.C. s 7491, to
improve visibility in those areas where the new national
standards prove ineffective. Id. at 38,679.

State and Business Petitioners' Claims

State and Business Petitioners urge us to vacate the pri-
mary NAAQS because EPA "did not apply any legal stan-
dard, much less the correct standard." State & Bus. Pet'rs'
Br. at 35. In support of this argument, they cite two
passages in the final PM2.5 rule. In one, Petitioners claim,
EPA asserted that it had no obligation to determine a "safe
level" of PM2.5 prior to adopting a primary NAAQS. Id. at
36 (internal quotation marks omitted). In the other, EPA
allegedly acknowledged that "its approach 'might result in

regulatory programs that go beyond those that are needed to
effectively reduce risks to public health.' " Id. at 38 (quoting
Particulate Matter NAAQS, 62 Fed. Reg. at 38,675 (emphasis
added)). As Petitioners see it, these "concessions" prove that
EPA failed to set the primary NAAQS at levels " 'requisite'--
that is, not lower or higher than ... necessary--to protect
the public health with an adequate margin of safety," as
mandated by Whitman. 531 U.S. at 475-76.

Petitioners' argument suffers from two significant flaws.
First, the final PM rule makes neither alleged concession. In
the first passage, which Petitioners cite as evidence that EPA
failed to identify a "safe level" of PM2.5, the Agency merely
disclaimed any obligation to set primary NAAQS by means of
a two-step process, identifying a "safe level" and then apply-
ing an additional margin of safety. Instead, EPA stated, 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." Particulate
Matter NAAQS, 62 Fed. Reg. at 38,688. Nothing in this
statement implies that EPA failed to determine "safe levels"
for fine particulate matter; indeed, the Agency's establish-
ment of new primary NAAQS demonstrates that it did reach
a conclusion regarding "safe" daily- and annual-average
PM2.5 levels. State and Business Petitioners obviously dis-
agree with that conclusion, but they have no basis for arguing
that EPA failed to identify levels of PM2.5 that the Agency
considers safe.

Viewed in its proper context, EPA's other alleged "conces-
sion"--that the new NAAQS "go beyond" what is necessary
to protect public health--proves equally chimerical. In the
final PM2.5 rule, EPA said only that "a number of ...
commenters [to the proposed NAAQS] strongly supported
standard levels more stringent than those proposed by" the
Agency, but that "setting such [lower] standards ... might
result in regulatory programs that go beyond those that are
needed to effectively reduce risks to public health." Id. at
38,675 (emphasis added). This passage in no way supports
Petitioners' argument that EPA failed to set the primary
PM2.5 NAAQS at levels " 'requisite' ... to protect the public

health with an adequate margin of safety." Whitman, 531
U.S. at 475-76. Instead, the passage documents EPA's rejec-
tion of lower standards, demonstrating that the Agency not
only recognized, but acted upon, its statutory obligation to set
the primary NAAQS at levels no lower than necessary to
reduce public health risks.

Petitioners' argument that EPA neither identified nor ap-
plied the proper legal standard also exaggerates the Agency's
obligation to quantify its decisionmaking. The argument
relies on two statements from the rulemaking and one from
ATA I: EPA's assertion that it need not "determine a 'safe
level' " of PM2.5 before calculating a margin of safety, State
& Bus. Pet'rs' Br. at 36 (quoting Particulate Matter NAAQS,
62 Fed. Reg. at 38,688 (internal quotation marks omitted));
the Agency's "disavow[al]" of certain "specific risk estimates,"
id. at 37 (citing Particulate Matter NAAQS, 62 Fed. Reg. at
38,656); and finally, the Agency's claim that "there is no
threshold 'amount of scientific information or degree of cer-
tainty' required to promulgate or revise a NAAQS," id.
(quoting Resp't's 1998 Br. at 49). According to Petitioners,
these three assertions prove that EPA failed to "describe[ ]
the standard under which [it] ... arrived at [its] conclusion"
regarding the appropriate level for the NAAQS, as required
by our decision in American Lung Ass'n v. EPA, 134 F.3d
388, 392-93 (D.C. Cir. 1998). Petitioners misread American
Lung Ass'n, however, if they think it requires EPA, prior to
setting primary NAAQS, to identify perfectly safe levels of
pollutants, to rely on specific risk estimates, or to specify
threshold amounts of scientific information.

Although we recognize that the Clean Air Act and circuit
precedent require EPA qualitatively to describe the standard
governing its selection of particular NAAQS, we have ex-
pressly rejected the notion that the Agency must "establish a
measure of the risk to safety it considers adequate to protect
public health every time it establishes a [NAAQS]." Natural
Res. Def. Council, Inc. v. EPA, 902 F.2d 962, 973 (D.C. Cir.
1990), vacated in part, 921 F.2d 326 (D.C. Cir. 1991) (vacating
a later part of the court's decision). Such a rule would
compel EPA to leave hazardous pollutants unregulated unless

and until it completely understands every risk they pose, thus
thwarting the Clean Air Act's requirement that the Agency
err on the side of caution by setting primary NAAQS that
"allow[ ] an adequate margin of safety[.]" 42 U.S.C.
s 7409(b)(1). The Act requires EPA to promulgate protec-
tive primary NAAQS even where, as here, the pollutant's
risks cannot be quantified or "precisely identified as to nature
or degree," Particulate Matter NAAQS, 62 Fed. Reg. at
38,653; see also Ozone NAAQS, 62 Fed. Reg. at 38,857
(explaining that section 109(b)(1)'s "margin of safety require-
ment was intended to address uncertainties associated with
inconclusive scientific and technical information ... as well as
to provide a reasonable degree of protection against hazards
that research has not yet identified"). For its part, Ameri-
can Lung Ass'n requires only that EPA "engage in reasoned
decision-making," 134 F.3d at 392, not that it definitively
identify pollutant levels below which risks to public health are
negligible.

Thus, EPA's inability to guarantee the accuracy or increase
the precision of the PM2.5 NAAQS in no way undermines the
standards' validity. Rather, these limitations indicate only
that significant scientific uncertainty remains about the health
effects of fine particulate matter at low atmospheric concen-
trations. As the exhaustive rulemaking process makes clear,
see supra pp. 15-21, EPA set the primary NAAQS notwith-
standing that uncertainty, just as the Act requires.

We are equally unpersuaded by State and Business Peti-
tioners' argument that EPA should have considered whether
reducing atmospheric concentrations of fine particles would
increase levels of " 'ozone or ... a different fine particle
component,' potentially increasing [overall] health risk[s]."
State & Bus. Pet'rs' Br. at 37 (quoting Advance NPRM, 61
Fed. Reg. at 65,768). Petitioners apparently believe EPA
may not regulate one pollutant without determining how that
regulation would affect the levels of all other pollutants with
which the first could react. Given the complexity of atmo-
spheric chemistry, see Advance NPRM, 61 Fed. Reg. at
65,768 (noting "multiple nonlinearities and positive and nega-
tive feedbacks"), however, imposing such a requirement

would hamstring the Agency, preventing it from complying
with the Clean Air Act's mandate to set protective primary
NAAQS. We might feel differently about EPA's obligations
in this regard had Petitioners pointed to clear evidence that
lowering atmospheric PM2.5 levels will necessarily increase
levels of other pollutants. Petitioners cite no such evidence,
however, instead relying on an obscure passage from the
Advance NPRM in which EPA said only that the complicated
interactions among different atmospheric pollutants, including
PM, make "integrated implementation" of pollution controls
"far from a straightforward exercise." Id. As EPA ob-
serves, this "general discussion of the interrelationship[s]"
among pollutants hardly constitutes a finding that regulating
PM2.5 levels will increase health risks from ozone and other
pollutants. Resp't's Particulate Matter Br. at 31-32 n.23.

State and Business Petitioners' remaining substantive
claims merit little discussion. They argue that EPA "failed
to determine whether attainment of the [old particulate mat-
ter] standard would leave unacceptable public health risk."
State & Bus. Pet'rs' Br. at 40. As EPA notes, however, the
final PM rule explicitly states:

[T]he extensive PM epidemiological data base provides
evidence of serious health effects (e.g., mortality, exacer-
bation of chronic disease, increased hospital admissions)
in sensitive populations (e.g., the elderly, individuals with
cardiopulmonary disease), as well as significant adverse
health effects (e.g., increased respiratory symptoms,
school absences, and lung function decrements) in chil-
dren. Moreover, these effects associations are observed
in areas or at times when the levels of the [old PM]
standards are met.

Particulate Matter NAAQS, 62 Fed. Reg. at 38,657 (emphasis
added). Most CASAC members, moreover, recognized the
need to strengthen "the health protection[s] provided by the
[old] PM standards." Particulate Matter NAAQS, 62 Fed.
Reg. at 38,666. Finally, in ATA I, we found "ample support
for EPA's decision to regulate coarse particulate pollution
above the 1987 levels," particularly given the lenient standard
of review. 175 F.3d at 1054. Though we referred only to

coarse particulate matter, the statement nevertheless indi-
cates our belief--which Petitioners give us no reason to
reconsider--that the Agency substantiated its judgment re-
garding the inadequacy of the old particulate matter stan-
dards.

ATA I also forecloses Petitioners' "confounder" argu-
ment--that factors "such as the presence of other pollutants
in the ambient air, temperature, humidity, and indoor air
pollution might account for some or all of the associations
reported in the studies on which EPA relied." State & Bus.
Pet'rs' Br. at 46-47. "[T]he growing empirical evidence
demonstrating a relationship between fine particle pollution
and adverse health effects," we said in ATA I, "amply justi-
fies establishment of new fine particle standards." 175 F.3d
at 1056. We could not have reached that conclusion had we
agreed with the Non-State Petitioners in ATA I that "EPA's
... [s]tudies failed adequately to address the confounding
effect of other pollutants." Non-State Pet'rs' 1998 Br. at 28.
Even if we did not think ourselves bound by ATA I, though,
we would defer to EPA's entirely plausible reasoning regard-
ing the confounder issue: According to the Agency, the
"consistency of PM effects across areas with widely varying
concentrations of potentially confounding copollutants," to-
gether with Agency staff's "extended analyses of the Philadel-
phia studies," amply justify the conclusion that "PM, alone or
in combination with other pollutants, is associated with ad-
verse effects at levels below those allowed by the current
standards." Particulate Matter NAAQS, 62 Fed. Reg. at
38,661.

Next, Petitioners argue that the PM2.5 NAAQS are arbi-
trary and capricious because "EPA conceded that public
health risks would decline with more stringent standards only
if" the Agency correctly assumed "that (1) there is 'a contin-
uum of health risks down to the lower end of the ranges of air
quality' at issue, and (2) 'the reported associations are, in fact,
causally related to PM2.5 at the lowest concentrations mea-
sured.' " State & Bus. Pet'rs' Br. at 45 (quoting Particulate
Matter NAAQS, 62 Fed. Reg. at 38,675). EPA made this
alleged concession, however, only in the context of rejecting
standards "more stringent than those proposed" in the

NPRM, Particulate Matter NAAQS, 62 Fed. Reg. at 38,675;
the Agency never "conceded" that the actual NAAQS rely on
questionable assumptions or associations.

Petitioners mount several specific challenges to the daily
and annual NAAQS. To begin with, they contend that EPA
failed to justify the daily PM2.5 standard, but because this
argument appears nowhere in their ATA I briefs, see Non-
State Pet'rs' 1998 Reply Br. at 1 ("We focus on the lack of
justification for setting the annual standard at 15 ug/m3.");
see also id. at 1 n.3 ("Of the two new primary standards, the
annual standard imposes the more stringent 'controlling' re-
quirement."), we decline to consider it. See Order of the
United States Court of Appeals for the District of Columbia
Circuit at 1 (May 29, 2001) (No. 97-1440) ("The [post-remand]
briefs shall address only 'preserved challenges' not hitherto
resolved by the Supreme Court or by this court[.]" (quoting
Whitman, 531 U.S. at 476)). Challenging the annual NAAQS,
Petitioners argue the record does not support setting the
standard at 15 ug/m3. Contrary to Petitioners' assertion,
however, EPA staff never recommended a higher, 20 ug/m3
level. Rather, staff recommended that EPA consider a range
of levels from 12.5 to 20 ug/m3, making no specific recommen-
dation within this range. Office of Air Quality Planning &
Standards, U.S. Envtl. Prot. Agency, EPA-452lE0R-96-013,
Review of the National Ambient Air Quality Standards for
Particulate Matter: Policy Assessment of Scientific and
Technical Information VII-36 to VII-37 (1996). Moreover,
EPA ultimately set the standard just below the range of
mean annual PM2.5 concentrations observed in studies show-
ing a statistically significant association between fine particu-
late matter and health effects. See supra p. 19. While we
cannot say those studies necessitated a standard level of 15
ug/m3, neither have we any basis for concluding that EPA's
decision was unreasonable or unsupported by the record. We
repeat: "That the evidence in the record may also support
other conclusions, even those that are inconsistent with the
Administrator's, does not prevent us from concluding that
[her] decisions were rational and supported by the record."
Lead Indus. Ass'n, 647 F.2d at 1160 (internal citations omit-
ted).

 This brings us finally to Petitioners' argument that EPA
"denied the public essential procedural rights" by failing to
obtain and make public the data underlying certain "key
studies" relating to the "confounder" issue. Claiming neither
that they were unable to obtain the studies, nor that the
studies were improperly published or peer reviewed, Petition-
ers instead urge us to impose a general requirement that
EPA obtain and publicize the data underlying published
studies on which the Agency relies. The Clean Air Act
imposes no such obligation; it merely directs EPA to include
in any notice of proposed rulemaking "data, information, and
documents ... on which the proposed rule relies." 42 U.S.C.
s 7607(d)(3) (emphasis added). Here, EPA explained that it
"relied on the scientific studies cited in the rulemaking rec-
ord, rather than on the raw data underlying" those studies.
Particulate Matter NAAQS, 62 Fed. Reg. at 38,689. In
addition, Agency counsel advised us at oral argument that on
those few occasions when EPA requested underlying data
from an investigator, the Agency included those data in the
record, Tr. of Oral Arg. at 74-75. More generally, we agree
with EPA that requiring agencies to obtain and publicize the
data underlying all studies on which they rely "would be
impractical and unnecessary." Particulate Matter NAAQS,
62 Fed. Reg. at 38,689. As EPA persuasively stated in
denying Petitioners' original request for the information:

If EPA and other governmental agencies could not rely
on published studies without conducting an independent
analysis of the enormous volume of raw data underlying
them, then much plainly relevant scientific information
would become unavailable to EPA for use in setting
standards to protect public health and the environ-
ment.... [S]uch data are often the property of scienti-
fic investigators and are often not readily available be-
cause of ... proprietary interests ... or because of
[confidentiality] arrangements [with study participants].

Id.

State and Business Petitioners' challenge to the secondary
NAAQS hinges on the proposition that the identical primary

NAAQS are arbitrary and capricious. Having just rejected
the latter argument, we need not consider the former.

Environmental Petitioners' Claims

Environmental Petitioners' central argument regarding the
primary NAAQS is straightforward: EPA should have set a
stricter daily PM2.5 NAAQS rather than relying almost ex-
clusively on the stringent annual standard. Petitioners con-
tend that because "adverse health effects [of] ... PM2.5
occur after single-day exposures[,] ... the NAAQS must
prevent most, if not all, such daily exposures in order to"
reduce health risks. Envtl. Pet'rs' Br. at 10. A strategy that
focuses on the annual rather than the daily standard, Peti-
tioners argue, will do little to prevent short-term pollution
"events," in which PM2.5 concentrations exceed healthy levels
for days or weeks. Questioning the level of the daily
NAAQS, Petitioners note that EPA evaluated three possible
levels (65, 50, and 25 ug/m3), ultimately choosing the highest
even though the risk assessment study indicates that in some
cities, a lower daily standard would greatly reduce the risks
of both short- and long-term exposures. Risk Assessment at
113-18. Finally, with respect to the form of the daily stan-
dard, Petitioners argue that focusing on the 98th percentile
value allows about seven days per year (two percent of 365
days) to escape regulation. The 15 ug/m3 annual standard,
Petitioners observe, will do almost nothing to limit PM2.5
levels on these seven otherwise unregulated days.

Responding to these arguments, EPA observes that there
are "significant uncertainties in identifying the extent of the
incremental risk associated with single peak exposures to
PM2.5 in areas where the annual standard is met." Particu-
late Matter NAAQS, 62 Fed. Reg. at 38,677. As we under-
stand the problem, existing data are insufficient to permit
EPA to separate health effects of long-term-average PM2.5
concentrations from those of short-term peak concentrations
because in all cities in which the Agency found a positive
correlation between short-term exposures and adverse health
effects, the long-term-average PM2.5 concentrations exceeded

the new annual NAAQS. Thus, EPA cannot determine "what
risks might have been associated with [short-term] peak
[PM2.5] levels had the long-term averages in these areas
been below that selected for the [new] annual standard." Id.
This uncertainty, together with evidence suggesting that "the
... risk over the course of a year associated solely with a
limited number of peak exposures is ... considerably smaller
than that associated with the entire air quality distribution,"
id. at 38,677, convinced EPA to focus its attention on the
annual NAAQS. We think this expert judgment worthy of
deference, at least until formerly polluted areas come into
compliance with the new annual PM2.5 standard and new
health effects data from those areas become available.

Moreover, EPA provides a convincing justification for its
decision to place little faith in the quantitative results of the
risk assessment, which Petitioners cite for the proposition
that setting a restrictive daily standard would provide bene-
fits beyond those of the annual standard. Although Petition-
ers' position finds some support in the numerical results of
the risk assessment, see Risk Assessment at 113-18 (detailing
the results of the study, which suggest that lowering the daily
standard would reduce health effects even if the annual
standard were held steady), EPA contends that uncertainties
in the assumptions underlying that study render the quantita-
tive predictions insufficiently "reliable to serve as the basis
for establishing any more stringent standards than were
warranted based directly on EPA's analysis of the epidemio-
logical evidence." Resp't's Particulate Matter Br. at 51; see
also Particulate Matter NAAQS, 62 Fed. Reg. at 38,656
("EPA emphasizes that it places greater weight on the overall
conclusions derived from the studies ... than on the specific
concentration-response functions and quantitative risk esti-
mates derived from them."). Specifically, in the relevant
portions of the risk assessment, Agency staff apparently used
a model that assumed imposition of a new daily standard
would prompt States to restrict PM-generating activities on
all days, not just days that would otherwise exceed the
standard. Consequently, stricter daily standards in the mod-
el drove down both short-term and annual-average PM2.5

concentrations. "In other words," EPA explains, "even
though the annual standard was held constant[,] ... the
model drove all daily values, including those at or below the
annual mean, to levels lower than those calculated from
compliance with the annual standard alone," and it was this
drop in annual PM2.5 levels, not the drop in daily levels, that
produced the projected health benefits. Resp't's Particulate
Matter Br. at 53. Not only do we owe deference to an
agency's determination regarding the reliability of scientific
evidence, but Petitioners give us no reason to question EPA's
judgment regarding the reliability of the risk assessment.
See Troy Corp. v. Browner, 120 F.3d 277, 283 (D.C. Cir. 1997)
("As we have said, we review scientific judgments of [an]
agency '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 Ethyl Corp. v. EPA, 541 F.2d 1, 36 (D.C.Cir. 1976))).

We recognize that EPA's reasoning leaves two unanswered
questions, but neither proves fatal. First, one might argue
from EPA's discussion of the risk assessment results that the
Agency should have adopted a lower annual NAAQS. On
this point, EPA persuasively explains that it discounted the
quantitative predictions of the risk assessment in light of
"inherent scientific uncertainties," including the "possibility of
... thresholds" below which PM2.5 has little or no effect.
Particulate Matter NAAQS, 62 Fed. Reg. at 38,675. In other
words, although the model predicted that dropping annual-
average PM2.5 levels below 15 ug/m3 would reduce adverse
health effects, those predictions relied on untested--and
hence unreliable--assumptions about the incidence of health
effects at low particulate matter levels.

Second, EPA's skepticism about the quantitative results of
the risk assessment, together with the Agency's repeated
assertions that the evidence does not justify a strict daily
NAAQS, lead one to wonder why a daily standard is even
necessary. EPA's position on this issue seems incongruous,

emphasizing on the one hand the need for a daily standard to
"provid[e] supplemental protection against peak exposures
not addressed by the annual standard," Particulate Matter
NAAQS, 62 Fed. Reg. at 38,677, and on the other the lack of
evidence establishing that "peak exposures not addressed by
the annual standard" pose "incremental" risks over and above
those associated with high annual-average PM2.5 levels, id.
None of the parties, however, challenges the daily NAAQS in
precisely these terms: State and Business Petitioners focus
on the annual NAAQS, while Environmental Petitioners pri-
marily cite the evidence supporting a stricter daily NAAQS.
Even if the parties had raised this issue, however, we would
neither vacate nor remand the daily NAAQS. Reviewing the
record ourselves, we find support for the following explana-
tion of the existence and form of the daily standard: (1) Some
daily NAAQS is necessary to prevent deadly, short-term
PM2.5 episodes of the sort that occurred during the famous
1952 London Fog, id. at 38,659; but (2) in areas that meet the
new 15 ug/m3 annual standard, such events, though "theoreti-
cally possible," are "unlikely," id. at 38,673; so (3) each year,
States should be permitted to address one or a few such
events through statutorily-authorized "emergency episode
plans" rather than longer-term pollution control measures, id.
at 38,673 & n.35; (4) otherwise States would have to design
their pollution control programs around "single high exposure
events that may be due to unusual meteorological conditions
alone," rendering the programs less "stable"--and hence, we
assume, less effective--than programs designed to address
longer-term average conditions, id. at 38,673. We cannot
fault this strategy. As Justice Breyer noted in his Whitman
concurrence, because "[a] rule likely to cause more harm to
health than it prevents is not a rule that is 'requisite to
protect the public health,' " the Clean Air Act must permit
EPA to "consider whether a proposed rule promotes safety
overall." 531 U.S. at 495 (Breyer, J., concurring) (emphasis
added). Therefore, to promote development of more "effec-
tive [pollution] control programs," EPA was entirely justified
in permitting a few days to escape regulation under the daily

NAAQS. Particulate Matter NAAQS, 62 Fed. Reg. at 38,673.
Having no basis to question EPA's choice of seven (rather
than, for example, four or ten) unregulated days, our deferen-
tial standard of review requires that we leave the daily
NAAQS in place.

Finally, unlike State and Business Petitioners, Environ-
mental Petitioners mount an independent challenge to the
secondary standards, contending that EPA expressly recog-
nized the standards were inadequate to improve visibility in
much of the western United States. See supra p. 21 (citing
Particulate Matter NAAQS, 62 Fed. Reg. at 38,680-81). In
ATA I, however, we concluded that "Congress did not intend
the secondary NAAQS to eliminate all adverse visibility ef-
fects and, therefore, that the EPA acted within the scope of
its authority in deciding to rely upon the regional haze
program to mitigate some of the adverse visibility effects
caused by" PM2.5. 175 F.3d at 1056-57. Moreover, EPA
provides a plausible explanation for its decision not to set
lower secondary NAAQS: National standards adequate to
improve visibility in the west "would have to be set at or even
below natural background levels in the East." Particulate
Matter NAAQS, 62 Fed. Reg. at 38,680. Petitioners give us
no reason to question this explanation.

IV.

Turning to the ozone standards, we again begin with a
summary of the rulemaking process, and then consider Peti-
tioners' challenges.

The Rulemaking

EPA first announced its plan to revise the ozone NAAQS in
late 1996, issuing a public notice of proposed rulemaking.
Ozone NAAQS, 62 Fed. Reg. at 38,858 (discussing NAAQS
for Ozone: Proposed Decision, 61 Fed. Reg. 65,716 (proposed
Dec. 13, 1996)). In that notice, EPA indicated that it intend-
ed to replace the existing, one-hour-average, primary ozone

standard of 0.12 ppm with a new, eight-hour-average stan-
dard. It sought comments on this change and also on three
possible levels for the new standard--0.09, 0.08, and 0.07
ppm. According to EPA, a level of 0.09 ppm would afford
about the same protection as the old, one-hour-average stan-
dard, id. at 38,858; 0.08 would replicate the lowest exposure
level actually tested in clinical studies, id. at 38,861; and 0.07
would be "highly precautionary in nature," id. at 38,858.

As in connection with the particulate matter NAAQS, EPA
took account of CASAC's recommendations, Agency staff's
assessment of the available information regarding human
exposure and risk, and tens of thousands of comments sub-
mitted in response to the NPRM. Id. at 38,858-59. EPA
also conducted risk assessments in nine representative urban
areas (the "nine area study") to estimate the correlation
between ozone exposure and health risks for the general
population and for two groups at higher risk, "outdoor work-
ers" and a group EPA calls "outdoor children." Id. at 38,860
(internal quotation marks omitted). Moreover, EPA devel-
oped quantitative risk estimates for those specific health
effects for which sufficient concentration-response data were
available. Id. Finally, in deciding when certain observed
physiological effects of ozone "become so significant that they
should be regarded as adverse" to individuals' health, EPA
looked to guidelines published by the American Thoracic
Society. Id.

In the end, EPA adopted an eight-hour-average, 0.08 ppm
standard. Ozone NAAQS, 62 Fed. Reg. at 38,856. Two
aspects of this new standard require discussion here: averag-
ing time and level.

Explaining its switch from a one- to an eight-hour averag-
ing time, EPA first noted that the one-hour-average standard
reflected the belief--since refuted--that the most serious
health effects of ozone result from short-term (one- to three-
hour) exposures. According to EPA, however, new studies
clearly "demonstrate[ ] associations between a wide range of
health effects and prolonged (... [six]- to [eight]-hour) expo-

sures" to concentrations less than that of the old standard.
Id. at 38,861. Citing the nine area study, EPA indicated that
an averaging time of eight hours not only reduces risk
associated with short-term exposures, id. at 38,862 & n.11,
but also limits cumulative exposure, id. at 38,861. The longer
averaging time, moreover, reduces variability in ozone levels
across geographic areas. Id. at 38,862. EPA also empha-
sized that CASAC unanimously agreed with the proposed
change. Id. at 38,861; see also U.S. Envtl. Prot. Agency,
EPA-SAB-CASAC-LTR-96-002, Letter Re: CASAC Clo-
sure on the Primary Standard Portion of the Staff Paper for
Ozone 2 (1995) ("CASAC Ozone Letter") ("The Panel was in
unanimous agreement that the present [one]-hour standard
be eliminated and replaced with an [eight]-hour standard.").
Acknowledging some individual CASAC members' comments
to the effect that "choosing between a [one]- or [eight]-hour
averaging time is a 'policy' choice" not dictated by science,
EPA declined to view "these individual statements during the
course of CASAC's review" as in any way "contradict[ing]
[ ]or supersed[ing] the clear and unanimous agreement" on
the issue "conveyed ... in [CASAC's] closure letter." Ozone
NAAQS, 62 Fed. Reg. at 38,862. In EPA's opinion, "the fact
that an averaging time of [eight] hours results in a signifi-
cantly more uniformly protective national standard than the
current [one]-hour standard is an important public health
policy consideration that supports the selection" of the former
averaging time. Id. at 38,862.

Turning to the 0.08 ppm level of the primary standard,
EPA first explained that because ozone is--or is thought to
be--a non-threshold pollutant, "it is not possible to select a
level below which absolutely no [health] effects are likely to
occur." Id. at 38,863. Nevertheless, EPA undertook to
select a standard level that would "reduce risk sufficiently to
protect public health with an adequate margin of safety." Id.
In deciding among the proposed levels (0.09, 0.08, and 0.07
ppm), EPA took into account a number of "key observations
and conclusions," id., including:

. CASAC's conclusion that the old standard "provided little,
if any, margin of safety," id. (internal quotation marks and
citations omitted);

. Numerous epidemiological studies attributing "excess hos-
pital admissions" to ozone exposure at concentrations
below that of the old standard, id. at 38,864;

. Evidence that an eight-hour-average, 0.09 ppm standard
would constitute only a modest improvement over the old
standard, id.;

. "[C]lear evidence from human clinical studies" that pro-
longed exposure to 0.08 ppm of ozone, at moderate exer-
tion, correlates positively with health effects like coughing,
pain with inhalation, and reduced lung function, id. at
38,863-64;

. Clinical studies suggesting that "[w]hile group mean re-
sponses ... at ... 0.08 ppm"--the "lowest exposure level
tested"--are usually "small or mild in nature, responses of
some sensitive individuals are sufficiently severe ... to be
considered adverse," id.;

. Indications from the nine area study that "statistically
significant reductions in exposure and risk ... result from
alternative [eight]-hour standards as the level [is lowered]
from 0.09 ppm to ... 0.07 ppm," but that "there is no ...
bright line that differentiates between acceptable and
unacceptable risks within this range," id. at 38,864; and
finally,

. Evidence from the same study that "a standard set at 0.09
ppm would allow approximately 40 percent to 65 percent
more outdoor children to experience [decreases in lung
function and pain with inhalation] than would a 0.08 ppm
standard," id. at 38,868.

Despite some "inherent uncertainties," EPA viewed these
observations and conclusions, taken together, as indicating
that the "public health impacts" of ozone at levels lower than
the one-hour, 0.12 ppm standard are "important and suffi-

ciently large as to warrant a standard set at a level of 0.08
ppm." Id.

Acknowledging that numerous public comments advocated
a level of 0.07 ppm, EPA offered several explanations for its
decision to reject a more stringent standard. Most impor-
tant, EPA pointed out that not one CASAC panel member
"supported a standard set lower than 0.08 ppm, specifically
after considering a range of alternative standards that includ-
ed 0.07 ppm." Ozone NAAQS, 62 Fed. Reg. at 38,868. In
addition, EPA contended that the "most certain" adverse
health effects of ozone "are transient and reversible" at low
ozone levels, while "the more serious ... impacts on health
are less certain," id.--presumably in part because no human
clinical studies have evaluated concentrations lower than 0.08
ppm. Finally, EPA noted that a 0.07 ppm standard "would
be closer to peak background levels that infrequently occur in
some areas due to nonanthropogenic sources of [ozone] pre-
cursors." Id.

One final aspect of EPA's discussion of the primary
NAAQS level is relevant here: The Agency's response to
certain comments questioning its reliance on specific field,
epidemiological, and clinical studies. According to EPA, the
comments "did not reflect an integrative assessment of the
evidence--the approach CASAC has historically urged [the
Agency] to follow--but rather a piecemeal look at each indi-
vidual study." Id. at 38,866. EPA therefore dismissed the
comments, arguing that such an incremental critique "tends
to miss the strength of the entire body of evidence taken
together." Id.

In setting the secondary NAAQS, EPA "focused on [ozone]
effects on vegetation[,] since these public welfare effects are
of most concern at [ozone] concentrations typically occurring
in the United States." Ozone NAAQS, 62 Fed. Reg. at
38,874-75. The NPRM identified two possible changes to the
secondary standard: replacing the old, one-hour-average 0.12
ppm standard with an eight-hour-average standard identical
to the new primary standard, or adopting an entirely new
"seasonal standard" that would regulate the cumulative sum

of hourly ozone concentrations during the three consecutive
months of the year with highest ozone levels. Id. at 38,858.
In deciding between these options, EPA again took account of
public comments, CASAC's recommendations, and Agency
staff's assessment of the available information. Id. at 38,874.
Citing statements by CASAC panel members, id. at 38,875,
EPA first concluded that "a secondary NAAQS more strin-
gent than the [old, 0.12 ppm] primary standard, was neces-
sary to protect vegetation from [ozone]," id. at 38,877 (inter-
nal quotation marks and citations omitted). "[I]nadequate
rural and remote [ozone] air quality data," however, limited
EPA's ability to evaluate the potential benefits of a seasonal
standard. Id. ("[G]iven the present limits of the scientific
evidence ..., the Administrator has decided that it is not
appropriate to move forward with a seasonal secondary stan-
dard at this time."). Ultimately, therefore, EPA rejected the
seasonal standard (at least until better data become avail-
able), choosing instead to set the secondary standard equal to
the primary standard. Id. at 38,877-78.

Ozone Petitioners' Claims

American Trucking Associations and the other Ozone Peti-
tioners challenge the NAAQS along several lines. To begin
with, just as State and Business Petitioners argued with
respect to the NAAQS for particulate matter, Ozone Petition-
ers contend that EPA neither identified nor applied "any
legal standard" in setting the ozone NAAQS. Ozone Pet'rs'
Br. at 38. Petitioners cite EPA's statements that it need not
identify a "safe level" of ozone, and that "margin of safety
determinations ... may not be amenable to quantification in
terms of what risk is 'acceptable' or any other metric," Ozone
NAAQS, 62 Fed. Reg. at 38,883, as evidence that EPA failed
to set the primary ozone NAAQS "at the level that is 'requi-
site' ... to protect the public health with an adequate margin
of safety," Whitman, 531 U.S. at 475-76. As we discussed
earlier, however, EPA has no obligation either to identify an
accurate "safe level" of a pollutant or to quantify precisely the
pollutant's risks prior to setting primary NAAQS. See supra
pp. 23-24. Rather, EPA must err on the side of caution, just

as it did here--setting the NAAQS at whatever level it deems
necessary and sufficient to protect the public health with an
adequate margin of safety, taking into account both the
available evidence and the inevitable scientific uncertainties.

Petitioners raise two specific arguments regarding the
primary ozone NAAQS. First, they assert that EPA "failed
to determine whether attainment of the [old, one-hour-
average, primary ozone] standard would leave unacceptable
public health risk," Ozone Pet'rs' Br. at 43, and relatedly, that
"none of the alternative [eight]-hour standards [considered by
EPA] is significantly more protective of the public health than
the [old] [one]-hour NAAQS," id. at 47-48. We disagree. As
noted earlier, not only is the record replete with references to
studies demonstrating the inadequacies of the old one-hour
standard, see supra p. 39, but EPA discussed at length the
advantages of a longer averaging time, including reduced risk
of prolonged exposures to unhealthy ozone levels and in-
creased uniformity of protection across different urban areas,
see Ozone NAAQS, 62 Fed. Reg. at 38,861. Moreover, EPA
specifically cited CASAC's "consensus ... that an [eight]-
hour standard [is] more appropriate for a human health-based
standard than a [one]-hour standard" and its recommendation
that "the present ... standard be eliminated and replaced
with an [eight]-hour standard." CASAC Ozone Letter at 2;
see also Ozone NAAQS, 62 Fed. Reg. at 38,861 ("In proposing
to change the averaging time of the primary standard from
[one] to [eight] hours, the Administrator was concurring with
the unanimous recommendation of CASAC."). Given this
record evidence, our deferential standard of review, and the
Clean Air Act's requirement that EPA must either follow
CASAC's advice or explain why the proposed rule "differs ...
from ... [CASAC's] recommendations," 42 U.S.C.
s 7607(d)(3), Petitioners cannot seriously expect us to second-
guess EPA's conclusion regarding the inadequacy of the old,
one-hour-average standard.

Though somewhat more persuasive, Petitioners' second
specific challenge also falls short. They argue that in select-
ing a level of 0.08 ppm rather than 0.09 or 0.07, EPA reached
"inconsistent conclusions regarding specific health risks,"

thus "demonstrat[ing] that [the Agency's] decision to revise
the NAAQS lacks a rational basis and therefore is arbitrary
and capricious." Ozone Pet'rs' Br. at 48. In support of this
point, Petitioners challenge EPA's three justifications for
selecting 0.08 rather than 0.07: that no CASAC member
supported a standard below 0.08; that health effects at ozone
levels below 0.08 are transient and reversible; and that 0.07
would be too close to peak background levels. See supra p.
37 (noting these arguments). As to the first point, Petition-
ers observe that "most members of the CASAC panel who
expressed an opinion on standard level supported a level
above ... 0.08 ppm," and that "CASAC ultimately concluded
that there is 'no bright line' distinguishing any of the alterna-
tive standards ... as significantly more protective of public
health." Ozone Pet'rs' Br. at 49-50 (quoting CASAC Ozone
Letter at 3). In addition, Petitioners point out, ATA I
expressly discredits the contention that ozone health effects
are more transient and reversible at concentrations below
0.08 ppm than at concentrations between 0.08 and 0.09. 175
F.3d at 1035 ("[The record evidence] does not make the
categorical distinction the dissent says it does, and it is far
from apparent that any health effects existing above [0.08
ppm] are permanent or irreversible."). Petitioners finally
note that proximity to peak background levels is an indeter-
minate standard that points to no particular level for the
primary NAAQS.

Although we think Petitioners' individual criticisms have
some force, we are satisfied that in selecting a level of 0.08
rather than 0.07 (or, for that matter, 0.09), EPA "engage[d] in
reasoned decision-making." American Lung Ass'n, 134 F.3d
at 392. For one thing, CASAC's inability to reach consensus
is hardly dispositive; EPA is entitled to give "significant
weight" to the fact that no committee member advocated a
level of 0.07 ppm, Ozone NAAQS, 62 Fed. Reg. at 38,868,
particularly as eight of the ten panel members who expressed
opinions advocated a level of 0.08 ppm or greater, while the
remaining two simply "endorsed the [0.07-0.09 ppm] range
presented by the Agency ... and stated that the [final]
selection should be a policy decision," CASAC Ozone Letter

at 3. Also, although relative proximity to peak background
ozone concentrations did not, in itself, necessitate a level of
0.08, EPA could consider that factor when choosing among
the three alternative levels. Most convincing, though, is the
absence of any human clinical studies at ozone concentrations
below 0.08. See id. at 38,863. This lack of data amply
supports EPA's assertion that the most serious health effects
of ozone are "less certain" at low concentrations, id. at 38,868,
providing an eminently rational reason to set the primary
standard at a somewhat higher level, at least until additional
studies become available. Cf. Natural Res. Def. Council,
Inc., 902 F.2d at 974 (declining to require EPA "to explain
the risk it considered tolerable" in setting the challenged
NAAQS "because of the uncertainty of the data upon which
the Administrator needed to rest his assessment"). Overall,
therefore, we disagree with Petitioners that in selecting 0.08
ppm rather than a lower or higher level, EPA reached
"inconsistent conclusions." The Agency could reasonably
conclude that existing data support a standard below 0.09 but
do not yet justify a standard below 0.08.

This brings us, finally, to Petitioners' challenges to the
secondary ozone NAAQS. According to Petitioners, the sec-
ondary NAAQS are unlawful because EPA failed to account
for factors other than ozone--including "temperature, rainfall,
and pests"--that affect crop-yield. Ozone Pet'rs' Br. at 56
(internal quotation marks and citations omitted). This is
unreasonable. EPA had no more obligation to consider cli-
matic conditions and pests in "evaluat[ing] whether its new
[secondary] standard would measurably improve crop yield,"
id., than to consider automobile accidents and malnutrition in
evaluating whether its new primary standard would measur-
ably improve public health. The Clean Air Act directs EPA
to protect public welfare from adverse effects of ozone and
other pollutants; the Agency cannot escape that directive
simply because ozone wreaks less havoc than temperature,
rainfall, and pests.

Citing various CASAC comments regarding the " 'lack of
relevant plant exposure studies,' " Petitioners next argue that
EPA estimated the crop-related risks of ozone "crude[ly]."

Id. (quoting U.S. Envtl. Prot. Agency, EPA-SAB-CASAC-
LTR-96-006, Letter Re: Closure by [CASAC] on the Sec-
ondary Standard Portion of the Staff Paper for Ozone 4
(1996) ("Second CASAC Ozone Letter")) (internal quotation
marks and citations omitted). CASAC, however, found more
than enough data to justify revising the existing secondary
NAAQS. Second CASAC Ozone Letter at 2 ("[I]t was
agreed that a secondary NAAQS, more stringent than the
present primary standard, was necessary to protect vegeta-
tion from ozone."). Moreover, nothing in the Clean Air Act
requires EPA to wait until it has perfect information before
adopting a protective secondary NAAQS. Rather, the Act
mandates promulgation of secondary standards requisite to
protect public welfare from any "anticipated adverse effects
associated with" regulated pollutants, 42 U.S.C. s 7409(b)(2)
(emphasis added), suggesting that EPA must act as soon as it
has enough information (even if crude) to "anticipate[ ]" such
effects--just as it did here.

V.

The petitions for review are denied except to the extent
ATA I, ATA II, and Whitman require further action by EPA.

So ordered.