United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued September 15, 2008 Decided February 24, 2009

No. 06-1410

AMERICAN FARM BUREAU FEDERATION AND NATIONAL PORK

PRODUCERS COUNCIL,

PETITIONERS

v.

ENVIRONMENTAL PROTECTION AGENCY,

RESPONDENT

AMERICAN CHEMISTRY COUNCIL, ET AL.,

INTERVENORS

Consolidated with 06-1411, 06-1415, 06-1416, 06-1417

On Petitions for Review of an Order 

of the Environmental Protection Agency

Michael J. Myers, Assistant Attorney General, Attorney

General’s Office of State of New York, argued the cause for

State Petitioners and State Amici. With him on the briefs were

Andrew M. Cuomo, Attorney General, Barbara D. Underwood,

Solicitor General, and Katherine Kennedy, Assistant Attorney

General, Terry Goddard, Attorney General, Attorney General’s

Office of the State of Arizona, Joseph P. Mikitish, Assistant
2

Attorney General, Edmund G. Brown, Jr., Attorney General,

Attorney General’s Office of the State of California, Theodora

Berger, Senior Assistant Attorney General, Susan L. Durbin,

Deputy Attorney General, Richard Blumenthal, Attorney

General, Attorney General’s Office of the State of Connecticut,

Kimberly Massicotte, Assistant Attorney General, Joseph R.

Biden, III, Attorney General, Attorney General’s Office of the

State of Delaware, Valerie S. Csizmadia, Deputy Attorney

General, Lisa Madigan, Attorney General, Attorney General’s

Office of the State of Illinois, Thomas Davis, Assistant Attorney

General, G. Steven Rowe, Attorney General, Attorney General’s

Office of the State of Maine, Gerald D. Reid, Assistant Attorney

General, Douglas F. Gansler, Attorney General, Attorney

General’s Office of the State of Maryland, Kathy M. Kinsey,

Roberta R. James, and Mary Raivel, Assistant Attorneys

General, Martha Coakley, Attorney General, Attorney General’s

Office for the Commonwealth of Massachusetts, William L.

Pardee, Assistant Attorney General, Kelly A. Ayotte, Attorney

General, Attorney General’s Office of the State of New

Hampshire, Maureen D. Smith, Senior Assistant Attorney

General, Ann Milgram, Attorney General, Attorney General’s

Office of the State of New Jersey, Kevin P. Auerbacher and

Jung Kim, Deputy Attorneys General, Gary King, Attorney

General, Attorney General’s Office of the State of New Mexico,

Stephen R. Farris, Assistant Attorney General, Tracy M.

Hughes, William G. Grantham, and Karen L. Reed, Special

Assistant Attorneys General, Hardy Myers, Attorney General,

Attorney General’s Office of the State of Oregon, Philip

Schradle, Special Counsel, Kristen M. Campfield, Associate

Counsel, The Commonwealth of Pennsylvania, Department of

Environmental Protection, Patrick C. Lynch, Attorney General,

Attorney General’s Office of the State of Rhode Island, Tricia

K. Jedele, Special Assistant Attorney General, William H.

Sorrell, Attorney General, Attorney General’s Office of State of

Vermont, Kevin O. Leske, Assistant Attorney General, Peter J.
3

Nickles, Interim Attorney General, Attorney General’s Office of

the District of Columbia, Todd S. Kim, Solicitor General, Donna

M. Murasky, Senior Assistant Attorney General, Kurt R. Wiese,

District Counsel, South Coast Air Quality Management District,

and Barbara Baird, Principal Deputy District Counsel. J. Jared

Snyder, Assistant Attorney General, Attorney General's Office

of State of New York, and Kristen C. Furlan, Attorney, The

Commonwealth of Pennsylvania, Department of Environmental

Protection, entered appearances.

Paul R. Cort argued the cause for Environmental

Petitioners. With him on the briefs were Erin M. Tobin,

Deborah S. Reames, and David S. Baron.

Denise W. Kennedy and Richard E. Schwartz argued the

cause for Industry Petitioners. With them on the briefs were

Kirsten L. Nathanson, Lawrence E. Volmert, Robert T. Connery,

John F. Shepherd, Gary H. Baise, and Julie Anna Potts. John

J. Zimmerman entered an appearance.

Hope M. Babcock was on the joint brief for Health Amici.

Norman L. Rave, Jr., and Brian H. Lynk, Attorneys, U.S.

Department of Justice, argued the cause for respondent. With

them on the brief were John C. Cruden, Deputy Assistant

Attorney General, and Steven E. Silverman and John T. Hannon,

Attorneys, U.S. Environmental Protection Agency.

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

Langworthy, Leslie A. Hulse, Stacy R. Linden, Richard S.

Wasserstrom, Robin S. Conrad, and Amar D. Sarwal were on

the joint brief of Fine PM Industry Intervenors in support of

respondent.

Richard E. Schwartz, Kirsten L. Nathanson, Peter S.
4

Glaser, Robert R. Gasaway, Ashley C. Parrish, Julie Anna Potts,

and Harold P. Quinn, Jr., were on the joint Coarse PM NAAQS

brief for industry intervenors in support of respondent.

Erin Tobin, Paul Cort, Deborah S. Reames, and David S.

Baron were on the brief for intervenors American Lung

Association and Environmental Defense.

Thomas J. Ward, Robert R. Gasaway, and Ashley C.

Parrish were on the brief of amicus curiae National Association

of Home Builders in support of respondent. Duane J. Desiderio

entered an appearance.

Before: GINSBURG, GARLAND, and GRIFFITH, Circuit

Judges.

Opinion for the Court filed PER CURIAM.

PER CURIAM: In these consolidated cases, we consider

several challenges to the Environmental Protection Agency’s

most recent revision of the National Ambient Air Quality

Standards for particulate matter. Because the agency

promulgated standards for fine particulate matter that were, in

several respects, contrary to law and unsupported by adequately

reasoned decisionmaking, we grant the petitions for review in

part and remand those standards to the agency for further

proceedings. We deny the petitions for review of the agency’s

standards for coarse particulate matter because those standards

are not arbitrary, capricious, or otherwise contrary to law.
5

I.

A. Background to the 2006 Rulemaking

Particulate Matter Pollution

This case is about the Environmental Protection Agency’s

(EPA) regulation of particulate matter (PM), an air pollutant.

PM includes “a broad class of chemically and physically diverse

substances that exist as discrete particles (liquid droplets or

solids) over a wide range of sizes.” Final Rule: National

Ambient Air Quality Standards for Particulate Matter, 71 Fed.

Reg. 61,143, 61,146 (2006). Within this general definition, PM

is classified based on factors such as particle size, origin, and

chemical composition. The EPA primarily uses particle size to

classify PM, distinguishing between “fine PM” and “coarse

PM.”

Fine and coarse PM differ in ways other than size. Fine PM

is produced chiefly by combustion processes and atmospheric

reactions of gaseous pollutants; sources include motor vehicles,

power generation, and residential fuel burning. Coarse PM

tends to result from mechanical processes or the resuspension of

dusts in the air; sources include construction and demolition

activities as well as agricultural and mining operations. The two

types of PM also exhibit different atmospheric behavior: while

fine PM can remain suspended for long periods of time and

travel great distances, coarse PM tends to deposit rapidly and

does not travel as far.

The EPA further notes that “it is appropriate to draw a

distinction between two general types of ambient mixes of

coarse particles: ‘urban’ and ‘non-urban.’” Id. at 61,185 n.66.

“Urban” coarse “characterizes the mix in more heavily

populated urban areas, where sources such as motor vehicles and
6

industry contribute heavily to ambient coarse particle

concentrations and composition.” Id. In contrast, “non-urban”

coarse “encompasses mixes in a variety of other locations

outside of urbanized areas, including mixes in rural areas which

are likely to be dominated by natural crustal materials.” Id. The

EPA cautions, however, that this is not a sharp distinction

because “some types of sources are present in both urban and

non-urban areas.” Id.

Studies have demonstrated that both fine and coarse PM can

have negative effects on public health and welfare. For

example, each is associated with increased mortality (premature

death) rates and morbidity (illness) effects such as

cardiovascular disease and decreased lung function.

Specifically, scientific evidence supports these associations for

long- and short-term exposure to fine PM as well as short-term

exposure to urban coarse PM. With regard to public welfare,

high levels of fine PM in the air can impair visibility, while both

fine and coarse PM can damage vegetation, disrupt ecosystems,

corrode metals, and erode paints and other building materials.

The Clean Air Act

The Clean Air Act (CAA), 42 U.S.C. §§ 7401–7671p

(2000), a comprehensive statutory scheme designed to reduce air

pollution, requires the EPA to set national ambient air quality

standards (NAAQS) for air pollutants such as PM. The statute

directs the agency to identify air pollutants that “may reasonably

be anticipated to endanger public health or welfare.” Id.

§ 7408(a)(1). Once a pollutant is identified, the agency staff

develop and issue air quality criteria, collected in a Criteria

Document, that “accurately reflect the latest scientific

knowledge useful in indicating the kind and extent of all

identifiable effects on public health or welfare which may be

expected from the presence of such pollutant in the ambient air.”
7

Id. § 7408(a)(2). Although not required by the statute, in

practice EPA staff also develop a Staff Paper, which discusses

the information in the Criteria Document that is most relevant to

the policy judgments the EPA makes when it sets the NAAQS.

For each pollutant identified, the EPA must propose and

promulgate two sets of NAAQS: (1) primary NAAQS, “the

attainment and maintenance of which in the judgment of the

Administrator, based on [the air quality criteria] and allowing an

adequate margin of safety, are requisite to protect the public

health,” id. § 7409(b)(1) (emphasis added); and (2) secondary

NAAQS specifying a level of air quality “the attainment and

maintenance of which in the judgment of the Administrator,

based on such criteria, is requisite to protect the public welfare

from any known or anticipated adverse effects associated with

the presence of such air pollutant in the ambient air,” id.

§ 7409(b)(2) (emphasis added). In setting both standards, the

EPA takes into account the Criteria Document, the Staff Paper,

and the recommendations of the Clean Air Scientific Advisory

Committee (CASAC), a seven-member, independent scientific

review committee appointed by the Administrator pursuant to 42

U.S.C. § 7409(d)(2). The EPA must review the air quality

criteria and the NAAQS and revise them as necessary at least

once every five years. Id. § 7409(d)(1).

Each NAAQS has four components: the indicator, the

level, the averaging time, and the form. The “indicator” defines

the parameters of the substance that the EPA will measure — for

example, the size or composition of the particles to which a PM

standard will apply. The “level” specifies the acceptable

concentration of that indicator in the air. The “averaging time”

specifies the span of time across which the amount of a pollutant

in the air will be averaged. For example, some NAAQS require

a certain average annual level, while others require a certain

average daily level. The “form” of a NAAQS describes how
8

compliance with the level will be determined within this

averaging time. A NAAQS with a daily averaging time, for

example, might require that the level not be exceeded on more

than one day each year.

Previous PM Rulemakings

In 1971, the EPA identified PM as an air pollutant under

the CAA and promulgated the first set of PM NAAQS. It

selected as the indicator “total suspended particulate” (TSP),

which is measured using a device that collects from an air

sample all PM up to a diameter of 25 to 45 μm (micrometers).

The EPA first revised the PM NAAQS in 1987. It changed the

indicator from TSP to PM10, which includes all particles with a

diameter less than or equal to 10 μm. The EPA also changed

both the level and the form of the primary annual standard,

setting the new standard at 50 μg/m3 (micrograms per cubic

meter) measured as an expected annual arithmetic (rather than

geometric) mean. It also changed the level of the 24-hour

standard from 260 to 150 μg/m3, but retained the form of one

exceedance per year. Finally, the EPA revised the secondary

standards to be identical to the new primary standards.

The next revision took place in 1997 and allowed separate

regulation of fine and coarse PM for the first time. The EPA

added a second PM indicator, PM2.5, which includes all particles

with a diameter less than or equal to 2.5 μm. It set the primary

and secondary NAAQS for fine PM at a level of 15 μg/m3

 based

on a three-year average of annual arithmetic mean

concentrations, and a level of 65 μg/m3

 based on a three-year

average of the 98th percentile of daily concentrations. For

coarse PM, the EPA revised the form of the primary and

secondary 24-hour PM10 standards to be based on the 99th

percentile of concentrations at each monitoring site within a

geographic area.
9

Several parties filed petitions for review in this court

challenging the 1997 revision to the NAAQS for PM and ozone.

In May 1999, we upheld the EPA’s decision to regulate fine PM

separately from coarse PM. See Am. Trucking Ass’ns, Inc. v.

EPA (ATA I), 175 F.3d 1027, 1055–56 (D.C. Cir. 1999). We

held, however, that the selection of PM10 as the indicator for

coarse PM was arbitrary and capricious. Because a PM10

indicator includes particles that are also part of a PM2.5 indicator,

the PM10 indicator was “inherently confounded,” and the EPA

had not explained how the two standards would work together

rather than lead to overregulation of the fine fraction of PM10

and underregulation of the coarse fraction. Id. at 1054. We also

held that the EPA’s interpretation of the CAA’s requirement to

set NAAQS at the “requisite” protective level lacked an

intelligible principle to guide the agency’s decisionmaking and

therefore “effect[ed] an unconstitutional delegation of legislative

power.” Id. at 1033–34. We subsequently granted rehearing in

part to revise a section of the original opinion dealing with the

NAAQS for ozone, but made no change to our conclusions

about PM. See Am. Trucking Ass’ns, Inc. v. EPA (ATA II), 195

F.3d 4, 10 (D.C. Cir. 1999).

In February 2001, the Supreme Court reversed our holding

as to unconstitutional delegation and resolved certain other

issues not relevant to this case. See Whitman v. Am. Trucking

Ass’ns, Inc., 531 U.S. 457 (2001). On remand, we considered

the remaining challenges to the NAAQS not resolved by ATA I

and II or Whitman. We rejected each of the challenges to the

new PM2.5 NAAQS. See Am. Trucking Ass’ns, Inc. v. EPA (ATA

III), 283 F.3d 355, 358 (D.C. Cir. 2002). At the conclusion of

the litigation, therefore, the EPA’s new PM2.5 NAAQS remained

intact and its revised PM10 NAAQS were rescinded; the

preexisting 1987 PM10 NAAQS remained in place.
10

B. 2006 Rulemaking

The EPA began the review process that resulted in the

revisions challenged here in October 1997, shortly after the 1997

NAAQS were promulgated. In 2003, several environmental

groups brought a lawsuit challenging the EPA’s failure to

complete its review within five years as required by the CAA.

Pursuant to a consent decree entered in that lawsuit, the EPA

published its proposed revisions to the PM NAAQS on January

17, 2006. The EPA proposed several changes to the existing

suite of PM standards: (1) reducing the level of the primary

daily fine PM standard from 65 to 35 μg/m3

; (2) changing the

indicator for coarse PM to PM10–2.5, in order to measure only

particles with a diameter between 2.5 and 10 μm; (3) qualifying

the coarse PM indicator to include ambient mixes of PM

“dominated by” the type of particles found in urban areas while

excluding ambient mixes “dominated by” particles typical of

rural areas; (4) revoking the annual coarse PM standards; (5)

reducing the level of the primary daily coarse PM standard from

150 to 70 μg/m3

 and changing the form to measure compliance

based on the 98th percentile of annual daily concentrations

averaged over three years; and (6) adjusting all secondary

standards to be identical to the revised primary standards.

The CASAC, along with medical and public health groups

who submitted comments, challenged the EPA’s proposal to

retain the existing level of the primary annual fine PM standard

at 15 μg/m3

. They urged the EPA to lower the level to

somewhere between 12 and 14 μg/m3. The CASAC and several

public commenters also objected to setting the secondary

standards for fine PM at the same level and averaging time as

the primary standards, arguing that they were insufficient to

protect against adverse visibility effects. The CASAC supported
11

the EPA’s revocation of the annual standard for coarse PM;

some commenters, however, objected.

While commentary on most of the proposals was split, the

EPA’s proposed qualification of the coarse PM indicator drew

an almost entirely negative response. Commenters challenged

the agency’s interpretation of the scientific evidence and its

conclusion that nonurban coarse PM is not associated with

adverse effects on public health. The EPA’s proposed approach

for distinguishing between urban and nonurban coarse PM came

in for criticism as well. Commenters argued that the EPA had

not adequately defined which substances were subject to

regulation, but had instead adopted an indicator defined

arbitrarily by the requirements for placing coarse PM monitors

in particular geographic areas.

The EPA issued its final rule on October 17, 2006.

Although it made no changes to its proposed fine PM standards,

the EPA reversed its position on the qualified coarse PM

indicator and retained an unqualified indicator. The EPA

selected PM10 rather than PM10–2.5 in an attempt to differentiate

indirectly between urban and rural coarse PM. As discussed

above, PM10 includes all particles with a diameter less than or

equal to 10 μm — including fine PM particles with a diameter

up to 2.5 μm. Because urban areas tend to have higher levels of

fine PM than rural areas, a coarse PM limit based on PM10

would effectively require urban areas to have lower levels of

true coarse PM — that is, particles between 2.5 and 10 μm in

diameter — than rural areas. The EPA thus believes the

standard will target protection toward urban areas, where it is

most needed. Finally, based on the recommendation by the EPA

staff and the CASAC that the existing level of protection from

coarse PM was appropriate, and on its decision to retain the

existing PM10 indicator, the EPA concluded that no change to

the level of the 24-hour coarse PM standard was necessary.
12

C. Petitions for Review

Three sets of petitioners, joined by several intervenors and

amici, filed petitions for review of the EPA’s final rule. The

American Lung Association, Environmental Defense, and the

National Parks Conservation Association (environmental

petitioners) challenge the primary annual and secondary

standards for fine PM as well as the elimination of the annual

standard for coarse PM. Several states and state agencies (state

petitioners) challenge the primary annual fine PM standard. The

American Farm Bureau Federation, the National Pork Producers

Council, the National Cattlemen’s Beef Association, and the

Agricultural Retailers Association (industry petitioners)

challenge the EPA’s retention of the PM10 indicator for coarse

PM and the 150 μg/m3

 level for the daily coarse PM standard.

We review the actions of the EPA to determine whether

they are “(A) arbitrary, capricious, an abuse of discretion, or

otherwise not in accordance with law; (B) contrary to

constitutional right, power, privilege, or immunity; [or] (C) in

excess of statutory jurisdiction, authority, or limitations.” 42

U.S.C. § 7607(d)(9). Although we perform a “searching and

careful” inquiry into the facts underlying the agency’s decisions,

we will “presume the validity of agency action as long as ‘a

rational basis for it is presented.’” ATA III, 283 F.3d at 362

(quoting Lead Indus. Ass’n v. EPA, 647 F.2d 1130, 1145 (D.C.

Cir. 1980)). We give an “extreme degree of deference to the

agency when it is evaluating scientific data within its technical

expertise,” reviewing the agency’s action to “ensure that the

EPA has examined the relevant data and has articulated an

adequate explanation for its action.” City of Waukesha v. EPA,

320 F.3d 228, 248 (D.C. Cir. 2003) (internal quotation marks

omitted). And with regard to the EPA’s interpretation of the

CAA — a statute that the agency administers — we follow the
13

rule announced in Chevron U.S.A., Inc. v. Natural Resources

Defense Council, 467 U.S. 837 (1984). We first consider

whether the Congress has directly addressed the question at

issue. If it has, “that is the end of the matter; for the court, as

well as the agency, must give effect to the unambiguously

expressed intent of Congress.” Id. at 842–43. If the statute is

silent or ambiguous on the issue, we ask “whether the agency’s

answer is based on a permissible construction of the statute.” Id.

In Part II we grant in part the petitions for review of the

primary annual fine PM standard, brought by the states and

environmental groups, and remand the standard to the EPA for

reconsideration. The EPA failed to explain adequately why an

annual level of 15 μg/m3

 is “requisite to protect the public

health,” including the health of vulnerable subpopulations, while

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

§ 7409(b)(1).

In Part III we grant in full the petition for review of the

secondary NAAQS for fine PM brought by the environmental

groups and remand them to the EPA for reconsideration. The

EPA unreasonably concluded that the NAAQS are adequate to

protect the public welfare from adverse effects on visibility.

In Part IV we deny the petitions for review of the primary

daily standards for coarse PM brought by the industry groups.

We do not reach the question raised by the amicus National

Association of Home Builders whether the EPA lawfully could

have distinguished between urban and nonurban coarse PM in

selecting the coarse PM indicator.

Finally, in Part V, we deny the petition for review of the

EPA’s revocation of the primary annual standard for coarse PM

brought by the environmental groups.
14

II.

The state and environmental petitioners claim the EPA’s

decision to set the primary annual NAAQS for PM2.5 at 15 μg/m3

pursuant to its authority under § 109(b)(1) of the CAA is

“arbitrary, capricious, an abuse of discretion, or otherwise not in

accordance with law.” 42 U.S.C. § 7607(d)(9). In assessing

their arguments, we defer to the EPA’s scientific judgment while

examining the record to ensure the agency has considered the

relevant factors and reasonably explained how it reached its

conclusions. See Carus Chem. Co. v. EPA, 395 F.3d 434, 441

(D.C. Cir. 2005) (under Administrative Procedure Act, court

defers to agency’s technical expertise but asks whether it “has

‘examined relevant data and has articulated a rational

explanation for its actions’”); Chem. Mfrs. Ass’n v. EPA, 28

F.3d 1259, 1263 (D.C. Cir. 1994) (substantive review under

CAA mirrors that under APA). 

We conclude the EPA failed adequately to explain why, in

view of the risks posed by short-term exposures and the

evidence of morbidity resulting from long-term exposures, its

annual standard is sufficient “to protect the public health [with]

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

Accordingly, we grant the petitions for review in part and

remand the annual NAAQS to the EPA for reconsideration. 

A. Risk of Short-Term Exposure

An agency’s failure adequately to consider a relevant and

significant aspect of a problem may render its rulemaking

arbitrary and capricious. See Chamber of Commerce of U.S. v.

SEC, 412 F.3d 133, 140 (D.C. Cir. 2005); see also Motor

Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins. Co., 463 U.S.

29, 43 (1983) (“agency must examine the relevant data and

articulate a satisfactory explanation for its action”). In setting
15

the primary annual NAAQS for PM2.5, the EPA relied

exclusively upon studies of long-term exposure, placing the

greatest weight upon the Harvard Six Cities and the American

Cancer Society (ACS) studies. See 2006 Final Rule, 71 Fed.

Reg. at 61,176/2. The state and environmental petitioners argue

the EPA did not adequately explain why, even if the studies of

long-term exposure were “most directly relevant” to setting the

annual standard, id. at 61,174/3, the studies of short-term

exposure were not at all relevant. 

In the EPA’s view, studies of long-term exposure are most

directly relevant to the annual standard because the human body

responds differently to long-term than to short-term exposure

and because a long-term study evaluates “periods of exposure”

closer to a year than the periods evaluated in a short-term study.

See id.; Proposed Rule: National Ambient Air Quality Standards

for Particulate Matter, 71 Fed. Reg. 2620, 2627/2–3 (2006); U.S.

ENVTL. PROT. AGENCY, AIR QUALITY CRITERIA FOR

PARTICULATE MATTER (CRITERIA DOCUMENT) § 8.2.1, at 8-18

(2004). Be that as it may, the EPA did not assert that short-term

studies provide no relevant information. The EPA concluded

merely that it would be “more appropriate to consider the shortterm exposure studies as a basis for . . . the 24-hour standard and

to consider the long-term exposure studies as a basis for the . . .

annual standard.” 2006 Final Rule, 71 Fed. Reg. at 61,174/2. 

Two premises underlie this conclusion: the agency can (1)

“appropriately . . . evaluate[]” where to set the level of the

annual standard based solely upon the long-term studies and (2)

“provide an appropriate degree of protection” from short-term

exposure through a daily standard alone. Id. at 61,174/3. The

petitioners challenge both. 

As to the first, the petitioners start by pointing out that the

staff and the CASAC, both of which relied upon three short-
16

*

 We will not consider the letter the CASAC sent to the EPA on September

29, 2006, shortly after the Administrator had signed the final rule, as it was

not part of the administrative record. See Citizens to Preserve Overton

Park, Inc. v. Volpe, 401 U.S. 402, 420 (1971) (generally judicial review is

limited to “record that was before the [agency] at the time [it] made [its]

decision”). 

term studies, proposed an annual standard lower than 15 μg/m3.

See OFFICE OF AIR QUALITY PLANNING AND STANDARDS, U.S.

ENVTL. PROT. AGENCY, REVIEW OF THE NATIONAL AMBIENT

AIR QUALITY STANDARDS FOR PARTICULATE MATTER: POLICY

ASSESSMENT OF SCIENTIFIC AND TECHNICAL INFORMATION

(STAFF PAPER) § 5.3.1.1, at 5-7 (2005); Letter from Dr. Rogene

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

3–4 (Mar. 21, 2006) (“Studies described in the PM Staff Paper

indicate that short term effects of PM2.5 persist in cities with

annual PM2.5 concentrations below [15 μg/m3

]”). By statute the

EPA must explain its rejection of the CASAC’s

recommendation, 42 U.S.C. § 7607(d)(3), and the staff’s

analysis is something we consider when determining whether

the EPA has adequately addressed the relevant considerations

and reasonably reached its conclusions, see Natural Res. Def.

Council v. EPA, 902 F.2d 962, 967–68, 970 (D.C. Cir. 1990)

(noting staff paper’s relevance as “bridge” over “gap” between

criteria document and EPA’s policy judgment).* 

We reject the EPA’s initial response, to wit, that its

approach is consistent with those recommendations. See 2006

Final Rule, 71 Fed. Reg. at 61,174/3 n.45. The CASAC directly

challenged the EPA’s proposal to retain the annual standard at

15 μg/m3

. See Letter from Dr. Rogene Henderson, CASAC, to

Administrator Stephen L. Johnson, EPA 7 (June 6, 2005). The

staff recommended that if the agency kept the annual standard

at 15 μg/m3

 while setting the daily standard with a 98th

percentile form, then it should set the daily standard at the

“middle to lower end” of the range from 25 to 35 μg/m3

. See
17

STAFF PAPER § 5.3.7, at 5-46; see also id. § 5.3.5.1, at 5-32

(“[S]taff continues to believe that an annual standard cannot be

expected to offer an adequate margin of safety against the

effects of all short-term exposures”). The EPA instead set the

daily standard at 35 μg/m3 with a 98thpercentile form, see 2006

Final Rule, 71 Fed. Reg. at 61,165/2; id. at 61,171/3, while

acknowledging it did not consider the short-term studies when

setting the annual standard. As both the CASAC and the staff

reasoned, the studies of Eight Canadian Cities, of Santa Clara

County (CA), and of Phoenix are relevant to setting an annual

standard because each reports adverse health effects associated

with short-term exposures in places where the annual PM2.5

concentration is below 15 μg/m3. See CASAC Mar. 21 Letter at

3–4. The EPA failed adequately to explain its reason for not

accepting the CASAC’s recommendations, instead stating only

that it did not “disagree with CASAC’s factual statements

regarding the findings of [the short-term studies],” but “believes

. . . it . . . more appropriate to consider the short-term exposure

studies as a basis for the level of the 24-hour standard.” 2006

Final Rule, 71 Fed. Reg. at 61,174/2.

The petitioners also point to the EPA’s unexplained change

of position since it last dealt with this issue, which was in 1997.

Then the EPA believed short-term studies were indeed relevant

to the setting of an annual standard, stating, “the strongest

evidence for short-term PM2.5 effects occurs at concentrations

near the long-term (e.g., annual) average.” See Final Rule:

National Ambient Air Quality Standards for Particulate Matter,

62 Fed. Reg. 38,652, 38,676/1 (1997). The agency did not

thereby commit itself irrevocably, however if the relevant facts

have changed or the EPA has reasonably made a different policy

judgment, then it need only explain itself and we will defer. See

State Farm, 463 U.S. at 57 (“An agency’s view of what is in the

public interest may change, either with or without a change in

circumstances” (internal quotation marks omitted)). 
18

The EPA’s only explanation for its change of position is

that it relied upon short-term studies in 1997 because the studies

of long-term exposure then available were less reliable than the

short-term studies. See 2006 Final Rule, 71 Fed. Reg. at

61,174/3. At the time, however, the agency also said, “an

annual standard that controls an area’s attainment status is likely

to reduce aggregate risks associated with both short- and longterm exposures with more certainty than a 24-hour standard.”

1997 Final Rule, 62 Fed. Reg. at 38,670/3. Even if the longterm studies available today are useful for setting an annual

standard, therefore, it is not clear why the EPA no longer

believes it useful to look as well to short-term studies in order to

design the suite of standards that will most effectively reduce the

risks associated with short-term exposure. 

We conclude the EPA failed adequately to explain its first

premise, viz. that an annual standard could be “appropriately . . .

evaluated based” solely upon long-term studies. See 2006 Final

Rule, 71 Fed. Reg. at 61,174/3. Therefore, we need not reach

the state petitioners’ argument that the EPA should have relied

in part upon the short-term studies because, unlike the ACS

Study, those studies do not underestimate the magnitude of risk

from exposure to PM2.5. That argument does not rest upon the

ACS Study having examined long-term rather than short-term

exposure, but instead questions whether the ACS Study involved

a representative cohort. If, however, the EPA can adequately

explain why studies of short-term effects are not relevant to

setting an annual standard, then it may disregard those studies

regardless whether the short-term studies are based upon a more

representative cohort than is the ACS Study.

Nor do we consider the EPA’s argument that the short-term

studies showed an annual standard would not provide more

protection against short-term exposure than would a daily
19

standard. That argument was advanced for the first time in its

brief and therefore is not properly before us. See SEC v.

Chenery Corp., 332 U.S. 194, 196 (1947) (“[A] reviewing court

. . . must judge . . . [administrative] action solely by the grounds

invoked by the agency” when it acted); Ashland Oil, Inc. v. FTC,

548 F.2d 977, 981 & n.6 (D.C. Cir. 1976) (court cannot rely

upon post hoc position unless “no special agency expertise is

involved”). 

The petitioners also challenge the EPA’s second premise,

that a primary daily standard for PM2.5 at 35 μg/m3

 with a 98th

percentile form could “provide an appropriate degree of

protection” from short-term exposure, thus eliminating the need

to calibrate the annual standard to address short-term exposure.

2006 Final Rule, 71 Fed. Reg. at 61,174/3; see id. at 61,165/2

(selecting percentile form of standard); id. at 61,171/3 (selecting

level of standard). The EPA’s daily standard will require each

state to make sure that the average of the seventh highest

concentration measured each year for three years does not

exceed 35 μg/m3, thus permitting approximately six days a year

to have concentrations above that level. See id. at 61,164–65

(describing form of standard). The petitioners argue the EPA’s

reliance upon the daily standard to address all the health risks

from short-term exposure was unreasonable for several reasons.

First, the environmental petitioners contend the record

shows that low- to mid-range concentrations of PM2.5 cause most

health effects from short-term exposure, noting that both the

staff and the CASAC warned of risks at ambient concentrations

below 35 μg/m3

. See STAFF PAPER § 5.3.5.1, at 5-31–32;

CASAC June 6 Letter at 7. In particular, the CASAC observed

that, because some cities have “relatively high annual PM

concentrations” but would “rarely” exceed ambient

concentrations of 35 μg/m3

, it would be necessary to lower the

annual standard below 15 μg/m3 to provide sufficient protection
20

from the risks associated with short-term exposure. CASAC

June 6 Letter at 7. The EPA maintains lowering the daily

standard to 35 μg/m3 will provide additional protection in all

areas where it is required, but as we discuss below, it is not clear

why the EPA believes areas that already meet the daily standard

either do not require additional protection or will enjoy

improved protection against short-term exposure. 

Second, the environmental petitioners question the EPA’s

methodology. The EPA focused upon the 98th percentile air

quality values in the short-term studies, concluding those values

would shed light upon which overall distributions of air quality

likely cause harm from short-term exposure. Because some

studies did not find a statistically significant association between

short-term exposure and health effects when the 98th percentile

values were below 35 μg/m3

, the EPA selected 35 μg/m3 as the

level, and the 98th percentile as the form, of the primary daily

standard. 2006 Final Rule, 71 Fed. Reg. at 61,169. The EPA

does not contend, however, that lowering the peak

concentrations to 35 μg/m3 would provide sufficient protection.

Indeed, the EPA acknowledges that to provide adequate

protection against short-term exposure, it must lower “a broad

distribution of PM2.5 air quality values in an area,” but explains

that lowering the daily standard will lower concentrations not

only at the peak of the distribution curve, but also those at the

trough because state implementation plans, “[a]lthough . . . not

yet defined, . . . are likely to” require the necessary

improvements in air quality. Id. at 61,168/3.

In support of this prediction the EPA points to a study

finding that as peak daily levels of PM2.5 decreased from “one

year to the next” in Philadelphia and Los Angeles, all off-peak

air quality values, or “low daily PM2.5 levels[,] decreased

proportionally.” See ABT ASSOCS.,INC.,PARTICULATE MATTER

HEALTH RISK ASSESSMENT FOR SELECTED URBAN AREAS (RISK
21

ASSESSMENT) 17 (2005); see also STAFF PAPER § 4.3.1.2, at 4-

18 (study showed “concentrations at different points in the

distribution of 24-hour PM2.5 values . . . decreased by

approximately the same percentage”). Although the EPA is

entitled to rely upon its experience, it must have a reasonable

explanation of how its experience supports its conclusion. See

DSE, Inc. v. United States, 169 F.3d 21, 30 (D.C. Cir. 1999)

(court “will defer to [agency’s] experience provided that the

agency has offered a reasoned explanation . . . [and] the result is

in accord with material facts . . . in the administrative record”).

The environmental petitioners rightly question the EPA’s

conclusion based upon past trends in Philadelphia and Los

Angeles; according to the same study, they point out, the

proportional decline in peak and off-peak daily levels of PM2.5

in those cities was “not . . . the result of a PM2.5 control strategy,

but likely result[ed] from control programs for PM10 and control

programs for other pollutants.” RISK ASSESSMENT at 17. The

EPA offers no basis in the experience of those two cities — thus

understood — for expecting controls on the peak levels of fine

particles will lead to a proportional decline in levels of PM2.5 at

off-peak times simply because controls for coarse particles and

for other pollutants did so in the past. The EPA may not and

perhaps need not fully understand why off-peak PM2.5 levels

decreased as peak PM2.5 levels decreased in the past, but it

nevertheless needs plausibly to explain why it believes future

controls for PM2.5 will, like other controls for other pollutants in

the past, trigger the phenomenon. 

Even assuming off-peak levels of PM2.5 will decrease in

proportion to decreases at peak levels, the environmental

petitioners argue there will be no such decline in areas that

already comply with the daily standard and therefore will not

have to reduce ambient daily concentrations. It is not enough

for the EPA to respond that, as it implied during the rulemaking,

those areas do not need further improvement; the agency itself
22

observed that short-term health effects occur across a “broad

distribution of PM2.5 air quality values.” See 2006 Final Rule, 71

Fed. Reg. at 61,168/3. 

We conclude the EPA has failed reasonably to explain why

it believes its daily standard will “provide an appropriate degree

of protection from health effects associated with short-term

exposures to PM2.5.” Id. at 61,174/3. We therefore remand the

annual standard to the EPA for further consideration of whether

it is set at a level requisite to protect the public health while

providing an adequate margin of safety from the risk of shortterm exposure to PM2.5. 

B. Morbidity Among Vulnerable Subpopulations

The state petitioners believe the EPA unreasonably

concluded morbidity studies did not provide a basis for setting

a primary annual standard for PM2.5 below 15 μg/m3

. First, they

argue the EPA arbitrarily refused to rely upon a study finding an

association between irreversible lung damage in children and

long-term exposure to PM2.5 at levels below 15 μg/m3. See W.

James Gauderman et al., Association Between Air Pollution and

Lung Function Growth in Southern California Children, 162

AM.J.RESPIRATORY & CRITICAL CARE MED. 1383 (2000). The

staff thought the Gauderman Study, if given “appreciable

weight,” indicated an annual standard of 13 μg/m3

. See STAFF

PAPER § 5.3.4.1, at 5-22–23. Although the CASAC did not

advert to the Gauderman Study, it recommended a level of 13 to

14 μg/m3

 based upon the risk assessment and the short-term

studies. See CASAC Mar. 21 Letter at 1. The EPA

acknowledged the Gauderman Study presented “important new

findings” but decided not to rely upon it because it was the only

study measuring “decreased growth in lung function” over time

and it focused solely upon children in one location. 2006 Final

Rule, 71 Fed. Reg. at 61,172/2-3.
23

 

The state petitioners complain that the EPA’s rejection of

the Gauderman Study was based upon the faulty premise that

“further study . . . would be needed to increase confidence in the

reported associations” because no other study in the record was

consistent with the results of the Gauderman Study. Id. at

61,172/3. According to the state petitioners, the findings of the

Gauderman Study are supported by Mark Raizenne et al., Health

Effects of Acid Aerosols on North American Children:

Pulmonary Function, 104 ENVTL. HEALTH PERSP. 506 (1996)

(24-Cities Study). The EPA disagreed: the 24-Cities Study

measured decreased lung function “at a single point in time”

while the Gauderman Study found “decreased growth in lung

function” over time. 2006 Final Rule, 71 Fed. Reg. at 61,172/3.

We believe the EPA’s approach to the Gauderman and 24-

Cities studies was unreasonable in light of the agency’s

obligation to explain how the annual standard it set would

protect “not only average healthy individuals, but also ‘sensitive

citizens.’” Am. Lung Ass’n v. EPA, 134 F.3d 388, 389 (D.C. Cir.

1998). As the American Lung Association argued during the

rulemaking — and the EPA agreed — with each breath a child

aspirates more pollution relative to its body weight than does an

adult. See American Lung Association (ALA) et al., Comments,

2006 Rulemaking, Pub. Dkt. No. OAR-2001-0017-1890.1, at 17

(filed Apr. 17, 2006); 2006 Proposed Rule, 71 Fed. Reg. at

2637/1 (“Several factors may make children susceptible to PMrelated effects, including the greater ventilation rate per

kilogram body weight in children”). We therefore doubt

whether the studies of adult mortality upon which the EPA

relied provide the necessary confidence concerning the risk of

morbidity in children. 

We doubt the reasonableness of the EPA’s analysis for a

second reason: the agency imputed undue significance to one
24

difference, despite many similarities, between the Gauderman

Study, which focused upon the growth over time of lung

function in children, and the 24-Cities Study, which focused

upon the level of lung function of children at a particular time.

Both studies found an association between exposure to PM2.5 at

levels below 15 μg/m3

 and impairment of lung function in

children, although the association in the 24-Cities Study was

“not statistically significant” below 15 μg/m3, 2006 Final Rule,

71 Fed. Reg. at 61,172/3. Indeed, the 24-Cities Study predicted

the children it examined might “continue on [a] track [of

reduced] . . . growth of their lung function, as suggested by

previous studies,” Raizenne et al. at 513, which was consistent

with the findings of the later Gauderman Study, see Gauderman

et al. at 1388. The EPA also relied upon its 1997 conclusion

that the 24-Cities Study does not show a statistically significant

association between health effects and annual average

concentrations of PM2.5 below 15 μg/m3

. See 2006 Final Rule,

71 Fed. Reg. at 61,172/3. Viewed in isolation, of course, the

studies are far from conclusive. Viewed together in the context

of the studies the EPA considered when deciding whether to

revise the standard for PM2.5, however, the conclusion reached

after comprehensive scientific review by the EPA’s National

Center for Environmental Assessment in the Criteria Document

seems the only reasonable one: the findings of the Gauderman

and 24-Cities studies are related and together indicate a

significant public health risk. See CRITERIA DOCUMENT

§ 8.4.6.4, at 8-314; cf. 2006 Final Rule, 71 Fed. Reg. at 61,154/1

(“[T]he Criteria Document found that new studies of a cohort of

children in Southern California have built upon earlier limited

evidence to provide fairly strong evidence that long-term

exposure to fine particles is associated with development of

chronic respiratory disease and reduced lung function growth”).

On this record, therefore, it appears the EPA too hastily

discounted the Gauderman and 24-Cities studies as lacking in

significance. See Am. Radio Relay League, Inc. v. FCC, 524
25

F.3d 227, 241 (D.C. Cir. 2008) (agency’s inadequate explanation

for dismissing empirical studies rendered decision arbitrary and

capricious); cf. ATA I, 175 F.3d at 1052–53 (EPA arbitrarily and

capriciously placed upon some studies “higher information

threshold” than it placed upon others). 

Second, the state petitioners argue the EPA should have

explained how an annual standard of 15 μg/m3 would provide an

adequate margin of safety for vulnerable subpopulations, such

as children, the elderly, or those with conditions that expose

them to greater risk from fine particles. The EPA said there was

insufficient evidence that vulnerable subpopulations will be

exposed to adverse health effects at annual levels of PM lower

than 15 μg/m3

. See 2006 Final Rule, 71 Fed. Reg. at 61,173/1.

Having concluded the EPA failed adequately to consider the

Gauderman and 24-Cities studies, however, we are constrained

to agree with the state petitioners that the EPA was unreasonably

confident that, even though it relied solely upon long-term

mortality studies, the revised standard would provide an

adequate margin of safety with respect to morbidity among

children. Notably absent from the final rule, moreover, is any

indication of how the standard will adequately reduce risks to

the elderly or to those with certain heart or lung diseases despite

(a) the EPA’s determination in its proposed rule that those

subpopulations are at greater risk from exposure to fine particles

and (b) the evidence in the record supporting that determination.

See 2006 Proposed Rule, 71 Fed. Reg. at 2637/1; STAFF PAPER

§ 3.3.2, at 3-19–22 & fig.3-2 (charting risk of morbidity for

persons with various diseases, but noting associations between

exposure to PM2.5 and certain heart diseases and stroke are not

“statistically significant”); CRITERIA DOCUMENT § 8.4.9, at 8-

327–28 (citing studies discussing risks to “older adults and

children”); ALA Comments at 52 (discussing risk to elderly at

level of 12 μg/m3

). We therefore grant the petition for review

and remand this matter for the EPA to explain why it believes
26

the NAAQS will provide, as required by the CAA, an adequate

margin of safety against morbidity in children and other

vulnerable subpopulations. See Am. Lung Ass’n, 134 F.3d at

393 (remanding because EPA had “failed adequately to explain

[why there is] . . . no public health threat” to asthmatics). 

C. Other Challenges 

Both groups of petitioners object also to the EPA’s analysis

of the long-term studies; the state petitioners, furthermore, claim

it was unreasonable of the agency not to rely upon the risk

assessment. We find neither objection persuasive.

First, the petitioners argue the EPA unreasonably focused

upon the long-term mean ambient concentrations of PM2.5 in the

ACS Study (17.7 μg/m3) and the Six Cities Study (18 μg/m3)

and then set a level below those concentrations to address longterm exposure. See 2006 Final Rule, 71 Fed. Reg. at

61,176/2–3. The state and environmental petitioners claim this

approach violates the requirement of the CAA that the NAAQS

provide an “adequate margin of safety,” 42 U.S.C. § 7409(b)(1),

which requires that the EPA “err on the side of caution.” Lead

Indus., 647 F.2d at 1154–55. Because the most recent data from

the ACS and the Six Cities studies showed adverse health effects

in years when the mean ambient concentration of PM2.5 was

below 15 μg/m3

, see DANIEL KREWSKI ET AL., REANALYSIS OF

THE HARVARD SIX CITIES STUDY AND THE AMERICAN CANCER

SOCIETY STUDY OF PARTICULATE AIR POLLUTION AND

MORTALITY: ASPECIAL REPORT OF THE INSTITUTE’S PARTICLE

EPIDEMIOLOGY REANALYSIS PROJECT 175 (2000), the

petitioners argue the EPA had to lower the level below 15 μg/m3

in order to address the problem created by long-term exposure.

They point to the staff’s conclusion that the ACS and the Six

Cities studies indicated an annual standard “somewhat below 15

μg/m3

.” STAFF PAPER § 5.3.4.1, at 5-23. As the staff and the
27

authors of the two studies explained, however, “it is not easy to

differentiate the role of historic exposures from more recent

exposures.” See id. § 3.6.5.4, at 3-53. In other words, a person

exposed to relatively high concentrations of PM2.5 in the past

may suffer long-term effects in the present even if he is exposed

to relatively low concentrations of PM2.5 in the present. The

EPA, mindful of its obligation to set a standard “not lower or

higher than is necessary . . . to protect the public health,”

Whitman, 531 U.S. at 475–76, reasonably decided to address

long-term exposure with an annual standard somewhat below

the long-term mean concentrations in the ACS and Six Cities

studies. See 2006 Final Rule, 71 Fed. Reg. at 61,172–73. We

approved a similar approach to assuring an adequate margin of

safety in ATA III and we do so again here. See 283 F.3d at 372

(denying petition for review of standard set “just below the

range of mean annual PM2.5 concentrations observed in studies

showing a statistically significant association between fine [PM]

and health effects”). We therefore deny the petitions for review

as to the EPA’s analysis of the key long-term studies.

Second, the state petitioners object on four grounds to the

EPA’s decision not to rely upon a quantitative risk assessment,

prepared for this rulemaking by Abt Associates, Inc. and

analyzed and summarized in the EPA’s staff paper, which

estimated how variations in the ambient concentration of PM

affect the incidence of adverse health effects. See 2006 Final

Rule, 71 Fed. Reg. at 61,171/2–3. First, if a threshold of 7.5

μg/m3 is assumed as the level below which exposure to PM is

not harmful, then the risk assessment predicts the EPA’s

standards will result in 3,700 premature deaths in the nine cities

studied, whereas lowering the primary annual standard to 12

μg/m3, as the petitioners suggest the EPA should do, may reduce

that number by 80%. See RISK ASSESSMENT at 117, E-17–31;

see also ALA Comments at 40 fig.4 (synthesizing data). The

EPA reasonably responds, however, that the comparison
28

between the petitioners’ preferred NAAQS and the one the EPA

selected is based upon unreliable assumptions; in particular, the

risk assessment is “based on studies that do not resolve the issue

[whether there is] a threshold” below which PM2.5 is harmless.

See 2006 Final Rule, 71 Fed. Reg. at 61,168/2. The risk

assessment therefore “necessarily predicts that ever lower

standards result in ever lower risks,” which is not supported by

statistically significant epidemiological evidence. Id. 

The state petitioners also claim the EPA is inconsistent

because it relied upon a risk assessment when setting the annual

standard for PM in 1997. Although the EPA stated the risk

assessment then before it contained “reasonable estimates . . .

given the available information,” it did not in fact use those

estimates to set the annual standard. See 1997 Final Rule, 62

Fed. Reg. at 38,656/2–3. Indeed, the EPA expressed the same

concern about the earlier risk assessment that it has about the

new one. We deferred to its decision then not to rely upon the

risk assessment in setting the level of the NAAQS. See ATA III,

283 F.3d at 374 (“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”). 

The state petitioners nonetheless argue we should not

similarly defer now because the current risk assessment is more

reliable than the one available in 1997; this time the staff used

“only health endpoints for which . . . the overall weight of the

evidence supports the conclusion that PM2.5 is likely causally

related.” STAFF PAPER § 4.3.4, at 4-34. They note both the

CASAC and the staff thought the risk assessment was reliable.

See CASAC Mar. 21 Letter at 3; STAFF PAPER § 5.3.7, at 5-46.

Even so we must defer to the EPA’s assessment of “scientific

data within its technical expertise” as long as the agency has

examined the data and adequately explained itself. City of
29

Waukesha, 320 F.3d at 247 (internal quotation marks omitted).

Here, unlike in its analysis of vulnerable subpopulations, the

EPA considered all aspects of the problem, catalogued its

concerns, and took the same decision — not to rely upon the risk

assessment — it took before; we deferred to that decision before

and we do so again now. 

Finally, the state petitioners argue the EPA contradicted

itself when it relied upon the ACS and the Six Cities studies but

not upon the risk assessment, which was based in part upon

those studies. The record, however, indicates the risk

assessment magnified the uncertainties inherent in the

underlying studies. See STAFF PAPER § 4.3.4, at 4-34 (reliance

of risk assessment upon single-city studies in addition to multicity studies introduces additional uncertainty). Moreover, the

EPA’s decision not to rely upon the lower mean concentrations

in the multi-city studies is consistent with its decision that the

risk assessment overestimated the risk of harm from lower

concentrations. We cannot say the EPA’s distinction between

the studies and the risk assessment was unreasonable. 

Because we believe the EPA reasonably analyzed the risk

assessment, we deny the petition for review in that respect. We

do not reach the industry intervenors’ defense of the EPA’s

action — except to note it was repudiated by the agency. See

State Farm, 463 U.S. at 50 (court can look only to “basis

articulated by the agency”); EPA, Responses to Significant

Comments, 2006 Rulemaking, Pub. Dkt. No. OAR-2001-0017-

3203, at 166–67 (filed Sept. 25, 2006). 

D. Conclusion

In sum, the EPA did not adequately explain why an annual

level of 15 μg/m3 is sufficient to protect the public health while

providing an adequate margin of safety from short-term
30

exposures and from morbidity affecting vulnerable

subpopulations. We therefore grant in part the petitions for

review filed by the States and by the environmental petitioners

(Nos. 06-1416 and 06-1411) and remand for reconsideration the

primary annual NAAQS for PM2.5. We deny the same petitions

insofar as they seek review of the EPA’s analysis of the longterm mortality studies and the agency’s decision not to rely upon

the risk assessment. We do not, however, vacate the annual

standard. First, the EPA’s failure adequately to explain itself is

in principle a curable defect. Second, vacating a standard

because it may be insufficiently protective would sacrifice such

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

good. See Allied-Signal, Inc. v. U.S. Nuclear Regulatory

Comm’n, 988 F.2d 146, 150–51 (D.C. Cir. 1993) (when

selecting remedy court should consider: “the seriousness of the

[rule’s] deficiencies . . . and the disruptive consequences of an

interim change that may itself be changed” (internal quotation

marks omitted)). 

III.

The environmental petitioners challenge as arbitrary and

capricious the EPA’s decision to set the secondary NAAQS for

fine PM, which protect the public welfare from adverse visibility

effects, at the same level as the primary NAAQS, which protect

public health. 

The EPA staff and the CASAC recommended a secondary

standard level of 20 to 30 μg/m3 of PM2.5, averaged across a 4-

or 8-hour midday period each day, and using the 92nd to 98th

percentile of these daily measurements. Both groups explained

that this standard would be shielded from the confounding

effects of humidity. Because both humidity and fine PM can

impair visibility, measuring PM concentration only at midday,

when humidity is lowest, would ensure that the standard is
31

targeting visibility effects caused by fine PM rather than by

humidity. The EPA staff and the CASAC based the

recommended ranges of levels and percentile forms on studies

in which participants rated the acceptability of visibility levels

as depicted in actual photographs or computer-generated

simulations. In each study, a majority of participants rated

visibility ranges of 40 to 60 kilometers as “acceptable.” The

EPA staff explained that the recommended standard was

“consistent with” this target range.

The EPA rejected these recommendations. Although it

agreed with its staff and the CASAC that a sub-daily averaging

time had “strong technical merit,” the EPA characterized the

evidence supporting the recommended level and form as

“limited and uncertain,” a conclusion based on “the generally

subjective nature of the public welfare effect involved.” 2006

Final Rule, 71 Fed. Reg. at 61,206–08. Having rejected the

recommended target level of visibility protection, the EPA did

not identify its own. Instead, it first considered the extent to

which the revised primary NAAQS would protect visibility and

compared this with the protection that would be provided by the

recommended secondary standard.

To make this comparison, the EPA looked to two tables in

the Staff Paper predicting the percentage of counties where the

existing air quality was unlikely to meet several potential

standards. One table showed that 27% of counties contained

PM monitors with a recent history of recorded fine PM levels

that would not meet the new primary PM NAAQS. The second

table showed that 24% of counties contained PM monitors with

a record of fine PM levels that would not meet one possible

standard within the range recommended by the EPA staff and

the CASAC. From this data, the EPA concluded that the

primary NAAQS would be more protective of visibility than the

standard recommended by its staff and the CASAC. Because a
32

greater percentage of counties would fail to meet those

standards, more counties would be required under the CAA to

reduce fine PM levels and, incidentally, improve visibility.

Accordingly, the EPA adopted secondary fine PM NAAQS

identical to the primary fine PM NAAQS.

The petitioners argue that the EPA’s decision lacks a

reasoned basis. First, they assert that the EPA never determined

what level of visibility is “requisite to protect the public

welfare.” 42 U.S.C. § 7409(b)(2). The petitioners argue that

the EPA unreasonably rejected the starting point provided by its

staff — visibility of 40 to 60 kilometers — based on the inherent

“uncertainty” in subjective value judgments about visibility.

Second, the petitioners challenge the EPA’s method of

comparing the protection expected from potential standards.

They contend that the EPA relied on a meaningless numerical

comparison, ignored the effect of humidity on the usefulness of

a standard using a daily averaging time, and unreasonably

concluded that the primary standards will achieve a level of

visibility roughly equivalent to the level the EPA staff and the

CASAC deemed “requisite to protect the public welfare.” Id.

The EPA responds that it did not need to identify a specific

level of visibility to achieve because the evidence did not

support a need for secondary standards to be more stringent than

the primary standards. Moreover, the EPA argues that it was

reasonable to reject the target level of visibility recommended

by the Staff Paper because it relied on the subjective

perspectives of study participants. The EPA characterizes this

subjective evidence as “uncertain” because the studies could not

identify the precise level or percentage of days of visibility

impairment at which there is an adverse effect on public welfare.

Such uncertain evidence, the EPA argues, cannot substitute for

the sound policy judgment of the EPA Administrator. Finally,

the EPA defends its reliance on the comparative analysis of the
33

protection afforded by different standards: it claims it did not

rely solely on the finding of equivalent protection, but also

found that the primary standards were in fact within the range

recommended by the EPA staff and the CASAC.

The EPA’s assertion that it need not determine what level

of visibility protection is requisite to protect the public welfare

fails under the plain language of the statute. The CAA provides:

“Any national secondary ambient air quality standard shall

specify a level of air quality the attainment and maintenance of

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

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

§ 7409(b)(2) (emphases added). The EPA’s failure to identify

such a level when deciding where to set the level of air quality

required by the revised secondary fine PM NAAQS is contrary

to the statute and therefore unlawful. Furthermore, the failure

to set any target level of visibility protection deprived the EPA’s

decisionmaking of a reasoned basis. Because the EPA failed to

identify any target level, we need not decide whether it was

reasonable for the agency to reject the target recommended by

the Staff Paper and the CASAC because it was based on

uncertain subjective evidence.

The EPA’s substitute for identifying a target level — its

analysis of the relative protection expected from the

recommended standards and the revised primary NAAQS —

cannot save its decision. The equivalence analysis fails on its

own terms. The EPA compared the predicted 27% of counties

that would not meet the revised primary fine PM standards with

the predicted 24% that would not meet a standard of 30 μg/m3

using a 4-hour averaging time and a 95th percentile form — one

standard within the range recommended by the CASAC. The

EPA concluded that because fewer counties would meet the

primary standards, and would therefore have to take steps to

reduce fine PM levels, that standard was more protective than
34

the one recommended by the CASAC. But the same table

shows that six other standards within the recommended range

would be more “protective” under the EPA’s definition than the

primary standards: from 47% to 85% of counties would not

meet these alternative standards. The alternative cited by the

EPA is one of only three standards that would offer equal or

greater protection than the primary standards. In other words,

two-thirds of the potential standards within the CASAC’s

recommended range would be substantially more protective than

the primary standards. The EPA failed to explain why it looked

only at one of the few potential standards that would be less

protective — and only slightly so — than the primary standards.

More fundamentally, however, the EPA’s equivalence

analysis demonstrates nothing about the relative protection

offered by the different standards. The CASAC put the EPA on

notice of this fact in a letter submitted in response to the

proposed revisions to the NAAQS: “The cited comparability

between percentages of counties not likely to meet a lenient subdaily secondary standard and the proposed 24-hour primary

standard is a numerical coincidence.” CASAC Mar. 21 Letter

at 5. Indeed, the tables both state that the “estimates are not

based on the same air quality data that would be used to

determine whether an area would attain a given standard or set

of standards” and “should be interpreted with caution.” STAFF

PAPER app. 5B at 2, app. 7A at 2. The tables simply offer no

valid information about the relative visibility protection

provided by the standards, and yet the EPA relied on them

almost exclusively in selecting the secondary NAAQS for fine

PM.

Finally, as the Staff Paper makes clear, a visibility standard

using a daily averaging time will be confounded by regional

differences in humidity. The EPA acknowledged this problem,

recognizing that a sub-daily averaging time, using the daylight
35

hours when humidity tends to be lowest, has “strong technical

merit.” 2006 Final Rule, 71 Fed. Reg. at 61,208. Yet the EPA’s

equivalence analysis — its basis for concluding that the primary

standards would be sufficiently protective of visibility — did not

address this issue at all.

The EPA’s decision to set secondary fine PM NAAQS

identical to the primary NAAQS was unreasonable and contrary

to the requirements of 42 U.S.C. § 7409(b)(2). Accordingly, we

grant the petition for review (No. 06-1411) and remand for

reconsideration the secondary NAAQS for fine PM.

IV.

The industry petitioners lodge three challenges to the

EPA’s regulation of coarse PM. First, they challenge the EPA’s

decision to retain a daily standard for all coarse PM, including

that in nonurban areas. Second, they claim that the 150 μg/m3

daily NAAQS that the EPA set is not requisite to protect public

health under Whitman v. American Trucking, 531 U.S. 457.

Finally, they argue that the PM10 indicator that the EPA chose to

retain for coarse PM is confounded and impermissible under this

court’s decision in ATA I, 175 F.3d 1027. We find the EPA’s

decisions on all three issues to be reasonable, and we therefore

deny these petitions (Nos. 06-1410, 06-1415, and 06-1417). 

A. Retention of a Daily Standard for All Coarse PM

In their challenge to the EPA’s decision to maintain a daily

NAAQS for all — not just urban — coarse PM, the industry

petitioners make two arguments, one contesting the EPA’s

authority to regulate nonurban coarse PM and the other

contesting its evidence of coarse PM’s dangerousness. 

First, the industry petitioners contend that, “to promulgate

a NAAQS for nonurban PM coarse, the EPA must show that
36

such particles present a significant risk to public health.”

Industry Reply Br. 4. To support this contention, the petitioners

rely on 42 U.S.C. § 7408(a)(1), which states: 

For the purpose of establishing national primary and

secondary ambient air quality standards, the Administrator

shall . . . publish, and shall from time to time thereafter

revise, a list which includes each air pollutant — (A)

emissions of which, in his judgment, cause or contribute to

air pollution which may reasonably be anticipated to

endanger public health or welfare.

On the basis of this section, the petitioners argue that the EPA

lacks authority to establish a NAAQS for coarse PM that

includes nonurban coarse PM unless the agency first makes a

finding that such nonurban coarse PM “may reasonably be

anticipated to endanger public health or welfare.” Id.

The petitioners misunderstand § 7408(a)(1). Under that

section, the EPA must make a dangerousness finding in order to

place a pollutant on the list of pollutants regulated by NAAQS.

But that requirement to find dangerousness does not apply when

the EPA sets the NAAQS for a pollutant already on the list. At

issue in this review is not the listing of PM, but rather the review

and revision of the PM NAAQS that was mandated by

§ 7409(d)(1). See id. § 7409(d)(1) (“Not later than December

31, 1980, and at five-year intervals thereafter, the Administrator

shall complete a thorough review of the criteria published under

section 7408 of this title and the national ambient air quality

standards promulgated under this section and shall make such

revisions in such criteria and standards and promulgate such

new standards as may be appropriate in accordance with section

7408 of this title and subsection (b) of this section”). 

In ATA I, this court confirmed that the establishment of an
37

indicator for PM2.5 was not a new listing under § 7408(a)(1)

because PM2.5 had previously been regulated as part of the PM10

indicator. ATA I, 175 F.3d at 1055. Similarly, nonurban PM10

has been regulated under the general PM10 standard, and

therefore the EPA’s continued regulation of all PM10 is not a new

listing that would require an endangerment finding under

§ 7408(a)(1). 

The industry petitioners also seek support for their lack-ofauthority claim in the Supreme Court’s opinion in Whitman. In

particular, they argue that Whitman’s citation of Industrial Union

Department, AFL-CIO v. American Petroleum Institute, 448 U.S.

607 (1980), a case that addressed a provision of the Occupational

Safety and Health (OSH) Act of 1970, means that “a NAAQS is

‘requisite’ to protect public health only if EPA has made a

threshold showing of real and significant risk.” Industry Br. 16.

But Whitman compared the specificity of the CAA to that of the

OSH Act — citing Industrial Union in the course of that

comparison — solely for the purpose of deciding whether the

CAA violated the nondelegation doctrine, Whitman, 531 U.S. at

473-74, a purpose not relevant here. It did not suggest that

Industrial Union has any implication for the EPA’s exercise of

its authority under section 109 of the CAA. The industry

petitioners’ contention that the EPA lacks authority to regulate

nonurban PM10 therefore lacks merit.

The industry petitioners’ second argument against the

EPA’s regulation of nonurban coarse is that scientific evidence

“shows that nonurban coarse PM is not associated with adverse

health effects.” Industry Br. 17. Drawing on statements in the

final rule, the petitioners argue that the scientific evidence shows

danger only from urban coarse, and not from nonurban coarse.

See 2006 Final Rule, 71 Fed. Reg. at 61,185-86. The petitioners

argue both that scientific evidence demonstrates that nonurban

coarse PM is not dangerous and that the EPA’s “cautious
38

approach” is unlawful absent evidence of nonurban coarse PM’s

dangerousness. Industry Br. 17-22. 

In assessing the scientific evidence, the petitioners have

mistakenly equated an absence of certainty about dangerousness

with the existence of certainty about safety. The petitioners

selectively quote from the final rule to bolster their argument that

nonurban coarse is not dangerous, see id. at 19, but they ignore

passages that weaken the force of their contention. For example,

the petitioners note that studies of exposure to coarse PM from

dust storms do not show significant health effects. 2006 Final

Rule, 71 Fed. Reg. at 61,186; Industry Br. 18. But the petitioners

ignore the EPA’s qualifier on the dust storm studies: people in

such situations may practice avoidance behavior and limit their

exposure to the dust. 2006 Final Rule, 71 Fed. Reg. at 61,191.

The petitioners similarly quote the portion of the final rule that

discusses the lack of health impacts observed in studies focusing

on volcanic ash from Mt. St. Helens, id.; Industry Br. 19, but

omit the subsequent discussion of possibilities for toxic

contamination in more typical nonurban coarse PM. 2006 Final

Rule, 71 Fed. Reg. at 61,191.

By contrast, the EPA has provided evidence that suggests

nonurban coarse PM likely is not safe. The EPA points to

dosimetric, toxicological, and occupational exposure studies that

all indicate danger from nonurban coarse. EPA Br. 95-97.

Dosimetric evidence shows that all types of coarse PM can

deposit in the “sensitive regions of the lung of most concern, the

tracheobronchial and alveolar regions.” 2006 Final Rule, 71 Fed.

Reg. at 61,197. Toxicological evidence demonstrates that all

types of coarse can be contaminated by motor vehicles and

industrial emissions, molds, fungi, endotoxins, polycyclic

aromatic hydrocarbons (PAHs), and resuspended fine PM. Id. at

61,189-92. Some rural coarse, such as that found in dry

lakebeds, may also “be highly contaminated with metals, salts,
39

and other toxic constituents.” Id. at 61,191. The EPA notes that

contamination means there is not a clear division between urban

and nonurban coarse; rather “there is a continuum of sources and

contamination, so that the difference between them is a question

of degree.” EPA Br. 96. In addition, occupational studies

showing health effects from nonurban coarse at occupational

exposure levels “lend[] further support to a cautious approach in

considering revisions to the standards affording protection from

thoracic coarse particles.” 2006 Final Rule, 71 Fed. Reg. at

61,191. 

Although the evidence of danger from coarse PM is, as the

EPA recognizes, “inconclusive,” id. at 61,193, the agency need

not wait for conclusive findings before regulating a pollutant it

reasonably believes may pose a significant risk to public health.

The evidence in the record supports the EPA’s cautious decision

that “some protection from exposure to thoracic coarse particles

is warranted in all areas.” Id. As this court has consistently

reaffirmed, the CAA permits the Administrator to “err on the

side of caution” in setting NAAQS. Lead Indus., 647 F.3d at

1155; see also ATA III, 283 F.3d at 369 (“The [Clean Air] Act

requires EPA to promulgate protective primary NAAQS even

where, as here, the pollutant’s risks cannot be quantified or

‘precisely identified as to nature or degree’” (quoting 1997 Final

Rule, 62 Fed. Reg. at 38,653)); Am. Petroleum Inst. v. Costle,

665 F.2d 1176, 1186 (D.C. Cir. 1981) (“In setting margins of

safety the Administrator need not regulate only the known

dangers to health, but may ‘err’ on the side of overprotection by

setting a fully adequate margin of safety”). 

This court’s role is “limited to determining if the

Administrator made a rational judgment” and is “not to weigh

the evidence anew and make technical judgments.” Am.

Petroleum Inst., 665 F.2d at 1185. On the basis of the evidence

in the record, we find the Administrator’s decision to regulate all
40

coarse PM to be rational and therefore deny the industry

petitioners’ claims on this issue. 

B. Level of the Daily Coarse PM NAAQS

The industry petitioners next challenge the level of the daily

coarse PM standard. The EPA used PM10 as the indicator for

coarse PM and set the 24-hour coarse PM standard at 150 μg/m3

,

the same level it retained in the 1997 NAAQS revision. See

1997 Final Rule, 62 Fed. Reg. at 38,679. It did not differentiate

between urban and nonurban coarse particles. 

The industry petitioners first contend that the agency should

have set “different limits for urban and nonurban PM coarse.”

Industry Br. 27. In response, the EPA maintains that it is not

possible, given the “current state of science,” to set different

standards for urban and nonurban areas because there is no

reliable way to determine which ambient mixes are urban and

which are nonurban. 2006 Final Rule, 71 Fed. Reg. at 61,195.

The agency explains that, “given the variety of sources

contributing to PM10-2.5 concentrations in different locations, a

wide variety of ‘ambient mixes’ are likely to exist, greatly

complicating the determination of the appropriate standard level

for each location.” Id. The agency further notes that “there is

insufficient evidence regarding coarse particle composition in

different areas to allow for the proper assignment of different

standard levels in different locations, and the technical

capabilities necessary to make such determinations are currently

lacking.” Id. This is a reasonable explanation for declining to

set different levels for “urban” and “nonurban” coarse. Because

we affirm the EPA’s decision not to set different urban and

nonurban coarse PM standards, we do not reach the argument of

the amicus National Association of Home Builders that the EPA

does not have the authority to set different NAAQS for urban

areas or for different emissions sources. See NAHB Amicus Br.
41

2-6.

The industry petitioners also contend that the EPA’s single

standard of 150 μg/m3 is not requisite to protect the public health.

Again, the EPA has a reasonable response. The EPA explains

that the Staff Paper undertook an examination of epidemiologic

studies, and that the studies showed mortality and morbidity

effects in areas that exceed the 150 μg/m3 standard. 2006 Final

Rule, 71 Fed. Reg. at 61,200 (“[T]he Staff Paper found little

basis for concluding that the degree of protection afforded by the

current PM10 standards in urban areas is greater than warranted,

since potential mortality effects have been associated with air

quality levels not allowed by the current 24-hour standard, but

have not been associated with air quality levels that would

generally meet that standard, and morbidity effects have been

associated with air quality levels that exceeded the current 24-

hour standard only a few times”); see also STAFF PAPER

§ 5.4.4.1, at 5-67. The Administrator agreed with the Staff

Paper’s findings, and on that basis concluded that the 150 μg/m3

standard did not provide more protection than necessary. 2006

Final Rule, 71 Fed. Reg. at 61,202. The Administrator further

concluded that the “very high degree of uncertainty in the

relevant population exposures implied by the morbidity studies

suggests there is little basis for concluding at this time that a

greater degree of protection is warranted.” Id.

The industry petitioners next argue that the 150 μg/m3

standard for PM10 will result in arbitrarily varying levels of

coarse PM, and that the agency should instead have used a PM10-

2.5 indicator. The EPA does not dispute that using the PM10

indicator will result in coarse PM levels that vary within the limit

of 150 μg/m3. As the EPA explains: “Because the PM10

indicator includes both coarse PM (PM10-2.5) and fine PM (PM2.5),

the concentration of PM10-2.5 allowed by a PM10 standard set at a

single level declines as the concentration of PM2.5 increases.
42

Thus, the level of coarse particles allowed varies depending on

the level of fine particles present.” Id. at 61,195. 

Although the EPA acknowledges that a PM10 indicator will

result in varying coarse PM levels, it does not agree that the

variance will be arbitrary. The EPA agrees with the industry

petitioners that protection from coarse particles should be

targeted at urban areas, where coarse particles have been shown

to pose the greatest danger. Id. at 61,194. But the agency argues

that targeting of urban areas is effectively accomplished by using

an indicator that permits the varying levels that the industry

petitioners challenge. As the EPA explains:

PM2.5 levels tend to be lower in rural areas and higher in

urban areas. Thus, to the extent that higher PM2.5 levels

lead to a lower allowable level of coarse particles in some

areas compared to others, this will occur in precisely those

locations — i.e. urban or industrial areas — where the

science has shown the strongest evidence of adverse health

effects associated with exposure to coarse particles. 

Id. at 61,195-96 (citations omitted). In other words: “The

varying levels of coarse particles allowed by a PM10 indicator

will therefore target protection in urban and industrial areas

where the evidence of adverse health effects associated with

exposure to coarse particles is strongest.” Id. 

The EPA also offers a further rationale for tying the

stringency of coarse PM regulation to increases in the level of

PM2.5. The EPA explains that the contamination of coarse

particles discussed above, which makes such particles more

dangerous to health, is linked to the presence of fine PM.

Specifically, “[m]any of these contaminants in PM10-2.5 come

originally from fine particles, which may become attached in the

atmosphere or be deposited and mixed into coarse materials on
43

the ground.” Id. Because contamination increases the health

risks posed by coarse particles, the EPA argues that it is “logical

to allow lower levels of coarse particles when fine particle

concentrations are high. . . . [I]nclusion of PM2.5 in the PM10

indicator for purposes of coarse particle protection would

appropriately reflect the contribution that contaminants emitted

in fine particle form can make to the overall health risk posed by

coarse particles.” Id. Thus, “inclusion of the PM2.5 fraction in

the PM10 indicator . . . ensures that this risk of contamination of

coarse particles by PM2.5 is addressed in the suite of fine and

coarse PM standards.” Id.

It is true that the EPA relies on a qualitative analysis to

describe the protection the coarse PM NAAQS will provide. But

the fact that the EPA’s analysis is qualitative rather than

quantitative does not undermine its validity as an acceptable

rationale for the EPA’s decision. As this court held in ATA III,

“[t]he [Clean Air] Act requires EPA to promulgate protective

primary NAAQS even where, as here, the pollutant’s risks

cannot be quantified or ‘precisely identified as to nature or

degree.’” 283 F.3d at 369 (quoting 1997 Final Rule, 62 Fed.

Reg. at 38,653). The EPA’s qualitative explanation for the

targeting capacity of the 150 μg/m3

 PM10 standard is reasonable

and warrants rejection of the petitioners’ challenge on this point.

In sum, we find that the EPA has provided a reasonable

explanation for its decisions not to set separate urban and

nonurban coarse PM standards, to set the single coarse PM

standard at 150 μg/m3

, and to utilize a standard that allows

targeted variance in coarse PM levels in an inverse relationship

to the amount of fine PM in the air. Consequently, we reject the

industry petitioners’ challenge to the level of the coarse PM

standard.
44

C. Choice of the PM10 Indicator 

The industry petitioners also level a direct challenge to the

EPA’s choice of the PM10 indicator — a challenge that

significantly overlaps with that addressed in Part IV.B above.

The EPA initially proposed using a PM10-2.5 indicator instead of

a PM10 indicator. See 2006 Proposed Rule, 71 Fed. Reg. at 2665-

68. The PM10-2.5 indicator would have measured only the coarse

fraction (PM10-2.5) of PM smaller than 10 μg, see id., and would

have disposed of concerns about the PM10 indicator that this

court raised in ATA I. See 175 F.3d at 1054-55. In response to

comments expressing concern with the PM10-2.5 indicator, see

2006 Final Rule, 71 Fed. Reg. at 61,188-94, the EPA’s final rule

reverted to the PM10 indicator that the agency had adopted in the

1997 NAAQS review. Id. at 61,194-99. 

The petitioners challenge the PM10 indicator on three

grounds. First, they argue that ATA I holds that the PM10

indicator is “inherently confounded,” and that no explanation by

the EPA for the indicator’s utility could make it acceptable.

Industry Reply Br. 13-14. For support, the petitioners rely on the

court’s statement that “it is the very presence of a separate PM2.5

standard that makes retention of the PM10 indicator arbitrary and

capricious.” ATA I, 175 F.3d at 1054. But ATA I did not

definitively preclude the use of a PM10 indicator. It held only

that the EPA had not offered an adequate explanation to “aid us

in understanding its decision” to select PM10 as the indicator for

coarse PM. Id. Without that explanation, we were constrained

to conclude the PM10 and PM2.5 indicators could lead to arbitrary

“‘double regulation’ of the PM2.5 component of PM10 and

potential underregulation of the PM10-2.5 component.” Id. As

recounted in Part IV.B, the EPA has now cured that failure of

explanation and provided a reasonable rationale for its choice of

PM10. See 2006 Final Rule, 71 Fed. Reg. at 61,193-97; EPA Br.

107. 
45

Second, the industry petitioners argue that the EPA’s choice

of a PM10 indicator also runs afoul of ATA I because it was based

solely on reasons of administrative convenience, which ATA I

found impermissible. ATA I, 175 F.3d at 1055 (“The

administrative convenience of using PM10 cannot justify

choosing an indicator poorly matched to the relevant pollution

agent”). This time, however, the EPA expressly disavowed

reliance upon administrative convenience, 2006 Final Rule, 71

Fed. Reg. at 61,195, and chose the PM10 indicator only after

considering and rejecting alternatives on the basis of flaws

discerned during the comment period. Id. at 61,193-97. For

example, the final rule explains that an unqualified PM10-2.5

indicator would not have been “requisite” because it would have

been too stringent in nonurban areas and insufficiently stringent

in urban areas. As discussed above, the agency rejected a

qualified indicator — one with different levels for urban and

nonurban areas — because “determining appropriate levels for

different kinds of ambient mixes is not feasible at this time.” Id.

at 61,195; see also id. at 61,193. Finally, as also discussed

above, the EPA explained its choice of PM10 on scientific rather

than administrative grounds: PM10 allows for targeting

regulation of PM10-2.5 concentrations in those areas that

experience high concentrations of PM2.5, which can contaminate

— and thus render more dangerous — coarse PM. Id. at 61,196-

97.

Third, the petitioners argue that the PM10 standard will

overregulate — or rather, double-regulate — fine particles

because such particles are also subject to the PM2.5 standard. The

discussion in Part IV.B, however, rebuts this argument as well.

As we concluded there, the EPA has reasonably explained that

the variance allowed by the PM10 indicator will target protection

by allowing less coarse PM in areas that experience high

concentrations of potentially contaminating fine PM. Id. at
46

61,196. This will not double-regulate PM2.5. Fine PM will

primarily be regulated via the newly tightened daily PM2.5

standard. Id. at 61,196 n.72. Any residual regulation of PM2.5 by

the PM10 standard will serve a different, “non-duplicative

purpose[] in providing requisite protection from thoracic coarse

particles” that can be contaminated by PM2.5. Id. at 61,196.

Moreover, the petitioners’ over-regulation argument wrongly

assumes that an area in violation of the PM10 coarse standard

must decrease the fine fraction of PM to achieve compliance

with the standard. Such an area could, however, simply choose

to decrease the amount of coarse PM (PM10-2.5), a choice that

would yield no additional “regulation” of fine PM.

For the foregoing reasons, we reject these and all of the

industry petitioners’ other objections to the EPA’s revised

NAAQS for coarse PM and deny their petitions for review.

V.

Finally, the environmental petitioners challenge the EPA’s

revocation of the primary annual standard for coarse PM. The

petitioners argue that the CAA prohibits the EPA from revoking

this standard. Alternatively, they argue that the EPA’s

justification for the revocation was arbitrary and capricious. We

reject both arguments.

The statutory provision at issue, 42 U.S.C. § 7513(d)(2),

was enacted as part of the 1990 amendments to the CAA. In

those amendments, the Congress created a detailed enforcement

scheme to require that areas not in compliance with the NAAQS

(nonattainment areas) make faster progress toward meeting that

goal. The amendments established a procedure for classifying

nonattainment areas and setting the date by which an area must

attain compliance with the NAAQS. An area that is classified as

“moderate nonattainment” for coarse PM may receive a one-year

extension of its attainment date if certain criteria are met:
47

Upon application by any State, the Administrator may

extend for 1 additional year . . . the date [for attainment] if:

 (1) the State has complied with all requirements and

commitments pertaining to the area in the applicable

implementation plan; and

(2) no more than one exceedance of the 24-hour

national ambient air quality standard level for PM-10 has

occurred in the area in the year preceding the Extension

Year, and the annual mean concentration of PM-10 in the

area for such year is less than or equal to the standard

level.

Id. § 7513 (emphasis added).

Despite the explicit statutory reference to an annual

standard level of PM10, the EPA decided to revoke the PM10

annual standards while leaving in place the 24-hour standards.

The EPA interpreted the CAA to permit this revocation. We

review the EPA’s interpretation under the familiar standard set

forth by the Supreme Court in Chevron. See 467 U.S. at 842–43.

The EPA argues that the CAA does not unambiguously

require an annual standard for coarse PM and that its reading of

the statute is reasonable under Chevron step two. The EPA relies

primarily on this court’s decision in ATA I, in which we held that

the 1990 amendments, which referenced the existing primary

ozone standards, did not eliminate the agency’s authority to

revise those standards. See 175 F.3d at 1047–48.

The petitioners argue that the EPA’s action fails at step one

of the Chevron analysis because the 1990 amendments to the

CAA codified the use of an annual averaging time for the coarse

PM standard. The petitioners argue that the reference in

§ 7513(d)(2) to a standard “annual mean concentration of
48

[PM10]” means that the EPA must have in place an annual PM10

standard.

The EPA is correct that this court’s precedent forecloses the

petitioners’ statutory argument; the decisive case, however, is not

ATA I but South Coast Air Quality Management District v. EPA,

472 F.3d 882 (D.C. Cir. 2006). Whereas ATA I dealt with the

EPA’s authority to revise the ozone standards in light of the 1990

amendments, South Coast considered the agency’s ability to

revoke a standard explicitly referenced by the text of those

amendments. The EPA had revoked the primary one-hour

standard for ozone and replaced it with a standard using an eighthour averaging time. The petitioners in South Coast pointed to

42 U.S.C. § 7511(a)(1), tbl.1, which sets out a scheme for

classifying areas into varying levels of nonattainment based on

the extent of their noncompliance with the one-hour ozone

standard in existence at the time. The petitioners argued that the

EPA could not revoke the one-hour standard because the

Congress had codified it as part of the 1990 amendments. See

South Coast, 472 F.3d at 899. We rejected this argument,

explaining that the “Congress contemplated . . . the possibility

that scientific advances would require amending the NAAQS.”

Id. Indeed, the 1990 amendments left intact § 7409(d)(1), which

directs the EPA to “complete a thorough review” of the NAAQS

every five years and to “make such revisions . . . as may be

appropriate.” Furthermore, another provision of the CAA

“regulates what EPA must do with revoked restrictions.” South

Coast, 472 F.3d at 899; see 42 U.S.C. § 7502(e) (requiring EPA

to promulgate so-called “anti-backsliding” regulations in the

event that the agency relaxes a NAAQS).

The petitioners here have failed to distinguish this case

from South Coast. As we explained there, it would frustrate the

purpose of the CAA to read the 1990 amendments as limiting the

EPA’s ability to revise the NAAQS based on advances in
49

scientific understanding. The EPA still must review the NAAQS

every five years and make appropriate revisions, including

revoking a standard no longer warranted by the current scientific

understanding. See 42 U.S.C. § 7409(d)(1). In South Coast, we

held that the amendments’ incorporation of the existing one-hour

ozone standard did not prevent the EPA from revoking that

standard and replacing it with one based on an eight-hour

averaging time. Likewise, in this case, the reference in the 1990

amendments to an annual mean standard for PM10 does not

require the EPA to maintain an annual standard in the face of

scientific evidence counseling revocation. We reject the

petitioners’ argument to the contrary.

The petitioners alternatively argue that, even if the EPA had

the authority to revoke the annual coarse PM standard, its

decision to do so was arbitrary and capricious because it was an

unreasonable departure from the agency’s past practice and was

not based on the record. The petitioners point to the EPA’s 1997

revision to the PM NAAQS, in which the agency explained that

the annual coarse PM standard would “provide substantial

protection against short-term as well as long-term exposures to

particles.” 1997 Final Rule, 62 Fed. Reg. at 38,676. The

petitioners argue that, in light of the EPA’s previous decision to

control both short- and long-term exposure through the annual

standard, it was unreasonable for the EPA to revoke the annual

standard based only on the lack of adverse effects from longterm exposure. The petitioners further assert that scientific

evidence still demonstrates an association between short-term

exposure to coarse PM and negative health effects.

But as the EPA points out, the petitioners have forfeited

their argument that an annual standard reduces the risk from

short-term exposure. The CAA provides that “[o]nly an

objection to a rule or procedure which was raised with

reasonable specificity during the period for public
50

comment . . . may be raised during judicial review.” 42 U.S.C.

§ 7607(d)(7)(B); see also Appalachian Power Co. v. EPA, 135

F.3d 791, 818 (D.C. Cir. 1998) (“The purpose of the exhaustion

requirement is to ensure that the agency is given the first

opportunity to bring its expertise to bear on the resolution of a

challenge to a rule”). In the comments submitted in response to

the EPA’s proposed revocation of the annual standard, the

environmental petitioners argued only that the record evidence

demonstrated adverse effects from long-term coarse PM

exposure; they did not raise their current argument that an annual

standard is necessary to prevent adverse effects from short-term

exposure.

The EPA reasonably decided that an annual coarse PM

standard is not necessary because, as the Criteria Document and

the Staff Paper make clear, the latest scientific data do not

indicate that long-term exposure to coarse particles poses a

health risk. The CASAC also agreed that an annual coarse PM

standard is unnecessary. We deny the petition for review (No.

06-1411) with regard to the revocation of the annual coarse PM

standard.

VI.

For the foregoing reasons, we grant in part the petitions for

review of the EPA’s primary annual fine PM standard. We grant

in full the environmental petitioners’ petition for review of the

EPA’s secondary fine PM NAAQS. In all other respects, the

petitions for review are denied.

So ordered.