American Trucking Associations, Inc. v. Environmental Protection Agency, Commonwealth of Massachusetts, Intervenors. American Trucking Associations, Inc. v. Environmental Protection Agency, Commonwealth of Massachusetts, Intervenors

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

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

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

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

These consolidated cases concern NAAQS for particulate matter and ozone, two ubiquitous atmospheric pollutants. The term "particulate matter," or "PM," refers to all "solid particles and liquid droplets found in air." OFFICE OF AIR & RADIATION, U.S. ENVTL. PROT. AGENCY, EPA-454/R-00-005, AIR QUALITY INDEX: A GUIDE TO AIR QUALITY AND YOUR HEALTH 11 (2000) ("EPA, AIR QUALITY INDEX"). Although these particles and droplets come in varying sizes, only particulate matter less than 2.5 micrometers in diameter — so-called "fine PM" or "PM2.5" — is relevant here. As originally filed, these cases also concerned "coarse" particulate matter, or particles and droplets between 2.5 and 10 micrometers in diameter, but we resolved all issues relating to this "coarse particulate matter" in our earlier ruling. See Am. Trucking Ass'ns v. EPA, 175 F.3d 1027, 1053-55 (D.C.Cir.1999) ("ATA I"), reh'g granted in part and denied in part, 195 F.3d 4 (D.C.Cir.1999) ("ATA II"), aff'd in part and rev'd in part, Whitman v. Am. Trucking Ass'ns, 531 U.S. 457, 121 S.Ct. 903, 149 L.Ed.2d 1 (2001) ("Whitman").

PM2.5 is associated with a range of adverse health effects such as coughing; shortness of breath; aggravation of existing respiratory conditions like asthma and chronic bronchitis; increased susceptibility to respiratory infections; and heightened risk of premature death. EPA, AIR QUALITY INDEX at 11. High PM2.5 concentrations also impair visibility, reducing people's "well-being ..., both where they live and work, and in places [like national parks and wilderness areas] where they enjoy recreational opportunities." Particulate Matter NAAQS, 62 Fed. Reg. at 38,680. Sources of fine PM include vehicle engines, power plants, and wood fires. EPA, AIR QUALITY INDEX at 11.

Unlike PM, ozone is a colorless, odorless gas. EPA, AIR QUALITY INDEX at 7. Not a direct product of human activity, ozone forms when other atmospheric pollutants — ozone "precursors" — react in the presence of sunlight. OFFICE OF AIR QUALITY PLANNING & STANDARDS, U.S. ENVTL. PROT. AGENCY, EPA/451-K-97-002, OZONE: GOOD UP HIGH, BAD NEARBY 2-3, (1997) ("EPA, OZONE FACTS"). Significant health effects associated with ozone pollution include coughing; throat irritation; aggravation of existing conditions like asthma, bronchitis, heart disease, and emphysema; and lung tissue damage. Id. Ozone pollution can also interfere with plants' ability to produce and store food, rendering them more susceptible to disease, insect pests, and other stressors. Id. EPA estimates that ozone "is responsible for 500 million dollars in reduced crop production in the United States each year." Id. Ozone levels tend to be highest in urban areas, in part because cars, power plants, and chemical solvents are the principal sources of ozone precursors. Id.

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

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

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

Soon after EPA issued the revised particulate matter and ozone NAAQS, various parties, including American Trucking Associations, other businesses and business associations, environmental groups, citizens, and several States, petitioned for review of the revised standards. Other parties intervened (some supporting Petitioners and others supporting EPA), four additional States filed amicus briefs supporting EPA, and Senator Orrin Hatch and Representative Thomas Bliley filed amicus briefs supporting Petitioners. Petitioners and supporting Intervenors and Amici challenged the NAAQS from all sides, arguing (among other things) that the Clean Air Act delegates excessive legislative authority to EPA in violation of Article I of the Constitution and that the Agency failed to consider certain relevant factors, including implementation costs, prior to setting the NAAQS. Invoking Clean Air Act section 307(d)(9), which directs federal appeals courts to vacate Agency action "found to be ... arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law," 42 U.S.C. § 7607(d)(9)(A), Petitioners also challenged specific aspects of the new particulate matter and ozone NAAQS ("the 307(d)(9) claims").

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

Having remanded the challenged NAAQS for other reasons, we never addressed Petitioners' § 307(d)(9) claims. We did, however, address Petitioners' more general claims, holding that EPA: may not consider either the implementation costs or the indirect environmental consequences of NAAQS prior to setting the standards, ATA I, 175 F.3d at 1040-41; has no obligation to comply with certain requirements of the National Environmental Policy Act, the Unfunded Mandates Reform Act, or the Regulatory Flexibility Act before issuing NAAQS, id. at 1041-45 (citing NEPA, 42 U.S.C. § 4332(2)(C)-(D); UMRA, 2 U.S.C. §§ 1532, 1535; and RFA, 5 U.S.C. §§ 603(a), 605(b)); should have considered any beneficial effects of ozone before revising the ozone NAAQS, id. at 1051-53; has only limited statutory authority to enforce the new ozone NAAQS, id. at 1046-51; see also ATA II, 195 F.3d at 8-10; need not identify the specific biological mechanism by which fine particulate matter affects human health, ATA I, 175 F.3d at 1055-56; and has no obligation to set secondary NAAQS at levels "sufficient to eliminate all adverse visibility effects" of a pollutant, id. at 1056-57 (emphasis added).

The Supreme Court granted certiorari with respect to four questions: whether the Clean Air Act violates the nondelegation doctrine; whether EPA may consider implementation costs in setting NAAQS; whether this court had jurisdiction to review the Agency's interpretation of the Clean Air Act provisions governing implementation of the ozone NAAQS; and if this court had jurisdiction, whether the Agency's interpretation of those provisions was permissible. Whitman, 531 U.S. at 462, 121 S.Ct. 903. Disagreeing with our nondelegation ruling, the Supreme Court held that the Clean Air Act requirement that EPA "set air quality standards at the level that is `requisite' — that is, not lower or higher than is necessary — to protect the public health with an adequate margin of safety, fits comfortably within the scope of [Agency] discretion permitted by [Court] precedent." Id. at 475-76, 121 S.Ct. 903. Writing for the Court, Justice Scalia next confirmed our long-held view that "`economic considerations [may] play no part in the promulgation of ambient air quality standards,'" id. at 464, 121 S.Ct. 903 (quoting Lead Indus. Ass'n v. EPA, 647 F.2d 1130, 1148 (D.C.Cir.1980)); see also ATA I, 175 F.3d at 1040-41 (making the same point), and agreed with ATA I that this court had jurisdiction to review EPA's interpretation of the Act's provisions governing implementation of the ozone NAAQS, Whitman, 531 U.S. at 477-80, 121 S.Ct. 903. Turning to the merits of the latter issue, the Court found the Agency's ozone NAAQS "implementation policy to be unlawful" and indicated that after this court's final disposition of the parties' § 307(d)(9) claims "it is left to the EPA to develop a reasonable interpretation of the nonattainment implementation provisions insofar as they apply to revised ozone NAAQS." Id. at 486, 121 S.Ct. 903.

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

In considering these section 307(d)(9) claims, we apply the same highly deferential standard of review that we use under the Administrative Procedure Act, 5 U.S.C. § 706(2)(A). See, e.g., Allied Local & Reg'l Mfrs. Caucus v. EPA, 215 F.3d 61, 68 (D.C.Cir.2000) ("To determine whether [the Agency's] rules are `arbitrary and capricious,' we apply the same standard of review under the Clean Air Act as we do under the Administrative Procedure Act."). Thus, we presume the validity of agency action as long as "a rational basis for it is presented." Lead Indus. Ass'n, 647 F.2d at 1145. That said, however, we perform a "searching and careful" inquiry into the underlying facts. Id. (internal quotation marks and citations omitted). In a similar case involving a challenge to ambient air quality standards, Lead Industries, we elaborated on this standard of review as follows:

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

Id. at 1160 (internal citations omitted).

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

Petitioners base this law of the case argument primarily on our statement in ATA II that although EPA's "petition for rehearing ... argue[d] that § 109 of the Clean Air Act contains [a] principle limiting the agency's discretion" — namely, that the NAAQS must be set at levels "necessary for public health protection: neither more nor less stringent than necessary, but `requisite'" — EPA never "identif[ied]" this principle "as a limit upon its discretion" anywhere in the challenged rules. 195 F.3d at 6-7 (internal quotation marks and citations omitted). In addition, Petitioners point to a later passage in ATA II in which we declined to "express [an] opinion upon the sufficiency of [the newly identified limiting] principle," choosing to wait until "after the [Agency] itself has applied [the principle] in setting a NAAQS" to determine "whether the principle, in practice, fulfills the purposes of the nondelegation doctrine." Id. at 7 (emphasis added). Finally, Petitioners cite the following sentences from ATA I:

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

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

Petitioners misread ATA I and ATA II. Both decisions address the broad claim that the Clean Air Act effects an unconstitutional delegation of legislative authority. To evaluate that claim, we looked first to the Act for an "intelligible principle" limiting the Agency's "application of the[] factors" it "uses in determining the degree of public health concern associated with different levels of ozone and PM[.]" ATA I, 175 F.3d at 1034 (internal quotation marks omitted). Finding none, we next looked to see whether the Agency developed "binding standards for itself." ATA I, 175 F.3d at 1038 (emphasis added). Again finding none, we remanded the NAAQS to EPA to give it "an opportunity to extract [from the Act] a determinate standard" that would "channel" its rulemaking authority. Id. at 1034, 1038; see also ATA II, 195 F.3d at 6-8 (affirming that holding). At no point in either ATA I or ATA II, however, did we address the narrower question now before us: Did EPA abuse its discretion or otherwise violate the Clean Air Act in setting the challenged NAAQS? Indeed, in ATA I we emphasized this very distinction:

[The Agency] cites prior decisions of this Court holding that when there is uncertainty about the health effects of concentrations of a particular pollutant within a particular range, EPA may use its discretion to make the "policy judgment" to set the standards at one point within the relevant range rather than another....

We agree. But none of those panels addressed the claim of undue delegation that we face here, and accordingly had no occasion to ask EPA for ... a "principle[]"... in making its "policy judgment."

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

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

We start with Petitioners' challenge to the PM2.5 NAAQS. Fulfilling our obligation to "undertake a `substantial inquiry' into the facts" underlying challenged agency actions, Lead Indus. Ass'n, 647 F.2d at 1146 (quoting Citizens to Preserve Overton Park v. Volpe, 401 U.S. 402, 415, 91 S.Ct. 814, 28 L.Ed.2d 136 (1971), overruled on other grounds by Califano v. Sanders, 430 U.S. 99, 105, 97 S.Ct. 980, 51 L.Ed.2d 192 (1977)), we first summarize the Agency's rulemaking process with respect to both the primary and secondary NAAQS and then consider State, Business, and Environmental Petitioners' specific arguments.

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

To facilitate public input on the proposed changes, EPA established a toll-free telephone hotline and a system for submission of comments via the Internet. Particulate Matter NAAQS, 62 Fed. Reg. at 38,654. In addition, Agency staff conducted public hearings in communities across the country and held two national satellite telecasts. Id. By the end of the comment period, EPA had received more than 50,000 comments. Id.

In setting the final NAAQS, EPA considered these public comments, in addition to CASAC's recommendations and Agency staff's "thorough review ... of the latest scientific information on known and potential human health effects associated with exposure to PM." Id. at 38,655-56. EPA also considered an independent (though partially Agency-funded) research institute's study of the correlation between particulate matter levels and mortality in six urban areas, the same institute's extended analyses of the mortality data for Philadelphia alone (the "Philadelphia studies"), see id. at 38,660, and Agency staff's quantitative assessment of other PM-related health risks in Philadelphia and Los Angeles ("risk assessment"), see id. at 38,656 (citing OFFICE OF AIR QUALITY PLANNING & STANDARDS, U.S. ENVTL. PROT. AGENCY, A PARTICULATE MATTER RISK ASSESSMENT FOR PHILADELAPHIA AND Los ANGELES (1996) ("RISK ASSESSMENT")). EPA emphasized, however, that it "place[d] greater weight on the overall conclusions derived from the[se] studies — that PM air pollution is likely causing or contributing to significant adverse effects at levels below those permitted by the [old] standards — than on the [more uncertain] concentration-response functions and quantitative risk estimates derived from them." Id.

Ultimately, EPA adopted the proposed annual PM2.5 standard of 15 μg/m³, explaining that it would assess compliance based on the three-year average of annual arithmetic mean PM2.5 concentrations at monitoring stations in a given area. See Particulate Matter NAAQS, 62 Fed. Reg. at 38,652. For the daily standard, EPA chose a less restrictive level than it had originally proposed, setting the standard at 65 μg/m³, with a "form" — or method of assessing compliance — based on the three-year average of the 98th percentile of daily PM2.5 concentrations at each monitoring site. Id. EPA provided a lengthy explanation of its selection of these new standards, four aspects of which are relevant here: its discussions of (1) the need to revise the old NAAQS for particulate matter; (2) the reasons for adopting both annual and daily standards; (3) the grounds for choosing 15 and 65 μg/m³, respectively, as the levels for the new standards; and (4) the rationale behind the new daily standard's rather peculiar form.

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

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

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

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

Explaining its decision to establish both annual and daily standards, EPA indicated that although "either standard could be viewed as providing both short-and long-term protection" from PM2.5, the use of two standards with very different averaging times would "serv[e] to address situations where the daily peaks and annual averages are not consistently correlated." Particulate Matter NAAQS, 62 Fed. Reg. at 38,669. In such situations, the annual standard would "lower[] both short and long-term PM2.5 concentrations," while the daily standard would "protect[] against ... localized `hot spots,' and ... seasonal emissions," neither of which would be "well controlled by a national annual standard" alone. Id.

EPA next made a number of key points about the levels it selected for the new NAAQS. Implementing its view that the annual standard should do most of the work in mitigating the risks of PM2.5, EPA selected the level of the standard "so as to protect against the range of effects associated with both short- and long-term exposures to PM.