This report provides a selective overview of court decisions that have shaped the U.S. Environmental Protection Agency's (EPA's) implementation of the Clean Air Act (CAA or Act). The subjects these decisions cover are broad, because almost every major EPA rule has been challenged in court. Challenges to EPA actions are filed largely in the U.S. Court of Appeals for the D.C. Circuit (D.C. Circuit), as the CAA requires for challenges to "nationally applicable regulations." Claims that EPA has failed to perform a nondiscretionary duty under the Act, such as meeting a statutory deadline, are challenged through "citizen suits" filed in federal district courts. Over the 46 years of the modern CAA's existence, more than a dozen circuit and district court decisions on the Act have yielded Supreme Court opinions on appeal. This report also highlights several court cases currently being litigated that address a number of unresolved questions related to EPA's CAA authority. A glossary of acronyms used in this report is provided on page 1 9 . The central construct of the CAA is the national ambient air quality standard (NAAQS), a maximum concentration for an air pollutant that all areas of the country must meet. To establish NAAQSs, CAA Section 108 directs EPA to issue air quality criteria — documents assessing the scientific evidence on a pollutant's effects—for pollutants that "may reasonably be anticipated to endanger public health or welfare," and "the presence of which in ambient air results from numerous or diverse mobile or stationary sources ...." Section 109 then requires EPA to set primary NAAQSs at a level requisite to protect public health, allowing an "adequate margin of safety," and secondary NAAQSs at a level requisite to protect the "public welfare" from any known or anticipated adverse effects. Finally, Section 110 requires states to adopt state implementation plans to attain or maintain each NAAQS (see following section). In the 1970s, EPA established NAAQSs for six pollutants, commonly referred to as "criteria" pollutants. The agency has not added any pollutants to the list since then, although it is required to review the existing standards at five-year intervals and promulgate revisions if appropriate. These reviews have raised continuing issues among stakeholders, including states, industrial and other sources of pollution, and nongovernmental organizations (NGOs) in the health and environmental fields, particularly over the role of cost and feasibility in determining the level at which standards should be set. In directing EPA to set and revise the NAAQSs, Sections 108 and 109 make no reference to the cost or feasibility of attaining the standards—in contrast to other CAA sections where cost, feasibility, or the role of demonstrated technology are specifically required to be considered. The silence of Sections 108 and 109 as to cost and feasibility led to a number of cases, all of which held that EPA must set NAAQSs without regard to their cost or technological feasibility. In Lead Industries Ass'n v. EPA , the D.C. Circuit addressed the primary and secondary NAAQSs for lead, holding that requiring EPA to consider cost and feasibility in setting NAAQSs was "totally without merit." Specifically rejected was the argument that the "adequate margin of safety" required by Section 109 to be factored into primary NAAQSs requires EPA to consider costs and feasibility. Persuasive to the court was that the CAA expressly provides for EPA to factor in costs and feasibility, and that even if the technology to achieve a NAAQS did not currently exist, the CAA was of a "technology forcing" character. In 2001, in Whitman v. American Trucking Ass'ns , the Supreme Court unanimously affirmed Lead Industries , finding it impermissible to consider costs in setting primary NAAQSs. The Whitman challengers had argued that the high costs of imposing a stringent primary NAAQS might injure health by closing down whole industries and impoverishing workers and consumers dependent on them. The Whitman challengers contended that EPA had to consider this effect on health, too. The Court disagreed, pointing out that CAA Section 110(f)(1) allowed EPA to waive compliance deadlines for stationary sources in certain circumstances, and that, as noted in Lead Industries , numerous CAA provisions allow EPA to take compliance costs into account. That being so, the Court refused to infer from the ambiguous language of Section 109 an authority for EPA to consider costs that elsewhere in the Act had been granted expressly. The CAA directs EPA to translate NAAQSs into emission limits for specific stationary sources. After EPA promulgates or revises NAAQS, CAA Section 110 requires each state to submit a state implementation plan (SIP) to EPA within three years. SIPs specify what mix of federal, state, and local air pollution control measures the state will implement in order to reach or maintain the NAAQSs. To be approved by EPA, the SIP must satisfy a long list of requirements in Section 110(a)(2). For example, the SIP must contain enforceable emission limitations, timetables for compliance, air quality monitoring, provisions addressing interstate pollution, and so on. Because Section 110(a)(2) does not require that SIPs be economically and technologically feasible, the Supreme Court concluded in Union Electric Co. v. EPA that EPA may not consider infeasibility when approving or disapproving a SIP. Further, the Supreme Court held that states may submit SIPs more stringent than the CAA requires and EPA must approve them if they meet the Section 110(a)(2) factors. In addition, as held in Michigan v. EPA , EPA may not condition SIP approval on a state's adoption of any specific control measure under the CAA. Finally, under Indiana & Michigan Elec tric Co . , EPA does not have to prepare an environmental impact statement under the National Environmental Policy Act (NEPA) when approving or disapproving a submitted SIP. Courts have held EPA actions generally to be exempt from NEPA requirements, because the agency's mission of protecting the environment duplicates that of NEPA. After EPA sets or revises a NAAQS, the governor of each state, using available monitoring data, must submit to EPA a list identifying each air quality control region in the state as either attainment, nonattainment, or unclassifiable for the pollutant in question. After reviewing these submissions, and often after negotiating over the boundaries of the listed areas, EPA formally promulgates a list of nonattainment areas . Affected states are then required to submit SIPs or SIP revisions specifying what mix of federal, state, and local air pollution control measures will be implemented for each of the areas to reach or maintain the NAAQS. In many states, particularly in the Northeast, air quality is so affected by emissions from other states (referred to as "transported" air pollution) that it is difficult or impossible for the state to demonstrate how all areas in the state will reach attainment. To assist these downwind states, the CAA contains several provisions dealing with transported air pollution. The most important (and most frequently litigated) is Section 110(a)(2)(D), the so-called "good neighbor" provision . Section 110(a)(2)(D)(i)(I) requires that a state prohibit stationary sources within the state from emitting air pollutants in amounts that will "contribute significantly" to NAAQS nonattainment, or "interfere with maintenance" of a NAAQS, in any other state. States must include such measures in the SIPs they submit to EPA. When EPA determines that existing SIPs must be revised to satisfy the good neighbor provision (or other CAA requirements), it issues a "SIP call," sometimes to many states at once. Litigation over the good neighbor provision generally has involved emissions of nitrogen oxides (NO x ), particulates, and sulfur dioxide (SO 2 ), all of which can remain in the atmosphere and travel long distances from the point of emission. These substances are not only pollutants in their own right; they also contribute (through atmospheric reactions) to the formation of ozone, particulates, acid deposition, and other widespread regional pollution problems. Litigation over the good neighbor provision has addressed how EPA defines when a state "contribute[s] significantly" to another state's pollution; what role cost-effectiveness can play in identifying control measures; whether regional cap-and-trade systems can substitute for emission control requirements imposed at specific sources; and the respective roles of EPA and the states in identifying required controls. In 2000, the D.C. Circuit held in Michigan v. EPA that nothing in Section 110(a)(2)(D) bars EPA from considering costs. Consequently, EPA had acted lawfully when it determined that a state would no longer be contributing "significantly" to a downwind state's NAAQS nonattainment if the state reduced the relevant emissions by the amount that could be eliminated using "highly cost-effective controls" (those that eliminate a ton of the relevant pollutant for less than $2,000 per ton). The court found that EPA may apply this standard uniformly to all the covered states, no matter the amount of each state's contribution. North Carolina v. EPA involved a challenge to EPA's Clean Air Interstate Rule (CAIR), issued in 2005. CAIR sought to reduce SO 2 and NO x precursor emissions in 28 states in order to reduce nonattainment of the NAAQSs for fine particulate matter and ozone in downwind states. The D.C. Circuit, however, found CAIR to be flawed.