	Introduction

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). 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." 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. Most important to the court, CAIR's emissions trading program, though aimed at reducing emission-control costs as approved in Michigan , did not assure some "measurable" emission reduction in each upwind state. The CAA, held the Court, does not require that states be given a second opportunity to file a SIP after EPA has quantified the state's emissions budget. Rather, Portland Cement Ass'n v. Ruckelshaus says that Section 111 "looks toward what may fairly be projected for the regulated future, rather than the state of the art at present ...." Still, Lignite Energy Council v. EPA cautions that "EPA may not base its determination that a technology is adequately demonstrated or that a standard is achievable on mere speculation or conjecture ...." The agency may compensate for the absence of emissions data in a new source category by, for example, "extrapolation of a technology's performance in other industries." (See later section on " Bubble Concept .") "Routine Maintenance"

As mentioned, NSPSs and new source review in PSD areas apply not just to new sources, but also to existing sources that undergo modifications (NSPS) or major modifications (PSD). A modification is subject to the same requirements as a new source. The CAA definition of "modification" has been interpreted by EPA and state pollution control agencies. The case established that determining whether the RMRR exemption applies to an existing-source renovation depends on a case-by-case weighing of four factors: the resulting increase in a plant's life expectancy, and the project's cost, nature, and magnitude. The following year, the D.C. Circuit held in National Mining Ass'n v. EPA that EPA may determine if a facility is a major source by aggregating all sources in a contiguous plant site under common control; EPA is not restricted to aggregating only sources within a single source category or under the same Standard Industrial Classification Code. Nothing in the statute, the court said, suggests that EPA may set emission levels only for listed hazardous air pollutants that are currently controlled with technology. The Court did allow, however, that EPA could impose one of these programs, requiring installation of Best Available Control Technology on new and modified stationary sources in PSD areas, to GHG emissions from certain stationary sources: those that emit New Source Review pollutants in amounts sufficient to come under the program independently. Pre-enforcement review by a court, it held, must be made available to the recipient of the order. Glossary of Acronyms

ACO: administrative compliance order
BACT: best available control technology
BSER: best system of emission reduction
CAA: Clean Air Act
CAIR: Clean Air Interstate Rule
CO 2 : carbon dioxide
CPP: Clean Power Plan
EPA: Environmental Protection Agency
GHG: greenhouse gas
HAP: hazardous air pollutant
MACT: maximum achievable control technology
NAAQS: national ambient air quality standard
NESHAP: national emission standard for hazardous air pollutant
NO x : nitrogen oxides
NEPA: National Environmental Policy Act
NSPS: new source performance standard
PSD: prevention of significant deterioration
RMRR: routine maintenance, repair, and replacement
SIP: state implementation plan
SO 2 : sulfur dioxide
This report provides a selective overview of court decisions that historically have most shaped EPA's program under the Clean Air Act (CAA or Act). Court decisions described in the report deal with the following:
National ambient air quality standards (NAAQSs), holding that in setting the standards EPA is not to consider economic and technological feasibility. State implementation plans for achieving NAAQSs, holding that EPA may not consider economic and technological feasibility in approving or disapproving such plans, or that the state plan is more stringent than necessary, or does not require an EPA-preferred control method. Interstate air pollution, holding that EPA may consider costs in applying the CAA "good neighbor" provision, but any emissions trading program must assure some emission reduction in each upwind state. Nor does the CAA require that states be given a second opportunity to file an implementation plan after EPA has quantified the state emissions budget; EPA may promulgate its own plan for the state immediately. New source performance standards (NSPSs), holding that while the Act requires NSPSs to be based on "adequately demonstrated" technology, EPA is allowed to consider technologies that will be fairly projected in the future so long as the technology is not speculative. New source review in areas not subject to NAAQSs, holding that EPA may override a state's determination of the "best available control technology" required for new stationary sources. EPA may require new source review for greenhouse gas emitters only if the new source will emit certain pollutants above threshold amounts. The "routine maintenance" exemption from NSPSs and new source review, created by EPA and accepted by the courts despite statutory silence. Courts hold that whether the exemption applies depends on the increase in a plant's expected life due to the project, and the project's cost, nature, and magnitude. Expansive interpretation of the exemption has been judicially rejected. The "bubble concept," an EPA approach that looks at net changes in the emissions of a pollutant from a facility, holding that its permissibility depends on statutory context. National standards for hazardous air pollutants, holding that EPA may determine if a facility triggers the Act's "maximum achievable control technology" requirement for such pollutants by aggregating emission sources in a contiguous plant under common control, not just sources within the same source category. EPA is not limited in setting emission standards to hazardous air pollutants currently controlled with technology. Greenhouse gas emissions, holding that the CAA generally covers them, and that EPA has to exercise that authority based on policy concerns. See, however, "new source review" above. Enforcement, holding that the recipient of an administrative compliance order must be allowed to seek pre-enforcement review of the order in court.