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programs to control air pollution from stationary sources (such as refineries and factories) and moving sources (such as cars and airplanes). 69 Stat. 322, as amended, 42 U. S. C. §§7401–7671q. One of these is the National Emissions Standards for Hazardous Air Pollutants Program—the hazardous-air-pollutants program, for short. Established in its current form by the Clean Air Act Amendments of 1990, 104 Stat. 2531, this program targets for regulation stationary-source emissions of more than 180 specified “hazardous air pollutants.” §7412(b).

For stationary sources in general, the applicability of the program depends in part on how much pollution the source emits. A source that emits more than 10 tons of a single pollutant or more than 25 tons of a combination of pollutants per year is called a major source. §7412(a)(1). EPA is required to regulate all major sources under the program. §7412(c)(1)–(2). A source whose emissions do not cross the just-mentioned thresholds is called an area source. §7412(a)(2). The Agency is required to regulate an area source under the program if it “presents a threat of adverse effects to human health or the environment. . .warranting regulation.” §7412(c)(3).

At the same time, Congress established a unique procedure to determine the applicability of the program to fossil-fuel-fired power plants. The Act refers to these plants as electric utility steam generating units, but we will simply call them power plants. Quite apart from the hazardous-air-pollutants program, the Clean Air Act Amendments of 1990 subjected power plants to various regulatory requirements. The parties agree that these requirements were expected to have the collateral effect of reducing power plants’ emissions of hazardous air pollutants, although the extent of the reduction was unclear. Congress directed the Agency to “perform a study of the hazards to public health reasonably anticipated to occur as a result of emissions by [power plants] of [hazardous air