The Clean Air Act (CAA), 42 U.S.C. § 7602(g), defined air pollutants as “any air pollution agent or combination of such agents . . . which is emitted into or otherwise enters the ambient air.” In Massachusetts v. EPA, 549 U.S. 497 (2007), the United States Supreme Court determined that the CAA-wide definition of air pollutant included greenhouse gases because the definition was so broad. However, although the CAA-wide definition was broad, the EPA consistently interpreted the term “air pollutant” much more narrowly when required by the context. Specifically, the CAA contained a permitting program that required facilities to get permits if they emitted either 100 or 250 tons or more of air pollutants per year, depending on the type of pollutant. In the context of this permitting program, the EPA had traditionally applied the permitting requirements only to conventional air pollutants and not to greenhouse gases. In 2009, the EPA established regulations for greenhouse-gas emissions from new motor vehicles. The EPA determined that these regulations automatically triggered the CAA permitting requirement for stationary sources that emitted greenhouse gases. However, the EPA recognized that so many facilities emit 250 tons or more per year of greenhouse gases that the permitting program would be unworkable if the CAA’s statutory threshold amounts applied to greenhouse-gas emissions. Thus, the EPA added a new threshold not found in the CAA, saying the CAA’s permitting program only applied if a facility emitted at least 100,000 tons of greenhouse gases per year. The EPA believed modifying the CAA threshold requirements was necessary to correctly implement the CAA. The EPA also interpreted the term pollutant in a way that meant facilities already subject to the permitting program (i.e., "anyway" sources) had to begin monitoring greenhouse-gas emissions, as well. The Utility Air Regulatory Group and others (plaintiffs) challenged the EPA’s determination that its greenhouse-gas emissions regulations automatically triggered the CAA’s permitting requirement. The court of appeals held that because of the Massachusetts holding that greenhouse gases were air pollutants under the CAA-wide definition, and because the CAA required permitting for all major sources of air pollutants, the CAA required permitting for all major sources that emitted greenhouse gases. The United States Supreme Court granted certiorari.