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MICHIGAN v. EPA Opinion of the Court

Council, Inc., 467 U. S. 837 (1984). Chevron directs courts to accept an agency’s reasonable resolution of an ambiguity in a statute that the agency administers. Id., at 842– 843. Even under this deferential standard, however, “agencies must operate within the bounds of reasonable interpretation. ” Utility Air Regulatory Group v. EPA, 573 U. S. ___, ___ (2014) (slip op., at 16) (internal quotation marks omitted). EPA strayed far beyond those bounds when it read §7412(n)(1) to mean that it could ignore cost when deciding whether to regulate power plants. A The Clean Air Act treats power plants differently from other sources for purposes of the hazardous-air-pollutants program. Elsewhere in §7412, Congress established cabined criteria for EPA to apply when deciding whether to include sources in the program. It required the Agency to regulate sources whose emissions exceed specified numerical thresholds (major sources). It also required the Agency to regulate sources whose emissions fall short of these thresholds (area sources) if they “presen[t] a threat of adverse effects to human health or the environment. . . warranting regulation.” §7412(c)(3). In stark contrast, Congress instructed EPA to add power plants to the program if (but only if) the Agency finds regulation “appropriate and necessary.” §7412(n)(1)(A). One does not need to open up a dictionary in order to realize the capaciousness of this phrase. In particular, “appropriate” is “the classic broad and all-encompassing term that naturally and traditionally includes consideration of all the relevant factors.” 748 F. 3d, at 1266 (opinion of Kavanaugh, J.). Although this term leaves agencies with flexibility, an agency may not “entirely fai[l] to consider an important aspect of the problem” when deciding whether regulation is appropriate. State Farm, supra, at 43. Read naturally in the present context, the phrase “ap-