468
Opinion of the Court
(refusing to infer in certain provisions of the CAA deadlines and enforcement limitations that had been expressly imposed elsewhere).
Accordingly, to prevail in their present challenge, respondents must show a textual commitment of authority to the EPA to consider costs in setting NAAQS under § 109(b)(1). And because § 109(b)(1) and the NAAQS for which it provides are the engine that drives nearly all of Title I of the CAA, 42 U. S. C. §§ 7401-7515, that textual commitment must be a clear one. Congress, we have held, does not alter the fundamental details of a regulatory scheme in vague terms or ancillary provisions—it does not, one might say, hide elephants in mouseholes. See MCI Telecommunications Corp. v. American Telephone & Telegraph Co., 512 U. S. 218, 231 (1994); FDA v. Brown & Williamson Tobacco Corp., supra, at 159-160. Respondents' textual arguments ultimately founder upon this principle.
Their first claim is that § 109(b)(1)'s terms "adequate margin" and "requisite" leave room to pad health effects with cost concerns. Just as we found it "highly unlikely that Congress would leave the determination of whether an industry will be entirely, or even substantially, rate-regulated to agency discretion—and even more unlikely that it would achieve that through such a subtle device as permission to 'modify' rate-filing requirements," MCI Telecommunications Corp. v. American Telephone & Telegraph Co., supra, at 231, so also we find it implausible that Congress would give to the EPA through these modest words the power to determine whether implementation costs should moderate national air quality standards. Accord, Christensen v. Harris County, 529 U. S. 576, 590, n. (2000) (Scalia, J., concurring in part and concurring in judgment) ("The implausibility of Congress's leaving a highly significant issue unaddressed (and thus 'delegating' its resolution to the administering agency) is assuredly one of the factors
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