Cite as: 531 U. S. 457 (2001)
Opinion of the Court
ary practices in the area, implicitly incorporated into the statute, were reflected in the regulations, 332 U. S., at 250. The idea that an agency can cure an unconstitutionally stand-ardless delegation of power by declining to exercise some of that power seems to us internally contradictory. The very choice of which portion of the power to exercise—that is to say, the prescription of the standard that Congress had omit-ted—would itself be an exercise of the forbidden legislative authority. Whether the statute delegates legislative power is a question for the courts, and an agency's voluntary self-denial has no bearing upon the answer.
We agree with the Solicitor General that the text of § 109(b)(1) of the CAA at a minimum requires that "[f]or a discrete set of pollutants and based on published air quality criteria that reflect the latest scientific knowledge, [the] EPA must establish uniform national standards at a level that is requisite to protect public health from the adverse effects of the pollutant in the ambient air." Tr. of Oral Arg. in No. 99-1257, p. 5. Requisite, in turn, "mean[s] sufficient, but not more than necessary." Id., at 7. These limits on the EPA's discretion are strikingly similar to the ones we approved in Touby v. United States, 500 U. S. 160 (1991), which permitted the Attorney General to designate a drug as a controlled substance for purposes of criminal drug enforcement if doing so was " 'necessary to avoid an imminent hazard to the public safety.' " Id., at 163. They also resemble the Occupational Safety and Health Act of 1970 provision requiring the agency to " 'set the standard which most adequately assures, to the extent feasible, on the basis of the best available evidence, that no employee will suffer any impairment of health' "—which the Court upheld in Industrial Union Dept., AFL-CIO v. American Petroleum Institute, 448 U. S. 607, 646 (1980), and which even then-Justice Rehnquist, who alone in that case thought the statute violated the nondelegation doctrine, see id., at 671 (opinion concurring in judgment), would have upheld if, like the statute
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