472
Opinion of the Court
III
Section 109(b)(1) of the CAA instructs the EPA to set "ambient air quality standards the attainment and maintenance of which in the judgment of the Administrator, based on [the] criteria [documents of § 108] and allowing an adequate margin of safety, are requisite to protect the public health." 42 U. S. C. § 7409(b)(1). The Court of Appeals held that this section as interpreted by the Administrator did not provide an "intelligible principle" to guide the EPA's exercise of authority in setting NAAQS. "[The] EPA," it said, "lack[ed] any determinate criteria for drawing lines. It has failed to state intelligibly how much is too much." 175 F. 3d, at 1034. The court hence found that the EPA's interpretation (but not the statute itself) violated the non-delegation doctrine. Id., at 1038. We disagree.
In a delegation challenge, the constitutional question is whether the statute has delegated legislative power to the agency. Article I, § 1, of the Constitution vests "[a]ll legislative Powers herein granted . . . in a Congress of the United States." This text permits no delegation of those powers, Loving v. United States, 517 U. S. 748, 771 (1996); see id., at 776-777 (Scalia, J., concurring in part and concurring in judgment), and so we repeatedly have said that when Congress confers decisionmaking authority upon agencies Congress must "lay down by legislative act an intelligible principle to which the person or body authorized to [act] is directed to conform." J. W. Hampton, Jr., & Co. v. United States, 276 U. S. 394, 409 (1928). We have never suggested that an agency can cure an unlawful delegation of legislative power by adopting in its discretion a limiting construction of the statute. Both Fahey v. Mallonee, 332 U. S. 245, 252-253 (1947), and Lichter v. United States, 334 U. S. 742, 783 (1948), mention agency regulations in the course of their nondelegation discussions, but Lichter did so because a subsequent Congress had incorporated the regulations into a revised version of the statute, ibid., and Fahey because the custom-
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