Cite as: 540 U. S. 461 (2004)
Opinion of the Court
gram is to "protect public health and welfare from any actual or potential adverse effect which in [EPA's] judgment may reasonably be anticipate[d] to occur from air pollution . . . notwithstanding attainment and maintenance of all national ambient air quality standards"). Before 1977, no CAA provision specifically addressed potential air quality deterioration in areas where pollutant levels were lower than the NAAQS. Alabama Power Co. v. Costle, 636 F. 2d 323, 346- 347 (CADC 1979). Responding to litigation initiated by an environmental group,2 however, EPA issued regulations in 1974 requiring that SIPs include a PSD program. Id., at 347, and n. 18 (citing 39 Fed. Reg. 42510 (1974)). Three years later, Congress adopted the current PSD program. See S. Rep. No. 95-127, p. 11 (1977) (Congress itself has "a responsibility to delineate a policy for protecting clean air").
The PSD program imposes on States a regime governing areas "designated pursuant to [42 U. S. C. § 7407] as attainment or unclassifiable." § 7471.3 An attainment area is one in which the air "meets the national primary or secondary ambient air quality standard for [a regulated pollutant]." § 7407(d)(1)(A)(ii). Air in an unclassifiable area "cannot be classified on the basis of available information as meeting or not meeting the national primary or secondary ambient air quality standard for the pollutant." § 7407(d)(1)(A)(iii). Northwest Alaska, the region this case concerns, is classified as an attainment or unclassifiable area for nitrogen dioxide, 40 CFR § 81.302 (2002); therefore, the PSD program applies to emissions of that pollutant in the region. In 2002, the Agency reported that "[a]ll areas of the country that once
2 Sierra Club v. Ruckelshaus, 344 F. Supp. 253 (DC 1972), aff'd per curiam, 4 E. R. C. 1815, 2 Env. L. Rep. 20656 (CADC 1972), aff'd by an equally divided court sub nom. Fri v. Sierra Club, 412 U. S. 541 (1973) (per curiam).
3 The PSD program also requires visibility control measures, 42 U. S. C. §§ 7491-7492, not at issue in this case.
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