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Section 111 of the Clean Air Act, 42 U.S.C. § 1857c-6 (1970) (now codified as amended at 42 U.S.C. § 7411). Since 1967 the centerpiece of the Clean Air Act has been the provision for setting National Ambient Air Quality Standards (NAAQS). NAAQS define ambient concentrations of harmful substances at levels determined to be necessary to protect public health and welfare. NAAQS are enforced by the states through emission limitations set on a source-by-source basis at a level which ensures that the ambient concentrations defined by NAAQS are not exceeded. By 1970 there was concern that NAAQS would constrain economic growth by limiting the construction of new sources as ambient concentrations of a designated substance approached the NAAQS, and by imposing costly requirements for retrofitting additional control technology on existing sources in cases where ambient concentrations of a substance did increase to meet or exceed NAAQS.
In addition, Congress was concerned that the NAAQS system of air quality regulation would place states with relatively cleaner air at an economic advantage, since these states could attract industry by setting less stringent emission limits. The Environmental Policy Division, Congressional Research Service, The Library of Congress, 93d Cong., 2d Sess., A ...
