he Clean Air Act, 42 U.S.C. 7401, allows each state to craft a state implementation plan to control the levels of certain air pollutants. Most state plans include “Prevention of Significant Deterioration” (PSD) programs, to prevent backsliding in “attainment areas” that meet or exceed the Act’s air quality standards, while allowing some new sources of pollution. A PSD program prevents designated sources from propelling the region’s aggregate emissions over specified limits. The Act establishes these limits by setting a baseline and a cap on pollutants above that baseline. The Act grandfathers sources operational before 1975: the baseline incorporates their emissions, with post-1975 sources counting against the allowance. Title V of the Act requires each covered stationary source to have an operating permit. In 2002 Georgia-Pacific asked Wisconsin to renew the Title V permit for its pre-1975 paper mill. Meanwhile, Georgia-Pacific modified a paper machine at the plant. The application was unopposed and the modification permit issued in February 2004. In 2011 Wisconsin reissued the plant’s operating permit. Objectors claim that modifications to any part of a plant require all emissions from the plant, including pre-1975 emissions incorporated into the baseline, to count against the state’s allowance, so that the whole plant might need to close for lack of available allowance. The U.S. EPA declined to object, concluding that Wisconsin’s approach is consonant with its understanding of the statute. The Seventh Circuit denied a petition for review. EPA presented a reasonable interpretation of an ambiguous statutory provision.