The Clean Air Act’s program of pollution reduction also seeks to protect natural resources and ecosystems, including those conserved by national parks. The Act generally directs the EPA to take regulatory action to reduce air pollution where emissions would endanger “public health and welfare.”

Moreover, the CAA explicitly protects the unique vulnerability of national parks in two provisions. In the Prevention of Significant Deterioration program, the Act requires a higher degree of air quality protection for units of the National Park System, designating them as either Class I or Class II areas, with Class I areas having the highest level of safeguards. 42 U.S.C. §§ 7472, 7474. Similarly, the Act provides special protections to prevent and remediate any impairment of visibility for national park units designated as mandatory Class I areas. 42 U.S.C. § 7491.

Read the court filing here.