The Clean Air Act requires EPA to promulgate and periodically revise national ambient air quality standards ('NAAQS') for each air pollutant identified by the agency as meeting certain statutory criteria. See Clean Air Act §§ 108-09, 42 U.S.C. §§ 7408-09. For each pollutant, EPA sets a 'primary standard'--a concentration level 'requisite to protect the public health' with an 'adequate margin of safety'--and a 'secondary standard'--a level 'requisite to protect the public welfare.' Id. § 7409(b).