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Areas that measure air quality above a National Ambient Air Quality Standard (NAAQS) are classified by the United States Environmental Protection Agency (U.S. EPA) as nonattainment. A nonattainment area is subject to more rigorous requirements under the federal Clean Air Act (CAA) than areas classified as attainment or unclassifiable. A nonattainment area’s classification does not change until it meets a specific set of criteria and is re-designated by U.S. EPA.