The federal Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to establish and maintain national air quality standards, including criteria for permits and other authorizations issued to (potential) emission sources by state or local air quality management agencies (with EPA itself as the default regulator if other agencies fail). Forms of authorization include permits for specified stationary emission sources, and equipment/emission standards for mobile sources and some components of stationary sources. Almost all requirements apply to “direct sources” – the equipment or activity that directly produces emissions.