The Clean Air Act (CAA) was founded on the principle that a maximum safe national ambient air quality standard (NAAQS) concentration existed for each pollutant. Once those concentrations were defined, emission limitations for individual plants could be determined that ensured those concentrations would not be exceeded in ambient air. The CAA gives the states the primary authority to define the emission limitations necessary to meet the NAAQS in their airsheds. The collection of emission limits forms the state implementation plan (SIP) that, when approved by the U.S. Environmental Protection Agency (EPA), becomes enforceable by EPA and the public as well as the state. Key features of SIP rules are (1) the emission limitations ensure compliance with, or attainment of, the NAAQS both within the state and in the air of its neighbors, and (2) the emission standards apply to all emission sources, both existing and new.