The Clean Air Act defines an air quality control region (AQCR) as a contiguous area where air quality, and thus air pollution, is relatively uniform. In those cases where topography is a factor in air movement, AQCRs often correspond with airsheds. AQCRs may consist of two or more cities, counties or other governmental entities, and each region is required to adopt consistent pollution control measures across the political jurisdictions involved. AQCRs may even cross state lines and, in these instances, the states must cooperate in developing pollution control strategies. Each AQCR is treated as a unit for the purposes of pollution reduction and achieving National Ambient Air Quality Standards. As of 1993, most AQCRs had achieved national air quality standards; however the remaining AQCRs where standards had not been achieved were a significant group, where a large percentage of the United States population dwelled. AQCRs involving major metro areas like Los Angeles, New York, Houston, Denver, and Philadelphia were not achieving air quality standards because of smog, motor vehicle emissions, and other pollutants.