Motor vehicles are Washington's largest source of air pollution. The Emission Check Program tests factory-installed emission control systems on motor vehicles, and identifies vehicles whose systems are not working properly. This program is estimated to prevent more than 149,000 tons of air pollution in Washington per year. Emission testing is required by federal law in areas that have had problems meeting health-based air quality standards. In Washington, emission testing is required in the urban areas of Clark, King, Pierce, Snohomish, and Spokane counties.Automobile emission check areas are designated by zip code where the vehicle is registered. In Clark County, three zip codes cover only partial areas: 98604 - Except north of NE 279th St.; 98629 - Except east of NE 50th Avenue; 98671 - Except Skamania County. A complete list is available at <http://www.ecy.wa.gov/programs/air/cars/zip_codes.htm>. More information on automobile emission testing can be found at <http://www.ecy.wa.gov/programs/air/airhome.html>.