From this site:
http://www.ecy.wa.gov/programs/air/c...tive_pages.htm
"Where is testing required?
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. "
http://www.ecy.wa.gov/programs/air/pdfs/Zip_codes07.pdf