The California Smog Check Program is a joint effort between the Bureau of Automotive Repair, Department of Motor Vehicles, and Air Resources Board to reduce vehicle emissions and air pollution. The program requires biennial smog tests for cars built in 1976 or earlier. Exceptions include electric, hybrid and pre-1998 diesel vehicles, and gasoline cars from 1975 or older. Smog tests monitor toxic gases like hydrocarbons, carbon monoxide, nitrogen oxides and carbon dioxide that are linked to health issues. The $10 billion annual program aims to improve air quality in California, which has the worst air quality of any US state.