In the Bay Area, as in the entire state of California, a certain amount of air pollution comes from stationary industrial sources, such as refineries and power plants. But a greater percentage of harmful air emissions come from cars and trucks, construction equipment, and other mobile sources. California has more cars per household (1.8) than any other state, along with a thriving business economy and a continually expanding population. All of these factors contribute to the state and regional air quality challenges.