Owing to geography, heavy reliance on automobiles, and the Los Angeles/Long Beach port complex, Los Angeles suffers from air pollution in the form of smog. The Los Angeles Basin and the San Fernando Valley are susceptible to atmospheric inversion, which holds in the exhausts from road vehicles, airplanes, locomotives, shipping, manufacturing, and other sources. Unlike other large cities that rely on rain to clear smog, Los Angeles gets only 15 inches (381 mm) of rain each year: pollution accumulates over many consecutive days. Issues of air quality in Los Angeles and other major cities led to the passage of early national environmental legislation, including the Clean Air Act. More recently, the state of California has led the nation in working to limit pollution by mandating low emission vehicles.
As a result, pollution levels have dropped in recent decades. The number of Stage 1 smog alerts has declined from over 100 per year in the 1970s to almost zero in the new millennium. Despite improvement, the 2006 annual report of the American Lung Association ranks the city as the most polluted in the country with short-term particle pollution and year-round particle pollution. In addition, the groundwater is increasingly threatened by MTBE from gas stations and perchlorate from rocket fuel. With pollution still a significant problem, the city continues to take aggressive steps to improve air and water conditions.
