California dominates the American Lung Association's list of cities with the poorest air quality in America. California has the nation's worst 4 cities for ozone pollution (including 6 of the top 10), worst 4 cities for year-round particle pollution (including 6 of the top 10), and worst 2 cities for short-term particle pollution (including 8 of the top 10). This is in spite of the states' aggressive environmental policies, which are commonly praised as the best in the nation. Some of the other factors that impact California's air quality include severe wildfires, mountain ranges that trap pollutants, and warm weather that generates ozone.