L.A. Ranked Nation's Sixth Most Toxic City

Criteria for the list range from air quality to the number of areas no longer in use that contain hazardous waste known as superfund sites. California took 4 spots on the list primarily because of smog, a chronic air quality problem in many major metropolitan cities.

Philadelphia tops the list for an excess of superfund sites and poor water quality, followed by Bakersfield, Fresno, New York, and Baton Rouge.

Riverside-San Bernardino rounded out the top ten.