"During the Revolutionary War Philadelphia served as one of America's first capital cities. These days, however, Philadelphia could be considered the capital of toxicity, since the city and its environs ranked No. 1 on our 2011 Most Toxic Cities list. One big reason: The sprawling Philadelphia-Camden-Wilmington Metropolitan Statistical Area (MSA), including parts of four states (Pennsylvania, New Jersey, Delaware and one county in Maryland), is pocked with more than 50 Superfund sites---areas no longer in use that contain hazardous waste.
While the East Coast metro, with its old industrial sites, grabbed the top spot, California metropolitan areas claimed four of the 10 spots on our Most Toxic list, primarily due to the chronic air quality problem known as smog."