"An urban skyline dotted with puffing smokestacks isn't the only measure of a city's cleanliness (or lack thereof). Most major cities suffer from a range of unseen hazards. Contaminants can seep into the ground from bygone chemical spills or shuttered steel mills. Invisible leaks at industrial complexes discharge harmful substances into the air, or the normal course of business requires factories to expel toxins that eventually find their way to the water supply."
The ranking was based on the number of superfund sites in each city, the number of facilities reporting toxic releases, the total weight of toxic releases made by those facilities, and a ranking based on the number of days in 2007 that overall air quality was at unhealthy levels.