8) Milwaukee,  Wisconsin

Decades of heavy industrial development along the Milwaukee River has contributed to significant amounts of PCBs and heavy metals polluting ground water and draining towards Lake Michigan. Pollution continues to trouble the residents of Milwaukee.

7) Houston, Texas

Cattle industry, oil industry, traffic congestion and processing industry are the main causes of pollution in Houston. Oil refineries in Houston release harmful chemicals to the environment. Meat processing plants also add to the pollution of this sprawling metropolis.