West Virginia State Police direct traffic at the entrance to Massey Energy's Upper Big Branch coal mine in Montcoal, W.Va., after an explosion April 5, 2010. Twenty-nine miners were killed in the blast. Jeff Gentner/AP hide caption

itoggle caption Jeff Gentner/AP

A haze can be seen at night hovering over the Asarco copper smelter, which turns copper ore into nearly pure copper bars. Emma Schwartz/Center for Public Integrity hide caption

itoggle caption Emma Schwartz/Center for Public Integrity

Jeff Galemore leans on his pickup truck near the Ash Grove Cement plant in Chanute, Kan. He and his family are concerned about the toxic emissions and are fighting for independent testing downwind. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

The Ash Grove Cement Kiln, as seen from an aerial photograph, sits on the northern edge of Chanute, Kan. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

The Continental Carbon plant sits on the southern outskirts of Ponca City, Okla. Residents blamed the plant, which produces a black dust known as carbon black, for polluting their city. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR