An evening view of the Exxon Mobil oil refinery complex in Baton Rouge, La. John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

The Houston Ship Channel is home to a wide range of heavy industry, including chemical processing plants and petrochemical refineries. Richard Harris/NPR hide caption

itoggle caption Richard Harris/NPR

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

The Continental Carbon plant sits on the southern outskirts of Ponca City, Okla. Until August, the plant was on an internal EPA "watch list," for violating rules of the Clean Air Act. 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