Much of the public health focus in Houston has been on the hazards posed by flood waters, but the city is also facing a crisis in air quality. Even under normal circumstances, “air quality is a big problem in Houston,” says Loren Raun, the chief scientist for the Houston Health Department. “We put out more emissions than any other city.”

Much of the pollution is the result of refineries shutting down and restarting, which releases petrochemicals into the air. The Environmental Defense Fund warns that about half the refineries in the region still are not operating at full capacity, and as they restart facilities, they’ll release more pollution.