The lawsuit claims an upgrade in the No. 4 blast furnace at the mill in 1990 increased the emissions of nitric oxide, sulfur dioxide and particulate matter. Company spokeswoman Jill Ritchie said U.S. Steel has already addressed those concerns.
The EPA also accused the company of failing to report emissions. The lawsuit also makes similar claims against U.S. Steel mills in Granite City, Ill., and Ecorse, Mich.