Several environmental and conservation groups are suing the U.S. Environmental Protection Agency (EPA) over air pollution in Minnesota’s Voyageurs National Park and Isle Royale, Minnesota Public Radio reported.

The suit filed in federal district court in St. Paul claims the pollution comes from Xcel Energy’s power plant in Sherburne County. Known locally as Sherco, it is the biggest plant in Xcel’s fleet; it burns three trainloads of coal a day.

Three years ago, the National Park Service determined the plant contributes to haze in Voyageurs and Isle Royale. Under the Clean Air Act, the EPA can force Xcel to install pollution controls. But it has not done so.

Kevin Reuther, an attorney representing the National Parks Conservation Association and other groups, said Xcel could install scrubbers, switch to natural gas or close the plant.

“We would love it if Xcel decided that they want to shut Sherco down and replace it with renewable energy sources and more efficiency,” Reuther said.

Reuther said it’s not just the parks that are being harmed.

“This is the stuff that we’re breathing,” Reuther said. “This causes lung disease, heart disease; there are major public health consequences from continuing to emit these pollutants.”

In a statement, Xcel Energy said Sherco does not cause haze in the parks, but the company is installing controls that will cut haze-causing pollution in half.