The Environmental Protection Agency expects Friday to finalize a rule requiring coal- and oil-fired power plants to cut emissions of 84 toxic chemicals, including mercury and other pollutants.

The rule comes 21 years after Congress amended the Clean Air Act to require the cleanup. The EPA estimates the changes will keep up to 17,000 people from dying prematurely each year and prevent 11,000 non-fatal heart attacks. It also estimates that for every $1 spent to clean up the plants, $3 to $5 will be saved in health costs.

“We think they will be historic clean air and public health achievements,” says John Walke, with the Natural Resources Defense Council, an environmental group.