The U.S. Court of Appeals has ruled that the EPA violated the Clean Air Act in 2005 when it exempted coal plants from the strictest emission controls for mercury and other toxic substances like arsenic, lead and nickel.

The EPA's "Clean Air Mercury Rule" would have created a "cap-and-trade" program to allow utilities to swap rights to emit mercury to comply with overall limits that would reduce nationwide emissions by 70 percent by 2018. Some 14 states, including New York and California, sued the EPA over the rules, along with environmental and public health groups. The ruling adds to the U.S. backlash against building coal-fired power plants, which are also a major source of heat-trapping carbon dioxide emissions.