US top court invalidates Obama administration mercury air pollution rule

The U.S. Supreme Court on Monday invalidated a key Obama administration environmental regulation aimed at limiting emissions of mercury and other hazardous pollutants mainly from coal-fired power plants.

The justices ruled in a 5-4 decision that the U.S. Environmental Protection Agency should have weighed the cost of compliance in deciding whether to regulate the pollutants.

The court sent the case back to the U.S. Court of Appeals for the District of Columbia Circuit, which will then ask the EPA to reconsider its rule-making.

Industry groups and some states appealed after an appeals court upheld the regulation in June 2014.

Among companies opposing the rule is Peabody Energy Corp, the nation's largest coal producer. Exelon Corp, the biggest U.S. nuclear power plant operator, is one of several power companies that support the rule.

The 2012 mercury regulation, which covers oil-fired plants as well as coal-burning ones, was targeted by Michigan and other states in addition to various industry groups, including the National Mining Association.