The fate of a key rule to limit toxic air pollution hinges on the interpretation of two words

A seemingly divided Supreme Court weighed the Obama administration's first-ever regulations aimed at reducing power plant emissions of mercury and other hazardous air pollutants that contribute to respiratory illnesses, birth defects and developmental problems in children.

The rule, known as Mercury and Air Toxic Standards (MATS), was the first federal limit on the amount of these types of toxic air pollutants.

The justices heard arguments Wednesday in a challenge brought by industry groups and Republican-led states arguing the EPA should have considered costs, which could reach $9.6 billion a year, when limiting emissions.

But the benefits are much greater, $37 billion to $90 billion annually, the EPA said. The savings stem from the prevention of up to 11,000 deaths, 4,700 nonfatal heart attacks and 540,000 lost days of work, the EPA said. Mercury accumulates in fish and is especially dangerous to pregnant or breastfeeding women, and young children, because of concern that too much could harm a developing brain.

The dispute stems from a sentence at the heart of the Clean Air Act: "The Administrator shall regulate electric utility steam generating units under this section, if the Administrator finds such regulation is appropriate and necessary after considering the results of the study required by this subparagraph."

The petitioners -which include Michigan and 20 other states as well as organizations like the National Mining Association - think the EPA misinterpreted the words "appropriate and necessary."

In their most recent petition to the court, they state the EPA "treats the word 'appropriate' as meaningless and ignores a factor - costs - that Congress intended [the] EPA to consider." The EPA also "unreasonably fail[ed] to give 'appropriate' any meaning beyond that already ascribed to 'necessary.'"

Several conservative justices questioned whether EPA should have taken costs into account when it first decided to regulate hazardous air pollutants from power plants, or whether health risks are the only consideration under the Clean Air Act. The EPA did factor in costs at a later stage when it wrote standards that are expected to reduce the toxic emissions by 90 percent.

Justice Antonin Scalia was critical of the agency's reading of the provisions of the anti-air pollution law at issue in the case throughout 90 minutes of arguments. "It's a silly way to read them," Scalia said.

The court's four liberal justices appeared more comfortable with EPA's position, leaving Justice Anthony Kennedy as the possible decisive vote.

Solicitor General Donald Verrilli Jr. said EPA followed the same process in deciding whether to regulate other sources of emissions, including from motor vehicles.

The case is the latest in a string of attacks against the Obama administration's actions to rein in pollution from coal-burning power plants that harms health and contributes to global warming. The administration is seeking to use the Clean Air Act for the first time to control mercury and carbon pollution from the nation's power plants.

But numerous states have already filed challenges to a proposed rule to curb the pollution linked to global warming from the nation's coal-burning power plants. And Congress is working on a bill that would allow states to opt out of any rules clamping down on heat-trapping carbon dioxide.

The legal and political challenges ahead could undermine US efforts to inspire other countries to control their emissions, as they head into negotiations in Paris on a new international treaty later this year.

A disproportionate share of the 600 affected power plants, most of which burn coal, are in the South and upper Midwest. Michigan Attorney General Bill Schuette, representing 21 states at the Supreme Court, said the law requires the EPA to take account of costs before deciding whether to step in. The states and industry groups also said the agency overstated the benefits of reducing mercury emissions.

Shuttering older plants or installing pollution-control equipment also will reduce emissions of particulate matter, such as dust, dirt and other fragments associated with a variety of respiratory ailments. The administration said it properly took those benefits into account, but the challengers argued that they are not relevant to the case.

Chief Justice John Roberts called the inclusion of those other benefits an "end run" around more stringent procedures EPA would have to follow to try to reduce emissions of particulate matter.

Several utilities that already have installed the equipment or that primarily rely on natural gas and nuclear power to make electricity said the EPA rules are economically practical. Moreover, they said that until the rules take effect their competitors who haven't yet complied with the rules have an unfair advantage. Another 16 states and several large cities also are backing the administration.

Congress first ordered the EPA to study the release of mercury among 180 toxic substances in 1990. The agency initially decided to go ahead with the limits on power plant emissions in 2000, the final year of the Clinton administration.

After President George W. Bush took office in 2001, the EPA tried to undo its earlier decision, but the U.S. Court of Appeals in Washington blocked that attempt. When Barack Obama became president in 2009, the agency again decided to move forward. It issued final rules in 2012, and the appeals court upheld them last year.

A decision is expected by the end of June.