In a victory for coal-fired power plants, the Supreme Court has accepted a case challenging plant emission rules. At issue: emissions of mercury and other toxic pollutants. 

There are about 600 power plants that burn coal or oil and send such toxins up their smokestacks. The EPA rule, expected to take effect next year, limits how much. Plants have to install expensive equipment to comply, or shut down.

It costs an estimated $10 billion dollars a year. Industry groups and more than 10 states argue EPA did not appropriately consider those costs during the regulatory process. They counter that they did, and that benefits to society in lives saved and illness averted outweigh the costs by at least 4:1.

If EPA wins, observers say it may embolden the agency to pursue more ambitious air quality rules in the future. If industry prevails, the implications may be less clear. More than 6 in 10 plants already have installed pollution controls that would comply with the rules.

And the bigger threat to coal plants is not the government, but cheaper natural gas.

“The fundamental challenge is just this massive supply of natural gas now coming out of the ground in the U.S affecting the price of electricity,” Bloomberg New Energy Finance analyst Ethan Zindler says. “That is the overriding trend that will be impossible of the coal industry to escape from.”

An industry win may have significant legal effect, potentially exposing all EPA pollution rules on pollutants such as soot, sulfur and carbon dioxide.

“Other air rules will be challengeable based on EPA’s process being improper,” says Brandon Barnes of Bloomberg Intelligence. “And utilities challenging these rules will be able to go back to the courts on existing rules and proposed rules.”

Separately, on Wednesday, the EPA issues new, more aggressive limits on ozone emissions, also known as smog. The coming year may bring CO2 limits on existing and future plants. Put together, EPA’s air pollution standards are considered a signature environmental initiative for this administration.