WASHINGTON -- Environmentalists argued before the Supreme Court on Wednesday that electric utilities knowingly cheated on federal pollution laws to keep dirty power plants operating, while the industry complained that it was being held to unreasonable and illegal standards in a suit accusing it of violating the Clean Air Act.

In a case that will affect such basic concerns as public health and the cost of electric power, lawyers for Duke Energy complained that the Environmental Protection Agency was using the wrong formula to calculate pollution from old generating units that the company refurbished in the 1980s.

At times, the arguments revolved around issues of jurisdiction and bureaucratic definitions that even the justices seemed to find murky.

When Sean Donahue, a lawyer for three environmental organizations, referred to some regulations as "clear on their face," Chief Justice John Roberts drew laughter from lawyers on both sides by remarking, "That's an audacious interpretation."

The case could affect 10 states where power companies are being sued for violating the act: Ohio, North Carolina, Alabama, Indiana, Kentucky, Pennsylvania, South Carolina, Tennessee, Virginia and West Virginia.

It grew out of Clinton administration lawsuits filed in 1999 and 2000 that accused owners of coal-burning power plants in the South and Midwest of cheating on Clean Air Act provisions.

Amendments to the law passed by Congress in 1977 had "grandfathered" old power plants out of new pollution control requirements. However, the companies were required to obtain EPA permits and install new pollution controls if the old plants were modernized and generated more pollution.

The lawsuits claimed the companies had expanded the plants while claiming the changes were only "routine maintenance." In doing so, the companies had generated more than 1 million tons of illegal pollution, former Attorney General Janet Reno charged.