The Environmental Protection Agency and the Justice Department filed lawsuits against seven of the nation's largest utility companies yesterday, alleging that for years they have illegally poured pollutants into the nation's air.

"Today we file the first complaints in one of the largest investigations in the history of EPA, launched two years ago and still ongoing," said EPA Administrator Carol M. Browner at a press conference. Attorney General Janet Reno added, "We are asking federal courts in five cities to force these utilities to install the pollution controls they should have installed years ago."

The government alleges that the companies--American Electric Power Co., Cinergy Corp., FirstEnergy Corp., Illinova Corp., Southern Co., TECO Energy Inc. and Southern Indiana Gas & Electric Co.--have been upgrading their coal-fired plants without adding modern pollution controls as required by the Clean Air Act of 1970. The EPA also filed an administrative order against the Tennessee Valley Authority, a government-owned corporation, charging similar violations.

The companies argue that they have merely been doing routine maintenance at the plants, hence the newer pollution controls are not required. The 32 plants in question are located in West Virginia, Georgia, Illinois, Indiana, Kentucky, Mississippi, Ohio, Tennessee and Alabama.

The companies could face fines of $25,000 per day for each violation before 1997, and $27,500 per day thereafter.

But Browner said in an interview that the agency was more interested in getting the plants to install controls than in collecting fines. "These pollutants don't stay in the neighborhood where the smokestack is located. They travel. The air we breathe in Washington is affected by these pollutants hundreds of miles away."

The companies disputed what they called EPA's "new interpretation" of the Clean Air Act. Dale Heydlauff, American Electric Power's vice president for environmental affairs, said the lawsuits "take decades of industry maintenance and repair practices previously deemed acceptable by EPA and retroactively converts them to unlawful activities."

Mike Casey, vice president for public affairs at the Washington-based Environmental Working Group, said that since 1992, the companies have been spewing out the equivalent of the smog produced by 6.8 million cars.

"These utilities have been cheating on a decades-old agreement with the American public," he said. "They were let off the hook in terms of key air pollution standards on the understanding they would not upgrade their facilities. And that is what they have been doing, but they have been categorizing it as routine maintenance."

CAPTION: EPA Administrator Carol M. Browner, at lectern, and Attorney General Janet Reno explain lawsuit.