WASHINGTON — The Environmental Protection Agency on Thursday issued new standards for power plants in 27 states that would sharply cut emissions of chemicals that have polluted forests, farms, lakes and streams across the Eastern United States for decades.

The agency said the regulations, which will take effect in 2012, would reduce emissions of compounds that cause soot, smog and acid rain from hundreds of power plants by millions of tons at an additional cost to utilities of less than $1 billion a year. The E.P.A. said the cleaner air would prevent as many as 34,000 premature deaths, 15,000 nonfatal heart attacks and hundreds of thousands of cases of asthma and other respiratory ailments every year.

Lisa P. Jackson, the E.P.A. administrator, said the new rule would improve air quality for 240 million Americans living in states where the pollution is produced or where it travels downwind.

“No community should have to bear the burden of another community’s polluters, or be powerless to prevent air pollution that leads to asthma, heart attacks and other harmful illnesses,” Ms. Jackson said. “This is a long-overdue step to protect the air we breathe.”