Washington ? The Bush administration on Wednesday made it easier for thousands of older power plants, refineries and factories to avoid having to install costly clean air controls when they replace aging equipment.

In a major revision to its air pollution rules, the Environmental Protection Agency will allow up to 20 percent of the costs of replacing each plant’s production system to be considered “routine maintenance” not requiring expensive anti-pollution controls, according to agency documents and interviews with EPA officials.

The new rule signed Wednesday by the EPA’s acting administrator, Marianne L. Horinko, could be applied to about 17,000 facilities nationwide and culminates decades of debate over a controversial Clean Air Act program. Electric utilities and oil companies have been urging the White House to revise the program, saying the costs prohibit them from making energy-efficiency improvements.

The change represents a fundamental shift away from a long-problematic 1971 maintenance standard. “We’re going to really, I think, create certainty going forward for industrial facilities, by spelling out what specific replacement is exempt,” Horinko told The Associated Press.

Environmentalists say the exemption will allow power plants in the Midwest and South to continue emitting millions of tons of pollutants that cause health problems for people living downwind.

New York Atty. Gen. Eliot Spitzer immediately threatened to sue the Bush administration in an effort he said would include other states. Spitzer and other attorneys general have already filed suits challenging earlier changes the administration made to the program.