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-efficient 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, particularly in the Northeast.

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

“If allowed to stand, this flagrantly illegal rule will ensure that, under the watch of the Bush administration, Americans will breathe dirtier air, contract more respiratory disease and suffer more environmental degradation caused by air pollution,” said Spitzer, a Democrat.

Jeff Holmstead, the EPA’s assistant administrator in charge of air quality, said the rule was meant to let a plant replace a piece of equipment with something identical or functionally equivalent, as long as the plant remains within its pollution permit limits and the basic operating design remains the same.

“We can say categorically that pollution will not increase as a result of this rule,” he said.

Congress put the Clean Air Act’s “new source review” program into law in 1977. The agency has had mixed success in enforcing the maintenance provision.

Until now, operators have been required to add more pollution-cutting devices if they do anything more than “routine maintenance” on a plant and cause emissions to increase significantly.

The White House-led reworking of the maintenance standard essentially allows industries – including manufacturers, chemical plants and pulp and paper mills – to modernize one-fifth of a facility’s essential production systems at a time.

They can do so even if the upgrades increase emissions, and with no apparent restrictions on time intervals between modernization. Horinko, Holmstead and other top EPA officials said the plants still must comply with overall pollution permit limits and other state and federal programs for pollutants.

Environmentalists and health advocates, however, said emissions could increase and still be within the plant’s permitted limits. They described the new changes as disastrous for people’s health and said the EPA ignored concerns expressed by hundreds of thousands of Americans opposed to the new regulations.

“EPA is throwing in the towel to industry just as its own enforcement of the existing rules has proven successful in the courts,” said John Kirkwood, president of the American Lung Association. “EPA policy should be based on protecting public health, not bolstering industry profits.”

During the Clinton administration, the federal government began suing 51 aging power plants and succeeded in forcing several to install hundreds of millions of dollars’ worth of pollution-control equipment.

Just this month, the first ruling against a utility in those cases came from a federal judge in Ohio who said Akron-based FirstEnergy Corp. violated the law when it upgraded seven coal-fired power plants in the name of routine maintenance without installing anti-pollution equipment.