A federal appeals court gave the Environmental Protection Agency the final go-ahead Tuesday to issue more stringent air quality health standards after a five-year legal fight that went all the way to the Supreme Court.

The appeals court rejected several remaining challenges to the new standards first issued by the EPA in 1997 and quickly protested by industry groups.

The standards will require states and local authorities to impose tougher controls on smog-causing chemicals and microscopic soot.

The changes have been in limbo because of the court challenges waged against the standards suggested they had no definitive scientific basis and would be exceedingly expensive to business.

The Supreme Court ruled a year ago that the EPA under the Clean Air Act had no requirement to take into account costs when issuing health standards and had acted reasonably in trying to protect certain population groups including small children, the elderly and people with respiratory problems.

In the latest court decision, a three-judge panel of the U.S.Court of Appeals rejected claims the EPA had acted arbitrarily in determining the level of the new standards. While technically, the decision could be appealed, lawyers expected no such action since the Supreme Court already had decided on the broader issues of the case.

"We are gratified that after more than four years of litigation, the court has affirmed these standards, implementation of which will improve the lives of millions of Americans who suffer asthma, bronchitis and other respiratory illnesses," said Tom Sansonetti, the Justice Department's assistant attorney general for environment and natural resource.

The American Lung Association has estimated that together the tougher requirements on smog-causing ozone and microscopic soot will prevent 15,000 premature deaths, 350,000 cases of aggravated asthma and perhaps as many as a million cases of children having decreased lung functions.

EPA Administrator Christie Whitman has embraced the new rules, which had been a top priority of her predecessor, Carol Browner, who headed the EPA during the Clinton administration.

After the Supreme Court upheld the rules in February, 2001, Whitman called them key to the EPA's "efforts to protect the health of millions of Americans from the dangers of pollution.

Environmentalists said they hoped the EPA will move swiftly to determine where state and local officials will have to take additional measures to assure their air meets the new federal standards.

Scores of counties that have been in compliance with the old standard will no longer meet the new federal air quality requirements, federal officials and environmentalists acknowledge. The EPA has waited for the court challenges to be resolved, before assessing where additional pollution control measures will be needed.

"This puts to rest finally any claim that the standards are not based on sound science and sound law," said Vickie Patton, an attorney for Environmental Defense. "The legal wrangling by industry lawyers has delayed critical progress in delivering cleaner, healthier air."

The long legal fight shows "how industry can throw everything it has against a public health standard and in the course of doing so they managed to delay this process for many years," added Howard Fox, an attorney for Earthjustice Legal Defense Fund, who represented the American Lung Association in the litigation.

The 1997 standards limited ozone, a major component of smog, to 0.08 parts per million instead of 0.12 parts per million under the old requirement. It also changed the monitoring period from 12 hours to eight hours to better reflect actual air quality.

States and local authorities also were required to limit microscopic soot from power plants, cars and other sources down to 2.5 microns in diameter, or 28 times smaller than the width of a human hair.

The standards had been challenged by the American Trucking Associations, the U.S. Chamber of Commerce, the National Association of Manufacturers and the states of Michigan, Ohio and West Virginia.