WASHINGTON – The nation’s premier environmental law withstood a major industry challenge Tuesday as the Supreme Court upheld the way the government sets air-quality standards under the Clean Air Act.

The court unanimously rejected industry arguments that the Environmental Protection Agency must consider financial cost as well as health benefits in writing standards.

The American Lung Association called the ruling “a victory for the Clean Air Act and for the health of the American people.”

Maine was one of nine states that had filed a friend-of-the-court brief supporting the clean-air rules, and Gov. Angus King praised the court’s ruling Monday.

“This decision is good news for air quality in Maine,” King said. “We supported EPA’s position in this case because a national public health approach to clean air is indispensable to the effort to reduce air pollutants that are transported into Maine.

“We will closely follow EPA’s response to the part of the decision requiring the agency to rewrite its implementation strategy. I strongly urge EPA to act swiftly to develop a new implementation plan to improve air quality in Maine and throughout the nation.”

The Clean Air Act became law in 1970, and the challenge by industry groups was viewed as the most significant environmental case before the Supreme Court in years.

The justices rejected industry arguments that the EPA took too much lawmaking power from Congress when it set tougher standards for ozone and soot in 1997.

Nevertheless, the court threw out the EPA’s policy for implementing new ozone rules and ordered the agency to come up with a more “reasonable” interpretation of the law.

Edward Warren, the lawyer for industry groups that challenged the law, said they retain a right to challenge the ozone and soot standards in a lower court under traditional legal rules. “There’s a good chance that both of these standards will fall,” he said.

The American Trucking Associations, leader of the industry group, said it was “clearly disappointed” by the ruling. It said its goal in the case was “to obtain clear, understandable legal standards to promote clean air in a sensible fashion.”

Carol Browner, the Clinton administration’s EPA administrator who pushed through the tougher rules, called the decision “a clear vindication of the work we did for public health on air pollution over the last eight years.”

New EPA head Christie Whitman supported the standards when she was governor of New Jersey.

Frank O’Donnell of the Clean Air Trust environmental advocacy group, called the decision a “huge victory for breathers.”

Justice Antonin Scalia wrote for the court that the clean-air law “unambiguously bars cost considerations” from the process of setting air-quality standards.

The federal law, which “we interpret as requiring the EPA to set air quality standards at the level that is ‘requisite’ – that is, not lower or higher than is necessary – to protect the public health with an adequate margin of safety, fits comfortably within the scope of discretion permitted by our precedent,” Scalia wrote.

All nine justices agreed on the result of the ruling, although sometimes for different reasons.

In setting air-quality standards, the EPA is required to use criteria that “accurately reflect the latest scientific knowledge” for identifying pollution’s effects on health.

Business groups that long have chafed under the clean-air law argued that the EPA was setting standards without clear criteria and without considering the financial costs of complying with them.

Scalia said that even though the law bars the EPA from considering economic costs in setting clean-air standards, the agency can consider costs in its instructions for implementing the rules.

A federal appeals court ruled that the EPA went too far, interpreting the federal law so loosely that it took over Congress’ lawmaking authority. But the U.S. Circuit Court of Appeals for the District of Columbia also refused to require the government to weigh financial costs against health benefits.

The Supreme Court decided the appeals court was right in ruling the EPA could not consider costs in setting air-quality standards, but wrong in saying the agency unlawfully usurped Congress’ authority.

Scalia said the EPA’s authority was similar to the Federal Communications Commission’s authority to regulate the airwaves in the “public interest.”

On ozone, the justices ruled against the EPA’s implementation of revised ozone standards, saying the agency ignored a section of law that restricted its decision-making authority.

In addition, the lower court ruled that in setting the ozone standard, the EPA must consider any beneficial health effects of ozone, such as protection against skin cancer.

The 1997 air 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. States also were required to limit soot from power plants, cars and other sources to 2.5 microns, or 28-times smaller than the width of a human hair.

Industry groups that challenged the clean-air rules included the American Trucking Associations, the U.S. Chamber of Commerce, the National Association of Manufacturers and three states – Michigan, Ohio and West Virginia.

The cases are Whitman vs. American Trucking Associations, 99-1257, and American Trucking Associations vs. Whitman, 99-1426.