WASHINGTON – A federal appeals court on Friday struck down the Bush administration’s strategy for reducing smog, which affects the health of more than half the nation’s population, mostly those prone to asthma and other respiratory illnesses.

The Environmental Protection Agency rules for forcing state cleanups of smog don’t meet Clean Air Act requirements, a three-judge panel ruled in a suit brought by a Southern California clean-air agency, environmental groups and some mid-Atlantic and Eastern states, including Maine, downwind of other states’ smog.

Circuit Judge Judith Rogers, writing for the panel of the U.S. Court of Appeals for the District of Columbia, said, “EPA has failed to heed the restrictions on its discretion set forth in the Act.” The court ordered the agency to come up with a new enforcement plan.

Smog is produced by nitrogen oxides reacting with other chemicals to create ground-level ozone, particularly in the summer months when the sun is hottest and brightest. Other major sources of the pollution are motor vehicle exhaust, gas vapors and chemical solvents.

In the stratosphere, ozone occurs naturally and is a beneficial shield against harmful ultraviolet rays.

The EPA in 2004 issued new smog standards requiring 474 counties with about 159 million people living in them to reduce pollution of smog-causing ozone and improve the air enough to meet federal health standards. But the agency gave them three to 17 years to do so.

The appeals court panel said that was too long. It also said the agency was allowing “backsliding” by states when it should have been directing them to order new emission controls on industrial plants, more public transportation, tougher vehicle inspection programs or cleaner-burning gasoline. States that fail to meet the standards risk losing federal highway dollars under the law.

Tougher standards for ground-level ozone and fine soot were ordered by the Clinton administration in 1997 after it determined that levels set in 1979 didn’t adequately protect children, the elderly and people with respiratory ailments. The Supreme Court in 2001 upheld the tougher standards after they were challenged by industry.

EPA spokeswoman Jennifer Wood said Friday that the agency will review the panel’s decision to determine whether it will ask the full appeals court to rehear the case. “EPA is committed to ensuring our nation’s ozone air quality standards are implemented to protect public health and the environment,” she said.

Sen. Barbara Boxer, D-Calif., who in January becomes chairwoman of the Senate Environment and Public Works Committee, said she was pleased the court had “seen through EPA’s transparent attempts to weaken” the clean air law. She promised that the Democratic Congress would closely monitor the agency as it comes up with a new plan for reducing smog.

“Smog kills people, it increases asthma and other respiratory illnesses, and it remains a major public health threat in many areas of the country,” Boxer said. “Sadly, we have once again had to rely on the court to tell EPA how to read the text of the Clean Air Act in a way that protects people, not polluters.”

Plaintiffs in the suit were California’s South Coast Air Quality Management, the American Lung Association, Environmental Defense, the Sierra Club, the Natural Resources Defense Council, the District of Columbia and the states of Massachusetts, Connecticut, Delaware, Maine, New York and Pennsylvania.