In a major blow to the tobacco industry, an EPA scientific advisory board formally endorsed an agency study that concluded breathing smoke from other people's cigarettes can cause lung cancer.

The panel, which evaluates the accuracy of Environmental Protection Agency studies, said Thursday the EPA risk assessment correctly interpreted available scientific evidence in finding "secondhand smoke" a cancer-causing agent in humans."The committee concurs with the judgment of the EPA that environmental tobacco smoke should be classified as a (known human) carcinogen," the Science Advisory Board told EPA Administrator William Reilly in a letter.

The panel also agreed with the study's finding that mothers who smoke around their children increase the youngsters' vulnerability to respiratory disease.

However, the board did not accept more specific findings of the EPA risk assessment, in particular, an estimate that about 3,700 American adults die each year from exposure to secondhand smoke.

Board members said the magnitude of the risk to non-smokers was not clear, but that occasional exposure - such as sitting in the smoking section of a plane - poses little serious threat.

"Occasional, light exposure is not likely to cause any harm," said Dr. Morton Lippmann, a specialist in the effects of toxic air pollution at New York University.

However, he said evidence showed sustained, long-term exposure - such as living with a smoker - measurably increased a non-smoker's risk of lung cancer.

"We're not saying the risk is enormous," Lippmann said. "We're saying there is a believable risk."