The surgeon general is urging Americans to test the air in their homes for elevated levels of radon, a natural radioactive gas believed to pose an increased cancer risk for at least 3 million households nationwide.

The health advisory was triggered by a new Environmental Protection Agency survey that found radon, previously believed to be a problem of a few Eastern states, is a nationwide health threat ranging from Massachusetts to Wyoming.Dr. Vernon Houk, assistant surgeon general, pointed out Monday that as many as 20,000 lung cancer deaths each year are caused by the colorless, odorless gas released by underground uranium deposits in rock and soil. Radon seeps into homes through cracks and openings in basements.

"Radon exposure raises the risk of lung cancer for a non-smoker to that of a smoker," Houk warned. "It is a serious national health problem. It exceeds by 10 times the threat posed by outdoor air pollution.

"I would not buy a home - I would not move into a home - without knowing what the radon content is in it."

Houk said radon poses even a more serious threat to smokers, who face cancer risks from the gas 15 times greater than non-smokers. He said while radon intake is normally exhaled, the gas can attach to smoke particles permanently lodged in smokers' lungs.

The radon problem was discovered in 1982 when a nuclear plant worker in eastern Pennsylvania set off radiation alarms when he reported for duty. After checking out all other causes, investigators found exceedingly high radon levels in his home.

Officials first thought the problem would affect only a limited area, but EPA Administrator Lee Thomas said the problem is national in scope.