More than a fifth of houses checked in a 10-state survey contained potentially dangerous levels of radon, a radioactive gas that can cause lung cancer, the Environmental Protection Agency reported yesterday.

EPA officials said the survey of 11,600 houses detected unsafe radon levels in every state, even in areas where soil characteristics were not considered conducive to radon formation. One percent of the houses showed radon levels of more than five times the EPA guideline, enough to suggest that homeowners should act quickly to reduce the risk of cancer.

"These findings indicate that radon may be a problem in virtually every state," said deputy administrator A. James Barnes. "It reinforces our earlier belief that people who think their homes may have a radon problem should test and take proper actions, if necessary."

The survey showed that elevated radon levels were most common in Colorado, where 39 percent of houses exceeded the EPA guideline, and least common in Alabama (6 percent) and Michigan (9 percent). The survey also covered Connecticut, Kansas, Kentucky, Rhode Island, Tennessee, Wisconsin and Wyoming.

Maryland, Virginia and the District of Columbia were not included in the EPA survey. Virginia has told the EPA it intends to conduct an independent survey. In a preliminary report released last week, the Fairfax County health department said it found radon levels above the EPA guideline in one-third of the houses it tested.

Radon is a colorless, odorless gas produced by the decay of uranium in the soil. As the gas decays, it gives off radioactive particles that can be inhaled and lodge in the lungs. The EPA estimates that 5,000 to 20,000 lung cancer deaths each year are caused by radon, which is believed to be the leading cause of lung cancer among non-smokers.

The gas occurs naturally outdoors, where it poses only a minimal health risk. Inside a house, however, the gas can seep in through foundation cracks or drains and accumulate to dangerous levels.

The EPA has set a guideline of 4 picocuries per liter as the level at which homeowners should consider taking action to reduce radon levels. A picocurie is a measurement of radiation. In terms of cancer risk, regular exposure to 4 picocuries is the equivalent of smoking about eight cigarettes a day, or enough to cause cancer in 13 to 50 of every 1,000 persons exposed to that level for a lifetime.

The risk increases sharply at higher radon levels. At 10 picocuries per liter, the cancer risk is as high as one in 10; at 200 picocuries per liter, it is as high as seven in 10.

"We feel it is a fairly significant health problem," said Richard Guimon, director of the EPA's radon action program.

EPA officials have previously estimated that 8 to 12 percent of the 75 million houses in the United States may have radon levels above the 4 picocurie-per-liter guideline. Despite the higher percentage found in the 10-state survey, Barnes said the agency thinks that its estimate is still valid because some of the readings would drop if averaged over a full year.

The EPA survey was done in the winter months, when radon levels are generally highest because of reduced ventilation during the heating season.

As expected, the survey found that elevated radon levels are most common in areas underlain with soil and rocks that are relatively high in uranium, including central and eastern Tennessee and much of Colorado and Wyoming.

But the agency said it found "hot spots" in some states where relatively few houses exceeded the EPA guideline. For example, one house in Alabama showed radon levels of 180 picocuries per liter -- the highest in the survey -- although the state had the lowest overall incidence of elevated radon.

EPA officials said the results suggest that while geology is a "good indicator of high-risk areas," there is no way to determine individual risk without testing individual houses.

"You can have two houses side by side, and one house may have a serious radon problem while the next has not much at all," Guimon said. "Testing the individual house is the only way for homeowners to be sure they don't have a radon problem."

The EPA has certified more than 350 firms that supply and analyze radon-testing equipment, at a cost of from $11 to $50.

Corrective measures may be as simple as sealing foundation cracks or as elaborate as installation of underground ductwork and ventilation fans.

In addition to Colorado, Alabama and Michigan, the surveyed states and their percentage of radon readings above the EPA guideline were: Wisconsin (27 percent), Wyoming (26), Kansas (21), Connecticut (19), Rhode Island (19), Kentucky (17) and Tennessee (16).