A George Mason University geologist has found indoor radon levels above the federally recommended safety level in up to half of the nearly 1,500 homes tested during the past year in Montgomery and Fairfax counties.

Radon, a naturally occurring radioactive gas that has become a major health concern in many parts of the United States, is considered by scientists and health officials to be the leading cause of lung cancer among nonsmokers. The Environmental Protection Agency has identified radon, which seeps into homes from the ground, as one of the nation's most serious environmental health hazards.

During the recent summer testing period, the study found that radon levels exceeded the EPA safety range in about one-fourth of the houses checked, according to Douglas Mose, the geology professor who has been conducting the independent study since November.

Mose said his study has generally found radon in the western sections of both counties to be at levels "twice as high" as those in the eastern sections.

In Fairfax County, higher radon levels were found in portions of McLean, Great Falls, Reston, Burke, Clifton and Fairfax City. In Montgomery County, potential locations for high radon levels were discovered in parts of Rockville, Gaithersburg, Kensington, Potomac, Boyds, Spencerville, Damascus and Clarksburg.

Radon levels are normally much lower in summer months, in part because homes are better ventilated, thereby allowing the gas to escape.

In addition, the "chimney effect" generally does not occur in homes in the summer. In the winter, warm indoor air rises and escapes to the outside. That creates a vacuum in the basement, and radon gases are drawn into the house from the ground.

Last winter, Mose found that 45 percent to 50 percent of the Washington area homes tested had radon levels exceeding the EPA safety range.

The Fairfax and Montgomery county homes with high radon levels contained concentrations of more than four picocuries per liter of air. A picocurie is a measurement of radioactivity.

A family living in a house with a radon level of four picocuries is exposed to the radiation equivalent of 400 chest X-rays a year. The EPA has estimated that if 1,000 people lived their entire lives in houses with radon levels of four picocuries, 25 could be expected to die from lung cancer caused by exposure to the gas.

The EPA also has recommended that corrective action be taken within one to two years if a home contains radon concentrations of four picocuries or more.

Mose also found that, depending on the season, between 2 percent and 17 percent of the homes tested had radon levels above 10 picocuries, equivalent to smoking about one pack of cigarettes a day or receiving about 1,000 chest X-rays in one year. Mose said he has been surprised by his findings because they have revealed radon concentrations higher than past EPA estimates for homes in the area. His test results, however, have been similar to those in studies funded by Montgomery and Fairfax counties and released in the past several months.

Richard J. Guimond, director of the EPA's radon program, said the George Mason geologist's radon figures could be higher than EPA estimates because Mose used voluntary participants instead of a random selection of homes. "I'm not trying to discredit the study because ... there's no question that there is plenty of radon {in the Washington area}," Guimond said. "There is clearly a fair amount of radon, and people are going to have to deal with it."

Radon is produced by the natural breakdown of uranium in the soil. It is odorless, tasteless and invisible. Radon is found in areas containing certain rock formations, as well as in soils contaminated with some kinds of industrial wastes.

Radon exists outdoors, but in relatively low levels. Indoors, however, radon gases accumulate, depending largely on an area's soil type and a home's construction. Radon breaks down and forms radioactive decay products, which can become lodged in the lungs.

The EPA has estimated that 5,000 to 20,000 lung cancer deaths each year in the United States are caused by the inert gas. Last year, the agency found that one in eight American homes has radon concentrations equal to the danger of smoking half a pack of cigarettes daily.

In the Mose study, higher radon levels were found in homes built above schist and phyllite, two types of rock known to emit substances that produce radon gas. Such rock formations are found in western Montgomery and Fairfax counties.

Mose acknowledged that not all health officials are convinced of the dangers posed by radon. Mose cautioned that homeowners should not panic over his test results because radon levels can vary even on a particular block, depending upon a home's construction and the rock type upon which it sits.

"The chances of finding that you don't have a radon problem is pretty good," said Mose, who is finishing the last part of his year-long study. "On the other hand, the health risks are significant enough that homeowners should be encouraged to at least get a measurement of their house."

Mose said such tests are simple and inexpensive, ranging from $25 to $50. He advised homeowners to test their houses over a period of several months in order to get an accurate, seasonally adjusted figure. Testing in the autumn or spring, Mose said, will usually give a reading that is close to the average for the year.

Homeowners can set up the testing devices themselves and send the results to a private company for analysis. Mose said the monitors should be placed in a home's basement, where radon levels are generally about one-third higher than those on the first floor.

If a radon problem exists, levels can be reduced easily and without much expense, Mose said. For existing homes, the fastest way is to openwindows and ventilate the building.

For longer-term remedies, homeowners should patch holes and cracks in foundations and caulk around pipes that lead from a house into the ground. In more serious cases, homeowners can install a small vacuum below the basement floor to expel radon gases. The pumps sell for about $200, not including installation.

Mose said radon levels also should be checked in homes under construction because problems can be solved more easily and less expensively before the building is finished