The Fairfax County health department has found indoor radon levels above the federally recommended minimum safety levels in nearly one-third of the homes it surveyed throughout the county.

Radon, which has become a major health concern throughout the United States in the last two years, has been found by scientists and health officials to cause lung cancer.

The preliminary report, released to the county's Board of Supervisors this week, found higher-than-acceptable radon concentrations in portions of Herndon, Reston, Vienna, Chantilly and Centreville, as well as at other spot locations throughout the county. However, Fairfax County health officials said that not all homes in those areas necessarily have a radon problem.

Homes in portions of Springfield, Mt. Vernon, Lorton and most areas east of Interstate 95 had generally lower concentrations of radon, according to the study.

In the study conducted last winter, county health investigators found that 32 percent of the 1,030 homes tested had radon levels in varying amounts above the recommended range set by the Environmental Protection Agency. Families living in those homes discovered to be above the federal safety limit are likely exposed to the equivalent of at least 400 chest X-rays per year.

A senior EPA official, when informed of the Fairfax County test results, said county residents should consider having their homes evaluated for the presence of radon.

"The figures are fairly high," said Richard J. Guimond, director of the agency's radon program. "They indicate that the area has quite a bit of radon if they're getting into those types of figures."

Steven Church, senior sanitarian of the toxic substance program of the county's health department, said that further testing by the county is needed before any conclusions or recommendations can be made. "The implications of this report are not yet clear," said Church, whose department conducted the study. "There are areas of the county we are concerned about, but we just don't know the extent of {the radon problem} yet."

Church said the second phase of the county's radon study will be conducted this winter.

The Fairfax County results closely mirror the findings from a similar report released last month of homes in Montgomery County. Such studies have been carried out in the Washington metropolitan area and elsewhere in recent years following the revelations by federal and state agencies of the dangers of radon gas.

Radon, which is created by the breakdown of uranium deposits in the soil, is a naturally produced, odorless, colorless gas. The EPA has said that radon is the leading cause of lung cancer in nonsmokers, and estimates that the inert gas is responsible for as many as 20,000 lung cancer deaths in the United States each year. The agency last year found that one in eight American homes has radon levels equal to the danger of smoking half a pack of cigarettes daily.

The Fairfax study revealed that one-third of the homes it tested had radon levels above four picocuries per liter of air, an amount at which the EPA recommends that corrective action be taken by homeowners within one to two years. A family living in a house with a radon level of four picocuries is exposed to the radiation equivalence of 400 chest X-rays per year.

The study also found that about 2 percent of the homes studied had radon levels higher than 20 picocuries, equivalent to smoking two packs of cigarettes a day or receiving about 2,000 chest X-rays in one year.

Investigators discovered one home in the county that contained radon levels at a concentration high enough to equal the risk of exposure to more than 20,000 chest X-rays per year, or about 400 cigarettes per day, according to EPA estimates. At that level, EPA recommends taking immediate action to remove the radon levels.

Fairfax County's Church declined to identify the location of that home. "I wouldn't want to label an area of the county {as a health hazard}," said Church, who added that the home's residents have since corrected the high radon levels.

Exposure to radon gas has long been considered a health hazard to mine workers, but has only emerged as a major health issue in the past few years. Concern among homeowners grew two years ago following the well-publicized case of a Pennsylvania resident who kept triggering radiation alarms at the Limerick nuclear power plant where he worked. Plant officials were stymied in their attempt to find the cause of the readings until tests of the worker's home revealed that it contained exceptionally high levels of radon.

Radon, an inert gas, usually seeps into homes through cracks or holes in the foundation, then breaks down, giving off radioactive particles that often lodge in the lungs.

Dr. Douglas Mose, a George Mason University geologist who has been conducting his own radon tests in the Washington area, said the implication of the Fairfax study "is that one of every three homes {contains} radon at high enough levels to warrant some radon lowering {action}".

During a similar period as the county's study last winter, Mose found that nearly 50 percent of the homes he tested had radon levels above the EPA guidelines. In some selected areas of Fairfax and Montgomery counties, those levels jumped to 75 percent.

"Even one year of exposure {above the EPA safety level} is estimated to have a small, but measureable, contribution to developing lung cancer," Mose said. He added that homeowners concerned about radon exposure should have their homes tested with inexpensive kits that sell for about $15.

The Fairfax report said that corrective measures should be proposed if next winter's tests reveal similar findings as those presented to the supervisors this week. Those proposals couldinclude rewriting county building regulations to incorporate some type of radon abatement procedure in the construction of new homes built in the county.

Theresa Brett, environmental health director at the National Capital Area's Lung Association, said the report "is something residents have a reason to be concerned about, but not alarmed."