Potentially dangerous levels of radon have been found at five Fairfax County schools, and officials said yesterday that they will take steps to ventilate the buildings if further monitoring confirms that a problem still exists.

At five additional schools, a first round of testing detected unhealthy amounts of radon, but a second measurement found the gas at a level not considered dangerous, according to results released by the Fairfax County Federation of Teachers.

Radon, a colorless and odorless gas produced by the breakdown of uranium deposits in the soil, seeps into buildings via walls or foundations and can build to dangerous levels, particularly in modern, airtight structures. Long-term exposure is considered hazardous, and radon is believed to be the leading cause of lung cancer in nonsmokers.

A recent survey in Fairfax found that one-third of homes tested had levels above the minimum the federal government considers safe.

The county tested its 178 schools between August 1986 and June, placing several charcoal canisters throughout each building to measure radon levels.

At 10 schools, the radon reading on one canister was higher than four picocuries per liter of air, which is the maximum level the U.S. Environmental Protection Agency considers safe.

Seven schools were retested, and readings at five of them dropped below the four picocuries level, according to results released by the teachers union.

Schools with two readings above four picocuries per liter will be monitored, and if the readings persist, "something will be done," said Alton C. Hlavin, assistant superintendent in charge of building maintenance.

Hlavin said the high readings generally came from canisters placed in unoccupied low-lying parts of buildings, such as boiler rooms, where the worst possible result might be expected.

None of the five schools that exceeded four picocuries showed readings above 6.1 picocuries, well below the 20 picocuries the EPA has set as the level requiring immediate corrective action.

James O. Bowman, the county's director of environmental health, said that even if the school system needs to correct the problem, which generally can be done by ventilating the building's foundation, the repair does not have to be done for a year or two because short-term radon exposure to that level of gas is thought to pose no problem.

Douglas G. Mose, a geology professor who heads a radon testing project at George Mason University, said he would recommend retesting even those schools with a one-time reading above four picocuries, because measurements vary depending on factors such as the season of the year, with the highest readings found in winter and the lowest in summer.

Mose said that if his child's school were tested, he would like to see continued readings no higher than two picocuries, "a fairly easy number to obtain for large buildings."

Rick Nelson, president of the Fairfax County Federation of Teachers, said he will ask school system officials to retest all schools with initially high readings.

Nelson said he also wants retesting in schools without air-conditioning, where testing may have been done during warm months, after windows were left open to ventilate the building, which lowers radon levels.

Alexandria tested its schools for radon last spring and found only one result above four picocuries, in the gymnasium of an administration building, a spokesman said yesterday.

Arlington school officials said they plan to test for radon this year. Testing plans are in the works but not final in Prince George's and Montgomery counties, spokesmen said.

Spokesmen for Loudoun and Anne Arundel county school systems say they have no plans to test for radon. Officials of other area school systems could not be reached for comment.

Fairfax has no immediate plans to test for radon in other government buildings such as health centers, offices and senior citizen centers because it is concentrating its resources on a testing program for 1,200 homes, Bowman said.Staff researcher Dianne Saenz contributed to this report.