Montgomery County officials said yesterday that slightly elevated but "nondangerous" levels of the odorless gas radon were detected in recent weeks at 31 school buildings in the county during systemwide testing. School Superintendent Harry Pitt said that additional testing will be done at the facilities and that foundation repairs will be made soon at one school -- Clarksburg Elementary School in the upper county, where the highest reading was taken.

Radon, a naturally occurring breakdown product of uranium, is believed to be responsible for 5,000 to 20,000 lung cancer deaths a year across the country, and to be second only to smoking as a cause of that disease. Students, who spend a limited time in classrooms, are not at serious risk from radon, said the county health director, Dr. Martin P. Wasserman. The effects of exposure are cumulative, making it largely dangerous to persons who have had lifetime exposure to elevated levels 75 to 90 percent of the time, according to EPA guidelines.

Tests at 174 school system buildings showed levels of between 4 and 25.3 picocuries in at least one room at 31 schools. The federal Environmental Protection Agency has issued guidelines indicating that no action need be taken at levels under 4 picocuries, and that additional testing should be done above that level, school officials said. EPA recommends taking remedial action, such as sealing foundation cracks and improving ventilation, within a few years if levels remain elevated.

Wasserman and county Environmental Protection Director John Menke stressed that children can receive their highest exposure to radon at home, and urged county residents to have their houses tested for the gas. Kits are available commercially for relatively low cost, Menke said.

Here are the schools where radon readings have been above 4.0 in at least one classroom:

Elementary schools: The former Radnor school (now housing the Bannockburn Elementary School while that facility is remodeled), Barnsley, Beall, Cannon Road, Clarksburg, Cresthaven, Damascus, East Silver Spring, Fairland, Forrest Knolls, Gaithersburg, Glen Haven, Glenallan, Highland View, Jackson Road, the Kay Center (now housing New Hampshire Estates and Rolling Terrace), Oak View, Oakland Terrace, Takoma Park.

Middle, junior and intermediate schools: Martin Luther King, Ridgeview, Westland, White Oak.

High schools: Springbrook, Walter Johnson.

Other facilities: Kingsley Wilderness; Mark Twain; Phoenix II; Smith, Rocking Horse Road and Taylor centers; Whittier Alternative; and Randolph and Bethesda Maintenance depots.