The Environmental Protection Agency has chosen Fairfax County as a model in developing national guidelines to test for potentially lethal radon in schools.

An EPA official said the agency selected Fairfax in part because of the well-publicized radon problem at West Springfield Elementary School, where high levels of the gas were detected in November in tests independently conducted for the school's PTA.

The school system conducted tests that followed EPA procedures for home radon testing because the federal agency has no specific guidelines for schools. The county tests had found that levels of the gas were safe at West Springfield and other schools throughout the system.

"It made the problem of the schools more visible," said Mike Mardis, an environmental scientist in the EPA's radon division.

Radon is a naturally occurring gas that is considered by scientists and health officials to be the leading cause of cancer among nonsmokers. The colorless gas, produced by the breakdown of uranium deposits in the soil, seeps into buildings through walls or foundations.

Agency officials said funds recently became available to set up procedures for measuring radon in the schools. The project in Fairfax will take four to six months to complete, according to school officials.

The agency plans to install hundreds of radon detectors in three or four Fairfax schools, which are to be selected at a later date. West Springfield will not be among the schools because efforts are already under way there to get rid of the gas.

"We want a school just as it is," said Melinda Ronca-Battista, an EPA health physicist. "The major question is how radon levels vary from room to room and how radon levels vary over time . . . . "

Radon detectors, which will include four types, are to be placed in every ground-level classroom. The experiment should determine the link between radon and such factors as the school's design, the heating and ventilation systems and the terrain surrounding the school.

Fairfax will be the only school district equipped with EPA-sponsored detectors, but Montgomery County school officials also have expressed a willingness to volunteer their radon test results, EPA officials said.

Based on the EPA's investigation in Fairfax, schools throughout the country will have a game plan to follow when testing radon levels.

To a great extent, the recent radon initiatives are the result of the skepticism and the aggressiveness of the PTA at West Springfield.

The elementary school was among 178 schools that the county tested for radon between August 1986 and June 1987. Although several schools, including West Springfield, initially had some radon readings at potentially unhealthy levels, subsequent test results showed the schools were safe.

The school's PTA wanted to be sure. "It was kind of like, 'Gee, do you think we ought to put a few more {test kits} around the school?' " said PTA President Judy Pensabene. The PTA placed 30 radon test canisters in classrooms, while the last county test in the school had used one canister in an identified problem area.

The results of the PTA-backed tests showed high levels of the radioactive gas in seven classrooms, some more than quadruple federal safety guidelines. "I was surprised. I did not expect that at all," said Pensabene, a mother of three children who has practiced as an oil and gas industry lawyer.

As a result, classrooms got additional monitors and students were moved from some classrooms to trailers. Children only recently returned to three classrooms where radon levels have been reduced by sealing cracks and ventilating rooms.

"It's a little frightening to think it might have gone undetected," Pensabene said.

PTAs from other schools since have made inquiries about radon retesting, but no problems have been discovered, said Doug Thorpe, who coordinates the school district's radon mitigation efforts.

The school system does not plan to retest all the schools as a result of the difference in the county and PTA-initiated test results, said Larry Byers, the school district's director of administrative services. But Byers added that schools are being retested that have measured over the action level of 4.0 picocuries per liter of air.

Officials said the cost of corrective action at West Springfield has not been determined.

The radon episode at West Springfield Elementary illustrates how "you have to take a good, hard look at a building," Thorpe said. "We've learned that there are more things to look at than the way we originally looked at it. It's evident we learned something from it." HOW SCHOOL DISCOVERED PROBLEM

SEPTEMBER 1987: Potentially harmful radon levels found at five Fairfax schools in county test.

OCTOBER: School officials announce that follow-up tests show schools are safe.

NOVEMBER: PTA of West Springfield conducts independent radon tests, placing 30 test canisters around school. The county had used just one canister in identified trouble spot.

DECEMBER: PTA-backed tests find potentially dangerous levels in several classrooms.

JANUARY 1988: About 75 pupils at West Springfield Elementary are moved to trailers while steps are taken to reduce radon levels in three classrooms. Students have since returned.