A potentially dangerous level of radon gas has been discovered in a Prince George's County middle school, and school officials said yesterday that all 171 county schools will be tested for the gas within three weeks.

The radon was discovered Tuesday in two rooms at Walker Middle School in Capitol Heights during a science project conducted by students in the school's talented and gifted magnet program and a television reporter, said Patricia Palmer, associate superintendent for supporting services.

The science project tests showed radon concentrations of more than 43 picocuries per liter of air in a teacher's lounge and one classroom, Palmer said. A subsequent test conducted yesterday by the Environmental Protection Agency showed a concentration of 13.5 picocuries per liter, said school spokeswoman Bonnie Jenkins. The EPA recommends that corrective action be taken when more than 4 picocuries of radon per liter are found in a building.

The students in the one classroom were moved to another room, where they will stay until some action is taken, school officials said yesterday. School officials sent the students home with letters about the discovery for their parents.

Radon is a colorless, odorless radioactive gas produced by the natural breakdown of bedrock. The gas seeps into buildings through walls or foundations and has been suspected as a cause of lung cancer in people exposed to it over many years.

Palmer said county school officials had long ago planned to test for radon but had not scheduled a meeting to discuss a start until March 2. That meeting is still scheduled and the testing will begin within two weeks after that and be completed in a month, she said.

The schools, working in conjunction with the county health department, will conduct the tests using state guidelines modeled after those used by Montgomery County.

Fairfax County was chosen by the EPA this month to be used as a model in developing national guidelines to test for radon in schools.

Prince George's school spokesman Brian J. Porter said the cost of testing had not been determined yet, but the system will have to buy equipment, install it and monitor it.