Beginning today, about 75 pupils at West Springfield Elementary School in Fairfax County will attend classes in trailers as school officials take steps to reduce the amount of radon that was found recently in several classrooms.

Testing conducted in November for the school's PTA showed a range from 4.4 to 18.4 picocuries of radon per liter of air. The school system conducted tests during the week of Dec. 15, finding high levels of the radioactive gas in three classrooms and prompting use of the temporary classrooms.

U.S. Environmental Protection Agency officials recommend that corrective measures be taken when more than 4 picocuries is found, but they have said that immediate action is not needed unless the levels measure above 200 picocuries.

Radon, a colorless, odorless gas produced by the breakdown of uranium deposits in the soil, is considered by scientists and health officials to be the leading cause of lung cancer among nonsmokers. The gas is thought to seep into buildings through walls and foundations.

Based on EPA recommendations, the school system has started work to seal the cracks and joints where the floors and the walls meet in the rooms that have shown high gas levels. In addition, exhaust and ventilation fans with fresh, heated air will be installed.

The repair work and retesting of the rooms are expected to take 14 days, school officials said in a statement last week. During that period, about 75 students will be moved to three trailers.

A second set of tests by the school system was conducted from Dec. 21 to 24 in three additional classrooms, but officials said they found lower levels of the gas.

The results of the new tests showed averages for classrooms to be 2.7, 1.9 and 3.1 picocuries of radon per liter of air. The ranges for the testing levels were not available during the weekend, but they will be supplied to the public at a later date, said school system spokeswoman Dolores Bohen.