Americans are exposed to only about a third as much radon inside their homes as monitoring kits indicate, and many people have probably spent money needlessly to get rid of the gas, a study suggests.

The Environmental Protection Agency recommends that homeowners put radon monitors in their basements, where levels of the odorless gas are likely to be highest.Now, a researcher has performed the first study to see how much radon people - rather than basements - are exposed to. She found human exposure was about 30 percent of the radon levels found in the basements.

"I don't think you should remediate a home based on basement levels," said Dr. Naomi H. Harley, a radiological health expert at New York University Medical Center. "A single basement reading is not representative of personal exposure."

She presented her findings Wednesday at a meeting sponsored by the American Cancer Society.

Radon gas is released during the decay of radium, which is present in soils and rocks in many places. It seeps into homes from the ground and can reach dangerous levels in unventilated spaces.

The EPA estimates that radon gas causes 20,000 cases of lung cancer annually in the United States. The figure is based on lung cancer rates among miners who are exposed to high levels of radon.

The EPA recommends that vents and fans be installed in homes with high levels, which costs about $1,000. Based on basement readings, the EPA estimates perhaps 20 percent of homes should have radon vents. She said the true figure should be about 7 percent.