Radon test kits are usually just a metal canister filled with activated charcoal or a piece of foam rubber. The charcoal absorbs gases from the air, including radon and its daughters, for 24-72 hours. The canister is sealed and mailed off to a lab. There are versions of this test that can be left out for 30, 60, 90, and 180 days, so that a long-term average can be determined.

The California map shows the risk of getting a certain amount of ionizing radiation from radon sources by county, The hot spot is Santa Barbara and Ventura Counties. In general, the southwestern U.S., the Rocky Mountain region, and the Appalachian region are the areas of the U.S. that seem to be most at risk of producing radon-infested houses.