The levels of radon in outside air varies markedly from place to place, however, people are mainly exposed to the dose from radon decay products indoors.

Radon concentrates in indoor air when buildings are, by and large, closed spaces. Once the gas gets in, by filtering up through floorboards from the ground or, to a much lesser extent, seeping out of the materials used to construct the building, it cannot get out. Modern construction techniques known for good airtightness and insulation, make it especially hard for radon to get out.