How Does Radon Get Into My House?
The following are excerpts from The Geology of Radon, from the US Geologic Survey.
Because radon is a gas, it has much greater mobility than uranium or radium, which are fixed in the solid matter in rocks and soils.
Radon can more easily leave the rocks and soils, by escaping into fractures and openings in rocks and into the pore
spaces between grains of soil. The ease and efficiency with which radon moves in the pore space or fracture affects
how much enters a house. If radon is able to move easily in the pore space, then it can travel a great distance before it decays,
and it is more likely to collect in high concentrations inside a building.
Radon moving through soil pore spaces and rock fractures near the surface of the earth usually escapes into the atmosphere.
Where a house is present, however, soil air often flows toward its foundation for three reasons: differences in air pressure
between the soil and the house, the presence of openings in the house's foundation, and increases in permeability around the
basement (if one is present).
Most houses draw less than one percent of their indoor air from the soil; the remainder comes from outdoor air,
which is generally quite low in radon. Houses with low indoor air pressures, poorly sealed foundations, and
several entry points for soil air, however, may draw as much as 20 percent of their indoor air from the soil.
Even if the soil air has only moderate levels of radon, levels inside the house may be very high.