All rocks contain some uranium, typically at concentrations of 1–3 ppm. The
uranium content of a soil will be about the same as the uranium content of the
rock from which the soil was derived. As radium-226, the immediate parent of
radon, is a decay product of uranium, the higher the uranium content of the soil
the greater the radium concentration and the higher the chance that houses built
on such soil will have high levels of indoor radon. The main source of indoor
radon is the radon produced by the decay of radium in the soil subjacent to a
house. Soil gas containing radon enters a house through cracks and fractures in
the foundations by pressure-driven flow, as the air in a house is generally warmer
and therefore at a lower pressure than the subjacent soil gas (13). Radon concentrations
in soil air/gas typically range from less than 10 000 Bq/m3 up to 100 000
Bq/m3. Most houses draw less than 1% of their indoor air from the soil; the remainder
comes from outdoor air, which is generally quite low in radon. Houses
with poorly sealed foundations, built on high-permeability ground and with several
entry points for soil gas may draw more than 10% of their indoor air from
the soil. Even if the soil air has only moderate levels of radon, levels inside such
houses may be very high.