Radon transport from soil to air

Radon generated within the upper few meters of the Earth's crust by the radioactive decay of radium can migrate during its brief lifetime from soil into the atmosphere. This phenomenon leads to a human health concern as inhalation of the short-lived decay products of radon causes irradiation of cells lining the respiratory tract. This paper reviews the factors that control the rate at which two radon isotopes, 222Rn and 220Rn, enter outdoor and indoor air from soil. The radium content of surface soils in the United States is usually in the range 10–100 Bq kg−1. The emanation coefficient, which refers to the fraction of radon generated in a material that enters the pore fluids, varies over a wide range with a typical value being 0.2. Radon in soil pores may be partitioned among three states: in the pore air, dissolved in the pore water, and sorbed to the soil grains. Except in the immediate vicinity of buildings, radon migrates through soil pores principally by molecular diffusion. Average reported flux densities from undisturbed soil into the atmosphere are 0.015–0.048 Bq m−2 s−1 for 222Rn and 1.6–1.7 Bq m−2 s−1 for 220Rn. Soil is the dominant source of radon in most buildings. Advective flow of soil gas across substructure penetrations is a key element in the transport process. The advective flow is driven by the weather (wind and indoor-outdoor temperature differences) and by the operation of building systems, such as heating and air conditioning equipment. A typical radon entry rate into a single-family dwelling of 10–15 kBq h−1 can be accounted for by weather-induced pressure-driven flow through moderately to highly permeable soils. The extent to which diffusion through soil pores contributes to radon entry into buildings is not known, but in buildings with elevated concentrations, diffusion is believed to be less important than advection.