Environmental Engineering Reference
In-Depth Information
Figure 2.8
Radon entry pathways into a residential building.
supplies, and building materials. Radon can enter building environments,
particularly residences, by pathways illustrated in
Figure 2.8
.
The dominant source of elevated radon levels in buildings is the soil
beneath and adjacent to the building. The potential for soils to emit radon-
222 depends on concentrations of uranium-238, thorium-232, and radium-
226, which are usually proportional (in concentration) to each other based
on the uranium decay series. The world average concentration for uranium-
238 and thorium-232 in soil is approximately 0.65 picocuries (pCi) per gram.
Local concentrations, however, vary widely from this average. The radon
source potential under an individual dwelling reflects concentrations of
radioactive parent isotopes in site soils.
As radon is produced, it moves through air spaces between soil particles.
The emanation power (radon that enters soil pores) of radon formed on or
in soil fragments depends on the soil type, pore volume, and water content.
Reported emanation powers vary from 1 to 80%. Soil gas measurements of
a few hundred to several thousand pCi/L have been reported.
Movement of radon through soil pores occurs by diffusion, convection,
or both. The movement of radon into building structures appears to be
primarily due to convection-induced pressure flows associated with
indoor-outdoor temperature differences and pressures associated with
wind speed.
Radon entry into buildings through building substructures is affected
by (1) soil radon production rates, (2) soil permeability, (3) cracks/fissures
in underlying geology, (4) the nature of the building substructure, and (5)
meteorological variables that cause indoor/outdoor pressure differences.
In general, higher radon levels can be expected in buildings constructed
on soils with high radon production rates. Though production rates are
