Several sources of indoor radon have been described; from all but outdoor air, the resulting indoor concentrations (and hence exposures) depend on the combination of the source strength and the ventilation rate of the building. As noted earlier, the stack and wind effects are primarily responsible for the natural ventilation of buildings, in addition to providing a driving force for radon transport into buildings. It is useful to provide a context for these flows.
The steady-state solution to the first-order differential equation that describes indoor radon concentrations illustrates the key variables:
Ci is the indoor concentration (Bq m-3), Co is the outdoor concentration (Bq m-3), S is the radon entry or production rate (Bq per unit time, t), V is the house volume (m3), R is the removal rate (t-1).
Here R can account for any method of removal and is just the sum of the individual removal terms. In this case, R is the air-exchange rate (AER). A typical single-story house has a "footprint" of 120 m2 and a corresponding volume of about 300 m3. Annual average natural ventilation rates are about 0.9 h-1. Using those values for V and AER, the air flow rate (the product of V and AER) through this house is 270 m3 h-1.
The radon entry rate corresponding to the Environmental Protection Agency (EPA) guideline concentration of 150 Bq m-3 can be estimated from equation 8.1. Neglecting any contribution from outdoor air and using the ventilation rate described above, the radon entry rate, S, is about 40,000 Bq h-1 (about 11 Bq s-1). That can be compared with the estimates of diffusive radon entry. Assuming a floor area of 120 m2, the entry rate due to diffusion is about 1 Bq s-1, a small fraction of what is needed to produce an indoor air concentration of 150 Bq m-3.
Similarly, the soil-gas flow to produce this indoor concentration can be estimated. Assuming a typical value of about 40,000 Bq m-3 for the concentration of radon in soil gas, the soil-gas entry rate is about 1 m3 h-1, which is about 0.4% of the overall air flow rate into the house.
Conceptually, there are two approaches for mitigating indoor radon concentrations (or most other indoor pollutants, for that matter): source control and