Modeling the effects of exhaust ventilation on 222Rn entry rates and indoor 222Rn concentrations.

Continuous mechanical exhaust ventilation of moderately air-tight residences is common in Scandinavia and has recently been employed in several U.S. residences. Exhaust ventilation decreases the indoor pressure and, thus, increases the pressure difference which drives 222Rn entry. Relatively simple analytical (i.e. closed form) models were developed and used to examine the impacts of exhaust ventilation on 222Rn entry rates and concentrations in houses with either a crawl space or a basement. A more complex finite difference model was also developed for the house with a basement and yielded soil gas entry rates within 16% of those predicted with the analytical model. The models indicate that exhaust ventilation is suitable, but not optimal (from the perspective of indoor 222Rn), for houses with a vented crawl space. Exhaust ventilation is also indicated to be suitable for houses with basements surrounded by relatively impermeable soil (10(-12) m2 or less). If soil permeabilities are between 10(-12) and 10(-10) m2, exhaust ventilation may cause substantial increases in indoor 222Rn when soil gas 222Rn concentrations are above average. If soil permeabilities are greater than 10(-10) m2, exhaust ventilation is indicated to be inappropriate, unless some procedure is employed to inhibit soil gas entry. These conclusions should be considered tentative, since extensive verification of the models is still required.