A model for indoor radon variations

Abstract

The model relates radon concentration to variations in source strength, air exchange rate, and meteorological factors. The diffusion source represents radon diffused from building materials or from soil. The pressure-difference driven flow represents radon flowing with soil pore air and driven by the stack effect. In a house with diffusion source, the radon concentration decreases when the air exchange rate increases due to increasing temperature differences, whereas the flow source causes an increasing concentration. This is due to the fact that the effect of the source strength increase is stronger than the decreasing effect of air exchange on concentration. The winter-summer concentration ratio depends on the combination of the two types of source. A pure pressure-difference driven flow gives a winter-summer ratio of 2–3 (winter-5°C, summer+15°C, wind speed 3 m/s). A strong contribution of a diffusion source is needed to cause a summer concentration higher than the winter concentration. The model is in agreement with the winter-summer concentration ratios measured. This ratio increases with the increasing winter concentration. The results indicate that radon concentration must be taken into account in analyses of seasonal variations of indoor radon. In houses with a diffusion source, the diurnal maximum occurs in the afternoon; in houses with a pressure-difference driven flow, the maximum is reached in the early morning.