Thoron exhalation from building material and related indoor concentrations.

Increased concentrations of thoron (220Rn) were recently measured in residential traditional dwellings in China (Tschiersch and Müsch, 2005, Shang et al., 2005) and India (Sreenath Reddy et al., 2004). Because of the short half-life (56 s) of the radioactive rare gas, the main source of the indoor concentration is the surrounding building material. In the traditional dwellings, this is the bare soil floor, either soil in cave dwellings or unburned adobe bricks and unplastered stone in above ground dwellings. The thoron exhalation of these materials is studied in dependence of the parameters soil thorium content and humidity. The resulting thoron indoor concentration is not only determined by the exhalation but also by the air exchange with the outdoor air. At different air exchange rates the indoor thoron concentration is discussed in dependence of the exhalation rate. Because of the short half live of thoron, the indoor concentration is not homogeneous but increases towards the walls (Tschiersch and Müsch, 2005). In a large model room, thoron concentration profiles are analysed in regard to exhalation rate and air exchange rate. Normalised profiles are generated and generalised