Development of an indoor thoron model.

Increased concentrations of thoron and its decay products were recently found in traditional Chinese clay buildings. Conditions which effect such concentrations were examined in a room which has been rebuilt to scale at Helmholtz Zentrum Muenchen, and in the laboratory. It could be shown that increased thoron exhalation rates can also occur with clay of average thorium content. At an increase of the air exchange rate, a strong decrease of the decay product inventory but an increase of the inventory of the thoron itself could be observed. Furthermore, the influence of the aerosol concentration on the unattached fraction of the thoron decay products was determined. Deposition to the walls, floor, and ceiling was investigated and deposition velocities of the decay products were measured. The obtained information was used to set up an indoor compartmental model of thoron and its decay products. With this model, indoor concentrations can be predicted in dependence on ventilation rate, exhalation rate of the building material, and aerosol particle concentration