The Complex Influence of Filtration Operation on Indoor 222Rn (radon) and 220Rn (thoron) Concentration

Since the proximate source of dose to the cells of the bronchial epithelium is caused by the airborne decay products of radon and thoron, it is in some situations more practical to remove the daughter products from the air in the form of filtration with pump and High Efficiency Particulate Air (HEPA) filters. However, the filtration process induced some complex influence on indoor radon, thoron concentration despite the high performance of removing the decay products of radon and thoron. It is found that in the test range of filtration rates from 0.2-0.8 h^-1, the thoron concentration was increased to similar extents, for radon, except the case of 0.2 h^-1, the higher filtration rates (0.4-C0.8 h^-1) slightly decreased the radon concentration, and the higher filtration rates, the lower average concentrations of the radon. Discrimination between the different responses of radon and thoron might be caused by their wide disparity in half-life thus varied mechanism of exhalation and spatial distribution in the case of filtration operation.