Inhalation of radon daughters is considered to be the major source of radiation exposure to man. The quantitative estimate of risk of radon-induced lung cancer is presently based on two complementary methods. In the first method risk coefficients are derived directly from epidemiological studies on radon-exposed uranium miners. In the second, dosimetric approach, a lung model is used to estimate doses to the bronchial epithelium. To express the dose from the radon daughter a-particles absorbed in the epithelium in terms of dose equivalent a quality factor, Q, is applied the value of which is recommended by international bodies (ICRP). The presently used value of Q appears to be of limited relevance for evaluating the risk of lung cancer.
We analyze the results of radiobiological experiments in order to derive a more realistic value of Q for radon exposure, basing our investigations on the microdosimetric analysis of measurements of oncogenic transformation per surviving C3H10T1/2 cells after doses of protons, deuterons, helium-3. We argue that the probability relevant to radiation protection is that of cell transformation per exposed cell, therefore probability of cell transformation must be considered in conjunction with that of cell survival. The RBE for cell transformation following irradiation of these cells by alpha particles should therefore decrease because some of the cells are killed and cannot undergo consecutive transformation. This conditional probability of cell transformation leads to a decrease of the value of Q for alpha particles, especially at the stopping end of the particle track. We therefore suggest that the value of Q applied presently in the dosimetric approach may be overestimated.