periments on smoking and radon progeny have not yielded strong evidence on the combined effects of the 2 exposures. The findings are inconsistent and dependent on the sequence of exposures. In the residential setting, exposure to cigarette smoke and exposure to radon progeny occur essentially simultaneously throughout adulthood. Among the miners, smoking and radon exposure can take place simultaneously or radon exposure can begin before or after smoking has started (Thomas and others 1994). The unique pattern of sustained smoking by humans, which has not been replicated in the animal experiments, is an additional barrier to extending the findings of the animal studies to humans.

THE DOSIMETRIC APPROACH TO RADON RISK ESTIMATION

The approach to domestic radon risk estimation taken in this report involves epidemiologic studies of people who have been exposed to radon. A different, and possibly complementary, approach is to estimate radon risks on the basis of people exposed primarily to sparsely ionizing radiation—largely the Japanese atomic-bomb survivor cohorts. This so-called dosimetric approach to radon risk assessment using ICRP quantities (ICRP 1991) has the following logic:

  • Use physical models to estimate a bronchoepithelial dose per Jhm-3 (WLM).

  • Convert that lung dose, with the specified radiation weighting factor (wR) (ICRP 1991), to an equivalent dose for radon-progeny alpha particles in the bronchial epithelium.

  • Convert the equivalent dose to an effective dose, with the appropriate tissue weighting factor for lung.

  • Use the best estimate for the lifetime-fatality probability coefficient per unit of effective dose to estimate the lifetime risk per Jhm-3 (WLM).

A more direct dosimetric approach could be to apply an appropriate alpha-particle RBE factor specifically for lung, rather than the general radiologic protection quantity wR (or "quality factor," Q). Then one could estimate the lifetime risk per Jhm-3 (WLM) from the lung-cancer-fatality probability coefficient per unit of absorbed dose of atomic-bomb survivor data extrapolated to low dose rate.

Assuming a quality factor of 20, Burchall and James (1994) used a dosimetric approach to estimate a risk from residential radon exposure and found the risk to be larger than estimated from the miner data (for example, Lubin and others 1994a) by a factor of 4–5. However, it is difficult to interpret the difference between the two approaches. In light of the uncertainties in many of the steps involved in arriving at both types of risk estimates, the difference is modest. One of the major uncertainties in the dosimetric approach is related to the current impossibility of estimating RBE directly in any realistic quantitative sense for relevant in vivo end points. Consequently, the rationale usually adopted is to



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