exposure in homes is lifelong for the population; and most of the miners studied have been smokers, but only a minority of U.S. adults in the general population are currently smokers. Those factors are potential determinants of the relationship between exposure to radon progeny (in working-level months) and equivalent dose of -energy (in sieverts) delivered to target cells in the respiratory tract. With models of the respiratory tract, the dose to target cells in the respiratory epithelium can be estimated for the circumstances of exposure of a miner in an underground mine and of a man, woman, or child in a home. Uncertainties arising from dosimetric differences between exposures in the two settings can be characterized by comparing the relationships between exposure and dose.

Before the BEIR IV report (National Research Council, 1988) was issued, comparative analyses of dosimetry had been made for the mining and indoor environments. The BEIR IV report included a qualitative assessment of uncertainty associated with differences in lung dosimetry in the two environments. The report's analysis of dosimetry was based on the value of "K," the ratio of dose to exposure in homes divided by the ratio of dose to exposure in mines. Values above unity indicate greater dose and hence greater risk for those exposed in homes than for those exposed in mines; values less than unity indicate lesser risk in homes. The BEIR IV report considered the determinants of K in a qualitative fashion and found the value of K to be 1. The report did not include a detailed assessment of the evidence on the determinants of K, nor did the committee develop its own model.

In followup to the BEIR IV report, EPA asked the National Research Council to study the dosimetry and related matters, considerations that affect the applications of lung-cancer risk estimates based on studies of miners to the general population. The resulting

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