RADON-EXPOSED UNDERGROUND MINERS AND INVERSE DOSE-RATE (PROTRACTION ENHANCEMENT) EFFECTS

Recent models for radon-induced lung cancer assume that at high levels of cumulative exposure, as experienced historically by many undergrou nd miners of uranium and other ores, the risk of lung cancer follows a n inverse dose-rate (protraction enhancement) pattern, That is, for eq ual total dose, a greater risk is incurred by those whose total dose i s accumulated at a lower rate over a longer duration than at a higher rate over a shorter duration. This inverse dose-rate effect is hypothe sized to be the consequence of multiple traversals of the nucleus of a target cell by alpha particles. It has recently been concluded, howev er, that for low total doses, as in most residential settings, the inv erse dose-rate effect should diminish and perhaps even disappear, sinc e at very low doses the probability that more than one alpha particle would traverse a cell is small and there would be no possibility for i nteractions from multiple hits. Pooling original data from 11 cohort s tudies of underground miners, including nearly 1.2 million person-y of observation and 2,701 lung cancer deaths, rye evaluate the presence o f an inverse dose-rate effect and its modification by total dose. An i nverse dose-rate effect was confirmed in each cohort, except one, and overall in the pooled data, There also appears to be a diminution of t he inverse dose-rate effect below 50 Working Level Months (WLM), altho ugh analyses were necessarily hampered by a limited range of exposure rates at low total WLM. These data support both the presence of an inv erse dose-rate effect, as well as its diminution at low total dose, As a consequence, assessment of risks of radon progeny exposure in homes (on average 15-20 WLM for a lifetime) using miner-based models should not assume an ever-increasing risk per unit dose, Rather, it is more appropriate to apply risk models that take into account protraction en hancement and its diminution.