range of miner exposure rates was applied for relevant indoor exposures without further adjustment.

Combined Effect of Smoking and Radon

Apart from the results of very limited in vitro and animal experiments, the only source of evidence on the combined effect of the 2 carcinogens (cigarette smoke and radon) was the data from 6 of the miner studies. Analysis of those data indicated a synergistic effect of the two exposures acting together, which was characterized as submultiplicative, i.e., less than the anticipated effect if the joint effect were the product of the risks from the two agents individually, but more than if the joint effect were the sum of the individual risks. The committee applied a full multiplicative relation of the joint effect of smoking and exposure to radon, as done by the BEIR IV committee, and also a submultiplicative relationship. Although the committee could not precisely characterize the joint effect of smoking and radon exposure, the submultiplicative relation was preferred by the committee because it was found to be more consistent with the available data.

Risks for Women

The risk model is based on epidemiologic studies of male miners. The effect of radon exposure on lung-cancer risk in women might be different from that in men because of differing lung dosimetry or other factors related to gender. The K factor was calculated separately for women and men, but did not differ by gender. The committee also could not identify strong evidence indicative of differing susceptibility to lung carcinogens by sex. Consequently, the model was extended directly to women, with the assumption that the excess risk imposed by radon progeny estimated from the male miners multiplies the background lung-cancer rates for women, which are presently substantially lower than for men.

Risks Associated with Exposures in Childhood

Evidence was available from only one study of miners on whether risk was different for exposures received during childhood, during adolescence, and during adulthood. There was not a clear indication of the effect of age at exposure. The committee made no specific adjustment for exposures received at earlier ages. The K factor for children aged 10 was calculated as 1 and the value for infants was only slightly higher (about 1.08).

Characterization of Radon Risks

In making its calculations, the committee used the latest data on lung-cancer mortality for 1985–1989 and for smoking prevalence for the U.S. in 1993. To

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