Republic, and in Port Radium, Beaverlodge, and Ontario in Canada; metal miners in Sweden; tin miners in China; and fluorspar miners in Canada. The data for 4 studies were updated with new information. These 11 studies offered a substantially greater data resource than had been available to the BEIR IV committee. The 11 epidemiologic studies covered a range of mining environments, times, and countries, and their methods of data collection differed in some respects.

The committee analyzed the data with a relative-risk model in which radon exposure has a multiplicative effect on the background rate of lung-cancer. In particular, the committee modeled the excess relative risk (ERR), which represents the multiplicative increment to the excess disease risk beyond background resulting from exposure. The model represents the ERR as a linear function of past exposure to radon. This model allows the effect of exposure to vary flexibly with the length of time that has passed since the exposure, with the exposure rate, and with the attained age. The mathematical form of the model for ERR is:

The parameter β represents the slope of the exposure-risk relationship for the assumed reference categories of the modifying factors. Exposure at any particular age has 4 components: exposure in the last 5 years—excluded as not biologically relevant to cancer risk—and exposures in 3 windows of past time, namely 5–14, 15–24, and 25 or more years previously. Those exposures are labeled w5–14, w15–24, and w25+, respectively, and each is allowed to have its own relative level of effect, θ5–14 (set equal to unity), θ15–24, and θ25+, respectively. With this weighting system, total exposure can be calculated as w* = w5–14 + θ15–24w15–24 + θ25+w25+. The rate of exposure also affects risk through the parametery, γz; thus, the effect of a particular level of exposure increases with decreasing exposure rate, as indexed either by the duration of exposure or the average concentration at which exposure was received. The ERR also declines with increasing age, as described by the parameter age.

Based on this analysis, the committee developed two preferred risk models referred to as the exposure-age-concentration model and the exposure-age-duration model. These two models differ only with respect to the parameter γz, which represents either duration of exposure or the average concentration over the time of the exposure. The models were equally preferred by the committee. The new models are similar in form to the BEIR IV model, but have an additional term for exposure rate and more-detailed categories for the time-since-exposure windows and for attained age.

Risk Assessment

The committee's risk models can be used to project the lung-cancer risk associated with radon exposure, both for individuals and for the entire US population. To extend the models that were developed from miner data to the general

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