(1994a) again found that the exposure-response relation varied with time since exposure and attained age, but they also found variation with exposure rate. Lower exposure rates were associated with increased risk. The BEIR VI committee used the work of Lubin and colleagues (1994a) as a starting point for the analyses described in this chapter.
This section considers the sources of data, methods of combining data from diverse populations, and assumptions that underlie the lung-cancer risk model developed by the committee in its analysis of miner data. The committee used a relative-risk model that relates lung-cancer rate in miners to their occupational exposure to radon.
In the analysis, exposure refers to occupational exposure to radon progeny during employment in underground mines, and relative risks refer to the additional risks associated with occupational exposure to radon progeny beyond the background risk from lung-cancer, which reflects other exposures, including indoor radon. Residential radon-progeny exposures of the miners are not considered in the analysis data and are implicitly assumed to be the same, on average, at all levels of occupational exposure. Any bias in the modeling due to ignoring nonmine exposures is likely to be small, because residential radon concentrations are generally much lower than mine concentrations.
The committee's model is based on a linear relationship between exposure and the relative risk of lung-cancer. This linear relationship was based on an empirical evaluation of the 11 individual miner studies. In analyzing the miner data, Lubin and others (1994a) explored various models for describing the form of the relative risk in relation to radon exposure. Within the range of exposures in miners, linear models provided an adequate characterization of each cohort except the Colorado Plateau uranium miners. In the Colorado data, the authors found a relative-risk pattern that was concave at high cumulative exposures. Accordingly, in the analysis of pooled data, data from the Colorado study were limited to exposures below 11.2 Jhm-3 (3,200 WLM), below which relative risks were consistent with linearity.
Pooled data from 11 cohort studies of radon-exposed underground miners were used to develop the committee's risk models; these data were derived from all the major studies with estimates of exposure for individual miners (Table 3-2). Data were available from 7 studies in addition to those considered by the BEIR IV committee. These data are described in detail in appendixes D and E.