Having thus considered the various alternative approaches, the committee chose to follow the general approach of the BEIR IV committee and of Lubin and others (1994a) to the analysis of pooled miner data and to base risk estimates on an empirically derived model (the third approach). This approach provided the committee with well-established databases and methods as a starting point.

Empirical or descriptive modeling of risk allows a unified approach for testing the validity of the form of the model and of the significance of model parameters. For example, the committee used a relative risk model rather than an absolute risk model to describe lung-cancer risk to radon exposure. It had been observed that a relative risk model, with time-varying covariates, provided a more parsimonious description of the miner data than an absolute excess-risk model (Lubin and others 1994a).

The flexibility of the modeling approach allowed the incorporation of specific biologically based patterns of risk. Two important choices in the committee's analysis are the incorporation of an inverse exposure-rate effect and the assumption of linearity of the exposure-response relationship at low cumulative exposure. For radon-induced lung-cancer, those choices have a plausible biologic rationale, as well as some experimental justification (see chapter 2).

PREVIOUS MODELS

A number of models have been previously developed for estimating lung-cancer risk posed by exposure to radon and its progeny. Models developed through the middle 1980s were described in the BEIR IV report (NRC 1988). These and other models are discussed in detail in appendix A to this report. With the exception of preliminary reports from 2 studies which later changed, these models have all assumed linearity of the exposure-response relationship. All models used risk estimates derived from the studies of miners.

The earliest risk models specified effects of exposure in terms of the absolute excess risk of lung-cancer from radon-progeny exposure. The absolute (excess) risk model represents lung-cancer mortality as r (x, z, w) = r0 (x) + g (z, w), where r0 (x) is the background lung-cancer rate and g (z, w) is an effect of exposure. (Here, w denotes cumulative exposure, x represents covariates that determine the background risk, and z denotes covariates that modify the exposure-response relationship.) The model proposed by the BEIR III committee allowed the absolute excess risk to vary by categories of attained age with allowance for different minimal latent periods for each category. A descriptive model for the absolute excess lung-cancer risk, proposed by Harley and Pasternack (1981), served as the basis of risk estimates in Reports 77 and 78 of the National Council of Radiation Protection and Measurements (NCRP 1984a,b). That model assumed that exposure has no effect on risk before age 40 years and that, after a latent period, the absolute excess risk declines exponentially with time since exposure. The model was proposed specifically to address risk associated with radon-progeny expo-



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