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Lung-Cancer Risk Posed by Inhalation of 222Rn Decay Products
There are five models for transporting lung cancer risk from the underground-mining studies to exposure in the environment: the historic NCRP (1984b; 1984a) model, the ICRP (1987) model, the model developed by the fourth National Research Council (National Research Council 1988) Committee on Biological Effects of Ionizing Radiations (BEIR IV), the National Cancer Institute model (Lubin and others 1994) derived from the pooling of 11 underground mining studies, and the model developed by the sixth National Research Council (National Research Council 1999) Committee on Biological Effects of Ionizing Radiations (BEIR VI). The BEIR VI model is similar to the NCI model, with updated data from the same 11 underground miner cohorts.
Although the 222Rn decay-product exposure data are universally weak in all the miner epidemiologic followup studies, they are the only human data available from which one can derive numerical estimates of occupational and domestic lung-cancer risk. Some generalizations are possible and necessary to quantitate this risk.
NCRP was the first to propose a model for environmental lung-cancer risk based on the miner data (NCRP 1984a). The model accounted for the fact that miners exposed for the first time when over 40 y old appeared to have higher lifetime risk of lung cancer than miners exposed for the first time in their 20s. The apparent lower lifetime risk for those exposed at young ages was assumed to be due to a reduction in risk with time since last exposure. Thus, earlier exposure was assumed to diminish because of cell death or repair of cells transformed by earlier exposure. The half-life for repair (or loss) was assumed to be 20 y.
One key factor noted by NCRP was that lung cancer is a rare disease before the age of 40, regardless of the population considered. Miners exposed when young did not generally appear as lung-cancer cases until the usual cancer ages were attained (50–70 y). That would account for an apparent increase in lifetime lung-cancer risk at higher ages because there would be a shorter time for transformed cells to be lost compared with the situation in persons exposed at lower ages. Miners were exposed, on the average, for less than 10 y in the Colorado, Ontario, and Czech cohorts. The total time for followup was 20 y or more, so the apparent reduction in risk with time after exposure could be observed.
The NCRP model took the form of an exponential reduction with time after exposure, with the stipulation that there was a minimal latent period of 5 y. Also, lung cancer could not appear before the age of 40. This model is known as a modified absolute-risk model. Risk is expressed after exposure without regard to