and damage; whether these considerations also hold for other carcinogens, such as x rays, was not an issue that was addressed by the committee.

In developing its risk model, the committee conducted analyses of the full miner data set and additional analyses limited to the exposure range of < 0.35 Jhm-3 (less than 100 WLM) and focused on < 0.175 Jhm-3 (less than 50 WLM) which is an exposure range involving less than one alpha-particle traversal per cell nucleus location depending on the assumed target cell. An exposure of 0.35 Jhm-3 (100 WLM) results in about 0.84 traversals for basal cell nuclei, about 1.88 traversals for bronchiolar secretory-cell nuclei, and about 3.25 traversals per bronchial secretory-cell nuclei. Additional details regarding alpha-particle traversals of basal- and secretory-cell nuclei can be found in Harley and others (1996) who used a large data base of bronchial-cell nuclei morphometry. The committee's preferred risk model describes lung-cancer risk with a linear nonthreshold relationship between exposure and risk. The model describes risk as time-dependent and age-dependent.

When the risk model is applied to the general population, possible differences between miners and the general population need consideration; that is, similar exposures to radon progeny in homes and miners may not result in the same doses of alpha energy for target cells. Following the approach of the BEIR IV committee, the BEIR VI committee used a mathematical model of the lung to calculate doses to target cells received by miners and by the general population for a given exposure. Those doses were used to determine whether an adjustment was needed in extending risks from miners to the general population.

The committee's model describes the increase in the relative risk of lung-cancer associated with exposure to radon progeny. To project the lung-cancer risks associated with exposure to indoor radon, the committee needed to assume a background rate of lung-cancer mortality. Assumptions will also be needed to extend the model to the full lifespan, to women, and to ever-smokers and never-smokers. The bases for these assumptions and associated uncertainties are described in the report.

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