cancer death risk from lifetime ingestion of radon dissolved in drinking water at a concentration of 1 Bq m-3 is 0.2 × 10-8. Stomach cancer is the major contributor to the risk. The actual risk from ingested radon could be as low as zero depending on the validity of the linear, no-threshold dose response hypothesis, however, the committee has estimated confidence limits on the ingestion risk (see chapter 4).

Inhalation Risk

Lung cancer arising from exposure to radon and its decay products is bronchogenic. The alpha-particle dose delivered to the target cells in the bronchial epithelium is necessarily modeled on the basis of physical and biologic factors. The dose depends particularly on the diameter of the inhaled ambient aerosol particles to which most of the decay products attach. These particles deposit on the airway surfaces and deliver the pertinent dose, and the dose can vary, because of changes in particle size, by about a factor of 2 in normal home conditions.

The dose from radon gas itself is smaller than the dose from decay products on the airways, mainly because of the location of the gas in the airway relative to the target cells—that is, the source-to-target geometry. The dose from radon gas that is soluble in body tissues is also smaller than the decay-product dose. Two of the underground-miner studies showed no statistically significant risk of cancer in organs other than the lung due to inhaled radon and radon decay products. The dosimetry supports that observation, although there is a need to continue the miner observations.

The risk of lung cancer associated with lifetime inhalation of radon in air at a concentration of 1 Bq m-3 was estimated on the basis of studies of underground miners. The values were based on risk projections from three follow-up studies: BEIR IV (National Research Council 1988), NIH (1994) and BEIR VI (National Research Council 1999). These three reports used data from 4 to 11 cohorts of underground miners in seven countries and developed risk projections of 1.0 × 10-4, 1.2 × 10-4, and 1.3 × 10-4 per unit concentration in air (1 Bq m-3 ), respectively. The three values were for a mixed population of smokers and nonsmokers. The value adopted by the committee is the rounded average derived from the two BEIR-VI model results and equals 1.6 × 10-4 per Bq m-3. The lung-cancer risk to smokers is statistically significantly higher than the risk to nonsmokers. Given the adopted transfer coefficient of 1 × 10-4, the risk of lung cancer (discussed in two reports of the National Research Council and one of the National Institutes of Health) posed by lifetime exposure to radon (222Rn) in water at 1 Bq m-3 was calculated to be 1.6 × 10-8.

Summary of Risk Estimates

The risk estimates developed by the committee for radon in drinking water are summarized in table ES-1. Although the committee was asked to estimate the

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