cells, the results might lead to reconsideration of the linear no-threshold assumption used at present.

Ingestion Risk

The cancer risk arising from ingestion of radon dissolved in water must be derived from calculations of the dose absorbed by the tissues at risk because no studies have quantified the risk. Studies of the behavior of radon and other inert gases have established that they are absorbed from the gastrointestinal tract and readily eliminated from the body through the lungs. The stomach, the portal of entry of ingested radon into the body, is of particular concern. The range of alpha particles emitted by radon and its short-lived decay products is such that alpha particles emitted within the stomach are unable to reach the cells at risk in the stomach wall. Thus, the dose to the wall depends heavily on the extent to which radon diffuses from the contents into the wall. Once radon has entered the blood, through either the stomach or the small intestine, it is distributed among the organs according to the blood flow to them and the relative solubility of radon in the organs and in blood. Radon dissolved in blood that enters the lung will equilibrate with air in the gas-exchange region and be removed from the body.

The committee found it necessary to formulate new mathematical models of the diffusion of radon in the stomach and the behavior of radon dissolved in blood and other tissues. The need for that effort arose from the lack of directly applicable experimental observations and from limitations in the extent to which one can interpret available studies. The diffusion of radon within the stomach was modeled to determine the expected time-integrated concentration of radon at the depth of the cells at risk. The result, based on a diffusion coefficient of 5 × 10-6 cm2 s-1, indicated that a conservative estimate of the integrated concentration in the wall was about 30% of that in the stomach content.

The committee also found it useful to set forth a physiologically-based pharmacokinetic (PBPK) model of the behavior of radon in the body. Various investigators have assessed the retention of inhaled and ingested radon in the body, but their observations do not relate directly to the distribution of radon among the tissues. The PBPK is formulated using information on blood flow to the tissues and on the relative solubility of radon in blood and tissue to determine the major tissue of deposition (which was adipose tissue) and retention within this tissue. The PBPK model is consistent with the observations regarding radon behavior in the body. Unlike previous estimates of the radiation dose, the committee's analysis also considered that each radioactive decay product formed from radon decay in the body exhibited its own behavior with respect to tissues of deposition, retention, and routes of excretion.

The committee's estimates of cancer risk are based on calculations with risk-projection models for specific cancer sites. The computational method was that described in EPA's Federal Guidance Report 13. An age-and gender-averaged

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