evidence for lack of significant wall transport. No experimental evidence was provided by any of the investigators as a basis for their assumptions concerning the movement or lack of movement of radon into the stomach wall.

Behavior of Radon in the Body

The investigations by Harley and colleagues (1994), Harly and Robbins (1994) and Hursh and others (1965) provide basic information on the behavior of radon in the body. Harley and coworkers fit their observations to a function involving five exponential terms and associated the terms with tissue compartments. Bernard and Snyder (1975) interpreted the Harley data using a mammillary model to derive their estimates of the distribution of radon among the tissues. It is possible to observe the tissue distribution of noble gases in medical studies using radioisotopes of krypton and xenon. Correia and others (1987) used nuclear-medicine instrumentation to observe the behavior of ingested 133Xe and then attempted to infer the fate of ingested radon. Presently, it is considered that compartment models describing the movement of a contaminant within the body which are consistent with anatomic and physiologic principles provide the best basis for interpretation of experimental observations. Models based on physiologic principles are referred to as physiologically based pharmacokinetic (PBPK) models. Details regarding this modeling practice are given in a review by Bischoff (1986).

The committee has used a PBPK model of ingested radon that was formulated using the blood-flow model of Leggett and Williams (1995) (see also Leggett and Williams 1991; Williams and Leggett 1989). The resulting model, shown in figure 4.2, is discussed in detail in appendix A. Briefly, radon is distributed by the blood flow to the organs, where its transfer depends on its solubility in tissue relative to that in blood—the partition coefficient. The blood volume of the body is apportioned among a number of compartments, which represent various blood pools. In figure 4.2, the compartment ''Large Veins'' represents the venous blood return from the systemic tissues, "Right Heart" and "Left Heart" the content of the heart chambers, "Pulmonary" the blood-exchanging gases in the lung, and "Large Arteries" represents the arterial blood flow to the systemic tissues. The compartment labeled "Gut Cont" in figure 4.2 is expanded in figure 4.3, where the GI tract is divided into four segments, the compartment "St Contents," representing the contents of the stomach, "SI Contents" that of the small intestine, "ULI Contents" that of the upper large intestine, and "LLI Contents" that of the lower large intestine. "St Wall," "SI Wall," "ULI Wall," and "LLI Wall" represent the walls of those segments. Ingested radon enters the stomach and is absorbed from the gut as indicated in the upper right of figure 4.3. In figures 4.2 and 4.3, dashed arrows denote the transfer of radon as a gas, and solid arrows correspond to the flow of radon dissolved in arterial (thicker arrows) and venous blood. As shown in figure 4.3, the gut is perfused by arterial blood,

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