the lumen. The latter value, derived with the diffusion model of appendix B, is the basis of the committee's recommendation regarding the risks posed by radon dissolved in water. For comparative purposes, dose coefficients were calculated as bounding cases corresponding to the situations of table 4.4; that is, the assumptions that radon does not diffuse into the stomach wall and that concentration in the wall is the same as that in the stomach contents represent the two limiting cases.

Fate of Radon Decay Products in the Body

The members of the decay series through 214Po are referred to as the short-lived decay products relative to the long-lived series headed by 210Pb. The half-life of the "short-lived" decay product 214Pb (26.8 min) is not short relative to physiological processes, inasmuch as, for example, during this half-life, blood passes through the heart more than 30 times. Thus, it is reasonable to assume that the 214Pb has its own fate within the body, a fate that is distinct from that of radon. The calculations performed here include explicit consideration of the fate of each decay product in the manner of recent ICRP publications (1989; 1988); for further details, see appendix A.

Dose Coefficients for Ingestion of Dissolved Radon

The dosimetric analysis presented here is based on the current ICRP method (ICRP 1989), which is consistent with the schema of the Medical Internal Radiation Dose Committee (MIRD) of the US Society of Nuclear Medicine (Loevinger and others 1988). Both ICRP and MIRD consider the mean absorbed dose to a target region as the fundamental dosimetric quantity. The mean absorbed dose in the target region is relevant to cancer induction to the extent that it is representative of the dose to the cells at risk. If the cells at risk are not uniformly distributed within the target region or if the stochastic nature of the energy deposition is such that mean values are of questionable validity, it might be necessary to address the stochastic nature of irradiation.

The ICRP method considers two sets of anatomic regions. The set of "source regions" specifies the location of radionuclides in the body, and the set of "target regions" consists of organs and tissues for which the radiation doses are to be calculated. The source regions are those anatomical regions involved in the behavior of the radionuclide (and subsequent decay products) within the body. It is assumed that the radionuclide is uniformly distributed within the volume of the source region.

The mean energy absorbed in the target region depends on the types of the radiations (including their energies and intensities) emitted in the source regions, the spatial relationships between the source and target regions, and the nature of tissues between the regions. The details of these considerations are embodied in



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