Risk Assessment of Radon in Drinking Water (1999) / Chapter Skim
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B A Model for Diffusion of Radon Through the Stomach Wall
B A Model for Diffusion of Radon Through the Stomach Wall
Pages 241-248
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From page 241... ...
The epithelial surface of the stomach wall is coated with a mucous layer that serves as a barrier between the acidic lumen and the tissues surrounding the stomach. Adjacent to the mucous layer is a region of tissue consisting of crypts that contain proliferating stem or progenitor cells that eventually reach the surface to perform the necessary functions.
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From page 242... ...
The radon concentration at the outer surface of the wall was considered to be zero at all times because of the removal of radon by blood flowing through the stomach wall. The time-dependent equation describing the concentration of radon can be derived from Fick's law: - = DV2C- SAC dt where C = the concentration of radon, B.1 D = the effective diffusion coefficient, V2 = the Laplacian operator, and = the radioactive-decay constant associated with 222Rn.
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From page 243... ...
B.5 B.6 Substituting U= r R into the spatial part of the equation transforms the spherical Laplacian operator into a simple second-order differential equation: 42U K dlJ dry D dr B.7 That is identical with the equation for one-dimensional diffusion through a slab, provided that the initial and boundary conditions reflected in U are and U(r,t)
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From page 244... ...
A solution can be obtained with Duhamel's theorem where the concentration in the wall is the convolution of the time derivative of a solution after a unit step function at t = 0 (Carslaw and Jaeger 1959~. The dimensions corresponding to the boundary conditions following an intake of 250 mL of water are as follows (see fig.
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From page 245... ...
The effective diffusion coefficient includes a retardation factor that accounts for absorption in the medium. There are no published values in the literature for the effective diffusion coefficient of radon in tissue.
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From page 246... ...
The concentration in the lumen is considered to be uniform and is assumed to decrease exponentially with a half-time of 20 min. The depth of the mucous layer is 50 ,um, and that of the stem cell population is 200 ,um.
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From page 247... ...
The depth of the mucous layer is 50 ,um and that of the stem cell population is 200 ,um. For the conditions described here, the fraction of radon released to the blood stream due to diffusion through the stomach wall is 20%.
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From page 248... ...
The model presented here assumes that there is no capillary involvement in the first 250 ,um of tissue below the mucous layer in the stomach wall. If such capillaries were present in the region between the surface cells and the crypts containing the stem cells, the capillary blood flow would reduce radon penetration into the wall.
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