of epithelial cells from unilateral sources on the surface. Ingestion of waterborne radon might, on first impression, similarly expose the cells of the stomach lining. After ingestion, however, radon travels as gas molecules with high mobility through cell membranes, and cells may receive a more uniform exposure. Stem cells and other proliferating cells of the stomach are found in bands at the bases of the necks of narrow invaginations of the stomach wall that constitute the secretory glands of the stomach wall (Nomura 1996). Stem cells and other proliferative cells of the stomach are major targets of radon alpha particles, but cells of the small intestine are also potential targets. After ingestion of water, radon passes into the small intestine with a half-time of about 15–20 minutes. Radon can therefore be absorbed into the bloodstream from both the stomach wall and the small intestine. The resulting exposures to most cells of the body will then be through bloodborne radon. From that point of view, the stomach might be at greatest risk of exposure from ingested, aqueous radon. The transfer of dissolved radon from water to air and its later inhalation constitute another route by which the lungs can be at risk.

Implicit in these scenarios is the idea that the cells most likely to become malignant are the stem cells and proliferative cells that retain the capacity for continued division and can fix and express permanent genetic change. Malignant cells often retain characteristic enzymatic and cellular features of their tissue of origin, so the differentiation and specialization programs of cells might be altered but not completely abrogated by the malignant-transformation process. Alpha-particle damage to genetic material becomes fixed as permanent alterations to gene structure and expression as a result of processes that involve DNA repair, replication, and cell division. The stem cells of epithelial tissues are embedded in crypts; this renders them relatively inaccessible to direct contact with ingested or inhaled radon. Stem cells will, however, still be exposed to alpha irradiation from the lumen or blood stream, from intercellular and intracellular water, and after inhalation from decay products that plate out and act as additional sources of radiation damage. An additional factor to be considered is the potential role of chronic stomach infections. A large fraction of the normal human population carry Helicobacterpylori infections in the stomach that can cause gastritis and, in severe cases, ulcers. The inflammation and proliferation associated with these infections can be a factor in the induction and progression of stomach cancer and have been regarded as risk factors (McFarlane and Munro 1997).

Cellular Damage Induced by Radon Alpha Particles

Alpha particles create dense ionization that leaves tracks of ion-pair clusters across cells and tissues. Cells that suffer an alpha-particle track through the nucleus are severely injured. At the low exposure conditions under consideration from waterborne and airborne radon in the home, however, less than 1% of the cells in the bronchial epithelium would experience an alpha-particle track per



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