Carcinogenic Risks Associated with Ingesting Naturally Occurring Radionuclides in Drinking Water, including the Contribution to Overall Risk from Radon-222

D.A. Falta; T.A. DeVol; and R.A. Fjeld (Clemson University)

An assessment of the carcinogenic risk associated with the ingestion of a radionuclide relies upon the use of an appropriate dose conversion factor (DCF) or cancer risk coefficient. A series of published reports by the International Commission for Radiological Protection (ICRP) provides DCFs as well as nominal probability coefficients, or slope factors, which allow for the conversion of estimated doses into cancer risk probabilities. Federal Guidance Report 13 introduces cancer risk coefficients that directly convert radionuclide uptakes associated with environmental low-level exposures into mortality or morbidity cancer risks. Neither the ICRP nor the Federal Guidance Report 13 provides any values for the radon isotopes. However, the maximum contaminant level proposed for radon in drinking water by the United States Environmental Protection Agency relies upon cancer risk coefficients given in a National Research Council (NRC) report entitled "Risk Assessment of Radon in Drinking Water". This NRC report also provides DCF values for radon. In our presentation, both NRC DCF and cancer risk coefficient values have been used to estimate carcinogenic risks due to ingestion of radon in groundwater for various regions of the United States. Cancer risks associated with the ingestion of other naturally occurring radionuclides have also been computed and compared to the risks associated with radon ingestion. The total fatal cancer risks summed from the contributions of uranium-234 and –238, radium-224, -226, and –228, radon-222, lead-210, and polonium-210 in typical combinations remain well below regulatory acceptable limits of 10-4 to 10-6. Although all risks are small, radon contributes between 25% and 75% of the overall mortality risk for all cancers combined, with radium-228 contributing most of the remaining risk. For both fatal total cancers and stomach cancer risks, if the radium-228 activity is negligible, radon-222 provides 80% of the overall risk.

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