Answer to Question #258 Submitted to "Ask the Experts"

Q

Do the following levels of radioactivity in a private water supply pose a skin contamination hazard: uranium 575 pCi per liter, ²²⁶Ra 32 pCi per liter (gross alpha 973 pCi per liter, radon 2680 pCi per liter, ²²⁸Ra 0.7 pCi per liter)? Do showering/tooth brushing pose a problem? Thanks.

A

Presumably the private water supply in question is not being used for drinking water but could provide water for other household uses. Skin penetration by uranium and radium radionuclides from bathing is not well established and no dose or risk estimates are available, but the hazard is expected to be low. The dominating risk would probably be that from radon transferred to indoor air. The risk from brushing teeth would probably not contribute significantly to the overall risk. Although the risk from other alpha emitters could be significant, radionuclide-specific contributions to the gross alpha level of 973 pCi per liter would have to be established to estimate their risk. If the water sample was not analyzed promptly, a substantial contribution from ²²⁴Ra could be missed. We note that the data presented are all for alpha emitters; the risks from beta and photon emitters could also be a consideration. An approximation for skin absorption can be made using the Environmental Protection Agency (EPA) equation for dermal absorption and associated default values for inorganics from the EPA handbook on dermal absorption⁽¹⁾. During a 15-minute shower (or bath), an estimated 0.16 pCi of ²²⁶Ra, 0.0034 pCi of ²²⁸Ra, and 2.79 pCi of uranium could be absorbed through the skin. These should be upper limits of expected absorption. While there are no risk coefficients for dermal absorption in Federal Guidance Report No. 13⁽²⁾, estimates can be made by dividing the activity absorbed by the f₁ for that element. This gives an ingestion equivalent for dermal absorption. The risk of fatal cancer from a daily 15-minute shower would be about 8×10^-8 per year for ²²⁶Ra, 5×10^-9 per year for ²²⁸Ra, and 2×10^-7 per year for the uranium isotopes absorbed through the skin. Dose and risk estimates for the stomach and colon would be overestimated since the radionuclide wasn't ingested, but for these cases, the effect is not substantial. Using external exposure dose coefficients from FGR 12⁽³⁾ and risk coefficients from FGR-13⁽²⁾, one can estimate the risk for immersion in water. For a daily 15-minute bath (or shower), the risk from external exposure to radiations from the radium isotopes would be about 3×10^-10 cancer deaths per year of exposure, for the uranium isotopes about 1×10^-10 cancer deaths per year of exposure. Presuming that household uses of water containing 1 pCi of radon per liter contributes 1×10^-4 pCi per liter of radon to the household air, the contribution to the radon level in the house would be about 0.3 pCi per liter. This is about 3/4 as great as the national ambient concentration of 0.4 pCi per liter. The risk of cancer death for 0.3 pCi per liter would be about 1×10^-5 per year of exposure. If 2 liters of water were ingested each day, the risk from ²²⁶Ra would be about 6×10^-6 cancer deaths per year of exposure, for ²²⁸Ra about 4×10^-7 cancer deaths per year of exposure, and for the uranium isotopes about 2×10^-5 cancer deaths per year of exposure. The effective intake–probably not more than a few milliliters per day–and associated risk from brushing teeth would be a small fraction of that from drinking the water. If the water is not used for drinking, cooking, or food preparation, the major source of risk would be from the release of radon in the water to the household. Of course any changes in usage would be reflected in changes in the dose and risk estimates. Values used in the calculations above are default values and the true risks are expected to be smaller. For a perspective of risk, the above numbers can be compared to the maximum contaminant level (MCL) for radium in drinking water which is set to limit the risk to about 1x10^-6 per year. Neal Nelson, Ph.D. Environmental Protection Agency References:

1. Dermal Exposure Assessment: Principles and Applications, EPA/600/8-9-91, U.S. EPA, Washington, DC; 1991.
    
2. Federal Guidance Report No. 13, Cancer Risk Coefficients for Environmental Exposure to Radionuclides, EPA 402-R-99-001, U.S. EPA, Washington, DC; 1999.
    
3. Federal Guidance Report No. 12, External Exposure to Radionuclides in Air, Water, and Soil, EPA 402-R-93-081, U.S. EPA, Washington, DC; 1993.