from the penetration of soil gas into homes, so only very high concentrations of radon in water will make an important contribution to the airborne concentration.
Even though water generally makes only a small contribution to the indoor airborne radon concentration, the risk posed by radon released from water, even at typical groundwater concentrations, is estimated to be larger than the risks posed by the other drinking water contaminants that have been subjected to regulation, such as disinfection by-products. Thus, in most homes, the risk to the occupants posed by indoor radon is dominated by the radon from soil gas, which is not subject to regulation, and a change in the radon in drinking water would produce a minimal change in the risk posed by airborne radon. This problem led to the suggestion that mitigation of radon in indoor air be considered an alternative means of achieving risk reduction equal to or greater than that which would be achieved by reducing the concentration of radon in drinking water.
The ingestion of radon in water also presents a possible risk. Questions were raised with respect to the ingestion risk assessment that EPA used in the 1991 proposed regulations and in the revised multimedia risk assessment of 1994. The questions were related to the applicability of some of the data used as the basis of the risk model and to the resulting assumptions that were used to estimate risk. The substantial uncertainties in the radon health risks other than those posed by inhalation add to the problems of setting an appropriate MCL to protect public health. Thus, a reevaluation of the ingestion risks was needed.
EPA contracted with NAS to address the issues cited above, and the committee on the Risk Assessment of Radon Gas in Drinking Water was formed in the National Research Council's Board on Radiation Effects Research. The specific tasks assigned to the committee were:
The final report includes: