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ering the larger reductions in risk that might be obtained by reducing radon concentrations originating from soil gas (EPA-SAB 1993a).
The Safe Drinking Water Act was amended again in 1996 (US Congress 1996). The proposed national primary drinking-water regulation for radon was withdrawn. Before proposing a new regulation for radon in water, EPA was instructed to ask the National Academy of Sciences to prepare a risk assessment for radon in drinking water on the basis of the best science available. The assessment was to consider each of the pathways associated with exposure to radon from drinking water at concentrations and conditions likely to be experienced in residential environments. The Academy was also asked to prepare an assessment of health-risk reductions that have been realized from various methods used to reduce radon concentrations in indoor air to provide a basis for considering alternative or multi-media mitigation schemes as opposed to mitigation of water alone.
Charge to the Committee
The Committee on the Risk Assessment of Exposure to Drinking Water in the National Research Council's Board on Radiation Effects Research began deliberations in July 1997. The specific tasks assigned to the committee were:
To examine the development of radon risk assessments for both inhalation of air and ingestion of water.
To modify an existing risk model if it were deemed appropriate or develop a new one if necessary.
To review the scientific data and technical methods used to arrive at risk coefficients for exposure to radon in water.
To assess potential health-risk reductions associated with various measures to reduce radon concentrations in indoor air.
The final report was to include:
Estimates of lung, stomach, and other potential cancer risks per unit concentration of radon in water.
Assessment of whether health effects of radon in drinking water could be estimated for various sub-populations at risk, such as infants, children, pregnant women, smokers, elderly persons, and seriously ill persons.
Examination of evidence for teratogenic and reproductive effects in men and women due to radon in water.
Estimates of the transfer coefficient that relates radon in water to radon in indoor air.
Population-weighted estimates of radon concentrations in ambient air.
Estimates of increases in health risks that could result from methods used to comply with regulations for radon in drinking water.