sure duration and the overall water-to-air transfer factor. The EPA reanalysis (EPA 1994b) used a direct tapwater consumption rate of 1 L d-1, an exposure time of 70 y, and assumed that 20% of the radon in the tapwater is released from the water in the process of transferring the water from the tap to the stomach (tapwater is defined as water ingested directly, without agitation or heating). The committee used an age-and gender-specific tapwater usage rate that corresponds to an age-and gender-average rate of 0.6 L d-1 and assumed all of the radon remained dissolved in the water during the transfer process. Both the EPA and the committee analyses used a transfer factor of 1 x 10-4 for purposes of estimating the contribution radon dissolved in water makes to the overall indoor air radon concentration.

The estimated number of cancer deaths per year from public exposure to radon are compared in table ES-3. Ranges estimated by this committee are approximate and are based on judgment using the best available information.

Uncertainty Analysis

Estimating potential human exposures to and health effects of radon in drinking water involves the use of large amounts of data and the use of models for projecting relationships outside the range of observed data. The data and models must be used to characterize population behaviors, engineered-system performance, contaminant transport, human contact, and dose-response relationships among populations in different areas, so large variabilities and uncertainties are associated with the resulting risk characterization. The report provides an evaluation of the importance of and methods for addressing the uncertainty and variability that arise in the process of assessing multiple-route exposures to and the health risks associated with radon.

Table ES-3

Comparison of estimated cancer deaths per year due to exposure to radon and estimated possible ranges due to uncertainty

Exposure Pathway

Committee Analysisa

Revised EPA Analysisc

Inhalation of radon progeny in indoor air

18.200b

(3,000–33,000)

13,600

Inhalation of radon progeny in outdoor air

(120–1300)

720

520

Inhalation of radon progeny derived from

the release of radon from drinking water

160

(25–290)d

86

Ingestion of radon in drinking water

(5–50)

23

100

a Based on the 1998 estimated U.S. population of 270 million.

b Based on data from BEIR VI (National Research Council 1999).

c Based on a U.S. population of 250 million (EPA 1994b).

d Values derived from rescaling the analysis of the EPA-SAB (1994b) report using 1998 population and mortality data and risk estimates from BEIR VI (National Research Council 1999).



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