risks to susceptible populations—such as infants, children, pregnant women, smokers, and elderly and seriously ill persons—there is insufficient scientific information to permit such estimation except for the lung-cancer risk to smokers, which is presented separately in the table. The adopted lifetime risk of lung cancer for a mixed population of smokers and nonsmokers, men and women, resulting from the air exposure to radon from a waterborne radon concentration of I Bq m-3 is 1.6 × 10-8. The adopted lifetime risk of stomach cancer for the same water concentration is 0.2 × 10-8; the committee could not make a distinction in ingestion risk for any specifically identified subpopulation other than the differences in gender.

Figure I (see Public Summary) puts the inhalation and ingestion risks into perspective by direct comparison of annual cancer deaths. The number of lung-cancer deaths in the United States is estimated to be 160,100 in 1998 (ACS 1998). Using the average of the two BEIR-VI risk models and adjusting for the 1998 increase in the number of lung-cancer deaths, the committee estimates there will be about 19,000 lung-cancer deaths in 1998 attributable to radon and the combination of radon and smoking. The committee estimated there might be about 20 stomach-cancer deaths in 1998 (with a subjectively determined uncertainty range from 1 to 50 deaths) attributable to the ingestion of radon in drinking water as compared to 13,700 stomach-cancer deaths that are estimated to develop in the United States in 1998 from all causes (ACS 1998). Based on an estimated national mean value of radon in drinking water, the committee estimates 160 lung cancer deaths in 1998 (with a subjectively determined range from 25 to 280 deaths) attributable to indoor radon (in air) resulting from the release of radon from household water. The committee's analysis indicates that most of the cancer risk posed by radon in drinking water arises from the transfer of radon into indoor air and the subsequent inhalation of the radon decay products, and not from the ingestion of the water.

Table ES-1

Committee Estimate of Lifetime Risk Posed by Exposure to Radon in Drinking Water at 1 Bq m-3

Exposure Pathway

Lifetime risk

 

Male

Female

U.S. Populationa

Inhalation (ever-smokers)b

3.1 × 10-8

2.0 × 10-8

2.6 × 10-8

Inhalation (never-smokers)b

0.59 × 10-8

0.4 × 10-8

0.50 × 10-8

Inhalation (population)b

2.1 × 10-8

1.2 × 10-8

1.6 × 10-8

Ingestion

0.15 × 10-8

0.23 × 10-8

0.19 × 10-8

Total Risk (inhalation and ingestion)

2.2 × 10-8

1.4 × 10-8

1.8 × 10-8

a These rounded values combine the various subpopulations, with appropriate weighting factors taken from the 1990 U.S. Census.

b Based on the radon decay product risks of BEIR VI Report (National Research Council 1999) and includes the incremental dose to showering with thc uncertainties in these estimates.



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