The committee was asked to review teratogenic and reproductive risks. There is no scientific evidence of teratogenic and reproductive risks associated with radon in tissues from either inhalation or ingestion.

Comparison of the Present Analysis with the Previous EPA Analyses

The committee's analysis results in a modest reduction of the overall risk associated with radon in drinking water compared with the two previous analyses conducted by the EPA. However, the magnitudes of the risks associated with the different exposure pathways are different, as shown in table ES-2. The committee's analysis estimates that the inhalation pathway accounts for about 89% of the estimated cancer risk and ingestion accounts for 11%. In contrast, EPA's 1994 analysis suggested that inhalation accounted for 47% of the overall risk and ingestion accounted for 53%.

Based on the committee's analysis, the estimated inhalation risk has increased while the estimated ingestion risk has decreased. The committee did not do any new analysis for the inhalation risk. An average risk value based on three studies: BEIR IV, NIH, and BEIR VI (NRC 1988; Lubin et al. 1994; NRC 1999; respectively) was adopted. The committee did conduct a new analysis of the ingestion risk, based on a model developed for this study. This model reduces the overall ingestion risk factor by about a factor of 5, and suggests that, in contrast with the previous EPA analysis, almost all of the ingestion risk is attributed to the stomach. The estimated ingestion risk factors for various organs are compared in table ES-2.

There are a number of factors underlying the analysis of the risk associated with radon in drinking water, in addition to the lifetime radiation risk factors described above. These include the amount of water ingested, the effective expo-

Table ES-2

Comparison of Individual Lifetime Risk Estimates Posed by Radon in Drinking Water at a Concentration of 1 Bq m-3

Exposure Pathway

Committee Analysisa

1991 EPA Proposed Ruleb

1994 Revised EPA Analysisc

(A) Radon progeny inhalationa

1.6 × 10-8

1.3 × 10-8

0.81 × 10-8

(B) Radon inhalation

 

0.05 × 10-8

0.054 × 10-8

(C) Ingestion

0.2 × 10-8

0.4 × l0-8

0.95 × 10-8

Stomach

1.6 × 10-9

2.0 × 10-9

4.9 × 10-9

Colon

0.059 × 10-9

0.46 × 10-9

1.4 × 10-9

Liver

0.058 × 10-9

0.33 × 10-9

0.25 × 10-9

Lung

0.034 × 10-9

0.55 × 10-9

1.2 × 10-9

General tissue

0.079 × 10-9

0.61 × 10-9

1.5 × 10-9

Total risk (A+B+C)

1.8 × 10-8

1.8 × 10-8

1.8 × 10-8

a Total for the U.S. population (averaging across sex and smoking stares).

b EPA 1991b.

c EPA 1994b.



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