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## Risk Assessment of Radon in Drinking Water (1999) Commission on Life Sciences (CLS)

### Citation Manager

. "7 Defining Key Variabilities and Uncertainties." Risk Assessment of Radon in Drinking Water. Washington, DC: The National Academies Press, 1999.

 Page 133

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that the average ambient radon concentration would most likely be 14-16 Bq m-3. Thus, it is the committee's recommendation to treat the value of the average ambient radon concentration as being represented as a uniform distribution of range 14-16 Bq m-3 with a most probable value of 15 Bq m-3.

#### Variability and Uncertainty in Transfer Factors

The committee considered and re-evaluated the variability in the transfer of radon gas from water to indoor air. Assessing the increment of airborne radon in a home that arises from the use of water that contains dissolved radon is a problem that involves both uncertainty and variability. It involves the solubility of radon in water, the amount of water used in the home, the volume of the home, and the home ventilation rate. The amount of radon from the water is not constant throughout a home, but is higher in areas of active water use, such as bathrooms and kitchens. Table 7.1 summarizes the recommended values of the transfer factor and the parameters used to construct it.

The resulting geometric mean value is 5.5 × 10-5 or 3.9 × 10-5 with a geometric standard deviation (GSD) of 3.5. These values can be compared with those of Nazaroff and others (1987) who reported a geometric mean of 6.5 × 10-5 and a GSD of 2.8, and EPA (1995), which reported a geometric mean of 6.5 × 10-5 and a GSD of 2.9. There was reasonable agreement between the geometric mean of the transfer coefficient estimated by the model and the estimated value calculated from the measured data. The average of the measurements was 8.7 × 10-5 with a standard error of 1.0 × 10-5. With the modeled geometric mean ventilation of 1.07 air changes per hour, the calculated transfer coefficient is the same value as the measurements. However, if we use the estimate of the geometric mean of the ventilation rate of 0.77, the resulting estimate of the transfer coefficient is 1.2 × 10-4. The committee feels that there are problems with both the measurements of the transfer coefficient and the measurements that are the input values into the model. The committee recommends that EPA continue to use 1.0 × 10 -4 as the

Table 7.1

Parameters of the Lognormal Distributions for the Parameters in the Transfer-Factor Calculation

 Committee's Values Parameter Geometric Mean Geometric Standard Deviation House volume per occupant, m3 person-1 115 2.0 Ventilation rate 0.77 or 1.07 2.3 Transfer efficiency 0.52 1.3 Water use per capita, m3 person-1 hr-1 9.4 10-3 1.8 Transfer coefficient 5.5 × 10-5 or 3.9 × 10-5 3.5
 Page 133
 Front Matter (R1-R14) Public Summary (1-7) Executive Summary (8-22) 1 Introduction (23-31) 2 Baseline Information on Indoor Radon and Radon in Water in the United States (32-49) 3 Transfer of Radon from Water to Indoor Air (50-58) 4 Dosimetry of Ingested Radon and its Associated Risk (59-81) 5 Dosimetry of Inhaled Radon and its Associated Risks (82-104) 6 Molecular and Cellular Mechanisms of Radon Induced Carcinogenesis (105-123) 7 Defining Key Variabilities and Uncertainties (124-140) 8 Mitigation (141-179) 9 Multimedia Approach to Risk Reduction (180-197) 10 Research Recommendations (198-199) References (200-222) Glossary (223-230) A Behavior of Radon and Its Decay Products in the Body (231-240) B A Model for Diffusion of Radon Through the Stomach Wall (241-248) C Water-Mitigation Techniques (249-253) D Risks Associated with Disinfection By-products Formed by Water Chlorination Related to Trihalomethanes (THMs) (254-256) E Gamma Radiation Dose from Granular-Activated Carbon (GAC) Water Treatment Units (257-259) F EPA Approach to Analyzing Uncertainty and Variability (260-268) Index (269-279)