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.

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