As described in chapter 2, the national average ambient air concentration was estimated to be 15 Bq m^{-3} with a high level of certainty that the value lies between 14 and 16 Bq m^{-3}. Therefore, the uncertainty in the estimation of M_{ambient} was represented by lower and upper bounds of 14 and 16 Bq m^{-3}, respectively.

Regarding the mean transfer factor, M_{TF}, the committee noted that the compiled measurement data had an estimated mean and standard error of 0.9 × 10^{-4} and 0.1 × 10^{-4}, respectively, on the basis of 154 observations, whereas the estimate derived from modeling was either 0.9 or 1.2 × 10^{-4} (see chapter 3). The committee's best estimate of the mean transfer factor was 1 × 10^{-4}.

The uncertainty in the estimation of M_{TF} was represented by lower and upper bounds of 0.8 × 10^{-4} and 1.2 × 10^{-4}, respectively. Therefore, on the basis of estimates of m_{ambient} (15 Bq m^{-3}) and m_{TF} (1 X 10^{-4} ) given above, the committee estimates the AMCL to be 150,000 Bq m^{-3}. The uncertainty of AMCL_{est} arising from the uncertainties in estimation of M_{ambient} and M_{TF} was estimated by considering the extremes of the bounds that were defined for the two input values. By propagating the upper and lower bounds on the numerator and denominator, the lower bound of the AMCL_{est} is 117,000 Bq m^{-3}, and the upper bound is 200,000 Bq m^{-3}.

EPA will set the MCL value on the basis of the committee's risk assessment in this report and its own policy considerations. However, for the examples in this chapter, it is necessary to assume a value of the MCL. It will be *assumed* that the MCL will be 25,000 Bq m^{-3} of water. The committee makes no recommendation or endorsement of a specific value and is using this assumption only in order to provide a framework for the following discussion of potential risk-reduction scenarios for implementing a multimedia mitigation program.

To estimate the health-risk reductions that are obtained by treating water to remove radon or mitigating homes to reduce indoor ^{222}Rn, it is necessary to consider all the associated risks. For example, the processing of water to remove radon would probably then require that the water be disinfected under the proposed rules (EPA 1992b). Disinfection could be performed through illumination with ultraviolet light for small systems or the addition of chlorine or ozone for larger systems. The risks arising from exposure to disinfection byproducts were discussed in chapter 8 and are estimated to be smaller than the risks arising from airborne radon. Thus, the incremental risk posed by disinfection byproducts will not be included in the risk-reduction analysis. The cancer risks to the body associated with ingested radon (2 × 10^{-9} Bq^{-1} m^{3}) are small but not negligible when compared with the risk to the lungs posed by the airborne decay products arising from radon released by water used in the home (1.6 × 10^{-8} Bq^{-1} m^{3}). Thus, the committee has assumed a mitigation of airborne radon equal to 113% of the airborne radon would provide an equivalent health-risk reduction to account for