Table F.2 summarizes the probability distributions used to represent value ranges for the parameters of this model. Each parameter has a variability distribution that is defined by two parameters, such as the mean and standard deviation. They are drawn from distributions that represent the uncertainty of the two parameters. EPA used two-dimensional Monte Carlo simulations to develop an outcome distribution for UD. Two models were used to develop a distribution for the transfer factor (TF)—a one compartment indoor-air model and a three-compartment indoor-air model. Similar results were obtained from the two models. Because of the lack of information on the uncertainty and variability in the inhalation risk factor for radon gas, EPA calculated the mean population risk, PR, as
PR = population risk of fatal cancer (cancers per year) posed by ingestion of radon gas in water.
UD = unit dose (pCi inhaled per year per pCi/L of radon in water).
RF = risk factor, lifetime risk of cancer per person per pCi inhaled per year.
Cmean = population mean concentration of radon in water, pCi/L.
N number of people in the population.
EPA used an inhalation risk factor, RF, of 1.1 × 10-12 cancer death per person per pCi/L of radon in water and a Cmean, of 246 pCi/L. We can use this equation with N = 1 and UD = 380, which is the median value with respect to uncertainty and the mean value with respect to variability, to estimate the per caput risk within the exposed population. This gives a per caput risk of 1.0 × 10-7, which is low compared with the mean individual risk (at median with respect to uncertainty) of 6.2 × 10-7 calculated for the ingestion pathway.
The EPA (1995) report provided a separate calculation of variability and uncertainty of risk associated with exposure to radon progeny attributable to radon releases from household uses of water. In the EPA uncertainty analysis for radon progeny, the basic equation used to calculate risk is based on uncertainty and variability of the unit dose factor. The unit dose factor for radon progeny released from water includes three factors:
UD= unit dose (WLM per year pCi/L)