With regard to uncertainty and variability about radon levels in ambient air and groundwater, there are several key questions, including:
National data on indoor radon, radon in water, and geologic radon potential indicate systematic differences in the distribution of radon across the United States. From geologic-radon potential maps and from statistical modeling of indoor radon exposures, it is clear that the northern United States, the Appalachian and Rocky Mountain states, and states in the glaciated portions of the Great Plains tend to have higher than average indoor radon (see chapter 2).
Available data on radon in water from public water supplies indicate that higher concentrations of radon in water occur in the New England, Appalachian, and Rocky Mountain states and in small areas of the Southwest and Great Plains. Available data also indicate that small water supplies have higher average radon concentrations than large ones. The reasonable agreement of water concentration variation among the various studies suggests that the Longtin (1990) data used by EPA (1995) are adequate for representing variations in water-supply radon concentrations.
The ambient concentration of radon outdoors varies with distance and height from its principal source in the ground (rocks and soil) and from other sources that can locally or regionally affect it, such as bodies of water, mine or mill tailings, vegetation, and fossil-fuel combustion. However, diurnal changes due to air stability and meteorologic events account for most of the variability. As reported in chapter 2 of this report, the committee does not believe that the available data are sufficiently representative to provide a population-weighted annual average ambient radon concentration. From the available data, the committee has obtained an unweighted average of 15 Bq m-3 with a standard error of 0.3 Bq m-3. The committee recommends this value as the best available national average ambient concentration. In reviewing all the other ambient-radon concentration data that are available for other specific sites, the committee concluded