The committee's report addresses each of those points, and its conclusions are summarized below. The order of presentation below follows that in the report.
National data on indoor radon, radon in water, and geologic radon potential indicate systematic differences in the distribution of radon across the United States. Geologic radon-potential maps and statistical modeling of indoor radon exposures make it 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 concentrations. Some smaller areas of the southern states also have higher than average indoor radon concentrations. Data on radon in water from public water supplies indicate that elevated concentrations of radon in water occur in the New England states, the Appalachian states, the Rocky Mountain states, and small areas of the Southwest and the Great Plains.
The ambient concentration of radon varies with distance from and height over its principal source in the ground (rocks and soil) and from other sources that can locally or regionally affect it, such as lakes, mine or mill tailings, vegetation, and fossil-fuel combustion. However, diurnal fluctuations due to changes in air stability and meteorologic events account for most of the variability. Average ambient radon concentrations were measured by EPA over nine seasons at 50 sites across the United States. Most, but not all, sites coincided with the capital city of the state but did not statistically represent the population across the U.S.,