others 1989), soil permeability, and foundation housing characteristics. It is a map of the land potential, not a map of exposure or risk. It was compiled from individual state geologic-radon potential maps (Gundersen and others 1993) that served as the basis of the Environmental Protection Agency (EPA) map of radon zones that has been incorporated into one of the national building codes (EPA 1993).
Figure 2.3 shows the most recent and most comprehensive map of indoor-radon potential and represents a prediction of the geometric mean of annual exposure to indoor radon. The elements used in the map include the radium content of the surficial soil derived from the aerial radiometric data collected by the National Uranium Resource Evaluation (summarized in Duval and others 1989), information on the geologic province that comprises most of the county (from the US Geological Survey), soil characteristics, the fraction of homes with basements and with living-area basements, and radon-concentration surveys conducted nationally and in each state from EPA and other sources. Those elements are used in a Bayesian mixed-effects regression model to provide predictions of the geometric mean indoor radon concentration by county. Additional details of the model are given in Price (1997). The predicted county means have standard errors of 15–30% for typical counties; the uncertainty in a given county depends on the number of radon measurements in the county and the level of detail in the geologic information.