dure were made by the second quarter. The authors (Hopper and others 1991) emphasize that the study does not statistically represent the distribution of ambient radon concentrations in the United States but indicates that estimates of annual average ambient radon concentrations and the associated error estimates can be derived for each site. The authors used only quarters 2–5 for their original report for the sake of timeliness, but provided our committee with the entire data set for this report (R. Hopper, private communication). The updated Citizens Guide To Radon (EPA 1992c) reported an average outdoor concentration of 14.8 Bq m-3 on the basis of the survey.
As can be seen from table 2.1, ambient radon concentrations above the average (all quarters, all sites, 14.8 Bq m-3) tend to occur in the Appalachian Mountains, the northern Midwest, and the northern western states. Sites in the southern and western coastal states, the Great Lakes states, and several of the central and southwestern states tend to be at or below the average. These trends probably reflect the geology or other sources at the sites and the proximity to large water bodies. The bar graph in figure 2.7 illustrates the average of all data from each site by season, showing the spring minima and fall maxima.
Statewide or regional studies have been conducted in California (Liu and others 1991), Nevada (Price and others 1994), Minnesota and Iowa (Steck and