National and regional distributions of airborne radon concentrations in U.S. homes.

The National Residential Radon Survey was conducted during 1989 and 1990 to provide data on the frequency distribution of annual average radon concentrations in U.S. residences nationwide, in U.S. Environmental Protection Agency defined Regions, and in subgroups of the housing stock. The National Residential Radon Survey also provided housing and demographic data and a preliminary assessment of the relationship of housing and geographical characteristics to residential radon concentrations. This paper focuses solely on the national and regional estimates of annual average radon concentrations. A stratified, three-stage sampling procedure was used to select housing units for the survey. Data were collected through personal interviews with residents and placement of alpha track detectors in each level of the residences for 12 mo. The survey found an arithmetic annual average radon concentration in U.S. homes of 46.3 +/- 4.4 Bq m-3 (1.25 +/- 0.12 pCi L-1). About 6.0 +/- 1.4% of homes (5.8 million) had radon levels greater than the U.S. Environmental Protection Agency's action level for mitigation of 148 Bq m-3 (4 pCi L-1). Concentrations varied significantly across Environmental Protection Agency Regions. A lognormal distribution was found to closely approximate the major distributions of radon concentrations.