Predicted geometric mean (GM) indoor radon concentration by county. These GMs are for annual average living area (AALA) indoor radon concentrations and were estimated based on a correlation of two national databases, one having AALA concentrations from about 5,000 homes selected nationally and the other having about 40,000 short-term (several- day) screening measurements, usually taken in the basement if there was one. Note that there is considerable variation in indoor concentrations within counties; also note that some county estimates are much more certain than others.

For several years, researchers in the High-Radon Project at the Center's Indoor Environment Program have been developing a statistical methodology for estimating regional indoor radon concentrations across the United States. The purpose of this work is to help state and other agencies identify high-radon counties and areas more precisely so that these authorities can focus their indoor monitoring and control efforts more effectively. The Project is a collaboration of the Indoor Environment Program, the U.S. Geological Survey, and the Statistics Department of UC Berkeley with funding from DOE and EPA.

Approach and Types of Results
The Project's basic approach is to examine the statistical correlation between available indoor radon monitoring data and information on physical factors affecting indoor concentrations for a state or region. These factors may include house structure types, radium concentrations in surface soils, other soil and geological characteristics, meteorological variables, and other predictive variables. The resulting correlation model is then used to predict local concentrations for counties (or smaller geographic units) that are more precise than would be possible using the monitoring data alone, because representative data are invariably relatively sparse [Summer 1994, p.4].

One result is a map of indoor radon for the state or region for which the analysis is performed, or for the U.S. as a whole if applied across the country. This indoor radon map can help identify with more reliability where indoor concentrations are expected to be substantially higher than average. The appropriate state or local agencies could then target surveys in these areas to find the houses with very high levels of indoor radon and give their occupants information and assistance to reduce indoor radon concentrations.

Many of the project's analytical results are presented as estimates and uncertainties in geometric mean (GM) indoor radon concentrations by state or county. When interpreting GMs, it is important to realize that within any region, some smaller areas will have higher GMs than others and that some individual homes will have indoor concentrations considerably higher than the region's GM.

We have conducted analyses for several states that were selected because of their particular distribution of radon concentrations or because of the relative availability of suitable predictive information. We have also performed analyses using a national dataset and a series of regional analyses that include most of the 48 contiguous states. In both the state and regional analyses, we have often found that our statistical models account for about 80% of the variability in mean county concentrations or, more precisely, in the logarithm of the county geometric mean indoor concentrations.

These analytical results are available from published journal articles and, for the regional analyses, from the Project's home page. Our results do not include predictions for smaller geographic scales such as census tracts or townships. Soon, users will be able to perform analyses themselves using monitoring data at these smaller scales. We will make the model subroutines and their documentation available through the Project's Web site. Encouraging the widespread use of these methods is a principal objective of the Project.

--Anthony Nero and Phil Price

Phil Price
Indoor Environment Program
(510) 486-7875; (510) 486-6658 fax

Visit the project's Web site.

This work is supported by DOE's Office of Health and Environmental Research and the Environmental Protection Agency's Office of Air and Radiation.