Figure 1. Indoor Radon Levels by County, 1990
Radon is a colorless, odorless, radioactive gas produced from the natural decay of radium, which in turn is a decay product of uranium. Radon is found in almost all types of soils, with the level of radon concentration depending on several factors: the composition underlying rock structures, soil porosity, moisture content, and permeability to gas movement.
Radon emitted from the ground typically enters homes and other building through cracks and openings in building foundations. Therefore, lower levels of buildings are more likely to have higher concentrations of radon. It is estimated that one out of every 15 homes in the United States have indoor radon concentrations exceeding the recommended level.
In the United States, federal and state officials urge homeowners to reduce their radon exposure if average levels exceed 4 picoCuries per liter (pCi/l); this value is approximately three times the average indoor-air level of radon. However, recent studies report varied results on the level of radon that poses a health risk. A Swedish study found that people exposed to radon levels between 3.8 and 10.8 pCi/l had a 30% greater risk of lung cancer than people exposed to levels less than 3.8 pCi/l. A Canadian study found no link between radon and lung cancer and suggested that the 20 pCi/l radon standard in Canada is justified.
North Carolina's Division of Radiation Protection (DRP) maintains two radon datasets. The first dataset contains information gathered during a joint survey by U.S. EPA and DRP in the spring of 1990. This survey was statistically based and included 1,290 homes that were randomly sampled. The criteria for inclusion in the survey were that homes had to be owner occupied, have a telephone number, and have one floor on or below ground level.
The second radon dataset is nonstatistically based and contains information gathered in 1991 from a variety of sources, including commercial vendors that do radon testing, homeowners using commercially-available radon testing kits, and research organizations using more extensive testing methods. This dataset contains approximately 7,000 observations and is upperwardly biased since most homeowners reported their results only if they had a high reading.
Because of the bias of the 1991 survey, this report will present information from the 1990 statistical survey to generate a “snapshot” view of indoor radon levels for North Carolina counties in 1990 (Figure 1).
Notice that the mountain areas of the state had the highest radon levels with four counties,
Ashe, Watauga, Henderson, and Transylvania having average radon concentrations in the
4.00-5.99 pCi/l range. The second highest level of radon concentrations, 3.00-3.99 pCi/l,
was also found in the mountains, namely, Avery, Swain, and Cherokee counties, with one
upper-piedmont county, Forsyth, having an average concentration in this range. The third
highest average concentrations, 2.00-2.99 pCi/l, were found mainly in the mountain counties
and in some of the bordering upper-piedmont counties. Finally, concentrations below 2.00
pCi/l are located almost exclusively in the middle and lower piedmont counties and coastal
areas of the state.