Regional levels of indoor radon in Virginia and Maryland

The levels of Rn-222 in homes located in Fairfax County, Virginia, and Montgomery County, Maryland, are currently being measured during four consecutive three-month seasonal intervals using alpha-track detectors. Significant variations occur between parts of northern Virginia and southern Maryland because the area is part of three very different geologic provinces. Results from the winter period in these three provinces show that the indoor radon levels were about twice as high as anticipated. Approximately 45 percent of the homes had winter indoor radon levels above 4 pCi/l, the EPA’s recommended action level, and in the spring period, more than 30 percent of the homes still had indoor levels above 4 pCi/l. Indoor radon variations due to seasonal control were about as significant as geological control. Worst-case combinations developed over some rock units in the winter, producing areas in which about 70 percent of the homes exceeded 4 pCi/l.