Colorado , Montana , North Dakota , South Dakota , Utah and Wyoming are among the highest-risk states for lung cancer from radon gas , according to the EPA . Radon concentrations are generally high in this area because uranium is concentrated in the granites and metamorphic rocks of the Rocky Mountains .
A map of geologic radon potential in the United States . The “ high ” potential includes all of eastern South Dakota and the ridge around the Black Hills . ( Courtesy of USGS )
Burns said he ’ s seen homes in Rapid City with levels above 100 pCi / L — he ’ s even worked on a home that had a reading of 1,000 pCi / L in the Black Hills area .
Custer County has the highest average radon concentration with an estimated mean of 30.1 pCi / L — over seven times the EPA recommendation , according to the American Lung Association .
Of South Dakota counties that had at least 10 reported tests between 2008 and 2017 , the lowest mean radon level was 4.9 pCi / L in Aurora County . Minnehaha County , with 1,920 tests , had an estimated mean radon level of 6.5 pCi / L .