Percentage of Homes, listed by County, that Exceed Radon Levels of 4.0 pCi/L: Cook: 25.50% DuPage: 39.50% Kane: 43.60% Kendall: 45.90% Lake: 30.90% Will: 41.10% DeKalb: 34% (Data compiled by the Illinois Emergency Management Agency's Department of Nuclear Safety) Please note that this data is not an indication of a particular house's potential to have elevated levels of radon. Radon is a house-by-house issue, and radon levels seasonally and diurnally fluctuate. Close to half of all Illinois homes test out elevated for radon gas! Our highest reading was 180 pCi/L!