Radon comes from uranium in Ohio's rock and glacial soils. EPA maps 53 of Ohio's 88 counties as Zone 1, including Hamilton, Butler, and Warren, and the rest as Zone 2. About 35 percent of the roughly 508,000 home tests reported to ODH since 2010 exceeded 4.0 pCi/L.
More detail
Every Ohio county has measurable radon potential, which is why EPA's 1993 map has no Ohio county in Zone 3, the low-potential category. The U.S. Geological Survey's 1993 assessment of Ohio identified the uranium-rich Ohio Shale as the state's largest source rock, noted that about two-thirds of the state is covered by glacial material, and found that permeability of that sediment strongly controls how much radon reaches the surface, with homes over glacial outwash and alluvium showing the highest measurements. Southwest Ohio sits on Ordovician limestone and shale, and the measured results here differ from the zone map in instructive ways. Ohio Department of Health data from licensed testers since 2010 shows about 22 percent of Hamilton County tests above 4.0 pCi/L, while Butler, Warren, and Clermont counties each run about 36 to 37 percent, even though Clermont is mapped as Zone 2. The highest shares in the state, above half of all tests, are in Fairfield, Pickaway, Madison, Coshocton, Licking, and Morrow counties. Within Hamilton County, the share climbs from single digits in parts of the urban core to nearly half of tests around Harrison. Our Ohio radon map by county lists every county's zone and measured results, and the radon levels by ZIP code table breaks Greater Cincinnati down further. A county or ZIP figure is only a starting point, so the next step is a test in your own home.