Ohio sits atop limestone and shale geology that releases radon as uranium decays. Hamilton, Butler, Warren, and Clermont counties are all designated EPA Radon Zone 1 (the highest classification), meaning average indoor levels exceed 4.0 pCi/L.
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The Cincinnati Arch is an Ordovician-period geologic uplift exposing limestone and shale at relatively shallow depth across southwest Ohio. Both rock types contain uranium-238 in trace concentrations sufficient to produce measurable radon as decay occurs. Ohio Department of Health publishes county-level data showing Hamilton County at the highest end of the state for residential testing averages, with Cuyahoga County (Cleveland) and Franklin County (Columbus) close behind. Roughly 80% of Ohio counties are EPA Zone 1; the entire state is at minimum Zone 2. Testing is recommended for every Ohio home regardless of county; mitigation is recommended for any home reading above 4.0 pCi/L. Cincinnati neighborhood-level variation: even within Hamilton County, certain neighborhoods test consistently higher than others. Pre-war stone-basement housing in Hyde Park, Norwood, and Walnut Hills shows the highest pre-mitigation averages (often 8-12 pCi/L). Newer slab-on-grade housing in Mason and West Chester typically tests lower (1.5-4.0 pCi/L). The Ohio Department of Health county-level averages mask this within-county variation. Cincinnati mitigators on our team have working knowledge of which Cincinnati ZIP codes consistently show high pre-mitigation readings and quote scope accordingly during the on-site assessment.