Ohio sits atop limestone and shale geology that releases radon as uranium decays. Franklin, Delaware, Licking, and Fairfield counties are all designated EPA Radon Zone 1 (the highest classification), meaning average indoor levels exceed 4.0 pCi/L.
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Central Ohio is underlain by Devonian-period limestone and shale (the Columbus Limestone, the Olentangy Shale, and the Ohio Shale) exposed at relatively shallow depth across Franklin County and the surrounding metro. 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 Franklin County (Columbus) among the higher-testing counties in the state for residential averages, alongside Hamilton County (Cincinnati) and Cuyahoga County (Cleveland). 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. Columbus neighborhood-level variation: even within Franklin County, certain neighborhoods test consistently higher than others. Pre-war stone-basement housing in Clintonville, Bexley, and Grandview Heights shows the highest pre-mitigation averages (often 8-12 pCi/L). Newer slab-on-grade housing in Dublin and New Albany typically tests lower (1.5-4.0 pCi/L). The Ohio Department of Health county-level averages mask this within-county variation. Columbus mitigators on our team have working knowledge of which Columbus ZIP codes consistently show high pre-mitigation readings and quote scope accordingly during the on-site assessment.