Geology. The uranium-rich Ohio Shale runs north to south beneath Franklin and Delaware counties, and glaciers spread shale and limestone debris across the region. Every Central Ohio county sits in EPA Radon Zone 1.
More detail
The USGS radon assessment for Ohio traces the pattern: counties averaging above 4 pCi/L tend to follow the Ohio Shale outcrop belt as glaciers redistributed it, along with soils formed from limestone. One finding surprises people. A study the USGS cites measured higher basement radon over the Columbus and Delaware limestones than over the shale nearby, likely because weathering concentrates uranium in iron-rich soil. Soil texture matters as well: sandy outwash and river alluvium let gas travel more easily than dense clay till or moraine ridges. The Olentangy Shale lies just beneath the Ohio Shale. For county numbers and the older state survey, see our Central Ohio radon zones table and the Ohio Shale explainer.