Ohio's radon problem isn't uniform across the state. Two distinct geological provinces — glacial till in the north and west, and the unglaciated Appalachian Plateau in the east and southeast — both produce elevated indoor radon, but for different reasons and through different mechanisms. Understanding which province your county sits in helps explain why your home tests high and what mitigation approach works best.
The Glacial Boundary: Ohio's defining geological line
Approximately 18,000 years ago, the Wisconsinan ice sheet — the most recent glaciation in the Pleistocene — covered roughly the northwestern two-thirds of Ohio. The ice sheet's southern boundary ran in a diagonal line from near Cincinnati through central Ohio up to the northeast corner of the state. Everything north and west of this line was buried under glacial till (a mix of clay, silt, sand, and rock fragments deposited by the receding ice). Everything south and east of the line was left as the unglaciated Appalachian Plateau — bedrock-exposed, hillier terrain.
This geological boundary is also a radon boundary. The two provinces produce radon through entirely different mechanisms.
Province 1: Glacial Till (Northern and Western Ohio)
Counties: Cuyahoga, Lucas, Lorain, Hamilton, Montgomery, Franklin, Stark, Mahoning, Butler, and most of central + western Ohio
Cities: Cleveland, Toledo, Columbus, Cincinnati, Dayton, Akron, Canton, Youngstown
In glacial-till country, the radon source is primarily the till itself. When the Wisconsinan ice sheet scraped across Ontario and the Canadian Shield, it picked up rock fragments containing trace uranium and thorium. As the ice retreated, it deposited that material across Ohio as till — a heterogeneous mix of clay, silt, sand, and rock fragments. The uranium content of glacial till is low individually, but the till covers vast areas and is extremely permeable to soil gas in some layers, sealed in others. This dual permeability profile means radon generated in the till can concentrate under home basements where the till's clay-rich layers act as a partial seal.
Most of Ohio's largest cities (Cleveland, Toledo, Columbus, Cincinnati, Dayton, Akron) sit on glacial till. Mitigation in these areas relies on standard active sub-slab depressurization with attention to sealing the till-foundation interface.
Province 2: Unglaciated Appalachian Plateau (Eastern and Southeastern Ohio)
Counties: Mahoning (eastern portion), Columbiana, Carroll, Tuscarawas, Harrison, Belmont, Jefferson, Monroe, Noble, Guernsey, Muskingum, Morgan, Washington, Athens, Meigs, Vinton, Hocking, Perry, Coshocton, Holmes (southern portion)
Cities: Steubenville, Marietta, Athens, Cambridge, Zanesville
In the Appalachian Plateau, the radon source is the exposed bedrock itself — primarily Mississippian-age sandstones (Berea Sandstone, Sharon Conglomerate, Logan Formation) and Devonian-age shales (Ohio Shale, Chagrin Shale, Olentangy Shale). These rocks contain trace uranium that decays to radium and then radon. Without glacial till cover, radon-rich soil gas has a direct path from bedrock to home foundations.
Plateau-region homes are often built into hillsides with partial basements, walkout basements, or stacked-foundation designs. Mitigation requires more careful design — multiple suction points may be needed to capture soil gas under different foundation segments, and crawl-space encapsulation may be combined with sub-slab depressurization.
Why both provinces are EPA Zone 1
EPA's Map of Radon Zones (originally published 1993, regularly updated) classifies counties into three zones based on predicted indoor radon levels. Zone 1 is the highest predicted radon potential (predicted average indoor radon > 4 pCi/L). The vast majority of Ohio counties fall into Zone 1, despite the two distinct geological provinces.
This is unusual — most states have a mix of Zone 1, Zone 2, and Zone 3 counties. Ohio's near-universal Zone 1 designation reflects the reality that both glacial till and Appalachian Plateau bedrock are radon-productive, just through different mechanisms.
Where the two provinces meet: the Transition Zone
Counties along the Glacial Boundary — Richland (Mansfield), Knox, Licking, Fairfield (Lancaster), Hocking — sit at the transition between the two provinces. Homes in these counties often have characteristics of both: some glacial till cover plus shallow exposure of underlying bedrock (often the uranium-rich Ohio Shale). The transition zone produces some of the most variable radon results in the state because the radon source can change house-by-house.
What this means for testing and mitigation
The geological lesson for Ohio homeowners: regardless of which province you're in, get your home tested. Both glacial till and Appalachian Plateau geology consistently produce indoor radon levels above the EPA action level. The mitigation strategy varies by foundation type (full basement, crawl space, slab, hillside) and bedrock province, which is why hiring an experienced NRPP + ODH-licensed contractor who understands Ohio geology matters.