Radon is a natural gas you cannot see, smell, or taste. It forms when uranium breaks down in soil and rock, and it can seep into homes through cracks, sump pits, and crawlspaces. Your risk depends heavily on which Ohio county you live in.
This guide answers that question directly: it ranks the Ohio counties with the highest radon levels, with a close look at Central Ohio and the Columbus metro area.
How the EPA Measures Radon Risk by County
The U.S. Environmental Protection Agency created the Map of Radon Zones in 1993. This map sorts every county in the country into one of three zones based on predicted average indoor radon levels.
Zone 1 counties carry the highest potential, with predicted averages above 4 picocuries per liter (pCi/L). Zone 2 counties fall between 2 and 4 pCi/L. Zone 3 counties are predicted to have averages below 2 pCi/L. The EPA sets 4 pCi/L as the action level, the point at which it recommends homeowners take action.
More than half of Ohio’s 88 counties fall into Zone 1, which makes Ohio one of the higher-risk states in the country.
Central Ohio Counties With the Highest Radon Levels
Central Ohio sits squarely in this higher-risk territory. Franklin, Delaware, Union, Fairfield, Pickaway, Licking, Knox, Madison, and Fayette counties all carry an EPA Zone 1 designation. Homeowners across Columbus, Delaware, Marysville, Lancaster, Circleville, Newark, and Mount Vernon all live in counties the EPA flags for elevated potential.
Radon Levels by County in Central Ohio
Researchers at the University of Toledo built the Ohio Radon Information System from tens of thousands of indoor readings collected across all 88 counties. An earlier University of Toledo geologic study used that same database to rank Ohio counties by geometric mean radon concentration. Several of the highest-ranking counties sit directly inside Ohio Radon Control’s service area.
| County | Geometric Mean Radon Level (pCi/L) | EPA Zone |
|---|---|---|
| Licking | 11.5 | Zone 1 |
| Pickaway | 7.5 | Zone 1 |
| Knox | 7.1 | Zone 1 |
| Franklin | 6.7 | Zone 1 |
| Ross | 6.2 | Zone 1 |
| Delaware, Union, Fairfield, Madison, Fayette | Above 4.0 | Zone 1 |
For comparison, the statewide average across tested Ohio homes runs 5.76 pCi/L, already above the EPA’s action level. Every county in the table above meets or exceeds that average, and Licking County runs more than double it.
Licking County's Radon Levels Stand Out
Licking County has the highest average radon levels in Ohio. University of Toledo geologic research puts its geometric mean at 11.5 pCi/L, nearly triple the EPA’s 4 pCi/L action level. No other Ohio county averages above 8 pCi/L.
Homeowners near Newark, Granville, and Hebron sit inside this high-reading zone. A short-term test remains the only reliable way to know your specific level, since a county average describes the region, not any single house.
Why Central Ohio Soil Drives High Radon
Central Ohio sits on glacial till layered over limestone and shale bedrock. These rock formations naturally contain uranium, and as it decays, it releases radon gas. The soil above this bedrock tends to be loose and permeable, which lets gas move upward easily.
Homes with basements pull air from the soil more strongly than slab homes, since basement pressure draws gas inward through foundation cracks. This combination of geology and common Central Ohio home construction explains why the region ranks this high.
What These Numbers Mean for Your Home
Radon is the second leading cause of lung cancer in the United States, and the leading cause among nonsmokers. A 2025 study in the Journal of Environmental Management links radon exposure to roughly 900 lung cancer deaths in Ohio every year.
The Ohio Department of Health reports that about one in three Ohio homes test at an elevated level. A county ranking raises the odds, but it does not tell you your home’s actual number. If you live in Licking, Pickaway, Knox, Franklin, or Ross County, a professional radon test is the only way to know for sure. A result at or above 4 pCi/L calls for a radon mitigation system.
Frequently Asked Questions
Licking County has the highest average radon levels in Ohio, with a geometric mean of 11.5 pCi/L. That is the only county average above 8 pCi/L in the state.
Franklin, Delaware, Union, Fairfield, Pickaway, Licking, Knox, Madison, and Fayette counties all carry an EPA Zone 1 designation, meaning their predicted average radon level exceeds 4 pCi/L.
Not necessarily. A county average describes the region, not any single house. The only way to know your home's actual level is a professional radon test.
The EPA sets 4 pCi/L as its action level. A reading at or above that level calls for a radon mitigation system to bring it back down to a safe range.
The Ohio Department of Health reports that about one in three Ohio homes tests at an elevated level, a rate that holds true even in neighborhoods that look identical from the outside.
Central Ohio sits on glacial till over limestone and shale bedrock, both of which naturally contain uranium. As that uranium decays, it releases radon gas into the loose, permeable soil above it.
Schedule Your Radon Test Today
Central Ohio’s geology puts homes across Franklin, Delaware, Union, Fairfield, Pickaway, Licking, and Knox counties at real risk for elevated radon. The only way to know where your home stands is to test it.
Ohio Radon Control offers professional testing and mitigation services throughout Columbus and Central Ohio, backed by licensed expertise under Ohio Department of Health credentials RC 105 and RS 199. Contact us today to schedule a test or request a free mitigation quote for your home.
