The EPA's radon zone map classifies every county in the United States into one of three risk categories based on predicted indoor radon screening levels. Hamilton, Butler, Warren, Boone, Campbell, and Kenton counties — covering essentially all of the Cincinnati metro — are classified as Zone 1, the highest risk designation.
What EPA Radon Zone 1 actually means
Zone 1 counties have a predicted average indoor radon screening level greater than 4.0 pCi/L. This designation is based on geological surveys, indoor radon measurements, and predictive modeling.
It's important to understand what Zone 1 does and does not mean:
- It does mean that the geology underlying the county is highly conducive to radon production and entry into homes
- It does mean that a typical home in the county is more likely than not to need mitigation
- It does not mean every home in the county tests above 4.0 pCi/L — individual home variation is significant
- It does not mean you can predict your home's level from the zone alone — testing is still required
Why Cincinnati is Zone 1
Three converging geological and structural factors:
1. Uranium-rich bedrock
Cincinnati sits on a thick layer of Ordovician-period limestone and shale that is exceptionally uranium-rich for its age. The Kope Formation and Fairview Formation, both widely exposed across the Cincinnati Arch, contain elevated uranium concentrations that decay to produce radon.
2. Fractured limestone and good gas mobility
Limestone is naturally permeable, with countless small fractures and dissolution channels. This means the radon gas produced in the bedrock can migrate easily through the soil profile up to the surface — and into foundations.
3. Older housing stock with basements
Most Cincinnati homes — particularly older neighborhoods like Hyde Park, Mariemont, Wyoming, Norwood, and Fort Thomas — have full basements. Basements are the highest-risk foundation type for radon entry because they're deepest into the soil profile and provide the most surface area for gas to penetrate.
Cincinnati-area Zone 1 counties
| County | State | EPA Zone | Key Suburbs |
|---|---|---|---|
| Hamilton | OH | 1 | Cincinnati, Hyde Park, Indian Hill, Mariemont, Wyoming, Madeira, Blue Ash, Anderson Twp |
| Butler | OH | 1 | West Chester, Liberty Twp, Fairfield, Hamilton, Monroe |
| Warren | OH | 1 | Mason, Loveland, Lebanon, Springboro |
| Clermont | OH | 2 | Milford, Amelia, Batavia, Goshen |
| Boone | KY | 1 | Florence, Burlington, Hebron, Union |
| Campbell | KY | 1 | Fort Thomas, Newport, Bellevue, Cold Spring, Alexandria |
| Kenton | KY | 1 | Fort Mitchell, Covington, Villa Hills, Edgewood, Erlanger |
| Dearborn | IN | 2 | Lawrenceburg, Aurora |
What Zone 1 designation means for homeowners
If you live in any of the seven Zone 1 counties listed above:
- Test your home, period. Don't assume the zone designation gives you your level. Test.
- Expect higher readings. If you've never tested, statistical likelihood of being above 4.0 pCi/L is 25-35%.
- Plan for radon in any real estate transaction. Buyers expect it; pre-list mitigation if you're selling.
- Don't panic. Zone 1 doesn't mean an emergency — it means an actionable, fixable, well-understood problem with a $900-$1,600 solution.