Radon levels in the Cincinnati metro are consistently among the highest in the eastern United States. According to the EPA's radon zone map, all four major Ohio counties in the metro — Hamilton, Butler, Warren, and Clermont (Clermont being Zone 2) — and the three primary Northern Kentucky counties (Boone, Campbell, Kenton) are Zone 1, the highest-risk classification.
What "EPA Zone 1" means
The EPA classifies US counties into three radon zones based on predicted indoor radon screening levels:
- Zone 1: Predicted average indoor screening level greater than 4.0 pCi/L
- Zone 2: Predicted average between 2.0 and 4.0 pCi/L
- Zone 3: Predicted average less than 2.0 pCi/L
Zone 1 doesn't mean every home tests high — but it does mean that the underlying geology is conducive to high radon, and individual homes can test dramatically higher than the county average.
Hamilton County radon data
The Ohio Department of Health has compiled radon test data for Hamilton County going back decades. Key findings:
- Approximately one in three tested homes (~33%) returns results above 4.0 pCi/L
- The county average screening level is approximately 5.2 pCi/L
- Older neighborhoods (Hyde Park, Mt. Lookout, Wyoming, Mariemont, Norwood) tend to test higher due to porous block-wall basements and older foundation construction
- Newer subdivisions are not immune — new construction in Mason, West Chester, and Liberty Township still routinely tests above 4.0 pCi/L
Butler County radon data
Butler County is one of Ohio's fastest-growing counties, with major suburban developments in West Chester, Liberty Township, Hamilton, Fairfield, and Monroe. Radon characteristics:
- EPA Zone 1
- Average screening level approximately 4.8 pCi/L
- About 30% of tested homes exceed 4.0 pCi/L
- Newer subdivisions sometimes test higher than expected because tighter modern construction traps radon more effectively
Warren County radon data
Warren County includes Mason — one of Cincinnati's most active real estate markets — plus Lebanon, Springboro, and Loveland. Radon characteristics:
- EPA Zone 1
- Average screening level approximately 4.5 pCi/L
- Approximately 28-32% of tested homes exceed 4.0 pCi/L
- Mason's housing boom has produced thousands of newer homes; radon testing is now routine in Warren County real estate transactions
Northern Kentucky radon data
Boone, Campbell, and Kenton counties all share Cincinnati's geological characteristics and all three are EPA Zone 1. Fort Thomas, Fort Mitchell, and Florence consistently test high due to older housing stock and hillside lots. The Kentucky Radon Program provides similar guidance to the Ohio Department of Health.
Seasonal variation in Cincinnati
Radon levels in Cincinnati homes typically vary 2-7× between summer and winter:
- Winter (Nov-Mar): Highest levels. Closed windows, indoor heating, and the stack effect maximize radon entry.
- Shoulder seasons (Apr, Oct): Moderate levels as homes balance ventilation and heating
- Summer (Jun-Aug): Lowest levels. Open windows, less stack effect, more natural ventilation.
This is why short-term tests done in summer often underestimate a home's true annual average — and why winter testing (or long-term alpha-track testing) is the most reliable.
What this means for Cincinnati homeowners
Three takeaways:
- Test your home, regardless of suburb or housing age. Zone 1 means the geology is hostile; the only question is whether your specific home has an entry route. Learn about professional testing.
- If you're buying or selling, expect radon to come up. Plan for it. See our real-estate transaction guide.
- If you test high, mitigation is straightforward. Most Cincinnati homes can be mitigated in a single day for $900-$1,600. Read about the mitigation process.