Average Radon Levels in Ohio: EPA Zone Data and What It Means
Ohio Is Entirely EPA Radon Zone 1
The EPA divides the United States into three radon zones based on predicted average indoor screening levels. Zone 1 is the highest category, defined as counties where the average indoor radon level is expected to exceed 4 pCi/L. Zone 2 ranges from 2 to 4 pCi/L. Zone 3 is below 2 pCi/L.
Ohio has no Zone 2 or Zone 3 counties. Every single county in the state is classified as Zone 1. That does not mean every home in Ohio has dangerous radon levels, but it does mean Ohio has more high-radon homes per capita than most of the country.
The Ohio Department of Health (ODH) estimates that roughly 40 percent of Ohio homes test above the EPA action level of 4 pCi/L. Some regions run considerably higher than that average. Testing is the only way to know what is happening in a specific house, because radon levels vary dramatically from one building to the next, even on the same street.
Why Ohio Has High Radon: The Geology
Radon comes from uranium and radium decaying in soil and rock. Ohio's elevated radon is a direct result of its geological history, and that history split the state into two distinct regions: the glaciated plains of northern and western Ohio, and the unglaciated Appalachian Plateau of southeastern Ohio.
Glaciated Northern and Western Ohio
During the last ice age, glaciers from the Canadian Shield repeatedly advanced across northern and western Ohio. As they moved, they scraped up uranium-bearing rock and deposited it across the landscape as glacial till, a dense mixture of clay, sand, gravel, and rock fragments. This till can be hundreds of feet deep in some parts of northwest Ohio.
When uranium-bearing till sits beneath a house, radon gas works its way up through the soil and enters through foundation cracks, floor drains, and crawl space openings. The thick, fine-grained nature of the glaciated till traps radon near the surface and funnels it toward structures. This is why northwest Ohio counties like Lucas (Toledo), Wood, Hancock, Allen, Mercer, and Auglaize consistently produce some of the highest radon readings in the state.
Southeastern Ohio: Appalachian Plateau and Shale
Southeast Ohio was not glaciated. The Appalachian Plateau counties, including Athens, Hocking, Vinton, Morgan, and Gallia, have different geology: Devonian shale, sandstone, and coal-bearing formations. Shale in particular can be high in uranium and thorium, and the natural fractures in Appalachian rock give radon an easy path into homes.
Radon levels in this region are less uniform than in glaciated Ohio. One home may test at 2 pCi/L while a neighbor on a different soil type tests at 12 pCi/L. This unpredictability makes testing especially important here, since regional averages are a poor guide to what is in any individual home.
Radon Levels by Region and County
| Region | Key Counties | Radon Risk Level | Primary Driver |
|---|---|---|---|
| Northwest Ohio | Lucas, Wood, Hancock, Allen, Mercer, Auglaize | Very High | Deep uranium-bearing glacial till |
| Northeast Ohio | Cuyahoga, Summit, Stark, Wayne | High | Glacial deposits from Lake Erie basin |
| Central Ohio | Franklin, Delaware, Union, Madison, Pickaway | High | Till plains, outwash deposits |
| Southwest Ohio | Montgomery, Greene, Clark, Warren, Butler | High | Glaciated till, limestone bedrock |
| Southeast Ohio | Athens, Hocking, Vinton, Gallia, Morgan | Variable but elevated | Devonian shale, sandstone fractures |
Northwest Ohio: The Highest-Risk Region
Lucas County, which includes Toledo, is among the highest-testing counties in Ohio. The city sits on the bed of ancient Lake Erie, surrounded by deep glacial till. Wood County to the south and Hancock County further south share similar geology. Allen County (Lima) and Mercer and Auglaize counties in the far northwest corner are also consistently high.
In these counties, homes with basements are at greatest risk because the basement sits in or near the glacial till layer where radon concentrations are highest. Homes on slabs can also have significant radon entry, particularly when built on clay-rich soil with poor natural drainage.
Northeast Ohio: Cleveland and Akron Metro
Cuyahoga County (Cleveland) and Summit County (Akron) are high-radon areas. Northeast Ohio was glaciated by the Lake Erie lobe of the Laurentide ice sheet, depositing material from the Canadian Shield across the region. Stark County (Canton) and Wayne County (Wooster) to the south also see consistently elevated levels.
The Cleveland metro real estate market treats radon testing as routine. Virtually every home inspection in suburban Cuyahoga County includes a radon test, and mitigation systems are a common sight on homes throughout the region.
Central Ohio: Columbus Metro
Franklin County (Columbus) and its surrounding counties, including Delaware to the north, Union to the northwest, Madison to the west, and Pickaway to the south, sit on the glaciated till plains of central Ohio. The Columbus metro is one of the fastest-growing markets in the Midwest, and radon disclosure comes up in nearly every transaction.
ODH data consistently shows that a substantial share of Columbus-area homes test above 4 pCi/L. Delaware County, which has seen explosive residential development, has a mix of newer construction (which can trap radon more efficiently due to tighter building envelopes) and older homes that were built before radon was well understood.
Southwest Ohio: Dayton and Cincinnati Corridor
Montgomery County (Dayton), Greene County, Clark County, Warren County, and Butler County in southwest Ohio are all Zone 1. The region sits on thick glacial deposits over limestone and dolostone bedrock. Average levels here tend to be somewhat lower than northwest Ohio but remain well above the national average.
Hamilton County (Cincinnati) is also Zone 1, though its position in the Great Miami and Ohio River valleys and its mix of urban, suburban, and hillside terrain produces more variability than the flat till plains to the north. Testing matters just as much here, even if the average risk is slightly lower.
Southeastern Ohio: Appalachian Plateau
Athens County, Hocking County, Vinton County, Morgan County, and Gallia County are the heart of unglaciated Ohio. Radon here comes from different geological sources than in the rest of the state: fractured Devonian shale, organic-rich black shale units, and coal seams can all off-gas radon into homes.
The house-to-house variation in this region is higher than anywhere else in Ohio. A home built on a ridge top with sandstone below may test very low. A home in a hollow with shale walls and a crawl space may test extremely high. Regional averages from ODH data are not reliable predictors for individual properties in this part of the state. Every home needs its own test.
Seasonal Variation in Ohio
Radon levels in Ohio homes typically rise in winter and fall in summer. Several factors drive this pattern:
- Homes are sealed more tightly in cold weather, reducing natural ventilation
- Frozen ground surface acts as a cap, directing more soil gas toward structures rather than releasing it into the atmosphere
- Heating systems create negative pressure inside homes, drawing in soil gas through foundation gaps
Short-term radon tests (charcoal canisters left for 48 to 96 hours) can vary by 30 to 50 percent depending on when they are conducted. Long-term tests (90 days to one year) give a more accurate picture of annual average exposure. For real estate transactions, the EPA recommends short-term testing under closed-house conditions, with a follow-up test if the first result is between 4 and 8 pCi/L.
The 4 pCi/L Action Level
The EPA recommends taking action when a home tests at or above 4 pCi/L. At that level, the lifetime lung cancer risk from radon exposure is estimated at about 7 in 1,000 for non-smokers, and substantially higher for smokers. The World Health Organization recommends action at 2.7 pCi/L, a threshold that would put a large majority of Ohio homes in the action range.
There is no radon level that is completely risk-free. The goal of mitigation is to get levels as low as reasonably achievable. Most Ohio homes treated with sub-slab depressurization reach levels below 2 pCi/L.
Test Before You Assume
Zone 1 classification tells you the statistical likelihood that a home in a county has elevated radon. It does not tell you what is in your specific home. Two identical houses on the same street can test at 1 pCi/L and 14 pCi/L, depending on soil permeability beneath the foundation, the presence of cracks or openings in the slab, and how the HVAC system moves air.
Testing is the only way to know. Ohio certified radon testers use devices calibrated to NRPP or NRSB standards, and their results are reliable guides for whether mitigation is needed. Find certified radon testers in Ohio to schedule a test.
Frequently Asked Questions
What EPA radon zone is Ohio?
Ohio is entirely EPA Radon Zone 1, the highest category. Every county in the state has a predicted average indoor radon level above 4 pCi/L, the EPA action level. Ohio is one of only a handful of states where no county falls into Zone 2 or Zone 3.
Which part of Ohio has the highest radon levels?
Northwest Ohio, particularly Lucas (Toledo), Wood, Hancock, Allen, Mercer, and Auglaize counties, consistently records the highest radon levels. This region sits on very deep uranium-bearing glacial till deposited by Canadian Shield glaciers. Columbus and Cleveland metro areas are also high-risk.
Does southeastern Ohio have lower radon than the rest of the state?
Not necessarily lower, but more variable. Appalachian Ohio (Athens, Hocking, Vinton, and surrounding counties) has different geology: fractured Devonian shale and coal-bearing formations. Individual homes in this region can test very high or relatively low depending on exactly what is under the foundation.
Why do radon levels go up in winter in Ohio?
In winter, Ohio homes are sealed tightly, reducing ventilation. Frozen ground pushes more soil gas toward structures instead of releasing it outdoors. Home heating systems also create negative pressure that draws radon up through foundation gaps. Long-term tests (90 days or more) average out seasonal swings and give a more accurate annual exposure estimate.
Does a newer home in Ohio have lower radon?
Not reliably. Newer homes built after radon-resistant construction techniques became common may have passive mitigation features, but a tighter building envelope can also trap more radon. Testing is the only way to know, regardless of the age of the home.