Ohio Shale radon: why Franklin and Delaware counties test high

You live in Delaware, Powell or somewhere in Franklin County, a neighbor just had a radon fan put in, and you want to know why this corner of Ohio keeps coming up whenever radon does. The short answer is a band of dark rock under your feet. The longer answer is more interesting, and it changes how you should read any map or average you come across.

The rock under Central Ohio

The main character is the Ohio Shale, a Late Devonian black shale made up of the Huron, Chagrin and Cleveland members. The U.S. Geological Survey's Ohio radon chapter puts its average uranium content at about 30 parts per million and calls it the largest source of uranium in the state. Radon is born from uranium's decay, so wherever this shale sits near the surface, the raw material for indoor radon sits there too.

Across Central Ohio the shale comes to the surface in a belt running north to south, and Franklin County lies over that band. Directly beneath it lies the Olentangy Shale. USGS found the shale's radon potential rises with its uranium, and the uranium follows the rock's organic carbon, which climbs as you move from east to west across Ohio.

Put together, the geology predicted high indoor radon along a north-south corridor through the middle of the state, and Franklin County sits on it. A random survey of Franklin County homes by Grafton (1990), cited in the USGS chapter, found 92 percent above 4 pCi/L on screening measurements.

Fractures and glaciers move the gas around

Two things keep the story from being as simple as "live on shale, get radon."

First, the Ohio Shale is split by vertical fractures. USGS notes those cracks let soil gas travel in bulk over long distances rather than creeping only by diffusion, so radon made deep in the rock has an easier road up.

Second, glaciers. Roughly two-thirds of Ohio lies under glacial material, and the ice that left it dragged shale debris along the way. The counties that average above 4 pCi/L tend to follow the shale band as the glaciers smeared it out, together with glacial and residual limestone soils.

At the surface, how easily gas moves through those deposits matters more than anything else. Work by Smith and Mapes (1989), summarized by USGS, found permeability of the glacial sediment to be the strongest control on surface radon. End moraines, the ridges of packed till, had the lowest indoor radon. Outwash and alluvium, the sand and gravel laid down along river valleys, had the highest. Around Columbus that describes the valley ground along the Scioto, the Olentangy, Alum Creek and Big Walnut Creek, while much of the region's clay-rich till moves gas slowly. Treat that as a tendency across many homes, not a verdict on any one street.

The limestone surprise

Here is the finding that trips up anyone who thinks the shale explains everything. In the same area, Harrell and colleagues (1991) measured higher basement radon over the Columbus and Delaware Limestones than over the Ohio Shale itself. The USGS explanation: as carbonate rock weathers, uranium can concentrate in the iron-rich soil it leaves behind.

Sitting on limestone rather than shale is not an exemption. If anything, this is the fact that should retire the idea that one side of a county is safe ground. Our FAQ on where Ohio radon comes from covers the decay chain itself, from uranium to the gas that reaches a basement.

What the numbers say, and what they do not

Two sets of numbers are worth knowing, and both come with caveats.

The 1988-89 EPA and state residential survey placed 2- to 7-day charcoal detectors in the lowest level of each home, under screening conditions, with small samples in each county:

• Franklin: of 170 homes, 64 percent read above 4 pCi/L. Median 5.4, mean 7.4, highest 46.0.
• Delaware: of 20 homes, 55 percent above 4. Median 5.6, mean 6.2.
• Licking: of 29 homes, 72 percent above 4. Median 5.3, mean 8.0.
• Fairfield: of 31 homes, 61 percent above 4. Median 8.1, mean 19.9, highest 238.5.

Look at Fairfield. One house at 238.5 drags the mean far above the typical result. The median describes the typical house; the mean tells you some houses are very high. Neither describes yours.

Newer figures come from the American Lung Association's 2022 Ohio report, which drew on CDC tracking data for 2008 through 2017. Franklin County logged more pre-mitigation tests than any other Ohio county, 16,142 of them, with an estimated mean of 9.5 pCi/L, against 6.5 for Ohio overall. Licking County had the state's highest testing rate, about 66 tests per 1,000 housing units. These are tests people chose to run, not a random sample of houses, so read them as a portrait of tested homes.

The Ohio Department of Health now publishes a dashboard of radon levels by ZIP code and county, built from pre-mitigation results that Ohio-licensed professionals have reported since 2010 and refreshed monthly. ODH is clear that those maps are for education and that testing is the only way to know a building's level. Our radon zones by ZIP guide gathers the county and ZIP picture for the Columbus area in one place.

Zone 1 in every direction

EPA's radon zone map puts every Central Ohio county in Zone 1, its highest category, meaning a predicted average indoor screening level above 4 pCi/L: Franklin, Delaware, Union, Madison, Licking, Fairfield, Pickaway, Knox and Morrow. The 1988-89 survey tested only 10 homes in Madison County and 6 in Union, and both came back with low averages. Samples that small say very little, and EPA mapped both counties as Zone 1 regardless.

EPA itself says the 1993 map should not be used to decide whether a particular home needs a test. Its advice is that every home should be tested.

Geology sets the odds; a test sets your number

Everything above explains why Delaware, Powell, Clintonville and Canal Winchester all sit in radon country. None of it tells you what your basement reads. Two houses on one street, one on a pocket of river gravel and one on tight till, one with an open sump and one without, can come back far apart.

What to do with the geology lesson:

• Test the lowest level you live on, or could live on, in a regularly used room.
• Use a free Ohio kit from ohio.radon.com, a hardware-store charcoal kit at $15 to $30, or a professional short-term test at $150 to $300 through our radon testing service.
• If the result is 4 pCi/L or higher, the Ohio protocol calls for a follow-up test before a decision, and the map never gets a vote.

Next step: order the free state kit this week or call us to set up a professional test, and let your own result, not the rock or the county average, decide what happens next.