Radon test results in Central Ohio, county by county and ZIP code by ZIP code
The Ohio Department of Health publishes the radon tests that radon professionals report to it, summarized by county and by ZIP code: how many tests, what share came back above 4 picocuries per liter, and the middle and highest results. This page sets out those figures for seven Central Ohio counties and 36 ZIP codes as the department's dashboard carried them in September 2026, together with what the department and the federal Environmental Protection Agency say about reading them, and why a basement that is about to become a bathroom is exactly the case the federal guide names. A county or ZIP figure describes tests of other houses; only a test of a house says anything about that house.
Last updated September 2026 · Radon figures are the Ohio Department of Health's Radon Levels by Zip Code and County dashboard, exported September 24, 2026, and the testing advice is the Environmental Protection Agency's · No prices on this page; the Estimate Builder carries those · Part of the knowledge base
What the Ohio Department of Health publishes
The department's radon data page, dated April 21, 2026, says its maps are for educational purposes only and that radon testing is the only way to know the radon levels in a home. It describes the figures behind its Radon Levels by Zip Code and County dashboard in four points: the statistics are based on radon levels prior to radon mitigation, the data runs from 2010 to the present, it is reported by radon professionals who hold an Ohio license, and it is updated monthly. The figures below are the dashboard's own export, taken on September 24, 2026, and printed as exported.
The unit is the picocurie per liter of air. The federal Environmental Protection Agency's A Citizen's Guide to Radon recommends: "Fix your home if your radon level is 4 picocuries per liter (pCi/L) or higher." The same guide adds that radon levels less than 4 pCi/L still pose a risk, and in many cases may be reduced. The department's dashboard reports, for each county and ZIP code, the share of tests above that 4 line.
County by county
| County | Number of tests | Percent of tests above 4 pCi/L | Geometric mean, pCi/L | Arithmetic mean, pCi/L | Highest single result, pCi/L |
|---|---|---|---|---|---|
| Franklin | 109,347 | 45.59 | 3.7 | 6.0 | 904 |
| Delaware | 22,470 | 49.82 | 3.9 | 6.0 | 260.9 |
| Licking | 10,952 | 51.92 | 4.7 | 8.9 | 479 |
| Fairfield | 7,251 | 53.8 | 4.5 | 7.6 | 414.2 |
| Union | 3,917 | 43.17 | 3.4 | 5.0 | 307 |
| Pickaway | 1,419 | 52.78 | 4.8 | 8.0 | 278.3 |
| Madison | 1,184 | 52.45 | 4.5 | 6.9 | 143 |
In every one of the seven counties, somewhere between 43 and 54 percent of reported tests came back above 4. Fairfield County has the highest share, 53.8 percent, and Union the lowest, 43.17. Franklin County carries by far the most tests, 109,347. The two means differ for a reason worth knowing: a handful of very high results, such as Franklin County's single highest of 904, pull an arithmetic mean up, while a geometric mean is far less sensitive to them, and in every one of the seven counties the arithmetic mean is the higher of the two.
ZIP code by ZIP code
The dashboard reports the same statistics for each ZIP code. The 36 below are a selection from Franklin County and the six counties around it, in numerical order. They include the county seats of Newark, Lancaster, Circleville, London and Marysville, and they leave out most Columbus city ZIP codes; the department's dashboard carries every ZIP code in the state. A ZIP code is a postal delivery area, not a town: its boundary does not follow a city line, as the page on water and sewer providers town by town shows for mailing addresses, so no town name is attached to any of them here.
| ZIP code | Number of tests | Percent of tests above 4 pCi/L | Median, pCi/L | Highest single result, pCi/L |
|---|---|---|---|---|
| 43004 | 3,259 | 48.67 | 3.8 | 190.3 |
| 43015 | 5,402 | 52.13 | 4.2 | 80.3 |
| 43016 | 4,233 | 45.43 | 3.4 | 274 |
| 43017 | 5,696 | 44.4 | 3.2 | 837 |
| 43021 | 1,589 | 54.12 | 4.4 | 66.6 |
| 43023 | 1,904 | 49.74 | 3.9 | 285.5 |
| 43026 | 6,935 | 48.67 | 3.8 | 74.2 |
| 43031 | 767 | 50.33 | 4 | 64.1 |
| 43035 | 4,012 | 45.44 | 3.4 | 63.3 |
| 43040 | 2,354 | 41.97 | 3.2 | 307 |
| 43054 | 4,260 | 38.24 | 2.7 | 147 |
| 43055 | 3,013 | 58.38 | 5.6 | 271 |
| 43062 | 2,645 | 49.6 | 3.9 | 407 |
| 43064 | 1,372 | 47.23 | 3.7 | 105.9 |
| 43065 | 6,198 | 51.4 | 4.1 | 260.9 |
| 43068 | 3,772 | 47.48 | 3.6 | 124.2 |
| 43074 | 1,521 | 44.18 | 3.4 | 76.9 |
| 43081 | 7,238 | 42.21 | 3 | 212.2 |
| 43082 | 4,400 | 49.32 | 3.8 | 192 |
| 43085 | 3,288 | 55.44 | 4.8 | 141 |
| 43110 | 2,144 | 54.66 | 4.7 | 157.5 |
| 43113 | 706 | 54.39 | 4.5 | 278.3 |
| 43119 | 2,414 | 44.61 | 3.1 | 67.7 |
| 43123 | 3,792 | 55.22 | 4.6 | 479 |
| 43125 | 690 | 56.96 | 5.5 | 81.9 |
| 43130 | 1,457 | 55.66 | 4.7 | 414.2 |
| 43140 | 610 | 52.79 | 4.35 | 143 |
| 43147 | 4,618 | 53.07 | 4.4 | 109.9 |
| 43206 | 2,783 | 47.75 | 3.8 | 904 |
| 43209 | 3,456 | 50.93 | 4.05 | 94 |
| 43212 | 1,985 | 41.26 | 3.3 | 68.2 |
| 43214 | 3,659 | 56.93 | 4.8 | 134 |
| 43220 | 3,667 | 48.21 | 3.7 | 125.4 |
| 43221 | 5,144 | 57.72 | 4.9 | 121.7 |
| 43228 | 3,064 | 37.3 | 2.5 | 66.2 |
| 43230 | 5,053 | 52.17 | 4.3 | 344 |
The highest share in the table is 58.38 percent above 4, in 43055, and the lowest is 37.3 percent, in 43228; even there, more than a third of reported tests came back above the line. And every ZIP code in the table carries at least one result far above it, from 63.3 in 43035 to 904 in 43206. Every one of the 36 also carries a lowest result of 0.1 or less. A ZIP code figure narrows the question without answering it.
Why a basement bathroom is the case the federal guide names
The federal guide lists how radon gets in: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, and the water supply. A bathroom added to a basement usually means cutting a floor for drain lines and bringing pipe through walls, which is to say working on several of the routes on that list.
The guide speaks to that case directly. Under the heading Radon and Home Renovations it says: "If you are planning any major structural renovation, such as converting an unfinished basement area into living space, it is especially important to test the area for radon before you begin the renovation." If the test indicates a radon problem, it says radon-resistant techniques can be inexpensively included as part of the renovation, and it closes: "Because major renovations can change the level of radon in any home, always test again after work is completed." The Ohio Department of Health's page gives the same advice for a house: retesting of single-family residences is recommended every two years, or after significant renovations which may affect the airflow inside the structure.
Ohio's residential code does not require any of this. As the page on what goes wrong in a Central Ohio house sets out, the Residential Code of Ohio contains no radon provision, and the same page carries the federal radon zone map and the 1988 to 1989 state survey. Those survey figures come from different tests of different houses decades apart, so they are not compared with the dashboard's figures here.
The short version. The Ohio Department of Health's dashboard, as exported on September 24, 2026, reports between 43 and 54 percent of radon tests above 4 picocuries per liter in each of seven Central Ohio counties, from 43.17 percent in Union to 53.8 in Fairfield, with 45.59 percent of Franklin County's 109,347 tests above the line. The 36 ZIP codes in the table run from 37.3 to 58.38 percent. Those are results before mitigation reported since 2010, and they describe other houses: the department says testing is the only way to know, and the federal guide says to test a basement before turning it into living space and again after the work.
Sources
Every figure, date and quotation on this page comes from one of these documents, read on the date given. Where two of them disagree, the page says so.
- Ohio Department of Health, Radon Education and Licensing Program, Radon Data, April 21, 2026; read September 2026.
- Ohio Department of Health, Radon Levels by Zip Code and County dashboard, ZIP code and county views, CSV export, exported September 24, 2026; read September 2026.
- United States Environmental Protection Agency, A Citizen's Guide to Radon, The Guide to Protecting Yourself and Your Family from Radon (EPA 402/K-12/002), 2025, as printed on its cover; read September 2026.
Common questions
What share of radon tests in Franklin County come back above 4 picocuries per liter?
In the Ohio Department of Health's dashboard, exported September 24, 2026, 45.59 percent of Franklin County's 109,347 reported tests were above 4 picocuries per liter. The department says these are results before mitigation, reported since 2010 by radon professionals who hold an Ohio license, and updated monthly. The same export gives a geometric mean of 3.7 and an arithmetic mean of 6.0 for the county, and a highest single result of 904.
Which Central Ohio county has the highest share of radon tests above 4 pCi/L?
Fairfield County, at 53.8 percent of 7,251 tests, in the Ohio Department of Health's dashboard as exported on September 24, 2026. Pickaway follows at 52.78 percent, Madison at 52.45, Licking at 51.92, Delaware at 49.82, Franklin at 45.59 and Union at 43.17. The spread among the seven counties is about ten points, and in every one of them somewhere between 43 and 54 percent of reported tests came back above the line.
Should a basement be tested for radon before it is turned into a bathroom?
The federal Environmental Protection Agency's A Citizen's Guide to Radon says that before any major structural renovation, such as converting an unfinished basement area into living space, it is especially important to test the area for radon before you begin, that radon-resistant techniques can be inexpensively included as part of the renovation if there is a problem, and that because major renovations can change the level of radon in any home, you should always test again after work is completed.
Where do Ohio's radon numbers by ZIP code come from?
From the Ohio Department of Health's Radon Levels by Zip Code and County dashboard. The department says the statistics are based on radon levels prior to mitigation, from 2010 to the present, as reported by radon professionals who hold an Ohio license, and updated monthly. For each ZIP code it reports the number of tests, the share above 4 picocuries per liter, the arithmetic and geometric means, the median, the standard deviation and the lowest and highest single results; for each county it reports the number of tests, the share above 4, the two means and the highest single result.
Does my ZIP code's radon number tell me the radon level in my house?
No. It summarizes tests of other houses in the same postal delivery area. The Ohio Department of Health says its radon maps are for educational purposes only and that radon testing is the only way to know the radon levels in a home. Every one of the 36 Central Ohio ZIP codes read for the page carries results far above 4 picocuries per liter alongside results below it, so a low share in a ZIP code does not clear a particular house and a high share does not condemn one.
How does radon get into a house?
The federal Environmental Protection Agency's A Citizen's Guide to Radon lists seven routes: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, and the water supply. It says radon from soil gas is the main cause of radon problems and that building materials rarely cause them by themselves. It also says any home may have a radon problem, whether new or old, well sealed or drafty, with or without a basement.
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