Radon levels by ZIP code: reading the Cincinnati numbers

Type "radon levels by ZIP code" into a search bar and you will find maps shaded red and orange, but very little help reading them. For Greater Cincinnati there is a better source than most: the Ohio Department of Health publishes results by ZIP code from the tests Ohio-licensed professionals have reported since 2010. Here is how to read those numbers, what they genuinely say about this region, and where they stop being useful.

Where the ZIP numbers come from

ODH's Radon Levels by ZIP Code and County dashboard pools pre-mitigation results reported by licensed radon testers and mitigation specialists, and it is updated monthly. Across Ohio that adds up to roughly 508,000 tests. ODH is careful to call the maps educational and to say that testing is the only way to know the level in a given home.

Two features of the data shape everything else. The results are pre-mitigation readings, so a house that has since been fixed still counts at its original number. And the tests are the ones people paid a licensed professional to run, which skews toward home sales and toward homeowners who already suspected a problem. None of that makes the data wrong. It makes it a picture of tested houses rather than every house.

Three numbers worth reading, and one to treat with care

For each ZIP code the dashboard reports several statistics. These are the ones that matter to a homeowner:

• Median. Half the reported tests fell below this value and half above. It is the best single description of a typical tested house.
• Share above 4.0 pCi/L. This is the closest thing to your odds, since 4.0 is EPA's action level.
• Number of tests. A ZIP with 2,000 results says far more than one with 40. Treat small counts as a rough hint.

The arithmetic mean is the one to handle carefully. A few basements in the dozens of picocuries drag it upward. Trenton, ZIP 45067 in Butler County, has a median of 4.8 pCi/L but a mean of 9.2, because its highest reported result was 79.3. When the mean runs far above the median, a handful of extreme houses are doing the talking.

What the Cincinnati numbers actually show

Laid side by side, the ZIP results trace a pattern many residents would not guess. The central city tests lowest. Downtown and Over-the-Rhine (45202) show a median of 0.9 pCi/L with about 9 percent of 950 tests above 4.0. Queensgate (45203), Lower Price Hill and Sedamsville (45204), and Camp Washington (45225) sit lower still, each with about 3 percent or less of tests above 4.0, though on samples of only 96 to 185 tests.

The older east side neighborhoods land in the low teens. Hyde Park and Mt. Lookout (45208) have a median of 1.9 with 14 percent of 2,419 tests above the action level. Norwood (45212) sits at 1.6 and 11 percent, and Oakley (45209) at 1.6 and 9 percent.

The share climbs as you move out. Anderson Township's eastern ZIP (45244) reaches 31 percent, Mason (45040) 34 percent, West Chester Township (45069) 36 percent, and Loveland (45140) 37 percent. The Maineville and Landen area of Warren County's Hamilton Township (45039) reaches 41 percent. Out west, Harrison (45030) posts a median of 3.7 and 48 percent of 783 tests above 4.0.

The data describes where high results were found. It does not explain why. Soils, foundation styles, the age of the housing, and which houses happened to be tested all play a part, and the dashboard separates none of them. Treat any story about why one ZIP runs higher as a hypothesis, not a finding.

Why the county zone and the data disagree

The other map people find is EPA's Map of Radon Zones, which assigns every county a zone based on a predicted average screening level. It was built in 1993 from geology, aerial radioactivity, soils, foundation types, and the indoor measurements available then, and EPA says plainly that it should not be used to decide whether an individual home needs a test.

Put the two side by side and the gaps are instructive. Hamilton County is Zone 1, the highest-potential category, yet about 22 percent of its 42,706 reported tests read above 4.0. Clermont County is Zone 2, yet about 36 percent of its 9,824 tests did. Adams County is Zone 1 with only about 12 percent above 4.0 across 281 tests. On the Kentucky side, EPA places Boone, Kenton, and Campbell counties in Zone 2, and Kentucky publishes no comparable ZIP-level dashboard. Our Ohio radon map by county lists every county's zone next to its measured results.

Two houses on one street

EPA's Citizen's Guide lists "a neighbor's test result is a good indication of whether your home has a problem" among its radon myths. Radon levels vary a great deal from house to house, and the reasons are mostly inside the walls.

• Foundation type. A full basement, a slab, a crawl space, and a split-level all meet the soil differently.
• Openings in the floor. An open sump, a cracked slab, or an unsealed floor drain gives soil gas an easy route that a tight neighbor lacks.
• Where and when the test ran. A basement kit in January and a first-floor kit in June are not measuring the same thing.
• How the house breathes. Furnace placement, exhaust fans, and how tightly the upper floors are sealed all change the pull on the soil.

So a neighbor at 1.5 and you at 9.0 is entirely possible, and so is the reverse.

Using your ZIP number the right way

Use the ZIP figure to set expectations, then let your own test make the decision. If your ZIP shows 35 or 40 percent of tests above 4.0, a high result is common rather than alarming, and it is worth testing sooner rather than later. If your ZIP shows 10 percent, you still have roughly a one-in-ten chance, which is why EPA recommends testing every home below the third floor.

Find your ZIP in our radon levels by ZIP code table, then follow our heating-season testing guide to place a kit correctly. If you would rather have a licensed professional run it, the radon testing page explains the options, and we are a call or a text away when the result comes back.