Michigan · county radon data
Radon in Washtenaw County, Michigan
41.3% of the 5,668 home radon tests on record here reached the EPA action level — among the higher-testing counties in the state, ranked #6 of 83 tested Michigan counties.
What the numbers say
Washtenaw County sits 24.7 points above the Michigan median (16.6% of tests over the limit across the state's counties). The median reading among its 5,668 reported tests was 3.2 pCi/L — and the single highest test on record reached 328.5 pCi/L, more than 82× the action level. EPA maps place the county in Radon Zone 1 — its highest-potential category, where average indoor screening levels are predicted above 4 pCi/L.
Why: the ground under Washtenaw County
Part of the south-central Michigan belt where Devonian–Mississippian black shales lie at shallow depth beneath glacial deposits — the combination behind Michigan's cluster of EPA Zone 1 counties. (Regional attribution is well supported; formation-level detail is coarser.) (Regional attribution here is well supported; unit-level detail is coarser — see methodology.)
Geology sources: EPA MI zone documentation · USGS WRI 98-4169.
The housing stock factor
The median Washtenaw County home dates to 1978 — decades before radon-resistant new construction entered building codes. With 369,822 residents largely housed in that older stock, testing is the only way any individual household knows where it stands.
ZIP codes in Washtenaw County
Michigan doesn't publish ZIP-level test results, so these ZIPs share the county figures above — the roster is here so you can confirm your address is covered by this data.
| ZIP | Place |
|---|---|
| 48103 | Ann Arbor |
| 48104 | Ann Arbor |
| 48105 | Ann Arbor |
| 48108 | Ann Arbor |
| 48109 | Ann Arbor |
| 48118 | Chelsea |
| 48130 | Dexter |
| 48158 | Manchester |
| 48176 | Saline |
| 48189 | Whitmore Lake |
| 48190 | Whittaker |
| 48191 | Willis |
| 48197 | Ypsilanti |
| 48198 | Ypsilanti |
Washtenaw County in the Michigan picture
| Washtenaw County | 41.3% over the limit | 5,668 tests |
| Michigan's highest: Lenawee County | 56.7% | 1,349 tests |
| Michigan county median | 16.6% | 83 counties ranked |
| Share of all Michigan tests from this county | 6.9% | |
Ranked nearby in Michigan
- Hillsdale County — #3, 54.8% of 1,177 tests over the limit
- Jackson County — #4, 49.7% of 1,989 tests over the limit
- Livingston County — #5, 43.1% of 3,306 tests over the limit
- Dickinson County — #7, 39.3% of 446 tests over the limit
- Clinton County — #8, 38.5% of 1,466 tests over the limit
- Otsego County — #9, 38.2% of 598 tests over the limit
(Ranking neighbors — counties adjacent in the Michigan test ranking, not necessarily on the map.)
High test? Fixes and real pricing: the Michigan mitigation cost guide. Every statistic here traces to the methodology (CDC EPHT (labs), 2008–2017; zones: EPA; year built & population: ACS 5-year).
Washtenaw County radon FAQ
- Is radon high in Washtenaw County?
- 41.3% of 5,668 home tests reported to the CDC came back at or above the EPA action level of 4 pCi/L — 24.7 points above the Michigan county median of 16.6%, ranking #6 of 83 tested counties. EPA classifies the county as Zone 1.
- Does an older home in Washtenaw County mean more radon risk?
- The county's median home was built in 1978 — before radon-resistant construction practices entered building codes in the 1990s, so most of the housing stock has no built-in radon protection. Construction age changes the odds, not the answer: only a test tells you about your house.
- Where do these Washtenaw County numbers come from?
- 5,668 aggregated home tests in the CDC's Environmental Public Health Tracking dataset (CDC EPHT 2008-2017). Median reading: 3.2 pCi/L; the highest recorded test hit 328.5 pCi/L. How we handle the data, including its limits, is documented on our methodology page.
- What's the highest radon reading recorded in Washtenaw County?
- 328.5 pCi/L — 82× the EPA action level — against a county median of 3.2 pCi/L across 5,668 tests. That spread is the point: neighboring homes routinely test wildly differently.