Wisconsin has some of the highest indoor radon in the country — but the risk isn’t uniform. Using radon test results reported to the CDC between 2008 and 2017, here’s how Wisconsin counties actually compare, which ones run highest, and why every home needs testing regardless of its county’s average.
The highest-radon counties in Wisconsin
Among the Wisconsin counties we serve, these have the highest average indoor radon levels (CDC, 2008–2017):
- Marathon County — 11.6 pCi/L (8.9× the U.S. average), covering Wausau. Marathon County radon →
- Waukesha County — 6.4 pCi/L (4.9× the U.S. average), covering Waukesha. Waukesha County radon →
- Rock County — 6.4 pCi/L (4.9× the U.S. average), covering Janesville. Rock County radon →
- Winnebago County — 6.2 pCi/L (4.8× the U.S. average), covering Oshkosh. Winnebago County radon →
- Outagamie County — 6.0 pCi/L (4.6× the U.S. average), covering Appleton. Outagamie County radon →
For context, the EPA recommends fixing any home at or above 4.0 pCi/L. Marathon County alone averages 11.6 pCi/L — 8.9× the U.S. average of 1.3 — and 13 of the 13 counties we serve average above the action level, meaning the typical home in them needs a mitigation system.
No Wisconsin county we serve averages below the action level
Every Wisconsin county in our service area averages at or above the EPA’s 4.0 pCi/L action level — which is exactly why testing is so important across the state.
How we know these numbers
These county averages come from the CDC National Environmental Public Health Tracking Network, which aggregates radon test results reported by laboratories. The figures reflect mean pre-mitigation indoor radon levels measured between 2008 and 2017 — real tested homes, not models or predictions. You can see the full county-by-county breakdown on our Wisconsin radon levels by county page.
What to do next
Wherever you live in Wisconsin: (1) test your home with a short-term kit or a professional measurement; (2) if your result is at or above 4.0 pCi/L, mitigate — an active sub-slab depressurization system typically cuts indoor radon by 70–99%; (3) re-test after install to confirm. Wisconsin Radon Experts routes you to an NRPP-certified partner contractor for any of these steps. Find your county’s radon level →