Wisconsin Radon Levels by County
The county map is the natural place to start when you want a sense of your radon risk in Wisconsin. Open the EPA Map of Radon Zones and you can see at a glance whether your county is painted higher or lower risk, and for most Wisconsin homeowners that one fact is enough to show radon is worth taking seriously. A zone, though, is a screening estimate for a whole county, built from geology and averages, not a measurement of the air inside your house. The only thing that settles your actual level is a test in your own home. This page explains how the zones are defined, where every area we cover falls, and why the number on the map is a starting point rather than an answer.
How EPA radon zones work
The EPA Map of Radon Zones sorts every county in the country into one of three categories based on its predicted average indoor radon level. Zone 1 counties have a predicted average at or above 4.0 pCi/L, the level at which the EPA recommends fixing a home. Zone 2 counties fall in the 2 to 4 pCi/L range. Zone 3 counties sit below 2 pCi/L. The map was assembled from indoor radon measurements, underlying geology, aerial radioactivity, soil traits, and foundation types, and it was designed to help state programs and builders target resources, not to predict the reading at any one address.
Here is the fact that reframes the whole map for Wisconsin residents: the state has no low-risk county at all. Every Wisconsin county is EPA Radon Zone 1 or Zone 2. The state has no Zone 3 county. Of the 72 counties, 29 of the 72 counties in Wisconsin are Zone 1, the highest category, and the rest are Zone 2. That places the entire state in the two designations where the EPA advises homeowners to test and, where levels warrant, to act. The geology behind that pattern shows up in testing too. Statewide, about one in 10 Wisconsin homes tests above the action level, and in higher-risk counties like Dane the odds run closer to one in five, according to Wisconsin DHS.
Wisconsin county radon zones
The table below shows the EPA Radon Zone for each area we cover, anchored on Madison and Dane County and reaching across central and western Wisconsin. It is drawn from the EPA Map of Radon Zones and cross-checked against Wisconsin DHS materials. Use it to locate your area, then read past it, because the paragraphs that follow explain what the label does and does not tell you.
| Area (anchor city) | County | Region | EPA Radon Zone |
|---|---|---|---|
| Madison | Dane | south-central Wisconsin | Zone 1 |
| Dane County | Dane | south-central Wisconsin | Zone 1 |
| Eau Claire | Eau Claire | west-central Wisconsin | Zone 2 |
| La Crosse | La Crosse | the western Wisconsin Coulee Region | Zone 2 |
| Wausau | Marathon | north-central Wisconsin | Zone 1 |
| Stevens Point | Portage | central Wisconsin | Zone 1 |
| Janesville | Rock | south-central Wisconsin | Zone 1 |
| Fond du Lac | Fond du Lac | east-central Wisconsin | Zone 1 |
| Appleton | Outagamie | the Fox Valley | Zone 2 |
| Green Bay | Brown | northeastern Wisconsin | Zone 2 |
Zone 1 means the EPA predicts an average indoor level at or above 4 pCi/L; Zone 2 means 2 to 4 pCi/L. Every Wisconsin county is EPA Radon Zone 1 or Zone 2. The state has no Zone 3 county. Zone is a countywide screening designation, not a reading for any one home, so check your address on the WI DHS radon results map and test to learn your level. Sources: EPA Map of Radon Zones and Wisconsin DHS.
The pattern in the data is easy to read. The south-central counties around Madison and Dane County sit in Zone 1. North-central areas such as Wausau are Zone 1 as well, with added well-water relevance in the granite belt, where private wells can carry radon of their own. Several western river communities, including Eau Claire and La Crosse, are mapped Zone 2. Even that split understates the caution warranted, because a Zone 2 designation still means a predicted county average of up to 4 pCi/L, and individual homes routinely test well above their county average. You may also see a local source list a western city as Zone 1 based on collected test results rather than the official map. Where that happens, the EPA map and the WI DHS data are the two references to weigh, and a test of your own home is what resolves it.
Why the county number is not your home number
A zone tells you what the EPA predicts for a county as a whole. It does not account for your soil, your foundation, the cracks and gaps that let soil gas in, how tightly the home is sealed, or the way you heat it. Two houses on the same street, even next door to each other, can test very differently, because radon entry depends on the specific path the gas finds into that specific building. This is why a Zone 1 label is not a diagnosis and a Zone 2 label is not an all-clear. The map narrows the odds; it does not read your basement.
It also helps to know where the county figures you find online actually come from. Many of the county-level averages and percentages circulating on the web are compiled by private test-kit vendors from the kits their own customers mail in, which is a self-selected sample rather than a random survey of the county. Those numbers can give a rough sense of scale, but they are not an official measurement of your county, and they are certainly not a measurement of your home. For Wisconsin, the WI DHS radon results map is the place to check what testing has actually found near you, because it draws on results reported to the state rather than to a marketer.
How to check your address
To move from the county map to your own ZIP code, start with the WI DHS radon results map. It lets you look up radon test results reported around the state by area, so you can see what homes near your address have measured, which is a far better signal than a countywide zone. From there, confirm your county designation on the EPA Map of Radon Zones, and read the broader picture, from testing through mitigation and home sales, in our Wisconsin radon guide. None of these tools replace testing your own home, but together they tell you how seriously to treat radon before your kit even arrives.
What to do if your area is Zone 1 or Zone 2
The next step is the same whether your county is Zone 1 or Zone 2: test, then decide. Start with a short-term charcoal kit or a continuous monitor, ideally in winter, when Wisconsin homes are closed up against the cold and radon has the best chance to accumulate. If the result comes back at or above 4.0 pCi/L, run a follow-up test to confirm before you spend on anything. Between 2 to 4 pCi/L, the EPA suggests you consider a fix, since there is no level with zero risk. You can read how each type of test works, and what a real-estate test requires, on our radon testing page.
If a confirmed result lands high, that is where Badger State Radon is useful. We connect you with independent local radon mitigation contractors so you can compare quotes and qualifications yourself. We are a free matching service, never a contractor, and there is no obligation to hire anyone we introduce. Whether the map put your county in Zone 1 or Zone 2, the practical path is identical: test your own home, confirm a high reading, and then line up a professional to bring the level down.