If you just got a radon test result back and you're staring at a number wondering whether it's bad, you're not alone. Radon has no smell, no color, and no taste — the only way to know your risk is the number on the report, and knowing what that number means is the whole ballgame.
What radon level is considered dangerous?
The U.S. Environmental Protection Agency sets 4.0 picocuries per liter (pCi/L) as its action level — the point at which it recommends fixing a home with a mitigation system. The World Health Organization uses a stricter reference level of about 2.7 pCi/L (100 Bq/m³), meaning a result your inspector calls "acceptable" by U.S. standards could still be flagged internationally.
That gap matters for Wisconsin buyers and sellers because real estate contracts almost always reference the EPA's 4.0 pCi/L threshold, not the WHO number. A result of 3.8 pCi/L will pass most inspection contingencies even though it's well above what the WHO considers a long-term health concern.
Is there a safe level of radon at all?
No — the EPA and the U.S. Surgeon General have stated there is no known safe level of radon exposure, since it's classified as a Group 1 human carcinogen and the leading cause of lung cancer among non-smokers in the United States. The 4.0 pCi/L figure is a practical action threshold, not a safety guarantee.
This is why some homeowners choose to mitigate even when a test comes back between 2 and 4 pCi/L, especially in homes where young children or non-smokers spend most of their time in a finished basement.
How do I know what the radon level is in my Wisconsin home?
The only reliable way to know is a test — either a short-term kit (2 to 90 days) or a continuous monitor placed by a certified tester, run under closed-house conditions with windows and exterior doors shut for at least 12 hours before and during the test. Guessing based on your neighbor's result or your home's age tells you nothing, because radon concentration depends on soil composition under your specific foundation, not the house next door.
Wisconsin's heating season — roughly October through April — tends to produce the highest readings because closed windows and furnace-driven stack effect pull more soil gas into the living space. A test run in July on a house with open windows can understate what that same home will show in January.
- Short-term test: fastest result, good for real estate transactions and Addendum S timelines
- Long-term test: more accurate picture of year-round average exposure
- Continuous monitor: used by certified testers, resistant to tampering, logs hour-by-hour data
For a full walkthrough of what a certified test actually involves in Wisconsin, see Certified Radon Testing Near Me in Wisconsin: What to Expect.
What should I do if my test comes back above 4 pCi/L?
A result at or above 4.0 pCi/L means it's time to install a radon mitigation system, typically a sub-slab depressurization setup with a vent pipe and inline fan that pulls soil gas from beneath the foundation and exhausts it above the roofline before it enters the home. This is not a DIY fix — Wisconsin requires radon mitigation work to be performed by a certified radon mitigation professional.
After installation, a follow-up test confirms the system brought levels below the action threshold. If you're buying or selling and this result is tied to a real estate transaction, it likely intersects with Wisconsin's Addendum S disclosure paperwork — see Addendum S Wisconsin: What This Radon Disclosure Form Means for how that timeline usually works. To find out whether a contractor is actually certified before you sign anything, read Certified Radon Mitigation Contractors in Wisconsin: How to Verify.
Are certain Wisconsin areas more prone to dangerous radon levels?
Yes — radon risk in Wisconsin varies significantly by county because it's driven by underlying soil and bedrock geology, and several counties across the state fall into the EPA's highest predicted-average radon zone. Homes in Brown County (including the Allouez-adjacent and Fort Howard area neighborhoods near Green Bay), Outagamie County (Appleton and the surrounding North Appleton and West Appleton areas), and Dane County (Madison and Maple Bluff) have all logged high readings in past county-level data.
Geology doesn't respect city limits, though — a low-risk zip code can still have an individual home with a high reading, and a high-risk county can have plenty of homes that test clean. County-level averages are a starting point for awareness, not a substitute for testing your own house. For a breakdown of how Wisconsin counties compare using CDC and state data, see Which Wisconsin Counties Have the Highest Radon? (CDC County Data).
If your home does need a mitigation system, the licensed operators in our network handle sub-slab depressurization installs across the state, and you can read what installation day actually looks like in Certified Radon Mitigation Near Me: What Installation Day Looks Like.
This article is educational and does not replace a professional radon test or the advice of a licensed radon professional. Local permitting, disclosure, and code requirements vary by municipality — check with your local building department or a certified radon mitigation contractor before making decisions based on this content.