What EPA's radon map says about Jefferson County

Jefferson County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Nationally that's the 93rd percentile for radon potential — Jefferson County outranks 93% of every county EPA has rated.

Where Jefferson County sits nationally

About 86,245 people live in Jefferson County, Wisconsin, spread over 556.494 square miles. Averaged across its land area, Jefferson County runs about 155 people per square mile. Among counties sharing Jefferson County's EPA zone, it outranks 80% of them by population. Geologically, Jefferson County is grouped into EPA Region 5 alongside 523 other counties that share its rock and soil setting.

Zone vs. an actual test

Jefferson County's predicted average sits above EPA's own action level of 4 pCi/L — the concentration at which EPA recommends fixing a home — versus a national average around 1.3 pCi/L indoors and 0.4 pCi/L outdoors. That doesn't mean every Jefferson County house tests high — a zone is a county-wide prediction, never a single-home reading — but it's a reason to test sooner rather than let it wait.

Next steps for a home in Jefferson County

Mitigation cost doesn't depend on Jefferson County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find Wisconsin's state radon program

Jefferson County's test kit answers today; a Airthings radon monitor keeps answering for Jefferson County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. Code RADONZONE-10OFF takes 10% off at checkout.

More on how this number was built

Jefferson County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.