Is Marathon County, WI a radon risk area?

EPA places Marathon County in Zone 1 - Highest Potential — a predicted county-wide average indoor radon level above 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Nationally, Marathon County ranks higher in radon potential than 95% of rated counties (the 95th percentile).

How Marathon County compares

About 139,091 people live in Marathon County, Wisconsin, spread over 1,545.249 square miles. Population density there runs about 90 people per square mile. Rank 145 by population within its EPA zone puts Marathon County at the 86th percentile among zone peers. Geologically, Marathon County is grouped into EPA Region 5 alongside 523 other counties that share its rock and soil setting.

What this actually means for a house in Marathon County

Marathon County carries a Zone 1 label because its predicted average clears 4 pCi/L outright — three-plus times the 0.4 pCi/L typical outdoors and above the 1.3 pCi/L most homes see. Some Marathon County homes will test comfortably under the action level despite the county's Zone 1 rating — the only way to know which kind of home you have is an actual test.

Testing and mitigation in Marathon County

Nothing about Marathon County's EPA zone changes the mitigation number: $800–$2,500 nationally (around $1,200 typical), set by the house's foundation rather than the county it sits in — and only relevant once a test actually clears 4 pCi/L.

Find Wisconsin's state radon program

Marathon County's test kit answers today; a Airthings radon monitor keeps answering for Marathon 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

The methodology guide shows exactly which EPA and Census sources feed Marathon County's numbers. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.