Winnebago County, IL radon potential
Winnebago County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Nationally, Winnebago County ranks higher in radon potential than 97% of rated counties (the 97th percentile).
Winnebago County by the numbers
Winnebago County, Illinois is home to about 283,790 people across 513.049 square miles. Population density there runs about 553 people per square mile. Within its own EPA zone, Winnebago County is bigger by population than 93% of the counties that share it. Winnebago County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.
What this actually means for a house in Winnebago County
Fixing a home becomes an EPA recommendation past 4 pCi/L, and that is precisely the side of the line Winnebago County's Zone 1 prediction lands on, well above the roughly 1.3 pCi/L most US homes average. That doesn't mean every Winnebago 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.
If Winnebago County tests above the action level
A short-term test kit is the standard first step and typically costs far less than mitigation itself. If a test comes back at or above 4 pCi/L, a radon mitigation system for a typical home runs roughly $800–$2,500 (around $1,200 for a common single-family install), driven mainly by foundation type, not by county.
Find Illinois's state radon program
For Winnebago County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Winnebago County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.
Where Winnebago County's figures come from
Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.