Is Williamson County, IL a radon risk area?

EPA places Williamson County in Zone 2 - Moderate Potential — a predicted county-wide average indoor radon level between 2 and 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Nationally, Williamson County ranks higher in radon potential than 57% of rated counties (the 57th percentile).

Where Williamson County sits nationally

About 67,064 people live in Williamson County, Illinois, spread over 420.216 square miles. Averaged across its land area, Williamson County runs about 160 people per square mile. Rank 277 by population within its EPA zone puts Williamson County at the 73rd percentile among zone peers. EPA's geology-based Region 5 groups Williamson County with 523 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Call it a middle case: above the 1.3 pCi/L most US homes average, below the 4 pCi/L line EPA uses to recommend mitigation — that's Williamson County's Zone 2 prediction. A Zone 2 rating is a moderate county-wide average, not a guarantee about any one house in Williamson County — testing is the only way to know an individual home's actual level.

If Williamson County tests above the action level

Nothing about Williamson 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 Illinois's state radon program

Williamson County's test kit answers today; a Airthings radon monitor keeps answering for Williamson 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 Williamson County's numbers. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.