Is St. Clair County, IL a radon risk area?

St. Clair County is mapped as Zone 2 - Moderate Potential by EPA — meaning the county-wide average indoor level is predicted between 2 and 4 pCi/L. Nationally that's the 63rd percentile for radon potential — St. Clair County outranks 63% of every county EPA has rated.

Where St. Clair County sits nationally

About 251,149 people live in St. Clair County, Illinois, spread over 657.748 square miles. Averaged across its land area, St. Clair County runs about 382 people per square mile. Rank 95 by population within its EPA zone puts St. Clair County at the 91st percentile among zone peers. EPA's geology-based Region 5 groups St. Clair County with 523 other counties nationally sharing its underlying rock and soil profile.

Why the zone isn't the answer by itself

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 St. Clair County's Zone 2 prediction. Nothing about a Zone 2 label lowers the case for testing in St. Clair County — EPA recommends it in every zone, moderate included.

Next steps for a home in St. Clair County

Nothing about St. Clair 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

St. Clair County's test kit answers today; a Airthings radon monitor keeps answering for St. Clair 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

St. Clair County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.