What EPA's radon map says about Clay County

EPA places Clay 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. Clay County sits at the 42nd percentile nationally for radon potential — above 42 of every 100 rated counties.

Where Clay County sits nationally

About 12,821 people live in Clay County, Illinois, spread over 468.351 square miles. Averaged across its land area, Clay County runs about 27 people per square mile. Among counties sharing Clay County's EPA zone, it outranks 26% of them by population. Geologically, Clay 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 Clay County

EPA's Zone 2 rating for Clay County sits below its own 4 pCi/L action level but above the national indoor average of about 1.3 pCi/L. Middle-of-the-pack doesn't mean risk-free for any one Clay County address — a moderate county average still leaves individual homes to test either side of it.

Next steps for a home in Clay County

Mitigation cost doesn't depend on Clay 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 Illinois's state radon program

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

Clay 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.