Henderson County, IL: EPA's radon zone, explained

EPA places Henderson 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. That puts Henderson County in the 72nd percentile nationally for radon potential — higher than 72% of the counties EPA has rated.

How Henderson County compares

Henderson County, Illinois is home to about 6,048 people across 378.793 square miles. That works out to roughly 16 people per square mile. By population, Henderson County is the 879th largest of every county sharing its EPA zone — the 17th percentile by population within that zone. Geologically, Henderson 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 Henderson County

Henderson 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. A Zone 1 rating means testing is worth doing sooner rather than later, but it is still county-wide average, not a reading for any single house — some homes in Henderson County will test well under the action level, and EPA's guidance is to test regardless of zone.

Next steps for a home in Henderson County

For a Henderson County homeowner, the order is simple: test first, since a kit costs a small fraction of what mitigation would. Only if the result clears 4 pCi/L does a system — typically $800–$2,500, about $1,200 for a common install — become worth pricing out.

Find Illinois's state radon program

For Henderson County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Henderson County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. Code RADONZONE-10OFF takes 10% off at checkout.

Where Henderson County's figures come from

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