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

Jefferson 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 52nd percentile for radon potential — Jefferson County outranks 52% of every county EPA has rated.

How Jefferson County compares

Jefferson County, Illinois is home to about 36,107 people across 571.237 square miles. Population density there runs about 63 people per square mile. By population, Jefferson County is the 432nd largest of every county sharing its EPA zone — the 58th percentile by population within that zone. Geologically, Jefferson County is grouped into EPA Region 5 alongside 523 other counties that share its rock and soil setting.

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 Jefferson County's Zone 2 prediction. A Zone 2 rating is a moderate county-wide average, not a guarantee about any one house in Jefferson County — testing is the only way to know an individual home's actual level.

Next steps for a home in Jefferson County

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

Where Jefferson 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.