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

Mason County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. That puts Mason County in the 77th percentile nationally for radon potential — higher than 77% of the counties EPA has rated.

Where Mason County sits nationally

Mason County, Illinois is home to about 12,543 people across 539.352 square miles. Population density there runs about 23 people per square mile. By population, Mason County is the 709th largest of every county sharing its EPA zone — the 33rd percentile by population within that zone. Geologically, Mason County is grouped into EPA Region 5 alongside 523 other counties that share its rock and soil setting.

Why the zone isn't the answer by itself

Mason 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. EPA's guidance doesn't change because of a Zone 1 label: test the house. What changes is the reason to not put it off in Mason County specifically.

If Mason County tests above the action level

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

Where Mason 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.