Mercer County, IL radon potential

EPA places Mercer 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 Mercer County in the 79th percentile nationally for radon potential — higher than 79% of the counties EPA has rated.

How Mercer County compares

Mercer County, Illinois is home to about 15,292 people across 561.24 square miles. That works out to roughly 27 people per square mile. Within its own EPA zone, Mercer County is bigger by population than 39% of the counties that share it. Mercer County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.

What this actually means for a house in Mercer County

Fixing a home becomes an EPA recommendation past 4 pCi/L, and that is precisely the side of the line Mercer County's Zone 1 prediction lands on, well above the roughly 1.3 pCi/L most US homes average. 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 Mercer County will test well under the action level, and EPA's guidance is to test regardless of zone.

If Mercer County tests above the action level

A short-term test kit is the standard first step and typically costs far less than mitigation itself. If a test comes back at or above 4 pCi/L, a radon mitigation system for a typical home runs roughly $800–$2,500 (around $1,200 for a common single-family install), driven mainly by foundation type, not by county.

Find Illinois's state radon program

For Mercer County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Mercer 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 Mercer County's figures come from

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