Marshall County, IL radon potential

EPA's own radon map rates Marshall County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Nationally that's the 77th percentile for radon potential — Marshall County outranks 77% of every county EPA has rated.

How Marshall County compares

Marshall County, Illinois is home to about 11,552 people across 386.822 square miles. Averaged across its land area, Marshall County runs about 30 people per square mile. Within its own EPA zone, Marshall County is bigger by population than 31% of the counties that share it. Marshall 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 Marshall County

Marshall County's predicted average sits above EPA's own action level of 4 pCi/L — the concentration at which EPA recommends fixing a home — versus a national average around 1.3 pCi/L indoors and 0.4 pCi/L outdoors. 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 Marshall County specifically.

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

Marshall County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. Whatever zone Marshall County carries, the zone explainer covers what it does and doesn't tell you about any one house.