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

EPA places McLean 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. Rank it against the rest of the country and McLean County lands in the 96th percentile for radon potential.

How McLean County compares

McLean County, Illinois is home to about 172,069 people across 1,183.248 square miles. That works out to roughly 145 people per square mile. By population, McLean County is the 111th largest of every county sharing its EPA zone — the 90th percentile by population within that zone. EPA's geology-based Region 5 groups McLean County with 523 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — McLean County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. That doesn't mean every McLean County house tests high — a zone is a county-wide prediction, never a single-home reading — but it's a reason to test sooner rather than let it wait.

Testing and mitigation in McLean County

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

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