What EPA's radon map says about Grundy County

Grundy County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Nationally that's the 91st percentile for radon potential — Grundy County outranks 91% of every county EPA has rated.

How Grundy County compares

About 53,835 people live in Grundy County, Illinois, spread over 418.133 square miles. Population density there runs about 129 people per square mile. Among counties sharing Grundy County's EPA zone, it outranks 72% of them by population. EPA's geology-based Region 5 groups Grundy County with 523 other counties nationally sharing its underlying rock and soil profile.

Why the zone isn't the answer by itself

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Grundy County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. 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 Grundy County will test well under the action level, and EPA's guidance is to test regardless of zone.

If Grundy County tests above the action level

Mitigation cost doesn't depend on Grundy County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find Illinois's state radon program

Grundy County's test kit answers today; a Airthings radon monitor keeps answering for Grundy County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.

More on how this number was built

The methodology guide shows exactly which EPA and Census sources feed Grundy County's numbers. Whatever zone Grundy County carries, the zone explainer covers what it does and doesn't tell you about any one house.