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

Cook County is mapped as Zone 2 - Moderate Potential by EPA — meaning the county-wide average indoor level is predicted between 2 and 4 pCi/L. Cook County sits at the 66th percentile nationally for radon potential — above 66 of every 100 rated counties.

Where Cook County sits nationally

Cook County, Illinois is home to about 5,182,617 people across 944.93 square miles. That works out to roughly 5,485 people per square mile. By population, Cook County is the 2nd largest of every county sharing its EPA zone — the 100th percentile by population within that zone. Geologically, Cook County is grouped into EPA Region 5 alongside 523 other counties that share its rock and soil setting.

Zone vs. an actual test

Neither at the 4 pCi/L action level nor down at the roughly 1.3 pCi/L national indoor norm, Cook County's Zone 2 prediction lands in the gap between the two. Middle-of-the-pack doesn't mean risk-free for any one Cook County address — a moderate county average still leaves individual homes to test either side of it.

Next steps for a home in Cook County

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

Cook County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.