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

McHenry 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. Nationally that's the 64th percentile for radon potential — McHenry County outranks 64% of every county EPA has rated.

How McHenry County compares

McHenry County, Illinois is home to about 315,959 people across 603.403 square miles. Averaged across its land area, McHenry County runs about 524 people per square mile. By population, McHenry County is the 73rd largest of every county sharing its EPA zone — the 93rd percentile by population within that zone. McHenry County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.

Why the zone isn't the answer by itself

Neither at the 4 pCi/L action level nor down at the roughly 1.3 pCi/L national indoor norm, McHenry County's Zone 2 prediction lands in the gap between the two. A county-wide moderate average, like McHenry County's, can still hide a specific house well above or below it — testing is what actually answers the question.

If McHenry County tests above the action level

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

McHenry County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.