St. Clair County, MI: EPA's radon zone, explained

EPA places St. Clair County in Zone 3 - Low Potential — a predicted county-wide average indoor radon level below 2 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. That puts St. Clair County in the 28th percentile nationally for radon potential — higher than 28% of the counties EPA has rated.

Where St. Clair County sits nationally

St. Clair County, Michigan is home to about 160,308 people across 721.522 square miles. That works out to roughly 222 people per square mile. By population, St. Clair County is the 160th largest of every county sharing its EPA zone — the 85th percentile by population within that zone. St. Clair 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 St. Clair County

St. Clair County's Zone 3 prediction undercuts the roughly 1.3 pCi/L most US homes average, nowhere near EPA's 4 pCi/L threshold for recommending mitigation. Don't read Zone 3 as a pass for St. Clair County — EPA's guidance to test every home makes no exception for counties predicted low.

Testing and mitigation in St. Clair County

For a St. Clair 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 Michigan's state radon program

For St. Clair County, an ongoing number beats a one-time test - a Airthings radon monitor tracks St. Clair County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.

Where St. Clair County's figures come from

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