St. Clair County, MO radon potential

St. Clair 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. St. Clair County sits at the 40th percentile nationally for radon potential — above 40 of every 100 rated counties.

Where St. Clair County sits nationally

St. Clair County, Missouri is home to about 9,848 people across 674.981 square miles. That works out to roughly 15 people per square mile. Within its own EPA zone, St. Clair County is bigger by population than 21% of the counties that share it. EPA's geology-based Region 7 groups St. Clair County with 411 other counties nationally sharing its underlying rock and soil profile.

What this actually means for a house in St. Clair County

Neither at the 4 pCi/L action level nor down at the roughly 1.3 pCi/L national indoor norm, St. Clair County's Zone 2 prediction lands in the gap between the two. A Zone 2 rating is a moderate county-wide average, not a guarantee about any one house in St. Clair County — testing is the only way to know an individual home's actual level.

Next steps for a home in St. Clair County

A short-term test kit is the standard first step and typically costs far less than mitigation itself. If a test comes back at or above 4 pCi/L, a radon mitigation system for a typical home runs roughly $800–$2,500 (around $1,200 for a common single-family install), driven mainly by foundation type, not by county.

Find Missouri'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. Use code RADONZONE-10OFF at checkout for 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. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.