Is St. Charles County, MO a radon risk area?

St. Charles 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. That puts St. Charles County in the 64th percentile nationally for radon potential — higher than 64% of the counties EPA has rated.

Where St. Charles County sits nationally

About 423,726 people live in St. Charles County, Missouri, spread over 560.313 square miles. Population density there runs about 756 people per square mile. Rank 62 by population within its EPA zone puts St. Charles County at the 94th percentile among zone peers. EPA's geology-based Region 7 groups St. Charles County with 411 other counties nationally sharing its underlying rock and soil profile.

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

St. Charles County's predicted average falls between EPA's 4 pCi/L action level and the national indoor average of about 1.3 pCi/L (outdoor air runs roughly 0.4 pCi/L). Middle-of-the-pack doesn't mean risk-free for any one St. Charles County address — a moderate county average still leaves individual homes to test either side of it.

Testing and mitigation in St. Charles County

Nothing about St. Charles County's EPA zone changes the mitigation number: $800–$2,500 nationally (around $1,200 typical), set by the house's foundation rather than the county it sits in — and only relevant once a test actually clears 4 pCi/L.

Find Missouri's state radon program

St. Charles County's test kit answers today; a Airthings radon monitor keeps answering for St. Charles 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

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.