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

St. Louis 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. Louis County in the 65th percentile nationally for radon potential — higher than 65% of the counties EPA has rated.

How St. Louis County compares

About 992,929 people live in St. Louis County, Missouri, spread over 508.525 square miles. Population density there runs about 1,953 people per square mile. Rank 17 by population within its EPA zone puts St. Louis County at the 98th percentile among zone peers. Geologically, St. Louis County is grouped into EPA Region 7 alongside 411 other counties that share its rock and soil setting.

Zone vs. an actual test

St. Louis County's Zone 2 prediction clears the 1.3 pCi/L national indoor average without reaching EPA's own 4 pCi/L point for recommending a fix. Middle-of-the-pack doesn't mean risk-free for any one St. Louis County address — a moderate county average still leaves individual homes to test either side of it.

Testing and mitigation in St. Louis County

Nothing about St. Louis 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. Louis County's test kit answers today; a Airthings radon monitor keeps answering for St. Louis 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

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