Is Washington County, MO a radon risk area?

EPA's own radon map rates Washington County zone 2 - moderate potential, predicting a county-wide average indoor level between 2 and 4 pCi/L. Nationally that's the 48th percentile for radon potential — Washington County outranks 48% of every county EPA has rated.

How Washington County compares

About 23,583 people live in Washington County, Missouri, spread over 759.866 square miles. Population density there runs about 31 people per square mile. Rank 571 by population within its EPA zone puts Washington County at the 45th percentile among zone peers. Washington County falls in EPA Radon Region 7, one of 412 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Washington 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). Washington County's Zone 2 rating sits in the middle nationally; EPA's guidance is unchanged by that — test every home regardless of its zone.

Next steps for a home in Washington County

Nothing about Washington 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

Washington County's test kit answers today; a Airthings radon monitor keeps answering for Washington County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. Code RADONZONE-10OFF takes 10% off at checkout.

More on how this number was built

Washington 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.