What EPA's radon map says about Snohomish County

EPA's own radon map rates Snohomish County zone 3 - low potential, predicting a county-wide average indoor level below 2 pCi/L. Nationally that's the 32nd percentile for radon potential — Snohomish County outranks 32% of every county EPA has rated.

How Snohomish County compares

About 864,113 people live in Snohomish County, Washington, spread over 2,086.518 square miles. Population density there runs about 414 people per square mile. Among counties sharing Snohomish County's EPA zone, it outranks 97% of them by population. EPA's geology-based Region 10 groups Snohomish County with 141 other counties nationally sharing its underlying rock and soil profile.

What this actually means for a house in Snohomish County

Snohomish 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. A Zone 3 label describes Snohomish County's average, not any one basement or slab in it — testing is still the only way an individual homeowner actually finds out.

If Snohomish County tests above the action level

Mitigation cost doesn't depend on Snohomish County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find Washington's state radon program

Snohomish County's test kit answers today; a Airthings radon monitor keeps answering for Snohomish 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

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.