Whatcom County, WA: EPA's radon zone, explained

EPA places Whatcom County in Zone 3 - Low Potential — a predicted county-wide average indoor radon level below 2 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. That puts Whatcom County in the 29th percentile nationally for radon potential — higher than 29% of the counties EPA has rated.

How Whatcom County compares

Whatcom County, Washington is home to about 234,954 people across 2,107.928 square miles. That works out to roughly 111 people per square mile. By population, Whatcom County is the 110th largest of every county sharing its EPA zone — the 89th percentile by population within that zone. EPA's geology-based Region 10 groups Whatcom County with 141 other counties nationally sharing its underlying rock and soil profile.

What this actually means for a house in Whatcom County

At Zone 3, Whatcom County's predicted county average is under the national indoor average (~1.3 pCi/L) and well below EPA's 4 pCi/L action level — outdoor air nationally runs about 0.4 pCi/L for comparison. Don't read Zone 3 as a pass for Whatcom County — EPA's guidance to test every home makes no exception for counties predicted low.

Testing and mitigation in Whatcom County

For a Whatcom County homeowner, the order is simple: test first, since a kit costs a small fraction of what mitigation would. Only if the result clears 4 pCi/L does a system — typically $800–$2,500, about $1,200 for a common install — become worth pricing out.

Find Washington's state radon program

For Whatcom County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Whatcom County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.

Where Whatcom County's figures come from

Whatcom County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. Whatever zone Whatcom County carries, the zone explainer covers what it does and doesn't tell you about any one house.