San Juan County, WA radon potential

San Juan County is mapped as Zone 3 - Low Potential by EPA — meaning the county-wide average indoor level is predicted below 2 pCi/L. Nationally that's the 12th percentile for radon potential — San Juan County outranks 12% of every county EPA has rated.

San Juan County by the numbers

San Juan County, Washington is home to about 18,668 people across 173.916 square miles. Population density there runs about 107 people per square mile. Within its own EPA zone, San Juan County is bigger by population than 37% of the counties that share it. Geologically, San Juan County is grouped into EPA Region 10 alongside 141 other counties that share its rock and soil setting.

Zone vs. an actual test

A Zone 3 county like San Juan County is predicted closer to the 0.4 pCi/L typical outdoors than to EPA's 4 pCi/L action level. Low doesn't mean zero: EPA's own data shows homes in Zone 3 counties like San Juan County testing above the action level often enough that skipping a test isn't the safe assumption.

If San Juan County tests above the action level

A short-term test kit is the standard first step and typically costs far less than mitigation itself. If a test comes back at or above 4 pCi/L, a radon mitigation system for a typical home runs roughly $800–$2,500 (around $1,200 for a common single-family install), driven mainly by foundation type, not by county.

Find Washington's state radon program

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

Where San Juan County's figures come from

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.