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

EPA places Island 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. Island County sits at the 25th percentile nationally for radon potential — above 25 of every 100 rated counties.

Island County by the numbers

Island County, Washington is home to about 86,478 people across 208.698 square miles. That works out to roughly 414 people per square mile. By population, Island County is the 237th largest of every county sharing its EPA zone — the 77th percentile by population within that zone. Island County falls in EPA Radon Region 10, one of 142 counties nationally EPA assigns to that same geologic province.

What this actually means for a house in Island County

Island 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. Don't read Zone 3 as a pass for Island County — EPA's guidance to test every home makes no exception for counties predicted low.

If Island County tests above the action level

For a Island 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 Island County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Island 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 Island 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.