What Zone 3 means in King County
The EPA still recommends testing every home. Zone 3 is the lowest predicted tier, but individual houses in any county can and do read above the action level.
The EPA's Map of Radon Zones (EPA-402-R-93-071) sorts all 3,143 US counties into three tiers by predicted indoor screening level. King County sits in Zone 3. The classification is a planning assumption for builders, inspectors and buyers, and it is the figure a home inspector or realtor will cite for this county.
How King County compares inside Washington
Of the 39 counties in Washington on the EPA map, 7 are Zone 1, 14 are Zone 2 and 18 are Zone 3. King County is one of the 18 Washington counties classified Zone 3.
Why your own home still needs a test
Radon comes out of the soil and enters through the foundation, so two houses on the same street can read very differently depending on foundation cracks, soil contact and how the house is ventilated. A Zone 3 classification tells you the county's planning assumption, not your reading. A short-term test kit costs about $15 to $30 and settles it. Many state radon programs also offer free or low-cost kits.
ZIP codes in King County
98001, 98002, 98003, 98004, 98005, 98006, 98007, 98008, 98009, 98010, 98011, 98013, 98014, 98015, 98019, 98022, 98023, 98024, 98025, 98027, 98028, 98029, 98030, 98031, 98032, 98033, 98034, 98035, 98038, 98039, 98040, 98041, 98042, 98045, 98047, 98050, 98051, 98052, 98053, 98055, 98056, 98057, 98058, 98059, 98062, 98063, 98064, 98065, 98068, 98070, 98071, 98072, 98073, 98074, 98075, 98083, 98089, 98092, 98093, 98101 and 60 more
Get the full dossier for your ZIP
The $9 King County radon report gives you the county's EPA zone, the expected indoor average, your home's estimated likelihood of testing above the 4 pCi/L action level adjusted for basement and home type, the mitigation cost band, and the EPA lifetime-risk figures, on one printable page you can hand to a buyer's inspector.
Related reading
- What EPA Zones 1, 2 and 3 actually mean
- How to test your home for radon yourself
- How radon mitigation systems work, and what they cost
- Selling a home with high radon
- Radon zones for other counties
Source
Zone classification: US EPA Map of Radon Zones, county table (EPA-402-R-93-071 / EPA 402/A-16/001). Population: US Census Bureau county population estimates, 2024 vintage. This page reports the EPA's own classification for King County exactly as published. We do not interpolate or adjust it.