What Zone 3 means in Sonoma 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. Sonoma 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 Sonoma County compares inside California
Of the 58 counties in California on the EPA map, 2 are Zone 1, 29 are Zone 2 and 27 are Zone 3. Sonoma County is one of the 27 California 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 Sonoma County
94922, 94923, 94926, 94927, 94928, 94931, 94951, 94952, 94953, 94954, 94955, 94972, 94975, 94999, 95401, 95402, 95403, 95404, 95405, 95406, 95407, 95409, 95412, 95416, 95419, 95421, 95425, 95430, 95431, 95433, 95436, 95439, 95441, 95442, 95444, 95446, 95448, 95450, 95452, 95462, 95465, 95471, 95472, 95473, 95476, 95480, 95486, 95487, 95492, 95497
Get the full dossier for your ZIP
The $9 Sonoma 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 Sonoma County exactly as published. We do not interpolate or adjust it.