Counties with the Highest Radon Potential in California
The top 25 of 58 California counties on GIS Map, ranked by EPA radon zone, where zone 1 carries the highest predicted indoor radon potential and zone 3 the lowest.
| # | County | EPA radon zone |
|---|---|---|
| 1 | Butte County | 3 |
| 2 | Colusa County | 3 |
| 3 | Del Norte County | 3 |
| 4 | Glenn County | 3 |
| 5 | Humboldt County | 3 |
| 6 | Imperial County | 3 |
| 7 | Kings County | 3 |
| 8 | Lake County | 3 |
| 9 | Lassen County | 3 |
| 10 | Marin County | 3 |
| 11 | Mendocino County | 3 |
| 12 | Merced County | 3 |
| 13 | Modoc County | 3 |
| 14 | Napa County | 3 |
| 15 | Orange County | 3 |
| 16 | Sacramento County | 3 |
| 17 | San Diego County | 3 |
| 18 | San Joaquin County | 3 |
| 19 | Shasta County | 3 |
| 20 | Siskiyou County | 3 |
| 21 | Solano County | 3 |
| 22 | Sonoma County | 3 |
| 23 | Stanislaus County | 3 |
| 24 | Sutter County | 3 |
| 25 | Tehama County | 3 |
Source: EPA Map of Radon Zones (September 2025). All counties with data are ranked. Estimates carry margins of error.
Citing this page: GIS Map, “Counties with the Highest Radon Potential in California”, https://gismap.land/rankings/california/highest-radon-counties. Figures from EPA Map of Radon Zones (September 2025).
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