California has 58 counties, with 2 classified as EPA Zone 1 (highest risk). With an average radon level of 5.91 pCi/L, California state testing data reveals significantly elevated risks across multiple counties. Testing is highly recommended.
California is the most populous state with 38 million people, or every one out of eight Americans. It is also the third largest state by surface area in the country, right behind Alaska and Texas.
Residents of the Alpine, Mono and Inyo counties are being offered free radon testing kits from the California Department of Public Health in association with the US EPA, CDPH and California Geological Survey. If you live in one of the aforementioned counties it is recommended that you take the opportunity to acquire a testing kit. The free test kits do include mailing supplies and an analysis of the test results.
Explore radon data organized by metro region, including average levels, hotspot cities, and local mitigation professionals.
Cities with the most user-submitted radon test data, ranked by number of submissions.
| City | Avg User Radon Level | Submissions |
|---|---|---|
| Los Angeles | 7.83 pCi/L | 6 |
| Mammoth Lakes | 10.80 pCi/L | 5 |
| San Jose | 4.20 pCi/L | 5 |
| Rancho Palos Verdes | 8.00 pCi/L | 4 |
| Woodland Hills | 7.67 pCi/L | 3 |
| South Lake Tahoe | 6.33 pCi/L | 3 |
| Palos Verdes Peninsula | 6.00 pCi/L | 3 |
| Bishop | 5.67 pCi/L | 3 |
| Sacramento | 4.33 pCi/L | 3 |
| Oakland | 3.33 pCi/L | 3 |
| Fresno | 10.00 pCi/L | 2 |
| Davis | 8.00 pCi/L | 2 |
| Palo Alto | 6.50 pCi/L | 2 |
| Studio City | 6.50 pCi/L | 2 |
| Menlo Park | 5.00 pCi/L | 2 |
| Truckee | 5.00 pCi/L | 2 |
| Walnut Creek | 4.00 pCi/L | 2 |
| San Diego | 3.50 pCi/L | 2 |
| San Francisco | 3.50 pCi/L | 2 |
| Belmont | 20.00 pCi/L | 1 |
All 58 California counties with their EPA radon zone classification and user-submitted average radon levels.
Browse radon data for individual cities across California. Each city page includes user-submitted radon levels, local contractors, and mitigation resources.