Radon levels in California: Low
The model predicts a population-weighted basement average of 1.5 pCi/L across California's 58 counties - about 17% below the US average of 1.8 pCi/L. Above-ground floors average 0.9 pCi/L.
Predicted (model estimate). The pCi/L figures on this page are long-term area averages from a peer-reviewed national model built on millions of real radon tests (2001-2021). They describe the typical home in this area - not your home, which can be several times higher or lower. The EPA radon zone is a separate, 1993 county-level classification. How the numbers work.
2 / 58
EPA Zone 1 counties (3%)
29 / 27
Zone 2 / Zone 3 counties
0
counties predicted ≥ 4 pCi/L
0%
of ZIP codes predicted ≥ 4 pCi/L
California counties by predicted radon
Sorted highest to lowest predicted basement average. 58 counties.
| County | Predicted basement | Upstairs | Grade | EPA zone | ZIPs |
|---|---|---|---|---|---|
| Mono County | 2.7 pCi/L | 2.4 | Moderate | EPA Zone 2 | 8 |
| Alpine County | 2.3 pCi/L | 1.7 | Moderate | EPA Zone 2 | 2 |
| Plumas County | 2.3 pCi/L | 1.5 | Moderate | EPA Zone 2 | 14 |
| Inyo County | 2.2 pCi/L | 1.5 | Moderate | EPA Zone 2 | 6 |
| Sierra County | 2.2 pCi/L | 1.4 | Moderate | EPA Zone 2 | 8 |
| Modoc County | 2.0 pCi/L | 1.0 | Moderate | EPA Zone 3 | 9 |
| Lassen County | 2.0 pCi/L | 1.2 | Moderate | EPA Zone 3 | 14 |
| Siskiyou County | 2.0 pCi/L | 0.9 | Low | EPA Zone 3 | 20 |
| Mariposa County | 1.9 pCi/L | 1.3 | Low | EPA Zone 2 | 7 |
| Nevada County | 1.9 pCi/L | 1.2 | Low | EPA Zone 2 | 7 |
| Amador County | 1.9 pCi/L | 1.3 | Low | EPA Zone 2 | 8 |
| El Dorado County | 1.9 pCi/L | 1.3 | Low | EPA Zone 2 | 21 |
| Tuolumne County | 1.9 pCi/L | 1.1 | Low | EPA Zone 2 | 10 |
| Calaveras County | 1.8 pCi/L | 1.1 | Low | EPA Zone 2 | 11 |
| San Bernardino County | 1.8 pCi/L | 1.0 | Low | EPA Zone 2 | 79 |
| Trinity County | 1.8 pCi/L | 0.9 | Low | EPA Zone 3 | 13 |
| Lake County | 1.7 pCi/L | 0.8 | Low | EPA Zone 3 | 11 |
| Riverside County | 1.7 pCi/L | 0.9 | Low | EPA Zone 2 | 69 |
| San Luis Obispo County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 18 |
| Ventura County | 1.6 pCi/L | 1.1 | Low | EPA Zone 1 | 26 |
| San Benito County | 1.6 pCi/L | 0.8 | Low | EPA Zone 2 | 3 |
| Butte County | 1.6 pCi/L | 0.9 | Low | EPA Zone 3 | 15 |
| Shasta County | 1.6 pCi/L | 0.8 | Low | EPA Zone 3 | 22 |
| Tehama County | 1.6 pCi/L | 0.8 | Low | EPA Zone 3 | 8 |
| Colusa County | 1.6 pCi/L | 0.7 | Low | EPA Zone 3 | 6 |
| Santa Barbara County | 1.6 pCi/L | 1.0 | Low | EPA Zone 1 | 22 |
| Madera County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 13 |
| Tulare County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 28 |
| Kern County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 43 |
| Monterey County | 1.6 pCi/L | 0.8 | Low | EPA Zone 2 | 25 |
| Placer County | 1.6 pCi/L | 0.9 | Low | EPA Zone 2 | 28 |
| Kings County | 1.5 pCi/L | 0.9 | Low | EPA Zone 3 | 6 |
| Santa Cruz County | 1.5 pCi/L | 0.8 | Low | EPA Zone 2 | 15 |
| Glenn County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 6 |
| Merced County | 1.5 pCi/L | 0.9 | Low | EPA Zone 3 | 17 |
| Sutter County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 8 |
| Yuba County | 1.5 pCi/L | 0.8 | Low | EPA Zone 2 | 11 |
| Los Angeles County | 1.5 pCi/L | 0.9 | Low | EPA Zone 2 | 274 |
| Yolo County | 1.5 pCi/L | 0.7 | Low | EPA Zone 3 | 17 |
| Mendocino County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 25 |
| Imperial County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 11 |
| Napa County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 8 |
| Orange County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 83 |
| San Diego County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 98 |
| Santa Clara County | 1.4 pCi/L | 0.8 | Low | EPA Zone 2 | 55 |
| San Mateo County | 1.4 pCi/L | 0.7 | Low | EPA Zone 2 | 27 |
| Fresno County | 1.4 pCi/L | 0.9 | Low | EPA Zone 2 | 48 |
| Alameda County | 1.4 pCi/L | 0.7 | Low | EPA Zone 2 | 47 |
| Sonoma County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 32 |
| Humboldt County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 30 |
| Marin County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 27 |
| San Joaquin County | 1.4 pCi/L | 0.9 | Low | EPA Zone 3 | 26 |
| Solano County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 14 |
| Stanislaus County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 23 |
| Contra Costa County | 1.4 pCi/L | 0.7 | Low | EPA Zone 2 | 39 |
| Del Norte County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 4 |
| Sacramento County | 1.3 pCi/L | 0.8 | Low | EPA Zone 3 | 52 |
| San Francisco County | 1.3 pCi/L | 0.6 | Low | EPA Zone 2 | 25 |
California radon questions
- Is radon high in California?
- California's population-weighted predicted basement average is 1.5 pCi/L, about 17% below the US average of 1.8. 0 of its 58 modeled counties are predicted at or above the EPA action level of 4 pCi/L, 7 are in the 2-4 "consider fixing" range and 51 are below 2. 0% of California ZIP codes are predicted at 4 or higher.
- Which California county has the highest radon?
- Mono County has the highest predicted basement average in California at 2.7 pCi/L (EPA Zone 2). The lowest is San Francisco County at 1.3 pCi/L. These are modeled area averages; individual homes in any county can test far higher.
- How many EPA Zone 1 counties are in California?
- 2 of California's 58 counties (3%) are EPA Zone 1 (highest potential), 29 are Zone 2 and 27 are Zone 3. The EPA zone map dates from 1993; the predicted values on this page come from a 2001-2021 model built on millions of actual tests, so the two can disagree for a given county.
- Should I test for radon in California?
- Yes. Even though California's predicted average is below 2 pCi/L, the EPA recommends every home be tested - radon varies house to house, and 9.3% of California ZIP codes are still predicted at 2 pCi/L or higher.
California radon program
Many states offer free or discounted test kits and keep lists of certified mitigators. Find the California radon office through the EPA state radon contacts page. Then read how to test and what mitigation costs.