California Radon Levels
2Zone 1 (High)
29Zone 2 (Moderate)
27Zone 3 (Low)
3% of counties in California are classified as EPA Zone 1 (highest radon potential), meaning predicted average indoor radon levels exceed 4 pCi/L — the EPA action level. The EPA recommends that all homes be tested for radon, regardless of zone classification.
| County | EPA Zone | Risk Level |
|---|---|---|
| Alameda County | Zone 2 | Moderate Potential |
| Alpine County | Zone 2 | Moderate Potential |
| Amador County | Zone 2 | Moderate Potential |
| Butte County | Zone 3 | Lowest Potential |
| Calaveras County | Zone 2 | Moderate Potential |
| Colusa County | Zone 3 | Lowest Potential |
| Contra Costa County | Zone 2 | Moderate Potential |
| Del Norte County | Zone 3 | Lowest Potential |
| El Dorado County | Zone 2 | Moderate Potential |
| Fresno County | Zone 2 | Moderate Potential |
| Glenn County | Zone 3 | Lowest Potential |
| Humboldt County | Zone 3 | Lowest Potential |
| Imperial County | Zone 3 | Lowest Potential |
| Inyo County | Zone 2 | Moderate Potential |
| Kern County | Zone 2 | Moderate Potential |
| Kings County | Zone 3 | Lowest Potential |
| Lake County | Zone 3 | Lowest Potential |
| Lassen County | Zone 3 | Lowest Potential |
| Los Angeles County | Zone 2 | Moderate Potential |
| Madera County | Zone 2 | Moderate Potential |
| Marin County | Zone 3 | Lowest Potential |
| Mariposa County | Zone 2 | Moderate Potential |
| Mendocino County | Zone 3 | Lowest Potential |
| Merced County | Zone 3 | Lowest Potential |
| Modoc County | Zone 3 | Lowest Potential |
| Mono County | Zone 2 | Moderate Potential |
| Monterey County | Zone 2 | Moderate Potential |
| Napa County | Zone 3 | Lowest Potential |
| Nevada County | Zone 2 | Moderate Potential |
| Orange County | Zone 3 | Lowest Potential |
| Placer County | Zone 2 | Moderate Potential |
| Plumas County | Zone 2 | Moderate Potential |
| Riverside County | Zone 2 | Moderate Potential |
| Sacramento County | Zone 3 | Lowest Potential |
| San Benito County | Zone 2 | Moderate Potential |
| San Bernardino County | Zone 2 | Moderate Potential |
| San Diego County | Zone 3 | Lowest Potential |
| San Francisco County | Zone 2 | Moderate Potential |
| San Joaquin County | Zone 3 | Lowest Potential |
| San Luis Obispo County | Zone 2 | Moderate Potential |
| San Mateo County | Zone 2 | Moderate Potential |
| Santa Barbara County | Zone 1 | Highest Potential |
| Santa Clara County | Zone 2 | Moderate Potential |
| Santa Cruz County | Zone 2 | Moderate Potential |
| Shasta County | Zone 3 | Lowest Potential |
| Sierra County | Zone 2 | Moderate Potential |
| Siskiyou County | Zone 3 | Lowest Potential |
| Solano County | Zone 3 | Lowest Potential |
| Sonoma County | Zone 3 | Lowest Potential |
| Stanislaus County | Zone 3 | Lowest Potential |
| Sutter County | Zone 3 | Lowest Potential |
| Tehama County | Zone 3 | Lowest Potential |
| Trinity County | Zone 3 | Lowest Potential |
| Tulare County | Zone 2 | Moderate Potential |
| Tuolumne County | Zone 2 | Moderate Potential |
| Ventura County | Zone 1 | Highest Potential |
| Yolo County | Zone 3 | Lowest Potential |
| Yuba County | Zone 2 | Moderate Potential |