Radon in California: radon map by county
EPA's Map of Radon Zones puts 2 of California's 58 counties in Zone 1 (predicted average indoor radon above 4 pCi/L), 29 in Zone 2 and 27 in Zone 3. Of 5,663 building tests that radon laboratories reported for 2008 to 2017, 10.7% were at or above EPA's 4 pCi/L action level, against 36.5% nationally (number 41 of 42 states with 1,000+ tests). The highest county share is El Dorado County (30.3% of 224 tests).
Radon by county in California
| County | EPA zone | Lab tests | Median | 4+ pCi/L |
|---|---|---|---|---|
| Alameda County | Zone 2 | 290 | 0.7 | 6.7% |
| Alpine County | Zone 2 | 13 | 4.5 | 63.2% |
| Amador County | Zone 2 | 23 | 2.3 | 27.8% |
| Butte County | Zone 3 | 20 | 0.8 | 11.4% |
| Calaveras County | Zone 2 | 31 | 1.7 | 17.6% |
| Colusa County | Zone 3 | 1 | 0.2 | 0% |
| Contra Costa County | Zone 2 | 177 | 0.9 | 8.8% |
| Del Norte County | Zone 3 | 1 | 0.1 | 0% |
| El Dorado County | Zone 2 | 224 | 1.6 | 30.3% |
| Fresno County | Zone 2 | 61 | 1.5 | 11% |
| Glenn County | Zone 3 | 1 | 2.8 | 0% |
| Humboldt County | Zone 3 | 11 | 1.3 | 20% |
| Imperial County | Zone 3 | 3 | 0.6 | 0% |
| Inyo County | Zone 2 | 13 | 4.0 | 50% |
| Kern County | Zone 2 | 57 | 1.8 | 11.6% |
| Kings County | Zone 3 | 2 | 1.7 | 0% |
| Lake County | Zone 3 | 8 | 0.9 | 12.5% |
| Lassen County | Zone 3 | 3 | 1.1 | 0% |
| Los Angeles County | Zone 2 | 1,367 | 0.7 | 9.5% |
| Madera County | Zone 2 | 25 | 1.4 | 14.7% |
| Marin County | Zone 3 | 68 | 0.8 | 6.3% |
| Mariposa County | Zone 2 | 5 | 1.3 | 16.7% |
| Mendocino County | Zone 3 | 7 | 0.7 | 0% |
| Merced County | Zone 3 | 3 | 2.1 | 0% |
| Modoc County | Zone 3 | n/a | n/a | n/a |
| Mono County | Zone 2 | 16 | 4.6 | 50% |
| Monterey County | Zone 2 | 44 | 0.8 | 14.9% |
| Napa County | Zone 3 | 30 | 0.9 | 10.5% |
| Nevada County | Zone 2 | 203 | 2.0 | 25% |
| Orange County | Zone 3 | 259 | 0.7 | 3.9% |
| Placer County | Zone 2 | 193 | 1.1 | 7.4% |
| Plumas County | Zone 2 | 16 | 2.1 | 38.1% |
| Riverside County | Zone 2 | 134 | 0.6 | 1.3% |
| Sacramento County | Zone 3 | 131 | 0.9 | 7.7% |
| San Benito County | Zone 2 | 5 | 0.7 | 0% |
| San Bernardino County | Zone 2 | 81 | 0.7 | 4.5% |
| San Diego County | Zone 3 | 406 | 0.2 | 2.6% |
| San Francisco County | Zone 2 | 174 | 0.5 | 2.8% |
| San Joaquin County | Zone 3 | 38 | 1.4 | 14.9% |
| San Luis Obispo County | Zone 2 | 52 | 1.0 | 12.5% |
| San Mateo County | Zone 2 | 280 | 0.9 | 9.2% |
| Santa Barbara County | Zone 1 | 207 | 1.2 | 17.6% |
| Santa Clara County | Zone 2 | 447 | 0.9 | 9.2% |
| Santa Cruz County | Zone 2 | 65 | 1.2 | 21.9% |
| Shasta County | Zone 3 | 12 | 2.0 | 21.7% |
| Sierra County | Zone 2 | 1 | 1.2 | 0% |
| Siskiyou County | Zone 3 | 6 | 1.1 | 25% |
| Solano County | Zone 3 | 16 | 0.9 | 15.8% |
| Sonoma County | Zone 3 | 74 | 0.5 | 0.8% |
| Stanislaus County | Zone 3 | 15 | 1.4 | 11.1% |
| Sutter County | Zone 3 | 5 | 1.9 | 42.9% |
| Tehama County | Zone 3 | 3 | 2.2 | 0% |
| Trinity County | Zone 3 | n/a | n/a | n/a |
| Tulare County | Zone 2 | 48 | 1.7 | 23.8% |
| Tuolumne County | Zone 2 | 56 | 1.4 | 23% |
| Ventura County | Zone 1 | 203 | 1.0 | 11.9% |
| Yolo County | Zone 3 | 22 | 0.9 | 2.1% |
| Yuba County | Zone 2 | 7 | 0.1 | 0% |
Lab tests: buildings tested by radon laboratories, 2008 to 2017; median in pCi/L; “4+ pCi/L” is the share of tests at or above EPA's action level. n/a: no value on file (no tests, or too few to report).
What the zones mean
Zone 1 counties have a predicted average indoor radon screening level above 4 pCi/L, Zone 2 from 2 to 4 pCi/L and Zone 3 below 2 pCi/L. The prediction is from 1993 indoor measurements, geology, aerial radioactivity, soils and foundation types. EPA: “This map should not be used to determine if individual homes need to be tested.” EPA recommends fixing a home at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of exposure.
Radon in California: questions
What radon zone is California?
EPA's 1993 map does not give states one zone; it zones each county. In California, 2 counties are Zone 1, 29 Zone 2 and 27 Zone 3. EPA says the map "should not be used to determine if individual homes need to be tested".
Which California counties have the highest radon levels?
By the share of laboratory tests at or above 4 pCi/L for 2008 to 2017 (counties with 20 or more tests): El Dorado County 30.3% (median 1.6 pCi/L, 224 tests); Amador County 27.8% (median 2.3 pCi/L, 23 tests); Nevada County 25% (median 2.0 pCi/L, 203 tests); Tulare County 23.8% (median 1.7 pCi/L, 48 tests); Tuolumne County 23% (median 1.4 pCi/L, 56 tests).
How common is high radon in California?
10.7% of the 5,663 tests in the laboratory data for California (2008 to 2017) were at or above 4 pCi/L; the national share was 36.5%. Buildings are tested by choice, often because of a sale or a concern, so the share describes the tests on file, not every home.
Should I test for radon in California?
Yes. EPA recommends testing every home no matter where it is, fixing it at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of radon exposure.
States bordering California
Other free hazard lookups
- Radon by county and ZIP codeEPA radon zone and the radon test results on file for any county, ZIP code or address.
- Fault lines near an addressUSGS-mapped earthquake faults within 5 miles, their age and slip rate, and the USGS seismic hazard value.
- Power lines near an addressHigh-voltage transmission lines within 1 mile from federal maps, with voltage and distance.
- Flood zone lookupThe FEMA flood zone mapped at any US address. Not an official flood zone determination.
- Superfund sites near an addressThe nearest EPA Superfund (NPL) sites and their status.
All of these, plus wetlands, FEMA hazard ratings and the federal contamination lists, are in the Environmental Records Report for any US address ($29).
Superfund sites in California · Radon map of every state
Data compiled 2026-10-20. County-level screening information, not a radon measurement of any building.