Updated November 2026 · Data: EPA Map of Radon Zones · Sources
California has a mix of radon risk: 2 counties are in EPA Zone 1 (highest potential), 29 in Zone 2 and 27 in Zone 3. Find your county below, then learn how to test your home and what to do if the result is high.
Your county’s zone is not your home’s radon level. Homes with high radon are found in every zone. The EPA recommends testing every home, whatever its zone.
Map of EPA radon zones by county
Radon zones by county in California
The table lists the EPA radon zone for each of the 58 counties in California. The zone shows the predicted average indoor radon screening level for homes in that county.
| County | EPA zone | Predicted average indoor level |
|---|---|---|
| Alameda | Zone 2 | 2 to 4 pCi/L |
| Alpine | Zone 2 | 2 to 4 pCi/L |
| Amador | Zone 2 | 2 to 4 pCi/L |
| Butte | Zone 3 | Below 2 pCi/L |
| Calaveras | Zone 2 | 2 to 4 pCi/L |
| Colusa | Zone 3 | Below 2 pCi/L |
| Contra Costa | Zone 2 | 2 to 4 pCi/L |
| Del Norte | Zone 3 | Below 2 pCi/L |
| El Dorado | Zone 2 | 2 to 4 pCi/L |
| Fresno | Zone 2 | 2 to 4 pCi/L |
| Glenn | Zone 3 | Below 2 pCi/L |
| Humboldt | Zone 3 | Below 2 pCi/L |
| Imperial | Zone 3 | Below 2 pCi/L |
| Inyo | Zone 2 | 2 to 4 pCi/L |
| Kern | Zone 2 | 2 to 4 pCi/L |
| Kings | Zone 3 | Below 2 pCi/L |
| Lake | Zone 3 | Below 2 pCi/L |
| Lassen | Zone 3 | Below 2 pCi/L |
| Los Angeles | Zone 2 | 2 to 4 pCi/L |
| Madera | Zone 2 | 2 to 4 pCi/L |
| Marin | Zone 3 | Below 2 pCi/L |
| Mariposa | Zone 2 | 2 to 4 pCi/L |
| Mendocino | Zone 3 | Below 2 pCi/L |
| Merced | Zone 3 | Below 2 pCi/L |
| Modoc | Zone 3 | Below 2 pCi/L |
| Mono | Zone 2 | 2 to 4 pCi/L |
| Monterey | Zone 2 | 2 to 4 pCi/L |
| Napa | Zone 3 | Below 2 pCi/L |
| Nevada | Zone 2 | 2 to 4 pCi/L |
| Orange | Zone 3 | Below 2 pCi/L |
| Placer | Zone 2 | 2 to 4 pCi/L |
| Plumas | Zone 2 | 2 to 4 pCi/L |
| Riverside | Zone 2 | 2 to 4 pCi/L |
| Sacramento | Zone 3 | Below 2 pCi/L |
| San Benito | Zone 2 | 2 to 4 pCi/L |
| San Bernardino | Zone 2 | 2 to 4 pCi/L |
| San Diego | Zone 3 | Below 2 pCi/L |
| San Francisco | Zone 2 | 2 to 4 pCi/L |
| San Joaquin | Zone 3 | Below 2 pCi/L |
| San Luis Obispo | Zone 2 | 2 to 4 pCi/L |
| San Mateo | Zone 2 | 2 to 4 pCi/L |
| Santa Barbara | Zone 1 | Above 4 pCi/L |
| Santa Clara | Zone 2 | 2 to 4 pCi/L |
| Santa Cruz | Zone 2 | 2 to 4 pCi/L |
| Shasta | Zone 3 | Below 2 pCi/L |
| Sierra | Zone 2 | 2 to 4 pCi/L |
| Siskiyou | Zone 3 | Below 2 pCi/L |
| Solano | Zone 3 | Below 2 pCi/L |
| Sonoma | Zone 3 | Below 2 pCi/L |
| Stanislaus | Zone 3 | Below 2 pCi/L |
| Sutter | Zone 3 | Below 2 pCi/L |
| Tehama | Zone 3 | Below 2 pCi/L |
| Trinity | Zone 3 | Below 2 pCi/L |
| Tulare | Zone 2 | 2 to 4 pCi/L |
| Tuolumne | Zone 2 | 2 to 4 pCi/L |
| Ventura | Zone 1 | Above 4 pCi/L |
| Yolo | Zone 3 | Below 2 pCi/L |
| Yuba | Zone 2 | 2 to 4 pCi/L |
What the EPA radon zones mean
- Zone 1 (highest potential): predicted average indoor screening level greater than 4 pCi/L. In California: 2 counties (3%).
- Zone 2 (moderate potential): predicted average between 2 and 4 pCi/L. In California: 29 counties (50%).
- Zone 3 (low potential): predicted average below 2 pCi/L. In California: 27 counties (47%).
The EPA built the map from indoor radon measurements, geology, soil permeability and type of house foundation. It was made to help governments and builders target resources, not to decide whether a single home needs testing.
California counties with the highest radon potential
2 counties in California are in Zone 1: Santa Barbara, Ventura.
How to test for radon in California
Testing is the only way to know your home’s radon level. Short-term test kits (2 to 90 days) are sold at hardware stores and online, and some state programs offer kits at low cost; check with the California Indoor Radon Program (CDPH). Test in the lowest lived-in level of the home, with windows and doors closed, and follow the kit instructions.
- Do-it-yourself kits: a simple, low-cost first step for your own home.
- Long-term tests (more than 90 days): give a better picture of your home’s year-round average.
- Certified professionals: look for certification from the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB), and ask the state program whether California requires a state license.
- Buying or selling a home: ask for a radon test as part of the home inspection.
If your radon level is high
The EPA recommends fixing your home if the radon level is 4 pCi/L or higher, and considering a fix between 2 and 4 pCi/L. The most common fix is a radon mitigation system (sub-slab depressurization), which pulls radon from under the foundation and vents it outside, above the roof line.
- The EPA says most homes can be fixed for about the same cost as other common home repairs; the price depends on the size and design of the home.
- Get estimates from more than one qualified, certified mitigation contractor. See our radon mitigation cost guide for typical prices.
- Test again after the system is installed, and retest every few years.
Learn more: safe radon levels · how to test · test kits compared · radon and home sales · radon in well water
California radon program
Radon rules and programs in California
- Program focus: the California Radon Program (CDPH) educates residents, encourages testing and mitigation in homes and schools, and promotes radon-resistant construction in high-potential areas.
- Phone: the program also lists a toll-free line, 1-800-745-7236.
Contact
The California Indoor Radon Program (CDPH) answers questions from residents about radon testing, results and certified professionals.
- Phone: 916-716-3697
- Website: www.cdph.ca.gov/Programs/CEH/DRSEM/Pages/EMB/Radon/Radon.aspx
Contact details as listed by the state radon program directory. Please confirm on the program’s website before calling.
Frequently asked questions
Is radon a problem in California?
2 of California’s 58 counties are in EPA Zone 1, where the predicted average indoor radon level is above 4 pCi/L, and 29 are in Zone 2 (2 to 4 pCi/L). High levels can be found in any home, so testing is the only way to know.
Which California counties have the highest radon potential?
The 2 counties in EPA Zone 1 have the highest predicted average levels. You can find them in the table above and in the Zone 1 list.
What radon level is safe?
There is no known safe level of radon. The EPA recommends fixing your home at 4 pCi/L or higher and considering a fix between 2 and 4 pCi/L.
Who can I contact about radon in California?
The California Indoor Radon Program (CDPH) answers questions about testing, results and certified professionals. Phone: 916-716-3697.
Sources
- SOSRadon.org (CRCPD/EPA): California state radon program fact sheet (2022)
- U.S. EPA: EPA Map of Radon Zones (county data file, June 2024)
- U.S. EPA: A Citizen’s Guide to Radon (checked November 2026)
- U.S. EPA: How much can a radon mitigation system cost? (updated September 16, 2026)
- California Indoor Radon Program (CDPH) (checked November 2026)
- SOSRadon.org (CRCPD): State radon contacts (checked November 2026)