Locations and reports
Radon Levels in California
What this means for you
- Your area: EPA rates 2 of California's 58 classified counties Zone 1, the highest of its three radon zones. Zones are predictions for whole counties, and homes with elevated levels occur in all three zones.
- Local tests: CDC publishes California's radon test results by county. Open your county below to see its figures.
- Your home: The only way to know your home’s level is to test it.
pCi/L (picocuries per liter) is the unit radon is measured in. EPA recommends fixing a home at 4 pCi/L or above, its action level.
For California, EPA's Map of Radon Zones places 2 of its 58 classified counties in Zone 1, highest predicted potential, a prediction for each county as a whole. Radon levels vary from home to home, so only a test measures the level in yours.
Predicted: EPA radon zone
58 of 58 counties classified
EPA classifies counties, not states. Of the 58 counties in California with a zone, 2 are in Zone 1, 29 in Zone 2 and 27 in Zone 3.
- Zone 1: Highest predicted potentialgreater than 4 pCi/L2 of 58 counties · 3.4%
- Zone 2: Moderate predicted potential2 to 4 pCi/L29 of 58 counties · 50.0%
- Zone 3: Lower predicted potentialless than 2 pCi/L27 of 58 counties · 46.6%
An EPA radon zone describes predicted potential across a whole county. It is not intended to determine whether any individual home should be tested, and homes with elevated levels occur in all three zones. EPA recommends testing every home regardless of zone, and testing is the only way to know the radon level in a specific building.
This is a county-level geologic prediction. It is not a measurement of any address, and houses with elevated radon are found in all three zones.
Predicted: EPA Map of Radon Zones
Measured: tests reported to CDC
CDC publishes California's radon test figures by county: 56 of its 58 counties have one. Each county's share is in the county list.
California, In figures
California in figures
What the published data says about California, in words. A zone and a share of tests describe an area, never the level in any one home.
Predicted
PredictedEPA places 2 of California's 58 counties in Zone 1, 29 in Zone 2 and 27 in Zone 3.
The Zone 1 counties: Santa Barbara County and Ventura County.
Compared
Calculated by Radon4113% of California's counties with a zone on file are in Zone 1, a smaller share than across all U.S. counties (34%). It is a share of counties, not of land, people or homes.
Reported tests
MeasuredCDC has no statewide testing figure for California on file here.
56 of the 58 counties have a published county figure; the most tests in each one's latest year were reported for Los Angeles County (1,367 in 2017), Santa Clara County (447 in 2017) and San Diego County (406 in 2017).
California's radon program is the California Indoor Radon Program, run by California Department of Public Health; its contacts are under Local resources.
California, Counties
California counties by EPA zone
PredictedAll 58 counties, by the zone EPA predicts for each. Every county has its own report.
Zone 1: highest predicted potential2
Zone 2: moderate predicted potential29
- Alameda County6.7% of reported tests at or above 4 pCi/L, 2017
- Alpine County63.2% of reported tests at or above 4 pCi/L, 2017
- Amador County27.8% of reported tests at or above 4 pCi/L, 2017
- Calaveras County17.6% of reported tests at or above 4 pCi/L, 2017
- Contra Costa County8.8% of reported tests at or above 4 pCi/L, 2017
- El Dorado County30.3% of reported tests at or above 4 pCi/L, 2017
- Fresno County11.0% of reported tests at or above 4 pCi/L, 2017
- Inyo County50.0% of reported tests at or above 4 pCi/L, 2017
- Kern County11.6% of reported tests at or above 4 pCi/L, 2017
- Los Angeles County9.5% of reported tests at or above 4 pCi/L, 2017
- Madera County14.7% of reported tests at or above 4 pCi/L, 2017
- Mariposa County16.7% of reported tests at or above 4 pCi/L, 2017
- Mono County50.0% of reported tests at or above 4 pCi/L, 2017
- Monterey County14.9% of reported tests at or above 4 pCi/L, 2017
- Nevada County25.0% of reported tests at or above 4 pCi/L, 2017
- Placer County7.4% of reported tests at or above 4 pCi/L, 2017
- Plumas County38.1% of reported tests at or above 4 pCi/L, 2017
- Riverside County1.3% of reported tests at or above 4 pCi/L, 2017
- San Benito County0.0% of reported tests at or above 4 pCi/L, 2017
- San Bernardino County4.5% of reported tests at or above 4 pCi/L, 2017
- San Francisco County2.8% of reported tests at or above 4 pCi/L, 2017
- San Luis Obispo County12.5% of reported tests at or above 4 pCi/L, 2017
- San Mateo County9.2% of reported tests at or above 4 pCi/L, 2017
- Santa Clara County9.2% of reported tests at or above 4 pCi/L, 2017
- Santa Cruz County21.9% of reported tests at or above 4 pCi/L, 2017
- Sierra County0.0% of reported tests at or above 4 pCi/L, 2017
- Tulare County23.8% of reported tests at or above 4 pCi/L, 2017
- Tuolumne County23.0% of reported tests at or above 4 pCi/L, 2017
- Yuba County0.0% of reported tests at or above 4 pCi/L, 2017
Zone 3: lower predicted potential27
- Butte County11.4% of reported tests at or above 4 pCi/L, 2017
- Colusa County0.0% of reported tests at or above 4 pCi/L, 2017
- Del Norte County0.0% of reported tests at or above 4 pCi/L, 2017
- Glenn County0.0% of reported tests at or above 4 pCi/L, 2017
- Humboldt County20.0% of reported tests at or above 4 pCi/L, 2017
- Imperial County0.0% of reported tests at or above 4 pCi/L, 2017
- Kings County0.0% of reported tests at or above 4 pCi/L, 2017
- Lake County12.5% of reported tests at or above 4 pCi/L, 2017
- Lassen County0.0% of reported tests at or above 4 pCi/L, 2017
- Marin County6.3% of reported tests at or above 4 pCi/L, 2017
- Mendocino County0.0% of reported tests at or above 4 pCi/L, 2017
- Merced County0.0% of reported tests at or above 4 pCi/L, 2017
- Modoc CountyReported testing not yet available
- Napa County10.5% of reported tests at or above 4 pCi/L, 2017
- Orange County3.9% of reported tests at or above 4 pCi/L, 2017
- Sacramento County7.7% of reported tests at or above 4 pCi/L, 2017
- San Diego County2.6% of reported tests at or above 4 pCi/L, 2017
- San Joaquin County14.9% of reported tests at or above 4 pCi/L, 2017
- Shasta County21.7% of reported tests at or above 4 pCi/L, 2017
- Siskiyou County25.0% of reported tests at or above 4 pCi/L, 2017
- Solano County15.8% of reported tests at or above 4 pCi/L, 2017
- Sonoma County0.8% of reported tests at or above 4 pCi/L, 2017
- Stanislaus County11.1% of reported tests at or above 4 pCi/L, 2017
- Sutter County42.9% of reported tests at or above 4 pCi/L, 2017
- Tehama County0.0% of reported tests at or above 4 pCi/L, 2017
- Trinity CountyReported testing not yet available
- Yolo County2.1% of reported tests at or above 4 pCi/L, 2017
California, Local resources
Local resources in California
Who to ask in California: the state's radon program, test kits and professionals.
California Indoor Radon Program
California Department of Public Health
California’s radon program answers questions about testing, mitigation and the rules where you live.
- Phone
- 1-800-745-7236
- Radonprogram@cdph.ca.gov
- Licensing of radon professionals
The Radon Program certifies all radon service providers operating in California.
Test kits
The program says: Discounted test kits can be purchased through our partner laboratory, Alpha Energy, for $14.95.
Radon professionals
3 professionals are listed in California.
Professionals in CaliforniaCalifornia, Questions
Questions about radon in California
Common questions9 questions, answered from the figures above
Is radon a problem in California?
EPA's Map of Radon Zones places 2 of the 58 California counties it classifies in Zone 1, the highest predicted potential, 29 in Zone 2 and 27 in Zone 3. An EPA radon zone describes predicted potential across a whole county. It is not intended to determine whether any individual home should be tested, and homes with elevated levels occur in all three zones. EPA recommends testing every home regardless of zone, and testing is the only way to know the radon level in a specific building.
Which California counties are in EPA radon Zone 1?
2 counties are in Zone 1: Santa Barbara County, Ventura County. Zone 1 is EPA's class for highest predicted potential, a predicted average indoor screening level greater than 4 pCi/L. A zone is a prediction for a county, not a measurement of any home.
How do I get a radon test in California?
EPA says you can test your home yourself or hire a qualified radon test company, and that either approach takes only a small amount of time and effort. The California radon program is the place to ask about certified testers and any test kit offer it runs.
How to test a homeFree and low-cost kits by stateCalifornia radon program
Who does radon mitigation in California?
Radon411's California directory lists 3 certified radon companies. EPA recommends having a qualified contractor fix a home with a radon problem, because lowering high radon levels takes technical knowledge and special skills, and some states require the work to be done by a licensed or certified professional.
How radon mitigation worksWhat mitigation costsRadon professionals in California
Which California counties have the highest radon?
58 of California's 58 counties have an EPA radon zone published here. Radon411 will not rank counties on data it does not hold, and a zone is a prediction for an area rather than a measurement of any building.
What share of radon tests in California are high?
CDC has published no statewide radon testing statistics for California, so we cannot give a figure. Where CDC does publish one, the percentage describes radon tests reported to CDC, not homes.
Which EPA region is California in?
California is in EPA Region 9: Pacific Southwest.
Do I need to test my home in California?
Yes, wherever you live in the state. Testing is the only way to know the radon level in a specific building. EPA recommends fixing a home when the radon level is 4 pCi/L or higher. EPA recommends considering a fix when the level is between 2 and 4 pCi/L.
Why does this page show coverage rather than one radon level for California?
A statewide figure averages counties with very different geology and very different numbers of reported tests, so it hides more than it shows. The county report is the level at which the EPA zone, the geologic potential and the reported testing statistics are actually published, which is why the county table above links to every one of them.
What these figures mean if you own, buy, sell or rent
The zone counts on this page describe counties across California. They are predictions for areas, and none of them measures your building.
- How to test a home
If you own your home
Test the lowest lived-in level; a short-term kit screens in days, a long-term kit reflects the year.
- Buying a house: radon guide
If you are buying
Ask for past results and any system's post-mitigation test, and test during the sale.
- Selling a house: radon guide
If you are selling
Testing before you list puts the timing in your hands; disclosure rules differ by state.
- Radon in rental homes
If you rent, or rent out a home
A tenant can test; a fix is the owner's to make, and duties differ by state.
How long a test has to run, under what conditions, and what EPA says a result means: the testing standard, in full.
California, Products
Testing and mitigation products
Whatever the figures for California show, only a test tells you about one home.
Testing
- Short-term test kitsPassive detectors exposed for a few days under closed-building conditions, then analyzed by a lab.
- Long-term test kitsDetectors exposed for more than 90 days, for a closer picture of the annual average.
- Continuous monitorsPowered devices that log radon on a repeating cycle, often used for transaction tests.
- Digital detectorsConsumer devices that display a running radon reading without a lab.
- Water radon testsKits for radon dissolved in well water, analyzed by a lab.
Mitigation
- Mitigation fansThe inline fan that keeps a soil depressurization system running, sized by the installer.
- System indicatorsU-tube manometers and fan alarms that show whether a mitigation system is still working.
- Sealing materialsCrack, joint and sump sealing used alongside a mitigation system, not instead of one.
California / Next steps
What to do next
Learn
Know what radon is
What radon is, how it gets indoors, and why a county zone is not a reading for your home.
Learn about radonTest
Test the building you live in
Short-term or long-term, where the device goes, and closed-house conditions.
How to testFix
Bring a high level down
How a mitigation system works, and the retest that confirms it.
How mitigation worksFind a pro
Hire a certified professional
Measurement and mitigation professionals by state, with their certifications.
Find a professional
Informational. Follow EPA guidance. EPA says all radon tests should be taken for a minimum of 48 hours, with windows and doors closed from 12 hours before a short-term test.