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California Radon Map and Levels by County

California's counties span all three EPA radon zones. Homes with elevated levels occur in all three zones, so only a test measures yours.

Data checked By Radon411Methodology

Predicted
2 of 58counties in Zone 1, EPA's highest radon zone
Measured
By countyCDC publishes California's test figures by county: 56 of 58 counties have one.
Your home
Test to knowNo area figure here is a reading of your home.
How to test this homeFind a certified tester

The data

California, At a glance

California at a glance

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.

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

CDC test figures, county by county

CDC publishes California's radon test figures by county: 56 of its 58 counties have one. Each one's share is in the county list.

California, Counties

California counties by EPA zone

Predicted

California's radon map, then all 58 counties by the zone EPA predicts for each, highest reported share of tests at or above 4 pCi/L first. Each has its own report.

California radon map by county: EPA radon zones. Map of California's 58 counties shaded by EPA radon zone: Zone 1 in red-orange, Zone 2 in amber, Zone 3 in pale sand. 2 in Zone 1, 29 in Zone 2, 27 in Zone 3.
California radon map by countyEPA radon zones for California's counties: 2 counties in Zone 1, 29 counties in Zone 2, 27 counties in Zone 3.Predicted potential for each county, not a reading of any home. Homes with elevated levels of radon have been found in all three zones.

Zone 1: highest predicted potential2

Local help

California, Local resources

Local resources in California

Who to ask in California: its radon program, test kits and professionals.

What California's program says
Phone
1-800-745-7236
Email
Radonprogram@cdph.ca.gov
Licensing of radon professionals
The Radon Program certifies all radon service providers operating in California.

Radon Action Brief

National Radon Action MonthPre-sale

The radon brief for your California county, in one dated file.

  • Your area's radon record, explained.
  • Your plan to test this week.
  • Fix costs and pros near you.

Save 64%Sale price: $8.99Price after the sale: $25Ends Jan 31Then $25

Get your California county's BriefSee a free sample

Area data, not a test of the building.

What it means

California, Questions

Questions about radon in California

Common questions

9 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.

California on the radon mapRadon levels by state

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.

Radon in Santa Barbara CountyRadon in Ventura County

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 4 radon businesses for testing, mitigation or both, each with the licensing list it was read from. 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.

If you own, buy, sell or rent

What these figures mean for each reader, with the guide that covers it

The zone counts on this page describe counties across California. They are predictions for areas, and none of them measures your building.

California in figures

The figures above, said in words

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

      Predicted

      EPA 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 Radon411

      3% 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

    Measured

    CDC 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 (2,363 tests over 2008 to 2017, laboratory), Santa Clara County (793 tests over 2008 to 2017, laboratory) and San Diego County (692 tests over 2008 to 2017, laboratory).

California's radon program is the California Indoor Radon Program, run by California Department of Public Health; its contacts are under Local resources.

Products

California, Products

Testing and mitigation products

Whatever the figures for California show, only a test tells you about one home.

Test kits and monitors42

See all products

As an Amazon Associate, Radon411 earns from qualifying purchases.

Best radon monitors for California homes, compared

Testing

5

Mitigation

3

Explore

California, Next steps

What to do next

  1. LearnKnow what radon isWhat radon is, how it gets indoors, and why a county zone is not a reading for your home.Learn about radon
  2. TestTest the building you live inShort-term or long-term, where the device goes, and closed-house conditions.How to test
  3. FixBring a high level downHow a mitigation system works, and the retest that confirms it.How mitigation works
  4. Find a proHire a certified professionalMeasurement 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.