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Radon zones in California

The EPA assigns every county in California to one of three radon potential zones. 2 are in Zone 1, the highest category, home to about 3% of the state's residents.

58
Counties
2
Zone 1 counties
Predicted average above 4 pCi/L
29
Zone 2 counties
Predicted average 2-4 pCi/L
27
Zone 3 counties
Predicted average below 2 pCi/L

The map

Radon potential across California

Every county in California, filled by the zone the EPA assigns it. Zones follow geology rather than county lines, so the colour usually changes where the bedrock does, not where the border is.

Zone 1 · 2Zone 2 · 29Zone 3 · 27

Hover a county · click to open it

County by county

All 58 California counties

Zone 1 · > 4 pCi/LZone 2 · 2 - 4 pCi/LZone 3 · < 2 pCi/L
CountyZonePopulationCOPDSmokingLung ca. deaths
Santa Barbara County1441k5.6%10.7%20.2
Ventura County1830k5.2%10.4%21.0
Alameda County21.6M4.1%9.3%20.6
Alpine County21,1416.4%11.0% -
Amador County242k7.1%12.2%30.3
Calaveras County247k8.4%13.4%31.2
Contra Costa County21.2M4.7%9.3%21.7
El Dorado County2192k6.0%10.4%20.1
Fresno County21.0M6.0%13.2%24.3
Inyo County219k6.9%12.0%16.4
Kern County2914k6.4%14.6%26.6
Los Angeles County29.7M4.9%11.2%20.5
Madera County2163k7.2%14.8%26.0
Mariposa County217k8.4%12.6%25.7
Mono County213k6.0%11.7% -
Monterey County2431k5.7%11.9%19.3
Nevada County2102k7.2%11.0%21.2
Placer County2424k5.2%9.4%23.8
Plumas County219k8.7%13.0%27.2
Riverside County22.5M5.4%11.5%25.0
San Benito County268k4.7%10.9%22.1
San Bernardino County22.2M5.2%12.3%25.3
San Francisco County2809k3.9%8.8%22.0
San Luis Obispo County2282k5.8%10.2%21.5
San Mateo County2726k4.0%8.0%18.6
Santa Clara County21.9M3.5%8.1%17.7
Santa Cruz County2262k5.4%10.0%18.8
Sierra County23,20010.2%15.1% -
Tulare County2479k6.1%14.0%24.5
Tuolumne County254k7.6%12.3%27.5
Yuba County286k6.5%14.5%37.3
Butte County3207k6.8%12.8%34.1
Colusa County322k6.4%13.4%25.5
Del Norte County327k8.1%15.0%37.8
Glenn County328k7.1%13.9%34.2
Humboldt County3134k7.6%14.2%33.1
Imperial County3179k5.7%12.9%18.0
Kings County3153k5.9%14.8%27.6
Lake County368k9.1%15.2%37.4
Lassen County329k7.6%15.9%25.4
Marin County3254k5.3%8.2%16.8
Mendocino County389k8.5%14.3%27.8
Merced County3292k6.1%14.1%25.3
Modoc County38,5009.9%14.8%43.1
Napa County3133k5.7%10.4%20.7
Orange County33.1M4.7%9.2%21.0
Sacramento County31.6M5.6%11.8%26.3
San Diego County33.3M4.7%10.0%22.7
San Joaquin County3801k5.5%12.6%25.5
Shasta County3180k7.6%13.8%34.9
Siskiyou County343k9.1%14.4%34.0
Solano County3449k5.3%11.3%29.6
Sonoma County3482k5.8%10.6%24.5
Stanislaus County3551k6.1%13.4%28.4
Sutter County398k6.8%13.9%28.7
Tehama County365k8.4%15.1%40.7
Trinity County316k11.3%16.9%28.2
Yolo County3221k4.7%9.8%24.9

Zones from the EPA Map of Radon Zones. COPD and smoking are CDC PLACES model-based estimates of adult prevalence. Lung cancer death rates are age-adjusted deaths per 100,000 per year, 2019-2023, from NCI State Cancer Profiles. A dash means the source does not report a value for that county.

Highest potential

Zone 1 counties in California

These 2 counties have a predicted average indoor radon screening level above the EPA action level. The EPA recommends every home in them be tested.

Other states

Radon zones elsewhere