Radon levels in California

Verified county-level radon indicators are available for 56 California counties. Select a county for its source period and sample details.

High results are most common in El Dorado County (30%) and least common in Sonoma County (1%)Share of tests at or above 4 pCi/L by county · Radon test results from laboratories, 2008–2017 · 56 of 58 counties have data
San Benito County: 0% of tests at or above 4 pCi/L · 19 testsShasta County: 22% of tests at or above 4 pCi/L · 23 testsSierra County: 0% of tests at or above 4 pCi/L · 1 testsFresno County: 11% of tests at or above 4 pCi/L · 100 testsAmador County: 28% of tests at or above 4 pCi/L · 36 testsEl Dorado County: 30% of tests at or above 4 pCi/L · 274 testsMonterey County: 15% of tests at or above 4 pCi/L · 87 testsSan Bernardino County: 4% of tests at or above 4 pCi/L · 178 testsTehama County: 0% of tests at or above 4 pCi/L · 6 testsNevada County: 25% of tests at or above 4 pCi/L · 260 testsTuolumne County: 23% of tests at or above 4 pCi/L · 74 testsSanta Barbara County: 18% of tests at or above 4 pCi/L · 421 testsTrinity County: no dataStanislaus County: 11% of tests at or above 4 pCi/L · 27 testsColusa County: 0% of tests at or above 4 pCi/L · 1 testsAlpine County: 63% of tests at or above 4 pCi/L · 19 testsSiskiyou County: 25% of tests at or above 4 pCi/L · 8 testsDel Norte County: 0% of tests at or above 4 pCi/L · 1 testsInyo County: 50% of tests at or above 4 pCi/L · 14 testsSan Joaquin County: 15% of tests at or above 4 pCi/L · 74 testsImperial County: 0% of tests at or above 4 pCi/L · 11 testsCalaveras County: 18% of tests at or above 4 pCi/L · 34 testsButte County: 11% of tests at or above 4 pCi/L · 35 testsMadera County: 15% of tests at or above 4 pCi/L · 34 testsVentura County: 12% of tests at or above 4 pCi/L · 335 testsOrange County: 4% of tests at or above 4 pCi/L · 431 testsKern County: 12% of tests at or above 4 pCi/L · 121 testsContra Costa County: 9% of tests at or above 4 pCi/L · 285 testsKings County: 0% of tests at or above 4 pCi/L · 2 testsMendocino County: 0% of tests at or above 4 pCi/L · 11 testsSonoma County: 1% of tests at or above 4 pCi/L · 131 testsYuba County: 0% of tests at or above 4 pCi/L · 7 testsMerced County: 0% of tests at or above 4 pCi/L · 5 testsTulare County: 24% of tests at or above 4 pCi/L · 80 testsSacramento County: 8% of tests at or above 4 pCi/L · 194 testsSolano County: 16% of tests at or above 4 pCi/L · 38 testsSan Francisco County: 3% of tests at or above 4 pCi/L · 246 testsMariposa County: 17% of tests at or above 4 pCi/L · 6 testsMarin County: 6% of tests at or above 4 pCi/L · 95 testsSutter County: 43% of tests at or above 4 pCi/L · 7 testsLassen County: 0% of tests at or above 4 pCi/L · 4 testsRiverside County: 1% of tests at or above 4 pCi/L · 226 testsPlumas County: 38% of tests at or above 4 pCi/L · 21 testsSanta Clara County: 9% of tests at or above 4 pCi/L · 793 testsPlacer County: 7% of tests at or above 4 pCi/L · 283 testsSan Luis Obispo County: 13% of tests at or above 4 pCi/L · 80 testsSan Mateo County: 9% of tests at or above 4 pCi/L · 468 testsMono County: 50% of tests at or above 4 pCi/L · 20 testsSan Diego County: 3% of tests at or above 4 pCi/L · 692 testsNapa County: 11% of tests at or above 4 pCi/L · 57 testsAlameda County: 7% of tests at or above 4 pCi/L · 478 testsYolo County: 2% of tests at or above 4 pCi/L · 48 testsLake County: 13% of tests at or above 4 pCi/L · 24 testsLos Angeles County: 9% of tests at or above 4 pCi/L · 2,363 testsGlenn County: 0% of tests at or above 4 pCi/L · 2 testsSanta Cruz County: 22% of tests at or above 4 pCi/L · 105 testsHumboldt County: 20% of tests at or above 4 pCi/L · 20 testsModoc County: no data
  • Under 10%
  • 10–20%
  • 20–30%
  • 30–40%
  • 40% or more
  • No data

Counties

CountyTests% ≥ 4 pCi/LMedian (pCi/L)EPA zone (1993)Source
Alameda County4787%0.7Zone 2Radon test results from laboratories through 2017
Alpine County1963%4.5Zone 2Radon test results from laboratories through 2017
Amador County3628%2.3Zone 2Radon test results from laboratories through 2017
Butte County3511%0.8Zone 3Radon test results from laboratories through 2017
Calaveras County3418%1.7Zone 2Radon test results from laboratories through 2017
Colusa County10%0.2Zone 3Radon test results from laboratories through 2017
Contra Costa County2859%0.9Zone 2Radon test results from laboratories through 2017
Del Norte County10%0.1Zone 3Radon test results from laboratories through 2017
El Dorado County27430%1.6Zone 2Radon test results from laboratories through 2017
Fresno County10011%1.5Zone 2Radon test results from laboratories through 2017
Glenn County20%2.8Zone 3Radon test results from laboratories through 2017
Humboldt County2020%1.3Zone 3Radon test results from laboratories through 2017
Imperial County110%0.6Zone 3Radon test results from laboratories through 2017
Inyo County1450%4.0Zone 2Radon test results from laboratories through 2017
Kern County12112%1.8Zone 2Radon test results from laboratories through 2017
Kings County20%1.7Zone 3Radon test results from laboratories through 2017
Lake County2413%0.9Zone 3Radon test results from laboratories through 2017
Lassen County40%1.1Zone 3Radon test results from laboratories through 2017
Los Angeles County2,3639%0.7Zone 2Radon test results from laboratories through 2017
Madera County3415%1.4Zone 2Radon test results from laboratories through 2017
Marin County956%0.8Zone 3Radon test results from laboratories through 2017
Mariposa County617%1.3Zone 2Radon test results from laboratories through 2017
Mendocino County110%0.7Zone 3Radon test results from laboratories through 2017
Merced County50%2.1Zone 3Radon test results from laboratories through 2017
Modoc County———Zone 3—
Mono County2050%4.6Zone 2Radon test results from laboratories through 2017
Monterey County8715%0.8Zone 2Radon test results from laboratories through 2017
Napa County5711%0.9Zone 3Radon test results from laboratories through 2017
Nevada County26025%2.0Zone 2Radon test results from laboratories through 2017
Orange County4314%0.7Zone 3Radon test results from laboratories through 2017
Placer County2837%1.1Zone 2Radon test results from laboratories through 2017
Plumas County2138%2.1Zone 2Radon test results from laboratories through 2017
Riverside County2261%0.6Zone 2Radon test results from laboratories through 2017
Sacramento County1948%0.9Zone 3Radon test results from laboratories through 2017
San Benito County190%0.7Zone 2Radon test results from laboratories through 2017
San Bernardino County1784%0.7Zone 2Radon test results from laboratories through 2017
San Diego County6923%0.2Zone 3Radon test results from laboratories through 2017
San Francisco County2463%0.5Zone 2Radon test results from laboratories through 2017
San Joaquin County7415%1.4Zone 3Radon test results from laboratories through 2017
San Luis Obispo County8013%1.0Zone 2Radon test results from laboratories through 2017
San Mateo County4689%0.9Zone 2Radon test results from laboratories through 2017
Santa Barbara County42118%1.2Zone 1Radon test results from laboratories through 2017
Santa Clara County7939%0.9Zone 2Radon test results from laboratories through 2017
Santa Cruz County10522%1.2Zone 2Radon test results from laboratories through 2017
Shasta County2322%2.0Zone 3Radon test results from laboratories through 2017
Sierra County10%1.2Zone 2Radon test results from laboratories through 2017
Siskiyou County825%1.1Zone 3Radon test results from laboratories through 2017
Solano County3816%0.9Zone 3Radon test results from laboratories through 2017
Sonoma County1311%0.5Zone 3Radon test results from laboratories through 2017
Stanislaus County2711%1.4Zone 3Radon test results from laboratories through 2017
Sutter County743%1.9Zone 3Radon test results from laboratories through 2017
Tehama County60%2.2Zone 3Radon test results from laboratories through 2017
Trinity County———Zone 3—
Tulare County8024%1.7Zone 2Radon test results from laboratories through 2017
Tuolumne County7423%1.4Zone 2Radon test results from laboratories through 2017
Ventura County33512%1.0Zone 1Radon test results from laboratories through 2017
Yolo County482%0.9Zone 3Radon test results from laboratories through 2017
Yuba County70%0.1Zone 2Radon test results from laboratories through 2017

Test your home

County figures show how often tests come back high, not the level in your home. Only a test tells you that: see how to test for radon.

Sponsored: we earn a commission on purchases through these links. We haven't tested Airthings monitors yet.

Prefer a digital monitor? See Airthings radon monitors, or enter code radonbycounty-10OFF at their discount store for 10% off.