Radon levels in California: Low
The model predicts a population-weighted basement average of 1.5 pCi/L (model estimate) across California's 58 counties - about 17% below the US average of 1.8 pCi/L. Above-ground floors average 0.9 pCi/L.
Predicted (model estimate). The pCi/L figures on this page are long-term area averages from a peer-reviewed national model built on millions of real radon tests (2001-2021). They describe the typical home in this area - not your home, which can be several times higher or lower. The EPA radon zone is a separate, 1993 county-level classification. How the numbers work.
2 / 58
EPA Zone 1 counties (3%)
29 / 27
Zone 2 / Zone 3 counties
0
counties predicted ≥ 4 pCi/L
0%
of ZIP codes predicted ≥ 4 pCi/L
Test California once anyway - radon varies house to house
The predicted basement average here is 1.5 pCi/L, below the EPA's 2 pCi/L "consider fixing" threshold. That's good news for the area - but the EPA still recommends every home be tested, because foundation cracks, soil pockets and ventilation make one house very different from its neighbor. One inexpensive kit settles it.
Monitor link goes to the Airthings store - use code RADONGRADE-10OFF at checkout for 10% off any Airthings monitor.
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What the data says about radon in California
On the EPA's 1993 radon zone map, 2 of California's 58 counties are EPA Zone 1 (3%), 29 are Zone 2 and 27 are Zone 3. Zone 2 (predicted screening average of 2-4 pCi/L) is the most common designation, with the 2 Zone 1 counties marking where the EPA expected the highest potential. That classification has not been revised since 1993; it describes counties, not homes, and elevated readings turn up in Zone 3 counties every year.
California models out at 1.5 pCi/L predicted basement average (low; 17% below the US average of 1.8). Of the 58 counties with predictions, 0 are predicted at or above 4 pCi/L, 7 in the 2-4 band and 51 below 2. The highest predicted county is Mono County at 2.7 pCi/L; the lowest is San Francisco County at 1.3 - a 1.4 pCi/L spread that means a statewide average tells you little about any one county.
At ZIP-code resolution, 0% of California ZIPs are predicted at 4 pCi/L or higher in the basement and 9.3% at 2 or higher - below the national shares of 4.2% and 48.7%. No ZIP in the state is predicted above 4 on average - but averages hide individual homes, which is why the EPA still asks everyone to test.
California counties by predicted radon
Sorted highest to lowest predicted basement average (model estimate). 58 counties.
| County | Predicted basement | Upstairs | Grade | EPA zone | ZIPs |
|---|---|---|---|---|---|
| Mono County | 2.7 pCi/L | 2.4 | Moderate | EPA Zone 2 | 8 |
| Alpine County | 2.3 pCi/L | 1.7 | Moderate | EPA Zone 2 | 2 |
| Plumas County | 2.3 pCi/L | 1.5 | Moderate | EPA Zone 2 | 14 |
| Inyo County | 2.2 pCi/L | 1.5 | Moderate | EPA Zone 2 | 6 |
| Sierra County | 2.2 pCi/L | 1.4 | Moderate | EPA Zone 2 | 8 |
| Modoc County | 2.0 pCi/L | 1.0 | Moderate | EPA Zone 3 | 9 |
| Lassen County | 2.0 pCi/L | 1.2 | Moderate | EPA Zone 3 | 14 |
| Siskiyou County | 2.0 pCi/L | 0.9 | Low | EPA Zone 3 | 20 |
| Mariposa County | 1.9 pCi/L | 1.3 | Low | EPA Zone 2 | 7 |
| Nevada County | 1.9 pCi/L | 1.2 | Low | EPA Zone 2 | 7 |
| Amador County | 1.9 pCi/L | 1.3 | Low | EPA Zone 2 | 8 |
| El Dorado County | 1.9 pCi/L | 1.3 | Low | EPA Zone 2 | 21 |
| Tuolumne County | 1.9 pCi/L | 1.1 | Low | EPA Zone 2 | 10 |
| Calaveras County | 1.8 pCi/L | 1.1 | Low | EPA Zone 2 | 11 |
| San Bernardino County | 1.8 pCi/L | 1.0 | Low | EPA Zone 2 | 79 |
| Trinity County | 1.8 pCi/L | 0.9 | Low | EPA Zone 3 | 13 |
| Lake County | 1.7 pCi/L | 0.8 | Low | EPA Zone 3 | 11 |
| Riverside County | 1.7 pCi/L | 0.9 | Low | EPA Zone 2 | 69 |
| San Luis Obispo County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 18 |
| Ventura County | 1.6 pCi/L | 1.1 | Low | EPA Zone 1 | 26 |
| San Benito County | 1.6 pCi/L | 0.8 | Low | EPA Zone 2 | 3 |
| Butte County | 1.6 pCi/L | 0.9 | Low | EPA Zone 3 | 15 |
| Shasta County | 1.6 pCi/L | 0.8 | Low | EPA Zone 3 | 22 |
| Tehama County | 1.6 pCi/L | 0.8 | Low | EPA Zone 3 | 8 |
| Colusa County | 1.6 pCi/L | 0.7 | Low | EPA Zone 3 | 6 |
| Santa Barbara County | 1.6 pCi/L | 1.0 | Low | EPA Zone 1 | 22 |
| Madera County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 13 |
| Tulare County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 28 |
| Kern County | 1.6 pCi/L | 1.0 | Low | EPA Zone 2 | 43 |
| Monterey County | 1.6 pCi/L | 0.8 | Low | EPA Zone 2 | 25 |
| Placer County | 1.6 pCi/L | 0.9 | Low | EPA Zone 2 | 28 |
| Kings County | 1.5 pCi/L | 0.9 | Low | EPA Zone 3 | 6 |
| Santa Cruz County | 1.5 pCi/L | 0.8 | Low | EPA Zone 2 | 15 |
| Glenn County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 6 |
| Merced County | 1.5 pCi/L | 0.9 | Low | EPA Zone 3 | 17 |
| Sutter County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 8 |
| Yuba County | 1.5 pCi/L | 0.8 | Low | EPA Zone 2 | 11 |
| Los Angeles County | 1.5 pCi/L | 0.9 | Low | EPA Zone 2 | 274 |
| Yolo County | 1.5 pCi/L | 0.7 | Low | EPA Zone 3 | 17 |
| Mendocino County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 25 |
| Imperial County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 11 |
| Napa County | 1.5 pCi/L | 0.8 | Low | EPA Zone 3 | 8 |
| Orange County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 83 |
| San Diego County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 98 |
| Santa Clara County | 1.4 pCi/L | 0.8 | Low | EPA Zone 2 | 55 |
| San Mateo County | 1.4 pCi/L | 0.7 | Low | EPA Zone 2 | 27 |
| Fresno County | 1.4 pCi/L | 0.9 | Low | EPA Zone 2 | 48 |
| Alameda County | 1.4 pCi/L | 0.7 | Low | EPA Zone 2 | 47 |
| Sonoma County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 32 |
| Humboldt County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 30 |
| Marin County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 27 |
| San Joaquin County | 1.4 pCi/L | 0.9 | Low | EPA Zone 3 | 26 |
| Solano County | 1.4 pCi/L | 0.7 | Low | EPA Zone 3 | 14 |
| Stanislaus County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 23 |
| Contra Costa County | 1.4 pCi/L | 0.7 | Low | EPA Zone 2 | 39 |
| Del Norte County | 1.4 pCi/L | 0.8 | Low | EPA Zone 3 | 4 |
| Sacramento County | 1.3 pCi/L | 0.8 | Low | EPA Zone 3 | 52 |
| San Francisco County | 1.3 pCi/L | 0.6 | Low | EPA Zone 2 | 25 |
Radon levels in California cities
The 207 most-populous California places among the 1,000 largest US cities (Census incorporated places and CDPs), by population. City figures are ZIP-land-weighted means of the model's ZIP predictions (model estimate).
| City | Population | Predicted basement | Upstairs | Grade | ZIPs ≥ 4 |
|---|---|---|---|---|---|
| Los Angeles | 3,820,914 | 1.6 pCi/L | 1.0 | Low | 0% |
| San Diego | 1,388,320 | 1.4 pCi/L | 0.8 | Low | 0% |
| San Jose | 969,655 | 1.5 pCi/L | 0.8 | Low | 0% |
| San Francisco | 808,988 | 1.4 pCi/L | 0.6 | Low | 0% |
| Fresno | 545,716 | 1.4 pCi/L | 0.9 | Low | 0% |
| Sacramento | 526,384 | 1.3 pCi/L | 0.7 | Low | 0% |
| Long Beach | 449,468 | 1.4 pCi/L | 0.7 | Low | 0% |
| Oakland | 436,504 | 1.4 pCi/L | 0.7 | Low | 0% |
| Bakersfield | 413,381 | 1.5 pCi/L | 1.0 | Low | 0% |
| Anaheim | 340,512 | 1.4 pCi/L | 0.8 | Low | 0% |
| Stockton | 319,543 | 1.4 pCi/L | 0.8 | Low | 0% |
| Riverside | 318,858 | 1.6 pCi/L | 1.0 | Low | 0% |
| Irvine | 314,621 | 1.4 pCi/L | 0.8 | Low | 0% |
| Santa Ana | 310,539 | 1.4 pCi/L | 0.8 | Low | 0% |
| Chula Vista | 274,333 | 1.4 pCi/L | 0.8 | Low | 0% |
| Fremont | 226,208 | 1.5 pCi/L | 0.8 | Low | 0% |
| Santa Clarita | 224,028 | 1.8 pCi/L | 1.2 | Low | 0% |
| San Bernardino | 223,728 | 1.9 pCi/L | 1.2 | Low | 0% |
| Modesto | 218,915 | 1.4 pCi/L | 0.8 | Low | 0% |
| Fontana | 215,465 | 1.6 pCi/L | 1.0 | Low | 0% |
| Moreno Valley | 212,392 | 1.7 pCi/L | 0.9 | Low | 0% |
| Oxnard | 198,488 | 1.5 pCi/L | 0.9 | Low | 0% |
| Huntington Beach | 192,129 | 1.4 pCi/L | 0.7 | Low | 0% |
| Glendale | 187,050 | 1.6 pCi/L | 1.0 | Low | 0% |
| Ontario | 182,457 | 1.7 pCi/L | 0.9 | Low | 0% |
| Elk Grove | 178,444 | 1.4 pCi/L | 0.8 | Low | 0% |
| Santa Rosa | 175,845 | 1.4 pCi/L | 0.7 | Low | 0% |
| Rancho Cucamonga | 174,405 | 1.6 pCi/L | 1.0 | Low | 0% |
| Oceanside | 170,020 | 1.6 pCi/L | 0.9 | Low | 0% |
| Garden Grove | 168,234 | 1.4 pCi/L | 0.7 | Low | 0% |
| Lancaster | 166,236 | 1.8 pCi/L | 1.0 | Low | 0% |
| Palmdale | 161,404 | 1.8 pCi/L | 1.1 | Low | 0% |
| Corona | 160,238 | 1.7 pCi/L | 1.0 | Low | 0% |
| Salinas | 159,506 | 1.6 pCi/L | 0.9 | Low | 0% |
| Roseville | 159,135 | 1.4 pCi/L | 0.7 | Low | 0% |
| Hayward | 155,675 | 1.5 pCi/L | 0.8 | Low | 0% |
| Sunnyvale | 151,967 | 1.5 pCi/L | 0.8 | Low | 0% |
| Escondido | 148,122 | 1.7 pCi/L | 0.9 | Low | 0% |
| Pomona | 145,502 | 1.6 pCi/L | 0.9 | Low | 0% |
| Visalia | 144,998 | 1.5 pCi/L | 1.0 | Low | 0% |
| Fullerton | 139,250 | 1.4 pCi/L | 0.8 | Low | 0% |
| Torrance | 139,224 | 1.5 pCi/L | 0.8 | Low | 0% |
| Victorville | 138,869 | 1.7 pCi/L | 1.0 | Low | 0% |
| Orange | 138,337 | 1.5 pCi/L | 0.8 | Low | 0% |
| Pasadena | 133,560 | 1.7 pCi/L | 1.1 | Low | 0% |
| Santa Clara | 131,062 | 1.5 pCi/L | 0.8 | Low | 0% |
| East Los Angeles | 126,496 | 1.4 pCi/L | 0.8 | Low | 0% |
| Clovis | 125,826 | 1.4 pCi/L | 0.8 | Low | 0% |
| Simi Valley | 125,113 | 1.7 pCi/L | 1.2 | Low | 0% |
| Thousand Oaks | 123,463 | 1.7 pCi/L | 1.1 | Low | 0% |
| Vallejo | 122,807 | 1.4 pCi/L | 0.6 | Low | 0% |
| Concord | 122,315 | 1.3 pCi/L | 0.7 | Low | 0% |
| Fairfield | 120,768 | 1.4 pCi/L | 0.7 | Low | 0% |
| Berkeley | 118,962 | 1.3 pCi/L | 0.7 | Low | 0% |
| Antioch | 117,096 | 1.3 pCi/L | 0.7 | Low | 0% |
| Richmond | 114,106 | 1.4 pCi/L | 0.7 | Low | 0% |
| Carlsbad | 113,495 | 1.6 pCi/L | 0.9 | Low | 0% |
| Menifee | 113,433 | 1.8 pCi/L | 0.9 | Low | 0% |
| Murrieta | 111,878 | 1.7 pCi/L | 0.8 | Low | 0% |
| Temecula | 110,682 | 1.7 pCi/L | 0.9 | Low | 0% |
| Santa Maria | 109,987 | 1.6 pCi/L | 0.8 | Low | 0% |
| Ventura | 109,058 | 1.6 pCi/L | 1.0 | Low | 0% |
| Downey | 108,816 | 1.4 pCi/L | 0.7 | Low | 0% |
| Costa Mesa | 108,354 | 1.4 pCi/L | 0.8 | Low | 0% |
| Jurupa Valley | 107,321 | 1.6 pCi/L | 1.0 | Low | 0% |
| West Covina | 105,617 | 1.5 pCi/L | 0.8 | Low | 0% |
| El Monte | 103,794 | 1.4 pCi/L | 0.7 | Low | 0% |
| Rialto | 103,391 | 1.6 pCi/L | 0.9 | Low | 0% |
| El Cajon | 102,991 | 1.5 pCi/L | 0.8 | Low | 0% |
| Inglewood | 102,865 | 1.4 pCi/L | 0.7 | Low | 0% |
| Burbank | 102,755 | 1.6 pCi/L | 0.8 | Low | 0% |
| Vacaville | 102,526 | 1.4 pCi/L | 0.7 | Low | 0% |
| San Mateo | 101,327 | 1.4 pCi/L | 0.6 | Low | 0% |
| Chico | 101,301 | 1.5 pCi/L | 0.8 | Low | 0% |
| Hesperia | 100,633 | 1.9 pCi/L | 1.1 | Low | 0% |
| Daly City | 99,833 | 1.4 pCi/L | 0.7 | Low | 0% |
| Vista | 98,344 | 1.5 pCi/L | 0.9 | Low | 0% |
| Norwalk | 98,078 | 1.4 pCi/L | 0.7 | Low | 0% |
| Tracy | 98,010 | 1.5 pCi/L | 0.9 | Low | 0% |
| San Marcos | 94,188 | 1.6 pCi/L | 0.9 | Low | 0% |
| Merced | 93,692 | 1.5 pCi/L | 0.9 | Low | 0% |
| Chino | 93,114 | 1.7 pCi/L | 0.9 | Low | 0% |
| Indio | 93,057 | 1.6 pCi/L | 0.9 | Low | 0% |
| Redding | 92,727 | 1.6 pCi/L | 0.8 | Low | 0% |
| Hemet | 92,368 | 1.7 pCi/L | 0.9 | Low | 0% |
| Arden-Arcade | 92,186 | 1.2 pCi/L | 0.8 | Low | 0% |
| Carson | 91,139 | 1.5 pCi/L | 0.8 | Low | 0% |
| Manteca | 91,059 | 1.4 pCi/L | 0.9 | Low | 0% |
| Compton | 90,986 | 1.4 pCi/L | 0.7 | Low | 0% |
| Mission Viejo | 90,624 | 1.6 pCi/L | 0.9 | Low | 0% |
| South Gate | 90,070 | 1.3 pCi/L | 0.7 | Low | 0% |
| Santa Monica | 89,922 | 1.3 pCi/L | 0.8 | Low | 0% |
| Westminster | 88,729 | 1.4 pCi/L | 0.7 | Low | 0% |
| Santa Barbara | 86,499 | 1.6 pCi/L | 1.1 | Low | 0% |
| Citrus Heights | 86,239 | 1.3 pCi/L | 0.8 | Low | 0% |
| Lake Forest | 85,840 | 1.5 pCi/L | 0.8 | Low | 0% |
| San Leandro | 85,784 | 1.4 pCi/L | 0.7 | Low | 0% |
| San Ramon | 84,929 | 1.4 pCi/L | 0.8 | Low | 0% |
| Folsom | 84,782 | 1.4 pCi/L | 0.7 | Low | 0% |
| Whittier | 84,143 | 1.5 pCi/L | 0.8 | Low | 0% |
| Hawthorne | 83,364 | 1.3 pCi/L | 0.7 | Low | 0% |
| Livermore | 82,908 | 1.6 pCi/L | 0.7 | Low | 0% |
| Newport Beach | 82,637 | 1.5 pCi/L | 0.8 | Low | 0% |
| Rancho Cordova | 82,605 | 1.4 pCi/L | 0.8 | Low | 0% |
| Buena Park | 81,958 | 1.4 pCi/L | 0.7 | Low | 0% |
| Mountain View | 81,785 | 1.4 pCi/L | 0.8 | Low | 0% |
| Redwood City | 80,996 | 1.4 pCi/L | 0.7 | Low | 0% |
| Perris | 80,603 | 1.7 pCi/L | 0.9 | Low | 0% |
| Alhambra | 79,776 | 1.4 pCi/L | 0.8 | Low | 0% |
| Upland | 78,699 | 1.7 pCi/L | 1.0 | Low | 0% |
| Lakewood | 78,135 | 1.4 pCi/L | 0.7 | Low | 0% |
| Tustin | 77,704 | 1.5 pCi/L | 0.8 | Low | 0% |
| Napa | 77,492 | 1.4 pCi/L | 0.8 | Low | 0% |
| Milpitas | 77,321 | 1.5 pCi/L | 0.7 | Low | 0% |
| Chino Hills | 77,212 | 1.6 pCi/L | 0.9 | Low | 0% |
| Pittsburg | 75,803 | 1.3 pCi/L | 0.7 | Low | 0% |
| Alameda | 75,353 | 1.3 pCi/L | 0.7 | Low | 0% |
| Bellflower | 75,122 | 1.3 pCi/L | 0.7 | Low | 0% |
| Apple Valley | 75,036 | 1.8 pCi/L | 1.2 | Low | 0% |
| Pleasanton | 74,653 | 1.6 pCi/L | 0.8 | Low | 0% |
| Rocklin | 73,472 | 1.5 pCi/L | 0.8 | Low | 0% |
| Lake Elsinore | 73,028 | 1.7 pCi/L | 0.9 | Low | 0% |
| Redlands | 72,556 | 2.1 pCi/L | 1.4 | Moderate | 0% |
| Turlock | 72,100 | 1.4 pCi/L | 0.8 | Low | 0% |
| Tulare | 71,092 | 1.6 pCi/L | 1.0 | Low | 0% |
| Eastvale | 70,510 | 1.7 pCi/L | 0.9 | Low | 0% |
| Camarillo | 69,514 | 1.6 pCi/L | 1.0 | Low | 0% |
| Walnut Creek | 69,152 | 1.3 pCi/L | 0.7 | Low | 0% |
| Dublin | 69,128 | 1.4 pCi/L | 0.7 | Low | 0% |
| Baldwin Park | 68,806 | 1.4 pCi/L | 0.8 | Low | 0% |
| Yuba City | 68,666 | 1.5 pCi/L | 0.8 | Low | 0% |
| Madera | 68,079 | 1.4 pCi/L | 0.9 | Low | 0% |
| Redondo Beach | 67,749 | 1.5 pCi/L | 0.8 | Low | 0% |
| Lodi | 67,679 | 1.4 pCi/L | 0.9 | Low | 0% |
| Yorba Linda | 66,147 | 1.7 pCi/L | 0.9 | Low | 0% |
| Palo Alto | 65,882 | 1.5 pCi/L | 0.9 | Low | 0% |
| Davis | 65,832 | 1.5 pCi/L | 0.7 | Low | 0% |
| Union City | 65,414 | 1.4 pCi/L | 0.8 | Low | 0% |
| Brentwood | 65,126 | 1.5 pCi/L | 0.8 | Low | 0% |
| Florence-Graham | 63,387 | 1.4 pCi/L | 0.8 | Low | 0% |
| Lynwood | 63,234 | 1.3 pCi/L | 0.6 | Low | 0% |
| South San Francisco | 63,123 | 1.5 pCi/L | 0.7 | Low | 0% |
| Laguna Niguel | 62,899 | 1.6 pCi/L | 1.1 | Low | 0% |
| Porterville | 62,876 | 1.5 pCi/L | 0.9 | Low | 0% |
| San Clemente | 62,313 | 1.6 pCi/L | 0.9 | Low | 0% |
| Carmichael | 61,762 | 1.3 pCi/L | 0.8 | Low | 0% |
| Santa Cruz | 61,501 | 1.5 pCi/L | 0.8 | Low | 0% |
| Castro Valley | 61,388 | 1.4 pCi/L | 0.8 | Low | 0% |
| Woodland | 61,123 | 1.6 pCi/L | 0.8 | Low | 0% |
| La Habra | 60,991 | 1.5 pCi/L | 0.8 | Low | 0% |
| Encinitas | 60,841 | 1.5 pCi/L | 0.8 | Low | 0% |
| La Mesa | 60,537 | 1.4 pCi/L | 0.8 | Low | 0% |
| Montebello | 60,015 | 1.5 pCi/L | 0.8 | Low | 0% |
| Hanford | 59,938 | 1.6 pCi/L | 0.9 | Low | 0% |
| San Rafael | 59,555 | 1.3 pCi/L | 0.6 | Low | 0% |
| Santee | 59,478 | 1.4 pCi/L | 0.8 | Low | 0% |
| Pico Rivera | 59,189 | 1.4 pCi/L | 0.8 | Low | 0% |
| Petaluma | 58,800 | 1.5 pCi/L | 0.7 | Low | 0% |
| Beaumont | 58,463 | 1.9 pCi/L | 1.1 | Low | 0% |
| Gardena | 58,377 | 1.4 pCi/L | 0.7 | Low | 0% |
| Gilroy | 58,250 | 1.5 pCi/L | 0.8 | Low | 0% |
| Monterey Park | 57,877 | 1.6 pCi/L | 0.8 | Low | 0% |
| Cupertino | 57,285 | 1.4 pCi/L | 0.7 | Low | 0% |
| South Whittier | 57,156 | 1.4 pCi/L | 0.8 | Low | 0% |
| Highland | 56,202 | 2.0 pCi/L | 1.4 | Low | 0% |
| West Sacramento | 55,842 | 1.4 pCi/L | 0.7 | Low | 0% |
| Fountain Valley | 55,468 | 1.4 pCi/L | 0.7 | Low | 0% |
| San Jacinto | 55,440 | 1.9 pCi/L | 1.0 | Low | 0% |
| National City | 55,236 | 1.2 pCi/L | 0.6 | Low | 0% |
| Lincoln | 54,538 | 1.6 pCi/L | 0.9 | Low | 0% |
| Arcadia | 54,157 | 1.5 pCi/L | 0.8 | Low | 0% |
| Hacienda Heights | 54,038 | 1.4 pCi/L | 0.8 | Low | 0% |
| Yucaipa | 53,947 | 2.1 pCi/L | 1.5 | Moderate | 0% |
| Colton | 53,357 | 1.6 pCi/L | 0.8 | Low | 0% |
| Cathedral City | 52,356 | 1.5 pCi/L | 0.8 | Low | 0% |
| Placentia | 52,192 | 1.6 pCi/L | 1.0 | Low | 0% |
| Diamond Bar | 52,041 | 1.7 pCi/L | 1.1 | Low | 0% |
| Palm Desert | 51,951 | 1.5 pCi/L | 0.8 | Low | 0% |
| Huntington Park | 51,942 | 1.4 pCi/L | 0.7 | Low | 0% |
| Novato | 51,722 | 1.4 pCi/L | 0.7 | Low | 0% |
| Delano | 51,500 | 1.5 pCi/L | 0.9 | Low | 0% |
| Paramount | 51,072 | 1.3 pCi/L | 0.7 | Low | 0% |
| Watsonville | 50,867 | 1.6 pCi/L | 0.8 | Low | 0% |
| Aliso Viejo | 50,263 | 1.5 pCi/L | 0.9 | Low | 0% |
| Glendora | 49,934 | 1.7 pCi/L | 1.1 | Low | 0% |
| Rosemead | 49,305 | 1.4 pCi/L | 0.7 | Low | 0% |
| San Luis Obispo | 49,244 | 1.6 pCi/L | 0.9 | Low | 0% |
| Rowland Heights | 48,993 | 1.6 pCi/L | 0.9 | Low | 0% |
| Cypress | 48,782 | 1.4 pCi/L | 0.7 | Low | 0% |
| Covina | 48,728 | 1.5 pCi/L | 0.9 | Low | 0% |
| Los Banos | 48,553 | 1.6 pCi/L | 0.9 | Low | 0% |
| Brea | 48,479 | 1.7 pCi/L | 0.9 | Low | 0% |
| Ceres | 48,397 | 1.4 pCi/L | 0.8 | Low | 0% |
| Azusa | 48,272 | 1.5 pCi/L | 1.0 | Low | 0% |
| Poway | 48,051 | 1.5 pCi/L | 0.9 | Low | 0% |
| Florin | 47,513 | 1.4 pCi/L | 0.8 | Low | 0% |
| Newark | 46,929 | 1.5 pCi/L | 0.7 | Low | 0% |
| Cerritos | 46,797 | 1.5 pCi/L | 0.8 | Low | 0% |
| Rancho Santa Margarita | 46,182 | 1.6 pCi/L | 0.9 | Low | 0% |
| Antelope | 45,770 | 1.3 pCi/L | 0.7 | Low | 0% |
| Oakley | 45,761 | 1.4 pCi/L | 0.7 | Low | 0% |
| La Mirada | 45,505 | 1.4 pCi/L | 0.7 | Low | 0% |
| Palm Springs | 45,218 | 1.6 pCi/L | 0.8 | Low | 0% |
| Hollister | 44,658 | 1.6 pCi/L | 0.8 | Low | 0% |
| Rohnert Park | 44,546 | 1.4 pCi/L | 0.6 | Low | 0% |
| Morgan Hill | 44,478 | 1.5 pCi/L | 0.8 | Low | 0% |
| El Centro | 43,772 | 1.4 pCi/L | 0.8 | Low | 0% |
Highest-radon ZIP codes in California
The 10 California ZIP codes with the highest predicted basement average (model estimate), out of 1,602 modeled. A ZIP average is still an area figure - neighboring homes routinely test several pCi/L apart.
| ZIP | County | Predicted basement | Upstairs | Grade |
|---|---|---|---|---|
| 93546 | Mono County | 3.1 pCi/L | 2.6 | Moderate |
| 95389 | Mariposa County | 2.8 pCi/L | 2.0 | Moderate |
| 93526 | Inyo County | 2.7 pCi/L | 2.0 | Moderate |
| 93541 | Mono County | 2.7 pCi/L | 2.2 | Moderate |
| 93529 | Mono County | 2.7 pCi/L | 2.0 | Moderate |
| 96150 | El Dorado County | 2.6 pCi/L | 2.7 | Moderate |
| 96107 | Mono County | 2.5 pCi/L | 2.0 | Moderate |
| 93514 | Mono County | 2.5 pCi/L | 2.3 | Moderate |
| 96106 | Plumas County | 2.5 pCi/L | 1.5 | Moderate |
| 95728 | Placer County | 2.5 pCi/L | 1.9 | Moderate |
California radon questions
- Is radon high in California?
- California's population-weighted predicted basement average is 1.5 pCi/L, about 17% below the US average of 1.8. 0 of its 58 modeled counties are predicted at or above the EPA action level of 4 pCi/L, 7 are in the 2-4 "consider fixing" range and 51 are below 2. 0% of California ZIP codes are predicted at 4 or higher.
- Which California county has the highest radon?
- Mono County has the highest predicted basement average in California at 2.7 pCi/L (EPA Zone 2). The lowest is San Francisco County at 1.3 pCi/L. These are area figures; individual homes in any county can test far higher.
- How many EPA Zone 1 counties are in California?
- 2 of California's 58 counties (3%) are EPA Zone 1 (highest potential), 29 are Zone 2 and 27 are Zone 3. The EPA zone map dates from 1993; the predicted values on this page come from a 2001-2021 model built on millions of actual tests, so the two can disagree for a given county.
- Should I test for radon in California?
- Yes. Even though California's predicted average is below 2 pCi/L, the EPA recommends every home be tested - radon varies house to house, and 9.3% of California ZIP codes are still predicted at 2 pCi/L or higher.
California radon program
California's radon program page is at www.cdph.ca.gov; it lists testing and certified-mitigator guidance for the state. The EPA also keeps a directory of every state radon contact: EPA state radon contacts page. Then read how to test and what mitigation costs.