Radon and home risks by county
Radon in California
2 of California's 58 counties are in EPA's highest radon zone, home to about 3% of the state's residents. Radon zones describe an area's potential; EPA recommends testing every home regardless of zone.
Zone 1 counties
2
high radon potential
Zone 2 counties
29
moderate potential
Zone 3 counties
27
low potential
Counties with water violations
52
since 2016; 33 with unresolved ones
Tap water reports for California cities
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Zone 1: high potential (2 counties)
| County | Residents | Water systems | Health violations since 2016 | Unresolved |
|---|---|---|---|---|
| Santa Barbara County | 444,500 | 63 | 0 | 0 |
| Ventura County | 835,427 | 68 | 14 | 6 |
Zone 2: moderate potential (29 counties)
| County | Residents | Water systems | Health violations since 2016 | Unresolved |
|---|---|---|---|---|
| Alameda County | 1,649,060 | 14 | 0 | 0 |
| Alpine County | 1,099 | 5 | 13 | 0 |
| Amador County | 42,026 | 20 | 4 | 0 |
| Calaveras County | 46,505 | 19 | 9 | 0 |
| Contra Costa County | 1,172,607 | 39 | 2 | 0 |
| El Dorado County | 192,823 | 19 | 1 | 0 |
| Fresno County | 1,024,125 | 102 | 128 | 1 |
| Inyo County | 18,485 | 44 | 25 | 1 |
| Kern County | 922,529 | 167 | 319 | 2 |
| Los Angeles County | 9,757,179 | 199 | 66 | 4 |
| Madera County | 165,432 | 68 | 172 | 1 |
| Mariposa County | 17,048 | 11 | 0 | 0 |
| Mono County | 12,991 | 18 | 21 | 2 |
| Monterey County | 436,251 | 157 | 17 | 0 |
| Nevada County | 102,195 | 21 | 1 | 0 |
| Placer County | 433,822 | 52 | 1 | 0 |
| Plumas County | 18,834 | 27 | 25 | 0 |
| Riverside County | 2,529,933 | 94 | 15 | 0 |
| San Benito County | 69,159 | 31 | 16 | 3 |
| San Bernardino County | 2,214,281 | 147 | 64 | 6 |
| San Francisco County | 827,526 | 5 | 2 | 0 |
| San Luis Obispo County | 281,843 | 72 | 53 | 0 |
| San Mateo County | 742,893 | 36 | 5 | 0 |
| Santa Clara County | 1,926,325 | 73 | 70 | 4 |
| Santa Cruz County | 262,406 | 52 | 24 | 0 |
| Sierra County | 3,113 | 8 | 0 | 0 |
| Tulare County | 483,546 | 91 | 211 | 2 |
| Tuolumne County | 53,893 | 49 | 9 | 1 |
| Yuba County | 87,469 | 22 | 0 | 0 |
Zone 3: low potential (27 counties)
| County | Residents | Water systems | Health violations since 2016 | Unresolved |
|---|---|---|---|---|
| Butte County | 208,334 | 42 | 4 | 3 |
| Colusa County | 22,074 | 9 | 34 | 0 |
| Del Norte County | 27,009 | 16 | 2 | 1 |
| Glenn County | 28,304 | 13 | 2 | 0 |
| Humboldt County | 132,380 | 44 | 10 | 3 |
| Imperial County | 181,724 | 29 | 38 | 2 |
| Kings County | 154,913 | 13 | 30 | 0 |
| Lake County | 67,764 | 43 | 38 | 3 |
| Lassen County | 28,340 | 15 | 2 | 2 |
| Marin County | 256,400 | 14 | 0 | 0 |
| Mendocino County | 89,175 | 42 | 35 | 2 |
| Merced County | 296,774 | 22 | 52 | 4 |
| Modoc County | 8,491 | 5 | 2 | 2 |
| Napa County | 132,727 | 28 | 30 | 3 |
| Orange County | 3,170,435 | 40 | 2 | 1 |
| Sacramento County | 1,611,231 | 62 | 9 | 1 |
| San Diego County | 3,298,799 | 82 | 148 | 3 |
| San Joaquin County | 816,108 | 93 | 16 | 0 |
| Shasta County | 181,121 | 60 | 2 | 1 |
| Siskiyou County | 42,498 | 32 | 13 | 3 |
| Solano County | 455,101 | 26 | 19 | 2 |
| Sonoma County | 485,375 | 129 | 126 | 2 |
| Stanislaus County | 556,972 | 61 | 139 | 2 |
| Sutter County | 98,545 | 8 | 26 | 0 |
| Tehama County | 64,451 | 49 | 13 | 1 |
| Trinity County | 15,642 | 17 | 5 | 2 |
| Yolo County | 225,251 | 16 | 2 | 2 |
How to read this table
Water systems counts community systems that list the county in their service area; one system can serve several counties. Health violations are breaches of a legal limit or treatment rule recorded by EPA since January 2016, not paperwork lapses. A violation stays unresolved until the state records it as fixed.
Other states
Sources: EPA Map of Radon Zones, EPA ECHO Safe Drinking Water Act data (2026Q3, published October 2026), US Census Bureau Vintage 2024 estimates. See the methodology.