California: 58 counties covered
Of 58 rated California counties, 2 are in EPA's highest radon-potential zone (Zone 1) — but every county page links to testing guidance regardless of zone, because a county rating is a county-wide average, not a reading for any one house.
- Los Angeles County
- San Diego County
- Orange County
- Riverside County
- San Bernardino County
- Santa Clara County
- Alameda County
- Sacramento County
- Contra Costa County
- Fresno County
- Kern County
- Ventura County
- San Francisco County
- San Joaquin County
- San Mateo County
- Stanislaus County
- Sonoma County
- Tulare County
- Solano County
- Santa Barbara County
- Monterey County
- Placer County
- Merced County
- San Luis Obispo County
- Santa Cruz County
- Marin County
- Yolo County
- Butte County
- El Dorado County
- Imperial County
- Shasta County
- Madera County
- Kings County
- Napa County
- Humboldt County
- Nevada County
- Sutter County
- Mendocino County
- Yuba County
- San Benito County
- Lake County
- Tehama County
- Tuolumne County
- Calaveras County
- Siskiyou County
- Amador County
- Lassen County
- Glenn County
- Del Norte County
- Colusa County
- Plumas County
- Inyo County
- Mariposa County
- Trinity County
- Mono County
- Modoc County
- Sierra County
- Alpine County