Radon in California

About 1 in 9 homes tested in California were at or above the EPA action level of 4 pCi/L.

76%13%11%

5,663 homes tested, 2008–2017

Below 2 pCi/L: 76%
2 to 4 pCi/L: 13%
4 pCi/L or more: 11%

The line marks the EPA action level, 4 pCi/L.

The EPA puts 2 of California's 58 counties in Zone 1, its highest radon category, 29 in Zone 2 and 27 in Zone 3.

Every county in California

Sorted by the share of homes tested at or above 4 pCi/L. Select a county for local details and costs.

County	Homes tested	At or above 4	Median	EPA zone
Alpine County	13 tests	–	–	2
Inyo County	13 tests	–	–	2
Mono County	16 tests	–	–	2
Sutter County	5 tests	–	–	3
Plumas County	16 tests	–	–	2
El Dorado County		30%	1.6 pCi/L	2
Amador County		28%	2.3 pCi/L	2
Nevada County		25%	2.0 pCi/L	2
Siskiyou County	6 tests	–	–	3
Tulare County		24%	1.7 pCi/L	2
Tuolumne County		23%	1.4 pCi/L	2
Santa Cruz County		22%	1.2 pCi/L	2
Shasta County	12 tests	–	–	3
Humboldt County	11 tests	–	–	3
Calaveras County		18%	1.7 pCi/L	2
Santa Barbara County		18%	1.2 pCi/L	1
Mariposa County	5 tests	–	–	2
Solano County	16 tests	–	–	3
Monterey County		15%	0.8 pCi/L	2
San Joaquin County		15%	1.4 pCi/L	3
Madera County		15%	1.4 pCi/L	2
Lake County	8 tests	–	–	3
San Luis Obispo County		13%	1.0 pCi/L	2
Ventura County		12%	1.0 pCi/L	1
Kern County		12%	1.8 pCi/L	2
Butte County		11%	0.8 pCi/L	3
Stanislaus County	15 tests	–	–	3
Fresno County		11%	1.5 pCi/L	2
Napa County		11%	0.9 pCi/L	3
Los Angeles County		10%	0.7 pCi/L	2
San Mateo County		9%	0.9 pCi/L	2
Santa Clara County		9%	0.9 pCi/L	2
Contra Costa County		9%	0.9 pCi/L	2
Sacramento County		8%	0.9 pCi/L	3
Placer County		7%	1.1 pCi/L	2
Alameda County		7%	0.7 pCi/L	2
Marin County		6%	0.8 pCi/L	3
San Bernardino County		5%	0.7 pCi/L	2
Orange County		4%	0.7 pCi/L	3
San Francisco County		3%	0.5 pCi/L	2
San Diego County		3%	0.2 pCi/L	3
Yolo County		2%	0.9 pCi/L	3
Riverside County		1%	0.6 pCi/L	2
Sonoma County		1%	0.5 pCi/L	3
Colusa County	1 tests	–	–	3
Del Norte County	1 tests	–	–	3
Glenn County	1 tests	–	–	3
Imperial County	3 tests	–	–	3
Kings County	2 tests	–	–	3
Lassen County	3 tests	–	–	3
Mendocino County	7 tests	–	–	3
Merced County	3 tests	–	–	3
San Benito County	5 tests	–	–	2
Sierra County	1 tests	–	–	2
Tehama County	3 tests	–	–	3
Yuba County	7 tests	–	–	2
Modoc County	No data	–	–	3
Trinity County	No data	–	–	3

"–" means fewer than 20 tests or no lab data, too few for a reliable figure.

Sources

EPA Map of Radon Zones (public domain).
CDC Environmental Public Health Tracking Network, radon tests from labs, 2008–2017. Pre-mitigation tests only.

Last updated October 2026. How we calculate these numbers.