California Radon Zone Map
Counties colored by EPA radon zone. Red = Zone 1, Orange = Zone 2, Green = Zone 3.
Highest-Risk Counties
Santa Barbara33.8% above action level
Ventura30.8% above action level
All 58 Counties — By Radon Risk
| County | EPA Zone | Tests | % Above 4 pCi/L |
|---|
| Santa Barbara |
High |
65 |
33.8% |
| Ventura |
High |
120 |
30.8% |
| Alameda |
Moderate |
212 |
24.5% |
| Alpine |
Moderate |
— |
— |
| Amador |
Moderate |
8 |
62.5% |
| Calaveras |
Moderate |
10 |
40% |
| Contra Costa |
Moderate |
177 |
28.8% |
| El Dorado |
Moderate |
37 |
40.5% |
| Fresno |
Moderate |
126 |
25.4% |
| Inyo |
Moderate |
— |
— |
| Kern |
Moderate |
96 |
31.2% |
| Los Angeles |
Moderate |
1,287 |
22.8% |
| Madera |
Moderate |
22 |
31.8% |
| Mariposa |
Moderate |
4 |
— |
| Mono |
Moderate |
— |
— |
| Monterey |
Moderate |
52 |
40.4% |
| Nevada |
Moderate |
24 |
45.8% |
| Placer |
Moderate |
73 |
31.5% |
| Plumas |
Moderate |
4 |
— |
| Riverside |
Moderate |
317 |
31.9% |
| San Benito |
Moderate |
7 |
— |
| San Bernardino |
Moderate |
240 |
31.2% |
| San Francisco |
Moderate |
129 |
30.2% |
| San Luis Obispo |
Moderate |
51 |
27.5% |
| San Mateo |
Moderate |
115 |
32.2% |
| Santa Clara |
Moderate |
249 |
26.5% |
| Santa Cruz |
Moderate |
42 |
31% |
| Sierra |
Moderate |
— |
— |
| Tulare |
Moderate |
42 |
38.1% |
| Tuolumne |
Moderate |
13 |
46.2% |
| Yuba |
Moderate |
11 |
45.5% |
| Butte |
Low |
41 |
36.6% |
| Colusa |
Low |
— |
— |
| Del Norte |
Low |
5 |
— |
| Glenn |
Low |
4 |
— |
| Humboldt |
Low |
23 |
34.8% |
| Imperial |
Low |
20 |
35% |
| Kings |
Low |
16 |
37.5% |
| Lake |
Low |
13 |
53.8% |
| Lassen |
Low |
4 |
— |
| Marin |
Low |
48 |
33.3% |
| Mendocino |
Low |
17 |
35.3% |
| Merced |
Low |
33 |
27.3% |
| Modoc |
Low |
— |
— |
| Napa |
Low |
26 |
34.6% |
| Orange |
Low |
439 |
26% |
| Sacramento |
Low |
221 |
29.9% |
| San Diego |
Low |
458 |
30.8% |
| San Joaquin |
Low |
93 |
33.3% |
| Shasta |
Low |
37 |
40.5% |
| Siskiyou |
Low |
5 |
80% |
| Solano |
Low |
72 |
29.2% |
| Sonoma |
Low |
88 |
39.8% |
| Stanislaus |
Low |
72 |
31.9% |
| Sutter |
Low |
15 |
40% |
| Tehama |
Low |
13 |
46.2% |
| Trinity |
Low |
4 |
— |
| Yolo |
Low |
26 |
34.6% |
Sources: EPA Map of Radon Zones, CDC Environmental Health Tracking Network
Frequently Asked Questions
How many counties in California have high radon risk?
California has 2 counties classified as EPA Zone 1 (high radon risk), 29 as Zone 2 (moderate), and 27 as Zone 3 (low risk).
Is radon a problem in California?
California has 2 high-risk counties where elevated radon is likely. The EPA recommends all homes in California be tested for radon, regardless of which zone they're in.
How do I check radon levels for my California address?
Enter your ZIP code in the search box above to see your county's EPA radon zone and available testing data. For your specific home's radon level, you'll need a radon test kit.