Short-term kit
RA400 short-term liquid scintillation radon test kit, lab fee included
- Lab fee included
Method: Liquid scintillation
In home: 48 to 96 hours
Locations and reports
Radon Levels in Sonoma County, California
EPA puts Sonoma County in Zone 3, its lowest radon zone, and most reported tests there come back low, though 0.8% of 131 (2008 to 2017, laboratory totals) reached 4 pCi/L. Homes with elevated levels occur in all three zones, so only a test measures yours.
Data checked By Radon411Methodology
Share at 4+ pCi/L
0.8%
9.5 pts below CA
- Sonoma
- 0.8%
- California
- 10.3%
131 tests, 2008 to 2017
Median result
0.5pCi/L
Below action level
- Sonoma
- 0.5 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2008 to 2017
Tests reported
131
17th of 33 by tests
- Sonoma
- 131
- Los Angeles
- 2,363
Reported to CDC before any fix, 2008 to 2017
EPA radon zone
Zone 3
Lower potential
- 1
- 2
- Zone 3
1 highest, 3 lowest
EPA predicts under 2 pCi/L indoors
Rank in California
33 of 33
Lowest quarter
- Sonoma
- 0.8%
- El Dorado
- 30.3%
By share, of 33 with 30+ tests
Read a result for Sonoma County
Starts at Sonoma County's median, 0.5 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 0.5 pCi/L
Below 2 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is below 2 pCi/L, the level at which EPA says a fix should be considered. Sonoma County's median reported result, 2008 to 2017.
The data
Sonoma County, At a glance
Sonoma County at a glance
Predicted: EPA radon zone
EPA Zone 3: lower predicted potential
USGS geologic potential: Low across 100% of the county's area
Predicted: EPA Map of Radon Zones
Measured: tests reported to CDC
Radon tests reported for Sonoma County, 2008 to 2017, laboratory
Under 2 pCi/L: 92%2 to 4 pCi/L: 7%4 pCi/L and above: 1%
Of 131 pre-mitigation tests reported to CDC, not homes in this area.
| Result band | Tests Measured | Share Measured |
|---|---|---|
| Under 2 pCi/L | 121 | 92.4% |
| 2 to 4 pCi/L | 9 | 6.9% |
| 4 pCi/L and above | 1 | 0.8% |
What these figures count
These are the 131 tests reported to CDC for Sonoma County over 2008 to 2017 (laboratory totals), many from homes being sold or already suspected of a problem: they describe the county's reported tests, not this house.
Measured: CDC tracking data. Figure above: 2008 to 2017, laboratory. Measured data period: 2005 to 2017 (every CDC figure on file).
Sonoma County, Compared
Sonoma County radon figures, compared
Calculated by Radon411Only 1 of 131 radon tests reported for Sonoma County over the 2008 to 2017 laboratory totals reached 4 pCi/L (0.8%). The median result, 0.5 pCi/L, sits below 2, well under EPA's 4 pCi/L action level, and the top reported result was 4.1 pCi/L.
Across 56 California counties pooled together, 10.3% of tests reached that level, so Sonoma County runs 9.5 points below its state. Against the national pool of 35.7%, the gap is 34.9 points lower. Its place in the state: 33rd of 33 California counties with 30 or more tests for the same period.
Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 0.8% share point the same way here. Of 3 neighbours with a figure, all 3 reported a larger share at or above 4 pCi/L; the spread is 6.3% (Marin County) to 15.8% (Solano County).
Below the action level, 121 tests (92.4%) came in under 2 pCi/L, against 76.8% statewide, and 9 tests (6.9%) fell between 2 and 4, against 12.9% statewide. USGS geology rates 100% of the county's land low radon potential.
CDC's ten-year totals for Sonoma County went from 100 tests (2005 to 2014) to 131 (2008 to 2017), the share going from 3.0% to 0.8%.
Pooled shares: Radon411's sum of CDC's county counts for one period.
Sonoma County, Geology
The ground under Sonoma County
PredictedUSGS's geologic map puts all of Sonoma County's mapped land in one class: low radon potential. All of it lies in the area the map calls Northern Coast Ranges, to which USGS gives a radon index of 6, in the low band (5 to 8).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Northern Coast Ranges | Metamorphosed marine sedimentary rocks and ultramafic rocks. | Low | 6 | 99.9% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Sonoma County beside the ground around it
Across California 0.1% of the land is rated high; none of Sonoma County's is. None of the 5 counties that border it has any moderate-potential ground either.
The map rates the ground, not the air in any home.
Local help
Sonoma County, Local resources
Radon testing and mitigation in Sonoma County
- California Indoor Radon ProgramCalifornia Indoor Radon Program, run by California Department of Public Health, licenses or certifies radon professionals and offers free or discounted test kits, and answers questions about testing and mitigation in California.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
Discounted test kits can be purchased through our partner laboratory, Alpha Energy, for $14.95.
- Compare kits
- Professionals in California4 professionals are listed in California.
What California's program says
- Phone
- 1-800-745-7236
- Radonprogram@cdph.ca.gov
- Licensing of radon professionals
The Radon Program certifies all radon service providers operating in California.
What it means
Sonoma County, Questions
Questions about radon in Sonoma County
Are radon levels high in Sonoma County?
Over the 2008 to 2017 laboratory totals, 0.8% of the radon tests reported to CDC for Sonoma County, California were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.5 pCi/L. Of 33 California counties with 30 or more tests for the same period, 32 reported a higher share; pooled, the state comes to 10.3%.
Which counties border Sonoma County?
Sonoma County borders 5 counties: Napa County, Marin County, Lake County, Solano County and Mendocino County.
Products
Sonoma County, Products
Testing a home in Sonoma County
In Sonoma County, 0.8% of 131 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 10.3% pooled across California. Zone 3 is the lowest label, yet homes at or above 4 pCi/L occur in every zone: a short-term kit (2 to 90 days) is the quick first check in Sonoma County.
Test kits and monitors3
See every kit and monitor
Long-term kit
RL416 long-term radon test kit, lab fee included
- Lab fee included
Method: Alpha track
In home: 90 days to 12 months
MonitorAEGTEST
HOUND-3786 rechargeable radon monitor with a 3.5-inch color screen
- C-NRPP 2024/25: Not included
- Battery
- Alarm
- Screen
Method: Ion chamber
First reading: Within 10 minutes
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Explore
Sonoma County, Bordering counties
Counties bordering Sonoma County
Calculated by Radon411- Radon levels in Solano CountyEPA zone 3
15.8%at or above 4 pCi/L
Median 0.9 pCi/L · 38 tests · 15.0 points above Sonoma County
- Radon levels in Lake CountyEPA zone 3
12.5%at or above 4 pCi/L
Median 0.9 pCi/L · 24 tests · 11.7 points above Sonoma County
- Radon levels in Napa CountyEPA zone 3
10.5%at or above 4 pCi/L
Median 0.9 pCi/L · 57 tests · 9.7 points above Sonoma County
- Radon levels in Marin CountyEPA zone 3
6.3%at or above 4 pCi/L
Median 0.8 pCi/L · 95 tests · 5.5 points above Sonoma County
- Radon levels in Mendocino CountyEPA zone 3
0.0%at or above 4 pCi/L
Median 0.7 pCi/L · 11 tests · 0.8 points below Sonoma County
Sonoma County, Cities and towns
Cities and towns in Sonoma County
Sonoma County's population has barely moved since 2020: 486,444 residents on July 1, 2025 against 488,514 then, by the Census Bureau's estimates. That works out to 309 people per square mile of land. Its largest town, Santa Rosa, has 179,437 residents, 36.9% of the total. Together its 9 incorporated places hold 73.0% of those residents; the other 27.0% live outside any of them. CDC's 131 reported tests for 2008 to 2017 work out to 0.3 per 1,000 residents (2025 estimate), close to California's 0.2.
From Santa Rosa to Cotati, CDC publishes no figure by town: each reads Sonoma County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Santa Rosa | 179,437 | +0.8% | 36.9% |
| Petaluma | 59,688 | −0.1% | 12.3% |
| Rohnert Park | 44,811 | +0.8% | 9.2% |
| Windsor | 25,864 | −1.8% | 5.3% |
| Healdsburg | 11,172 | −1.6% | 2.3% |
| Sonoma | 10,537 | −1.8% | 2.2% |
| Cloverdale | 8,701 | −3.4% | 1.8% |
| Sebastopol | 7,532 | −0.8% | 1.5% |
| Cotati | 7,347 | −3.3% | 1.5% |
9 incorporated places, as listed in the U.S. Census Bureau’s 2020 place reference files. Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025).
Sonoma County, Homes
Homes in Sonoma County
Of Sonoma County's 206,319 homes, 67% are detached houses, against 57% across California; 14% are in buildings of five or more homes. Half of its homes were built before 1979, close to California's 1976: 21% date from before 1960 and 16% from 2000 or later. 62% of occupied homes are lived in by their owner (California: 56%); in the rest a tenant can run a test, but a fix is the landlord's to make.
| Homes | Sonoma County | California |
|---|---|---|
| Housing units | 206,319 | 14,532,683 |
| Detached houses | 67% | 57% |
| In buildings of 5 or more | 14% | 24% |
| Mobile homes | 5% | 4% |
| Built before 1960 | 21% | 27% |
| Built since 2000 | 16% | 18% |
| Middle year built | 1979 | 1976 |
| Owner-occupied | 62% | 56% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.