Short-term kit
RA100 short-term liquid scintillation radon test kit
- Lab fee extra
Method: Liquid scintillation
In home: 48 to 96 hours
Locations and reports
Radon Levels in Napa County, California
EPA puts Napa County in Zone 3, its lowest radon zone, and most reported tests there come back low, though 10.5% of 57 (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
10.5%
0.2 pts above CA
- Napa
- 10.5%
- California
- 10.3%
57 tests, 2008 to 2017
Median result
0.9pCi/L
Below action level
- Napa
- 0.9 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2008 to 2017
Tests reported
57
27th of 33 by tests
- Napa
- 57
- 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
18 of 33
Middle half
- Napa
- 10.5%
- El Dorado
- 30.3%
By share, of 33 with 30+ tests
Read a result for Napa County
Starts at Napa County's median, 0.9 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 0.9 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. Napa County's median reported result, 2008 to 2017.
The data
Napa County, At a glance
Napa 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 Napa County, 2008 to 2017, laboratory
Under 2 pCi/L: 81%2 to 4 pCi/L: 9%4 pCi/L and above: 10%
Of 57 pre-mitigation tests reported to CDC, not homes in this area.
| Result band | Tests Measured | Share Measured |
|---|---|---|
| Under 2 pCi/L | 46 | 80.7% |
| 2 to 4 pCi/L | 5 | 8.8% |
| 4 pCi/L and above | 6 | 10.5% |
What these figures count
These are the 57 tests reported to CDC for Napa 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).
Napa County, Compared
Napa County radon figures, compared
Calculated by Radon41110.5% of its 57 tests over the 2008 to 2017 laboratory totals were at or above 4 pCi/L. That is 0.2 points above the 10.3% pooled from 56 California counties.
Ordered from highest share down, it comes 18th of 33 California counties with 30 or more tests for the same period. Nationally, every county with a figure for the same period pools to 35.7%; Napa County sits 25.2 points below it.
Next door, the shares run from 0.8% in Sonoma County to 15.8% in Solano County, with Yolo County and Sonoma County below Napa County. Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 10.5% share point the same way here.
Half the tests came in under 0.9 pCi/L and half over it; the single highest reached 37.3 pCi/L. The rest split 80.7% under 2 pCi/L and 8.8% from 2 to 4 (California: 76.8% and 12.9%).
CDC's ten-year totals for Napa County went from 54 tests (2005 to 2014) to 57 (2008 to 2017), the share going from 14.8% to 10.5%.
Pooled shares: Radon411's sum of CDC's county counts for one period.
Napa County, Geology
The ground under Napa County
PredictedUSGS's geologic map puts all of Napa 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 | 99.6% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Napa County beside the ground around it
Across California 0.1% of the land is rated high; none of Napa County's is. None of the 4 counties that border it has any moderate-potential ground either.
The map rates the ground, not the air in any home.
Local help
Napa County, Local resources
Radon testing and mitigation in Napa 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
Napa County, Questions
Questions about radon in Napa County
Are radon levels high in Napa County?
Over the 2008 to 2017 laboratory totals, 10.5% of the radon tests reported to CDC for Napa County, California were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.9 pCi/L. Of 33 California counties with 30 or more tests for the same period, 17 reported a higher share; pooled, the state comes to 10.3%.
Which counties border Napa County?
Napa County borders 4 counties: Solano County, Yolo County, Sonoma County and Lake County.
Products
Napa County, Products
Testing a home in Napa County
In Napa County, 10.5% of 57 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 Napa County.
Test kits and monitors3
See every kit and monitor
Long-term kit
RL116 long-term radon test kit
- Lab fee extra
Method: Alpha track
In home: 90 days to 12 months
Monitor
lüft plug-in radon and indoor air quality monitor
- C-NRPP 2024/25: Approved
- Plug-in
- Wi-Fi, Bluetooth
- App only
First reading: Within 90 minutes
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Explore
Napa County, Bordering counties
Counties bordering Napa 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 · 5.3 points above Napa County
- Radon levels in Lake CountyEPA zone 3
12.5%at or above 4 pCi/L
Median 0.9 pCi/L · 24 tests · 2.0 points above Napa County
- Radon levels in Yolo CountyEPA zone 3
2.1%at or above 4 pCi/L
Median 0.9 pCi/L · 48 tests · 8.4 points below Napa County
- Radon levels in Sonoma CountyEPA zone 3
0.8%at or above 4 pCi/L
Median 0.5 pCi/L · 131 tests · 9.7 points below Napa County
Napa County, Cities and towns
Cities and towns in Napa County
About 132,949 people live in Napa County by the Census Bureau's estimate for July 1, 2025, 4,532 fewer than in 2020 (−3.3%). That works out to 178 people per square mile of land. Napa alone is home to 76,562 of them, 57.6% of the total. Together its 5 incorporated places hold 84.8% of those residents; the other 15.2% live outside any of them. Testing is reported here more often than across the state: CDC's 57 reported tests for 2008 to 2017 come to 0.4 for every 1,000 residents (2025 estimate), against 0.2 for California.
From Napa to Yountville, CDC publishes no figure by town: each reads Napa County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Napa | 76,562 | −3.5% | 57.6% |
| American Canyon | 22,773 | +4.5% | 17.1% |
| St. Helena | 5,194 | −4.2% | 3.9% |
| Calistoga | 4,954 | −5.2% | 3.7% |
| Yountville | 3,198 | −6.6% | 2.4% |
5 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).
Napa County, Homes
Homes in Napa County
Of Napa County's 55,601 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 1976, close to California's 1976: 27% date from before 1960 and 18% from 2000 or later. 64% 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 | Napa County | California |
|---|---|---|
| Housing units | 55,601 | 14,532,683 |
| Detached houses | 67% | 57% |
| In buildings of 5 or more | 14% | 24% |
| Mobile homes | 7% | 4% |
| Built before 1960 | 27% | 27% |
| Built since 2000 | 18% | 18% |
| Middle year built | 1976 | 1976 |
| Owner-occupied | 64% | 56% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.