Radon Levels & Zone Map in Napa County, CA
Direct Answer for basement and lowest-level tests: Napa County has a lower countywide signal, but that does not clear an individual home. A real test still beats the map.
Low county risk is not a home-level clear signal
Do not let a Zone 3 label talk you out of the first test. House-by-house differences still matter.
Lower countywide map signal, but house-by-house spikes still happen.
2.0-3.9 pCi/L usually means retest or track. 4.0+ is where EPA action and quote planning start to matter.
Low zone is not a free pass. Use a kit before you rule radon out.
Measured Radon Data
Napa County evidence before the next step
Napa County, CA has more than the EPA map: CDC Tracking Network exposes 30 reported tests, 2.1 pCi/L county average, 0.9 pCi/L median, 10.5% of reported tests at or above 4.0 pCi/L, and 37.3 pCi/L high-end signal for 2008-2017.
Source window
2008-2017
Processed verdict
Borderline measured burden
Solid confidence - 84/100
Primary result
2.1 pCi/L
59th percentile in-state
4.0+ signal
10.5%
50th percentile in-state
High-end signal
37.3 pCi/L
77th percentile in-state
County-specific verdict
Napa County is a retest-and-watch market, not a dismiss-it market.
Napa County is a confirmation case because 2.1 pCi/L average, 0.9 pCi/L median, and 10.5% of reported tests at or above 4.0. In-state rank: 59th percentile for average, and 50th percentile for 4.0+ share. A 2.0-3.9 result should not be treated as final without a follow-up test or longer-term read.
Real-estate use
Buyer or seller use: use the county signal to justify a test contingency or retest, then reserve credits for confirmed 4.0+ home results.
Napa County is a split-decision county: no reading means test first, 2.0-3.9 means retest or track, and 4.0+ means cost planning starts.
Choose Next StepBorderline-intent answer
What does a borderline radon result mean in Napa County?
Napa County is a split-decision county. 10.5% of reported tests at or above 4.0 pCi/L, 2.1 pCi/L primary measured result, and 37.3 pCi/L high-end signal means a 2.0-3.9 pCi/L home result should be retested or tracked instead of dismissed.
Pick the situation that matches you
You should not need to read the whole guide before clicking one of these. Start with the lane that matches your current stage, then come back for the deeper reference only if you still need it.
Jump into a prefilled Napa County action plan based on the result you already have, instead of starting from a generic cost page.
I have not tested yet
Do not price mitigation blind. Get the first number, then decide whether you need monitoring, quotes, or nothing at all.
My result is 2.0-3.9
Usually retest or track first. If the reading keeps showing up, use the local action plan to decide whether pricing makes sense.
My result is 4.0+
This is the EPA action line. Use the local cost page before calling contractors so you know the likely scope, timing, and budget.
I am buying or selling
Turn the reading into a credit or repair number before negotiation starts. This is the faster path than arguing from a generic article.
Already tested once and just want to watch the number trend?
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County Evidence Snapshot
Napa County testing context
Napa County has a lower predicted countywide zone signal, so direct testing matters more than the map label.
EPA map signal
Zone 3
County-level predicted indoor screening range, not a home-level test result.
Housing base
55,588
45th percentile among 58 CA counties with data.
Older housing share
41.1%
24th percentile in-state; older homes often need clearer test placement decisions.
Median home value
$794,500
Used as context for whether mitigation is a small maintenance item or a negotiation issue.
Measured Radon Data
CDC Environmental Public Health Tracking Network Radon Tests from Labs
2008-2017
Average result
2.1 pCi/L
At or above 4.0
10.5%
Maximum reported
37.3 pCi/L
10-year tested
30
CDC county summaries are based on national radon testing laboratories and participating state feeds; they are not a statistically designed survey of every home.
RadonVerdict Processed Verdict
Borderline measured burden
Primary result rank
59th percentile
2.1 pCi/L
4.0+ rank
50th percentile
10.5% at or above 4.0
High-end rank
77th percentile
37.3 pCi/L
Test volume rank
52nd percentile
30 over 10 years
How to use this county data
Data source
National tracking data
CDC Tracking provides comparable county-level measurement fields; state-specific sources still outrank it when they expose stable county tables.
What the numbers show
Fuller county picture
This is the most useful setup: county average, 4.0+ share, high-end readings, and 30 reported tests/properties can be read together instead of relying on one number.
Nearby comparison
Nearby comparison: Closest counties by county average: Lake County (2.1 pCi/L) is just lower, and Tehama County (2.1 pCi/L) is just higher.
How this helps
Use this for 2.0-3.9 pCi/L results, where retesting is usually smarter than dismissing the issue.
What the data says
Napa County, CA is measurement-backed for 2008-2017. The measured average is 2.1 pCi/L, and 10.5% of reported results are at or above 4.0 pCi/L. The high-end signal reaches 37.3 pCi/L.
Napa County, CA sits at the 59th percentile for measured average, 50th percentile for 4.0+ share, 77th percentile for high-end readings, and 52nd percentile for test volume among 58 measured counties in the state. Closest counties by county average: Lake County (2.1 pCi/L) is just lower, and Tehama County (2.1 pCi/L) is just higher.
What to do with it
Napa County is a split-decision county: no reading means test first, 2.0-3.9 means retest or track, and 4.0+ means cost planning starts.
Retest trigger: a 2.0-3.9 pCi/L home result is exactly the gray zone for this county; retest before ignoring it or paying for mitigation.
Solid confidence (84/100) from CDC Tracking Network based on about 30 reported tests/properties plus comparable county-level measurement fields.
No reading yet
No reading yet: start with a test kit; the county data is context, not a substitute for the home result.
2.0-3.9 result
2.0-3.9 pCi/L: retest or monitor before paying for mitigation, then escalate if the level repeats or rises.
4.0+ result
4.0+ pCi/L: use the result for mitigation quotes, repair scope, or seller-credit negotiation; the county signal is no longer the deciding input.
Source hierarchy: CDC Tracking Network is used for this county, with EPA zone and Census housing data kept as supporting context. CDC Tracking provides comparable county-level measurement fields; state-specific sources still outrank it when they expose stable county tables.
Direct Answer
What radon risk level should homeowners assume in Napa County?
Napa County is currently categorized as EPA Zone 3 (Lower Predicted Average Risk). Testing is still recommended because home-level variance can be high.
| Evidence | Value |
|---|---|
| Area | Napa County, CA |
| EPA Zone | Zone 3 |
| Primary Recommendation | Perform direct radon testing in the lowest livable level |
Your Radon Reading
Enter your home's measured level; the starting value is only a planning example until you have your own result.
Lower Concern Range
Your reading is below the common action reference levels. Both the EPA (4.0) and WHO (2.7) thresholds are not exceeded. Mitigation is usually not the next immediate step after a confirmed low result. If you have never tested your home, start with a short-term kit first. If this is already a confirmed low reading, a digital monitor can help you keep an eye on seasonal changes.
Understanding Radon Levels: Complete Reference
Below 2.0 pCi/L - Lower Concern, Keep Testing
Below both the EPA (4.0) and WHO (2.7) action reference levels. This usually means mitigation is not the next immediate step after a confirmed result. The average outdoor radon level is approximately 0.4 pCi/L, and there is no known risk-free indoor level. Periodic testing is still recommended because levels can change over time due to seasonal variations, changes in home ventilation, or foundation settling.
-4.0
2.0 - 4.0 pCi/L - Elevated, Consider Action
Exceeds the World Health Organization's reference level of 2.7 pCi/L but falls below the US EPA action threshold. The EPA states that homeowners should "consider fixing" homes in this range, especially if the home has a basement used as living space, if children are present, or in connection with a real estate transaction. Practical next step: run a confirmatory long-term test, then compare mitigation quotes if levels remain elevated.
-8.0
4.0 - 8.0 pCi/L - Action Recommended
Exceeds the EPA action level of 4.0 pCi/L. The EPA and Surgeon General strongly recommend mitigation within a few months. At this level, prioritize confirmatory testing and contractor planning. Standard sub-slab depressurization systems typically reduce indoor levels by 80-99%.
Above 8.0 pCi/L - Urgent Action Required
At these levels, the EPA recommends expedited mitigation - ideally within weeks, not months. Occupants should minimize time in lower-level rooms until the system is installed. Use a certified mitigator and request priority scheduling to shorten high-exposure time. Many mitigators offer priority scheduling for homes above 8.0 pCi/L.
Why Radon is Still Relevant in Napa County
Napa County is classified as EPA Zone 3, with a lower predicted indoor screening range below 2.0 pCi/L. This does not mean radon is absent - it means the countywide map signal is low.
The EPA has documented elevated radon readings in every state and in homes in every zone classification. Local geological anomalies - pockets of granite, shale intrusions, or fractured bedrock - can create localized high-radon areas even within an otherwise low-risk county.
The Surgeon General and EPA recommend testing all homes, regardless of geographic zone. A simple short-term test kit ($15-$30) provides results within a few days and can give you peace of mind.
Radon & Health: What the Science Says
lung cancer
from radon
4.0 pCi/L
Radon is a Class A carcinogen - the same classification as asbestos and tobacco smoke. The National Academy of Sciences estimates that radon causes approximately 21,000 lung cancer deaths annually in the United States, making it the leading environmental cause of cancer death.
Unlike smoking, radon exposure is involuntary and often invisible. There is no safe level of radon - risk increases linearly with exposure. The good news: radon mitigation systems are highly effective, typically reducing indoor levels by 80-99% within hours of activation.
Source: US Environmental Protection Agency, "A Citizen's Guide to Radon" (EPA 402/K-12/002). National Academy of Sciences, Biological Effects of Ionizing Radiation (BEIR VI) Report, 1999.
Step 1: Test Your Home
Testing is the only way to know your home's radon level. Zone data tells you the regional risk, but your home could be significantly higher or lower than the countywide pattern. For most homeowners, the right first purchase is a low-cost short-term test kit.
A digital monitor is a better fit after your first result, for seasonal re-checks, or to keep tracking levels after mitigation.
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Already Know Your Level?
If your test shows 4.0 pCi/L or higher, get an itemized cost estimate specific to Napa County - including regional labor rates and permit requirements.
Get Mitigation Cost Estimate ->CA Radon Regulations
California requires sellers to complete the Transfer Disclosure Statement (TDS), which covers known environmental hazards including radon.
California does not require specific radon licensing. Look for NRPP or AARST-certified professionals.
How to Test for Radon in Napa County
Buy a Test Kit
Purchase a short-term charcoal test kit online or from a local hardware store. Cost: $15-$30. Place it in the lowest livable level of your home.
Wait 2-7 Days
Keep doors and windows closed (except normal entry/exit) during the test period. Avoid running whole-house fans. Mail the kit to the lab provided.
Read Your Results
If results are below 4.0 pCi/L, re-test every 2 years or use a monitor for ongoing tracking. If above 4.0, use our cost calculator to see mitigation options.
Related Radon Resources for Napa County
Explore Other CA Counties
Official State Resource
California radon program and rules
Use the state program link to verify local radon guidance, disclosure language, and contractor credential expectations before you act on an estimate.
Disclosure rule tracked
California requires sellers to complete the Transfer Disclosure Statement (TDS), which covers known environmental hazards including radon.
Credential note
California does not require specific radon licensing. Look for NRPP or AARST-certified professionals.
Sources & Methodology
Radon zone classifications for Napa County are sourced from the EPA's Map of Radon Zones, which uses geological surveys, indoor radon measurements, and soil permeability data to assign each county a risk tier.
Disclaimer: Zone data represents county-level screening ranges and cannot predict the radon level in any specific home. Testing is the only reliable method to determine your home's radon concentration. This content is for informational purposes and is not medical advice.
Content review: Source-level retrieval dates
Editorial and Data Transparency
- Author
- RadonVerdict Data Team (Public Data and Cost Modeling)
- Content Review
- Source-level dates shown below
- Data Retrieved At
- 2026-02-24
Primary Sources
- EPA Map of Radon Zones (retrieved 2026-02-21)
- EPA A Citizen's Guide to Radon (retrieved 2026-05-05)
- CDC Environmental Public Health Tracking - Radon Testing (retrieved 2026-05-05)
- Official CA radon program
- US Census Bureau, 2018-2022 American Community Survey 5-Year Estimates (retrieved 2026-02-24)