California Radon Map, Levels & Testing Guide
Browse the 19 listed county pages surfaced for California. Open a listed county to compare its EPA zone, source caveats, testing meaning, and when mitigation pricing becomes worth checking.
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Official Evidence in California
19 of 19 listed counties have official evidence
Open a county page to see the processed verdict: source confidence, local burden, state percentile, and the next step for no reading, 2.0-3.9, or 4.0+ pCi/L.
100%
covered
Measured
19
State source
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CDC source
19
Needs source detail
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Measured Risk Leaders in California
County rankings from actual reported radon tests
These lists rank the visible California county pages by measured radon signals, not by the EPA zone label alone. Use them to spot where the state hub has a real data story before opening individual county pages.
19
measured counties
State-level verdict
This hub has measured high-burden counties, so the first click should be evidence-led.
The state hub is now doing a decision job: it separates first-click counties, retest counties, and lower-signal counties from 19 visible county measurement rows. 4 counties cross the high measured-burden band, so those pages should answer testing and 4.0+ action questions most directly.
First-click counties
Open Nevada County first when you need the strongest local answer. It is tagged Test-now from 25.0% 4.0+ - 7.4 pCi/L primary - 203 tests. 5 visible measured counties are elevated or high enough to review before lower-signal counties.
Buyer/seller lane
Buyer/seller lane: start with the elevated or high counties, require a fresh lowest-level test, and turn any 4.0+ property result into quote or credit math. The hub has 19 CDC-backed county rows to support that routing.
Retest lane
Retest lane: 2.0-3.9 pCi/L deserves more caution in the elevated/high county set than in lower-signal counties. The hub should send those users to county pages before product or cost paths.
Best county pages to open first
Start with these local evidence pages
Ranked by measured burden, 4.0+ share, test volume, and whether the EPA zone understates the measurement signal.
Nevada County
CDC Tracking
25.0% 4.0+ - 7.4 pCi/L primary - 203 tests
Nevada County is a first-click page: 25.0% of reported tests at or above 4.0 and 7.4 pCi/L primary result. Route no-reading users to a test now and 4.0+ users to quote or credit planning.
El Dorado County
CDC Tracking
30.3% 4.0+ - 4.1 pCi/L primary - 224 tests
El Dorado County is a first-click page: 30.3% of reported tests at or above 4.0 and 4.1 pCi/L primary result. Route no-reading users to a test now and 4.0+ users to quote or credit planning.
Kern County
CDC Tracking
11.6% 4.0+ - 4.6 pCi/L primary - 57 tests
Kern County is a first-click page: 11.6% of reported tests at or above 4.0 and 4.6 pCi/L primary result. Route no-reading users to a test now and 4.0+ users to quote or credit planning.
Santa Barbara County
CDC Tracking
17.6% 4.0+ - 4.1 pCi/L primary - 207 tests
Santa Barbara County is a first-click page: 17.6% of reported tests at or above 4.0 and 4.1 pCi/L primary result. Route no-reading users to a test now and 4.0+ users to quote or credit planning.
Highest 4.0+ share
Primary result 4.1 pCi/L - high-end 39.0 pCi/L
CDC Tracking
Primary result 7.4 pCi/L - high-end 1100.0 pCi/L
CDC Tracking
Primary result 3.0 pCi/L - high-end 27.2 pCi/L
CDC Tracking
Primary result 2.5 pCi/L - high-end 15.2 pCi/L
CDC Tracking
Primary result 4.1 pCi/L - high-end 86.3 pCi/L
CDC Tracking
Highest high-end reading
4.0+ share 25.0% - primary result 7.4 pCi/L
CDC Tracking
4.0+ share 9.5% - primary result 1.9 pCi/L
CDC Tracking
4.0+ share 11.6% - primary result 4.6 pCi/L
CDC Tracking
4.0+ share 6.7% - primary result 1.6 pCi/L
CDC Tracking
4.0+ share 17.6% - primary result 4.1 pCi/L
CDC Tracking
Most reported tests
4.0+ share 9.5% - primary result 1.9 pCi/L
CDC Tracking
4.0+ share 9.2% - primary result 2.0 pCi/L
CDC Tracking
4.0+ share 6.7% - primary result 1.6 pCi/L
CDC Tracking
4.0+ share 9.2% - primary result 2.0 pCi/L
CDC Tracking
4.0+ share 30.3% - primary result 4.1 pCi/L
CDC Tracking
Measured pattern
Among 19 visible counties with measurement tables, 4 land in the high measured-burden band and 5 land in high or elevated measured-burden bands. That lets this hub rank counties by observed test distribution instead of repeating the EPA map.
Map vs measurements
4 elevated measured counties are not EPA Zone 1. Those are the pages where the actual test distribution matters more than a map-only answer.
Source confidence
The measured rows shown here use CDC Tracking. This is still official evidence, but the hub should keep source caveats visible until a stable state table is available.
Already Tested
View California Cost Estimates
If you already have a radon result, skip the directory and move straight to local price guidance.
Understand the Number
2.0 vs 4.0 vs 8.0 pCi/L
Use the parent levels guide if you need the plain-English meaning of a radon result before browsing counties.
Need a Number First
Read the Testing Guide
Use the fastest valid radon test setup before you compare quotes or ask whether mitigation is worth it.
Browse Counties
Jump to the Directory
Open your county to see its EPA zone, testing meaning, and the point where mitigation becomes worth pricing.
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.
Already tested? Get your itemized mitigation cost estimate.
View California Mitigation Cost Estimates ->