California Radon Map, Levels & Testing Guide
Browse the 5 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.
High Risk
Moderate
Low Risk
Official Evidence in California
5 of 5 listed counties have official evidence
Open a county page to see the official source context: measurement fields, local burden, state percentile, and the next step for no reading, 2.0-3.9, or 4.0+ pCi/L.
100%
covered
Measured
5
State source
0
CDC source
5
Needs source detail
0
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.
5
measured counties
State-level evidence read
This hub has official measurement coverage, but most visible counties need home-specific confirmation.
The state hub is now doing a decision job: it separates first-click counties, retest counties, and lower-signal counties from 5 visible county measurement rows. None of the visible measured counties cross the elevated band, so the hub should emphasize home-specific testing rather than implied statewide danger.
First-click counties
Open Monterey County first when you need the strongest local answer. It is tagged Retest lane from 14.9% 4.0+ - 1.9 pCi/L primary - 44 tests. 0 visible measured counties are elevated or high enough to review before lower-signal counties.
Buyer/seller lane
Buyer/seller lane: use the hub to pick the county page, but do not negotiate from statewide context alone. A property result still controls quotes and credits.
Retest lane
Retest lane: with 5 measured county rows and no elevated statewide cluster, 2.0-3.9 pCi/L is mostly a confirm-or-monitor decision until a home repeats higher.
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.
Monterey County
CDC Tracking
14.9% 4.0+ - 1.9 pCi/L primary - 44 tests
Monterey County belongs in the retest lane: 14.9% of reported tests at or above 4.0 and 1.9 pCi/L primary result. A 2.0-3.9 home result should be confirmed before the user dismisses it.
Santa Clara County
CDC Tracking
9.2% 4.0+ - 2.0 pCi/L primary - 447 tests
Santa Clara County belongs in the retest lane: 9.2% of reported tests at or above 4.0 and 2.0 pCi/L primary result. A 2.0-3.9 home result should be confirmed before the user dismisses it.
Los Angeles County
CDC Tracking
9.5% 4.0+ - 1.9 pCi/L primary - 1,367 tests
Los Angeles County is lower at county level: 9.5% of reported tests at or above 4.0 and 1.9 pCi/L primary result. Keep the page home-specific rather than presenting it as a statewide alarm.
San Bernardino County
CDC Tracking
4.5% 4.0+ - 1.2 pCi/L primary - 81 tests
San Bernardino County is lower at county level: 4.5% of reported tests at or above 4.0 and 1.2 pCi/L primary result. Keep the page home-specific rather than presenting it as a statewide alarm.
Highest 4.0+ share
Primary result 1.9 pCi/L - high-end 20.3 pCi/L
CDC Tracking
Primary result 1.9 pCi/L - high-end 363.7 pCi/L
CDC Tracking
Primary result 2.0 pCi/L - high-end 43.2 pCi/L
CDC Tracking
Primary result 1.2 pCi/L - high-end 10.1 pCi/L
CDC Tracking
Primary result 1.0 pCi/L - high-end 28.6 pCi/L
CDC Tracking
Highest high-end reading
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 2.8% - primary result 1.0 pCi/L
CDC Tracking
4.0+ share 14.9% - primary result 1.9 pCi/L
CDC Tracking
4.0+ share 4.5% - primary result 1.2 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 2.8% - primary result 1.0 pCi/L
CDC Tracking
4.0+ share 4.5% - primary result 1.2 pCi/L
CDC Tracking
4.0+ share 14.9% - primary result 1.9 pCi/L
CDC Tracking
Measured pattern
Among 5 visible counties with measurement tables, 0 land in the high measured-burden band and 0 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
The strongest measured signals mostly align with the EPA zone structure, so the county pages can use the map as support while still leading with test data.
Source strategy
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 ->