California note
Radon is checked with a test, not a guess
Testing is the only way to know a home's radon level; CDPH says real-estate transaction testing should use a certified tester.
Radon is easy to miss because you cannot see or smell it. A map can show areas where radon has been found, but it cannot tell you the exact level in one living room, basement, or slab-on-grade house.
CDPH says testing is the only way to know. Homeowners can use a test kit or hire a certified tester; for a real-estate transaction, CDPH says to use a certified tester.
Do not treat one neighbor’s result like your own. Houses on the same street can test differently. EPA and CDPH strongly recommend fixing a home at 4 picocuries per liter or higher, and say to seriously consider action between 2 and 4. Use the official testing steps to confirm the result and find qualified mitigation help.
Where to see it
CDPH indoor radon pages and county or local environmental health information.
Official sources
Official pages
Reviewed July 14, 2026
Current maps, alerts, conditions, and public safety guidance.
Conditions and public guidance can change quickly. Check the current page before you act.
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