Radon Levels in California
California is classified as an EPA Zone 3 state with a low risk for radon. Average indoor radon levels range from 0.5 to 2.5 pCi/L, and approximately 6% of California homes test above the EPA action level of 4.0 pCi/L.
California Radon Summary
Key Facts About Radon in California
- Most of California is EPA Zone 3 (low potential)
- Some inland and mountain areas have higher geological uranium concentrations
- California requires radon-resistant construction in new homes in Zone 1 counties
California Testing Requirements & Recommendations
No universal mandate. CDPH recommends testing all homes built before 1990.
Coastal climate and open construction styles naturally dilute radon, but testing remains important.
Major Cities in California
Radon levels can vary significantly within California depending on local geology. All residents — regardless of city — should test their specific home, as neighborhood-level radon varies independently of broader state and county averages.
What Should California Residents Do?
Test Your Home
Use a short-term charcoal test kit ($15–$25) for an initial reading, or a continuous radon detector for ongoing monitoring. Test in your basement or lowest livable area.
Understand Your Result
If your result is below 2.0 pCi/L: monitor periodically. Between 2.0–4.0 pCi/L: consider mitigation. Above 4.0 pCi/L: the EPA recommends mitigation. Above 8.0 pCi/L: take action promptly.
Hire a Certified Contractor if Needed
Look for NRPP or NRSB-certified radon mitigation contractors in California. Mitigation systems typically cost $800–$2,500 and can reduce radon levels by 50–99%.
Verify with a Follow-Up Test
After mitigation, test again to confirm the system is working. Retest every 2 years, or after significant home renovations.
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