San Diego County, CA: EPA's radon zone, explained

EPA places San Diego County in Zone 3 - Low Potential — a predicted county-wide average indoor radon level below 2 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Nationally that's the 33rd percentile for radon potential — San Diego County outranks 33% of every county EPA has rated.

San Diego County by the numbers

San Diego County, California is home to about 3,298,799 people across 4,210.884 square miles. Population density there runs about 783 people per square mile. By population, San Diego County is the 2nd largest of every county sharing its EPA zone — the 100th percentile by population within that zone. EPA's geology-based Region 9 groups San Diego County with 94 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

A Zone 3 county like San Diego County is predicted closer to the 0.4 pCi/L typical outdoors than to EPA's 4 pCi/L action level. Low doesn't mean zero: EPA's own data shows homes in Zone 3 counties like San Diego County testing above the action level often enough that skipping a test isn't the safe assumption.

Testing and mitigation in San Diego County

For a San Diego County homeowner, the order is simple: test first, since a kit costs a small fraction of what mitigation would. Only if the result clears 4 pCi/L does a system — typically $800–$2,500, about $1,200 for a common install — become worth pricing out.

Find California's state radon program

For San Diego County, an ongoing number beats a one-time test - a Airthings radon monitor tracks San Diego County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. Code RADONZONE-10OFF takes 10% off at checkout.

Where San Diego County's figures come from

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.