Marin County, CA: EPA's radon zone, explained

EPA's own radon map rates Marin County zone 3 - low potential, predicting a county-wide average indoor level below 2 pCi/L. Nationally that's the 30th percentile for radon potential — Marin County outranks 30% of every county EPA has rated.

Where Marin County sits nationally

Marin County, California is home to about 256,400 people across 520.141 square miles. That works out to roughly 493 people per square mile. By population, Marin County is the 100th largest of every county sharing its EPA zone — the 90th percentile by population within that zone. EPA's geology-based Region 9 groups Marin County with 94 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

At Zone 3, Marin County's predicted county average is under the national indoor average (~1.3 pCi/L) and well below EPA's 4 pCi/L action level — outdoor air nationally runs about 0.4 pCi/L for comparison. Low doesn't mean zero: EPA's own data shows homes in Zone 3 counties like Marin County testing above the action level often enough that skipping a test isn't the safe assumption.

Next steps for a home in Marin County

For a Marin 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 Marin County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Marin 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 Marin 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. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.