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

EPA's own radon map rates Ventura County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. That puts Ventura County in the 100th percentile nationally for radon potential — higher than 100% of the counties EPA has rated.

How Ventura County compares

Ventura County, California is home to about 835,427 people across 1,840.78 square miles. Averaged across its land area, Ventura County runs about 454 people per square mile. By population, Ventura County is the 15th largest of every county sharing its EPA zone — the 99th percentile by population within that zone. Ventura County falls in EPA Radon Region 9, one of 95 counties nationally EPA assigns to that same geologic province.

Why the zone isn't the answer by itself

Ventura County's predicted average sits above EPA's own action level of 4 pCi/L — the concentration at which EPA recommends fixing a home — versus a national average around 1.3 pCi/L indoors and 0.4 pCi/L outdoors. Being in Zone 1 raises the odds Ventura County homes test above the action level, but it does not mean any specific house does — EPA recommends every home be tested, in every zone, because the only way to know is to test.

Next steps for a home in Ventura County

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

Where Ventura 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.