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

EPA's own radon map rates Imperial County zone 3 - low potential, predicting a county-wide average indoor level below 2 pCi/L. Nationally that's the 28th percentile for radon potential — Imperial County outranks 28% of every county EPA has rated.

Imperial County by the numbers

Imperial County, California is home to about 181,724 people across 4,175.453 square miles. That works out to roughly 44 people per square mile. By population, Imperial County is the 142nd largest of every county sharing its EPA zone — the 86th percentile by population within that zone. Geologically, Imperial County is grouped into EPA Region 9 alongside 94 other counties that share its rock and soil setting.

What this actually means for a house in Imperial County

At Zone 3, Imperial 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. Don't read Zone 3 as a pass for Imperial County — EPA's guidance to test every home makes no exception for counties predicted low.

Testing and mitigation in Imperial County

For a Imperial 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 Imperial County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Imperial 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 Imperial County's figures come from

The methodology guide shows exactly which EPA and Census sources feed Imperial County's numbers. Whatever zone Imperial County carries, the zone explainer covers what it does and doesn't tell you about any one house.