What EPA's radon map says about Contra Costa County

EPA's own radon map rates Contra Costa County zone 2 - moderate potential, predicting a county-wide average indoor level between 2 and 4 pCi/L. That puts Contra Costa County in the 66th percentile nationally for radon potential — higher than 66% of the counties EPA has rated.

Where Contra Costa County sits nationally

About 1,172,607 people live in Contra Costa County, California, spread over 716.901 square miles. That works out to roughly 1,636 people per square mile. Among counties sharing Contra Costa County's EPA zone, it outranks 99% of them by population. Contra Costa 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

Call it a middle case: above the 1.3 pCi/L most US homes average, below the 4 pCi/L line EPA uses to recommend mitigation — that's Contra Costa County's Zone 2 prediction. Middle-of-the-pack doesn't mean risk-free for any one Contra Costa County address — a moderate county average still leaves individual homes to test either side of it.

Next steps for a home in Contra Costa County

Mitigation cost doesn't depend on Contra Costa County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find California's state radon program

Contra Costa County's test kit answers today; a Airthings radon monitor keeps answering for Contra Costa County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. Code RADONZONE-10OFF takes 10% off at checkout.

More on how this number was built

The methodology guide shows exactly which EPA and Census sources feed Contra Costa County's numbers. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.