Is Napa County, CA a radon risk area?

EPA places Napa 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, Napa County ranks higher in radon potential than 27% of rated counties (the 27th percentile).

Where Napa County sits nationally

About 132,727 people live in Napa County, California, spread over 751.976 square miles. Averaged across its land area, Napa County runs about 177 people per square mile. Rank 182 by population within its EPA zone puts Napa County at the 82nd percentile among zone peers. Geologically, Napa County is grouped into EPA Region 9 alongside 94 other counties that share its rock and soil setting.

Zone vs. an actual test

Napa County's Zone 3 prediction undercuts the roughly 1.3 pCi/L most US homes average, nowhere near EPA's 4 pCi/L threshold for recommending mitigation. Even at Zone 3, Napa County homes are not guaranteed a low reading — zone is a county-wide average, and EPA recommends testing every home regardless of zone.

Next steps for a home in Napa County

Nothing about Napa County's EPA zone changes the mitigation number: $800–$2,500 nationally (around $1,200 typical), set by the house's foundation rather than the county it sits in — and only relevant once a test actually clears 4 pCi/L.

Find California's state radon program

Napa County's test kit answers today; a Airthings radon monitor keeps answering for Napa 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

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.