Is Mariposa County, CA a radon risk area?

Mariposa County is mapped as Zone 2 - Moderate Potential by EPA — meaning the county-wide average indoor level is predicted between 2 and 4 pCi/L. Nationally that's the 44th percentile for radon potential — Mariposa County outranks 44% of every county EPA has rated.

Where Mariposa County sits nationally

About 17,048 people live in Mariposa County, California, spread over 1,448.84 square miles. Population density there runs about 12 people per square mile. Rank 672 by population within its EPA zone puts Mariposa County at the 35th percentile among zone peers. Mariposa County falls in EPA Radon Region 9, one of 95 counties nationally EPA assigns to that same geologic province.

What this actually means for a house in Mariposa County

EPA's Zone 2 rating for Mariposa County sits below its own 4 pCi/L action level but above the national indoor average of about 1.3 pCi/L. A county-wide moderate average, like Mariposa County's, can still hide a specific house well above or below it — testing is what actually answers the question.

Next steps for a home in Mariposa County

Nothing about Mariposa 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

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

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. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.