Is Sierra County, CA a radon risk area?

EPA places Sierra County in Zone 2 - Moderate Potential — a predicted county-wide average indoor radon level between 2 and 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Rank it against the rest of the country and Sierra County lands in the 35th percentile for radon potential.

How Sierra County compares

About 3,113 people live in Sierra County, California, spread over 953.168 square miles. That works out to roughly 3 people per square mile. Rank 983 by population within its EPA zone puts Sierra County at the 5th percentile among zone peers. EPA's geology-based Region 9 groups Sierra County with 94 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Sierra County's predicted average falls between EPA's 4 pCi/L action level and the national indoor average of about 1.3 pCi/L (outdoor air runs roughly 0.4 pCi/L). Sierra County's Zone 2 rating sits in the middle nationally; EPA's guidance is unchanged by that — test every home regardless of its zone.

Next steps for a home in Sierra County

Nothing about Sierra 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

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

Sierra County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. Whatever zone Sierra County carries, the zone explainer covers what it does and doesn't tell you about any one house.