Is Lemhi County, ID a radon risk area?

EPA places Lemhi County in Zone 1 - Highest Potential — a predicted county-wide average indoor radon level above 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Nationally that's the 74th percentile for radon potential — Lemhi County outranks 74% of every county EPA has rated.

How Lemhi County compares

About 8,397 people live in Lemhi County, Idaho, spread over 4,563.704 square miles. That works out to roughly 2 people per square mile. Rank 812 by population within its EPA zone puts Lemhi County at the 24th percentile among zone peers. EPA's geology-based Region 10 groups Lemhi County with 141 other counties nationally sharing its underlying rock and soil profile.

What this actually means for a house in Lemhi County

Fixing a home becomes an EPA recommendation past 4 pCi/L, and that is precisely the side of the line Lemhi County's Zone 1 prediction lands on, well above the roughly 1.3 pCi/L most US homes average. That doesn't mean every Lemhi County house tests high — a zone is a county-wide prediction, never a single-home reading — but it's a reason to test sooner rather than let it wait.

If Lemhi County tests above the action level

Nothing about Lemhi 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 Idaho's state radon program

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

Lemhi County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.