Custer County, ID: EPA's radon zone, explained

EPA's own radon map rates Custer County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Nationally that's the 70th percentile for radon potential — Custer County outranks 70% of every county EPA has rated.

Custer County by the numbers

Custer County, Idaho is home to about 4,597 people across 4,922.177 square miles. That works out to roughly 1 people per square mile. By population, Custer County is the 935th largest of every county sharing its EPA zone — the 12th percentile by population within that zone. Custer County falls in EPA Radon Region 10, one of 142 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Custer County's predicted average sits above EPA's own action level of 4 pCi/L — the concentration at which EPA recommends fixing a home — versus a national average around 1.3 pCi/L indoors and 0.4 pCi/L outdoors. That doesn't mean every Custer 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 Custer County tests above the action level

For a Custer County homeowner, the order is simple: test first, since a kit costs a small fraction of what mitigation would. Only if the result clears 4 pCi/L does a system — typically $800–$2,500, about $1,200 for a common install — become worth pricing out.

Find Idaho's state radon program

For Custer County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Custer County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.

Where Custer County's figures come from

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.