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

EPA places Custer 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. Rank it against the rest of the country and Custer County lands in the 77th percentile for radon potential.

Where Custer County sits nationally

Custer County, Montana is home to about 11,964 people across 3,783.309 square miles. That works out to roughly 3 people per square mile. By population, Custer County is the 719th largest of every county sharing its EPA zone — the 33rd percentile by population within that zone. EPA's geology-based Region 8 groups Custer County with 290 other counties nationally sharing its underlying rock and soil profile.

What this actually means for a house in Custer County

Custer County carries a Zone 1 label because its predicted average clears 4 pCi/L outright — three-plus times the 0.4 pCi/L typical outdoors and above the 1.3 pCi/L most homes see. Some Custer County homes will test comfortably under the action level despite the county's Zone 1 rating — the only way to know which kind of home you have is an actual test.

Next steps for a home in Custer County

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 Montana'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

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