What EPA's radon map says about Stillwater County

Stillwater County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Nationally, Stillwater County ranks higher in radon potential than 75% of rated counties (the 75th percentile).

Where Stillwater County sits nationally

About 9,450 people live in Stillwater County, Montana, spread over 1,796.533 square miles. Population density there runs about 5 people per square mile. Among counties sharing Stillwater County's EPA zone, it outranks 27% of them by population. Geologically, Stillwater County is grouped into EPA Region 8 alongside 290 other counties that share its rock and soil setting.

Zone vs. an actual test

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Stillwater County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. Some Stillwater 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.

Testing and mitigation in Stillwater County

Mitigation cost doesn't depend on Stillwater County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find Montana's state radon program

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

More on how this number was built

The methodology guide shows exactly which EPA and Census sources feed Stillwater County's numbers. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.