What EPA's radon map says about Carter County

EPA's own radon map rates Carter County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Nationally that's the 66th percentile for radon potential — Carter County outranks 66% of every county EPA has rated.

How Carter County compares

About 1,399 people live in Carter County, Montana, spread over 3,340.477 square miles. Population density there runs about 0 people per square mile. Among counties sharing Carter County's EPA zone, it outranks 1% of them by population. Geologically, Carter County is grouped into EPA Region 8 alongside 290 other counties that share its rock and soil setting.

Why the zone isn't the answer by itself

Fixing a home becomes an EPA recommendation past 4 pCi/L, and that is precisely the side of the line Carter County's Zone 1 prediction lands on, well above the roughly 1.3 pCi/L most US homes average. EPA's guidance doesn't change because of a Zone 1 label: test the house. What changes is the reason to not put it off in Carter County specifically.

If Carter County tests above the action level

Mitigation cost doesn't depend on Carter 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

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

Carter 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.