Is Valley County, MT a radon risk area?

EPA's own radon map rates Valley County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Rank it against the rest of the country and Valley County lands in the 73rd percentile for radon potential.

Where Valley County sits nationally

About 7,434 people live in Valley County, Montana, spread over 4,926.115 square miles. Population density there runs about 2 people per square mile. Rank 832 by population within its EPA zone puts Valley County at the 22nd percentile among zone peers. EPA's geology-based Region 8 groups Valley County with 290 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Valley County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. Being in Zone 1 raises the odds Valley County homes test above the action level, but it does not mean any specific house does — EPA recommends every home be tested, in every zone, because the only way to know is to test.

If Valley County tests above the action level

Nothing about Valley 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 Montana's state radon program

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

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. Whatever zone Valley County carries, the zone explainer covers what it does and doesn't tell you about any one house.