Is Phillips County, MT a radon risk area?

EPA places Phillips 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. Nationally that's the 70th percentile for radon potential — Phillips County outranks 70% of every county EPA has rated.

Phillips County by the numbers

About 4,214 people live in Phillips County, Montana, spread over 5,140.422 square miles. Averaged across its land area, Phillips County runs about 1 people per square mile. Rank 946 by population within its EPA zone puts Phillips County at the 11th percentile among zone peers. Geologically, Phillips County is grouped into EPA Region 8 alongside 290 other counties that share its rock and soil setting.

What this actually means for a house in Phillips County

Phillips 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 Phillips 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 Phillips County tests above the action level

Nothing about Phillips 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

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

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