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

EPA's own radon map rates Granite County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Nationally that's the 69th percentile for radon potential — Granite County outranks 69% of every county EPA has rated.

How Granite County compares

Granite County, Montana is home to about 3,602 people across 1,727.162 square miles. Averaged across its land area, Granite County runs about 2 people per square mile. By population, Granite County is the 965th largest of every county sharing its EPA zone — the 9th percentile by population within that zone. Geologically, Granite 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 Granite County

Where the national indoor average runs about 1.3 pCi/L, Granite County's Zone 1 rating puts its predicted county average above EPA's 4 pCi/L action level itself. Being in Zone 1 raises the odds Granite 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.

Testing and mitigation in Granite County

For a Granite 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 Granite County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Granite County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. Use code RADONZONE-10OFF at checkout for 10% off.

Where Granite County's figures come from

The methodology guide shows exactly which EPA and Census sources feed Granite County's numbers. Whatever zone Granite County carries, the zone explainer covers what it does and doesn't tell you about any one house.