Big Horn County, MT: EPA's radon zone, explained

EPA's own radon map rates Big Horn County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. That puts Big Horn County in the 78th percentile nationally for radon potential — higher than 78% of the counties EPA has rated.

How Big Horn County compares

Big Horn County, Montana is home to about 12,796 people across 4,997.778 square miles. Population density there runs about 3 people per square mile. By population, Big Horn County is the 703rd largest of every county sharing its EPA zone — the 34th percentile by population within that zone. Big Horn County falls in EPA Radon Region 8, one of 291 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Where the national indoor average runs about 1.3 pCi/L, Big Horn County's Zone 1 rating puts its predicted county average above EPA's 4 pCi/L action level itself. 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 Big Horn County specifically.

Next steps for a home in Big Horn County

For a Big Horn 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 Big Horn County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Big Horn 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 Big Horn County's figures come from

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.