Montana: 56 counties covered
Of 56 rated Montana counties, 49 are in EPA's highest radon-potential zone (Zone 1) — but every county page links to testing guidance regardless of zone, because a county rating is a county-wide average, not a reading for any one house.
- Yellowstone County
- Gallatin County
- Missoula County
- Flathead County
- Cascade County
- Lewis and Clark County
- Ravalli County
- Silver Bow County
- Lake County
- Lincoln County
- Park County
- Hill County
- Sanders County
- Glacier County
- Jefferson County
- Big Horn County
- Custer County
- Fergus County
- Carbon County
- Richland County
- Roosevelt County
- Beaverhead County
- Deer Lodge County
- Madison County
- Stillwater County
- Dawson County
- Broadwater County
- Rosebud County
- Valley County
- Powell County
- Blaine County
- Teton County
- Pondera County
- Chouteau County
- Musselshell County
- Mineral County
- Toole County
- Phillips County
- Sweet Grass County
- Granite County
- Sheridan County
- Fallon County
- Judith Basin County
- Meagher County
- Wheatland County
- Liberty County
- Powder River County
- McCone County
- Daniels County
- Carter County
- Garfield County
- Prairie County
- Wibaux County
- Golden Valley County
- Treasure County
- Petroleum County