Updated November 2026 · Data: EPA Map of Radon Zones · Sources
Radon is a widespread concern in Montana: 50 of its 57 counties are in EPA Zone 1, the group with the highest predicted indoor radon levels. Find your county below, then learn how to test your home and what to do if the result is high.
Your county’s zone is not your home’s radon level. Homes with high radon are found in every zone. The EPA recommends testing every home, whatever its zone.
Radon zones by county in Montana
The table lists the EPA radon zone for each of the 57 counties in Montana. The zone shows the predicted average indoor radon screening level for homes in that county.
| County | EPA zone | Predicted average indoor level |
|---|---|---|
| Beaverhead | Zone 1 | Above 4 pCi/L |
| Big Horn | Zone 1 | Above 4 pCi/L |
| Blaine | Zone 1 | Above 4 pCi/L |
| Broadwater | Zone 1 | Above 4 pCi/L |
| Carbon | Zone 1 | Above 4 pCi/L |
| Carter | Zone 1 | Above 4 pCi/L |
| Cascade | Zone 1 | Above 4 pCi/L |
| Chouteau | Zone 1 | Above 4 pCi/L |
| Custer | Zone 1 | Above 4 pCi/L |
| Daniels | Zone 1 | Above 4 pCi/L |
| Dawson | Zone 1 | Above 4 pCi/L |
| Deer Lodge | Zone 1 | Above 4 pCi/L |
| Fallon | Zone 1 | Above 4 pCi/L |
| Fergus | Zone 1 | Above 4 pCi/L |
| Flathead | Zone 1 | Above 4 pCi/L |
| Gallatin | Zone 1 | Above 4 pCi/L |
| Garfield | Zone 1 | Above 4 pCi/L |
| Glacier | Zone 1 | Above 4 pCi/L |
| Golden Valley | Zone 2 | 2 to 4 pCi/L |
| Granite | Zone 1 | Above 4 pCi/L |
| Hill | Zone 1 | Above 4 pCi/L |
| Jefferson | Zone 1 | Above 4 pCi/L |
| Judith Basin | Zone 1 | Above 4 pCi/L |
| Lake | Zone 1 | Above 4 pCi/L |
| Lewis and Clark | Zone 1 | Above 4 pCi/L |
| Liberty | Zone 1 | Above 4 pCi/L |
| Lincoln | Zone 1 | Above 4 pCi/L |
| McCone | Zone 1 | Above 4 pCi/L |
| Madison | Zone 1 | Above 4 pCi/L |
| Meagher | Zone 1 | Above 4 pCi/L |
| Mineral | Zone 1 | Above 4 pCi/L |
| Missoula | Zone 1 | Above 4 pCi/L |
| Musselshell | Zone 2 | 2 to 4 pCi/L |
| Park | Zone 1 | Above 4 pCi/L |
| Petroleum | Zone 2 | 2 to 4 pCi/L |
| Phillips | Zone 1 | Above 4 pCi/L |
| Pondera | Zone 1 | Above 4 pCi/L |
| Powder River | Zone 1 | Above 4 pCi/L |
| Powell | Zone 1 | Above 4 pCi/L |
| Prairie | Zone 1 | Above 4 pCi/L |
| Ravalli | Zone 1 | Above 4 pCi/L |
| Richland | Zone 1 | Above 4 pCi/L |
| Roosevelt | Zone 1 | Above 4 pCi/L |
| Rosebud | Zone 1 | Above 4 pCi/L |
| Sanders | Zone 1 | Above 4 pCi/L |
| Sheridan | Zone 1 | Above 4 pCi/L |
| Silver Bow | Zone 1 | Above 4 pCi/L |
| Stillwater | Zone 1 | Above 4 pCi/L |
| Sweet Grass | Zone 2 | 2 to 4 pCi/L |
| Teton | Zone 1 | Above 4 pCi/L |
| Toole | Zone 1 | Above 4 pCi/L |
| Treasure | Zone 2 | 2 to 4 pCi/L |
| Valley | Zone 1 | Above 4 pCi/L |
| Wheatland | Zone 2 | 2 to 4 pCi/L |
| Wibaux | Zone 1 | Above 4 pCi/L |
| Yellowstone | Zone 2 | 2 to 4 pCi/L |
| Yellowstone National Park, MT | Zone 1 | Above 4 pCi/L |
What the EPA radon zones mean
- Zone 1 (highest potential): predicted average indoor screening level greater than 4 pCi/L. In Montana: 50 counties (88%).
- Zone 2 (moderate potential): predicted average between 2 and 4 pCi/L. In Montana: 7 counties (12%).
- Zone 3 (low potential): predicted average below 2 pCi/L. In Montana: 0 counties (0%).
The EPA built the map from indoor radon measurements, geology, soil permeability and type of house foundation. It was made to help governments and builders target resources, not to decide whether a single home needs testing.
Montana counties with the highest radon potential
50 counties in Montana are in Zone 1: Beaverhead, Big Horn, Blaine, Broadwater, Carbon, Carter, Cascade, Chouteau, Custer, Daniels, Dawson, Deer Lodge, Fallon, Fergus, Flathead, Gallatin, Garfield, Glacier, Granite, Hill, Jefferson, Judith Basin, Lake, Lewis and Clark, Liberty, Lincoln, McCone, Madison, Meagher, Mineral, Missoula, Park, Phillips, Pondera, Powder River, Powell, Prairie, Ravalli, Richland, Roosevelt, Rosebud, Sanders, Sheridan, Silver Bow, Stillwater, Teton, Toole, Valley, Wibaux, Yellowstone National Park, MT.
How to test for radon in Montana
Testing is the only way to know your home’s radon level. Short-term test kits (2 to 90 days) are sold at hardware stores and online, and some state programs offer kits at low cost; check with the Montana DEQ Radon Program. Test in the lowest lived-in level of the home, with windows and doors closed, and follow the kit instructions.
- Do-it-yourself kits: a simple, low-cost first step for your own home.
- Long-term tests (more than 90 days): give a better picture of your home’s year-round average.
- Certified professionals: look for certification from the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB), and ask the state program whether Montana requires a state license.
- Buying or selling a home: ask for a radon test as part of the home inspection.
If your radon level is high
The EPA recommends fixing your home if the radon level is 4 pCi/L or higher, and considering a fix between 2 and 4 pCi/L. The most common fix is a radon mitigation system (sub-slab depressurization), which pulls radon from under the foundation and vents it outside, above the roof line.
- The EPA says most homes can be fixed for about the same cost as other common home repairs; the price depends on the size and design of the home.
- Get estimates from more than one qualified, certified mitigation contractor. See our radon mitigation cost guide for typical prices.
- Test again after the system is installed, and retest every few years.
Montana radon program
Radon rules and programs in Montana
- Home sales: Montana requires a general radon warning statement and disclosure of known radon test results to buyers of real property.
Contact
The Montana DEQ Radon Program answers questions from residents about radon testing, results and certified professionals.
- Phone: 1-800-546-0483
- Website: deq.mt.gov/energy/programs/radon
Contact details as listed by the state radon program directory. Please confirm on the program’s website before calling.
Frequently asked questions
Is radon a problem in Montana?
50 of Montana’s 57 counties are in EPA Zone 1, where the predicted average indoor radon level is above 4 pCi/L, and 7 are in Zone 2 (2 to 4 pCi/L). High levels can be found in any home, so testing is the only way to know.
Which Montana counties have the highest radon potential?
The 50 counties in EPA Zone 1 have the highest predicted average levels. You can find them in the table above and in the Zone 1 list.
What radon level is safe?
There is no known safe level of radon. The EPA recommends fixing your home at 4 pCi/L or higher and considering a fix between 2 and 4 pCi/L.
Who can I contact about radon in Montana?
The Montana DEQ Radon Program answers questions about testing, results and certified professionals. Phone: 1-800-546-0483.
Sources
- SOSRadon.org (CRCPD/EPA): Montana state radon program fact sheet (2022)
- U.S. EPA: EPA Map of Radon Zones (county data file, June 2024)
- U.S. EPA: A Citizen’s Guide to Radon (checked November 2026)
- U.S. EPA: How much can a radon mitigation system cost? (updated September 16, 2026)
- Montana DEQ Radon Program (checked November 2026)
- SOSRadon.org (CRCPD): State radon contacts (checked November 2026)