Radon in Montana: radon map by county
EPA's Map of Radon Zones puts 49 of Montana's 56 counties in Zone 1 (predicted average indoor radon above 4 pCi/L), 7 in Zone 2 and 0 in Zone 3. Of 6,612 building tests that radon laboratories reported for 2008 to 2017, 45.6% were at or above EPA's 4 pCi/L action level, against 36.5% nationally (number 8 of 42 states with 1,000+ tests). The highest county share is Richland County (78.6% of 29 tests).
Radon by county in Montana
| County | EPA zone | Lab tests | Median | 4+ pCi/L |
|---|---|---|---|---|
| Beaverhead County | Zone 1 | 26 | 5.0 | 59.5% |
| Big Horn County | Zone 1 | 10 | 2.0 | 28.6% |
| Blaine County | Zone 1 | 6 | 1.2 | 6.7% |
| Broadwater County | Zone 1 | 23 | 4.1 | 56% |
| Carbon County | Zone 1 | 34 | 4.2 | 52.2% |
| Carter County | Zone 1 | n/a | n/a | n/a |
| Cascade County | Zone 1 | 136 | 1.7 | 29.4% |
| Chouteau County | Zone 1 | 7 | 5.5 | 54.5% |
| Custer County | Zone 1 | 19 | 3.4 | 40.7% |
| Daniels County | Zone 1 | 2 | 2.5 | 0% |
| Dawson County | Zone 1 | 46 | 3.5 | 47.5% |
| Deer Lodge County | Zone 1 | 10 | 3.4 | 30.8% |
| Fallon County | Zone 1 | 4 | 4.7 | 62.5% |
| Fergus County | Zone 1 | 57 | 7.2 | 72.4% |
| Flathead County | Zone 1 | 575 | 3.0 | 37.8% |
| Gallatin County | Zone 1 | 1,295 | 3.2 | 42.8% |
| Garfield County | Zone 1 | n/a | n/a | n/a |
| Glacier County | Zone 1 | 5 | 2.6 | 14.3% |
| Golden Valley County | Zone 2 | n/a | n/a | n/a |
| Granite County | Zone 1 | 7 | 2.1 | 25% |
| Hill County | Zone 1 | 20 | 2.1 | 14.8% |
| Jefferson County | Zone 1 | 47 | 6.3 | 71% |
| Judith Basin County | Zone 1 | 26 | 5.9 | 64.3% |
| Lake County | Zone 1 | 90 | 2.9 | 40.3% |
| Lewis and Clark County | Zone 1 | 333 | 5.1 | 61.8% |
| Liberty County | Zone 1 | 1 | 3.8 | 33.3% |
| Lincoln County | Zone 1 | 51 | 4.0 | 51.4% |
| Madison County | Zone 1 | 41 | 3.9 | 50% |
| McCone County | Zone 1 | 2 | 1.5 | 0% |
| Meagher County | Zone 1 | 1 | 3.6 | 0% |
| Mineral County | Zone 1 | 18 | 5.1 | 56.7% |
| Missoula County | Zone 1 | 1,441 | 3.4 | 44.6% |
| Musselshell County | Zone 2 | 14 | 2.6 | 29.4% |
| Park County | Zone 1 | 100 | 5.3 | 60% |
| Petroleum County | Zone 2 | n/a | n/a | n/a |
| Phillips County | Zone 1 | n/a | n/a | n/a |
| Pondera County | Zone 1 | 5 | 1.1 | 0% |
| Powder River County | Zone 1 | 7 | 4.5 | 62.5% |
| Powell County | Zone 1 | 17 | 4.2 | 58.6% |
| Prairie County | Zone 1 | 3 | 7.6 | 66.7% |
| Ravalli County | Zone 1 | 577 | 3.5 | 44.6% |
| Richland County | Zone 1 | 29 | 5.9 | 78.6% |
| Roosevelt County | Zone 1 | 12 | 8.7 | 93.8% |
| Rosebud County | Zone 1 | 4 | 2.9 | 40% |
| Sanders County | Zone 1 | 24 | 4.0 | 50% |
| Sheridan County | Zone 1 | 59 | 6.5 | 75.7% |
| Silver Bow County | Zone 1 | 106 | 6.0 | 70.1% |
| Stillwater County | Zone 1 | 40 | 4.8 | 55.9% |
| Sweet Grass County | Zone 2 | 2 | 11.9 | 88.9% |
| Teton County | Zone 1 | 7 | 3.1 | 28.6% |
| Toole County | Zone 1 | 8 | 3.7 | 45.5% |
| Treasure County | Zone 2 | 1 | 7.5 | 100% |
| Valley County | Zone 1 | 16 | 4.5 | 54.2% |
| Wheatland County | Zone 2 | 1 | 12.8 | 100% |
| Wibaux County | Zone 1 | 4 | 3.0 | 50% |
| Yellowstone County | Zone 2 | 1,243 | 3.4 | 42.4% |
Lab tests: buildings tested by radon laboratories, 2008 to 2017; median in pCi/L; “4+ pCi/L” is the share of tests at or above EPA's action level. n/a: no value on file (no tests, or too few to report).
What the zones mean
Zone 1 counties have a predicted average indoor radon screening level above 4 pCi/L, Zone 2 from 2 to 4 pCi/L and Zone 3 below 2 pCi/L. The prediction is from 1993 indoor measurements, geology, aerial radioactivity, soils and foundation types. EPA: “This map should not be used to determine if individual homes need to be tested.” EPA recommends fixing a home at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of exposure.
Radon in Montana: questions
What radon zone is Montana?
EPA's 1993 map does not give states one zone; it zones each county. In Montana, 49 counties are Zone 1, 7 Zone 2 and 0 Zone 3. EPA says the map "should not be used to determine if individual homes need to be tested".
Which Montana counties have the highest radon levels?
By the share of laboratory tests at or above 4 pCi/L for 2008 to 2017 (counties with 20 or more tests): Richland County 78.6% (median 5.9 pCi/L, 29 tests); Sheridan County 75.7% (median 6.5 pCi/L, 59 tests); Fergus County 72.4% (median 7.2 pCi/L, 57 tests); Jefferson County 71% (median 6.3 pCi/L, 47 tests); Silver Bow County 70.1% (median 6.0 pCi/L, 106 tests).
How common is high radon in Montana?
45.6% of the 6,612 tests in the laboratory data for Montana (2008 to 2017) were at or above 4 pCi/L; the national share was 36.5%. Buildings are tested by choice, often because of a sale or a concern, so the share describes the tests on file, not every home.
Should I test for radon in Montana?
Yes. EPA recommends testing every home no matter where it is, fixing it at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of radon exposure.
States bordering Montana
Other free hazard lookups
- Radon by county and ZIP codeEPA radon zone and the radon test results on file for any county, ZIP code or address.
- Fault lines near an addressUSGS-mapped earthquake faults within 5 miles, their age and slip rate, and the USGS seismic hazard value.
- Power lines near an addressHigh-voltage transmission lines within 1 mile from federal maps, with voltage and distance.
- Flood zone lookupThe FEMA flood zone mapped at any US address. Not an official flood zone determination.
- Superfund sites near an addressThe nearest EPA Superfund (NPL) sites and their status.
All of these, plus wetlands, FEMA hazard ratings and the federal contamination lists, are in the Environmental Records Report for any US address ($29).
Superfund sites in Montana · Radon map of every state
Data compiled 2026-10-20. County-level screening information, not a radon measurement of any building.