Long-term kit
RL116 long-term radon test kit
- Lab fee extra
Method: Alpha track
In home: 90 days to 12 months
Locations and reports
Radon Levels in Stillwater County, Montana
EPA puts Stillwater County in Zone 1, its highest radon zone, and high results are common there: 55.9% of the 59 radon tests reported to CDC (2008 to 2017, laboratory totals) came back at or above 4 pCi/L. Homes with elevated levels occur in all three zones, so only a test measures yours.
Data checked By Radon411Methodology
Share at 4+ pCi/L
55.9%
10.4 pts above MT
- Stillwater
- 55.9%
- Montana
- 45.5%
59 tests, 2008 to 2017
Median result
4.8pCi/L
Action level reached
- Stillwater
- 4.8 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2008 to 2017
Tests reported
59
15th of 23 by tests
- Stillwater
- 59
- Missoula
- 2,613
Reported to CDC before any fix, 2008 to 2017
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
EPA predicts over 4 pCi/L indoors
Rank in Montana
11 of 23
Middle half
- Stillwater
- 55.9%
- Richland
- 78.6%
By share, of 23 with 30+ tests
Read a result for Stillwater County
Starts at Stillwater County's median, 4.8 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 4.8 pCi/L
At or above 4 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is at or above 4 pCi/L. EPA recommends fixing a home when the radon level is 4 pCi/L or higher. Stillwater County's median reported result, 2008 to 2017.
The data
Stillwater County, At a glance
Stillwater County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Stillwater County in
USGS geologic potential: Moderate across 100% of the county's area
Predicted: EPA Map of Radon Zones
Measured: tests reported to CDC
Radon tests reported for Stillwater County, 2008 to 2017, laboratory
Under 2 pCi/L: 29%2 to 4 pCi/L: 15%4 pCi/L and above: 56%
Of 59 pre-mitigation tests reported to CDC, not homes in this area.
| Result band | Tests Measured | Share Measured |
|---|---|---|
| Under 2 pCi/L | 17 | 28.8% |
| 2 to 4 pCi/L | 9 | 15.3% |
| 4 pCi/L and above | 33 | 55.9% |
What these figures count
These are the 59 tests reported to CDC for Stillwater County over 2008 to 2017 (laboratory totals), many from homes being sold or already suspected of a problem: they describe the county's reported tests, not this house.
Measured: CDC tracking data. Figure above: 2008 to 2017, laboratory. Measured data period: 2005 to 2017 (every CDC figure on file).
Stillwater County, Compared
Stillwater County radon figures, compared
Calculated by Radon411More than half: 55.9% of the 59 radon tests reported to CDC for Stillwater County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 11th of 23 Montana counties with 30 or more tests for the same period. That is 10.4 points above the 45.5% pooled from 51 Montana counties.
The median result, 4.8 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 38.6 pCi/L. The rest split 28.8% under 2 pCi/L and 15.3% from 2 to 4 (Montana: 29.2% and 25.2%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 55.9% share point the same way here.
1 of its 3 bordering counties with figures ran higher, led by Park County at 60.0%; Yellowstone County reported 42.4%. Statewide for the same period, Richland County reported the highest share (78.6%) and Cascade County the lowest (29.4%).
CDC's ten-year totals for Stillwater County went from 43 tests (2005 to 2014) to 59 (2008 to 2017), the share going from 60.5% to 55.9%.
Nationally, every county with a figure for the same period pools to 35.7%; Stillwater County sits 20.2 points above it. CDC also counts 12 tests taken after mitigation in Stillwater County for the same period: tests in homes where a fix was already in.
Pooled shares: Radon411's sum of CDC's county counts for one period.
Stillwater County, Geology
The ground under Stillwater County
PredictedUSGS's geologic map puts all of Stillwater County's mapped land in one class: moderate radon potential. The map puts 82.5% of Stillwater County in the area it calls Plains and Mountains and 17.5% in Absaroka Volcanic Field. USGS gives both a radon index of 10, in the moderate band (9 to 11).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Plains and Mountains | Marine and continental shale, siltstone, and sandstone of Cretaceous and Tertiary ages, and volcanic rocks. | Moderate | 10 | 82.5% |
| Absaroka Volcanic Field | Intrusive and extrusive volcanic rocks and overlying sediments. | Moderate | 10 | 17.5% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Stillwater County beside the ground around it
Across Montana 78.5% of the land is rated high; none of Stillwater County's is. Of the 5 counties that border it, 4 have more high-potential ground, Carbon County the most at 18.0%.
The map rates the ground, not the air in any home.
Local help
Stillwater County, Local resources
Radon testing and mitigation in Stillwater County
- Radon Control ProgramRadon Control Program, run by Montana DEQ, does not license radon professionals and offers free or discounted test kits, and answers questions about testing and mitigation in Montana.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
Energy Bureau staff manage the radon information toll-free hotline and distribute subsidized radon test kits to Montana residents.
- Compare kits
- See the Montana directoryNo professionals are listed in Montana yet. The state program can point you to registered testers and mitigators.
What Montana's program says
- Phone
- 1-800-546-0483
- Licensing of radon professionals
Montana DEQ does not certify or require proof of certification for the contractors listed.
What it means
Stillwater County, Questions
Questions about radon in Stillwater County
Are radon levels high in Stillwater County?
Over the 2008 to 2017 laboratory totals, 55.9% of the radon tests reported to CDC for Stillwater County, Montana were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.8 pCi/L. Of 23 Montana counties with 30 or more tests for the same period, 10 reported a higher share; pooled, the state comes to 45.5%.
Which counties border Stillwater County?
Stillwater County borders 5 counties: Carbon County, Sweet Grass County, Yellowstone County, Golden Valley County and Park County.
Products
Stillwater County, Products
Testing a home in Stillwater County
In Stillwater County, 55.9% of 59 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 45.5% pooled across Montana. In Zone 1 the year-round level in Stillwater County is the one to know: a long-term test (more than 90 days) or a monitor, with a short-term kit for a first answer.
Test kits and monitors3
See every kit and monitor
Monitor
RadonScan USB-powered radon monitor with a color screen
- C-NRPP 2024/25: Not included
- Plug-in
- Alarm
- Screen
First reading: Within 5 minutes
Short-term kitKidde
442020 short-term charcoal radon test kit
- Lab fee included
Method: Charcoal
In home: 3 days
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Stillwater County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Stillwater County
Explore
Stillwater County, Bordering counties
Counties bordering Stillwater County
Calculated by Radon411- Radon levels in Sweet Grass CountyEPA zone 2
88.9%at or above 4 pCi/L
Median 11.9 pCi/L · 9 tests · 33.0 points above Stillwater County
- Radon levels in Park CountyEPA zone 1
60.0%at or above 4 pCi/L
Median 5.3 pCi/L · 150 tests · 4.1 points above Stillwater County
- Radon levels in Carbon CountyEPA zone 1
52.2%at or above 4 pCi/L
Median 4.2 pCi/L · 46 tests · 3.7 points below Stillwater County
- Radon levels in Yellowstone CountyEPA zone 2
42.4%at or above 4 pCi/L
Median 3.4 pCi/L · 1,520 tests · 13.5 points below Stillwater County
- Radon levels in Golden Valley CountyEPA zone 2
No CDC figure for 2008 to 2017; its page reads it against its own neighbours.
Stillwater County, Cities and towns
Cities and towns in Stillwater County
The Census Bureau estimates 9,445 residents in Stillwater County for July 1, 2025, 447 more than in 2020 (+5.0%). Spread over its land, that is 5.3 people to the square mile. Columbus, its one incorporated place, has 1,964 residents, 20.8% of the total. CDC's 59 reported tests for 2008 to 2017 work out to 6.2 per 1,000 residents (2025 estimate), close to Montana's 8.6.
Columbus is the one incorporated place here; CDC publishes no figure by town, so it reads Stillwater County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Columbus | 1,964 | +5.3% | 20.8% |
One incorporated place, as listed in the U.S. Census Bureau’s 2020 place reference files. Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025).
Stillwater County, Homes
Homes in Stillwater County
Stillwater County is a place of houses on their own lots: 83% of its 4,657 homes are detached single-family houses, against 69% across Montana. Half of its homes were built before 1985, close to Montana's 1980: 25% date from before 1960 and 30% from 2000 or later. Owners live in 81% of the occupied homes (Montana: 69%), so for most households a test and a fix are their own to order.
| Homes | Stillwater County | Montana |
|---|---|---|
| Housing units | 4,657 | 522,939 |
| Detached houses | 83% | 69% |
| In buildings of 5 or more | 2% | 9% |
| Mobile homes | 11% | 10% |
| Built before 1960 | 25% | 26% |
| Built since 2000 | 30% | 27% |
| Middle year built | 1985 | 1980 |
| Owner-occupied | 81% | 69% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.