Long-term kit
RL416 long-term radon test kit, lab fee included
- Lab fee included
Method: Alpha track
In home: 90 days to 12 months
Locations and reports
Radon Levels in Custer County, Montana
EPA puts Custer County in Zone 1, its highest radon zone, and only 27 radon tests from it are on file with CDC (2008 to 2017, laboratory totals). Homes with elevated levels occur in all three zones, so only a test measures yours.
Data checked By Radon411Methodology
Share at 4+ pCi/L
38.9%
9.1 pts below MT
- Custer
- 38.9%
- Montana
- 48.0%
36 tests, 2005 to 2014
Median result
3.3pCi/L
Below action level
- Custer
- 3.3 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2005 to 2014
Tests reported
36
24th of 24 by tests
- Custer
- 36
- Missoula
- 1,903
Reported to CDC before any fix, 2005 to 2014
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
21 of 24
Lowest quarter
- Custer
- 38.9%
- Richland
- 76.7%
By share, of 24 with 30+ tests
Read a result for Custer County
Starts at Custer County's median, 3.3 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 3.3 pCi/L
Between 2 and 4 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is between 2 and 4 pCi/L. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. Custer County's median reported result, 2005 to 2014.
The data
Custer County, At a glance
Custer County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Custer County in
USGS geologic potential: High across 95% of the county's area
Predicted: EPA Map of Radon Zones
Measured: tests reported to CDC
Radon tests reported for Custer County, 2008 to 2017, laboratory
CDC holds 27 reported tests for Custer County over the 2008 to 2017 laboratory totals, with a median of 3.4 pCi/L: too few to describe a county (Radon411 reads a share from 30 tests up). Custer County beside the counties around it is how this page reads it.
What these figures count
Only 27 tests were reported to CDC for Custer County over 2008 to 2017 (laboratory totals), and tests are often run on homes being sold or already suspected of a problem: so few describe neither the county nor this house.
Measured: CDC tracking data. Figure above: 2008 to 2017, laboratory. Measured data period: 2005 to 2017 (every CDC figure on file).
Custer County, Compared
Custer County radon figures, compared
Calculated by Radon411In Custer County, about 4 in 10 reported tests over the 2005 to 2014 laboratory totals (14 of 36) were at or above EPA's action level. Against the national pool of 36.0%, the gap is 2.9 points higher. Across 52 Montana counties pooled together, 48.0% of tests reached that level, so Custer County runs 9.1 points below its state.
The newest figure, over the 2008 to 2017 laboratory totals, counts only 27 tests (40.7% at or above 4 pCi/L), too few to compare, so this reading uses the 36-test figure.
By share, Custer County is 21st of 24 Montana counties with 30 or more tests for the same period.
Half the tests came in under 3.3 pCi/L and half over it; the single highest reached 12.3 pCi/L. Below the action level, 9 tests (25.0%) came in under 2 pCi/L, against 27.4% statewide, and 13 tests (36.1%) fell between 2 and 4, against 24.6% statewide.
Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 38.9% share point the same way here. USGS geology rates 95% of the county's land high radon potential.
CDC's ten-year totals for Custer County went from 36 tests (2005 to 2014) to 27 (2008 to 2017), the share going from 38.9% to 40.7%.
Pooled shares: Radon411's sum of CDC's county counts for one period.
Custer County, Geology
The ground under Custer County
PredictedMost of the ground under Custer County is one class on USGS's geologic map: 95.0% of its land is high radon potential, with 5.0% moderate. 95.0% of it lies in the area the map calls Great Plains, to which USGS gives a radon index of 12, in the high band (12 to 17).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Great Plains | Tertiary sandstone, siltstone, shale, coal. | High | 12 | 95.0% |
| Eastern Great Plains | Cretaceous marine shale and marine and nearshore sandstone, siltstone. | Moderate | 11 | 5.0% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Custer County beside the ground around it
Custer County has more high-potential ground than Montana as a whole: 95.0% of its land against 78.5%. Of the 6 counties that border it, 1 has more high-potential ground, Prairie County the most at 98.7%.
The map rates the ground, not the air in any home.
Local help
Custer County, Local resources
Radon testing and mitigation in Custer 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
Custer County, Questions
Questions about radon in Custer County
Are radon levels high in Custer County?
Over the 2008 to 2017 laboratory totals, 40.7% of the radon tests reported to CDC for Custer County, Montana were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.4 pCi/L. Of 24 Montana counties with 30 or more tests for the same period, 20 reported a higher share; pooled, the state comes to 48.0%.
Which counties border Custer County?
Custer County borders 6 counties: Prairie County, Fallon County, Rosebud County, Powder River County, Carter County and Garfield County.
Products
Custer County, Products
Testing a home in Custer County
In Custer County, 38.9% of 36 tests reported to CDC over the 2005 to 2014 laboratory totals reached 4 pCi/L, against 48.0% pooled across Montana. In Zone 1 the year-round level in Custer 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
MonitorAEGTEST
HOUND-1085 palm-sized rechargeable radon detector
- C-NRPP 2024/25: Not included
- Battery
- Alarm
- Screen
Method: Semiconductor sensor
First reading: Within 6 hours
Short-term kit
Radon Short Term Test Kit short-term charcoal radon test kit, lab fee included
- Lab fee included
Method: Charcoal
In home: 2 to 7 days
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Custer County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Custer County
Explore
Custer County, Bordering counties
Counties bordering Custer County
Calculated by Radon411- Radon levels in Fallon CountyEPA zone 1
71.4%at or above 4 pCi/L
Median 5.2 pCi/L · 7 tests · 32.5 points above Custer County
- Radon levels in Powder River CountyEPA zone 1
71.4%at or above 4 pCi/L
Median 4.5 pCi/L · 7 tests · 32.5 points above Custer County
- Radon levels in Rosebud CountyEPA zone 1
55.6%at or above 4 pCi/L
Median 5.9 pCi/L · 9 tests · 16.7 points above Custer County
- Radon levels in Prairie CountyEPA zone 1
28.6%at or above 4 pCi/L
Median 3.3 pCi/L · 7 tests · 10.3 points below Custer County
- Radon levels in Carter CountyEPA zone 1
No CDC figure for 2005 to 2014; its page reads it against its own neighbours.
- Radon levels in Garfield CountyEPA zone 1
No CDC figure for 2005 to 2014; its page reads it against its own neighbours.
Custer County, Cities and towns
Cities and towns in Custer County
The Census Bureau estimates 12,034 residents in Custer County for July 1, 2025, 174 more than in 2020 (+1.5%). Spread over its land, that is 3.2 people to the square mile. Miles City alone is home to 8,450 of them, 70.2% of the total. Together its 2 incorporated places hold 70.4% of those residents; the other 29.6% live outside any of them. Reported testing is thin here: 36 tests over 2005 to 2014, or 3.0 for every 1,000 residents (2025 estimate), where Montana as a whole comes to 6.7.
From Miles City to Ismay, CDC publishes no figure by town: each reads Custer County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Miles City | 8,450 | +1.3% | 70.2% |
| Ismay | 21 | Level | 0.2% |
2 incorporated places, 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).
Custer County, Homes
Homes in Custer County
Of Custer County's 5,770 homes, 69% are detached houses, against 69% across Montana; 12% are in buildings of five or more homes. The homes are older than Montana's as a whole: half were built before 1963 (1980 for Montana), and 48% date from before 1960. 68% of occupied homes are lived in by their owner (Montana: 69%); in the rest a tenant can run a test, but a fix is the landlord's to make.
| Homes | Custer County | Montana |
|---|---|---|
| Housing units | 5,770 | 522,939 |
| Detached houses | 69% | 69% |
| In buildings of 5 or more | 12% | 9% |
| Mobile homes | 11% | 10% |
| Built before 1960 | 48% | 26% |
| Built since 2000 | 12% | 27% |
| Middle year built | 1963 | 1980 |
| Owner-occupied | 68% | 69% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.