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Radon Levels in Jefferson County, Montana

EPA puts Jefferson County in Zone 1, its highest radon zone, and high results are common there: 71.0% of the 62 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

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  • Share at 4+ pCi/L

    71.0%

    25.5 pts above MT

    Jefferson
    71.0%
    Montana
    45.5%

    62 tests, 2008 to 2017

  • Median result

    6.3pCi/L

    Action level reached

    Jefferson
    6.3 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    62

    14th of 23 by tests

    Jefferson
    62
    Missoula
    2,613

    Reported to CDC before any fix, 2008 to 2017

  • EPA radon zone

    Zone 1

    Highest potential

    1. Zone 1
    2. 2
    3. 3

    1 highest, 3 lowest

    EPA predicts over 4 pCi/L indoors

  • Rank in Montana

    4 of 23

    Top quarter

    Jefferson
    71.0%
    Richland
    78.6%

    By share, of 23 with 30+ tests

Read a result for Jefferson County

Starts at Jefferson County's median, 6.3 pCi/L

6.3pCi/L

EPA’s own thresholds; the figures are the area’s, never this home’s.

EPA’s reading of 6.3 pCi/L

At or above 4 pCi/L

  1. Under 20 to 1.9 pCi/L
  2. 2 to 4consider a fix
  3. 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. Jefferson County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Jefferson County, At a glance

Jefferson County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Jefferson County in and predicts an average indoor of greater than 4 pCi/L (greater than 150 Bq/m³) across the county. That is one forecast for the whole county, not a reading of any address in it; homes above EPA's turn up in all three zones.

USGS geologic potential: High across 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Jefferson County, 2008 to 2017, laboratory

Reported results by concentration band, Jefferson County, 2008 to 2017, laboratory
Under 2 pCi/L: 18%2 to 4 pCi/L: 11%4 pCi/L and above: 71%

Of 62 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Jefferson County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L1117.7%
2 to 4 pCi/L711.3%
4 pCi/L and above4471.0%
What these figures count

These are the 62 tests reported to CDC for Jefferson 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).

How to read these figures

Jefferson County, Compared

Jefferson County radon figures, compared

Calculated by Radon411

More than half: 71.0% of the 62 radon tests reported to CDC for Jefferson County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 4th of 23 Montana counties with 30 or more tests for the same period. That is 25.5 points above the 45.5% pooled from 51 Montana counties.

The median result, 6.3 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 34.0 pCi/L. The rest split 17.7% under 2 pCi/L and 11.3% from 2 to 4 (Montana: 29.2% and 25.2%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 71.0% share point the same way here.

No bordering county with a figure for the same period reported a higher share; the closest is Silver Bow County at 70.1%, the lowest Gallatin County at 42.8%. 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 Jefferson County went from 54 tests (2005 to 2014) to 62 (2008 to 2017), the share going from 75.9% to 71.0%.

Nationally, every county with a figure for the same period pools to 35.7%; Jefferson County sits 35.3 points above it.

Jefferson County against Montana, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Jefferson County
Jefferson County6271.0%6.3 pCi/LThis county
Montana, pooled51 counties, pooled9,89345.5%Not pooled−25.5 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−35.3 points
Richland Countyhighest in the state4278.6%5.9 pCi/L+7.6 points
Cascade Countylowest in the state23529.4%1.7 pCi/L−41.6 points
Silver Bow County, MTborders it17470.1%6.0 pCi/L−0.9 points
Lewis and Clark County, MTborders it46861.8%5.1 pCi/L−9.2 points
Powell County, MTborders it2958.6%4.2 pCi/L−12.4 points
Broadwater County, MTborders it2556.0%4.1 pCi/L−15.0 points
Madison County, MTborders it5850.0%3.9 pCi/L−21.0 points
Gallatin County, MTborders it1,60042.8%3.2 pCi/L−28.2 points
Deer Lodge County, MTborders it1330.8%3.4 pCi/L−40.2 points
Where Jefferson County's reported results fell, 2008 to 2017, laboratory, beside Montana and the United States
ResultJefferson County testsJefferson County shareMontana shareU.S. shareAgainst Montana
Under 2 pCi/L1117.7%29.2%39.6%−11.5 points
2 to 4 pCi/L711.3%25.2%24.7%−13.9 points
4 pCi/L and above4471.0%45.5%35.7%+25.5 points
Jefferson County ranked among the 23 Montana counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Richland County1 of 234278.6%5.9 pCi/L
Sheridan County2 of 237075.7%6.5 pCi/L
Fergus County3 of 237672.4%7.2 pCi/L
Jefferson County4 of 236271.0%6.3 pCi/L
Silver Bow County5 of 2317470.1%6.0 pCi/L
Judith Basin County6 of 234264.3%5.9 pCi/L
Lewis and Clark County7 of 2346861.8%5.1 pCi/L
Park County8 of 2315060.0%5.3 pCi/L
Beaverhead County9 of 233759.5%5.0 pCi/L
Mineral County10 of 233056.7%5.1 pCi/L
Stillwater County11 of 235955.9%4.8 pCi/L
Jefferson County ten-year laboratory totals, against Montana's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestMontana, pooledChange from the row before
2008 to 20176271.0%6.3 pCi/L34.0 pCi/L45.5%+1.0 points
2007 to 20166070.0%6.3 pCi/L34.0 pCi/L46.6%+3.9 points
2006 to 20156266.1%6.4 pCi/L34.0 pCi/L47.4%−9.8 points
2005 to 20145475.9%6.8 pCi/L34.0 pCi/L48.0%First on file

Pooled shares: Radon411's sum of CDC's county counts for one period.

Jefferson County, Geology

The ground under Jefferson County

Predicted

USGS's geologic map puts all of Jefferson County's mapped land in one class: high radon potential. All of it lies in the area the map calls Northern Rocky Mountains, to which USGS gives a radon index of 13, in the high band (12 to 17).

USGS's geologic areas under Jefferson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Northern Rocky MountainsGranitic, metaigneous, metasedimentary rocks, valley fill colluvium and alluvium.High13100%

Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.

Jefferson County beside the ground around it

Jefferson County has more high-potential ground than Montana as a whole: 100% of its land against 78.5%. Of the 7 counties that border it, none has as large a share of high-potential ground; Broadwater County comes nearest at 100%.

Share of land by USGS radon potential: Jefferson County, the 7 counties bordering it and Montana
AreaEPA zoneLow potentialModerate potentialHigh potential
Jefferson CountyZone 1NoneNone100%
Silver Bow CountyZone 1NoneNone100%
Broadwater CountyZone 1NoneNone100%
Deer Lodge CountyZone 1NoneNone100%
Madison CountyZone 1NoneNone100%
Powell CountyZone 1NoneNone100%
Lewis and Clark CountyZone 1NoneNone100%
Gallatin CountyZone 1None21.0%79.0%
Montana56 counties, pooled by land areaVariesNone21.5%78.5%

The map rates the ground, not the air in any home.

Local help

Jefferson County, Local resources

Radon testing and mitigation in Jefferson County

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

Jefferson County, Questions

Questions about radon in Jefferson County

Are radon levels high in Jefferson County?

Over the 2008 to 2017 laboratory totals, 71.0% of the radon tests reported to CDC for Jefferson County, Montana were at or above 4 pCi/L, and the median reported pre-mitigation result was 6.3 pCi/L. Of 23 Montana counties with 30 or more tests for the same period, 3 reported a higher share; pooled, the state comes to 45.5%.

Which counties border Jefferson County?

Jefferson County borders 7 counties: Silver Bow County, Broadwater County, Deer Lodge County, Madison County, Powell County, Lewis and Clark County and Gallatin County.

Products

Jefferson County, Products

Testing a home in Jefferson County

In Jefferson County, 71.0% of 62 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 Jefferson 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

Jefferson County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Jefferson County

Explore

Jefferson County, Bordering counties

Counties bordering Jefferson County

Calculated by Radon411

Jefferson County, Cities and towns

Cities and towns in Jefferson County

Jefferson County is growing: the Census Bureau puts it at 13,329 residents on July 1, 2025, up 9.8% from 12,141 in 2020. Spread over its land, that is 8.0 people to the square mile. No town dominates: Boulder, the largest, has 1,305 residents, 9.8% of the total. Together its 2 incorporated places hold 18.3% of those residents; the other 81.7% live outside any of them. Reported testing is thin here: 62 tests over 2008 to 2017, or 4.7 for every 1,000 residents (2025 estimate), where Montana as a whole comes to 8.6.

From Boulder to Whitehall, CDC publishes no figure by town: each reads Jefferson County's.

The cities and towns in Jefferson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Boulder1,305+8.3%9.8%
Whitehall1,131+12.2%8.5%

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).

Jefferson County, Homes

Homes in Jefferson County

Jefferson County is a place of houses on their own lots: 84% of its 5,388 homes are detached single-family houses, against 69% across Montana. Much of Jefferson County is recent building: 30% of its homes have gone up since 2000 and the middle year built is 1989, against 1980 for Montana. Owners live in 83% of the occupied homes (Montana: 69%), so for most households a test and a fix are their own to order.

Homes in Jefferson County, Montana, 2019 to 2023
HomesJefferson CountyMontana
Housing units5,388522,939
Detached houses84%69%
In buildings of 5 or more2%9%
Mobile homes10%10%
Built before 196018%26%
Built since 200030%27%
Middle year built19891980
Owner-occupied83%69%

Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.