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Radon Levels in Flathead County, Montana

EPA puts Flathead County in Zone 1, its highest radon zone, and 37.8% of its 891 reported radon tests (2008 to 2017, laboratory totals) were 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

    37.8%

    7.7 pts below MT

    Flathead
    37.8%
    Montana
    45.5%

    891 tests, 2008 to 2017

  • Median result

    3.0pCi/L

    Below action level

    Flathead
    3.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    891

    5th of 23 by tests

    Flathead
    891
    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

    22 of 23

    Lowest quarter

    Flathead
    37.8%
    Richland
    78.6%

    By share, of 23 with 30+ tests

Read a result for Flathead County

Starts at Flathead County's median, 3.0 pCi/L

3.0pCi/L

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

EPA’s reading of 3.0 pCi/L

Between 2 and 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 between 2 and 4 pCi/L. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. Flathead County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Flathead County, At a glance

Flathead County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Flathead 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 Flathead County, 2008 to 2017, laboratory

Reported results by concentration band, Flathead County, 2008 to 2017, laboratory
Under 2 pCi/L: 33%2 to 4 pCi/L: 29%4 pCi/L and above: 38%

Of 891 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Flathead County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L29833.4%
2 to 4 pCi/L25628.7%
4 pCi/L and above33737.8%
What these figures count

These are the 891 tests reported to CDC for Flathead 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

Flathead County, Compared

Flathead County radon figures, compared

Calculated by Radon411

In Flathead County, about 4 in 10 reported tests over the 2008 to 2017 laboratory totals (337 of 891) were at or above EPA's action level. Against the national pool of 35.7%, the gap is 2.1 points higher. Across 51 Montana counties pooled together, 45.5% of tests reached that level, so Flathead County runs 7.7 points below its state.

Of 5 neighbours with a figure, all 5 reported a larger share at or above 4 pCi/L; the spread is 40.3% (Lake County) to 61.8% (Lewis and Clark County). By share, Flathead County is 22nd of 23 Montana counties with 30 or more tests for the same period.

Half the tests came in under 3.0 pCi/L and half over it; the single highest reached 209.6 pCi/L. Below the action level, 298 tests (33.4%) came in under 2 pCi/L, against 29.2% statewide, and 256 tests (28.7%) fell between 2 and 4, against 25.2% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 37.8% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 26 tests taken after mitigation in Flathead County for the same period: tests in homes where a fix was already in.

CDC's ten-year totals for Flathead County went from 856 tests (2005 to 2014) to 891 (2008 to 2017), the share going from 39.3% to 37.8%.

Flathead County against Montana, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Flathead County
Flathead County89137.8%3.0 pCi/LThis county
Montana, pooled51 counties, pooled9,89345.5%Not pooled+7.7 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−2.1 points
Richland Countyhighest in the state4278.6%5.9 pCi/L+40.8 points
Cascade Countylowest in the state23529.4%1.7 pCi/L−8.4 points
Lewis and Clark County, MTborders it46861.8%5.1 pCi/L+24.0 points
Powell County, MTborders it2958.6%4.2 pCi/L+20.8 points
Lincoln County, MTborders it7451.4%4.0 pCi/L+13.6 points
Sanders County, MTborders it3850.0%4.0 pCi/L+12.2 points
Missoula County, MTborders it2,61344.6%3.4 pCi/L+6.8 points
Lake County, MTborders it14940.3%2.9 pCi/L+2.5 points
Teton County, MTborders it728.6%3.1 pCi/L−9.2 points
Glacier County, MTborders it714.3%2.6 pCi/L−23.5 points
Pondera County, MTborders it50.0%1.1 pCi/L−37.8 points
Where Flathead County's reported results fell, 2008 to 2017, laboratory, beside Montana and the United States
ResultFlathead County testsFlathead County shareMontana shareU.S. shareAgainst Montana
Under 2 pCi/L29833.4%29.2%39.6%+4.2 points
2 to 4 pCi/L25628.7%25.2%24.7%+3.5 points
4 pCi/L and above33737.8%45.5%35.7%−7.7 points
Flathead 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
Lincoln County13 of 237451.4%4.0 pCi/L
Madison County14 of 235850.0%3.9 pCi/L
Sanders County14 of 233850.0%4.0 pCi/L
Dawson County16 of 235947.5%3.5 pCi/L
Missoula County17 of 232,61344.6%3.4 pCi/L
Ravalli County18 of 231,09044.6%3.5 pCi/L
Gallatin County19 of 231,60042.8%3.2 pCi/L
Yellowstone County20 of 231,52042.4%3.4 pCi/L
Lake County21 of 2314940.3%2.9 pCi/L
Flathead County22 of 2389137.8%3.0 pCi/L
Cascade County23 of 2323529.4%1.7 pCi/L
Flathead County ten-year laboratory totals, against Montana's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestMontana, pooledChange from the row before
2008 to 201789137.8%3.0 pCi/L209.6 pCi/L45.5%−0.4 points
2007 to 201692138.2%2.9 pCi/L147.9 pCi/L46.6%+0.2 points
2006 to 201592438.0%2.8 pCi/L147.9 pCi/L47.4%−1.3 points
2005 to 201485639.3%2.9 pCi/L188.4 pCi/L48.0%First on file

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

Flathead County, Geology

The ground under Flathead County

Predicted

USGS's geologic map puts all of Flathead 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 Flathead 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.

Flathead County beside the ground around it

Flathead County has more high-potential ground than Montana as a whole: 100% of its land against 78.5%. Of the 9 counties that border it, none has as large a share of high-potential ground; Lake County comes nearest at 100%.

Share of land by USGS radon potential: Flathead County, the 9 counties bordering it and Montana
AreaEPA zoneLow potentialModerate potentialHigh potential
Flathead CountyZone 1NoneNone100%
Lake CountyZone 1NoneNone100%
Glacier CountyZone 1NoneNone100%
Lincoln CountyZone 1NoneNone100%
Sanders CountyZone 1NoneNone100%
Pondera CountyZone 1NoneNone100%
Missoula CountyZone 1NoneNone100%
Teton CountyZone 1NoneNone100%
Powell CountyZone 1NoneNone100%
Lewis and Clark CountyZone 1NoneNone100%
Montana56 counties, pooled by land areaVariesNone21.5%78.5%

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

Local help

Flathead County, Local resources

Radon testing and mitigation in Flathead 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

Flathead County, Questions

Questions about radon in Flathead County

Are radon levels high in Flathead County?

Over the 2008 to 2017 laboratory totals, 37.8% of the radon tests reported to CDC for Flathead County, Montana were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.0 pCi/L. Of 23 Montana counties with 30 or more tests for the same period, 21 reported a higher share; pooled, the state comes to 45.5%.

Which counties border Flathead County?

Flathead County borders 9 counties: Lake County, Glacier County, Lincoln County, Sanders County, Pondera County, Missoula County, Teton County, Powell County and Lewis and Clark County.

Products

Flathead County, Products

Testing a home in Flathead County

In Flathead County, 37.8% of 891 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 Flathead 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

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

Explore

Flathead County, Bordering counties

Counties bordering Flathead County

Calculated by Radon411

Flathead County, Cities and towns

Cities and towns in Flathead County

Flathead County is growing: the Census Bureau puts it at 115,429 residents on July 1, 2025, up 10.1% from 104,873 in 2020. That works out to 23 people per square mile of land. Its largest town, Kalispell, has 31,851 residents, 27.6% of the total. Together its 3 incorporated places hold 40.7% of those residents; the other 59.3% live outside any of them. CDC's 891 reported tests for 2008 to 2017 work out to 7.7 per 1,000 residents (2025 estimate), close to Montana's 8.6.

From Kalispell to Columbia Falls, CDC publishes no figure by town: each reads Flathead County's.

The cities and towns in Flathead County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Kalispell31,851+26.1%27.6%
Whitefish9,358+18.7%8.1%
Columbia Falls5,753+6.6%5.0%

3 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).

Flathead County, Homes

Homes in Flathead County

Of Flathead County's 50,948 homes, 73% are detached houses, against 69% across Montana; 9% are mobile homes. Much of Flathead County is recent building: 33% of its homes have gone up since 2000 and the middle year built is 1989, against 1980 for Montana. 74% 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 in Flathead County, Montana, 2019 to 2023
HomesFlathead CountyMontana
Housing units50,948522,939
Detached houses73%69%
In buildings of 5 or more8%9%
Mobile homes9%10%
Built before 196016%26%
Built since 200033%27%
Middle year built19891980
Owner-occupied74%69%

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