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Radon in Great Falls, Montana

EPA puts Cascade County, where Great Falls is, in Zone 1, its highest radon zone, and 29.4% of its 235 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

    29.4%

    16.1 pts below MT

    Cascade
    29.4%
    Montana
    45.5%

    Cascade, 23rd of 23 in MT, 2008 to 2017

  • Median result

    1.7pCi/L

    Below action level

    Cascade
    1.7 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of Cascade's tests, 2008 to 2017

  • Nearest county

    61.8%

    Lewis and Clark, 58 miles

    Lewis and Clark
    61.8%
    Cascade
    29.4%

    468 tests, 2008 to 2017

  • EPA radon zone

    Zone 1

    Highest potential

    1. Zone 1
    2. 2
    3. 3

    1 highest, 3 lowest

    Cascade, where Great Falls is

  • Tests reported

    235

    In Cascade

    Cascade
    235
    Lewis and Clark
    468

    Reported to CDC before any fix, 2008 to 2017

Read a result for Great Falls

Starts at Cascade County's median, 1.7 pCi/L

1.7pCi/L

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

EPA’s reading of 1.7 pCi/L

Below 2 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 below 2 pCi/L, the level at which EPA says a fix should be considered. Cascade County's median reported result, 2008 to 2017.

Interpret this reading

The data

Great Falls, What is published

What is published for Great Falls

Information:

Every figure for Great Falls is a county's

Radon data is published by county, not by city, so for Great Falls it is Cascade County's: its EPA zone (Zone 1) and the 235 tests reported to CDC for 2008 to 2017, laboratory. Only a test tells you about one home.

Great Falls covers 23 square miles of land, all of it in Cascade County. About 60,208 people live there, 2,617 to the square mile.

Great Falls, Counties

The county Great Falls lies in

Measured

Cascade County's EPA zone and the figures CDC publishes for it, 2008 to 2017, laboratory.

Cascade CountyZone 1Highest predicted potential

29.4%
Reported tests at or above 4 pCi/L
1.7 pCi/L
Median reported result
235 tests
Tests reported to CDC
None
Yearly figures on file

Cascade County ranks 23rd of 23 Montana counties with 30 or more tests for that period, 16.1 points below the 45.5% pooled across Montana's counties, 6.3 points below the national pool of 35.7%.

Measured from the middle of Great Falls, the nearest counties with a CDC figure for 2008 to 2017, laboratory are Lewis and Clark County (58 miles, 61.8% of 468 tests), Judith Basin County (58 miles, 64.3% of 42 tests) and Fergus County (99 miles, 72.4% of 76 tests). Across the 8 nearest, the share at or above 4 pCi/L runs from 40.3% to 72.4%; the farthest of them, Gallatin County, is 136 miles away at 42.8%.

No business in the directory gives Great Falls as its address; the Montana list covers it. Montana's other cities with a page of their own: Bozeman, Kalispell, Billings and Boulder.

The counties around Great Falls

By distance from the middle of Great Falls; counties with 30 or more tests.

Nearest counties to Great Falls with 30 or more tests
CountyDistanceTestsAt or above 4 pCi/LMedian
Lewis and Clark County58 mi46861.8%5.1 pCi/L
Judith Basin County58 mi4264.3%5.9 pCi/L
Fergus County99 mi7672.4%7.2 pCi/L
Jefferson County101 mi6271.0%6.3 pCi/L
Missoula County128 mi2,61344.6%3.4 pCi/L
Silver Bow County128 mi17470.1%6.0 pCi/L
Lake County131 mi14940.3%2.9 pCi/L
Gallatin County136 mi1,60042.8%3.2 pCi/L

Predicted: EPA Map of Radon Zones, by county. Measured: CDC tracking data, by county, 2008 to 2017, laboratory.

Great Falls, Geology

The ground under Great Falls

Predicted

USGS's geologic map puts all of Great Falls' mapped land in one class: high radon potential. The map puts 70.6% of Great Falls in the area it calls Northern Rocky Mountains and 29.4% in Great Plains (Glaciated). USGS gives both a radon index of 13, in the high band (12 to 17).

USGS's geologic areas under Great Falls
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Northern Rocky MountainsGranitic, metaigneous, metasedimentary rocks, valley fill colluvium and alluvium.High1370.6%
Great Plains (Glaciated)Glacial deposits derived from Cretaceous shales containing higher than average amounts of uranium.High1329.4%

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

Great Falls beside its county and Montana

The ground under Great Falls is close to Cascade County's as a whole: 100% rated high against 96.9%. Great Falls has more high-potential ground than Montana as a whole: 100% of its land against 78.5%.

Share of land by USGS radon potential: Great Falls, the county it lies in and Montana
AreaEPA zoneLow potentialModerate potentialHigh potential
Great FallsBy countyNoneNone100%
Cascade CountyZone 1None3.1%96.9%
Montana56 counties, pooled by land areaVariesNone21.5%78.5%

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

Great Falls, Homes

Homes in Great Falls

Of Great Falls' 28,333 homes, 66% are detached houses, against 67% across Cascade County; 17% are in buildings of five or more homes. Half of its homes were built before 1965, close to Cascade County's 1969: 42% date from before 1960 and 12% from 2000 or later. 68% of occupied homes are lived in by their owner (Cascade County: 70%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Great Falls, Cascade County, Montana, 2019 to 2023
HomesGreat FallsCascade CountyMontana
Housing units28,33339,248522,939
Detached houses66%67%69%
In buildings of 5 or more17%13%9%
Mobile homes4%7%10%
Built before 196042%37%26%
Built since 200012%16%27%
Middle year built196519691980
Owner-occupied68%70%69%

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

Local help

Great Falls, Local resources

Local resources for Great Falls

Who to ask in Montana: its radon program, test kits and professionals.

The program's own words on licensing are on the Montana page.

What it means

Great Falls, Questions

Questions about radon in Great Falls

What radon zone is Great Falls in?

Great Falls has no zone of its own: it shares Cascade County's, Zone 1, the map's highest class: a predicted county average above 4 pCi/L.

What are radon levels in Great Falls?

Around Great Falls, 29.4% of 235 tests reported from Cascade County for 2008 to 2017 (laboratory totals) reached 4 pCi/L or more, so a high result is no surprise here. The median reported result was 1.7 pCi/L.

Should I test my home in Great Falls?

Yes. Cascade County reported about 69 of its 235 tests at or above 4 pCi/L. That is below Montana's pooled 45.5%, and still not a reason to skip it, and ranks Cascade County 23rd of the 23 Montana counties with 30 or more tests. Which homes they were, no map can say. How to test a home.

What ground is Great Falls built on, as far as radon goes?

70.6% of Great Falls sits on what USGS's map calls "Northern Rocky Mountains", rated high for radon potential, radon index 13; another 29.4% is "Great Plains (Glaciated)", in the same class.

Is radon higher in the counties around Great Falls?

Higher, in every direction measured: all 8 of the nearest counties with a figure report a larger share of high tests than Cascade County's 29.4%, up to Fergus County (72.4%, 99 miles off).

The answers use the published figures of Cascade County, the county Great Falls lies in.

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