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Radon Levels in Fremont County, Idaho

EPA puts Fremont County in Zone 1, its highest radon zone, and high results are common there: 49.5% of the 101 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

    49.5%

    8.0 pts above ID

    Fremont
    49.5%
    Idaho
    41.5%

    101 tests, 2008 to 2017

  • Median result

    3.9pCi/L

    Below action level

    Fremont
    3.9 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    101

    19th of 37 by tests

    Fremont
    101
    Ada
    3,030

    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 Idaho

    13 of 37

    Middle half

    Fremont
    49.5%
    Bear Lake
    78.1%

    By share, of 37 with 30+ tests

Read a result for Fremont County

Starts at Fremont County's median, 3.9 pCi/L

3.9pCi/L

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

EPA’s reading of 3.9 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. Fremont County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Fremont County, At a glance

Fremont County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Fremont 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 68% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Fremont County, 2008 to 2017, laboratory
Under 2 pCi/L: 25%2 to 4 pCi/L: 26%4 pCi/L and above: 49%

Of 101 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Fremont County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L2524.8%
2 to 4 pCi/L2625.7%
4 pCi/L and above5049.5%
What these figures count

These are the 101 tests reported to CDC for Fremont 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

Fremont County, Compared

Fremont County radon figures, compared

Calculated by Radon411

In Fremont County, about 5 in 10 reported tests over the 2008 to 2017 laboratory totals (50 of 101) were at or above EPA's action level. Against the national pool of 35.7%, the gap is 13.8 points higher. Across 44 Idaho counties pooled together, 41.5% of tests reached that level, so Fremont County runs 8.0 points above its state.

Of 7 neighbours with a figure, 5 reported a larger share at or above 4 pCi/L and 2 a smaller one; the spread is 42.8% (Gallatin County, MT) to 67.1% (Jefferson County). By share, Fremont County is 13th of 37 Idaho counties with 30 or more tests for the same period.

Half the tests came in under 3.9 pCi/L and half over it; the single highest reached 61.4 pCi/L. Below the action level, 25 tests (24.8%) came in under 2 pCi/L, against 31.5% statewide, and 26 tests (25.7%) fell between 2 and 4, against 26.9% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 49.5% share point the same way here. USGS geology rates 68% of the county's land high radon potential.

CDC's ten-year totals for Fremont County went from 67 tests (2005 to 2014) to 101 (2008 to 2017), the share going from 56.7% to 49.5%.

Fremont County against Idaho, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Fremont County
Fremont County10149.5%3.9 pCi/LThis county
Idaho, pooled44 counties, pooled12,96141.5%Not pooled−8.0 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−13.8 points
Bear Lake Countyhighest in the state3278.1%7.8 pCi/L+28.6 points
Payette Countylowest in the state614.9%1.5 pCi/L−44.6 points
Clark County, IDborders it475.0%14.1 pCi/L+25.5 points
Jefferson County, IDborders it20767.1%5.7 pCi/L+17.6 points
Madison County, IDborders it31463.7%6.2 pCi/L+14.2 points
Beaverhead County, MTborders it3759.5%5.0 pCi/L+10.0 points
Teton County, IDborders it47353.3%4.4 pCi/L+3.8 points
Madison County, MTborders it5850.0%3.9 pCi/L+0.5 points
Teton County, WYborders it27844.6%3.4 pCi/L−4.9 points
Gallatin County, MTborders it1,60042.8%3.2 pCi/L−6.7 points
Where Fremont County's reported results fell, 2008 to 2017, laboratory, beside Idaho and the United States
ResultFremont County testsFremont County shareIdaho shareU.S. shareAgainst Idaho
Under 2 pCi/L2524.8%31.5%39.6%−6.7 points
2 to 4 pCi/L2625.7%26.9%24.7%−1.2 points
4 pCi/L and above5049.5%41.5%35.7%+8.0 points
Fremont County ranked among the 37 Idaho counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Cassia County8 of 3712159.5%5.4 pCi/L
Blaine County9 of 3735257.7%5.3 pCi/L
Teton County10 of 3747353.3%4.4 pCi/L
Bingham County11 of 3744652.0%4.2 pCi/L
Adams County12 of 373151.6%4.0 pCi/L
Fremont County13 of 3710149.5%3.9 pCi/L
Elmore County14 of 3715848.1%3.8 pCi/L
Caribou County15 of 375743.9%3.4 pCi/L
Bannock County16 of 3773241.9%3.2 pCi/L
Bonneville County17 of 371,11740.1%3.4 pCi/L
Idaho County18 of 379040.0%2.9 pCi/L
Fremont County ten-year laboratory totals, against Idaho's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestIdaho, pooledChange from the row before
2008 to 201710149.5%3.9 pCi/L61.4 pCi/L41.5%−1.0 points
2007 to 20169350.5%4.1 pCi/L61.4 pCi/L40.8%−4.0 points
2006 to 20156654.5%4.5 pCi/L61.4 pCi/L41.3%−2.2 points
2005 to 20146756.7%4.6 pCi/L61.4 pCi/L41.1%First on file

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

Fremont County, Geology

The ground under Fremont County

Predicted

USGS's geologic map splits Fremont County: 68.2% of its land is high radon potential and 31.8% moderate. The map puts 61.5% of Fremont County in the area it calls Middle Rocky Mts./Yellowstone and 31.8% in Eastern Snake river plain and Malheur-Boise-King Section, with one smaller area beside them. USGS gives the first a radon index of 12, in the high band (12 to 17), and the second 10, in the moderate band (9 to 11).

USGS's geologic areas under Fremont County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Middle Rocky Mts./YellowstoneExtrusive volcanics, rhyolite flows, variable composition.High1261.5%
Eastern Snake river plain and Malheur-Boise-King SectionBasalt flows.Moderate1031.8%
Northern Rocky Mountain ProvinceDiorite and granite of the Idaho batholith.High126.6%

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

Fremont County beside the ground around it

Fremont County has more high-potential ground than Idaho as a whole: 68.2% of its land against 50.7%. Of the 8 counties that border it, 3 have more high-potential ground, Madison County, MT the most at 100%.

Share of land by USGS radon potential: Fremont County, the 8 counties bordering it and Idaho
AreaEPA zoneLow potentialModerate potentialHigh potential
Fremont CountyZone 1None31.8%68.2%
Teton CountyZone 2None60.3%39.7%
Madison CountyZone 2None41.9%58.1%
Clark CountyZone 1None33.3%66.7%
Teton County, WYZone 1None58.3%41.7%
Jefferson CountyZone 2None99.2%0.8%
Gallatin County, MTZone 1None21.0%79.0%
Madison County, MTZone 1NoneNone100%
Beaverhead County, MTZone 1NoneNone100%
Idaho44 counties, pooled by land areaVaries8.0%41.3%50.7%

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

Local help

Fremont County, Local resources

Radon testing and mitigation in Fremont County

What Idaho's program says
Phone
800-445-8647
Email
radon@dhw.idaho.gov

What it means

Fremont County, Questions

Questions about radon in Fremont County

Are radon levels high in Fremont County?

Over the 2008 to 2017 laboratory totals, 49.5% of the radon tests reported to CDC for Fremont County, Idaho were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.9 pCi/L. Of 37 Idaho counties with 30 or more tests for the same period, 12 reported a higher share; pooled, the state comes to 41.5%.

Which counties border Fremont County?

Fremont County borders 8 counties: Teton County, ID, Madison County, ID, Clark County, ID, Teton County, WY, Jefferson County, ID, Gallatin County, MT, Madison County, MT and Beaverhead County, MT.

Products

Fremont County, Products

Testing a home in Fremont County

In Fremont County, 49.5% of 101 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 41.5% pooled across Idaho. In Zone 1 the year-round level in Fremont 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

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

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Fremont County, Bordering counties

Counties bordering Fremont County

Calculated by Radon411

Fremont County, Cities and towns

Cities and towns in Fremont County

Fremont County is growing: the Census Bureau puts it at 14,208 residents on July 1, 2025, up 5.9% from 13,420 in 2020. Spread over its land, that is 7.6 people to the square mile. Its largest town, St. Anthony, has 3,990 residents, 28.1% of the total. Together its 8 incorporated places hold 46.6% of those residents; the other 53.4% live outside any of them. CDC's 101 reported tests for 2008 to 2017 work out to 7.1 per 1,000 residents (2025 estimate), close to Idaho's 6.4.

From St. Anthony to Warm River, CDC publishes no figure by town: each reads Fremont County's.

The cities and towns in Fremont County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
St. Anthony3,990+8.1%28.1%
Ashton977−0.7%6.9%
Teton831+5.2%5.8%
Newdale329−0.6%2.3%
Parker289−1.7%2.0%
Island Park190−1.6%1.3%
Drummond15+7.1%0.1%
Warm River1LevelUnder 0.1%

8 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).

Fremont County, Homes

Homes in Fremont County

Fremont County is a place of houses on their own lots: 84% of its 8,669 homes are detached single-family houses, against 74% across Idaho. The homes are older than Idaho's as a whole: half were built before 1979 (1990 for Idaho), and 19% date from before 1960. Owners live in 84% of the occupied homes (Idaho: 72%), so for most households a test and a fix are their own to order.

Homes in Fremont County, Idaho, 2019 to 2023
HomesFremont CountyIdaho
Housing units8,669776,683
Detached houses84%74%
In buildings of 5 or more1%9%
Mobile homes12%7%
Built before 196019%18%
Built since 200025%35%
Middle year built19791990
Owner-occupied84%72%

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