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Radon Levels in Fremont County, Wyoming

EPA puts Fremont County in Zone 1, its highest radon zone, and high results are common there: 55.7% of the 618 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

    55.7%

    11.4 pts above WY

    Fremont
    55.7%
    Wyoming
    44.3%

    618 tests, 2008 to 2017

  • Median result

    4.7pCi/L

    Action level reached

    Fremont
    4.7 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    618

    4th of 22 by tests

    Fremont
    618
    Laramie
    1,834

    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 Wyoming

    8 of 22

    Middle half

    Fremont
    55.7%
    Sheridan
    69.5%

    By share, of 22 with 30+ tests

Read a result for Fremont County

Starts at Fremont County's median, 4.7 pCi/L

4.7pCi/L

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

EPA’s reading of 4.7 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. 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 93% 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: 18%2 to 4 pCi/L: 26%4 pCi/L and above: 56%

Of 618 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/L11017.8%
2 to 4 pCi/L16426.5%
4 pCi/L and above34455.7%
What these figures count

These are the 618 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

More than half: 55.7% of the 618 radon tests reported to CDC for Fremont County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 8th of 22 Wyoming counties with 30 or more tests for the same period. That is 11.4 points above the 44.3% pooled from 23 Wyoming counties.

The median result, 4.7 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 212.5 pCi/L. The rest split 17.8% under 2 pCi/L and 26.5% from 2 to 4 (Wyoming: 25.5% and 30.2%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 55.7% share point the same way here.

1 of its 8 bordering counties with figures ran higher, led by Hot Springs County at 55.9%; Washakie County reported 38.5%. Statewide for the same period, Sheridan County reported the highest share (69.5%) and Uinta County the lowest (24.6%).

CDC's ten-year totals for Fremont County went from 623 tests (2005 to 2014) to 618 (2008 to 2017), the share going from 56.7% to 55.7%.

Nationally, every county with a figure for the same period pools to 35.7%; Fremont County sits 20.0 points above it. CDC also counts 17 tests taken after mitigation in Fremont County for the same period: tests in homes where a fix was already in.

Fremont County against Wyoming, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Fremont County
Fremont County61855.7%4.7 pCi/LThis county
Wyoming, pooled23 counties, pooled7,63844.3%Not pooled−11.4 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−20.0 points
Sheridan Countyhighest in the state84669.5%5.8 pCi/L+13.8 points
Uinta Countylowest in the state19924.6%2.2 pCi/L−31.1 points
Hot Springs County, WYborders it5955.9%4.2 pCi/L+0.2 points
Natrona County, WYborders it1,05455.7%4.3 pCi/LLevel
Sublette County, WYborders it6050.0%4.0 pCi/L−5.7 points
Sweetwater County, WYborders it40949.6%3.9 pCi/L−6.1 points
Teton County, WYborders it27844.6%3.4 pCi/L−11.1 points
Park County, WYborders it50542.8%3.4 pCi/L−12.9 points
Carbon County, WYborders it8041.3%3.0 pCi/L−14.4 points
Washakie County, WYborders it13038.5%3.2 pCi/L−17.2 points
Where Fremont County's reported results fell, 2008 to 2017, laboratory, beside Wyoming and the United States
ResultFremont County testsFremont County shareWyoming shareU.S. shareAgainst Wyoming
Under 2 pCi/L11017.8%25.5%39.6%−7.7 points
2 to 4 pCi/L16426.5%30.2%24.7%−3.7 points
4 pCi/L and above34455.7%44.3%35.7%+11.4 points
Fremont County ranked among the 22 Wyoming counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Weston County3 of 223265.6%6.3 pCi/L
Converse County4 of 2213062.3%4.8 pCi/L
Crook County5 of 225158.8%5.1 pCi/L
Hot Springs County6 of 225955.9%4.2 pCi/L
Natrona County7 of 221,05455.7%4.3 pCi/L
Fremont County8 of 2261855.7%4.7 pCi/L
Sublette County9 of 226050.0%4.0 pCi/L
Sweetwater County10 of 2240949.6%3.9 pCi/L
Teton County11 of 2227844.6%3.4 pCi/L
Campbell County12 of 2227143.9%3.5 pCi/L
Park County13 of 2250542.8%3.4 pCi/L
Fremont County ten-year laboratory totals, against Wyoming's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestWyoming, pooledChange from the row before
2008 to 201761855.7%4.7 pCi/L212.5 pCi/L44.3%−0.3 points
2007 to 201660956.0%4.7 pCi/L212.5 pCi/L44.6%−0.3 points
2006 to 201563156.3%4.7 pCi/L212.5 pCi/L45.3%−0.4 points
2005 to 201462356.7%4.7 pCi/L212.5 pCi/L45.5%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

Most of the ground under Fremont County is one class on USGS's geologic map: 92.7% of its land is high radon potential, with 7.3% moderate. The map puts 42.1% of Fremont County in the area it calls Wind River Mountains and 37.8% in Wind River Basin, with 3 smaller areas beside them. USGS gives both a radon index of 12, in the high band (12 to 17).

USGS's geologic areas under Fremont County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Wind River MountainsPrecambrian granite, glacial till in valleys.High1242.1%
Wind River BasinCretaceous and Tertiary uranium-bearing marginal-marine and fluvial sandstones.High1237.8%
Big Horn MountainsPrecambrian granitic and metamorphic rocks locally overlain by marine clastic and carbonate sedimentary rocks.High127.7%
Absaroka Volcanic FieldIntrusive and extrusive volcanic rocks and overlying sediments.Moderate106.9%
Green River BasinCretaceous and Tertiary uranium-bearing marginal-marine and fluvial sandstones.High125.0%

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 Wyoming as a whole: 92.7% of its land against 72.4%. Of the 8 counties that border it, 2 have more high-potential ground, Washakie County the most at 100%.

Share of land by USGS radon potential: Fremont County, the 8 counties bordering it and Wyoming
AreaEPA zoneLow potentialModerate potentialHigh potential
Fremont CountyZone 1None7.3%92.7%
Hot Springs CountyZone 1None11.4%88.6%
Washakie CountyZone 1NoneNone100%
Sublette CountyZone 1None12.2%87.8%
Natrona CountyZone 1None39.4%60.6%
Park CountyZone 1None51.9%48.1%
Sweetwater CountyZone 1None0.4%99.6%
Teton CountyZone 1None58.3%41.7%
Carbon CountyZone 1None29.2%70.8%
Wyoming23 counties, pooled by land areaVariesNone27.6%72.4%

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 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, 55.7% of the radon tests reported to CDC for Fremont County, Wyoming were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.7 pCi/L. Of 22 Wyoming counties with 30 or more tests for the same period, 7 reported a higher share; pooled, the state comes to 44.3%.

Which counties border Fremont County?

Fremont County borders 8 counties: Hot Springs County, Washakie County, Sublette County, Natrona County, Park County, Sweetwater County, Teton County and Carbon County.

Products

Fremont County, Products

Testing a home in Fremont County

In Fremont County, 55.7% of 618 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 44.3% pooled across Wyoming. 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

The Census Bureau estimates 39,464 residents in Fremont County for July 1, 2025, 272 more than in 2020 (+0.7%). Spread over its land, that is 4.3 people to the square mile. Its largest town, Riverton, has 10,910 residents, 27.6% of the total. Together its 6 incorporated places hold 51.8% of those residents; the other 48.2% live outside any of them. CDC's 618 reported tests for 2008 to 2017 work out to 15.7 per 1,000 residents (2025 estimate), close to Wyoming's 13.0.

From Riverton to Pavillion, 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
Riverton10,910+2.1%27.6%
Lander7,379−0.3%18.7%
Dubois984+8.1%2.5%
Shoshoni509+6.9%1.3%
Hudson436Level1.1%
Pavillion228+1.8%0.6%

6 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

Of Fremont County's 17,471 homes, 71% are detached houses, against 68% across Wyoming; 17% are mobile homes. Half of its homes were built before 1978, close to Wyoming's 1980: 22% date from before 1960 and 20% from 2000 or later. 71% of occupied homes are lived in by their owner (Wyoming: 72%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Fremont County, Wyoming, 2019 to 2023
HomesFremont CountyWyoming
Housing units17,471275,131
Detached houses71%68%
In buildings of 5 or more6%9%
Mobile homes17%12%
Built before 196022%24%
Built since 200020%24%
Middle year built19781980
Owner-occupied71%72%

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