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Radon Levels in Lemhi County, Idaho

EPA puts Lemhi County in Zone 1, its highest radon zone, and high results are common there: 65.2% of the 112 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

    65.2%

    23.7 pts above ID

    Lemhi
    65.2%
    Idaho
    41.5%

    112 tests, 2008 to 2017

  • Median result

    5.2pCi/L

    Action level reached

    Lemhi
    5.2 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    112

    18th of 37 by tests

    Lemhi
    112
    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

    4 of 37

    Top quarter

    Lemhi
    65.2%
    Bear Lake
    78.1%

    By share, of 37 with 30+ tests

Read a result for Lemhi County

Starts at Lemhi County's median, 5.2 pCi/L

5.2pCi/L

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

EPA’s reading of 5.2 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. Lemhi County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Lemhi County, At a glance

Lemhi County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

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

Reported results by concentration band, Lemhi County, 2008 to 2017, laboratory
Under 2 pCi/L: 12%2 to 4 pCi/L: 23%4 pCi/L and above: 65%

Of 112 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Lemhi County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L1311.6%
2 to 4 pCi/L2623.2%
4 pCi/L and above7365.2%
What these figures count

These are the 112 tests reported to CDC for Lemhi 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

Lemhi County, Compared

Lemhi County radon figures, compared

Calculated by Radon411

More than half: 65.2% of the 112 radon tests reported to CDC for Lemhi County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 4th of 37 Idaho counties with 30 or more tests for the same period. That is 23.7 points above the 41.5% pooled from 44 Idaho counties.

The median result, 5.2 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 36.1 pCi/L. The rest split 11.6% under 2 pCi/L and 23.2% from 2 to 4 (Idaho: 31.5% and 26.9%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 65.2% share point the same way here.

1 of its 5 bordering counties with figures ran higher, led by Custer County at 72.7%; Valley County reported 27.3%. Statewide for the same period, Bear Lake County reported the highest share (78.1%) and Payette County the lowest (4.9%).

CDC's ten-year totals for Lemhi County went from 200 tests (2005 to 2014) to 112 (2008 to 2017), the share going from 57.5% to 65.2%.

Nationally, every county with a figure for the same period pools to 35.7%; Lemhi County sits 29.5 points above it.

Lemhi County against Idaho, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Lemhi County
Lemhi County11265.2%5.2 pCi/LThis county
Idaho, pooled44 counties, pooled12,96141.5%Not pooled−23.7 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−29.5 points
Bear Lake Countyhighest in the state3278.1%7.8 pCi/L+12.9 points
Payette Countylowest in the state614.9%1.5 pCi/L−60.3 points
Butte County, IDborders it2975.9%12.8 pCi/L+10.7 points
Clark County, IDborders it475.0%14.1 pCi/L+9.8 points
Custer County, IDborders it3372.7%6.8 pCi/L+7.5 points
Beaverhead County, MTborders it3759.5%5.0 pCi/L−5.7 points
Ravalli County, MTborders it1,09044.6%3.5 pCi/L−20.6 points
Idaho County, IDborders it9040.0%2.9 pCi/L−25.2 points
Valley County, IDborders it12827.3%2.3 pCi/L−37.9 points
Where Lemhi County's reported results fell, 2008 to 2017, laboratory, beside Idaho and the United States
ResultLemhi County testsLemhi County shareIdaho shareU.S. shareAgainst Idaho
Under 2 pCi/L1311.6%31.5%39.6%−19.9 points
2 to 4 pCi/L2623.2%26.9%24.7%−3.7 points
4 pCi/L and above7365.2%41.5%35.7%+23.7 points
Lemhi 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
Bear Lake County1 of 373278.1%7.8 pCi/L
Custer County2 of 373372.7%6.8 pCi/L
Jefferson County3 of 3720767.1%5.7 pCi/L
Lemhi County4 of 3711265.2%5.2 pCi/L
Madison County5 of 3731463.7%6.2 pCi/L
Shoshone County6 of 378163.0%5.7 pCi/L
Kootenai County7 of 372,95561.0%6.0 pCi/L
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
Lemhi County ten-year laboratory totals, against Idaho's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestIdaho, pooledChange from the row before
2008 to 201711265.2%5.2 pCi/L36.1 pCi/L41.5%+4.4 points
2007 to 201615360.8%5.1 pCi/L37.6 pCi/L40.8%+2.1 points
2006 to 201520858.7%4.8 pCi/L49.6 pCi/L41.3%+1.2 points
2005 to 201420057.5%4.8 pCi/L49.6 pCi/L41.1%First on file

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

Lemhi County, Geology

The ground under Lemhi County

Predicted

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

USGS's geologic areas under Lemhi County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Northern Rocky Mountain ProvinceDiorite and granite of the Idaho batholith.High1299.9%

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

Lemhi County beside the ground around it

Lemhi County has more high-potential ground than Idaho as a whole: 100% of its land against 50.7%. Of the 7 counties that border it, none has as large a share of high-potential ground; Custer County comes nearest at 100%.

Share of land by USGS radon potential: Lemhi County, the 7 counties bordering it and Idaho
AreaEPA zoneLow potentialModerate potentialHigh potential
Lemhi CountyZone 1NoneNone100%
Beaverhead County, MTZone 1NoneNone100%
Custer CountyZone 1NoneNone100%
Ravalli County, MTZone 1NoneNone100%
Valley CountyZone 1NoneNone100%
Idaho CountyZone 120.7%0.1%79.3%
Butte CountyZone 1None46.9%53.1%
Clark CountyZone 1None33.3%66.7%
Idaho44 counties, pooled by land areaVaries8.0%41.3%50.7%

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

Local help

Lemhi County, Local resources

Radon testing and mitigation in Lemhi County

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

What it means

Lemhi County, Questions

Questions about radon in Lemhi County

Are radon levels high in Lemhi County?

Over the 2008 to 2017 laboratory totals, 65.2% of the radon tests reported to CDC for Lemhi County, Idaho were at or above 4 pCi/L, and the median reported pre-mitigation result was 5.2 pCi/L. Of 37 Idaho counties with 30 or more tests for the same period, 3 reported a higher share; pooled, the state comes to 41.5%.

Which counties border Lemhi County?

Lemhi County borders 7 counties: Beaverhead County, MT, Custer County, ID, Ravalli County, MT, Valley County, ID, Idaho County, ID, Butte County, ID and Clark County, ID.

Products

Lemhi County, Products

Testing a home in Lemhi County

In Lemhi County, 65.2% of 112 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 Lemhi 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

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

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Lemhi County, Bordering counties

Counties bordering Lemhi County

Calculated by Radon411

Lemhi County, Cities and towns

Cities and towns in Lemhi County

Lemhi County is growing: the Census Bureau puts it at 8,474 residents on July 1, 2025, up 6.5% from 7,955 in 2020. Spread over its land, that is 1.9 people to the square mile. Its largest town, Salmon, has 3,286 residents, 38.8% of the total. Together its 2 incorporated places hold 39.9% of those residents; the other 60.1% live outside any of them. Testing is reported here more often than across the state: CDC's 112 reported tests for 2008 to 2017 come to 13.2 for every 1,000 residents (2025 estimate), against 6.4 for Idaho.

From Salmon to Leadore, CDC publishes no figure by town: each reads Lemhi County's.

The cities and towns in Lemhi County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Salmon3,286+6.1%38.8%
Leadore99+5.3%1.2%

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).

Lemhi County, Homes

Homes in Lemhi County

Of Lemhi County's 4,636 homes, 75% are detached houses, against 74% across Idaho; 16% are mobile homes. The homes are older than Idaho's as a whole: half were built before 1979 (1990 for Idaho), and 21% date from before 1960. Owners live in 81% of the occupied homes (Idaho: 72%), so for most households a test and a fix are their own to order.

Homes in Lemhi County, Idaho, 2019 to 2023
HomesLemhi CountyIdaho
Housing units4,636776,683
Detached houses75%74%
In buildings of 5 or more4%9%
Mobile homes16%7%
Built before 196021%18%
Built since 200017%35%
Middle year built19791990
Owner-occupied81%72%

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