Skip to content
RADON411

Locations and reports

Radon Levels in Custer County, Idaho

EPA puts Custer County in Zone 1, its highest radon zone, and high results are common there: 72.7% of the 33 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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    72.7%

    31.2 pts above ID

    Custer
    72.7%
    Idaho
    41.5%

    33 tests, 2008 to 2017

  • Median result

    6.8pCi/L

    Action level reached

    Custer
    6.8 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    33

    33rd of 37 by tests

    Custer
    33
    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

    2 of 37

    Top quarter

    Custer
    72.7%
    Bear Lake
    78.1%

    By share, of 37 with 30+ tests

Read a result for Custer County

Starts at Custer County's median, 6.8 pCi/L

6.8pCi/L

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

EPA’s reading of 6.8 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. Custer County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Custer County, At a glance

Custer County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

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

Reported results by concentration band, Custer County, 2008 to 2017, laboratory
Under 2 pCi/L: 6%2 to 4 pCi/L: 21%4 pCi/L and above: 73%

Of 33 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Custer County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L26.1%
2 to 4 pCi/L721.2%
4 pCi/L and above2472.7%
What these figures count

These are the 33 tests reported to CDC for Custer 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

Custer County, Compared

Custer County radon figures, compared

Calculated by Radon411

More than half: 72.7% of the 33 radon tests reported to CDC for Custer County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 2nd of 37 Idaho counties with 30 or more tests for the same period. That is 31.2 points above the 41.5% pooled from 44 Idaho counties.

The median result, 6.8 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 33.0 pCi/L. The rest split 6.1% under 2 pCi/L and 21.2% from 2 to 4 (Idaho: 31.5% and 26.9%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 72.7% share point the same way here.

No bordering county with a figure for the same period reported a higher share; the closest is Lemhi County at 65.2%, the lowest Boise County at 22.4%. 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 Custer County went from 37 tests (2005 to 2014) to 33 (2008 to 2017), the share going from 75.7% to 72.7%.

Nationally, every county with a figure for the same period pools to 35.7%; Custer County sits 37.0 points above it.

Custer County against Idaho, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Custer County
Custer County3372.7%6.8 pCi/LThis county
Idaho, pooled44 counties, pooled12,96141.5%Not pooled−31.2 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−37.0 points
Bear Lake Countyhighest in the state3278.1%7.8 pCi/L+5.4 points
Payette Countylowest in the state614.9%1.5 pCi/L−67.8 points
Butte County, IDborders it2975.9%12.8 pCi/L+3.2 points
Lemhi County, IDborders it11265.2%5.2 pCi/L−7.5 points
Blaine County, IDborders it35257.7%5.3 pCi/L−15.0 points
Elmore County, IDborders it15848.1%3.8 pCi/L−24.6 points
Valley County, IDborders it12827.3%2.3 pCi/L−45.4 points
Boise County, IDborders it5822.4%2.0 pCi/L−50.3 points
Where Custer County's reported results fell, 2008 to 2017, laboratory, beside Idaho and the United States
ResultCuster County testsCuster County shareIdaho shareU.S. shareAgainst Idaho
Under 2 pCi/L26.1%31.5%39.6%−25.4 points
2 to 4 pCi/L721.2%26.9%24.7%−5.7 points
4 pCi/L and above2472.7%41.5%35.7%+31.2 points
Custer 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
Custer County ten-year laboratory totals, against Idaho's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestIdaho, pooledChange from the row before
2008 to 20173372.7%6.8 pCi/L33.0 pCi/L41.5%−1.0 points
2007 to 20163873.7%6.8 pCi/L42.7 pCi/L40.8%+0.7 points
2006 to 20153773.0%6.7 pCi/L42.7 pCi/L41.3%−2.7 points
2005 to 20143775.7%6.8 pCi/L42.7 pCi/L41.1%First on file

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

Custer County, Geology

The ground under Custer County

Predicted

USGS's geologic map puts all of Custer 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 Custer County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Northern Rocky Mountain ProvinceDiorite and granite of the Idaho batholith.High12100%

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

Custer County beside the ground around it

Custer County has more high-potential ground than Idaho as a whole: 100% of its land against 50.7%. Of the 6 counties that border it, none has as large a share of high-potential ground; Lemhi County comes nearest at 100%.

Share of land by USGS radon potential: Custer County, the 6 counties bordering it and Idaho
AreaEPA zoneLow potentialModerate potentialHigh potential
Custer CountyZone 1NoneNone100%
Lemhi CountyZone 1NoneNone100%
Blaine CountyZone 1None36.8%63.2%
Butte CountyZone 1None46.9%53.1%
Valley CountyZone 1NoneNone100%
Boise CountyZone 10.1%None99.9%
Elmore CountyZone 1None34.9%65.1%
Idaho44 counties, pooled by land areaVaries8.0%41.3%50.7%

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

Local help

Custer County, Local resources

Radon testing and mitigation in Custer County

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

What it means

Custer County, Questions

Questions about radon in Custer County

Are radon levels high in Custer County?

Over the 2008 to 2017 laboratory totals, 72.7% of the radon tests reported to CDC for Custer County, Idaho were at or above 4 pCi/L, and the median reported pre-mitigation result was 6.8 pCi/L. Of 37 Idaho counties with 30 or more tests for the same period, 1 reported a higher share; pooled, the state comes to 41.5%.

Which counties border Custer County?

Custer County borders 6 counties: Lemhi County, Blaine County, Butte County, Valley County, Boise County and Elmore County.

Products

Custer County, Products

Testing a home in Custer County

In Custer County, 72.7% of 33 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 Custer 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

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

Explore

Custer County, Bordering counties

Counties bordering Custer County

Calculated by Radon411

Custer County, Cities and towns

Cities and towns in Custer County

Custer County is growing: the Census Bureau puts it at 4,636 residents on July 1, 2025, up 8.6% from 4,269 in 2020. Spread over its land, that is 0.9 people to the square mile. Its largest town, Challis, has 959 residents, 20.7% of the total. Together its 4 incorporated places hold 34.0% of those residents; the other 66.0% live outside any of them. CDC's 33 reported tests for 2008 to 2017 work out to 7.1 per 1,000 residents (2025 estimate), close to Idaho's 6.4.

From Challis to Clayton, CDC publishes no figure by town: each reads Custer County's.

The cities and towns in Custer County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Challis959+6.8%20.7%
Mackay480+8.6%10.4%
Stanley124+6.0%2.7%
Clayton11+10.0%0.2%

4 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).

Custer County, Homes

Homes in Custer County

Of Custer County's 3,164 homes, 67% are detached houses, against 74% across Idaho; 23% are mobile homes. Half of its homes were built before 1987, close to Idaho's 1990: 11% date from before 1960 and 18% from 2000 or later. Owners live in 78% of the occupied homes (Idaho: 72%), so for most households a test and a fix are their own to order.

Homes in Custer County, Idaho, 2019 to 2023
HomesCuster CountyIdaho
Housing units3,164776,683
Detached houses67%74%
In buildings of 5 or more2%9%
Mobile homes23%7%
Built before 196011%18%
Built since 200018%35%
Middle year built19871990
Owner-occupied78%72%

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