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Radon Levels in Franklin County, Idaho

EPA puts Franklin County in Zone 2, its middle radon zone, and 34.7% of its 75 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

    34.7%

    6.8 pts below ID

    Franklin
    34.7%
    Idaho
    41.5%

    75 tests, 2008 to 2017

  • Median result

    3.1pCi/L

    Below action level

    Franklin
    3.1 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    75

    23rd of 37 by tests

    Franklin
    75
    Ada
    3,030

    Reported to CDC before any fix, 2008 to 2017

  • EPA radon zone

    Zone 2

    Moderate potential

    1. 1
    2. Zone 2
    3. 3

    1 highest, 3 lowest

    EPA predicts 2 to 4 pCi/L indoors

  • Rank in Idaho

    19 of 37

    Middle half

    Franklin
    34.7%
    Bear Lake
    78.1%

    By share, of 37 with 30+ tests

Read a result for Franklin County

Starts at Franklin County's median, 3.1 pCi/L

3.1pCi/L

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

EPA’s reading of 3.1 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. Franklin County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Franklin County, At a glance

Franklin County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

Franklin County is a county: EPA's forecast for it as a whole is an average indoor of 2 to 4 pCi/L (75 to 150 Bq/m³). It describes no single address, and homes above EPA's are found in every zone, this one included.

USGS geologic potential: Moderate across 68% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Franklin County, 2008 to 2017, laboratory
Under 2 pCi/L: 28%2 to 4 pCi/L: 37%4 pCi/L and above: 35%

Of 75 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Franklin County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L2128.0%
2 to 4 pCi/L2837.3%
4 pCi/L and above2634.7%
What these figures count

These are the 75 tests reported to CDC for Franklin 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

Franklin County, Compared

Franklin County radon figures, compared

Calculated by Radon411

In Franklin County, about 3 in 10 reported tests over the 2008 to 2017 laboratory totals (26 of 75) were at or above EPA's action level. Against the national pool of 35.7%, the gap is 1.0 points lower. Across 44 Idaho counties pooled together, 41.5% of tests reached that level, so Franklin County runs 6.8 points below its state.

Of 4 neighbours with a figure, all 4 reported a larger share at or above 4 pCi/L; the spread is 41.9% (Bannock County) to 78.1% (Bear Lake County). By share, Franklin County is 19th of 37 Idaho counties with 30 or more tests for the same period.

Half the tests came in under 3.1 pCi/L and half over it; the single highest reached 29.1 pCi/L. Below the action level, 21 tests (28.0%) came in under 2 pCi/L, against 31.5% statewide, and 28 tests (37.3%) fell between 2 and 4, against 26.9% statewide.

Zone 2 on EPA's map (predicted average between 2 and 4 pCi/L) and a 34.7% share point the same way here. USGS geology rates 68% of the county's land moderate radon potential.

CDC's ten-year totals for Franklin County went from 65 tests (2005 to 2014) to 75 (2008 to 2017), the share going from 24.6% to 34.7%.

Franklin County against Idaho, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Franklin County
Franklin County7534.7%3.1 pCi/LThis county
Idaho, pooled44 counties, pooled12,96141.5%Not pooled+6.8 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+1.0 points
Custer Countyhighest in the state3372.7%6.8 pCi/L+38.0 points
Payette Countylowest in the state614.9%1.5 pCi/L−29.8 points
Bear Lake County, IDborders it3278.1%7.8 pCi/L+43.4 points
Oneida County, IDborders it2774.1%6.5 pCi/L+39.4 points
Cache County, UTborders it1,33956.8%4.7 pCi/L+22.1 points
Caribou County, IDborders it5743.9%3.4 pCi/L+9.2 points
Bannock County, IDborders it73241.9%3.2 pCi/L+7.2 points
Rich County, UTborders it20.0%1.4 pCi/L−34.7 points
Where Franklin County's reported results fell, 2008 to 2017, laboratory, beside Idaho and the United States
ResultFranklin County testsFranklin County shareIdaho shareU.S. shareAgainst Idaho
Under 2 pCi/L2128.0%31.5%39.6%−3.5 points
2 to 4 pCi/L2837.3%26.9%24.7%+10.4 points
4 pCi/L and above2634.7%41.5%35.7%−6.8 points
Franklin 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
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
Franklin County19 of 377534.7%3.1 pCi/L
Bonner County20 of 3732133.0%2.3 pCi/L
Clearwater County21 of 373330.3%1.8 pCi/L
Gooding County22 of 376728.4%2.4 pCi/L
Valley County23 of 3712827.3%2.3 pCi/L
Power County24 of 374126.8%2.5 pCi/L
Franklin County ten-year laboratory totals, against Idaho's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestIdaho, pooledChange from the row before
2008 to 20177534.7%3.1 pCi/L29.1 pCi/L41.5%+4.4 points
2007 to 20167630.3%2.8 pCi/L29.1 pCi/L40.8%+1.9 points
2006 to 20157428.4%2.8 pCi/L29.1 pCi/L41.3%+3.8 points
2005 to 20146524.6%2.6 pCi/L29.1 pCi/L41.1%First on file

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

Franklin County, Geology

The ground under Franklin County

Predicted

USGS's geologic map splits Franklin County: 68.4% of its land is moderate radon potential and 31.6% high. The map puts 68.4% of Franklin County in the area it calls Basin and Range and 31.6% in Wasatch Range. USGS gives the first a radon index of 11, in the moderate band (9 to 11), and the second 14, in the high band (12 to 17).

USGS's geologic areas under Franklin County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Basin and RangeCenozoic to Paleozoic sedimentary rocks.Moderate1168.4%
Wasatch RangePrecambrian granite and gneiss and Tertiary igneous rocks that have relatively low uranium concentrations.High1431.6%

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

Franklin County beside the ground around it

Idaho as a whole has more high-potential ground than Franklin County: 50.7% of its land, against 31.6% here. Of the 6 counties that border it, 2 have more high-potential ground, Rich County, UT the most at 66.7%.

Share of land by USGS radon potential: Franklin County, the 6 counties bordering it and Idaho
AreaEPA zoneLow potentialModerate potentialHigh potential
Franklin CountyZone 2None68.4%31.6%
Bear Lake CountyZone 2None96.7%3.3%
Cache County, UTZone 2None55.3%44.7%
Bannock CountyZone 2None100%None
Oneida CountyZone 2None99.6%0.4%
Caribou CountyZone 2None100%None
Rich County, UTZone 2None33.3%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

Franklin County, Local resources

Radon testing and mitigation in Franklin County

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

What it means

Franklin County, Questions

Questions about radon in Franklin County

Are radon levels high in Franklin County?

Over the 2008 to 2017 laboratory totals, 34.7% of the radon tests reported to CDC for Franklin County, Idaho were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.1 pCi/L. Of 37 Idaho counties with 30 or more tests for the same period, 18 reported a higher share; pooled, the state comes to 41.5%.

Which counties border Franklin County?

Franklin County borders 6 counties: Bear Lake County, ID, Cache County, UT, Bannock County, ID, Oneida County, ID, Caribou County, ID and Rich County, UT.

Products

Franklin County, Products

Testing a home in Franklin County

In Franklin County, 34.7% of 75 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 41.5% pooled across Idaho. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in Franklin County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Franklin County homes, compared

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Franklin County, Bordering counties

Counties bordering Franklin County

Calculated by Radon411

Franklin County, Cities and towns

Cities and towns in Franklin County

Franklin County is growing: the Census Bureau puts it at 15,889 residents on July 1, 2025, up 11.2% from 14,284 in 2020. That works out to 24 people per square mile of land. Its largest town, Preston, has 6,177 residents, 38.9% of the total. Together its 6 incorporated places hold 56.5% of those residents; the other 43.5% live outside any of them. CDC's 75 reported tests for 2008 to 2017 work out to 4.7 per 1,000 residents (2025 estimate), close to Idaho's 6.4.

From Preston to Oxford, CDC publishes no figure by town: each reads Franklin County's.

The cities and towns in Franklin County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Preston6,177+11.3%38.9%
Franklin1,155+12.2%7.3%
Weston586+14.2%3.7%
Dayton545+4.6%3.4%
Clifton466+12.3%2.9%
Oxford43+10.3%0.3%

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).

Franklin County, Homes

Homes in Franklin County

Franklin County is a place of houses on their own lots: 85% of its 5,122 homes are detached single-family houses, against 74% across Idaho. The homes are older than Idaho's as a whole: half were built before 1972 (1990 for Idaho), and 38% date from before 1960. Owners live in 75% of the occupied homes (Idaho: 72%), so for most households a test and a fix are their own to order.

Homes in Franklin County, Idaho, 2019 to 2023
HomesFranklin CountyIdaho
Housing units5,122776,683
Detached houses85%74%
In buildings of 5 or more2%9%
Mobile homes4%7%
Built before 196038%18%
Built since 200026%35%
Middle year built19721990
Owner-occupied75%72%

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