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Radon Levels in Lewis County, Idaho

EPA puts Lewis County in Zone 3, its lowest radon zone, and only 6 radon tests from it are on file with CDC (2008 to 2017, laboratory totals). 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

  • Highest bordering

    40.0%

    Idaho County

    Idaho
    40.0%
    Nez Perce
    22.1%

    3 of 3 neighbours have 30+ tests, 2008 to 2017

  • Nearest county

    22.1%

    Nez Perce County, 17 miles

    Nez Perce
    22.1%
    Idaho
    41.5%

    Share at 4+ pCi/L, 149 tests, 2008 to 2017

  • Ground above low

    None

    92.0 pts below Idaho

    Lewis
    0.0%
    Idaho
    92.0%

    Share of land USGS rates moderate or high potential

  • Tests reported

    10

    Too few for a share

    Lewis
    10
    Needed
    30

    2006 to 2015, laboratory

  • EPA radon zone

    Zone 3

    Lower potential

    1. 1
    2. 2
    3. Zone 3

    1 highest, 3 lowest

    EPA predicts under 2 pCi/L indoors

Read a result for Lewis County

Starts at EPA's action level

4.0pCi/L

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

EPA’s reading of 4.0 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.

Interpret this readingWhat a fix costs

The data

Lewis County, At a glance

Lewis County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for Lewis County as a whole is an average indoor of less than 2 pCi/L (less than 75 Bq/m³). A low zone clears no address in it: homes above EPA's turn up in all three zones.

USGS geologic potential: Low across 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

CDC holds 6 reported tests for Lewis County over the 2008 to 2017 laboratory totals, with a median of 3.8 pCi/L: too few to describe a county (Radon411 reads a share from 30 tests up). Lewis County beside the counties around it is how this page reads it.

What these figures count

Only 6 tests were reported to CDC for Lewis County over 2008 to 2017 (laboratory totals), and tests are often run on homes being sold or already suspected of a problem: so few describe neither the county nor 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

Lewis County, Compared

Lewis County radon figures, compared

Calculated by Radon411

CDC's largest figure for Lewis County rests on 10 tests over the 2006 to 2015 laboratory totals, 3 of them at or above 4 pCi/L, so each test moves its share by 10.0 points and no rank or comparison is drawn from it. Results reached as high as 7.7 pCi/L, with a middle value of 1.1 pCi/L.

Of the counties bordering it, 3 have a figure over the 2008 to 2017 laboratory totals: Idaho County reported the largest share at or above 4 pCi/L, 40.0%, and Nez Perce County the smallest, 22.1%, with Clearwater County at 30.3% between. Farther out, Latah County (42 miles away) at 26.0%, Shoshone County (81 miles away) at 63.0% and Adams County (93 miles away) at 51.6% have figures too. Pooled across 44 Idaho counties, 41.5% of reported tests reached that level; across every county in the country with a figure, 35.7%.

Statewide for the same period, Bear Lake County reported the highest share (78.1%) and Payette County the lowest (4.9%). EPA's 1993 map puts Lewis County in Zone 3, its lowest label: a predicted county average under 2 pCi/L. USGS geology rates 100% of the county's land low radon potential.

Idaho, the United States and the counties around Lewis County, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedian
Idaho, pooled44 counties, pooled12,96141.5%Not pooled
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled
Bear Lake Countyhighest in the state3278.1%7.8 pCi/L
Payette Countylowest in the state614.9%1.5 pCi/L
Idaho County, IDborders it9040.0%2.9 pCi/L
Clearwater County, IDborders it3330.3%1.8 pCi/L
Nez Perce County, IDborders it14922.1%2.3 pCi/L
The nearest Idaho counties to Lewis County with 30 or more tests, 2008 to 2017, laboratory
CountyDistanceTestsAt or above 4 pCi/LMedian
Nez Perce County17 mi14922.1%2.3 pCi/L
Latah County42 mi15026.0%1.7 pCi/L
Clearwater County48 mi3330.3%1.8 pCi/L
Idaho County54 mi9040.0%2.9 pCi/L
Shoshone County81 mi8163.0%5.7 pCi/L
Adams County93 mi3151.6%4.0 pCi/L
Kootenai County100 mi2,95561.0%6.0 pCi/L
Valley County110 mi12827.3%2.3 pCi/L
Lewis County ten-year laboratory totals, against Idaho's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestIdaho, pooledChange from the row before
2008 to 2017650.0%3.8 pCi/L7.7 pCi/L41.5%+16.7 points
2007 to 2016933.3%1.2 pCi/L7.7 pCi/L40.8%+3.3 points
2006 to 20151030.0%1.1 pCi/L7.7 pCi/L41.3%+7.8 points
2005 to 2014922.2%1.1 pCi/L7.7 pCi/L41.1%First on file

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

Lewis County, Geology

The ground under Lewis County

Predicted

USGS's geologic map puts all of Lewis County's mapped land in one class: low radon potential. All of it lies in the area the map calls Seven Devils and Tri-State Uplands, to which USGS gives a radon index of 8, in the low band (5 to 8).

USGS's geologic areas under Lewis County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Seven Devils and Tri-State UplandsColumbia River basalt flows.Low8100%

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

Lewis County beside the ground around it

Across Idaho 50.7% of the land is rated high; none of Lewis County's is. Of the 3 counties that border it, 2 have more high-potential ground, Clearwater County the most at 81.2%.

Share of land by USGS radon potential: Lewis County, the 3 counties bordering it and Idaho
AreaEPA zoneLow potentialModerate potentialHigh potential
Lewis CountyZone 3100%NoneNone
Nez Perce CountyZone 399.5%0.5%None
Clearwater CountyZone 118.8%None81.2%
Idaho CountyZone 120.7%0.1%79.3%
Idaho44 counties, pooled by land areaVaries8.0%41.3%50.7%

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

Local help

Lewis County, Local resources

Radon testing and mitigation in Lewis County

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

What it means

Lewis County, Questions

Questions about radon in Lewis County

Are radon levels high in Lewis County?

Over the 2008 to 2017 laboratory totals, 50.0% of the radon tests reported to CDC for Lewis County, Idaho were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.8 pCi/L. Of the counties bordering it, 3 have a figure over the 2008 to 2017 laboratory totals: Idaho County reported the largest share at or above 4 pCi/L, 40.0%, and Nez Perce County the smallest, 22.1%, with Clearwater County at 30.3% between.

Which counties border Lewis County?

Lewis County borders 3 counties: Nez Perce County, Clearwater County and Idaho County.

Products

Lewis County, Products

Testing a home in Lewis County

No CDC figure names Lewis County; the nearest county with one, Nez Perce County (17 miles away), reported 22.1% of its tests at or above 4 pCi/L over the 2008 to 2017 laboratory totals, the only reading nearby until a test of this home. Zone 3 is the lowest label, yet homes at or above 4 pCi/L occur in every zone: a short-term kit (2 to 90 days) is the quick first check in Lewis County.

Test kits and monitors3

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Best radon monitors for Lewis County homes, compared

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Lewis County, Bordering counties

Counties bordering Lewis County

Calculated by Radon411

Lewis County, Cities and towns

Cities and towns in Lewis County

The Census Bureau estimates 3,704 residents in Lewis County for July 1, 2025, 170 more than in 2020 (+4.8%). Spread over its land, that is 7.7 people to the square mile. Kamiah, the largest place on its list (1,134 residents in all), extends beyond Lewis County, so no share of the total is worked out for it. Reported testing is thin here: 10 tests over 2006 to 2015, or 2.7 for every 1,000 residents (2025 estimate), where Idaho as a whole comes to 5.7.

From Kamiah to Reubens, CDC publishes no figure by town: each reads Lewis County's.

The cities and towns in Lewis County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Kamiahextends beyond Lewis County1,134+1.5%Counted whole
Nezperce471+2.8%12.7%
Craigmont469+2.0%12.7%
Winchester363+2.3%9.8%
Reubens52+10.6%1.4%

5 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). A place that extends beyond Lewis County is counted whole, so it has no share.

Lewis County, Homes

Homes in Lewis County

Lewis County is a place of houses on their own lots: 80% of its 1,823 homes are detached single-family houses, against 74% across Idaho. The homes are older than Idaho's as a whole: half were built before 1965 (1990 for Idaho), and 46% date from before 1960. 74% of occupied homes are lived in by their owner (Idaho: 72%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Lewis County, Idaho, 2019 to 2023
HomesLewis CountyIdaho
Housing units1,823776,683
Detached houses80%74%
In buildings of 5 or more2%9%
Mobile homes13%7%
Built before 196046%18%
Built since 20008%35%
Middle year built19651990
Owner-occupied74%72%

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