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Radon Levels in Lewis County, Washington

EPA puts Lewis County in Zone 3, its lowest radon zone, and only 10 radon tests from it are on file with CDC (2020). 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

    2.8%

    12.2 pts below WA

    Lewis
    2.8%
    Washington
    15.0%

    71 tests, 2014

  • Median result

    0.4pCi/L

    Below action level

    Lewis
    0.4 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2014

  • Tests reported

    71

    12th of 14 by tests

    Lewis
    71
    King
    689

    Reported to CDC before any fix, 2014

  • 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

  • Rank in Washington

    10 of 14

    Middle half

    Lewis
    2.8%
    Klickitat
    48.5%

    By share, of 14 with 30+ tests

Read a result for Lewis County

Starts at Lewis County's median, 0.4 pCi/L

0.4pCi/L

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

EPA’s reading of 0.4 pCi/L

Below 2 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 below 2 pCi/L, the level at which EPA says a fix should be considered. Lewis County's median reported result, 2014.

Interpret this reading

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, 2020

Share of reported tests at or above 4 pCi/L, by year, Lewis County
Share of reported tests at or above 4 pCi/L, by year, Lewis County
YearShare of reported tests at or above 4 pCi/L
20142.8%
201527.3%
20160.0%
20170.0%
201811.1%
201920.0%
202070.0%

2020: 70.0%. Across these years the share ran from 0.0% to 70.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

Only 10 tests were reported to CDC for Lewis County over 2020, 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: 2020. Measured data period: 2005 to 2020 (every CDC figure on file).

How to read these figures

Lewis County, Compared

Lewis County radon figures, compared

Calculated by Radon411

Only 2 of 71 radon tests reported for Lewis County in 2014 reached 4 pCi/L (2.8%). The median result, 0.4 pCi/L, sits below 2, well under EPA's 4 pCi/L action level, and the top reported result was 10.8 pCi/L.

The newest figure, in 2020, counts only 10 tests (70.0% at or above 4 pCi/L), too few to compare, so this reading uses the 71-test figure.

Across 36 Washington counties pooled together, 15.0% of tests reached that level, so Lewis County runs 12.2 points below its state. Against the national pool of 34.9%, the gap is 32.1 points lower. Its place in the state: 10th of 14 Washington counties with 30 or more tests for the same period.

Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 2.8% share point the same way here. Of 3 neighbours with a figure, all 3 reported a larger share at or above 4 pCi/L; the spread is 6.3% (Thurston County) to 22.7% (Cowlitz County).

Below the action level, 61 tests (85.9%) came in under 2 pCi/L, against 72.8% statewide, and 8 tests (11.3%) fell between 2 and 4, against 12.2% statewide. USGS geology rates 100% of the county's land low radon potential.

The busiest year on file was 2014, with 71 tests reported; the quietest, 2017, had 2.

Lewis County against Washington, the United States and nearby counties, 2014
AreaTestsAt or above 4 pCi/LMedianDifference from Lewis County
Lewis County712.8%0.4 pCi/LThis county
Washington, pooled36 counties, pooled3,15315.0%Not pooled+12.2 points
United States, pooled925 counties, pooled517,29334.9%Not pooled+32.1 points
Klickitat Countyhighest in the state3348.5%3.4 pCi/L+45.7 points
Skagit Countylowest in the state300.0%0.5 pCi/L−2.8 points
Grays Harbor County, WAborders it425.0%2.1 pCi/L+22.2 points
Skamania County, WAborders it825.0%2.7 pCi/L+22.2 points
Cowlitz County, WAborders it13222.7%1.0 pCi/L+19.9 points
Pierce County, WAborders it21515.3%0.9 pCi/L+12.5 points
Thurston County, WAborders it1126.3%0.6 pCi/L+3.5 points
Pacific County, WAborders it40.0%1.1 pCi/L−2.8 points
Wahkiakum County, WAborders it10.0%0.1 pCi/L−2.8 points
Yakima County, WAborders it210.0%0.8 pCi/L−2.8 points
Where Lewis County's reported results fell, 2014, beside Washington and the United States
ResultLewis County testsLewis County shareWashington shareU.S. shareAgainst Washington
Under 2 pCi/L6185.9%72.8%40.0%+13.1 points
2 to 4 pCi/L811.3%12.2%25.1%−0.9 points
4 pCi/L and above22.8%15.0%34.9%−12.2 points
Lewis County ranked among the 14 Washington counties with 30 or more tests, by share at or above 4 pCi/L, 2014
CountyRankTestsAt or above 4 pCi/LMedian
Clark County4 of 1457118.6%1.1 pCi/L
Pierce County5 of 1421515.3%0.9 pCi/L
Whitman County6 of 147812.8%0.7 pCi/L
Thurston County7 of 141126.3%0.6 pCi/L
Benton County8 of 141135.3%0.7 pCi/L
Snohomish County9 of 141303.8%0.5 pCi/L
Lewis County10 of 14712.8%0.4 pCi/L
King County11 of 146892.2%0.4 pCi/L
Whatcom County12 of 14851.2%0.5 pCi/L
Kitsap County13 of 14820.0%0.4 pCi/L
Skagit County13 of 14300.0%0.5 pCi/L
Lewis County radon tests by year, against Washington's pooled share
YearTestsAt or above 4 pCi/LMedianHighestWashington, pooledChange from the row before
20201070.0%15.6 pCi/L22.0 pCi/L19.0%+50.0 points
2019520.0%0.4 pCi/L17.9 pCi/L20.9%+8.9 points
2018911.1%2.4 pCi/L4.0 pCi/L16.1%+11.1 points
201720.0%1.8 pCi/L3.5 pCi/L19.1%Level
201640.0%0.8 pCi/L3.9 pCi/L16.6%−27.3 points
20151127.3%0.1 pCi/L8.0 pCi/L17.4%+24.5 points
2014712.8%0.4 pCi/L10.8 pCi/L15.0%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. The map puts 67.2% of Lewis County in the area it calls Cascade Mountains and 19.0% in Coastal Hills, with one smaller area beside them. USGS gives both a radon index of 6, in the low band (5 to 8).

USGS's geologic areas under Lewis County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Cascade MountainsNorthern Cascade Mountains are Mesozoic metamorphic rocks intruded by Mesozoic and Tertiary granitic rocks. Southern Cascade Mountains are composed primarily of Tertiary and Holocene volcanic rocks.Low667.2%
Coastal HillsCretaceous and Tertiary continental and marine sedimentary rocks and pre-Miocene volcanic rocks.Low619.0%
Puget LowlandsContinental and marine sediments overlain by sandy and silty glacial deposits.Low613.8%

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 Washington 13.7% of the land is rated high; none of Lewis County's is. Of the 8 counties that border it, 3 have more moderate-potential ground, Yakima County the most at 80.3%.

Share of land by USGS radon potential: Lewis County, the 8 counties bordering it and Washington
AreaEPA zoneLow potentialModerate potentialHigh potential
Lewis CountyZone 3100%NoneNone
Cowlitz CountyZone 396.6%2.1%None
Pierce CountyZone 398.4%NoneNone
Thurston CountyZone 396.8%NoneNone
Skamania CountyZone 175.5%24.0%None
Wahkiakum CountyZone 392.5%NoneNone
Pacific CountyZone 397.4%NoneNone
Grays Harbor CountyZone 399.4%NoneNone
Yakima CountyZone 219.7%80.3%None
Washington38 counties, pooled by land areaVaries46.4%39.0%13.7%

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 Washington's program says
Phone
360-236-3200
Email
doh.radon@doh.wa.gov

What it means

Lewis County, Questions

Questions about radon in Lewis County

Are radon levels high in Lewis County?

In 2020, 70.0% of the radon tests reported to CDC for Lewis County, Washington were at or above 4 pCi/L, and the median reported pre-mitigation result was 15.6 pCi/L. Of 14 Washington counties with 30 or more tests for the same period, 9 reported a higher share; pooled, the state comes to 15.0%.

Which counties border Lewis County?

Lewis County borders 8 counties: Cowlitz County, Pierce County, Thurston County, Skamania County, Wahkiakum County, Pacific County, Grays Harbor County and Yakima County.

Products

Lewis County, Products

Testing a home in Lewis County

In Lewis County, 2.8% of 71 tests reported to CDC in 2014 reached 4 pCi/L, against 15.0% pooled across Washington. 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

See every kit and monitor

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

Lewis County is growing: the Census Bureau puts it at 88,062 residents on July 1, 2025, up 6.7% from 82,551 in 2020. That works out to 37 people per square mile of land. Its largest town, Centralia, has 19,375 residents, 22.0% of the total. Together its 9 incorporated places hold 40.9% of those residents; the other 59.1% live outside any of them. Testing is reported here more often than across the state: CDC's 71 reported tests for 2014 come to 0.8 for every 1,000 residents (2025 estimate), against 0.4 for Washington.

From Centralia to Pe Ell, 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
Centralia19,375+5.4%22.0%
Chehalis7,764+3.7%8.8%
Winlock2,640+43.8%3.0%
Napavine2,153+13.4%2.4%
Morton1,102+6.1%1.3%
Mossyrock806+4.4%0.9%
Toledo776+22.0%0.9%
Vader730+17.7%0.8%
Pe Ell669+4.0%0.8%

9 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).

Lewis County, Homes

Homes in Lewis County

Of Lewis County's 36,037 homes, 70% are detached houses, against 62% across Washington; 17% are mobile homes. Half of its homes were built before 1979, close to Washington's 1984: 28% date from before 1960 and 21% from 2000 or later. 74% of occupied homes are lived in by their owner (Washington: 64%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Lewis County, Washington, 2019 to 2023
HomesLewis CountyWashington
Housing units36,0373,262,667
Detached houses70%62%
In buildings of 5 or more7%22%
Mobile homes17%6%
Built before 196028%21%
Built since 200021%28%
Middle year built19791984
Owner-occupied74%64%

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