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Radon Levels in Cowlitz County, Washington

EPA puts Cowlitz County in Zone 3, its lowest radon zone, and only 24 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

    22.7%

    7.7 pts above WA

    Cowlitz
    22.7%
    Washington
    15.0%

    132 tests, 2014

  • Median result

    1.0pCi/L

    Below action level

    Cowlitz
    1.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2014

  • Tests reported

    132

    5th of 14 by tests

    Cowlitz
    132
    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

    3 of 14

    Top quarter

    Cowlitz
    22.7%
    Klickitat
    48.5%

    By share, of 14 with 30+ tests

Read a result for Cowlitz County

Starts at Cowlitz County's median, 1.0 pCi/L

1.0pCi/L

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

EPA’s reading of 1.0 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. Cowlitz County's median reported result, 2014.

Interpret this reading

The data

Cowlitz County, At a glance

Cowlitz County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for Cowlitz 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 97% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Cowlitz County, 2020

Share of reported tests at or above 4 pCi/L, by year, Cowlitz County
Share of reported tests at or above 4 pCi/L, by year, Cowlitz County
YearShare of reported tests at or above 4 pCi/L
201422.7%
20156.1%
20169.8%
201722.2%
20180.0%
201921.1%
20204.2%

2020: 4.2%. Across these years the share ran from 0.0% to 22.7%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

Only 24 tests were reported to CDC for Cowlitz 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

Cowlitz County, Compared

Cowlitz County radon figures, compared

Calculated by Radon411

Cowlitz County's 132 reported tests in 2014 had a middle value of 1.0 pCi/L and a maximum of 18.4 pCi/L, and 22.7% reached 4 pCi/L. Washington as a whole (36 Washington counties, pooled) comes to 15.0%: 7.7 points lower than here.

The newest figure, in 2020, counts only 24 tests (4.2% at or above 4 pCi/L), too few to compare, so this reading uses the 132-test figure.

Among its state's counties it ranks 3rd of 14 Washington counties with 30 or more tests for the same period. Statewide for the same period, Klickitat County reported the highest share (48.5%) and Skagit County the lowest (0.0%).

Below the action level, 87 tests (65.9%) came in under 2 pCi/L, against 72.8% statewide, and 15 tests (11.4%) fell between 2 and 4, against 12.2% statewide. The busiest year on file was 2014, with 132 tests reported; the quietest, 2018, had 17.

EPA's 1993 zone map puts Cowlitz County in Zone 3, a predicted county average under 2 pCi/L. USGS geology rates 97% of the county's land low radon potential.

Next door, the shares run from 2.8% in Lewis County to 33.3% in Columbia County, OR, with Clark County and Lewis County below Cowlitz County. Over 3 years of annual figures the share fell 12.9 points, from 22.7% in 2014 to 9.8% in 2016; its peak was 22.7% in 2014.

Share of reported tests at or above 4 pCi/L by year, Cowlitz County against Washington2014 to 2016, years with 30 or more tests: Cowlitz County 22.7% to 9.8%, the state's pooled share 16.6% in 2016.
Cowlitz CountyWashington, pooled
Share of reported tests at or above 4 pCi/L by year, Cowlitz County against Washington: data table
YearTestsCowlitz County, at or above 4 pCi/LWashington, pooled
201413222.7%15.0%
2015336.1%17.4%
2016619.8%16.6%
Cowlitz County against Washington, the United States and nearby counties, 2014
AreaTestsAt or above 4 pCi/LMedianDifference from Cowlitz County
Cowlitz County13222.7%1.0 pCi/LThis county
Washington, pooled36 counties, pooled3,15315.0%Not pooled−7.7 points
United States, pooled925 counties, pooled517,29334.9%Not pooled+12.2 points
Klickitat Countyhighest in the state3348.5%3.4 pCi/L+25.8 points
Skagit Countylowest in the state300.0%0.5 pCi/L−22.7 points
Columbia County, ORborders it3933.3%1.8 pCi/L+10.6 points
Skamania County, WAborders it825.0%2.7 pCi/L+2.3 points
Clark County, WAborders it57118.6%1.1 pCi/L−4.1 points
Lewis County, WAborders it712.8%0.4 pCi/L−19.9 points
Wahkiakum County, WAborders it10.0%0.1 pCi/L−22.7 points
Where Cowlitz County's reported results fell, 2014, beside Washington and the United States
ResultCowlitz County testsCowlitz County shareWashington shareU.S. shareAgainst Washington
Under 2 pCi/L8765.9%72.8%40.0%−6.9 points
2 to 4 pCi/L1511.4%12.2%25.1%−0.8 points
4 pCi/L and above3022.7%15.0%34.9%+7.7 points
Cowlitz 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
Klickitat County1 of 143348.5%3.4 pCi/L
Spokane County2 of 1460034.7%2.2 pCi/L
Cowlitz County3 of 1413222.7%1.0 pCi/L
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
Cowlitz County radon tests by year, against Washington's pooled share
YearTestsAt or above 4 pCi/LMedianHighestWashington, pooledChange from the row before
2020244.2%1.1 pCi/L5.0 pCi/L19.0%−16.9 points
20191921.1%0.7 pCi/L11.5 pCi/L20.9%+21.1 points
2018170.0%0.6 pCi/L2.7 pCi/L16.1%−22.2 points
20172722.2%0.5 pCi/L6.2 pCi/L19.1%+12.4 points
2016619.8%0.7 pCi/L7.4 pCi/L16.6%+3.7 points
2015336.1%1.3 pCi/L8.6 pCi/L17.4%−16.6 points
201413222.7%1.0 pCi/L18.4 pCi/L15.0%First on file

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

Cowlitz County, Geology

The ground under Cowlitz County

Predicted

Most of the ground under Cowlitz County is one class on USGS's geologic map: 96.6% of its land is low radon potential, with 2.1% moderate. The map puts 74.6% of Cowlitz County in the area it calls Cascade Mountains and 21.5% in Coastal Hills, with 2 smaller areas beside them. USGS gives both a radon index of 6, in the low band (5 to 8).

USGS's geologic areas under Cowlitz 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.Low674.6%
Coastal HillsCretaceous and Tertiary continental and marine sedimentary rocks and pre-Miocene volcanic rocks.Low621.5%
Columbia River GorgeRiver and lacustrine delta deposits and volcanic debris flows.Moderate92.1%
Puget LowlandsContinental and marine sediments overlain by sandy and silty glacial deposits.Low60.5%

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

Cowlitz County beside the ground around it

Across Washington 13.7% of the land is rated high; none of Cowlitz County's is. Of the 5 counties that border it, 3 have more moderate-potential ground, Columbia County, OR the most at 91.6%.

Share of land by USGS radon potential: Cowlitz County, the 5 counties bordering it and Washington
AreaEPA zoneLow potentialModerate potentialHigh potential
Cowlitz CountyZone 396.6%2.1%None
Clark CountyZone 126.0%70.9%None
Columbia County, ORZone 24.2%91.6%None
Lewis CountyZone 3100%NoneNone
Wahkiakum CountyZone 392.5%NoneNone
Skamania CountyZone 175.5%24.0%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

Cowlitz County, Local resources

Radon testing and mitigation in Cowlitz County

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

What it means

Cowlitz County, Questions

Questions about radon in Cowlitz County

Are radon levels high in Cowlitz County?

In 2020, 4.2% of the radon tests reported to CDC for Cowlitz County, Washington were at or above 4 pCi/L, and the median reported pre-mitigation result was 1.1 pCi/L. Of 14 Washington counties with 30 or more tests for the same period, 2 reported a higher share; pooled, the state comes to 15.0%.

Which counties border Cowlitz County?

Cowlitz County borders 5 counties: Clark County, WA, Columbia County, OR, Lewis County, WA, Wahkiakum County, WA and Skamania County, WA.

Products

Cowlitz County, Products

Testing a home in Cowlitz County

In Cowlitz County, 22.7% of 132 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 Cowlitz County.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Cowlitz County homes, compared

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Cowlitz County, Bordering counties

Counties bordering Cowlitz County

Calculated by Radon411

Cowlitz County, Cities and towns

Cities and towns in Cowlitz County

The Census Bureau estimates 114,885 residents in Cowlitz County for July 1, 2025, 3,871 more than in 2020 (+3.5%). That works out to 101 people per square mile of land. Its largest town, Longview, has 38,362 residents, 33.4% of the total. Testing is reported here more often than across the state: CDC's 132 reported tests for 2014 come to 1.1 for every 1,000 residents (2025 estimate), against 0.4 for Washington.

From Longview to Castle Rock, CDC publishes no figure by town: each reads Cowlitz County's.

The cities and towns in Cowlitz County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Longview38,362+1.3%33.4%
Kelso12,801+0.6%11.1%
Woodlandextends beyond Cowlitz County6,611+1.0%Counted whole
Kalama3,165+6.4%2.8%
Castle Rock2,442−0.2%2.1%

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 Cowlitz County is counted whole, so it has no share.

Cowlitz County, Homes

Homes in Cowlitz County

Of Cowlitz County's 45,884 homes, 68% are detached houses, against 62% across Washington; 14% are in buildings of five or more homes. The homes are older than Washington's as a whole: half were built before 1975 (1984 for Washington), and 31% date from before 1960. 66% 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 Cowlitz County, Washington, 2019 to 2023
HomesCowlitz CountyWashington
Housing units45,8843,262,667
Detached houses68%62%
In buildings of 5 or more14%22%
Mobile homes10%6%
Built before 196031%21%
Built since 200019%28%
Middle year built19751984
Owner-occupied66%64%

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