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Radon Levels in King County, Washington

EPA puts King County in Zone 3, its lowest radon zone, and most reported tests there come back low, though 5.0% of 503 (2020) reached 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

    5.0%

    14.0 pts below WA

    King
    5.0%
    Washington
    19.0%

    503 tests, 2020

  • Median result

    0.3pCi/L

    Below action level

    King
    0.3 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2020

  • Tests reported

    503

    2nd of 10 by tests

    King
    503
    Spokane
    721

    Reported to CDC before any fix, 2020

  • 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

    7 of 10

    Middle half

    King
    5.0%
    Spokane
    40.6%

    By share, of 10 with 30+ tests

Read a result for King County

Starts at King County's median, 0.3 pCi/L

0.3pCi/L

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

EPA’s reading of 0.3 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. King County's median reported result, 2020.

Interpret this reading

The data

King County, At a glance

King County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for King 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 98% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for King County, 2020

Share of reported tests at or above 4 pCi/L, by year, King County
Share of reported tests at or above 4 pCi/L, by year, King County
YearShare of reported tests at or above 4 pCi/L
20142.2%
20150.8%
20164.8%
20172.8%
20184.5%
20192.4%
20205.0%

2020: 5.0%. Across these years the share ran from 0.8% to 5.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 503 tests reported to CDC for King County over 2020, 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: 2020. Measured data period: 2005 to 2020 (every CDC figure on file).

How to read these figures

King County, Compared

King County radon figures, compared

Calculated by Radon411

King County reports fewer high tests than Washington overall. 5.0% of its 503 tests in 2020 were at or above 4 pCi/L. That is 14.0 points below the 19.0% pooled from 37 Washington counties.

Ordered from highest share down, it comes 7th of 10 Washington counties with 30 or more tests for the same period. Nationally, every county with a figure for the same period pools to 25.5%; King County sits 20.5 points below it.

Next door, the shares run from 1.0% in Snohomish County to 5.3% in Yakima County, with Pierce County and Snohomish County below King County. Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 5.0% share point the same way here.

Half the tests came in under 0.3 pCi/L and half over it; the single highest reached 15.4 pCi/L. The rest split 89.3% under 2 pCi/L and 5.8% from 2 to 4 (Washington: 67.6% and 13.3%). Year by year, 2014 opened at 2.2% and 2020 closed at 5.0% (up 2.8 points), with 2020 the high at 5.0%.

The busiest year on file was 2015, with 708 tests reported; the quietest, 2018, had 223.

Share of reported tests at or above 4 pCi/L by year, King County against Washington2014 to 2020, years with 30 or more tests: King County 2.2% to 5.0%, the state's pooled share 19.0% in 2020.
King CountyWashington, pooled
Share of reported tests at or above 4 pCi/L by year, King County against Washington: data table
YearTestsKing County, at or above 4 pCi/LWashington, pooled
20146892.2%15.0%
20157080.8%17.4%
20166194.8%16.6%
20175772.8%19.1%
20182234.5%16.1%
20194972.4%20.9%
20205035.0%19.0%
King County against Washington, the United States and nearby counties, 2020
AreaTestsAt or above 4 pCi/LMedianDifference from King County
King County5035.0%0.3 pCi/LThis county
Washington, pooled37 counties, pooled2,78119.0%Not pooled+14.0 points
United States, pooled915 counties, pooled405,35525.5%Not pooled+20.5 points
Spokane Countyhighest in the state72140.6%2.7 pCi/L+35.6 points
Kitsap Countylowest in the state500.0%0.4 pCi/L−5.0 points
Kittitas County, WAborders it520.0%1.0 pCi/L+15.0 points
Chelan County, WAborders it2015.0%1.7 pCi/L+10.0 points
Yakima County, WAborders it575.3%0.7 pCi/L+0.3 points
Pierce County, WAborders it4204.3%0.7 pCi/L−0.7 points
Snohomish County, WAborders it1021.0%0.2 pCi/L−4.0 points
Where King County's reported results fell, 2020, beside Washington and the United States
ResultKing County testsKing County shareWashington shareU.S. shareAgainst Washington
Under 2 pCi/L44989.3%67.6%51.6%+21.7 points
2 to 4 pCi/L295.8%13.3%22.9%−7.5 points
4 pCi/L and above255.0%19.0%25.5%−14.0 points
King County ranked among the 10 Washington counties with 30 or more tests, by share at or above 4 pCi/L, 2020
CountyRankTestsAt or above 4 pCi/LMedian
Spokane County1 of 1072140.6%2.7 pCi/L
Stevens County2 of 104537.8%1.9 pCi/L
Walla Walla County3 of 103420.6%1.5 pCi/L
Clark County4 of 1049017.6%1.2 pCi/L
Thurston County5 of 10466.5%0.4 pCi/L
Yakima County6 of 10575.3%0.7 pCi/L
King County7 of 105035.0%0.3 pCi/L
Pierce County8 of 104204.3%0.7 pCi/L
Snohomish County9 of 101021.0%0.2 pCi/L
Kitsap County10 of 10500.0%0.4 pCi/L
King County radon tests by year, against Washington's pooled share
YearTestsAt or above 4 pCi/LMedianHighestWashington, pooledChange from the row before
20205035.0%0.3 pCi/L15.4 pCi/L19.0%+2.6 points
20194972.4%0.3 pCi/L20.9 pCi/L20.9%−2.1 points
20182234.5%0.4 pCi/L26.8 pCi/L16.1%+1.7 points
20175772.8%0.4 pCi/L14.9 pCi/L19.1%−2.0 points
20166194.8%0.6 pCi/L58.7 pCi/L16.6%+4.0 points
20157080.8%0.4 pCi/L72.7 pCi/L17.4%−1.4 points
20146892.2%0.4 pCi/L50.0 pCi/L15.0%First on file

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

King County, Geology

The ground under King County

Predicted

USGS's geologic map puts all of King County's mapped land in one class: low radon potential. The map puts 70.8% of King County in the area it calls Cascade Mountains and 27.3% in Puget Lowlands. USGS gives both a radon index of 6, in the low band (5 to 8).

USGS's geologic areas under King 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.Low670.8%
Puget LowlandsContinental and marine sediments overlain by sandy and silty glacial deposits.Low627.3%

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

King County beside the ground around it

Across Washington 13.7% of the land is rated high; none of King County's is. Of the 5 counties that border it, 3 have more moderate-potential ground, Yakima County the most at 80.3%.

Share of land by USGS radon potential: King County, the 5 counties bordering it and Washington
AreaEPA zoneLow potentialModerate potentialHigh potential
King CountyZone 398.1%NoneNone
Pierce CountyZone 398.4%NoneNone
Snohomish CountyZone 399.7%NoneNone
Kittitas CountyZone 222.5%77.5%None
Chelan CountyZone 396.5%3.5%None
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

King County, Local resources

Radon testing and mitigation in King County

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

What it means

King County, Questions

Questions about radon in King County

Are radon levels high in King County?

In 2020, 5.0% of the radon tests reported to CDC for King County, Washington were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.3 pCi/L. Of 10 Washington counties with 30 or more tests for the same period, 6 reported a higher share; pooled, the state comes to 19.0%.

Which counties border King County?

King County borders 5 counties: Pierce County, Snohomish County, Kittitas County, Chelan County and Yakima County.

Products

King County, Products

Testing a home in King County

In King County, 5.0% of 503 tests reported to CDC in 2020 reached 4 pCi/L, against 19.0% 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 King County.

Test kits and monitors3

See every kit and monitor

Best radon monitors for King County homes, compared

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King County, Bordering counties

Counties bordering King County

Calculated by Radon411

King County, Cities and towns

Cities and towns in King County

The Census Bureau estimates 2,344,939 residents in King County for July 1, 2025, 70,635 more than in 2020 (+3.1%). Packed into its land, that is 1,109 people per square mile. Its largest town, Seattle, has 784,777 residents, 33.5% of the total. Reported testing is thin here: 503 tests over 2020, or 0.2 for every 1,000 residents (2025 estimate), where Washington as a whole comes to 0.3.

From Seattle to Skykomish, CDC publishes no figure by town: each reads King County's.

The 12 largest cities and towns in King County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Seattle784,777+6.0%33.5%
Bellevue154,193+1.6%6.6%
Kent134,871−0.4%5.8%
Renton104,975−1.6%4.5%
Federal Way99,729−1.3%4.3%
Kirkland97,410+5.0%4.2%
Auburnextends beyond King County86,301−2.2%Counted whole
Redmond82,506+12.4%3.5%
Shoreline66,542+13.4%2.8%
Sammamish66,110−2.0%2.8%
Bothellextends beyond King County52,199+8.4%Counted whole
Burien51,859−0.3%2.2%
  • Issaquah
  • SeaTac
  • Des Moines
  • Maple Valley
  • Mercer Island
  • Kenmore
  • Tukwila
  • Covington
  • Woodinville
  • Snoqualmie
  • Lake Forest Park
  • Enumclaw (part)
  • Newcastle
  • Duvall
  • Milton (part)
  • North Bend
  • Pacific (part)
  • Black Diamond
  • Normandy Park
  • Algona
  • Clyde Hill
  • Medina
  • Carnation
  • Yarrow Point
The other 3 places in King County
  • Hunts Point
  • Beaux Arts Village
  • Skykomish

39 incorporated places, as listed in the U.S. Census Bureau’s 2020 place reference files. “(part)” marks a place that extends into another county. Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025). A place that extends beyond King County is counted whole, so it has no share.

King County, Homes

Homes in King County

King County has 988,330 homes, and many share a building: 37% are in buildings of five or more, 51% are detached houses, against 62% across Washington. Half of its homes were built before 1983, close to Washington's 1984: 23% date from before 1960 and 28% from 2000 or later. 56% 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 King County, Washington, 2019 to 2023
HomesKing CountyWashington
Housing units988,3303,262,667
Detached houses51%62%
In buildings of 5 or more37%22%
Mobile homes2%6%
Built before 196023%21%
Built since 200028%28%
Middle year built19831984
Owner-occupied56%64%

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