Locations and reports
Radon in Seattle, Washington
EPA puts King County, where Seattle is, 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
Share at 4+ pCi/L
5.0%
14.0 pts below WA
- King
- 5.0%
- Washington
- 19.0%
King, 7th of 10 in WA, 2020
Median result
0.3pCi/L
Below action level
- King
- 0.3 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of King's tests, 2020
Nearest county
0.0%
Kitsap, 16 miles
- Kitsap
- 0.0%
- King
- 5.0%
50 tests, 2020
EPA radon zone
Zone 3
Low potential
- 1
- 2
- Zone 3
1 highest, 3 lowest
King, where Seattle is
Tests reported
503
In King
- King
- 503
- Kitsap
- 50
Reported to CDC before any fix, 2020
Read a result for Seattle
Starts at King County's median, 0.3 pCi/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
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 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.
The data
Seattle, What is published
What is published for Seattle
Every figure for Seattle is a county's
Radon data is published by county, not by city, so for Seattle it is King County's: its EPA zone (Zone 3) and the 503 tests reported to CDC for 2020. Only a test tells you about one home.
Seattle covers 83.8 square miles of land, all of it in King County. Its 784,777 residents live 9,361 to the square mile.
Seattle, Counties
The county Seattle lies in
MeasuredKing County's EPA zone and the figures CDC publishes for it, 2020.
King CountyZone 3Lower predicted potential
- 5.0%
- Reported tests at or above 4 pCi/L
- 0.3 pCi/L
- Median reported result
- 503 tests
- Tests reported to CDC
- 7
- Annual years on file
King County ranks 7th of 10 Washington counties with 30 or more tests for that period, 14.0 points below the 19.0% pooled across Washington's counties, 20.5 points below the national pool of 25.5%.
Measured from the middle of Seattle, the nearest counties with a CDC figure for 2020 are Kitsap County (16 miles, 0.0% of 50 tests), Snohomish County (41 miles, 1.0% of 102 tests) and Pierce County (43 miles, 4.3% of 420 tests). Across the 8 nearest, the share at or above 4 pCi/L runs from 0.0% to 37.8%; the farthest of them, Stevens County, is 214 miles away at 37.8%.
No business in the directory gives Seattle as its address; the Washington list covers it. Washington's other cities with a page of their own: Vancouver.
The counties around Seattle
By distance from the middle of Seattle; counties with 30 or more tests.
Predicted: EPA Map of Radon Zones, by county. Measured: CDC tracking data, by county, 2020.
Seattle, Geology
The ground under Seattle
PredictedUSGS's geologic map puts all of Seattle's mapped land in one class: low radon potential. The map puts 81.6% of Seattle in the area it calls Puget Lowlands and 15.4% in Cascade Mountains. USGS gives both a radon index of 6, in the low band (5 to 8).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Puget Lowlands | Continental and marine sediments overlain by sandy and silty glacial deposits. | Low | 6 | 81.6% |
| Cascade Mountains | Northern 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. | Low | 6 | 15.4% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Seattle beside its county and Washington
Neither Seattle nor King County as a whole has any land USGS rates above low potential. Across Washington 13.7% of the land is rated high; none of Seattle's is.
The map rates the ground, not the air in any home.
Seattle, Homes
Homes in Seattle
Seattle has 382,933 homes, and many share a building: 51% are in buildings of five or more, 38% are detached houses, against 51% across King County. The homes are older than King County's as a whole: half were built before 1974 (1983 for King County), and 39% date from before 1960. Most occupied homes here are rented: owners live in 44% (King County: 56%). A tenant can run a test; a fix is the landlord's to make.
| Homes | Seattle | King County | Washington |
|---|---|---|---|
| Housing units | 382,933 | 988,330 | 3,262,667 |
| Detached houses | 38% | 51% | 62% |
| In buildings of 5 or more | 51% | 37% | 22% |
| Mobile homes | under 1% | 2% | 6% |
| Built before 1960 | 39% | 23% | 21% |
| Built since 2000 | 31% | 28% | 28% |
| Middle year built | 1974 | 1983 | 1984 |
| Owner-occupied | 44% | 56% | 64% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.
Local help
Seattle, Local resources
Local resources for Seattle
Who to ask in Washington: its radon program, test kits and professionals.
- Radon ProgramRun by Washington State Department of Health, phone 360-236-3200.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
We have a limited supply of free radon test kits.
- Compare kits
- Professionals in WashingtonRadon testers and mitigators in Washington, each with how its listing was checked.
The program's own words on licensing are on the Washington page.
What it means
Seattle, Questions
Questions about radon in Seattle
What radon zone is Seattle in?
EPA's map puts King County, and with it Seattle, in Zone 3, the map's lowest class: a predicted county average under 2 pCi/L. A low zone does not clear a house: homes over the action level are found in all three.
What are radon levels in Seattle?
Reported levels around Seattle run low: 5.0% of 503 tests from King County for 2020 reached 4 pCi/L. The median reported result was 0.3 pCi/L. Low is not none.
Should I test my home in Seattle?
Yes, even here. King County reported 25 of 503 tests at or above 4 pCi/L. That is below Washington's pooled 19.0%, and still not a reason to skip it, and ranks King County 7th of the 10 Washington counties with 30 or more tests. Testing found those homes; the county's low share did not. How to test a home.
What ground is Seattle built on, as far as radon goes?
81.6% of Seattle sits on what USGS's map calls "Puget Lowlands", rated low for radon potential, radon index 6; another 15.4% is "Cascade Mountains", in the same class.
Is radon higher in the counties around Seattle?
It depends which way you go. 5 of the 8 nearest counties with a figure report a larger share than King County's 5.0% and 3 a smaller one: from Kitsap County (0.0%, 16 miles off) to Stevens County (37.8%, 214 miles off).
The answers use the published figures of King County, the county Seattle lies in.
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