Radon in Washington

Radon in Washington varies from house to house, so the county figures below describe areas, not homes. EPA places 7 of the state's 39 counties in Zone 1, its highest band of predicted indoor radon, where about 15% of residents live; 18 are in Zone 3. On the water side, EPA has recorded health-based violations at 216 of the state's 2,309 community water systems since 2016.

Zone 1 counties
7
15% of residents
Zone 2 counties
14
12% of residents
Zone 3 counties
18
73% of residents
Water systems with violations
216
of 2,309, since 2016

What Washington's radon program offers

The Department of Health mails free test kits from a limited supply to people who fill in an online form; delivery takes 8 to 10 weeks. Radon line: 360-236-3200.

Under the state residential building code, new buildings in high radon potential (zone 1) counties must follow the radon control methods of Appendix F.

The same radon control methods apply anywhere in the state when a building uses the unvented crawl space method of Section R408.3.

The Department of Health points residents to the NRPP and NRSB national search tools for certified professionals.

Read on Washington State Department of Health, Washington State Indoor Radon Program pages, checked 2026-10-29: state radon program.

Radon in Washington by county

The Washington radon map below groups counties by EPA zone; zones predict Washington radon levels from geology and past surveys and are not home measurements. The largest county, King County (2,340,211 residents), is in Zone 3.

Zone 1: high potential, 7 counties
CountyResidentsWater systemsViolations since 2016Unresolved
Clark County527,2692540
Ferry County7,54311770
Okanogan County44,9424340
Pend Oreille County14,3321530
Skamania County12,6601330
Spokane County555,94787160
Stevens County49,01533100
Zone 2: moderate potential, 14 counties
CountyResidentsWater systemsViolations since 2016Unresolved
Adams County21,0393010
Asotin County22,523200
Benton County218,19044440
Columbia County4,025200
Douglas County45,7951820
Franklin County101,23831180
Garfield County2,404330
Grant County104,71781340
Kittitas County48,1723710
Klickitat County24,1242340
Lincoln County11,8621610
Walla Walla County62,06833470
Whitman County48,3991900
Yakima County258,5238960
Zone 3: low potential, 18 counties
CountyResidentsWater systemsViolations since 2016Unresolved
Chelan County81,2284650
Clallam County77,9586350
Cowlitz County113,9823151
Grays Harbor County77,8933540
Island County86,4782343002
Jefferson County33,9442421
King County2,340,211156160
Kitsap County281,420150100
Lewis County87,0495190
Mason County69,632120120
Pacific County24,24517160
Pierce County941,170227740
San Juan County18,66840690
Skagit County132,7363930
Snohomish County864,11311950
Thurston County302,912196531
Wahkiakum County4,800400
Whatcom County234,954102770

Washington tap water: violations and PFAS

Washington has 2,309 active community water systems serving about 9,565,771 people. Since January 2016, EPA has recorded at least one health-based violation at 216 of them (2% of the people served), and 5 have one still listed as unresolved. The most frequent cause was the rule on coliform bacteria in the distribution pipes, at 72 systems, followed by total trihalomethanes, a byproduct of chlorine disinfection (39) and nitrate, mostly from fertilizer and septic systems (33).

EPA tested 241 Washington systems for PFAS between 2023 and 2025. 34 had a PFOA or PFOS result above the 4 ng/L limit that applies from 2029; the highest was Artondale at 153.6 ng/L.

Seattle780,995 residents

Records: EPA Map of Radon Zones, EPA ECHO drinking water data (2026Q3), EPA UCMR 5, Washington State Department of Health, Washington State Indoor Radon Program, US Census Bureau Vintage 2024. How zones and violations are counted: methodology · all states.