Radon levels in Washington

About radon in Washington

Radon in Washington depends heavily on which side of the state you live in. In the test records behind our county pages, 12,629 of 64,932 tests recorded from 2000 to 2025 were at or above the EPA's 4 pCi/L action level, about 19% statewide (Washington State Department of Health). That average hides a sharp divide.

The highest shares are in the northeast. In Pend Oreille County, 46% of tests were at or above 4 pCi/L; in Stevens County, 44%; and in Spokane County, where 15,679 tests make it one of the best-measured counties in the state, 40%. Around Puget Sound, high results are rare: 1.3% of tests in Kitsap County, 1.6% in Snohomish County and 3.8% in King County. Clark County, in the southwest, sits in between at 21%. Because these records come from tests people chose to order, they describe tested homes rather than every home, and a low county share does not mean a particular house is low.

Washington's building code reflects the same pattern. Seven counties are listed as high radon potential (Clark, Ferry, Okanogan, Pend Oreille, Skamania, Spokane and Stevens), and new homes there must follow the code's radon-resistant construction requirements; unvented crawl spaces are not allowed in those counties (WAC 51-51-60101).

When a home is sold, the seller disclosure statement asks whether the seller knows of substances that may be environmental concerns, including radon gas (RCW 64.06.020). Washington does not run its own radon license; the Department of Health points residents to professionals certified by the National Radon Proficiency Program or the National Radon Safety Board, and offers a limited supply of free test kits (Washington State Department of Health).

Pick your county below to see its own results and how it compares with its neighbours.

Written and reviewed by Ray Roman · 2026-11-03