Why Spokane Is EPA Radon Zone 1
Spokane's radon potential isn't a modeled guess. It's a documented geological pattern with real historical test data behind it, and a regulatory contrast with North Idaho no page-one competitor covers.
What EPA Radon Zone 1 Actually Means
The EPA's Map of Radon Zones defines Zone 1 with specific, quantified language: "predicted average indoor radon screening levels greater than 4 pCi/L," the same level at which EPA recommends fixing a home. Spokane County carries this designation, corroborated independently at the state level: WAC 51-51-60101 Table AF101(1) lists Spokane among Washington's seven "high radon potential (zone 1) counties," alongside Clark, Ferry, Okanogan, Pend Oreille, Skamania, and Stevens. It's important to be precise about what this designation is and isn't. Zone 1 is a county-level prediction built from geology and historical sampling, not a guarantee that any individual home will test high. It does mean elevated readings are common enough across the county that testing every home, not just some, is the responsible default.
The Geology Behind Spokane's Radon Potential
Two geological factors combine to produce Spokane's radon potential, and neither alone would fully explain it. Uranium-bearing granitic and metamorphic bedrock in the Okanogan Highlands and the hills north of Spokane is the ultimate radon source; as uranium decays, it produces radon gas as one step in its decay chain. On its own, that source wouldn't necessarily translate into elevated indoor levels. What amplifies it is the second factor: thick, highly permeable glaciofluvial outwash gravel deposited by the ice age glacial Lake Missoula floods roughly 18,000 to 30,000 years ago, the same gravel that underlies the Spokane Valley and forms the Spokane Valley-Rathdrum Prairie Aquifer. USGS and EPA radon-potential studies for Washington repeatedly identify soil permeability, not soil-gas radon concentration alone, as the primary amplifier: the same excessively well-drained gravel that makes the aquifer such an effective water source also lets radon gas move easily into homes built on it.
Real Historical Radon Data for Spokane County
449 Spokane County homes sampled. Average 9.9 pCi/L. 59 percent above the 4 pCi/L action level. 13 percent above 20 pCi/L, the highest county average in the Eastern Washington dataset.
Neighboring counties in the same survey show a consistent pattern tied to the same geology: Pend Oreille at 7.2 pCi/L, Whitman at 5.9 pCi/L, Stevens at 5.1 pCi/L. This is a real, sourced, primary dataset, not a fabricated statistic, though it's worth stating its age plainly: it's a 30-plus-year-old sample from a specific survey round, not a current official average. At least one third-party radon-industry source cites a different "Spokane County average of 7.3 pCi/L" without clearly documenting its methodology or vintage; we're presenting both figures rather than silently picking one, and using the primary EPA/State-survey figure as the more verifiable of the two.
Kootenai County, Idaho Is Zone 1 Too
The geology behind Spokane's radon potential doesn't stop at the Washington-Idaho state line. Kootenai County, Idaho, home to Coeur d'Alene and Post Falls, is independently designated EPA Radon Zone 1 via EPA's own Idaho radon zone map, the same highest-potential category as Spokane County. That's consistent with what you'd expect from the underlying science: EPA zone designations are drawn from geology and historical sampling, not political boundaries, and the granite bedrock and glacial outwash gravel that elevate Spokane's radon potential extend into North Idaho's landscape as well. If you own property in the Coeur d'Alene or Post Falls area, the same testing guidance that applies throughout this site applies to your home too; nothing about the state line changes the physical risk.
Washington vs. Idaho: Same Zone, Different Rules
Here's the single most differentiated fact this site publishes: Washington and Idaho share the same geology and the same Zone 1 designation, but regulate new construction completely differently. Washington's building code, WAC 51-51 Appendix F, makes radon-resistant construction mandatory for new homes in Spokane County and six other Zone 1 counties. Idaho has no statewide equivalent, and no source shows Kootenai County, Coeur d'Alene, or Post Falls has locally adopted one. A new home built today in Zone 1 Spokane County is legally required to include a passive radon system; an equally Zone 1 new home in Kootenai County, across the state line, currently is not. See our new construction page for the full breakdown of what Washington's code actually requires.
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Zone 1 Questions, Answered Straight
What does it mean that Spokane is EPA Radon Zone 1?
Zone 1 is the EPA's highest radon potential category, meaning the average predicted indoor radon screening level for the county exceeds 4 pCi/L, the same level at which EPA recommends fixing a home. It doesn't mean every home is guaranteed to test high, but it does mean elevated readings are common enough to test for.
Why does Spokane have such high radon potential?
Two geological factors combine here: uranium-bearing granite bedrock in the hills north of Spokane supplies the radon source, and the thick, highly permeable glacial outwash gravel left by the ice age Missoula Floods, the same material that forms the Spokane Valley-Rathdrum Prairie Aquifer, lets soil gas move easily into homes above it.
Is there real historical data on Spokane's radon levels?
Yes. A 1991-1992 EPA and Washington State residential radon survey sampled 449 Spokane County homes and found an average of 9.9 pCi/L, with 59 percent above the EPA's 4 pCi/L action level. It's an older dataset, not a current statistic, but the geology behind it hasn't changed, and other Spokane sources report levels of similar magnitude.
Is Kootenai County, Idaho also Zone 1?
Yes. Kootenai County, home to Coeur d'Alene and Post Falls, is also designated EPA Radon Zone 1, the same highest-potential category as Spokane County. The two counties share similar underlying geology, though Washington and Idaho regulate new-construction radon protection very differently, covered in more detail on our new construction page.
Does a Zone 1 designation mean my specific home has high radon?
No, and this is an important distinction. Zone 1 is a county-level prediction based on geology and past sampling, not a guarantee about any individual house. EPA itself says the zone map shouldn't be used to decide whether to test a specific home; every home, in any zone, should be tested directly to know its actual level.
See Washington's construction requirements
What Spokane County requires and how North Idaho differs.