Clark County, WA Radon Zone Data

Clark County, WA radon potential

EPA places Clark County in Zone 1 - Highest Potential — a predicted county-wide average indoor radon level above 4 pCi/L, based on the geology, soil and aerial radioactivity data behind EPA's national map. Rank it against the rest of the country and Clark County lands in the 99th percentile for radon potential.

How Clark County compares

Clark County, Washington is home to about 527,269 people across 628.509 square miles. Averaged across its land area, Clark County runs about 839 people per square mile. Within its own EPA zone, Clark County is bigger by population than 96% of the counties that share it. Clark County falls in EPA Radon Region 10, one of 142 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Clark County carries a Zone 1 label because its predicted average clears 4 pCi/L outright — three-plus times the 0.4 pCi/L typical outdoors and above the 1.3 pCi/L most homes see. That doesn't mean every Clark County house tests high — a zone is a county-wide prediction, never a single-home reading — but it's a reason to test sooner rather than let it wait.

If Clark County tests above the action level

A short-term test kit is the standard first step and typically costs far less than mitigation itself. If a test comes back at or above 4 pCi/L, a radon mitigation system for a typical home runs roughly $800–$2,500 (around $1,200 for a common single-family install), driven mainly by foundation type, not by county.

Find Washington's state radon program

For Clark County, an ongoing number beats a one-time test - a Airthings radon monitor tracks Clark County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. RADONZONE-10OFF at checkout knocks 10% off.

Where Clark County's figures come from

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. Whatever zone Clark County carries, the zone explainer covers what it does and doesn't tell you about any one house.