What EPA's radon map says about Clark County

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. Nationally that's the 70th percentile for radon potential — Clark County outranks 70% of every county EPA has rated.

Where Clark County sits nationally

About 3,974 people live in Clark County, South Dakota, spread over 957.512 square miles. Population density there runs about 4 people per square mile. Among counties sharing Clark County's EPA zone, it outranks 10% of them by population. Geologically, Clark County is grouped into EPA Region 8 alongside 290 other counties that share its rock and soil setting.

What this actually means for a house in Clark County

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

Mitigation cost doesn't depend on Clark County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find South Dakota's state radon program

Clark County's test kit answers today; a Airthings radon monitor keeps answering for Clark County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. Use code RADONZONE-10OFF at checkout for 10% off.

More on how this number was built

The methodology guide shows exactly which EPA and Census sources feed Clark County's numbers. Whatever zone Clark County carries, the zone explainer covers what it does and doesn't tell you about any one house.