What EPA's radon map says about Lyman County

Lyman County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. That puts Lyman County in the 69th percentile nationally for radon potential — higher than 69% of the counties EPA has rated.

How Lyman County compares

About 3,721 people live in Lyman County, South Dakota, spread over 1,642.285 square miles. Population density there runs about 2 people per square mile. Among counties sharing Lyman County's EPA zone, it outranks 10% of them by population. Lyman County falls in EPA Radon Region 8, one of 291 counties nationally EPA assigns to that same geologic province.

What this actually means for a house in Lyman County

Lyman County's predicted average sits above EPA's own action level of 4 pCi/L — the concentration at which EPA recommends fixing a home — versus a national average around 1.3 pCi/L indoors and 0.4 pCi/L outdoors. EPA's guidance doesn't change because of a Zone 1 label: test the house. What changes is the reason to not put it off in Lyman County specifically.

If Lyman County tests above the action level

Mitigation cost doesn't depend on Lyman 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

Lyman County's test kit answers today; a Airthings radon monitor keeps answering for Lyman 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

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.