Is Sully County, SD a radon risk area?

Sully County is mapped as Zone 1 - Highest Potential by EPA — meaning the county-wide average indoor level is predicted above 4 pCi/L. Nationally that's the 66th percentile for radon potential — Sully County outranks 66% of every county EPA has rated.

Sully County by the numbers

About 1,468 people live in Sully County, South Dakota, spread over 1,006.727 square miles. That works out to roughly 1 people per square mile. Rank 1050 by population within its EPA zone puts Sully County at the 1st percentile among zone peers. EPA's geology-based Region 8 groups Sully County with 290 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Sully 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. That doesn't mean every Sully 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.

Testing and mitigation in Sully County

Nothing about Sully County's EPA zone changes the mitigation number: $800–$2,500 nationally (around $1,200 typical), set by the house's foundation rather than the county it sits in — and only relevant once a test actually clears 4 pCi/L.

Find South Dakota's state radon program

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

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. Whatever zone Sully County carries, the zone explainer covers what it does and doesn't tell you about any one house.