Marshall County, SD: EPA's radon zone, explained

EPA's own radon map rates Marshall County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Rank it against the rest of the country and Marshall County lands in the 70th percentile for radon potential.

How Marshall County compares

Marshall County, South Dakota is home to about 4,395 people across 838.085 square miles. Averaged across its land area, Marshall County runs about 5 people per square mile. By population, Marshall County is the 939th largest of every county sharing its EPA zone — the 12th percentile by population within that zone. Marshall County falls in EPA Radon Region 8, one of 291 counties nationally EPA assigns to that same geologic province.

Zone vs. an actual test

Outdoor air nationally averages about 0.4 pCi/L, indoor air about 1.3 pCi/L — Marshall County's Zone 1 prediction clears both of those and EPA's fix-it threshold besides. Some Marshall County homes will test comfortably under the action level despite the county's Zone 1 rating — the only way to know which kind of home you have is an actual test.

Testing and mitigation in Marshall County

For a Marshall County homeowner, the order is simple: test first, since a kit costs a small fraction of what mitigation would. Only if the result clears 4 pCi/L does a system — typically $800–$2,500, about $1,200 for a common install — become worth pricing out.

Find South Dakota's state radon program

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

Where Marshall County's figures come from

Marshall County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.