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

EPA places Marshall 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, Marshall County ranks higher in radon potential than 89% of rated counties (the 89th percentile).

How Marshall County compares

Marshall County, Indiana is home to about 46,464 people across 443.618 square miles. Averaged across its land area, Marshall County runs about 105 people per square mile. By population, Marshall County is the 334th largest of every county sharing its EPA zone — the 69th percentile by population within that zone. Marshall County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.

Why the zone isn't the answer by itself

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.

If Marshall County tests above the action level

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 Indiana'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. Use code RADONZONE-10OFF at checkout for 10% off.

Where Marshall 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. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.