What EPA's radon map says about Marshall County

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. Marshall County sits at the 88th percentile nationally for radon potential — above 88 of every 100 rated counties.

How Marshall County compares

About 40,392 people live in Marshall County, Iowa, spread over 572.503 square miles. That works out to roughly 71 people per square mile. Among counties sharing Marshall County's EPA zone, it outranks 65% of them by population. EPA's geology-based Region 7 groups Marshall County with 411 other counties nationally sharing its underlying rock and soil profile.

Zone vs. an actual test

Where the national indoor average runs about 1.3 pCi/L, Marshall County's Zone 1 rating puts its predicted county average above EPA's 4 pCi/L action level itself. 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

Mitigation cost doesn't depend on Marshall 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 Iowa's state radon program

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

Marshall County's zone, rank and region all trace back to named EPA and Census sources — the methodology guide lists exactly which ones. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.