What EPA's radon map says about Marion County

EPA's own radon map rates Marion County zone 1 - highest potential, predicting a county-wide average indoor level above 4 pCi/L. Nationally that's the 86th percentile for radon potential — Marion County outranks 86% of every county EPA has rated.

How Marion County compares

About 34,048 people live in Marion County, Iowa, spread over 554.49 square miles. Averaged across its land area, Marion County runs about 61 people per square mile. Among counties sharing Marion County's EPA zone, it outranks 59% of them by population. Geologically, Marion County is grouped into EPA Region 7 alongside 411 other counties that share its rock and soil setting.

Why the zone isn't the answer by itself

Marion 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 Marion 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 Marion County

Mitigation cost doesn't depend on Marion 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

Marion County's test kit answers today; a Airthings radon monitor keeps answering for Marion 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. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.