Iowa is the only US state where the EPA's foundational radon survey — the 1993 State Indoor Radon Survey (SIRS) — ranked it #1 in the nation. With a modeled statewide average of 8.5 pCi/L, Iowa more than doubles the EPA action level of 4 pCi/L. Why? Four geological and climatic factors converge in Iowa that don't converge anywhere else in the United States.
Factor 1: Uranium-bearing glacial till
Most of Iowa sits on glacial till — sediment ground out and deposited by Pleistocene glaciers between 2.5 million and 12,000 years ago. The till covering Iowa is unusually rich in uranium-bearing minerals because it includes scoured material from the Canadian Shield and the Precambrian basement of the Upper Midwest, plus phosphate-rich rocks from the Western Interior Seaway. Uranium-238 decays through a chain that produces radon-222 (the form of radon that reaches indoor air). Higher uranium content in the soil = more radon gas generated continuously.
Per Iowa State University Department of Geological and Atmospheric Sciences, Iowa's glacial-till uranium concentrations average 2-4 parts per million across most of the state — roughly double the US average. The northern third of Iowa (Iowa's most heavily glaciated region) has the highest till uranium concentrations.
Factor 2: Fractured limestone bedrock in eastern Iowa's Driftless Area
The Driftless Area of northeast Iowa — Allamakee, Clayton, Fayette, Winneshiek, and Howard counties — was never glaciated. Instead, the region exposes Cambrian-Ordovician limestone and dolomite bedrock that's been fractured by hundreds of millions of years of tectonic stress and karst dissolution. These fractures provide direct gas transport pathways from the bedrock to the soil surface, then into homes through foundation cracks.
The Driftless Area's combination of fractured limestone + thin soil overburden makes radon transport unusually efficient. Dubuque County (Driftless Area) and adjacent eastern Iowa counties consistently report some of the highest individual home radon readings in the state.
Factor 3: Basement-heavy housing stock
Iowa's housing stock is overwhelmingly basement-foundation: roughly 87% of single-family homes have a full basement. Compare to states like Florida (under 5% basement) or California (around 12% basement). Basements are the lowest occupied level of a home — they're where radon entering from soil reaches indoor air first, and where it concentrates because it's denser than warm air above.
Iowa's high basement penetration is itself partly geological: the state's water table is generally low and soils are well-drained, making full-basement foundations practical and code-standard. The same conditions that make basements possible also make them effective radon collectors.
Factor 4: Cold-winter stack effect
Iowa winters are cold. Average January overnight lows in Des Moines are 13°F; in northern Iowa they routinely drop below zero. Cold winters drive a powerful stack effect: heated indoor air rises through upper floors, creating negative pressure in the basement. That negative pressure literally sucks soil gas — including radon — through any foundation crack or penetration.
Iowa winter indoor radon readings typically test 30-50% higher than summer readings in the same home. This is why EPA recommends winter (closed-house) testing as the standard for accurate radon assessment.
What this means for Iowa homeowners
The four factors compound multiplicatively. Iowa isn't slightly above average in any single factor — it's simultaneously elevated across all four. That's why approximately 5 in 7 Iowa homes test above the EPA action level of 4 pCi/L (compared to roughly 1 in 15 US homes nationally).
The practical implication: radon exposure isn't a hypothetical risk for Iowa homeowners — it's the statistical baseline. The EPA estimates radon contributes to ~21,000 US lung cancer deaths annually, and Iowa's elevated state radon levels mean Iowans face a measurably higher per-capita lung cancer risk attributable to radon than residents of most other states.
What you can do
Test your home. Iowa HHS, the American Lung Association, and hardware-store retailers all offer short-term radon test kits in the $15-30 range. Use our free Iowa radon calculator to check your test result against the EPA action level, then estimate mitigation cost if elevated. For an in-depth guide to interpreting your test result, see our Radon Test Results Guide.
If your reading is above 4 pCi/L, choose an NRPP-certified, IDPH-registered Iowa mitigator — both credentials are required by Iowa law (Iowa Administrative Code Chapter 44).
Sources: EPA State Indoor Radon Survey (1993), Iowa State University Department of Geological and Atmospheric Sciences, Iowa Department of Health and Human Services Radon Program, EPA Map of Radon Zones.