Why 7 in 10 Iowa Homes Test High for Radon — and What the Cedar Valley’s Glacial Soil Has to Do With It

Here is the number that should get every Iowa homeowner’s attention: 71.6% of Iowa homes test at or above the EPA’s radon action level of 4.0 pCi/L — per the Iowa Radon Survey, the largest percentage of homes above the action level of any state in the country. EPA surveys put it more simply: about 7 in 10 Iowa homes. The average Iowa home measures around 8.5 pCi/L against a national average of about 1.3 — more than six times higher. And the EPA’s Map of Radon Zones places every one of Iowa’s 99 counties, Black Hawk included, in Zone 1, its highest-risk category. No other state is painted that uniformly red.

Why? The answer is under your basement floor, and it got there in an ice age.

Glaciers Ground the Rock to Powder

Over hundreds of thousands of years, repeated glaciations pushed down across what is now Iowa. Ice sheets are bulldozers: they scraped up uranium-bearing rock — from the Canadian Shield to the north and from local bedrock — and ground it against itself for millennia. What they left behind when they melted is the loose glacial material that blankets most of the state.

That grinding is the whole story. Uranium in solid bedrock releases its radon slowly, because the gas has to escape from deep inside intact rock. Grind that same rock into fine particles and you multiply its surface area enormously — the trace uranium is now everywhere near a particle surface, and the radon it produces escapes into the soil efficiently. Then, because the ground is loose rather than solid, the gas moves through it easily. Iowa’s soil is, in effect, engineered by glaciers to produce radon and deliver it upward.

The Cedar Valley’s Particular Version

Waterloo and Cedar Falls sit on the Iowan Surface, the distinctive landform region of northeast Iowa. During the last glacial period, roughly 16,500–21,000 years ago, intense tundra-climate erosion — freeze-thaw cycles, slopewash, and wind — scoured this region into the gently rolling surface you drive across today, leaving glacial deposits topped with a thin layer of windblown loess and studded with granite boulders carried down from Canada (you have seen them in farm fields and parks around Black Hawk County). Different glacial history than central Iowa, same consequence: uranium-bearing glacial material sitting directly under Cedar Valley foundations. The county’s numbers bear it out — Black Hawk County’s Board of Health reports that almost 40% of radon tests run in the county from 2000 to 2024 came back at or above 4.0 pCi/L.

Basements Turn Geology Into Exposure

Geology loads the gun; housing stock pulls the trigger. Iowa homes overwhelmingly have basements — cold-climate construction plus tornado-country common sense — and Iowans finish them and live in them. A basement puts hundreds of square feet of concrete in direct contact with radon-producing soil, and every slab crack, floor-wall joint, sump opening, and block core is a pathway. In winter, a sealed, heated house pulls air upward like a chimney (the stack effect), actively drawing soil gas in through the foundation — which is why winter radon readings run higher, and why a winter test tells you the number you most need to know.

High Odds, Cheap Test, Solvable Problem

None of this is a reason to fear your house. It is a reason to spend $10: Black Hawk County Public Health sells test kits at their Waterloo office, lab analysis included. If your number comes back at 4.0 or higher — as it does in almost 4 of 10 county tests — a mitigation system installed by a state-certified professional will bring it down, usually in a day, typically for $800–$2,500. The geology took twenty thousand years. The fix takes an afternoon.

Call (319) 319-9943