The EPA maps every county in the country into three radon zones based on geology and measured results. Polk County and nearly all of Iowa are Zone 1, the highest-potential category, with predicted average indoor levels above the 4 pCi/L action level. Here is what that does and does not mean for your house.
Local geology and test history
Iowa's radon comes from glacial till — rock ground fine by ice sheets and left as the state's subsoil. Ground-up rock has enormous surface area, which lets radon escape into soil gas efficiently. That is why Iowa's average indoor levels run among the highest in the nation.
Zone 1 is a probability statement, not a measurement of your house. It means testing here is not optional due diligence the way it might be in a low-radon state — it is the expected first step.
Basements and slabs
Central Iowa construction leans hard on basements, many of them finished, and a below-grade room concentrates whatever the soil delivers. Sump pits and drain tile — near-universal here because of clay soils and spring water — are also open pathways for soil gas unless they are sealed or put under suction.
Newer construction is not exempt. Tighter houses hold radon better, and even homes built with passive radon rough-ins often test high until a fan is added.
Why every house still needs its own test
Side-by-side houses on the same street routinely test wildly differently — one at 3, the neighbor at 15 — because entry depends on the soil under that footprint, the cracks in that slab, and how that house breathes.
So the zone tells you to test; only the test tells you your number. A charcoal kit or a 48-hour monitor is cheap, and it converts a county-level probability into a decision you can actually act on.
Using the zone information well
Treat Zone 1 as the reason to spend twenty dollars on a test, not as a verdict about your house. Plenty of Zone 1 homes test low; the zone just says don't assume yours is one of them.
If you are house shopping in Des Moines, the practical translation is: test during every inspection period, and treat an existing mitigation system with documentation as a small plus, not a red flag — it means the problem is already solved.
And if your neighbor's result scared you, remember it cuts both ways: their 15 does not condemn your house, and their 2 does not clear it.
