Why Minnesota has high radon
Radon is a naturally-occurring radioactive gas produced by the breakdown of uranium in soil and rock. Minnesota's geology — particularly the glacial till and bedrock across the metro — contains elevated uranium concentrations. As that uranium decays, radon gas seeps upward through soil and into homes through micro-cracks in foundations, slab penetrations, sump pits, and crawl spaces.
Once inside, radon accumulates because modern homes are increasingly air-sealed. The same energy-efficiency improvements that keep your heating bill down also trap radon at higher concentrations than older, leakier homes.
EPA radon zones and Hennepin County
The EPA classifies counties into three zones based on predicted average indoor radon levels:
- Zone 1 — Highest potential. Predicted average above 4.0 pCi/L.
- Zone 2 — Moderate potential. Predicted average 2.0–4.0 pCi/L.
- Zone 3 — Lowest potential. Predicted average below 2.0 pCi/L.
Hennepin County is Zone 1. So is almost every county in the southern half of Minnesota. The Twin Cities Metro is one of the highest-radon urban areas in the country.
What the data actually shows for Plymouth
The Minnesota Department of Health reports that roughly 40% of Minnesota homes test above the EPA action level of 4.0 pCi/L. Some Plymouth-area testing data suggests local averages run higher than the state average — possibly due to the specific uranium concentration in western Hennepin County's glacial deposits.
What this means practically: you cannot guess. The only way to know your home's radon level is to test.
How radon testing works
The gold standard is a continuous radon monitor — a calibrated electronic device that logs hourly readings over a minimum 48-hour period under EPA closed-house protocol. Continuous monitors capture variation across the test (which charcoal canisters cannot) and produce tamper-evident data.
What to do if your test is elevated
Radon mitigation is straightforward and effective. The most common system in Plymouth is sub-slab depressurization: a small fan pulls air from beneath the basement slab and vents it above the roofline. Cost typically runs $1,200–$2,500. Radon reduction is typically 80–95%.
Post-mitigation testing confirms the result. Annual or biennial follow-up testing maintains assurance.