Published: June 9, 2025 | Closer Look Home Inspectors · InterNACHI Certified · Mankato, MN

Radon Zones in Southern Minnesota: A County-by-County Guide

By Closer Look Home Inspectors | Updated June 2025 | (507) 721-3820

Radon is the leading cause of lung cancer among non-smokers in the United States, responsible for approximately 21,000 deaths annually. Southern Minnesota sits in one of the highest-risk radon regions in the country, with virtually every county in our service area classified as EPA Zone 1, the highest risk designation. As home inspectors who perform radon testing across the Mankato area, we believe every homeowner and home buyer should understand the radon risk in their specific county.

Understanding EPA Radon Zones

The EPA classifies counties into three radon zones based on predicted average indoor radon screening levels:

• Zone 1 (Highest Risk): Predicted average indoor radon level greater than 4.0 pCi/L (picocuries per liter). Radon-resistant construction features are recommended for all new construction.
• Zone 2 (Moderate Risk): Predicted average between 2.0 and 4.0 pCi/L
• Zone 3 (Lowest Risk): Predicted average below 2.0 pCi/L

The EPA action level is 4.0 pCi/L, meaning the EPA recommends mitigation for any home testing at or above this level. However, there is no safe level of radon exposure, and the EPA suggests considering mitigation for levels between 2.0 and 4.0 pCi/L.

Southern Minnesota County Radon Zones

Blue Earth County - Zone 1

Home to Mankato, Eagle Lake, Mapleton, and Good Thunder. Blue Earth County has some of the highest radon readings in the state. The glacial geology with uranium-containing granite deposits in the soil parent material produces significant radon generation. In our testing across Blue Earth County, approximately 40 to 50 percent of homes test above the 4.0 pCi/L action level. We have measured readings exceeding 20 pCi/L in some Mankato neighborhoods.

Nicollet County - Zone 1

Including St. Peter, Nicollet, and North Mankato (partly). Similar geological conditions to Blue Earth County produce elevated radon levels. St. Peter's older housing stock, built primarily on the river terrace deposits, tends to show higher radon levels due to soil characteristics and older, less sealed foundations.

Le Sueur County - Zone 1

Le Sueur, Cleveland, Kasota, and surrounding communities. The county's mix of river valley and upland terrain creates variable radon levels, but the overall county average exceeds 4.0 pCi/L. Homes in the river valley areas often test highest.

Waseca County - Zone 1

Waseca, Janesville, New Richland. Waseca County's glacial drift soils contain sufficient uranium-bearing minerals to place it firmly in Zone 1. Lakefront properties around Clear Lake and other Waseca County lakes are not immune; radon enters from the soil regardless of proximity to water.

Brown County - Zone 1

New Ulm, Springfield, Sleepy Eye. The county's geological parent material, including glacial deposits from multiple ice advances, produces consistent radon generation across the county.

Faribault County - Zone 1

Blue Earth, Winnebago, Wells. Another Zone 1 county with high testing rates above the action level.

Martin County - Zone 1

Fairmont, Trimont, Sherburn. Lake-region homes and rural properties alike show elevated radon levels.

Steele County - Zone 1

Owatonna, Medford, Blooming Prairie. Zone 1 classification with testing results consistent with the broader southern Minnesota pattern.

Why Radon Is So High in Southern Minnesota

Three geological factors combine to create our elevated radon risk:

1. Glacial geology: Multiple glacial advances deposited uranium-bearing granite fragments and till across the region. These materials continuously release radon gas as uranium decays through radium to radon.
2. Soil permeability: While our clay soils restrict surface water drainage, they have sufficient permeability at depth for radon gas to migrate toward the surface and enter buildings through foundation cracks and openings.
3. Winter conditions: Our long heating season creates a stack effect where warm indoor air rises and exits through the upper levels, pulling soil gases including radon in through the lower levels. Frozen ground caps the soil surface, concentrating radon migration pathways toward building foundations.

When to Test for Radon

Every home in southern Minnesota should be tested for radon. The Minnesota Department of Health recommends testing in these situations:

• When buying or selling a home, as part of the home inspection
• If you have never tested your current home
• After any foundation modification or repair
• Every two years as a routine precaution
• If you are finishing a basement or converting a lower level to living space

Visit our radon testing page for detailed information about our testing process and pricing.

Radon Mitigation

If testing reveals elevated radon levels, mitigation systems are effective and relatively affordable. A sub-slab depressurization system, the most common approach, typically costs $800 to $1,500 installed in the Mankato area and reduces radon levels by 80 to 99 percent. These systems use a fan and PVC piping to draw radon from beneath the foundation slab and vent it above the roofline where it dissipates harmlessly.

Need radon testing for your Mankato area home? Call Closer Look Home Inspectors at (507) 721-3820 to schedule professional testing.