Why is radon so high in Rochester compared to other parts of Minnesota?

The rock under the city

Most of Rochester sits on Galena Group limestone, a sedimentary rock laid down when the area was a shallow tropical sea about 450 million years ago. That same rock is exposed in Quarry Hill Park, where the city used to quarry it. It is the same rock that built the Mayo Clinic Plummer Building and a lot of older Rochester foundations.

Limestone is a porous rock. As radium in the rock decays, it produces radon gas, and the gas finds its way up through fractures, joints, and the small spaces between bedding planes. In the rural parts of Olmsted, Dodge, and Fillmore counties, the rock sits even closer to the surface, with thinner soil cover on top.

South and west of Rochester, the bedrock turns into karst country. Sinkholes, caves, underground streams, and visible bluffs are common across the Driftless Area that covers most of southeast Minnesota. Karst geology channels soil gas in ways that show up clearly in the radon data. Dodge County (Kasson, Mantorville) has the highest share of tested homes at or above 4 pCi/L of any of the four counties we serve.

The winter pulls the gas up

A Minnesota house in February is a pressure machine. The furnace runs, warm air leaves the top of the house through ceiling fixtures and the chimney, and the home tries to make up the lost air. Some of that replacement air comes from leaks around windows and doors. Some of it comes from the basement floor, through cracks in the slab and the gap where the slab meets the foundation wall.

That last part is the radon-carrying part. Air moving up through the floor brings soil gas with it. Radon levels in a Minnesota home run about 26% higher in winter than in summer, according to the Minnesota Department of Health 2014 to 2023 dataset. The winter mean is 4.3 pCi/L. The summer mean is 3.4 pCi/L.

What the local data shows

At the county level, the percentages are striking. From the MDH testing dataset for 2014 to 2023:

• Dodge County: 59.5% of tested homes at or above 4 pCi/L. Ranks #20 of 87 MN counties.
• Fillmore County: 54.9% at or above 4 pCi/L. Ranks #31.
• Goodhue County: 49.6% at or above 4 pCi/L. Ranks #41.
• Olmsted County: 42.3% at or above 4 pCi/L. Ranks #54.

All four are above the U.S. national average. Statewide, 62 of 87 Minnesota counties have a county-wide radon average at or above the EPA action level. Zero are below 2 pCi/L.

See the full data set for the picture by city and by Rochester census tract.

Why some Rochester neighborhoods are higher than others

The citywide Rochester median is 3.2 pCi/L, below the EPA action level on average. That number hides a lot. Across the 28 Rochester census tracts in the MDH data, tract-level medians range from 2.2 to 5.3 pCi/L. The highest tract sits at about 2.4 times the lowest.

The high-radon Rochester tracts cluster along the south and east sides of the city, where the soil cover over the limestone is shallower and the housing is mixed in age. The low-radon tracts are in the central and west areas with more newer construction and thicker overburden on top of the rock.

Across the rest of our service area, the small towns south and west of Rochester are higher than Rochester itself. Mantorville is the highest at a median of 6.4 pCi/L. Spring Valley, Chatfield, and Preston all come in above 5. Pine Island and Byron are the only two cities below 4.

What changes the answer for an individual home

• Foundation type. A full basement, a walkout, a slab on grade, or a crawl space each move air differently.
• Slab condition. Cracks, the gap at the foundation wall, plumbing penetrations, and sump pit covers all matter.
• Year built. Homes built after 2009 have a passive radon-control layout under the slab as required by Minnesota code. Older homes do not.
• HVAC setup. A negatively-pressured basement (large bathroom exhaust, dryer, or open combustion appliance) pulls more soil gas in than a balanced one.
• Where in town. Tract-level medians vary by more than 3 pCi/L across Rochester. They tell you a baseline, not a prediction.

Sources

• Minnesota Department of Health, Radon Testing public dataset, 2014 to 2023.
• Olmsted County Public Health, indoor radon program.
• U.S. Geological Survey, geology of southeast Minnesota.
• U.S. Environmental Protection Agency, EPA Radon Map (Zone 1).