Should I test for radon in winter or in summer?
The seasonal pattern in Minnesota
The Minnesota Department of Health 2014 to 2023 testing dataset shows a clear seasonal pattern in statewide averages:
- Winter: 4.3 pCi/L average
- Fall: 4.2 pCi/L average
- Spring: 4.1 pCi/L average
- Summer: 3.4 pCi/L average
The winter mean is about 26% higher than the summer mean. The same home can read 3.0 in July and 4.2 in February without anything actually changing inside.
Why winter pushes the number up
Two physical effects work together in a Minnesota winter.
The first is the stack effect. Warm indoor air rises and leaves the top of the house through ceiling fixtures, the chimney, attic openings, and gaps around upper-floor windows. The home then tries to make up the lost air. Some of that replacement air comes from leaks around lower-floor 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.
The second is closed-house operation. With outside doors and windows closed for months at a time, indoor air does not get diluted by outdoor air. Whatever the basement floor is letting in stays in the house longer.
Real estate testing is its own thing
For a home sale, the rules are the same year-round. The test follows the closed-house protocol regardless of season:
- Windows and outside doors closed except for normal use, for at least 12 hours before the test starts and during the test.
- Heating and cooling running as they normally would.
- The test sits on the lowest level of the home that is regularly occupied.
A summer real estate test under closed-house conditions is a valid test. It just reads a little lower than the same home would in February.
Short-term test or long-term test
Short-term (2 to 7 days)
Right tool for real estate timelines and for getting a fast read on what is going on. Most charcoal kits and continuous monitors fall here. Most sensitive to season and weather.
Long-term (90 days or more)
Averages across multiple weather cycles and gives the most honest year-round number. The EPA recommends a long-term test before any mitigation decision when there is no time pressure. A long-term test that spans both heating and cooling seasons is the best single number a homeowner can have.
What changes the answer
- Test placement. Test the lowest level of the home that gets regular use. Off the floor, away from drafts, away from exterior walls.
- House operation during the test. A whole-house fan running, a bathroom window cracked, or an exterior door propped open will all push the reading lower than reality.
- Test type. A short-term charcoal kit measures the average across its window. A continuous radon monitor logs hour by hour and shows the real swing across the test period.
- Recent changes to the home. A new HVAC system, a remodel that touched the basement, or a new sump pit cover can move the number from one test to the next.
Sources
- Minnesota Department of Health, Radon Testing public dataset, 2014 to 2023, seasonal averages.
- Olmsted County Public Health, indoor radon program.
- U.S. Environmental Protection Agency, Citizen's Guide to Radon (testing protocols).