Researchers followed 12 mitigated homes for ten years and watched what happened to the radon as fans, owners and houses changed.
If you have a radon mitigation system, or you are buying a house that has one, a test from the day it was installed does not tell you it is working now. Retest at least every two years, and again whenever the fan, the house or the owner changes.
In a ten-year study of 12 mitigated homes in Spokane, Washington and Coeur d’Alene, Idaho, fans failed, occupants turned systems down or off, and two houses sold to new owners whose sellers never told them about the radon system. Where one fan failed, the basement’s yearly average rose from about 3 to about 18 pCi/L.
The study
R. Prill of Washington State University in Spokane and W.J. Fisk of Lawrence Berkeley National Laboratory installed radon systems in 12 houses during the 1985 to 1986 heating season, then followed indoor radon quarterly and yearly for ten years. The occupants placed long-term detectors mailed to them and sent back comment cards on noise, fan settings, maintenance and remodeling. The U.S. Department of Energy funded the work, published as report LBNL-50041 in March 2002.
Seven houses got pipes under the basement slab. One pulled soil air out from under the slab. The other six pushed outdoor air under it instead, because suction had proved less effective in the area’s highly air-permeable soils. Five more houses used a fan to pressurize the basement. The target was 148 Bq/m3, which is 4 pCi/L, the level at which EPA recommends fixing a home.
What they found
All six slab-pressurization systems went over the target in at least one measurement period.
Debris carried in with the outdoor air built up where the pipes met the gravel under the slab. Cleaning it out did not last, and in year five the researchers dug pits at the pipe ends in five of the homes.
In the basement-pressurization homes, fan airflow had dropped 20 to 25 percent after two years. Three of those five homes did poorly at keeping radon under the target.
Two of the under-slab fans failed outright. Basement yearly averages rose from 111 to 259 Bq/m3 in one home and from 111 to 666 Bq/m3 in the other: roughly 3 to 7 pCi/L, and 3 to 18 pCi/L.
The fans were eventually replaced. The pCi/L figures are our conversion of the report’s Bq/m3 values, at 37 Bq/m3 per pCi/L.
The occupants played a part too: turning fans off, turning them down, defeating the pressurization, closing crawl space vents and remodeling. One home read 2,146 Bq/m3, about 58 pCi/L, in the year of an extensive remodel.
Two homes sold during the study, and the sellers did not tell the new owners about the radon system. That is the detail a buyer should hold on to.
Why it would happen
A fan-powered radon system works only while it holds a pressure difference under the house. A 2008 Minnesota study puts the risk in one line: “Mitigation system performance can change through fan failure, blockages, and leaks.” Any of those can happen without anyone in the house noticing a change in the air.
The limits, stated plainly
Twelve homes, in the early years of radon mitigation. The authors call the systems experimental, tuned to sit just under the target and “intentionally not over-sized nor particularly robust.” Eleven of the 12 used pressurization designs. The 2008 study below calls suction from under the slab the most common system, particularly in the Upper Midwest.
The one house in this study that used suction stayed under the target for the whole study except one measurement, with no fan repair or replacement reported by the occupant. And this is a national laboratory report, not a peer-reviewed journal article.
Professionally installed systems from the 2000s did much better. In that 2008 Minnesota study, D.J. Steck of St. John’s University had owners test about 130 professionally mitigated houses whose systems were 6 months to 7 years old.
Only 3% still averaged above 4 pCi/L, and 6% had at least one reading above it.
That paper is a conference proceeding, not a journal article, and its homes came from the client lists of contractors who agreed to take part.
What survives: most well-installed systems keep working, a few do not, and a test is how you tell which one you have.
What gets lost in “install it and forget it”
EPA compares a radon system to a furnace or chimney: it needs occasional maintenance. Fans may last five years or more, manufacturer warranties tend not to exceed five years, and a replacement fan runs around $200 to $350 including parts and labor (EPA page last updated June 23, 2026, read October 8, 2026).
The warning device on the pipe tells you the fan is pulling. It does not measure the radon in your bedroom. That is our explanation, not a finding of either study.
And people stop testing. In the Minnesota study, for systems more than two years old, the median time since the last radon test was three years.
What the data supports
- Retest at least every two years. That is EPA’s recommendation for homes with a system.
- Retest after a change. A fan replacement, a remodel, a basement finish, or anything that changes how air moves in the house.
- Buying a home with a radon pipe? Ask for the most recent test result and its date. If it is more than two years old, or nobody knows, test.
- Check the warning device regularly, as EPA advises, and remember what it does and does not tell you.
Bottom line
A radon system that tested fine once is not proof it is working now. In 12 homes around Spokane and Coeur d’Alene, fan failures and debris pushed radon back over the target, and occupant changes and a remodel came with high readings. Retest every two years, and whenever the house changes hands.
First step: find the date of your last radon test.
illumenair does indoor air quality testing in Portland and Vancouver homes.
We measure what is in the air and explain what we found. Indoor air quality and mold inspection in Vancouver, WA and Portland, Oregon
971-363-5626
References
Prill R, Fisk WJ. “Long Term Performance of Radon Mitigation Systems.” Lawrence Berkeley National Laboratory, report LBNL-50041, March 2002.
Steck DJ. “Post-Mitigation Radon Concentrations in Minnesota Homes.” Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium, Las Vegas, NV.
US EPA. “How do I know if my radon mitigation system is working properly?” https://www.epa.gov/radon/how-do-i-know-if-my-radon-mitigation-system-working-properly, last updated June 23, 2026, read October 8, 2026.