Researchers sampled the air in buildings 24 to 48 hours after the remediation contractor called the job done. Successful jobs had a clear pattern. The raw numbers, on their own, did not.
If you are paying for mold remediation, the mold testing afterward should compare rooms. The cleaned room should read lower than an untouched room in the same building and lower than the outdoor air, all sampled the same day.
That is what successful jobs looked like in a study of air testing after mold remediation. In failed jobs, the cleaned room had more than twice the live mold of the untouched room, and about as much as the outdoor air.
The study
G.T. Kleinheinz, B.M. Langolf and E. Englebert of the University of Wisconsin Oshkosh published the study in Microbiological Research in 2006. The samples were analyzed at the Wisconsin State Laboratory of Hygiene.
At each site the team took three air samples within 30 minutes of each other, 24 to 48 hours after the contractor declared the job complete: the cleaned room, a room in the same building not connected to it by ducts or hallways, and outdoors within 10 meters of the entrance. Each sample ran 5 minutes.
They used two methods side by side. One grew live mold on culture plates and counted colonies (CFU, colony forming units). The other was a spore trap, which counts all spores under a microscope, live or dead.
A job counted as successful if five things were true: no visible mold, the water source fixed, normal moisture in all building materials, surfaces cleaned with at least 20 air changes through a high-efficiency filter, and no mold coming back within 3 months. A job was also counted as failed if the cleaned room tested higher than the untouched room.
What they found
In 26 successful jobs, the cleaned room averaged 337 CFU per cubic meter of live mold, against 664 in the untouched room and 1,148 outdoors.
The spore trap, used at 36 successful sites, showed the same order: 1,702 spores per cubic meter in the cleaned room, 3,725 in the untouched room and 12,273 outdoors. In the authors’ words, cleaned rooms had “approximately one-half of the levels found in the non-complaint sample at the same location and less than one-third of the outdoor samples,” and this held for both methods.
In 13 failed jobs, the cleaned room averaged 1,576 CFU, against 675 in the untouched room and 1,592 outdoors.
That is more than twice the untouched room, our arithmetic from the published averages, and a statistically significant difference. On the spore trap, failed jobs read 13,741 in the cleaned room against 5,437 in the untouched room and 15,523 outdoors, but that gap was not statistically significant.
Here is the part that matters for anyone handed a lab report. The raw numbers overlapped. One successfully cleaned room read 2,198 CFU, above the average for failed rooms. The authors concluded that “just suggesting that mold levels need to be below a certain number is ineffective in determining the effectiveness of a project.”
Why it happens
The authors’ explanation: remediation contractors run the air through high-efficiency filters, so a properly cleaned room should read low. Mold that is still growing keeps releasing spores into the room.
The mix of mold types pointed the same way. On the culture plates, Penicillium made up 81% of the live mold in cleaned rooms, while outdoors Cladosporium made up 66%. Reading that difference as a sign of an indoor source is our inference. The authors also suggest that high live counts may mean growth is still active, while high spore counts with low live counts may mean growth stopped but spores escaped the cleanup.
The limits, stated plainly
This is one study, from 2006, of buildings. The paper does not say they were homes, how many buildings there were, or where they were. Only 13 jobs were classed as failed.
The biggest limit: the researchers partly defined a failed job by the same comparison they report, a cleaned room testing higher than the untouched room. So the gap between good and failed jobs is partly built in. What is not built in: absolute mold levels were not used to classify any job, and they still overlapped between good and failed jobs.
Each spot was sampled once, for 5 minutes, in a room kept under positive pressure below 30% humidity right after cleanup. That is a snapshot of a controlled room, not a lived-in house weeks later. Results varied widely from site to site. The paper reports averages and p-values, not confidence intervals. No health outcome was measured.
What survives: after cleanup, compare the cleaned room with an untouched room and outdoors on the same day.
What the guidance says
The US Environmental Protection Agency’s mold remediation guide (web page last updated April 29, 2026) lists how you know a job is finished. The water or moisture problem must be completely fixed. Visible mold, mold-damaged materials and moldy odors should not be present. If you sampled, “the kinds and concentrations of mold and mold spores in the building should be similar to those found outside.” People should be able to reoccupy the space without health complaints. And: “Ultimately, this is a judgment call; there is no easy answer.”
In this study, right after cleanup, matching the outdoor air is what the failed jobs looked like. The successful ones were well below it.
EPA also expects a background level of spores to remain, “roughly equivalent to or lower than the level in outside air,” and says those spores will not grow if the moisture problem has been fixed. That is why the water source comes first.
What gets lost in the mold marketing
Clearance reports often carry a single pass line. EPA says no EPA or other federal threshold limits have been set for mold. The recommended upper limits the authors reviewed ranged from 200 to more than 1,000 CFU per cubic meter, and the authors note many such recommendations rest on consensus or on data from a limited sample area. One successfully cleaned room in this study read above that whole range.
“Black mold” is the name that sells remediation. In this study, Stachybotrys turned up more often outdoors than indoors, and no live Stachybotrys was recovered from any cleaned room.
Killing mold is not the goal either. EPA: “Dead mold is still allergenic, and some dead molds are potentially toxic.” EPA does not recommend chlorine bleach or other biocides as a routine part of remediation.
What the data supports
- Before work starts: agree how the job will be checked. Ask for same-day air samples from the cleaned room, an unaffected room not connected to it by ducts or hallways, and outdoors.
- Before anyone samples: confirm the water source is fixed and the materials read dry.
- Reading the results: expect the cleaned room to read lower than the untouched room. If it reads higher, ask why before you sign off.
- Afterward: look again over the next 3 months for mold coming back.
- Our view, not the study’s: the person checking the job ideally is not the one paid to do it.
Bottom line
After mold remediation, the cleaned room should test lower than an untouched room and the outdoor air on the same day. A low number on its own does not show that.
First step: if you are getting a remediation quote, ask how the job will be tested when it is done.
illumenair does mold testing and indoor air quality testing in Portland and Vancouver.
We test the air and explain what we found. Mold inspection in Portland and Vancouver, WA
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Sources: Kleinheinz GT, Langolf BM, Englebert E. “Characterization of airborne fungal levels after mold remediation.” Microbiological Research, 2006;161(4):367-376. doi:10.1016/j.micres.2006.01.002. US Environmental Protection Agency. “Mold Remediation in Schools and Commercial Buildings Guide: Chapter 3.” Based on EPA 402-K-01-001. Web page last updated April 29, 2026; accessed September 30, 2026.