Researchers passed clean air across moldy agar and moldy ceiling tile and counted every particle that came off. Alongside the spores was a second population: smaller, immunologically reactive, and outnumbering the spores by as much as 320 to one.
Almost every conversation about mold in a house eventually turns into a conversation about spore counts. It is an understandable shortcut. Spores are what a microscope can see, what a lab can identify by species, and what a report can put a number on. The trouble is that a spore count answers a narrower question than most people think it is answering.
What they did
Rafał Górny, Tiina Reponen, Klaus Willeke, Detlef Schmechel, Enric Robine, Marjorie Boissier and Sergey Grinshpun published this work in Applied and Environmental Microbiology in July 2002.
They built an aerosolization chamber. Clean, HEPA-filtered air was drawn across a moldy surface at a precisely controlled speed while an optical particle counter measured everything that came off, particle by particle, over 30 minutes. Two surfaces were tested: malt extract agar, and the white mineral-fiber ceiling tile commonly hung in American buildings. Three fungal species were grown on them, two of them ordinary indoor residents: Aspergillus versicolor, Cladosporium cladosporioides and Penicillium melinii.
Four air speeds were used, each chosen to represent a real situation: 0.3 meters per second for still indoor air, 1.4 and 5.8 for outdoors, and 29.1 for the inside of a ventilation duct. Some runs added vibration at 1 Hz and 14 watts, which the authors describe as representing the disturbance of a slammed door or children jumping.
The second population
The size distributions revealed something the team had not gone looking for. Below the peak where the spores sat was a second, separate peak in the submicrometer range. The researchers set the dividing line at 1.6 micrometers: above it, count it as a spore; below it, count it as a fragment.
From agar, fragments came off at 160 to 1,400 particles per square centimeter. Spores, from the same surface at the same moment, came off at 1 to 70.
By species, the fragments outnumbered the spores 11 to 320 times for A. versicolor, 17 to 170 times for C. cladosporioides, and 7 to 270 times for P. melinii. Every one of those differences was statistically significant. From ceiling tile at duct speed, fragment release peaked at 570,000 particles per square centimeter.
Scanning electron microscopy confirmed what the counter was seeing: intact spores in one image, broken spores and hyphal fragments in the next.
The part that matters for testing
Fragments and spores do not leave a surface in a fixed ratio. At the low, indoor-typical air speed, the correlation between the two was not statistically significant for two of the three species. Only P. melinii showed a relationship, and even that explained about half the variation (r² = 0.508).
The authors’ conclusion is unambiguous: in indoor environments, the number of released spores is generally not a reliable indicator for the number of released fragments, and future studies of mold-problem buildings should measure both.
Which means a spore count is a measurement of spores. It is a real, useful measurement. It is simply not a measurement of the total fungal particle load in a room.
Are the fragments biologically relevant?
The team ran ELISA tests using three monoclonal antibodies raised against Aspergillus and Penicillium species. Fragments and spores shared antigens, which confirmed that the fragments really were fungal in origin. More striking, the fragment samples reacted more strongly than the spore samples every single time.
Optical densities for fragment samples ran 3.7 to 5.1 times higher than spore samples for A. versicolor, and 2.0 to 3.2 times higher for P. melinii.
Small, numerous, immunologically reactive, and below the size that traditional bioaerosol sampling could resolve. Earlier work, restricted to fragments large enough to see under a light microscope, had put indoor fungal fragments at 29 to 146 pieces per cubic meter, up to 6.3% of all fungal propagules indoors. On the evidence of this chamber, that was the visible tip of the population.
Timing changes the answer
Within the first 10 minutes of airflow, up to 86% of the aerosolizable fragments left the surface at duct speed, and up to 53% at indoor speed - Table 2 puts the duct-speed fragment range at 66 to 86%. Vibration alone increased fragment release at the low air speed for two of the three species. Airborne concentration in a real room therefore depends heavily on when the sample is taken relative to the last disturbance - a door closing, a furnace kicking on, someone pulling back a piece of baseboard.
The limits, stated plainly
This is a chamber, not a house. No person was exposed and no health outcome was measured in this work; the health concern is imported from the wider literature on damp buildings and on fine particles, not demonstrated here. Optical particle size is not the same as what a lung sees. The counter’s lower limit was 0.3 micrometers, so anything finer went uncounted, which makes these fragment numbers undercounts rather than overcounts. The ceiling tiles were incubated for 6 to 12 months at near-saturated humidity, producing abundant growth rather than a faint patch. And “up to 320 times” is a maximum across conditions, not a typical living room.
What survives all of that is consistent and narrow: from a colonized surface, sub-spore particles outnumbered spores in every species, at every air speed, with vibration and without.
What gets lost in the mold-testing marketing
The fragment literature gets pulled in two directions, and both distort it. Companies selling remediation sometimes cite it to argue that any spore count is worthless and only removal will do. Companies selling a single air sample sometimes present the resulting spore number as a complete picture of exposure. This paper supports neither position. It says that spores are one fraction of a larger released population, and it asks that investigations of moldy buildings measure both.
What the data supports
Judge a home by its moisture history first and by any single air number second. Water intrusion, condensation and elevated humidity are the upstream cause; growth is downstream of them.
Handle suspect material gently. Vibration alone raised fragment release at indoor air speeds, and prodding, cutting or vacuuming a colonized surface is a far larger disturbance than a slammed door.
Treat any single air sample for what it is: one moment, one room, one fraction of what is present. A pattern of readings across rooms, paired with moisture measurements, tells you more than one number ever will.
The bottom line. Killing what you can see does not settle what is airborne, and counting spores does not fully answer it either. The moisture that allowed the growth is the part that keeps producing. The one small first step: find where the water is getting in.
Before you decide what is in your air, measure the moisture, not just the spores
illumenair measures moisture, humidity and airborne mold room by room in Portland and Vancouver homes. We measure air. We do not diagnose or remediate.
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Sources: Górny RL, Reponen T, Willeke K, Schmechel D, Robine E, Boissier M, Grinshpun SA. “Fungal Fragments as Indoor Air Biocontaminants.” Applied and Environmental Microbiology, 2002;68(7):3522-3531.