I hope you're doing well and enjoying the end of summer. I also hope that you'll find the following article
about fungal aerosols by Dr. Harriet Burge both interesting and useful.
With best wishes,
By Dr. Harriet Burge, EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board
The development of a strategy or plan for documenting fungal growth in a building depends on a knowledge
of how, where, and why fungi grow in buildings. With this knowledge one can usually find fungal growth
and possibly go directly to recommendations for its removal. However, in some cases it is necessary to
collect aerosol samples. Reasons for collecting samples of fungal aerosols include a need to actually
document exposure, to assess whether or not identified fungal growth is producing aerosols, to check the
possibility that hidden growth has occurred, or to satisfy a client (or his/her attorney) that you have
done a complete investigation.
If you are going to understand the data recovered from air samples, it is essential that you have an
understanding of the nature of fungal aerosols, their sources and mechanisms for aerosolization.
The nature of fungal aerosols
Aerosols are suspensions of solid particles or liquid droplets in air (or other gas) that remain airborne
for at least minutes. Fungal aerosols are parts of fungal bodies that are small enough to become airborne.
They are usually spores, but other parts of the fungal body may be a part of fungal aerosols (Reponen, et
al., 2007). Fungal spores, especially those of ascomycetes and basidiomycetes, comprise a large proportion
of the outdoor coarse particle aerosol (1-10µm). In tropical rainforests, these fungi make up 35% of
this aerosol on average, and estimates for extra-tropical areas are similar (Elbert, et al., 2007). Although
studies have not been done for dry spores such as Cladosporium, percentages in this particle size
range are likely to be similar given the recorded concentrations of these fungi.
How long the particles remain airborne depends on their density, their aerodynamic diameters, air movement,
and the nature of the immediate environment.
We usually assume that spores have "unit density," which means that they have the same density
as water. This sounds reasonable since water fills the interior of the spore. However, this assumption
introduces a variable error in estimations of removal rates (Reponen, et al., 2001).
Aerodynamic diameter is the diameter of a smooth sphere of unit density that would fall in still air at
the same rate as the particle (spore) in question. If you have a perfectly spherical spore with no surface
ornamentation, then its actual diameter is usually close to its aerodynamic diameter. Few spores are either
completely spherical or smooth. Thus, generally, aerodynamic diameter must be determined experimentally.
One way this is done is to use cascade impactors that are designed to collect particles in specific ranges
of aerodynamic diameters. Data from several sources using this method indicate that the common fungi
involved in indoor growth have aerodynamic diameters in the range of 1.5-7µm. Many common
Aspergillus species fall into the lower part of this range (Meklin, et al 2002). Practically, we
often use the smallest diameter of a spore as its aerodynamic diameter. This assumes that the spore becomes
aligned with its long axis parallel to forces affecting its movement (McCartney, et al., 1993). We often
ignore surface ornamentation, assuming that this introduces only small errors. However, empirical data to
test this hypothesis is not available.
Apparently most fungal spores do not increase in volume with increasing relative humidity until the
humidity reaches near 100%. Then, the increase is relatively small (Reponen, et al., 1996). However, Madelin
and Johnson (1992) report an increase in aerodynamic diameter when spores are passed through air at 98% RH
and 38°C, compared to 40% RH and 20°C. This difference points out the importance of considering
comparability of methods used when evaluating different studies, as well as the kinds of spores.
In addition to spores, fungal aerosols include fragments of spores and mycelia (Reponen, et al., 2007).
However, tests documenting the concentrations of these particles have involved aerosolization from either
cultures or building materials using air speeds much higher than would be likely in an indoor environment
(20 l/m) and sample collection modalities that also produced high flow rates that could have caused
fragmentation. It is also not clear that spores are being fragmented rather than mycelium. Whether this
makes a difference with respect to exposure to fungal agents of human diseases (ie. allergens or toxins)
is unknown. Obviously, fungal fragments smaller than cell size are not important with respect to infectious
disease (Seo, et al., 2007 & 2009).
Sources for fungal aerosols
Outdoor fungal aerosols are derived primarily from fungal growth on plant surfaces (the "phylloplane"
fungi) or from fungi growing on the surface of the soil (e.g. mushrooms, puffballs, and cup fungi). Fungi
that grow on leaf and stem surfaces are often pathogenic for the host plants. The kinds of fungi in the
air depend on the kinds of plants that are common and nearby. Intensive (or extensive) agriculture may
provide vast acres of plants susceptible to a particular fungus, which will then produce spores that may
dominate the outdoor fungal aerosol, especially for those crops that are harvested after the leaves have
died (Mitikakis, et al., 2001). Likewise, fungi that grow on the ground are often dependent on symbiotic
relationships with particular plants and will be abundant in areas where those plants grow.
Fungi that grow within the soil (the so-called "soil fungi") may become airborne during windstorms
or when the soil is actively disturbed, such as during construction activities. Coccidioides immitis
is a human pathogen that becomes airborne from desert dust during windstorms, causing epidemics of human
disease. The composition of soil fungal communities is related in part to precipitation. In dry weather it
appears that fungal communities become diverse and stable, while abundant rainfall will tend to destabilize
the communities, and a few fungi will become dominant. This fact has implications for changes in carbon
cycling in the soil as a result of climate change (Hawkes, et al., 2011).
Composting processes utilize soil fungi and bacteria to decay organic material that is often gathered into
concentrated piles over many acres of ground. Measurements of fungi released from actively disturbed compost
(as occurs during turning) range from 104 to 105 spores/m3 (Taha, et al., 2007).
Sources for fungal aerosols indoors include outdoor air (the predominant source) and growth on indoor materials.
If indoor spaces received as much water as outdoors, then our buildings would be filled with fungi. We all
know that just a relatively small leak can initiate fungal growth and flooding, such as happened in New
Orleans and is occurring now along the Mississippi River, provides sufficient water to stimulate growth on
all interior surfaces. Although we think of indoor fungi being different than those outdoors, they really
are not. All indoor fungi came originally from outdoor sources. The reason some fungi do especially well
indoors is that they find food under conditions of relatively little competition.
The generation of fungal aerosols
The first step in aerosol generation is the formation and release of fungal spores. For many (perhaps most)
spores, internal clocks have evolved that control the timing of these processes. This insures that spores
will be produced and released at times that are most likely to provide appropriate conditions for spore
transport to a suitable substrate for growth (Bell-Pedersen, et al., 1996). Pathways to spore production
are beyond the scope of this article. However, methods by which spores are released from spore bearing
structures are an integral part of aerosol formation.
External mechanisms for spore release
External mechanisms for spore release involve the nature of the physical attachment of the spore to the
fungal body, and bonding forces that tend to keep spores attached to the surface where the fungus is growing.
Sharp drops in relative humidity appear to play a large role in release of spores that do not have internal
spore release mechanisms (Jones & Harrison 2004). Wind also plays a role in breaking bonds between spores
and spore-bearing structures as well as those that bind spores to leaf surfaces after release.
Additional forces that can cause "passive" spore release are rain splash, mechanical activity (such
as occurs during harvesting of field crops), or by remediation activities. Splash dispersal involves the action
of raindrops hitting a reservoir of fungal spores and actually splashing them into the air. Cladosporium
carpophilum, a pathogenic species of peach and other fruits, is splash dispersed from infected twigs (Lan
& Scherm 2003). Spores of puffballs (basidiomycetes) are also splash dispersed. Indoors, splash dispersal
might occur when water sprays are directed toward moldy surfaces, or within humidifiers where fungi are
growing on surfaces at the air/water interface.
Indoors, it appears that air speed is generally too low to release most spores from their growth sites.
The infamous fungus, Stachybotrys chartarum, has been studied for air speeds that will release
spores from conidiophores on surfaces (Tucker et al., 2007). Micronewton forces were needed to dislodge
spores from undisturbed colonies. Using a microflow apparatus, they determined that most spores that are
released at a certain airflow enter the aerosol within 5 minutes, and that following this time 99% of the
spores remain attached. They point out that airflows in indoor environments are generally much lower (in
the nanonewton range) and air movement is unlikely to be effective in removing Stachybotrys spores
from a colonized surface.
Penicillium and Aspergillus spores may be released at relatively low air velocities (0.4 m/s).
However, Cladosporium spores are not released at this speed. Kildeso et al. (2003), studied release of
spores from wet building materials. They determined that such release is dependent on many factors including
humidity, nutrients, interaction between microorganisms, and possible peak airflows. They conclude that because
all of these factors are involved, area covered by fungal growth does not predict potential exposure. Kanaani
et al. (2009), provide a good review of the laboratory studies evaluating relationships between environmental
conditions and release of fungal spores from culture.
Intrinsic spore release mechanisms
Intrinsic spore release mechanisms are powerful, and can shoot small (micron sized) spores distances of 1000
times the spore diameter (Schmale et al., 2005). Water relationships play an important part in most intrinsic
spore release mechanisms.
Basidiospores have evolved a unique bilaterally symmetrical shape that facilitates an especially interesting
method of spore discharge mechanism. This mechanism can shoot the spores as far as 2mm, which is far enough
to place the spore in the space between gills so that it can fall into the moving air stream (Fischer et al.,
2010). The mechanism is powered by "Buller's drop" which forms at the base of the asymmetric spore
and moves rapidly across the spore surface leading to a catapult motion that discharges the spore
(Stolze-Rybczynski et al., 2009). Noblin et al. (2009), performed elegant experiments that elucidate the
stages in this process.
Ascomycetes, on the other hand, often use a rocket mechanism (in essence, pressurized squirt guns) that shoot
spores as far as 2 or more meters (Yafetto et al., 2009). The limiting factor for the distance that spores
can travel after forcible discharge is drag, which is essentially the resistance caused by rapid travel
through air. The larger the spore, and the more aerodynamic the shape, the further the spore can travel
(Yafetto et al., 2008). Ascospore shapes have evolved to minimize drag. Roper et al. (2008), have experimentally
determined that spore shapes in ascomycetes have evolved so that drag does not exceed 1% of the minimum possible.
Thus many ascospores are oval or threadlike. The actual mechanism that triggers the squirt gun process of many
ascomycetes varies. Osmosis certainly plays a role by rapidly filling the ascospore sacs with water, which
ruptures the apex, causing the spores to shoot out. Some ascospores are released in groups, which increases the
mass of the projectile, enabling it to travel further. On the other hand, the ascomycete Venturia inaequalis,
a plant pathogen, the spores of which are often abundant in outdoor air, releases spores in response to
raindrop-induced vibrations of the leaves on which the fruiting bodies are growing (Alt & Kollar 2010).
Sclerotinia sclerotiorum synchronously discharges ascospores in a way that sufficient airflow is
produced to propel the spores through the boundary layer of still air that surrounds the fruiting body and
into the moving air stream (Roper et al., 2010).
The asexual spores also have intrinsic mechanisms for spore release. Electrostatics plays an important role
in discharge of many fungi, including Stemphylium botryosum, Pyricularia oryzae, and
Drechslera turcica (Leach 1980). Leach documented this in part by neutralizing spore discharge using
positive ions. Adams et al. (1986), documented that asexual powdery mildew spores were released with rapid
drops in relative humidity, indicating some kind of intrinsic mechanism. The explosive formation of gas
bubbles may launch spores that then travel millimeters away from the parent fruiting body (Meredith 1963;
Fischer et al., 2010).
Aerosol transport and concentrations
Factors affecting outdoor transport and concentration changes
Maximum aerosol concentrations will be seen close to sources. These concentrations are dependent on diurnal
patterns of spore production and release, as well as meteorological and other factors that force spores into
the air. The transport of fungal spores depends first on the escape of the released spore from its immediate
surroundings. Studies of soybean pathogens indicate that the spore escape rate depends on turbulence in and
above the canopy of plants surrounding the release site, and the filtration accomplished by impaction on
surfaces within the canopy (Andrade et al., 2009).
Once outside the canopy, spore concentrations depend on transport mechanisms (wind) and deposition mechanisms
(gravity, drag, rain, and impaction on surfaces). Gravity has only a relatively small impact compared to
other factors on most spores outdoors. Aerodynamic drag rapidly slows small spores in still air, but in
windy conditions the spores can be transported long distances. Rain essentially washes dry spores from the
air. Wet spores are probably also removed, but the strong activation of sources during rain may mask this effect.
Fungal spores clearly can be transported long distance and can survive for many (even hundreds) of years.
Charles Darwin collected dust from over the Atlantic Ocean during his travels during the 19th century, and
that dust, analyzed in the 21st century produced culturable fungi and bacteria that had been transported
from distant land masses (Gorbushina et al., 2007).
Distances traveled can be very small or very large. Spore transport is likely to follow prevailing winds.
For the plant pathogen Leptosphaeria maculans, spore deposition as measured by infection rates
declined 50% within 12 meters. This measurement was, of course, affected by viability. In other words,
if spores die quickly, their transport will not be measured by this method (Guo & Fernando 2005). On
the other hand, the relatively large sporangia of Phytophthora infestans (potato blight) were
collected using spore trapping methods 500 meters downwind of an infected site (Aylor et al., 2011).
Clearly, meteorological variables strongly affect spore transport as well as spore release as discussed
above. This topic has been extensively reviewed by Jones and Harrison (2004). Their paper is strongly
recommended to those who are especially interested in the topic of fungal aerosols.
Indoor/outdoor relationships have long been of interest to those studying indoor mold concentrations.
In fact, one of the most common ways for estimating the probability of indoor growth has been the use
of indoor/outdoor ratios in one form or another. However, studies that actually document how spores move
from outdoors to indoors are not common. It is generally true that indoor and outdoor spore concentrations
and populations are correlated with the indoor concentrations usually lower than those outdoors. Indoor
concentrations that exceed those outdoors may result from indoor growth, but also from past penetrations
of concentrated outdoor aerosols. On the other hand, indoor concentrations that are lower than those
outdoors may, in fact, indicate a "clean" indoor environment, but may also be related to slow
penetration of extant outdoor aerosols. Li & Kendrick (1996) used path analysis to study the
relationships between different kinds of fungal spores indoors and out. They determined that, overall,
indoor and outdoor concentrations were strongly correlated, although Aspergillus and Penicillium
did not appear to be correlated. Indoor/outdoor relationships differ between May-October and November-April.
Spore penetration into a full-scale model chamber was found to depend on pressure differences, rather
than air leakage unless a clear path was present (e.g., open windows or doors) (Airaksinen et al., 2004).
Those authors point out the importance of these facts with respect to ventilation design.
Aerosol transport and concentration dynamics indoors
The transport of fungal aerosols within a building and changes in concentration over time are dependent
on the source of the aerosols, aerodynamic spore sizes, patterns of airflow within the building, and
opportunities for deposition. Aerosols from surface growth within a room are likely to be most
concentrated within that room, and may be completely absent from other rooms that are at a positive
pressure with respect to the moldy room. Even if the moldy room is positive with respect to the rest of
the building, the concentrations will still tend to decrease as distance from the source decreases.
With adequate ventilation of a moldy room, spore concentrations will gradually decrease as the source
strength diminishes over time.
Few studies have actually documented transport of fungal aerosols through buildings. Slightly moldy
studs did not release a significant number of spores under a variety of conditions designed to encourage
their release (Rao et al., 2009). Kanaani et al. (2008), studied deposition (removal) rates for fungal
spores in a room-sized chamber and found that spore size was important as well as ventilation. It is
interesting to think about these facts in light of the more common recovery of Penicillium and
Aspergillus spores, than those of larger-spored fungi (e.g., Alternaria, Epicoccum,
etc.) known to grow on indoor materials. This is one of the biases imposed on the use of air sampling
to determine the status of a building with respect to fungal growth.
The information presented here represents merely the tip of the iceberg of the information available
on fungal spore aerosols. Most of the available literature has resulted from plant pathology research,
although in most cases, the principles and the models developed are probably widely applicable.
Clearly, the indoor environment has been poorly studied with respect to factors that lead to the
formation and transport of fungal aerosols. This should be a fruitful field of research for new investigators.
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