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B. Destructive Activities of Saprobic Fungi 3. Indoor Air Pollution
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3.
Indoor Air Pollution:
Sick
building syndrome is a condition we hear much about today; both in our
homes and in our workplaces. This may be caused by indoor air pollution
that may be in the form of volatiles that produce odors and can cause
allergic responses, smoke, and other house dust problems. One of the
greatest causes, however, has been the buildup of fungal spores in indoor
air. Some obvious signs of a mold/mildew problem are the appearance of
surface molds on walls, furniture, and clothing; and the staining or
puckering of wall coverings; Fig. 9-1; Fig. 9-2)
Fig. 9-1. Deterioration of vinyl wall covering as a result of mold growth in the walls.
Fig. 9-2. Staining of wall covering as a result of molds. By removing stained or loosened wall coverings, a regular fungal garden can often be found (Fig. 9-3) with an array of different fungal colonies.
To fully understand mold/mildew problems in our homes
and work places, we must understand the organisms that cause such
problems. We have studied already what fungi are and how they differ
from other organisms. They were shown to be uniquely different from the
other groups of living organisms in that most are composed of
filamentous threads (mycelium) surrounded by a wall composed of largely
chitin and glucans. They do not have chlorophyll and, therefore, must
live as some type of parasite or saprobe. Having walled cells with no
mouth-parts, fungi must secure their nutrients in other ways. They do so
by secreting enzymes into the substrate on or in which they are growing,
the enzymes dissolve nutrients that can then be absorbed by the fungus.
Fungi reproduce by spores. Spores are highly variable in size, shape,
color, and septation. With proper growth conditions, most fungi will
form millions of spores. Terrestrial fungi form windblown spores that
can travel considerable distances and become ever present in our
atmosphere. There are four things that are essential for active fungal growth: (a) the presence of fungi, (b) suitable nutrients, (c) proper temperature, and (d) adequate moisture (Fig. 09-4). a. Presence of Fungi (Fig. 9-4):
Fig. 9-4. The four things essential for mold and mildew problems. Mycologists have estimated that there are perhaps more than 250,000 species of fungi. The 1995 Dictionary of Fungi (Hawksworth et al. CMI, Kew, Surrey, England) records more than 72,000 species, with almost 1,000 new species described annually. Over 90% of these species develop windborne spores. A single mushroom or molded orange, for example, may release several millions of spores. Spore production is seasonal and populations vary depending on the amount of substrate and moisture available. Studies on indoor air quality in more than 150 homes and public buildings in Florida reveal that average spore count outside is more than 1,100 spores per cubic meter of air and approximately 450 inside of homes and 280 inside of public buildings. Thus, the fungi are ever-present (Fig. 9-5) and their populations may vary from season to season. During the summer months when air-conditioning keeps down the humidity
within buildings, there is a relative low number of spores in the air;
whereas, in mid-winter when humidity levels increase, the spore levels
increase.
Fig. 9-5. Fungi in indoor environments. b. Nutrients. Fungi have developed a wide array of enzyme systems. As a result, they are able to invade all kinds of substrates, although their preferences are plants and plant products such as sugars, starches, cellulose, fats & oils, and complex hydrocarbons (Fig. 9-6).
Fig. 9-6. A variety of substrates that fungi can use as food.
Fig. 9-7. Common materials in the home that can serve as fungal nutrients. As we look around our homes and public buildings, we find a great variety of suitable organic substrates on which fungi can grow (Fig. 9-7). Many of our textiles are of cotton, flax, wool, hemp, and other plant derived materials. The major component of wood is cellulose, dry wall backing is of cellulose with organic adhesives. Soaps and other household products have oil bases; fungi love them (Fig. 9-8)!
Fig. 9-8. Shower stalls with an abundance of soap scum with oils, and moisture, provide an outstanding place for fungal growth. In recent years there have been major problems with certain polyvinyls used as outdoor floor covering. Several years ago it was discovered that a fungus, Cladosporium resinae, would grow in tanks of jet fuel, clogging carburetor jets, resulting in aircraft losses. Aureobasidium pullulans will invade a number of complex hydrocarbons such as those in polyvinyl. Lint and house dust is made up largely of organic particles. That is why it is important to keep filters and ductwork clean of debris. A large accumulation of “crud” means that there are high levels of nutrients mixed with large numbers of fungal spores. A sure sign of excessive spore levels is when occupants will suffer from watery eyes, scratchy throats, runny noses, wheezing, and oftentimes rashes. Allergy patients will experience great discomfort and may require increase dosages of medication. The presence of moldy odors is a good sign of active fungal growth. A look around a motel room or your home will reveal a number of good substrates for fungal growth. c. Temperature. Most fungi are mesophilic,
i.e. they will grow best at 75-95° F, but can grow at from 50-100° F.
A few fungi are thermophilic
(heat loving) and will grow at 75-120° F; while fewer are psychrophilic (cold loving) and will grow at 30-40° F. Most fungi
enjoy the temperatures that we enjoy and, therefore, we can rule out the
use of extreme temperatures to lower the population of fungi indoors. d. Moisture. The most vital factor in fungal growth is moisture. Why is this
so? Reflect back to our earlier discussions, fungi have an absorptive
nutritional system. Water is necessary to trigger enzyme production, it
is necessary to carry the enzymes through the fungal cell walls, and
whenever enzyme action is complete and the substrate is dissolved, water
is the basic carrier of nutrients back into the cells. Most fungi grow
best at a relative humidity (RH) of 70% or above. A RH of less than 55%
will eliminate most active fungal growth.
Low RH delays spore germination and greatly reduces mycelial
growth and spore production. Various substrates can absorb substantial
moisture from the air whenever the RH is high. A recently constructed
hotel near Kissimmee, FL had 30-40% moisture levels in the walls, all
due to faulty construction.
Granular carbohydrates such as found in cereals should be maintained at
a moisture level below 15%. The optimum moisture level for most
wood-rotting fungi is around 40%. The “dry rot fungi” will grow
whenever the moisture level is between 20-25%. Warm air will hold more
moisture than cool air. Air at 80° F can hold twice the moisture as air
at 60° F. Thus, one of the keys to reducing or eliminating mold/mildew
growth in the indoor environment is to maintain cool, dry conditions.
That is why in a hot and humid climate, such as Florida, air-conditioning
is necessary, not only for our comfort, but to prevent mold/mildew
problems.
Fig. 9-9. Moisture requirements for fungi. There are a number of things that can be done to control
moisture. During hot, humid seasons, keep windows and doors closed
as much as possible and have good seals around doors and windows to
prevent moisture encroachment. Proper “sizing” of air conditioners
is vital. If a unit is too small, the compressor will run constantly and
yet have ineffective cooling and dehumidification, also resulting in
higher energy cost. If an air conditioner
is “oversized”, i.e. too large for the volume of air that needs to
be cooled, another problem occurs. With a very high cooling capacity,
the thermostat quickly tells the unit it’s cool enough, stop! When the
compressor stops, dehumidification stops, moisture encroaches into the
building, condensing on the cool surfaces inside. Once the thermostat
calls for more cooling, the unit does so quickly and stops again. Thus,
within a particular timeframe, very little dehumidification takes place
and a minor level of moisture condensation can occur between each cycle.
Many substrates within can reach moisture levels that will support
fungal growth Another problem in homes and public buildings is the use of vinyl wall covering or other material that will serve as a vapor barrier. Moisture encroaches through the walls of buildings, even those that are well constructed, and under most conditions would diffuse into the rooms where the air handling system will cool and dehumidify. If there is a vapor barrier, however, the warmer moist air will diffuse through the wall until it reaches the cold vapor barrier where it will condense. Soon moisture levels of 30-40% can be reached, and with cellulose-backed drywall and wall cover adhesives, all the necessities for a fungal garden are present. That is what usually occurs. Fungal spores were trapped there at the time the wall covering was applied, only waiting until conditions were right for germination. Signs of fungal problems within walls include odors and stains, or the puckering of the wall covering. In severe cases the vinyl wall covering may become completely detached because adhesives will not dry, or the fungus may have eaten the adhesive (Fig. 9-10)!
Fig. 9-10. A fungal garden behind wall covering where moisture is too high. As
fungi release their enzymes and break down the substrates on which they
are growing, stains and odors are often produced.
The presence and kinds of stains that are produced by a
particular fungus is influenced by the available nutrients; for example,
the same fungus on different media may display different colors (Fig.
9-11).
Mildew growth on the outer wall surface is an indication that moisture
is condensing within the room and there is ample organic material on the
walls to support fungal growth.
Fig. 9-11. The same fungus on two different media, yet producing different pigmentation. There are several thousand genera and almost 100,000 species of fungi. At any one time, dozens of species can be found during routine air sampling (Fig. 9-12).
Fig. 9-12. A typical air sample may have a great variety of fungi. Black pepper shaken over a fungal growth medium will yield an even greater variety of fungal species. The following genera have species common to the indoor environment and are listed in order of their abundance:
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Cladosporium : There are approx. 500 species of Cladosporium that have been described. Many of these species are based upon the plant host or substrate on which they were found, and may not be valid species. Species of Cladosporium make up 60% of the spore population outside, but may represent as much as 90% of the indoor population. Most are greyish-green to olive-brown in colony color (Fig. 9-13). They are characterized microscopically by the formation of chains of brownish, one-celled to septate conidia (Fig. 9-14).
Fig. 9-13. Cladosporium herbarum.
Fig. 9-14. A drawing of different species of Cladosporium. They are the most common cause of mildew of textiles
and fabrics within the home.
Penicillium : More than 300 species of Penicillium have been described (Fig. 9-15). It is one of our most cosmopolitan groups, growing on all sorts of substrates. They cause decay of fruits, vegetables, and other foodstuff, grow on fabrics and building materials, and contaminate liquids around the home and reagents in our laboratories. They make up close to 30% of the spores found in the indoor environment. It is important that species are identified correctly because certain species are pathogenic to man, causing, as we will see in our chapter on medical mycology, eye, ear, nose, throat, and lung infections referred to as Penicilliosis. P. notatum and P. chrysogenum are noteworthy for their production of the antibiotic penicillin.
Fig. 9-15. Some species of Penicillium involved in indoor air pollution.
Aspergillus : While spores of Aspergillus are found rather consistently in air samples, they are not as frequent as those of Cladosporium and Penicillium. Close to 150 species have been described. Like Penicillium, they are found growing in all types of substrates. Again, species should be identified correctly because some of them can also cause a disease in man called Aspergillosis, which is similar to Penicilliosis. Most species are green to yellow, sometimes brown or darker in colony (Fig. 9-16), and produce their spores on inflated heads (Fig. 9-17).
Fig. 9-16. Aspergillus flavus, of concern because it forms aflotoxins.
Fig. 9-17. A scanning electron micrograph of an Aspergillus. Species of Aspergillus
are a major problem in the
storage of grains, not only from the deterioration of the grains, but
from the secretion of carcinogenic metabolites we refer to as aflatoxins,
named for A. flavus in which
the toxins were first discovered. A number of species of Aspergillus are
used in the commercial production of enzymes, organic acids, and other
chemicals.
Alternaria: Species of Alternaria are
found on living and decaying plant material. Many cause leaf spots and
cankers in the field; others get into our indoor environment and find
suitable substrates on which to grow. They were some of the first fungi
proven to be allergenic. Their colonies, like Cladosporium,
are olive-brown to black (Fig. 9-18) and their spores are large,
multicelled, obclavate and have both vertical and horizontal septa (Fig.
9-19).
Fig. 9-18. A culture of Alternaria.
Fig. 9-19. A drawing of Alternaria cheiranthi.
Curvularia:
Species of Curvularia, like Alternaria, cause leaf spots on plants and grow saprobically on a
large variety of plant debris. Their colonies are blackish to
brownish-black, but Curvularia
spores have horizontal septa only and are usually curved. They are
able to grow and amplify in the indoor environment if growth conditions
are suitable.
Dreschlera,
(Fig. 9-20) Bipolaris,
& Exerohilum
: Species of
these genera until recent years were described in the genus Helminthosporium.
They, like Alternaria, are
mostly plant parasites, causing leaf spots and blights. They are found
less frequently in the indoor environment. All three genera have
colonies that look much like those of Alternaria,
but their spores are brown and elongate with several cross septa.
Fig. 9-20. A drawing of conidia of Dreschlera.
Epicoccum: Species of Epicoccum are common airborne fungi that produce dark brown, globose, multicelled spores in clusters on short stalks. In culture, they begin as a bronze colony that later become darker as spores mature (Fig. 9-21).
Fig. 9-21. A culture of Epicoccum nigrum.
They are weakly parasitic on plants and grow mainly as saprobes on
decaying leaves and wood.
Trichoderma: Species of Trichoderma form a quickly spreading growth over the substrates on which they grow. They are frequently found on damp building materials and are some of our most widespread contaminants. They are white to greenish in culture and form their spores in whorls of three stalks (Fig. 9-22).
Fig. 9-22. A culture of Trichoderma viride. Some species
are mycostatic, i.e. inhibit or slow the growth of other fungi,
and have been used in the biocontrol of more important plant pathogens.
Stachybotrys : For some reason, species of Stachybotrys prefer very wet cellulosic substrates. Indoors, they are common on the cellulosic backing of drywall and on suspended ceiling tiles that have become wet. They produce a dark black growth on such surfaces and in culture (Fig. 9-23; Fig. 9-24).
Fig. 9-23. A culture of Stachybotrys chartarum.
Fig. 9-24. Wet drywall heavily contaminated with Stachybotrys chartarum. In buildings with improperly sized air conditioners, this fungus is found around vent covers on which moisture condenses and diffuses into drywall or ceiling tiles. Microscopically, they somewhat resemble species of Penicillium, except the spores are darkly pigmented and ornamented (Fig. 9-25).
Fig. 9-25. A drawing of different species of Stachybotrys. Some species of Stachybotrys are
of particular concern because they cause a disease in animals called Stachybotryosis, and it has been shown that they can emit volatile
mycotoxins (see later under the section on mycotoxins). There were 23
children in Cleveland, Ohio who were impacted by this fungus as reported
in an Associated Press report from the Center of Disease and Prevention
in Atlanta in 1996. Volatiles of the fungus caused a disease, pulmonary hemosiderosis, with bleeding in the lungs, and was fatal
to at least 3 children. All of these children lived in old, run-down
houses with extensive water damage. It is not known at this time whether
the disease is cause by inhaling volatile toxins from the growth of the
mold, or from inhaling high levels of spores that would have these
toxins.
Others: Several other genera of fungi are less frequently isolated from the indoor environment. They include: Acremonium, Aureobasidiuim (Fig. 9-26),
Fig. 9-26. A culture of Aureobasidiuim pullulans. Botrytis, Fusarium, Mucor, Paecilomyces, Scopulariopsis (Fig. 9-27), Sporobolomyces, and Rhizopus. In dwellings in which there is considerable animal hair, dermatophytes such as Trichophyton may be found.
Fig. 9-27. A culture of Scopulariopsis brevicaulis.
(Images of fungi in culture are from Gravesen et al. 1994. Microfungi. Munksgaard, Netherlands, 168 pp)
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