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Fungi and Bacteria in Ventilation Systems
Fungi growing in ventilation systems may contaminate indoor
environments and cause a variety of problems. Some fungi can
cause lung infections. Many fungi can cause allergic reactions in
susceptible people and respiratory irritation in non-allergic
people.Inhalation of fungal spores by highly susceptible people
can have fatal consequences. Some environmental bacteria can
grow in ventilation systems, but these are raely a threat to
healthy people. They can, however, be a nosocomial problem.
Low levels of airborne fungi can be a primary or contributing
cause of Sick Building Syndrome (SBS) and poor Indoor Air
Quality (IAQ). The photomicrograph at the right shows lung tissue infected with a growing
mycelium of aspergillus.
Fungi differ significantly, in certain respects, from most other airborne pathogens, such as
bacteria, viruses, and protozoa. Fungi do not cause secondary contagious infections; only
the person inhaling the fungi is at risk. Fungi can exist outdoors and enter the building
through the air intakes. No other respiratory pathogens can exist outdoors -- viruses and
bacteria are carried and transmitted indoors by human or animal hosts, with anthrax being
the one exception. Fungi are normally harmless and non-parasitic. Fungal infections
inevitably result from fungi being in the wrong place, often as the result of poor
cleanliness or improper design of ventilation system components.
Fungi are actually plants that contain no chlorophyll -- see the chart above. The true fungi
are the Eumycetes. Some of the fungi, the mushrooms, yeasts and some of the molds are
extremely beneficial to us. They assist the production of cheese, antibiotics, yogurt, wine
and beer. Some fungi, like the blights, can cause extensive crop damage. Dutch Elm
disease is, in fact, a fungus. In buildings, the ones that cause problems when they get into
the wrong place are usually certain of the ascomycetes.
Airborne Pathogenic Fungi
Only certain fungi can produce infections, and only a few of these have been noted to
travel via the airborne route, or become entrained in the airflow of intake ducts. The
following chart list those fungi which are of primary concern:
FUNGI
DISEASE / SYMPTOM
Aspergillus
Aspergillosis
Absidia
-
Rhizopus
-
Mucor
-
Cryptococcus neoformans
Cryptococcosis
Histoplasma capsulatum
Histoplasmosis
Blastomyces dermatitidis
Blastomycosis
Coccidioides immitis
Coccidioidomycosis
Penicillium sp.
Respiratory irritation, allergic reactions
Micromonospora faeni
Respiratory irritation, allergic reactions
Thermoactinomyces vulgaris Respiratory irritation, allergic reactions
Alternaria
Respiratory irritation
Cladosporium
Respiratory irritation
Helminthosporium
Respiratory irritation
Fungi from Outdoors
Fungi produce spores, in much the same way as bacteria do, and this enables them to
survive harsh conditions while they travel or lie dormant. Spores are usually what enter
the building air intakes and what can travel through the
ventilation airstream. Fungal spores are smaller than fungal
cells and can vary in size from 1 micron to 100 microns. A wellmaintained HEPA filter should be capable of intercepting the
vast majority of fungal spores. At the right is an image of a
colony of Candida albicans that has produced a number of large
and small spores.
Fungi are ubiquitous in the outdoors, but occur in high
concentrations only in hot Southern climates, especially during dry spells. Florida,
Louisiana, Texas, New Mexico and southern California often experience high seasonal
mold spore levels. Generally, when the ground dries after a
period of moisture, the winds can overturn the top layers of
soil and disperse large quantities of mold spores. These can
be carried aloft into urban areas, where they are drawn into
air intakes and building ventilation systems. The photo at left
shows the long growing branches of mycelium that are
characteristic of Nocardia asteroides, from a sputum sample of
an infected patient. Nocardia are bacteria called
actinomycetes, which greatly resemble fungi in characteristics,
and they also produce spores.
Even though the problem is more common in southern states, it only takes the right
conditions for microscopic quantities of fungi to gain a foothold in a ventilation system.
This situation has occurred across the US, regardless of climate. Many, if not most, cases
of poor IAQ and SBS can be tied directly to the occurrence of mold spores either in the
ventilation ducts, or in the walls of buildings. Sometimes, fungi are merely a contributing
factor when the ventilation is inadequate -- normal levels of airborne fungi are not
removed from the building air.
Dealing with Fungi in Ventilation Systems
Filtration provides the primary defense against fungal spores entering a building
ventilation system. Pre-filters can be effective against most fungi, even when in the spore
form. If a higher degree of protection is required, HEPA filters
can be very effective, provided they are tightly installed, and well
maintained. Fungus can grow on HEPA filters as well as other
ventilation components and, if unchecked, can actually contribute
to the problem. The image at right shows a layer of actinomyces
mycelium growing on a surface.
Fungus or fungal spores from the outdoors can be dealt with
easily, as described above. If, however, the fungus is already growing inside the building
or ventilation system, the problem becomes somewhat more difficult. Fungi require
moisture for growth. The source of the moisture must be identified and then controlled.
Cooling coils, drains pans, and water pans for humidifiers are
likely locations for fungal growth, especially when there is
standing water. These must be treated as necessary with proper
disinfectants. Some systems provide built-in UVGI lights for
continuous disinfection. These components should be
disassembled and cleaned with a strong disinfectant, such as
chlorine, when fungal or bacterial growth is found. Clogged
drains are often a cause for standing water.
Condensation on ductwork or other components is another likely
source of moisture. The ductwork must be inspected for fungal
growth and cleaned with a disinfectant. The cause of the
condensation must be identified. Often, it results from inadequate insulation, or leakage
into, or out of, the ductwork. Sometimes return air can leak into the supply air duct and
result in localized condensation. Sometimes the insulation itself can absorb and hold
moisture, resulting in fungus growth that
may then directly or indirectly produce
contamination of the building air. Smoke
tests, or airflow measurements, and/or
pressure tests can determine duct leakage.
In the absence of water they may reduce to
spore form, which makes them even more
subject to air entrainment. Therefore, a cycle
of condensation and dehydration may
exacerbate a fungal dispersion problem. In
this case, the problem might be perplexing to
isolate -- sometimes the duct and
components will appear dry, while cases of
respiratory irritation or infection may occur in irregular cycles that could ultimately depend
on humidity variations. Every situation can be unique and must be studied carefully.
Air Sampling and Testing
Sampling of airborne microorganisms can be inconclusive. There are no absolute
standards, and decisions on whether a building has a fungus problem or not are often
made arbitrarily. Methods of collection can give divergent results and are therefore heavily
subject to interpretation. Swabbing a sample from a duct or an exhaust grille will yield
some concentration of fungal or bacterial cells, but doesn't
exactly correlate with airborne concentrations.
Measuring airborne concentrations can likewise produce results
that depend on interpretation. Often, the testing agencies will
not identify the specific microorganisms, but will merely state
that colonies were formed, or that there is a potential
contamination problem. Most fungi are unique and have
distinctive characteristics. The photomicrograph at right shows a colony of actinomyces, in
which the radiating rays of the mycelium are clearly visible around the central granule.
Any studies contracted to be performed, such as on schools or office buildings, should be
required both to state the types of microorganisms discovered, their probable airborne
concentrations, and how these compare with standards or typical concentrations in
normal, or "healthy," buildings.
Fungi in Ventilation Systems Bibliography
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