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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 rarely
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.
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.
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 well-maintained 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.
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