Biological Contamination in the HVAC System
The heating, ventilating, and airconditioning system (HVAC) in a building is very
similar to the respiratory system in a human body. The HVAC system
provides conditioned air to building occupants and is essential to the comfort of building
occupants, and necessary to the operation of the building. The respiratory system
delivers oxygenated air to the blood stream in a human body and is essential to the
survival of humans. The importance of these two systems is clearly significant to
humans.
To provide sound attenuation and to conserve energy, the HVAC system,
including the air handling unit and the airduct, is often insulated with fiberglass and
other insulation materials. Some airducts are insulated internally or externally. There
are also airducts made of glass fiberboard. Internal insulation materials with a rough
porous surface will trap particles and particulates from the air stream. Materials trapped
include pollen grains, plant matter (including decayed leaves, plant hairs, or fern spores),
fungal spores, insect parts, skin flakes, paper fibers, and other organic matter. These
materials are often hygroscopic and can absorb moisture from the air. With sufficient
moisture content in the accumulated dust, fungal spores germinate and grow. During the
cooling season, condensation from cooling coils and water in the drain pans allow fungi
and bacteria to proliferate.
The HVAC systems are designed for cooling during hot months and for heating during
cold months. During the cooling cycle, warm air is cooled by passing through the
cooling coils. Excessive moisture in the warm air is condensed into liquid water when
passing through the cooling coils. Air discharged from the cooling coils often has
elevated relative humidity. Therefore, the cooling coils, condensate drainage pan, and
adjacent areas are primary amplification sites for fungal and bacterial growth.
Humidifiers in certain systems also provide moisture for microbial growth. Several cases
of excessive fungal and bacterial growth and reported illnesses were associated with cold
water humidifiers in the HVAC system. Humidifiers were found to have bacterial growth
and endotoxin levels. As with many IAQ problems, it is difficult to directly link
environmental data with actual illnesses.
Biological Contaminants in the HVAC System
Various biocontaminants may be found in the HVAC system. The primary
biocontaminants are fungi and bacteria. Secondary biocontaminants may include mites,
insects, or nematodes. Many fungi and bacteria are saprophytes and very adaptable to the
environment as long as there are organic nutrients and sufficient moisture. Secondary
contaminants are often the result of fungal growth. Fungal contaminants produce
allergens; mycotoxins; beta1,3glucans; and fungal volatile organic chemicals (VOCs).
Bacterial contaminants may produce allergenic proteins, toxins (endotoxins in particular),
and bacterial VOCs. Occupants in buildings with a contaminated HVAC system
occasionally report musty odors. These odors can often be traced to microbial
amplification in the system. Some bacteria, such as Pseudomonas aeruginosa, may cause
opportunistic infections. These biocontaminants can produce adverse health effects in
exposed building occupants.
Moistureloving fungi, such as Acremonium spp., Aureobasidium pullulans, Exophiala
spp., Phoma spp., Sporobolomyces spp., Rhodotorula spp., and yeasts, are common
inhabitants of cooling coils and drain pans. In addition, Gramnegative bacteria and
endotoxins at elevated levels have been detected and reported from drain pans and
contaminated humidifiers. A second group of fungi can be found thriving in insulated
ductwork downstream and adjacent to cooling coils. They are mostly Cladosporium spp.
(primarily C. cladosporioides), Penicillium spp. (primarily P. corylophilum), and
occasionally, Aspergillus spp. These fungi grow well at a moderately high water activity
near 0.85 and 0.90 (water activity measures available water in substrates for microbial
growth; the highest water activity is 1.0). Gramnegative bacteria are seldom detected in
large numbers with these fungi.
Besides fungi and bacteria, signs of mites and insects have been observed among fungal
growth on a few occasions. Intact fungal spores have been seen in insect and mite fecal
pellets, suggesting that insects and mites fed on fungal spores. In one instance,
nematodes and a nematodetrapping fungus, Harposporium anguillae, were recovered
from slime in a drainage pan in the HVAC system of a large building. Endotoxin
(produced by Gramnegative bacteria) exposures were associated with cases of sick
building syndrome in an office. The source of endotoxins in that instance was identified
to be contaminated humidifiers and ventilation ducts.
There are several approaches to address concerns of biocontamination in the HVAC
system.
1. Upgrade filtration efficiency of the system. A mechanical engineer should be
consulted regarding pressure drop from higher filtration efficiency. Filters
should be replaced or cleaned according to the manufacturer’s
recommendations.
2. Clean and maintain regularly the cooling coil and drainage pan,
preferably quarterly or twice a year. The frequency should be decided upon
depending on the age, operation, uses, and history of the system.
3. Maintain insulation in the air handling unit and the ductwork to minimize
conditions for the amplification and accumulation of biocontaminants. The
maintenance should include upgrading filters, making sure that filters are
installed properly and do not allow bypass, changing filters on a timelybasis,
inspecting cooling coils and drainage pans, and cleaning the insulation materials
when necessary.
4. Consider cleaning the HVAC system when there are obvious signs of microbial
growth and/or heavy dust accumulation. Consult a competent environmental
professional for an evaluation.
5. Design the HVAC system so that the air stream does not contact
internal insulation with a rough porous surface.
6. Inspect and maintain humidifiers regularly. Avoid installing and
using cold water humidifiers.
7. Place outside air intakes away from street level, cooling towers, and loading
dock areas. Air intakes should be cleaned to remove litter and dirt.
Microbial contamination in buildings and its impact on indoor air quality (IAQ) are well
documented. Several recent articles suggest that as much as one third of IAQ problems
may be microberelated. Microbial contamination was the primary IAQ concern in
approximately 29% of more than 150 office buildings studied by the State of Minnesota.
It is important to note that no environmental and clinical correlation was established in
the Minnesota studies. The etiologic agents, whether allergens, mycotoxins, endotoxins,
or microbial VOCs, that were responsible for IAQ complaints were not identified.
However, complaints of poor IAQ in the buildings involved disappeared, subsided, or
were greatly reduced after biocontamination was abated.
Porous materials are commonly used for acoustical and thermal insulation inside the
HVAC system. Microbial growth and amplification are known to occur in soft, porous
insulation materials. Poorly maintained humidifiers, cooling coils and drainage pans
provide a wet environment for microbial amplification. Many biocontaminants have
been identified in HVAC systems. They can cause adverse health effects to building
occupants. As outlined above, steps can be taken to prevent these problems.
Also, there is usually mixed growth of various molds and bacteria together in
a wet and damp environment. An actively growing mold may provide or release various
bioactive agents, such as allergens, mycotoxins, microbial volatile organic compounds,
and glucan. People with differing levels of immunity may suffer varying reactions
different to these agents. building operators, managers, and
occupants is not to allow microbial growth and amplification to occur in the building; if
you can prevent colonization and growth, then you do not need to worry about the
various health effects caused by microbes.
The following microbes may be associated with the HVAC system, including cooling
towers.
1. Legionella pneumophila, a common waterborne bacterium, may be detected in
cooling tower water or the building potable water system (particularly hot
water). It is known to cause Legionnaire’s disease and pontiac fever.
2. Pseudomonas aeruginosa, a common waterborne bacterium, may cause
opportunistic infection. It grows in water, from potable water to stagnant water.
3. Cladosporium cladosporioides,
a common mold found outdoors, is a common colonizer of a dirty HVAC
system, particularly just downstream from cooling coils. Spores of
Cladosporium are potentially allergenic and Clado sporium has been associated
with hypersensitivity pneumonitis.
4. Penicillium corylyphilum is another common colonizer of a dirty HVAC
system. Spores of Penicillium are potentially allergenic.
5. Endotoxin is a cell wall component
of Gram negative bacteria, which is
common and abundant in any water. It is released into the environment when
bacteria grow, divide, or die.
Endotoxin is known to cause various adverse
health effects related to the respiratory system. |
