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Indoor Mold Growth: Health hazards and remediation

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Wladyslaw J. Kowalski at Sanuvox
Wladyslaw J. Kowalski
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September 2000 – HPAC Heating/Piping/AirConditioning Engineering
DATA DIGEST
Indoor Mold Growth
Health Hazards and Remediation
By W. J. Kowalski, MS, PE
The Pennsylvania State University
Department of Architectural Engineering
Many different fungi grow indoors as mold in the presence of moisture. Some of these fungi can cause allergic
or toxic reactions, while a few may cause infections in susceptible individuals. A comprehensive treatment of this
complex topic would take volumes, but this article provides a synopsis for engineers to help clarify the mysteries
of indoor mold growth.
First, here’s a little mycology regarding human pathogenic fungi. In general, fungi grow as either molds or
yeasts. In the environment, where available nutrients, moisture and temperature conditions may be marginal,
these fungi normally grow as molds. In infected tissue, where conditions are more ideal, these fungi usually
grow as yeasts. The yeast form of growth greatly resembles the colony formation of bacteria.
Most pathogenic species of fungi reproduce asexually. Asexual spores produced in the mold phase serve to
disseminate the fungus. Spores are considerably more resistant to the elements than the mold or yeast forms,
and some spores have even been known to survive in space on the exterior of spacecraft.
HEALTH PROBLEMS CAUSED BY MOLD
No contagious diseases are caused by fungi. Respiratory infections such as aspergillosis and histoplasma are
caused by inhalation, usually from long-term exposure (Samson 1994, Howard and Howard 1983). Allergic
alveolitis, rhinitis, and hypersensitivity pneumonitis may result from long-term exposure in the workplace by
individuals who have no allergies (Pope et al 1993). Many fungi cause infections that are unique to the species
like cryptococcosis and blastomycosis.
Asthma can be aggravated or even induced by exposure to certain fungal species. Allergic rhinitis can
occur in sensitive individuals who are regularly exposed to both fungal agents and other allergens such as
pollen, dust mites, and animal dander.
Some fungi cause infections of the skin, including ringworm and athlete’s foot. Inhalation of certain
species can cause toxic reactions. Stachybotris atra (chartarum) is alleged to have caused several infant
fatalities and has been frequently isolated growing indoors (Woods et al 1997). Fungal infections that pose no
threat to healthy individuals can be fatal to those suffering immunodeficiency, or recovering from burns or
surgery.
SICK BUILDING SYNDROME
Some 15-30% of cases of building related illness have been associated, if not directly linked, to indoor fungal or
bacterial contamination. Certain fungi produce VOCs and odors, from which long-term exposure can result in
impaired health or contribute to sick building syndrome (Godish 1995, Lacey and Crook 1988, Samson 1994).
In addition to human health problems, damage to building materials, books, clothes, and stored foods can occur
from mold growth.
How common is the problem of mold growth indoors? Mold growth can occur from water damage,
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condensation, leaks, or even the mere presence of high humidity (i.e. >90%) since nutrient and temperature
conditions are invariably satisfied indoors. Some molds, like mildew on clothing and bathtubs, rarely pose any
hazards. Some potentially hazardous fungi, like certain species of Aspergillus and Penicillium, predominate
wherever mold growth occurs.
The relationship between building dampness, fungal growth, and health complaints has seen much recent
study. In England, some 30% of houses were found to be damp while 47% had mold growth (Platt et al 1989).
High correlations between health problems and dampness or mold growth were observed, especially among
children.
In a Canadian study, 38% of houses were either damp or had mold growth. The presence of lower
respiratory symptoms was approximately 50% higher in these homes while upper respiratory symptoms were
almost 25% higher (Dales et al 1991). The presence of Aspergillus above 50 CFU/m3 was associated with
coughs, colds, and eye and skin irritation.
A study in Finland found that 52-58% of houses had moisture problems and that this was associated with a
higher risk of respiratory ailments, especially in children (Koskinen et al 1996).
WHERE FUNGAL SPORES COME FROM
Fungal spores normally and ultimately hail from environmental sources. In the North, spores appear seasonally
with peaks in the dry periods of summer and lows during snow covered winters. Outdoor levels typically vary
between 100-1000 cfu/m3. Geography can determine the make-up of outdoor spore concentrations.
In new buildings, indoor levels of spores are lower than outdoors, even with natural ventilation. Buildings
that foster fungal growth may generate spore levels higher than outdoors. Cases of such problem buildings may
require special treatment, including elimination of moisture sources and water-damaged materials.
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What fungal spores are common indoors? Table 1 lists all the major pathogenic and allergenic fungi, by
genus, that have been found growing indoors (Kowalski et al 1998). This list is by no means exclusive, as
previously unknown hazards may become recognized.
CAUSES OF INDOOR MOLD GROWTH
Normal indoor conditions provide a suitable environment for the growth of a wide range of fungal spores. The
presence of moisture or high relative humidity is a sufficient catalyst for the germination and growth of fungal
spores. Figure 1 shows the results of a study done on how the growth rates of various types of fungi are affected
by indoor conditions.
Materials normally present in buildings provide nutrients for fungal growth. These include building
materials like wood or cellulose and organic materials found in rugs and curtains. Water damage to rugs will
sometimes result in rapid mold growth due to the fact that mold spores have settled or been tracked into the rug
over time. Cleaning rugs periodically and exposing them to direct sunlight can help.
Dust can provide a nutrient base on which fungi can grow. In HVAC systems, dust that collects on
surfaces or in crevices is sufficient to support fungal growth in the presence of moisture from condensation.
Bacteria can influence the growth of fungi. Environmental bacteria can grow biofilms, and thereby provide
fungal spores a nutrient base.
REMEDIAL MEASURES
Filtration of the intake air provides the best means of preventing airborne spores from entering a building,
although spores may still be tracked or carried in by other means. Filtration of return air can also control indoor
airborne levels, but if spores are being generated indoors then this problem should perhaps be dealt with at the
source.
Typical dust filters may be insufficient to intercepting fungal spores since the most common ones tend to
be in the 1– 4 micron size range. Excellent removal rates can be attained with simple high-efficiency ASHRAE
filter. HEPA filters would be overkill in this regard. For example, a 35-40% ASHRAE filter will remove 57% of
Aspergillus and 83% of Stachybotris spores (Kowalski et al 1999).
Figure 1: Limiting Temperature and Relative Humidity conditions below
which growth will not occur. Based on data from Clarke et al (1998).
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Control of humidity provides one means of preventing or limiting growth once spores have entered a
building, but humidity levels of 60% or less are no guarantee, since moisture content of buildings materials is a
more critical factor. Controlling humidity inside an air handling unit (AHU) below about 90% may be impossible
but another approach is keeping HVAC systems clean of dust and keeping drain pans unclogged.
A novel method for controlling fungal growth on surfaces involves the cycling of cooling systems to
alternate periods of moisture and dehumidification. Spores germinate in the presence of moisture, but then their
resistance to dehydration becomes reduced. A normal cycle of daytime cooling (i.e. >90% RH to germinate
spores) and night-time dehumidification has the potential to significantly reduce fungal growth inside AHUs
(Sakuma and Abe 1996).
Ultraviolet germicidal irradiation (UVGI) can control microbial growth on cooling coils and internal duct
surfaces through continuous exposure. Some recent studies have shown that UVGI can improve operational
efficiency and produce savings through reduced energy consumption (Shaughnessey et al 1999).
Inspections should be performed whenever a mold growth problem is suspected. AHUs and building
areas subject to condensation or water damage should be examined for visible mold growth. The presence of
mold growth is sufficient reason to undertake remedial measures, regardless of what species is found.
Air sampling isn’t recommended unless fungal growth is observed or occupant complaints are high. Air
sampling of general indoor areas and the air supply registers can be used to determine if overall spore levels
inside are high or exceed those outdoors. Bulk sampling can be performed before and after remediation to
assess effectiveness of any measures taken, and any problem species can be assessed individually. As a
general guideline, indoor spore levels should be less than about 100 cfu/m3 or less than outdoor levels,
whichever is lower. The species mix found indoors should not differ significantly from that of the outdoors. Health
care facilities should seek much lower levels than these (i.e. 10 CFU/m3 or less), depending on facility type.
Allergic individuals may also want to target indoor levels lower than the minimum outdoors.
Surface sampling can determine the presence of fungal growth on the inside of ductwork or cooling coils
but is not necessarily an indicator of an actual problem, since low levels of growth may be tolerable.
Determination of the specific species is not always essential, especially if occupants do not report a high
incidence of health problems.
Duct cleaning usually involves vacuuming or scrubbing mold growth. In addition, a variety of commercial
cleaning agents or disinfectants are used, including ordinary water or a 10% bleach solution.
REFERENCES
Clarke, J. A., C. M. Johnstone, N. J. Kelly, R. C. McLean, J. A. Anderson, N. J. Rowan, and J. E. Smith. (1998). “A
technique for the prediction of the conditions leading to mould growth in buildings.” Building and Environment 34: 515-
521.
Dales, R. E., R. Burnett, H. Zwanenburg. (1991). “Adverse health effects among adults exposed to home dampness and
molds.” Am. Rev. Resp. Dis. 143: 505-509.
Godish, T. (1995). Sick Buildings : Definition, Diagnosis and Mitigation. Boca Raton, Lewis Publishers.
Heinemann, S., H. Beguin, N. Nolard. (1994). Biocontamination in air-conditioning. Health implications of fungi in indoor
environments. R. A. Samson. Amsterdam, Elsevier: 179.
Howard, D. H. and L. F. Howard (1983). Fungi pathogenic for Humans and Animals. New York, Marcel Dekker, Inc.
Koskinen, O., T. Husman, T. Maklin, A. Nevalainen. (1996). The relationship between moisture observations in houses
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and inhabitants' state of health, Part I: Adults. Indoor Air '96, Nagoya, Japan.
Kowalski, W. and W. P. Bahnfleth (1998). “Airborne respiratory diseases and technologies for control of microbes.”
HPAC 70(6).
Kowalski, W. J., W. P. Bahnfleth, T. S. Whittam (1999). “Filtration of Microorganisms: Modeling and prediction.”
ASHRAE Transactions 105(2): 4-17.
Lacey, J. and B. Crook (1988). “Fungal and actinomycete spores as pollutants of the workplace and occupational illness.”
32: 515-533.
Platt, S. D., C. J. Martin, S. M. Hunt, and C. W. Lewis. (1989). “Damp housing, mould growth, and symptomatic health
state.” Brit. Med. J. 298: 1673-1678.
Pope, A. M., R. Patterson, and H. Burge, Eds. (1993). Indoor Allergens. Washington, DC, National Academy Press.
Sakuma, S. and K. Abe (1996). Prevention of fungal growth on a panel cooling system by intermittent operation. The 7th
International Conference on IAQ and Climate, Nagoya, Japan, Indoor Air '96.
Samson, R. A., editor. (1994). Health Implications of Fungi in Indoor Environments. Amsterdam, Elsevier.
Shaughnessy, R., E. Levetin, and C. Rogers. (1999). “The effects of UV-C on biological contamination of AHUs in a
commercial office building: Preliminary results.” Indoor Environment '99: 195-202.
Sugawara, F. (1997). Components of dust and microbial proliferation in ducts of air conditioning systems. Healthy
Buildings/IAQ '97, Bethesda, MD, ASHRAE.
Woods, J. E., D. T. Grimsrud, N. Boschi. (1997). Healthy Buildings / IAQ '97. Washington, DC, ASHRAE.
This article was published in HPAC September 2000, Vol. 72, No. 9, p80-83. Reprinted with permission from HPAC Enginering. Some
additions (the three photographs) and some other minor textual differences exist between this and published version.
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Sick Buildings : Definition, Diagnosis and Mitigation
  • Jan 1995
  • T Godish
Godish, T. (1995). Sick Buildings : Definition, Diagnosis and Mitigation. Boca Raton, Lewis Publishers.
Biocontamination in air-conditioning. Health implications of fungi in indoor environments
  • Jan 1994
  • 179
  • S Heinemann
  • H Beguin
  • N Nolard
Heinemann, S., H. Beguin, N. Nolard. (1994). Biocontamination in air-conditioning. Health implications of fungi in indoor environments. R. A. Samson. Amsterdam, Elsevier: 179.