Risk
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Indoor Mold Growth Health hazards and remediationBy W. J.
Kowalski, MS, PE 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. 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 mold or yeast. In the environment, where available nutrients, moisture and temperature conditions may be marginal, these fungi normally grow as mold. In infected tissue, where conditions are more ideal, these fungi usually grow as yeast. 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 are the mold or yeast forms; in fact, some spores have 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.1, 2 Allergic alveolitis, rhinitis, and hypersensitivity pneumonitis may result from long-term exposure in the workplace by individuals who have no allergies.3 Many fungi, such as cryptococcosis and blastomycosis, cause infections that are unique to the species. 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.4 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 to 30 percent of cases of building-related illness have been associated, if not directly linked, to indoor fungal or bacterial contamination. Certain fungi produce volatile organic compounds (VOCs) and odors. Long-term exposure to such substances can result in impaired health or contribute to Sick Building Syndrome.1, 5, 6 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, condensation, leaks, or even the mere presence of high humidity (i.e. more than 90 percent) because 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 percent of houses were found to be damp while 47 percent had mold growth.7 High correlations between health problems and dampness or mold growth were observed, especially among children. In a Canadian study, 38 percent of houses were either damp or had mold growth. The presence of lower respiratory symptoms was approximately 50 percent higher in these homes while upper respiratory symptoms were almost 25 percent higher.8 The presence of Aspergillus above 50 cfu per cu m was associated with coughs, colds, and eye and skin irritation. A study in Finland found that 52 to 58 percent of houses had moisture problems and that this was associated with a higher risk of respiratory ailments, especially in children.9 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 to 1000 cfu per cu m. Geography can determine the makeup of outdoor spore concentrations. In new buildings, indoor levels of spores are lower than outdoor levels, even with natural ventilation. Buildings that foster fungal growth may generate spore levels higher than those outdoors. Cases of such problem buildings may require special treatment, including elimination of moisture sources and water-damaged materials. What fungal spores are common indoors? Table 1 lists all of the major pathogenic and allergenic fungi, by genus, that have been found growing indoors.10 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 (rh) 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.
Figure 1: Effect of Temperature and Relative Humidity on Mold Growth Rates. Based on data from Clarke et al (1998). Materials normally present in buildings provide nutrients for fungal growth. These include building materials such as wood or cellulose and organic materials found in rugs and curtains. Water damage to rugs sometimes will 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 still may be tracked or carried in by other means. Filtration of return air also can control indoor airborne levels, but if spores are being generated indoors, then this problem should be dealt with at the source. Typical dust filters may be insufficient to intercept fungal spores since the most common ones tend to be in the 1 to 4 micron size range. Excellent removal rates can be attained with simple high-efficiency ASHRAE filters. HEPA filters would be overkill in this regard. For example, a 35 to 40 percent ASHRAE filter will remove 57 percent of Aspergillus and 83 percent of Stachybotris spores.11 Control of humidity provides one means of preventing or limiting growth once spores have entered a building, but humidity levels of 60 percent or less are no guarantee, since the moisture content of buildings materials is a more critical factor. Controlling humidity inside of an air-handling unit (AHU) below about 90 percent may be impossible. However, another approach is keeping HVAC systems clean of dust and keeping drain pans unclogged. A novel method of 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. greater than 90 percent rh to germinate spores) and night-time dehumidification has the potential to significantly reduce fungal growth inside of AHUs.12 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.13 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 the effectiveness of any measures taken. Any problem species can be assessed individually. As a general guideline, indoor spore levels should be less than about 100 cfu per cu m 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 per cu m or less), depending on facility type. Allergic individuals may 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. REFERENCES
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