inlet are defective. If rain or snow is carried over into the air-handling unit (AHU), water or rust in the mixed-air plenum may occur.
In air-conditioned residences without outdoor air inlets (the majority of the residential building stock), pollen and other atmospheric allergens are excluded by filtration in the envelope itself or by physical factors such as sedimentation. In large buildings, air from the mixed-air plenum is usually cleaned by one or more sets of particulate air cleaners before it is thermally treated (i.e., heated, humidified, cooled).
The most common type of filter provided for most residential and commercial HVAC systems is installed primarily to prevent dirt and debris from depositing on the heating and cooling coils (Morey, 1988; Woods, 1989b; Woods and Krafthefer, 1986). These filters, rated in terms of weight arrestance (ASHRAE, 1976), have little effectiveness in removing respirable particles from the air moving through the HVAC system (Morey and Shattuck, 1989). The capacities of filters of varying efficiencies to remove bioaerosols (size 1 to 5 µm) from air have been studied (H. Decker et al., 1963). Roughing or prefilters were shown to remove 10 to 60 percent of bacterial particles from the airstream. Medium- and high-efficiency filters (including bag filters) removed 60 to 99 percent of bacterial particles. Kuehn et al. (1991) published a recent review of the filtration of bioaerosols. Figure 7-2 shows the approximate particle sizes of various potential contaminants.
In HVAC systems that contain filters, the air to be used for room ventilation passes through a filter "dust cake." The dust cake often contains contaminants such as human skin scales, fungal spores, pollen, tobacco smoke components, and atmospheric dust and debris. Filters that become moist or wet can function as significant amplification sites for microorganisms, especially fungi (Morey, 1984; Schicht, 1972). Fungal populations in filters can amplify by 2 to 4 orders of magnitude when incubated at 96 percent relative humidity for 10 days (Pasanen et al., 1991). The protection of filters from moisture and careful, periodic replacement of the filters (i.e., without leaving residue from its dust cake in the system) are essential for controlling potential allergen emissions from this portion of the HVAC system.
Dirt, debris, and fungi can be expected to accumulate in AHUs and main and branch supply air ducts, especially in HVAC systems with inefficient filters, where filters do not fit properly in filter frames, or in poorly designed filter banks where significant volumes of air can bypass the filter bank. One study in Kuopio, Finland, found that pollen from outdoor air made up 9 percent of the weight of supply air duct dust (Laatikainen et al., 1991). Dust and debris in supply air duct systems can be expected to be