pans and exits the AHU through drain lines. During this cooling and dehumidification process, microorganisms can amplify in the heat exchanger section because of the presence of stagnant water in drain pans. A biofilm or slime on pan or coil surfaces is an indicator of microbiological amplification. Moisture that can promote the growth of microorganisms in locations downstream from the cooling coils can originate from water droplets being blown off coil surfaces when the air velocity through the coils is too great. Organic dust in the airstream, especially in HVAC systems with inefficient filter banks (e.g., most residences), can impact on moist surfaces or settle in moist duct work and provide nutrients for the amplification of microorganisms, especially fungi.

Water spray systems (air washers) with recirculated chilled water (in place of cooling coils) represent another type of system that can be used to extract heat and moisture from the airstream. These systems, which are found in some office buildings (Hodgson et al., 1987) and in industrial operations (Reed et al., 1983), can become strong microbiological amplifiers because the dirt and debris that are also extracted or scrubbed from the airstream serve as nutrients for the microorganisms present in the sumps of these systems. Water spray systems used for air conditioning were originally designed to use sterile water or water that was disinfected by biocides (Yaglou and Wilson, 1942). The aerosolization of biocidal chemicals into the ventilation airstream is unacceptable because of their potential adverse health effects on occupants (CFR, 1987).

Another form of air conditioning, prevalent in hot, dry areas of the United States, is achieved with evaporative coolers. In these systems, outdoor air is drawn through mats or pads that are wetted by recirculating water from a sump. In these systems, the dry-bulb temperature of the air entering the unit is cooled adiabatically to approach the wet-bulb temperature, and the air exiting the unit is thus nearly saturated with moisture. Amplification of microorganisms can occur on the evaporative mats or in the sumps of these units (Macher and Girman, 1989). One case of hypersensitivity pneumonitis has been attributed to the presence of thermophilic actinomycetes found in the straw evaporative mat of the unit and in the house dust of the residence where illness occurred (Marinkovich and Hill, 1975).


Humidifiers are used to add moisture to the air. These devices are usually installed in the supply air plenum or duct work, downstream from the heating coils. In some residential installations, however, they may be found in the return air. Injection nozzles of humidifiers should be located in areas of AHUs or duct work that are devoid of porous insulation (Morey,

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