Most commercial buildings in North America are mechanically ventilated with sealed exterior shells. In this type of highly automatically controlled indoor environment, microbial contamination of the HVAC system often contributes to building-related illness and symptoms in building occupants, such as outbreaks of rhinitis, humidifier fever, asthma, hypersensitivity problems, and so on. Contamination of bacteria, fungi, and protozoa has been found on various parts of HVAC systems such as air cooling units, cooling coils, and drip pans.