Study on the Behavior and Control of Microbial Contamination in an Air-conditioning System

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Most so-called building related illness are related to the microbial contamination in an air conditioning system. It is well known that indoor air pollution especially caused by microorganism is strongly influenced by the contaminant source in an air conditioning system. Since the temperature and humidity inside of an air-conditioning system are suitable for growth of microorganism, poor maintenance will causes an air conditioning system to a pollution source of indoor air.The goal of this study is to clarify the actual status of microbial contamination in an air conditioning system, and to establish a control method from the standpoint of architectural hygiene. A national scale questionnaire on maintenance of an air conditioning system, in situ experiments, and laboratory studies were conducted from April 31, 2003 to March 31, 2006. The main results are shown as follows.Actual status of microbial contamination(1)By the results obtained from the questionnaire on maintenance of an air c … More onditioning system, it has been clarified that almost air conditioning systems, include air filter, heat exchange coil, humidifier, air duct are not cleaned periodically.(2)By measurement of the growth speed of Alternari alternate, Eurotium herbariorum, Aspergillus penicillioides which used as a biosensor in this study, it is shown clearly that the environment down stream of a coil is favorable environment for the microbes of the 70-95% of the minimum relative humidity for growing.(3)On some object surfaces of an air filter, a coil, and a humidifier, when many, 10,000 microbes per square centimeter were measured out.Control method(1)It was proved that the method of controlling humidity environment can suppress the microbial growth speed.(2)The results obtained from the in situ experiments about the filtration efficiency of low- and medium- efficiency particulate air filter over both suspended particles and airborne microbes under real operating conditions of three air-handling units and one package air-conditioner located in Tokyo, show that there are significant correlations of filtration efficiency between airborne microbes and distributed suspended particles. Each of the filtration efficiency over airborne staphylococcus aureus, bacteria, and fungal spores is almost equal to the filtration efficiency over suspended particulates larger than 1 μm, 2 μm, and 5 μm in diameter, respectively. This result indicates that it is possible to design indoor air cleanliness on bio-aerosol by using the filtration efficiency of the air filter over distributed suspended particles instead of airborne microbes such as bacteria and fungal spores in office buildings.(3)Based on the results about the filtration efficiency over airborne microbes under actual operating conditions throughout summer season in 2005, it has been clarified that the filtration efficiency of the air filters (pre-filter 74% with weight method ; main filter 65% with dust spot method) over bacteria and fungi were 87±4.9% (AHU), 96±2.5% (OAHU) and 90±8.8% (AHU), 97±2.7% (OAHU), respectively. Furthermore, as the fact that the same sp.of microbes such as Cladosporium sp.and Penicillum sp.were measured out both of upstream and downstream of the air filters, it can be suggested that the same sp.of airborne microbe may show the different behavior according to aerodynamic diameter in indoor air.(4)Although the concentration of positive air ion and negative air ion raised up to 2.3 times and 1.9 times, respectively, by the operation of the ion generator, there was no reduction of the concentration in airborne microorganisms.(5)There were no remarkable differences in the number of adhesive bacteria and fungal between the enzyme filter and control filters.(6)Maintenance, such as cleaning coil, humidifier, and air duct periodically, of an air conditioning system is most important to reduce air pollution caused by microorganism from an air conditioning system. Less