In order to control indoor air pollutions caused by microorganisms and design the indoor air cleanliness appropriately, it is necessary to quantify the filtration efficiency over airborne microbes of an air filter installed in an air-conditioning system. This paper reports the in situ experimental results about the filtration efficiency of low- and medium-efficiency particulate air filter over both suspended particled and airborne microbes under real operating conditions of three air-handling units and one package air-conditioner located in Tokyo. Results 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 1μm, 2μm, and 5μm in diameter, respectively. This result indicates that it is possible to design indoor air cleanliness on microorganisms 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.
