nutrient, proper humidity and temperature (Maus et al. 2001). The colonization of bacteria and

fungi in the air filter used in the HVAC system was observed in previous studies (Kemp et al.

1995a; Kemp et al. 1995b; Simmons and Crow 1995). Furthermore, the survival of bacteria in

various types of surgical masks and respirator filters was also documented (Brosseau L. M. et al.

1997). It has been also shown that building materials such as wallpaper and gypsum boards can

be a source of microbial air contamination when the growth of microorganisms is supported by

sufficient moisture and nutrients (Nielsen et al. 1998). Studies on the growth and survival of

microorganisms on two different air filtration media reported the survival of a wide range of

fungal species and bacteria on the fiberglass medium that had significantly high water content.

After 6 weeks of use, the accumulation of dust on the multi-layered polymer material provided

nutrients for the growth of microorganisms (Foarde et al. 1996). However, the growth of

microorganisms can be inhibited when the growth medium dries out, suggesting better survival

of microorganisms at a favorable RH (Heldal et al. 1996). Other researchers observed that

microorganisms did not multiply in unused filter media at low RH (RH < 70%) but the growth of

microorganisms was induced only where sufficient moisture and nutrients were possessed

(Kemp et al. 1995a). Simmons and Crow (1995) observed fungal growth at HRH (> 70-80 %),

which was supported by the presence of air dust and/or cellulose fibers in the filter. Regarding

respirators, rapid fungal growth in the respirator made of cellulose was observed in the humid

storage environment (Pasanen et al. 1993). When the respirator is worn for several hours, the

humidity and nutrients in the respirator may be increased due to exhalation and saliva containing

various components, which can be either nutrient or antimicrobial agents (Wang et al. 1999).

 Microorganisms collected on filter are of great concern due to microbial contamination of

ambient air by releasing byproducts or by their re-entrainment along with the adverse health