AN INVESTIGATION ON THE OCCURRENCE OF FUNGI AND BACTERIA IN THE MVAC SYSTEM IN AN OFFICE PREMISE

Transcription

1 AN INVESTIGATION ON THE OCCURRENCE OF FUNGI AND BACTERIA IN THE MVAC SYSTEM IN AN OFFICE PREMISE PK Chow, WY Chan *, LLP Vrijmoed Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, People s Republic of China ABSTRACT Microbial contaminations in the Mechanical Ventilation and Air Conditioning (MVAC) system are known to lower the indoor air quality. We have investigated the microbial propagules (fungi and bacteria) inside a MVAC system in a typical Hong Kong office premise in June Air samples were collected by using a Reuter Centrifugal Air Sampler (RCS) at three different locations within the system mixing chamber (MC), beyond the cooling coils (CC) and the diffuser outlets (DO). Microbial readings were also obtained at the corresponding indoor and ambient environments. The investigations were conducted under three different modes of operation system on, system off during week days and on Monday morning after the system was switched off for more than 24 hours. The efficiency of a commercial used duct cleaning method was also evaluated. Different microbial counts were obtained under different modes of operation. When system was on, lower fungal counts were obtained at MC, and then increased at CC and decreased in DO and in indoor area. For bacteria, higher counts were obtained at MC, and then decreased at CC and DO, and increased to same levels of MC in indoor area. The comparatively higher relative humidity recorded at CC were probably favored microbial growth. Highest bacterial counts were recorded at MC as expected since it is the site of mixing of outdoor and indoor microbial sources. When the system was off, the fungal (at CC) and bacterial (at MC) counts recorded were 3880 and 865 CFU/m 3 respectively, which were at least 14 and 3 times higher among the other modes, since the relatively higher temperature and relative humidity with stagnant airflow promote the microbial growth inside the system. Slightly higher microbial counts (1.1 to 1.5 times higher) were obtained in Monday morning when compared with the system on mode. The dominant fungi collected were Aspergillus spp., Cladosporium spp., Penicillium spp. and Fusarium spp.. Over 86% of bacteria samples were gram positive, and the dominant genera were Micrococcus spp., Staphylococcus spp., and Bacillus spp. Our investigations also shows that only Cladosporium was found remaining on the duct surface, though over 94% of fungal load were removed after mechanical duct cleaning. INDEX TERMS Viable sampling; Microbials; Duct cleaning INTRODUCTION Exposure to bioaeorsols has been proved to be the cause of allergic, respiratory symptoms and diseases (Pastuszka, 2000; Fischer and Dott, 2003). People spend more than 70% of their lives indoor (HKEPD, 2001). Over 90% of commercial buildings in Hong Kong equipped with Mechanical Ventilation and Air Conditioning (MVAC) system. The MVAC system consists different major components including mixing chamber, cooling coils, condensate drainage pan and ductwork. The temperature and relative humidity of outdoor air are adjusted after passing through the cooling coils. Excessive moisture in the outdoor air is condensed into water when passing through the cooling coils. Additionally, the organic and inorganic matters are accumulated in the system, which provide a favorable condition with moisture and plenty of nutrients for the microbial growth (Kemp et al., 2001). Microbial contaminations in the MVAC system are known to lower the indoor air quality (Chang et al., 1996; Pejtersen, 1999; Butala et al., 2003). Therefore, in this study, the microbial contamination at different units of the typical MVAC system in a Hong Kong office premise is investigated. The objectives of this study were (1) to investigate the microbial contaminations inside a typical MVAC system, and (2) to evaluate the efficiency of commercial mechanical duct cleaning method in reducing the microbial load on the duct surface. RESEARCH METHODS * Corresponding author @student.cityu.edu.hk 1096

2 Microbial investigations (fungi and bacteria) of Mechanical Ventilation and Air Conditioning (MVAC) system were conducted in a typical Hong Kong office premise (10 years old) in June The MVAC system is consisted of a packaged air handling unit in a mechanical plant room with air flow rate of 4 m 3 /s. A water cooling tower installed on roof, and a ductwork distribution network of more than 150m length. It was designed to serve an office area of 600 m 2 with 40 occupants when fully occupied. Sampling locations Air sampling Air samples were collected at three different locations within the system mixing chamber (MC), beyond the cooling coils (CC) and the diffuser outlets (DO). Microbial readings were also obtained at the corresponding indoor and ambient environments. Investigations were conducted under three different modes of operation system on, system off during week days and on Monday morning after the system was switched off for more than 24 hours. The Biotest Reuter Centrifugal Sampler (RCS) Plus Air Sampler, agar strips were with Malt Extract Agar (MEA) and Rose Bengal (Biotest) was used as sampling media for fungi and Tryptic Soy Agar (Biotest) was used for bacteria (Biotest Company, 1996). After sampling, the agar strips were incubated at 25 C for 4 to 5 days for fungi and 30 C for 2 to 3 days for bacteria, without disturbance until they are ready for reading. The fungi and bacteria counts were calculated as CFU/m 3 and the colonies were isolated and cultivated for identification using 2 % MEA and Nutrient Agar (NA) respectively. Swab sampling The efficiency of a commercial used duct cleaning method was evaluated by using swab sampling on the duct surface before and after cleaning. Swab samples were collected on a 5 X 5 cm 2 of galvanized sheet metal duct surface. The duct surface was swabbed with sterile cotton swabs which soaked with sterilized Ringer solution (Oxoid) (AIHA, 1996). A serial dilution of the samples was prepared and each dilution was spreaded on 2% MEA, and then incubated at 25 C. The fungal counts were calculated as CFU/m 2 and the colonies were isolated and cultivated for identification. RESULTS Temperature and relative humidity The temperature and relative humidity recorded fluctuation inside the MVAC system and different under different mode of operation. Under system on and Monday morning, the temperature and relative humidity recorded at MC, indoor and ambient environment are similar with average 23.5 C and 67.6%. Lowest temperature and highest relative humidity was recorded at CC, average temperature and relative humidity was 16 C and 84.7%. Slightly higher temperature and lower relative humidity was recorded at DO, which was 20 C and 73%. Under system on condition, the temperature and relatively humidity recorded at all parts in the MVAC system are similar with average of 26.5 C and 80%. Microbial counts Different microbial counts were obtained at different sampling locations and under different modes of operation. 1097

3 The fungi and bacteria counts obtained at different locations and modes of operations were presented in Figure 1 and 2. Figure 1 and 2. Fungi and Bacteria count (CFU/m 3 ) obtained at ambient (AM), mixing chamber (MC), cooling cools (CC), diffuser outlet (DO) and indoor (IN) under three different modes of operations system on (S-ON), system off (S-OFF) and Monday (MON) morning after the system has been turn off for 24 hours. As presented in Fig. 1, under both system on and off modes, lower fungi counts were recorded at MC and then increased at CC and decreased at DO. When the system was off, higher fungal counts were recorded at all sampling sites, which were 3 to 14 times higher when compared with system on mode. Highest fungal count was obtained at DO on Monday morning. The fungal counts recorded at MC, CC and DO under system on mode were 75, 257 and 70 CFU/m 3 ; under system off mode, the counts were 1050, 3880 and 150 CFU/m 3 ; and on Monday morning, the counts were 32, 90 and 398 CFU/m 3. The Indoor/Outdoor (I/O) ratio of measured inside the MVAC system was higher than 1 which indicated that the fungal sources are present inside the system. The bacteria counts recorded were presented in Fig. 2, under all conditions, highest bacteria counts were recorded at MC, then decreased at CC and DO, and obtained at the same level of MC in indoor area. Similarly, higher bacteria counts were obtained under system off mode (~ 3 times) when compare with the modes of system on and Monday morning. The bacteria counts recorded at MC, CC and DO under system on mode were 258, 146 and 64 CFU/m 3 ; under system off mode, the counts were 865, 110 and 155 CFU/m 3 ; and on Monday morning, the counts were 300, 110 and 92 CFU/m 3. The Indoor/Outdoor (I/O) ratio of measured inside the MVAC system was higher than 1 which indicated that the bacterial sources are present inside the system as well as the indoor environment. Microbial composition Figure 3 Figure % abundance % abundance Fusarium 20 Bacillus Staphylococc 20 Penicillium Cladosporiu 0 S-ON S-OFF MON Micrococcus 0 S-ON S-OFF MON Aspergillus Mode Mode Figure 3 and 4. Composition of fungi and bacteria under three different modes of operations system on (S-ON), system off (S-OFF) and Monday (MON) morning after the system has been turn off for 24 hours. 1098

4 The composition of bacteria found in the MVAC system is presented in Fig. 4. There are three bacterial genera found, namely, Bacillus, Micrococcus and Staphylococcus. In general, over 86% of isolated bacteria are gram-positive. Micrococcus spp. (49, 65 and 42% under system on, system off and Monday morning respectively) is the most dominant bacteria under all the sampling condition, and followed by Bacillus spp. under system on condition (29, 5 and 5%) and Staphylococcus spp. (23, 29 and 53%) when under system off and Monday morning. Efficiency of duct cleaning method Surface cleaning by mechanical contact vacumming was able to reduce 94% fungi counts on the galvanized sheet metal duct surface, which decreased from 3.7 X 10 8 to 2.2 X 10 7 CFU/m 3. Cladosporium is the only fungal genus collected on the duct surface before and after the cleaning. DISCUSSIONS In this study, the fungal and bacterial counts are relatively low, except for the fungal count collected at CC under system off condition. The counts are lower than the recommended guideline published by HKEPD (2001). The composition of fungi collected in this study agreed with previous studies of microorganisms in the office premises in Hong Kong (Wong, 1997; Law et al., 2000; Chan et al., 2003), as well as those in different parts of the world (Takahashi, 1997). Relatively higher fungal counts were recorded at CC, it may probably due to the higher relative humidity (Chang et al., 1996). Especially for the dominant fungal species, higher relative humidity would favor their growth and germination (Wong, 1997). The fungal count was 14 times higher under the system off condition. Under system off condition, water droplets are condensed on the duct surface. The relatively higher temperature and relative humidity as well as the stagnant air flow favor the fungal amplification inside the MVAC system. In this study, Micrococcus spp., Staphylococcus spp. and Bacillus spp. are obtained, which are similar to other studies (Flannigan et al., 1991; Maroni et al., 1993; Pastuszka et al., 2000). Micrococcus spp. and Staphylococcus spp. are known as human shed bacteria and are caused by normal shed of skin. Therefore, it is common in the indoor environment. The relatively higher bacterial counts recorded at MC may probably due to the combination of bacteria imported from the ambient as well as generated in the indoor environment. Although the I/O ratio of comparing the indoor and the ambient environment is less than 1, the I/O ratios of comparing the MVAC system and the ambient were larger than 1, it indicated that there are fungal and bacterial amplification inside the system. Therefore frequency cleaning of duct is suggested to avoid microbial accumulation. In this study, over 94% of fungal counts reduced after cleaning. However, the fungal count is still 100 times higher than the proposed guideline of 1.6 X 10 5 CFU/m 2 by the National Air Duct Cleaning Association (NADCA, 1996). According Forade et al. (1997), microorganisms can grow both on the surface dirt and deep in the material and vacuum cleaning was only able to remove the surface microbes temporarily. Therefore, more investigations should be conducted to improve the nowadays commercial cleaning methods in order to minimize the microbial accumulations in the MVAC system thus improve the indoor air quality. CONCLUSIONS This study suggests that microbials contamination were observed at different locations within the MVAC system. The fungi accumulated at cooling coil (CC) and bacteria accumulated at the mixing chamber (MC). Both fungal and bacterial counts amplified under system off condition. The dominant fungal genera are Aspergillus, Cladosporium, Fusarium and Penicillium, and dominant bacterial genera are Micrococcus, Staphylococcus and Bacillus. The commonly duct cleaning may not completely remove the microbes on the duct surface effectively. ACKNOWLEDGEMENTS We would like to thank the Department of Biology and Chemistry of City University of Hong Kong for the financially support. REFERENCES Butala M., Crnigoj M., Zalar P., Butala V Isolation and identification of filamentous fungi from HVAC system. Proceedings of International Conference of Healthy Buildings 2003, Singapore, Vol. 1, pp Chan WY., Law KM. and Vrijmoed LLP A comparative study of qualitative sampling methods for the analysis of the indoor air molds. Proceedings of International Conference of Healthy Buildings 2003, Singapore, Vol. 1, pp

5 Chang JCS., Foarde KK. and VanOsdell DW Assessment of fungal (Penicillium Chrysogenum) growth on three HVAC duct materials. Environmental International 22(4), Fischer G. and Dott W Relevance of airborne fungi and their secondary metabolites for environmental, occupational and indoor hygiene. Achieves of Microbiology 179, Foarde K., VanOsdell D., Meyers E. and Chang J Investigation of contact vacuuming for remediation of fungally contaminated duct materials. Environmental International 23(6), Flannigan B., McCabe EM., McGarry F Allergenic and toxigenic microorganisms in houses. Journal of Applied Bacteriology 70(Suppl), HKEPD (Hong Kong Environmental Protection Department) Indoor Air Quality. Indoor air quality information center. Kemp PC., Neumeister-Kemp HG., Lysek G., and Murray F Survival and growth of microorganism on air filtration media during initial loading. Atmospheric Environment 35(8), Maroni M., Bersani M., Cavallo D., Anversa A., Alcni D Microbial contamination in buildings: comparison between seasons and ventilation systems. In: Kalliokoski, P., Jantunen, M., Seppanen, O. (Eds.), Proceedings of Indoor Air 93, Vol. 4. Helsinki, Finland, pp NADCA (National Air Duct Cleaning Association) Understanding microbial contamination in HVAC systems. Washington, DC: National Air Duct Cleaners Association. Pastuszka JS., Kyaw Tha Paw U., Lis DO., Wlazlo A. and Ulfig K Bacterial and fungal aerosol in indoor environment in Upper Silesia, Poland. Atmospheric Environment 34, Pejtersen J Sensory pollution and microbial contamination of ventilation filters. Indoor Air 6, Takahashi, T Airborne fungal colony-forming units in outdoor and indoor environments in Yokohama, Japan. Mycopathologia 139, Wong MC Indoor Airborne Fungi of Hong Kong: Biodiversity, Physiology and Ecology. MPhil thesis, City University of Hong Kong, Hong Kong. 1100