Contribution of bioaerosol emission from air-conditioning and mechanical ventilation system to indoor bioaerosol concentration

Abstract

Human exposure to indoor bioaerosols has been a major health concern indoors. Bioaerosol emission from air-conditioning and mechanical ventilation (ACMV) systems has a potential to be a significant sources of indoor bioaerosols. In order to understand impacts of bioaerosol emission from the ACMV system on indoor occupants, quantification and species identification of bioaerosols emitted from the ACMV system are necessary. Airborne bacteria and fungi are common bioaerosols indoors. This work investigates bacterial and fungal emissions from an ACMV system in a tropical indoor environment. A series of experiments employing both surface and air sampling are conducted to quantify the emission rates of different bioaerosol sources that contribute to indoor bioaerosol concentrations. Surface sampling is conducted to quantify the loadings of bacteria and fungi on the surfaces of indoor environment and the various components of the ACMV system, and the results indicate that bacterial and fungal accumulation on surfaces of fan blades are the highest among the sampled surfaces. The average loadings of bacteria and fungi on fan blades are more than six and eleven times that on a fresh air duct. Air sampling is performed to measure indoor airborne bacterial and fungal concentrations as well as species. Material balance model is applied to quantify the contribution from ACMV system to indoor bioaerosol concentration. Air sampling results show that both the AC unit and the fresh air duct can emit bioaerosols into the indoor environment. In addition, the experimental results indicate that emissions from the ACMV system and occupants contribute significantly to indoor airborne bacterial concentrations. Indoor airborne fungal pollution is predominantly contributed by the infiltration of outdoor airborne fungi. Results of simulation of cleaning the ACMV system show that the average indoor airborne bacterial and fungal concentrations can drop by 45% and 34%, respectively, when bioaerosol emission from the ACMV system is removed. This investigation provides direct support that bioaerosol emission from ACMV systems can degrade indoor air quality and that some species could potentially be pathogens for indoor occupants. Natural ventilation is also found to be more effective at lowering indoor airborne bacterial concentrations and less effective at lowering indoor airborne fungal concentrations than ACMV in the indoor space according to air sampling results. Simulation results indicate that the ACMV system can be more effective in lowering both airborne bacterial and fungal concentrations than the natural ventilation system by cleaning the ACMV system employed in the ACMV. The scheme developed in this study to quantify bioaerosol emission from ACMV system, as well as other sources, overcome the limitation of quantification of the bioaerosol emission by resuspension model, which are difficult to be applied on surfaces of non-uniform distribution of microorganisms, for example, most surfaces in air path in real-site ACMV system. The scheme developed in this study to species-identify bioaerosol emission from ACMV system also provide a more comprehensive bioaerosol emission profile compared to the surfaces sampling method on limited surfaces in the air path, which are widely applied in previous studies. The results of this study will serve as the basis for future indoor bioaerosol exposure estimation and to develop effective control measures against the bioaerosol emission from ACMV systems. In addition, the results of this study also contribute to understanding of applicability of natural ventilation as an alternative ventilation strategy compared to ACMV system, which has a potential to save the significant electricity energy cost by ACMV system. Future work will be conducted to investigate impacts of environmental parameters on comparison of ACMV and natural ventilation. In addition, acoustic agglomeration effects will be investigated as a novel cleaning technology of ACMV system, which has the potential to remove bioaerosol pollution from ACMV system effectively while saving the electricity energy cost.