Seasonal Difference of Airborne Bacteria and Fungi in Commuter Buses

Size distributions of aerobiological concentrations in commuter buses in northern Taiwan were examined from December 2006 until May 2007. Two six-stage (>7, 4.7-7, 3.3-4.7, 2.1-3.3, 1.1-2.1, and 0.65-1.1 mu m) cascade impactors were used to collect viable bacteria and fungi. Results showed that bacterial concentrations ranged from 113 to 601 colony forming units (CFU)/m(3) and averaged 282 CFU/m(3). Fungal concentrations ranged from 42 to 2,193 CFU/m(3) and averaged 674 CFU/m(3). Additionally, the highest fractions occurred in the fourth stage (2.1-3.3 mu m) for both bacteria and fungi. Respirable fractions (particles smaller than 4.7 mu m in aerodynamic diameter) for bacteria and fungi were 68% and 78%, respectively, which are higher than those in other studies. The number median diameters of the total bacterial and fungal concentration were 1.67 and 2.67 mu m in winter and 3.02 and 2.30 mu m in spring, respectively. Airborne microorganisms are often associated with asthma, rhinitis, sick building syndrome, and hypersensitivity pneumonitis. Although total bacterial or fungal concentrations did not exceed the recommended standard (1,000 CFU/m(3)) in Taiwan, we found a relatively high respirable fraction in commuter buses. The relationship between bioaerosol aerodynamic diameter and a greater health risk for sensitive commuters, such as those with asthma, warrants further investigation in the future. The small sample size is a limitation in this study and the results indicated that there were no statistical significances found among bacterial/fungal concentrations, fungal respirable fraction, and seasons (p>0.05). However, statistical significance was found between bacterial respirable fraction, the fourth fraction, and seasons (p<0.05). Bioaerosol concentration measurements in commuter buses performed by this study provide basic information for further research on exposure assessment.