Concentrations and size-resolved I/O ratios of household airborne bacteria and fungi in Nanjing, southeast China

This work quantitatively investigated the concentration relationships between indoor and outdoor bioaerosols in residence. We measured bacteria and fungi from air in 28 residential homes throughout Nanjing, southeast China during summer and winter. The culture-based method was applied and the ratios of indoor to outdoor concentrations, expressed in I/O, were calculated. Results revealed that concentrations of indoor bioaerosols were significantly associated with outdoor bioaerosols, and their size distributions showed similar single-peak pattern. Spearman correlation analysis indicated that I/O ratios were positively related to indoor microbial concentration. The I/O ratios differed insignificantly from seasons, but significantly higher I/O ratios were observed in airborne bacteria rather than airborne fungi during the same season. For bacteria, as much as 89% of homes in summer and 67% of homes in winter have I/O ratios >1, while for fungi, more than half of homes have I/O ratios <1, i.e. 50% in summer and 73% in winter. The different size distribution of bacteria and fungi could make a difference in I/O ratios. The greater percentage of I/O > 1 was observed in smaller size of bacteria and greater percentage of I/O < 1 was observed in larger size of fungi. The significant difference of I/O ratios between bacteria and fungi revealed that airborne bacteria being mainly originated from indoor sources, but airborne fungi dominantly from outdoor sources. Among various housing operations, the window opening period per day, the usage of air cleaner, and indoor planting played significant roles on I/O ratios, but the frequency of HVAC filter cleaning and the housing cleaning as well as the occurrence of mold played insignificant roles in this study.