Temporal changes in the size resolved fractions of bacterial aerosols in urban and semi-urban residences.

Despite the significant amount of time spent in the domestic environment, culture-independent size distribution data of bioaerosols are largely missing. This study investigated the temporal changes in size-resolved bacterial aerosols in urban and semi-urban residential settings. Overall, airborne bacterial taxa identified in both sites were dispersed across particles of various sizes. qPCR analysis showed that outdoors bacteria dominated particles > 8 μm, whilst indoor bacterial loadings were greater with 1-2 μm (winter) and 2-4 μm (summer) ranges. Indoor and outdoor aerosols harboured distinct bacterial communities due to the dominance of human-associated taxa (Staphylococcus, Micrococcus, Corynebacterium) in indoor air. The aerosol microbiome exhibited significant temporal variation, with Actinobacteria, Gammaproteobacteria and Bacilli predominant indoors, whereas Actinobacteria, Alphaproteobacteria and Gammaproteobacteria were the most abundant taxa outdoors. The variation between the two residences was mostly driven by particles < 2 μm, whereas differences between indoors and outdoors were mostly influenced by particles > 2 μm. Source-tracking analysis estimated that household surfaces accounted for the greatest source proportion of bacteria, surpassing that of outdoor air, which varied due to natural ventilation throughout the year. Our findings provide new insights into the factors governing the aerosol microbiome in residential environments which are crucial for exposure assessment.