The advent of high‐throughput sequencing methods allowed researchers to fully characterize microbial community in environmental samples, which is crucial to better understand their health effects upon exposures. In our study, we investigated bacterial and fungal community in indoor and outdoor air of nine classrooms in three elementary schools in Seoul, Korea. The extracted bacterial 16S rRNA gene and fungal ITS regions were sequenced, and their taxa were identified. Quantitative polymerase chain reaction for total bacteria DNA was also performed. The bacterial community was richer in outdoor air than classroom air, whereas fungal diversity was similar indoors and outdoors. Bacteria such as Enhydrobacter, Micrococcus, and Staphylococcus that are generally found in human skin, mucous membrane, and intestine were found in great abundance. For fungi, Cladosporium, Clitocybe, and Daedaleopsis were the most abundant genera in classroom air and mostly related to outdoor plants. Bacterial community composition in classroom air was similar among all classrooms but differed from that in outdoor air. However, indoor and outdoor fungal community compositions were similar for the same school but different among schools. Our study indicated the main source of airborne bacteria in classrooms was likely human occupants; however, classroom airborne fungi most likely originated from outdoors.

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Indoor air quality is important for people's health and quality of life because we spend most of our time indoors. 1,2 Indoor air quality (IAQ) is one of the top five public health concerns of the US Environmental Protection Agency (EPA). 3 Indoor air can be contaminated with chemicals and particulate matters (PMs) that include bioaerosols. Furthermore, indoor exposure to bioaerosols has been associated with development or exacerbation of asthma and respiratory symptoms. [4][5][6] Particularly, pathogenic bioaerosols including viruses can result in infectious diseases upon exposure. 7,8 As of late February 2022, there were more than 400 million confirmed coronavirus disease 2019 (COVID-19) cases owing to indoor infection with severe acute respiratory syndrome coronavirus 2

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