HVAC systems have a significant impact on the indoor environment, and microbial contamination in HVAC systems has a significant effect on the indoor air quality. In this study, to gain a better understanding of the microbial contamination inside ACs, we used NGS to analyze the 16S rRNA gene of bacteria adhering to AC filters, cooling coils, fans, and air outlet surfaces. The five phyla in terms of the highest relative abundance were Proteobacteria, Firmicutes, Actinobacteria, Cyanobacteria, and Bacteroidetes. The surface of an AC filter provides a history of indoor airborne bacterial contamination, and of the 10 bacterial genera we detected with the highest abundance (in the following order: Pseudomonas > Staphylococcus > Paracoccus > Corynebacterium > Acinetobacter > Streptococcus > Methylobacterium > Enhydrobacter > Sphingomonas > Actinotignum) on the filter surface, the top 6 genera were Gram-negative bacteria. Furthermore, the seventh-most abundant genus adhering to the filter surface (Methylobacterium) was the second-most abundant genus on the cooling coil and fan, and the ninth-most abundant genus on the air filter (Sphingomonas) was the third-most abundant genus on the cooling coil. Various factors impact the bacterial flora inside AC units, including the location of the house, AC unit usage, and occupant activity.
The air and the surface fungal communities were collected from a cool/heat pit and two offices during the 2019 heating and the 2021 cooling seasons. A total of 16 samples were analyzed with ITS2. The results showed that 92.2% (heating season) and 82.3% (cooling season) of each collected fungi were classified under the phylum Basidiomycota and Ascomycota. Genera Malassezia, Cladosporium and Mycosphaerella were predominantly detected in both seasons. Schizophyllum commune and Aspergillus fumigatus were predominantly detected indoors. Fungi in the pit during the cooling season can affect offices and countermeasures are needed. This does not occur in the heating season.
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