Pre-proofs version of paper.
Practical Implications:The damp surfaces of AC units are known to promote bacterial and fungal growth. The resulting microbial load has been shown to diminish heat transfer efficiencies and increase fan power requirements, and it may also result in microbial emissions into occupied building spaces. This study demonstrated that microbial concentrations in well-maintained AC systems vary significantly from unit to unit, and this variability may be largely explained by environmental and building parameters including climate (dew point temperature of outdoor air) and upstream filter nominal efficiency. The fungal and bacterial coil concentration data and the association between microbial loading and environmental factors provide a baseline understanding for future <efforts to reduce microbial fouling of AC heat exchanger surfaces.