Air conditioning (AC) systems cool and dehumidify air. The process deposits moisture on the cooling coils, creating an environment conducive to microbial growth. We know very little, however, about the microbes that grow on AC units or how these microbes affect and interact with the microbial populations of the buildings they cool. This grant supports Jordan Peccia, associate professor of environmental engineering at Yale, who will lead a multidisciplinary team in a pilot study examining how the microbial and chemical emissions of commercial air conditioning units impact the microbiome of occupied spaces. Over two years, Peccia and his team will characterize the bacterial and fungal communities present on the cooling coil surfaces of commercial air conditioners, estimate the microbial volatile organic compound (MVOC) emission rates from commercial AC units, and quantify the influence that AC emissions have on the indoor air and surface microbiome of occupied spaces. The team will initially sample 40 different commercial air conditioning units and use these samples to examine how microbial population structure is affected by a host of environmental variables, including outdoor climate, coil moisture, and coil temperature. They will then measure AC microbial emission rates and the characteristics of emitted microbes to study how these correlate with the surface and air microbiome composition in the buildings these units cool.