Urbanisation reduces the abundance and diversity of airborne microbes - but what does that mean for our health? A systematic review.

Over half of people live in cities and while urban environments offer myriad social, cultural and economic benefits, they alter the microbial communities to which people are exposed: with potentially important but underexplored health impacts. In particular, higher rates of asthma and allergies in urban areas have been linked to urban-altered microbial communities - including aerial microbial communities. To date, however, there has been no synthesis of the disparate literature on the impacts of urbanisation on aerial microbial communities, making it difficult to ascertain potential health impacts. We fill this knowledge gap by systematically examining studies that compare the characteristics (e.g. microbial abundance/diversity) and/or health effects of airborne fungal and bacterial communities (hereafter referred to as 'aerobiomes') across urban and rural locations. We included 19 studies, with 31 distinct urban-rural comparisons, in our analysis. We found that rural aerobiomes more often have a greater abundance of microbes (57% of studies). Aerobiome diversity was under-reported but when comparisons were made, rural aerobiome diversity was often higher (67%). Only two studies experimentally examined the impact of urban and rural aerobiomes on human health outcomes; both found rural aerobiomes shifted immune function away from allergic (Th2-type) responses. Overall, we conclude that significant gaps remain in our understanding of how urbanisation impacts aerobiomes and the health implications of those changes. We highlight the need to standardise methods and make aerobiome data open access to facilitate cross-study comparisons. Further mechanistic studies are urgently needed to examine the impact of aerobiome composition on immune function to demonstrate how urban-driven changes to the aerobiome impact human health - ultimately facilitating the development of healthier cities.