The Earth’s atmosphere supports microorganisms and they include potential pathogens and microbial allergens. Whilst indoor environments have been well studied, relatively little is known of bio-aerosols in outdoor locations and their potential influence on human health, particularly with regard to urban development. Hong Kong provides an ideal model system for testing hypotheses related to the impact of urbanization on bio-aerosols, with a well-defined gradient of urbanization and large population. This thesis describes work to establish the biodiversity and spatio-temporal dynamics of outdoor bio-aerosols in Hong Kong. A comprehensive study of multi-domain microbial diversity and allergen levels in urban aerosols over a contiguous annual timescale and along a gradient of urbanization was carried out. A comprehensive suite of climatic and pollutant variables were also recorded during the sampling interval. Terminal restriction fragment length polymorphism (T-RFLP) was employed to investigate variations in bacterial and eukaryal assemblages, followed by phylogenetic assessment using high-throughput sequencing. The results revealed a strong seasonality in both bacterial and eukaryal assemblages, with Archaea forming a negligible part of the urban bio-aerosols. The most abundant bacteria were proteobacteria but community shifts were seen due to increases in algae in summer, and betaproteobacteria and cyanobacteria in winter. This was most parsimoniously explained by considering the backward trajectory analysis of air mass. A greater abundance of marine-associated phylotypes such as Bacillariophyta and Chlorophyta were identified when the dominant air mass arriving in Hong Kong in the summer originated from oceanic sources. In contrast, betaproteobacteria, which indicated soil sources were prevalent when the origin of air mass was from terrestrial sources. A trend in fungal phylotypes was also apparent, with summer samples dominated by basidiomycetous Agaricales, and winter samples by the ascomycete genus Cladosporium. This was likely due to favourable climatic conditions during wetter summer months enhancing release of fungal basidiospores. A range of airborne human pathogens was also detectable at low levels including pathogenic bacteria such as Acinetobacter baumannii, Clostridium perfringens, Escherichia coli O157:H7, and Ricinus communis, and the pathogenic fungus Aspergillus terreus. Microbial allergens including bacterial endotoxins and fungal glucans were also quantified with immunological assays. These generally followed variations in biomass, and during some months were recorded at levels that may impact human health upon chronic exposure. Carbon dioxide levels were the only climatic or pollutant variable that correlated with allergen levels. Conversely changes in microbial assemblages were strongly correlated to several climatic variables including temperature, rainfall, air pressure and relative humidity, but not with the degree of urbanization or airborne pollutants. This study highlights the importance of including microbial assessments in future bio-surveillance of urban aerosols. / published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy