The objective of the proposed study was to compare seasonal aerosol samples collected during Spring, Summer, Fall and Winter from 3 US cities with high prevalence of asthma, Boston, Philadelphia and Detroit, to determine the effect of seasonal variation on bacterial urban aerosol composition. In addition, comparisons were made between aerosol bacterial communities from these three cities and those present in San Francisco in the Spring. To determine if there were any similarities between aerosol and airway bacterial communities comparison were also made between aerosol and respiratory samples from asthmatic patients (collected in a separate multi-center US trial). Finally analysis of commonalities and distinctions in urban aerosols in East coast versus West coast samples were examined to determine whether geographic location impacted the bacterial community present at these sites. Bacterial community composition varied across sites but appeared, at least in the samples analyzed, to be more associated with specific geographic location than by seasonality. Phyla detected in these samples represented a vast diversity of bacteria and included many members of the Firicutes and Bacteroidetes, two of the primary phyla colonizing the human gastrointestinal tract as well as members of the Sphingobacteria and Actinobacteria that have previously been detected in the respiratory tract of patients with inflammatory airway disease. A compairson of bacterial communities from Boston, Philadelphia and Detroit (collected in the Spring) to those detected in the same season in San Francisco revealed a core of bacteria common to all samples.