Background culturable bacteria aerosols were collected and identified in two large public buildings located in Minneapolis, Minnesota and Seattle, Washington over a period of 5 months and 3 months, respectively. The installed particulate air filters in the ventilation systems were used as the aerosol sampling devices at each location. Both pre and final filters were collected from four air handing units at each site to determine the influence of location within the building, time of year, geographical location and difference between indoor and outdoor air. Sections of each loaded filter were eluted with 10 ml of phosphate buffered saline (PBS). The resulting solutions were cultured on blood agar plates and incubated for 24 h at 36°C. Various types of growth media were then used for subculturing, followed by categorization using a BioLog MicroStation (Biolog, Hayward, CA, USA) and manual observation. Environmental parameters were gathered near each filter by the embedded on-site environmental monitoring systems to determine the effect of temperature, humidity and air flow. Thirty nine different species of bacteria were identified, 17 found only in Minneapolis and 5 only in Seattle. The hardy spore-forming genus Bacillus was the most commonly identified and showed the highest concentrations. A significant decrease in the number of species and their concentration occurred in the Minneapolis air handling unit supplying 100% outdoor air in winter, however no significant correlations between bacteria concentration and environmental parameters were found.