The concentrations of total culturable bacterial and fungal aerosols in underground subway systems were determined by quantitative measurements. In addition, concentrations of airborne bacteria and fungi were observed at different locations in a subway station. We found that bacterial aerosol concentration in the subway systems varied throughout the seasonal transitions from spring to summer and from fall to winter. Bacterial aerosol concentrations increased by more than three times from March to April, and decreased by more than two times from October to November. The results of t-test showed that these changes were statistically significant. The number of passengers had a small effect on the concentration of bacterial aerosols in the subway station. However, fungal aerosols are relatively unaffected by seasonal changes and human presence. These experimental results provide useful and basic guidelines for analyzing existing methods and developing new methods to control airborne microorganisms in underground subway stations by providing information on initial air quality conditions. (C) 2015 Elsevier Ltd. All rights reserved.