This study presents the results of bioaerosol analyses conducted in the area of 9 wastewater treatment plants (WTTPs) in the Wielkopolska Region (Poland) with different capacities (from 500 to 200,000 m³/day). The abundance of mesophilic and psychrophilic bacteria, Staphylococcus, Actinobacteria, Pseudomonas fluorescens, coliform bacteria, and microscopic fungi in different plant sites and the control site were analysed. Microscopic fungi exhibited the highest contribution to air contamination and their highest median value was found in WWTP No. 5, which reached 6,700 colony forming units (CFU/m³) of air. Psychrophiles and mesophiles were the most abundant bacterial groups. It was established that the highest concentration of bioaerosols was observed during the mechanical treatment of wastewater at sewage sludge treatment sites and near bioreactors. The R_av parameter, which represents the ratio of average annual microbial abundance in the air to the average annual microbial abundance in the background at the control site, was a very good indicator of air contamination rate. This parameter may be analysed with respect to bacteria (R_{av B}), microscopic fungi (R_{av F}), and all microorganisms (R_{av M}).