Wastewater treatment plants (WWTPs) have been recognized as a source of odors and microbial pathogens to the outdoor air. The results of many studies revealed that high amounts of microorganisms are not only present in the stream of wastewater or sludges but also in the bioaerosols that are generated during the different stages of the wastewater treatment. Hence, possible migration of biological contaminants into the interiors cannot be excluded. However, there is a knowledge gap in an assessment of the microbiological indoor air quality of the facilities located at WWTPs. The aim of this study was to evaluate the level of outdoor microbial air contamination upon the indoor environment based on the determined outdoor to indoor (O/I) ratios. The sampling of airborne bacteria and fungi was conducted in three replications with the sedimentation and impaction method, during a one-year survey in ten technological and office buildings as well as their vicinity, at the municipal wastewater treatment plant of Lublin (Poland). Moreover, the cleanliness of hand contact surfaces in staff rooms was examined (Rodac plates). Additionally, API identification of bacteria and fungi was carried out. The highest concentration of total bacteria count (3617 CFU/m3) and fungi in bioaerosols (5386 CFU/m3) was detected in the air around the sewage pumping station, close to the aeration tanks. P. fluorescens was found in the air around the grit chamber (78 CFU/m3). The majority of the examined indoor air samples were characterized with different levels of microbiological contamination – from non-polluted to moderately polluted. The number of total bacteria counts ranged from 180 to 4679 CFU/m3. The highest estimated indoor fungi concentration was 4022 CFU/m3. The controlled surfaces were mostly contaminated with the Actinomycetes and Coliform bacteria. No Salmonella sp. were detected. The bacteria from the Enetrobacteriaceae family were commonly isolated from the indoor and outdoor air samples. The obtained data can be used to devise further guidelines facilitating control and management of WWTP to avoid or minimize the staff exposure.