Aims: The purpose of this study was to evaluate the degree of bacterial contamination generated by three Italian composting plants (1, 2 and 3) in two different seasons and to assess the health risk for the employees. Methods and Results: Aerosols samples were collected with an agar impact sampler. Several plant sites and external upwind and downwind controls were examined. Total colony-forming counts of mesophilic and thermophilic bacteria, actinomycetes and streptomycetes, Gram-negatives, coliforms and sulfite-reducers were determined. Selective media were used in order to isolate pathogenic bacteria. The levels of total mesophilic and thermophilic micro-organisms ranged between 33 and > 40 000 CFU m(-3) in plant 1, 39 and 18 700 CFU m(-3) in plant 2 and 261 and 6278 CFU m(-3) in plant 3. Strains of Escherichia coli, Staphylococcus aureus and Clostridium petfringens were also found. Conclusions: The plants monitored in this study have proved to be sources of aerosolized bacteria. The activities involving mechanical movement of the composting mass and the indoor activities were of greatest potential risk. In all the studied plants, a statistically significant dependence was found between the bacterial contamination and the season for some or almost all the analysed parameters, but a clear seasonal trend Could not be observed. Significance and Impact of the Study: This Study provides broad evidence of bacterial aerosol dispersion and site-related biological hazards that may be useful to the regional government to implement regulations on worker safety in composting plants.