Bioaerosol release from organic waste treatment plants has become an important public concern because of the potential health impacts on workers and local residents. Airborne microorganisms were monitored at seven different composting facilities with different operating systems and treating a range of organic wastes. Aspergillus fumigatus and mesophilic bacteria were used as the principal monitoring parameters. The composting plants all showed levels of both airborne microorganisms in the 10(3) to 10(5) cfu m(-3) range in the operating area, making it advisable for the staff to use protective masks. The highest bioaerosol release occurred in the composting plant treating green waste using turned windrows, The main operational factors affecting airborne microorganism dispersion are discussed as well as the systems proposed for dispersion control. Bioaerosol release was reduced at enclosed systems through the use of biofilters, and the dispersion depended on meteorological conditions. In all cases the airborne levels 300 m downwind were similar to those measured at background locations.