Summary In recent years, growing ecological awareness has significantly increased people’s interest in environmental protection. Behaviours that promote sustainable development and long-term preservation of the environment, such as composting, are now widespread. By its very nature, composting requires the action of microorganisms. Each phase of composting involves its own characteristic microorganism populations and, as a result, the release of corresponding bioaerosols. A range of different gases may also be present in the ambient air during composting activities. This means that composting workers may be exposed to chemical and biological agents. The study compared the concentrations of contaminants found in the ambient air of three composting plants that processed different kinds of organic materials—i.e., organic waste sorted at source from a three-stream collection system, manure from a dairy farm, and carcasses and animal tissues from a hog farm—in order to assess the related health risks. Differences in concentrations and types of both microorganisms and gaseous compounds were observed at the three plants. The study showed that compost promotes biological activity by producing a significant increase in cultivable microorganism concentrations in the ambient air of composting plants. The polymerase chain reaction (PCR) method revealed the presence of Legionella spp. and Legionella pneumophila in the air at one of the plants, that of Saccharopolyspora rectivirgula in some instances and the sustained presence of Mycobacterium spp. in all of the plants. These results show that molecular biology analyses using specific markers can make a valuable contribution to microbial risk assessments. The use of these markers could enable quick, specific, better-targeted assessment of microbial risk in composting plants. The evaluation of biodiversity established the existence of preferential aerosolization of the phylum Actinobacteria and the genus Mycobacterium spp. This means that even though they are not the most prevalent bacteria in compost, conditions seem to favour their presence in the air over other groups of microbes. Examination of particle size distribution showed that the fluorescent and total particles measured less than 10 µm in aerodynamic diameter. These two types of particles are therefore able to penetrate deep into the lungs. The pH and water content are the only parameters that can be linked to microbial contaminant concentrations. The exposure of composting plant workers to microorganisms and gases has been demonstrated. Although in most cases the length of exposure is limited, respiratory protection may be desirable.