Peat moss is organic matter colonized by large numbers of microorganisms. Storage prior to its processing may result in massive microbial growth. These biological contaminants can become airborne during processing. Our goals were (a) to evaluate concentrations of bioaerosols (inhalable dust, molds, bacteria) in peat moss processing plants that used dust removing systems, and (b) to evaluate the presence of these microorganisms in peat moss. Fourteen plants from Eastern Canada were visited; 3 plants operated all year (all-year mixing plants), and 11 plants functioned only during summer months (seasonal). Air samples were taken throughout the day at different work sites using IOM cassettes for inhalable dust and All-Glass Impinger-30 samplers and Andersen six-stage impactors for microorganisms. Samples of nonprocessed and bagged peat moss (solid material) were also taken and analyzed. A total of 25 work sites for air sampling and 33 solid material samples were analyzed. Air samples contained up to 441.7 mg/m3 of inhalable dust and up to 1.0 x 10(8) CFU/m3 mesophilic molds and 3.3 x 10(5) CFU/m3 bacteria. Seasonal plants were more contaminated with molds and dust than all-year mixing plants. Sieving sites were the most highly contaminated work sites. Airborne dust concentration was significantly correlated with molds and bacteria. Up to 3.8 x 10(7) CFU/g (dry weight) and 4.8 x 10(7) CFU/g (dry weight) molds and bacteria, respectively, were found in the solid material samples. Airborne contaminants did not correlate with solid material content. Despite the use of dust removing systems, peat moss processing plants contain very large amounts of microbially contaminated bioaerosols that do not correlate with the quality of the processed peat. Efficiency of dust removing systems could influence the contamination levels.