This report presents the results of a study of chemical and biological contaminants and ergonomic hazards in two (A and B) household waste treatment centers (the only ones in the province). The goals of this study were to document the nuisances (chemical and biological) in a composting and sorting plant, and to interpret these data by comparing them to the standards and recommendations reported in the scientific and technical literature. Microbial samples of air were collected using the methodology recommended by the ASTM (American Society for Testing and Materials) in their protocol E 884-82. Chemical contaminants were measured using the IRSST's standard methods. In the reception areas of centers A and B, and in the fermentation buildings, the total bacteria concentrations were higher than the maximum suggested level of 10,000 colonies per cubic meter of air (cfu/m³) set for this type of activity. When the concentrations of Gram-negative bacteria are compared to the maximum level of 1,000 cfu/m³ of air, no sampling station exceeded this value. The maximum concentration of thermophilic actinomycetes (15,000 ±150 cfu/m³ of air) wasfound in center B's fermentation building. Concentrations of Aspergillus fumigatus were significantly higher (p £ 0.05) than in the outdoor air in all workstations of plant B's and in plant A's fermentation building. Total mold concentrations were significantly higher (p £ 0.05) than in the outdoor air at center A's reception area, and in both centers' fermentation buildings. The concentrations of chemical contaminants in center B's fermentation building were 50% less than the regulated exposure value, and higher than this 50% of the regulated exposure value for ammonia, carbon dioxide and hydrogen sulfide in center A. Finally, the quality of the outdoor air, 100 meters downwind, does not seem to be affected by the operations performed in these centers.