Construction workers are exposed to 4,4’-diphenylmethane diisocyanate (MDI) when they spray a mixture of MDI polymers and a resin compound, which form a rigid polyurethane foam used as thermal insulation for buildings. Studies have shown that this process releases significant concentrations of MDI. The Regulation respecting occupational health and safety indicates that this substance is a sensitizer. This study documented the practices relating to occupational health and safety (OHS) under a variety of conditions under which insulation work is done, and evaluated the MDI concentrations in the work areas. The workers run a risk of exposure to MDI in aerosol form, particularly when they do this work indoors. The report’s authors note that nothing indicates that OHS practices are completely respected on Québec construction sites and recommend that the subject of respiratory and skin protection be integrated into the continuing training of workers.