Summary In recent years, some Québec construction workers have been "sensitized" to 4,4'- diphenylmethane diisocyanate (MDI). This substance is emitted during spraying of a polyisocyanate (a mixture of MDI polymers) and a polyhydroxyl compound (resin) to form a rigid polyurethane foam used for thermal insulation in buildings. The permissible exposure value (PEV) for MDI (monomer form) in Québec is 0.051 mg/m3 (51 μg/m3). This substance has been identified as a sensitizer (S) in the Regulation respecting occupational health and safety (RROHS) (Règlement sur la santé et la sécurité du travail RSST), which also mentions that the exposure should be kept at a minimum (ME). Field studies (on-site spraying) reported in the scientific literature have shown significant concentrations of MDI during this process, particularly in the residential construction sector. In fact, the Lesage study (2007) confirmed the need to sample MDI aerosols and to use a collection medium that allows their optimal collection, considering the short polymerization time in this process. The Canadian Urethane Foam Contractors Association (CUFCA) and the Alliance for the Polyurethane Industry (API) offer a training program for installers (or appliers) of this foam. The program covers the spraying methods available to obtain a quality product, which will therefore depend on the workers’ skill. The safety aspect during spraying operations is hardly or not addressed at all. Initially, this study documented occupational health and safety practices under a wide range of conditions where insulation work is carried out, including commercial or residential construction, the location of the work (indoors or outdoors), and the season (summer or winter). The study then evaluated MDI by means of stationary sampling in the work areas. These concentrations (average concentration over the duration of the work and weighted over an 8-hour period, TWAEV) were analyzed according to the determining factors influencing the workers' exposure to MDI. The details of the work were examined in order to estimate, with a fair degree of confidence, the highest risk scenarios during foam spraying operations. The risk of exposure to MDI in monomer form (aerosols) for workers performing spraying operations is high, particularly when such operations are performed indoors where the average of the concentrations in the spraying zone reach 288 μg/m3. In addition to these characteristics, individual and team (team consisting of an installer (or applier) and his assistant) work practices explain certain variations in the data collected. Despite the training program for installers (or appliers) offered by an accreditation authority, nothing indicates that occupational health and safety practices are being completely followed on Québec construction sites. In the context of competency training, occupational health and safety practices as well as respiratory and skin protection need to be rigorously addressed and should be part of the workers' continuing training process.