The mandate of the Comité aviseur sur l'amiante (exposure measurement) consisted in assessing the pertinence and the feasibility of assessing asbestos exposure in the general population, particularly in public buildings such as schools. [...] At the same time, work continued on other relevant aspects: the criteria and standards in effect in Quebec, the sampling of asbestos fibres in ambient air, the situation in workplaces such as mines and processing plants, and finally, exposure levels in mining towns and in ambient air in general. [...] The main sources of indoor air contamination are linked to the presence of friable asbestos materials such as the sprayed asbestos applied to the ceilings and walls of some public buildings up to the late 1970s. [...] Principle of the method The principle of the method consists in first carrying out a meticulous examination with a low- magnification stereomicroscope (10 to 60x) in order to verify the homogeneity of the sample, reveal the different components of the sample, and estimate the content. [...] MEASUREMENTS OF AIRBORNE FIBRE CONCENTRATION (5) Phase-contrast optical microscopy Principle of the method To evaluate fibre concentration in the atmosphere of a workstation and compare the results to the values prescribed in the Regulation Respecting the Quality of the Work Environment (RQMT), fibres are collected on a membrane filter and counted using phase-contrast optical microscopy (PCOM).