Cited as a reference document in the Regulation respecting occupational health and safety (RROHS), an essential tool for hygienists, this guide describes the strategy and methods for measuring workers' exposure to gases, fumes, vapours or dusts so that means of control can be implemented. The modifications and additions to this eighth edition take into account the recent evolution in workplaces and the Québec health and safety regulations. The Sampling Strategy chapter has been enhanced by the addition of new references and reorganized to make its content comply with the RROHS. In the Instruments and Sampling Techniques chapter, a new section called Compliance with the RROHS's permissible exposure values (PEVs) clearly states the invariable nature of PEVs in relation to temperature and pressure. This change led to substantial modifications to the chapters on pumps, flowmeters and calibration. The sections on direct-reading instruments, analysis of respirable compressed air, microbiology and surface tests were also updated.