Isocyanates, widely used in Quebec, are the leading cause of occupational asthma. Several companies use direct-reading monitors to evaluate the concentrations of airborne isocyanates. In theory, these monitors, designed to produce real-time read-outs of ambient concentrations, should facilitate rapid detection of increases in concentration, identification of emission sources, and rapid emergency response. However, these devices have never been evaluated and their reliability was unknown at the time this study was undertaken. The IRSST, in collaboration with the International Isocyanate Institute (III), evaluated the five most common direct-reading monitors used to measure various isocyanates (PMDI). Commercial calibration under typical usage conditions was found to lead to inaccurate PMDI estimates in four of the five monitors, with observed values ranging from 50% to 1000% of the true value. Monitor response was particularly dependent on manufacturers calibration, which was often inadequate, humidity levels, and the isocyanate species present. These results highlight the need for prudence in interpreting results obtained from direct-reading monitors. Despite their observed limitations, these monitors may nevertheless prove for qualitative exposure monitoring, e.g. during modifications of industrial processes, spills, and sudden leaks.
