The aim of this study was to evaluate the efficacy of anaesthetic gases monitoring in the operating theatre. From January 1997 to December 2007, in compliance with the Ministerial Circular on Professional anaesthetic exposure in operating theatres (5/89), we conducted an environmental monitoring of nitrous oxide (N2O) in 71 operating rooms of 31 public hospitals to determine the respect of limits established by circular (50 ppm). The results show that number of surgery rooms with airborne concentrations of nitrous oxide outside normative limits reduced varying approximately from 40% without monitoring activity, to 15% after a cycle of 10 monitorings. This study demonstrate that the environmental monitoring is crucial, efficacy and should be the first step in developing work practices and worker education programs. To the best of our knowledge, this study demonstrates the efficacy of anaesthetic gases monitoring in the operating theatre was evaluated.