The double mask, a new concept for local scavenging of anaesthetic gases escaping around face masks and from endotracheal tubes, is described. The system consists of a flexible silicone mask into which the anaesthetic gas mixture is led through a flow distributor which is necessary for adequate scavenging. A rigid transparent polysulphone shell surrounds the inner mask, and escaping gas is evacuated via the 3-4 mm slot between the two masks by a fan, centrally located outside the operating-room. With this system, the anaesthetist's exposure to nitrous oxide during mask anaesthesia was reduced from 145 +/- 29 to 15 +/- 3 ppm (mean +/- s.e. mean, P less than 0.001) and to halothane from 2.9 +/- 1.1 to 0.5 +/- 0.1 ppm (mean +/- s.e. mean, P less than 0.05), as measured in 32 patients aged 1-74 years. The double-mask system provided safe and adequate mask anaesthesia, as judged from inspired concentrations of anaesthetic and fresh gases and transcutaneous and blood gases in 24 patients. The noise level was low with a mean value under regular operating-room conditions around 45 dB(A) in the low frequency range. It is concluded that using the scavenging conditions provided by the system, the Swedish limits for anaesthesiologists' gas exposure could be reduced to the levels recommended by the US National Institute of Occupational Safety and Health (25 ppm for nitrous oxide and 0.5 ppm for halothane).