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Although there is a recognised need for routine monitoring of the exposure of operating theatre staff to waste anaesthetic gases, there is as yet no standard method for carrying out such measurements. We have assessed a commercial version of one proposed sampling system--the evacuated canister--by testing it in the laboratory and also using it to carry out pilot surveys in operating theatres. We encountered many difficulties in using the system; some of these were caused by design faults in the commercial equipment, but others are inherent in the concept of using an evacuated canister for personal sampling.

A method is described for preparing static standards of per cent concentrations of volatile anaesthetic agents, that is designed to minimize adsorptive losses in the container used for the standards. The extent to which this aim has been realized was investigated for atmospheres of halothane, enflurane and isoflurane, both by monitoring the relative concentration within the container over a period of 30 min after the standard was prepared, and by measuring the absolute concentration of the volatile agents. Gas chromatography was used for both sets of measurements, with liquid standards being used for the absolute measurements. No discernible loss of volatile agent occurred over the 30-min period, and the measured concentrations agreed closely with the predicted values. The standard mixtures prepared by the method were suitable for calibrating a gas chromatograph, and allowed accurate determinations of volatile anaesthetics to be made. The relative standard error of the predicted concentration for a single standard mixture was less than 0.5%, and the calibration accuracy could be further improved by taking the mean chromatograph response to several standard mixtures prepared in succession.

The design and performance of an apparatus for producing accurately known trace concentrations of anaesthetic agents in air is described. The generator was constructed to facilitate the testing and calibration of equipment used in measuring the personal exposure of operating theatre staff to waste anaesthetic gases, and operates on the principle of the controlled injection of the pure anaesthetic agent into a diluting air stream. It is compact, simple to construct and easy to operate. Furthermore, it has a short stabilization time and can provide a wide range of predetermined concentrations with very good repeatability. It appears to be considerably more flexible than systems which have been described previously and should be of interest to groups involved in monitoring pollution in the operating theatre.

A new monitor for volatile anaesthetics, the Datex Normac, was assessed in the laboratory and operating theatre. The instrument had a warm up time of 90 minutes from cold, but only 5 minutes from standby. Zero drift was less than 0.01% over 7 hours, and repeated measurements of gain showed a coefficient of variation of less than 1%. However, gain drift of 6% occurred when the instrument was switched off between periods of use. Signal noise was less than 0.02 vol% on a new instrument, but was about six times greater on an instrument that had been in use for a year. The response time of 550 ms is short enough to allow breath by breath monitoring. When the instrument was calibrated on the enflurane setting, measurements made on the enflurane, halothane and isoflurane settings were accurate to within 6% of the reading. This is satisfactory for clinical purposes, but not for more demanding applications. Calibration canisters are available, and two that we tested were accurate to better than 0.1 vol%. Nitrous oxide, carbon dioxide and water vapour have very small effects on the monitor. The instrument provided trouble-free monitoring in theatre.