To the Editor:—

We read with interest the investigation revealing causes of nitrous oxide contamination in operating rooms. 1The authors observed occupational exposure to trace amounts of a waste anesthetic gas, nitrous oxide, and showed a number of sources that were responsible for abnormally high workplace concentrations. In addition to insufficient or lacking air conditioning systems and scavenging devices, inhalational mask induction and leakage during use of uncuffed tubes have widely been proved as the most important factors with regard to exposure to both nitrous oxide and volatile agents. 2,3 

However, we feel some points of the recent study require further discussion. The air samples were taken at the air conditioning exhaust grill at a distance of approximately 3 m from the sources of contamination. Therefore, the measurements only reflect air contamination at a given point, not actual exposure of an individual, which is far more important in the evaluation of workplace safety and eventual health hazards. Actual exposure to an individual was not measured because anesthetic gases are distributed within the room and thus—depending on the distance from the source of contamination—are diluted in significant manner. 3,4 

To estimate dilution of nitrous oxide, we checked leakage 62 wall-mounted gas outlet sockets (Draeger, Luebeck, Germany) that provide nitrous oxide from the high-pressure central gas system to the anesthesia machines in 17 operating rooms in our hospital. All rooms were well air conditioned by laminar flow and an air exchange rate ranging from 19.2–21.3/h without recirculation of exhaust air. Measurements were taken continuously for 6 min with a directly displaying infrared spectrometer (Brüel & Kjaer 1302, Naerum, Denmark) at a distance of 1 cm from the sockets. To determine the baseline contamination of the operating rooms, levels of nitrous oxide were measured in the middle of the rooms, approximately 3.5 m away from the sockets.

Our measurements revealed that no socket was absolutely gas-tight. Mean leakage was 181.6 ± 624.0 ppm (range, 1.6–4,730.2 ppm). Despite high concentrations of nitrous oxide close to the sockets, overall room contamination was low, which can be attributed to dilution. In the center of the operating rooms, nitrous oxide levels were only 4.38 ± 2.30 ppm and showed that at a distance from the leakage comparable to that in which Kanmura et al.  took their air samples, the concentrations were approximately 40-fold lower than those measured close to the leakage. Therefore, in the recent study, exposure of personnel who worked close to the sources of contamination could have been much higher than the concentrations actually measured at the exhaust grill.

Although having already undergone distribution and dilution, concentrations of nitrous oxide at the exhaust grill were disproportionately high when compared with recent studies conducted in equally climatized operating rooms. 3–5Such high exposure may be plausible only in cases of undetected massive leakage from the central gas system or malfunction of air conditioning or scavenging devices, 6which resulted in relatively high baseline contaminations of the operating rooms close to the 50-ppm threshold set by the authors. 1During use of nitrous oxide, relatively low concentrations from anesthesia itself added to the high baseline level and triggered alarm. Furthermore, high concentrations may result from the careless use of nitrous oxide by anesthesiologists. 3,6 

Nonetheless, the recent study shows that there is still a lot of work to be done from the standpoint of occupational health and workplace safety. Furthermore, our results show that there are undetected sources of leakage that led to occupational exposure. Because exposure to naturally produced nitrous oxide is estimated to be only approximately 0.31 ppm, 7occupational exposure in contaminated rooms caused by leakage in our study was approximately 12 times as high. Thus, air-conditioning systems and scavenging devices, as well as anesthetic equipment, should be checked in short intervals to prevent or repair malfunction and leakage. Only increased awareness of this problem will keep occupational exposure as low as possible.

Kanmura YK, Sakai J, Yoshinaka H, Shirao K: Causes of nitrous oxide contamination in operating rooms. A NESTHESIOLOGY 1999; 90: 693–6
Hobbhahn J, Hoerauf K, Wiesner G, Schroegendorfer K, Taeger K: Waste gas exposure during desflurane and isoflurane anaesthesia. Acta Anaesthesiol Scand 1998; 42: 864–7
Byhahn C, Westphal K, Wilke HJ, Lischke V: Occupational exposure during mask induction and use of different types of endotracheal tubes. Anaesth Intensivmed 1998; 39: 627–32
Hoerauf K, Koller C, Wiesner G, Taeger K, Hobbhahn J: Nitrous oxide exposure of operating room personnel in intubation anaesthesia. Gesundh-Wes 1995; 57: 92–6
Westphal K, Wilke HJ, Strouhal U: Exposure of surgeon to desflurane and nitrous oxide in intraoral operative procedures (letter). Acta Anaesthesiol Scand 1998; 42: 745
Imberti R, Preseglio I, Imbriani M, Ghittori S, Cimino F, Mapelli A: Low flow anaesthesia reduces occupational exposure to inhalational anaesthetics. Acta Anaesthesiol Scand 1995; 39: 586–91
Daten zur Umwelt, 4th Edition. Edited by Umweltbundesamt. Berlin, Schmidt Publishers, 1992, p 19