Epidemiologic studies have shown that trace concentrations of inhalation anaesthetics polluting the air of operating theatres could have deleterious effects on the personnel's health. Nitrous oxide (N2O) oxidises vitamin B12 and thus decreases DNA production by inactivation of methionine synthase. Therefore, the United States and most European health authorities recommend threshold values to protect against potential health risks. These values range from 25 to 100 ppm, expressed as time-weighted averages (TWA). There is a lack of data concerning measurements of trace concentrations under defined conditions. The aim of this study was to quantify levels of N2O in a recovery room (RR) with an air conditioning system. METHODS. Trace concentrations of N2O were determined in the main RR of the University Hospital of Regensburg (Germany). Measurements were taken for 5 days from 8:00 a.m. to 8:00 p.m. Trace concentrations of N2O were measured directly by means of a highly sensitive photoacoustic infrared spectrometry analyser. The lower detection limit was 0.03 ppm. Samples of room air were taken continuously from six different places in the recovery room, five of which had a distance of 50 cm to the patients' heads. One point represented the nurses' desk 5 m away from the patients. TWAs were calculated for each day and location. RESULTS. All values were below 5 ppm TWA at each location. Typical TWA (range) values recorded at day 2 were for point 1:3.5 ppm (0.4-8.9), point 2:3.2 (0.5-7.3), point 3:3.0 (0.5-5.4), point 4:3.7 (0.5-21.2), point 5:3.2 (0.6-6.6), and at the nurses' desk 3.3 (0.5-6.3). Peak concentrations of nearly 25 ppm were reached for at least 10 min. Significant differences between the days and locations could not be found (P < 0.05, Wilcoxon test). CONCLUSION. Exposure to N2O in a climatised RR is determined by several factors: (1) efficacy of air conditioning, with 10.7 changes per hour without recirculation; (2) recovery room size; (3) transport of the patients takes about 15 min, during which some quantities of N2O leave the patient; and (4) high numbers of patients staying 2 and more hours in the recovery room and exhaling smaller concentrations of N2O into the room air. Because of these factors, all measured values are significantly below the standard international threshold values. Under other conditions of room design, such as ventilation and size, measured values may be higher.