Under extreme conditions, ambulance attendants and drivers could be exposed to nitrous oxide administered to transported patients in concentrations causing acute effects. Special arrangements are necessary to prevent such exposure, which is influenced by travelling speed, local exhaust ventilation and the use of an excess gas transfer tube evacuating expired air and overflow gas from the face mask to the outside. The separate eliminative effects of travelling speed and local exhaust varied considerably with the experimental conditions. The excess gas transfer tube reduced the levels of nitrous oxide in the air by 86 to 97% inside the ambulance at different experimental conditions. The combination of excess gas transfer tube and local exhaust resulted in a relatively constant reduction of the airborne nitrous oxide levels by about 98% when the ambulance was at a standstill and 99% when it was running.