To elucidate a limit to the efficiency of the upper airway mucosa as an air conditioner, the temperatures of the inspiratory air and mucosa were measured in the cervical trachea. Both of them were affected only minimally by change of atmospheric air temperature during resting nose breathing, but were affected greatly by change of mode of breathing. During hyperventilation through the mouth, when the atmospheric air temperature was 1 °C, a temperature difference of 9 °C was noted between inspiratory air in the cervical trachea and body temperature, together with a mucosal temperature fall by 1.86 ± 0.61 °C. Wearing of a mask caused a rise of 3 °C in the inspiratory air temperature in the cervical tra