The air we breathe must be moist for the efficient working of the lungs (see RESPIRATION). Humidification, or moistening of the air, is achieved largely by the NOSE, which acts as an air-conditioner, warming, moistening, and filtering the 10,000 litres of air which we inhale daily; in this process it produces around 1.5 litres of secretion daily.

Those suffering from chronic BRONCHITIS are particularly susceptible to dry air, as are those individuals with disorders of the EYE because the secretions that bathe the eyes and keep them moist are unnaturally dried. (See also VENTILATION.)