The efficiency of this defense mechanism is critical in reducing the incidence of respiratory infection while optimizing gas exchange. This is reliant on the coordination and beat frequency of the cilia, and the viscosity of the mucus (which in turn is heavily influenced by the level of humidity to which the mucosa is exposed).
The airway surface contributes heat and moisture to inspired gas until it reaches 37 °C, 44 mg/L. The lower the humidity of the inspired gas, the further it needs to travel down the airway before this temperature and humidity are reached.
Williams R, Rankin N, Smith T, Galler D, Seakins P. Relationship between the humidity and temperature of inspired gas and the function of the airway mucosa. Critical Care Medicine 1996; 24(11):1920-1929.