These defense mechanisms include: (a) ciliary escalator system—the mucus carpet carrying microorganisms and small foreign particles at a normal rate of about 1 cm/min from bronchioles and bronchi to the larynx entrance, (b) coughing, and (c) phagocytosis—macrophages in alveoli clearing via interstitial spaces (lymphatics) and upward via the ciliary escalator system. Maintaining the soft mucus carpet of the ciliary escalator system requires systemic hydration and tracheal gas with 100% relative humidity (RH) at body temperature. Normally, the nasal mucosa warms even cold dry gas to 37°C and fully humidifies it (100% RH; 44 g H2O/m3). The oropharynx is less efficient in this regard. Bypassing the nose with a tracheal tube and inhaling dry gas brings movement of the carpet to a halt. Thus when the nasal air passages are bypassed, inhaled gas should be supplied with at least 100% RH at 37°C.