The ventilation of the lung ranges between 1,000 and 21,000 liters per 24 hours in humans depending on body size and physical activity. This exposes the extensive epithelial surface of the respiratory tract between the nose and the alveoli to a large burden of inorganic and organic particulate and gaseous material with potentially injurious actions. A series of defense systems have evolved that protect the airways. One of these is the mucociliary apparatus of the tracheobronchial tree. The mucociliary apparatus has three major functions. First, it serves as a mechanical barrier by trapping particulates in the surface liquid covering the airway epithelium and clearing them from the tracheobronchial tree by ciliary action. Second, the surface liquid acts as a chemical screen; for example, airway mucus has antioxidant properties. Third, the surface liquids provide a biologic barrier function by interacting with microorganisms and luminal inflammatory cells, thereby preventing them from adhering to and migrating through the airway epithelium. For normal mucociliary function, the surface liquids have to exhibit certain chemical and physical characteristics to fulfill their barrier functions and to interact with cilia for mucus clearance. It has been well established that physical and chemical injury impair airway mucociliary function, and that various forms of