Particle clearance is promoted by mucociliary transport and cough. Cilia beat in a coordinated manner at frequencies of between 600 and 900 beats/min. Ciliary beating moves a blanket of mucus along the tracheobronchial tree in a cephalad direction. Two layers comprise the mucous blanket. The sol layer is adjacent to the cilia and a more viscous gel layer is located at the luminal surface. The gel layer consists of a complex polymer of mucopolysaccharides produced by goblet and mucus-secreting cells. Debris deposited on the mucous blanket move toward the oropharynx at a rate 6–20 mm/min.