Effect of Particles on Mucus and Mucociliary ClearanceThis chapter deals with how the mucus reacts to particles and particulate loads, thereby altering its pattern and rate of clearance. Particles can stimulate secretion and modulate ciliary function either due to mechanical factors or irritation or via the chemicals they contain or are capable of releasing. Particles have a direct effect on the physical properties of mucus, even when acting as “neutral filler,” where their contribution to viscoelasticity is a simple function of their volume fraction. Beyond this, particles may interact with the mucous gel network and add multiple cross-link points.