The key mechanisms influencing pulmonary aerosol deposition are inertial impaction, sedimentation and diffusion. These three mechanisms operate within a range of various combinations when certain aerosolized drugs that target different areas within the lung are used. Inertial impaction occurs when a large aerosol, usually three microns and larger, is inhaled and deposited in the process of the oropharynx and larger airways. Smaller sized aerosols diffuse into the lower airways, making it the dominant mechanism if aerosol particles produce it