The mouth, the pharynx and the larynx are potential sites of aerosol deposition in the upper airway during inhalation of aerosolized drugs. The right angle bend of the lumen at the back of the mouth, the position of the tongue, the variable size and shape of the lumen in the pharynx and the larynx, and the breathing pattern could increase aerosol deposition in the upper airway and decrease lung deposition.
Nikander, K., Von Hollen, D., & Larhrib, E. H. (2017). The size and behavior of the human upper airway during inhalation of aerosols. Expert Opinion on Drug Delivery, 14, 1-10. https://doi.org/10.1080/17425247.2016.1227780