Fig. 5. Size-dependent aerosol deposition mechanisms to sites in the respiratory tract.

(A) Main deposition mechanisms and corresponding airflow regimes in different regions of the human respiratory tract. Large aerosols tend to deposit in the nasopharyngeal region as a result of inertial impaction, whereas small aerosols tend to deposit in the tracheobronchial and alveolar regions on the basis of gravitational sedimentation and Brownian diffusion. An enlarged view of tracheobronchial and alveolar regions illustrates the deposition mechanism. (B) The deposition efficiency of aerosols at different regions of the respiratory tract as a function of aerosol diameter based on the ICRP lung deposition model is shown (116). The majority of large aerosols deposit in the nasopharyngeal region; only aerosols that are sufficiently small can reach and deposit in the alveolar region.