In order to understand where and how inhaled particles are deposited onto the airway wall, it is necessary to appreciate the structure of the airways and how this has bearing on gas velocity. Secondly it is of great use to reflect on the mass of particles and their likely fate when they are subjected to both gravitational forces and to airflow in the airways. Now turn your attention to the presentation on the right.
When large particles (>10-20 µm) are airborne, they can be inhaled. These large particles usually impact on the walls of the nose and pharynx. Their relatively large mass, which varies as diameter³, and high velocity result in high mass inertia. The velocity of a particle is similar to the velocity of the air, which equals flow divided by cross-sectional area of the airway. If the peak inspiratory flow is 0.6 L/s (600 cm³ per second, the peak flow during normal tidal breathing in an adult), and the cross-sectional surface area of the nose is 1 cm², then the average velocity of a particle is 6 metre/s or nearly 22 km/h; that is pretty fast.
The larger the particles, the greater their mass (which varies as diameter³) and hence the greater the mass inertia. The nose represents a narrow airway passage which is rather tortuous. The very high mass inertia of the largest particles prevents these particles from passing the tortuous nasal passage without impacting against the wall into the sticky mucosa. Even small hygroscopic particles may get trapped in the nasal passage since humidification within the nose can result in these particles swelling to greater size and mass.
The rate of particle impaction increases with the velocity of the particle, the angle of deflection, and the square of the (aerodynamic) diameter of the particle. A small percentage of particles <10 µm passes through the nose and nasopharynx (upper airways) to enter the lower airways. Less than 2% of the larger particles reach the lower (intrathoracic) airways.