The likelihood of impaction depends on the air velocity and the particle mass. The result is that the particles will settle on a surface of the lung. This type of deposition is most common in the bronchi, and the bronchioles. Using aerodynamic diameters allows occupational hygiene specialists to compare particles of different sizes, shapes and densities in terms of how they will settle out of the air flow stream. The smaller the particle size, the more vigorous the movement is.
Diffusion is the most important mechanism for deposition in the small airways and alveoli.
This mechanism is prominent because of the high air speed and the many turns in the nasopharyngeal air way. The changes in airflow direction cause many particles to hit the walls of the air passage and so the particles deposit or settle in this region. The air is essentially calm. Particulates that make it this far into the lungs are usually 1 m or smaller. They enter the lung essentially by randomly landing on the membrane or other parts of the lung.