Mechanisms influencing deposition of particles dependent on their size

Deposition of particles in different regions of the respiratory tract depends on particle size and inspiration profile. Large particles (>8 µm) are predominately deposited in the extrathoracic area (oropharyngeal). Smaller particles with a size between 4 to 10 µm reach the tracheobronchial system, while very small particles (1-5 µm) may even arrive at peripheral alveoli.

Impaction is the physical phenomenon, which mainly influences the deposition of larger particles. With growing size particles become more inert, setting rate increases and their ability to follow the respiratory flow is reduced proportional to velocity of flow. Thus larger particles precipitate in flexions and narrow parts of the respiratory tract, i.e. nose, larynx and bifurcation of the larger airways.

The second important mechanism leading to particle deposition is sedimentation, i.e. gravitational attraction. This process is proportional to aerodynamic particle size and to the period during which the particles stay in the lungs. Therefore a short arrest at the end of inhalation increases the likelihood of lung deposition. Both impaction and sedimentation result in deposition of particles larger than 3-5 µm before reaching the alveoli.

For particles smaller than 0.5 µm deposition is mainly influenced by diffusion. This mechanism increases inversely proportional with particle size and directly proportional with length of stay in the lungs.


Voshaar T. Therapie mit Aerosolen. 2005. Uni-Med: Bremen