Due to the much longer residence times of the residual air in the lungs, the low intrinsic particle displacements of 0.1 to 1 µm particles within such trapped volumes of inhaled tidal air become sufficient to cause their deposition by sedimentation and/or diffusion over the course of successive breaths.
The essentially particle-free residual lung air that accounts for about 15% of the expiratory tidal flow tends to act like a clean-air sheath around the axial core of distally moving tidal air, such that particle deposition in the respiratory acinus is concentrated on interior surfaces such as airway bifurcations, while interbranch airway walls have little deposition.
It reflects the minimal lung deposition between 0.1 and 1 µm, where deposition is determined largely by the exchange, in the deep lung, between tidal and residual lung air.
Deposition increases below 0.1 µm as diffusion becomes more efficient with decreasing particle size.
Particles larger than about 2 µm in aerodynamic diameter (diameter of a unit density sphere having the same terminal settling (Stokes) velocity) can have significant momentum and deposit by impaction at the relatively high velocities present in the larger airways.
Particles larger than about 1 µm can deposit by sedimentation in the smaller conductive airways, where flow velocities are very low.
The likelihood that a particle is cleared relatively slowly by the mucociliary system appears to depend on its physical size.