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.
) is subject to fast clearance in the layer of fluid that covers these airways.
These discrete pathways are represented by the compartment model shown in figure 10.3 . They correspond to the anatomic compartments illustrated in figure 10.1, and are summarized in table 10.1 , along with those of other groups providing guidance on the dosimetry of inhaled particles.
The complex structure and numerous functions of the human respiratory tract have been summarized concisely by a Task Group of the International Commission on Radiological Protection (ICRP 1994), as shown in figure 10.1 .
The conductive airways, also known as the respiratory dead space, occupy about 0.2 litres.
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.