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Finally, particles with diameters between 0.1 and 1 µm, which have a very low probability of depositing during a single tidal breath, can be retained within the approximately 15% of the inspired tidal air that is exchanged with residual lung air during each tidal cycle.This volumetric exchange occurs because of the variable time-constants for airflow in the different segments of the lungs.The system can be abused and overwhelmed by severe insults such as high concentrations of cigarette smoke and industrial dust, or by low concentrations of specific pathogens which attack or destroy its defence mechanisms, or cause them to malfunction.Its ability to overcome or compensate for such insults as competently as it usually does is a testament to its elegant combination of structure and function.

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.Material deposited in the AI region is subdivided among three compartments (AI Figure 10.4 depicts the predictions of the ICRP (1994) model in terms of the fractional deposition in each region as a function of the size of the inhaled particles.The ICRP deposition model is used to estimate the amount of inhaled material that enters each clearance pathway.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.

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