ABSTRACT
During inhalation, particles are transported with the inspired air through the extrathoracic airways and the bifurcating tracheobronchiolar system to the gasexchanging region of the lung. A certain number of these particles are caught in the respiratory system by touching the wet airspace surfaces, a phenomenon generally referred to as particle deposition. Therefore, with exhalation, not all particles are recovered. Figure 1A shows the fraction of particles deposited in the respiratory system (total deposition) during quiet mouth breathing as a function of the particle diameter. This fraction is small for particles in the size range between 0.1 and 1 m m. But it becomes larger for smaller and larger particles reaching almost 100% for 0.01-or 10-m m particles. However, the particle size determines not only how many particles are deposited, but also in which region of the respiratory tract these particles are deposited (regional deposition, see Fig. 1B-D). Total and regional deposition are modified further by other physical properties of the inhaled particles (particle density and shape), by the breathing pattern (tidal volume, breathing frequency, and flow rate; Fig. 2), and by the lung geometry (airspace dimensions).
