Particle Deposition in Human Lung Airways: Effects of Airflow, Particle Size, and Mechanisms

Characterizing particle deposition in the airways of the human lungs is essential to evaluate the health effects of particulate air pollution. However, lung deposition is rather complicated, and its main influencing factors remain unclear. Hence, this study applied computational fluid dynamics (CFD) to investigate the roles of airflow (Reynolds number [Re] = 100–2000) and particle size (1–10 µm) in deposition using a human tracheobronchial airway model (G3–G6). We calculated the deposition efficiency (DE) based on two mechanisms, inertial impaction (DEi) and gravitational sedimentation (DEg), which produced hot spots around the bifurcations and uniform distributions along the tube, respectively. Furthermore, as the particle size increased, DEi grew rapidly, whereas DEg grew log-linearly. Particles that were less than 2 µm in diameterµ only penetrated deep in the lungs where the airflow rate was low, but 3 µm particles were more likely to settle in this region owing to the combination of gravitational sedimentation and inertial impaction. Larger particles, on the other hand, mainly deposited in the proximal bifurcations as a result of inertial impaction. Additionally, the deposition due to inertial impaction and that due to gravitational sedimentation primarily depended on the Stokes number (St) and the ratio of St to Re2, respectively. The orientation of the human body was another potential factor in the pattern of deposition, although the upright and lateral positions exhibited similar deposition efficiencies for 3 µm particles regardless of Re. These findings identify the critical Reynolds number at which the particle deposition mechanism for a specific size shifts from gravitation to inertia.