A mathematical model is presented that describes the health-endangering interaction of fibrous particles deposited in the human alveoli with alveolar fluids, cells, and tissues. As suggested by the theoretical approach, short fibers (diameter: 0.5 μm, length: 2-10 μm) are preferably ingested by alveolar macrophages and removed from the alveolar surface 10-15 days after exposure. Long (diameter: 0.5 μm, length: 10-50 μm) biopersistent fibers are not effectively cleared from the alveoli due to the repeated process of frustrated phagocytosis. Long biosoluble fibers also undergo a frustrated phagocytosis, with processes of extensive lysis leading to their significant shortening. The decrease in length causes the initiation of those clearance mechanisms that are efficient for short fibers.