Hypothetical model of asbestos-induced pulmonary fibrosis. Asbestos elicits a macrophage response to phagocytize and clear the fibers, but this response results in reactive oxygen species (ROS) production by a Rac1-dependent mechanism as well as the release of cytokines and growth factors. Asbestos-induced alveolar macrophage (AM) and alveolar epithelial cell (AEC) mitochondrial ROS production promotes AEC apoptosis that may be important for myofibroblast differentiation, collagen deposition by myofibroblasts, and ultimately pulmonary fibrosis. Abbreviations: PDGF, platelet-derived growth factor; IL, interleukin; Rac1, Ras-related C3 botulinum toxin substrate 1; TGF, transforming growth factor; TNF, tumor necrosis factor.