Asbestos fibers may exert their carcinogenic effects on mesothelial cells and bronchial epithelial cells by direct and indirect mechanisms. Direct effects can occur following the physical interaction of fibers with target cells or by the generation of free radicals from the fiber surface; indirect effects, following the interaction of fibers with inflammatory cells can result in the production of cellular mediators such as cytokines and various reactive oxygen species. As a result, target cells may be induced to proliferate and/or sustain genetic alterations, which lead to tumor development.