Asbestos-related mesothelioma: evidence for a threshold in animals and humans.

A threshold for mesothelioma for the major asbestos fiber types becomes not only plausible but also very likely in view of the existence of a distinct background incidence of spontaneously occurring and non-asbestos-related mesotheliomas; the high occupational doses associated with the appearance of mesotheliomas in humans; and the large number of "tumorigenic" fibers required to produce significant numbers of mesotheliomas in animals. Even when the duration of exposure associated with the appearance of mesotheliomas in humans has been brief, the exposure itself has been intense. The review of the relevant animal and human literature cited herein supports the concept of mesothelioma threshold.