Asbestos and metals as carcinogens.

Increased incidences of lung carcinoma and pleural mesothelioma in humans exposed to asbestos have been irrefutably established. Different forms of asbestos may have different tumorigenic activities, depending on surface properties, durability, and size of the fibers. A number of metals, such as nickel, chromium, and arsenic, are known to be carcinogenic to humans; for beryllium and cadmium the epidemiologic evidence is less extensive. All these metals also induce genetic toxicity in vitro. For chromium the molecular mechanism of metal tumorigenesis has been extensively investigated; the hexavalent form is generally a much more potent mutagen than is chromium (III). Solubility seems to be necessary for the genotoxicity of nickel. At present it cannot be concluded that all metals act by the same carcinogenic mechanism, even though direct modification of DNA seems to be the common experimental finding.