Arsenic may be released into the environmental through industrial processes and through the generation of power from coal. It is also widely used in agriculture and was formerly used extensively in medicine. For the general population, exposure to arsenic occurs mainly through the ingestion of foodstuffs containing inorganic and organic arsenicals. Trivalent arsenicals are regarded as being primarily sulfhydryl reagents with the result they inhibit a number of thiol-dependent enzymic systems in various tissues. Arsenite also has an effect on DNA synthesis and DNA repair. Owing to its lower affinity for hydroxy and thiol groups, pentavalent arsenate inhibits fewer enzymic systems. Although there is no reliable evidence that arsenic produces tumors in experimental animals, epidemiological studies show that the incidence of epidermoid carcinomas of the skin and lungs, and of pre-cancerous dermal keratoses, is significantly increased in human subjects who have been chronically exposed to arsenic compounds by oral or respiratory routes. Arsenic appears to be one of the only teratogenic members of the Group V metals. Most of the studies performed on the mutagenic activity of arsenic have provided positive results. They involve experiments on microorganisms, plant material and Drosophila as well as observations on the ability of this metal to induce, in vitro and in vivo, chromosomal aberrations in mammalian cells.