Nickel is widely used in the metallurgical industry, and although not released extensively into the environment, may represent a hazard to human health. Owing to their low absorption from the gastrointestinal tract, nickel compounds, except nickel carbonyl, are essentially non-toxic after ingestion. Epidemiological investigations and experimental studies have demonstrated that certain nickel compounds are extremely potent carcinogens after inhalation, but also that the carcinogenic risk is limited to conditions of occupational exposure. The relatively small number of mutagenicity studies performed up to now do not yet allow definite conclusions as to whether nickel is mutagenic. Nickel can cross the placenta and has embryotoxic and teratogenic properties. The principal hazard of nickel to man, beside its carcinogenicity, however, is its ability to provoke reactions of sensitization.