Nickel is found in hydrogenated oils, shellfish, air pollution, cigarette smoke and plating.
While the biological role of nickel has been recently recognized (starting largely in the 1970s), nickel is an allergen for many individuals and many nickel compounds are toxic. In particular, nickel sulfide fumes are believed to be carcinogenic, raising concerns about other nickel compounds.
Nickel levels are often found to be high in breast cancer patients. Nickel levels in hair samples often correlate with local soil conditions.
Sensitized individuals may show an allergy to nickel affecting their skin and is an important cause of contact allergy, partly due to its use in jewellery (earrings). It was voted Allergen of the Year in 2008 by the American Contact Dermatitis Society.