[1][2][3] Initial sensitisation frequently occurs from jewellery such as ear studs and other body piercings,[3] and nickel allergy is more prevalent among women than men.

[1][4] Once sensitised, an individual can develop contact dermatitis from shorter term contact with nickel-containing products:[4] this is a particular problem given the use of nickel in coinage,[5][6] such as the European one- and two-euro coins[7] and the Canadian five-cent piece.

This led to moves by two European countries to prevent the initial sensitisation of jewellery wearers by limiting the use of nickel in piercing studs and other products which are in prolonged contact with the skin, and then to the European Union Nickel Directive in 1994.

The Nickel Directive imposes limits on the amount of nickel that may be released from jewellery and other products intended to come into direct and prolonged contact with the skin.

These limits, known as migration limits, are: Nickel release is measured by a test method known as EN 1811, which involves placing the object in an artificial sweat solution for one week, then measuring nickel by atomic absorption spectroscopy or any other appropriate technique (e.g. ICP-MS).