Nickel is a chemical element used in a majority of metal products we employ because it is cheap, long lasting and it gives a nice gloss. Most metal items are plated or have covers. A nickel alloy is often used as the base metal and can be plated afterwards. Once the overlay or plating wears thin, even in an imperceptible way, nickel salts can have a direct contact with the skin and might lead to irritation. 

The European Union limits in REACH the amount of nickel set loose by metal objects, like earrings and body piercing jewelry, must be less than 0.2μg/cm²/week; this amount must be less than 0.5μg/cm²/week for other objects in direct contact with the skin for a long time.