An allergic reaction (contact dermatitis) usually begins within hours to days after exposure to nickel. The reaction may last as long as two to four weeks. The reaction tends to occur only where your skin came into contact with nickel, but sometimes may appear in other places on your body.

Nickel allergy signs and symptoms include:

• Rash or bumps on the skin

• Itching, which may be severe

• Redness or changes in skin color

• Dry patches of skin that may resemble a burn

• Blisters and draining fluid in severe cases

The exact cause of nickel allergy is unknown. As with other allergies, nickel allergy develops when your immune system views nickel as a harmful, rather than harmless substance. Normally, your immune system only reacts to protect your body against bacteria, viruses or toxic substances.

Once your body has developed a reaction to a particular agent (allergen) — in this case, nickel — your immune system will always be sensitive to it. That means anytime you come into contact with nickel, your immune system will respond and produce an allergic response.

Your immune system's sensitivity to nickel may develop after your first exposure or after repeated or prolonged exposure. Sensitivity to nickel may, in part, be inherited.

Sources of nickel exposure

Common items that may expose you to nickel include:

• Jewelry for body piercings

• Other jewelry, including rings, bracelets, necklaces and jewelry clasps

• Watchbands

• Clothing fasteners, such as zippers, snaps and bra hooks

• Belt buckles

• Eyeglass frames

• Coins

• Metal tools

• Cellphones

• Keys

• Military "dog-tag" ID

• Chalk

• Medical devices

• Laptops or computer tablets

• E-cigarettes

Certain factors may increase your risk of developing a nickel allergy, including:

• Having ear or body piercings. Because nickel is common in jewelry, nickel allergy is most often associated with earrings and other body-piercing jewelry containing nickel.

• Working with metal. If you work in an occupation that constantly exposes you to nickel, your risk of developing an allergy may be higher than it is for someone who doesn't work with the metal.

In addition, people who have regular exposure to nickel while doing "wet work" — as a result of either sweat or frequent contact with water — may be more likely to develop nickel allergy. These people may include bartenders, people who work in certain food industries and domestic cleaners.

Other people who may have an increased risk of nickel allergy include metalworkers, tailors and hairdressers.

• Being female. Females are more likely to have a nickel allergy than are males. This may be because females tend to have more piercings. A recent study found that overweight women seem to have an even higher risk of nickel allergy.

• Having a family history of nickel allergy. You may have inherited a tendency to develop a nickel allergy if other people in your family are sensitive to nickel.

• Being allergic to other metals. People who have a sensitivity to other metals may also be allergic to nickel.