ZURICH, Switzerland (CNN) - The euro currency has always been unpopular with traditionalists who prefer francs, shillings or deutschemarks, and now there's another complaint against some euro coins -- they cause skin irritation in people who are sensitive to nickel.

Scientists at the University of Zurich studied the phenomenon by taping one- and two-euro coins to the skin of patients with nickel allergies. After 48 to 72 hours, all the patients showed a strong allergic reaction, including redness and blisters.

Two-euro coin

The researchers say these coins cause more irritation than other coins with similar levels of nickel, and they think they know why.

The one- and two-euro coins are made with a ring of one metallic alloy surrounding a central "pill" of another alloy. Both alloys contain nickel.

When the coins are exposed to sweaty hands, ions flow between the two compounds, which generates a tiny electrical charge, and makes both metals corrode faster than they would by themselves.

The amount of nickel released can be more than 300 times the levels permitted under European Union regulations. The scientists confirmed the effect by soaking coins in artificial human sweat.

The reaction also takes its toll on the coins: They changed color and showed signs of corrosion.

Researchers from the departments of dermatology and metallurgy worked together on the study. The findings were published in a letter to the journal Nature.