Two-toned euro coins shed metallic allergen. (Money Allergies).

Holding on to your cash might be good for your finances but not for your skin--at least in Europe.

A new study concludes that 1-euro and 2-euro coins release up to 320 times as much nickel as European standards permit for prolonged contact with the metal, a common skin allergen.

The culprit is the coins' two-toned structure, the researchers report. Unlike other euro coins, the 1-euro and 2-euro pieces contain a ring of one nickel-containing alloy surrounding a center made from a different nickel alloy. Euro coins began circulating in 12 European countries in January.

Dermatologist Frank O. Nestle of the University of Zurich Medical School and his coworkers began the study because patients in the school's allergy unit complained about skin irritation after handling euros. To determine whether the coins actually caused these reactions, Nestle taped them to the backs of seven patients known to have nickel allergies. After a couple of days, all seven had red rashes and small blisters under the coins, say Nestle and his colleagues, Hannes Speidel and Markus O. Speidel of the Institute of Metallurgy at the Swiss Federal Institute of Technology Zurich. They report their results in the Sept. 12 Nature.

In another experiment, the researchers submerged 1-euro and 2-euro coins in artificial sweat and found that they release more nickel than do samples of pure nickel.

The researchers determined that an electric current of about 40 millivolts flows between the two alloys in each coin. It's "like a minibattery," says Nestle. The current leads to corrosion and the release of nickel ions.

"Nickel is one of the commonest contact allergens," says Bruce A. Brod of the University of Pennsylvania School of Medicine in Philadelphia. Up to 15 percent of women have the allergy, which is often caused by jewelry and results in a rash that looks like eczema.

And while it's most likely for an allergic person to develop a nickel reaction during prolonged contact, short or repetitive exposures can also be problematic. "People don't tape euros to their backs," says David E. Cohen of the New York University School of Medicine, but "coins are not an insignificant exposure.... You may be a guy who jingles your change in your pocket." What's more, Cohen adds, "you can imagine all the retailers and bank clerks out there."

"It is ironic that this has occurred in Europe," says Brod. "Historically, the Europeans have been much more vigilant about nickel than us here in the U.S."

Economist Johan Verhaeven of the European Commission in Brussels, which coordinated the coins' production, says the risk of nickel allergies was discussed when the coins were designed 5 years ago. Subsequent studies suggested that the coins shouldn't elicit allergic reactions because they would only contain small amounts of nickel locked safely inside the coins, he says.

However, Verhaeven says, "if there is new evidence coming up, we'll certainly have a look at it."