**1. Introduction**

Dental metal allergy is the general term used to describe allergic diseases caused by reactions to dental metal materials. Recently, allergic symptoms involving other dental materials, such as organic compounds, have been reported, and these allergic diseases need to be referred to as either a dental allergy or dental material allergy. When safety evaluations involving biomaterials are performed, various kinds of risk factors, including the potential for cytotoxicity and/or allergization, need to be taken into consideration (Geurtsen, 2002, Wataha, 2000).

At the present time, even ordinal dental treatment requires the use of many kinds of metallic and organic materials, some of which are known to cause allergic symptoms. The first clinical cases of dental metal allergy involved a mercurial allergy to intraoral amalgam fillings that led to stomatitis and dermatitis around the anus (Fleischmann, 1928). Previous studies in many countries have reported a variety of symptoms to be associated with different metals (Hubler&Hubler, 1983, Lundstrom, 1984, Magnusson et al., 1982, Wiesenfeld et al., 1984). Nickel, chromium, mercury, palladium, and cobalt are typical of metals used in dentistry that have caused allergies, which have included reactions to these materials not only in the mucosa of the oral cavity, but also on the skin of the hands, feet, and/or entire body (Gawkrodger, 2005, Hamano et al., 1998, Yanagi et al., 2005).

Typical allergies reported to be associated with dental materials have included contact dermatitis, systemic contact dermatitis, and contact dermatitis syndrome. Since most of the intraoral dental materials cannot be removed from home environments, these allergic reactions tend to be intractable, with repetitions of symptomatic treatments, such as external medications, found in many of these cases. Sometimes general and local dermatitis is found in the skin apart from the intraoral dental material, and it exhibits pathognomonic symptoms of the allergy that are different from those noted in other contact dermatitis.
