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Contact dermatitis is a localized rash or skin inflammation caused by direct contact with a substance. Contact dermatitis falls into two broad categories, allergic contact dermatitis (ACD) and the more common irritant contact dermatitis (ICD). Apart from the differences in origin, ACD and ICD are often clinically indistinguishable.

People who suffer from contact dermatitis may not experience symptoms the first time they are exposed to a substance. However, as they use or are exposed to the substance repeatedly, the skin becomes sensitized to it over time. Examples of such sensitization may include:

• Use of nail polish remover
• Preservatives in contact lens solutions
• Repeated contact with metals in earring posts or the metal backs of watches

Though the two main categories of contact dermatitis resemble each other in morphology (appearance and cell structure) and the resulting rashes are very similar, they differ in origin. ICD is caused by exposure to a naturally irritating substance to human skin. ACD is caused by a normally harmless substance, which triggers an exaggerated immune system reaction.

In addition, outbreaks of contact dermatitis are more likely to affect certain populations than others. For example, hospital workers who use latex gloves are more likely to suffer from latex-based contact dermatitis, and women are more likely to experience facial contact dermatitis than men because they use facial cosmetics more frequently.