Many skin conditions are considered a type of eczema. Atopic dermatitis is one type. Other types include hand dermatitis, nummular dermatitis, and seborrheic dermatitis. Dandruff is a mild type of seborrheic dermatitis. Diaper rash and the rash that many people get after coming into contact with poison ivy are other types of eczema.

What causes some types of eczema is clear-cut. One type of eczema, irritant contact dermatitis, develops after frequent exposure to a mild irritant such as a detergent or brief exposure to a strong irritant such as battery acid. Another type, allergic contact dermatitis develops when an allergen touches the skin. Common allergens include poison ivy and nickel. A nickel allergy is actually one of the most common causes of allergic contact dermatitis. Many everyday objects contain nickel, including coins, buttons, jewelry, and eyeglass frames.

The exact cause of other types of eczema is not fully understood. Researchers believe that atopic dermatitis develops when many factors combine. These factors include inheriting certain genes, having an overactive immune system, and having something that dermatologists call a "barrier defect." A barrier defect is a term that means "gaps in the skin." These gaps allow the skin to lose water too quickly. The gaps also allow germs and other things too small to see with the naked eye to enter the body.