Causes

Contact dermatitis is caused by a substance you're exposed to that irritates your skin or triggers an allergic reaction. The substance could be one of thousands of known allergens and irritants. Some of these substances may cause both irritant contact dermatitis and allergic contact dermatitis.

Irritant contact dermatitis is the most common type. This nonallergic skin reaction occurs when a substance damages your skin's outer protective layer.

Some people react to strong irritants after a single exposure. Others may develop signs and symptoms after repeated exposures to even mild irritants. And some people develop a tolerance to the substance over time.

Common irritants include:

• Solvents
• Rubbing alcohol
• Bleach and detergents
• Shampoos, permanent wave solutions
• Airborne substances, such as sawdust or wool dust
• Plants
• Fertilizers and pesticides

Allergic contact dermatitis occurs when a substance to which you're sensitive (allergen) triggers an immune reaction in your skin. It usually affects only the area that came into contact with the allergen. But it may be triggered by something that enters your body through foods, flavorings, medicine, or medical or dental procedures (systemic contact dermatitis).

You may become sensitized to a strong allergen such as poison ivy after a single exposure. Weaker allergens may require multiple exposures over several years to trigger an allergy. Once you develop an allergy to a substance, even a small amount of it can cause a reaction.

Common allergens include:

• Nickel, which is used in jewelry, buckles and many other items
• Medications, such as antibiotic creams and oral antihistamines
• Balsam of Peru, which is used in many products, such as perfumes, cosmetics, mouth rinses and flavorings
• Formaldehyde, which is in preservatives, disinfectants and clothing
• Personal care products, such as deodorants, body washes, hair dyes, cosmetics and nail polish
• Plants such as poison ivy and mango, which contain a highly allergenic substance called urushiol
• Airborne substances, such as ragweed pollen and spray insecticides
• Products that cause a reaction when you're in the sun (photoallergic contact dermatitis), such as some sunscreens and oral medications

Children develop the condition from the usual offenders and also from exposure to diapers, baby wipes, sunscreens, clothing with snaps or dyes, and so on.