Contact dermatitis is skin inflammation caused by direct contact with a particular substance. The rash is very itchy, is confined to a specific area, and often has clearly defined boundaries.
Substances can cause skin inflammation by one of two mechanisms—irritation (irritant contact dermatitis) or allergic reaction (allergic contact dermatitis).
Irritant contact dermatitis, which accounts for 80% of all cases of contact dermatitis, occurs when a chemical substance causes direct damage to the skin; symptoms are more painful than itchy. Typical irritating substances are acids, alkalis (such as drain cleaners), solvents (such as acetone in nail polish remover), strong soaps, and plants (such as poinsettias and peppers). Some of these chemicals cause skin changes within a few minutes, whereas others require longer exposure. People vary in the sensitivity of their skin to irritants. Even very mild soaps and detergents may irritate the skin of some people after frequent or prolonged contact.
Allergic contact dermatitis is a reaction by the body's immune system to a substance contacting the skin. Sometimes a person can be sensitized by only one exposure, and other times sensitization occurs only after many exposures to a substance. After a person is sensitized, the next exposure causes itching and dermatitis within 4 to 24 hours, although some people, particularly older people, do not develop a reaction for 3 to 4 days.
Thousands of substances can result in allergic contact dermatitis. The most common include substances found in plants such as poison ivy, rubber (latex), antibiotics, fragrances, preservatives, and some metals (such as nickel and cobalt). About 10% of women are allergic to nickel, a common component of jewelry. People may use (or be exposed to) substances for years without a problem, then suddenly develop an allergic reaction. Even ointments, creams, and lotions used to treat dermatitis can cause such a reaction. People may also develop dermatitis from many of the materials they touch while at work (occupational dermatitis).
Sometimes contact dermatitis results only after a person touches certain substances and then exposes the skin to sunlight (photoallergic or phototoxic contact dermatitis). Such substances include sunscreens, aftershave lotions, certain perfumes, antibiotics, coal tar, and oils.
Common Causes of Allergic Contact Dermatitis