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:
This type of dermatitis, which accounts for 80% of all cases of contact dermatitis, occurs when a chemical substance causes direct damage to the skin. Irritant contact dermatitis is 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). Skin can also be irritated by body fluids (such as urine and saliva). 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:
This type of 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 intense 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, certain antibiotics applied to the skin, coal tar, and oils.
Regardless of cause or type, contact dermatitis results in itching and a rash. The itching is usually severe when allergic, but the rash varies from a mild, short-lived redness to severe swelling and large blisters. Most commonly, the rash contains tiny blisters. The rash develops only in areas contacted by the substance. However, the rash appears earlier in thin, sensitive areas of skin, and later in areas of thicker skin or on skin that had less contact with the substance, giving the impression that the rash has spread. Touching the rash or blister fluid cannot spread contact dermatitis to other people or to other parts of the body that did not make contact with the substance.
Determining the cause of contact dermatitis is not always easy. The person's occupation, hobbies, household duties, vacations, clothing, topical drug use, cosmetics, and household members' activities must be considered. Most people are unaware of all the substances that touch their skin. Often, the location of the initial rash is an important clue, particularly if it occurs under an item of clothing or jewelry or only in areas exposed to sunlight. However, many substances that people touch with their hands are unknowingly transferred to the face, where the more sensitive facial skin may react even if the hands do not.
The use test, in which a suspected substance is applied far from the original area of contact dermatitis (usually on the forearm), is useful when perfumes, shampoos, or other substances used in the home are suspected.
If a doctor suspects contact dermatitis and a process of elimination does not pinpoint the cause, patch testing can be done. For this test, small patches containing substances that commonly cause dermatitis are placed on the skin for 1 to 2 days to see whether a rash develops beneath one of them. Although useful, patch testing is complicated. People may be sensitive to many substances, and the substance they react to on a patch may not be the cause of their dermatitis. A doctor must decide which substances to test based on what a person might have been exposed to.
Contact dermatitis can be prevented by avoiding contact with the causative substance. If contact does occur, the material should be washed off immediately with soap and water. If circumstances risk ongoing exposure, gloves and protective clothing may be helpful. Barrier creams are also available that can block certain substances, such as poison ivy and epoxy resins, from contacting the skin. Desensitization with injections or tablets of the causative substance is not effective in preventing contact dermatitis.
Treatment is not effective until there is no further contact with the substance causing the problem. Once the substance is removed, the redness usually disappears after a week. Blisters may continue to ooze and form crusts, but they soon dry. Residual scaling, itching, and temporary thickening of the skin may last for days or weeks.
Itching and blisters can be relieved with a number of drugs applied to the skin or taken by mouth (see see Treatment). In addition, small areas of dermatitis can be soothed by applying pieces of gauze or thin cloth dipped in cool water or aluminum acetate (Burow solution) several times a day for an hour. Larger areas may be treated with short, cool tub baths with or without colloidal oatmeal.
Often a corticosteroid is applied to the affected skin. Over-the-counter hydrocortisone may help. If not, doctors prescribe a corticosteroid cream. If the rash is particularly severe, a corticosteroid can be taken by mouth. If itching is severe, hydroxyzine or diphenhydramine (both antihistamines) taken by mouth can be tried.
Last full review/revision October 2012 by Karen McKoy, MD, MPH