(See also Overview of Dermatitis.)
Substances can cause skin inflammation by one of two mechanisms:
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.
Some of these substances cause skin changes within a few minutes, whereas others require longer exposure. Even very mild soaps and detergents may irritate the skin of some people after frequent or prolonged contact. People vary in the sensitivity of their skin to irritants. The person's age (very young or very old) and environment (low humidity or high temperature) are other factors that influence whether irritant contact dermatitis develops.
This type of dermatitis is a reaction by the body's immune system to a substance contacting the skin. When the skin first comes into contact with the substance, the skin becomes sensitized to that substance. 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
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—see Chemical photosensitivity). Such substances include
With photoallergic dermatitis, the reaction may spread to areas of skin that were not exposed to the sun.
Regardless of cause or type, contact dermatitis results in itching and a rash.
Allergic contact dermatitis usually causes severe itching.
Irritant contact dermatitis causes more pain than itching.
For both, 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.
The rash often occurs in a pattern that suggests exposure to a specific substance. For instance, poison ivy causes line-like streaks to form on the skin. 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, travel, clothing, use of topical drugs (drugs applied to the skin), cosmetics, and household members' activities must be considered. Most people are unaware of all the substances that touch their skin. Often, the location and pattern of the initial rash provide an important clue, particularly if the rash 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 substances that cause dermatitis (causative substance). If contact does occur, the substance should be washed off immediately with soap and water. If people are at risk of 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 that contain the causative substance is not effective in preventing contact dermatitis.
Treatment of contact dermatitis 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. 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. 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.