Contact dermatitis is an irritation of the skin caused by contact with certain items. It affects twice as many women as men, and common causes include plants, metal, rubber, latex, cosmetics, fragrances, fabrics, solvents, cleaning agents, soaps, shampoos, foods, preservatives, food coloring, and adhesives.

Irritant contact dermatitis, a common version, is generally caused by contact with chemicals, acids, industrial solvents, and cleaning products. For some people, however, everyday soaps, detergents, and lotions can be problems. Handling foods such as chopped chili peppers or garlic can also be irritating.

The length of contact time with the skin affects the extent of skin damage. Mild irritants such as detergents often require multiple exposures before symptoms appear, but strong irritants such as drain cleaners can produce symptoms immediately. Hands are affected most often, but irritant contact dermatitis on the face, neck, arms, and legs is also common. For example, "trouser dermatitis" on the thighs and knees is usually linked to the formaldehyde used in dry cleaning.

Allergic contact dermatitis, another subtype, is an allergic reaction. To develop the allergy, the person must encounter a substance once, which sets off a series of events called sensitization. During the initial contact, the skin absorbs some of the substance (called the allergen). The body recognizes it as foreign and mounts an attack, producing antibodies. The first contact does not result in a reaction. The body's immune cells "remember" this invader, and, when contact happens again, create an allergic reaction. For example, up to 70%of people are allergic to urushiol, an oil found in poison ivy, oak, and sumac. Although most people remember their first itchy rash after touching the plant, few can remember their first contact, which sensitized them but did not result in a rash.

For some substances, sensitization may require more than 1 exposure, but once it occurs, a reaction appears within 4 to 24 hours of reexposure. In some people, especially the elderly, reaction may take up to 4 days. Once sensitized to 1 substance, some people develop allergies to different but chemically similar substances. This is called cross sensitization. For example, people who are allergic to poison ivy may also become allergic to Florida holly, mango rind, or cashew nut resin. Likewise, people who are allergic to fragrances may become allergic to cinnamon, vanilla, and tomatoes.

Symptoms

Irritant and allergic contact dermatitis often produce similar symptoms, listed in Table 1. There are, however, some differences. Allergic contact dermatitis is always confined to the contact area, but skin damage from irritants may be more widespread. Itchy skin and hives are mostly seen in allergic reactions, whereas pain, redness, and blisters are usually associated with irritant contact dermatitis. Not everyone has all symptoms, and neither condition is contagious.

Diagnosis

It is not always easy to know the difference between irritant dermatitis and allergic dermatitis. Identifying the irritating substance can be difficult, especially with commercial products, because labels are often vague. For example, makeup sponges contain rubber, and mascara contains pine resin. Up to 10% of women are allergic to nickel, but nickel lurks in nonobvious sources (Table 2).

If your contact dermatitis is frequent or long-lasting, see an allergist. If you suspect a particular substance, bring a sample with you. Patch testing may be required. Your doctor's initial evaluation will determine which of the 3700 known agents should be selected for testing. Patch testing involves 3 visits. Small adhesive patches containing different allergens and other substances are placed on your back. You will be asked to return 2 days later, and again in another 2 or 3 days. Your doctor will examine your back to see if you reacted to anything and will tell you about treatment options.

Treatment

Wash immediately and thoroughly when you come into contact with known allergens or irritants. This minimizes exposure. Avoid scratching or rubbing; this can thicken or harden the skin. For mild reactions, lukewarm oatmeal baths and calamine lotion help relieve itching. Always pat the skin dry, never rub. For blisters, cold moist compresses applied 3 times a day for 30 minutes are recommended.

Your pharmacist can help you select an appropriate over-the-counter antihistamine or topical corticosteroid cream for more serious allergic reactions. These products do not require a prescription, but each can cause side effects. Follow the instructions carefully?long-term use of topical corticosteroid creams on the face can thin the skin and raise blood vessels, and overuse of topical creams can cause other skin problems.

Call your doctor for severe reactions. If your doctor prescribes oral medication, take it for the prescribed number of days, even if your symptoms disappear; otherwise, they may come back.

Prevention

Contact dermatitis symptoms can be treated, but not cured. After that, avoiding irritants or allergens is critical. Wear protective gloves and clothing when handling known irritants. You can also use protective cream barriers such as petroleum jelly. Because dry skin, especially on the hands, is more susceptible to irritants, it is important to use mild soaps and moisturizing creams. Along with avoidance, knowing what to do when a reaction occurs can promote faster healing.

Dr. Zanni is a psychologist and health-systems consultant based in Alexandria,Va.