Contact dermatitis is a broad term referring to an inflammatory condition of the skin caused by direct contact with an external substance. Irritant contact dermatitis is the most common type of contact dermatitis and involves inflammation resulting from contact with acids, alkaline materials (such as soaps and detergents), solvents, or other chemicals. People who wash their hands repeatedly or wear occlusive gloves for long periods of time may develop intense irritation of the hands.
In cases of irritant contact dermatitis, the skin appears dry, red, and chapped. Allergic contact dermatitis is less common and is caused by exposure to a substance that the person has previously become allergic to, such as poison oak or poison ivy (Table 10). As in other types of allergic dis-eases, repeated exposure to the substance over time is required before allergic contact dermatitis can develop.
Following reexposure to the substance, allergic contact reactions usually begin after 12 to 24 hours and reach a peak between 48 and 72 hours after exposure. The skin inflammation in allergic contact dermatitis varies from mild redness and itching to weeping blisters, dependin on the nature of the substance, the body part affected, and the individual’s level of sensitivity. In addition to plants, the most common causes of allergic contact dermatitis are metals (e.g., nickel), medications (e.g., antibiotics), rubber, and cosmetics. Some products cause a reaction only when they contact the skin and are exposed to sunlight (i.e., photocontact dermatitis).
These include shaving lotions, sunscreens, sulfa ointments, some perfumes, and coal tar products. The diagnosis of both irritant and allergic contact dermatitis is primarily based on the history of exposure to an irritant or an allergen and the appearance of the skin. For confirmation purposes, particularly when multiple different substances are suspected, diagnostic patch testing is very useful. The test is performed by applying a series of materials in small patches to the back.
The patches are available in a commercial kit or are brought to the doctor’s office by the patient. They are removed and the skin is assessed after 48 hours and again at 72 hours, preferably by a clinician with experience in these procedures and interpretation of the results.