Contact dermatitis is a localized rash or irritation of the skin caused by contact with a foreign substance. Only the superficial regions of the skin are affected in contact dermatitis. Inflammation of the affected tissue is present in the epidermis (the outermost layer of skin) and the outer dermis (the layer beneath the epidermis). Unlike contact urticaria, in which a rash appears within minutes of exposure and fades away within minutes to hours, contact dermatitis takes days to fade away. Even then, contact dermatitis fades only if the skin no longer comes in contact with the allergen or irritant. Contact dermatitis results in large, burning, and itchy rashes, and these can take anywhere from several days to weeks to heal. Chronic contact dermatitis can develop when the removal of the offending agent no longer provides expected relief.
Toxicodendron diversilobum (syn. Rhus diversiloba; Western Poison-oak or Pacific Poison-oak) is a plant best known for its ability to cause allergic rashes after contact. Western Poison-oak is found only on the Pacific Coast of the United States and of Canada. It is extremely common in that region, where it is the predominant species of the genus; the closely related Atlantic Poison-oak (T. pubescens) occurs on the Atlantic Coast. The hyphenated form “Poison-oak” is used, rather than “Poison Oak” to clearly indicate that it is not a variety of oak, just as “Poison-ivy” is not a variety of ivy.