Allergic contact rash (dermatitis) is caused by contact with oil called urushiol which is found in sap. It is a colorless or pale yellow oil that oozes from any cut or crushed part of the plant, including the roots, stems and leaves. After exposure to air, urushiol turns brownish-black, making it easier to spot. Contact with urushiol can occur in three ways:
Direct contact – touching the sap of the toxic plant.
Indirect contact – touching something to which urushiol has spread. The oil can stick to the fur of animals, garden tools, sports equipment, or any object that has come into contact with a crushed or broken plant.
Airborne urushiol particles, such as from burning plants, that may come in contact with your skin.
Once urushiol touches the skin, it begins to penetrate in minutes. In those who are sensitive, a reaction appears as a line or streak of rash, usually within 12 to 48 hours. Redness and swelling occur, often followed by blisters and severe itching. In a few days, the blisters may become crusted and begin to scale. The rash takes 10 days or longer to heal.
The rash can affect almost any part of your body, especially where your skin is thin, such as on your face. A rash develops less often on the soles of your feet and palms of your hands, where the skin is thicker. The rash does not spread, although it may seem to when it breaks out in new areas. This may happen because urushiol absorbs more slowly in to skin that is thicker, such as on your forearms, legs, and trunk.
The dermatologist may order blood tests, scrape the skin or take a sample from one of the spots (skin biopsy) and examine it under a microscope to make the diagnosis.