Poison Ivy, Oak, and Sumac

  • Basics

    Poison ivy, poison oak, and poison sumac are plants that produce an allergic skin reaction (contact dermatitis) in some people who come into contact with them. Poison ivy, poison oak, and poison sumac are the most common causes of allergic reactions in this country. Every year, millions of Americans develop the characteristic red, itchy, blistering rash after being exposed to any one of the varieties of these poisonous plants belonging to the genus Rhus. Most of the time, poison ivy, poison oak, and poison sumac are more bothersome than serious. Occasionally, however, the itching is so severe that it disrupts sleep, or the swelling (particularly when outbreaks occur on the face) is so intense that it is disfiguring.

    Poison ivy grows as a vine that wraps around trees or snakes along the ground; it also grows as a shrub. Poison oak takes the shape of a low shrub in the eastern US and a high shrub in the western US. Poison sumac typically grows as tall as a shrub or tree. Most poisonous plants have three leaflets per leaf. Poison sumac, however, can have as many as 13 leaflets per leaf.

    • Poison Ivy (Toxicodendron radicans) grows as a stand-alone bush or as a vine. Each leaf has three leaflets: a bigger one in the center, flanked by two smaller ones. In the fall, the leaves turn a dull red color.
    • Rydberg’s Poison Ivy (Toxicodendron rydbergii) grows as a bush that is about three feet high. Most plants have the classic three-leaflet formation.
    • Eastern Poison Oak (Toxicodendron toxicarium) grows as a bush in sandy soil and in areas that have been burnt by a fire. This variety also has three leaflets.
    • Western Poison Oak (Toxicodendron diversilobum) grows as a thick bush or vine at most elevations — from sea level to 5,000 feet. It too has three leaflets.
    • Poison Sumac (Toxicodendron vernix) grows in standing water in peat bogs or swamps. Each leaf has as many as 13 leaflets.

  • Causes

    Urushiol — an oil in the sap of poison ivy, poison oak, and poison sumac — is what causes the allergic reaction. The allergy can be triggered directly by touching the plant or indirectly by touching something (a pet, a piece of clothing, a garden tool) that has touched the plant. Poison ivy, poison oak, and poison sumac can also be picked up from the air if the plants are being burned. The rash itself does not spread, nor can you catch it from another person’s rash.

    The offending substance that causes the rash is urushiol, a clear or slightly yellow oil in the sap of the plant genus Rhus, to which poison ivy, poison oak, and poison sumac belong. Similar substances occur in several other plants and trees, including the Japanese lacquer tree, the cashew nut tree (nutshell), the mango (rind, leaves, sap), and the fruit or pulp of the female ginkgo. Just one billionth of a gram is all that is needed to cause relentless itching in some people. People are not born with a sensitivity to poisonous plants, but can develop one after the first exposure (which usually does not cause a rash). As many as 85% of the population will get a rash after the second brush with a poisonous plant.

    In order to catch poison ivy, poison oak, or poison sumac, you need to touch the oil that has oozed from a broken plant root, stem, or leaf. Keep in mind that the oil can stay active for up to five years on any unwashed surface, including pets, tools, sports equipment, sneakers, shoelaces, rugs — even dead plants — and so on.

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