Poison ivy

Poison ivy is a type of allergenic plant in the genus Toxicodendron native to Asia and North America. Formerly considered a single species, Toxicodendron radicans, poison ivies are now generally treated as a complex of three separate species: T. radicans, T. rydbergii, and T. orientale. They are variable in appearance and habit, and despite its common name, it is not a "true" ivy (Hedera), but rather a member of the cashew and pistachio family (Anacardiaceae).

Though T. radicans is commonly eaten by many animals and the seeds are consumed by birds, poison ivy is most often thought of as an unwelcome weed. It is well known for causing urushiol-induced contact dermatitis, an itchy, irritating, and sometimes painful rash, in most people who touch them. The rash is caused by urushiol, a clear liquid compound in the plant's sap.

Poison ivies can grow as small plants, shrubs, or climbing vines. They are commonly characterized by clusters of leaves, each containing three leaflets, hence the common expression "leaves of three, let it be". These leaves can vary between an elliptic to egg shape and will have either smooth, lobed, or toothed margins. Additionally, the leaf clusters are alternate on the stem. Clusters of small, greenish flowers bloom from May to July and produce white berries in the fall a few millimeters in diameter.

Three species of poison ivy are generally recognised; they are sometimes considered subspecies of Toxicodendron radicans:

Urushiol-induced contact dermatitis is the allergic reaction caused by poison ivy. In extreme cases, a reaction can progress to anaphylaxis. Around 15 to 25 percent of people have no allergic reaction to urushiol, but most people have a greater reaction with repeated or more concentrated exposure.

Over 350,000 people are affected by urushiol annually in the United States.

The oozing fluids released by scratching blisters do not spread the poison. The fluid in the blisters is produced by the body and it is not urushiol itself. The appearance of a spreading rash indicates that some areas received more of the poison and reacted sooner than other areas or that contamination is still occurring from contact with objects to which the original poison was spread.^{[citation needed]}

Those affected can unknowingly spread the urushiol inside the house, on phones, door knobs, couches, counters, desks, and so on, thus in fact repeatedly coming into contact with poison ivy and extending the length of time of the rash. If this happens, the surfaces should be wiped with bleach or a commercial urushiol removal agent. The blisters and oozing result from blood vessels that develop gaps and leak fluid through the skin; if the skin is cooled, the vessels constrict and leak less. If plant material with urushiol is burned and the smoke then inhaled, this rash will appear on the lining of the lungs, causing extreme pain and possibly fatal respiratory difficulty. If poison ivy is eaten, the mucus lining of the mouth and digestive tract can be damaged.