Poison Ivy & Other Poisonous Plants

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Questions to Ask Your Doctor about Allergies
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Overview

Poison ivy (Rhus toxicodendron or Toxicodendron radicans) is a climbing plant that contains an oily substance called urushiol. Contact with this substance can cause a severe, itchy skin rash (allergic contact dermatitis). Other plants that are related to poison ivy and also can cause an allergic reaction include poison oak (Rhus radicans, Rhus diversiloba, or Toxicodendron diversiloba) and poison sumac (Toxicodendron vernix). These plants are native to North America.

Poison ivy grows as a vine in some parts of the United States (e.g., East, South) and as a shrub in other areas (e.g., West, North). Poison ivy branches each contain three leaves, and each leaf contains three leaflets. The center leaf is on a stalk that is longer than the other two. In the spring, poison ivy leaves usually are shiny (glossy) and red in color. In the summer, newer leaves may be shiny and reddish, and older leaves are dull and green. In the autumn, poison ivy leaves often turn orange, yellow, red, or brown.

Poison oak usually grows as a shrub, but it may also grow as a vine. This plant also has three leaves, each with three leaflets, and its leaves resemble oak leaves. Poison sumac grows as a tall shrub, usually in very wet soil. The leaves of this plant are paired, and each leaf contains seven to 13 leaflets. The center leaflet is on a longer stalk than the others. Poison sumac has yellow-green flowers and clusters of white-green fruits.

Incidence and Prevalence
Exposure to poison ivy, poison oak, and poison sumac causes a reaction in about 60–85% of people. In many cases, repeated exposure produces increasingly severe reactions. Approximately 15% of people are resistant to poisonous plants (i.e., do not develop an allergic reaction following exposure).

Sensitivity to poisonous plants often decreases with age. People who experience severe reactions as children may outgrow this sensitivity, even without repeated exposure.

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