Poison ivy, poison oak, and poison sumac are plants that contain an irritating, oily sap called urushiol. Urushiol triggers an allergic reaction when it comes into contact with skin, resulting in an itchy rash, which can appear within hours of exposure or up to several days later. A person can be exposed to urushiol directly or by touching objects -- such as gardening tools, camping equipment, and even a pet's fur -- that have come into contact with the sap of one of the poison plants.

Urushiol is found in all parts of these plants, including the leaves, stems, and roots, and is even present after the plant has died. Urushiol is absorbed quickly into the skin. It can also be inhaled if the poison plants are burned. The smoke may expose not only the skin to the chemical but also the nasal passages, throat, and lungs. Inhaled urushiol can cause a very serious allergic reaction.

The rash that results from the poison plants is a form of allergic contact dermatitis. (Dermatitis is swelling and irritation of the skin.) Skin is not automatically sensitive to urushiol. Sensitivity builds up after the skin is exposed to the substance. When initially exposed to urushiol, the skin alerts the immune system of the presence of the irritating chemical. (Usually, no visible reaction will occur the first time a person comes in contact with a poison plant.) The immune system then prepares a defensive reaction for the next time the skin encounters the substance. This sensitizes the skin so that new contact with urushiol causes an allergic reaction.

Poison ivy, poison oak, and poison sumac can be found in most areas of the U.S., except Alaska, Hawaii, and the deserts of the Southwest. In some areas of the country (East, Midwest, and South), poison ivy grows as a vine. In the northern and western U.S., and around the Great Lakes, it grows as a shrub. Each poison ivy leaf has three leaflets.

Poison oak closely resembles poison ivy, although it is usually more shrub-like, and its leaves are shaped somewhat like oak leaves. The undersides of the leaves are always a much lighter green than the surface and are covered with hair. Poison oak is more common in the western U.S.

Poison sumac grows as a woody shrub, with each stem containing 7 to 13 leaves arranged in pairs. Poison sumac can be distinguished from harmless sumac by its drooping clusters of green berries. Harmless sumac has red, upright berry clusters. Poison sumac is more common in wet, swampy areas.

What Are the Symptoms of a Poison Plant Reaction?

The symptoms of a poison plant reaction are similar since they all contain the same chemical, urushiol. Symptoms generally occur in the following phases:

Redness and itching of the skin.
A rash erupts on the skin, often in a pattern of streaks or patches from where the plant has come into contact with the skin.
The rash develops into red bumps, called papules, or large, oozing blisters.