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 three to five 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?

All three plants contain the same chemical, urushiol, and cause the same reaction, which generally occurs 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.
How Common Are Poison Plant Allergies?

Poison ivy, poison oak, and poison sumac are three of the most common causes of allergic contact dermatitis in North America. Some experts estimate that three out of four people are sensitive to the chemical found in these plants, although the degree of sensitivity varies. Some people are very sensitive and will have a quick reaction upon contact with a small amount of urushiol. For those who are less sensitive, exposure to a large amount of urushiol is necessary before a reaction develops. Cases of poison plant allergy occur most frequently during the spring, summer, and early fall when people spend more time outdoors.

How Are Reactions to Poison Plants Diagnosed?

An allergic reaction to a poison plant is diagnosed based on the typical pattern of symptoms and the appearance of the rash.

How Are Reactions to Poison Plants Treated?

An allergic reaction to a poison plant cannot be cured, but the symptoms can be treated. You may take cool showers and apply an over-the-counter lotion -- such as calamine lotion -- to help relieve the itch. If your reaction is more severe or involves mucus membranes (membranes found in the eyes, nose, mouth, and genitals), you may need a prescription medication, such as prednisone, to help control the reaction.

How Long Does a Poison Plant Rash Last?

Most rashes caused by poison ivy, poison oak, or poison sumac are mild and last from 5 to 12 days. In severe cases, the rash can last for 30 days or longer.

Does Immunotherapy Help With Poison Plant Allergies?

Immunotherapy is not available for allergies to poison ivy, poison oak, and poison sumac.

How Can Poison Plant Reactions Be Prevented?

You can take steps to prevent exposure.

Learn to identify poison ivy, poison oak, and poison sumac, and avoid contact with them.
Remove these plants from around your home, especially in areas where you may be working or playing.
When walking in the woods or working in areas where these plants may grow, cover your skin as much as possible by wearing long pants, long-sleeves, shoes, and socks.
Do not let pets run in wooded areas where they may be exposed to the poison plants. They can carry urushiol back home on their fur.
Is a Poison Plant Rash Contagious?

Many people think a poison plant rash can be spread from one part of the body to another or from person to person. In general, this is not true. You can spread the rash only if you have urushiol on your hands. Also, it can take longer for the rash to appear on certain areas of the body, especially areas such as the soles of the feet where the skin is thicker. This may give the appearance that the rash has spread from one part of the body to another. You can also be re-exposed to the urushiol by touching gardening tools, sports equipment, or other items that were not cleaned after being in contact with the plants. Scratching or touching the rash and fluid from blisters will not cause the rash to spread because urushiol is not present in the blister fluid.

What Should I Do if I am Exposed to a Poison Plant?

If you think you may have been exposed to a poison plant:

Remove your clothes.
Wash all exposed areas with cool running water. Use soap and water if you can. Be sure to clean under your fingernails. In the woods, the water of a running stream can be an effective cleanser.
Wash your clothing and all gardening tools, camping gear, sports equipment, and other objects that came into contact with the plants.
Bathe pets exposed to the plants.
When Should I Call the Doctor?

If any of the following occurs, seek immediate medical attention:

You have symptoms of a severe reaction, such as severe swelling and/or difficulty breathing.
You have been exposed to the smoke of burning poison ivy, poison oak, or poison sumac.
The rash covers more than one quarter of your body.
The rash occurs on your face, lips, eyes, or genitals.
The initial treatment does not relieve the symptoms.
You develop a fever and/or the rash shows signs of infection, such as increased tenderness, pus or yellow fluid oozing from the blisters, and an odor coming from the blisters.

Reviewed by the doctors at The Cleveland Clinic Department of Pulmonary, Allergy and Critical Care Medicine.

Edited by Charlotte E. Grayson, MD, Feb. 2004, WebMD.

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