Bruce P.Robinson, MD

Poison Ivy, Poison Oak, & Poison Sumac

Poison ivy, poison oak, and poison sumac are the most common cause of allergic reactions in the United States. Each year 10 to 50 million Americans develop an allergic rash after contact with these poison plants.

Poison ivy, poison oak, and poison sumac grow almost everywhere in the United States, except Hawaii, Alaska, and some desert areas in the Western U.S. poison ivy usually grows east of the Rocky Mountains and in Canada. Poison oak grows in the Western United States, Canada, Mexico (western poison oak), and in the Southeastern states (eastern poison oak). poison sumac grows in the Eastern states and southern Canada.

Poison Oak

In the West, this plant may grow as a vine but usually is a shrub. In the East, it grows as a shrub. It has three leaflets to form its leaves.

Poison Ivy

Grows as a vine in the East, Midwest and South. In the far Northern and Western United States, Canada and around the Great Lakes, it grows as a shrub. Each leaf has three leaflets.

Poison Sumac

Grows in standing water in peat bogs in the Northeast and Midwest and in swampy areas in parts of the Southeast. Each leaf has seven to 13 leaflets.

A Poison Plant Rash

Poison Plant rash is an allergic contact dermatitis caused by contact with oil called urushiol. Urushiol is found in the sap of poison plants like Poison Ivy, Poison Oak, and Poison Sumac. It is colorless or pale yellow oil that oozes from any cut or crushed part of the plant, including the roots, stems, and leaves. After exposure to air, urushiol turns brownish-black. Damaged leaves look like they have spots of black enamel paint making it easier to recognize and identify the plant. Contact with urushiol can occur in three ways:

  • Direct contact - touching the sap of the toxic plant.
  • Indirect contact - touching something on which urushiol is present. The oil can stick to the fur of animals, to garden tools or sports equipment, or to any objects that have come into contact with it.
  • Airborne contact - burning poison plants put urushiol particles into the air

When urushiol gets on the skin, it begins to penetrate in minutes. A reaction appears usually within 12 to 48 hours. There is severe itching, redness, and swelling, followed by blisters. The rash is often arranged in streaks or lines where the person brushed against the plant. In a few days, the blisters become crusted and take 10 days or longer to heal.

Poison plant dermatitis can affect almost any part of the body. The rash does not spread by touching it, although it may seem to when it breaks out in new areas. This may happen because urushiol absorbs more slowly into skin that is thicker such as on the forearms, legs, and trunk.

Who is sensitive and who is not?

Sensitivity develops after the first direct skin contact with urushiol oil. An allergic reaction seldom occurs on the first exposure. A second encounter can produce a reaction which may be severe. About 85 percent of all people will develop an allergic reaction when adequately exposed to poison ivy. This sensitivity varies from person to person. People who reach adulthood without becoming sensitive have only a 50 percent chance of developing an allergy to poison ivy. However, only about 15 percent of people seem to be resistant.

Recognizing Poison Plants

Identifying the poison ivy plant is the first step in avoiding the rash. The popular saying "leaves of three, beware of me" is a good rule of thumb for Poison Ivy and Poison Oak but is only partly correct. A more exact saying would be "leaflets of three, beware of me," because each leaf has three leaflets. Poison sumac, however, has a row of paired leaves. The middle or end leaf is on a longer stalk than the other leaves. This differs from most other three-leaf look alikes.

Poison Ivy has different forms. It grows as vines or low shrubs. Poison Oak, with its oak-like leaves, is a low shrub in the East and can be a low or high shrub in the West. Poison sumac is a tall shrub or small tree. The plants also differ in where they grow. Poison Ivy grows in fertile, well-drained soil. Western Poison Oak needs a great deal of water, and Eastern Poison Oak prefers sandy soil but sometimes grows near lakes. Poison Sumac tends to grow in standing water, such as peat bogs.

These plants are common in the spring and summer. When they grow, there is plenty of sap and the plants bruise easily. The leaves may have black marks where they have been injured. Although Poison Ivy rash is usually a summer complaint, cases may occur in winter when people are cleaning their yards and burning wood with urushiol on it, or when cutting Poison Ivy vines to make wreaths.

It is important to recognize these toxic plants in all seasons. In the early fall, the leaves can turn colors such as yellow or red when other plants are still green. The berry-like fruit on the mature female plants also changes color in fall, from green to off-white. In the winter, the plants lose their leaves. In the spring, Poison Ivy has yellow-green flowers.

Prevention of Poison Ivy

Prevent the misery of Poison Ivy by looking out for the plant and staying away from it. You can destroy these plants with herbicides in your own backyard, but this is not practical elsewhere. If you are going to be where you know poison ivy likely grows, wear long pants, long sleeves, boots, and gloves. Remember that the plant's nearly invisible oil, urushiol, sticks to almost all surfaces, and does not dry. Do not let pets run through wooded areas since they may carry home urushiol on their fur. Because urushiol can travel in the wind if it burns in a fire, do not burn plants that look like Poison Ivy.

Barrier skin creams such as a lotion containing bentoquatum offer some protection before contact with Poison Ivy, Poison Oak, or Poison Sumac. Over-the-counter products prevent urushiol from penetrating the skin. Ask your dermatologist for details.


If you think you've had a brush with Poison Ivy, Poison Oak, or Poison Sumac, follow these simple ste

  • Wash all exposed areas with cold running water as soon as you can reach a stream, lake, or garden hose. If you can do this within five minutes, the water may keep the urushiol from contacting your skin and spreading to other parts of your body. Within the first 30 minutes, soap and water are helpful.

Common Myths about Poison Ivy

Scratching Poison Ivy blisters will spread the rash.
False. The fluid in the blisters will not spread the rash. The rash is spread only by urushiol. For instance, if you have urushiol on your hands, scratching your nose or wiping your forehead will cause a rash in those areas even though leaves did not contact the face. Avoid excessive scratching of your blisters. Your fingernails may carry bacteria that could cause an infection.

Poison Ivy rash is "catchy."
. The rash is a reaction to urushiol. The rash cannot pass from person to person; only urushiol can be spread by contact.

Once allergic, always allergic to Poison Ivy.
A person's sensitivity changes over time, even from season to season. People who were sensitive to Poison Ivy as children may not be allergic as adults.

Dead Poison Ivy plants are no longer toxic.
Urushiol remains active for up to several years. Never handle dead plants that look like Poison Ivy.

Rubbing weeds on the skin can help.
Usually, prescription cortisone preparations are required to decrease the itching.

One way to protect against poison ivy is by keeping yourself covered outdoors.
However, urushiol can stick to your clothes, which your hands can touch, and then spread the oil to uncovered parts of your body. For uncovered areas, barrier creams are sometimes helpful. Learn to recognize poison ivy so you can avoid contact with it.

  • Relieve the itching of mild rashes by taking cool showers and applying over-the-counter preparations like Calamine Lotion or Burow's Solution. Soaking in a lukewarm bath with an oatmeal or baking soda solution may also ease itching and dry oozing blisters. Over-the-counter hydrocortisone creams are not strong enough to have much effect on poison ivy rashes. Prescription cortisonecan halt the reaction if used early. If you know you have been exposed and have developed severe reactions in the past, consult your dermatologist. He or she may prescribe cortisone or other medicines that can prevent blisters from forming. If you receive treatment with a cortisone drug, you should take it longer than six days, or the rash may return.
  • Wash your clothing in a washing machine with detergent. If you bring the clothes into your house, be careful that you do not transfer the urushiol to rugs or furniture. You may also dry clean contaminated clothes. Because urushiol can remain active for months, wash camping, sporting, fishing, or hunting gear that was in contact with the oil.
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