General Dermatology

Poison Oak & Ivy

Poison Oak Treatment | Poison Ivy Treatment | Palo Alto CA | Redwood City CAPoison Oak and Ivy are the single most common cause of allergic reactions in the United States and will affect ten to 50 million Americans every year. These plants grow practically everywhere in the United States, except Hawaii, Alaska and some desert areas of Nevada. Poison oak grows in the West and poison sumac and ivy in the South.

Poison oak rash is really an allergic contact dermatitis caused by a substance called urushiol, (you-ROO-shee-ol), found in the sap of poison ivy, poison oak and poison sumac. Urushiol is a colorless or slightly yellow oil that oozes from any cut, or crushed part of the plant, including the stem and the leaves. You may develop a rash without ever coming into contact with poison oak, because the urushiol is so easily spread. Sticky, and virtually invisible, it can be carried on the fur of animals, on garden tools, or sports equipment, or on any objects that have come into contact with a crushed or broken plant. After exposure to air, urushiol turns brownish-black, making it easier to spot. It can be neutralized to an inactive state by water.

Once it touches the skin, the urushiol begins to penetrate in a matter of minutes. In those who are sensitive, a reaction will appear in the form of a line or streak of rash (sometimes resembling insect bites) within 12-48 hours. Redness and swelling will be followed by blisters and severe itching. In a few days, the blisters become crusted and begin to scale. The rash will usually take about ten days to heal, sometimes leaving small spots, especially noticeable in dark skin. The rash can affect almost any part of the body, especially areas where the skin is thin; the soles of the feet and palms of the hands are thicker and less susceptible.

What causes sensitivity?

Sensitivity to poison oak is not something we are born with. It develops only after several encounters with the plants, and sometimes over many years. Studies have shown that approximately 85 percent of the population will develop an allergic reaction if exposed to poison oak. This sensitivity varies from person to person. Although they are not sure why, scientists believe that an individual's sensitivity to poison oak changes with time and tends to decline with age. The first bout of poison oak usually occurs in children between the ages of 8 and 16, and can be quite severe. If there is no repeated exposure to poison oak, or urushiol, sensitivity will probably decrease by half by the time these individuals reach their thirties.

Investigators have found that people who reach adulthood without becoming sensitized have only a 50 percent chance of developing an allergy to poison oak. Those who were once allergic may lose their sensitivity later in life. However, dermatologists say you should not assume that you are one of the few people who are not sensitive-- only 10 to 15 percent of the population is believed to be resistant. That same percentage (25-40 million people) is thought to be very susceptible to poison oak. These people will develop a rash and extreme swelling on the face, arms and genitals. In such severe cases, treatment by a dermatologist will be required.

Identifying the Plant

Identifying the plant is the first step toward avoiding poison oak. The popular saying "leaves of three, let them be," is a good rule of thumb, but it's only partially correct. Poison oak or poison ivy will take on a different appearance depending on the environment. The leaves may vary from groups of three, to groups of five, seven, or even nine. Poison oak is found in the West and Southwest, poison ivy usually grows east of the Rockies, and poison sumac east of the Mississippi River. The plants grow near streams and lakes, and wherever there are warm, humid summers. Poison ivy grows as a low shrub, vine or climbing vine. It has yellow green flowers and white berries. Poison oak is a low shrub or small tree with clusters of yellow berries and the oak-like leaves. Poison sumac grows to a tall, rangy shrub producing 7-13 smooth edged leaves, and cream-colored berries.

These weeds are most dangerous in the spring and summer. That's when there is plenty of sap and urushiol content is high, and the plants are easily bruised. Although poison oak is usually a summer complaint, cases are sometimes reported in winter, when the sticks may be used for firewood, and the vines for Christmas wreaths. The best way to avoid these toxic plants is to know what what they look like in your area and where you work, and to learn to recognize them in all seasons.


Prevention is the best cure. The best way to avoid the misery of poison ivy is to be on the look-out for the plant whenever you are outdoors. Know what you are looking for, and stay away from it. The weeds can be destroyed with herbicides in your own backyard, but this is not a practical solution for forest preserves and other natural areas. If you are going to be in areas where you know poison oak or ivy is likely to grow, wear long pants and long sleeves, and, whenever possible, gloves and boots. Remember that the plant's virtually invisible, oily resin-- urushiol--sticks to almost all surfaces, and can even be carried in the wind if it is burned in a fire. Studies have shown that a sensitive person may develop an internal inflammation from inhaling urushiol. In addition, don't let pets run through wooded areas since urushiol may be carried home on their fur.

Barrier creams offer little hope against poison oak and ivy, although new products may offer some protection. These may soon be marketed throughout the United States. Dermatologists are also working on a skin treatment to prevent urushiol from penetrating the skin. Ask your physician about new treatments available that might help to protect you.


If you think you have had a brush with poison ivy, poison oak, or poison sumac, follow this simple procedure:

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 will neutralize or deactivate the urushiol in the plant's sap and keep it from spreading to other parts of the body. Soap is not necessary, and may even spread the oil.

When you return home, wash all clothing outside, with a garden hose, before bringing it into the house, where resin could be transferred to rugs or to furniture. Handle the clothing as little as possible until it is soaked. Since urushiol can remain active for months, it's important to wash all camping, sporting, fishing or hunting gear that may also be carrying the resin.

If you do develop a rash, avoid scratching the blisters. Although the fluid in the blisters will not spread the rash, fingernails may carry germs that could cause an infection.

Cool showers will help ease the itching and over-the-counter- preparations, like calamine lotion or Burow's solution, will relieve mild rashes. Soaking in a lukewarm bath with an oatmeal or baking soda solution is often recommended to dry oozing blisters and offer some comfort. Over-the-counter hydrocortisone creams will not help. Dermatologists say they aren't strong enough to have any effect on poison ivy rashes.

In severe cases, prescription corticosteroid drugs can halt the reaction if taken soon enough. If you know you have been exposed and have developed severe reactions in the past, be sure to consult your dermatologist. He or she may prescribe steroids, or other medications, which can prevent blisters from forming.


Investigators have found that most people could be immunized against poison ivy through prescription pills. These pills contain gradually increasing amounts of active extract from the plants. However, this procedure can take four months to achieve a reasonable degree of hyposensitization.

In addition, the medication must be continued over a long period of time and it can often cause uncomfortable side effects. This procedure is recommended only if the doses are given before contact with the plant, and only for individuals, such as firefighters, who must live or work in areas where they come into constant contact with poison ivy. Consult your dermatologist for his or her advice on whether you should consider immunization.

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