Poison Ivy Allergies
Reaction and how exposure occurs
Part one of three

Allergic reactions to three native American plants -- poison ivy, poison oak, and poison sumac, all members of the plant genus Toxicodendron -- have been sources of misery for many centuries. Native Americans warned the early settlers about the ill effects of these plants, and Captain John Smith described them in his journal, thus making the first report of an allergic disease in America. According to experts at the American Academy of Dermatology, approximately 85 percent of the population will develop an allergic reaction to poison ivy, oak, or sumac if exposed to them.

What is an allergic reaction?
In general, an allergic reaction is a sensitivity or overreaction by the body to a substance that does not cause a problem in most people. The substances, known as allergens, include mold spore, dust, food, or pollen. The reaction may take many forms; for example, the sneezing and runny nose of hay fever (pollen allergy) or the itching and skin rash of hives. When these allergic reactions develop within minutes of exposure to an allergen, they are called immediate hypersensitivity reactions.

What are poison ivy and related allergies?
The allergic response to poison ivy, oak, and sumac is not immediate, but occurs 12 to 48 hours after exposure. This type of response is known as delayed hypersensitivity. In susceptible persons, contact with these plants leads to a skin rash known as allergic contact dermatitis. (Dermatitis means inflammation of the skin). Such a rash also may be caused by other plants as well as by things such as cosmetics, medicines applied to the skin, synthetic fabrics, latex, and metals such as nickel, which is often found in jewelry.

A person who is sensitive to poison ivy, oak, or sumac also may react to other plants in the same family (Anacardiaceae or cashew family). This family includes several trees-cashew, mango, Japanese lacquer, and Indian marking nut. The substance causing the dermatitis may be found in different forms and in different parts of the plants. For example, skin rashes have followed contact with wooden and lacquered articles produced in China and Japan and with the oil from the cashew nut shell.

Usually, no visible reaction will occur the first time a person comes in contact with poison ivy, oak, or sumac. In fact, one may not even realize that contact has taken place. If a reaction (rash) does appear, it may be 7 to 10 days after the first exposure. Many low-level exposures over a period of years are generally necessary for an individual to reach this level of sensitization, and many people never develop allergic contact dermatitis from poison ivy, oak, or sumac plants.

How does exposure occur?
Poison ivy, oak, and sumac are three of the most common causes of allergic contact dermatitis in North America. The resin of all three contains an active oily ingredient known as urushiol. While the urushiols of the three plants are not identical, they are sufficiently similar in chemical composition for a person allergic to one to react to all three.

Because urushiol is in the resin of the plant, rubbing or crushing the plant or a leaf provides sufficient contact for an allergic reaction. Very small amounts of the chemical can provoke a serious reaction in susceptible people.

Cases of poison ivy allergy occur most frequently during the spring, summer, and early fall when people spend more time outdoors. A reaction to poison ivy, however, can develop even during winter because urushiol is still present in dead or decaying Toxicodendron plants. The chemical can be carried in smoke particles from burning plants, including branches of poison sumac trees accidentally gathered for firewood. The smoke may expose not only the skin of susceptible persons to the chemical but also their nasal passages, throat, and lungs.

Urushiol is transferred easily from one object to another, so clothing or tools that touch the plants, or pets that rub against them, can pick up the plant oil and pass it directly to a person. The chemical can remain active for at least one year and sometimes much longer. Therefore, exposed objects or pets should be thoroughly cleaned with water after contact with the plants.

Contrary to common belief, scratching or spontaneous oozing of the rash and blisters will not cause them to spread because urushiol is not present in the blister fluid. If the chemical has not been completely washed off the skin, touching the affected areas and then another part of the body can transfer the chemical and thus the rash. Similarly, the rash cannot be passed between persons, unless urushiol is still present on the skin of the affected individual.

Symptoms and treatments

Links, information and more for you

Skin Care soothers and remedies
Nature's Revenge: The Secrets of Poison Ivy, Poison Oak, Poison Sumac, and Their Remedies (book)
Directory of health articles
Directory of all articles

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About this information: This material was prepared by the Office of Communications and Public Liaison, National Institute of Allergy and Infectious Diseases at the National Institutes of Health.