Poison Ivy: The Most Common of Allergens

Taking a hike through the woods typically isn't dangerous, but it could leave you itching if you are not careful.

The old saying, "leaves of three, let them be," still applies as a reminder to steer clear of poison ivy, said Thomas Ray, MD, Professor Emeritus of Dermatology at the University of Iowa.

Poison ivy is common throughout the United States, particularly around lakes and streams, as well as backyards and pastures in the Midwest and the East. Its ability to grow in a variety of places makes our susceptibility to it even greater.

"Of all the possible allergens, poison ivy is the most common one to worry about," Ray said.


Urushiol produces symptoms in three out of four people. Often the symptoms begin as a severe itching of the skin. Redness, burning, swelling and blistering follow the itching. Severity of the symptoms depends on each individual's sensitivity and reaction to the allergen.

"If you have never been exposed, or are not yet allergic to poison ivy, it may take 10 to 21 days for a reaction to occur the first time," Ray said. "Once allergic to poison ivy, however, most people break out 48 to 72 hours after contact with the plant. Typically, individuals have been exposed at least once, if not several times, before they break out with a rash."

Allergic reactions on the body are often regional. Tough layers of skin--the palms of the hands and the soles of the feet, for example--tend to be more resistant and have less of a reaction than thinner layers of skin, such as the eyelids and underarms, which are very sensitive.

"The severity of the reaction partially depends on the ability of the sap to penetrate the skin," Ray said.

One major misconception about poison ivy is its transmission from person to person. The blister fluid associated with poison ivy is a product of the body and actually contains very little or no resin.

"Someone who has broken out cannot transfer the poison ivy allergen to someone else through the fluid found on the rash," Ray said.

If a rash continues to break out on new areas of the body, it is usually due to repeated exposure or different regions of the skin being exposed and reacting at different times to the resin. While an allergic reaction usually occurs as a result of direct exposure, it can also occur because of indirect exposure. Clothes, shoes, sporting and gardening equipment, and pets can cause indirect exposure.

Treatment for an allergic reaction to poison ivy can usually be done at home. Over-the-counter medications, such as hydrocortisone creams or ointments and antihistamines, can help to relieve some of the symptoms. Lotions containing calamine and menthol are good for relieving the itch. Some remedies containing such chemicals as topical diphenhydramine or benzocaine can be skin sensitizers themselves and cause another break out, Ray said.

Home treatment can also involve wet and dry compresses to reduce itching and redness. However, if any signs of infection occur, such as warmth, redness or pain, or poison ivy exposure involves facial or genital areas or large areas of skin, see your dermatologist or family doctor. Prescription medications may be needed to control the reaction and relieve the itching.

Healing from poison ivy varies on the severity of the reaction. A mild case may last a week, while a severe case may last three to four weeks. Keeping the rash clean is important. Any bandages placed over the rash should be changed frequently and applied loosely in order for oxygen to reach the surface of the skin.

Avoiding poison ivy is the best way to prevent an allergic reaction. Knowing what the plant looks like during the seasons is one good way. During the spring, the plant may contain yellow or green flowers and white berries. In the summer the leaves are green; in the fall they become red. The leaves are often shiny, and deceptively attractive, especially in autumn.

Wearing long pants and long-sleeved shirts when in a wooded area is important; wearing gloves when gardening in an area that may have poison ivy also is a good idea. Pets that are loose in wooded areas should be watched carefully. They may rub up against the plant, acquire the resin on their fur, and then indirectly pass the allergen to humans who handle the animal.

If contact has been made, it is important to wash everything right away. This includes the skin and garments or items exposed.

"The skin absorbs the active compounds of the sap within the first few minutes of contact," Ray said. "The sooner you wash it off the less severe the reaction."

Heat is the key to deactivating the urushiol oil in affected clothing. An allergic reaction is possible a year or more after the initial exposure if the garment was never washed. A hot wash cycle is one way to deactivate the urushiol oil. Another way is to place the garment through the hot cycle of a clothes dryer, Ray said.

What you wear, where you walk, and how you wash are all important, but according to Ray, one final thing to be aware of is the smoke from burning wood, vines, weed, or pasture grass contaminated with poison ivy.

"Airborne exposure should also be taken into consideration," Ray said. "The resin in the plant is released in the smoke during burning and can cause an allergic reaction on the eyelids, face or exposed skin surfaces."

University of Iowa Health Science Relations
Thomas Ray, MD
Professor Emeritus of Dermatology