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Department of Dermatology
Poison Ivy: The Most Common of Allergens
University of Iowa Health Science Relations and
Thomas Ray, MD
Professor of Dermatology
First Published: November 2000
Peer Review Status: Internally Peer Reviewed
- 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 in the UI Department of Dermatology.
- 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
- "Of all the possible allergens, poison ivy is the most common
one to worry about," Ray said.
- Poison ivy is coated with a resin-like sap, which contains a
chemical called urushiol. Every part of the plant contains the
resin: leaves, stems and roots. Most people only come in contact
with the leaves, but it is important to remember the entire plant
can produce an allergic reaction, 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
- "Someone who has broken out cannot transfer the poison ivy
allergen to someone else through the fluid found on the rash," Ray
- 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
- 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
- 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
- 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
- "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
- 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."