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There's nothing that can ruin an outdoor experience faster than the stinging itch of Poison Ivy. The single most common cause of allergic reaction in the United States is that of Poison Ivy. Each year 10-50 million Americans develop an allergic rash after contact with this poisonous plant. Before reaching for another application of calamine, read more about this weed and how to protect yourself against its maddening symptoms.

The Poison Ivy plant exists everywhere in the United States, except Hawaii, Alaska and some desert areas of Nevada. In the Eastern, Midwestern and Southern regions of the U.S., Poison Ivy grows as a crawling vine. In the far Northern and Western areas of the U.S. and Canada, it grows as a shrub.

Poison Ivy has a large range of growth, able to survive and thrive on sandy, stony or rocky shores of streams, rivers and lakes, and equally capable of sprouting in thickets along the borders of woods and wooded openings. These weeds are most dangerous in the spring and summer months when sap is fertile and plants tend to easily bruise. An injured Poison Ivy plant will present itself with large black marks on the leaves or stem areas.

Poison Ivy is a perennial that spreads by dropping seeds and woody rhizomes. Each plant or bush has three leaflets per stem that are easily identifiable.

The easiest way to prevent a Poison Ivy reaction is to avoid the plant altogether. Your best defense is knowledge.

1. Know what Poison Ivy looks like so that you are better able to avoid coming in contact with it. Look at pictures of Poison Ivy and familiarize yourself with it. Poison Ivy has three very distinct leaftlets per stem.

2. Treat yourself immediately. If you know that you have come in contact with Poison Ivy, acting fast can make all the difference in the world. Within six hours of exposure, remove all clothing and shoes that have touched the plant. Wash skin with mild soap and water. Apply rubbing alcohol with cotton balls to the parts of skin that are affected.

Poison Ivy rash is an allergic contact rash (also known as dermatitis) which is caused by contact with an oil, urushiol. Urushiol is found in the sap of the Poison Ivy plant, and is a colorless or pale yellow oil that will ooze from any cut or crushed part of the plant, including roots, stems and leaves. After simple exposure to air, urushiol turns brownish-black, making it easy to spot.

Contact with Poison Ivy and its sap can occur in three ways:

1. Direct contact: touching the sap of the plant.
2. Indirect contact: touching something on which urushiol has spread.
3. Airborne Urushiol particles.

Once urushiol touches the skin, it begins to penetrate within minutes. In those who are hypersensitive, a reaction appears as a streak or line rash within 12-48 hours. Redness and swelling occur, followed by blisters and severe itching. Within 2-3 days, the blisters become crusted and begin to scale. A typical Poison Ivy rash takes 10 days or longer to heal completely.

Poison Ivy rash can affect any part of the body, but is especially common on areas where skin is thin, such as the face. The rash is less commonly found on the soles of feet or palms of the hand, where skin is much thicker.

Poison Ivy rash does not spread, contrary to common belief, New rashes may occur at a later time on thicker skinned areas because it takes longer for usushiol to absorb. Common late sites for rashes include forearms, legs and the trunk.

Sensitivity to Poison Ivy develops after the first direct skin contact with the oil urushiol. An allergic reaction seldom occurs on the first exposure. A second or third encounter however, will generally produce a reaction, which may be severe. About 85 percent of the population will develop an allergic reaction to Poison Ivy in their lifetime.

Sensitivity varies widely from person to person. People who reach adulthood without becoming sensitive, have a much smaller chance of developing hypersensitivity to the plant. Sensitivity also declines with age. Severe sensitivity, which manifests itself through extreme swelling on the face, arms, legs and genitals, require medical attention.

If you think you've come into contact with Poison Ivy, be on the look out for:

Burning feeling

Once you've come in contact with Poison Ivy, it should be treated immediately.

1. Wash all exposed areas with cold running water.
2. Wash all clothes and shoes. Urushiol can remain active for months and can be transported from person to person via furniture, clothing, pets or the air.
3. Relieve itching. Cool showers and over the counter preparations like calamine lotion work well to stop bothersome itching. Soaking in an oatmeal or baking soda bath will also help to dry blisters. In severe cases, cortisone creams are helpful.

Like all things, Poison Ivy is easier to avoid, than treat. If hiking or traveling with pets, remember that they, too, can be affected by Poison Ivy.