Poison ivy, oak and sumac are plants that contain an oil that may cause a rash on many people's skin. Urushiol, the irritant oil in these plants, need only be present in a billionth of a gram to cause a rash. It is the most common allergy -- affecting half of the population, according to the Poison Ivy, Oak & Sumac Information Center.
Here are some other things to know about these poisonous plants.
*Rashes from poison ivy and the other plants is not contagious. The only way it spreads is if the urushiol oil is still on the site.
*Poison ivy may have three leaves per cluster, but poison sumac can have 7 to 13 leaves on a branch.
*The oil that causes the effects of poison ivy can remain on any surface for up to five years, including dead plants.
*The more frequently one is exposed to urushiol oil, the shorter the time an allergic reaction will occur.
*Poison ivy and poison oak account for an estimated 10 percent of lost work time among U.S. Forest Service workers.
*Poison ivy can grow along the ground but is often found as a vine creeping up trees. Poison sumac and oak grow more often as bushes.
*Some topical and prescription medications exist to help relieve the uncomfortable symptoms that result after exposure to a poisonous plant.