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.