Poison ivy is a plant that is harmful to humans, though it is stated that poison ivy does not affect animals. There are two types of poison ivy, one type has the ability to climb and the other does not. The ability to climb simply means the poison ivy is able to grow up along trees or land formations similar to the way a vine grows; while the eastern species of poison ivy is only able to grow out the ground as a normal plant would.

Both species of plant give the same allergen effect, which ranges from a simple rash to the need for medical attention. A human exposed to the urushiol oil of poison ivy on their skin will generally be plagued with a rash that can last up to three weeks. While serious exposure to the oil, such as the oil entering a small cut or breathing in smoke produced by burning poison ivy, requires the affected person to seek medical attention or face the risk of dying from exposure to urushiol oil. The most common effect from poison ivy is an irritating rash, which may need skin cream and extra care to cure. The oil causes the rash by seeping through the layers of skin and producing an allergic reaction from the skin. The oil is tough to remove, not only from the skin but from clothing and animal fur as well. While animals are not susceptible to the allergic reaction, coming into contact with oil trapped in the dog's fur or paws may induce an allergic reaction identical to the allergic reaction produced by handling poison ivy. Clothing, gardening tools and footwear may retain the oil of a poison ivy plant for years and induce an allergic reaction if it the oil is touched.

Poison ivy grows nearly everywhere in the United States, though the species varies depending on the location. The western states provide the habitat for the climbing species of poison ivy, while the eastern states offer the environment for the non-climbing species of poison ivy. The only areas that do not support the growth of poison ivy are the far west states, deserts and locations with high altitudes. The plants may be found along roads or highways, and along the edges of fields. The poison ivy plants need a certain amount of light and carbon dioxide to flourish. The U.S. Department of Agriculture states that deforestation and increases in the Earth's carbon dioxide levels have inspired abnormal growth of poison ivy. It was reported that poison ivy has began to flourish in areas where the forest has been disrupted or destroyed, allowing for increased growth along with poison ivy growing to larger sizes than before.

The general phrase used to reply to one's question about what poison ivy looks like is "if its three, let it be." This simply means that one should beware any plants they meet that produce sets of three leaves, though there are many plants that grow sets of three leaves. To further separate poison ivy from other plants, one should look for areas of red that grow along the outer edges to the middle of the leaf. The leaves are also particularly shaped with only two broad curves forming the frame of the leaves.

In conclusion, poison ivy grows nearly everywhere in the United States and specifically affects humans. Exposure may lead to a horrid rash, or hospitalization depending on the severity. While some people may b immune, it's only a small percent; which leaves about fifty million people to suffer from the allergen effects. A simple green or greenish-red plant with sets of three leaves should be let be.