Poison ivy (and its cousins poison oak and poison sumac) contains an oily substance called urushiol. About 1/3 of people are allergic to urushiol. The leaves and branches of the poison ivy plant contain urushiol, and breaking the plant releases additional urushiol to the surface. Burning the poison ivy plant will release the urushiol into the air, possibly causing serious inflammation of the throat and lungs. Also, unlike some allergens, repeated exposure to urushiol can increase a person's sensitivity.

If you think you may have touched a poison ivy plant, as soon as possible you should wash the affected area with soap and cold water. Hot water will disperse the urushiol over a larger area, possibly making it much worse. You can buy specially formulated soaps like Tecnu that will bind to the urushiol, allowing it to be washed off harmlessly.

Also, note that dogs and cats and other animals will happily play in poison ivy without being affected, but they will get the urushiol on their fur. If you then touch the animal, you get urushiol on your hands, which you can then spread to other parts of your body or to other people.