A clear, sticky oil called urushiol (pronounced yoo-roo-shee-ol) puts the “poison” in poison ivy. This oil oozes from the plant whenever it gets cut or crushed. It also causes an allergic reaction in almost nine out of every ten people who touch it.
Even the tiniest droplets of urushiol from poison ivy can leave you with an itchy rash that will last for ten to fourteen days. Most of this oil gets produced in the spring or summer, so now’s the time to be extra careful. The best way to ditch the itch is to avoid poison ivy to begin with. Unfortunately, this plant doesn’t come with a warning label. So on your next walk in the woods, beware of plants with white berries, yellow or green flowers, and shiny green leaves that turn red in the fall. This old saying may also come in handy when dodging poison ivy: “leaves of three, let them be.” That’s because poison ivy leaves often grow in clusters of three (but can be seen in groups of as many as nine).
Answer provided by Discovery.com