Urushiol is an oily substance found in poison ivy, poison oak, and poison sumac that causes an itchy, blistering rash when it touches your skin. The name comes from urushi, the Japanese word for lacquer, because a very similar substance appears in the sap of Asian lacquer trees.

When you brush against poison ivy on a hike, urushiol transfers from the plant's leaves to your skin. Your immune system treats it like an invader and launches an attack, which creates the maddening itch and red bumps that can last for weeks. The oil is incredibly persistent: it stays active on clothing, tools, and even pet fur for months, so you can get a rash from touching contaminated objects long after the original contact.

About 85% of people are allergic to urushiol, making it one of the most common causes of allergic skin reactions. The oil is so potent that an amount smaller than a grain of salt can cause a rash. It doesn't spread from person to person, but scratching can spread the oil to other parts of your body if it's still on your skin. Washing with soap and cool water within minutes of exposure can help prevent the rash, but once urushiol bonds to your skin, the damage is done.