 Over 350,000 people are affected by urushiol annually in the United States.The pentadecyl catechols of the oleoresin within the sap of poison ivy and related plants causes the allergic reaction; the plants produce a mixture of pentadecylcatechols, which collectively is called urushiol.After injury, the sap leaks to the surface of the plant where the urushiol becomes a blackish lacquer after contact with oxygen.