Poison Oak is known for causing itching and allergic rashes in many humans, after contact by touch or smoke inhalation. Pacific poison oak leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. However, most, if not all, will become sensitized over time with repeated or more concentrated exposure to urushiol.
Fun Fact Californian Native Americans used the plant's stems and to make baskets, the sap to cure ringworm, and leaves as a poultice applied to rattlesnake bites. An infusion of dried roots or buds were eaten by some native peoples for an immunity from the plant poisons. Chumash peoples used sap to remove warts and calluses; to cauterize sores; and to stop bleeding.