For the PCT, poison ivy is not seen in California but covers the Pacific Northwest. Poison oak is a major problem in California, disabling one third of forest fire fighters because it is widespread and aerosolizes in fires. The villain in poison ivy poisoning is urushiol, an oil similar to carbolic acid. It is the active agent in plant sap, which causes the contact dermatitis. The initial contact with the toxin does not cause a rash, but over time sensitization develops.
Most cases are mild, but some are severe.
Rash onset may occur as early as six to twelve hours after exposure, but is usually at 24 to 48 hours. It may begin as late as a week after contact. The rash may last about a week in mild cases, but in highly sensitive cases it can last two to three weeks and may require hospitalization. It's a myth that only the leaves of poison ivy, oak, and sumac are poisonous. Stems, roots, flowers, and berries are also poisonous.