It secretes an oil called urushiol, which is also produced by poison oak (T. diversilobum), the poison primrose (Primula obconica), and the lacquer tree (T. vernicifluum). When urushiol comes in contact with the skin, it initiates the contact hypersensitivity reaction.

…of the lacquer tree is urushiol (from urushi, the Japanese word for lacquer), a substance that can cause contact dermatitis if the lacquer touches the skin before it has cured. The cured lacquer produces a hard, glossy coating that is highly prized in traditional decorative arts. See also lacquerwork.

The toxic principle, urushiol, is produced in the resinous juice of the resin ducts of the leaves, flowers, fruits, and bark of stems and roots but not in the pollen grains. Being almost nonvolatile, the urushiol may be carried from the plant on clothing, shoes, tools, or soil…

These species contain urushiol, and contact with the leaves and sap can cause a severe, itchy, and painful inflammation of the skin. Like many other lobe-leafed plants commonly called “oak,” neither species of poison oak is a true oak of the genus Quercus.

…on exposure to air, contains urushiol and is extremely irritating to the skin for many people. The plant is considered more allergenic than the closely related poison ivy (Toxicodendron radicans) or western poison oak (T. diversilobum). The itchy and painful inflammation, known as contact dermatitis, can persist for days or…