Poison Oak

by Brendan Bane

Plants have evolved alongside animals for hundreds of millions of years. Many are beautiful, some are delicious, and others can kill. While poison oak (Toxicodendron diversilobum) isn’t a killer, its toxic oils may make you wish otherwise.

This deciduous oak grows throughout forests and woodlands along the Pacific coast of North America. It thrives in a range of habitats, from mountains to valleys. Its form is also variable—it can be a tree, a shrub or a vine, depending on the amount of sunlight and water available.

Lobed leaves are clustered in triplets and spaced apart. They are green and glossy in spring, and are often accompanied by small white flowers, which turn to champagne berries. Green fades to yellow in summer and bright pink in the fall. Hiding within those tissues is the substance for which the plant is most famous.

Poison oak is covered in a toxic oil, urushiol, which sticks to everything it touches (the name comes from the Japanese word for "lacquer"). The same substance is found in poison ivy and poison sumac. Urushiol alone does no damage. Instead, it tricks the body into harming itself.

When urushiol touches flesh, it alters proteins embedded in skin cell membranes. Those proteins normally help the body to recognize skin cells as part of the self. But when manipulated, the immune system recognizes them as foreign and begins attacking the tissues it’s supposed to protect.

About 24 hours after contact with urushiol, symptoms set in. Rashes and inflammation come first, blistering second, and oozing third. Sensitivity varies per person. Some are immune while others suffer blisters after being exposed to less than one ten-millionth of an ounce of urushiol.

It’s best to immediately wash the contact area with cold, soapy water (so as to keep pores closed and oil out). Symptoms subside in one to two weeks. Be sure to wash anything that may have touched the plant—clothing and all.

The rhyme to know is "leaves of three, let it be."