No Ill Nature: The Surprising History and Science of Poison Ivy and Its Relatives

Chemist Rikou Majima with his students. Majima discovered the chemical structure of the toxin found in both the lacquer tree and poison ivy. (Museum of Osaka University)

Chemist Rikou Majima with his students. Majima discovered the chemical structure of the toxin found in both the lacquer tree and poison ivy. (Museum of Osaka University)

An Oily Tale

In the 20th century botanists began to reclassify the poisonous plants. Poison ivy, poison oak, and poison sumac were originally placed in the genus Rhus (the sumacs). By the 1930s botanists often separated the irritating plants from the other sumacs, assigning them to the appropriately named genus Toxicodendron (Greek for “poison tree”). There are two species of poison ivy: T. radicans (formerly Rhus toxicodendron or Rhus radicans), the familiar trailing or climbing vine that is widespread in the United States and Canada east of the Rocky Mountains, and T. rydbergii (nonclimbing poison ivy), a shrub found throughout North America except in the southeastern states. The two species of poison oak—T. pubescens, or Atlantic poison oak, and T. diversilobum, western or Pacific poison oak—earn their common name from their oak-shaped leaves. Poison sumac (T. vernix), an eastern swamp-dwelling shrub or small tree, is far more toxic than its relatives.

Given the long history of the toxicodendrons as health hazards and their economic role in making lacquer, chemists soon became interested in isolating their noxious compounds. Because smoke from burning plants can cause allergic reactions, even inside the lungs, many believed the active ingredient was a volatile oil that the plants exuded into the air. Chemists soon abandoned that idea and instead wrongly identified the toxin as a carbohydrate (that is, a sugar).

The chemist who finally got it right, identifying and characterizing urushiol as the key irritant, was from Japan, appropriately enough. Rikou Majima was among the first Japanese scientists to receive most of his education in his home country following the end of sakoku, Japan’s 200-year period of isolationism. Sakoku had left Japan far behind the West in terms of science and innovation. In order to compete with Western chemists Majima focused his research on local natural products, studying the lacquer tree from 1907 to 1922. Building on previous work by Japanese and Western scientists, he collected sap from a lacquer tree on a nearby plantation and extracted the toxin and other components with alcohol. After a series of filtrations, distillations, and extractions he obtained a sample of the allergic substance pure enough for chemical analysis. His dermatologist colleague Ikuzo Toyama then tested the samples, presumably on himself and courageous volunteers. For his next steps Majima turned to modern analytical methods and instrumentation imported from the West, such as ozonolysis, which used ozone produced by running high-voltage electricity through purified oxygen to systematically break down organic compounds. He eventually established the exact chemical structure of the toxin, which he named urushiol after urushi, the Japanese word for lacquer.

Majima found that urushiol is not a single substance but rather a mixture of closely related compounds known as alkyl catechols. These elongated molecules have a head containing a ringed structure (the catechol) and a long, greasy, water-repellent tail of either 15 or 17 carbon atoms (the alkyl chain). Catechols are common in nature and include such familiar molecules as adrenaline and dopamine. The greasy alkyl chain allows urushiol to penetrate skin and to remain on clothing and other surfaces for months or even years. Once inside the skin oxygen activates the catechol head, which links up with skin proteins. White blood cells recognize these urushiol-studded proteins as invaders and activate the body’s immune defenses, causing the familiar and painful rash, blisters, and itching. Little can be done to stop the allergic response once it begins, although immunosuppressive drugs, such as cortisone or prednisone, can help. Applying hot water or calamine lotion provides only partial relief.

The study of these intriguing molecules continues. In 2012 researchers at the University of California, Santa Cruz, engineered a molecule that reacts with urushiol to form a fluorescent compound, allowing even minute amounts of urushiol to be detected with ultraviolet light—perhaps a future boon to hikers and campers.

Best in Show

One California community has learned to “celebrate” poisonous plants, using humor to deal with a persistent nuisance. The Annual Poison Oak Show in Columbia, a historic mining town in the foothills of the Sierra Nevada mountains, began in 1982. The festival includes flower show–type competitions for the best poison oak arrangements (displayed behind yellow caution tape), for “Most Potent Looking” red and green leaves, for best poison oak jewelry and accessories, and even for the best photograph of a poison oak rash. As a local floral designer noted dryly in 2007, “It would be a great fall accent—if you could use it. The trouble is, you couldn’t count on re-orders.”

Finally, a few brave and patient horticulturists in Japan and the United States use poison ivy in a unique way, training the plants into bonsai, miniature trees in containers. With shiny green leaves that turn a brilliant red in the fall, thick woody stems, and pale green-white flowers, these poison ivy bonsai are as lovely as the maples and junipers more commonly made into tiny trees. Their owners would surely agree with Captain Smith and Governor Butler that even “a poysoned weed” has “no ill nature.”