Urushiol

 ( Catechols ) 

André-Ignace-Joseph Dufresnoy was one of the first to come up with a medicinal use for this chemical in 1780 when he boiled poison ivy to produce an infusion for internal use. This led to a distilled extract of poison ivy which he prescribed to many people suffering from skin problems and even paralysis. He claimed this treatment to have yielded several positive results.

For many years, poison ivy was thought to fall into the Rhus genus; however, in the 1900s, it was reclassified into a more appropriate genus, Toxicodendron, meaning poison tree. There were many documented cases of irritations and allergic reactions from the plant, and its prevalence in medicinal use quickly dwindled. After this new categorization, scientists began attempts to determine what it was that rendered plants of this genus noxious, starting with a hypothesis of a volatile oil present in the plants. While this proved incorrect, Rikou Majima from Japan was able to determine that the chemical urushiol was the irritant. Further, he determined that the substance was a type of alkyl catechol, and due to its structure it was able to penetrate the skin and survive on surfaces for months to years.

When urushiol comes in contact with oxygen, under certain conditions it becomes a black lacquer and has been named urushi lacquer. Urushiol's ability to polymerise into a hard glossy coating is the chemical basis for traditional lacquerware in many Asian countries.

Urushiol is a mixture of several closely related . Each consists of a catechol substituted in the 3 position with a hydrocarbon chain that has 15 or 17 carbon atoms. The hydrocarbon group may be saturated or unsaturated. The exact composition of the mixture varies, depending on the plant source. Whereas western poison oak urushiol contains chiefly catechols with C₁₇ side-chains, poison ivy and poison sumac contain mostly catechols with C₁₅ sidechains.

The likelihood and severity of allergic reaction to urushiol is dependent on the degree of unsaturation of the hydrocarbon chain. Less than half of the general population experience a reaction with the saturated urushiol alone, but over 90% do so with urushiol that contains at least two degrees of unsaturation (double bonds). Longer side chains tend to produce a stronger reaction.

Urushiol is an Resin contained within the sap of poison ivy and related plants, and after injury to the plant, or late in the fall, the sap leaks to the surface of the plant, where under certain temperature and humidity conditions the urushiol becomes a blackish lacquer after being in contact with oxygen.

Within the United States, urushiol-containing plants are widely distributed. Poison ivy can be found in all states except California, Alaska, and Hawaii. Poison oak can be found on the west coast or in some states in the southeast, while poison sumac can be found only in the eastern half of the country.

These plants all have distinguishing features that will help in identification. Poison ivy always grows with groups of three glossy, pointed leaflets on leaves that alternate along the branch (in contrast to similar looking leaves of boxelder which has opposite leaves). Poison oak has a similar appearance, but with larger and more rounded lobes, with a light, velvety Indumentum and grow in groups of 3, 5, or 7. Poison sumac grows in groups of 7 to 13 leaflets, always an odd number. The leaflets are ovate-lanceolate and glossy.