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For information on urushiol poisoning, see Urushiol-induced contact dermatitis.

Urushiol (IPA pronunciation: [ʊˈɹuʃiˌɔl]) is an oil found in plants of the Family Anacardiaceae, especially Toxicodendron spp. (e.g. poison oak, poison ivy, and poison sumac). It is also found in the nut shell of cashew fruit (Anacardium occidentale). It causes an allergic skin rash on contact, known as urushiol-induced contact dermatitis. The name comes from the Japanese word urushi, which denotes a lacquer produced in East Asia from the sap of kiurushi trees (Lacquer Tree). The oxidation and polymerization of urushiol in the tree's sap in the presence of moisture allows it to form a hard lacquer, which is used to produce traditional Chinese and Japanese lacquerwares.

Urushiol is a yellow liquid with a boiling point of 200-210 °C. It is soluble partially in alcohol and ether, but nearly insoluble in water. Chemically, urushiol is a mixture of several closely related organic compounds. Each consists of a catechol substituted with an alkyl chain that has 15 or 17 carbon atoms. The alkyl group may be saturated or unsaturated, urushiol oil is a mixture of the saturated and unsaturated urushiol molecules. The exact mixture depends on the species of the plant. For example, poison oak urushiol contains mostly catechols with C₁₇ side chains, but poison ivy and poison sumac contain mostly catechols with C₁₅ side chains. The allergic reaction is dependent on the degree of unsaturation of the alkyl chain. Less than half of the general population reacts with the saturated urushiol alone, but over 90% react with urushiol containing at least two degrees of unsaturation (double bonds).

R = (CH2)14CH3 or R = (CH2)7CH=CH(CH2)5CH3 or R = (CH2)7CH=CHCH2CH=CH(CH2)2CH3 or R = (CH2)7CH=CHCH2CH=CHCH=CHCH3 or R = (CH2)7CH=CHCH2CH=CHCH2CH=CH2 and others

• Burow's solution, for treating the toxin
• Hapten
• Mangoes, the skin
• Poison ivy
• Toxin
• Western Poison-oak

• Poison Oak at Wayne's Word
• The Poison Ivy Tutorial