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Urushiol (a ortho-diphenol with a long hydrocarbon chain) is an white/light yellow oil found mainly in the Rhus genus of plant life. The irritant in Poison Oak, Poison Ivy. Urushiol binds to the skin, and causes rashes to form in about 85% of all humans. Frequent exposure increases sensitivity.

Interestingly enough, Urushiol is also found in the shells of cashew nuts.