"Leaves of three, let them be," runs the standard advice on how to avoid poison ivy and its equally villainous cousins, poison oak and poison sumac. But those who are allergic to this relative of the cashew - many as 85 percent of all Americans - find that no amount of armor or vigilance can protect them.
The chemical that gives these plants their poisonous reputation is an oily resin called urushiol. And what makes it truly diabolical is that it can hitchhike on clothing, dog's fur, even garden tools. If you come into contact with poison ivy, wash the oil off (preferably with brown soap and water) within 20 to 30 minutes, before it soaks into the skin. Since the residue can remain potent for a year or more, scrub tainted items as well.