Explanation: All three "poison" plants dreaded by campers, kids, and gardeners -- poison ivy, poison oak, and poison sumac -- have the same itch-producing chemical in their sap, urushiol. About 85% of people develop an allergy to urushiol after a few exposures. If you've touched one of the plants, wash the area with hot, soapy water. Wash your clothes and shoes, too. To help control the itch, you may want to try calamine lotion or over-the-counter hydrocortisone or diphenhydramine.