with poison ivy poison oak and poison sumac the sap in the leaves stems and roots of these plants contain an oil called urushiol (pronounced yoo-roo-shee-all). many people have an allergic reaction when urushiol touches the skin which produces the rash. you may be caught by surprise thinking you haven't touched any of these poisonous plants -- thinking only of the leaves -- but you may have unwittingly come in contact with their roots or stems. urushiol quickly penetrates the skin often leaving red lines that show where you brushed against the plant. from 12 to 72 hours later symptoms appear. scratching the itchy rash doesn't cause it to spread but can prolong skin healing and cause a secondary infection. the rash isn't contagious so you won't spread it to others by going to school or work. three types of transmission can occur: direct contact when you touch or brush your skin directly against the plant. indirect contact when you touch pets gardening tools sports equipment or other objects that had direct contact with the plant. airborne contact from burning these plants which releases particles of urushiol into the air that can penetrate your skin eyes nose throat or respiratory system.
what causes the rashes from poison ivy oak and sumac?