|This is a tough myth to dispel because scratching the red area appears to spread the rash to other areas. Actually, the reaction develops over a period of time - usually taking hours or days to occur. The occurrence of a new rash, say further up the arm, doesn't mean the rash was spread but that that area was slower to respond to the poisonous oil.
Once the oil triggers the reaction, our own bodies release defensive chemicals that cause the redness, itching, and blistering. The blister fluid doesn't come from the plant at all but is produced by our own