After several hours of exposure to air, this sap turns black. That s why so many people think just touching poison ivy is dangerous. Highly sensitive people can begin reacting in minutes . The results will be the same if they handle the plant or launder the camp clothes that carry black residue. A poison ivy-caused rash can t spread .
Broken blisters can t do it.
Skin doesn t swell to show it s happening. When a reaction to poison ivy worsens, the culprit tends to be the victim . In turn, the rash can seem to spread quickly. So, toxic oil often remains on the skin and makes the rash appear to spread over time. The sap is what s called marginally soluble in water.