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BRIEF: Can you tell one poison rash from the other?

3 or 4 of us in my family have run into a poison ivy. The others think they have something stronger. I contend that ivy, oak & sumac all have the same effect & you can't tell one from the other by looking at ones skin. Correct or no ?


I think you are correct, although the toxin does vary among these three plants in the length of the sidearm chain 17 carbons for oak, 15 for ivy and I forget what poison sumac has. The actual lesion that forms will look alike, but slightly different solubilities in the skin may change how lomg you have to wash off the oil before it has a chance to trigger the allergic reaction. My impression has been that poison ivy is a little less virulent and easier to wash off (no scientific evidence, just an impression), which would fit with the shorter, slightly less lipid soluble ivy urushiok.