At this time of year I start to see the same plant appearing regularly for my Google Alerts including the word poison. The reason I mention this is because it is not a poisonous plant.
Rhus radicans, also known as Rhus toxicodendron or Toxicodendron radicans, is usually referred to by its common name, poison ivy. Contact with the plant transfer an oil called urishiol to the skin and, in the majority of cases, results in the symptoms of dermatitis; itching, blistering and pigmentation change. Repeated exposure can produce sensitization so that the effects become much worse.
These effects may not, however, be poisoning but rather an allergic response.
There may not be a straightforward way to distinguish between the two. The simple starting point is to say that everyone gets affected in the same way by a poison but only some of the population responds to an allergen. So, I wrote ‘the majority of cases’, above, because it is estimated that 30% of people have no reaction to urishiol.
That simple definition breaks down a little because some poisons produce tolerance so that people have little or no reaction to them. The morphinist who visited Thomas Edison’s laboratory could consume huge quantities of the products from Papaver somniferum, opium poppy, and Marie Jeanneret Monday developed such a tolerance to the atropine from Atropa belladonna, deadly nightshade, that no amount of it produced any symptoms. See blog for 27th June 2011
But such cases are rare and, in any event, they are an acquired tolerance whereas people who are not allergic to a substance will have no response to it even on first contact.
That question of tolerance is another means of separating poison from allergen. Though repeated exposure to a poison will, often, build up tolerance to it, so that increasing amounts are required to see any affect, allergens produce sensitisation so that smaller amounts produce the same reaction or the same amount produces greater problems.
A may seem a little pedantic to be concerned about people using the name poison ivy rather ‘allergenic ivy’ because, surely, if the name keeps them away from the plant that is what matters.
My problem with that approach is that it can lead people to think that other, truly poisonous, plants are simply allergens.
If you are allergic to it, it can result in a rash with small bumps under the skin and a burning sensation.
My worry is that this could make people think that giant hogweed and poison ivy act in the same way and, if they know themselves to be unaffected by the latter, they may think they can safely handle the former.