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Giant hogweed is a highly invasive plant which has spread throughout the whole of Great Britain, primarily favouring river banks but also other areas such as parks, cemeteries and wasteland. Its sap contains toxic chemicals known as furanocoumarins. When these come into contact with the skin, and in the presence of sunlight, they cause a condition called phyto-photodermatitis, which is a reddening of the skin, often followed by severe burns and blistering. The burns can last for several months and even once they have died down the skin can remain sensitive to light for many years. See > Link for a wider view.