English: An afternoon walk down the canal. Good job these plants are on the other side. A clear signal to keep away! The sap of Giant Hogweed 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: 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.