The plant contains furocoumarins (psoralens) which produce changes in the cell structure of the skin reducing its protection against the effects of UV radiation. These can be released from the plant simply by brushing against it. Exposure to sunlight after contact causes severe skin rashes and/or blistering and burns but the effects may not start until about twenty four hours after contact. It may take several years for the skin to return to normal during which time any renewed exposure to even quite dull daylight will produce new burns.
Depending on the extent of the contact, the victim may suffer a reddening of the skin, blisters or burns requiring hospital treatment.
In some cases, a permanent change in skin pigmentation occurs.
Whenever Heracleum mantegazzianum is being discussed there will be those who claim that the case against it is over-stated and that many other plants are more dangerous. That point of view seems to be opposed to the findings of a 1996 Swiss study of 29 years of plant poisoning reports. Though only 18 reports in that time concerned giant hogweed producing 'serious' consequences that made it the second most dangerous plant with only Atropa belladonna, deadly nightshade, at 42 cases, exceeding it.
Typical contact comes from brushing through a stand of plants when found on a riverbank, strimming a patch of rough ground in early spring without realising that the young plant is present and even contact with pets which have had contact with the plant.
It was introduced to the UK by the Victorians who thought its size would make a dramatic statement in large gardens. It escaped and has spread rapidly to be a major problem on river banks in some areas. The Wildlife and Countryside Act 1981 makes it an offence to plant or cause giant hogweed to grow in the wild.