The sap from the leaves and particularly the stem is highly toxic and contact with the skin can lead to severe scars. Contact with the eyes can lead to temporary or, in some cases, permanent blindness. The sap renders skin photo-sensitive which means that exposure to sunlight following contact causes blisters and burns. If you do come into contact with the plant, and especially the sap, you are advised to wash the affected areas immediately, keep them out of direct sunlight and seek medical advice. Everyone learns to treat nettles with respect from a very early age but most people are blissfully unaware of the dangers associated with these impressive giants.
Every year there are reports of people sustaining horrific injuries as a result of their ignorance.
British Rail used to publish posters warning their staff explicitly about the dangers, railway embankments usually have all the right conditions to make a hogweed very comfortable. Local authorities do sometimes make an effort to clear the plants where they pose a threat to the public, if this isn't possible warning signs may be erected. The riverside country park at Teston, Kent, near where I live is situated right in the middle of the plants local stronghold but persistence by the council kept it clear last year. Stray a few hundred yards at either end of the park and its a different story. This page serves two purposes then, a warning to treat these impressive plants with caution, but also to highlight a magestic alien which has made its home in our fields and hedgerows.