So what is this danger? ..
If your skin should come into contact with the sap of this plant, within about 24 hours blistering will occur. Depending upon the degree of contact, you may end up with . .
The blistering can last for many months, but the overall effect will last for about 2-3 years provided you always cover up when you go outside If you do not do so, the sunlight will reactivate the blisters and you will be back to “square one”. Permanent effects can include scarring and/or a change in pigmentation of the skin. The sap of the whole plant is dangerous, but the lower part of the hollow stems and petioles, hollow hairs on the plant, foliage, stem, flower and fruit (seed) have the highest concentration and so most serious adverse effects.
This Plant is difficult to miss when fully grown, due to its size, but early in the season it is easily overlooked in undergrowth; it is commonly found along river banks (although in other places too) and spreads easily as it may have as many as 50,000 seeds which can float downstream, or be easily dispersed by the wind, and create more colonies of the plant.
So . . . please BE AWARE and TAKE CARE. (This plant affects animals too)
Have a look at the following links for more information :
Video and very comprehensive information - http://www.thepoisongarden.co.uk/atoz/heracleum_mantegazzianum.htm
Other links –