According to Wikipedia: "Giant hogweed is a phototoxic plant. Its sap can cause phytophotodermatitis (severe skin inflammations) when the skin is exposed to sunlight or to ultraviolet rays. Initially, the skin colours red and starts itching. Then blisters form as it burns within 48 hours. They form black or purplish scars that can last several years. Hospitalisation may be necessary. Presence of minute amounts of sap in the eyes can lead to temporary or even permanent blindness. These reactions are caused by the presence of linear derivatives of furocoumarin in its leaves, roots, stems, flowers and seeds. These chemicals can get into the nucleus of the epithelial cells, forming a bond with the DNA, causing the cells to die."
