I have had a bout with phytophotodermatitis, which lasted quite a long time (nasty itchy rash). I was told by my dermatologist that there are substances in any plant in the umbelliferae or apiaceae family (carrots, parsley, angelica, dill, anything with that firework type bloom to it) that can be activated by sunlight to cause a reaction. He also said each plant may be more potent at different stages of its lifespan, which makes it difficult to pin down one culprit over the other. As I recall, I treated the rash with some cortisone cream and was advised to keep my affected parts out of the sun until it cleared up. That was the most difficult part, as it was midsummer and the rash was on my lower legs.
Not sure if the giant hogsweed falls in this family or not, but it certainly looks like it might.
