Well, I would definitely avoid cow parsnip even though Giant Hogweed is supposed to be much worse. The blisters didn't hurt but they left scars after they healed that took a few years to fade. And I heard about a woman who had to be evacuated by helicopter from a hiking trip because she had most of her legs and a lot of her torso exposed. She was also hiking on a glacier so UV exposure was probably pretty high, too.

Fortunately, we only have cow parsnip around here. (So far, the qualifier we always have to use with invasive species.) From what I understand, Giant Hogweed looks very similar, only larger.