We’ve got loads of giant hogweed over here it’s a risk on a lot of the sites we work on, as far I know it’s only giant hogweed that burns, cow parley and common hogweed are everywhere and I’ve never seen them cause anyone any harm.
@Gorman; I don't know the answer to your question yet, but I found this cool PDF by New York State that tells a bit more about it, with great photos, etc. Also, I just noticed in that Wikipedia article that it says multiple species within the genus cause this burning reaction. Here's a quote from the article. "In all, 29 species of the Heracleum genus have been reported to contain furanocoumarin, a chemical compound that sensitizes human skin to sunlight."