Ah, right, diacetyl. I'd forgotten about that, actually. However from what I've read, popularized by that "popcorn lung" story, it's only of particular harm when inhaled in larger quantities by people who work at the factories that make either the flavouring or the microwave popcorn. I recall that case of the guy who tried suing some microwave popcorn company because he developed popcorn lung because he ate an obscene amount of the stuff, and he turned out to be full of crap.
Not that I'm saying inhaling diacetyl isn't bad of course, but I think since it's really only being used as an additive and in small quantities I don't think it should be particularly harmful. Plus, how else am I going to make a Danish butter cookie flavour?
