Is acute exposure to diacetyl (in popcorn butter) harmful, or only long-term, cumulative exposure?

Possibly... Any "chemical" runs the risk of being potentially dangerous. Now i would never deprive you of your popcorn; but i cannot foresee the future; I am not aware of any studies to date on this topic...But just follow my standard advice; 'anything in moderation' and we'll see what the future holds(besides; i can't skip to the end of this movie and ruin it for ya) cheers!