I think I read somewhere that its the flavouring thats the main issue, at the time they were going off of guides for safe chemicals but most of those are for eating, not smoking. While they're safe to eat (popcorn) lungs are a different system. I think it was lung macrophages that accumulated these compounds and they're not sure how to process them so over time they just keep accumulating and cause disease. I'm not sure where the science is now I read that a few years back.