Comment by theophrastus

In synthetic organic labs we often pass relatively small compounds (very much like the favorants in vape juice) over heated elements in order to generate intermediate "activated"/free-radical compounds for the next step of a complicated reaction sequence. So no organic chemist i know is a bit surprised that vaping exposes the user's lungs directly to significant levels of toxic compounds like diacetyl. And they are often heard to wonder out loud: "Why has the FDA not stepped in to regulate these? I wouldn't let any of my family do this!"