Heating glycerin can produce acrolein and formaldehyde, and this has been reported in ecigs in several studies. The amounts are minuscule compared to cigarettes and don't seem to show up in second-hand vapor, but it's plausible that this could cause some long term harm.

This is why I really wish someone would sell an ultrasonic vape, which the original patent included. That should be far safer, but so far, nobody seems to be making them.