there are studies out there if you search for them. ive ran across a handful and so far everyhing is positive. havent yet run into a study that came up with any real, hard negitave results.
further, one study determined that PG vapor sterilizes the air (!). two groups of mice were kept in separate chambers, both with the flu virus in the air. one group was given PG vapor pumped into the tank. the group without the vapor all died, the group with the vapor had zero deaths.
i think that if anything were to be found potentially hazardous, it would be a particular element of a flavoring. PG and VG have been around for a while, and even in some medical settings they are inhaled. also fog machines have been pumping vapor at us for numerous years. there are some strange elements in some of our flavorings. diacetyl is a hazardous one that got nipped in the bud right quick by every vendor i do business with. its used widely in the artificial butter flavoring realm: microwave popcorn beign a large patron. its relatively safe to injest by eating, but not by inhaling. youd end up with "popcorn lung", a disease that afflicts popcorn factory workers (and 1 very unlucky man who had a compulsion to inhale vapors from a fresh bag of microwave popcorn daily).
ive been told that vapng oils is not very safe either. some flavorings do contain oils, like peppermint oil for example. so itll be interesting to follow and track the results of the numerous tests that are bound to be conducted.