E-cigarettes have an history of 3 or 4 years I guess. So I don't think it is wise to claim VG and PG to be totally harmless. No one knows about their long term effects when inhaled. Correct me if I am wrong please.
Actually, people have been inhaling vaporized pg for a very long time, over 100 years, in theatrical fog. Vg, too, has a long history although not as long as pg. Both substances have had a lot of clinical study done with them on the inhalation front. Confidence is quite high that, unless you're allergic or asthmatic, these substances are generally safe to inhale, even in the long term. (Vg can produce a toxic substance if heated beyond a certain temperature, but most attys don't get that hot.)
The only component of
juice that hasn't been well studied and established to be reasonably safe is the flavorings.