How dangerous are nicotine free electronic cigarretes/vapers compounds?

Unclear: Certainly, on balance, it is likely that e-cigs are safer than smoking tobacco in whatever form (pipes, cigars, cigarettes), and without the nicotine there is no longer a substance which causes both the physical dependency and the cardiovascular problems. However, there is usually a vehicle in the e-cig such as propylene glycol, and the safety of this when smoked is unknown. Ditto for additives.