Well, really, no one is certain 100%. There have been studies on inhaling PG, and I think glycerin/VG to some extent, but most e-liquids also contain some trace level of other things..plus, or as part of, the flavorings. They are of course, generally regarded as safe for consumption/ingestion, but it's unknown if there have been any specific inhalation longer term tests on most flavorings.
The general 'community consensus' is for flavorings - go all natural whenever possible, avoid dyes if possible, but it would seem quite likely to at worse, be far healthier than analogs. YMMV, you've got to make up your own mind here, really..