PG's wiki page goes over how Propylene glycol was fully tested on ingestion and inhalation in the 40's... This is why the FDA has approved it as a food additive and VG as an additive in beauty and medical products. They know what it is they know what it does they don't want to test it effectively and publicly though cause then they wouldn't be able to say "oh PG is used in antifreeze" so idiot people will assume it would be as healthy as inhaling antifreeze.
PG has a very low freezing temp due to it's natural compound build up so naturally it would be a cheap and effective substance to use to cut the other chemicals with used in antifreeze. Don't know how the FDA could indirectly tell people that they might as well continue to puff on something proven to kill you because there may be some elements of smoking vapor that's not healthy or there might not be, it's just too unimportant of a thing to bother testing...
PG has a very low freezing temp due to it's natural compound build up so naturally it would be a cheap and effective substance to use to cut the other chemicals with used in antifreeze. Don't know how the FDA could indirectly tell people that they might as well continue to puff on something proven to kill you because there may be some elements of smoking vapor that's not healthy or there might not be, it's just too unimportant of a thing to bother testing...