> > The other is almost entirely a mixture of Propylene Glycol and Vegetable Glycerin.
> What an extraordinary meaningless statement.
Not at all. The point being made is that we're dealing with a product composed primarily (99%) of two chemicals that are very clearly established as safe for human consumption (propylene glycol is commonly _injected_). As such, it seems unreasonable to assume absent evidence that the chemicals themselves would pose any significant threat to health.
That leaves other additives (which I noted) and the inhalation itself (which I also noted). As I stated in my previous comment, chronic inhalation of anything is likely to be bad for your health.
You really seem to have missed the context of the question I was responding to - it asked not about absolute safety, but rather safety relative to cigarette consumption.
Yes, as you note, there is always the potential for chemical reactions when mixtures are heated. Given the particular chemical composition and temperatures involved, I'm not personally concerned absent solid evidence. None of the byproducts listed in the study you linked appear particularly worrisome to me. Compared to cigarette, weed, or wood smoke, the contents of vaping such aerosols appears downright healthy. (Only in comparison, of course!)
Rereading my previous comment, something I failed to mention is the risk of inhaling things that are too hot. Depending on what temperature the aerosol is heated to, it could burn you internally (but then we're comparing this to inhaling burning plant matter, so... meh).
> I believe the current scientific consensus is that more research is needed.
To establish what, their precise negative health effects? Of course. But not to establish that they are safer than smoking cigarettes - there's absolutely no question about that.