The answer I saw was from a poster, NOT a MD. The one that blew up in the guy's face was posted all over the internet. He allegedly tampered with a cigalike, and didn't get the result he was looking for. I have also read rumors about antifreeze being in e-juice. What they are using though is polypropylene, and not propylene glycol USP, if that makes sense. There is a difference between the two. Propylene glycol is used in hospital ventilation systems to sanitize the air, to my understanding. Propylene glycol, (medical grade) is also used in medical inhalers. If you feel unsafe vaping, then I suggest finding medical grade and DIY.
Here I go butting in...again... I see this statement all the time and it's not exactly true. PG is used as a (liquid) carrier for whatever sanitizing fluids are being used - the same is true for inhalers as far as I know (and I'm no doctor). PG is used because it has no inherent harmful properties beyond the possibility of person specific allergies, but it is not some kind of air cleaner on its own. It's just recognized as being a safe carrier.
OK... I'll go back to my cave now