If I were you I'd stop using those carts and just direct drip. Then you don't have to worry about heating up whatever is in those cartridges. As for Propylene Glycol, it's also in a lot of foods, asthma inhalers and fog machines. You've no doubt been ingesting it all of your life.
Here's a link of applications for Propylene Glycol. Propylene Glycol applications
If you're worried about PG, you can get juices that are made with vegetable glycerin instead of PG. Personally, I'd rather inhale PG or VG than the arsenic and other 4000 deadly chemicals in a cigarette.
if you have any more questions don't hesitate to ask, the people here are quite friendly and helpful