Well, in the short term, the FDA regulation describes it as generally accepted as safe for humans. That comes from propylene glycol being used as a delivery mechanism in many of the asthma inhalers, which is where we've got the human exposure data from. But again, with an asthma inhaler, the person sprays maybe once or twice a day, as opposed to puffing back or whiffing on that e-cigarette through the whole day and becoming a chain user of it. You may be getting yourself a very large exposure to propylene glycol, for which we don't have any understanding whether it is harmful or not.
The same thing applies if somebody drinks alcohol in a small quantity for a standard drink every other day: their risk of harm is small. Of course, they could be harmed if they tripped, fell, and hurt themselves, but clearly, the risk of harm to somebody who's consuming a whole 40-ouncer every day is going to be much greater. It's not that the quality of the alcohol has shifted; it's the amount of exposure that makes a difference.