Short answer: we don’t know. That should terrify you. Short guess: the added aldehydes.
Longer answer: Most states are scrambling to just identify e-cigarettes, to decide on when kids should be able to get them, and to regulate them in some way. It’s the Wild West out there, with ages anywhere from 18–21, and many states not classifying vaping in the same way as tobacco use. A cynical person might be suspicious that vaping is an end run by certain manufacturers to entice and addict more young smokers. In my blog post on popcorn lung I connect the dots and - no surprise - vaping is funded and owned by big tobacco.
The two solvents most commonly used in vaping, PG and glycerine, are relatively nontoxic, but the flavoring agents they are often combined with can be extremely toxic. We have almost no regulation of these, as many are considered safe for eating. But the chemistry changes when you vaporize something. One study found “detectable levels of over one hundred fifteen volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs) from a single 40mL puff.”
A volatile compound is one that interacts with your lung tissue easily. One or more of these compounds could easily be the culprit, but it’s more likely that many of these compounds interact in new and terrible ways. Until we have studies on the prolonged inhalation of these compounds (picture mice vaping for days), every person who vapes is potentially damaging their lungs. Some countries have banned vaping for this reason.
But shouldn’t the FDA be regulating these things? They’ve tried, but vaping doesn’t fall under existing laws. So in 2016 the FDA ruled that they could regulate vaping. And the vaping manufacturers sued the FDA, cases still pending.
If the FDA does win, they are only regulating the nicotine, not the solvents or additives. So the problem of severe lung disease will continue due to: “(v)arious chemical substances and ultrafine particles known to be toxic, carcinogenic, and/or to cause respiratory and cardiac disease have been identified in e-cigarette aerosols, cartridges, refill liquids, and environmental emissions.”So it’s not one compound, it’s many. Just like cigarettes cause cancer because of a combination of carcinogens.