It's primarily been because of the contaminant diacetyl, which is the cause of popcorn lung per se.
Some studies out of Italy suggest that people who have chronic lung disease or asthma and are unable or unwilling to stop smoking may experience an improvement when they switch to vaping devices, but given the short duration of exposure that we are seeing, we're not having reports of harm from the general population if they vape, other than anecdotal case reports.
Again, as I said, the concerns are not so much with associations with the lung disease but with regard to the cardiovascular effects of these products. Again, what we really need to focus on is the relative risk for people who are tobacco-addicted or smoking-addicted to these products. From a lung perspective, other than the diacetyl, it's not clear that you could end up with popcorn lung, and diacetyl is a flavouring.