Acetyl Proprionyl, Diacetyl, Acetoine are all ingredients that are used to create flavors such as creamy, caramel, butterscotch, any of the custard flavors and a few others I cannot think of. They can cause irreversible damage to the lungs. Not all e-liquids have these ingredients, but many do. It requires special testing to determine if those ingredients are present and then find a substitute. There are a number of highly ethical e-liquid manufactures that have done the testing and have made the changes, but I would say, and this is just speculation on my part, that most have not.