To be clear, there are no documented cases of bronchiolitis obliterans caused by vaping. Historically, there were cases among workers in a microwave popcorn factory who were exposed to large quantities of the flavoring chemical diacetyl. Some early e-liquid flavors contained diacetyl, and while the level of exposure was drastically lower than that of a popcorn factory worker, people were concerned about it, and that concern led e-liquid manufacturers to stop using it.