The United States National Institute for Occupational Safety and Health has suggested diacetyl, when used in artificial butter flavoring (as used in many consumer foods), may be hazardous when heated and inhaled over a long period.
Workers in several factories that manufacture artificial butter flavoring have been diagnosed with bronchiolitis obliterans, a rare and serious disease of the lungs. The cases found have been mainly in young, healthy, nonsmoking males. As with other end-stage lung diseases, transplantation is currently the most viable treatment option. However, lung transplant rejection is very common and happens to be another setting in which bronchiolitis obliterans is known to occur.
While several authorities have called the disease "popcorn worker's lung", a more accurate term suggested by other doctors may be more appropriate, since the disease can occur in any industry working with diacetyl: diacetyl-induced bronchiolitis obliterans.
Granted these guys were inhaling a lot more for a longer period of time, but I took up vaping to reduce harmful effects. Diacetyl is used in e-liquid, typically custards and cream type juices. I'm not saying everyone uses them but it doesn't hurt to ask.