Diacetyl (butanedione or butane-2,3-dione) is an organic compound with the chemical formula (CH3CO)2. It is a yellow or green liquid with an intensely buttery flavor. It is a vicinal diketone (two C=O groups, side-by-side) with the molecular formula C4H6O2. Diacetyl occurs naturally in alcoholic beverages and is added to some foods to impart its buttery flavor.
A 2014 publication found that diacetyl was present in samples of many sweet-flavored e-liquids, though at levels 100 times lower than that of traditional tobacco smoke. According to that research, diacetyl is approved for food use, but is associated with respiratory disease when inhaled. Although diacetyl was initially linked to a condition known as popcorn lung (formally bronchiolitis obliterans) among flavored popcorn factory workers, the condition is incredibly rare and likely only occurs with prolonged exposure to pure diacetyl in the air with little or no protection. Despite levels of diacetyl being much higher in traditional cigarettes than it is in e-liquids, bronchiolitis obliterans has never been linked to traditional smoking, yet mass media used the trace-detectable presence of the common flavoring additive in e-liquids to spread the rumour that it was causing popcorn lung.