Diacetyl or butane-2,3-dione ((CH3CO)2) is an organic compound that is a by-product of fermentation.
Diacetyl is naturally present in alcoholic beverages and is used as a buttery flavor in the food industry. This flavor is also used in the elaboration of receipes of e-liquids.
Despite innocuousness when ingested, diacetyl has proven harmfulness upon inhalation. Some employees of manufactures using artificial butter flavoring have been diagnosed with bronchiolitis obliterans, a rare and serious disease of the lungs associated to this compound.
Together with acetylpropionyl, diacetyl is of legal use as a flavoring substance in the European food industry but their use is controversial in e-liquids for their potential harmfulness (Farsalinos et al., 2014).