Diacetyl (2,3-butanedione) is a natural ingredient in foodstuffs which is not generally regarded health risk to consumers. Nevertheless, when manufactured for use as a synthetic flavouring/additive in processed foods (e.g. microwave popcorn), it poses a human health threat at the workplace. Its pneumotoxic action consists of inflammation, obstruction and restriction in the distal respiratory tract. One of the factors causing bronchiolitis obliterans is also recognised to be diacetyl. The scientific literature mostly describes human exposure to diacetyl in factory settings where functional disorders and structural changes of the respiratory system have been recorded, particularly bronchiolitis obliterans. Moreover, differential diagnosis shows pathological changes in the distal respiratory tract and in the pneumotoxic actions of diacetyl.