Diacetyl, the buttery-like additive used in microwave popcorn could be the causative agent in so-called "popcorn worker's lung," or more formally bronchiolitis obliterans syndrome (BOS). BOS is a severe occupational lung disease first observed in 2001 among workers at US plant that makes microwaveable popcorn. Frits van Rooyof the University of Utrecht in the Netherlands and colleagues examined a population of workers at a chemical plant that produced diacetyl (a key component of butter flavoring). "Our study found a cluster of [previously undiagnosed] BOS cases in a diacetyl production plant," van Rooy explains, "This supports the conclusion that an agent in the diacetyl production process has caused BOS." The findings were reported in the September issue of the American Journal of Respiratory and Critical Care Medicine. The novel finding of four cases of BOS in workers at the diacetyl plant has important implications for practicing physicians and public health officials, although a definitive connection between diacetyl itself and the onset of BOS is yet to be obtained.