Scientists at the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health, said they conducted the study because of implications that the chemical, which is used in popcorn and confectionery production, may lead to the lung disease bronchiolitis obliterans syndrome (BOS), also known as obliterative bronchiolitis (OB).
BOS, an incurable disease that causes thickening and scarring of the lungs, was diagnosed in a number of workers at US popcorn manufacturing plants last year.
Daniel Morgan, head of the Respiratory Toxicology Group at the NIEHS, said that mice subjected to inhalation of the chemical later developed lymphocytic bronchiolitis, a condition that can lead to BOS.
"This is one of the first studies to evaluate the respiratory toxicity of diacetyl at levels relevant to human health," he said in a prepared statement. "Mice were exposed to diacetyl at concentrations and durations comparable to what may be inhaled at some microwave popcorn packaging plants."
The authors concluded that "these findings suggest that workplace exposure to diacetyl contributes to the development of BOS in humans", but added that more research on the topic is needed.
The National Toxicology Program, headquarted at the NIEHS now plans to compile more scientific data that will then be shared with public health and regulatory agencies, Morgan said.
Diacetyl fears first arose after researchers from the University of Utrecht in the Netherlands found undiagnosed BOS cases amongst a group of popcorn workers in the US.
They then traced 196 former workers who were still living and who had been employed at the diacetyl production plant between 1960 and 2003, when the plant closed.
Of the 102 process workers considered to be at the highest risk of exposure, researchers positively identified three cases of BOS, and later, a fourth, in a worker who had initially declined to participate in the research.
US popcorn giants Pop Weaver and ConAgra then announced they were eliminating diacetyl in their butter flavours in 2007.
In Europe, diacetyl was last assessed in 1999, when the former Committee of Experts on Flavouring Substances of the Council of Europe (CEFS) concluded that it is safe for human consumption at the quantities used. The assessment looked only at consumption and not inhalation.
However, in October, the European Food Safety Authority (EFSA) said that it was taking the US findings "seriously". EFSA's scientific panel on food additives and flavourings (AFC) said it is already evaluating the chemical along with other flavourings as part of a larger study. Editorial: Toxicological Sciences "Respiratory Toxicity of Diacetyl in C57BI/6 mice" DOI: 10.1093/toxsci/kfn016 Authors: D. Morgan, G. Flake, P Kirby, S Palmer.