Exposures to Flavoring Chemicals

Exposure to Flavoring Chemicals

soybean oil tank

What are Flavorings?

Flavorings are often complex mixtures of natural and man-made substances. The U.S. Food and Drug Administration (FDA) evaluates flavoring ingredients to determine whether they are “generally recognized as safe” (GRAS) to be eaten. Even if they are safe to eat, these ingredients might still be harmful to breathe in the forms and amounts to which food and chemical industry workers may be exposed. Given the complexity of flavorings mixtures, and the lack of health data for many of the component materials, identifying the relative contributions of individual substances to causing flavoring-induced lung disease is a difficult challenge. As noted in the NIOSH Alert: Preventing Lung Disease in Workers Who Use or Make Flavorings, the flavorings industry has estimated that over a thousand flavoring ingredients have the potential to be respiratory hazards due to possible volatility and irritant properties (alpha, beta-unsaturated aldehydes and ketones, aliphatic aldehydes, aliphatic carboxylic acids, aliphatic amines, and aliphatic aromatic thiols and sulfides).


Diacetyl is a chemical that was found to be a prominent volatile constituent in butter flavoring and air at the microwave popcorn plant initially investigated by NIOSH. Diacetyl is also known as the alpha-diketone, 2,3-butanedione, or by its Chemical Abstracts Service (CAS) number, 431-03-08. NIOSH has published a comprehensive document describing diacetyl and 2,3 pentanedione entitled Criteria for a Recommended Standard: Occupational Exposure to Diacetyl and 2,3-Pentanedione.

Workers in microwave popcorn manufacturing are exposed to many materials besides diacetyl. Thus, NIOSH’s initial studies in a total of six microwave popcorn plants were not able to definitely determine if diacetyl exposure contributed to lung disease or was a marker for other hazardous substances that contributed to disease. Still, NIOSH studies in the initial plant documented a relationship between cumulative exposure to diacetyl vapor over time and having abnormal lung function as measured by spirometry. Higher cumulative exposure to diacetyl in this plant was associated with having a lower level of forced expiratory volume in 1 second (FEV 1), an important measure of lung function. Across all six microwave popcorn plants studied by NIOSH, working as a mixer of butter flavorings and heated soybean oil was associated with higher exposure to diacetyl vapor than working in other areas of the plants. People who had ever worked as mixers had more chest symptoms and poorer lung function as measured by lower FEV 1 than people who had never worked as mixers. People who had worked as mixers for more than 12 months had more shortness of breath with exertion and lower FEV 1 than people who had worked as mixers for less than 12 months.

Subsequent studies have helped to clarify the role of diacetyl in substance toxicity. Toxicology studies have shown that vapors from heated butter flavorings can cause damage to airways in animals (Hubbs et al. 2002external icon). Studies in both rats and mice demonstrate that the cells lining the airways can be damaged by inhaling diacetyl vapors as a single agent exposure in both acute and sub-chronic studies (Hubbs et al. 2008external icon, Morgan et al. 2008external icon).  In mice, aspiration of diacetyl alone caused a pattern of injury that replicates some of the features of human obliterative bronchiolitis (Morgan et al. 2008external icon).  In addition, inhaling either diacetyl or the related flavoring, 2,3-pentanedione can cause an obliterative bronchiolitis-like condition in rats (Morgan et al. 2016external icon).

Furthermore, diacetyl reacts with proteins and a recent study demonstrating striking changes in protein homeostasis in the airways of diacetyl-exposed mice strongly implicates protein damage in airway epithelium as the mechanism for diacetyl-induced airway injury (Hubbs et al. 2016external icon).  Dosimetry studies indicate that at a given exposure concentration, a much greater concentration of diacetyl can reach the deep lung of humans than reaches the deep lung of rats (Gloede et al. 2011external iconMorris and Hubbs. 2009external icon). These findings support the hypothesis that diacetyl vapors are an inhalation hazard in the workplace. Also, chemical workers in a plant that manufactured diacetyl and coffee workers exposed to diacetyl developed the same type of lung disease as microwave popcorn workers (van Rooy et al. 2007 and 2009). The chemical workers had less complicated exposures than microwave popcorn workers. Overall, current evidence points to diacetyl as one agent that can cause flavorings-related lung disease.


The alpha-diketone, 2,3-pentanedione, has received attention as a flavoring substitute for diacetyl. It is also known as acetyl propionyl or by CAS number 600-14-6. It is structurally very similar to diacetyl since 2,3-pentanedione is a 5-carbon alpha-diketone and diacetyl is a 4-carbon alpha-diketone. Recent mechanistic studies implicate the alpha-diketone functional group in the airway toxicity of diacetyl (Morgan et al. 2016external icon, Hubbs et al. 2016external icon).

Reports of 2,3-pentanedione toxicity were first published in 2010 (Hubbs et al. 2010aexternal icon,  Morgan et al. 2010). A follow-up full length publication demonstrated that acute inhalation exposures to 2,3-pentanedione cause airway epithelial damage that is similar to diacetyl in laboratory studies (Hubbs et al. 2012external icon). In longer 2-week inhalation studies in rats, researchers from the National Institute of Environmental Health Sciences (NIEHS) found that repeated exposures to either 2,3-pentanedione or diacetyl can cause airway fibrosis, including obliterative bronchiolitis-like changes, in rats (Morgan et al. 2016external icon). In the acute inhalation study of 2,3-pentanedione, changes in gene expression were noted in the brain (Hubbs et al. 2012external icon). Similarly, diacetyl can cause changes in gene expression and other markers of damage in the olfactory bulb of the mouse brain (Hubbs et al. 2016external icon).

Other Alpha-diketones

As a group, these publications raise concerns that the toxicologic effects of diacetyl may be shared with close structural analogs used in food manufacturing such as 2,3-hexanedione and 2,3-heptanedione (Day et al. 2011external icon). The 6-carbon alpha-diketone, 2,3-hexanedione is less chemically reactive than diacetyl or 2,3-pentanedione, and only 2 of 12 rats had bronchial fibrosis after a 2-week inhalation exposure to 2,3-hexanedione (Morgan et al. 2016external icon).

Page last reviewed: October 3, 2017