Flavoring Chemicals in E-Cigarettes: Diacetyl, 2,3-Pentanedione, and Acetoin in a Sample of 51 Products, Including Fruit-, Candy-, and Cocktail-Flavored E-Cigarettes

The World Health Organization (WHO) reports that $3 billion was spent on electronic cigarettes (e-cigarettes) in 2013 in the United States alone, with sales expected to increase 17-fold in 15 years. The CDC (Centers for Disease Control and Prevention) estimates that 1.78 million children tried e-cigarettes as of 2012, with 160,000 of them reporting that they had not used tobacco cigarettes (CDC 2013). E-cigarettes are not currently regulated; the U.S. Food and Drug Administration (FDA), which has the authority to regulate certain tobacco and nicotine-containing products under the Family Smoking Prevention and Tobacco Control Act (2009) has issued a proposed rule to include e-cigarettes under this Act (FDA 2014). Although the popularity and use of e-cigarettes continues to increase, data are lacking on the exposures and potential human health effects of the use of e-cigarettes.

Concerns regarding e-cigarettes primarily focus on nicotine exposure, secondhand exposure, the potential for e-cigarettes to be a gateway to cigarette use, and renormalization/social acceptance of smoking (Bell and Keane 2014; Coleman et al. 2014; Goniewicz et al. 2014a; Long 2014; McMillen et al. 2015; Trehy et al. 2011). Other recent investigations have focused on the chemical content of the e-cigarettes beyond nicotine, with researchers finding that users of e-cigarettes are exposed to carbonyl compounds, aldehydes, fine particulate matter, metals, propylene glycol, glycerol, formaldehyde, VOCs, and other additives (Bekki et al. 2014; Callahan-Lyon 2014; Cheng 2014; Goniewicz et al. 2014b; Hutzler et al. 2014; Jensen et al. 2015; Orr 2014; Pellegrino et al. 2012; Uchiyama et al. 2013; Williams et al. 2013). However, despite > 7,000 flavors of e-cigarettes currently marketed (Zhu et al. 2014), only three papers have been published that focus on exposure to flavoring chemicals specifically (Farsalinos et al. 2015; Hutzler et al. 2014; Behar et al. 2014), and one opinion piece in JAMA that highlights the potential respiratory health effects from using flavored e-cigarettes (Barrington-Trimis et al. 2014).

The use of flavorings in food products gained public attention in the early 2000s because of reports of serious lung disease in microwave popcorn workers (Hilts 2001). The flavoring chemicals involved were on the Generally Recognized As Safe (GRAS) list that applies only to ingestion, but exposures were occurring via inhalation and very little was known about potential inhalation hazards of these chemicals at that time (FDA 2015). In May 2000, eight persons who had previously worked at a microwave popcorn–processing plant were reported to have severe bronchiolitis obliterans (Kreiss et al. 2002), an irreversible loss of pulmonary function that can become so severe that the only treatment option may be a lung transplant (OSHA 2007). Researchers from the National Institute of Occupational Safety and Health (NIOSH) Division of Respiratory Disease Studies conducted an investigation at the facility where the affected workers were employed. The NIOSH investigation included medical testing (including pulmonary function testing, medical questionnaires and work history documentation) and industrial hygiene exposure measurements (including grab samples, use of direct reading instruments, and full shift samples). NIOSH determined that workers at this plant had > 2 times the expected rates of chronic cough, shortness of breath, asthma, and chronic bronchitis, and nonsmokers had > 10 times the expected prevalence of airway obstruction (CDC 2007; Kreiss et al. 2002). A strong association was found between this excess of lung disease, including bronchiolitis obliterans, and airborne exposures to butter-flavoring chemicals in the facility. Diacetyl was the most prominent chemical in the butter flavorings. Two other flavoring compounds of interest, acetoin and 2,3-pentanedione, were present in significant amounts or not sampled, respectively. Workers in the area where diacetyl-containing butter flavoring was added into heated mixing vats were exposed to volatilized flavor chemicals, and a significant, positive dose response relationship was identified (CDC 2007; Kreiss et al. 2002). A follow-up investigation at six other microwave popcorn manufacturing facilities found that, in five of six plants, mixers of butter flavoring and packaging-area employees working near tanks of heated oil, with exposure to diacteyl as low as 0.2 ppm, had fixed airway obstruction consistent with bronchiolitis obliterans (Kanwal et al. 2006). Based on its occurrence in microwave popcorn manufacturing plants, bronchiolitis obliterans (and some related respiratory diseases of the small airways) became commonly known as “popcorn lung.” The findings of adverse health effects in workers at microwave popcorn plants prompted additional investigations. The CDC identified seven additional cases of bronchiolitis obliterans in workers at a flavoring manufacturing company (CDC 2007).

Diacetyl is contained in a variety of flavors in addition to butter flavor (Table 1) (OSHA 2010), and its use is not limited to microwave popcorn facilities or food flavoring production facilities. Diacetyl, 2,3-pentanedione (a structurally related replacement for diacetyl), and acetoin are used in the manufacture of many other foods for a wide range of flavors beyond butter flavorings (e.g., caramel, butterscotch, pina colada, strawberry). Many of these same flavors are common in e-cigarette flavor cartridges, and are often sold with names that we consider to be potentially appealing to children, teenagers, and young adults: Cupcake, Fruit Squirts, Waikiki Watermelon, Cotton Candy, Tutti Frutti, Double Apple Hookah, Blue Water Punch, Oatmeal Cookie, and Alien Blood. Further, e-cigarettes use a battery-driven nicotine delivery system in which an atomizer produces an aerosol (and vapors of evaporated liquids) through the heating of e-cigarette liquids contained in replaceable cartridges or re-fillable wells (Burstyn 2014; Jensen et al. 2015).

The heating, vaporization, and subsequent inhalation of these flavoring chemicals in e-cigarettes makes an exposure pathway for these flavorings that has significant similarities to those of the workers at the microwave popcorn facilities. In microwave popcorn manufacturing, flavorings, salt, and colorants are added to heated soybean oil (57–60°C). Kullman et al. (2005) reported that aerosols and flavoring ingredient vapors were found in these mixing rooms. The aerosol found to have a combustible fraction that ranged from 70% to 85% by weight (average 79%) and a noncombustible fraction of 21%. The aerosol was identified as salt particles and oil-coated salt particles, and much of the aerosol was of respirable size. The mixing rooms were where the highest air concentration of flavorings was found (Kullman et al. 2005).

Given the widespread use of these food flavors across many industries and the knowledge that specific chemicals/artificial flavors were developed to mimic certain natural flavors commonly used in e-cigarettes, we hypothesized that these compounds are likely used in the manufacturing of flavored e-cigarettes. We sought to expand the state of knowledge on flavoring chemicals in e-cigarettes with a particular focus on e-cigarettes sold by the largest cigarette companies and also those flavors that we deem would be appealing to children, teenagers, and young adults.

The total mass per e-cigarette (micrograms per e-cigarette) of the flavoring chemicals diacetyl, 2,3-pentanedione, and acetoin are presented in Table 2. Diacetyl was above the LOD in 39 of the 51 flavors tested, ranging from < LOQ to 239 μg/e-cigarette. 2,3-Pentanedione and acetoin were detected in 23 and 46 of the 51 flavors tested at concentrations up to 64 and 529 μg/e-cigarette, respectively.

At least one of the flavoring chemicals was detected in 47 of the 51 unique flavors tested (92%). This includes several e-cigarette flavors that are not candy or fruit flavored, such as “classic” and “menthol.” Diacetyl and 2,3-pentanedione were detected simultaneously in 21 unique flavors, suggesting that 2,3-pentanedione may not be only a replacement for diacetyl but is often used in conjunction with diacetyl in e-cigarettes. Similarly, 2,3-pentanedione and acetoin were also detected simultaneously in 22 flavors. Diacetyl and acetoin were simultaneously detected in an even greater number of flavors (n = 38).

Figure 2 depicts the distributions of the chemical masses of the e-cigarette samples, including replicates, according to flavor type. The three compounds were detected in all flavor types, except for 2,3-pentanedione in dairy-flavored e-cigarettes, which only had two samples. The median masses of the flavor types did not have a consistent ranking from one chemical to the next. For example, tobacco flavors had the second to lowest median mass for diacetyl compared with the fourth lowest for acetoin. The cocktail-flavored e-cigarettes had the highest median masses and largest range for all three compounds, but none of the differences in the mean masses of each flavor type were statistically significant, using the Bonferroni correction for multiple comparisons.

Diacetyl—a flavoring compound associated with the development of “popcorn lung” in workers after inhalation exposure—was detected in 39 of the 51 flavored e-cigarettes tested in this study, including flavors that have particular appeal to children, teenagers, and young adults. Forty-seven of the 51 flavors tested in our study had at least one of the three flavoring compounds detected (diacetyl, 2,3-pentandedione, acetoin). These compounds were ubiquitous among flavor types: “tobacco” and “menthol” flavored e-cigarettes contained diacetyl despite not being listed on OSHA’s list of flavors that likely contain diacetyl (Table 1).

The health concerns related to inhaling diacetyl and other flavoring chemicals are now well recognized by OSHA and the flavoring industry. OSHA established a National Emphasis Program (NEP) in 2007 focused on respiratory disease in workers at microwave popcorn–processing facilities, and a NEP in 2009 focused on “Facilities that Manufacture Food Flavorings Containing Diacetyl” (OSHA 2007, 2009). The Flavoring and Extract Manufacturers Association of the United States released a report in April 2012 on respiratory health and safety in the food-manufacturing workplace that highlighted the potential risks associated with inhaling diacetyl and a long list of other food flavoring chemicals (FEMA 2012). FEMA recommends the following warning for “Any compounded flavors (liquid, dry or powdered) containing any flavoring substances listed in Table 1 in any concentrations if the compounded flavor or any of its individual flavoring substances will be heated during processing.” [The Table 1 referenced is specific to diacetyl.]

Unlike these efforts by OSHA and the flavor industry to raise awareness of the hazards associated with inhaling flavoring chemicals, our review of the websites and packaging for the flavored e-cigarette brands in our study did not identify any similar notifications regarding diacetyl specifically or flavorings generally. Two companies explicitly stated that their products do not contain diacetyl in written communication, yet in our testing we did find diacetyl in their product.

Rules for labeling do not currently exist for e-cigarettes, because—unlike tobacco products, which are regulated by the FDA under authority of the Family Smoking Prevention and Tobacco Control Act (2009), a statute that authorizes the FDA to require warning labels on packages and advertisements and bans flavored cigarettes—e-cigarettes are not currently regulated. However, this may be changing. In 2014, the FDA issued a proposed rule that seeks to expand the legal definition of tobacco products to include e-cigarettes and other nicotine-containing products (FDA 2014). If finalized, the rule may include minimum age and identification requirements and proposed addictiveness warnings. Specifically related to the research presented here, and our opinion that many flavors are appealing to youth, the FDA (2014) states that “some tobacco products, such as e-cigarettes and certain cigars, are being marketed with characterizing flavors, and that these flavors can be especially attractive to youth.” The FDA then acknowledges that the existing Family Smoking Prevention and Tobacco Control Act of 2009 prohibiting flavors currently applies only to cigarettes, not e-cigarettes, and they are seeking additional information regarding the effects e-cigarettes have on public health. The data presented in this manuscript on the presence of flavoring chemicals in e-cigarettes that have been previously associated with severe respiratory disease are a step toward addressing this information gap.

As a result of the toxicological and epidemiological studies by NIOSH, inhalation exposure limits for adult workers have been established for several food-flavoring compounds, including diacetyl and its structurally similar replacement, 2,3-pentanedione (Table 3). However, there are no health-based standards for diacetyl inhalation for the general public, and no standards for children. We agree with a recent response to an article by NIOSH investigators (Hubbs et al. 2015) and an advisory released by FEMA (2015) that there are important considerations in interpreting analyses that use occupational health limits for estimating risk for e-cigarette smokers. First, these occupational health limits are set for healthy workers, not the general population, and e-cigarette users are not exclusively workers. Second, the U.S. regulatory agencies accept greater risk for workers than for the general population. For example, “acceptable” risk for workers is generally 1 in 1,000 to 1 in 10,000 risk of an adverse event (Hubbs et al. 2015), whereas the U.S. EPA (Environmental Protection Agency) uses 1 in 100,000 to 1 in 1,000,000 as “acceptable” for the general population (Castorina and Woodruff 2003). By applying occupational health limits to the general population of flavored e-cigarette smokers, we would thus be accepting a higher risk than typical. Third, the occupational limits are based on an 8-hr period, 5 days per week, and come with the assumption that a worker will have 16 hr of recovery time between shifts, and 2-day recovery on the weekend, which is not applicable to e-cigarette users. Fourth, these exposure limits are for adults, not children, who on average have a smaller body weight compared with typical adult workers, resulting in a greater overall dose per e-cigarette for children and adolescents. Fifth, we do not know whether the dose–response relationships observed for workers would be similar for children, who can be more susceptible to some environmental exposures. Last, the occupational exposure limits are not “bright lines”; values below the limit should not automatically be interpreted as “safe.” In fact, there is guidance for interpreting values below the occupational health limits. The AIHA first uses the upper confidence limit of the 95th percentile exposure value when comparing exposure measurements to an occupational limit, not just a point estimate or mean. They then use a “control banding” to establish where the exposure fits within five AIHA exposure control categories (Ignacio and Bullock 2006). Even when the 95th percentile exposure estimate is 10% of the occupational limit, at a minimum hazard communication is a typical control response to the exposure (Hewett et al. 2006). At 50% of the occupational limit, additional controls are also typical, including exposure surveillance, medical surveillance, and work practice evaluation (Hewett et al. 2006).

Our findings confirm the presence of diacetyl and other flavoring chemicals in flavored e-cigarettes. Because of the associations between diacetyl and bronchiolitis obliterans and other severe respiratory diseases among workers inhaling heated vapors containing diacetyl, urgent action is recommended to further evaluate the extent of this new exposure to diacetyl and related flavoring compounds in e-cigarettes.