Exposure to e-cigarette flavouring chemicals and liquids can cause significant inflammation to monocytes, a type of white blood cell – and many flavouring compounds are also toxic, with cinnamon, vanilla and buttery flavours among the worst. That’s the finding of new research, which also found that mixing e-cigarette flavours has a much worse effect than exposure to just one. The study adds to growing evidence on the harmful health effects of e-cigarettes.
The use of e-cigarettes has exploded in the past decade as traditional cigarette consumption has declined. In the US alone, more than 500 e-cigarette brands with almost 8,000 uniquely flavoured e-juices are available to consumers.
Vaping exposes the lungs to flavouring chemicals when the e-liquids are heated and inhaled. Since the flavouring chemicals are considered safe to eat, e-cigarettes are often considered – and advertised – as a healthier alternative to traditional cigarettes. However, the health effects of inhaling these chemicals are not well understood.
This new study, led by researchers at the University of Rochester Medical Centre in the US, wanted to test the assumption that vaping nicotine-free flavoured e-liquids is safer than smoking conventional cigarettes. Previous studies show that flavours used in e-cigarettes cause inflammatory and oxidative stress responses in lung cells. Users of e-cigarettes also show increased levels oxidative stress markers in the blood compared to non-smokers. The new study extends this to assess the effects of commonly used flavouring chemicals, as well as e-liquids without nicotine, directly on immune cells – namely, a type of white blood cell called monocytes.
Exposure to the e-cigarette flavouring chemicals and e-liquids led to higher production of two well-established bio-markers for inflammation and tissue damage mediated by oxidative stress. Furthermore, many of the flavouring chemicals caused significant cell death – with some flavours being more toxic than others.
The study’s first author, Dr Thivanka Muthumalage says that while the flavouring compounds tested may be safe for ingestion, these results show they are not safe for inhalation. “Cinnamon, vanilla and butter flavouring chemicals were the most toxic but our research showed that mixing flavours of e-liquids caused by far the most toxicity to white blood cells.”
Senior author, Dr Irfan Rahman, says he hopes this new data will provide insights into understanding the harmful effects of flavoured e-juices without nicotine. “Currently, these are not regulated, and alluring flavour names, such as candy, cake, cinnamon roll and mystery mix, attract young vapers,” he says. “Our scientific findings show that e-liquid flavours can, and should, be regulated and that e-juice bottles must have a descriptive listing of all ingredients. We urge regulatory agencies to act to protect public health.”
This study directly exposed monocytic blood cells to e-liquids. The authors plan to undertake further research to simulate live vaping, by exposing cells to e-liquid aerosols in an air-liquid interface system. They also call for further long-term human studies to assess the harmful effects of e-cigarettes.
Background: The respiratory health effects of inhalation exposure to e-cigarette flavoring chemicals are not well understood. We focused our study on the immuno-toxicological and the oxidative stress effects by these e-cigarette flavoring chemicals on two types of human monocytic cell lines, Mono Mac 6 (MM6) and U937. The potential to cause oxidative stress by these flavoring chemicals was assessed by measuring the production of reactive oxygen species (ROS). We hypothesized that the flavoring chemicals used in e-juices/e-liquids induce an inflammatory response, cellular toxicity, and ROS production.
Methods: Two monocytic cell types, MM6 and U937 were exposed to commonly used e-cigarette flavoring chemicals; diacetyl, cinnamaldehyde, acetoin, pentanedione, o-vanillin, maltol and coumarin at different doses between 10 and 1,000 μM. Cell viability and the concentrations of the secreted inflammatory cytokine interleukin 8 (IL-8) were measured in the conditioned media. Cell-free ROS produced by these commonly used flavoring chemicals were also measured using a 2′,7′dichlorofluorescein diacetate probe. These DCF fluorescence data were expressed as hydrogen peroxide (H2O2) equivalents. Cytotoxicity due to the exposure to selected e-liquids was assessed by cell viability and the IL-8 inflammatory cytokine response in the conditioned media.
Results: Treatment of the cells with flavoring chemicals and flavored e-liquid without nicotine caused cytotoxicity dose-dependently. The exposed monocytic cells secreted interleukin 8 (IL-8) chemokine in a dose-dependent manner compared to the unexposed cell groups depicting a biologically significant inflammatory response. The measurement of cell-free ROS by the flavoring chemicals and e-liquids showed significantly increased levels of H2O2 equivalents in a dose-dependent manner compared to the control reagents. Mixing a variety of flavors resulted in greater cytotoxicity and cell-free ROS levels compared to the treatments with individual flavors, suggesting that mixing of multiple flavors of e-liquids are more harmful to the users.
Conclusions: Our data suggest that the flavorings used in e-juices can trigger an inflammatory response in monocytes, mediated by ROS production, providing insights into potential pulmonary toxicity and tissue damage in e-cigarette users.
Thivanka Muthumalage, Melanie Prinz, Kwadwo O Ansah, Janice Gerloff, Isaac K Sundar, Irfan Rahman