Vaping may seem like a healthier alternative to cigarettes, but the list of its specific potential risks keeps growing. A study published Thursday in the journal Nicotine & Tobacco Research uncovers new evidence that some common flavoring chemicals in e-liquids are not as benign as they seem.

In the paper, researchers at Duke University and Yale University show that the flavoring chemicals undergo reactions with other compounds in the e-liquid before a vape even heats up. In particular, they looked at the reaction between flavoring chemicals called aldehydes and propylene glycol, which forms the base of many e-liquids. Aldehyde-based vape flavors have previously been criticized for their potential hazards to human health.

Sven Eric Jordt, Ph.D., an associate professor of anesthesiology at Duke University and the corresponding author on the new study, says that his team’s research adds to the growing body of evidence that e-cigarettes are different from — but not necessarily better than — conventional cigarettes.

“E-cigarette vendors often state that e-cigarettes are inherently more safe since they contain only a few ingredients, flavors, nicotine, and a solvent, compared to traditional cigarettes that produce smoke with 1,000s of chemicals in it,” he told Gizmodo. “We found that the e-liquids vaporized by e-cigarettes are in fact chemically unstable and that, after the mixing of components, the flavor chemicals are changed into new chemicals (the acetals) with unknown toxic effects.”

Vape life

To model how the chemicals react in containers of e-liquid, the team mixed together propylene glycol (PG) with benzaldehyde, cinnamaldehyde, citral, vanillin, and ethylvanillin — aldehydes commonly used as flavorings. Within two weeks, they reacted to form chemicals called “flavor aldehyde PG acetals,” which the researchers have also observed in store-bought vape liquids.

“This study demonstrates the potential chemical instability of e-liquids,” the authors write. “This reaction starts almost immediately and continues over days.” As the team later found, the acetals are present in the vapor formed by an e-cigarette, which means people inhale them while vaping.

Of course, none of this would matter if acetals were harmless, but as the team shows, human cells exposed to acetals in a petri dish displayed an irritation response. It’s too early to say how that interaction would play out in the human body, but these results strongly suggest that these flavor aldehyde PG acetals are probably not good for us.

This work gives strong evidence that there are far more chemicals in e-cigarette vapor than just the ones that are listed on an ingredients label — if a product even has an ingredients label. The study’s authors call for manufacturers and regulators to further investigate the full range of chemicals that end up in a person’s body when they vape.

“To fully assess the risk potential of e-liquid use for regulatory purposes,” they write, “it is imperative that the compounds that a user will actually be exposed to are reported and evaluated, and not only the initial ingredients combined during manufacturing.”


Introduction: “Vaping” electronic cigarettes (e-cigarettes) is increasingly popular with youth, driven by the wide range of available flavors, often created using flavor aldehydes.The objective of this study was to examine whether flavor aldehydes remain stable in e-cigarette liquids or whether they undergo chemical reactions, forming novel chemical species that may cause harm to the user. Methods: Gas chromatography was used to determine concentrations of flavor aldehydes and reaction products in e-liquids and vapor generated from a commercial e-cigarette. Stability of the detected reaction products in aqueous media was monitored by ultraviolet spectroscopy and nuclear magnetic resonance spectroscopy, and their effects on irritant receptors determined by fluorescent calcium imaging in HEK-293T cells.