Chemical changes in e-cigarette liquids can make them irritants, study says

"Flavor chemicals and the solvents, the liquids, that are in electronic cigarettes, they are forming new chemical compounds," said Sven Jordt, one of the authors of the study published Thursday in the journal Nicotine and Tobacco Research and an associate professor of anesthesiology, pharmacology and cancer biology at the Duke University School of Medicine.

These new compounds can cause irritation in the airways of e-cigarette users, the researchers found.

"Our nose, mouth and throat contain nerve endings that sense painful and irritating chemicals in the inhaled air. For example, the burning, stinging feeling elicited by smoke inhalation is mediated by these nerve endings. They also trigger sneezing and cough, basically to defend the lungs from inhaling toxic chemicals," Jordt said.

"The new chemicals we identified in e-cigarettes activate these nerve endings more strongly. Especially when activated over a longer period, as in smokers, and potentially in e-cigarette users, these mechanisms have been shown to cause inflammation and asthma and contribute to emphysema."

E-cigarettes work by heating a pure liquid called e-juice -- composed of flavorings, propylene glycol, glycerin and often nicotine -- until it vaporizes.

The researchers vaporized and then chemically analyzed 10 e-liquids, with two flavors and five different propylene glycol ratios for each flavor. A propylene glycol ratio is the amount of solvent used in the e-liquid. The e-liquids were purchased from the website AmericaneLiquidStore. The researchers also created and analyzed their own liquids that included the flavors (also known as aldehydes) and solvents often used by the e-cigarette industry.

These analyses showed that 40% of the flavored aldehydes turned into new compounds called aldehyde PG acetals. "PG" represents the solvent used in the liquid, propylene glycol.

The flavorings used in the study were vanilla, cherry and cinnamon: vanillin and ethylvanillin, benzaldehyde and cinnamaldehyde, respectively.

When the liquids that the researchers created were turned to e-vapor, between 50% and 80% of the acetals were carried over. This means "a significant proportion of the aldehyde PG acetal will reach the airways during vaping," the study notes.

These new compounds are also stable in water and other physiological solutions, so they can stay in the body for a period of time and have effects that not much is known about, Jordt said.

These acetals are not listed on e-liquid ingredient labels, the authors said, as they are produced after the product has been mixed.

Jordt says that one major claim made by e-cigarette companies is that they use well-known chemicals, nicotine and solvents to make the liquids and that nothing changes when they are mixed together. Thus, the user believes that they know what they are being exposed to.

"We welcome additional science in this area," Gregory Conley, president of the American Vaping Association wrote in an email. The group is a nonprofit organization that advocates for sensible regulation of vaping products but is sponsored by companies that make vaping products.

"It is critical that adult smokers understand that nothing in this study will change the conclusions of respected organizations like the Royal College of Physicians and Public Health England, both of which have estimated that vaping is at least 95% less risky than smoking," said Conley. The research he referenced is counter to studies that have found vaping to be more harmful than traditional cigarettes.

The research chemically confirms what many suspected, according to Ilona Jaspers, professor in pediatrics, microbiology, immunology and environmental science and engineering at the University of North Carolina at Chapel Hill.

"These are very reactive chemicals, and it would be naïve to think that they would stay in isolation and not react and cause secondary and tertiary products once they are put in the mixture," said Jaspers, who was not involved with the study.

"One of the big items to take away from this here is that we have to look at these e-liquids as a dynamic mixture potentially, a chemical mixture that is not necessarily stagnant and may change over time," she said.

Users need to pay attention to the original ingredients as well as the possibility of what they could become, she says. However, this is not always as easy as it appears.

"The problem is, even with some of the products that are currently on the market that have an ingredient list, the consumer absolutely has no idea what they ultimately are being exposed to," Jaspers said.