How Much Do We Really Know About Flavored Vaping Products?

Analysis reveals previously undetected chemical byproducts.

Young woman holding smart phone and an E-Cigarette for vaping in hands

According to a recent Yale study, inhaling vapor from flavored e-cigarette liquid exposes users to previously undetected chemical byproducts called acetals. As a result of the study, the authors hope the FDA will begin to study the short- and long-term effects of inhaled acetals.

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Parents now have another reason to worry about their kids’ vaping.

According to a recent Yale study, inhaling vapor from flavored e-cigarette liquid—also called “vape juice,” “e-juice,” “e-liquid,” or “vape liquid”—exposes users to previously undetected chemical byproducts. These byproducts, called acetals, form when solvents that make up a large portion of vaping liquid mingle with flavoring agents, such as vanilla or almond. It is not yet known if this has negative effects on the body.

A key aspect of chemistry is understanding how substances interact with one another, as well as with heat and other processes. “People often assume that these e-liquids are a final product once they are mixed,” says Hanno Erythropel, PhD, lead author of the study and an associate research scientist at Yale’s chemical and engineering department. “But chemical reactions create new molecules in the e-liquids.” He notes that this is not just something that happens in e-liquids from small vape shops but also in those from one of the country’s biggest e-cigarette companies, referring to San Francisco-based Juul Labs.

In September 2018, the Food and Drug Administration (FDA) asked Juul and other e-cigarette makers to submit plans detailing how they would help reverse what the organization has described as a vaping epidemic among teens in the country. Despite the awareness campaigns and educational programs the FDA has undertaken, nearly 21 percent of U.S. high schoolers currently vape, according to the latest National Youth Tobacco Survey. 

For this study, Yale researchers chose to investigate Juul’s vaping liquid because of its high levels of vegetable glycerin (also called glycerol). Glycerol and its chemical cousin, propylene glycol, form the clear liquid base of most vape juice. These solvents turn to vapor when heated and serve as a carrier of flavor compounds (vanilla, in this particular study) and nicotine, Erythropel says. “As chemists familiar with such reactions, we expected that the vegetable glycerin would react with vanillin (a flavor compound) to form acetals, and that’s what we found,” Erythropel says. The team published similar findings after analyzing other brands and flavors of e-cigarette liquids. 

Based on those earlier discoveries, the researchers looked for certain flavoring agents (vanilla, almond, citrus, and cinnamon) in all eight of Juul’s available flavors. But they only found vanillin in Juul’s “crème brûlée” flavor.   

Flavoring chemicals (called aldehydes), like the vanillin used in Juul’s crème brûlée-flavored liquid, are widely used in the flavor, food, and baking industries, Erythropel explains. The effects that aldehydes might have on the body have been studied and safety standards were established accordingly. The researchers have observed that acetals can be stronger irritants than the flavor chemicals they are derived from and are concerned about the health effects of these acetals in e-liquids in Juul and other e-cigarettes. 

Besides analyzing the aldehyde vanillin, the group also measured menthol levels in each of Juul’s flavors. Menthol is a compound made synthetically or extracted from mint oils. As expected, they found high levels of menthol in flavors such as “menthol” and “mint.” But researchers also found menthol in “fruit” and “cucumber” flavors, though not in the other four flavors. 

“We did not expect to find menthol in a flavor labeled ‘fruit,’” says Julie Zimmerman, PhD, who is the corresponding author of the study, professor of chemical and environmental engineering, and principal investigator of the analytical lab core of Yale’s Center for Tobacco Regulatory Science (TCORS)

Menthol has long been used in cigarettes because its well-known cooling and soothing effects help mask the irritation caused by smoking, says Sven-Eric Jordt, PhD, a TCORS member and associate professor of anesthesiology, pharmacology, and cancer biology at Duke University. “Since the 1970s, the tobacco industry has been using synthetic menthol in cigarettes because it covers up the naturally irritating effects of nicotine,” Jordt says.

Nicotine, the addictive ingredient in cigarettes and some vape pods, is a naturally bitter substance that automatically irritates fine nerve endings in our throats, nasal passages, and respiratory tract, Jordt explains. 

Menthol interacts with cold-sensing nerve endings in the respiratory tract and skin to create an analgesic or numbing effect. That’s why menthol is used in everything from cough drops to pain relief ointment for sore muscles. 

“We believe that menthol acts the same way in e-cigarette vapor as it does in regular cigarettes” in that it dulls the body’s knee-jerk reaction to reject the harsh bitterness of nicotine and irritating flavor chemicals, Jordt says. 

Early marketing campaigns touted e-cigarettes as less harmful than combustible ones, but this has not been proven. A comprehensive report published by the National Academy of Science, Engineering and Medicine states that “whether e-cigarettes have an overall positive or negative impact on public health is currently unknown.”

For his part, Jordt says he hopes that, as a result of this study, the FDA begins to study the short- and long-term effects of inhaled acetals. “We want these companies to be more transparent about what’s in their liquids,” he says. 

“These findings raise further concerns about the potential toxicity of e-cigarettes, especially for youth,” says Suchitra Krishnan-Sarin, PhD, professor of psychiatry, who leads TCORS, which provided funding for this research.

To learn more visit yalemedicine.org.