If you vape, then you may have heard that e-cigarettes are safer than tobacco cigarettes. There might be some truth to that, but as a study shows, most e-cigarette users don’t really know what they’re inhaling, even if they take the time to look at the ingredients on the label. 
Study author Sven Jordt, an associate professor of anesthesiology, pharmacology and cancer biology at the Duke University School of Medicine, said:
“Flavor chemicals and the solvents, the liquids, that are in electronic cigarettes, they are forming new chemical compounds.”
These chemicals can become irritants that agitate your airways, the study shows.
“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 e-cigarette users, these mechanisms have been shown to cause inflammation and asthma and contribute to emphysema.”
E-cigarettes work by heating liquid called “e-juice” or “e-liquid,” which contains flavorings, propylene glycol, glycerine, and often nicotine until it vaporizes.
Analyzing the Juice
Researchers analyzed the chemical composition of 10 e-liquids, with 2 flavors and 5 different propylene glycol ratios for each flavor. A propylene glycol ratio refers to the amount of solvent found in e-liquid. The vape juices were purchased from an online store.
Specifically, Jordt and his colleagues analyzed vanilla (vanillin and ethylvanillin), cherry (benzaldehyde), and cinnamon (cinnamaldehyde).
The team also created and analyzed their own e-juice that included the flavors (aldehydes) and solvents commonly used in the e-cigarette industry.
The research shows that 40% of the flavored aldehydes became new compounds called aldehyde PG (propylene glycol) acetals when mixed.
When the scientists vaporized the liquids they had created, 50%-80% of the acetals were carried over, which means that a significant amount of aldehyde PG acetal reaches the airways when the liquids are vaped.
These substances can stick around in the body for quite some time as they are stable in both water and other physiological solutions. It’s not clear what long-term effects they might have on health, according to Jordt.
And since these compounds are only created when they’re mixed together, they are not listed on e-liquid ingredient labels. E-cigarette users think they know what they’re being exposed to because e-cigarette companies claim to use well-known chemicals, nicotine, and solvents, and that they remain stable when they are mixed together. The study shows that is not the case.
Ilona Jaspers, professor in pediatrics, microbiology, immunology, and environmental science and engineering at the University of North Carolina at Chapel Hill, said that many scientists suspected that the e-cigarette manufacturers’ claims were not true.
“These are very reactive chemicals, and it would be naive to think that they would stay in isolation and not react and cause secondary and tertiary products once they are put in the mixture.
One of the big items to take away from this here is that we have to look at this e-liquids as a dynamic mixture potentially, a chemical mixture that is not necessarily stagnant and may change over time.”
The U.S. Food and Drug Administration (FDA) is currently considering regulating flavored e-liquids, citing 2 main concerns: 
- There is evidence that e-cigarettes serve as a gateway to smoking among teens and young adults. Vaping is popular among young people, in no small part because of the more than 7,000 e-liquid flavors on the market.
- The FDA says that many of the flavors used in e-liquids are safe to eat, but not enough is known about them to say whether they are safe for inhaling. Some research suggests that cinnamon-flavored e-liquid may harm lung cells.
Jordt pointed out that if the chemicals created by mixing e-liquids were severely irritating, you would see people giving up vaping en masse. The real concern is that chronic, low-level stimulation of the airways’ irritation receptors could lead to some sort of more serious lung damage in e-cigarette users.
“If they are more continuously activated they can lead to inflammation, chronic cough, they also promote asthma. That’s why we are concerned.”
The study is published in the journal Nicotine & Tobacco Research.
 The Verge