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Ecigarettes have exploded in popularity among teens. According to the US Food and Drug Administration, their use by US high school students surged from 12% in 2017 to 21% in 2018, despite federal regulations barring anyone under 18 years of age from buying e-cigarettes.

In particular, e-cigarette maker Juul has dominated the market, as well as news coverage, this year. “The whole Juul thing is completely blowing up,” says Portland State University chemist James Pankow, referring to the product’s growing use and press attention. While the company says that its product is intended to help tobacco smokers quit, “Juuling” has taken off among young consumers, who are drawn to the product’s discreet design and appealing e-liquid flavors, such as mint and mango, surveys suggest. In May, Pankow and Portland State colleagues David Peyton and Anna K. Duell reported another explanation for Juul’s success: chemistry.

Related: E-cigarettes' chemistry may explain their popularity among teens

Using nuclear magnetic resonance spectroscopy, the team measured the amounts of two major forms of nicotine, protonated and free-base nicotine, in a variety of e-liquid brands and found that Juul e-liquids had the lowest amounts of free-base nicotine, which is associated with harsh throat sensations when inhaled (Chem. Res. Toxicol. 2018, DOI: 10.1021/acs.chemrestox.8b00097).

The researchers think that Juul products are popular in part because they deliver high amounts of nicotine in a way that’s easily inhalable. The group says another important aspect of the aerosolized e-liquids that scientists should study is the particles’ size. Smaller particles penetrate deeper into the respiratory system than larger ones and are more easily absorbed by the body, possibly making it easier to get a bigger nicotine hit from each puff, Pankow explains.

E-cigarette devices are also used to deliver cannabidiol (CBD), a nonpsychoactive cannabinoid that is legal with a prescription in most states to treat conditions like insomnia and pain. Hundreds of CBD e-liquids are available for purchase online and are completely unregulated, says Virginia Commonwealth University’s Justin L. Poklis.

Poklis and his team analyzed nine CBD e-liquids from one manufacturer after receiving a tip from an outside source questioning their purity. By using mass spectrometry, the group detected a synthetic cannabinoid called 5F-ADB, reported to have euphoric and other psychoactive effects, in four of the products. In one of the products, the researchers detected dextromethorphan, a compound found in cough medicine (Forensic Sci. Int. 2018, DOI: 10.1016/j.forsciint.2018.10.019).

The study doesn’t report the amount of the contaminants in the e-liquids, but Poklis says his team plans to publish those data in a follow-up study.

“Until you know how much of the material there is and compare it to its bioactivity and toxicity, it’s hard to know how significant” these findings are, Portland’s Peyton says. However, he says, if the manufacturer intentionally added these compounds, that would be alarming.

Related: Mass spectrometry measures chemical exposures in e-cigarette users’ mouths