Some e-cigarette ingredients are more toxic than others

As e-cigarette use becomes more popular, particularly among teens and young adults, a new study shows that e-liquids are potentially far from harmless and contain ingredients that can vary wildly from one type of e-cigarette to another. The study, publishing March 27 in the open access journal, PLOS Biology, by UNC School of Medicine scientists led by Robert Tarran, created a new screening technique to deduce the different toxicity levels of the more than 7,700 types of e-liquid flavors available to consumers.

Recent surveys suggest that roughly 15-25 percent of U.S. high school 11th and 12th graders have used e-cigarettes. Other surveys showed that 10-15 percent of U.S. adults have used the products. These numbers rise every year. Yet so far, the health effects of vaping remain unclear for the majority of the commercially available e-liquids.

Tarran and colleagues, including co-first author M. Flori Sassano, developed a system for the rapid evaluation of e-liquid toxicity, based on a standard toxicology approach. Their system uses plastic plates arrayed with hundreds of tiny wells in which fast-growing human cells are exposed to different e-liquids. The more these liquids reduce the cells' growth rates, the greater their toxicity.

"We found that e-liquid ingredients are extremely diverse, and some of them are more toxic than nicotine alone and more toxic than just the standard base ingredients in e-cigarettes – propylene glycol and vegetable glycerin," said study senior-author Robert Tarran.

The main ingredients of e-liquids, propylene glycol and vegetable glycerin, have been considered non-toxic when consumed orally, but of course e-cigarette vapors are inhaled. The UNC scientists found that even in the absence of nicotine or flavorings, small doses of these two organic compounds significantly reduced the growth of the test cells, thus indicating high toxicity.

Besides these base ingredients, e-liquids include nicotine, plus flavoring compounds, and are sold under innocuous-sounding names such as "Candy Corn," "Chocolate Fudge," and "Berry Splash." The scientists found that these ingredients varied tremendously across the e-liquid products tested, and overall, more ingredients meant greater toxicity.

The toxicity results remained largely the same when the researchers used other cell types, including human lung and upper airway cells. Toxicity results were also the same when the researchers exposed the cells to vaporous puffs of e-liquids, which is how the lung cells would be exposed when vaping. These experiments confirmed the reliability of using standard toxicology cell cultures and e-liquids in liquid forms as a relatively fast initial screening method.

"We have this tool and it's very fast and reliable, and we now plan to use it on a wider scale," Sassano said. "There are more than 7,700 e-liquid products out there, and regulators as well as the general public should know more about the ingredients they contain and how toxic they might be."

The authors report that the FDA is only now beginning to regulate e-liquid ingredients, and this work aims to better inform their efforts. To aid in disseminating such results, Tarran, Sassano and their colleagues have set up a database of e-liquid ingredients and toxicity data at http://www.eliquidinfo.org. The National Institutes of Health and the Federal Drug Administration funded this research.

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In your coverage please use this URL to provide access to the freely available article in PLOS Biology: http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2003904

Citation: Sassano MF, Davis ES, Keating JE, Zorn BT, Kochar TK, Wolfgang MC, et al. (2018) Evaluation of e-liquid toxicity using an open-source high-throughput screening assay. PLoS Biol 16(3): e2003904. https://doi.org/10.1371/journal.pbio.2003904

Funding: FDA/NIH https://prevention.nih.gov/tobacco-regulatory-science-program/research-portfolio/centers#UNC2 (grant number P50 HL 120100). Research reported in this publication was supported by NIH and the Family Smoking Prevention and Tobacco Control Act. This funding was received by Dr. Robert Tarran. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH or the Food and Drug Administration.

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Related Journal Article

http://dx.doi.org/10.1371/journal.pbio.2003904