A nice bowl of banana pudding may hit the spot as dessert, but when banana pudding hits the lungs as an e-cigarette flavoring the results may not be so pleasant.

E-cigarettes come in thousands of flavors, but these flavors are not yet regulated. In an effort to develop more rigorous standards for the flavored liquids used in e-cigarettes, researchers exposed lung epithelial cells to e-cigarette liquids of various flavors and examined their effects on the health of the cells. The work was presented today at the 2015 American Thoracic Society International Conference.

"The outcomes seem to be very flavor-specific," lead author Temperance Rowell of the University of North Carolina at Chapel Hill said in an interview. "Some of the chemical flavorings that are used are FDA approved, but that's more for ingestion than inhalation."

Epithelial cells line the respiratory tract and, among other functions, act as a physical barrier against environmental insults and regulate gas exchange. One sign of damage that the researchers measured was the rate of cell growth and division, or cell proliferation. Exposure to certain flavors significantly reduced cell proliferation, which is a sign of cell damage.

"Over the course of the 24 hour exposure, cell proliferation decreased greatly in a dose-dependent manner," says Rowell.

The researchers exposed the cells to 13 different "representative" flavors of e-cigarette liquid from the company the Vapor Girl. As a control they also exposed some cells only to the e-cigarette liquid base, which is made of propylene glycol and vegetable glycerin, but reported no toxic effects from this treatment.

"Of the 13 that we tried, Hot Cinnamon Candies, Banana Pudding (Southern Style), and Menthol Tobacco were the ones that we found to be most harmful," says Rowell. "Peanut Butter Cookie and Captain Zack Cigar were the least harmful."

The enormous variety of e-cigarette flavors has made studying their unique effects challenging. Rowell hopes that developing a screening process that identifies particular chemical components that may be common to a variety of flavors as harmful will help to streamline the process of regulating these products.

"Some of the outcomes we've seen are flavor-specific and it could be from any chemical constituent, and this kind of screening experiment allows us to come up with a way to try to identify those proactively," she says. 

While the results of these preliminary experiments suggest that certain flavors are more harmful, further tests will be necessary.

"We don't know exactly if this is going to be the same exposure that we're going to see from the heated e-cigarette liquid flavors," says Rowell.

Next steps for this research will include developing a protocol for exposing the cells to puffs of smoke.

"We want to create a protocol so that we can evaluate more realistically the effects of those different flavors on lung cells," Rowell says.

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