The chemicals in e-products come under scrutiny as federal regulators crack down on the vaping industry.

While federal regulators are cracking down on e-cigarettes, the makers of so-called “smoke-free” tobacco alternatives continue to insist their products are safer than smoking traditional cigarettes.

They also say vaping is an effective way for smokers to wean themselves off the regular cigarettes.

The industry got some backup this past spring when a report by the Royal College of Physicians in England concluded e-cigarettes are safer than traditional cigarettes.

Nonetheless, the U.S. Food and Drug Administration (FDA) enacted new rules in August that give it authority over e-cigarette use as well as prohibiting the sale of these products to people under 18 years of age.

FDA officials enforced those rules this week when they announced warning letters had been issued to 55 tobacco retailers for selling e-cigarettes and e-liquids to minors.

“We’re helping protect the health of America’s youth by enforcing restrictions that make it illegal to sell tobacco products to minors — including e-cigarettes, e-liquids, and cigars. Retailers play a vital role in keeping harmful and addictive tobacco products out of the hands of children and we urge them to take that responsibility seriously,” said Mitch Zeller, J.D., director of the FDA’s Center for Tobacco Products, in a press release.

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Last December, researchers at the Harvard T.H. Chan School of Public Health released a study that concluded that 75 percent of flavored e-cigarettes contain a chemical linked to cases of a severe respiratory disease.

A few months earlier, research presented at the American Thoracic Society’s international conference concluded that certain e-cigarette liquid flavors are actually toxic to lung cells.

Researcher Temperance Rowell, a graduate student in the Cell Biology and Physiology Department at the University of North Carolina at Chapel Hill, exposed cultures of human lung epithelial cells to 13 different e-cigarette flavors.

Five flavors were found to decrease the cells’ ability to reproduce or perform their basic cellular functions. Epithelial cells make up an important protective layer in the airway and lungs.

“Because these e-liquids are not FDA regulated, we don’t know what chemical constituents make up all of these flavors,” Rowell said. “E-cigarettes are marketed as a safer alternative to traditional cigarettes because they don’t contain tobacco or tar, but the effects of inhaling many of the chemical constituents that create the flavor haven’t been tested.”

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E-cigarette liquid is usually composed of propylene glycol and vegetable glycerin, which easily vaporizes and serves as a vehicle for nicotine and, in many cases, the user’s preferred flavor.

Rowell’s experiments concluded that cells exposed to Vapor Girl flavors of hot cinnamon candies, menthol tobacco, banana pudding southern style, vanilla tobacco, and kola showed a decreased number of viable cells and decreased cell proliferation when compared to cells only exposed to the glycerin substance.

In essence, the cells exposed to flavoring either died or couldn’t reproduce.

With the exception of vanilla tobacco, these flavors also induced a physiologic process known as calcium signaling in the cells. This process is associated with cells’ defense mechanisms and usually triggers protective responses that increase mucus secretion and cell movement to protect the lungs from potentially toxic or invasive organisms like allergens or bacteria.

“We were interested in calcium signaling because published papers have shown cigarette smoke-induced calcium signaling affects airway surface hydration, which could contribute to symptoms of chronic bronchitis found in COPD,” Rowell said.

COPD, or chronic obstructive pulmonary disease, is most commonly diagnosed in cigarette smokers. One of the features of the disease is chronic mucus buildup that limits breathing. Rowell’s findings hint that certain flavors of e-cigarette liquid could have similar effects on the lungs.

“Several flavors did not elicit these responses in the same experiments, suggesting that we could group flavors into categories of potentially ‘nontoxic’ and ‘toxic’,” Rowell said.

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Editor’s Note: The original story was published on May 18, 2015 and was updated on September 16, 2016.