Recent research published in the journal Frontiers in Physiology examines the effect of electronic cigarette vapors on two types of white blood cells. The findings suggest that the compounds that give e-cigarettes their flavor are toxic, with some flavors being worse than others.

Despite the fact that electronic cigarettes (e-cigarettes) help some people to quit smoking conventional ones, the devices contain many other non-nicotine chemicals, the health effects of which are still being investigated.

Studies have been providing updates on all the latest scientific discoveries when it comes to unraveling the complex effects of using e-cigarettes, or "vaping."

For example, a couple of studies have suggested that e-cigarettes may have adverse cardiovascular effects, and that they may slow down heart rate.

That said, some of these studies are either observational—and thus unable to explain causality—or performed in mice.

New research, however, takes things to the laboratory. Scientists at the University of Rochester Medical Center in New York set out to test the hypothesis that vaping e-cigarettes that do not contain nicotine would be less harmful than conventional cigarettes.

To this end, the researchers—who were led by senior author Dr. Irfan Rahman—focused on "the immuno-toxicological and the oxidative stress effects by these e-cigarette flavoring chemicals on two types of human monocytic cell lines."

Oxidative stress is a process in which oxygen radicals are produced in excess, resulting in a series of damaging effects, including increased toxicity, damage to our DNA, or even cancer.

Monocytes are a type of white blood cell that play a critical role in our immune response to inflammation. Therefore, the results of the new study are key for our understanding of the relationship between e-cigarettes and our immune system.

Cinnamon, vanilla, buttery flavors the worst

To assess the flavorings' potential for causing oxidative stress, the team measured the production of so-called reactive oxygen species (ROS).

"We hypothesized," the authors write, "that the flavoring chemicals used in e-juices/e-liquids induce an inflammatory response, cellular toxicity, and ROS production."

As expected, the cytotoxicity tests performed by first author Dr. Thivanka Muthumalage and colleagues revealed that treatment with these chemicals increased inflammation and tissue damage. All of this was done by increasing the levels of oxidative stress.

Also, "Mixing a variety of flavors resulted in greater cytotoxicity and cell-free ROS levels compared to the treatments with individual flavors, suggesting that mixing of multiple flavors of e-liquids are more harmful to the users," the researchers add.

The authors conclude by saying, "Our data suggest that the flavorings used in e-juices can trigger an inflammatory response in monocytes, mediated by ROS production, providing insights into potential pulmonary toxicity and tissue damage in e-cigarette users."

"Cinnamon, vanilla, and butter flavoring chemicals were the most toxic, but our research showed that mixing flavors of e-liquids caused by far the most toxicity to white blood cells."

Dr. Thivanka Muthumalage

Dr. Rahman comments on the significance of these findings, saying, "Currently, [nicotine-free e-cigarettes] are not regulated, and alluring flavor names, such as candy, cake, cinnamon roll, and mystery mix, attract young vapers."

"Our scientific findings show that e-liquid flavors can, and should, be regulated and that e-juice bottles must have a descriptive listing of all ingredients. We urge regulatory agencies to act to protect public health," he adds.