Teenagers who use e-cigarettes are ingesting many of the same carcinogens found in regular cigarettes, according to a study published Monday in the medical journal Pediatrics. And those who use fruit-flavored e-cigs may be exposed to even more cancer-causing chemicals than those who use the tobacco-flavored ones.

The study was the first to measure toxins directly in the saliva and urine of teenage smokers.

“The presence of harmful ingredients in e-cigs is well established,” says Mark Rubinstein, M.D., a researcher at the University of California, San Francisco’s Tobacco Control Center, and the study’s lead author. “We can now say that those toxins are making their way into the bodies of teenagers who use these products.”

E-cigarettes are often promoted as a less toxic alternative for adults struggling to kick the regular cigarette habit. But critics say that message has led to soaring e-cig use among teenagers.

According to a 2016 survey by the U.S. Department of Health and Human Services, more than twice as many teenagers use e-cigs as traditional cigarettes, and that many of them choose vaping because they believe it’s safe. “They think the vapor is just water vapor,” Rubinstein says. “And some of the advertising around these products reinforces that belief.”

More On E-Cigs

Rubinstein and his colleagues tested the saliva and urine of 103 teenagers who were divided into three distinct groups: those who smoke both e-cigs and regular cigarettes; those who smoke e-cigs only; and those who don’t smoke either. Their tests measured levels of several toxins found in both e-cigs and regular cigarettes, including some known to cause cancer. 

Those toxins were generally most prevalent among the teenagers who smoked both regular cigarettes and e-cigs, and lowest among those who did not smoke either product. Some chemicals were three times higher in the “dual use” group than they were in the "e-cig only" group.  And there were chemicals found to be three times higher in the "e-cig only" group than they were in the non-smoking group.

Nicotine-Free E-cigs Still Have Risks

Some e-cigs contain nicotine, and some don't. Studies indicate that many teens think nicotine-free e-cigs are safe and unlikely to increase the risk of cancer. But the new study found that, among the teenagers who only smoked e-cigarettes (the "e-cig only" group), several cancer-causing compounds were just as high in users of nicotine-free e-cigs as in those who vaped with e-cigs that contained nicotine. 

Previous studies have shown that e-cigs contain additives and solvents that can become carcinogenic when heated. The current study confirms that many of those chemicals are equally prevalent in both nicotine-containing and nicotine-free e-cigs. 

Fruit Flavors May Increase E-cig Use

Overall, fruit-flavored e-cigs were the most popular type of among teenagers who participated in the study: 55% of the “e-cig only” group, and 67% of the “dual use” group, said they used fruit flavored e-cigs most often.

Those who preferred fruit flavors tended to use e-cigs more frequently, and also had significantly higher levels of one toxin in particular (a cancer-causing chemical known as acrylonitrile). It's unclear if those higher concentrations were due to the increased amount of e-cig consumption, or because of the flavoring itself. But the finding indicates that despite what many teens may think, fruit-flavored e-cigs come with their own risks, and are not necessarily safer than tobacco-flavored ones. 

Some E-cigs May Be Worse Than Others

Previous studies have suggested that toxin exposure can vary significantly by the brand and type of e-cig product. For example, e-cigarette devices with higher voltages (or those that can be modified to increase the voltage) are riskier, because higher voltages tend to produce more toxins.

The current study was not able to measure the differences between various e-cig products, but the authors have follow-up studies planned that may address this question. 

In the meantime, this much already seems clear: According to health officials, study authors, and Consumer Reports medical experts, no e-cigarettes are safe. The best way to protect against cancer and other diseases is to not smoke at all.