A study conducted at the University of California, San Francisco, found a presence of cancer-causing chemicals in e-cigarette users as well as people who smoked traditional cigarettes. The study examined urine samples from 104 adolescents. Sixty-seven of them used e-cigarettes only, 17 used e-cigarettes and traditional cigarettes, and 20 did not smoke or vape. Those who used both types of cigarettes showed levels of toxic compounds three times higher than those who only used e-cigarettes, who in turn showed levels of toxic compounds three times higher than the non-smokers.

The results held whether the e-cigarettes contained nicotine or not. Mark Rubinstein, the lead author of the study, said “the vapor produced by e-cigarettes is not harmless water vapor, but actually contains some of the same toxic chemicals found in smoke from traditional cigarettes.” According to Rubinstein, the toxicity of non-nicotine e-cigarettes is caused by the propylene glycol and glycerin used to preserve the flavors.

Manufacturers of e-cigarettes claim they are a safe alternative to tobacco, but studies on the effects of using e-cigarettes have been inconclusive at best. And while they’ve been successful in helping some nicotine-addicted adults stop using tobacco products, the number of teens who smoke has soared since their introduction.