Teenagers who use e-cigarettes are exposed to significant levels of potentially cancer-causing chemicals also found in regular cigarettes, even when they do not contain nicotine, according to a new study published in the journal Pediatrics.

Researchers at the University of California, San Francisco (UCSF) studied urine samples from 104 adolescents in the Bay Area with an average age of 16.4. Sixty-seven used e-cigarettes only and 17 used both e-cigarettes and traditional tobacco cigarettes. They were compared with a control group of 20 non-smoking teens.

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Lead author Mark L. Rubinstein, M.D., a professor of pediatrics at UCSF, said in a press release, "Teenagers need to be warned that 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."

“Teenagers should be inhaling air, not products with toxins in them," he added.

Teenagers who used the e-cigarettes had as much as three times higher level of toxic chemicals in their urine than the non-smoking teens. And, the groups that used both e-cigarettes and smoked tobacco cigarettes also had three times higher level of toxic chemicals than in the e-cigarette-only group.

"E-cigarettes are marketed to adults who are trying to reduce or quit smoking as a safer alternative to cigarettes,” said Rubinstein. “While they may be beneficial to adults as a form of harm reduction, kids should not be using them at all."

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Some of the potentially cancer-causing compounds found in those who used e-cigarettes were acrylonitrile, acrolein, propylene oxide, acrylamide and crotonaldehyde. Researchers discovered some of the chemicals were also found in adolescents who used flavored e-cigarettes without nicotine.

The study cautioned that e-cigarettes are often promoted as a safer, healthier alternative to traditional tobacco smoking. However, the Centers for Disease Control and Prevention warns there is enough evidence that vaping can be harmful to teens.