A debate is raging between e-cig proponents who say that the devices merely provide an alternative to people who already smoke and are trying to quit, and critics who worry that vaping mainly attracts new users who might never have started smoking anything otherwise. Now a new study suggests that, at least among teenagers, e-cigs are a lure for new users. 

In the new study, researchers at the University of Southern California compared tobacco use among high schoolers who graduated in 1995, 1998, 2001, 2004, and 2014. They found that tobacco use among teens had declined significantly, from 19 percent in 1995 to 9 percent in 2004.

But the group of 11th and 12th graders from 2014 were unique—they were the first cohort who also had e-cigs at their disposal. And when the question was expanded to ask whether the students smoked or vaped, the numbers jumped up to 14 percent.

Jessica Barrington-Trimis, Ph.D., the lead author of the study, says that the increase stems mainly from teens who started vaping, perhaps because e-cigs are "perceived as less harmful and less dangerous than combustible cigarettes." But, she warns, "the increase in vaping, possibly followed by increases in smoking, could erode the progress that has been made over the last several decades in tobacco control."

To be sure, the findings come with some caveats. It’s not clear whether the numbers, which come exclusively from schools in southern California, can be extrapolated to the rest of the country. And because the authors measured only once between 2004 and 2014, it’s not clear exactly when the numbers went up, or what factors besides the popularity of vaping, might have contributed to the increase.

“We don’t know what happened in the intervening years,” says Douglas Kamerow, M.D., a professor of clinical family medicine at Georgetown University in Washington, D.C., and a former assistant surgeon general, who was not involved in the study.

The potential public health problems posed by vaping and  e-cigarettes are numerous, the study’s authors write, and particularly bad for children. Early exposure to nicotine can stunt cognitive development. Nicotine dependence in later adolescence and early adulthood can lead to lifelong nicotine dependence, and to higher rates of addiction. And the evidence so far suggests that e-cigs may contain aldehydes and other toxic chemicals, not to mention flavoring additives that may cause respiratory problems.

In May, the FDA announced that e-cig manufacturers would have two years to apply for approval from the agency for all products—including e-cigs and e-cig batteries—that came on the market after Feb. 15, 2007. The new rules also prohibit retailers from selling vaping devices to anyone younger than 18, either in stores or online, and restricts e-cig vending machines to adult-only establishments like bars and casinos.

The question of whether e-cigs are triggering an uptick in tobacco use among teens is an important one, Kamerow says. “But we don’t need to know the answer to that to know that we don’t want kids smoking them," he says.