Nicotine in electronic cigarettes may be responsible for turning teens into regular smokers and vapers.

Researchers surveyed 181 high school students from Los Angeles who vaped. Those who used e-cigs with higher nicotine concentrations were more likely to keep smoking e-cigs and regular tobacco cigarettes later on, according to a study published today in JAMA Pediatrics.

Studies have shown that young people who vape are more likely to pick up the smoking habit, but why this happens isn’t entirely clear. Today’s study shows that nicotine concentrations may play a role.

E-cigs have been on the market for about a decade, and are increasingly popular — especially among teenagers. More than 2 million middle and high school students vaped in 2016, including 11 percent of all high school students in the US. That’s a “major public health concern,” according to the US surgeon general. Though vaping is largely considered a healthier alternative for people who smoke regular tobacco cigarettes, some in the medical community fear that vaping could be a gateway to tobacco smoking, especially among teens. And vaping isn’t completely risk-free: research has shown that e-cigs might increase your risk of heart disease, just like regular cigarettes; e-cigs were also found to produce harmful chemicals, including some that are believed to cause cancer.

Last year, the US Food and Drug Administration banned the sale of e-cigs to minors, but because they can be bought online, e-cigs remain widely available. E-cigs also often contain flavors, which appeal to young people. Over 80 percent of young e-cig users say that they vape because flavors are available, research shows. Electronic cigarettes can also contain nicotine, which is highly addictive. (Nicotine also harms brain development in kids, and it might increase vapers’ risk of heart disease.)

To figure out whether nicotine concentrations in e-cigs play a role in encouraging kids to smoke more, researchers looked at 181 10th graders from 10 high schools in Los Angeles. All teenagers were e-cig users, and were asked to report the amount of nicotine in their e-cigs, which ranged from zero to over 18 mg/mL. (For reference, most tobacco cigarettes contain 10 to 15 mg of nicotine per cigarette.) After six months, the teens were surveyed again on their vaping and smoking habits. The findings: teens vaping on e-cigs with higher nicotine concentrations were more likely to keep smoking e-cigs and regular tobacco cigarettes after six months; they were also vaping more. And compared to teens who didn’t vape nicotine, those who used e-cigs with high nicotine concentrations smoked 14 times as many cigarettes per day.

The study has some limitations: it only included 181 students, a relatively small number. It also relied on teens self-reporting their smoking habits, which can be unreliable. Still, it shows how much nicotine is inside e-cigs may have an effect on whether young people keep vaping and smoking.

JAMA Pediatrics suggests parents talk to their kids about electronic cigarettes, as well as how dangerous nicotine is. “Some teenagers benefit from discussing how nicotine companies try to target teenagers via advertising or child-friendly flavors to get them hooked at an early age,” according to Megan Moreno, associate professor of pediatrics at the University of Washington, “so avoiding nicotine can be a way to ‘rebel’ against these companies.”