While popular belief holds that teens use electronic cigarettes as a substitute for cigarettes, it may in fact be a new habit for those who didn’t smoke in the first place.

A new study by the University of Southern California found that the percentage of students who reported smoking or vaping has increased from previous years. In 2014, 14 percent of 12th-graders reported having used cigarettes or E-cigarettes within the past 30 days. In 2004, prior to the release of electronic cigarettes, only around 9 percent of surveyed students within the same age group reported smoking.

The study, published in Pediatrics, was conducted on 5,490 high school juniors and seniors in Southern California. According to the study, in 2011 only about 1.5 percent of students revealed having vaped in the past 30 days; in 2015, this number was up to around 16 percent.

E-cigarettes, which are battery-powered devices, vaporize liquids that can contain flavorings, chemicals, and nicotine. The user then inhales the vapors, a process otherwise known as vaping.

Researchers said they saw a downward trend in cigarette use from 1995 to 2004, but no further decrease in 2014.

“The combined e-cigarette and cigarette use in 2014 far exceeded what we would have expected if teens were simply substituting cigarettes with e-cigarettes,” lead study author, Jessica Barrington-Trimis, a postdoctoral scholar research associate in the Department of Preventive Medicine at the Keck School of Medicine at USC, said in a press release. “The data suggest that at least some of the teens who are vaping would not have smoked cigarettes.”

A separate study published in June by Barrington-Trimis and her team at USC asserts that teens who experiment with e-cigarettes are six times more likely to try normal cigarettes than teens who have never vaped.

Researchers also worry about potential nicotine addiction in young who are vaping. While some electronic liquid providers state that their products do not contain nicotine, the electronic cigarette industry has not been thoroughly regulated. However, the United States Food and Drug Administration released new plans to regulate tobacco products, including electronic cigarettes, last May.

“Because e-cigarettes are perceived as less harmful and less dangerous than combustible cigarettes, another concern is that teens may be introduced to nicotine use via e-cigarettes,” Barrington-Trimis said in the news release. “In California, where smoking rates are among the lowest in the country, 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.”

Tobacco use is currently the leading cause of preventable deaths in the United States, with approximately 480,000 deaths annually, according to the Centers for Disease Control and Prevention.