More teenagers in the United States have used vaporizing e-cigarettes (known as vaping) instead of smoking tobacco, according to a new study that raises concern among researchers of the rise of the prevalence of vaping and its dangers to young people.

According to the annual Monitoring the Future survey, which involves tens of thousands of American students and is nationally representative, 35.8 percent of senior high school students have tried vaping during the 2016-2017 school year, compared to 26.6 percent who smoked a cigarette.

Of those who vaped, more than 50 percent responded that they were only inhaling flavoring, while 33 percent said they vaporized nicotine. However, the study’s researchers said these numbers could vary since teenagers do not always know what a vaping product contains.

Vaping is more popular among teenagers because vaporized nicotine and marijuana are scentless, and therefore are easier to hide from adults. According to a report by the U.S. surgeon general, the most common reasons young people begin using e-cigarettes is curiosity, avoiding indoor smoking restrictions, and as an alternative to traditional tobacco products.

While e-cigarettes do not contain as many harmful substances as cigarettes, young people who use any tobacco product are at risk of becoming addicted to nicotine. Wilson Compton, the deputy director of the U.S. National Institute on Drug Abuse, told CBS News that vaping “could be an entree into what we know can be a lifelong, extraordinarily harmful habit. Kids that start with vaping do transition to smoked tobacco more often than those who’ve never used e-cigarettes.”

A research team at the University of Rochester in New York found that vaping can lead to gum disease.

Rob McConnell, a medicine specialist at the University of Southern California in Los Angeles, stated: “There are a lot of potentially harmful substances in e-cigarettes. If you’re a teen with your whole life in front of you, why take that risk?”