Around 5.3% of middle-school students reported using e-cigarettes in 2015, which is up from 3.9% in 2014, according to the Centers for Disease Control and Prevention. E-cigarette usage among high school students has hit 16%, and that’s significantly up from the 1.5% who vaped in 2011.

Such is the popularity of e-cigarettes, that it has become the most used tobacco product among teenagers, with around 3 million reportedly using then. Overall, around 4.7 million middle and high school students are using any form of tobacco product, and when dissected, roughly one in four high school students and one in 13 middle school students use some kind of tobacco product.

The study of tobacco usage among youth is important, as while smoking rates have declined in the past decade, 80% of adult smokers first try smoking by the age of 18. What remains unclear, however, is whether vaping leads to smoking. In the CDC’s latest report, the rise in e-cig usage is in tandem with a gradual decrease in the use of normal cigarettes—among all high school students, that has declined from 15.8% in 2011 to 9.3% last year.

It provides some proof that e-cigarettes, which are generally regarded as less toxic than cigarettes, may not lead to teens picking up the smoking habit. “We do not have any strong evidence that it is encouraging smoking among kids but neither do we have good evidence that it won’t over time,” Kenneth E. Warner, a professor of public health at the University of Michigan, told the New York Times.