Vaping is more popular with teens than ever, with more than one-third of high school students reporting having tried e-cigarettes. And teens aren’t always using e-cigs for nicotine, according to a new US Centers for Disease Control and Prevention report that dug into teen vaping behavior.

To evaluate e-cig use, the CDC and the US Food and Drug Administration poured through surveys filled out by 17,000 middle and high school students across the US in 2015. About 38 percent of high school students and 13 percent of middle school students reported that they’ve tried e-cigarettes. That could be an underestimate, too, since the students were reporting their own behavior, and surveys based on self-reports are known to be unreliable.

Nearly 38 percent of high school students have tried e-cigarettes

The CDC is interested in vaping is because we still don’t know exactly how using e-cigarettes could affect a teen’s development. A medical group in the UK lauded e-cigs as useful tools to help current smokers quit, but the CDC said in a statement there’s no evidence that they work. What’s more, e-cig use during adolescence could kickstart an addiction, and the US Surgeon General Vivek Murthy warns that nicotine in any form is unsafe for teenagers. Still, more than 3 million teens used e-cigs in 2015, a tenfold increase over four years that Murthy called a public health crisis. But to stop it, the CDC has to understand it better.

In today’s report, one-third of e-cigarette users reported using their devices for something other than nicotine. This was more common for male white and Hispanic students than non-Hispanic black students. The survey didn’t get into what exactly the students were using their vape pens for, if not nicotine. But other studies point to pot as the most likely substance.

Ads promise independence and sex appeal to manipulate people into buying

More than half of the e-cig users stuck to reusable electronic cigarettes — the ones you can refill with new liquid nicotine cartridges — as opposed to the disposable kind. Although most of the students didn’t know what brand they were using, the ones who did used blu and VUSE most frequently.

Both of these brands are owned by big tobacco companies, and are among the most heavily advertised. Millions of teens are exposed to ads for e-cigarettes online and in stores. These ads take a leaf out of big-tobacco’s book, promising independence and sex appeal to manipulate people into buying. And they work: more exposure to e-cig advertisements corresponds with more e-cig use in young adults, according to previous CDC research.

The CDC has repeatedly called for restricting e-cig marketing, but they have no control over advertisements. But regulation of the devices is growing; just this year, the FDA ruled that e-cigarettes and vape pens fall under the regulatory umbrella of tobacco products, which means the agency can ban sales to people under 18. We’ll see if the numbers of teenage users drop when the CDC analyzes the data from 2016.