The Centers for Disease Control and Prevention are onto the fact that young people are using e-cigarettes to vape cannabis. Now, thanks to a new study, the CDC has a better sense for just how common it is.

The findings are from the 2016 National Youth Tobacco Survey of more than 20,000 middle school and high school students. More than 5,200 students reported having tried e-cigarettes, the study reports today in the journal JAMA Pediatrics. But the researchers were especially interested in what else kids were vaping with the device. About one out of every 11 students surveyed, or 9 percent, answered, “Yes, I have used an e-cigarette device with marijuana, THC [tetrahydrocannabinol] or hash oil, or THC wax.”

One out of 11 students

If you apply those estimates across the US, that means more than 2 million young people might have tried vaping cannabis with an e-cigarette, the study says. And the percentage increases if you just look at the students who admitted to using e-cigarettes: about one third of those high schoolers reported having used e-cigarettes to vape some form of pot. For e-cigarette users in middle school, it was about a quarter.

“The use of marijuana in these products is of particular concern because cannabis use among youth can adversely affect learning and memory and may impair later academic achievement and education,” Katrina Trivers, CDC epidemiologist and lead author of the study, told The Verge in an email. The National Academies of Sciences, Engineering, and Medicine agree, warning in a report last year that getting blazed can dull memory and learning at least over the next day or so, the report says.

“For young, inexperienced users, acute cannabis intoxication is a concern.”

But there’s still a lot to learn about the long-term risks — particularly of vaping cannabis products, says Gideon St. Helen, a clinical pharmacologist at the University of California, San Francisco who was not involved in the CDC survey. There are compounds in cannabis, for example, “that we have no idea what happens to them when heated in an e-cigarette,” St. Helen says in an email to The Verge. There also tend to be higher levels of THC in cannabis extracts, hash oil, and waxes compared to the regular plant, he says: “For young, inexperienced users, acute cannabis intoxication is a concern.”

There are some limitations to the study: for one thing, the students self-reported their e-cigarette and cannabis use. And self reports are notoriously unreliable because people forget or might fudge their answers. The study also didn’t go into what sort of vaporizers the students were using, St. Helen says. Were they modifying e-cigarettes intended for nicotine, or using vaporizers designed for cannabis products?

So learning more, like where young people are getting these products, how often they use them, and what it means for substance use later in life could help inform public health policy going forward, Trivers says. And cluing into just how many teens are using e-cigs for cannabis is a start.