More teenagers are using marijuana e-cigarettes than ever before, according to two new studies published in the Journal of the American Medical Association.

Vaping rates among adolescents have been on the rise for years, and the U.S. Centers for Disease Control and Prevention has recently linked many cases in a spate of vaping-related lung illnesses to Vitamin E acetate, which is found in vaping pods containing THC, the psychoactive ingredient in cannabis. But the size of this increase is especially concerning, said Hongying Dai, an associate professor at the University of Nebraska Medical Center College of Public Health.

“I expected marijuana use in e-cigarettes to increase, but was surprised at the magnitude of increase.… This translates to millions of adolescents,” said Dai, the author of a study that looked at teen e-cigarette use in 2017 and 2018.

Using data from the National Youth Tobacco Survey, Dai found that 14.7% of adolescents reported using marijuana e-cigarettes in 2018, up from 11.1% the year before.

The CDC’s National Youth Tobacco Survey includes self-reported tobacco use data from 38,000 students in grades six through 12.

A second study, by researchers at the University of Michigan, found the trend of youth marijuana vaping continued this year, especially among older teens.

In 2019, 14% of high school seniors reported marijuana vaping in the last 30 days, compared with 7.5% the year before, according to an analysis of data from Monitoring the Future, a self-reported survey of more than 42,000 teenagers by the National Institute on Drug Abuse.

The Nebraska study was supported by the National Cancer Institute and the U.S. Food and Drug Administration. The Michigan study was supported by the National Institute of Drug Abuse.

Dai said the findings point to the need for more education for teens and parents about the risks of vaping and marijuana. Nicotine and marijuana can have lasting effects on teenagers’ developing brains.

The CDC has urged people to not use THC vaping liquids, especially those bought on the street or from online retailers.

More than 2,400 cases of severe lung illness confirmed or suspected to be related to vaping have been reported, including 111 in Pennsylvania and 96 in New Jersey. Each state has reported one death.

The CDC has linked Vitamin E acetate, an ingredient used in vaping liquids containing THC, to many of the patient samples tested. The agency continues to look for other ingredients or products making people sick.