CHICAGO (Reuters) - More than 20% of U.S. high school seniors reported vaping marijuana in 2019, the second largest single-year increase in any substance of abuse ever recorded in the annual study of national drug use trends, researchers reported on Wednesday.

FILE PHOTO: A man uses a vape device in this illustration picture, September 19, 2019. REUTERS/Adnan Abidi/Illustration/File Photo

Results of the study bit.ly/34tuccc funded by the National Institutes of Health are alarming as federal officials continue to investigate fatal lung injuries associated with vaping.

Earlier this week, U.S. officials reported another four vaping-related deaths from a nationwide outbreak of a respiratory illness, taking the toll to 52 deaths and more than 2,400 who have been hospitalized. Most of the injuries have occurred in people who vaped THC, the high-producing ingredient in marijuana.

Compiled by researchers at the University of Michigan and published in the medical journal JAMA, the “Monitoring the Future” study found that 21% of 12th graders, 19% of 10th graders and 7% of 8th graders reported vaping THC in the last year.

The increases in all three grades translate into at least 1 million additional THC vapers in 2019 versus 2018, researchers said.

Among 12th graders, that amounted to a 7.7 percentage-point increase, the second-largest for any substance in the study’s 45-year history. The largest occurred last year, when it reported a 10.9 percentage-point gain in nicotine vaping, which the U.S. Food and Drug Administration characterized as a national epidemic.

“Whatever teens can vape has increased dramatically in the last few years,” Richard Miech, who led the study, said in emailed comments.

Part of that is because vaping devices are sleek, and easy to hide. Smoking marijuana is much harder to conceal, Miech said.

“Vaping allows teens to get around the policies and procedures put in place to prevent teen drug use,” he said.

The number of 12th graders who said they had vaped marijuana in the past month doubled from the 2018 survey to 14%. Rates also are rising among younger teens.

The survey found 12.6% of 10th graders reported vaping THC in the past month, up 5.6%, while 3.9% of 8th graders, who are typically about 13-years-old, reported vaping THC in the past month, a 1.3% increase.

‘THE BERMUDA TRIANGLE OF SUBSTANCE ABUSE’

Stanton Glantz, a tobacco control expert at the University of California San Francisco who was not involved in the study, said kids who try vaping nicotine are more prone to vaping THC or smoking cigarettes.

“It’s like the Bermuda Triangle of substance abuse. There’s good research out there showing any kid who does any of those is more likely to do the other ones,” Glantz said in a phone interview.

The 2019 study also showed another gain in nicotine vaping, which rose 5.6 percentage points from a year ago to 35% of 12th graders.

Asked why they vape, 60% of teens said they wanted to see what it was like, 37% said to relax, and 29% said to feel good or get high. Another 8.1% said they vaped because they are hooked, a figure that rose 4.5% from a year ago.

This year’s survey measured daily marijuana vaping for the first time, defined as 20 or more times a month, and found it occurs in 3.5% of 12th graders, 3% of 10th graders and 0.8% of eighth graders.

Dr. Nora Volkow, director of the National Institute on Drug Abuse, found those numbers especially troubling.

“When you use marijuana regularly, your risk of becoming addicted is close to 50 percent, particularly if you’re a young person,” she said in a phone interview.

The daily THC vaping question was added before the practice was tied to the nationwide lung injury outbreak. It remains to be seen whether that will encourage some teens to quit.

“We predict that’s what will happen,” Volkow said. “We don’t know.”