NEW YORK (AP) — Teen use of marijuana remains high and vaping appears to be one reason, a new U.S. survey shows.

One in 10 high school seniors said they had vaped marijuana at least once in the past year. It was the first time the annual survey asked about marijuana vaping and “it’s much higher than I expected,” said Richard Miech, the University of Michigan researcher who leads the study.

For years, teen marijuana use in the U.S. has been seen as relatively flat. Some experts have been expecting it to rise as states loosened marijuana laws, more adults used it and fewer kids considered marijuana harmful.

This year’s survey showed marijuana use was up overall about 1 percent, with nearly a quarter saying they’d vaped, smoked or eaten marijuana in the previous year. Included in the survey: about 45,000 students in grades 8, 10 and 12 in schools across the country.

Marijuana use was significantly higher than cigarette smoking. About 5 percent of teens said they’d smoked in the last 30 days, compared to nearly 15 percent who said they’d used marijuana.

The results released Thursday also show that the use of cocaine, heroin and other illicit drugs continues to decline overall, to the lowest levels seen in the 43 years of the survey.

Vaporizers, including electronic cigarettes, are used with a wide assortment of liquids, including oils that contain the active ingredients of marijuana. The oil is heated to create a vapor that is inhaled.

It’s becoming very popular, said Chris McAboy, co-owner of The Novel Tree, a marijuana retail shop in Bellevue, Washington, where recreational sales to adults is legal.

“They’re extremely discreet and they’re very convenient” and aren’t as messy or smelly as traditional joints, he said.

Experts say vaping marijuana is probably less harmful to the lungs than smoking joints, though they know less about its long-term effects and worry about its potency.

Even in states that allow recreational marijuana use, it’s illegal to sell weed to anyone younger than 21. And federal law prohibits sale of e-cigarettes to minors. But survey data suggests many teens are able to get the supplies to vape marijuana, often by ordering online.