Almost 10 percent of 11th and 12th graders are using e-cigarettes, and other alternative tobacco products are increasingly popular, according to a new study.

Young people often use multiple tobacco products at once, the researchers found.

"From this analysis we can say that there are two groups of youth that are concerning: those who are likely using e-cigs and hookah concurrently and those youth who are likely using three or more products," said lead author Tamika D. Gilreath of the University of Southern California in Los Angeles.

Researchers used data from about 2,000 11th and 12th graders in the Southern California Children's Health Study. Participants self-reported if they had ever used cigarettes, cigars, cigarillos, e-cigarettes, hookah or chewing tobacco, and which they had used during the past 30 days.

More than 10 percent of the kids said they had used hookah or waterpipes over the previous 30 days, and were therefore "current users." Almost as many said they were currently using e-cigarettes or vape pens. Only 2 percent said they were currently using chewing tobacco, the authors reported in the Journal of Adolescent Health.

"This is a burgeoning area of research but it is likely tied to the increase in social use of alternative products as exemplified by hookah bars and vape shops popping up all over the country," Gilreath told Reuters Health by email.

Hookah and e-cigarette use is not categorized as unhealthy in popular media and culture, and there is no stigma attached to them, she said.

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"Are these products safer?" Gilreath said. "In my opinion, long-term studies are needed before the safety question can be answered definitively."

E-cigarettes could potentially normalize the use of cigarettes, or could lead to eventual cigarettes use, she said.

"We are winning the battle against the cigarette and prevalence of smoking is going down and the social norm among young people now mostly rejects the cigarette, but tobacco and nicotine dependence is coming through the backdoor by allowing hookah and e-cigarettes to be smoked in public places and changing the social norm so that it is becoming 'cool' to smoke these products," said Dr. Wael Al-Delaimy of UC San Diego School of Medicine, who was not part of the new study.

"These products are nicotine which is addictive and no known dose is safe or non-toxic," said Tracey Barnett of the University of Florida department of behavioral science and community health, who was also not part of the new study.

Tracking polytobacco use is important, said Dr. Michael C. Fiore, the University of Wisconsin Hilldale Professor of Medicine and director of the Center for Tobacco Research and Intervention.

"It highlights a really remarkable development in our society in terms of the use of noncombustive cigarettes," Fiore told Reuters Health by phone.

Exposure to nicotine alters the adolescent brain, even without the tar of a traditional cigarette, he said.

"I believe these products should be regulated the same way cigarettes are," Gilreath said. "Age restrictions and enforcements of violations are a good start."