Some two million US high school students tried e-cigarettes last year, a rate that tripled in just one year, US health authorities said Thursday.

The 2014 survey by the US Centers for Disease Control and Prevention found that 13.4 percent of high school students said they had smoked an e-cigarette in the past month, up from 4.5 percent from 2013.

In middle school, some 3.9 percent of kids (about 450,000 students) said they had tried vaping in the past month according to the 2014 findings, up from 1.1 percent in 2013.

E-cigarettes are battery powered cylinders that heat a nicotine-containing liquid into a vapor that is inhaled, much like a conventional cigarette but without the flame.

Some health experts are concerned about the rising popularity of the devices, which are unregulated in the marketplace and contain liquid nicotine cartridges that are flavored like candy and fruit.

"We want parents to know that nicotine is dangerous for kids at any age, whether it's an e-cigarette, hookah, cigarette or cigar," said CDC director Tom Frieden.

"Adolescence is a critical time for brain development. Nicotine exposure at a young age may cause lasting harm to brain development, promote addiction, and lead to sustained tobacco use."

The report marked the first time since 2011 when researchers started collecting data on e-cigarettes that "current e-cigarette use has surpassed current use of every other tobacco product overall, including conventional cigarettes."

It also found hookah use had nearly doubled in a year's time, going from 5.2 percent in 2013 (about 770,000 high school students) to 9.4 percent in 2014 (about 1.3 million students).

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