Some two million U.S. high school students tried e-cigarettes last year, a rate that tripled in just one year, U.S. health authorities said. The 2014 survey by the U.S. 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.