But opinions were mixed on why the devices had caught on. A significant share said they were using the devices to quit smoking cigarettes or marijuana, while others said they had never smoked but liked being part of the trend and enjoyed the taste. Two favorite flavors of teenagers interviewed were Sweet Tart and Unicorn Puke, which one student described as “every flavor Skittle compressed into one.”

Image A 15-year-old with an e-cigarette in Peekskill, N.Y. Their use by teenagers tripled from 2013 to 2014. Credit... Bryan Thomas for The New York Times

James, 17, the senior in Virginia, said he and his friends started using e-cigarettes when he was 13, after his father abandoned the devices in a failed effort to quit smoking.

“It was something for us to do that was edgy and exciting,” said James, who asked that his last name not be used because he did not want his smoking habits to be on public display. He liked the smoke tricks that his friends had become good at, like blowing out the vapor so that it spun like a tornado. His favorite flavor is called Hawk Sauce, which he described as “a berry menthol kind of thing.”

He has never smoked cigarettes and said he could not imagine ever starting. “There’s a harshness to cigarettes,” he said. “Girls think they’re gross.”

E-cigarette use had grown exponentially in previous years, but from such a low base that the numbers had been relatively small. But last year’s rise, which was captured in the Centers for Disease Control and Prevention’s annual youth tobacco survey of about 20,000 schoolchildren, lifted e-cigarette use above that of traditional cigarettes, prompting an outcry from anti-tobacco advocates. They warned that e-cigarettes were undoing years of progress among the country’s most vulnerable citizens by making the act of puffing on a tobacco product normal again, and by introducing nicotine, an addictive substance, to a broad population of teenagers.

“This is a really bad thing,” said Dr. Thomas R. Frieden, the director of the C.D.C., who noted that research had found that nicotine harms the developing brain. “This is another generation being hooked by the tobacco industry. It makes me angry.”