Use of electronic cigarettes by high school students tripled over three years, according to a new government report released Thursday.

In a large national survey last year, 4.5 percent of high school students said they had used e-cigarettes in the previous month. That's up from 1.5 percent in 2011 and 2.8 percent in 2012.

It's not known, though, how many were repeatedly using e-cigarettes and how many only tried it once during that month and didn't do it again.

E-cigarettes began to appear in the United States in late 2006, but marketing has exploded in recent years. The devices heat liquid nicotine into a vapor. They are often described as a less dangerous alternative to regular cigarettes, but experts say nicotine - including the nicotine in e-cigarettes - is especially harmful to children.

Dozens of states outlaw the sale of e-cigarettes to minors, and federal officials have proposed a nationwide ban on such sales.

The report's e-cigarette findings are disheartening, said Dr. Patrick T. O'Gara, president of the American College of Cardiology. Smoking rates has slowly been declining over the last several decades, but "we risk going backwards if a new generation of smokers becomes addicted to nicotine," O'Gara said, in a statement.

The Centers for Disease Control and Prevention report comes from a survey of more than 18,000 high school and middle school students.

The CDC survey also found 13 percent of high school students recently smoked regular cigarettes, and that about 23 percent used some form of tobacco product - be it cigarettes, e-cigarettes, flavored cigars, hookahs or something else.

In contrast to the high school rate of 4.5 percent, the adult rate for use of e-cigarettes is lower - 2.6 percent of U.S. adults were current users last year, according to the CDC.

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