More U.S. teens are now using e- cigarettes than traditional tobacco ones or any other tobacco product, according a national study released Tuesday by the U.S. National Institute on Drug Abuse.

The results are part of the 2014 Monitoring the Future survey, which studied 40,000 to 50,000 students in the 8th, 10th and 12th grades throughout the United States to track trends in substance use in American teens. Use of e-cigarettes has been measured for the first time in the study, now in its 40th year.

"As one of the newest smoking-type products in recent years, e- cigarettes have made rapid inroads into the lives of American adolescents," said University of Michigan Professor Richard Miech, a senior investigator of the study. "Part of the reason for the popularity of e-cigarettes is the perception among teens that they do not harm health."

The survey found that 9 percent of the 8th-graders reported using an e-cigarette in the past 30 days, while only 4 percent reported using a tobacco cigarette.

In the 10th grade, 16 percent reported using an e-cigarette and 9 percent reported using a tobacco cigarette. Among the 12th- graders, 17 percent reported e-cigarette use and 14 percent reported use of a tobacco cigarette.

On the other hand, only 15 percent of the 8th-graders said there is a great risk of harm with regular use of e-cigarettes, compared with 62 percent of the 8th-graders who said there is a great risk by smoking one or more packs of tobacco cigarettes a day.

The study did not determine whether youth who use e-cigarettes exclusively were more likely to later go on to become tobacco cigarette smokers, which is of substantial concern to the public health community.

But it did found cigarette smoking reached historical lows among teens in 2014 in all three grades. For the three grades combined, 28 percent reported any smoking in the prior month in 1998, the recent peak year, but that rate was down to 8 percent this year.

E-cigarettes are battery-powered devices with a heating element, which produce an aerosol of nicotine and other chemicals. The liquid that is vaporized in e-cigarettes comes in hundreds of flavors, including bubble gum and milk chocolate cream to attract younger teens.

In April, the U.S. Food and Drug Administration (FDA) proposed rules that could eventually ban the sale of e-cigarettes to individuals under the age of 18.

"We are concerned about the levels of e-cigarette use among teens that we are seeing," said University of Michigan Professor Lloyd Johnston, the study's principal investigator. "It would be a tragedy if this product undid some of the great progress made to date in reducing cigarette smoking by teens."