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

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

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

E-cigarettes are battery powered cylinders that heat a nicotine-containing liquid into a vapour 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 flavoured like sweets 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 per cent in 2013 (about 770,000 high school students) to 9.4 per cent in 2014 (about 1.3 million students).

Meanwhile, there was no decline in overall tobacco use among middle or high school students.

A separate study out Thursday by the University of California, San Diego, followed 1000 adult smokers for one year, and found that e-cigarette users were less likely to cut down on traditional cigarettes than tobacco smokers who did not use the battery-powered devices.

"Smokers who used e-cigarettes were 49 per cent less likely to decrease cigarette use and 59 per cent less likely to quit smoking compared to smokers who never used e-cigarettes," said the study in the American Journal of Public Health.

"Based on the idea that smokers use e-cigarettes to quit smoking, we hypothesised that smokers who used these products would be more successful in quitting," said study author Wael Al-Delaimy, professor and chief of the Division of Global Public Health at UC San Diego.

"But the research revealed the contrary. We need further studies to answer why they cannot quit. One hypothesis is that smokers are receiving an increase in nicotine dose by using e-cigarettes."