Nearly 30 million people across Europe now use e-cigarettes and are most likely to be aged between 15 and 24, who smoke tobacco regularly and are trying to quit, a new analysis shows.
The rising number of users has led to "staggering" growth in the availability of e-cigarettes with around 10 new brands coming to market every month, a second study showed.
Writing in the journal Tobacco Control, researchers from the United States said their findings underlined the size and speed of growth in the market for e-cigarettes, and the need for more research into their potential risks and benefits.
"As e-cigarettes represent an emerging market in which the tobacco industry has extensively invested, it is imperative to identify the population subgroups that are more likely to use them and the subsequent implications this might have on public health," said Constantine Vardavas, a senior research scientist at Harvard School of Public Health who led the study.
"These new findings show that millions - including many young people and smokers trying to quit - are trying e-cigarettes, which underscores the importance of assessing their potential harm or benefits."
E-cigarettes are battery-operated devices designed to deliver a similar nicotine hit and sensory sensation to conventional cigarettes, but without the harmful carcinogens that come from burning tobacco.
They are seen by many as a healthier alternative to cigarettes and as a way to help smokers quit. A group of 53 top scientists warned the World Health Organization (WHO) last month not to classify and regulate e-cigarettes as tobacco products, arguing that doing so would jeopardize a vital opportunity to reduce smoking-related disease and deaths.
Smoking kills half of all those who do it, giving a death toll of around 6 million people a year, according to the WHO.
The tobacco industry has invested heavily in the rapidly emerging e-cigarette market and some public health experts worry they could be a new route to nicotine addiction and a gateway to cigarette smoking.
A second study published in the same journal on Monday found that the number and type of e-cigarettes available online has soared in the past few years, with around 10 new brands and more than 240 new flavors coming to market every month.
Researchers in that study said the large number of brands and "the variety of flavors (is) staggering" and noted a shift in marketing tactics from promoting them as cigarette substitutes to styling them as nicotine delivery systems offering consumer choice in models and flavors.
In the first study, which sought to gauge perceptions of e-cigarettes and their use across Europe, the researchers analyzed data from a survey known as Eurobarometer 2012 which involved more than 26,500 adults from 27 countries in the region.
Respondents were asked if they had ever tried e-cigarettes, and how often; whether they had ever heard of them; and whether they thought they were harmful or not to health.
People who smoked were also asked if they had tried to quit during the previous 12 months, and whether they had used e-cigarettes to do this. And they were asked what factors influenced their choice of cigarette brand.
The results showed that e-cigarette users were more likely to be young - aged 15 to 24 - current smokers of up to 20 cigarettes a day, and to have made at least one attempt to kick their habit over the past year. Would-be quitters over the past year were twice as likely to have tried an e-cigarette as those who had not tried to quit.
Extrapolating the figures to the whole EU population suggests that 29.3 million adults across the 27 countries have tried e-cigarettes, the researchers said.
"Our study's implications are strategically important for European policymakers," they wrote. "On the one hand, quitting tobacco use at an earlier age would substantially benefit individuals and public health. However, the re-normalization of smoking ... or maintained nicotine addiction may significantly hinder efforts to stop tobacco use."