WHAT'S IN THEM: The ingredients in the liquid used in most e-cigarettes include nicotine, water, glycerol, propylene glycol and flavorings. Propylene glycol is a viscous fluid sometimes used in antifreeze but also as a food ingredient.
SELLING POINTS: Users say e-cigarettes address both the addictive and behavioral aspects of smoking. Smokers get their nicotine without the thousands of chemicals found in regular cigarettes. And they get to hold something shaped like a cigarette, while puffing and exhaling something that looks like smoke without the ash, odor and tar.
THE WORRIES: So far, there's not much scientific evidence showing e-cigarettes help smokers quit or smoke less, or to say whether they are safe. Some are concerned that e-cigarette marketing could tempt young people to adopt the habit.
GROWING MARKET: The industry has rocketed from thousands of users in 2006 to several million worldwide, leading to the rise of more than 200 brands. Sales have been estimated to reach nearly $2 billion in 2013.
ARRAY OF FLAVORS: While some e-cigarette makers are limiting offerings to tobacco and menthol flavors, others are selling candy-like flavors like cherry and strawberry — barred for use in regular cigarettes because of the worry that the flavors are used to appeal to children.