The Food and Drug Administration just proposed a set of rules that would ban sales to minors and require e-cigarette manufacturers to disclose their products' ingredients. If passed, the regulations would bring some much-needed order to an uncontrolled industry. "Right now, because there are about 250 different kinds of e-cigarettes and no regulations, it's very much the Wild West," says Tom Glynn, director of science and trends at the American Cancer Society. "Some e-cigarettes are pretty much just liquid nicotine, propylene glycol, and flavorings. But others have lots of contaminants and carcinogens, including heavy metals like nickel and cadmium. It's very difficult for consumers to determine which is which."

The proposed regulations still don't address the biggest issue with e-cigarettes: Their long-term health effects are completely unknown. Experts agree that, compared to regular cigarettes – which contain 7,000 chemicals, 60 of them cancer-causing – e-cigarettes are the lesser of two evils. However, there's mounting fear that even those e-cigarettes that contain only a few, "safe" ingredients may prove to be hazardous. "Since they've only been available in the U.S. since 2007 and didn't really catch on until 2010, there's been no opportunity to do long-term study on their impacts," says Glynn.

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The first big question surrounds the safety of liquid nicotine, which can cause vomiting, nausea, and eye irritation if too much is inhaled, ingested, or absorbed through the skin. And since it's impossible to know how much nicotine you're getting per drag – or how much is even in an e-cigarette – poisoning has become a real issue. According to a new report from the Centers for Disease Control and Prevention, the number of calls received by poison control centers regarding liquid nicotine poisoning skyrocketed from one per month in September 2010 to 215 per month this February. Forty-two percent of these incidents involved people age 20 and over.

But even just continually inhaling nicotine vapor may be dangerous. "Our big concern is that we don't know how breathing in nicotine vapor will affect people long-term," says David Abrams, executive director of the Schroeder National Institute for Tobacco Research and Policy Studies at the Legacy Foundation.

Health experts are also concerned about the propylene glycol in e-cigarettes. Although the FDA deems this ingredient safe for use in foods, heating it into a vapor and inhaling it may be a different story. "We don't know whether inhaling heated propylene glycol is safe, especially if done continually," Glynn says. "If you take 100 drags a day over 10 years, it could be completely harmless. Or it could be Pandora's box." At the end of the day, no cigarette – electronic or otherwise – is truly safe.