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CNN - The popularity of e-cigarettes is growing. Thursday, the New York City council is set to vote on a proposal that could banthem in some public places just like traditional cigarettes. But are they really harmful?

Here's how they work: liquid nicotine is heated up by a battery-charged coil. There's no tobacco burned. Users inhale and instead of smoke, there's a steam-like vapor.

So, are e-cigarettes safe and perhaps the biggest innovation yet to kick smoking...or an addictive health hazard?

"With a product like e-cigarettes, you are guilty until proven innocent," Dr. Thomas Frieden, the director of the CDC, said. "We need to know that these things are safe and ok to use."

They've been in the U.S. less than a decade, and increasingly big tobacco companies are manufacturing them. Limited research has been done on the health impact, and there are conflicting studies on whether or not nicotine alone is harmful.

Some states have age requirements on sales, but not all. CDC data shows nearly 2 million middle and high school students tried e-cigarettes last year -- more than double the number in 2011.

They're not regulated by any federal body, and they're not an FDA-approved method to quit smoking. Critics point out they can keep users hooked on nicotine.

A Wells Fargo tobacco industry analyst estimates the market for e-cigarettes in the U.S. will hit $1.8 billion this year. And, she believes consumption of e-cigarettes could surpass that of traditional cigarettes within the next decade.