Nearly six in 10 adults in the U.S. are aware of the battery-powered devices that heat a liquid nicotine solution and create vapor that users inhale, according the first study to assess the change in awareness and use of electronic cigarettes on a national level. The CDC report published in the journal Nicotine & Tobacco Research also said about one in five current smokers reported having used an electronic cigarette.

"These finding sort of tantalizingly underscore the need for more rigorous study of patterns of use of e-cigarettes and impacts," Dr. Tim McAfee said in an interview with The Associated Press. "Until there's regulatory authority and oversight, it's going to be more difficult to be certainly reassuring around things like toxic effects."

Some of the nation's largest tobacco companies have gotten into the e-cig market as part of the industrywide push to diversify beyond the traditional cigarette business. Reynolds American Inc., the second-biggest U.S. cigarette maker, has begun limited distribution of its first electronic cigarette under the Vuse brand. Lorillard Inc., the nation's third-biggest tobacco company, acquired e-cigarette maker Blu Ecigs in April. Some e-cigarettes are made to look like a cigarette with a tiny light on the tip that glows like the real thing.

The Food and Drug Administration says e-cigarettes have not been fully studied. The federal agency is expected to assert regulatory authority over e-cigarettes later this year to treat them the same as traditional cigarettes and other tobacco products.

The FDA has said its tests found that the liquid in some electronic cigarettes contained toxins besides nicotine as well as cancer-causing substances that occur naturally in tobacco. But some public health experts say the level of carcinogens was comparable to those found in nicotine replacement therapy, because the nicotine in all of the products is extracted from tobacco.

According to the results of a series of online surveys, the CDC reported that awareness of electronic cigarettes increased from 40 percent in 2010 to nearly 60 percent in 2011, and use among adults doubled to 6 percent during that time. The share of current smokers who said they have used an e-cig grew from 10 percent to about 21 percent, and from 2.5 percent to 7.4 percent for former smokers.

McAfee said the motivations behind e-cigarette use remain unclear. For example, a former smoker could be experimenting with them as a new nicotine delivery system, or could have used an electronic cigarette to quit smoking regular cigarettes. Current smokers might be using e-cigs in places where tobacco smoking isn't allowed.

More than 45 million Americans smoke cigarettes, and about half of smokers try to quit each year.

First marketed overseas in 2002, e-cigarettes didn't become easily available in the U.S. until late 2006. Now, the industry has grown from the thousands of users in 2006 to several million worldwide. Analysts estimate sales could double to $1 billion in 2013. Some companies have even started running TV commercials.

Devotees tout e-cigs as a way to break addiction to real cigarettes. They insist the devices address both the nicotine addiction and the behavioral aspects of smoking — the holding of the cigarette, the puffing and exhaling something that looks like smoke — without the more than 4,000 chemicals found in cigarettes.

"The bottom line is ... the impact of e-cigarettes on public health remains uncertain," McAfee said.

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Michael Felberbaum can be reached at http://www.twitter.com/MLFelberbaum.