The Food and Drug Administration unveiled long-awaited proposed rules for electronic cigarettes Thursday, recommending mandatory ingredient reporting, a nicotine warning label and a nationwide ban on sales of the vapor-producing devices to minors.

E-cigarette advocates are somewhat relieved by the 241-page proposal. For months, a group of Democratic senators and other state and federal lawmakers insisted that the FDA move to ban popular flavors, which they say may appeal to children, as well as online sales.

The new proposal would prohibit e-cigarette makers from making health claims – something companies already do to avoid medical device regulations – and also would ban public vending machines and free samples.

States currently set their own age limits for e-cigarette sales. Trade and consumer groups have supported age-restriction legislation at the state level.

Unlike conventional cigarettes, which burn tobacco leaves, e-cigarettes vaporize a liquid that's usually a blend of propylene glycol and/or vegetable glycerin, combined with flavoring and tobacco-derived nicotine.

The devices, supporters say, are a less harmful alternative to their combustible cousins.

“FDA’s action today appears to lay the foundation for a new and comprehensive nicotine policy that will save lives,” Charles Connor, past president of the American Lung Association, said in a statement.

Connor, currently a paid senior adviser to the Electronic Cigarette Industry Group (ECIG), told U.S. News in September his support for the new technology stems from watching his mother die of lung failure in 2012 after a lifetime of smoking cigarettes. He sees the devices as a possible public health breakthrough.

“The problem we face is that the smoking rate has gone down a lot, but it plateaued around 20 percent,” Connor said in the release. “The cardinal rule moving forward should be to recognize the distinctions between combustible cigarettes and electronic cigarettes.”

The FDA will accept comments from the public for 75 days. The agency can then either issue a final rule or issue new proposed rules.

The Consumer Advocates for Smoke-free Alternatives Association (CASAA), a powerful grass-roots lobbying group that rallies e-cigarette users to political action, hasn’t yet developed a position on the FDA proposal.

“We would like to avoid being part of the whole 10-minute news cycle,” says Carl V. Phillips, CASAA's scientific director.

“There’s certainly some relief” that flavors and online sales do not appear to be targeted, Phillips says, “[but] this is a very complicated process and there is a lot of reading between the lines to be done.”

After CASAA has fully assessed the rules package, it will issue a call to action to its 12,000 members and to the broader e-cigarette community.

“We expect that there will be tens of thousands, if not hundreds of thousands, of comments from individuals on this,” Phillips says.

FDA spokesman Jeff Ventura tells U.S. News that Internet sales won’t be altered by the current package of rules. Online sellers would not be required to take additional age-verifying steps, as brick-and-mortar retailers would have to do with photo ID checks. But such changes may be in the not-too-distant future.

“On Internet sales, FDA must first bring the products under our authority,” Ventura says. “Then, FDA may put in place restrictions on the sale and distribution of the newly deemed products. These restrictions require separate rule-making and comment.”

E-cigarette sales in the U.S. were projected to exceed $1.7 billion in 2013, more than doubling 2012 sales.

Some surveys, such as the annual National Youth Tobacco Survey conducted by the Centers for Disease Control and Prevention, show an increase in e-cigarette use among high school students. From 2011 to 2012, the percentage of high school students who reported to the CDC ever trying an e-cigarette doubled from 4.7 percent to 10 percent. More than 92 percent of high school students who tried an e-cigarette had also smoked a tobacco cigarette, but the CDC did not ask which they tried first. The "current" use rate among high school students in 2012 – defined as use in the preceding month – was 2.8 percent for e-cigarettes. In 2011, by comparison, the CDC found 18.1 percent of high school students were "current" conventional cigarette users.

Skeptical lawmakers say the increase in non-adult use suggests e-cigarettes may be emerging as a gateway to combustible cigarettes, possibly encouraged by flavors and advertising. E-cigarette advocates say there's no evidence of the gateway theory, and that candy and fruit flavors help adults transition to a healthier alternative to smoking.

“[The FDA] notes the increase in e-cigarette use by youth and the availability of fruit and candy-flavored e-cigarette liquid,” the rules proposal says. “We do not currently have sufficient data about these products to determine what effects e-cigarettes have on the public health. Accordingly, FDA is seeking comment in this proposed rule as to how such products should be regulated.”

A group of Democratic lawmakers who had urged the FDA to take more aggressive action fumed Thursday.

“Today, after years of waiting for the FDA to act, we are extremely disappointed by its failure to take comprehensive action to prevent e-cigarette companies from continuing to deploy marketing tactics aimed at luring children and teenagers into a candy-flavored nicotine addiction,” said a statement issued by Sens. Richard Blumenthal of Connecticut, Sherrod Brown of Ohio, Richard Durbin of Illinois, Ed Markey of Massachusetts, Jack Reed of Rhode Island, Jay Rockefeller of West Virginia and Rep. Frank Pallone of New Jersey.

“Prohibiting sales of these products to minors is a positive step, but it isn’t enough,” they said. “As long as e-cigarette companies continue to take pages from Big Tobacco’s old and cynical marketing playbook, our children will remain vulnerable to the grave dangers of nicotine addiction.”

As e-cigarettes surge in popularity, several cities have added their use to existing smoking bans. In New York City, officials extended the strict municipal smoking ban to e-cigarettes in December over the objections of prominent retired health professionals, including Connor and former U.S. Surgeon General Dr. Richard Carmona. Los Angeles and Chicago quickly followed suit.

Carmona, chairman of e-cigarette maker NJOY’s Scientific Advisory Board, said in a statement released by the company that the "draft regulations are a good first step."

"Cigarettes have killed millions of Americans and while smoking rates have decreased they have plateaued and still too many adults and young people smoke today," Carmona said. "By providing better alternatives for nicotine delivery we can reduce the incidence of cancer and heart disease and improve the health and well-being of Americans."

Craig Weiss, NJOY’s president and CEO, praised the rules package as a victory.

“By resisting calls to regulate ahead of – and indeed in opposition to – the science and data, today the FDA has brought NJOY a giant step closer to achieving its corporate mission of obsoleting cigarettes," he said. "There are encouraging signs that 10 years from now, this date will be remembered as the beginning of the end of the tobacco epidemic.”

Eric Criss, president of the ECIG trade group, said the FDA proposal is “potentially a step in the right direction.” But, he said, “It will take some time to analyze the potential impact of these proposed rules in a meaningful way.”

There have not been long-term studies on the health effects of e-cigarette use, but preliminary studies suggest the inhaled vapor may be less harmful.

A 2012 study by University of Perugia researchers in Italy found the particulate matter of exhaled e-cigarette vapor is significantly lower than tobacco smoke. An analysis released in August by Drexel University researcher Igor Burstyn found the chemical composition of various e-cigarette liquids poses a negligible risk to secondhand inhalers and that "[t]here are no known toxicological synergies among compounds in the aerosol, and mixture of the contaminants does not pose a risk to health."

Vegetable glycerin and propylene glycol aren't known to be carcinogenic when ingested, but it’s unclear if those ingredients or flavoring additions are harmful when inhaled.

“[T]here is not adequate evidence that e-cigarette use is a safe alternative to conventional cigarette smoking,” the FDA says in its proposal. “[M]any consumers have strong, but to date unsubstantiated, beliefs that e-cigarettes are a safe and effective way for quitting cigarette use, and many consumers start consuming e-cigarettes because of those unsubstantiated beliefs.”