Congressional staffers seeking to wean themselves off combustible cigarettes may soon be breaking the rules if they sneak a puff of nicotine-laced vapor in the hall.

Six Democratic senators and one congressman wrote to the Senate Committee on Rules and Administration and the House Office Building Commission Tuesday asking that electronic cigarette use be banned in public spaces of the Capitol, its adjacent office buildings and within 25 feet of building entrances.

“In the absence of evidence demonstrating the safety of e-cigarettes, particularly for individuals exposed to their vapor, we ask [that e-cigarettes be included] in the existing prohibition on smoking,” the members of Congress wrote. “This is an appropriate precautionary step to promote public health and maintain a safe environment for staff and visitors of the institution and its grounds.”

Sens. Richard Blumenthal of Connecticut, Sherrod Brown of Ohio, Dick Durbin of Illinois, Ed Markey of Massachusetts, Tom Harkin of Iowa and Barbara Boxer of California signed the letter, as did Rep. Henry Waxman, D-Calif.

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

The members of Congress cited a 2009 Food and Drug Administration analysis of an electronic cigarette cartridge that allegedly contained diethylene glycol, an ingredient in antifreeze.

Several of the letter-signers have been aggressively lobbying for restrictions on the booming e-cigarette industry.

Blumenthal urged the FDA to ban e-cigarette flavors and online sales last year, claiming flavors that are popular with adult users also appeal to children. Brown alleged that e-cigarette makers are attempting to recruit a new “crop of customers” for conventional tobacco companies. The FDA is expected to announce proposed regulations early this year.

Not everyone is pleased with the latest proposed e-cigarette ban.

“Unfortunately, certain politicians have shown that they are willing to take any opportunity to score political points by attacking smoke-free electronic cigarettes, even if their misinformation may cause harm to smokers by encouraging them to continue to smoke,” says Gregory Conley, a research fellow at the Heartland Institute and an independent e-cigarette industry consultant.

“No scientific evidence supports banning e-cigarette use where smoking is banned and the legislators' attempt to even ban e-cigarettes in outdoor environments is especially absurd,” says Conley, formerly legislative director of the Consumer Advocates for Smoke-free Alternatives Association. “The reliance on one discredited study that is nearly five years old should signal that the legislators have not undertaken any serious study of the science on e-cigarettes or its users."

Several cities have added electronic cigarettes to existing smoking bans, notably including New York City, where officials extended the strict municipal smoking ban to e-cigarettes in December over the objections of prominent retired health professionals.

"By impulsively and inaccurately lumping vaping in with traditional cigarette smoking, a ban on these game-changing devices will discourage other smokers from trying a positive alternative," Charles Connor, former president and CEO of the American Lung Association, cautioned New York officials. "Electronic cigarettes are poised to revolutionize the tobacco industry by one day making traditional cigarettes obsolete,” he said.

Former U.S. Surgeon General Richard Carmona said in a December statement: "I'm extremely concerned that a well-intentioned but scientifically unsupported effort like the current proposal to include electronic cigarettes in New York's current smoking ban, could constitute a giant step backward in the effort to defeat tobacco smoking.”

Connor is a paid consultant for the Electronic Cigarette Industry Group and Carmona is a board member at e-cigarette maker NJOY.

The long-term effects of smoking e-cigarettes have not been studied, but even anti-tobacco hawks acknowledge the devices are likely healthier than their combustible cousins.

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, conceivably reducing or eliminating the risk of secondhand inhalation. A large-scale study by New Zealand researchers, published in September by The Lancet, found e-cigarettes are as effective as nicotine patches in helping smokers quit. Various nicotine concentrations are offered by e-cigarette companies, allowing users to slowly reduce their dependence on the drug.