Thanks to electronic cigarettes, it may now be easier to get away with smoking on an airplane than with using a cellphone. The devices are beginning to reverse decades-long efforts to ban smoking in public places, experts say, and may also be contributing to a rise in underage smoking, marijuana use, and other illegal activities.

E-cigarettes, which use liquid nicotine solution and batteries in place of paper and flame, emit vapor instead of smoke, and don’t produce the telltale cigarette odor or ash. Proponents say that makes the devices a discreet (and less harmful) alternative to smoking, enabling people to smoke whatever they want, wherever they want, without attracting attention.

But lawmakers and public health officials fear that e-cigarettes are helping tobacco sneak back into non-smoking zones as well as the hands of children. And because it’s hard to determine what’s actually in the devices, they also worry that e-cigarettes may not contain nicotine at all, but illegal drugs.

“Do you want to see a 15-year-old with a vaporizer making like he has an e-cigarette but there’s grass in it, the liquid version of marijuana?” says Massachusetts State Rep. Jeffrey Sanchez, a Democrat, who authored a bill prohibiting youth from buying e-cigarettes and adding the devices to public and workplace smoking bans. “You could vaporize anything if you put it in liquid form,” Sanchez says.

Now that 20 states, along with Washington, D.C., have legalized medical marijuana, Massachusetts and others are bracing for the arrival of dispensaries, which often sell cannabis solutions for vaporization. While marijuana vaporizers have been on the market much longer than e-cigarettes, there is debate over whether the products can (or should) be used interchangeably to inhale either pot or tobacco. The Tobacco Vapor Electronic Cigarette Association, which represents that industry, says none of its members make marijuana paraphernalia: “It’s a whole different animal altogether,” says CFO Thomas Kiklas.

That may be beside the point, as marijuana vaporizers and electronic cigarettes are indistinguishable. Some companies are even marketing vaporizers for both purposes. Rapid Fire Marketing RFMK 0.00%  , for one, makes the CannaCig vaporizer, but is rebranding the device to appeal more to tobacco users than cannabis users, says spokesperson Rick Lutz. “The reality is, whether it’s cannabis or tobacco, it can be used in a vaporizer in order to convert what normally is smoke into a vapor,” Lutz says. “You could conceivably walk out of a restaurant, have cannabis and the effects of cannabis, without the smoke.”

Indeed, the ability of e-cigarette users to “vape” undetected everywhere from office buildings to bars to airplanes, where smoking is generally banned, has prompted efforts to regulate the new industry. Under current Massachusetts law, “in the hallway of the school, I can’t light up a cigarette, but I can light up my e-cigarette,” Sanchez says, adding that he wouldn’t even be able to prevent members of the House public health committee, of which he is chairman, from smoking during meetings in his office.

Concern over e-cigarettes has especially focused on youth, who in many states can buy the products without being carded, while sales of traditional cigarettes and tobacco are restricted to ages 18 and over. The number of minors who have used e-cigarettes more than doubled to almost 2 million middle- and high-school students between 2011 and 2012, according to a report last week from the Centers for Disease Control and Prevention.

But where laws do restrict smoking, some people are using e-cigarettes to openly flout them. While actor Alec Baldwin was kicked off a flight in 2011 after defying the electronic device ban by playing smartphone game Words with Friends in the airplane’s bathroom, passengers report that they’ve had no problem smoking an e-cigarette in plane lavatories—or in their seat, for that matter. Rob Fontano, owner and president of Fort Myers Vapor, which sells high-end e-cigarettes out of its Florida showroom as well as online, says he has used them in more than 20 airports, including Chicago O’Hare and Dallas-Fort Worth without being stopped—even while waiting in line to board and in airplane restrooms. “I have a pilot for a customer who will use it in the cockpit from time to time, on a major airline,” he says.

Smoking has been banned on U.S. flights (and their bathrooms) for more than a decade, with penalties ranging from thousands of dollars to arrest, but electronic cigarettes have landed in a legal gray area. While the Department of Transportation believes the existing rule also applies to e-cigs, it proposed an amendment in 2011 to explicitly prohibit them, says spokesperson Bill Mosley, “as there has been some confusion over whether the department’s ban on smoking includes a ban on use of e-cigarettes.”

But until the DOT puts an official ban on the books (it plans to by mid-2014, Mosley says), e-cigarette users are coming up with their own rules, or leaving enforcement up to individual carriers. American Airlines’ non-smoking policy also prohibits the “activation” of e-cigarettes, but some European airlines may allow them.“They’re not banned on airplanes at this point—it’s a wandering policy,” says Kiklas, the e-cigarette industry spokesman. “E-cigarette users can use the e-cigarettes on a plane just by holding it in their hand.”

Fontano, for his part, says he primarily hides his use on planes as a courtesy to other passengers. “If somebody saw a cloud of vapor on an airplane and didn’t know what it was, they would probably lose their mind,” he says. “I don’t really recommend using them on an airplane, but you could, and people do.” Indeed, in response to Fontano’s recent blog post on the subject of plane “vaping,” a commenter by the name of Mike B. wrote on Aug. 5, “I vape on planes all the time. I choose a window seat and am inconspicuous about it.” He’s used e-cigarettes on a dozen planes in the past few months, he bragged in the post: “I don’t even worry about it.”

###

By Jen Wieczner