Lots of questions remain about the effects that using electronic cigarettes have on adolescents, including whether they are a “gateway” drug to regular cigarettes, several speakers said Wednesday.

Most young people who use them also use other forms of tobacco.

“Do e-cigarettes cause children to progress to other tobacco products that are extremely harmful?” Andrea Villanti, PhD, MPH, director for regulatory science and policy at the American Legacy Foundation’s Schroeder Institute for Tobacco Research and Policy Studies here, asked at a briefing on public health challenges related to e-cigarette usesponsored by George Washington University. “To know that … we need to track patterns over time. We need to know if ‘never users’ stayed never users, transferred to combustible use, dual use, or e-cigarette use, and what happened over time.”

For example, the University of Michigan’s “Monitoring the Future” survey for 2014 found that among 12th graders, 62% had used neither e-cigarettes nor conventional cigarettes, while 4% had used e-cigarettes only, 21% had used conventional cigarettes only, and 13% had used both.

E-cigarettes have gone through several iterations since they were first introduced, explained Naomi Freedner-Maguire, principal researcher at ICF International, a Fairfax, Va., consulting firm. While first-generation products tried to mimic the look of conventional cigarettes, the next generation, also known as “personal vaporizers,” were two-part devices with a tank and a separate battery, and offered thousands of flavor choices to users.

Today, there is a third generation of devices available, with the biggest difference being “mods” — components that can be modified to change the vaping experience, Freedner-Maguire said. “This is a much more effective nicotine delivery system, and it’s moved into a subculture of e-cigarette users and ‘vapers’ that are really committed to creating mods that will increase the power of the device or the flavor.”

“Sub-ohming” — modifying the coils in the atomizer to produce more vapor — is a popular pastime among e-cigarette users, she continued. “There’s a whole vaping culture dedicated to producing as much vapor as possible.”

Some people call themselves “cloud-chasers,” said Freedner-Maguire, “and there are cloud-chasing competitions where people square off back-to-back and inhale, and release the biggest plume of vapor that they can. Somebody stands with a yardstick on the other side and measures the size and density of the plume, and there are cash prizes for things like this.”

“Dripping” is another popular practice, in which “nicotine e-liquid is dripped directly onto wicked material in the atomizer to simulate more closely the experience of smoking a cigarette,” she said. “You can find these things on blogs and all over online about how people inhale, and the difference between mouth inhalation versus lung inhalation — a ‘lung hit.'”

Surveying young people about their e-cigarette use is complicated for many reasons, saidJennifer Pearson, PhD, MPH, a research investigator at the Legacy Foundation’s Schroeder Institute. For one thing, “E-cigarettes have many names. What’s a vape pen to someone in California is an e-hookah to someone in the Northeast, and who knows what to someone in Texas,” she said.

In addition, “young adults might call products different things than older adults,” Pearson continued. “If it’s an older adult, they might call it an e-cigarette and a younger adult might call it an e-hookah.”

Then there’s the issue of dosing. “It’s easy to ask about cigarettes per day, but with e-cigarettes it’s not sensible to ask that — even disposable ones can last several days,” said Pearson. “We’re telling people a ‘use episode’ is when you use an e-cigarette, then put it down and don’t intend to pick it back up for a while.”

With conventional cigarettes, “They’re very much compelled to smoke the entire thing,” said Niaura. “That means the way nicotine is delivered into the brain is very different; you’re getting a very intense bolus of inhaled nicotine. With e-products, if you work at it, you can do that, but is that how people really use them?”

Instead, “you can take a puff, put it in your pocket … and walk around, then take another couple of puffs. I don’t know the liability [of that] for producing long-term addiction.”

Do e-cigarettes help people quit smoking conventional cigarettes? “The evidence there is mixed,” he said. “There are a couple of randomized trials, but they’re not slam-dunk positive and not slam-dunk negative.”