Electronic cigarettes appear to be safer than ordinary cigarettes for one simple — and simply obvious — reason: people don't light up and smoke them.
With the e-cigarettes, there is no burning tobacco to produce myriad new chemicals, including some 60 carcinogens.
But new research suggests that, even without a match, some popular e-cigarettes get so hot that they, too, can produce a handful of the carcinogens found in cigarettes and at similar levels.
A study to be published this month in the journal Nicotine and Tobacco Research found that the high-power e-cigarettes known as tank systems produce formaldehyde, a known carcinogen, along with the nicotine-laced vapor that their users inhale. The toxin is formed when liquid nicotine and other e-cigarette ingredients are subjected to high temperatures, according to the study. A second study that is being prepared for submission to the same journal points to similar findings.
The long-term effects of inhaling nicotine vapor are unclear, but there is no evidence to date that it causes cancer or heart disease as cigarette smoking does. Indeed, many researchers agree that e-cigarettes will turn out to be much safer than conventional cigarettes, an idea that e-cigarette companies have made much of in their advertising.
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The website for Janty, a company that manufactures popular tank systems, says the benefits of e-cigarettes include having "no toxins associated with tobacco smoking."
Nonetheless, the new research suggests how potential health risks are emerging as the multibillion-dollar e-cigarette business rapidly evolves, and how regulators are already struggling to keep pace. While the Food and Drug Administration last month proposed sweeping new rules that for the first time would extend its authority to e-cigarettes, the F.D.A. has focused largely on what goes into these products — currently, an unregulated brew of chemicals and flavorings — rather than on what comes out of them, as wispy plumes of flavored vapor.
The proposed rules give the F.D.A. the power to regulate ingredients, not emissions, although the agency said it could consider such regulations in the future. Even so, some experts contend that the current approach is akin to examining the health risks associated with tobacco leaves rather than with cigarette smoke.
"Looking at ingredients is one thing, and very important," said Maciej L. Goniewicz, who led the first study, which is scheduled to be published on May 15. "But to have a comprehensive picture, you have to look at the vapor."
Both studies focused on tank systems, fast-growing members of the e-cigarette family. Unlike disposable e-cigarettes, which tend to mimic the look and feel of conventional smokes, tank systems tend to be larger devices heated with batteries that can vary in voltage, often resembling fountain pens or small flashlights. Users fill them with liquid nicotine, or e-liquid, and the devices are powerful enough to vaporize that fluid quickly, producing thick plumes and a big nicotine kick.
Dr. Goniewicz, an assistant professor of oncology at the Roswell Park Cancer Institute in Buffalo, said people using the systems "want more nicotine, but the problem is they're also getting more toxicants."
Complicating the issue is that the tank systems are made by a variety of manufacturers, many overseas, and then sometimes tinkered with and modified by retail shops or users. Still, e-cigarette makers should be measuring emissions, said Josh Rabinowitz, chief scientist at NJoy, a maker of more traditional e-cigarettes, not tank systems.
"We don't just evaluate the purity of what goes in, but also the purity of the emitted vapor because that's what the user is exposed to," said Dr. Rabinowitz. He said NJoy had significantly lower levels of formaldehyde than regular cigarettes.
Like Dr. Goniewicz's study, which has been subjected to peer review, the second study also centered on the impact of increased heat generated by the systems. In that study, the focus was on how people use the devices to create more — and more potent — vapor. Rather than fill their tank systems with e-liquid, experienced users often trickle drops of the fluid directly onto the device's heating element, a practice known as "dripping."
But with dripping, the e-liquid heats with such intensity that formaldehyde and related toxins "approach the concentration in cigarettes," said Dr. Alan Shihadeh, a project director at the Virginia Commonwealth University's Center for the Study of Tobacco Products and an associate professor of mechanical engineering at American University in Beirut, who led the research.
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Both studies point to the same phenomenon: Intense heat can change the composition of e-liquids, creating new chemicals. Importantly, the researchers said, the chemical reactions apply not only to the liquid nicotine, but also to two other crucial ingredients in most e-liquids: vegetable glycerin and propylene glycol.
Precisely what level of heat causes the reaction is difficult to pinpoint. The Roswell research found, generally, that when battery voltage increased to 4.8 volts from 3.2 volts, toxin levels increased markedly.
"This finding suggests that in certain conditions, E.C.s might expose their users to the same or even higher levels of carcinogenic formaldehyde as tobacco smoke," the Roswell study says.
Both studies examined only a handful of carcinogens. Traditional cigarettes, by contrast, create thousands of chemicals and dozens of carcinogens, according to Prue Talbot, professor of cell biology at the University of California, Riverside. E-cigarettes do not tend to generate enough heat to create combustion, which is a big reason that many public health officials and researchers predict they will prove less harmful than cigarettes.
"If I was in a torture chamber and you said I had to puff on something, I'd choose an e-cigarette over a regular cigarette," Dr. Shihadeh said. "But if you said I could choose an e-cigarette or clean air, I'd definitely choose clean air."
He added: "And I definitely wouldn't drip."
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Whatever the health effects of the new systems, they are exploding in popularity and reshaping the market, according to a report issued in March by Bonnie Herzog, a tobacco-industry analyst for Wells Fargo Securities. In 2013, Ms. Herzog projected domestic sales of e-cigarettes at $1.7 billion a year but, based on surveys with makers and marketers of e-cigarettes, she now expects "in excess of $2 billion."
Demand for tank systems, Ms. Herzog wrote, is "accelerating" and "undeniably a key growth driver." She estimated that the new systems now represent 35 percent of the market, but they are growing twice as fast as the previous models.
Experts say tank systems appeal to young people who like products that can be customized with flavors and power, and are comfortable with fast-changing technology.
At Sky City Vapor in San Diego, the owner, Chris Hayek, said consumers clearly are gravitating toward the more powerful systems and away from the "ciga-likes."
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"The ones shaped like a cigarette, that style doesn't do it for them," Mr. Hayek said. "It's not harsh, it doesn't produce as much vapor." He said that only about 5 percent of his customers are regular "drippers" but that the trend in the industry is toward tank systems that produce large amounts of strong vapor.
Some health data seems promising for e-cigarette users, such as a previous study led by the researchers at Roswell Park Cancer Institute. That research, published in 2012 in the journal Tobacco Control, found that the level of some toxic substances in 12 e-cigarette brands was significantly lower than in cigarette smoke. However, Dr. Goniewicz said that the technology is moving so quickly that many of the products tested are outdated — and some are no longer even on the market.
"Technology is way ahead of the science," Dr. Shihadeh concurred. "We're creating this stuff, and we don't understand the implications."
