The verdict on whether electronic cigarettes are safer than traditional cigarettes is still very much out. However, a recent study found e-cigarette emissions contain a variety of concerning chemicals, including some considered to be probable carcinogens.

In a study published in July in Environmental Science & Technology, researchers found significant levels of 31 harmful chemical compounds in e-cigarette vapors, including two that had yet to be detected: propylene oxide and glycidol, both of which health researchers have described as reasonably anticipated to be human carcinogens. Researchers also found chemical emission differences based on the voltage of the e-cig vaporizer and how many puffs users take. Those differences are a particularly interesting takeaway because they touch on ways that manufacturers, or even users, may be able to minimize potentially harmful exposures.

However, study co-author Hugo Destaillats, a staff scientist at the Lawrence Berkeley National Lab and deputy leader of its Indoor Environment Group, stressed that while the study’s findings are concerning, they are not a definitive statement as to whether e-cigarettes are less, just as or more harmful to human health than regular cigarettes.

“I don’t want to be seen as scaremongering,” Destaillats told me. “It may be that (e-cigarettes) are better than traditional cigarettes, especially for people who want to quit (cigarettes) but can’t. …But there are decades of research on smoking and little on vaping, so I wouldn’t be surprised if people find health effects we didn’t consider.”

To conduct the study, researchers simulated vaping using three different e-liquids (the substances heated to produce vapor) and two different vaporizers, and then used a method known as gas and liquid chromatography to determine the contents of the e-cigarette emissions. They found several volatile ingredients in the e-liquids, including two solvents (propylene glycol and vegetable glycerin), nicotine, propylene oxide, ethanol and acetol. When the two solvents, which the study noted are found in most e-liquids, were heated and began to decompose, it led to emissions of acrolein, a known irritant, and formaldehyde, a known human carcinogen. Researchers also said that propylene oxide, a likely impurity of propylene glycol, is probably present in most e-liquids now on the market, which is concerning because propylene oxide is also considered a probable carcinogen as well as a known respiratory and eye irritant.

The study also found big differences in the emissions produced by the first and last puffs. To imitate how people use e-cigarettes in real life, researchers used an apparatus that took puffs lasting five seconds every 30 seconds. Emissions were significantly higher once the vaporizers reached a steady temperature (what researchers called “steady-state”) at around 20 puffs, as compared to the first five to 10 minutes of puffing when the temperature was still rising. In fact, researchers found that in some cases, emission levels increased by a factor of 10 or more between initial puffs and steady-state puffs. For example, levels of the eye and respiratory irritant acrolein went from 0.46 micrograms to 8.7 micrograms per puff between initial temperature and steady-state temperature.

“When we look at the chemical composition in the first couple puffs versus the final puffs, there were dramatic changes,” Destaillats said. “Even the same device with the same e-liquid can give different emissions depending on how you use it.”

Researchers also found big emission differences between vaporizers with a single coil and double coil. (Destaillats explained that when the same voltage is split between two coils, as opposed to just one, fewer emissions are produced.) On the issue of voltage, the study found that as the battery power output increased, the average vapor temperature reached at a steady state was higher. As a result, as voltage went up, the amount of e-liquid consumed per puff was higher too.

Here again Destaillats emphasized that the findings don’t mean that lower temperatures make for safer vaping, saying: “By emitting less, the exposure may be less harmful…but we cannot say it’s safer or it’s healthier.”

Destaillats and his colleagues also examined how the age of a vaporizer affected emissions. In using a single vaping device for nine consecutive rounds of 50 puffs — similar to how an e-cigarette user would vape in real life — researchers found that aldehyde emissions increased by more than 60 percent, with greater contributions of formaldehyde, acetaldehyde and acrolein. The effect was likely due to residue buildup inside the vaporizer, or what users call “coil gunk.”

Destaillats and study co-authors Mohamad Sleiman, Jennifer Logue, V. Nahuel Montesinos, Marion Russell, Marta Litter and Lara Gundel write:

Since harmful chemical emissions are primarily due to thermal decomposition of e-liquid constituents, reducing these temperatures is a promising approach to limiting the harm caused by e-cigarettes. Proper maintenance or more frequent replacement of coils may also reduce emissions by avoiding accumulation of polymeric residues. From the regulatory point of view, particularly in light of recent U.S. (Food and Drug Administration) regulations issued in May and European Union regulations from 2014, it should be highlighted that toxic emissions originate primarily from heating the solvents propylene glycol and glycerin, which are the constituents most commonly found in e-liquid formulations.

Destaillats told me that this study is far from comprehensive, as the market is home to hundreds of different e-liquids. But he said it does tease out the particular problem of solvents, which are a common e-liquid ingredient that when heated up do, indeed, emit harmful chemicals. Destaillats and colleagues will soon publish another study on secondhand e-cigarette exposures, attempting to measure the composition of e-cig vapors that people exhale.

He noted that because e-cigarette use among young people is growing so quickly and health officials worry such trends threaten to reverse hard-fought declines in traditional smoking, understanding the true harms posed by e-cigarettes is crucial.

“Yes, there is some urgency to this kind of research,” Destaillats said. “But we’re often behind because the technology changes so fast.”

To request a full copy of the e-cigarette emissions study, visit Environmental Science & Technology.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for nearly 15 years.