E-cigarette use is growing, but much remains unknown about the devices, the chemicals they emit, and the risk those chemicals pose to consumers. Are the products truly safer than cigarettes, as proponents claim? Is there a way to make them safer? What aspect of e-smoking should regulators focus on?

In a study published today—one day after the Childhood Nicotine Poisoning Prevention Act took effect—a team of scientists at the Lawrence Berkeley National Laboratory in Berkeley, Calif. offers some partial answers to those questions.

The study found 31 different chemicals in e-vapor, many of them possible or probably carcinogens. And, surprisingly, it traced most of those chemicals back to the solvents used in e-liquids, not the flavor-giving chemicals or other more obscure additives. 

The researchers tested three different e-liquids, in two different vaporizers, at a range of power settings. (They purchased the products from retail stores in Berkeley, but did not say which brands). One vaporizer was “fairly cheap,” and contained just a single heating coil; the other was more expensive and contained two parallel coils. They analyzed the contents of the vapor produced by each, at the beginning of a smoke session and again after the devices had fully heated. They also tested emissions across the life of each device.

Here’s a quick breakdown of what they found.

Common Chemicals Likely Pose Biggest Danger

Researchers had previously known that e-cig vapor can contain dangerous chemicals, but were uncertain of the precise source of those toxins. The new study suggests that they come from propylene glycol and glycerin, the most common solvents used in the devices. Both those solvents are considered safe for use in foods and beverages (they're also the same chemicals used for making artificial smoke on concert stages). But hardly any studies have been done on the effects of heating or inhaling them, says Hugo Destaillats, Ph.D., a chemist at Lawrence Berkeley National Laboratory and the study’s lead author.

It turns out that when propylene glycol and glycerin burn they produce a range of toxic chemicals. The researchers found 31 in all, including two that have not previously been found in e-cig vapor—propylene oxide and glycidol. Both of those chemicals (along with several others the researchers found) have been classified by the World Health Organization as probable carcinogens.

E-cigarettes also emit some of the same chemicals as traditional cigarettes, albeit at much lower concentrations. For example, a traditional cigarette emits about 400 to 650 micrograms of acrolein, an eye and respiratory irritant, whereas an equivalent amount of e-cigarette emits 90 to 100 micrograms. But that difference doesn't necessarily mean that e-cigarettes are safe. "Regular cigarettes are super unhealthy, e-cigarettes are just unhealthy," Destalliats says.

Older, Cheaper E-Cigs May Pose Bigger Risks

Cheaper devices, which contain a single heating coil, produced more emissions than double-coiled ones in the current study. That was especially true with formaldehyde, one of the chemicals the researchers detected.

And any given device is likely to produce more emissions as it ages, because of the residues that accumulate (users commonly refer to this build up as “coil gunk”). In the current study, researchers used one of the e-cigarette devices for nine consecutive 50-puff cycles without cleaning it. Emissions of some chemicals, like formaldehyde and acrolein increased by as much as 60 percent from the first cycle to the last.

Temperature and Voltage Matter

Vapor temperatures increase during the first 20 or so puffs of a smoke session, and then level out. As the temperature increases, the amount of chemicals emitted in the vapor increases in tandem. One of the main reasons that single-coil smokers produce more emissions is that they tend to get hotter than double-coil devices.

In general, the researchers found that higher voltages led to higher vapor temperatures, which in turn led to greater amounts of e-liquid consumed, and more toxins emitted. For example, the amount of formaldehyde formed at the highest voltage (4.8) was an order of magnitude greater than the amount formed at the lowest voltage (3.3). Temperature is also determined by smoker behavior; longer and more frequent puffs transfer more heat. 

Regulators Should Focus on Common Chemicals, Device Design

The authors write that because most toxic emissions come from the solvents (which are found in every e-liquid formulation) it may make sense to focus regulation, at least initially, on those ingredients rather than individual additives or flavors.

They also argue that because higher temperatures are the ultimate driver of higher emissions from e-cigarettes, devices that limit the maximum temperature of e-liquids in some way (double coils and lower voltages, for example), offer what the authors describe as “a promising approach to limiting the harm caused by e-cigarettes.”

The same team of scientists plans to publish a follow-up study examining the health impacts of these initial findings.