While e-cigarettes are used as a "safer" alternative to smoking traditional cigarettes, there are still toxic emissions associated with usage.
Electronic cigarettes (e-cigarettes) are battery-powered products that typically deliver nicotine with flavorings and other chemicals to users in vapor from liquid instead of smoke.
According to the Centers for Disease Control and Prevention (CDC), around 3.7 percent of adults in the United States use e-cigarettes on a regular basis, which represents more than 9 million adult consumers.
While e-cigarettes have been marketed as a safer alternative to traditional cigarettes - which deliver nicotine by burning tobacco - little is known about the health risks of using these devices.
Most e-cigarettes work by using three components: a cartridge that holds a liquid solution, a heating device (the vaporizer), and a power source, which is usually a battery.
Cigarette smoking is responsible for over 400,000 deaths in the U.S. each year. While e-cigarettes seem to be a safer, less toxic alternative - due to not burning tobacco - e-cigarettes still contain nicotine and other potentially harmful chemicals.
The health consequences of repeated exposure to chemicals in e-cigarettes have been unclear.
The study, published in Environmental Science and Technology, finds that the thermal decomposition of two solvents found the liquid substance (e-liquid) vaporized by the e-cigarette - propylene glycol and glycerin - lead to toxic chemicals such as acrolein and formaldehyde.
"Advocates of e-cigarettes say emissions are much lower than from conventional cigarettes, so you're better off using e-cigarettes," says Berkeley Lab researcher and co-author Hugo Destaillats.
"I would say, that may be true for certain users - for example, long-time smokers that cannot quit - but the problem is, it doesn't mean that they're healthy. Regular cigarettes are super unhealthy. E-cigarettes are just unhealthy," he adds.
Destaillats and team simulated vaping in two types of e-cigarette - an inexpensive device with one coil, and a more expensive device with two parallel coils - and three types of e-liquid.
Association found between increasing temperature and emission levels
The first puffs and later puffs once the device had heated up and reached a "steady state" were examined to determine the content of the vapor.
Vaping emulating apparatus drew "puffs" from the e-cigarettes lasting 5 seconds, every 30 seconds. Over the course of the first 5-10 minutes of vaping, the vapor temperature increased rapidly. The temperature reached the steady state at around the 20th puff.
Emission levels between the first puffs and steady state rose by a factor of 10 or more in some devices. A single-coil e-cigarette operated at 3.8 volts emitted 0.46 micrograms of acrolein per puff in the first five puffs. However, at the steady state, it emitted 8.7 micrograms per puff.
"When you apply the same voltage to the double-coil e-cigarette you see a lot less emissions," says co-author and Berkeley Lab researcher Lara Gundel. "We think it has to do with lower temperatures at each of the coil surfaces."
Using a single device over nine consecutive 50-puff cycles without cleaning tested the consequences of the age of the device. Emissions of formaldehyde, acetaldehyde, and acrolein - all carcinogens or respiratory irritants - increased with usage.
Co-author Mohamad Sleiman notes that the increase in aldehyde levels was 60 percent in some cases between cycles 1-9.
The accumulation of residues on or near the coil would provide a secondary source of volatile aldehydes, the authors note in the paper.
Two new toxic compounds detected in e-liquid
Also observed was that as the voltage of the e-cigarette increased, so did the amount of e-liquid consumed per puff and the vapor temperature.
"We found there are emissions of toxic chemicals at any temperature at which you use the device. And the higher the temperature, the more emissions."
The researchers examined the solvents in the e-liquids of all the e-cigarettes, finding that almost all of them use a combination of propylene glycol and glycerin in varying proportions as a solvent.
"Both are used for making artificial smoke on stage," says Destaillats. "The ratio between the two determines things like the volume of vapor cloud that you produce. They are considered safe for food."
There have been few studies on the safety of heating and inhaling propylene glycol and glycerin.
Experiments were conducted to vaporize liquids composed of the solvents to confirm if they were the source of the emissions. A total of 31 harmful chemical compounds were detected, two of which - propylene oxide and glycidol - were not previously found in e-cigarette vapor. The authors propose that these could be carcinogens.
"Understanding how these compounds are formed is very important," says Destaillats. "One reason is for regulatory purposes, and the second is, if you want to manufacture a less harmful e-cigarette, you have to understand what the main sources of these carcinogens are."