According to researchers at the Lawrence Berkeley National Laboratory, most of the toxic chemicals found in vaporizer smoke comes from the thermal decomposition of glycol and glycerin -- two food-safe solvents commonly used in e-cigarette liquid. When vaporized, however, researchers found that the liquid emits toxic chemicals. Among them are propylene oxide and glycidol, two probable carcinogens.

That said, not all e-cigarettes are equal: researchers found that when it tested the emissions from a vaporizer with two heating elements, it had a lower concentration of harmful chemicals than the output of a single-coil model. The team also found that vaporizers that ran at a higher voltage tended to make smoke with more toxins, as did units that weren't cleaned on a regular basis. This doesn't mean that lower-temperature vaporizers are healthier, though -- all e-cigarettes emit toxic chemicals.

The uptick of the research mostly lies with what it means for the future. Understanding where the toxins in e-cigarettes come from will allow regulators and manufacturers to create less harmful vaporizers -- and provide a less dangerous alternative to cigarettes. Even so, the study co-author Hugo Destaillats is careful to avoid calling e-cigarettes 'healthy.' "Regular cigarettes are super unhealthy," he says. "E-cigarettes are just unhealthy."