The main component of e-liquid is usually propylene glycol and/or glycerin, which together can comprise up to 90 percent of e-liquid. Propylene glycol is “generally recognized as safe” according to the CDC, but information regarding the health effects of inhaling propylene glycol are limited.
The breakdown of propylene glycol after it is heated can produce formaldehyde, a carcinogen also found in traditional cigarettes. One study found that when using an identical e-cigarette at a “high” voltage (5.0 V) rather than “low” voltage (3.3 V), formaldehyde was released at detectable levels.
Flavor Additives and Other Compounds
Though flavoring of traditional cigarettes is generally banned in the U.S, e-cigarettes are available in a number of flavors, such as cotton candy, vanilla, traditional tobacco or whiskey. Recent evidence suggests that the chemicals used to create these flavors may be cause for concern, especially with regard to inhalation.
One study found that e-cigarette flavoring made up between 1 and 4 percent of e-liquid (by weight) and included vanillin, menthol and benzaldehyde, the last of which is known to irritate the lungs. These ingredients were typically not placed on the labels, according to the study. An analysis of e-liquid similarly found a number of harmful chemicals, including formaldehyde and acetone, at significant levels across dozens of samples from popular brands.
Some researchers have raised concern that most of these chemicals have not been assessed for safety by inhalation and note that further analysis is needed to help draw broader conclusions. In addition, e-cigarette users have been reported to mix multiple e-liquids to create their own unique flavors, further complicating safety assessments.
A primary difference between e-cigarettes and traditional cigarettes is that e-cigarettes are non-combustible, and thus create an aerosol (vapor) rather than smoke. Thus, some researchers have suggested that this difference alone makes e-cigarettes less harmful both to the user and people nearby through secondhand exposure.
Evidence suggests that secondhand e-cigarette vapor complies with regulatory standards for clean air. In addition, the existing literature has thus far shown that vapor is safer for exposure than traditional cigarette smoke.
Analysis of the aerosol generated by e-cigarettes—across several studies— shows low concentrations of the most potentially harmful chemicals and concluded that secondhand exposure posed little risk to human health for both adults and children. Though limited evidence exists to determine the amount of nicotine generated by secondhand vapor—especially longterm—some reports show that the air concentration of nicotine generated secondhand is about one-tenth of that generated by traditional cigarettes but is significant compared to clean air.
Risk to E-Cigarette User
One study showed that after adjusting for puff volume, dilution and quantity, e-cigarette vapor causes less stress to human lung cells than traditional cigarettes but is more harmful than clean air. Some compounds generated during vaporization, such as formaldehyde and acetaldehyde, may have adverse health effects, especially as the voltage of the e-cigarette device increases. Some studies suggest that vapor inhalation may pose minimal risk. Literature concerning metals (copper, lead and nickel) emitted from e-cigarettes, found that levels in e-cigarette vapor were within U.S. safety standards.