Multiple studies demonstrate that propylene glycol and vegetable glycerin thermally degrade during vaporization to form toxic aldehydes, specifically including formaldehyde and acetaldehyde.
Every retrieved paper examining the thermal degradation or emissions of propylene glycol and glycerin (glycerol) in electronic delivery systems reports the formation of formaldehyde, acetaldehyde, and other toxic carbonyls. The evidence is unanimous and robust, supporting the claim entirely.