Propylene Glycol is one of the primary ingredients in the majority of e-liquids and e-cigarette cartridges today. Most e-liquid contains at least 40% propylene glycol. This is the ingredient that produces the smoke like vapour when the e-liquid is vaporised.
Propylene Glycol is much thinner than its counterpart vegetable glycerin (VG), meaning it produces much more of a ‘throat hit’ than a VG based e-liquid does. Some believe this helps to simulate the feel of smoking more.