E-liquids, the solutions vaporized by electronic cigarettes, deliver nicotine and flavorings using two main carriers: Vegetable Glycerin (VG) and Propylene Glycol (PG). The ratio of these ingredients influences the user experience, determining vapor cloud density and the sensation known as “throat hit.” Assessing the safety of vaping requires separating the effects of the stable liquids from the products of their thermal decomposition when inhaled as a heated aerosol.
Understanding the Base Liquids: VG and PG
Propylene Glycol (PG) is a synthetic organic alcohol that is colorless, odorless, and thin. It is widely used across numerous industries as a solvent and humectant, meaning it helps retain moisture. Outside of vaping, PG is an additive in many processed foods, pharmaceuticals, and cosmetics, where it is generally recognized as safe (GRAS) for ingestion and topical application.
Vegetable Glycerin (VG) is also an alcohol, but it is derived from plant oils. In contrast to PG, VG is notably thicker, possessing a syrupy viscosity and a faint, sweet taste. Its humectant qualities lead to its use in food products as a sweetener and preservative, and in personal care products like lotions and soaps. While both substances have a long history of safe use in products designed for consumption or skin contact, their inhalation as a heated aerosol is a relatively new form of exposure.
Health Implications of Propylene Glycol
The immediate health effects of inhaling Propylene Glycol are related to its property as a hygroscopic agent. Being hygroscopic means PG readily absorbs water from its surroundings, including the moist tissues of the mouth and throat. This moisture-absorbing action is responsible for common, temporary side effects such as throat dryness, dry cough, and thirst.
Studies examining PG inhalation, including those involving theatrical fog machines, show that exposure can cause acute irritation of the eyes and upper respiratory tract. While these effects are generally mild and transient, the lack of long-term data means the full chronic impact of daily inhalation remains an active area of investigation. Systemic toxicity from the stable PG compound itself is considered low at typical device temperatures.
Health Implications of Vegetable Glycerin
Vegetable Glycerin’s high viscosity introduces inhalation concerns. This thickness contributes to the production of large, dense vapor clouds, which is desirable for many users. However, the thick nature of the aerosol particles may interfere with the lungs’ natural clearance mechanisms. The respiratory system relies on cilia to move mucus and foreign particles out of the airways, and the constant deposition of a highly viscous substance could potentially impede this function.
A recurring concern involves the potential for VG to cause lipoid pneumonia, a condition resulting from fat-like substances entering the lungs. Pure VG, or glycerol, is chemically classified as an alcohol, not a true lipid (fat), and is water-soluble. Experts suggest that cases of lipoid pneumonia in vapers are more likely caused by the inhalation of oil-based flavorings or contaminants, which are true lipids, rather than the stable VG itself. Nevertheless, VG aerosol inhalation has been associated with lung inflammation and adverse effects on bronchial cells in laboratory settings.
The Dangers of Thermal Breakdown Products
The most significant safety concern related to both VG and PG arises when the e-liquid is subjected to excessive heat, a process known as thermal degradation or pyrolysis. This chemical breakdown occurs when the heating element, or coil, overheats the liquid, often due to insufficient liquid saturation, commonly called a “dry hit.” Under these high-temperature conditions, the stable base liquids transform into toxic carbonyl compounds.
Propylene Glycol is more susceptible to thermal decomposition than Vegetable Glycerin, primarily generating formaldehyde. Formaldehyde is a known human carcinogen and is considered a serious public health hazard. Conversely, the thermal degradation of Vegetable Glycerin is the main source of acrolein, a highly reactive aldehyde. Acrolein is a severe irritant to the respiratory tract and is toxic to lung tissue.
The concentration of these toxic byproducts increases exponentially as device wattage and coil temperature rise. Studies show a sharp increase in formaldehyde and acrolein generation at power levels above 40 watts. Therefore, the safety of the vaping experience is less dependent on the stable, unheated components and more dependent on preventing this chemical transformation. Minimizing exposure requires preventing overheating through proper device maintenance, using appropriate wattage settings, and ensuring the coil is always adequately saturated with e-liquid.