Propylene Glycol (PG) and Vegetable Glycerin (VG) are common liquids found in a vast array of consumer products, leading to questions regarding their safety. PG is a synthetic organic compound, while VG is derived from plant oils, most often coconut or palm. These compounds are extensively used as solvents and carriers in various formulations. The safety profile of these substances depends heavily on whether they are ingested, applied topically, or aerosolized and inhaled.
Defining Propylene Glycol and Vegetable Glycerin
Propylene Glycol is a synthetic compound belonging to the alcohol family, characterized by two hydroxyl groups. It is a clear, odorless liquid with a slightly sweet taste that functions primarily as a humectant and a solvent. Vegetable Glycerin (glycerol) is a naturally sourced, plant-derived compound with three hydroxyl groups, making it more viscous than PG. This naturally sweet, colorless liquid also acts as a humectant, a solvent, and an emollient for soothing skin.
Both PG and VG have a long history of use in non-inhalation products due to their functional properties. They are found as food additives, pharmaceuticals like syrups, and cosmetic bases such as lotions and toothpaste. Regulatory bodies, including the U.S. Food and Drug Administration (FDA), have classified both compounds as “generally recognized as safe” (GRAS) for ingestion and topical application.
Safety Profile for Ingestion and Topical Use
The established safety record of Propylene Glycol and Vegetable Glycerin is largely based on their consumption and skin application. When PG is ingested, the body efficiently metabolizes it, primarily breaking it down into lactic acid, a compound naturally processed by the body. Similarly, ingested VG is metabolized, typically turning into glucose and triglycerides. The non-toxic nature of these compounds at typical consumer product levels has been repeatedly confirmed by regulatory agencies.
Although PG and VG are generally well-tolerated, rare instances of allergic reactions or skin irritation are possible, particularly for individuals with high sensitivity. For topical use, VG is often considered a superior emollient, helping to soothe and soften the skin. The low toxicity of both compounds means they are not known to cause cancer, genetic damage, or interfere with reproductive health at the exposure levels found in standard foods and cosmetics.
Assessing Safety Through Inhalation
The safety profile of Propylene Glycol and Vegetable Glycerin changes significantly when they are heated and aerosolized for inhalation, a method of exposure not commonly used before the development of e-cigarettes. When PG and VG are heated to high temperatures, such as those that can occur in certain electronic cigarette devices, they undergo thermal degradation. This process breaks down the parent compounds and can lead to the formation of toxic carbonyl compounds.
Specific toxic byproducts, including formaldehyde, acetaldehyde, and acrolein, are generated when PG and VG degrade thermally. The amount of these toxic substances can increase substantially under “dry puff” conditions, where the heating element is not sufficiently saturated with liquid. Formaldehyde and acetaldehyde are known irritants and potential carcinogens, and their presence raises concerns about the safety of high-temperature aerosolization.
Inhaling PG and VG aerosols can cause immediate, short-term respiratory effects, with common reports including throat dryness, coughing, and irritation in the nose or eyes. Scientific studies have indicated that the aerosolized PG/VG mixture can damage the airway epithelia and have adverse effects on respiratory function. Exposure to these aerosols has been shown to reduce the function of ion channels that are crucial for hydrating the airways and maintaining effective mucociliary clearance.
Longer-term inhalation exposure has been linked to potential disruptions in lung function. Evidence suggests PG/VG aerosols can cause inflammation and impair the lung’s immune response. In vitro and animal studies have demonstrated that PG/VG exposure can disrupt the metabolism of airway epithelial cells and reduce the defensive properties of the lung barrier. The current scientific consensus is that while PG and VG are safe for ingestion, their repeated inhalation, especially when heated, presents a distinct and evolving set of risks that require further, long-term study.