Propylene glycol (PG) is a synthetic organic compound that serves as a humectant and solvent in many consumer products. It is widely recognized as safe for ingestion and is found in foods, pharmaceuticals, and cosmetics. However, its presence in tobacco products is a matter of concern, as its function and the way it is consumed change between traditional cigarettes and modern vaping liquids. Propylene glycol is indeed present in both combustion and electronic nicotine delivery systems, but the health implications are closely tied to what happens to the compound when it is heated.
Propylene Glycol’s Function in Tobacco Products
Propylene glycol serves a distinct purpose in traditional combustible cigarettes, where it is classified as a tobacco additive. Its primary role is that of a humectant, maintaining the desired moisture content of the cut tobacco filler. Adding PG prevents the tobacco from drying out, which preserves the product’s freshness and texture. This moisture retention also affects how the cigarette burns and how flavor compounds are delivered to the user.
In electronic nicotine delivery systems, or e-cigarettes, PG is one of the two primary base ingredients, alongside vegetable glycerin (VG), that make up the e-liquid. PG acts as an effective solvent and carrier for both the nicotine and the various flavorings included in the liquid formulation. The compound is also physically thinner than VG, which some users report enhances flavor delivery and provides a desirable “throat hit” sensation similar to traditional smoking. When the e-liquid is heated by the device’s coil, the PG is aerosolized, creating the vapor that the user inhales.
Chemical Transformation During Heating
The safety of propylene glycol changes when it is subjected to the high temperatures involved in smoking or vaping. While PG is recognized as safe for oral consumption, heating it causes a process called thermal degradation, or pyrolysis. This chemical breakdown can start at temperatures as low as 133–175 °C when oxygen is available, and certainly occurs at temperatures above its boiling point of 188 °C, which are easily reached in both combustion and electronic devices.
When PG is heated past its stability point, it fragments into highly reactive carbonyls. These thermal degradation products include formaldehyde, acetaldehyde, and acrolein. Formaldehyde and acetaldehyde are known human carcinogens, while acrolein is a severe respiratory irritant. These toxic byproducts are generated through incomplete combustion or oxidation, where the PG molecule reacts with oxygen at high temperatures. The presence of metal heating elements, like those in e-cigarettes, can also significantly lower the temperature required for this decomposition to occur.
Inhalation and Health Effects
The inhalation of propylene glycol and its thermal breakdown products poses health risks to the respiratory system. Inhaling PG directly can lead to short-term, localized effects, such as irritation of the eyes and throat, which is a commonly reported symptom among e-cigarette users. This irritation is attributed to the hygroscopic nature of PG, meaning it absorbs moisture, which can cause a sensation of dry mouth and throat.
Health concerns are primarily linked to the inhalation of the toxic carbonyl compounds generated upon heating. Formaldehyde, acetaldehyde, and acrolein are known to cause inflammation and damage to lung tissue. Chronic exposure to these aldehydes increases the risk of developing respiratory diseases, including chronic obstructive pulmonary disease (COPD). Furthermore, formaldehyde and acetaldehyde are classified as carcinogens, meaning their inhalation is associated with an elevated risk of cancer.