The liquid, often called e-liquid or vape juice, is a complex chemical solution that turns into an inhalable aerosol when heated by the electronic device. Vaping offers a smokeless alternative to traditional tobacco products. The specific composition of the liquid determines the user experience, from the intensity of the nicotine delivery to the size of the vapor cloud produced.
The Primary Base Components
The vast majority of vape liquid, typically 90% or more, is made up of two primary carrier liquids: propylene glycol (PG) and vegetable glycerin (VG). Both compounds are colorless, odorless alcohols that serve as the foundational solvents for the other ingredients. While both are generally recognized as safe for ingestion by food and drug authorities, their safety profile for repeated inhalation has not been fully established.
Propylene glycol is a thinner liquid that serves as a highly effective flavor carrier and produces a sensation in the throat known as a “throat hit,” which mimics the feeling of inhaling traditional cigarette smoke. Vegetable glycerin, conversely, is a much thicker, slightly sweet compound derived from vegetable oils. VG is primarily responsible for producing the dense, visible clouds of vapor that are characteristic of many vaping devices.
The ratio of these two components, expressed as a PG/VG percentage, significantly impacts the user experience and device compatibility. A higher PG ratio, such as 70/30, enhances flavor intensity and throat sensation, while a higher VG ratio, like 30/70, results in a smoother inhale and maximum vapor production. E-liquids with a balanced 50/50 ratio are common, offering a compromise between flavor delivery and cloud density, and are typically well-suited for smaller, low-power devices.
Nicotine and Flavoring Agents
Nicotine is the active, addictive component in most e-liquids. Its concentration is highly variable, ranging from nicotine-free (0 mg/mL) up to 50 mg/mL or more, and is typically measured in milligrams per milliliter (mg/mL) or as a percentage. Users select concentrations based on their previous tobacco use habits to satisfy cravings.
Nicotine exists in two main forms: freebase and nicotine salts. Freebase nicotine is more alkaline and can be harsh on the throat at higher concentrations, making it suitable for lower-strength liquids. Nicotine salts are created by treating the freebase nicotine with an acid, most commonly benzoic acid, which lowers the pH. This chemical alteration allows for the inhalation of much higher nicotine concentrations with a significantly smoother, less irritating throat sensation, leading to faster and more efficient nicotine delivery.
Flavoring Agents
Flavoring agents are intentional additives, with thousands of varieties available, from fruit to dessert profiles. These compounds are largely food-grade chemicals, approved for consumption in foods, but this “Generally Recognized As Safe” (GRAS) status does not extend to inhalation safety.
Research has consistently shown that many food-grade flavorings, such as diacetyl, are known respiratory hazards when inhaled, having been linked to severe lung diseases like bronchiolitis obliterans, or “popcorn lung.” The sheer complexity of these flavor mixtures is also a concern, as chemical reactions can occur between the flavorants and the PG/VG solvents, even before heating, to create new, unlisted compounds like acetals.
Hazardous Additives and Thermal Degradation Products
Harmful substances in the final aerosol fall into two categories: specific additives and thermal breakdown products. Unregulated e-liquids, particularly those containing THC, have sometimes been cut with Vitamin E Acetate. This viscous oil was strongly linked to the 2019 outbreak of e-cigarette or vaping product use-associated lung injury (EVALI), as it damages lung tissue when inhaled.
Heavy metals can leach into the e-liquid from the device’s heating coil, particularly in older or poorly constructed devices. Studies have found toxic metals, including lead, nickel, and chromium, in the aerosol. These metals are shed when the e-liquid contacts the metallic components, especially if the liquid is acidic or the device is used at high temperatures.
The heating process itself generates toxic chemicals that were not present in the original liquid. When the base liquids, PG and VG, are heated at high temperatures or with insufficient liquid on the coil (a “dry puff”), they undergo thermal degradation. This breakdown creates volatile carbonyl compounds, including formaldehyde, acetaldehyde, and acrolein. Formaldehyde and acetaldehyde are known carcinogens, while acrolein is a highly reactive irritant. Propylene glycol is generally more susceptible to this thermal decomposition than vegetable glycerin, leading to the generation of these harmful byproducts.