Electronic cigarettes, commonly known as vapes, are a type of electronic nicotine delivery system (ENDS) designed to simulate smoking without burning tobacco. The core mechanism involves heating a liquid solution, referred to as e-liquid or vape juice, until it aerosolizes into a fine mist that the user inhales. This aerosol is often mistakenly called “water vapor,” but it is a complex mixture of chemicals derived from the heated liquid and the device itself. Understanding what is inhaled requires examining the unheated liquid components, additives, and byproducts created during the heating process.
Primary Components of the E-Liquid Base
The foundation of nearly all e-liquids consists of two primary carrier agents: Propylene Glycol (PG) and Vegetable Glycerin (VG). These clear, odorless liquids are classified as sugar alcohols and typically account for 80% to 95% of the total volume. While they are commonly found in food and cosmetics and are generally recognized as safe for ingestion, their long-term effects when inhaled are still under scientific review.
Propylene Glycol is the thinner liquid, playing a significant role in flavor delivery and providing the “throat hit” that mimics traditional cigarette smoke. Liquids with a higher PG ratio offer a more intense flavor experience but produce less visible vapor. Conversely, Vegetable Glycerin is a thicker liquid derived from vegetable oils, and its primary function is to generate dense, visible clouds of aerosol.
The ratio of these components, such as 50/50 or 70/30 (VG/PG), affects the user’s experience. A higher VG concentration results in a smoother inhale and larger vapor production. A higher PG content delivers a stronger sensation in the throat, requiring manufacturers to balance these properties based on device type and user preferences.
Nicotine and Flavoring Agents
Nicotine is the highly addictive component added to the e-liquid base, available in a wide range of concentrations, including nicotine-free options. Nicotine exists in two main forms: freebase nicotine and nicotine salts. Freebase nicotine is the traditional, purer form that is highly alkaline, which causes a harsh sensation in the throat at higher strengths. This typically limits concentrations to between 3 mg/ml and 18 mg/ml.
Nicotine salts are a newer formulation created by combining freebase nicotine with an organic acid, such as benzoic acid, to lower the pH. This adjustment makes the nicotine less irritating, allowing for much higher concentrations, often between 20 mg/ml and 50 mg/ml, to be inhaled comfortably. Nicotine salts are absorbed into the bloodstream more quickly, offering a fast and intense nicotine delivery that closely resembles that of a traditional cigarette.
Flavoring agents are synthetic food-grade chemicals, frequently esters, added to the base liquid to create thousands of available flavors, such as fruit, dessert, or candy profiles. While these chemicals are considered safe for ingestion, their safety when heated and inhaled is a significant concern. One widely discussed chemical is diacetyl, used to impart a buttery flavor, which has been linked to bronchiolitis obliterans, or “popcorn lung,” in workers who inhaled it. Other flavoring compounds, like 2,3-pentanedione and acetoin, are also used in e-liquids and have raised similar health concerns regarding lung function and damage to the airway’s cilia.
Chemical Byproducts and Metallic Contaminants
The aerosol inhaled by the user is chemically different from the unheated e-liquid due to the thermal degradation that occurs at the heating coil. When Propylene Glycol and Vegetable Glycerin are heated to high temperatures, they can break down into toxic carbonyl compounds. These compounds are not present in the original liquid and include known respiratory irritants and carcinogens.
The thermal breakdown of PG and VG specifically generates formaldehyde, acetaldehyde, and acrolein. Formaldehyde and acetaldehyde are classified as probable human carcinogens, while acrolein is a potent respiratory irritant and is also used as a weed killer. Propylene glycol is more susceptible to this thermal decomposition than vegetable glycerin, often leading to a greater quantity of volatile carbonyls in the resulting aerosol.
The heating element, or atomizer coil, within the device is another source of contaminants in the aerosol. These coils, often made from alloys like nichrome or kanthal, contain metals such as nickel, chromium, and iron. Microscopic metallic particles can leach from the coil and other internal components, like soldered joints, into the e-liquid and subsequently into the aerosol the user inhales.
Studies have detected toxic metals like nickel, chromium, lead, and manganese in the aerosol, sometimes at levels that exceed public health-based limits. These metallic contaminants are a concern because inhaling heavy metals can lead to systemic toxicity, affecting the lungs, cardiovascular system, and neurological development. The type of metal and its concentration can vary widely depending on the device’s design, the coil material, and the chemical composition of the e-liquid itself.