Vape liquid contains four core ingredients: propylene glycol, vegetable glycerin, nicotine, and flavorings. That’s the basic recipe, but the full picture is more complicated. When those ingredients are heated, they can break down into new compounds. Metals can leach from the device itself. And some additives that are perfectly safe to eat have never been tested for safety when inhaled.
The Two Base Liquids
Propylene glycol (PG) and vegetable glycerin (VG) make up the bulk of any vape liquid, typically accounting for 80% to 95% of the mixture. Both are thick, colorless liquids widely used in food, cosmetics, and pharmaceuticals. In vape liquid, they serve different roles. PG is thinner and produces a stronger throat hit, the sensation at the back of your throat that mimics smoking. VG is thicker, produces larger visible clouds, and gives the vapor a slightly sweeter, smoother quality.
Most commercial e-liquids use a blend of the two. Common ratios include 50/50 PG/VG, 70/30, or even pure VG for people who find propylene glycol irritating. Research comparing these ratios found that higher PG concentrations increase nicotine delivery slightly but can make the liquid feel less pleasant. For most users, the ratio is a matter of personal preference rather than a major factor in how much nicotine they absorb.
Nicotine: Freebase vs. Salt
Most vape liquids contain nicotine, though some are sold nicotine-free. The nicotine comes in two forms: freebase and nicotine salt. Freebase nicotine has been the standard for years, typically sold at concentrations between 3 and 20 milligrams per milliliter. It becomes harsh on the throat at higher concentrations, which limits how strong it can practically be.
Nicotine salts changed that equation. By combining nicotine with a mild acid (usually benzoic acid), manufacturers create a form that causes less throat irritation when inhaled, allowing concentrations as high as 40 or even 50 mg/mL. Pod-style devices like JUUL popularized this format. At equal concentrations, nicotine salt and freebase deliver similar amounts of nicotine into the bloodstream, but because salt formulas can go much higher without discomfort, they make it easy to consume large amounts of nicotine quickly.
Flavorings and Their Unknowns
The flavoring chemicals in vape liquid are borrowed almost entirely from the food industry. Hundreds of different compounds are used to create flavors ranging from tobacco and menthol to mango, cotton candy, and crème brûlée. These compounds are approved for eating. They have not been evaluated for breathing.
This distinction matters more than most people realize. The FDA’s “Generally Recognized as Safe” (GRAS) label applies exclusively to substances added to food and evaluated for oral consumption. The Flavor and Extract Manufacturers Association, the industry group that manages the GRAS program, has clarified that its safety assessments cover only ingested ingredients, not inhaled ones. The FDA’s own guidance to e-cigarette manufacturers states plainly: “e-liquid is not food and is not intended for oral ingestion; therefore, the fact that some substances have been designated as GRAS for food does not mean that they are safe for inhalation.” Despite this, some vape brands still use GRAS labeling on their products.
One of the better-studied examples is diacetyl, a buttery flavoring compound linked to severe lung disease in factory workers who inhaled it while manufacturing microwave popcorn. The condition, bronchiolitis obliterans, became known as “popcorn lung.” A study testing 51 flavored e-cigarettes found diacetyl in 39 of them. It showed up not just in butter or caramel flavors but also in fruit, candy, and cocktail flavors. Two structurally similar compounds, 2,3-pentanedione (a diacetyl replacement) and acetoin, were also widely detected. No inhalation safety limits exist for the general public or for children.
Sweeteners and What They Produce
Many e-liquids contain added sweeteners to enhance taste. These include sucralose (the same artificial sweetener found in diet drinks), maltol, ethyl maltol, and natural sweeteners like stevia extracts. Of these, sucralose has drawn the most concern from researchers because of what happens when it’s heated.
When vape liquid containing sucralose is heated by the coil, the sucralose molecule breaks apart. This produces several types of harmful byproducts: aldehydes (including acetaldehyde and acrolein), organochlorine compounds, and chloropropanols. Concentrations of sucralose as low as 0.24% by weight in e-liquid were enough to significantly increase the production of these degradation products. The breakdown also releases hydrochloric acid and chloride into the aerosol. Beyond health concerns, sucralose is notorious among vapers for gunking up coils quickly, which is a visible sign of this same thermal decomposition happening on the heating element.
What Forms When the Liquid Is Heated
Even without sweeteners or flavorings, heating the base liquids themselves generates new compounds. Vegetable glycerin, in particular, breaks down into reactive aldehydes when vaporized. This process was first described scientifically in the 19th century, and modern research confirms it happens inside e-cigarettes. The key compounds formed include formaldehyde, acetaldehyde, and acrolein, all of which are toxic when inhaled. An intermediate compound called glycidol, which is itself harmful, forms as a first step before converting into formaldehyde or acrolein.
How much of these compounds you’re exposed to depends heavily on temperature. At normal operating temperatures, the amounts are far lower than in cigarette smoke. But when a coil overheats, whether from high wattage settings, a dry wick, or chain vaping, production spikes dramatically. This is the “dry hit” that experienced vapers learn to avoid, and it represents a real increase in exposure to toxic aldehydes.
Testing of e-cigarette aerosols has also detected volatile organic compounds including benzene and toluene, both known carcinogens. These were found at low levels, parts per billion, and not consistently across all products. Benzene appeared more frequently than toluene in tested samples.
Metals From the Device Itself
Vape liquid picks up metals from the heating coil and other device components, and this contamination increases the longer the liquid sits in contact with the coil. Testing of 17 e-liquid samples found nickel and chromium in every sample after coil contact. Lead was present at lower levels in many samples. Cadmium was rarely detected above measurable thresholds.
The amount of metal leaching depends on the coil material, the chemical properties of the liquid (including its pH, nicotine concentration, and whether it uses freebase or salt nicotine), and how long the liquid has been in the tank. Some coil and liquid combinations produced nickel concentrations over 12,000 micrograms per kilogram and chromium levels above 800 micrograms per kilogram. These metals transfer into the aerosol you inhale.
Illicit Additives and the EVALI Crisis
In 2019, a wave of severe lung injuries swept across the United States, eventually affecting thousands of people. The condition was named EVALI (e-cigarette or vaping product use-associated lung injury). Investigators identified vitamin E acetate as the primary culprit. It was found in the lung fluid of 94% of EVALI patients tested, 48 out of 51 cases across 16 states. It was not found in any healthy comparison subjects.
Vitamin E acetate is an oily compound that was being used as a thickening agent and cutting agent in black-market THC vape cartridges. It made diluted THC oil look and feel like a higher-quality product. When inhaled, it coats the lungs and interferes with their function. The outbreak was traced almost entirely to illicit THC products rather than commercial nicotine e-liquids, but it revealed how unregulated additives can turn vape products into something far more dangerous than their labeled ingredients suggest.
What Manufacturers Are Required to Disclose
In the United States, any e-liquid legally sold must go through the FDA’s premarket tobacco product application process. This requires manufacturers to list every ingredient by its chemical name and CAS number, state the function of each ingredient, report its quantity, and identify its supplier and purity grade. They must also test and report on harmful and potentially harmful constituents in both the liquid and the aerosol it produces, including reaction products that form during heating or aging.
This level of disclosure goes to the FDA, not to you. What appears on the product label is far less detailed. Most commercial e-liquids list their PG/VG ratio, nicotine strength, and flavor name. The specific flavoring chemicals, sweeteners, and other minor ingredients are rarely itemized on packaging. If you want to know exactly what’s in a particular product, the label alone won’t tell you.