A typical vape contains a liquid made from two base solvents, nicotine, and flavoring chemicals. When that liquid is heated, the aerosol you inhale also picks up toxic byproducts from the heating process and trace metals from the device itself. So “what’s in a vape” is really two questions: what’s in the liquid before you use it, and what’s in the aerosol after the device heats it up.
The Two Base Liquids
Every e-liquid starts with a blend of two colorless, odorless solvents: propylene glycol (PG) and vegetable glycerin (VG). These make up the bulk of the liquid, often 90% or more of the bottle. Each one does something different. Propylene glycol carries flavor more effectively and produces a sharper throat hit, closer to the sensation of smoking a cigarette. Vegetable glycerin is thicker, adds a slight sweetness, and is responsible for the visible cloud you exhale.
Most e-liquids use a blend of the two. A 50/50 split is one of the most common ratios, giving a balance of flavor, throat sensation, and vapor production. Liquids designed for large clouds push the VG ratio to 70%, 80%, or even higher, though flavor and throat hit drop off significantly at those levels. Liquids aimed at people transitioning from cigarettes tend to lean heavier on PG for a stronger throat hit.
Nicotine Types and Strengths
Nicotine in e-liquid comes in two forms: freebase nicotine and nicotine salts. Freebase nicotine is the older form, typically sold in concentrations of 3, 6, 12, or 18 milligrams per milliliter. At higher concentrations, freebase nicotine becomes harsh and uncomfortable to inhale.
Nicotine salts changed that. By adding a mild acid to nicotine, manufacturers created a form that’s smoother on the throat at much higher concentrations. Disposable vapes and pod systems commonly use nicotine salts at 50 or 60 mg/mL, several times stronger than a typical freebase e-liquid. This is the same chemical form of nicotine found naturally in tobacco leaves. The smoothness is what makes high-nicotine disposable vapes so easy to inhale, and so easy to develop a dependence on.
Flavoring Chemicals
The flavors in e-liquid come from food-grade chemical compounds, sometimes dozens of them in a single recipe. While these chemicals are approved for eating, “food-grade” doesn’t mean they’re safe to inhale. Your lungs and your digestive system process substances very differently.
One flavoring chemical that drew early concern is diacetyl, a buttery-tasting compound linked to serious lung damage when inhaled at high levels. That connection was first established in workers at microwave popcorn factories who breathed in diacetyl vapor on the job. A related compound, acetyl propionyl, has also been shown to damage lungs in animal studies. Some manufacturers have removed diacetyl from their products, but hundreds of other flavoring compounds remain in wide use without long-term inhalation safety data.
What Heating Creates
The liquid’s ingredient list doesn’t tell the whole story. When propylene glycol and vegetable glycerin hit the heating coil, they break down and produce new chemicals that weren’t in the bottle. The most concerning are a group of compounds called aldehydes: formaldehyde, acetaldehyde, and acrolein. All three are toxic, and all three are also found in cigarette smoke, where they contribute to cardiovascular and lung disease.
The amount of these byproducts depends heavily on how hot the coil gets. In lab testing, increasing the power output of a vape device from 11.7 watts to 16.6 watts multiplied formaldehyde production roughly sixfold and acetaldehyde production more than twentyfold. Glycerin breaks down into acrolein, while propylene glycol produces acetaldehyde and formaldehyde. This is why “dry hits,” where the wick runs low on liquid and the coil overheats, taste so harsh. That burning sensation is partly these aldehydes spiking to much higher levels.
Heavy Metals From the Coil
The heating element inside a vape is a small coil or strip of resistance wire, often made from metal alloys containing nickel, chromium, or other metals. A wick, usually organic cotton, draws liquid onto this coil. Because the e-liquid sits in direct contact with the metal, trace amounts of those metals leach into the liquid and end up in the aerosol.
Research has detected arsenic, chromium, nickel, and lead in vape aerosol. Both nickel and chromium are established inhalation carcinogens, meaning they can cause cancer when breathed into the lungs repeatedly over time. Copper, another metal found in some coils, has been shown to be particularly damaging to the cells lining the lungs when combined with certain flavoring compounds like ethyl maltol, a sweetener used in many e-liquids.
Volatile Organic Compounds
Beyond aldehydes and metals, vape aerosol contains volatile organic compounds (VOCs) including benzene and toluene. Both are established toxicants. Benzene is a known human carcinogen. These compounds appear at lower levels than in cigarette smoke, but they’re present, and they add to the overall chemical load your lungs process with each puff.
What Manufacturers Are Required to Disclose
In the United States, any vape product legally sold must go through the FDA’s premarket review process. As part of that application, manufacturers must list every ingredient by its chemical name, its function, its quantity, and its purity. They also have to test and report the constituents in the aerosol, including harmful compounds and reaction products from heating or metal leaching. The pH of the liquid and the specific form of nicotine (freebase or salt) must be disclosed to the FDA as well.
In practice, this means the FDA has detailed chemical profiles of authorized products. But most vapes actually on the market, particularly disposables, have never received FDA authorization. For those products, no one is verifying that what’s on the label matches what’s inside.