A typical vape contains four main ingredients: a liquid base of propylene glycol and vegetable glycerin, nicotine (or THC in cannabis products), and chemical flavorings. But that’s just what’s listed on the label. When the liquid is heated, additional compounds form in the aerosol you inhale, including metals from the heating coil and toxic byproducts created by high temperatures.
The Liquid Base: Propylene Glycol and Vegetable Glycerin
Nearly every e-liquid starts with two carrier liquids: propylene glycol (PG) and vegetable glycerin (VG). These are the substances that produce the visible cloud when you exhale. PG is thinner and produces a stronger “throat hit,” while VG is thicker and creates denser vapor. Most e-liquids blend the two in ratios ranging from 50/50 to 30/70 PG-to-VG, though some use pure VG.
PG evaporates roughly 2,000 times more readily than VG, which is why the ratio matters. A higher PG mix produces a more volatile aerosol that dissipates quickly. A higher VG mix creates heavier, lingering clouds. Neither ingredient is toxic to swallow in small amounts (both are common food additives), but inhaling them as a heated aerosol is a different story, because both break down into harmful compounds at high temperatures.
Nicotine: Two Very Different Forms
Most vapes contain nicotine, but the form varies significantly. Older-style e-liquids use freebase nicotine, which becomes harsh on the throat above about 18 mg/mL. That harshness kept concentrations relatively low for years.
Nicotine salt changed that. By adding an acid (most commonly benzoic acid) to freebase nicotine, manufacturers lower the pH of the liquid, making it far less irritating to inhale. The result is that nicotine salt e-liquids can reach 50 mg/mL or higher without feeling harsh. This is why devices like JUUL, which popularized nicotine salts, deliver nicotine so efficiently. A single pod at 50 mg/mL contains roughly as much nicotine as a pack of cigarettes. The benzoic acid itself contributes to the flavor profile and may affect how quickly nicotine absorbs into your bloodstream.
Flavoring Chemicals
Thousands of flavoring compounds are used in e-liquids, borrowed largely from the food industry. Many are considered safe to eat but have never been studied for inhalation safety. Three compounds in particular have raised alarms: diacetyl, acetyl propionyl, and acetoin. Diacetyl gives a buttery or creamy taste and is linked to a serious lung condition called bronchiolitis obliterans, sometimes called “popcorn lung” after factory workers who inhaled it developed the disease.
Some manufacturers removed diacetyl but continued using acetoin as a substitute. Research has since shown that acetoin spontaneously converts into diacetyl inside e-liquid over time, meaning vapers are still exposed. All three compounds are avoidable, and researchers have recommended manufacturers stop using them entirely, though no regulation requires it.
What Forms When the Liquid Is Heated
The ingredients in e-liquid are only part of the picture. When propylene glycol and vegetable glycerin hit the heating coil, they break down into a group of chemicals called carbonyls. The most concerning are formaldehyde, acetaldehyde, and acrolein. Formaldehyde is a known carcinogen. Acrolein irritates the nasal cavity and damages lung tissue. Both form at temperatures above 215°C (419°F), which many vape devices easily reach.
Temperature and voltage matter enormously here. One study found no detectable formaldehyde at 3.3 volts but measured 380 micrograms of formaldehyde-releasing compounds per 10 puffs at 5.0 volts. Glycerin is the bigger offender: at 318°C, each milligram of glycerin produced about ten times more formaldehyde than the same amount of propylene glycol. Higher wattage settings, longer puffs, and dry coils all push temperatures up and increase exposure to these byproducts.
Small amounts of volatile organic compounds also appear in the aerosol, including benzene (a known carcinogen) and toluene, though at levels measured in parts per billion.
Metals From the Heating Coil
The heating element inside a vape is typically a small metal coil, often made of nichrome (a nickel-chromium alloy). As it heats, trace amounts of those metals transfer directly into the aerosol you inhale. Studies have found chromium and nickel at higher concentrations in the aerosol than in the liquid sitting in the tank, confirming that the metals come from the coil itself during heating rather than from contamination of the e-liquid.
Copper and lead have also been detected, likely from solder joints and other metal components in the device. The amounts increase with more puffs and at higher power settings. These are not metals you want in your lungs at any concentration. Chronic low-level inhalation of nickel and chromium is associated with respiratory damage and increased cancer risk.
Trace Impurities in the Nicotine
Because the nicotine in e-liquids is extracted from tobacco, small amounts of tobacco-specific compounds can carry over. Researchers testing commercial e-liquids have found trace levels of tobacco-specific nitrosamines, a group of potent carcinogens. The concentrations are extremely low, measured in nanograms per gram, and far below what’s found in a cigarette. But their presence means e-liquid is not entirely free of tobacco’s most dangerous chemical fingerprints.
THC Vapes Contain Different Risks
Cannabis vape cartridges use an entirely different formulation. Instead of propylene glycol and vegetable glycerin, THC oil is the primary ingredient. Legitimate products from licensed dispensaries typically contain THC distillate with added terpenes for flavor. The serious concern with THC vapes comes from illicit or black-market products.
During the 2019 EVALI outbreak, which caused thousands of hospitalizations and dozens of deaths, the CDC identified vitamin E acetate as the primary culprit. This oily substance was used as a cutting agent to dilute THC oil in counterfeit cartridges. When inhaled, it coats the lungs and interferes with their ability to function. In one investigation, 52% of THC products submitted by hospitalized patients contained vitamin E acetate. None of the nicotine-containing products tested positive for it. The outbreak was overwhelmingly tied to unregulated THC cartridges, not standard nicotine vapes.
What’s Not on the Label
The FDA requires tobacco product manufacturers to report harmful and potentially harmful constituents, but enforcement has been inconsistent, and many products on the market have never received FDA authorization. Independent lab tests routinely find discrepancies between what’s listed on the label and what’s actually inside. Nicotine concentrations can be higher or lower than advertised. Flavoring chemicals may not be disclosed at all. And the byproducts created by heating are never listed, because they don’t exist until you use the device.
What you inhale from a vape is ultimately a combination of what the manufacturer put in the bottle and what the device creates in the moment: a cocktail of carrier liquids, nicotine, flavoring chemicals, metal particles, and thermal breakdown products that varies with every puff depending on your device, your settings, and how hot the coil gets.